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13.8% Efficiency Hybrid Si/Organic Heterojunction Solar Cells with MoO3 Film as Antireflection and Inversion Induced Layer Ruiyuan Liu, Shuit-Tong Lee, and Baoquan Sun* Low temperature processed organic-inorganic hybrid solar cells based on Si are currently under intensive investigation for their unique advantages compared with traditional high temperature p-n junction counterparts: easy fabrication, simple device structure, and potentially low cost. The Si-organic hybrid structures combining the advantageous characteristics of crystalline Si and transparent conductive organics have emerged as promising candidates for cost-effective photovoltaics.[1–9] In principle, light is predominately absorbed by Si. Electron-hole pairs are then generated and separated with the driving force of a built-in electric field. The main functions of the organic layer are the formation of a heterojunction with Si, as hole/ electron transporting path, as optical window, and sometimes serving as an antireflection coating, such as poly(3,4-ethylene dioxythiophene):poly-styrenesulfonate (PEDOT:PSS).[10–14] Over the past few years, considerable progress has been achieved in the study of hybrid solar cells and power conversion efficiencies (PCEs) of over 10% have been reported by many groups. Most recently, we have achieved a PCE of over 12% in n-Si/ PEDOT:PSS hybrid solar cells by using methyl groups to passivate the Si surface in order to suppress the surface recombination velocity, as well as inserting a thin layer of wide band gap organic semiconductor 8-hydroxyquinolinolato-lithium (Liq) between the rear side of the Si substrate and Al in order to improve rear contact as well as reduce charge recombination.[15] At this high point, any improvement of the open-circuit voltage (Voc), the short-circuit current density (Jsc) and the fill factor (FF) can enhance the solar cell performance. Most currently reported Voc values of this kind of hybrid solar cell are well below 600 mV for the planar structures and even poorer in nanostructured ones due to inferior quality of silicon/polymer junction. Further improvement of Voc may be achieved by improving the silicon surface passivation or tuning the work function of PEDOT:PSS. Planar Si substrates offer good contact with polymers and less surface recombination, thus yielding superior Voc and FF. The high reflectivity of over 35% leads to R. Liu, S.-T. Lee, B. Sun Institute of Functional Nano and Soft Materials (FUNSOM) Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices and Collaborative Innovation Center of Suzhou Nano Science and Technology Soochow University Suzhou 215123, P. R. China Tel: 0086–512–65880951 E-mail: [email protected]
DOI: 10.1002/adma.201402076
Adv. Mater. 2014, DOI: 10.1002/adma.201402076
inferior Jsc value although the transparent PEDOT:PSS layer can somehow act as an antireflection layer. Reducing the reflection and increasing the absorption of light is the key to achieving high Jsc in a solar cell, which can actually be realized by fabricating surface structures or adopting antireflection coating layers. Silicon nanowire arrays integrating with 1,1-bis[(di4-tolylamino)phenyl]cyclohexane and PEDOT:PSS achieve the highest PCE of 13.01% with a high Jsc of 34.4 mA/cm2 and a relative low Voc of 540 mV.[13] An n-Si-PEDOT:PSS hybrid device using Si nanocones as the antireflection structure achieves an efficiency of 11.10% with a Jsc of 29.6 mA/cm2, but a lower Voc of 550 mV even by employing a high temperature process (950 °C) to make back surface doping.[16] Another recent work applies novel Si nanotube arrays with an aspect ratio of 1 as a light absorber shows a similar Jsc of 29.9 mA/cm2, a Voc of 510 mV and exhibited a PCE of 10.03%.[17] To date, there are few reports on the device with both superior Jsc and Voc, which restrict the best efficiency below ∼13%. As a transition metal oxide with non-toxicity and high work function properties, MoO3 is more traditionally utilized as a hole transport layer in organic photovoltaic (OPV) devices or organic light-emitting diodes.[18–21] The ambient stable MoO3 also has the additional advantage of protecting the device from decaying in air. In this report we find that depositing a thin layer of MoO3 film on the PEDOT:PSS layer can improve both the Jsc and the Voc of the hybrid solar cell by creating an antireflection layer on the front surface as well as inducing an inversion layer in Si (Figure 1(a)). This simple post-processing leads to an enhanced efficiency of 13.8% for the n-Si/ PEDOT:PSS hybrid solar cells, with a Voc of 630 mV, a Jsc of 29.2 mA/cm2 and a FF of 75% under air mass (AM) 1.5G illumination. The enhancement of Jsc is confirmed by reflectance and external quantum efficiency (EQE) measurements that MoO3 film with proper thickness can act as antireflection layer on the top of PEDOT:PSS. Scanning Kelvin probe microscope (SKPM), capacitance-voltage (C-V) and current-voltage (J–V) measurements reveal that the high work function MoO3 on PEDOT:PSS induces an inversion layer in front silicon underneath. A remarkable Voc of as large as 650 mV, which is comparable with that of conventional high temperature diffused p-n junction emitter solar cells, has been demonstrated due to the increased built-in voltage (Vbi), which is formed when metallic PEDOT:PSS contacts with n-type Si. To probe the effect of film thickness on Si/PEDOT:PSS performance, we fabricated different devices with thicknesses varying from 0 nm to 50 nm. Figure 2(a) shows the J–V characteristics for each thickness of MoO3 and Table S1 summarizes the average photovoltaic parameters. It can be clearly observed
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Figure 1. (a) Device structure of the MoO3/Ag Grids/PEDOT:PSS/n-Si/ Liq/Al hybrid solar cells. The thickness of the silicon layer is not proportional to the real device. (b) Optical image of the Si/PEDOT:PSS cell with (bottom left) and without (top right) MoO3 layer, the scale bar is 200 µm. (c) Cross-sectional SEM image of the cell with 15-nm-thick MoO3.
