Graphene Transparent Conductive Electrodes for Highly Efficient

May 31, 2013 - graphene/P3HT/SiNH array have achieved power conversion efficiencies of. 9.94% and ... for SiNW and SiNH arrays-based hybrid solar cell...
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Graphene Transparent Conductive Electrodes for Highly Efficient Silicon Nanostructures-Based Hybrid Heterojunction Solar Cells Yiming Wu,† Xiaozhen Zhang,† Jiansheng Jie,* Chao Xie, Xiwei Zhang, Baoquan Sun,* Yan Wang, and Peng Gao Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China S Supporting Information *

ABSTRACT: In comparison to conventional metallic electrodes, graphene possesses superior properties in terms of higher optical transmittance, tunable work function, excellent stability in air, etc. Here, we demonstrate the use of graphene as transparent conductive electrodes for constructing highly efficient hybrid heterojunction solar cells based on nanostructured silicon, including silicon nanowire (SiNW) and silicon nanohole (SiNH) arrays. Poly(3-hexylthiophene) (P3HT) is adopted as hole transport layer in the hybrid heterojunction. It also offers a large offset between lowest unoccupied molecular orbital of the organic and the conduction band minimum of Si to reduce the electron recombination at graphene anode. The roles of graphene layer number, silicon surface modification, as well as P3HT layer thickness are systemically investigated. After sufficient device optimization, the devices based on graphene/P3HT/SiNW array and graphene/P3HT/SiNH array have achieved power conversion efficiencies of 9.94% and 10.34%, respectively. Considering the simple and low-cost solution processed capability for both graphene and P3HT layers, we believed that graphene/organic/silicon is a viable low-temperature technique for highly efficient silicon solar cell.

1. INTRODUCTION Although current photovoltaics (PVs) production is dominated by single junction solar cells based on silicon wafers including single-crystalline (c-Si) and polycrystalline silicon (mc-Si), latest advances have shown a great potential for organic and organic/inorganic hybrid solar cells.1−4 Their versatility in production methods, properties, and applications looks very promising for the future of solar energy. In particular, conjugated polymer/inorganic semiconductor hybrid solar cells, which harness the advantageous properties of both organic and inorganic materials, such as high carrier mobility of inorganic materials and the large light absorption coefficient, low-cost solution processability, and soft mechanical behavior of conjugated polymer,5 have attracted much attention in recent years and shown the potential as low-cost and highefficiency PV devices. For instance, we have achieved devices with a power conversion efficiency (PCE) of 9.7% for organic layer and Si nanowires (SiNWs) hybrid structures.6 Lu et al. have reported the poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS)/SiNWs core/shell structured solar cells with a PCE of 6.35%,7 and a high PCE of 11.1% was recently achieved for the PEDOT:PSS/Si nanocones solar cells.8 It was found that the appropriate surface modification for suppressing the carrier recombination as well as the utilization of nanostructured Si for enhancing the light absorption and carrier transportation were crucial to achieve the high-performance hybrid heterojunction solar cells.9 © XXXX American Chemical Society

Nevertheless, further improvement of the device efficiency is impeded by the anode materials; normally, metallic electrodes such as Cu, Ag, Pd, and Au thin films were used as the top electrodes in the hybrid devices.8−11 However, their inferior optical transmittance, poor air/chemical stability, and low work function usually resulted in the discount of device performance. The device fabrication also became more complicated due to the high-vacuum metal deposition process. The rise of graphene inspired a tremendous effort to explore its potential applications in energy-related fields, such as solar cells, lithium ion batteries, and supercapacitors.12 In comparison to the conventional transparent electrodes such as metals and oxides, graphene possesses superior performance in terms of high optical transparency, large sheet conductivity, outstanding mechanical properties, and excellent physical/chemical stability.13 The development of large-scale synthesis and transfer techniques for graphene films also facilitates its applications in transparent conductive electrodes.14 By taking advantage of graphene anode, organic photovoltaic (OPV) devices have shown a substantial performance enhancement.15,16 On the other hand, graphene sheets have been directly deposited on silicon to fabricate Schottky junction solar cells. Although the initial studies demonstrated a very limited Received: March 13, 2013 Revised: May 17, 2013

