Si nanowires hybrid solar cells

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High-performance conducting polymer/Si nanowires hybrid solar cells using multilayer-graphene transparent conductive electrode and back surface passivation layer Dong Hee Shin, Ju Hwan Kim, and Suk-Ho Choi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b03005 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Sustainable Chemistry & Engineering

High-performance conducting polymer/Si nanowires hybrid solar cells using multilayer-graphene transparent conductive electrode and back surface passivation layer Dong Hee Shin†, Ju Hwan Kim†, and Suk-Ho Choi,* Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Korea.

ABSTRACT Polymer/Si hybrid solar cells (HSCs) are promising for mass production due to their high performance and low cost, but several problems such as big reflective index of Si, low aperture ratio of the mesa-type metal transparent conductive electrodes (TCEs), and large recombination loss at Si rear contact should be solved for the practical applications. Here, we

first

report

a

HSC

structure

of

ethylenedioxythiophene):poly(styrenesulfonate)/Si

multi-layer

graphene

nanowires/n-Si/TiOx

TCE/poly(3,4(back

surface

passivation layer) to cope with aforementioned problems. Resulting maximum power conversion efficiency (PCE) is 12.10 %, much larger than that of the planar-Si-type HSC (10.11 %) as a control sample, mainly due to the lowered reflectance (increased absorption) and recombination loss. As the active area increases from 14 to 50 mm2, the PCE decreases by only 2.5 % from 12.10 to 9.60 %, possibly resulting from the area-dependent change in the uniformity of the Si NWs. The PCE shows only a 10% decrease for 30 days under 25 °C temperature and 40% humidity.

Keywords: Si nanowire, conducting polymer, graphene, hybrid solar cell, titanium oxide, passivation, reflectance

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†

Two authors contributed equally to this work. Corresponding author: [email protected]

*

Introduction Recent concerns with global warming and depletion of fossil fuels have prompted a lot of studies to focus on developing environment-friendly renewable energy sources, among which solar cell is one of the most efficient device architectures harvesting solar energy.1,2 To date, crystalline-Si solar cells have occupied a major portion in the photovoltaic market due to their abundant resources, long-term stability, and high power conversion efficiency (PCE). However, high vacuum/high temperature processes are critically required for commercial Si-wafer solar cells, resulting in high production cost, thereby limiting their use.3,4 Currently, any approach is useless for further reducing the cost of the power generation in Si solar cells because the fabrication of high-purity Si ingot is a principal high-cost process of the solar cell production. Over the past decade, many researchers have focused on organic materials/Si hybrid solar cells (HSCs) to reduce the production cost of Si-based solar cells.5-12 Polymer or organic materials are very excellent in their electrical properties including charge carrier mobility.9-11 Especially, poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a conducting polymer, is not only a antireflective layer for HSCs but also a hole transporting/optical window that serves as a passivation layer.12 Recent several reports show remarkable enhancement in the photovoltaic parameters of PEDOT:PSS/Si HSCs, including open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and PCE, compared to Si cells.13,14 As one approach for further increase of the PCE, it is necessary to find a method of enhancing the

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light absorption of Si, for example, by lowering the refractive index. Si-based nanostructures such as Si nanowires (NWs) and porous Si are useful for reducing the reflectivity, resulting in higher light absorption than bulk Si.15 Nanostructured semiconductors are also known to exhibit fast electronic transport properties and provide large interface area, useful for optical trapping. On the other hand, indium tin oxide or metal mesa-type electrodes used as top electrodes for Si solar cells should be replaced because of their high cost, low transmittance in visible light region, and low aperture ratio. Therefore, it is important to find low-cost transparent conductive electrodes (TCEs) having high transparency and low sheet resistance (Rs). Among various kinds of electrode materials, graphene is regarded as a promising TCE candidate for optoelectronic devices due to its high transmittance, good chemical stability, and easy adjustment of Fermi level.16,17 However, single layer graphene produced by chemical vapor deposition (CVD) is limited in achieving large PCE due to the high Rs. Multilayer graphene (MLG) is known to be useful as a TCE to reduce Rs while maintaining transparency.18 Another crucial issue for improving the operation of the solar cells is the passivation for reducing the Schottky barrier at the metal contact/rear Si interface. Traditionally, the Si-metal ohmic contacts are formed by annealing up to 1000 °C during high-dose doping, which requires the use of hazardous doping gases, thereby raising the operational and environmental problems. Recently, titanium oxide (TiOx) is highly attractive as an electron selective contact layer or a hole blocking layer effective at the cathode interface of a hybrid solar cell for suppressing the rear recombination.19,20 Here, we first report MLG/PEDOT:PSS/Si NWs/n-Si/TiOx/InGa HSCs for various layer number (Ln) of MLG to solve the aforementioned obstacles for low-cost Si hybrid solar cells. The HSC 3



