Materials 2015, 8, 5922-5932; doi:10.3390/ma8095283
OPEN ACCESS
materials ISSN 1996-1944 www.mdpi.com/journal/materials Article
Light Absorption Enhancement of Silicon-Based Photovoltaic Devices with Multiple Bandgap Structures of Porous Silicon Kuen-Hsien Wu * and Chong-Wei Li Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, No. 1, Nan-Tai Street, Yungkang Dist., Tainan 710, Taiwan; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-6-253-3131 (ext. 3627); Fax: +886-6-234-2912. Academic Editor: Teen-Hang Meen Received: 29 June 2015 / Accepted: 1 September 2015 / Published: 7 September 2015
Abstract: Porous-silicon (PS) multi-layered structures with three stacked PS layers of different porosity were prepared on silicon (Si) substrates by successively tuning the electrochemical-etching parameters in an anodization process. The three PS layers have different optical bandgap energy and construct a triple-layered PS (TLPS) structure with multiple bandgap energy. Photovoltaic devices were fabricated by depositing aluminum electrodes of Schottky contacts on the surfaces of the developed TLPS structures. The TLPS-based devices exhibit broadband photoresponses within the spectrum of the solar irradiation and get high photocurrent for the incident light of a tungsten lamp. The improved spectral responses of devices are owing to the multi-bandgap structures of TLPS, which are designed with a layered configuration analog to a tandem cell for absorbing a wider energy range of the incidental sun light. The large photocurrent is mainly ascribed to an enhanced light-absorption ability as a result of applying nanoporous-Si thin films as the surface layers to absorb the short-wavelength light and to improve the Schottky contacts of devices. Experimental results reveal that the multi-bandgap PS structures produced from electrochemical-etching of Si wafers are potentially promising for development of highly efficient Si-based solar cells. Keywords: porous silicon; multi-layer; multi-bandgap; photoresponse; photovoltaic device; solar cell
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1. Introduction Since the report of room temperature photoluminescence in porous silicon (PS) [1], this material has been extensively investigated for the development of optoelectronic devices. PS is constituted by a nanocrystalline skeleton (quantum sponge) immersed in a network of pores and can be obtained simply by electrochemical etching of silicon in aqueous hydrofluoric (HF) [2,3]. Due to its many special characteristics such as extremely large surface-area-to-volume ratio, modulated energy bandgap, absorption spectrum within the solar irradiation, surface roughening with low reflection losses, and high photoconductivity, PS is very suitable for applications in photovoltaic devices [4]. In particular, PS structures have been intensively introduced in the device design of solar cells to increase the cell’s conversion efficiency [5–7]. In most of the related research, PS films acted as the surface texturing and anti-reflection (AR) layers to reduce the light-reflection losses or as the surface passivation layers to decrease surface carrier recombination velocity. On the other hand, PS can potentially serve as the parts of the active layer to effectively generate photo-induced carriers because of its wide energy bandgap and high photoconduction in response to solar irradiation [8,9]. However, a primary drawback of PS for use of the light-absorption layer in a photovoltaic device is its comparatively narrow spectral photoresponses. Therefore, to get higher and broader spectral photoresponses within the spectrum of the solar irradiation is a primary requirement for the fabrication of efficient PS-based solar cells. An important scheme to increase the light absorbance range of devices is using cell structures with multi-junctions (i.e., the “tandem cell” structures). Tandem cells are solar cells with multiple p–n junctions made of different semiconductor materials with different bandgap energy. Because each semiconductor material is sensitive to a different part of the solar spectrum, tandem cells can obtain enhanced overall sunlight photoresponses and achieve higher conversion efficiency. However, these tandem cells are usually not suitable for mass production of silicon (Si) solar cells since their fabrication processes are so complicated and are not Si-compatible [10,11]. A promising Si-based tandem-like structure is the PS multilayer (PSML) structure composed of stacked PS layers with different porosity. In the past, PSML had received much interest due to its tunable optical and electronic properties [4]. Because the refractive index of PS can be tuned through the porosity using controlled electrochemical-etching conditions, PSML with modulated refractive index can be fabricated by stacking PS layers of different porosity. These PSML structures can be used to make optical filters, waveguides, resonating cavities and mirrors for device applications [12,13]. Most of the recent studies about applications of PSML in solar cells mainly focused at the optimization of AR coatings [14,15]. There are few reports on cells with light-absorption areas based on PSML structures. Since the optical bandgap of PS can be tuned by its porosity, it is feasible to broaden the photoresponse spectra of Si photovoltaic devices by use of multi-bandgap PSML structures. Based on the fact that a PS layer of higher porosity has higher bandgap energy, a PSML structure composed of triple stacking layers with high-, medium- and low-porosity (from top to bottom) will absorb the sun light in the short-, medium- and long-wavelength bands respectively. Such a “tandem-like” PSML structure can potentially harvest a broader range of the sun’s energy to make a Si-based solar cell more efficient. Utilization of PSML as the light-absorption region of a device will face a dilemma in choosing the porosity of the upmost PS layer. To absorb light of shorter wavelength, the upmost PS layer must be designed with higher porosity. However, a PS layer with higher porosity has higher resistance, lower
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hardness and higher density of defects [4,5]. Therefore, using a high-porosity PS surface layer will result in high series resistance and degradation of metal contacts. To solve this problem, we proposed a special nanoporous-Si (NPS) thin film for serving as the surface active layer. The NPS layer associated with two stacked PS layers of different porosity will construct a triple-layered PS (TLPS) structure with multiple bandgap energy. Such a TLPS structure can be produced on Si substrates by tuning the etching parameters in an electrochemical anodization process. In this paper, we report the preparation and characterization of TLPS structures and demonstrated the improvements on the spectral photoresponses and light-absorption capability of Si-based photovoltaic devices fabricated with the developed TLPS structures. 2. Experimental Section 2.1. Formation of Triple-Layered Porous Silicon N+ -type (100) silicon wafers with resistivity of 5–10 mΩ¨ cm were used as the starting substrates. PS layers were prepared on Si substrates by electrochemical etching in an anodization process with HF-methanol etching solutions in a Teflon cell with a Pt electrode. To generate a TLPS structure, the electrochemical-etching parameters were successively changed in the anodization process. Different HF concentration of etching solution, etching current density and etching time were used for formation of different PS stacked layers with varied morphology and porosity. Table 1 lists the etching parameters used in the anodization process for preparation of the triple stacked PS layers. To obtain a NPS thin film as the upmost (the first) layer, lower etching current density of 10 mA/cm2 was used during the anodic process. The HF concentration of the etching solution is the key parameter to control the sizes and distribution of Si-nanocrystals in the NPS films. In order to get smaller sizes of Si-nanocrystals and higher reproducibility, the concentration of HF was set as 30% of the etching solution in this work. Table 1. Electrochemical-etching parameters used in the anodization process for preparation of the triple stacked PS layers. Etching Parameters
First Layer (Upmost)
Second Layer (Middle)
Third Layer (Bottom)
HF Concentration Etching Current Density (mA/cm2 ) Etching Time (s)
30% 10 60
10% 60 60
10% 30 90
To create the following PS layers, HF concentration of the etching solution was reduced insitu to 10% and the etching current density was increased to 60 and 30 mA/cm2 for formation of a high-porosity PS (HP-PS) layer as the second layer and a low-porosity PS (LP-PS) layer as the third layer, respectively. Because etching only occurs at the PS-Si interface, the previously formed PS layer will be left unaffected. After the formation of PS layers, samples with the prepared TLPS were rapid-thermally annealed in N2 at 800 ˝ C for 60 s to stabilize the PS structures. Figure 1 illustrates the scanning-electron-microscope (SEM) photo-image of the cross-sectional view of the produced TLPS structure on a Si substrate. It can be observed that a triple-layered structure with an NPS, an HP-PS and a LP-PS layer stacking from top to bottom are formed on a Si substrate. The thickness of each layer is about 0.85 µm, 1.85 µm and 1.85 µm, respectively. The upmost NPS layer has a different morphological structure from that of the HP-PS or the LP-PS layer. The NPS layer is
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aparticulate particulatethin thin film consisting uniformly distributed Si-nanoparticles (SNP’s), while the HP-PS particulate thin film consisting ofof uniformly distributed Si-nanoparticles (SNP’s), while the HP-PS HP-PS and film consisting of uniformly distributed Si-nanoparticles (SNP’s), while the and and LP-PS layersare are with with structures micro-rods and micro-holes. For the the LP-PS layers are structurescontaining containingpipe-shaped pipe-shaped micro-rods and micro-holes. micro-holes. the LP-PS layers structures containing pipe-shaped micro-rods and determining the porosity of the intermediate PS layers, individual PS layer waswas alsoalso prepared by For determining determining the porosity porosity ofthree the three three intermediate PS layers, layers, individual PS layer layer was also prepared For the of the intermediate PS individual PS prepared use of the etching parameters listedlisted in Table 1. The of theof HP-PS and LP-PS layer by use use of same the same same etching parameters listed in Table Table 1.porosity The porosity porosity ofNPS, the NPS, NPS, HP-PS HP-PS and LP-PS LP-PS by of the etching parameters in 1. The the and is estimated by weight measurements to be about 20%, 40%, layer estimated by weight weight measurements to be be about about70% 20%,and 70% andrespectively. 40%, respectively. respectively. layer isis estimated by measurements to 20%, 70% and 40%,
Figure The SEM SEM photo-image photo-image of the cross-sectional Figure 1. 1. The photo-image of of the the cross-sectional cross-sectional view view of of the the formed formed triple-layered triple-layered Figure 1. view of the formed triple-layered porous-silicon porous-silicon(TLPS) (TLPS)structure structureon onaaaSi Sisubstrate. substrate.The Thetriple triplelayers layersinclude includeaaananoporous-Si nanoporous-Si porous-silicon (TLPS) structure on Si substrate. The triple layers include nanoporous-Si (NPS), (NPS),aaahigh-porosity high-porosityPS PSand andaaalow-porosity low-porosityPS PSlayer layerfrom fromtop topto tobottom. bottom. (NPS), high-porosity PS and low-porosity PS layer from top to bottom.
