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High Efficiency Silicon/Organic Heterojunction Solar Cells with Improved Junction Quality and Interface Passivation Jian He, Pingqi Gao, Zhaoheng Ling, Li Ding, zhenhai yang, Jichun Ye, and Yi Cui ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b07511 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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High Efficiency Silicon/Organic Heterojunction Solar Cells with Improved Junction Quality and Interface Passivation Jian He,1 Pingqi Gao,1* Zhaoheng Ling,1 Li Ding,1 Zhenhai Yang,1 Jichun Ye,1* Yi Cui2* 1

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences,

Ningbo 315201, China *E-mail: [email protected]; [email protected] 2

Department of Materials Science and Engineering, Stanford University, Stanford, California

94305, USA *E-mail: [email protected]

Abstract Silicon/organic heterojunction solar cells (HSCs) based on conjugated polymers, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and n-type silicon (n-Si) have attracted wide attention due to their potential advantages in high-efficiency and low-cost. However, the state-of-the-art efficiencies are still far from satisfactory due to the inferior junction quality. Here, facile treatments were applied by pre-treating the n-Si wafer in tetramethylammonium hydroxide (TMAH) solution and capping copper iodide (CuI) layer upon the PEDOT:PSS layer to achieve a high quality Schottky junction. Detailed photo-electric characteristics indicated that the surface recombination was greatly suppressed after TMAH pre-treatment that produced an increased thickness of interfacial oxide layer. Furthermore, the CuI capping layer induced a strong inversion layer at near n-Si surface, resulting in an excellent field effect passivation. With the collaborative improvements in the interface chemical and electrical passivation, a competitive open-circuit voltage of 0.656 V and a high fill factor of 78.1% were achieved, leading to a stable efficiency of over 14.3% for the planar n-Si/PEDOT:PSS HSCs. Our findings suggest promising strategies to further exploit the full voltage as well as efficiency potentials for Si/organic solar cells. KEYWORDS: Si/PEDOT:PSS, hybrid solar cells, surface passivation, inversion layer, TMAH, CuI

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Silicon based heterojunction solar cells (HSCs) using functional materials as carrier-selective emitters have received great attention in recent years due to their advantages of low-temperature fabrication processes, simple device structures, and promising power conversion efficiencies.1-4 From band alignment viewpoint, the electron and hole selective heterojunctions on n-type crystalline silicon (n-Si) substrates formed with electron-transporting layers such as titanium oxide (TiOx),5-7 lithium fluoride (LiF),8,

9

etc. and hole-transporting layers of molybdenum oxide

(MoOx),9-11 tungsten oxide (WOx),12, 13 poly(3,4-ethylenedioxythiophene):polystyrene (PEDOT:PSS),14-16 respectively, have been successfully developed. These approaches embrace advantages over the conventional diffused PN junction in suppression of parasitic optoelectronic losses which are associated with the relatively high doping level in emitters. For the solution-processed Si/PEDOT:PSS HSCs, unlike MoOx or WOx that can almost not yield chemical passivation when directly coated on n-Si, with facile spin-coating of aqueous PEDOT:PSS solution on a n-type Si substrate and a subsequent low-temperature (~ 130 oC) annealing process, the PEDOT:PSS is able to deliver a good level of passivation (both chemical and electrical passivation) on the planar n-Si surface and enable the Si/PEDOT:PSS devices to reach a theoretical photovoltage of around 700 mV.17,

18

The evolvement in efficiency of the

Si/PEDOT:PSS HSCs in the past five years is shown in Figure S1. For the planar Si/PEDOT:PSS HSCs, an efficiency over 15% is achieved by modifying the PEDOT:PSS layer and employing advanced antireflection design.19 For the structured Si/PEDOT:PSS HSCs, power conversion efficiency (PCE) of 14.1% has been reported by incorporating a silane chemical in PEDOT:PSS solution to improve the contact quality in between Si and PEDOT:PSS.20 For the HSCs, the open-circuit voltage (VOC) is mainly determined by the quasi-Fermi level splitting of electrons and holes under illumination, which in Si/PEDOT:PSS HSCs is affected by the band alignment in between the n-Si substrate and the PEDOT:PSS layer, and also the defect states at the hetero-interface.21 Since the high quality n-Si substrates with a fixed carrier concentration have negligible bulk recombination, interface property is therefore become the major factor to determine


