nanomaterials Article
Influence of Quantum Dot Concentration on Carrier Transport in ZnO:TiO2 Nano-Hybrid Photoanodes for Quantum Dot-Sensitized Solar Cells Francis S. Maloney *, Uma Poudyal, Weimin Chen and Wenyong Wang Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA; [email protected] (U.P.); [email protected] (W.C.); [email protected] (W.W.) * Correspondence: [email protected]; Tel.: +1-307-766-1121 Academic Editor: Nimai Mishra Received: 15 July 2016; Accepted: 14 October 2016; Published: 25 October 2016
Abstract: Zinc oxide nanowire and titanium dioxide nanoparticle (ZnO:TiO2 NW/NP) hybrid films were utilized as the photoanode layer in quantum dot-sensitized solar cells (QDSSCs). CdSe quantum dots (QDs) with a ZnS passivation layer were deposited on the ZnO:TiO2 NW/NP layer as a photosensitizer by successive ion layer adsorption and reaction (SILAR). Cells were fabricated using a solid-state polymer electrolyte and intensity-modulated photovoltage and photocurrent spectroscopy (IMVS/PS) was carried out to study the electron transport properties of the cell. Increasing the SILAR coating number enhanced the total charge collection efficiency of the cell. The electron transport time constant and diffusion length were found to decrease as more QD layers were added. Keywords: quantum dot sensitized solar cell; photovoltage spectroscopy
carrier transport;
intensity modulated
1. Introduction Since the development of the dye-sensitized solar cells (DSSCs) in 1991 [1], both dye-sensitized and quantum dot sensitized solar cells (QDSSCs) have attracted enormous attention in alternative energy research in both academic and industrial laboratories. To convert sunlight into energy, sensitized solar cells make use of a semiconducting photoanode coated with a light absorbing layer. Upon absorption, this sensitizing layer will generate an exciton (electron-hole pair), leading to the injection of an electron to the photoanode, and a hole to the electrolyte. Quantum dots (QDs) can be used as sensitizers because they are generally small band-gap materials which can inject electrons into a large band-gap material such as TiO2 or ZnO [2,3]. Unlike molecular dyes, inorganic QDs represent the next-generation of solar cell sensitizers and have demonstrated many advantages over dyes including a size-dependent band-gap tunable to the infrared (IR) range, high stability and resistance to oxidation, possibilities for multi-layering of materials, an ability to generate multiple excitons, and the many permutations of materials that can comprise them. [4,5] QDs can also be grown directly onto a photoanode using a variety of techniques. Some methods include chemical bath depositions (CBD), pulsed laser deposition (PLD), spin-casting, dip-coating or successive ionic layer adsorption and reaction (SILAR) [5–7]. SILAR deposition has several advantages over these previous methods in that it allows easy control over the size and density of the QDs, since the dot synthesis takes place during the cycling. It is also relatively fast (the longest of depositions in this study lasting on the order of hours). SILAR is also considered the best method for depositing multilayers over QD cores by alternating the injection of cationic and anionic precursors [5]. QDs can be coupled with a variety of semiconducting photocatalysts to make QDSSCs. TiO2 has been the photocatalyst of choice for most DSSCs and QDSSCs due to its relatively low cost, low toxicity,
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chemical stability, and high efficiency in separating and transferring photogenerated charges to some other material interface [8]. Zinc oxide is another widely used photocatalyst with a similar band-gap Nanomaterials 2016, 6, 191 2 of 8 to TiO 2 (~3.2–3.4 eV at room temperature), though ZnO does not share the same chemical stability. Anotherstability, interesting alternative is in theseparating combination of the twophotogenerated oxides into a ZnO/TiO 2 coupled chemical and high efficiency and transferring charges to some polycrystalline system. Due to the low electron affinity and higher negativity of the ZnO conduction other material interface [8]. Zinc oxide is another widely used photocatalyst with a similar band-gap to bandTiO edge compared TiOtemperature), separation interface facilitated and the carrier 2 (~3.2–3.4 eV at to room though ZnO does at notthe share the sameischemical stability. 2 , electron-hole lifetime can be enhanced In the this system beenoxides used into as aaphotocatalytic reagent, Another interesting[9–11]. alternative is past, the combination of has the two ZnO/TiO2 coupled polycrystalline system. Due to the low electron affinity and higher negativity of the ZnO conduction where electron-hole pairs generated from light absorption and help trigger a redox reaction with a band to TiO2, on electron-hole separation the interface is facilitated and the carrier phenol or edge acid compared group adsorbed the surface [11–14].at The same ZnO/TiO 2 interface mechanism lifetime can be enhanced [9–11]. In the past, this system has been used as a photocatalytic where has also been used to reduce the recombination rate between the electrode andreagent, the electrolyte in electron-hole pairs generated from light absorption and help trigger a redox reaction with a phenol DSCCS and QDSSCs [11,15]. Electrons in ZnO nanoparticles (NPs) or nanowires (NWs) can thereby or acid group adsorbed on the surface [11–14]. The same ZnO/TiO2 interface mechanism has also been be shielded from reacting with oxidized electrolyte species by an energy barrier established by the used to reduce the recombination rate between the electrode and the electrolyte in DSCCS and TiO2 ,QDSSCs causing[11,15]. an increase in carrier lifetime and (NPs) an overall improvement in the performance Electrons in ZnO nanoparticles or nanowires (NWs) can thereby be shieldedof the device. Very few studies have utilized zinc oxide fusedbarrier with established TiO2 NPs as from reacting with oxidized electrolyte species byNWs an energy bythe the photoanode TiO2, causing layer in DSSCs [16]. an increase in carrier lifetime and an overall improvement in the performance of the device. Very few studies utilized oxide charge NWs fused with TiO2in NPs as the photoanode layerthe in DSSCs [16]. In thishave work, we zinc studied transport QDSSCs that utilize zinc titanium oxide this work, we studied charge transport in QDSSCs that utilize the zincelectrolyte. titanium oxide (ZnO:TiOIn nanostructures as the photoanode and a solid-state polymer 2 ) hybrid (ZnO:TiO2) hybrid nanostructures as the photoanode and a solid-state polymer electrolyte.
2. Results 2. Results
Figure 1a shows the scanning electron microscopy (SEM) image of the as-grown ZnO nanowires, Figure 1a shows the scanning electron microscopy (SEM) image of the as-grown ZnO nanowires, while Figure 1b shows the sample after sintering with TiO2 . Before the TiO2 deposition, the average while Figure 1b shows the sample after sintering with TiO2. Before the TiO2 deposition, the average NW length was found to be ~10–15 µm and the average width was ~200 nm. After sintering, it can be NW length was found to be ~10–15 μm and the average width was ~200 nm. After sintering, it can be seen seen that the nanoparticles fused and spread mostly evenly over the surface of the NWs, however thatTiO the2 TiO 2 nanoparticles fused and spread mostly evenly over the surface of the NWs, the width distribution greatly increased. however the width distribution greatly increased.
Figure 1. (a) Scanning electron microscopy (SEM) image of the as-grown ZnO nanowires. (b) Hybrid
Figure 1. (a) Scanning electron microscopy (SEM) image of the as-grown ZnO nanowires. (b) Hybrid film after sintering with TiO2. (c) Schematic of cell structure. film after sintering with TiO2 . (c) Schematic of cell structure.