shows a maximum PCE of 12.10 % thanks to the increased optical trapping capacity and reduced carrier recombination, approximatively 20 % larger than what obtained from the planar-type counterpart (absolutely 10.11 %) composed of MLG/PEDOT:PSS/bulk Si/TiOx/InGa, named as a reference solar cell (RSC).

Experimental section Preparation of Si nanowires To prepare the anodized aluminium (Al) oxide (AAO) membrane, the anodizing treatment was first performed on the Al plate loaded on the electrochemical cell filled with sulfuric acid under an external bias of 25 V for 24 hours. Subsequently, the Al oxide disk formed on the Al plate was selectively removed in chromic acid for 24 h at 50 °C for the embossed structure of Al to be exposed with 6-fold alignments. A thin AAO disc of 300400 µm thickness, a membrane of 55 nm pore diameter (see Supporting Information, Fig. S1a), was then formed through a second anodization process for 24 h. A 25 nm thick Au film was then deposited on the bottom of the AAO film using metal sputtering. The AAO membrane coated with Au was then suspended on the surface of 1M NaOH solution to remove the oxide membrane. The resulting 25 nm thick Au mesh on the surface of the aqueous solution were transferred onto the Si substrate without any structural disintegrations (see Supporting Information, Fig. S1b). The Si NW arrays were fabricated by metal-assisted chemical etching using the Au mesh as a catalyst. The Au mesh/Si wafer was then immersed in a mixture solution of HF, H2O2, and H2O (the volume ratio = 1:0.5:1) for a suitable time, resulting in the formation of uniform Si NW arrays of 55 nm diameter

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(see Supporting Information, Fig. S1c-d). Finally, the Si NW arrays were dipped in aqua regia (mixture of nitric acid/hydrochloric acid with a volume ratio of 1/3) for a few min to remove the Au mesh and particles. Preparation of graphene. Large area single-layer graphene was synthesized on 70 µm Cu foil by CVD at 1000 °C under flow of CH4 (30 sccm) as a carbon source and H2 (10 sccm) as a reduction gas. After the growth of graphene on the Cu foil, poly(methyl methacrylate) (PMMA) solution was drop-coated onto the graphene sheet/Cu foil at 5000 rpm for 60 s, and then annealed at 180 oC for 1 min. The sample was further annealed at 90 ºC for 30 min, followed by the lamination of a thin layer of freestanding poly(dimethylsiloxane) (PDMS) film (about 0.2 mm thick) on top of PMMA. The Cu foil was etched away using an FeCl3 etchant for 2 h to obtain a PDMS/PMMA/graphene film. These steps were repeated up to four times to obtain single to quadruple layers of graphene (named as 1LG to 4LG), based on layer-by-layer stacking method.21 Fabrication of MLG/polymer/Si NWs/Si/TiOx HSCs. TiOx precursor in isopropanol were prepared according to literature procedures.22 Titanium isopropoxide (369 µL) in isopropyl alcohol (IPA) (2.53 mL) and HCl (35 µL, 2 M) in IPA (2.53 mL) were prepared respectively. Then, HCl/IPA solution was dropped slowly into the titanium isopropoxide/IPA solution with stirring. After stirring two hours, the precursor solution was diluted by diﬀerent amount of IPA, and filtered with 0.2 µm polytetrafluoroethylene filter. The concentration of titanium isopropoxide/IPA solutions was 1.0 mg/mL. The TiOx precursor solution was then spin coated onto Si rear side at 3000