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PCE of ∼2.86%,17,18 a higher efficiency of 8.6% was achieved recently by performing graphene doping as well as passivating the Si surface with a native oxide layer.19 On the other hand, carbon nanotubes (CNTs) have also been utilized for Si-based hybrid heterojunction cells where CNTs function similarly to graphene.20 However, the inherent disadvantages of Schottkytype solar cells such as the large current leakage resulted by low junction height will inevitably restrict the performance improvement of the Schottky-type solar cells. As an alternative, the insertion of an appropriate organic to form graphene/ organic/inorganic hybrid heterojunction may offer a feasible way to overcome the limitation. In this study, we demonstrated highly efficient poly(3hexylthiophene) (P3HT)/Si nanoarrays hybrid solar cells by utilizing graphene as the transparent electrodes. Besides SiNW arrays, Si nanohole (SiNH) arrays were first used because they showed the advantages of larger contact area and better support to the graphene, while retaining the capability of high light absorption. Through passivating the Si surface with organic functionality termination, adjusting the thickness of polymer layer, and controlling the graphene doping level and layer number, substantial improvement of device performance was investigated, leading to optimum PCEs of 9.94% and 10.34% for SiNW and SiNH arrays-based hybrid solar cells, respectively. Our results demonstrate that graphene on organic/silicon has great potential for the low-cost, highefficiency solar cell applications.

min, followed by cleaning in DI water for 1 min. The silicon oxide-terminated Si (denoted as SiOx−Si) nanoarrays were fabricated by simply exposing the fresh H−Si nanoarrays in air for 1.5 h. The thickness of SiOx layer was determined to be ∼1.5 nm by fixed angle ellipsometry (Alpha-SE). Methyl groupterminated Si (denoted as CH3−Si) nanoarrays were prepared following a two-step chlorination/alkylation method.23,24 At first, H−Si nanoarrays were dipped into a saturated solution of PCl5 in chlorobenzene (CB) at 90 ± 10 °C to terminate the Si surface with chlorinated groups (denoted as Cl−Si). The Cl−Si nanoarrays were then rinsed sequentially with CB and tetrahydrofuran (THF) three times. After that, the Cl−Si nanoarrays were immersed into a solution of CH3MgCl (1 M) in THF for at least 8 h at 70 ± 10 °C and then rinsed with THF and methanol. The above-mentioned processes were conducted in a glovebox under the protection of N 2 atmosphere. After modification, the CH3−Si nanoarrays were taken out of the glovebox and immersed into DI water for 10 min, followed by rinsing with acetone and ethanol, respectively. 2.4. Growth and Transfer of Graphene Films. Monolayer graphene (MLG) films were prepared at 1000 °C by using a mixed reaction gas of CH4 (40 sccm) and H2 (20 sccm) via a CVD growth method with 25 μm thick Cu foil as the catalytic substrate.25 The as-prepared graphene film was spin-coated with 8 wt % polymethylmethacrylate (PMMA), and subsequently the underlying Cu substrate was etched away in the Marble’s reagent solution (CuSO4:HCl:H2O = 10 g:50 mL:50 mL). The PMMA/graphene film was cleaned with DI water several times and was ready for use. 2.5. Device Construction. Core−shell structured PV devices were constructed by coating the P3HT thin layer on the surface-modified Si nanoarrays, where P3HT acted as the hole transport layer and the electron blocking layer as well.11 First, P3HT dissolved in CB solution was dripped onto the Si nanoarrays and was left stewing for 30 s approximately. A spincoating process at 3000 rpm for 60 s also was employed to form a uniform P3HT layer on the Si nanoarrays. After that, the P3HT-coated nanoarrays were baked in N2 atmosphere at 150 °C for 20 min. The thickness of P3HT layer was controlled by adjusting the P3HT concentration in solution, while the other conditions were kept constant. In this work, P3HT layer with approximate thickness of 5, 10, 20, 40 nm on Si nanoarrays was obtained under P3HT concentrations of 0.5, 2.5, 5, and 10 mg/ mL, respectively. Second, Ti/Au (5 nm/50 nm) electrode was deposited on the SiO2 layer nearby the exposed Si window as the electrode contact to graphene film by e-beam evaporation using a shadow mask. Eventually, the PMMA-supported MLG film was directly transferred onto the top of the Si nanoarrays and dried at 100 °C for 10 min, followed by removing the PMMA in acetone. To enhance the conductivity of the graphene film, few-layer graphene (FLG) film was fabricated by layer-by-layer transferring the MLG film until the desired layer number was achieved. Indium−gallium (In−Ga) alloy was pasted on the back side of the n-Si substrate to achieve ohmic contact to the substrate. 2.6. Materials and Devices Characterizations. Morphologies of the Si nanoarrays were investigated by scanning electron microscopy (SEM, FEI Quanta 200FEG). The components of P3HT-coated SiNH array were detected by energy dispersive X-ray spectrometry (EDS) in SEM. The reflection spectra of planar Si, SiNW arrays, and SiNH arrays were detected by UV−vis spectrometer (Perkin-Elmer LAMBDA 750) equipped with an integrating sphere, which