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Thediagram diagram Figure 1c shows the conceived schematic(not (nottotoscale) scale)ofofthe thecell cell structure diagramininFigure Figure1c 1cshows showsthe theconceived conceivedschematic cellstructure structure The including the ZnO-TiO electrode with a QD coating, fluorine-doped tin oxide/platinum (FTO/Pt) the ZnO-TiO 2 electrode with a QD coating, fluorine-doped tin oxide/platinum (FTO/Pt) including the ZnO-TiO2 2electrode with a QD coating, fluorine-doped tin oxide/platinum (FTO/Pt) electrode and and polymer Incident-photon-to-electron-conversionefficiency efficiency (IPCE) (IPCE) and polymer polymer electrolyte. electrolyte. Incident-photon-to-electron-conversion Incident-photon-to-electron-conversion efficiency electrode electrolyte. measurements were performed on the finished cells and the results are shown in Figure 2. Devices results are shown in Figure 2. Devices measurements were performed on the finished cells and the results are shown in Figure 2. Devices coatedwith withone one through four cycles showed weak photocell performance subjecttotolarge large device onethrough throughfour fourcycles cyclesshowed showedweak weakphotocell photocellperformance performancesubject largedevice device coated variations. At four or more cycles, the photovoltaic response becomes more prominent and indicates variations. variations. At four or more cycles, the photovoltaic response becomes more prominent and indicates thelimit limit QD coverage necessaryfor forefficient efficientlight lightharvesting harvestingand andphoto-generation photo-generationofofelectrons. limitofof ofQD QDcoverage coveragenecessary electrons. the The spectra show red-shift with increasing SILAR cycles, corresponding theincrease increaseininthe the QD Thespectra spectrashow showaaared-shift red-shiftwith withincreasing increasingSILAR SILARcycles, cycles,corresponding correspondingtotothe theQD QD The size and thus the reduction of the band-gap. size and thus the reduction of the band-gap. size and thus the reduction of the band-gap.
Figure2.2. 2.Incident-photon-to-electron-conversion Incident-photon-to-electron-conversionefficiency efficiency(IPCE) (IPCE)signals signalsasas asaaafunction functionofof ofincreasing increasing Figure Figure Incident-photon-to-electron-conversion efficiency (IPCE) signals function increasing successive ion layer adsorption and reaction (SILAR) cycle number. successive successiveion ionlayer layeradsorption adsorptionand andreaction reaction(SILAR) (SILAR)cycle cyclenumber. number.
Short circuitcurrents currents (J)sc)were were estimatedby by integratingthe the areaunder under theIPCE IPCE curves.These These Short Shortcircuit circuit currents(J(Jscsc ) wereestimated estimated byintegrating integrating thearea area underthe the IPCEcurves. curves. These 2 values were~1.0, ~1.0, 10.3,18.4, 18.4, 14.5,95.3 95.3 and127.0 127.0 μA/cm 1–6cycles, cycles,respectively. respectively. 2 for1–6 values 2 for 1–6 valueswere were ~1.0,10.3, 10.3, 18.4,14.5, 14.5, 95.3and and 127.0μA/cm µA/cmfor cycles, respectively. Figure 3a shows the Nyquist diagram of the intensity modulated photovoltagespectra spectra(IMVS) (IMVS) Figure of the the intensity intensitymodulated modulatedphotovoltage photovoltage Figure3a 3ashows shows the the Nyquist Nyquist diagram diagram of spectra (IMVS) of of the QDSSCs under 1 sun intensity for 1–6 SILAR cycle coatings. High to low frequency reads from ofthe theQDSSCs QDSSCs under 1 sun intensity 1–6 SILAR cycle coatings. High low frequency reads from under 1 sun intensity forfor 1–6 SILAR cycle coatings. High to to low frequency reads from left lefttotoright. right.The Therecombination recombinationand andtransport transporttime timeconstants constants(τ(τ rec and τtrs, respectively) can be derived left rec and τtrs, respectively) can be derived to right. The recombination and transport time constants (τrec and τtrs , respectively) can be derived using the equation using usingthe theequation equation (1) τrec (or(or τtrs min τ(or )1/(2πf = 1/(2πf (1) (1) τrec τtrs )τ)=trs=1/(2πf ) )min ) rec min
(a) (a) 1515
1 cycle 1 cycle 2 cycles 2 cycles 3 cycles 3 cycles 4 cycles 4 cycles 5 cycles 5 cycles 6 cycles 6 cycles
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-Im
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-Im
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-Im (10-2-2mV/mWcm )2 -Im (10 mV/mWcm )
wherefmin min is the characteristic frequency taken from the minimum the imaginary componentofof ofthe the where ffmin the characteristic frequencytaken takenfrom fromthe theminimum minimumofof ofthe theimaginary imaginarycomponent component the where is is the characteristic frequency spectrum [17]. The resultant τ are shown in Figure 3b. rec spectrum[17]. [17].The Theresultant resultantττrecrecare areshown shownininFigure Figure3b. 3b. spectrum
0.000
0.000 0.025 0.050 0.075 0.100 2 0.025 0.050 0.075 0.100 Re (mV/mWcm ) 2 Re (mV/mWcm )
600 600
800 800 )
1000 1000
2
2 Re(mV/mW (mV/mW cm Re cm )
Figure 3. (a)Intensity Intensity modulatedphotovoltage photovoltage spectroscopy(IMVS) (IMVS) Nyquistplot plot forZnO:TiO ZnO:TiO2 Figure Figure3.3. (a) (a) Intensitymodulated modulated photovoltagespectroscopy spectroscopy (IMVS)Nyquist Nyquist plotfor for ZnO:TiO2 2 nanowire photoanode coated with CdSe quantum dots (QDs) by increasing SILAR deposition cycles nanowire nanowirephotoanode photoanodecoated coatedwith withCdSe CdSequantum quantumdots dots(QDs) (QDs)by byincreasing increasingSILAR SILARdeposition depositioncycles cycles under 1 sun illumination. (b) Recombination time constants calculated from IMVS data. under under11sun sunillumination. illumination.(b) (b)Recombination Recombinationtime timeconstants constantscalculated calculatedfrom fromIMVS IMVSdata. data.