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rpm for 40 s. PEDOT:PSS (Clevios PH 1000, with 1 wt% Triton and 5 wt% Dimethyl sulfoxide) solution was spin-coated onto the side of the Si NWs at 5000 rpm for 1 min, and annealed at 130 °C for 30 min, and the MLG was then transferred on top of the PEDOT:PSS. Finally, the InGa films were deposited on the top of the MLG and the bottom of the Si substrate to complete the device structure of the HSC. The schematic sequence of the preparation for the HSC was summarized elsewhere (see Supporting Information, Fig. S2). The PDMS/PMMA/graphene was compressed for better contact by applying appropriate external pressure on the Si NWs. Here, we named SSC as the MLG/Si NWs Schottky junction solar cells fabricated without PEDOT:PSS and TiOx back passivation layers. The illumination area of 14, 30, and 50 mm2 were fabricated by adjusting the mask size. Characterizations. The reflectance of Si and Si NWs was analyzed using ultraviolet (UV)-visible- near infrared (NIR) optical spectroscopy. The sheet resistance, transmittance, and work function of MLG films were measured by the 4 probe van der Pauw method, UV-visible-NIR optical spectroscopy, and Kelvin probe force microscopy, respectively. Scanning electron microscopy (SEM) and Raman spectroscopy were used to confirm the formation of PEDOT: PSS on the Si NWs surface. The atomic bonding states of the Si/TiOx were characterized by X-ray photoelectron spectroscopy (XPS) using an Al Ka line of 1486.6 eV. The current density-voltage (J-V) characteristics of the solar cells were measured with a Keithley 2400 source meter under the illumination of 1 Sun (100 mW-cm-2, AM 1.5G) in air. The external quantum efficiency (EQE) was measured under short circuit conditions while the cells were

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illuminated by a light source system with a monochromator.

Results and discussion Figure 1 shows schematics and reflectance spectra of planar Si and Si NWs. By forming Si NWs on Si wafer, the reflectance is significantly reduced with the surface area being greatly increased, more useful for harvesting solar energy. Figure 2a shows schematic diagram of a typical MLG/PEDOT:PSS/Si NWs/Si HSC. The uniformity of the PEDOT:PSS layer formed in the Si NWs was confirmed by SEM and Raman scattering (see Supporting Information, Fig. S3). From the Raman spectra, the presence or absence of PEDOT:PSS in the Si NWs was clearly distinguished, as previously reported.23,24 With increasing Ln, both the G and 2D bands are redshifted and the intensity ratio of G to 2D peaks, named as I(G/2D), gradually increases (see Supporting Information, Fig. S4a), resulting from the shift of the electronic band structure towards that of graphite, consistent with previous results.25,26 The work function/mobility of MLG (and PEDOT:PSS/MLG) monotonically increased/decreased from -4.65 ± 0.024 eV (-4.82 ± 0.014 eV)/1305 ± 30 cm2/V-s (542 ± 24 cm2/V-s) to -4.91 ± 0.020 eV (-4.95 ± 0.01 eV)/165 ± 18 cm2/V-s (45 ± 8 cm2/V-s) with increasing Ln to 4, respectively (see Supporting Information, Fig. S4b), indicating Lndependent changes of the band structure and carrier density, consistent with the previous results.27,28 These results suggest that the holes separated from the Si NWs/n-Si are smoothly transferred to the graphene TCE. As Ln increased from 1 to 4, the Rs of graphene/PMMA/PDMS was reduced from 585 ± 35 to 245 ± 15 Ω/sq (see Supporting

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Information, Fig. S4c). The transmittance was also monotonically reduced with increasing Ln (see Supporting Information, Fig. S4d). The attenuation coefficient of the transmittance per layer at a wavelength of 550 was estimated to be 2.5 %, very close to the theoretical value of 2.3%.29 Figure 2b shows energy band diagram of the HSC. When sunlight is irradiated, free electrons and holes are generated in Si wafer as well as in Si NWs, and transported/extracted towards InGa metal and MLG TCE, respectively. The J-V curves of the solar cells were measured for various Ln under AM 1.5G illumination at 100 mW-cm-2, as shown in Figure 2c-d and Table 1. All photovoltaic parameters of both the HSC and SSC are gradually enhanced as Ln increases to 2, but they are degraded above Ln = 2. These results suggest that the Ln dependence of the electrical and optical properties such as transmittance, work function, Rs, and carrier mobility greatly strongly affects the photovoltaic parameters of the HSC and SSC. In addition, the PEDOT:PSS layer also has a significant effect on the photovoltaic parameters, as shown in Figure 2c-d and Table 1. The highest occupied molecular orbital level of the PEDOT: PSS is at -5.2 eV, very close to the valence band maximum of the Si NWs, thereby giving optimum electron blocking effect. This explains why the photovoltaic properties of the HSCs are improved with the PEDOT:PSS layer. The best HSC/SSC shows 0.495/0.431 Voc, 29.84/23.05 mA·cm-2 Jsc, 68.66/51.35 % FF, and 10.14/5.10 % PCE at Ln = 2, respectively. Figure 2e shows EQE spectra of HSC and SSC at Ln =2 (For Ln-dependent EQE spectra, see Supporting Information, Fig. S5). The EQE of HSC is higher than that of SSC in a broad wavelength range from 300 to 1100 nm, very much consistent with the change of Jsc (see Supporting Information, Table S1).30 These results suggest that the presence of the PEDOT:PSS layer 8