from the data that devices with an appropriate thickness of MoO3 (10 nm, 15 nm, 20 nm) show enhanced output characteristics compared with the ones without MoO3. It is interesting to find that all devices with MoO3 show a higher Voc but only the ones with a proper thickness can improve the Jsc. A further increase in the thickness will dramatically decrease the Jsc, as is evident from the device with 50-nm-thick MoO3. For devices with MoO3 thickness of 15 nm, PCE in excess of 13% is achieved with a remarkable Voc of 650 mV and an enhanced Jsc of 28.4 mA/cm2, representing a 14.4% improvement in PCE. It is worth noting that this simple post-processing allows straightforward comparison of the same devices just before
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and immediately after MoO3 layer deposition, thus persuasive enough to confirm the role of MoO3. We first investigate how the MoO3 film affects the Jsc by measuring the reflection and transmission spectra. The planar Si/PEDOT:PSS cell surface appears sandy-beige before MoO3 deposition, which indicates moderate reflection suppression compared with the planar Si (Figure 1(b)). In contrast, the color changes into bluish violet after deposition of 15-nm-thick MoO3, implying a further reflection reduction. Figure 2(b) shows the reflectance spectra of polished bare Si, Si with PEDOT:PSS and Si/PEDOT:PSS with different thicknesses of MoO3 films. The planar polished bare Si exhibits an average reflectance of more than 35% over a wavelength range from 400 nm to 1100 nm while the PEDOT:PSS coated one shows a reduced reflectance of 20∼35%. It is clear that the MoO3 films with a proper thickness between 10 nm and 20 nm can significantly decrease the reflectance in almost the whole region; and reflectance as low as 6% has been achieved at wavelengths around 500 nm. Though nearly zero reflectance has been achieved for the devices with 50-nm-thick MoO3 at a wavelength range from 850 nm to 950 nm, the high reflection in shorter wavelength ranges makes it unsuitable for antireflection layer. Overall, the measured reflectance spectra are well consistent with the Jsc of the corresponding devices, which demonstrates the antireflection role of the MoO3 film. Although MoO3 is transparent in thin film, transmittance measurements reveal that a 50-nm-thick MoO3 film shows less transmittance than both the 10 nm and 20 nm ones (Figure S1). The worse transmittance can be part of the reason for the dramatic decrease of the Jsc in the corresponding device. The
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Figure 2. (a) Current-voltage curves of Si/PEDOT:PSS hybrid solar cells with varying thicknesses of MoO3. (b) Reflectance spectra of polished Si and Si/PEDOT:PSS/MoO3 surfaces. (c) Current-voltage curves and (d) EQE characteristics of the champion Si/PEDOT:PSS hybrid solar cell with and without MoO3 layer.
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Table 1. The average and the champion electrical output characteristics for 20 devices with the 15 nm MoO3 layer.
All Devices Champion Device
Voc (mV)
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Fill factor (%)
PCE (%)
629 ± 11
27.6 ± 0.7
72.2 ± 2.1
12.6 ± 0.4
630
29.2
74.9
13.8
Adv. Mater. 2014, DOI: 10.1002/adma.201402076
with an average Jsc over 27 mA/cm2 and a good FF of 72%, the average PCE of 12.6% indicates the high quality and performance stability of the hybrid solar cell. Figure 2(c) shows the J–V characteristics and Figure 2(d) shows the EQE of the champion device with 15-nm-thick MoO3 and the reference one. The highest PCE of 13.8% corresponds to an enhanced Voc of 630 mV, a Jsc of 29.2 mA/cm2 and a FF of 74.9% under AM 1.5G illumination. This makes it not only the best performing Si/PEDOT:PSS hybrid solar cell to date but also one of the best Si-based organic-inorganic hybrid devices.[23–26] The high efficiency originates from the good junction quality of planar Si and PEDOT:PSS as well as MoO3 contribution. The higher EQE reaching ∼85% in a broad range peaking at ∼500 nm is consistent with the reflection spectra, which clarifies the antireflection effect of MoO3. The Jsc calculated by integrating the EQE curve with an AM 1.5G reference spectrum is 29.4 mA/cm2, which is consistent well with the corresponding Jsc obtained from the J–V curves. The angle dependence electrical output characteristics were measured under AM 1.5 G illumination at 100 mW/cm2 with different incidence angles and plotted in Figure S4. As the angle of incidence increased, the Jsc and the total PCE dropped because the amount of light arriving at the devices decreased. The total reflection became stronger for both devices. The device with MoO3 shows a decreased dropping tendency of the Jsc and the PCE compares with the one without that layer, especially in the angles range from 10 ∼ 40 degree, which is believed to be a benefit of the double-layer antireflection coating. The contact of n-Si/PEDOT:PSS hybrid devices has been investigated as a Schottky junction for its high similarity with the metal-semiconductor (MS) junction,[27] where PEDOT:PSS acts as the function of metal due to its metallic conducting properties. In a typical Schottky junction, when an n-type semiconductor contacts with a high work function metal, electrons will flow from the low electrostatic (high electron) potential energy semiconductor to the higher electrostatic (lower electron) potential energy metal to establish electronic equilibrium thus forming an energy barrier for electrons to cross from metal into semiconductor. The Schottky barrier height, ΦBn, is equal to the difference between the semiconductor Fermi energy (EF) and electron affinity (EC), and a built-in potential Vbi, which is equal to the difference between the metal work function (Φm) and the Fermi energy with a relationship: ΦBn = Vbi + |EC – EF|. The high defect density states at the interface sometimes lead to Fermi level pinning and a majority-carrier-dominated device of which the photogenerated minority carriers have a short lifetime and high recombination rate.[28] If the ΦBn is large enough with the concentration of holes in the depletion layer exceeding the dopant concentration in bulk, strong inversion occurs with low bulk majority carrier density. Metal-insulatorsemiconductor (MIS) inversion layer solar cells have been demonstrated decades ago by inserting a transparent dielectric layer between the metal and semiconductor, yielding record Voc values up to 655 mV which was higher than any other contemporaneously reported Si solar cell.[29] A heterojunction photovoltaic device consisting of a crystalline Si (c-Si) absorber with an amorphous hydrogenated Si (a-Si:H) film as well-passivating layer without back-surface field or surface texturing had an efficiency of 14.1% and an Voc of 655 mV.[30] A recent work has
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distinctly different antireflection behaviors of MoO3 on polished Si and on PEDOT:PSS coated Si represent two different mechanisms: single-layer antireflection (SLAR) coatings and double layer antireflection (DLAR) coatings, respectively.[22] For a SLAR coating and normal light incidence, a quarter-wavelength-thick film gives the minimum reflection with a thickness of dAR = λ/ (4nAR), where λ is the incident light wavelength and nAR is the refractive index of the coating material. When single 50-nmthick MoO3 layer (with a refractive index of ∼2.1) is deposited onto a polished Si substrate, the lowest reflection wavelength valley is located at about 420 nm according to reflection equation, which matches well with our experimental results (Figure S2). Further decrease of the thickness of MoO3 shifts the valley to shorter wavelengths. The optimized DLAR coatings with PEDOT:PSS and MoO3 show a better average antireflection effect due to the combined advantages of two minimum reflection valleys. Previous studies have shown that for this kind of AR coating, the minimum reflection will occur when the thicknesses of the low index (about 1.6 for PEDOT:PSS in this case) and high-index (2.1 for MoO3 in this case) materials are about half-a-quarter wavelength or λ/8nAR, which differs from the λ/4nAR SLAR.