2.2. Fabrication Fabricationof ofPhotovoltaic PhotovoltaicDevices Devices 2.2. Fabrication of Photovoltaic Devices 2.2. Afterthe thepreparation preparationof ofTLPS, TLPS,aluminum aluminum(Al) (Al)was wasdeposited depositedon onthe theback backside sideof ofthe thesample sampleand andthen then After was deposited on the back side of the ˝ sinteredatat350 350°C °C to make make the the back back electrode electrodeof ofOhmic Ohmiccontact. contact. Thereafter, Thereafter,grid gridAl Almetals metalswere wereformed formed sintered C to onthe thefront frontside sideof ofthe thesample sampletotoserve serveas asthe thefront frontelectrodes electrodesof ofSchottky Schottkycontact. contact. Figure Figure 22 shows showsthe the on serve as the front electrodes of Schottky contact. schematic structure structure of of the the developed developed Si-based Si-based photovoltaic photovoltaicdevice devicewith withthe theTLPS TLPSserving servingas asthe themain main schematic developed photovoltaic device with the TLPS serving as the of the the light-absorption region. region. By By means means of of the the Tauc’s Tauc’s plots plots [16], [16], the the optical optical bandgap bandgap energy energy(Eg (Egopt opt)) of light-absorption By means the region. energy (Eg opt individualas-prepared as-preparedPS PSlayer layer isis estimated estimated to to be be 2.5 2.5 eV, eV,1.8 1.8eV, eV,and and1.6 1.6eV eVfor forthe theNPS, NPS,the theHP-PS, HP-PS, individual eV, individual as-prepared eV, and 1.6 eV for the NPS, andthe theLP-PS LP-PSlayer, layer,respectively. respectively.Like Likeaatandem tandemcell, cell,the thedeveloped developeddevice devicehas hasaalight-absorption light-absorptionregion region and respectively. Like device has withmultiple multipleoptical opticalbandgaps bandgapsand andthe thebandgap bandgapof ofeach eachlayer layerin inthe thelight-absorption light-absorptionzone zonedecreases decreasesfrom from with with multiple optical bandgaps and the bandgap of each layer in the light-absorption zone decreases from thedevice’s device’ssurface surfaceto toits itsinterior. interior. the device’s surface to its interior. the
Figure 2. Schematic structure of the developed Si-based photovoltaic device with a Figure 2. 2. Schematic Schematic structure structure of of the the developed developed Si-based Si-based photovoltaic photovoltaic device device with with aa Figure triple-layered porous-Si (TLPS) structure as the light-absorption region. triple-layered porous-Si porous-Si (TLPS) (TLPS) structure structure as as the the light-absorption light-absorption region. region. triple-layered
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3. 3. Results Results and and Discussion Discussion Figure Figure 3a 3a shows shows the the zoom-in zoom-in SEM SEM photo-image photo-image of of the theprepared preparedNPS NPSlayer layerininFigure Figure1.1. ItIt can can be be observed that the NPS layer has a special film-like structure, unlike the beneath HP-PS layer which observed that the NPS layer has a special film-like structure, unlike the beneath HP-PS layer which has has common morphology mesomicro-PSlayers layersgenerated generatedfrom from electrochemical-etching electrochemical-etching of the the common morphology of of mesoorormicro-PS of Si Si wafers. wafers. Figure Figure 3b 3b shows shows aa high-magnification high-magnification SEM SEM image image of of the the surface surface of of the the NPS NPS film. film. As As shown shown in in this figure, the NPS film is embedded with uniformly distributed Si-nanocrystals and nano-sized holes. this figure, the NPS film is embedded with uniformly distributed Si-nanocrystals and nano-sized holes. Average these Si-nanocrystals Si-nanocrystals are are estimated estimatedtotobe beabout about3–10 3–10nm nminin diameters. According to Average sizes sizes of of these diameters. According to the the reports of Lee al. [17], the bandgap energy of material a PS material with quantum-sized Si-crystallites reports of Lee et al.et[17], the bandgap energy of a PS with quantum-sized Si-crystallites of 3 nmofis 3about nm is2.5 about 2.5 eV. This is close to that was predicted by Suemune al. conforms [18] and conforms eV. This value is value close to that was predicted by Suemune et al. [18]etand to the Egto opt the Eg value of the NPS film measured from the Tauc’s plots in this work. The Eg of the beneath opt opt value of the NPS film measured from the Tauc’s plots in this work. The Egopt of the beneath HP-PS HP-PS layerporosity with porosity of 70% andLP-PS the LP-PS layer porosity 30% gotfrom fromthe theTauc’s Tauc’s plots plots is layer with of 70% and the layer withwith porosity of of 30% got is about eV,eV, respectively. These values are consistent with those from thefrom optical about 1.8 1.8eV eVand and1.61.6 respectively. These values are consistent with obtained those obtained the absorption measurements in other studies [17]. optical absorption measurements in other studies [17].
Figure 3. (a) The zoom-in SEM photo-image of the prepared NPS layer shown in Figure 1; Figure 3. (a) The zoom-in SEM photo-image of the prepared NPS layer shown in Figure 1; (b) A high-magnification SEM image of the surface of the prepared NPS film. (b) A high-magnification SEM image of the surface of the prepared NPS film. Figure Figure 44 shows shows the the spectral spectral photoresponses photoresponses of of the the individual individual PS PS (including (including NPS, NPS, HP-PS HP-PS and and LP-PS) LP-PS) layer layer prepared prepared on on aa Si Si substrate. substrate. The photoresponses were were measured measured with with aa TRIAX-320 TRIAX-320 spectrometer spectrometer (HORIBA, (HORIBA, Kyoto, Kyoto, Japan). Japan). To exclude exclude as as much much as as possible possible the the influence influence of of light light absorption absorption in in the the Si Si substrate, substrate, the the responsivity responsivity was was measured measured by by using using planar planar electrodes electrodes formed formed on on the the surface surface of of the the PS PS layer, layer, as as indicated indicated in in the the inserted inserted drawing. drawing. For comparison purposes, purposes, photoresponses photoresponses of of the the bare bare Si Si substrate substrate are are also also shown shown in in the the figure. figure. As shown shown in in the the figure, figure, the the photoresponses photoresponses of of the the LP-PS LP-PS layer layer mainly mainly appear appear within withinthe theranges rangesofofwavelength wavelengthbetween between650 650nm nmand and850 850nm, nm,which whichcorresponds corresponds to to the the 22 optical band band from from red of 62 62 mA/cm mA/cm at at 750 750 nm. nm. The The optical red to to infra-red infra-red (IR), (IR), and and has has aa maximum maximum responsivity responsivity of LP-PS layer layer displays displays the the common common photoresponsive photoresponsivecharacteristics characteristicsof ofaaPS PSmaterial, material,which which has has aa narrower narrower LP-PS and blue-shifted blue-shifted response response spectrum spectrum with with smaller smaller responsivity responsivityas ascompared comparedtotothat thatofofthe thebare bareSi. Si. The The and responses of of the the HP-PS HP-PS device device shift shift further further toward toward the the band band of of shorter-wavelength shorter-wavelength between between 550 550 nm nm responses 2 2 (green) and and 700 700 nm nm (red) (red) with with aa much much small small peak peak responsivity responsivity of of 55 mA/cm mA/cm at at 670 670 nm. nm. We We thought thought the the (green) change in in the the region region of of the the spectral spectral responses responses isis due due to to the the higher higher porosity porosity of of layers layers which which result result in in aa change larger Eg Egopt The low low responsivity responsivity is is ascribed ascribed to to not not only only its its high high resistivity resistivity and and the the high high defect defect density, density, opt.. The larger but also the large amount of the surface states and the high metal-contact resistance. By improving the
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Materials 2015, 8 6 but also the large amount of the surface states and the high metal-contact resistance. By improving the metal contacts contacts and PS can can be be metal and the the surface surface passivation passivation on on the the PS PS layer, layer, the the responsivity responsivity of of high-porosity high-porosity PS largely enhanced enhanced [19,20]. [19,20].Particularly Particularlyininthis thiswork, work,the theHP-PS HP-PSlayer layer was was made made under under the the NPS NPS surface largely layer in the the fabricated fabricated TLPS-based device. As As aa result, result, the the surface surface of of the the HP-PS HP-PS isis effectively effectively passivated passivated by the NPS layer and the quality of metal-contact is thus not degraded by the HP-PS layer since the electrodes are formed on the NPS layer that has a surface surface with with better better quality. quality. Therefore, the actual photoresponsivity of such a middle HP-PS layer in the TLPS structure is expected to be higher than that of an individual HP-PS layer as a surface layer on a Si substrate.