2. EXPERIMENTAL SECTION 2.1. Preparation of SiNW Arrays. SiNW arrays on planar Si substrates were prepared by using a Ag-assisted chemical etching method according to the previous report.21 Briefly, opening windows (0.2 cm × 0.2 cm) were first defined on the clean SiO2 (300 nm)/n-Si (resistivity 1−10 Ω cm−1) substrates by adhesive tape, and then the SiO2 insulative layers in the opening windows were removed by a buffered oxide etch (BOE) solution until the underlying Si substrates were exposed. The SiNW arrays were fabricated by immersing the substrates into an aqueous solution of HF (5 M) and AgNO3 (0.02 M) for 10 min at room temperature. After etching, the substrates were dipped into the aqueous solution of HNO3 (30% w/w) to remove any residual silver. SiO2 on the SiNW array surface was removed by wet chemical etching in aqueous HF (5% w/w) for an additional 10 min. Eventually, the SiNW arrays were rinsed with deionized (DI) water and then dried by the nitrogen gas stream. 2.2. Preparation of SiNH Arrays. SiNH arrays were prepared in electrolyte solution via electrochemical etching.22 Similar to the SiNW arrays, SiO2 (300 nm)/n-Si (resistivity 1− 10 Ω cm−1) substrates with 0.2 cm × 0.2 cm opening windows were used. The substrates were immersed in the electrolyte solution composed of ethanol and HF (≥40%, w/w) with a volume ratio of 1:2. A Pt plate served as the cathode electrode. After etching at a current of 50 mA for 3−4 min in the electrolyte solution, the substrates were taken out and then dipped into a diluted NaOH solution (5 M) for 10−12 s to remove the amorphous Si layer formed on the inner walls of the SiNH arrays. After that, the SiNH arrays were cleaned by the DI water and dried in the N2 gas stream. 2.3. Surface Modification of Si Nanoarrays. The hydrogen-terminated Si (denoted as H−Si) nanoarrays were obtained by immersing the as-prepared SiNW and SiNH arrays into an aqueous HF (5 M) solution with gentle shaking for 10 B

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was also used to detect the transmittance of the graphene film. The core−shell structures of P3HT/SiNW were investigated by transmission electron microscopy (TEM, FEI Tecnai G2 F20). The MLG film was characterized by Raman (Jobin Yvon/ LabRAM HR800) with 514 nm laser excitation. Work function of the pristine MLG film was determined by ultraviolet photoelectron spectroscopy (UPS, ULTRA DLD). The photovoltaic characteristics of the graphene/P3HT/Si nanoarray hybrid heterojunction solar cells were evaluated by a Keithley 2612 source meter in an ambient environment. Newport 91160 solar simulator equipped with a 300 W xenon lamp and an air mass (AM) 1.5 filter was used to generate simulated AM 1.5G solar spectrum irradiation. The irradiation intensity was 100 mW cm−2 calibrated by a Newport standard Si solar cell 91150.