[22] In this condition, strong reflections occur on each of three interfaces and then interfere to eliminate each other. Here the thickness of the spin-coated PEDOT:PSS is about 70 nm, thus the calculated reflection valley will occur at a wavelength of about 896 nm according to the upper equation. So for the λ/8nAR DLAR, the optimized thickness of MoO3 for the wavelength of 896 nm should be about 53 nm, which means that 70 nm PEDOT:PSS and 53 nm MoO3 on planar Si will produce near zero reflection at 896 nm wavelength. The calculated results agree well with the fact that the reflection of 50-nm-thick MoO3 film on PEDOT:PSS is lowest at around 900 nm. However, for the cell responds to wide wavelengths rather than narrow ones, this combination does not lead to a higher Jsc due to the fact that it shows much stronger reflection over the range of the shorter wavelengths. For MoO3 films with thicknesses less than 50 nm, the expected minimum reflection point will subsequently decrease and yield new wavelength valley where the reflection is low. In this experiment, the thickness of PEDOT:PSS is fixed because it is an optimized spincoating process. Considering the average lowest reflection in the whole spectrum, the optimal thickness of MoO3 should be ∼15 nm, as evidenced by the largest Jsc of the devices. To confirm the effects and reproducibility, over 20 devices were fabricated and tested with the optimized MoO3 thickness of 15 nm. The average electrical output characteristics of the devices before and after MoO3 deposition are shown in Table 1 and the histograms of the cell performance characteristics are shown in Figure S3. The average Voc values as high as 629 mV are achieved with good reproducibility, which are much higher than most state-of-the-art Si hybrid devices.[1,5,16] Combined
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presented that PEDOT:PSS/n-Si junctions operate in inversion over a wide range of Si substrate doping densities.[31] Methods including improving interface passivation or modifying the work function of semiconductor/polymer materials can further push semiconductor into inversion. Here the highest Voc of 650 mV indicates stronger inversion has been achieved, indicating the high work function MoO3 layer on PEDOT:PSS not only acts as an antireflection coating but also induces an inversion layer in Si underneath, just being analogous with the role of SiOx in MIS junction. In order to verify the increasing of PEDOT:PSS work function with MoO3, the surface potentials between PEDOT:PSS and PEDOT:PSS/MoO3 films were probed using scanning Kelvin probe microscopy (SKPM). The PEDOT:PSS film was partly covered with a 15 -nm-thick MoO3 layer and the boundary was visually clear due to the obvious difference of the colors. Figure 3(a) and 3(b) show the surface potentials of the PEDOT:PSS and MoO3 films. Although the surface potential was not so uniform near the interface, which resulted from the diffusion of MoO3 during the evaporation process, it gradually became continuous away from the boundary. The surface potential of the MoO3 layer on the PEDOT:PSS film was about 85 mV higher than that of pristine PEDOT:PSS. This result is consistent with the previous observation, where a very thin layer of MoO3 has been demonstrated to improve the NiOx contact by increasing its work function.[32] The NiOx/MoO3 bi-layer structure with vacuum deposited MoO3 with thicknesses of 0.5/1.0/2.0/5.0/10.0 nm display corresponding work functions of 4.9/5.0/5.4/6.2/6.6 eV. They attribute this work function increase to an electron transfer from NiOx to MoO3 with formation of an interface dipole between the two materials. OPV
devices built on this bi-layer show superior performance compared to devices built on single layer of these two oxides. The performance enhancement is explained by the combination of a high built-in field due to the work function of the MoO3 and reduced recombination due to the doping of NiOx by MoO3. Here the potential of the interface between PEDOT:PSS and Si might be different from the top surface, but the work function increase trend for the bi-layer should be similar as discussed above. To investigate the formation of the stronger inversion layer, C-V measurement was carried out on the devices and shown as plots of A2/C2-V (Figure 3(c)) where A is the device area and V is the applied voltage, ranging from −1 V to 0.25 V. A metal oxide semiconductor (MOS) capacitor model was used to extract Vbi, from the extrapolation of the linear portion of the A2/C2-V plots. While under strong inversion with the MoO3 layer, the concentration of carriers in the inversion layer exceeds the doping concentration in bulk, causing the mismatch of majority carrier distribution and the abrupt doping concentration. The different interface states could probably be responsible for the variation of the two slopes. The as extracted Vbi, calculated depletion layer width (WD) and measured Voc results of devices with and without 15 nm MoO3 layer are summarized in Table 2. The extracted Vbi of devices with MoO3 layer are 0.08 V higher than that without the layer, accompanied by the increasing WD from 0.061 µm to 0.064 µm, and Voc from 600 mV to 640 mV. All of these prove that the existence of MoO3 layer on PEDOT:PSS induced Si into stronger inversion. As a consequence, charge carriers are more efficiently swept out of the junction with stronger driving force as well as the wider depletion layer.
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Figure 3(d) demonstrates the J–V characteristics under dark conditions for the same devices investigated by the C-V measurements, and better light rectification is observed for the device with MoO3. The diode saturation current density, J0, which is extracted from the intercept of the linear region of ln(J)-V, falls from 3.0 × 10−7 A/cm2 for a device without MoO3 to 1.3 × 10−8 A/cm2 for a device with 15 nm MoO3, with the ideality factor (n) being 1.65 and 1.44, respectively. It is commonly accepted that Voc strongly depends on the properties at the interface where a low J0 indicates high junction quality.[33] The decrease of J0 subsequently favors a more efficient charge separation at the interface and leads to the increase of Voc, which is consistent with stronger inversion and higher Vbi analyzed in the above C-V results. A strong evidence of the decreasing recombination rate is the increasing of the minority carrier lifetime. To investigate the effect of MoO3 on the surface recombination, a microwave photoconductance decay (μ-PCD) technique (WT-2000PVN, Semilab) was used to perform the spatial mapping of minority carrier lifetime. The minority carrier lifetime of a silicon solar cell follows as the equation below:
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Table 2. Vbi, WD, n, J0 and Voc of the devices with and without MoO3 layer.
1 1 2S = + τ eff τ bulk w where τeff is the effective lifetime, τbulk is the bulk recombination lifetime, S is the surface recombination rate, and W is the wafer thickness. Since the bulk lifetime is fixed for the same silicon wafer, the measured lifetime will reflect the surface recombination rate. As shown in Figure 4, the average lifetime of silicon wafer with PEDOT:PSS is about 3.96 µs while the one with PEDOT:PSS/MoO3 is 7.98 µs. As described above, the increasing minority carrier lifetime indicates lower surface recombination rate benefiting from the inversion effect. It is worth noting that efficiency enhancement of MoO3 film can also work on silicon nanopillar arrays (SiNPs) device. Due to the excellent light absorption of SiNPs, the nonoptimized device without MoO3 layer shows a superior Jsc of 30.9 mA/cm2, which is much higher than planar ones, but an obviously lower Voc of 510 mV and FF of 0.64 as a result of surface recombination, and yields a PCE of 10.2% (Figure S6(a)). After the deposition of a 15 nm MoO3 film, the Voc increases to 560 mV and Jsc boosts to 32.3 mA/cm2, representing improvements of 9.8% of the Voc and 4.5% of the Jsc, respectively, contributing to a PCE of 12.0%, which is 17.9% (PCE) higher than the counterpart. Considering the enhancements are achieved via the same device, it is evident that the MoO3 film does affect the performance of nanostructure-based hybrid solar cells in the same way as the planar ones. In conclusion, we have demonstrated a simple post-processing method to achieve efficiency enhancement in n-Si/
Figure 4. (a)-(b) Spatial mapping of the minority carrier lifetime for the samples with and without MoO3 layer.