Figure 4. The spectral photoresponses of the individual PS (NPS, HP-PS or LP-PS) layer Figure 4. The spectral photoresponses of the individual PS (NPS, HP-PS or LP-PS) layer prepared on a Si substrate, measured with planar electrodes on the PS surface as indicated in prepared on a Si substrate, measured with planar electrodes on the PS surface as indicated in the inserted drawing. For comparison purposes, the measured data of the bare Si substrate the inserted drawing. For comparison purposes, the measured data of the bare Si substrate are also shown. are also shown. As shown shownininFigure Figure4, 4, NPS not only exhibits short-wavelength responses As thethe NPS layerlayer not only exhibits much much higherhigher short-wavelength responses within 2 2 within 400–600 nm blue in theband blue(with band a(with a maximum responsivity 72 mA/cm at nm), 500 nm), but also at 500 but also has 400–600 nm in the maximum responsivity of 72of mA/cm has apparent responses in the band red-to-IR. supposedthat thatthe thegreatly greatly enhanced enhanced blue-shifted blue-shifted apparent responses in the band of of red-to-IR. It Itis issupposed responses come from the SNP’s embedded in the NPS films, which has Eg Egopt larger than 2 eV due to the The red-to-IR red-to-IR responses responses are are ascribed ascribed to to the the light light absorption in the micro-sized Si quantum-size effects. The Besides, itit is is interesting interesting to to find find that the value of crystals around the SNP’s and the beneath Si substrate. Besides, the peak responsivity responsivity of the NPS, HP-PS, and LP-PS layer corresponds corresponds respectively respectively to a photon energy 2.48 eV, eV, 1.77 eV, and 1.65 eV, single PS PS layer layer respectively. respectively. of 2.48 eV, which which is is close to the value of Egopt opt of the single This fact indicates that the Egopt its photoresponses. photoresponses. opt of a PS layer has significant influences on its spectral photoresponses photoresponses of the photovoltaic photovoltaic devices with a TLPS structure on Figure 5 illustrates the spectral a Si substrate.
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Figure 5. The spectral photoresponses of the developed photovoltaic devices based on the Figure 5. The spectral photoresponses of the developed photovoltaic devices based on the TLPS TLPS structure structure and and the the individual individual PS PS (NPS, (NPS, HP-PS HP-PS or or LP-PS) LP-PS) layer. layer. The The summed summed data data (blue dotted line) were obtained by directly summing the three individual responses of (blue dotted line) were obtained by directly summing the three individual responses of the the PS layer in Figure 4, while the summed (with fitting) data (red dotted line) were obtained PS layer in Figure 4, while the summed (with fitting) data (red dotted line) were obtained by by using a 10-fold responses of HP-PS. The spectrum of solar irradiation is also shown in the using a 10-fold responses of HP-PS. The spectrum of solar irradiation is also shown in the figure figure for for comparison comparison purposes. purposes.
It is is obvious than thatthat of of an It obvious that that the the TLPS-based TLPS-baseddevice deviceexhibits exhibitsa abroader broaderphotoresponse photoresponsespectrum spectrum than individual PS PS layer andand thethe bare Si Sisubstrate. importantly, thethe developed an individual layer bare substrate.Most Most importantly, developeddevice deviceachieved achieveda of the the solar solar abroadened broadenedand andflat flatphotoresponse photoresponsespectrum spectrumthat that matches matches much much more more closely closely to to that that of irradiance. That That is is aa preferable preferable property property for for development development of of highly highly efficient efficient solar solar cell. cell. From From the the spectral spectral irradiance. distribution of ofphotoresponses photoresponsesofofthe thethree threeindividual individual layer in Figure is reasonable to suppose distribution PSPS layer in Figure 4, it4,isitreasonable to suppose that thatoverall the overall responses of a TLPS are primarily composed the generated responses the the responses of a TLPS devicedevice are primarily composed of theofgenerated responses in the in three three PS layers of a structure. TLPS structure. Tothis verify this assumption, wethe plotted the responses summed responses PS layers of a TLPS To verify assumption, we plotted summed (denoted (denoted as “Summed”) that wereby obtained directlythe summing the three responses individualofresponses of the as “Summed”) that were obtained directlyby summing three individual the PS layer in PS layer Figure 4. responses The shown responses from the datadown were twofold scaled down twofold for based curve Figure 4. in The shown from the summed datasummed were scaled for curve fitting, fitting, based fact thatofthe individual PSthan layerthat is smaller thanIt that of a TLPS. on the fact thaton thethe resistance theresistance individualofPSthe layer is smaller of a TLPS. is apparent that It is apparent that the summed response spectra bend down between 550 nm and 750 nm as a result of the summed response spectra bend down between 550 nm and 750 nm as a result of the relatively small the relatively small values of the individual HP-PS layer. However, expected to exhibit values of the individual HP-PS layer. However, the HP-PS is expectedthe to HP-PS exhibit is higher responses as a higher layer responses a middle layer TLPS structure the passivation of the response NPS layer. middle in theas TLPS structure dueinto the the passivation of thedue NPStolayer. By using a 10-fold of By using 10-fold response of HP-PS summing, the plotted summed response spectra (denoted as HP-PS forasumming, the plotted summedfor response spectra (denoted as “Summed (with fitting)”) become “Summed fitting)”) become flatter and more fitting to that of asupport TLPS-based device. These much flatter(with and more fitting to thatmuch of a TLPS-based device. These results the expectation that results support the layer expectation that the middle HP-PS layer than can generate photoresponses the middle HP-PS can generate higher photoresponses those of higher a surface HP-PS layer than and those of the a surface HP-PSthat layer indicatewide-band the assumption that the measured wide-band photoresponses indicate assumption theand measured photoresponses come from a synergistic function come from a synergistic function of the three PS layers in the TLPS structure is valid. of the three PS layers in the TLPS structure is valid. Furthermore, to to check check whether whether the the multilayered multilayered PS PS structure structure on on top top of of the the Si Si substrate substrate causes causes an an Furthermore, interference effect effect on on the interference the photoresponses photoresponses of of the the TLPS TLPS device, device,the thespectral spectralreflectance reflectanceofofthe theTLPS TLPSonona the average average aSiSisubstrate substratewas wasmeasured measuredand andshown shown inin Figure Figure 6. 6. From Fromthis thisfigure, figure, we we can can find find that that the reflectivity of of samples samples with that of of thethe bare Si substrate in the reflectivity with TLPS TLPS on onSi Siisisabout abouttwo twotimes timeslower lowerthan than that bare Si substrate in
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the visible band. verifies TLPS structureenhanced enhancedthe thelight lightabsorption. absorption.There Thereare are no no obvious obvious visible band. ThatThat verifies thethe TLPS structure interference patterns patterns appearing appearing in in the the spectral spectral reflectance. reflectance. It It indicates indicates that that the the layered layered structure structure of of TLPS TLPS interference in this work has a negligible interference effect on the generation of the spectral responses. responses. Therefore, the improved spectral responses of the TLPS-based device are mainly owing to the multi-bandgap TLPS structures in which different layers with different Egopt absorb different portions of the incident light and contribute all together to the whole photoresponses.
Figure 6. The spectral reflectance of the TLPS structure on a Si substrate and the bare Si substrate. Figure 6. The spectral reflectance of the TLPS structure on a Si substrate and the bare Si substrate. In aa tandem tandem cell, cell, layers layers with with different different bandgap bandgap are are stacked stacked on on top top of of one one another another with with the the highest highest In bandgap layer layer upmost. upmost. Since Since layers layers with with higher higher bandgap bandgap will will absorb absorb light light of of shorter shorter wavelength, wavelength, the the bandgap device’s conversion is optimally optimally device’s conversion efficiency efficiency can can be be largely largely increased increased if if bandgap bandgap energy energy of of each each layer layer is chosen. According Accordingtotorelated related researches, light-absorption spectra of with PS with higher porosity usually chosen. researches, light-absorption spectra of PS higher porosity usually shift shift towards shorter wavelength because of the larger bandgap energy of PS [21]. These results accorded towards shorter wavelength because of the larger bandgap energy of PS [21]. These results accorded with the the experimental experimental ones ones in in this this work. In other other words, words, PS PS layers layers with with larger larger bandgap bandgap energy energy would would with work. In absorb light light of of shorter shorter wavelength. wavelength. For For the the developed developed TLPS TLPS devices, devices, the the Eg Egopt value of of each each layer layer (from (from opt value absorb surface to to the the interior) interior) in in the the light-absorption light-absorption zone zone was was 2.5 2.5 eV eV (NPS), (NPS), 1.8 1.8 eV eV (HP-PS), (HP-PS), 1.6 1.6 eV eV (LP-PS) (LP-PS) surface and 1.1 1.1 eV eV (bulk (bulk Si), Si), respectively. respectively. Since Since these these Eg Egopt values spanned spanned the the main main ranges ranges of of photon photon energy energy opt values and in the the solar solar spectrum, spectrum, the the TLPS TLPS devices devices exhibited exhibited broadband broadband photoresponses photoresponses and and were were able able to to harvest harvest in more solar solar energy. energy. more To assess the influences influences of the improved improved photoresponses the performance performance of of the the photovoltaic photovoltaic To assess the of the photoresponses on on the devices, photo- and dark current-voltage characteristics of a TLPS-based device were measured and plotted in in Figure Figure7. 7. For Forcomparison comparisonpurposes, purposes,similar similardevices devicesbased basedon onaasingle singlePS PS(NPS, (NPS, LP-PS LP-PS or or HP-PS) plotted layer with a comparable thickness of about 4.5 µm m were also fabricated and measured. The photocurrent 2 of devices was measured under an incident irradiation of 100 mW/cm by using a 10-W tungsten lamp emitting light light of of wavelength wavelength ranging rangingfrom from300 300nm nmtoto1100 1100nm nmand anda apeak peakwavelength wavelengthof of550 550 nm. nm. with emitting The reason for choosing such a tungsten lamp as the light source is that it has an irradiation spectrum As observed observed inin the the figure, figure, the the TLPS-based TLPS-based device device obtains obtains the largest similar to that of the sun. As photocurrent and smallest dark current among these fabricated devices, indicating it has the most significant photoconduction property. property. Because the photoresponse spectrum of a TLPS device matched much more closely to that of the incident light, the higher photocurrent is mainly attributed to its better
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light-absorption The increase increase in in the the photocurrent light-absorption ability ability coming coming from from its its improved improved photoresponses. photoresponses. The photocurrent indicates higher short-circuit short-circuit current current and and higher higher conversion conversion efficiency indicates higher efficiency will will be be obtained obtained by by application application of of the TLPS structures. structures. At At aa reverse reverse bias bias of of 55 V, V, the the TLPS the TLPS TLPS device device has has about about one-order one-order higher higher photocurrent photocurrent than that of one based on a single LP-PS layer with porosity of 40%, which was applied as than that of one based on a single LP-PS layer with porosity of 40%, which was applied as the the surface surface film That imply imply that film in in aa Si-based Si-based solar solar cell cell to to achieve achieve optimum optimum device device performance performance [8]. [8]. That that the the TLPS TLPS structures structures have have good good potential potential to to improve improve the the performance performance of of aa Si-based Si-based photovoltaic photovoltaicdevice. device.