Figure 2. Reflection spectra of planar Si, SiNW array, SiNH array, CH3−SiNW array, and CH3−SiNH array.

comparison. We note that the Si nanoarrays show excellent antireflection performance in a wide spectrum range compared to the planar Si. The efficient light harvesting of the Si nanoarrays could be ascribed to their distinct array structures. Before modification, light absorption of SiNW array is stronger than the SiNH array. However, the opposite result is observed after modification. This result is consistent with the SEM investigation and attributed to the morphology change of the SiNW array after modification. 3.2. Device Structure and Band Analysis. Figure 3a and b shows the schematic illustrations and the photographs of

3. RESULTS AND DISCUSSION 3.1. Characterizations of the Si Nanoarrays. In comparison with planar Si, nanostructured silicon materials, such as SiNW and SiNH arrays, provide unique advantages in terms of strong light absorption arising from the array structure and efficient charge separation/transport due to the large surface area. Figure 1a and c shows the typical SEM images of

Figure 1. SEM images of (a) SiNW array, (b) methylated SiNW array, (c) SiNH array, and (d) methylated SiNH array. Scale bars in the insets are 10 μm.

the SiNW and SiNH arrays, respectively, revealing the NW diameter of 200−300 nm and height of ∼20 μm for the SiNW array, and a hole diameter of 0.8−1 μm and depth of ∼20 μm for the SiNH array. To reduce the surface carrier recombination, in this work, SiNW and SiNH arrays were further modified with methyl groups, which have been demonstrated to be highly effective to reduce the carrier recombination velocity.26,27 The recombination velocity of methyl-terminated Si could be as low as 45 cm/s, in contrast to the 500 cm/s for the hydrogen-terminated Si.27 After modification, it is observed that the SiNW array becomes very sparse due to the etching effect of PCl5 solution (Figure 1b). However, the SiNH array retains very well after modification except for a small increase in the hole size (Figure 1d). Figure 2 depicts the reflection spectra of Si nanoarrays before and after modification, along with the spectrum of planar Si for

Figure 3. Schematic illustrations and photographs of (a) graphene/ P3HT/SiNW array and (b) graphene/P3HT/SiNH array hybrid devices. Scale bars are 5 mm. (c) Band diagram of the hybrid solar cells.

graphene/P3HT/SiNW array and graphene/P3HT/SiNH array hybrid solar cells, respectively, in which the transparent graphene films serve as the anode electrodes and cover the top of the devices. CVD fabricated graphene films were utilized in this work because they are of high quality and have a large area capability. Raman spectrum of the graphene film reveals Gband and 2D band scattering peaks located at their normal positions of 1592 and 2653 cm−1, respectively (Supporting Information, Figure S1(a)). Monolayer graphene is also C

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Figure 4. Electric output characteristics of 4L-Gr/P3HT (10 nm)/SiNW array hybrid devices fabricated from SiNW arrays with different surface terminations. (a) J−V curves and (b) ln(J)−V curves of devices under dark condition. (c) J−V curves of devices measured under AM 1.5G light irradiation at 100 mW/cm2. (d) EQE spectra of the devices before and after graphene doping.