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PEDOT:PSS hybrid solar cells using a MoO3 film. MoO3 with proper thickness on the PEDOT:PSS film forms double layer antireflection coatings on Si which show superior antireflection effect, thus improving the Jsc. The high work function MoO3 also induces an inversion layer in Si underneath, leading to a higher built-in field and favoring charge separation and collection. The best planar device shows a Voc of 630 mV, a Jsc of 29.2 mA/cm2, a FF of 75% and a record PCE of 13.8%. The proposed devices enable cost-effective by low temperature and simple fabrication process for solar cell fabrication.
Experimental Section Device Fabrication: All the devices were fabricated on 300 µm thick, single-side polished, n-type (100)-oriented single crystal silicon wafers with resistivity of 0.05 – 0.1 Ω·cm. Samples cut to 1.2 × 1.5 cm2 were sequentially ultrasonically cleaned in acetone, ethanol and deionized (DI) water for 20 min., followed by immersing into a boiling solution of H2SO4/H2O2 for over 1 h to ensure surfaces without any residual material. The native oxide was removed by soaking the samples in 5% HF solution for 10 min and then the H-terminated samples were transferred immediately into glove box. The method used to fabricate the methyl group-terminated (Si-CH3) substrate was followed as previous report.[8] Highly conductive PEDOT:PSS (Heraeus PH 1000) solution mixed with 5%(wt.) dimethyl sulfoxide (DMSO) and 1%(wt.) Triton (Sigma Aldrich) was spin-coated onto the samples at a speed of 3000 r/min, followed by annealing at 125 °C for 30 min in N2 atmosphere. A 200-nm-thick Ag front electrode grid with an active area of 1.0 × 0.8 cm2 defined by a shadow mask was thermally deposited on top of the PEDOT:PSS layer. 1-nm-thick Liq and 200-nm-thick Al were sequentially thermally evaporated onto the backside of the Si samples as a rear electrode. MoO3 films were deposited on the front surface without covering the point where the probe touched the Ag electrode, in order to avoid any contact problem. Device Characterization: The optical and cross-section images of the solar cell were obtained by an optical microscope (Laica DM 4000M) and high-resolution scanning electron microscope (SEM) (Carl Zeiss Suppra 55), respectively. Reflection and transmission spectra were measured using an integrating sphere (Perkin-Elmer Lambda 700). Solar cell characteristics were tested by a source meter (Keithley 2612) and a solar simulator (Newport 91160) under AM 1.5G condition at 100 mW/cm2, calibrated by a Newport standard Si solar cell (91150). Newport monochromator 74125 and power meter 1918 with Si detector 918D were used to measure the external quantum efficiency (EQE). The morphologies and surface potentials of the PEDOT:PSS/MoO3 film were obtained by using an atomic force microscope (AFM) and scanning Kelvin probe microscope (SKPM) (Veeco Multimode V). The capacitance versus voltage (C-V) measurements were carried out with a Keithley 4200-SCS at 100 kHz applied frequency.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Basic Research Program of China (973 Program) (2012CB932402), National Natural Science Foundation of China (91123005, 61176057, 61211130358), the
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Priority Academic Program Development of Jiangsu Higher Education Institutions. Received: May 8, 2014 Revised: June 11, 2014 Published online:
[1] C.-Y. Liu, Z. C. Holman, U. R. Kortshagen, Nano Lett. 2008, 9, 449. [2] S.-C. Shiu, J.-J. Chao, S.-C. Hung, C.-L. Yeh, C.-F. Lin, Chem. Mater. 2010, 22, 3108. [3] S. Avasthi, S. Lee, Y. L. Loo, J. C. Sturm, Adv. Mater. 2011, 23, 5762. [4] G. Kalita, S. Adhikari, H. R. Aryal, R. Afre, T. Soga, M. Sharon, W. Koichi, M. Umeno, J. Phys. D: Appl. Phys. 2009, 42, 115104. [5] L. He, C. Jiang, D. Lai, H. Wang, Appl. Phys. Lett. 2011, 99, 021104. [6] L. He, C. Jiang, H. Wang, D. Lai, Appl. Phys. Lett. 2012, 100, 073503. [7] X. Shen, B. Sun, F. Yan, J. Zhao, F. Zhang, S. Wang, X. Zhu, S. Lee, ACS Nano 2010, 4, 5869. [8] F. Zhang, B. Sun, T. Song, X. Zhu, S. Lee, Chem. Mater. 2011, 23, 2084. [9] S. B. Darling, F. You, T. Veselka, A. Velosa, Energy Environ. Sci. 2011, 4, 3133. [10] F. Zhang, T. Song, B. Sun, Nanotechnology 2012, 23, 194006. [11] X. Shen, B. Sun, D. Liu, S.-T. Lee, J. Am. Chem. Soc. 2011, 133, 19408. [12] Q. Liu, M. Ono, Z. Tang, R. Ishikawa, K. Ueno, H. Shirai, Appl. Phys. Lett. 2012, 100, 183901. [13] P. Yu, C.-Y. Tsai, J.-K. Chang, C.-C. Lai, P.-H. Chen, Y.-C. Lai, P.-T. Tsai, M.-C. Li, H.-T. Pan, Y.-Y. Huang, ACS Nano 2013, 7, 10780. [14] M. Sharma, P. R. Pudasaini, F. Ruiz-Zepeda, D. Elam, A. A. Ayon, ACS Appl. Mater. Interfaces 2014. [15] Y. Zhang, F. Zu, S.-T. Lee, L. Liao, N. Zhao, B. Sun, Adv. Energy Mater. 2014, 4, 10.1002/aenm.201300923. [16] S. Jeong, E. C. Garnett, S. Wang, Z. Yu, S. Fan, M. L. Brongersma, M. D. Mcgehee, Y. Cui, Nano Lett 2012, 12, 2971. [17] H. Jeong, H. Song, Y. Pak, I. K. Kwon, K. Jo, H. Lee, G. Y. Jung, Adv. Mater. 2014, 26, 3445. [18] S. Tokito, K. Noda, Y. Taga, J. Phys. D: Appl. Phys. 1996, 29, 2750. [19] Y. Sun, C. J. Takacs, S. R. Cowan, J. H. Seo, X. Gong, A. Roy, A. J. Heeger, Adv. Mater. 2011, 23, 2226. [20] M. T. Greiner, M. G. Helander, W.-M. Tang, Z.-B. Wang, J. Qiu, Z.-H. Lu, Nat. Mater. 2012, 11, 76. [21] Y.-C. Tseng, A. U. Mane, J. W. Elam, S. B. Darling, Sol. Energy Mater. Sol. Cells 2012, 99, 235. [22] J. Zhao, M. A. Green, IEEE T Electron Dev. 1991, 38, 1925. [23] Y. Jia, A. Cao, X. Bai, Z. Li, L. Zhang, N. Guo, J. Wei, K. Wang, H. Zhu, D. Wu, Nano Lett. 2011, 11, 1901. [24] Y. Jung, X. Li, N. K. Rajan, A. D. Taylor, M. A. Reed, Nano Lett. 2012, 13, 95. [25] X. Miao, S. Tongay, M. K. Petterson, K. Berke, A. G. Rinzler, B. R. Appleton, A. F. Hebard, Nano Lett. 2012, 12, 2745. [26] E. Shi, L. Zhang, Z. Li, P. Li, Y. Shang, Y. Jia, J. Wei, K. Wang, H. Zhu, D. Wu, Sci. Rep. 2012, 2, 884. [27] M. G. Walter, X. Liu, L. E. O’leary, B. S. Brunschwig, N. S. Lewis, J. Phys. Chem. C 2013, 117, 14485. [28] J. Bardeen, Phys. Rev. 1947, 71, 717. [29] R. Godfrey, M. Green, Appl. Phys. Lett. 1979, 34, 790. [30] N. Jensen, R. M. Hausner, R. B. Bergmann, J. H. Werner, U. Rau, Prog. Photovolt. Res. Appl. 2002, 10, 1. [31] A. S. Erickson, A. Zohar, D. Cahen, Adv. Energy Mater. 2014, DOI: 10.1002/aenm.201301724. [32] P. Schulz, S. R. Cowan, Z.-L. Guan, A. Garcia, D. C. Olson, A. Kahn, Adv. Funct. Mater. 2014, 24, 701. [33] N. Li, B. E. Lassiter, R. R. Lunt, G. Wei, S. R. Forrest, Appl. Phys. Lett. 2009, 94, 023307.