Figure 7. Photo and dark current-voltage (I–V) characteristics of photovoltaic devices based Figure 7. Photo and dark current-voltage (I–V) characteristics of photovoltaic devices based on the TLPS structures and the individual PS (NPS, LP-PS or HP-PS) layer on a Si substrate. on the TLPS structures and the individual PS (NPS, LP-PS or HP-PS) layer on a Si substrate. The photocurrent of an NPS-based NPS-based device device is is about about half halfof ofthat thatof ofthe theTLPS TLPSone oneatat55V. V.From From this this data and the photoresponses of the individual NPS layer shown in Figure 4, we deduced that the NPS layer contributes the most part part of of the the photo-induced photo-induced current current to to aa TLPS TLPS device. device. In fact, there are many advantages by incorporating incorporating the the NPS NPS layer layer as as the the surface surface layer layer in in the the device. device. First, the NPS layer significantly increases the responses in the short-wavelength short-wavelength ranges around 450–550 nm that are just the portions portions including including the the maximum maximum irradiance irradiance of of the the incident incident light light (or (or the the sun sun light), light), as as we we can can observed observed in in Figures 44 and texturedsurface surfacestructure structure to to reduce reduce Figures and 5. 5. Second, Second,the themicro-holes micro-holesininthe theNPS NPSlayer layerprovide providea atextured the light-reflection as mentioned above, the the light-reflection losses losses and and augment augmentthe thelight lightabsorption absorptionofofdevices. devices.Third, Third, as mentioned above, NPSNPS layerlayer effectively passivates thethelower middle-band the effectively passivates lowerHP-PS HP-PSlayer layer for for complementing complementing the the middle-band photorespnses. Four, the smooth smooth and and dense dense surface surface of of the the NPS NPS layer layer ensures ensures the the formation formation of of aa Schottky Schottky photorespnses. Four, the contact with with high high quality quality between between the the metal metal electrodes electrodes and and the the PS PS layer. layer. This can be be realized realized contact This advantage advantage can from the the comparison comparison of of the themeasured measureddark darkcurrent currentbetween betweenthe theTLPS TLPSand andthe theHP-PS HP-PSdevices. devices.Although Althougha from a HP-PS device larger series resistance of a TLPS device, has higher dark current as HP-PS device hashas larger series resistance thanthan that that of a TLPS device, it has ithigher dark current as shown shown in 7. Figure For a Schottky device, high of density at the metal-semiconductor (M-S) in Figure For a7.Schottky device, high density defectsofatdefects the metal-semiconductor (M-S) interface interface willthe reduce the Schottky barrier of the M-S junction, thus causing a decrease the photocurrent will reduce Schottky barrier of the M-S junction, thus causing a decrease in theinphotocurrent and and increase the reverse (or leakage) In a photovoltaic leakage current increase in theinreverse (or leakage) current current [22]. In [22]. a photovoltaic device, thedevice, leakagethecurrent dominates dominates the dark current. is, a PS-based photovoltaic device a surface layerofofhigher-porosity higher-porosity the dark current. That is,That a PS-based photovoltaic device withwith a surface PSPS layer usually had had lower lower photocurrent photocurrent and and higher higher dark dark current current due due to to the the degraded degraded Schottky Schottky contact. contact. Therefore, Therefore, usually the large thethe contrary, duedue to the large amount amount of of defects defects on onthe thesurface surfaceHP-PS HP-PSlayer layercause causehigh highdark darkcurrent. current.OnOn contrary, thethe incorporation of of anan NPS good to incorporation NPSsurface surfacelayer, layer,the theTLPS TLPSdevice devicewith withaa middle middle HP-PS HP-PS layer layer has has aa good Schottky contact contact to generate aa higher higher photocurrent photocurrent and and retain retain lower lower dark dark current. current. Since Since the the photo-to-dark photo-to-dark Schottky to generate current ratio (PDCR) is an important figure of merit for the performance of a photovoltaic device,
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current ratio (PDCR) is an important figure of merit for the performance of a photovoltaic device, the large PDCR of the fabricated TLPS-based device demonstrates that the multi-bandgapped PS structure has high potential in development of Si-based photovoltaic device. 4. Conclusions By successively altering the etching parameters in an electrochemical anodization process, TLPS structures with NPS/HP-PS/LP-PS stacked layers can be produced on Si substrates. The three stacked PS layers in the formed TLPS structure have varied optical bandgap energy decreasing from top to bottom and provide the TLPS structure with multiple bandgaps. Experimental results show that the fabricated photovoltaic devices based on TLPS structures exhibit broadband photoresponses within the spectrum of the solar irradiation and get high photocurrent for the incident light of a tungsten lamp. The improved spectral responses are ascribed to the multi-bandgap structures of TLPS in which the separate PS layer with distinct optical bandgap energy absorbs different energy portions of the incident light and contributes all together to the total photoresponses. Most notably, the use of NPS films as the surface layers greatly enhances the light-absorption ability of devices by supplying greatly increased short-wavelength responses and providing good Schottky contacts to get large photocurrents and reduce the leakage current. Therefore, the developed TLPS structures have high potential in fabrication of Si-based solar cells with enhanced conversion efficiency. Acknowledgments The author acknowledge financial support from the Ministry of Science and Technology of Taiwan, R.O.C. under contract No. NSC 102-2221-E-218-038. Author Contributions Kuen-Hsien Wu directed the study, coordinated the research, analyzed the data and wrote the paper; Chong-Wei Li performed the experiments. Conflicts of Interest The authors declare no conflict of interest. References 1. Canham, L.T. Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers. Appl. Phys. Lett. 1990, 57, 1046–1048. [CrossRef] 2. Smith, R.L.; Collins, S.D. Porous silicon formation mechanisms. J. Appl. Phys. 1992, 71, 1–22. [CrossRef] 3. Granitzer, P.; Rumpf, K. Porous silicon—A versatile host material. Materials 2010, 3, 943–998. [CrossRef] 4. Bisi, O.; Ossicini, S.; Pavesi, L. Porous silicon: A quantum sponge structure for silicon based optoelectronics. Surf. Sci. Rep. 2000, 38, 1–126. [CrossRef] 5. Menna, P.; Francia, G.D.; Ferrara, V.L. Porous silicon in solar cells: A review and a description of its application as an AR coating. Sol. Energy Mater. Sol. Cells 1995, 37, 13–24. [CrossRef]
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6. Striemera, C.C.; Fauchet, P.M. Dynamic etching of silicon for broadband antireflection applications. Appl. Phys. Lett. 2002, 81, 2980–2982. [CrossRef] 7. Vitanov, P.; Kamenova, M.; Tyutyundzhiev, N.; Delibasheva, M.; Goranova, E.; Peneva, M. High-efficiency solar cell using a thin porous silicon layer. Thin Solid Film 1997, 297, 299–303. [CrossRef] 8. Duttagupta, S.P.; Fauchet, P.M.; Ribes, A.C.; Tiedje, H.F. Photovoltaic device applications of porous microcrystalline silicon. Sol. Energy Mater. Sol. Cells 1998, 52, 271–283. [CrossRef] 9. Yerokhov, V.Y.; Melnyk, I.I. Porous silicon in solar cell structures: A review of achievements and modern directions of further use. Renew. Sustain. Energy Rev. 1999, 3, 291–322. [CrossRef] 10. Yamaguchi, M.; Luque, A. High efficiency and high concentration in photovoltaics. IEEE Trans. Electron Devices 1999, 46, 2139–2144. [CrossRef] 11. Carmody, M.; Mallick, S.; Margetis, J.; Kodama, R.; Biegala, T.; Xu, D.; Bechmann, P.; Garland, J.W.; Sivananthan, S. Single-crystal II–VI on Si single-junction and tandem solar cells. Appl. Phys. Lett. 2010, 96, 153502:1–153502:3. [CrossRef] 12. Karacali, T.; Alanyalioglu, M.; Efeoglu, H. Single and double fabry–pérot structure based on porous silicon for chemical sensors. IEEE Sens. J. 2009, 12, 1667–1672. [CrossRef] 13. Torres-Costa, V.; Agulló-Rueda, F.; Martín-Palma, R.J.; Martínez-Duart, J.M. Porous silicon optical devices for sensing applications. Opt. Mater. 2005, 27, 1084–1087. [CrossRef] 14. Selj, J.H.; Thøgersen, A.; Foss, S.E.; Marstein, E.S. Optimization of multilayer porous silicon antireflection coatings for silicon solar cells. J. Appl. Phys. 2010, 107, 074904:1–074904:10. [CrossRef] 15. Osorio, E.; Urteaga, R.; Acquaroli, L.N.; García-Salgado, G.; Juaré, H.; Koropecki, R.R. Optimization of porous silicon multilayer as antireflection coatings for solar cells. Sol. Energy Mater. Sol. Cells 2011, 95, 3069–3073. [CrossRef] 16. Tauc, J. Amorphous and Liquid Semiconductor; Plenum Press: New York, NY, USA, 1974; p. 159. 17. Lee, W.H.; Lee, C.; Jang, J. Quantum size effects on the conductivity in porous silicon. J. Non-Cryst. Solids 1996, 198–200, 911–914. [CrossRef] 18. Suemune, I.; Noguchi, N.; Yamanishi, M. Photoirradiation effect on photoluminescence from anodized porous silicons and luminescence mechanism. Jpn. J. Appl. Phys. 1992, 31, L494–L497. [CrossRef] 19. Martín-Palma, R.J.; Guerrero-Lemus, R.; Moreno, J.D.; Martínez-Duart, J.M. Determination of the spectral behaviour of porous silicon based photodiodes. Solid-State Electron. 1999, 43, 1153–1157. [CrossRef] 20. Lee, M.K.; Wang, Y.H.; Chu, C.H. High-sensitivity porous silicon photodetectors fabricated through rapid thermal oxidation and rapid thermal annealing. IEEE J. Quantum Electron. 1997, 33, 2199–2202. [CrossRef] 21. Halimaoui, A. Porous Si: From single porous layers to porosity superlattices. In Porous Silicon Science and Technology; Vial, J.C., Derrien, J., Eds.; Springer-Verlag: New York, NY, USA, 1995; pp. 33–50. 22. Torchynska, T.V.; Hernandez, A.V.; Polupan, G.; Sandoval, S.J.; Cueto, M.E.; Sierra, R.P.; Rubio, G.R.P. Photoluminescence and photocurrent in porous silicon Schottky barriers. Thin Solid Films 2005, 492, 327–331. [CrossRef] © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).