Figure4a4ashows showsthe theNyquist Nyquistplot plotofofthe theintensity intensitymodulated modulatedphotocurrent photocurrentspectra spectra(IMPS). (IMPS).Figure Figure Figure 4b shows the electron transport times, τ trs, as a function of SILAR cycle number. The diffusion 4b shows the electron transport times, τtrs, as a function of SILAR cycle number. The diffusion
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coefficients, Dn, and diffusion lengths, LD were also examined. The value for Dn was estimated using 4 of 9 the equation
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-Im (10
-4
2 -4 2 -Im (10 mA/mW cm )mA/mWcm )
d2 D (2) n the intensity modulated photocurrent spectra (IMPS). Figure 4a the Nyquist plot 2.35τ coefficients, Dn,shows and diffusion lengths, LDof were alsotrsexamined. The value for Dn was estimated using Figure 4b shows the electron transport times, τtrs , as a function of SILAR cycle number. The diffusion the equation where d is the film thickness [18]. also The examined. electron diffusion length, , of each deviceusing was coefficients, Dn photoanode , and diffusion lengths, LD were The value for DLnDwas estimated 2 calculated using Dn, and τrec using [19]: d the equation Dn (2) 2.35τ trs d2 LDDn =Dn τ rec (2) (3) 2.35τtrs where d is the photoanode film thickness [18]. The electron diffusion length, LD, of each device was The are plottedfilm as athickness function[18]. of SILAR cycle number in length, Figure L5a,b, The where d isresults the photoanode The electron diffusion each device was D , ofrespectively. calculated using Dn, and τrec using [19]: decreasingusing valueDof LD after calculated τrec four usingcycles [19]: implies that when more QD layers are added, the electron n , and LD D (3) transport path is hindered. Though the QD layers improve the photocurrent, they introduce more pn τ rec L = D τ (3) disorder regarding the transport path. The charge collection efficiency, ηc (Figure 5c) was calculated n rec D The results are plotted as a function of SILAR cycle number in Figure 5a,b, respectively. The using [20] decreasing valueare of Lplotted D after four implies that when QD layers are added, the electron The results as acycles function of SILAR cyclemore number in Figure 5a,b, respectively. τ transport path is hindered. Though the QD layers improve the photocurrent, they introduce more The decreasing value of LD after four cyclesηcimplies that when more QD layers are added, the electron 1 trs (4) τ disorder regarding the transport path. collection efficiency, ηc (Figure 5c)introduce was calculated transport path is hindered. Though theThe QDcharge layersred improve the photocurrent, they more using [20] disorder regarding the transport path. The charge collection efficiency, ηc (Figure 5c) was calculated using [20] τ ηηc = 1 1 −trs τtrs (4) (a) (4) c τ redτ red 1 cycle 2 cycles 3 cycles 4 cycles 5 cycles 6 cycles
10
1 cycle 2 cycles 3 cycles 4 cycles 5 cycles 6 cycles
(a) 5 10
05
0.0
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Figure 4. (a) Intensity modulated photocurrent spectroscopy (IMPS) Nyquist plot for ZnO:TiO2 0 nanowire CdSe2.5 quantum dots (QDs) by increasing SILAR deposition cycles 0.0 photoanode 0.5 1.0coated 1.5 with 2.0 2 under 1 sun illumination. (b) Transport time constants, τtrs, calculated from the IMPS data for 1 to 6 Re (mA/mWcm ) SILAR cycles. Figure 4.4. (a) (a) Intensity Intensity modulated modulated photocurrent photocurrent spectroscopy spectroscopy (IMPS) (IMPS) Nyquist Nyquist plot plot for for ZnO:TiO ZnO:TiO2 Figure 2 nanowire photoanode coated with CdSe quantum dots (QDs) by increasing SILAR deposition cycles The transport time constants for the collection of injected electrons into the ZnO:TiO core shell nanowire photoanode coated with CdSe quantum dots (QDs) by increasing SILAR deposition 2cycles under sun illumination. (b) time ττtrs, calculated from IMPS data film is on the order of microseconds, whiletime the constants, recombination time constants are significantly under 11 sun illumination. (b) Transport Transport constants, fromthe the IMPS datafor for11tolonger; to6 trs, calculated SILAR cycles. 6 SILAR on the ordercycles. of seconds. The resulting charge collection efficiency is therefore very high for devices
made using four or more cycles. The transport time constants for the collection of injected electrons into the ZnO:TiO2 core shell film is on the order of microseconds, while the recombination time constants are significantly longer; on the order of seconds. The resulting charge collection efficiency is therefore very high for devices made using four or more cycles.
Figure 5. Cont.
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Figure 5.5.(a) (a)Calculated Calculateddiffusion diffusion coefficients (b) Diffusion lengths 1–6 SILAR cycles. (c) Figure coefficients andand (b) Diffusion lengths for 1–6for SILAR cycles. (c) Charge Charge collection efficiencies calculated for 1–6 SILAR cycles. collection efficiencies calculated for 1–6 SILAR cycles.
3. Discussion The transport time constants for the collection of injected electrons into the ZnO:TiO2 core shell Devices less than 4 cycles showed weak photovoltage responses. This is likely longer; due to film is on the coated order ofbymicroseconds, while the recombination time constants are significantly insufficient of CdSe QDs on charge the photoanodes required for photosensitization. At devices 1 cycle, on the ordercoverage of seconds. The resulting collection efficiency is therefore very high for we seeusing an extremely fast recombination time on the order of 10 μsec. Though TiO2 nanoparticles ought made four or more cycles. to aid in the passivation of surface states on the ZnO NW surface [12], and thus decrease the 3. Discussion time, the fast recombination time observed here implies that the cell is still suffering recombination considerable recombination loss.4This may be attributed to the fact that the steady-state white light Devices coated by less than cycles showed weak photovoltage responses. This is likely due to backgroundcoverage and red light perturbation have a negligible required effect on TiO 2 and ZnO, which absorb in the insufficient of CdSe QDs on the photoanodes for photosensitization. At 1 cycle, near-ultraviolet (UV)fast range [21–23]. In other words, for three less Though cycles charge generation is we see an extremely recombination time on the order of 10orµsec. TiO2 nanoparticles simply not occurring at any observable scale, and the true IMVS response may be clouded by the the ought to aid in the passivation of surface states on the ZnO NW surface [12], and thus decrease resistance-capacitance (RC) of the circuit at highhere frequencies [24,25]. recombination time, the fastattenuation recombination time observed implies that the cell is still suffering At four or more cycles the photo-response of the cell increases significantly. The white near semiconsiderable recombination loss. This may be attributed to the fact that the steady-state light circular shape of the IMVS Nyquist plots in the complex plane indicate that the diffusion D, background and red light perturbation have a negligible effect on TiO2 and ZnO, whichlength, absorbLin has near-ultraviolet exceeded the photoanode film thickness, which is onefor criterion efficient and the (UV) range [21–23]. In other words, three orfor less cyclescharge chargecollection generation is injection [26]. Recombination times for cycles four through six decrease with increasing cycle number. simply not occurring at any observable scale, and the true IMVS response may be clouded by the The decrease in τrec may be understood the following way. As electrons resistance-capacitance (RC) attenuation of theincircuit at high frequencies [24,25]. migrate through the film At to the transparent charge collecting fluorine-doped SnO (FTO) electrode, they undergo many four or more cycles the photo-response of the cell increases significantly. The near semi-circular trapping and de-trapping events, either through recombination with electron acceptors in the shape of the IMVS Nyquist plots in the complex plane indicate that the diffusion length, LD , has oxidized QDs or with redox electrolyte Since recombination occurs primarily through exceeded the photoanode film thickness,species which[20]. is one criterion for efficient charge collection and trap-states than through conduction band states, the reduction of electron recombination times can injection [26]. Recombination times for cycles four through six decrease with increasing cycle number. be attributed to surface passivation induced by the increase of QD coverage [27–29]. This The decrease in τrec may be understood in the following way. As electrons migrate through can the suppress recombination pathways between the nanostructured photoanode and the electrolyte by film to the transparent charge collecting fluorine-doped SnO (FTO) electrode, they undergo many acting as a radial energy barrier [10,16]. Electron injection into the conduction band of the ZnO:TiO trapping and de-trapping events, either through recombination with electron acceptors in the oxidized2 nanostructured anode is therefore more in devicesoccurs with more QD coverage and this is QDs or with redox electrolyte species [20].encouraged Since recombination primarily through trap-states observed as a reduction of the recombination time, τ rec. As the QD coverage is increased, however, than through conduction band states, the reduction of electron recombination times can be attributed to we would expect the creation of increase trap states at the QD-ZnO:TiO interface [6,30–32]. The slower surface passivation induced by the of QD coverage [27–29]. 