15% Efficiency Ultrathin Silicon Solar Cells with Fluorine-Doped

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15% Efficiency Ultrathin Silicon Solar Cells with Fluorine Doped Titanium Oxide and Chemically Tailored Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) as Asymmetric Heterocontacts Jian He, Md Anower Hossain, Hao Lin, Wenjie Wang, Siva Krishna Karuturi, Bram Hoex, Jichun Ye, Pingqi Gao, James Bullock, and Yimao Wan ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b01754 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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15% Efficiency Ultrathin Silicon Solar Cells with Fluorine Doped Titanium Oxide and Chemically Tailored Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) as Asymmetric Heterocontacts Jian He,1,2,* Md. Anower Hossain,3 Hao Lin,4 Wenjie Wang,1 Siva Krishna Karuturi,1 Bram Hoex,3 Jichun Ye,4 Pingqi Gao,5,* James Bullock,2 Yimao Wan 1 1

Research School of Engineering, The Australian National University, Canberra, ACT

2602, Australia. 2

Electrical & Electronic Engineering Department, University of Melbourne,

Melbourne, VIC 3052, Australia. 3

School of Photovoltaic and Renewable Energy Engineering, University of New South

Wales, Sydney, NSW 2052, Australia. 4

Ningbo Institute of Material Technology and Engineering, Chinese Academy of

Sciences, Ningbo 315201, China. 5

School of Materials, Sun Yat-sen University, Guangzhou 510275, China.

E-mail: [email protected]; [email protected]

Abstract In order to achieve a high performance-to-cost ratio to photovoltaic devices, the development of crystalline silicon (c-Si) solar cells with thinner substrates and simpler fabrication routes are important steps. Thin-film heterojunction solar cells (HSCs) with dopant-free and carrier-selective configurations look like ideal candidates in this respect. Here, we investigated the application of n-type silicon/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HSCs on periodic nano-pyramids textured, ultrathin c-Si (~ 25 m) substrates. A fluorine-doped titanium oxide film was used as an electron-selective passivating layer, showing excellent interfacial passivation (surface recombination velocity ~10 cm/s) and contact property (contact resistivity ~20 m/cm2). High efficiency of 15.10% was finally realized by

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optimizing the interfacial recombination and series resistance at both the front and rearsides, showing a promising strategy to fabricate high-performance ultrathin c-Si HSCs with a simple and low-temperature procedure.

KEYWORDS: ultrathin c-Si, asymmetric heterocontacts, passivating contact, titanium oxide, electron-selective contact

Ultrathin crystalline silicon (c-Si) solar cells with thicknesses below 50 m featuring low material usage, light weight, and flexibility have received significant attention in recent years. Theoretical power conversion efficiency (PCE) of over 29% is achievable for thin c-Si solar cells if light harvesting and surface passivation can both approach their ideal limits.1 In terms of optical design, surface texturing using periodic nanostructured arrays like nanowires,2 nanocones,3 nanopencils,4 and nanopyramids,5 have been used for improved optical design in ultrathin c-Si absorbers with the potential to approach the Lambertian absorption limit.6, 7 As for surface passivation, thus far the highest efficiency >50 µm cells have utilized heavily-diffused, directly-metalized contacts, which are known to introduce surface recombination losses.8 Passivated contacts, such as dopant free contacts are more ideally suited to this application. Multiple studies have shown that combining hole-selective materials like molybdenum oxide (MoOx),9,

10

vanadium oxide (VOx),11-13 and poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)14,

15

with electron-

selective materials such as lithium fluoride (LiFx),9, 16 magnesium oxide (MgOx),17 and titanium oxide (TiOx)18 can achieve competitive efficiencies. Ultimately this approach aims to utilize simple heterocontact deposition techniques and is not depend on passivating interlayers such as the commonly used hydrogenated amorphous silicon (aSi:H). In this paper, we test the extremes of these two trends in tandem. Dopant-free asymmetric heterocontacts (DASHs) were integrated with thin freestanding c-Si absorbers (~25 m), without the use of dedicated passivating interlayers, as shown in Figure 1a. Hexagonally ordered pyramid arrays with a period of 1.4 μm, depicted in Figure 1b, were textured on the frontside for improved light harvesting. These reduce


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the average reflectance from 40% to 14% in the whole wavelength range (as shown in Supporting Information, Figure S1). A chemically tailored PEDOT:PSS bilayer film is applied to the front surface texturing forming a conformal hole heterocontact, as shown in Figure 1c. On the planar rear-side, a TiOx/LiFx/Al electron heterocontact is employed, showing excellent interfacial passivation and contact property. All cell fabrication were performed at low temperatures (≤ 250 oC) and simplified steps (less than 7 steps), resulting a highest PCE of 15.10%, demonstrating the potential of this approach.