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materials ISSN 1996-1944 www.mdpi.com/journal/materials Article
Light Absorption Enhancement of Silicon-Based Photovoltaic Devices with Multiple Bandgap Structures of Porous Silicon Kuen-Hsien Wu * and Chong-Wei Li Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, No. 1, Nan-Tai Street, Yungkang Dist., Tainan 710, Taiwan; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-6-253-3131 (ext. 3627); Fax: +886-6-234-2912. Academic Editor: Teen-Hang Meen Received: 29 June 2015 / Accepted: 1 September 2015 / Published: 7 September 2015
Abstract: Porous-silicon (PS) multi-layered structures with three stacked PS layers of different porosity were prepared on silicon (Si) substrates by successively tuning the electrochemical-etching parameters in an anodization process. The three PS layers have different optical bandgap energy and construct a triple-layered PS (TLPS) structure with multiple bandgap energy. Photovoltaic devices were fabricated by depositing aluminum electrodes of Schottky contacts on the surfaces of the developed TLPS structures. The TLPS-based devices exhibit broadband photoresponses within the spectrum of the solar irradiation and get high photocurrent for the incident light of a tungsten lamp. The improved spectral responses of devices are owing to the multi-bandgap structures of TLPS, which are designed with a layered configuration analog to a tandem cell for absorbing a wider energy range of the incidental sun light. The large photocurrent is mainly ascribed to an enhanced light-absorption ability as a result of applying nanoporous-Si thin films as the surface layers to absorb the short-wavelength light and to improve the Schottky contacts of devices. Experimental results reveal that the multi-bandgap PS structures produced from electrochemical-etching of Si wafers are potentially promising for development of highly efficient Si-based solar cells. Keywords: porous silicon; multi-layer; multi-bandgap; photoresponse; photovoltaic device; solar cell
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1. Introduction Since the report of room temperature photoluminescence in porous silicon (PS) [1], this material has been extensively investigated for the development of optoelectronic devices. PS is constituted by a nanocrystalline skeleton (quantum sponge) immersed in a network of pores and can be obtained simply by electrochemical etching of silicon in aqueous hydrofluoric (HF) [2,3]. Due to its many special characteristics such as extremely large surface-area-to-volume ratio, modulated energy bandgap, absorption spectrum within the solar irradiation, surface roughening with low reflection losses, and high photoconductivity, PS is very suitable for applications in photovoltaic devices [4]. In particular, PS structures have been intensively introduced in the device design of solar cells to increase the cell’s conversion efficiency [5–7]. In most of the related research, PS films acted as the surface texturing and anti-reflection (AR) layers to reduce the light-reflection losses or as the surface passivation layers to decrease surface carrier recombination velocity. On the other hand, PS can potentially serve as the parts of the active layer to effectively generate photo-induced carriers because of its wide energy bandgap and high photoconduction in response to solar irradiation [8,9]. However, a primary drawback of PS for use of the light-absorption layer in a photovoltaic device is its comparatively narrow spectral photoresponses. Therefore, to get higher and broader spectral photoresponses within the spectrum of the solar irradiation is a primary requirement for the fabrication of efficient PS-based solar cells. An important scheme to increase the light absorbance range of devices is using cell structures with multi-junctions (i.e., the “tandem cell” structures). Tandem cells are solar cells with multiple p–n junctions made of different semiconductor materials with different bandgap energy. Because each semiconductor material is sensitive to a different part of the solar spectrum, tandem cells can obtain enhanced overall sunlight photoresponses and achieve higher conversion efficiency. However, these tandem cells are usually not suitable for mass production of silicon (Si) solar cells since their fabrication processes are so complicated and are not Si-compatible [10,11]. A promising Si-based tandem-like structure is the PS multilayer (PSML) structure composed of stacked PS layers with different porosity. In the past, PSML had received much interest due to its tunable optical and electronic properties [4]. Because the refractive index of PS can be tuned through the porosity using controlled electrochemical-etching conditions, PSML with modulated refractive index can be fabricated by stacking PS layers of different porosity. These PSML structures can be used to make optical filters, waveguides, resonating cavities and mirrors for device applications [12,13]. Most of the recent studies about applications of PSML in solar cells mainly focused at the optimization of AR coatings [14,15]. There are few reports on cells with light-absorption areas based on PSML structures. Since the optical bandgap of PS can be tuned by its porosity, it is feasible to broaden the photoresponse spectra of Si photovoltaic devices by use of multi-bandgap PSML structures. Based on the fact that a PS layer of higher porosity has higher bandgap energy, a PSML structure composed of triple stacking layers with high-, medium- and low-porosity (from top to bottom) will absorb the sun light in the short-, medium- and long-wavelength bands respectively. Such a “tandem-like” PSML structure can potentially harvest a broader range of the sun’s energy to make a Si-based solar cell more efficient. Utilization of PSML as the light-absorption region of a device will face a dilemma in choosing the porosity of the upmost PS layer. To absorb light of shorter wavelength, the upmost PS layer must be designed with higher porosity. However, a PS layer with higher porosity has higher resistance, lower
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hardness and higher density of defects [4,5]. Therefore, using a high-porosity PS surface layer will result in high series resistance and degradation of metal contacts. To solve this problem, we proposed a special nanoporous-Si (NPS) thin film for serving as the surface active layer. The NPS layer associated with two stacked PS layers of different porosity will construct a triple-layered PS (TLPS) structure with multiple bandgap energy. Such a TLPS structure can be produced on Si substrates by tuning the etching parameters in an electrochemical anodization process. In this paper, we report the preparation and characterization of TLPS structures and demonstrated the improvements on the spectral photoresponses and light-absorption capability of Si-based photovoltaic devices fabricated with the developed TLPS structures. 2. Experimental Section 2.1. Formation of Triple-Layered Porous Silicon N+ -type (100) silicon wafers with resistivity of 5–10 mΩ¨ cm were used as the starting substrates. PS layers were prepared on Si substrates by electrochemical etching in an anodization process with HF-methanol etching solutions in a Teflon cell with a Pt electrode. To generate a TLPS structure, the electrochemical-etching parameters were successively changed in the anodization process. Different HF concentration of etching solution, etching current density and etching time were used for formation of different PS stacked layers with varied morphology and porosity. Table 1 lists the etching parameters used in the anodization process for preparation of the triple stacked PS layers. To obtain a NPS thin film as the upmost (the first) layer, lower etching current density of 10 mA/cm2 was used during the anodic process. The HF concentration of the etching solution is the key parameter to control the sizes and distribution of Si-nanocrystals in the NPS films. In order to get smaller sizes of Si-nanocrystals and higher reproducibility, the concentration of HF was set as 30% of the etching solution in this work. Table 1. Electrochemical-etching parameters used in the anodization process for preparation of the triple stacked PS layers. Etching Parameters
First Layer (Upmost)
Second Layer (Middle)
Third Layer (Bottom)
HF Concentration Etching Current Density (mA/cm2 ) Etching Time (s)
30% 10 60
10% 60 60
10% 30 90
To create the following PS layers, HF concentration of the etching solution was reduced insitu to 10% and the etching current density was increased to 60 and 30 mA/cm2 for formation of a high-porosity PS (HP-PS) layer as the second layer and a low-porosity PS (LP-PS) layer as the third layer, respectively. Because etching only occurs at the PS-Si interface, the previously formed PS layer will be left unaffected. After the formation of PS layers, samples with the prepared TLPS were rapid-thermally annealed in N2 at 800 ˝ C for 60 s to stabilize the PS structures. Figure 1 illustrates the scanning-electron-microscope (SEM) photo-image of the cross-sectional view of the produced TLPS structure on a Si substrate. It can be observed that a triple-layered structure with an NPS, an HP-PS and a LP-PS layer stacking from top to bottom are formed on a Si substrate. The thickness of each layer is about 0.85 µm, 1.85 µm and 1.85 µm, respectively. The upmost NPS layer has a different morphological structure from that of the HP-PS or the LP-PS layer. The NPS layer is
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aparticulate particulatethin thin film consisting uniformly distributed Si-nanoparticles (SNP’s), while the HP-PS particulate thin film consisting ofof uniformly distributed Si-nanoparticles (SNP’s), while the HP-PS HP-PS and film consisting of uniformly distributed Si-nanoparticles (SNP’s), while the and and LP-PS layersare are with with structures micro-rods and micro-holes. For the the LP-PS layers are structurescontaining containingpipe-shaped pipe-shaped micro-rods and micro-holes. micro-holes. the LP-PS layers structures containing pipe-shaped micro-rods and determining the porosity of the intermediate PS layers, individual PS layer waswas alsoalso prepared by For determining determining the porosity porosity ofthree the three three intermediate PS layers, layers, individual PS layer layer was also prepared For the of the intermediate PS individual PS prepared use of the etching parameters listedlisted in Table 1. The of theof HP-PS and LP-PS layer by use use of same the same same etching parameters listed in Table Table 1.porosity The porosity porosity ofNPS, the NPS, NPS, HP-PS HP-PS and LP-PS LP-PS by of the etching parameters in 1. The the and is estimated by weight measurements to be about 20%, 40%, layer estimated by weight weight measurements to be be about about70% 20%,and 70% andrespectively. 40%, respectively. respectively. layer isis estimated by measurements to 20%, 70% and 40%,
Figure The SEM SEM photo-image photo-image of the cross-sectional Figure 1. 1. The photo-image of of the the cross-sectional cross-sectional view view of of the the formed formed triple-layered triple-layered Figure 1. view of the formed triple-layered porous-silicon porous-silicon(TLPS) (TLPS)structure structureon onaaaSi Sisubstrate. substrate.The Thetriple triplelayers layersinclude includeaaananoporous-Si nanoporous-Si porous-silicon (TLPS) structure on Si substrate. The triple layers include nanoporous-Si (NPS), (NPS),aaahigh-porosity high-porosityPS PSand andaaalow-porosity low-porosityPS PSlayer layerfrom fromtop topto tobottom. bottom. (NPS), high-porosity PS and low-porosity PS layer from top to bottom.