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the final VOC. A thin interfacial oxide layer (SiOx) inevitably forms during the device fabrication process, and the thickness of this SiOx layer is important to final device property. If the thickness of this insulating layer is too thin, it cannot produce sufficient chemical passivation, while it would severely hinder the carriers transport when it is a little bit too thick.22, 23 Chemical modifications of the Si/PEDOT:PSS interface,

including

grafting

of

methyl

group,24,

25

3-glycidoxypropyltrimethoxydsilane,20 etc., were attempted to improve the properties of the interfacial SiOx, but satisfactory results are still lacking. From electrical passivation perspective, PEDOT:PSS has a high work function and once it contacts to n-Si, a large conduction band offset is formed to reject the electron to reach the interface and thus a good electrical passivation is formed. Obviously, if the band bending level at the junction region can be enlarged by tuning the work function of PEDOT:PSS, the built-in potential can be increased and the inversion effect can be strengthened, which is rationally interpreted as a driving force that abets the hybrid device similar to a common P+N junction.21 For example, high work function layer of MoOx was deposited by Sun et al. upon the PEDOT:PSS layer to induce a stronger inversion layer in the underneath Si, resulting in enhanced VOC and PCE of 630 mV and 13.8%, respectively.25 Similar enhancement in inversion effect was also observed by Yu et al. through inserting 2 nm WO3 thin layer in between the frond silver-grid and PEDOT:PSS.12 However, the work function of these transition metal oxides are very sensitive to carbon contaminations and to air exposure, which causes an unreliable tuning of the inversion effect and might even lead to a fatal effect on the performance of the Si/PEDOT:PSS HSCs. Thus, achieving the concurrent improvements in chemical and electrical passivation is not an easy work, wherein various impacts of minimizing interfacial trapping states, optimizing the carriers separation and collection, and precisely tuning the work function of PEDOT:PSS must be taken into account simultaneously. New strategies using highly controllable interface chemical treatments and stable functional materials with the capability of enhancing the band-offset are eagerly needed to improve the junction quality and performance of Si/PEDOT:PSS solar cells.


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In this report, the Si wafers were pre-treated in tetramethylammonium hydroxide (TMAH) solution to reduce the interface defects and adjust the thickness of interfacial oxide layer. In comparison with conventional HF treatment, the surface terminations caused by TMAH treatment can improve the wettability of the Si surface and also produce a more satisfactory SiOx tunneling layer with noticeable improvements in passivation and carrier collection. Moreover, a transparent p-type inorganic semiconductor material of copper iodide (CuI),26 which has energy gap (3.1 eV) wider than Si and work function (~5.4 eV) higher than PEDOT:PSS, was post-deposited on the as-fabricated solar device on the PEDOT:PSS side, as a functional layer to modify the band off-set near junction region. Remarkably, in combination with the effects from TMAH treatment and CuI capping layer, we succeed in demonstrating a competitive performance with a best open-circuit voltage of 0.660 V and a fill factor (FF) of 78.1%, for the planar Si/PEDOT:PSS HSC that employed a non-passivated Si/metal rear contact, which are comparable to the VOC and FF for the traditional Si based solar cells with back-surface-field (BSF) structure. The best PCE of this kind of planar hybrid cells is thus enhanced from 11.9% to 14.3%, under air mass (AM) 1.5G illumination. More importantly, these results are statistically significant, indicating that the two approaches for better chemical and electrical passivation presented in this letter can be used as the general bases for further improvement in Si/PEDOT:PSS HSCs. RESULTS AND DISCUSSION Figure 1a illustrates the schematic diagram of the Si/PEDOT:PSS heterojunction solar cell, including an interfacial oxide layer and a CuI capping layer. The cross-sectional scanning electron microscopy (SEM) image of the Si/PEDOT:PSS/CuI stacks is illustrated in Figure S2. The SEM image was taken at a titled angle in order to partially observe the surface topography of the CuI capping layer, and thus the thickness of different layers cannot be strictly determined in accordance with the scale bar. The thickness of the PEDOT:PSS layer is optimized at about 80 nm for the best performance of planar devices in our previous research,14, 15 and the thickness of the CuI capping layer is chosen at 20 nm for optimal antireflection according to optical