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Adv. Mater. 2014, DOI: 10.1002/adma.201402076
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13.8% Efficiency Hybrid Si/Organic Heterojunction Solar Cells with MoO3 Film as Antireflection and Inversion Induced Layer Ruiyuan Liu, Shuit-Tong Lee, and Baoquan Sun* Low temperature processed organic-inorganic hybrid solar cells based on Si are currently under intensive investigation for their unique advantages compared with traditional high temperature p-n junction counterparts: easy fabrication, simple device structure, and potentially low cost. The Si-organic hybrid structures combining the advantageous characteristics of crystalline Si and transparent conductive organics have emerged as promising candidates for cost-effective photovoltaics.[1–9] In principle, light is predominately absorbed by Si. Electron-hole pairs are then generated and separated with the driving force of a built-in electric field. The main functions of the organic layer are the formation of a heterojunction with Si, as hole/ electron transporting path, as optical window, and sometimes serving as an antireflection coating, such as poly(3,4-ethylene dioxythiophene):poly-styrenesulfonate (PEDOT:PSS).[10–14] Over the past few years, considerable progress has been achieved in the study of hybrid solar cells and power conversion efficiencies (PCEs) of over 10% have been reported by many groups. Most recently, we have achieved a PCE of over 12% in n-Si/ PEDOT:PSS hybrid solar cells by using methyl groups to passivate the Si surface in order to suppress the surface recombination velocity, as well as inserting a thin layer of wide band gap organic semiconductor 8-hydroxyquinolinolato-lithium (Liq) between the rear side of the Si substrate and Al in order to improve rear contact as well as reduce charge recombination.[15] At this high point, any improvement of the open-circuit voltage (Voc), the short-circuit current density (Jsc) and the fill factor (FF) can enhance the solar cell performance. Most currently reported Voc values of this kind of hybrid solar cell are well below 600 mV for the planar structures and even poorer in nanostructured ones due to inferior quality of silicon/polymer junction. Further improvement of Voc may be achieved by improving the silicon surface passivation or tuning the work function of PEDOT:PSS. Planar Si substrates offer good contact with polymers and less surface recombination, thus yielding superior Voc and FF. The high reflectivity of over 35% leads to R. Liu, S.-T. Lee, B. Sun Institute of Functional Nano and Soft Materials (FUNSOM) Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices and Collaborative Innovation Center of Suzhou Nano Science and Technology Soochow University Suzhou 215123, P. R. China Tel: 0086–512–65880951 E-mail: [email protected]
DOI: 10.1002/adma.201402076
Adv. Mater. 2014, DOI: 10.1002/adma.201402076
inferior Jsc value although the transparent PEDOT:PSS layer can somehow act as an antireflection layer. Reducing the reflection and increasing the absorption of light is the key to achieving high Jsc in a solar cell, which can actually be realized by fabricating surface structures or adopting antireflection coating layers. Silicon nanowire arrays integrating with 1,1-bis[(di4-tolylamino)phenyl]cyclohexane and PEDOT:PSS achieve the highest PCE of 13.01% with a high Jsc of 34.4 mA/cm2 and a relative low Voc of 540 mV.[13] An n-Si-PEDOT:PSS hybrid device using Si nanocones as the antireflection structure achieves an efficiency of 11.10% with a Jsc of 29.6 mA/cm2, but a lower Voc of 550 mV even by employing a high temperature process (950 °C) to make back surface doping.[16] Another recent work applies novel Si nanotube arrays with an aspect ratio of 1 as a light absorber shows a similar Jsc of 29.9 mA/cm2, a Voc of 510 mV and exhibited a PCE of 10.03%.[17] To date, there are few reports on the device with both superior Jsc and Voc, which restrict the best efficiency below ∼13%. As a transition metal oxide with non-toxicity and high work function properties, MoO3 is more traditionally utilized as a hole transport layer in organic photovoltaic (OPV) devices or organic light-emitting diodes.[18–21] The ambient stable MoO3 also has the additional advantage of protecting the device from decaying in air. In this report we find that depositing a thin layer of MoO3 film on the PEDOT:PSS layer can improve both the Jsc and the Voc of the hybrid solar cell by creating an antireflection layer on the front surface as well as inducing an inversion layer in Si (Figure 1(a)). This simple post-processing leads to an enhanced efficiency of 13.8% for the n-Si/ PEDOT:PSS hybrid solar cells, with a Voc of 630 mV, a Jsc of 29.2 mA/cm2 and a FF of 75% under air mass (AM) 1.5G illumination. The enhancement of Jsc is confirmed by reflectance and external quantum efficiency (EQE) measurements that MoO3 film with proper thickness can act as antireflection layer on the top of PEDOT:PSS. Scanning Kelvin probe microscope (SKPM), capacitance-voltage (C-V) and current-voltage (J–V) measurements reveal that the high work function MoO3 on PEDOT:PSS induces an inversion layer in front silicon underneath. A remarkable Voc of as large as 650 mV, which is comparable with that of conventional high temperature diffused p-n junction emitter solar cells, has been demonstrated due to the increased built-in voltage (Vbi), which is formed when metallic PEDOT:PSS contacts with n-type Si. To probe the effect of film thickness on Si/PEDOT:PSS performance, we fabricated different devices with thicknesses varying from 0 nm to 50 nm. Figure 2(a) shows the J–V characteristics for each thickness of MoO3 and Table S1 summarizes the average photovoltaic parameters. It can be clearly observed
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Figure 1. (a) Device structure of the MoO3/Ag Grids/PEDOT:PSS/n-Si/ Liq/Al hybrid solar cells. The thickness of the silicon layer is not proportional to the real device. (b) Optical image of the Si/PEDOT:PSS cell with (bottom left) and without (top right) MoO3 layer, the scale bar is 200 µm. (c) Cross-sectional SEM image of the cell with 15-nm-thick MoO3.