2This can suppress recombination recombination rate the could also indicate photoanode that the increase of QDs is directly relatedastoa an increase in pathways between nanostructured and the electrolyte by acting radial energy photogenerated charge carriers, whereby the extra carriers fill these interfacial trap states, reduce the barrier [10,16]. Electron injection into the conduction band of the ZnO:TiO2 nanostructured anode is trapping more eventsencouraged and accelerate electron transport [19,33]. therefore in devices with more QD coverage and this is observed as a reduction of the The Nyquist plot of the IMPS (Figure 3a) spectra yielded a single semicircle, suggests recombination time, τrec . As the QD coverage is increased, however, we would expectwhich the creation of electron transport was dominated by one type of process. Based on previous work [18], we trap states at the QD-ZnO:TiO2 interface [6,30–32]. The slower recombination rate could alsoattribute indicate this the process to surface to the particle-like (rather than purely that increase of QDs trap-state is directly assisted related topercolation an increasedue in photogenerated charge carriers, whereby nanowire-like) morphology of the ZnO:TiO 2 film. In our case, the nanostructured anode is comprised the extra carriers fill these interfacial trap states, reduce the trapping events and accelerate electron of single-crystalline transport [19,33]. ZnO nanowires fused under a film of TiO2 nanoparticles. The ZnO nanowire will have a higher conductance than the TiO2 nanoparticles and we would expect that electrons would seek out the path of least resistance through the film (i.e. the path with most ZnO) [16]. This kind of
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The Nyquist plot of the IMPS (Figure 3a) spectra yielded a single semicircle, which suggests electron transport was dominated by one type of process. Based on previous work [18], we attribute this process to surface trap-state assisted percolation due to the particle-like (rather than purely nanowire-like) morphology of the ZnO:TiO2 film. In our case, the nanostructured anode is comprised of single-crystalline ZnO nanowires fused under a film of TiO2 nanoparticles. The ZnO nanowire will have a higher conductance than the TiO2 nanoparticles and we would expect that electrons would seek out the path of least resistance through the film (i.e. the path with most ZnO) [16]. This kind of transport mechanism would then dominate over any direct transport down pure single-crystalline nanowires. As the number of SILAR increases, τtrs also increases. This phenomenon can be explained by considering the reduction of the conduction band edge difference between the QDs and the ZnO:TiO2 as the average QD size is increased with increasing cycle number. Since the electron injection efficiency depends on the number of unfilled surface traps, the loss of the conduction band difference would slow down the transfer process by creating more unfilled traps. When there are more unfilled traps, the number of trapping and de-trapping events increases and τtrs also increases. Regarding the rather low Jsc ’s, the reason may be related to the polymer electrolyte. The effect of this polysulfide polymer electrolyte on the Pt film integrity is not yet well known. Electrolytes that utilize Sx 2− are known to interact with Pt. Sulfides can adsorb onto thin Pt surfaces causing a decrease in electrical conductivity [34]. Given that the Pt layer is rather thin (~200 nm), surface-adsorbed sulfide species may significantly influence the electro-catalytic activity of the cell. Pt-free counter-electrodes, such as Au or Cu2 S, may be used in future studies to further enhance cell performance. 4. Materials and Methods ZnO NWs were grown using a vapor-liquid-solid technique in a 1-in horizontal tube furnace using Zn foil (99.98%, Alfa Aesar, Ward Hill, MA, USA) and oxygen gas. TiO2 nanoparticles (NPs) (99.5%, Organics Aeroxide™ P25, ACROS Organics, Fair Lawn, NJ, USA) were spincoated from an ethanol solution onto the as-grown NWs and annealed for 8 hours at 850 ºC until the two materials were uniformly sintered. CdSe quantum dots were deposited on the ZnO:TiO2 anode layer by a successive ionic layer adsorption and reaction (SILAR) method. 0.03 M Cd(NO3 )2 (99% Fluka, Sigma-Aldrich, St. Louis, MO, USA) in ethanol was used as the cadmium ion source solution. The selenide ion source solution was prepared from SeO2 (99.9%, Sigma-Aldrich, St. Louis, MO, USA) and NaBH4 (99%, Sigma-Aldrich, St. Louis, MO, USA) also dissolved in ethanol. The ZnO:TiO2 anode was immersed in the Cd ionic solution, rinsed with pure ethanol and allowed to dry one minute. This was followed by immersion in the Se ionic solution, followed by the same rinsing and drying process. These six total steps comprised one cycle. Samples were coated with a final layer of ZnS (via immersion in a Zn2+ and S2− ionic solution) to passivate the QD surface. All solution preparation and SILAR cycling was carried out in an argon filled glove box. Solar cells were fabricated by adhering platinum coated fluorine-doped tin oxide (FTO) glass to the QD sensitized ZnO:TiO2 anode using a polymer electrolyte consisting of S/tetramethylammonium sulfate (S/TMAS) (99.5%, Sigma-Aldrich, St. Louis, MO, USA) redox additive to a poly(ethylene oxide)-Poly(vinylidene fluoride) (PEO-PVDF) (99%, Sigma-Aldrich, St. Louis, MO, USA) polymer base [35]. The anode was first coated with the electrolyte and heated in a drying oven at 80 ◦ C for several minutes; until the electrolyte became viscous. The Pt-FTO glass was then pressed to the anode and baked at the same temperature for several hours to allow the electrolyte to dry completely and effectively glue the two electrodes together. SEM images were obtained using FEI Quanta FEG 450 field-emission scanning electron microscope (FEI, Hillsboro, OR, USA). IPCE data was obtained using data was obtained using a monochromatic light source consisting of a 50 W tungsten halogen lamp and a monochromator (Newport Corporation, Irvine, CA, USA). The light beam was modulated by a chopper and a lock-in amplifier (Stanford Research SR830, Sunnyvale, CA, USA). Intensity modulated photocurrent and photovoltage (IMPS/VS)
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measurements were carried out under a constant white light background with a sinusoidal perturbation of red light (λ = 625 nm). The white light was supplied by a light-emitting diode (LED) array powered from a Yokogawa 7651 low-noise direct current (DC) supply (Newnan, GA, USA). The red light intensity was modulated by the output of a Stanford SR781 dynamic signal analyzer (Stanford Research SR830, Sunnyvale, CA, USA). The transfer function module of the SR780 detected the IMPS/VS signal, while a Stanford SR570 current preamplifier amplified the photocurrent response from the cell. The intensity of the red light output was maintained at less than 10% of the white light background intensity. The red light frequency was scanned from 0.1 Hz to 100 kHz. All measurements were made in the dark. 5. Conclusions The electron transport properties of ZnO:TiO2 NW/NP CdSe QDSSCs were studied. The CdSe QDs were deposited by varying SILAR cycles numbers and coated with a ZnS passivation layer. Device performance was dependent on the cycle number. The photocurrent of the device increased with successive SILAR coatings and the highest photocurrent was observed for devices coated with 6 SILAR cycles. IMVS and IMPS measurements were carried out and the transport and recombination time constants and charge collection efficiencies were obtained. Both the recombination time and transport time were found to decrease with increasing SILAR cycles, indicating surface state passivation due to QD coverage. Due to its low toxicity and potential for optimizing the band alignment between the QD and electrode, ZnO:TiO2 systems may be an important active material in future QDSSC studies. Acknowledgments: This work was supported by NASA EPSCoR under Award NNX10AR90A. Author Contributions: Francis S. Maloney, Weimin Chen and Uma Poudyal conceived and designed the experiments, Francis S. Maloney and Uma Poudyal performed the experiments; Francis S. Maloney analyzed the data, Wenyong Wang contributed materials/analysis tools, Francis S. Maloney wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Gujarro, N.; Lana-Villareal, T.; Shen, Q.; Toyoda, T.; Gómez, R. Sensitization of titanium dioxide photoanodes with cadmium selenide quantum dots prepared by SILAR: Photoelectrochemical and carrier dynamics. J. Phys. Chem. C 2010, 114, 21928–21937. [CrossRef] Yeh, M.H.; Lee, C.P.; Chou, C.Y.; Lin, L.Y.; Wei, H.Y.; Chu, C.W. Conducting polymer-based counter electrode for a quantum-dot-sensitized solar cell (QDSSC) with a polysulfide electrolyte. Electrochim. Acta 2011, 57, 277–284. [CrossRef] Yang, Y.; Wang, W. A new polymer electrolyte for solid-state quantum dot sensitized solar cells. J. Power Sources 2015, 285, 70–75. [CrossRef] © 2016 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 (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