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simulation in Supplementary Note 1 and Figure S3. Light current density-voltage (J-V) measurements for the three types of HSCs with HF treatment, TMAH treatment, and TMAH treatment together with CuI capping layer, respectively, are shown in Figure 1b, and the corresponding electrical output characteristics are summarized in Table 1. For the HF treated devices based on Ag-grid/PEDOT:PSS/n-Si/InGa structure, a mean PCE of 11.6% with mean VOC, JSC, FF of 0.615 V, 25.8 mA/cm2, and 73.6%, respectively, is achieved. After the pre-treating in TMAH solution with 60 seconds, the corresponding devices show an elevated performance statistically, with a mean VOC of 0.621 V, a mean FF of 75.4%, and a mean PCE of 12.1%. Photovoltaic characteristics of the Si/PEDOT:PSS heterojunction solar cells with different treating time in TMAH solution are also present in Table S1. The improvements in VOC and FF are believed to be directly related to the effective suppression of the carrier recombination at the interface between PEDOT:PSS and Si, demonstrating better passivating properties of the TMAH treated samples over the HF treated counterparts. After further capping with CuI layer upon the PEDOT:PSS film with device structure of CuI/Ag-grid/PEDOT:PSS/n-Si/InGa (marked as TMAH+CuI), a significant PCE boost from a mean value of 11.6% to over 14.0% is achieved, with a mean JSC of 27.9 mA/cm2, a mean VOC of 0.649 V, and a mean FF of 77.3%. It is worth noting that the increase in JSC is mainly from the antireflection effect of the CuI capping layer. Regardless of the limited light-trapping at front and the poor passivation at rear side, a champion efficiency of 14.3% with a VOC of 0.656 V and a FF of 78.1% is eventually realized, which is comparable to the VOC and FF for the traditional Si based solar cells with back-surface-field (BSF) structure. The significant voltage and efficiency advances for the CuI-capped devices might be rooted in the enhancement in self-built potential near junction region, which will be further discussed later. External quantum efficiency (EQE) and reflectance spectra of the HF treated and (TMAH+CuI) treated samples are shown in Figure 1c. The calculated JSC by integrating the EQE data for the two types of devices is 26.4 mA/cm2 and 28.7 mA/cm2, respectively, well corroborating the measured JSC values from J-V tests. The improvement in JSC for the (TMAH+CuI) treated devices are generally due to the


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improved junction quality and the anti-reflectance property at wavelength range of 500-1200 nm. Capacitance-Voltage (C-V) measurements are used to investigate the electronic performance of the HSCs and the plots of 1/C2-V are shown in Figure 1d. While extracting the built-in potential (Vbi), a metal oxide semiconductor model at frequency of 100 kHz is used. For the HSCs with TMAH+CuI treatment, large Vbi of 0.78 V is achieved, showing an enhancement of over 0.10 V and 0.14 V, separately, when compared with the HF treated and TMAH treated references. The enhanced Vbi results in a stronger internal electrical field for the easy separation of photo-generated carriers at Si/PEDOT:PSS heterojunction. For typical solar cells, the device performances are largely determined by the diode qualities.24, 27, 28 Steady-state J-V characteristics under dark condition are also shown in Figure S4 for the HSCs with different treatments. The corresponding diode saturation current density (J0), barrier height (φB), ideality factor (n) and also the C-V measured Vbi are listed in Table 2. For the devices with TMAH+CuI treatment, the J0 was obviously suppressed from 4.0 × 10−8 A/cm2 to 3.0 × 10−10 A/cm2, accompanied with the increase of φB from 0.873 eV to 0.998 eV and the decrease of n from 1.77 to 1.41, in comparison with the HF treated one. These improved diode parameters, consistent with the Vbi value measured by C-V system, indicate a well improved junction quality and minimized interfacial recombination, which will result in a more superior VOC at light J-V measurement. Note that the heterojunction property inherent to the Si/PEDOT:PSS device will not only vary with the preconditioning of the substrate and humidity of the atmosphere during preparation, but also with the time until measurement. So the results obtained above must be statistically significant, in particular for the open-circuit voltages, which possess the most sensitivity to the interface variations. The statistical VOC values over 20 devices for the individual HF treated, TMAH treated and (TMAH+CuI) treated devices are presented in Figure 2a-c. The mean (and best) VOC for the three types devices are of 0.615 V (0.625 V), 0.621 V (0.634 V), 0.649 V (0.660 V), respectively, confirming that the performance improvement related to TMAH treatment and CuI capping layer are significantly reproducible. The superior performance for the TMAH treated devices over the HF treated ones