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enhances the EQE due to the increased harvest efficiency, charge injection/transfer efficiency, and charge collection efficiency, consistent with the previous reports.31 To further investigate why the photovoltaic parameters such as Voc, Jsc, FF, and PCE are best at Ln = 2, the dark current was measured and analyzed. The Voc was calculated based on the following equation32 to check its difference between the MLG/Si NWs solar cells without/with PEDOT:PSS:

V =

ln( + 1)

(1)

, where J0, n, k, T, and q are reverse saturation current density, diode ideality factor, Boltzmann constant, absolute temperature, and elementary charge, respectively. The Voc of the HSC is calculated to be 0.489 eV at Ln = 2, larger than that of SSC (0.437 eV), consistent with the change of Voc (~ 640 mV increase), as shown in Table 1. To further understand the device properties, the shunt resistance (Rsh), series resistance (Rse), and the ideality factor (n) were extracted from the dark J-V curves, as shown in Figure 3a. The dark J-V characteristics in low (region I), intermediate (region II), and high voltage (region III) ranges are generally determined by Rsh, n, and Rse.33 The Rsh/Rse ratios of HSC in region I (< 0.1 V) and region III (V > 0.4 V) show the highest and lowest values of 3929 ± 92 and 4.4 ± 0.8 ohm-cm2 at Ln = 2, as summarized in Figure 3b and Table 1, consistent with the highest FF at Ln = 2. In region II (V = 0.1 ~ 0.4 V), the J-V characteristics of the nonideal diode based on the thermionic emission model can be expressed by the equation:33 J = Js [exp (eV/nkT) -1]

(2)

9



, where Js is the ideal reverse saturation current density and n can be estimated from the slope of the curves derived through curve fitting, as shown in Figure 3a. The n of HSC at Ln = 2 is estimated to be 3.52, smallest one, as shown in Figure 3b, indicating best diode quality. The best electrical and photovoltaic performances at Ln = 2 result from the Lndependent trade-off correlation between work function, mobility, Rs, and transmittance, as explained above. The Ln-dependent statistical deviations of the PCE averaged for 12 HSCs without/with PEDOT:PSS also show that the highest PCE (9.83 ± 0.31 %) is achieved at Ln = 2 with PEDOT:PSS (see Supporting Information, Fig. S6). These results strongly suggest that the insertion of the PEDOT:PSS effectively improves the performance of the MLG/Si NWs solar cells. Based on these results, the Ln was fixed at 2 for further studying the effect of back surface passivation layer on the performance. It is believed that the pendent bonding of the Si surface is partially terminated by the titanium isopropoxide precursor, thereby forming a chemical bond of Si-O-Ti.34 To check this, the XPS spectrum was analyzed (see Supporting Information, Fig. S7). The bond energy of the Si 2p core level was measured to be 101.7 eV, indicating a Si-O-Ti bond whose energy is lower than that of SiOx (102.7 eV).19,34 The formation of the Si-O-Ti bonds can significantly reduce the pendent bonding of the Si surface, thereby suppressing the surface charge recombination. Figure 4a shows a schematic of the HSC with TiOx layer for the back surface passivation. The conduction band of TiOx (4.0 eV) is close to that (4.05 eV) of Si,19 meaning ignorable band offset for electrons transporting from Si to TiOx layer, as shown in Figure 4b. In contrast, a large valence band energy offset exists between Si (5.17 eV) and TiOx (> 7.0 eV), which could block holes from Si to the cathode, thereby suppressing the 10