Influence of Quantum Dot Concentration on Carrier Transport in ZnO:TiO2 Nano-Hybrid Photoanodes for Quantum Dot-Sensitized Solar Cells Francis S. Maloney *, Uma Poudyal, Weimin Chen and Wenyong Wang Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA; [email protected] (U.P.); [email protected] (W.C.); [email protected] (W.W.) * Correspondence: [email protected]; Tel.: +1-307-766-1121 Academic Editor: Nimai Mishra Received: 15 July 2016; Accepted: 14 October 2016; Published: 25 October 2016
Abstract: Zinc oxide nanowire and titanium dioxide nanoparticle (ZnO:TiO2 NW/NP) hybrid films were utilized as the photoanode layer in quantum dot-sensitized solar cells (QDSSCs). CdSe quantum dots (QDs) with a ZnS passivation layer were deposited on the ZnO:TiO2 NW/NP layer as a photosensitizer by successive ion layer adsorption and reaction (SILAR). Cells were fabricated using a solid-state polymer electrolyte and intensity-modulated photovoltage and photocurrent spectroscopy (IMVS/PS) was carried out to study the electron transport properties of the cell. Increasing the SILAR coating number enhanced the total charge collection efficiency of the cell. The electron transport time constant and diffusion length were found to decrease as more QD layers were added. Keywords: quantum dot sensitized solar cell; photovoltage spectroscopy
carrier transport;
intensity modulated
1. Introduction Since the development of the dye-sensitized solar cells (DSSCs) in 1991 [1], both dye-sensitized and quantum dot sensitized solar cells (QDSSCs) have attracted enormous attention in alternative energy research in both academic and industrial laboratories. To convert sunlight into energy, sensitized solar cells make use of a semiconducting photoanode coated with a light absorbing layer. Upon absorption, this sensitizing layer will generate an exciton (electron-hole pair), leading to the injection of an electron to the photoanode, and a hole to the electrolyte. Quantum dots (QDs) can be used as sensitizers because they are generally small band-gap materials which can inject electrons into a large band-gap material such as TiO2 or ZnO [2,3]. Unlike molecular dyes, inorganic QDs represent the next-generation of solar cell sensitizers and have demonstrated many advantages over dyes including a size-dependent band-gap tunable to the infrared (IR) range, high stability and resistance to oxidation, possibilities for multi-layering of materials, an ability to generate multiple excitons, and the many permutations of materials that can comprise them. [4,5] QDs can also be grown directly onto a photoanode using a variety of techniques. Some methods include chemical bath depositions (CBD), pulsed laser deposition (PLD), spin-casting, dip-coating or successive ionic layer adsorption and reaction (SILAR) [5–7]. SILAR deposition has several advantages over these previous methods in that it allows easy control over the size and density of the QDs, since the dot synthesis takes place during the cycling. It is also relatively fast (the longest of depositions in this study lasting on the order of hours). SILAR is also considered the best method for depositing multilayers over QD cores by alternating the injection of cationic and anionic precursors [5]. QDs can be coupled with a variety of semiconducting photocatalysts to make QDSSCs. TiO2 has been the photocatalyst of choice for most DSSCs and QDSSCs due to its relatively low cost, low toxicity,
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chemical stability, and high efficiency in separating and transferring photogenerated charges to some other material interface [8]. Zinc oxide is another widely used photocatalyst with a similar band-gap Nanomaterials 2016, 6, 191 2 of 8 to TiO 2 (~3.2–3.4 eV at room temperature), though ZnO does not share the same chemical stability. Anotherstability, interesting alternative is in theseparating combination of the twophotogenerated oxides into a ZnO/TiO 2 coupled chemical and high efficiency and transferring charges to some polycrystalline system. Due to the low electron affinity and higher negativity of the ZnO conduction other material interface [8]. Zinc oxide is another widely used photocatalyst with a similar band-gap to bandTiO edge compared TiOtemperature), separation interface facilitated and the carrier 2 (~3.2–3.4 eV at to room though ZnO does at notthe share the sameischemical stability. 2 , electron-hole lifetime can be enhanced In the this system beenoxides used into as aaphotocatalytic reagent, Another interesting[9–11]. alternative is past, the combination of has the two ZnO/TiO2 coupled polycrystalline system. Due to the low electron affinity and higher negativity of the ZnO conduction where electron-hole pairs generated from light absorption and help trigger a redox reaction with a band to TiO2, on electron-hole separation the interface is facilitated and the carrier phenol or edge acid compared group adsorbed the surface [11–14].at The same ZnO/TiO 2 interface mechanism lifetime can be enhanced [9–11]. In the past, this system has been used as a photocatalytic where has also been used to reduce the recombination rate between the electrode andreagent, the electrolyte in electron-hole pairs generated from light absorption and help trigger a redox reaction with a phenol DSCCS and QDSSCs [11,15]. Electrons in ZnO nanoparticles (NPs) or nanowires (NWs) can thereby or acid group adsorbed on the surface [11–14]. The same ZnO/TiO2 interface mechanism has also been be shielded from reacting with oxidized electrolyte species by an energy barrier established by the used to reduce the recombination rate between the electrode and the electrolyte in DSCCS and TiO2 ,QDSSCs causing[11,15]. an increase in carrier lifetime and (NPs) an overall improvement in the performance Electrons in ZnO nanoparticles or nanowires (NWs) can thereby be shieldedof the device. Very few studies have utilized zinc oxide fusedbarrier with established TiO2 NPs as from reacting with oxidized electrolyte species byNWs an energy bythe the photoanode TiO2, causing layer in DSSCs [16]. an increase in carrier lifetime and an overall improvement in the performance of the device. Very few studies utilized oxide charge NWs fused with TiO2in NPs as the photoanode layerthe in DSSCs [16]. In thishave work, we zinc studied transport QDSSCs that utilize zinc titanium oxide this work, we studied charge transport in QDSSCs that utilize the zincelectrolyte. titanium oxide (ZnO:TiOIn nanostructures as the photoanode and a solid-state polymer 2 ) hybrid (ZnO:TiO2) hybrid nanostructures as the photoanode and a solid-state polymer electrolyte.
2. Results 2. Results
Figure 1a shows the scanning electron microscopy (SEM) image of the as-grown ZnO nanowires, Figure 1a shows the scanning electron microscopy (SEM) image of the as-grown ZnO nanowires, while Figure 1b shows the sample after sintering with TiO2 . Before the TiO2 deposition, the average while Figure 1b shows the sample after sintering with TiO2. Before the TiO2 deposition, the average NW length was found to be ~10–15 µm and the average width was ~200 nm. After sintering, it can be NW length was found to be ~10–15 μm and the average width was ~200 nm. After sintering, it can be seen seen that the nanoparticles fused and spread mostly evenly over the surface of the NWs, however thatTiO the2 TiO 2 nanoparticles fused and spread mostly evenly over the surface of the NWs, the width distribution greatly increased. however the width distribution greatly increased.
Figure 1. (a) Scanning electron microscopy (SEM) image of the as-grown ZnO nanowires. (b) Hybrid
Figure 1. (a) Scanning electron microscopy (SEM) image of the as-grown ZnO nanowires. (b) Hybrid film after sintering with TiO2. (c) Schematic of cell structure. film after sintering with TiO2 . (c) Schematic of cell structure.