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suggests different levels of interface passivation depending on the preparation method. It is therefore important to gain more detailed insight into the factors which control the chemical properties of the interface passivation layers. The hydrophilic characteristics on the silicon surfaces after the HF and TMAH treatments are present in Figure S5. The TMAH treated Si wafer shows much better wettability than the HF treated one, due to the stable silanoles (Si-OH) terminations. The improved wettability thus allows the polymer of PEDOT:PSS to be better dispersed on the substrate during the spin-coating process. In addition, the different surface terminations may cause different interfacial oxide layers, which usually play a key role in passivating the Si surface. Figure 3a shows the transmission electron microscope (TEM) characterization of the TMAH treated interfacial oxide layer. One can see the pre-treatment of the Si wafer with TMAH solution can produce an uniform oxide layer in between the Si and PEDOT:PSS interface. Figure 3b shows the x-ray photoelectron spectroscopy (XPS) spectra in the Si 2p region for the Si substrate immediately after the HF treatment, the buried heterojunction of the hybrid cells with HF treatment and TMAH treatment, respectively, from bottom to top. Immediately after the HF treatment, almost no oxide related signal is detected on the Si surface. For both of the different oxidation processes essentially for the TMAH treatment, an increase and a broadening of the SiOx peak is present. SiOx-layer thickness can be computed from the intensity ratios of the Si 2p XPS peak attributed to the oxide and the elemental Si 2p peak attributed to the substrate at different incident angles.23, 29 Here the TMAH treated sample shows a thicker oxide layer of 0.53 nm over the 0.32 nm for HF treated one. Regardless of the inconsistent to TEM results in absolute value, the XPS results tend to give the conclusion that a thicker oxide layer will be resulted by TMAH treatment. In order to make a direct connection between such oxide layers at the Si/PEDOT:PSS interfaces to the passivation properties, microwave photoconductance decay technique based minority carrier lifetime mapping measurement is further used to characterize the PEDOT:PSS coated Si substrates (strictly comply with the device fabrication procedure) with HF and TMAH treatments, as illustrated in Figure 3c-d. Because


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these samples have similar bulk lifetime and wafer thickness, the variations of the measured minority carrier lifetime thus well represents the surface recombination properties at the Si/PEDOT:PSS interface. The average minority carrier lifetimes for the HF and TMAH treated samples are 50.4 µs and 80.6 µs, respectively, indicating that the TMAH treatment can produce a thin oxide layer with an optimal thickness and an improved quality. Minority carrier lifetime of the Si/PEDOT:PSS heterojunction with different TMAH treating time are also present in Figure S6. To help understand the effect of CuI capping layer, ultraviolet photoelectron spectroscopy (UPS) measurement is used to investigated the electronic structure of PEDOT:PSS, PEDOT:PSS/CuI and CuI, as shown in Figure 4a. All of the measured samples are deposited on the indium-tin oxide (ITO) substrates, and the PEDOT:PSS/CuI one represents a sample with the PEDOT:PSS film spin-coated on top of the thermally deposited CuI film. The work functions of the pristine PEDOT:PSS and CuI are 5.07 eV and 5.38 eV, respectively, while for the PEDOT:PSS/CuI the secondary electron cutoff of the PEDOT:PSS film shifts toward higher work function about 0.10 eV. For the Si substrate with a fixed doping concentration, this enhanced work function of the PEDOT:PSS film will result in a stronger inversion effect and thus a higher built-in potential for the Si/PEDOT:PSS heterojunction. Scanning Kelvin probe microscopy (SKPM) is used to measure the surface potentials of PEDOT:PSS and PEDOT:PSS/CuI films, as shown in Figure 4b. The surface potential of the CuI layer is about 90 mV higher than that of pristine PEDOT:PSS layer, further confirming the work function enhancement of PEDOT:PSS layer when coated with CuI layer. Metal-semiconductor junction model is widely accepted for the Si/PEDOT:PSS HSCs, where the PEDOT:PSS layer act as metal part due to its metallic conducting properties. In this junction model, electrons will flow from the low electrostatic potential energy semiconductor to the higher electrostatic potential energy metal to establish electronic equilibrium and thus forming an energy barrier for electrons to cross from metal into semiconductor.25 Because the Si substrates have same doping concentrations, the Schottky barrier height of the HSC is mainly determined by the