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carrier recombination at the interface. Therefore, the back deformation of the TiOx layer is expected to improve Voc and Jsc. Figure 4c shows typical J–V curves of the HSCs without/with TiOx under illumination, indicating great improvement in the photovoltaic performances with the TiOx film, possibly resulting from the suppressed recombination loss. The TiOx-based HSC exhibits the largest photovoltaic parameters of 12.10 % PCE, 0.526 V Voc, 31.75 mA cm2 Jsc and 72.48 % FF, as shown in Table 2. The EQE was also enhanced by the use of TiOx film, as shown in Figure 4d, due to the smoother charge injection/transfer on the cathode side. The Jsc values calculated from the integration of EQE spectra are 28.37/30.24 mA/cm2 for the HSC without/with TiOx, respectively, and are well consistent with those extracted from the J-V curves (see Supporting Information, Table S1). These results suggest that the TiOx layer improves the charge collection efficiency of the Si backside. In comparison, the RSC showed lower-quality photovoltaic parameters: 10.11 % PCE, 0.516 V Voc, 27.35 mA-cm2 Jsc, and 71.80 % FF (see Supporting Information, Fig. S8). Here, it should be noted that the Jsc is remarkably enhanced with the use of Si NWs, as can be compared in Table 2, mainly due to the structure-dependent difference of the reflectance, as shown in Figure 1. Figure 5a shows the J-V curves for three HSCs with active areas of 14, 30, and 50 mm2. The active area was adjusted using a mask and measured under the same conditions. As the active area increased three times, the FF decreased from 72.48 to 63.20 % whilst Voc and Jsc are almost the same. Since charge carriers move through the graphene layer to be collected by the external electrode, the area of graphene is an important factor affecting the collection efficiency, resulting in smaller FF at larger active area (see Supporting Information, Table S2). Even so, the PCE is reduced by only absolutely 2.5 % from 12.10 11



(14 mm2) to 9.60 % (50 mm2), relatively small compared to the graphene/bulk Si solar cells, possibly resulting from the short movement of the carriers generated in Si NW, as reported before.35,36 We also investigated the long-term stability of the HSC, SSC, and RSC with 50 mm2 active area by measuring the PCE for 30 days under 25 oC temperature and 40 % humidity. Figure 5b shows the change of the PCE for 30 days, indicating only 10 % loss of its initial PCE value from 9.60 to 8.42%. The PCE losses of SSC and RSC were about 2 and 9 % from 4.37 and 9.10 % to 4.28 and 8.31 %, respectively (see Supporting Information, Figure S9).

Conclusion Highest photovoltaic parameters including 10.14% PCE were achieved from MLG/PEDOT:PSS/Si NWs HSCs without TiOx at Ln = 2, and explained based on the Lndependent trade-off correlations between the structural, electrical, and optical properties, and the electron blocking effect of the PEDOT:PSS layer. A TiOx layer was deposited on the backside of the Si to form a Si-O-Ti chemical bond, resulting in remarkable suppression of the carrier recombination, thereby further enhancing all the photovoltaic parameters including the increase of the PCE to 12.10%. The Si NWs were especially useful for enhancing Jsc due to the remarkable effect of lowering the reflectance of the HSC, resulting in higher PCE. The HSC showed relatively small (~ 10 %) degradation in its PCE while preserved for 30 days under 25 oC temperature and 40 % humidity. These results may provide a route for developing high-performance, low-cost, and long-term-stability polymer/Si HSCs.

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ASSOCIATED CONTENT Supporting Information Figures S1–S9. This material is available free of charge via the Internet at http://pubs.acs.org. SEM images, Raman spectrum, work function, sheet resistance, transmittance, EQE spectra, XPS spectrum, average efficiency, and long-term stability data. AUTHOR INFORMATION Corresponding Author [email protected] Author Contributions D. H. Shin and J. H. Kim performed device design, device fabrication, and characterization for hybrid solar cells. S.-H. Choi initiated, supervised the work, and wrote the paper. All authors discussed the results and commented on the manuscript. Funding Sources This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1A2B3006054). Notes The authors declare no competing financial interests.