identified from the high I2D/IG intensity ratio of ∼2.7. From the transmittance spectra of 1−6 layers FLG films (Supporting Information, Figure S2(a)), high transmittances of 95.9%, 92.5%, 91.1%, 86.6%, 84.8%, and 82.1% at 550 nm are observed for 1−6 layers FLG films, respectively. We further detected the sheet resistances of the FLG films with varied layer number (Supporting Information, Figure S2(b)), and HNO3 doping was utilized to improve the graphene conductivity. It is seen that the 1-layer graphene (MLG, also denoted as 1L-Gr) film possesses large sheet resistances of 683 and 279 Ω/□ before and after doping in HNO3 vapor for 2 min, respectively, while the sheet resistance decreases remarkably with the increase of layer number and reaches the minimum for 6L-Gr film (149 and 61.5 Ω/□ before and after doping, respectively). The opposite tendency of optical transmittance and sheet conductivity of graphene film indicates that there is a tradeoff between them for achieving high-efficiency graphene-based solar cells. The charge separation and transport in the graphene/P3HT/ Si nanoarrays hybrid solar cells can be understood from the energy band diagram in Figure 3c. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) for P3HT are 3.2 and 5.1 eV, respectively, as compared to the conduction band minimum (Ec) and valence band maximum (Ev) of 4.05 and 5.17 eV, respectively, for Si. Upon irradiation, electron−hole pairs generated in Si would diffuse to the P3HT/Si interface and then be separated by the strong built-in electric field of the heterojunction. Electrons in the conduction band of Si were preferentially collected by the In/Ga electrode (cathode), while injection of electrons from Si to graphene anode was prevented by the P3HT layer due to the large Ec-LUMO offset. On the other hand, holes were readily injected into the HOMO level of P3HT because of the negligible Ev-HOMO offset, and then collected by the graphene anode. Therefore, the P3HT layer cannot only act as the hole transport layer, but also serve as an electrons blocking layer for

reducing the carrier recombination at anode. This should lead to a lower saturation current density and hence a larger opencircuit voltage for the device. It is noted that the pristine graphene possesses a relative small work function of ∼4.55 eV as that determined by UPS detection (Supporting Information, Figure S3), while it could be further enhanced to ∼5.0 eV after HNO3 doping.16 The tunable work function of graphene, along with its high optical transmittance, ensures a large photocurrent in the device. In addition, by taking advantage of SiNW and SiNH arrays as the light absorption materials, a unique core− shell organic/inorganic hybrid heterojunction structure can be formed after coating with P3HT; light can be harvested along the long axial direction, while generated carriers are separated in the short radial direction, reducing the charge recombination velocity and increasing the charge separation/transport efficiency. Therefore, high-efficiency hybrid solar cells are expected to be achieved by combing the graphene anodes and the Si nanostructure arrays. 3.3. Effects of Different Surface Modifications. Suffering from a large amount of surface defects, such as dangling bonds, Si-based solar cells usually show poor performance in the case of without any surface passivation due to the rapid surface recombination. As compared to planar Si, surface recombination in Si nanostructures will be more severe due to the large surface area-to-volume ratio, and hence appropriated surface passivation is critical to achieve a high-efficiency Si nanostructure-based solar cell. In this study, graphene/P3HT/ SiNW array hybrid solar cells with different Si surface passivation, that is, H-, SiOx-, and CH3-terminations, were systemically studied, as shown in Figure 4. The layer number of FLG and thickness of P3HT were tentatively optimized to be 4 layers and ∼10 nm, which will be discussed in detail later. From the current density versus voltage (J−V) characteristics of the devices in the dark, Figure 4a, it is seen that all of the devices exhibit excellent rectifying behavior, signifying the high quality of the P3HT/Si hybrid heterojunctions. Nevertheless, the diode D

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Table 1. Electric Output Characteristics of Hybrid Devices with Different Surface Terminationsa JSC (mA/cm2)

VOC (V)

a

FF

PCE (%)

device structures

pristine

doped

pristine

doped

pristine

doped

pristine

doped

4L-Gr/P3HT/H−SiNWs 4L-Gr/P3HT/SiOx−SiNWs 4L-Gr/P3HT/CH3−SiNWs

0.37 0.42 0.44

0.47 0.49 0.49

0.33 6.84 30.6

17.3 26.4 31.4

0.12 0.20 0.44

0.29 0.48 0.63

0.015 0.59 5.99

2.35 6.16 9.73

Layer number of FLG films and thickness of P3HT are fixed at 4 layers and ∼10 nm for all of the devices.