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difference between the metal work function and the Fermi energy of Si. Figure 4c presents the calculated energy band diagram of Si/PEDOT:PSS HSCs with and without CuI capping layer (solid line and dash line, respectively). After coating with CuI layer, the 0.10 eV enhancement in work function of PEDOT:PSS layer results in an additional band bending of about 0.1 eV at Si surface region, which is believed to be responsible for the overall increase of 0.026 V in VOC . The stability of the PV parameters for the Si/PEDOT:PSS HSCs with and without CuI capping layer is also compared in Figure 5. The tests are carried out on the HSCs without any additional sealing at temperature of about 25 ºC while maintaining the open-circuit state in the dark. For the reference devices, the PV performance, especially the VOC and FF values, deteriorated rapidly in the air storage experiments due to the strong hygroscopicity of the PSS group.19, 30 After coated with an inorganic CuI layer, the devices are well protected from decaying in air. Even at harsh ambient humidity, the devices remain keeping 65% of the initial PCE. The stability improvement of the CuI coated devices can be attributed to the hydrophobicity of the CuI film, which keeps the underneath PEDOT:PSS film away from moisture. CONCLUSIONS In conclusion, we have demonstrated a simple treatment of the Si substrate with organic alkali, TMAH, can deliver improved interface passivation that enhances the performance of the Si/PEDOT:PSS heterojunction hybrid solar cell. In combination with an additional effect from CuI capping layer on the as-prepared device, we succeed in demonstrating a competitive performance on VOC and FF, with a best VOC of 0.660 V and a FF of 78.1%, for the Si/PEDOT:PSS HSCs without any rear-sided passivation at the contact of Si/metal. Moreover, the pre-treatment using TMAH and post-capping with CuI layer are further experimentally identified to be responsible for the high performance Si/PEDOT:PSS solar cells with efficiency as high as 14.3% via collaborative improvements in the interface chemical and electrical passivation. The methodologies demonstrated in this work could be general ways towards even higher efficiency for Si/organic hybrid solar cells, completely compatible with the solution-based processing.


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Experimental Section Device Fabrication: Single-side polished n-type silicon wafers with thickness of 270 ± 10 µm and resistivity of 0.05-0.1 Ω·cm were used for the heterojunction solar cells fabrication. Before spin-coating of the p-type conducting polymer layer, the wafers were firstly cleaned with standard RCA procedure and followed with dilute HF dip to remove the native oxide layer. For the TMAH treated one, the wafers were immersed in 1% TMAH solution for different time and rinsed with DI water. After all of the cleaning process, PEDOT:PSS (PH1000 from Clevios) solution mixed with 7 wt% ethylene glycol and 0.25 wt% Triton-100 (from Aldrich) was spin-coated on the polished side of the wafer, and then was annealed at 135 ºC to remove the solvents to form a highly conductive p-type organic thin film. Then, Ag-grid electrode was formed by screen-printing silver paste with top metal coverage ≈7% and InGa alloy paste was used to form a back contact. CuI films with thickness of 20 nm were then deposited by thermal evaporation on the front surface. Device Characterization: The morphological analyses of the samples were conducted by scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, Tecnai F20). X-ray photoelectron spectroscopy (XPS) was performed using a monochromatic Al Kα X-ray source and a hemispherical analyser in ultra high vacuum with a base pressure of 1 × 10

-10

mbar (Kratos AXIS

Ultra DLD). Secondary electron cutoff and valence band measurements were performed using X-ray excitation, with an added bias to extract the cutoff edge. For the work function measurements, the CuI and PEDOT:PSS layers were characterized directly on clean indium tin oxides substrates to reveal the electronic structure near the valence band edge. For the interfacial oxide layer thickness measurement, the spin-coated PEDOT:PSS layer was peel off by adhesive tape before measurement. The surface potentials of the PEDOT:PSS/CuI film were obtained by using scanning Kelvin probe microscope (SKPM) (Veeco Dimension3100V). The reflectance of the samples was measured by spectrophotometer (Helios LAB-re, with an integrating sphere) in the wavelength range of 400-1100 nm. The PV performance was measured


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with a solar simulator (Oriel®, Sol3ATM) under AM 1.5 illumination (1000W/m2) in the standard testing condition. The cells were shielded by an opaque mask with a designated aperture area of 0.5 cm2, and the temperature was actively controlled at 25 ± 0.5 ºC during the measurements. The external quantum efficiency curves were measured by a quantum efficiency measurement system (Oriel®, IQE-200TM). The capacitance versus voltage (C-V) measurements were carried out with keithley 4200-scs semiconductor parameter analyzer. The minority carrier lifetime was measured by a microwave photoconductivity decay system (WT-2000 µPCD, Semilab) using transient (PCD) methods. Double-side polished double-side PEDOT:PSS coated Si wafers with resistivity of 1-3 Ω•cm are used for minority carrier lifetime measurement.