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Thediagram diagram Figure 1c shows the conceived schematic(not (nottotoscale) scale)ofofthe thecell cell structure diagramininFigure Figure1c 1cshows showsthe theconceived conceivedschematic cellstructure structure The including the ZnO-TiO electrode with a QD coating, fluorine-doped tin oxide/platinum (FTO/Pt) the ZnO-TiO 2 electrode with a QD coating, fluorine-doped tin oxide/platinum (FTO/Pt) including the ZnO-TiO2 2electrode with a QD coating, fluorine-doped tin oxide/platinum (FTO/Pt) electrode and and polymer Incident-photon-to-electron-conversionefficiency efficiency (IPCE) (IPCE) and polymer polymer electrolyte. electrolyte. Incident-photon-to-electron-conversion Incident-photon-to-electron-conversion efficiency electrode electrolyte. measurements were performed on the finished cells and the results are shown in Figure 2. Devices results are shown in Figure 2. Devices measurements were performed on the finished cells and the results are shown in Figure 2. Devices coatedwith withone one through four cycles showed weak photocell performance subjecttotolarge large device onethrough throughfour fourcycles cyclesshowed showedweak weakphotocell photocellperformance performancesubject largedevice device coated variations. At four or more cycles, the photovoltaic response becomes more prominent and indicates variations. variations. At four or more cycles, the photovoltaic response becomes more prominent and indicates thelimit limit QD coverage necessaryfor forefficient efficientlight lightharvesting harvestingand andphoto-generation photo-generationofofelectrons. limitofof ofQD QDcoverage coveragenecessary electrons. the The spectra show red-shift with increasing SILAR cycles, corresponding theincrease increaseininthe the QD Thespectra spectrashow showaaared-shift red-shiftwith withincreasing increasingSILAR SILARcycles, cycles,corresponding correspondingtotothe theQD QD The size and thus the reduction of the band-gap. size and thus the reduction of the band-gap. size and thus the reduction of the band-gap.
Figure2.2. 2.Incident-photon-to-electron-conversion Incident-photon-to-electron-conversionefficiency efficiency(IPCE) (IPCE)signals signalsasas asaaafunction functionofof ofincreasing increasing Figure Figure Incident-photon-to-electron-conversion efficiency (IPCE) signals function increasing successive ion layer adsorption and reaction (SILAR) cycle number. successive successiveion ionlayer layeradsorption adsorptionand andreaction reaction(SILAR) (SILAR)cycle cyclenumber. number.
Short circuitcurrents currents (J)sc)were were estimatedby by integratingthe the areaunder under theIPCE IPCE curves.These These Short Shortcircuit circuit currents(J(Jscsc ) wereestimated estimated byintegrating integrating thearea area underthe the IPCEcurves. curves. These 2 values were~1.0, ~1.0, 10.3,18.4, 18.4, 14.5,95.3 95.3 and127.0 127.0 μA/cm 1–6cycles, cycles,respectively. respectively. 2 for1–6 values 2 for 1–6 valueswere were ~1.0,10.3, 10.3, 18.4,14.5, 14.5, 95.3and and 127.0μA/cm µA/cmfor cycles, respectively. Figure 3a shows the Nyquist diagram of the intensity modulated photovoltagespectra spectra(IMVS) (IMVS) Figure of the the intensity intensitymodulated modulatedphotovoltage photovoltage Figure3a 3ashows shows the the Nyquist Nyquist diagram diagram of spectra (IMVS) of of the QDSSCs under 1 sun intensity for 1–6 SILAR cycle coatings. High to low frequency reads from ofthe theQDSSCs QDSSCs under 1 sun intensity 1–6 SILAR cycle coatings. High low frequency reads from under 1 sun intensity forfor 1–6 SILAR cycle coatings. High to to low frequency reads from left lefttotoright. right.The Therecombination recombinationand andtransport transporttime timeconstants constants(τ(τ rec and τtrs, respectively) can be derived left rec and τtrs, respectively) can be derived to right. The recombination and transport time constants (τrec and τtrs , respectively) can be derived using the equation using usingthe theequation equation (1) τrec (or(or τtrs min τ(or )1/(2πf = 1/(2πf (1) (1) τrec τtrs )τ)=trs=1/(2πf ) )min ) rec min
(a) (a) 1515
1 cycle 1 cycle 2 cycles 2 cycles 3 cycles 3 cycles 4 cycles 4 cycles 5 cycles 5 cycles 6 cycles 6 cycles
1010
1.00
-4
(10
)
2
mV/mW cm
200 200
400 400
0.25
0.25 0.00 -0.25
(10
-4
00
1 cycle 1 cycle2 cycles 2 cycles
0.50
0.50
0.00 -0.25 -0.50 -0.50
00
(a) 0.75 (a)
0.75
-Im
55
mV/mW cm
2
)
1.00
-Im
2
-Im (10-2-2mV/mWcm )2 -Im (10 mV/mWcm )
wherefmin min is the characteristic frequency taken from the minimum the imaginary componentofof ofthe the where ffmin the characteristic frequencytaken takenfrom fromthe theminimum minimumofof ofthe theimaginary imaginarycomponent component the where is is the characteristic frequency spectrum [17]. The resultant τ are shown in Figure 3b. rec spectrum[17]. [17].The Theresultant resultantττrecrecare areshown shownininFigure Figure3b. 3b. spectrum
0.000
0.000 0.025 0.050 0.075 0.100 2 0.025 0.050 0.075 0.100 Re (mV/mWcm ) 2 Re (mV/mWcm )
600 600
800 800 )
1000 1000
2
2 Re(mV/mW (mV/mW cm Re cm )
Figure 3. (a)Intensity Intensity modulatedphotovoltage photovoltage spectroscopy(IMVS) (IMVS) Nyquistplot plot forZnO:TiO ZnO:TiO2 Figure Figure3.3. (a) (a) Intensitymodulated modulated photovoltagespectroscopy spectroscopy (IMVS)Nyquist Nyquist plotfor for ZnO:TiO2 2 nanowire photoanode coated with CdSe quantum dots (QDs) by increasing SILAR deposition cycles nanowire nanowirephotoanode photoanodecoated coatedwith withCdSe CdSequantum quantumdots dots(QDs) (QDs)by byincreasing increasingSILAR SILARdeposition depositioncycles cycles under 1 sun illumination. (b) Recombination time constants calculated from IMVS data. under under11sun sunillumination. illumination.(b) (b)Recombination Recombinationtime timeconstants constantscalculated calculatedfrom fromIMVS IMVSdata. data.