Results and Discussion To initially characterize the performance of the hole and electron heterocontacts, tests were performed on thick silicon substrates. For the n-type c-Si/PEDOT:PSS hole heterocontact studied here, a bi-layer of PEDOT:PSS based films with separate additives,

for

promoting

glycidoxypropyl)trimethoxysilane

conformality (GOPS)

and and

conductivity,

of

(3-

1-ethyl-3-methylimidazolium

tricyanomethanide (EMIM/TCM), respectively were used.19 Detailed characterizations of the bi-layer PEDOT:PSS films and their effects on photovoltaic performance is presented in our previous publications.20, 21 Applying GOPS into the PEDOT:PSS film as buffer layer can effectively avoid the formation of nano-caves between nanostructured c-Si and PEDOT:PSS film.19 This was also found to be correlated with the quality of interfacial passivation. Figure 2a presents the effective minority carrier lifetimes of nano-pyramid textured n-type c-Si wafers coated with GOPS incorporated PEDOT:PSS films on both sides, showing improvements in surface passivation as the concentration of GOPS in PEDOT:PSS film increases from 0 to 3 wt%. Though the incorporation of GOPS in PEDOT:PSS film can improve the contact conformality and passivation of the nanostructured c-Si substrate, the presence of insulated silane in PEDOT:PSS film reduces it’s electrical conductivity, thus affecting the charge transfer and collection in PEDOT:PSS film. Figure 2b presents the trade-off between effective surface recombination velocity (defined as Sfront) and the sheet resistance of the PEDOT:PSS film (defined as Rsheet) with different concentrations of GOPS in PEDOT:PSS film. Increasing the concentration of GOPS from 0 to 3 wt% can decrease


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the Sfront from over 700 cm/s to below 200 cm/s, but increase the Rsheet from 200 /sq to 500 /sq. As for the rear-side electron contact, atomic layer deposited TiOx films were used to passivate the c-Si surface and a low work function, thermally evaporated LiFx / Al electrode encourages the collection of electrons. As shown in Figure 2c, improved surface passivation quality was found for thicker TiOx films deposited on the c-Si surface. The lowest surface recombination velocity (defined as Srear) was achieved for 5.5-nm TiOx film, obtaining a value below 10 cm/s. Figure 2d also shows the contact properties of n-type c-Si/TiOx/LiFx/Al contact with different TiOx thicknesses. Typical current-voltage (I-V) curves used for contact resistivity (defined as ρcontact) extraction are provided in Figure S2. Compared with the non-Ohmic contact for Al/Si and Al/TiOx/Si samples, Ohmic contact can be achieved by additionally introducing a thin LiFx layer between the TiOx layer and Al film, with extremely low ρcontact about 20 m/cm2 even for the TiOx films thicker than 5 nm. To explore the potential of these contacts on ultrathin c-Si substrate, the dependence of photovoltaic performance on the front- and rear-side heterocontact properties was theoretically simulated. Detailed simulation information was shown in supporting information note. Figures 2e and 2f show PCEs of ultrathin n-type c-Si DASH solar cells as function of Sfront / Rsheet and Srear / ρcontact, respectively. Their influences on other photovoltaic parameters, like short-circuit current density (JSC), VOC, and fill factor (FF), are also presented in Figure S3 and S4. For the front-side contact, it can be found that the Sfront and Rsheet have very limited influence on the JSC. That is because the high hole concentration at the front surface would reduce the electron concentration, and thus reducing the front side carrier recombination even under a condition of large Sfront. The VOC is strongly sensitive to the Sfront but not the Rsheet. That is because heterojunction interface with high Sfront will recombine lots of photogenerated electron-hole pairs and thus reduce the built-in potential of the junction. Conversely, the FF is mainly affected by the Rsheet, which will affect the series resistance of the whole device. As for the rear-side contact, a rapid decrease of JSC and VOC values can be found when enlarging the Srear. Figures 2e and 2f also show the