Conflict of Interest: The authors declare no competing financial interest.

Acknowledgment: This work was financially supported by Zhejiang Provincial Natural Science Foundation (LY14F040005, LR16F040002), National Natural Science Foundation of China (No.61674154, 61404144), Natural Science Foundation of Ningbo (2015A610040), Major Project and Key S&T Program of Ningbo (No. 2016B10004, 2014B10026), International S&T Cooperation Program of Ningbo (No. 2015D10021).

Supporting Information Available: Cross-sectional scanning electron micrograph of the

CuI/PEDOT:PSS/Si

configuration.

Simulated photocurrent loss

of the

Si/PEDOT:PSS heterojunction solar cells with different thickness of PEDOT:PSS film and CuI capping layer. Dark current density-voltage characteristics of the Si/PEDOT:PSS heterojunction solar cells with HF treatment, TMAH treatment and TMAH+CuI treatment. Photographs of water drop on the silicon surfaces after the different treatment of HF and TMAH. Minority carrier lifetime of the Si/PEDOT:PSS heterojunction with different TMAH treating time. Photovoltaic characteristics of the Si/PEDOT:PSS heterojunction solar cells with different TMAH treating time. This


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Nanohole Solar Cells Using MoOx and LiF Films. Nano Lett. 2016, 16, 981-987. 10. Bullock, J.; Cuevas, A.; Allen, T.; Battaglia, C. Molybdenum Oxide MoOx: A Versatile Hole Contact for Silicon Solar Cells. Appl. Phys. Lett. 2014, 105, 232109. 11. Battaglia, C.; Yin, X. T.; Zheng, M.; Sharp, I. D.; Chen, T.; McDonnell, S.; Azcatl, A.; Carraro, C.; Ma, B. W.; Maboudian, R.; Wallace, R. M.; Javey, A. Hole Selective MoOx Contact for Silicon Solar Cells. Nano Lett. 2014, 14, 967-971. 12. Mu, X. H.; Yu, X. G.; Xu, D. K.; Shen, X. L.; Xia, Z. H.; He, H.; Zhu, H. Y.; Xie, J. S.; Sun, B. Q.; Yang, D. R. High Efficiency Organic/Silicon Hybrid Solar Cells with Doping-Free Selective Emitter Structure Induced by A WO3 Thin Interlayer. Nano Energy 2015, 16, 54-61. 13. Gerling, L. G.; Mahato, S.; Voz, C.; Alcubilla, R.; Puigdollers, J. Characterization of Transition Metal Oxide/Silicon Heterojunctions for Solar Cell Applications. Appl. Sci.-Basel 2015, 5, 695-705. 14. He, J.; Gao, P. Q.; Liao, M. D.; Yang, X.; Ying, Z. Q.; Zhou, S. Q.; Ye, J. C.; Cui, Y. Realization of 13.6% Efficiency on 20 µm Thick Si/Organic Hybrid Heterojunction Solar Cells via Advanced Nanotexturing and Surface Recombination Suppression. ACS Nano 2015, 9, 6522-6531. 15. He, J.; Yang, Z.; Liu, P.; Wu, S.; Gao, P.; Wang, M.; Zhou, S.; Li, X.; Cao, H.; Ye, J. Enhanced Electro-Optical

Properties

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Nanocone/Nanopillar

Dual-Structured

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Ultrathin

Silicon/Organic Hybrid Solar Cell Applications. Adv. Energy Mater. 2016, 6, 1501793. 16. Jeong, S.; Garnett, E. C.; Wang, S.; Yu, Z. G.; Fan, S. H.; Brongersma, M. L.; McGehee, M. D.; Cui, Y. Hybrid Silicon Nanocone-Polymer Solar Cells. Nano Lett. 2012, 12, 2971-2976.