Figure 5. (a) Typical TEM image of a P3HT encapsulated SiNW. (b) J−V curves of the 4L-Gr/P3HT/CH3−SiNW array hybrid devices with different P3HT thicknesses under AM 1.5G light of 100 mW/cm2. Devices were doped in HNO3 vapor for 2 min prior to measurements. Plots of (c) VOC, JSC and (d) FF, PCE as functions of P3HT layer thickness.

performance of CH3−SiNW array is much better than that of SiOx−SiNW and H−SiNW arrays; the diode ideality factor (n) of CH3−SiNW array is deduced to be 1.94, Figure 4b, while the values are 2.71 and 5.47 for SiOx−SiNW and H−SiNW arrays, respectively, indicating a lower interface recombination in the CH3−SiNW array. Because of the effective surface passivation, CH3−SiNW array-based device also shows a superior photovoltaic performance upon light irradiation, as shown in Figure 4c. It possesses a short circuit current density (JSC) of 31.4 mA/ cm2, open circuit voltage (VOC) of 0.49 V, and fill factor (FF) of 0.63, yielding an efficiency as high as 9.73% after graphene doping, in contrast to the efficiencies of 2.35% and 6.16% for H−SiNW and SiOx−SiNW device counterparts, respectively. The detailed comparison of the device performance can be found in Table 1. Moreover, the external quantum efficiency (EQE) values of the graphene/P3HT/SiNW array hybrid devices were detected, as shown in Figure 4d. It is clear that EQE values of these three devices have the same tendency with their photovoltaic characteristics. Upon graphene doping, the EQE values of the hybrid devices are remarkably enhanced in visible and near-infrared range. The CH3−SiNW array shows a high EQE of ∼70% in the wavelength range of 400−800 nm, which is much higher than that for the SiOx−SiNW and CH3− SiNW arrays, indicating that carrier transfer and collection should be more efficient in the CH3−SiNW array-based device.28

To further study the important role of surface passivation on the device performance of SiNW array-based photovoltaic devices, device counterparts based on planar Si were fabricated and compared, as shown in Figure S4. Notably, the SiNW array device with poor surface passivation, that is, H-termination, shows lower efficiency than that of the H-terminated planar Si device (3.9%) due to the large surface area and consequently a higher recombination ratio in the SiNW array as compared to the planar Si. However, after surface passivation with SiOx- and CH3-terminations, the SiNW arrays exhibit higher PCEs than that of SiOx-terminated planar Si (5.06%) and CH3-terminated planar Si (7.19%). This result indicates that the advantages of the SiNW array in light absorption can emerge only when the apporpriate surface passivation is performed. 3.4. Effects of P3HT Layer Thickness. P3HT organic layer with different thicknesses of ∼5, ∼10, ∼20, and ∼40 nm was fabricated by adjusting the P3HT concentration in solution. TEM image in Figure 5a discloses that the SiNW is fully encapsulated by a continued P3HT layer, forming an inorganic/organic core−shell hybrid heterojunction structure. Figure S5 in the Supporting Information also shows the TEM images of SiNWs with varied P3HT layer thickness, proving that P3HT thickness could be readily controlled via this solution-based coating method. Figure 5b plots the photovoltaic characteristics of the 4L-Gr/P3HT/CH3−SiNW arrays with varied P3HT thickness, and thickness-dependent output characteristics are displayed in Figure 5c and d. 4L-Gr/CH3− E