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experimental points (extracted from Figure 2b and 2d) with different combinations of Sfront / Rsheet and Srear / Rcontact, respectively, indicating that using 2 wt% GOPS doped PEDOT:PSS as hole-transporting layer and 5.5-nm TiOx/1-nm LiFx stacking layer as electron-transporting layer can achieve the best PCE. The LiFx / Al electrode is well known to be useful in collecting electrons in a range of devices, the mechanism of which is commonly ascribed to its low work function.22 To investigate whether other factors also contribute here, high-resolution transmission electron microscopy (HR-TEM) of the n-type c-Si/TiOx/LiFx/Al stack structure is performed and shown in Figure 3a, with typical thicknesses of 5.5 nm and 1.0 nm for TiOx and LiFx films, respectively. Clear interfaces appear between all layers, suggesting that a large degree of intermixing does not take place in the as-deposited state. The presence of an interlayer, likely SiOx, is clearly seen at the TiOx / c-Si interface. Energydispersive X-ray spectroscopy (EDX) line scan of the Al, F, Ti, O, and Si elemental distributions across the interface (red line in HR-TEM image) is also presented in Figure 3a, confirming the identity of the layers. The small atomic weight of Li makes it difficult to be detected by EDX. We note that the non-abrupt gradients in concentration across the interfaces are believed to be due to the resolution of the EDX scan rather than intermixing. The F signal is an exception to this, as a broad platform appears not only in the LiFx region but also in the TiOx region, suggesting the potential diffusion of F into the TiOx layer. To further investigate this, time-of-flight secondary ion mass spectrometry (ToFSIMS) characterization was also performed. Figure 3b shows the ToF-SIMS depth profile of an LiFx/TiOx/c-Si stack, where the four distinct materials are differentiated by their respective ions Al+, Li+/F-, TiO-, and Si2+. Firstly, at the TiOx/c-Si interface, a signal corresponding to SiO2- is observed, confirming that a thin silicon oxide interlayer has formed during the TiOx deposition process. This also suggests that the chemical passivation likely comes from SiOx at the TiOx/c-Si interface. As for the Li and F species, while knock-on sputtering makes diffusion into the TiOx layer difficult to evaluate, a slight separation of Li+ and F- signals can be found in Figure 3b. This may further support the migration of F into TiOx. X-ray photoelectron spectroscopy (XPS)


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measurement was also used to study the elemental composition of TiOx/c-Si and LiFx/TiOx/c-Si contacts, as shown in Figure S5. For Ti 2p and Si 2p spectra in Figure S5a and S5b, no obvious difference was shown for the TiOx/n-Si and LiFx/TiOx/n-Si contacts. As to the O 1s spectra in Figure 3c, a more pronounced shoulder at a higher binding energy level (531.8 eV) is shown for LiFx/TiOx stack, which may relate oxygen interaction with the lithium.23, 24 Ti 3s and Li 1s spectra from 40 eV to 65 eV are shown in Figure 3d. For the TiOx film, a peak at 61.5 eV can be found corresponding to the presence of Ti3+ in the non-stoichiometric TiOx film. As to the LiFx/TiOx stacking film, new sharp peak at 55.2 eV and a broad peak at lower binding energy level are presented which relate to the Li-F and Li-O bonds, respectively. It is evident from the characterizations of the c-Si/TiOx/LiFx/Al samples that the F element (even Li element) may be incorporated into the TiOx layer, which could in turn influence the electronic properties of TiOx. Here, density functional theory (DFT) calculations were performed considering Li, F and Al as potential dopants for the TiO2 film. The density of states (DOS) and projected density of states (PDOS) plots of a pure TiO2 shows (Figure 3e) defect-free band gap with an energy bandgap value of 2.63 eV, lower than the experimentally known value of 3.2 eV because the Perdew-BurkeErnzerhof (PBE) functional underestimate the band gap in DFT calculations.25 It shows that the valence band is mainly comprised of O 2p states and trace contribution from Ti 3d states, however, the conduction band is primarily comprised of Ti 3d states and a little contribution of O 2p states. Incorporation of dopants results in a change of electronic properties, such as incorporation of defect states and change of band gap energies. Therefore, the band gap value upon substitutional Al incorporation into the TiO2 lattice at the Ti site was reduced to 2.55 eV. However, it remains the same for the substitutional F doping at O site. Because of smaller atomic size, the Li was mainly considered as interstitial dopants where the interstitial Li will acts as an electron donor as suggested by Kronic Vink defect analyses. Li-doping is known to improve carrier transport properties of TiO2 at least one order higher than that of the undoped TiO2-based device.23, 26 In our calculations, as shown in Figure S6a and S6b, the incorporation of Li dopant in TiO2 mainly creates