2.2. Fabrication Fabricationof ofPhotovoltaic PhotovoltaicDevices Devices 2.2. Fabrication of Photovoltaic Devices 2.2. Afterthe thepreparation preparationof ofTLPS, TLPS,aluminum aluminum(Al) (Al)was wasdeposited depositedon onthe theback backside sideof ofthe thesample sampleand andthen then After was deposited on the back side of the ˝ sinteredatat350 350°C °C to make make the the back back electrode electrodeof ofOhmic Ohmiccontact. contact. Thereafter, Thereafter,grid gridAl Almetals metalswere wereformed formed sintered C to onthe thefront frontside sideof ofthe thesample sampletotoserve serveas asthe thefront frontelectrodes electrodesof ofSchottky Schottkycontact. contact. Figure Figure 22 shows showsthe the on serve as the front electrodes of Schottky contact. schematic structure structure of of the the developed developed Si-based Si-based photovoltaic photovoltaicdevice devicewith withthe theTLPS TLPSserving servingas asthe themain main schematic developed photovoltaic device with the TLPS serving as the of the the light-absorption region. region. By By means means of of the the Tauc’s Tauc’s plots plots [16], [16], the the optical optical bandgap bandgap energy energy(Eg (Egopt opt)) of light-absorption By means the region. energy (Eg opt individualas-prepared as-preparedPS PSlayer layer isis estimated estimated to to be be 2.5 2.5 eV, eV,1.8 1.8eV, eV,and and1.6 1.6eV eVfor forthe theNPS, NPS,the theHP-PS, HP-PS, individual eV, individual as-prepared eV, and 1.6 eV for the NPS, andthe theLP-PS LP-PSlayer, layer,respectively. respectively.Like Likeaatandem tandemcell, cell,the thedeveloped developeddevice devicehas hasaalight-absorption light-absorptionregion region and respectively. Like device has withmultiple multipleoptical opticalbandgaps bandgapsand andthe thebandgap bandgapof ofeach eachlayer layerin inthe thelight-absorption light-absorptionzone zonedecreases decreasesfrom from with with multiple optical bandgaps and the bandgap of each layer in the light-absorption zone decreases from thedevice’s device’ssurface surfaceto toits itsinterior. interior. the device’s surface to its interior. the
Figure 2. Schematic structure of the developed Si-based photovoltaic device with a Figure 2. 2. Schematic Schematic structure structure of of the the developed developed Si-based Si-based photovoltaic photovoltaic device device with with aa Figure triple-layered porous-Si (TLPS) structure as the light-absorption region. triple-layered porous-Si porous-Si (TLPS) (TLPS) structure structure as as the the light-absorption light-absorption region. region. triple-layered
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3. 3. Results Results and and Discussion Discussion Figure Figure 3a 3a shows shows the the zoom-in zoom-in SEM SEM photo-image photo-image of of the theprepared preparedNPS NPSlayer layerininFigure Figure1.1. ItIt can can be be observed that the NPS layer has a special film-like structure, unlike the beneath HP-PS layer which observed that the NPS layer has a special film-like structure, unlike the beneath HP-PS layer which has has common morphology mesomicro-PSlayers layersgenerated generatedfrom from electrochemical-etching electrochemical-etching of the the common morphology of of mesoorormicro-PS of Si Si wafers. wafers. Figure Figure 3b 3b shows shows aa high-magnification high-magnification SEM SEM image image of of the the surface surface of of the the NPS NPS film. film. As As shown shown in in this figure, the NPS film is embedded with uniformly distributed Si-nanocrystals and nano-sized holes. this figure, the NPS film is embedded with uniformly distributed Si-nanocrystals and nano-sized holes. Average these Si-nanocrystals Si-nanocrystals are are estimated estimatedtotobe beabout about3–10 3–10nm nminin diameters. According to Average sizes sizes of of these diameters. According to the the reports of Lee al. [17], the bandgap energy of material a PS material with quantum-sized Si-crystallites reports of Lee et al.et[17], the bandgap energy of a PS with quantum-sized Si-crystallites of 3 nmofis 3about nm is2.5 about 2.5 eV. This is close to that was predicted by Suemune al. conforms [18] and conforms eV. This value is value close to that was predicted by Suemune et al. [18]etand to the Egto opt the Eg value of the NPS film measured from the Tauc’s plots in this work. The Eg of the beneath opt opt value of the NPS film measured from the Tauc’s plots in this work. The Egopt of the beneath HP-PS HP-PS layerporosity with porosity of 70% andLP-PS the LP-PS layer porosity 30% gotfrom fromthe theTauc’s Tauc’s plots plots is layer with of 70% and the layer withwith porosity of of 30% got is about eV,eV, respectively. These values are consistent with those from thefrom optical about 1.8 1.8eV eVand and1.61.6 respectively. These values are consistent with obtained those obtained the absorption measurements in other studies [17]. optical absorption measurements in other studies [17].
Figure 3. (a) The zoom-in SEM photo-image of the prepared NPS layer shown in Figure 1; Figure 3. (a) The zoom-in SEM photo-image of the prepared NPS layer shown in Figure 1; (b) A high-magnification SEM image of the surface of the prepared NPS film. (b) A high-magnification SEM image of the surface of the prepared NPS film. Figure Figure 44 shows shows the the spectral spectral photoresponses photoresponses of of the the individual individual PS PS (including (including NPS, NPS, HP-PS HP-PS and and LP-PS) LP-PS) layer layer prepared prepared on on aa Si Si substrate. substrate. The photoresponses were were measured measured with with aa TRIAX-320 TRIAX-320 spectrometer spectrometer (HORIBA, (HORIBA, Kyoto, Kyoto, Japan). Japan). To exclude exclude as as much much as as possible possible the the influence influence of of light light absorption absorption in in the the Si Si substrate, substrate, the the responsivity responsivity was was measured measured by by using using planar planar electrodes electrodes formed formed on on the the surface surface of of the the PS PS layer, layer, as as indicated indicated in in the the inserted inserted drawing. drawing. For comparison purposes, purposes, photoresponses photoresponses of of the the bare bare Si Si substrate substrate are are also also shown shown in in the the figure. figure. As shown shown in in the the figure, figure, the the photoresponses photoresponses of of the the LP-PS LP-PS layer layer mainly mainly appear appear within withinthe theranges rangesofofwavelength wavelengthbetween between650 650nm nmand and850 850nm, nm,which whichcorresponds corresponds to to the the 22 optical band band from from red of 62 62 mA/cm mA/cm at at 750 750 nm. nm. The The optical red to to infra-red infra-red (IR), (IR), and and has has aa maximum maximum responsivity responsivity of LP-PS layer layer displays displays the the common common photoresponsive photoresponsivecharacteristics characteristicsof ofaaPS PSmaterial, material,which which has has aa narrower narrower LP-PS and blue-shifted blue-shifted response response spectrum spectrum with with smaller smaller responsivity responsivityas ascompared comparedtotothat thatofofthe thebare bareSi. Si. The The and responses of of the the HP-PS HP-PS device device shift shift further further toward toward the the band band of of shorter-wavelength shorter-wavelength between between 550 550 nm nm responses 2 2 (green) and and 700 700 nm nm (red) (red) with with aa much much small small peak peak responsivity responsivity of of 55 mA/cm mA/cm at at 670 670 nm. nm. We We thought thought the the (green) change in in the the region region of of the the spectral spectral responses responses isis due due to to the the higher higher porosity porosity of of layers layers which which result result in in aa change larger Eg Egopt The low low responsivity responsivity is is ascribed ascribed to to not not only only its its high high resistivity resistivity and and the the high high defect defect density, density, opt.. The larger but also the large amount of the surface states and the high metal-contact resistance. By improving the
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Materials 2015, 8 6 but also the large amount of the surface states and the high metal-contact resistance. By improving the metal contacts contacts and PS can can be be metal and the the surface surface passivation passivation on on the the PS PS layer, layer, the the responsivity responsivity of of high-porosity high-porosity PS largely enhanced enhanced [19,20]. [19,20].Particularly Particularlyininthis thiswork, work,the theHP-PS HP-PSlayer layer was was made made under under the the NPS NPS surface largely layer in the the fabricated fabricated TLPS-based device. As As aa result, result, the the surface surface of of the the HP-PS HP-PS isis effectively effectively passivated passivated by the NPS layer and the quality of metal-contact is thus not degraded by the HP-PS layer since the electrodes are formed on the NPS layer that has a surface surface with with better better quality. quality. Therefore, the actual photoresponsivity of such a middle HP-PS layer in the TLPS structure is expected to be higher than that of an individual HP-PS layer as a surface layer on a Si substrate.