Figure4a4ashows showsthe theNyquist Nyquistplot plotofofthe theintensity intensitymodulated modulatedphotocurrent photocurrentspectra spectra(IMPS). (IMPS).Figure Figure Figure 4b shows the electron transport times, τ trs, as a function of SILAR cycle number. The diffusion 4b shows the electron transport times, τtrs, as a function of SILAR cycle number. The diffusion
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coefficients, Dn, and diffusion lengths, LD were also examined. The value for Dn was estimated using 4 of 9 the equation
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-Im (10
-4
2 -4 2 -Im (10 mA/mW cm )mA/mWcm )
d2 D (2) n the intensity modulated photocurrent spectra (IMPS). Figure 4a the Nyquist plot 2.35τ coefficients, Dn,shows and diffusion lengths, LDof were alsotrsexamined. The value for Dn was estimated using Figure 4b shows the electron transport times, τtrs , as a function of SILAR cycle number. The diffusion the equation where d is the film thickness [18]. also The examined. electron diffusion length, , of each deviceusing was coefficients, Dn photoanode , and diffusion lengths, LD were The value for DLnDwas estimated 2 calculated using Dn, and τrec using [19]: d the equation Dn (2) 2.35τ trs d2 LDDn =Dn τ rec (2) (3) 2.35τtrs where d is the photoanode film thickness [18]. The electron diffusion length, LD, of each device was The are plottedfilm as athickness function[18]. of SILAR cycle number in length, Figure L5a,b, The where d isresults the photoanode The electron diffusion each device was D , ofrespectively. calculated using Dn, and τrec using [19]: decreasingusing valueDof LD after calculated τrec four usingcycles [19]: implies that when more QD layers are added, the electron n , and LD D (3) transport path is hindered. Though the QD layers improve the photocurrent, they introduce more pn τ rec L = D τ (3) disorder regarding the transport path. The charge collection efficiency, ηc (Figure 5c) was calculated n rec D The results are plotted as a function of SILAR cycle number in Figure 5a,b, respectively. The using [20] decreasing valueare of Lplotted D after four implies that when QD layers are added, the electron The results as acycles function of SILAR cyclemore number in Figure 5a,b, respectively. τ transport path is hindered. Though the QD layers improve the photocurrent, they introduce more The decreasing value of LD after four cyclesηcimplies that when more QD layers are added, the electron 1 trs (4) τ disorder regarding the transport path. collection efficiency, ηc (Figure 5c)introduce was calculated transport path is hindered. Though theThe QDcharge layersred improve the photocurrent, they more using [20] disorder regarding the transport path. The charge collection efficiency, ηc (Figure 5c) was calculated using [20] τ ηηc = 1 1 −trs τtrs (4) (a) (4) c τ redτ red 1 cycle 2 cycles 3 cycles 4 cycles 5 cycles 6 cycles
10
1 cycle 2 cycles 3 cycles 4 cycles 5 cycles 6 cycles
(a) 5 10
05
0.0
0.5
1.0
1.5
2.0
2.5
2
Re (mA/mWcm
)
Figure 4. (a) Intensity modulated photocurrent spectroscopy (IMPS) Nyquist plot for ZnO:TiO2 0 nanowire CdSe2.5 quantum dots (QDs) by increasing SILAR deposition cycles 0.0 photoanode 0.5 1.0coated 1.5 with 2.0 2 under 1 sun illumination. (b) Transport time constants, τtrs, calculated from the IMPS data for 1 to 6 Re (mA/mWcm ) SILAR cycles. Figure 4.4. (a) (a) Intensity Intensity modulated modulated photocurrent photocurrent spectroscopy spectroscopy (IMPS) (IMPS) Nyquist Nyquist plot plot for for ZnO:TiO ZnO:TiO2 Figure 2 nanowire photoanode coated with CdSe quantum dots (QDs) by increasing SILAR deposition cycles The transport time constants for the collection of injected electrons into the ZnO:TiO core shell nanowire photoanode coated with CdSe quantum dots (QDs) by increasing SILAR deposition 2cycles under sun illumination. (b) time ττtrs, calculated from IMPS data film is on the order of microseconds, whiletime the constants, recombination time constants are significantly under 11 sun illumination. (b) Transport Transport constants, fromthe the IMPS datafor for11tolonger; to6 trs, calculated SILAR cycles. 6 SILAR on the ordercycles. of seconds. The resulting charge collection efficiency is therefore very high for devices
made using four or more cycles. The transport time constants for the collection of injected electrons into the ZnO:TiO2 core shell film is on the order of microseconds, while the recombination time constants are significantly longer; on the order of seconds. The resulting charge collection efficiency is therefore very high for devices made using four or more cycles.
Figure 5. Cont.
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Figure 5.5.(a) (a)Calculated Calculateddiffusion diffusion coefficients (b) Diffusion lengths 1–6 SILAR cycles. (c) Figure coefficients andand (b) Diffusion lengths for 1–6for SILAR cycles. (c) Charge Charge collection efficiencies calculated for 1–6 SILAR cycles. collection efficiencies calculated for 1–6 SILAR cycles.
3. Discussion The transport time constants for the collection of injected electrons into the ZnO:TiO2 core shell Devices less than 4 cycles showed weak photovoltage responses. This is likely longer; due to film is on the coated order ofbymicroseconds, while the recombination time constants are significantly insufficient of CdSe QDs on charge the photoanodes required for photosensitization. At devices 1 cycle, on the ordercoverage of seconds. The resulting collection efficiency is therefore very high for we seeusing an extremely fast recombination time on the order of 10 μsec. Though TiO2 nanoparticles ought made four or more cycles. to aid in the passivation of surface states on the ZnO NW surface [12], and thus decrease the 3. Discussion time, the fast recombination time observed here implies that the cell is still suffering recombination considerable recombination loss.4This may be attributed to the fact that the steady-state white light Devices coated by less than cycles showed weak photovoltage responses. This is likely due to backgroundcoverage and red light perturbation have a negligible required effect on TiO 2 and ZnO, which absorb in the insufficient of CdSe QDs on the photoanodes for photosensitization. At 1 cycle, near-ultraviolet (UV)fast range [21–23]. In other words, for three less Though cycles charge generation is we see an extremely recombination time on the order of 10orµsec. TiO2 nanoparticles simply not occurring at any observable scale, and the true IMVS response may be clouded by the the ought to aid in the passivation of surface states on the ZnO NW surface [12], and thus decrease resistance-capacitance (RC) of the circuit at highhere frequencies [24,25]. recombination time, the fastattenuation recombination time observed implies that the cell is still suffering At four or more cycles the photo-response of the cell increases significantly. The white near semiconsiderable recombination loss. This may be attributed to the fact that the steady-state light circular shape of the IMVS Nyquist plots in the complex plane indicate that the diffusion D, background and red light perturbation have a negligible effect on TiO2 and ZnO, whichlength, absorbLin has near-ultraviolet exceeded the photoanode film thickness, which is onefor criterion efficient and the (UV) range [21–23]. In other words, three orfor less cyclescharge chargecollection generation is injection [26]. Recombination times for cycles four through six decrease with increasing cycle number. simply not occurring at any observable scale, and the true IMVS response may be clouded by the The decrease in τrec may be understood the following way. As electrons resistance-capacitance (RC) attenuation of theincircuit at high frequencies [24,25]. migrate through the film At to the transparent charge collecting fluorine-doped SnO (FTO) electrode, they undergo many four or more cycles the photo-response of the cell increases significantly. The near semi-circular trapping and de-trapping events, either through recombination with electron acceptors in the shape of the IMVS Nyquist plots in the complex plane indicate that the diffusion length, LD , has oxidized QDs or with redox electrolyte Since recombination occurs primarily through exceeded the photoanode film thickness,species which[20]. is one criterion for efficient charge collection and trap-states than through conduction band states, the reduction of electron recombination times can injection [26]. Recombination times for cycles four through six decrease with increasing cycle number. be attributed to surface passivation induced by the increase of QD coverage [27–29]. This The decrease in τrec may be understood in the following way. As electrons migrate through can the suppress recombination pathways between the nanostructured photoanode and the electrolyte by film to the transparent charge collecting fluorine-doped SnO (FTO) electrode, they undergo many acting as a radial energy barrier [10,16]. Electron injection into the conduction band of the ZnO:TiO trapping and de-trapping events, either through recombination with electron acceptors in the oxidized2 nanostructured anode is therefore more in devicesoccurs with more QD coverage and this is QDs or with redox electrolyte species [20].encouraged Since recombination primarily through trap-states observed as a reduction of the recombination time, τ rec. As the QD coverage is increased, however, than through conduction band states, the reduction of electron recombination times can be attributed to we would expect the creation of increase trap states at the QD-ZnO:TiO interface [6,30–32]. The slower surface passivation induced by the of QD coverage [27–29]. 