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defect states in the valence band side, suggesting that these shallow impurity states can enhance its electronic properties, i.e., the conductivity of the TiO2 layer. In the other hand, the interstitial Li-defect acts as an electron donor into the TiO2 lattice and creates states close to, or into the conduction band. Therefore, the interstitial Li-doped TiO2 believed to perform significantly higher electron transporting property than that of the undoped TiO2 electrodes. The substitutional Al-doped TiO2 also shows defect states close to the valence band (Figure S6c), suggesting the acceptor states because of the lower valence of Al than that of the Ti. When substitutional doping of F atom at O site, one unpaired electron located in the surrounds of the F will excess in the TiO2 structure. The previous study has shown the probability of electron transfer from F to its neighbors or the unpaired electron delocalizes within the conduction band.27, 28 In our calculations for the F-doped TiO2, as shown in Figure 3f and S7, the Fermi level is displaced into the conduction band, suggesting the delocalization of the electrons. The F states contribute to the conduction band in the dopant system, thereby, should enhance carrier transport performance. The photovoltaic properties of the ultrathin c-Si DASH solar cells with optimized front/rear-side contacts are shown in Figures 4a and 4b. As shown in Figure 4a, for a planarized n-Si substrate (Thin Flat), the lack of light trapping results in a relatively poor averaged PCE of 9.10%, with a VOC of 0.565 V, JSC of 22.7 mA/cm2, and FF of 0.710. After applying the ordered pyramid array as a light trapping strategy and bi-layer PEDOT:PSS film as hole-transporting layer (Thin Pyramid), a moderate PCE of 12.40% was achieved, with a VOC of 0.592 V, JSC of 28.5 mA/cm2, and FF of 0.735. This enhancement is attributed to the improved light utilization and better heterojunction interface at front-side. In the final structure, a 5.5-nm TiOx and 1-nm LiFx stack is applied between the c-Si absorber and the Al electrode (Thin Pyramid + TiOx + LiFx), an averaged PCE of 14.90% was obtained on 25-m thick ultrathin c-Si DASH solar cells, with VOC of 0.623 V, JSC of 31.8 mA/cm2, and FF of 0.752. The corresponding photovoltaic parameters are summarized in Table 1 and the statistical distribution of the photovoltaic parameters are shown in Figure S8. These results clearly show that with simultaneous optimization of the heterocontacts by advanced light-trapping and


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effective contact passivation, as well as reduced interfacial contact resistivity, the performance of n-type c-Si DASH solar cells can be significantly improved. External quantum efficiency (EQE) and reflectance curves of the above mentioned ultrathin n-type c-Si DASH solar cells are also presented in Figure 4b. For thin flat DASH cell, the EQE values were limited to less than 65% across the whole wavelength range mainly due to the strong reflection of the front flat surface. By introducing the ordered pyramid array the reflectance can be effectively reduced, improving the EQE value across the whole wavelength range. Though the insertion of TiOx and LiFx stack at the rear-side does not further increase light absorption, the presence of interfacial passivating layer can suppress carrier recombination at n-type c-Si/Al interface effectively. This resulted in an improvement in the EQE value especially for long wavelength light, corresponding to an integrated JSC of about 31.2 mA cm−2. The energy power loss of ultrathin n-type c-Si DASH solar cells using the optimal experimental contact conditions in this paper is shown in Figure S9. Typical efficiency evolvement of ultrathin m c-Si solar cells is also presented in Figure S10. Compared with ultrathin c-Si solar cells with doping configurations, there is still plenty of room for improvement in efficiency for DASHs. The electrical power loss for this ultrathin n-type c-Si DASH solar cell mainly comes from the front-side n-Si/PEDOT:PSS contact, due to the limitation of PEDOT:PSS film. Further researches will focus on searching hole-selective contact materials which can provide excellent surface passivation and carrier transport simultaneously.