Figure 4. The spectral photoresponses of the individual PS (NPS, HP-PS or LP-PS) layer Figure 4. The spectral photoresponses of the individual PS (NPS, HP-PS or LP-PS) layer prepared on a Si substrate, measured with planar electrodes on the PS surface as indicated in prepared on a Si substrate, measured with planar electrodes on the PS surface as indicated in the inserted drawing. For comparison purposes, the measured data of the bare Si substrate the inserted drawing. For comparison purposes, the measured data of the bare Si substrate are also shown. are also shown. As shown shownininFigure Figure4, 4, NPS not only exhibits short-wavelength responses As thethe NPS layerlayer not only exhibits much much higherhigher short-wavelength responses within 2 2 within 400–600 nm blue in theband blue(with band a(with a maximum responsivity 72 mA/cm at nm), 500 nm), but also at 500 but also has 400–600 nm in the maximum responsivity of 72of mA/cm has apparent responses in the band red-to-IR. supposedthat thatthe thegreatly greatly enhanced enhanced blue-shifted blue-shifted apparent responses in the band of of red-to-IR. It Itis issupposed responses come from the SNP’s embedded in the NPS films, which has Eg Egopt larger than 2 eV due to the The red-to-IR red-to-IR responses responses are are ascribed ascribed to to the the light light absorption in the micro-sized Si quantum-size effects. The Besides, itit is is interesting interesting to to find find that the value of crystals around the SNP’s and the beneath Si substrate. Besides, the peak responsivity responsivity of the NPS, HP-PS, and LP-PS layer corresponds corresponds respectively respectively to a photon energy 2.48 eV, eV, 1.77 eV, and 1.65 eV, single PS PS layer layer respectively. respectively. of 2.48 eV, which which is is close to the value of Egopt opt of the single This fact indicates that the Egopt its photoresponses. photoresponses. opt of a PS layer has significant influences on its spectral photoresponses photoresponses of the photovoltaic photovoltaic devices with a TLPS structure on Figure 5 illustrates the spectral a Si substrate.
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Figure 5. The spectral photoresponses of the developed photovoltaic devices based on the Figure 5. The spectral photoresponses of the developed photovoltaic devices based on the TLPS TLPS structure structure and and the the individual individual PS PS (NPS, (NPS, HP-PS HP-PS or or LP-PS) LP-PS) layer. layer. The The summed summed data data (blue dotted line) were obtained by directly summing the three individual responses of (blue dotted line) were obtained by directly summing the three individual responses of the the PS layer in Figure 4, while the summed (with fitting) data (red dotted line) were obtained PS layer in Figure 4, while the summed (with fitting) data (red dotted line) were obtained by by using a 10-fold responses of HP-PS. The spectrum of solar irradiation is also shown in the using a 10-fold responses of HP-PS. The spectrum of solar irradiation is also shown in the figure figure for for comparison comparison purposes. purposes.
It is is obvious than thatthat of of an It obvious that that the the TLPS-based TLPS-baseddevice deviceexhibits exhibitsa abroader broaderphotoresponse photoresponsespectrum spectrum than individual PS PS layer andand thethe bare Si Sisubstrate. importantly, thethe developed an individual layer bare substrate.Most Most importantly, developeddevice deviceachieved achieveda of the the solar solar abroadened broadenedand andflat flatphotoresponse photoresponsespectrum spectrumthat that matches matches much much more more closely closely to to that that of irradiance. That That is is aa preferable preferable property property for for development development of of highly highly efficient efficient solar solar cell. cell. From From the the spectral spectral irradiance. distribution of ofphotoresponses photoresponsesofofthe thethree threeindividual individual layer in Figure is reasonable to suppose distribution PSPS layer in Figure 4, it4,isitreasonable to suppose that thatoverall the overall responses of a TLPS are primarily composed the generated responses the the responses of a TLPS devicedevice are primarily composed of theofgenerated responses in the in three three PS layers of a structure. TLPS structure. Tothis verify this assumption, wethe plotted the responses summed responses PS layers of a TLPS To verify assumption, we plotted summed (denoted (denoted as “Summed”) that wereby obtained directlythe summing the three responses individualofresponses of the as “Summed”) that were obtained directlyby summing three individual the PS layer in PS layer Figure 4. responses The shown responses from the datadown were twofold scaled down twofold for based curve Figure 4. in The shown from the summed datasummed were scaled for curve fitting, fitting, based fact thatofthe individual PSthan layerthat is smaller thanIt that of a TLPS. on the fact thaton thethe resistance theresistance individualofPSthe layer is smaller of a TLPS. is apparent that It is apparent that the summed response spectra bend down between 550 nm and 750 nm as a result of the summed response spectra bend down between 550 nm and 750 nm as a result of the relatively small the relatively small values of the individual HP-PS layer. However, expected to exhibit values of the individual HP-PS layer. However, the HP-PS is expectedthe to HP-PS exhibit is higher responses as a higher layer responses a middle layer TLPS structure the passivation of the response NPS layer. middle in theas TLPS structure dueinto the the passivation of thedue NPStolayer. By using a 10-fold of By using 10-fold response of HP-PS summing, the plotted summed response spectra (denoted as HP-PS forasumming, the plotted summedfor response spectra (denoted as “Summed (with fitting)”) become “Summed fitting)”) become flatter and more fitting to that of asupport TLPS-based device. These much flatter(with and more fitting to thatmuch of a TLPS-based device. These results the expectation that results support the layer expectation that the middle HP-PS layer than can generate photoresponses the middle HP-PS can generate higher photoresponses those of higher a surface HP-PS layer than and those of the a surface HP-PSthat layer indicatewide-band the assumption that the measured wide-band photoresponses indicate assumption theand measured photoresponses come from a synergistic function come from a synergistic function of the three PS layers in the TLPS structure is valid. of the three PS layers in the TLPS structure is valid. Furthermore, to to check check whether whether the the multilayered multilayered PS PS structure structure on on top top of of the the Si Si substrate substrate causes causes an an Furthermore, interference effect effect on on the interference the photoresponses photoresponses of of the the TLPS TLPS device, device,the thespectral spectralreflectance reflectanceofofthe theTLPS TLPSonona the average average aSiSisubstrate substratewas wasmeasured measuredand andshown shown inin Figure Figure 6. 6. From Fromthis thisfigure, figure, we we can can find find that that the reflectivity of of samples samples with that of of thethe bare Si substrate in the reflectivity with TLPS TLPS on onSi Siisisabout abouttwo twotimes timeslower lowerthan than that bare Si substrate in
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the visible band. verifies TLPS structureenhanced enhancedthe thelight lightabsorption. absorption.There Thereare are no no obvious obvious visible band. ThatThat verifies thethe TLPS structure interference patterns patterns appearing appearing in in the the spectral spectral reflectance. reflectance. It It indicates indicates that that the the layered layered structure structure of of TLPS TLPS interference in this work has a negligible interference effect on the generation of the spectral responses. responses. Therefore, the improved spectral responses of the TLPS-based device are mainly owing to the multi-bandgap TLPS structures in which different layers with different Egopt absorb different portions of the incident light and contribute all together to the whole photoresponses.