from the data that devices with an appropriate thickness of MoO3 (10 nm, 15 nm, 20 nm) show enhanced output characteristics compared with the ones without MoO3. It is interesting to find that all devices with MoO3 show a higher Voc but only the ones with a proper thickness can improve the Jsc. A further increase in the thickness will dramatically decrease the Jsc, as is evident from the device with 50-nm-thick MoO3. For devices with MoO3 thickness of 15 nm, PCE in excess of 13% is achieved with a remarkable Voc of 650 mV and an enhanced Jsc of 28.4 mA/cm2, representing a 14.4% improvement in PCE. It is worth noting that this simple post-processing allows straightforward comparison of the same devices just before
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and immediately after MoO3 layer deposition, thus persuasive enough to confirm the role of MoO3. We first investigate how the MoO3 film affects the Jsc by measuring the reflection and transmission spectra. The planar Si/PEDOT:PSS cell surface appears sandy-beige before MoO3 deposition, which indicates moderate reflection suppression compared with the planar Si (Figure 1(b)). In contrast, the color changes into bluish violet after deposition of 15-nm-thick MoO3, implying a further reflection reduction. Figure 2(b) shows the reflectance spectra of polished bare Si, Si with PEDOT:PSS and Si/PEDOT:PSS with different thicknesses of MoO3 films. The planar polished bare Si exhibits an average reflectance of more than 35% over a wavelength range from 400 nm to 1100 nm while the PEDOT:PSS coated one shows a reduced reflectance of 20∼35%. It is clear that the MoO3 films with a proper thickness between 10 nm and 20 nm can significantly decrease the reflectance in almost the whole region; and reflectance as low as 6% has been achieved at wavelengths around 500 nm. Though nearly zero reflectance has been achieved for the devices with 50-nm-thick MoO3 at a wavelength range from 850 nm to 950 nm, the high reflection in shorter wavelength ranges makes it unsuitable for antireflection layer. Overall, the measured reflectance spectra are well consistent with the Jsc of the corresponding devices, which demonstrates the antireflection role of the MoO3 film. Although MoO3 is transparent in thin film, transmittance measurements reveal that a 50-nm-thick MoO3 film shows less transmittance than both the 10 nm and 20 nm ones (Figure S1). The worse transmittance can be part of the reason for the dramatic decrease of the Jsc in the corresponding device. The
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Figure 2. (a) Current-voltage curves of Si/PEDOT:PSS hybrid solar cells with varying thicknesses of MoO3. (b) Reflectance spectra of polished Si and Si/PEDOT:PSS/MoO3 surfaces. (c) Current-voltage curves and (d) EQE characteristics of the champion Si/PEDOT:PSS hybrid solar cell with and without MoO3 layer.
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Table 1. The average and the champion electrical output characteristics for 20 devices with the 15 nm MoO3 layer.
All Devices Champion Device
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Fill factor (%)
PCE (%)
629 ± 11
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72.2 ± 2.1
12.6 ± 0.4
630
29.2
74.9
13.8
Adv. Mater. 2014, DOI: 10.1002/adma.201402076
with an average Jsc over 27 mA/cm2 and a good FF of 72%, the average PCE of 12.6% indicates the high quality and performance stability of the hybrid solar cell. Figure 2(c) shows the J–V characteristics and Figure 2(d) shows the EQE of the champion device with 15-nm-thick MoO3 and the reference one. The highest PCE of 13.8% corresponds to an enhanced Voc of 630 mV, a Jsc of 29.2 mA/cm2 and a FF of 74.9% under AM 1.5G illumination. This makes it not only the best performing Si/PEDOT:PSS hybrid solar cell to date but also one of the best Si-based organic-inorganic hybrid devices.[23–26] The high efficiency originates from the good junction quality of planar Si and PEDOT:PSS as well as MoO3 contribution. The higher EQE reaching ∼85% in a broad range peaking at ∼500 nm is consistent with the reflection spectra, which clarifies the antireflection effect of MoO3. The Jsc calculated by integrating the EQE curve with an AM 1.5G reference spectrum is 29.4 mA/cm2, which is consistent well with the corresponding Jsc obtained from the J–V curves. The angle dependence electrical output characteristics were measured under AM 1.5 G illumination at 100 mW/cm2 with different incidence angles and plotted in Figure S4. As the angle of incidence increased, the Jsc and the total PCE dropped because the amount of light arriving at the devices decreased. The total reflection became stronger for both devices. The device with MoO3 shows a decreased dropping tendency of the Jsc and the PCE compares with the one without that layer, especially in the angles range from 10 ∼ 40 degree, which is believed to be a benefit of the double-layer antireflection coating. The contact of n-Si/PEDOT:PSS hybrid devices has been investigated as a Schottky junction for its high similarity with the metal-semiconductor (MS) junction,[27] where PEDOT:PSS acts as the function of metal due to its metallic conducting properties. In a typical Schottky junction, when an n-type semiconductor contacts with a high work function metal, electrons will flow from the low electrostatic (high electron) potential energy semiconductor to the higher electrostatic (lower electron) potential energy metal to establish electronic equilibrium thus forming an energy barrier for electrons to cross from metal into semiconductor. The Schottky barrier height, ΦBn, is equal to the difference between the semiconductor Fermi energy (EF) and electron affinity (EC), and a built-in potential Vbi, which is equal to the difference between the metal work function (Φm) and the Fermi energy with a relationship: ΦBn = Vbi + |EC – EF|. The high defect density states at the interface sometimes lead to Fermi level pinning and a majority-carrier-dominated device of which the photogenerated minority carriers have a short lifetime and high recombination rate.[28] If the ΦBn is large enough with the concentration of holes in the depletion layer exceeding the dopant concentration in bulk, strong inversion occurs with low bulk majority carrier density. Metal-insulatorsemiconductor (MIS) inversion layer solar cells have been demonstrated decades ago by inserting a transparent dielectric layer between the metal and semiconductor, yielding record Voc values up to 655 mV which was higher than any other contemporaneously reported Si solar cell.[29] A heterojunction photovoltaic device consisting of a crystalline Si (c-Si) absorber with an amorphous hydrogenated Si (a-Si:H) film as well-passivating layer without back-surface field or surface texturing had an efficiency of 14.1% and an Voc of 655 mV.[30] A recent work has