Conclusion In summary, periodic nano-pyramid array textured ultrathin n-type c-Si DASH solar cells with thickness ~25 µm were fabricated, using a chemical tailored bi-layer PEDOT:PSS film as hole-transporting layer and doped TiOx film as electrontransporting layer. Benefited from the improved interfacial passivation and contact properties both on front and rear side, best PCE of 15.10% was finally achieved. Theoretical simulations and experimental characterizations show that substitutional fluorine doping in TiOx film plays important role in tuning the contact property of TiOx


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film by reducing its Fermi level to make it beneficial for electron transport. These results highlight an effective way to design high-efficiency ultrathin (≤ 25 µm) c-Si DASH solar cells via simple and low-temperature fabrication procedures. Experimental Section Fabrication of periodic nano-pyramid array. Free standing ultrathin c-Si wafers were thinned from one side polished n-type (100) c-Si wafers (1-3 Ω.cm, 270 μm thickness, Czochralski-grown), using 50% concentrated KOH solution at 80 oC. Periodic nano-pyramid arrays were fabricated on the polished side of the ultrathin Si wafers by colloidal lithography and anisotropic wet-etching techniques, as reported in our previous papers.4, 7 Syringe injection was used to form closely packed polystyrene (PS) array with a periodicity of 1.4 μm. Subsequently, the PS monolayer coated c-Si wafers were transferred to reactive ion etching (RIE) system to reduce the diameter of the PS nanospheres. After RIE process, the Si wafers were anisotropically etched in NaOH: IPA: H2O (mass ratio of 1:1:3) solution at 60°C for 12 min to form a periodic nano-pyramid array. Following that the samples were immersed in methylbenzene solution to remove the PS nanospheres. Finally, samples were etched with 1 vol% tetramethylammonium hydroxide (TMAH) solution at room temperature for 30 s to smooth the surface of the nano-pyramids. Heterojunction solar cells. For the rear-side electron-transporting layer, TiOx layers with thicknesses of 5.5 nm were ALD-deposited (Beneq TFS 200) by sequential exposure of titanium isopropoxide (TTIP) and H2O at a temperature of 230 °C. Then ~1 nm of LiFX and 200 nm of Al were sequentially deposited by thermal evaporation. For the hole-transporting layer, PEDOT:PSS PH1000 solution mixed with 5 wt% DMSO and 1 wt% Triton-X100 was used for flat cells and a bi-layer of PEDOT:PSS based films (highly adhesive PEDOT:PSS film and highly conductive PEDOT:PSS film) were used for nanostructured cells. For the highly adhesive PEDOT:PSS film, PEDOT:PSS was firstly mixed with 5 wt% DMSO and 1 wt% Triton-X100, and then added 3 wt% concentrated (3-glycidoxypropyl)trimethoxysilane (GOPS). For the highly conductive PEDOT:PSS solution, 1-ethyl-3-methylimidazolium tricyanomethanide (EMIM/TCM) was added into the raw PEDOT:PSS solution with a concentration of 1.5 wt%. After spin-coating and annealing these films, an Ag front electrode (~8% coverage) with a thickness of 300 nm was deposited by thermal evaporation. Photovoltaic performance measurement was performed by Class AAA solar simulator (Newport Oriel) with a Xe arc lamp under Air-mass 1.5 illumination (1000 W/m2) in the standard testing condition. The illumination intensity was calibrated using a standard c-Si cell from Newport Corporation, and the test temperature was actively controlled at 25 ± 0.5 °C during the measurements. The cells were shielded by an opaque mask with a designated aperture area of 1 cm2. The external quantum efficiency curves were measured by the quantum efficiency measurement system (Newport Oriel, IQE-200). The reflectance was measured by a spectrophotometer (Helios LAB-re, with an integrating sphere) in the wavelength range of 370–1100 nm.


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During the reflectance measurement, the rear-side of the samples was deposited with an Al film of 150 nm. Materials characterization. Quasi-steady-state photo-conductance method transient photoconductance (QSS-PCD) method is used to characterize the surface passivation, using both sides PEDOT:PSS coated symmetrical nano-pyramid array structure and TiOx/LiFx deposited planar structure. Surface recombination velocity (S) was obtained 𝟏