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17. Zielke, D.; Pazidis, A.; Werner, F.; Schmidt, J. Organic-Silicon Heterojunction Solar Cells on N-type Silicon Wafers: The BackPEDOT Concept. Sol. Energy Mater. Sol. Cells 2014, 131, 110-116. 18. Schmidt, J.; Titova, V.; Zielke, D. Organic-Silicon Heterojunction Solar Cells: Open-Circuit Voltage Potential and Stability. Appl. Phys. Lett. 2013, 103, 183901. 19. Liu, Q. M.; Ishikawa, R.; Funada, S.; Ohki, T.; Ueno, K.; Shirai, H. Highly Efficient Solution-Processed

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ACS Nano

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Figure captions. Figure 1. (a) Device configuration of the Si/PEDOT:PSS heterojunction solar cell with CuI capping layer. (b) Current density-voltage curves of Si/PEDOT:PSS heterojunction solar cells with HF treatment, TMAH treatment and TMAH + CuI treatment, respectively. (c) Reflectance spectra and external quantum efficiency curves of the Si/PEDOT:PSS heterojunction solar cells with different treatments. (d) Capacitance-voltage measurements of the Si/PEDOT:PSS heterojunction solar cells with different treatments. Figure 2. Statistical distribution of the VOC values on 20 cells with (a) HF treatment, (b) TMAH treatment and (c) TMAH + CuI treatment. Figure 3. (a) Transmission electron microscope image of the Si/PEDOT:PSS heterojunction interface with TMAH treatment. Scale bar is 5 nm. (b) X-ray photoelectron spectroscopy spectra in Si 2p region collected from the bared Si surface immediately after the HF treatment, and the interfacial Si oxide tunneling layers of the Si/PEDOT:PSS devices combining

HF and TMAH treatment, respectively. (c)-(d)

Spatial mappings of the minority carrier lifetime for the different samples with HF and TMAH treatment. Double-side PEDOT:PSS coated Si wafers were used for minority carrier lifetime measurement. Figure 4. (a) Ultraviolet photoelectron spectroscopy measured work function of the as-prepared PEDOT:PSS, PEDOT:PSS on CuI substrate, and CuI. (b) Cross-sectional line profile of the surface potential image of the PEDOT:PSS and CuI film on PEDOT:PSS. The inset shows the surface potential mapping image. (c) Calculated energy band diagram of the Si/PEDOT:PSS heterojunction solar cells. Figure 5. Stability tests of the Si/PEDOT:PSS heterojunction solar cells with (a) planar reference and (b) CuI capping layer in dark condition up to 300 hours. Table 1. Photovoltaic characteristics of the Si/PEDOT:PSS heterojunction solar cells with HF treatment, TMAH treatment and TMAH + CuI treatment. Table 2. List of the reversed saturation current (J0), the ideality factor (n) and the barrier height (φB) obtained from dark J-V curves and the built-in potential (Vbi) measured from C-V characterization based on Si/PEDOT:PSS heterojunction solar


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cells with HF treatment, TMAH treatment and TMAH + CuI treatment.

Figure 1

Figure 2


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Figure 3

Figure 4

Figure 5


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a

VOCb

JSCb

FFb

PCEb

(V)

(mA/cm2)

(%)

(%)

0.618

26.0

74.6

11.9

(0.615 ± 0.010)

(25.8 ± 0.3)

(73.6 ± 0.6)

(11.6 ± 0.7)

0.629

26.3

77.0

12.7

(0.621 ± 0.013)

(26.0 ± 0.3)

(75.4 ± 0.8)

(12.1 ± 0.6)

0.656

28.0

78.1

14.3

(0.649 ± 0.011)

(27.9 ± 0.2)

(77.3 ± 0.8)

(14.0 ± 0.4)

Samples

HF Treatment

TMAH Treatment

TMAH + CuI a

Data and statistics based on 20 cells of each condition. bNumbers in bold are the

maximum efficiency values of the devices. Table 1

J0 Samples

(A/cm2)

n

φB

Vbi

(eV)

(V)

HF Treatment

4.0 × 10-8

1.77

0.873

0.64

TMAH Treatment

7.0 × 10-9

1.54

0.926

0.69

TMAH + CuI

3.0 × 10-10

1.41

0.998

0.78

Table 2


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ToC graphic


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