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distinctly different antireflection behaviors of MoO3 on polished Si and on PEDOT:PSS coated Si represent two different mechanisms: single-layer antireflection (SLAR) coatings and double layer antireflection (DLAR) coatings, respectively.[22] For a SLAR coating and normal light incidence, a quarter-wavelength-thick film gives the minimum reflection with a thickness of dAR = λ/ (4nAR), where λ is the incident light wavelength and nAR is the refractive index of the coating material. When single 50-nmthick MoO3 layer (with a refractive index of ∼2.1) is deposited onto a polished Si substrate, the lowest reflection wavelength valley is located at about 420 nm according to reflection equation, which matches well with our experimental results (Figure S2). Further decrease of the thickness of MoO3 shifts the valley to shorter wavelengths. The optimized DLAR coatings with PEDOT:PSS and MoO3 show a better average antireflection effect due to the combined advantages of two minimum reflection valleys. Previous studies have shown that for this kind of AR coating, the minimum reflection will occur when the thicknesses of the low index (about 1.6 for PEDOT:PSS in this case) and high-index (2.1 for MoO3 in this case) materials are about half-a-quarter wavelength or λ/8nAR, which differs from the λ/4nAR SLAR.[22] In this condition, strong reflections occur on each of three interfaces and then interfere to eliminate each other. Here the thickness of the spin-coated PEDOT:PSS is about 70 nm, thus the calculated reflection valley will occur at a wavelength of about 896 nm according to the upper equation. So for the λ/8nAR DLAR, the optimized thickness of MoO3 for the wavelength of 896 nm should be about 53 nm, which means that 70 nm PEDOT:PSS and 53 nm MoO3 on planar Si will produce near zero reflection at 896 nm wavelength. The calculated results agree well with the fact that the reflection of 50-nm-thick MoO3 film on PEDOT:PSS is lowest at around 900 nm. However, for the cell responds to wide wavelengths rather than narrow ones, this combination does not lead to a higher Jsc due to the fact that it shows much stronger reflection over the range of the shorter wavelengths. For MoO3 films with thicknesses less than 50 nm, the expected minimum reflection point will subsequently decrease and yield new wavelength valley where the reflection is low. In this experiment, the thickness of PEDOT:PSS is fixed because it is an optimized spincoating process. Considering the average lowest reflection in the whole spectrum, the optimal thickness of MoO3 should be ∼15 nm, as evidenced by the largest Jsc of the devices. To confirm the effects and reproducibility, over 20 devices were fabricated and tested with the optimized MoO3 thickness of 15 nm. The average electrical output characteristics of the devices before and after MoO3 deposition are shown in Table 1 and the histograms of the cell performance characteristics are shown in Figure S3. The average Voc values as high as 629 mV are achieved with good reproducibility, which are much higher than most state-of-the-art Si hybrid devices.[1,5,16] Combined
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presented that PEDOT:PSS/n-Si junctions operate in inversion over a wide range of Si substrate doping densities.[31] Methods including improving interface passivation or modifying the work function of semiconductor/polymer materials can further push semiconductor into inversion. Here the highest Voc of 650 mV indicates stronger inversion has been achieved, indicating the high work function MoO3 layer on PEDOT:PSS not only acts as an antireflection coating but also induces an inversion layer in Si underneath, just being analogous with the role of SiOx in MIS junction. In order to verify the increasing of PEDOT:PSS work function with MoO3, the surface potentials between PEDOT:PSS and PEDOT:PSS/MoO3 films were probed using scanning Kelvin probe microscopy (SKPM). The PEDOT:PSS film was partly covered with a 15 -nm-thick MoO3 layer and the boundary was visually clear due to the obvious difference of the colors. Figure 3(a) and 3(b) show the surface potentials of the PEDOT:PSS and MoO3 films. Although the surface potential was not so uniform near the interface, which resulted from the diffusion of MoO3 during the evaporation process, it gradually became continuous away from the boundary. The surface potential of the MoO3 layer on the PEDOT:PSS film was about 85 mV higher than that of pristine PEDOT:PSS. This result is consistent with the previous observation, where a very thin layer of MoO3 has been demonstrated to improve the NiOx contact by increasing its work function.[32] The NiOx/MoO3 bi-layer structure with vacuum deposited MoO3 with thicknesses of 0.5/1.0/2.0/5.0/10.0 nm display corresponding work functions of 4.9/5.0/5.4/6.2/6.6 eV. They attribute this work function increase to an electron transfer from NiOx to MoO3 with formation of an interface dipole between the two materials. OPV
devices built on this bi-layer show superior performance compared to devices built on single layer of these two oxides. The performance enhancement is explained by the combination of a high built-in field due to the work function of the MoO3 and reduced recombination due to the doping of NiOx by MoO3. Here the potential of the interface between PEDOT:PSS and Si might be different from the top surface, but the work function increase trend for the bi-layer should be similar as discussed above. To investigate the formation of the stronger inversion layer, C-V measurement was carried out on the devices and shown as plots of A2/C2-V (Figure 3(c)) where A is the device area and V is the applied voltage, ranging from −1 V to 0.25 V. A metal oxide semiconductor (MOS) capacitor model was used to extract Vbi, from the extrapolation of the linear portion of the A2/C2-V plots. While under strong inversion with the MoO3 layer, the concentration of carriers in the inversion layer exceeds the doping concentration in bulk, causing the mismatch of majority carrier distribution and the abrupt doping concentration. The different interface states could probably be responsible for the variation of the two slopes. The as extracted Vbi, calculated depletion layer width (WD) and measured Voc results of devices with and without 15 nm MoO3 layer are summarized in Table 2. The extracted Vbi of devices with MoO3 layer are 0.08 V higher than that without the layer, accompanied by the increasing WD from 0.061 µm to 0.064 µm, and Voc from 600 mV to 640 mV. All of these prove that the existence of MoO3 layer on PEDOT:PSS induced Si into stronger inversion. As a consequence, charge carriers are more efficiently swept out of the junction with stronger driving force as well as the wider depletion layer.