2This can suppress recombination recombination rate the could also indicate photoanode that the increase of QDs is directly relatedastoa an increase in pathways between nanostructured and the electrolyte by acting radial energy photogenerated charge carriers, whereby the extra carriers fill these interfacial trap states, reduce the barrier [10,16]. Electron injection into the conduction band of the ZnO:TiO2 nanostructured anode is trapping more eventsencouraged and accelerate electron transport [19,33]. therefore in devices with more QD coverage and this is observed as a reduction of the The Nyquist plot of the IMPS (Figure 3a) spectra yielded a single semicircle, suggests recombination time, τrec . As the QD coverage is increased, however, we would expectwhich the creation of electron transport was dominated by one type of process. Based on previous work [18], we trap states at the QD-ZnO:TiO2 interface [6,30–32]. The slower recombination rate could alsoattribute indicate this the process to surface to the particle-like (rather than purely that increase of QDs trap-state is directly assisted related topercolation an increasedue in photogenerated charge carriers, whereby nanowire-like) morphology of the ZnO:TiO 2 film. In our case, the nanostructured anode is comprised the extra carriers fill these interfacial trap states, reduce the trapping events and accelerate electron of single-crystalline transport [19,33]. ZnO nanowires fused under a film of TiO2 nanoparticles. The ZnO nanowire will have a higher conductance than the TiO2 nanoparticles and we would expect that electrons would seek out the path of least resistance through the film (i.e. the path with most ZnO) [16]. This kind of
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The Nyquist plot of the IMPS (Figure 3a) spectra yielded a single semicircle, which suggests electron transport was dominated by one type of process. Based on previous work [18], we attribute this process to surface trap-state assisted percolation due to the particle-like (rather than purely nanowire-like) morphology of the ZnO:TiO2 film. In our case, the nanostructured anode is comprised of single-crystalline ZnO nanowires fused under a film of TiO2 nanoparticles. The ZnO nanowire will have a higher conductance than the TiO2 nanoparticles and we would expect that electrons would seek out the path of least resistance through the film (i.e. the path with most ZnO) [16]. This kind of transport mechanism would then dominate over any direct transport down pure single-crystalline nanowires. As the number of SILAR increases, τtrs also increases. This phenomenon can be explained by considering the reduction of the conduction band edge difference between the QDs and the ZnO:TiO2 as the average QD size is increased with increasing cycle number. Since the electron injection efficiency depends on the number of unfilled surface traps, the loss of the conduction band difference would slow down the transfer process by creating more unfilled traps. When there are more unfilled traps, the number of trapping and de-trapping events increases and τtrs also increases. Regarding the rather low Jsc ’s, the reason may be related to the polymer electrolyte. The effect of this polysulfide polymer electrolyte on the Pt film integrity is not yet well known. Electrolytes that utilize Sx 2− are known to interact with Pt. Sulfides can adsorb onto thin Pt surfaces causing a decrease in electrical conductivity [34]. Given that the Pt layer is rather thin (~200 nm), surface-adsorbed sulfide species may significantly influence the electro-catalytic activity of the cell. Pt-free counter-electrodes, such as Au or Cu2 S, may be used in future studies to further enhance cell performance. 4. Materials and Methods ZnO NWs were grown using a vapor-liquid-solid technique in a 1-in horizontal tube furnace using Zn foil (99.98%, Alfa Aesar, Ward Hill, MA, USA) and oxygen gas. TiO2 nanoparticles (NPs) (99.5%, Organics Aeroxide™ P25, ACROS Organics, Fair Lawn, NJ, USA) were spincoated from an ethanol solution onto the as-grown NWs and annealed for 8 hours at 850 ºC until the two materials were uniformly sintered. CdSe quantum dots were deposited on the ZnO:TiO2 anode layer by a successive ionic layer adsorption and reaction (SILAR) method. 0.03 M Cd(NO3 )2 (99% Fluka, Sigma-Aldrich, St. Louis, MO, USA) in ethanol was used as the cadmium ion source solution. The selenide ion source solution was prepared from SeO2 (99.9%, Sigma-Aldrich, St. Louis, MO, USA) and NaBH4 (99%, Sigma-Aldrich, St. Louis, MO, USA) also dissolved in ethanol. The ZnO:TiO2 anode was immersed in the Cd ionic solution, rinsed with pure ethanol and allowed to dry one minute. This was followed by immersion in the Se ionic solution, followed by the same rinsing and drying process. These six total steps comprised one cycle. Samples were coated with a final layer of ZnS (via immersion in a Zn2+ and S2− ionic solution) to passivate the QD surface. All solution preparation and SILAR cycling was carried out in an argon filled glove box. Solar cells were fabricated by adhering platinum coated fluorine-doped tin oxide (FTO) glass to the QD sensitized ZnO:TiO2 anode using a polymer electrolyte consisting of S/tetramethylammonium sulfate (S/TMAS) (99.5%, Sigma-Aldrich, St. Louis, MO, USA) redox additive to a poly(ethylene oxide)-Poly(vinylidene fluoride) (PEO-PVDF) (99%, Sigma-Aldrich, St. Louis, MO, USA) polymer base [35]. The anode was first coated with the electrolyte and heated in a drying oven at 80 ◦ C for several minutes; until the electrolyte became viscous. The Pt-FTO glass was then pressed to the anode and baked at the same temperature for several hours to allow the electrolyte to dry completely and effectively glue the two electrodes together. SEM images were obtained using FEI Quanta FEG 450 field-emission scanning electron microscope (FEI, Hillsboro, OR, USA). IPCE data was obtained using data was obtained using a monochromatic light source consisting of a 50 W tungsten halogen lamp and a monochromator (Newport Corporation, Irvine, CA, USA). The light beam was modulated by a chopper and a lock-in amplifier (Stanford Research SR830, Sunnyvale, CA, USA). Intensity modulated photocurrent and photovoltage (IMPS/VS)
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measurements were carried out under a constant white light background with a sinusoidal perturbation of red light (λ = 625 nm). The white light was supplied by a light-emitting diode (LED) array powered from a Yokogawa 7651 low-noise direct current (DC) supply (Newnan, GA, USA). The red light intensity was modulated by the output of a Stanford SR781 dynamic signal analyzer (Stanford Research SR830, Sunnyvale, CA, USA). The transfer function module of the SR780 detected the IMPS/VS signal, while a Stanford SR570 current preamplifier amplified the photocurrent response from the cell. The intensity of the red light output was maintained at less than 10% of the white light background intensity. The red light frequency was scanned from 0.1 Hz to 100 kHz. All measurements were made in the dark. 5. Conclusions The electron transport properties of ZnO:TiO2 NW/NP CdSe QDSSCs were studied. The CdSe QDs were deposited by varying SILAR cycles numbers and coated with a ZnS passivation layer. Device performance was dependent on the cycle number. The photocurrent of the device increased with successive SILAR coatings and the highest photocurrent was observed for devices coated with 6 SILAR cycles. IMVS and IMPS measurements were carried out and the transport and recombination time constants and charge collection efficiencies were obtained. Both the recombination time and transport time were found to decrease with increasing SILAR cycles, indicating surface state passivation due to QD coverage. Due to its low toxicity and potential for optimizing the band alignment between the QD and electrode, ZnO:TiO2 systems may be an important active material in future QDSSC studies. Acknowledgments: This work was supported by NASA EPSCoR under Award NNX10AR90A. Author Contributions: Francis S. Maloney, Weimin Chen and Uma Poudyal conceived and designed the experiments, Francis S. Maloney and Uma Poudyal performed the experiments; Francis S. Maloney analyzed the data, Wenyong Wang contributed materials/analysis tools, Francis S. Maloney wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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