𝟏

by using the equation: 𝝉𝐞𝐟𝐟 = 𝝉𝐛𝐮𝐥𝐤 +

𝟐𝑺 𝑾,

where τbulk is the bulk lifetime of the substrate

and W is the wafer thickness. In order to estimate bulk lifetimes from each substrate type, a well-established procedure using iodine ethanol solution after an exhaustive cleaning has been used to passivate surfaces, assuming that a negligible surface recombination velocity has been obtained with this method. The front-side sheet resistances were extracted from four probe resistance test and the contact resistivities were measured by Cox and Strack model29 with the test structure of Al(round electrodes with different diameters)/LiFx/TiOx/n-Si/LiFx/Al. The morphology of the cell’s front surface was analyzed by scanning electron microscopy (Hitachi S-4800). For the rear side contact, high-resolution transmission electron microscopy (HR-TEM) and energydispersive X-ray (EDX) spectroscopic analysis were performed in a probe-side aberration corrected FEI Titan Tecnai F20 TEM microscope operated at 200 kV. XPS characterization was performed using a monochromatic Al Kα X-ray source and a hemispherical analyzer (Kratos AXIS Ultra DLD). Time-of-flight secondary ion mass spectroscopy (ToF-35 SIMS) depth profiling (ION-TOF Model IV, Ion-Tof GmbH, Germany) was performed by sputtering samples with AuGa ions (1keV) under a high vacuum. Acknowledgements J.H. and Y.W. acknowledge the support of the Australian Renewable Energy Agency (ARENA) Research and Development Program (2017/RND007). J.Y. acknowledge the support of the National Natural Science Foundation of China (61874177). P.G. acknowledge the support of Zhejiang Provincial Natural Science Foundation (LR19E020001) and National Natural Science Foundation of China (61674154). The computational resources were provided by the research computing facility at Texas A&M University in Qatar, and the UNSW’s “HCP@National Computational Infrastructure (NCI) in Australia”. Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Simulation details; Reflectance of ultrathin c-Si substrate with polished surface and ordered pyramid array texturing; Typical current-voltage curves used for contact resistivities measurement; Simulated photovoltaic parameters of thin film nSi/PEDOT:PSS HSCs as a function of Sfront, Rsheet, Srear and contact; X-ray photoelectron spectra of Ti 2p and Si 2p for TiOx film and TiOx/LiFx stacking structure; DOS and PDOS plots of TiO2 doping with Li, Al, F element; Band structure plots along the X-R-Z-G-M-A- path for TiO2 doping with Li, Al, F element; Statistical distribution


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of the photovoltaic parameters; Energy power loss of thin film n-Si/PEDOT:PSS HSC; Typically efficiency evolvement of thin film c-Si solar cells over time. References 1. Tiedje, T.; Yablonovitch, E.; Cody, G. D.; Brooks, B. G. Limiting Efficiency of Silicon Solar Cells. IEEE T. Electron Dev. 1984, 31, 711. 2. Garnett, E.; Yang, P. Light Trapping in Silicon Nanowire Solar Cells. Nano Lett. 2010, 10, 1082. 3. Wang, K. X.; Yu, Z.; Liu, V.; Cui, Y.; Fan, S. Absorption Enhancement in Ultrathin Crystalline Silicon Solar Cells with Antireflection and Light-Trapping Nanocone Gratings. Nano Lett. 2012, 12, 1616. 4. He, J.; Yang, Z.; Liu, P.; Wu, S.; Gao, P.; Wang, M.; Zhou, S.; Li, X.; Cao, H.; Ye, J. Enhanced Electro-Optical Properties of Nanocone/Nanopillar Dual-Structured Arrays for Ultrathin Silicon/Organic Hybrid Solar Cell Applications. Adv. Energy Mater. 2016, 6, 1501793. 5. Mavrokefalos, A.; Han, S. E.; Yerci, S.; Branham, M. S.; Chen, G. Efficient Light Trapping in Inverted Nanopyramid Thin Crystalline Silicon Membranes for Solar Cell Applications. Nano Lett. 2012, 12, 2792. 6. Han, S. E.; Chen, G. Toward the Lambertian Limit of Light Trapping in Thin Nanostructured Silicon Solar Cells. Nano Lett. 2010, 10, 4692. 7. Gao, P.; He, J.; Zhou, S.; Yang, X.; Li, S.; Sheng, J.; Wang, D.; Yu, T.; Ye, J.; Cui, Y. Large-Area Nanosphere Self-Assembly by A Micro-Propulsive Injection Method for High Throughput Periodic Surface Nanotexturing. Nano Lett. 2015, 15, 4591. 8. Wang, A.; Zhao, J.; Wenham, S.; Green, M. 21.5% Efficient Thin Silicon Solar Cell. Prog. Photovolt: Res. Appl. 1996, 4, 55. 9. Bullock, J.; Hettick, M.; Geissbühler, J.; Ong, A. J.; Allen, T.; Sutter-Fella Carolin, M.; Chen, T.; Ota, H.; Schaler, E. W.; De Wolf, S.; Ballif, C.; Cuevas, A.; Javey, A. Efficient Silicon Solar Cells with Dopant-Free Asymmetric Heterocontacts. Nat. Energy 2016, 1, 15031. 10. Um, H. D.; Kim, N.; Lee, K.; Hwang, I.; Seo, J.; Seo, K. Dopant-Free All-BackContact Si Nanohole Solar Cells Using MoOx and LiF Films. Nano Lett. 2016, 16, 981. 11. Almora, O.; Gerling, L.; Voz, C.; Alcubilla, R.; Puigdollers, J.; Garcia-Belmonte, G. Superior Performance of V2O5 as Hole Selective Contact Over Other Transition Metal Oxides in Silicon Heterojunction Solar Cells. Sol. Energy Mater. Sol. C. 2017, 168, 221. 12. Wu, W.; Bao, J.; Jia, X.; Liu, Z.; Cai, L.; Liu, B.; Song, J.; Shen, H. Dopant-Free Back Contact Silicon Heterojunction Solar Cells Employing Transition Metal Oxide Emitters. Phys. Status Solidi-R. 2016, 10, 662. 13. Wu, W.; Lin, W.; Bao, J.; Liu, Z.; Liu, B.; Qiu, K.; Chen, Y.; Shen, H. DopantFree Multilayer Back Contact Silicon Solar Cells Employing V2Ox/Metal/V2Ox as An Emitter. RSC Adv. 2017, 7, 23851. 14. Yoon, S.; Khang, D. High Efficiency (> 17%) Si-Organic Hybrid Solar Cells by Simultaneous Structural, Electrical, and Interfacial Engineering via Low-Temperature Processes. Adv. Energy Mater. 2018, 8, 1702655.