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Figure 3(d) demonstrates the J–V characteristics under dark conditions for the same devices investigated by the C-V measurements, and better light rectification is observed for the device with MoO3. The diode saturation current density, J0, which is extracted from the intercept of the linear region of ln(J)-V, falls from 3.0 × 10−7 A/cm2 for a device without MoO3 to 1.3 × 10−8 A/cm2 for a device with 15 nm MoO3, with the ideality factor (n) being 1.65 and 1.44, respectively. It is commonly accepted that Voc strongly depends on the properties at the interface where a low J0 indicates high junction quality.[33] The decrease of J0 subsequently favors a more efficient charge separation at the interface and leads to the increase of Voc, which is consistent with stronger inversion and higher Vbi analyzed in the above C-V results. A strong evidence of the decreasing recombination rate is the increasing of the minority carrier lifetime. To investigate the effect of MoO3 on the surface recombination, a microwave photoconductance decay (μ-PCD) technique (WT-2000PVN, Semilab) was used to perform the spatial mapping of minority carrier lifetime. The minority carrier lifetime of a silicon solar cell follows as the equation below:
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Table 2. Vbi, WD, n, J0 and Voc of the devices with and without MoO3 layer.
1 1 2S = + τ eff τ bulk w where τeff is the effective lifetime, τbulk is the bulk recombination lifetime, S is the surface recombination rate, and W is the wafer thickness. Since the bulk lifetime is fixed for the same silicon wafer, the measured lifetime will reflect the surface recombination rate. As shown in Figure 4, the average lifetime of silicon wafer with PEDOT:PSS is about 3.96 µs while the one with PEDOT:PSS/MoO3 is 7.98 µs. As described above, the increasing minority carrier lifetime indicates lower surface recombination rate benefiting from the inversion effect. It is worth noting that efficiency enhancement of MoO3 film can also work on silicon nanopillar arrays (SiNPs) device. Due to the excellent light absorption of SiNPs, the nonoptimized device without MoO3 layer shows a superior Jsc of 30.9 mA/cm2, which is much higher than planar ones, but an obviously lower Voc of 510 mV and FF of 0.64 as a result of surface recombination, and yields a PCE of 10.2% (Figure S6(a)). After the deposition of a 15 nm MoO3 film, the Voc increases to 560 mV and Jsc boosts to 32.3 mA/cm2, representing improvements of 9.8% of the Voc and 4.5% of the Jsc, respectively, contributing to a PCE of 12.0%, which is 17.9% (PCE) higher than the counterpart. Considering the enhancements are achieved via the same device, it is evident that the MoO3 film does affect the performance of nanostructure-based hybrid solar cells in the same way as the planar ones. In conclusion, we have demonstrated a simple post-processing method to achieve efficiency enhancement in n-Si/
Figure 4. (a)-(b) Spatial mapping of the minority carrier lifetime for the samples with and without MoO3 layer.
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PEDOT:PSS hybrid solar cells using a MoO3 film. MoO3 with proper thickness on the PEDOT:PSS film forms double layer antireflection coatings on Si which show superior antireflection effect, thus improving the Jsc. The high work function MoO3 also induces an inversion layer in Si underneath, leading to a higher built-in field and favoring charge separation and collection. The best planar device shows a Voc of 630 mV, a Jsc of 29.2 mA/cm2, a FF of 75% and a record PCE of 13.8%. The proposed devices enable cost-effective by low temperature and simple fabrication process for solar cell fabrication.
Experimental Section Device Fabrication: All the devices were fabricated on 300 µm thick, single-side polished, n-type (100)-oriented single crystal silicon wafers with resistivity of 0.05 – 0.1 Ω·cm. Samples cut to 1.2 × 1.5 cm2 were sequentially ultrasonically cleaned in acetone, ethanol and deionized (DI) water for 20 min., followed by immersing into a boiling solution of H2SO4/H2O2 for over 1 h to ensure surfaces without any residual material. The native oxide was removed by soaking the samples in 5% HF solution for 10 min and then the H-terminated samples were transferred immediately into glove box. The method used to fabricate the methyl group-terminated (Si-CH3) substrate was followed as previous report.[8] Highly conductive PEDOT:PSS (Heraeus PH 1000) solution mixed with 5%(wt.) dimethyl sulfoxide (DMSO) and 1%(wt.) Triton (Sigma Aldrich) was spin-coated onto the samples at a speed of 3000 r/min, followed by annealing at 125 °C for 30 min in N2 atmosphere. A 200-nm-thick Ag front electrode grid with an active area of 1.0 × 0.8 cm2 defined by a shadow mask was thermally deposited on top of the PEDOT:PSS layer. 1-nm-thick Liq and 200-nm-thick Al were sequentially thermally evaporated onto the backside of the Si samples as a rear electrode. MoO3 films were deposited on the front surface without covering the point where the probe touched the Ag electrode, in order to avoid any contact problem. Device Characterization: The optical and cross-section images of the solar cell were obtained by an optical microscope (Laica DM 4000M) and high-resolution scanning electron microscope (SEM) (Carl Zeiss Suppra 55), respectively. Reflection and transmission spectra were measured using an integrating sphere (Perkin-Elmer Lambda 700). Solar cell characteristics were tested by a source meter (Keithley 2612) and a solar simulator (Newport 91160) under AM 1.5G condition at 100 mW/cm2, calibrated by a Newport standard Si solar cell (91150). Newport monochromator 74125 and power meter 1918 with Si detector 918D were used to measure the external quantum efficiency (EQE). The morphologies and surface potentials of the PEDOT:PSS/MoO3 film were obtained by using an atomic force microscope (AFM) and scanning Kelvin probe microscope (SKPM) (Veeco Multimode V). The capacitance versus voltage (C-V) measurements were carried out with a Keithley 4200-SCS at 100 kHz applied frequency.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Basic Research Program of China (973 Program) (2012CB932402), National Natural Science Foundation of China (91123005, 61176057, 61211130358), the
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Priority Academic Program Development of Jiangsu Higher Education Institutions. Received: May 8, 2014 Revised: June 11, 2014 Published online:
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Adv. Mater. 2014, DOI: 10.1002/adma.201402076