Figure 6. The spectral reflectance of the TLPS structure on a Si substrate and the bare Si substrate. Figure 6. The spectral reflectance of the TLPS structure on a Si substrate and the bare Si substrate. In aa tandem tandem cell, cell, layers layers with with different different bandgap bandgap are are stacked stacked on on top top of of one one another another with with the the highest highest In bandgap layer layer upmost. upmost. Since Since layers layers with with higher higher bandgap bandgap will will absorb absorb light light of of shorter shorter wavelength, wavelength, the the bandgap device’s conversion is optimally optimally device’s conversion efficiency efficiency can can be be largely largely increased increased if if bandgap bandgap energy energy of of each each layer layer is chosen. According Accordingtotorelated related researches, light-absorption spectra of with PS with higher porosity usually chosen. researches, light-absorption spectra of PS higher porosity usually shift shift towards shorter wavelength because of the larger bandgap energy of PS [21]. These results accorded towards shorter wavelength because of the larger bandgap energy of PS [21]. These results accorded with the the experimental experimental ones ones in in this this work. In other other words, words, PS PS layers layers with with larger larger bandgap bandgap energy energy would would with work. In absorb light light of of shorter shorter wavelength. wavelength. For For the the developed developed TLPS TLPS devices, devices, the the Eg Egopt value of of each each layer layer (from (from opt value absorb surface to to the the interior) interior) in in the the light-absorption light-absorption zone zone was was 2.5 2.5 eV eV (NPS), (NPS), 1.8 1.8 eV eV (HP-PS), (HP-PS), 1.6 1.6 eV eV (LP-PS) (LP-PS) surface and 1.1 1.1 eV eV (bulk (bulk Si), Si), respectively. respectively. Since Since these these Eg Egopt values spanned spanned the the main main ranges ranges of of photon photon energy energy opt values and in the the solar solar spectrum, spectrum, the the TLPS TLPS devices devices exhibited exhibited broadband broadband photoresponses photoresponses and and were were able able to to harvest harvest in more solar solar energy. energy. more To assess the influences influences of the improved improved photoresponses the performance performance of of the the photovoltaic photovoltaic To assess the of the photoresponses on on the devices, photo- and dark current-voltage characteristics of a TLPS-based device were measured and plotted in in Figure Figure7. 7. For Forcomparison comparisonpurposes, purposes,similar similardevices devicesbased basedon onaasingle singlePS PS(NPS, (NPS, LP-PS LP-PS or or HP-PS) plotted layer with a comparable thickness of about 4.5 µm m were also fabricated and measured. The photocurrent 2 of devices was measured under an incident irradiation of 100 mW/cm by using a 10-W tungsten lamp emitting light light of of wavelength wavelength ranging rangingfrom from300 300nm nmtoto1100 1100nm nmand anda apeak peakwavelength wavelengthof of550 550 nm. nm. with emitting The reason for choosing such a tungsten lamp as the light source is that it has an irradiation spectrum As observed observed inin the the figure, figure, the the TLPS-based TLPS-based device device obtains obtains the largest similar to that of the sun. As photocurrent and smallest dark current among these fabricated devices, indicating it has the most significant photoconduction property. property. Because the photoresponse spectrum of a TLPS device matched much more closely to that of the incident light, the higher photocurrent is mainly attributed to its better
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light-absorption The increase increase in in the the photocurrent light-absorption ability ability coming coming from from its its improved improved photoresponses. photoresponses. The photocurrent indicates higher short-circuit short-circuit current current and and higher higher conversion conversion efficiency indicates higher efficiency will will be be obtained obtained by by application application of of the TLPS structures. structures. At At aa reverse reverse bias bias of of 55 V, V, the the TLPS the TLPS TLPS device device has has about about one-order one-order higher higher photocurrent photocurrent than that of one based on a single LP-PS layer with porosity of 40%, which was applied as than that of one based on a single LP-PS layer with porosity of 40%, which was applied as the the surface surface film That imply imply that film in in aa Si-based Si-based solar solar cell cell to to achieve achieve optimum optimum device device performance performance [8]. [8]. That that the the TLPS TLPS structures structures have have good good potential potential to to improve improve the the performance performance of of aa Si-based Si-based photovoltaic photovoltaicdevice. device.
Figure 7. Photo and dark current-voltage (I–V) characteristics of photovoltaic devices based Figure 7. Photo and dark current-voltage (I–V) characteristics of photovoltaic devices based on the TLPS structures and the individual PS (NPS, LP-PS or HP-PS) layer on a Si substrate. on the TLPS structures and the individual PS (NPS, LP-PS or HP-PS) layer on a Si substrate. The photocurrent of an NPS-based NPS-based device device is is about about half halfof ofthat thatof ofthe theTLPS TLPSone oneatat55V. V.From From this this data and the photoresponses of the individual NPS layer shown in Figure 4, we deduced that the NPS layer contributes the most part part of of the the photo-induced photo-induced current current to to aa TLPS TLPS device. device. In fact, there are many advantages by incorporating incorporating the the NPS NPS layer layer as as the the surface surface layer layer in in the the device. device. First, the NPS layer significantly increases the responses in the short-wavelength short-wavelength ranges around 450–550 nm that are just the portions portions including including the the maximum maximum irradiance irradiance of of the the incident incident light light (or (or the the sun sun light), light), as as we we can can observed observed in in Figures 44 and texturedsurface surfacestructure structure to to reduce reduce Figures and 5. 5. Second, Second,the themicro-holes micro-holesininthe theNPS NPSlayer layerprovide providea atextured the light-reflection as mentioned above, the the light-reflection losses losses and and augment augmentthe thelight lightabsorption absorptionofofdevices. devices.Third, Third, as mentioned above, NPSNPS layerlayer effectively passivates thethelower middle-band the effectively passivates lowerHP-PS HP-PSlayer layer for for complementing complementing the the middle-band photorespnses. Four, the smooth smooth and and dense dense surface surface of of the the NPS NPS layer layer ensures ensures the the formation formation of of aa Schottky Schottky photorespnses. Four, the contact with with high high quality quality between between the the metal metal electrodes electrodes and and the the PS PS layer. layer. This can be be realized realized contact This advantage advantage can from the the comparison comparison of of the themeasured measureddark darkcurrent currentbetween betweenthe theTLPS TLPSand andthe theHP-PS HP-PSdevices. devices.Although Althougha from a HP-PS device larger series resistance of a TLPS device, has higher dark current as HP-PS device hashas larger series resistance thanthan that that of a TLPS device, it has ithigher dark current as shown shown in 7. Figure For a Schottky device, high of density at the metal-semiconductor (M-S) in Figure For a7.Schottky device, high density defectsofatdefects the metal-semiconductor (M-S) interface interface willthe reduce the Schottky barrier of the M-S junction, thus causing a decrease the photocurrent will reduce Schottky barrier of the M-S junction, thus causing a decrease in theinphotocurrent and and increase the reverse (or leakage) In a photovoltaic leakage current increase in theinreverse (or leakage) current current [22]. In [22]. a photovoltaic device, thedevice, leakagethecurrent dominates dominates the dark current. is, a PS-based photovoltaic device a surface layerofofhigher-porosity higher-porosity the dark current. That is,That a PS-based photovoltaic device withwith a surface PSPS layer usually had had lower lower photocurrent photocurrent and and higher higher dark dark current current due due to to the the degraded degraded Schottky Schottky contact. contact. Therefore, Therefore, usually the large thethe contrary, duedue to the large amount amount of of defects defects on onthe thesurface surfaceHP-PS HP-PSlayer layercause causehigh highdark darkcurrent. current.OnOn contrary, thethe incorporation of of anan NPS good to incorporation NPSsurface surfacelayer, layer,the theTLPS TLPSdevice devicewith withaa middle middle HP-PS HP-PS layer layer has has aa good Schottky contact contact to generate aa higher higher photocurrent photocurrent and and retain retain lower lower dark dark current. current. Since Since the the photo-to-dark photo-to-dark Schottky to generate current ratio (PDCR) is an important figure of merit for the performance of a photovoltaic device,
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current ratio (PDCR) is an important figure of merit for the performance of a photovoltaic device, the large PDCR of the fabricated TLPS-based device demonstrates that the multi-bandgapped PS structure has high potential in development of Si-based photovoltaic device. 4. Conclusions By successively altering the etching parameters in an electrochemical anodization process, TLPS structures with NPS/HP-PS/LP-PS stacked layers can be produced on Si substrates. The three stacked PS layers in the formed TLPS structure have varied optical bandgap energy decreasing from top to bottom and provide the TLPS structure with multiple bandgaps. Experimental results show that the fabricated photovoltaic devices based on TLPS structures exhibit broadband photoresponses within the spectrum of the solar irradiation and get high photocurrent for the incident light of a tungsten lamp. The improved spectral responses are ascribed to the multi-bandgap structures of TLPS in which the separate PS layer with distinct optical bandgap energy absorbs different energy portions of the incident light and contributes all together to the total photoresponses. Most notably, the use of NPS films as the surface layers greatly enhances the light-absorption ability of devices by supplying greatly increased short-wavelength responses and providing good Schottky contacts to get large photocurrents and reduce the leakage current. Therefore, the developed TLPS structures have high potential in fabrication of Si-based solar cells with enhanced conversion efficiency. Acknowledgments The author acknowledge financial support from the Ministry of Science and Technology of Taiwan, R.O.C. under contract No. NSC 102-2221-E-218-038. Author Contributions Kuen-Hsien Wu directed the study, coordinated the research, analyzed the data and wrote the paper; Chong-Wei Li performed the experiments. Conflicts of Interest The authors declare no conflict of interest. References 1. Canham, L.T. Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers. Appl. Phys. Lett. 1990, 57, 1046–1048. [CrossRef] 2. Smith, R.L.; Collins, S.D. Porous silicon formation mechanisms. J. Appl. Phys. 1992, 71, 1–22. [CrossRef] 3. Granitzer, P.; Rumpf, K. Porous silicon—A versatile host material. Materials 2010, 3, 943–998. [CrossRef] 4. Bisi, O.; Ossicini, S.; Pavesi, L. Porous silicon: A quantum sponge structure for silicon based optoelectronics. Surf. Sci. Rep. 2000, 38, 1–126. [CrossRef] 5. Menna, P.; Francia, G.D.; Ferrara, V.L. Porous silicon in solar cells: A review and a description of its application as an AR coating. Sol. Energy Mater. Sol. Cells 1995, 37, 13–24. [CrossRef]
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