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15. Shen, X.; Sun, B.; Liu, D.; Lee, S. Hybrid Heterojunction Solar Cell Based on Organic-Inorganic Silicon Nanowire Array Architecture. J. Am. Chem. Soc. 2011, 133, 19408. 16. Zhang, Y.; Liu, R.; Lee, S.; Sun, B. The Role of A LiF Layer on The Performance of Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Si Organic-Inorganic Hybrid Solar Cells. Appl. Phys. Lett. 2014, 104, 4. 17. Wan, Y.; Samundsett, C.; Bullock, J.; Hettick, M.; Allen, T.; Yan, D.; Peng, J.; Wu, Y.; Cui, J.; Javey, A.; Cuevas, A. Conductive and Stable Magnesium Oxide Electron-Selective Contacts for Efficient Silicon Solar Cells. Adv. Energy Mater. 2017, 7, 1601863. 18. Yang, X.; Bi, Q.; Ali, H.; Davis, K.; Schoenfeld, W.; Weber, K. High-Performance TiO2 -Based Electron-Selective Contacts for Crystalline Silicon Solar Cells. Adv. Mater. 2016, 28, 5891. 19. Wu, S.; Cui, W.; Aghdassi, N.; Song, T.; Duhm, S.; Lee, S.; Sun, B. Nanostructured Si/Organic Heterojunction Solar Cells with High Open-Circuit Voltage via Improving Junction Quality. Adv. Funct. Mater. 2016, 26, 5035. 20. He, J.; Wan, Y.; Gao, P.; Tang, J.; Ye, J. Over 16.7% Efficiency Organic-Silicon Heterojunction Solar Cells with Solution-Processed Dopant-Free Contacts for Both Polarities. Adv. Funct. Mater. 2018, 28, 1802192. 21. Wang, X.; Liu, Z.; Yang, Z.; He, J.; Yang, X.; Yu, T.; Gao, P.; Ye, J. Heterojunction Hybrid Solar Cells by Formation of Conformal Contacts Between PEDOT: PSS and Periodic Silicon Nanopyramid Arrays. Small 2018, 14, 1704493. 22. Bullock, J.; Zheng, P.; Jeangros, Q.; Tosun, M.; Hettick, M.; Sutter-Fella, C.; Wan, Y.; Allen, T.; Yan, D.; Macdonald, D. Lithium Fluoride Based Electron Contacts for High Efficiency n-Type Crystalline Silicon Solar Cells. Adv. Energy Mater. 2016, 6, 1600241. 23. Giordano, F.; Abate, A.; Baena, J.; Saliba, M.; Matsui, T.; Im, S.; Zakeeruddin, S.; Nazeeruddin, M.; Hagfeldt, A.; Graetzel, M. Enhanced Electronic Properties in Mesoporous TiO2 via Lithium Doping for High-Efficiency Perovskite Solar Cells. Nat. Commun. 2016, 7, 10379. 24. Södergren, S.; Siegbahn, H.; Rensmo, H.; Lindström, H.; Hagfeldt, A.; Lindquist, S. Lithium Intercalation in Nanoporous Anatase TiO2 Studied with XPS. J. Phys. Chem. B 1997, 101, 3087. 25. Perdew, J.; Density Functional Theory and The Band Gap Problem. Int. J. Quantum Chem. 1985, 28, 497. 26. Lan, C.; Luo, J.; Lan, H.; Fan, B.; Peng, H.; Zhao, J.; Sun, H.; Zheng, Z.; Liang, G.; Fan, P. Enhanced Charge Extraction of Li-Doped TiO2 for Efficient ThermalEvaporated Sb2S3 Thin Film Solar Cells. Mater. 2018, 11, 355. 27. González-Torres, J.; Poulain, E.; Domínguez-Soria, V.; García-Cruz, R.; OlveraNeria, O. C-, N-, S-, and F-Doped Anatase TiO2 (101) with Oxygen Vacancies: Photocatalysts Active in the Visible Region. Int. J. Photoenergy 2018, 1-12. 28. Di Valentin, C.; Finazzi, E.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Czoska, A.; Paganini, M.; Giamello, E. Density Functional Theory and Electron Paramagnetic Resonance Study on The Effect of N− F Codoping of TiO2. Chem. Mater. 2008, 20,