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Table 1. Photovoltaic parameters of the MLG/Si NWs/Si solar cells with without and with PEDOP:PSS for various Ln. PEDOT Ln :PSS

without

with

Voc (V)

Jsc (mA/cm2)

FF (%)

Best PCE (%)

1

0.395

20.39

41.57

3.35

3.06 ± 0.29 832 ± 63 14.1 ± 1.9 9.2 ± 1.1

2

0.431

23.05

51.35

5.10

4.90 ± 0.20 1632 ± 45 11.3 ± 1.8 7.7 ± 0.9

3

0.416

22.37

49.59

4.61

4.36 ± 0.25 1432 ± 45 12.2 ± 1.4 7.8± 1.0

4

0.407

21.50

49.01

4.29

4.08 ± 0.21 1355 ± 37 12.6 ± 2.2 7.9 ± 1.0

1

0.475

27.20

63.00

8.14

7.87 ± 0.27 2805 ± 135 5.9 ± 1.0 5.1 ± 0.9

2

0.495

29.84

68.66

10.14

9.83 ± 0.31 3929 ± 92 4.4 ± 0.8 3.6 ± 0.5

3

0.49

28.66

66.98

9.41

9.09 ± 0.32 3605 ± 95 4.6 ± 0.9 3.9 ± 0.7

4

0.49

27.77

62.67

8.53

8.18 ± 0.35 3284 ± 80 4.7 ± 0.8 4.1 ± 0.7

Average PCE (%)

Rse Rsh 2 (ohm-cm ) (ohm-cm2)

Table 2. Photovoltaic parameters of the HSCs without and with TiOx layer. Jsc FF Best PCE TiOx Voc (V) (mA/cm2) (%) (%) Without

0.495

29.84

68.66

10.14

With

0.526

31.75

72.48

12.10

18


n

Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Figure 1. Schematics of (a) planar Si and (b) Si NWs. (c) Reflection spectra of planar Si and Si NWs.

Figure 2. (a)-(b) Schematic and energy band diagram of a typical MLG/PEDOT:PSS/Si NWs/n-Si/InGa HSC. Photo J–V curves of (c) MLG/Si NWs and (d) MLG/PEDOT:PSS/Si NWs HSCs. (e) EQE spectra and integrated Jsc of the solar cells without and with PEDOT:PSS at Ln = 2, respectively. Figure 3. (a) Dark J–V curves of MLG/Si NWs solar cells without/with PEDOT:PSS at Ln = 2. Regions I, II, and III illustrate how the different components (shunt resistance, ideality factor, and series resistance) of the solar cell equivalent circuit dominate the J-V response at different voltages. (b) Summary of Rsh, Rse, and n of solar cells for various Ln. Figure 4. (a) Schematic and (b) energy band diagram of a typical MLG/PEDOT:PSS/Si NWs/n-Si/TiOx/InGa HSC. (c) Photo J-V curves of the HSCs without and with TiOx under simulated AM 1.5 illumination at 100 mW/cm2. (d) EQE spectra of the HSCs without and with TiOx. Figure 5. (a) Photo J-V curves of the HSCs with TiOx layer for active areas of 14, 30, and 50 mm2. The inset shows photos of the HSCs. (b) Degradation of the PCE for the HSC of 50 mm2 active area under 25 oC temperature and 40 % humidity during 30 days.

19



(a)

(b)

Conventional Si

Si NWs

(c) 50 40 Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

30

Planar Si 20 10

Si NWs 0 300

400

500

600 700 800 900 1000 1100 Wavelength (nm)

Figure 1

Figure 2

20


Page 21 of 23

-3

10

-4

10

-5

10

2

With Without

Rsh (Ohm-cm )

4 2

Ι

2

15

Ⅱ

Ⅲ

Without With

Rse (Ohm-cm )

10 5 12

With Without

9

-6

10

6 3

-7

10 -0.5

6

3

Rsh (x10 )

Ln = 2

Rse

2

(b)

-2

10

n

(a)

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 0.5 Voltage (V)

1.0

1

2

Ln

3

4

Figure 3

Figure 4

21



Page 22 of 23

Figure 5

22


Page 23 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of contents

Schematic diagram of a typical hybrid solar cell and its current density-voltage curves showing the enhanced photovoltaic characteristics.

23


Si nanowires hybrid solar cells

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