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3706. 29. Cox, R.; Strack, H. Ohmic Contacts for GaAs Devices. Solid-State Electron. 1967, 10, 1213.

Figures:

Figure 1. (a) Configuration of ultrathin n-type c-Si DASH solar cell with front-side bilayer PEDOT:PSS film and rear-side TiOX/LiFX stacking structure. (b) SEM image of free-standing ultrathin c-Si substrate with ordered pyramid array texturing. (c) SEM image of c-Si and PEDOT:PSS interface.

Figure 2. (a) Minority carrier lifetimes of PEDOT:PSS/n-Si/PEDOT:PSS contact with different concentrated GOPS in PEDOT:PSS film. (b) Relationship of Sfront and Rsheet of PEDOT:PSS film with different concentrated GOPS. (c) Minority carrier lifetimes


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of LiFX/TiOX/n-Si/TiOX/LiFX contact with different thickness of TiOX. (d) Relationship of Srear and Rcontact on different thickness of TiOX. Simulated PCE of ultrathin n-type cSi DASH solar cells as a function of (e) Sfront and Rsheet, and (f) Srear and contact. Additional data points in each figure are experimental points extracted from Figure 2b and 2d.

Figure 3. (a) HR-TEM image of Al/LiFX/TiOX/n-Si stacking structure with EDX line scan of the Al, F, Ti, O, and Si signals (K edges) across the interface (red line). (b) ToFSIMS depth profile of the LiFX/TiOX/n-Si hetero-contact. X-ray photoelectron spectra of (c) O 1s, and (d) Ti 3s/Li 1s for TiOx film and TiOx/LiFx stacking structure. DOS and PDOS plots of (e) defect-free TiO2, (f) substitutional F-doped TiO2. The vertical


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red dotted lines show the Fermi level.

Figure 4. (a) Typical light J-V curves of ultrathin n-type c-Si DASH solar cells with flat, pyramid, and pyramid + TiOX/LiFX configurations. (b) Corresponding EQE and reflectance curves.

Table 1. Photovoltaic output parameters of ultrathin n-type c-Si DASH solar cells with flat, pyramid, and pyramid + TiOX/LiFX configurations. VOC b)

JSC b)

(V)

(mA/cm2)

0.571

22.8

0.722

9.40

0.565 ± 0.018

22.7 ± 0.3

0.710 ± 0.018

9.10 ± 0.30

0.597

28.4

0.743

12.60

0.592 ± 0.015

28.5 ± 0.2

0.735 ± 0.012

12.40 ± 0.20

Thin Pyramid + TiOx +

0.626

31.9

0.756

15.10

LiFx

0.623 ± 0.012

31.8 ± 0.2

0.752 ± 0.012

14.90 ± 0.20

Samples

a)

PCE b) FF b) (%)

Thin Flat

Thin Pyramid

a)

Data and statistics based on six cells of each condition; best values of each condition.


b)

Numbers in bold are the

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ToC：


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15% Efficiency Ultrathin Silicon Solar Cells with Fluorine-Doped

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