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Figure 6. Electric output characteristics of the graphene/P3HT (10 nm)/CH3−SiNW array hybrid devices with varied graphene layer number. (a) Typical J−V curves of the devices under AM 1.5G light of 100 mW/cm2. (b) Typical EQE spectra for 1L-Gr- and 5L-Gr-based devices. Plots of (c) VOC, JSC and (d) FF, PCE as functions of graphene layer number. Average values from three devices for each sample were used.

the organic layer, that is, ∼10 nm for P3HT layer.29−31 We note that the experimental result in this work is in good agreement with the expectation. 3.5. Effects of Graphene Layer Number. Figure 6a depicts the typical J−V curves of the hybrid devices with varied graphene layer number (N) measured under AM 1.5G light of 100 mW/cm2. It is noted that, to gain statistical significance, three samples for each device were measured, and their average device parameters were plotted in Figure 6c and d. When N is less than 5, Voc changes little in the range of 0.48−0.49 V with the increase of N, while JSC and FF enhance remarkably from 28.2 mA/cm2 and 0.44 for N = 1 to 31.6−34.9 mA/cm2 and 0.59−0.63 for N = 3−5. Correspondingly, average PCE values of 9.62−9.87% are yielded for the devices fabricated from 3 to 5 layer graphene. This result can be attributed to the enhanced sheet conductivity of graphene at larger layer number. Nevertheless, N > 5 leads to an obvious deterioration of the device performance due to the inferior optical transmittance. Table S2 summarizes the electric output characteristics of the devices with different graphene layer number. It is seen that the series resistance (Rseries) first decreases with increasing the graphene layer number (10 nm), a strong light absorption as well as a serious bulk recombination in P3HT will happen due to its poor electrical conductivity. As a result of enhanced carrier recombination, the shunt resistance (Rshunt) has remarkably decreased from 27 255.03 Ω for 10 nm P3HT device to 14 700.73 Ω for the 20 nm P3HT device, and further to 2501.41 Ω for the 40 nm device (Table S1). The remarkable decrease of Rshunt thus leads to the decrease of VOC as well as FF values with increasing P3HT thickness (>10 nm). Therefore, there is an optimum organic thickness for the hybrid devices, which is approximately equal to the excition diffusion length in F

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Figure 7. (a) Typical J−V curves of P3HT (10 nm)/CH3−SiNW array hybrid devices with 13 nm Cu, 13 nm Au, and 5L-Gr electrodes, respectively, under AM 1.5G light of 100 mW/cm2. (b) Transmittance spectra of various electrodes, including 13 nm Cu, 13 nm Au, and 5L-Gr.

3.7. SiNH Array-Based Hybrid Solar Cell with Graphene Anode. Figure S7 in the Supporting Information displays the EDS analysis on the P3HT-coated SiNH array. The strong carbon signals from the SiNH surface and side walls clearly indicate the successful coating of P3HT layer on the SiNH array. Figure 8a depicts the typical J−V curve of 5L-Gr/ P3HT (10 nm)/CH3−SiNH array hybrid device measured under AM 1.5 G light. JSC, VOC, and FF of the device are deduced to be 37.81 mA/cm2, 0.48 V, and 0.57, respectively, yielding a PCE as high as 10.34%. We note that JSC of this device is much higher than that for SiNW array-based device and close to the best value (42.7 mA/cm2) obtained for the conventional single-crystalline Si solar cells.32−34 Moreover, EQE spectra in Figure 8b reveal a high EQE value for the SiNH array, which is ∼85% at a wide spectrum range. In particular, the EQE of SiNH array at near-infrared wavelength range is remarkably enhanced as compared to SiNW array. This result is in good agreement with the observation in Figure 2; the less reflection of SiNH array at near-infrared range signifies a stronger light absorption in this wavelength range. As a result, the SiNH array-based device shows improved EQE and JSC. On the other hand, due to the unique hole structure with flat top surface, the SiNH array can offer better support to the graphene films than can the SiNW array, leading to a larger effective contact area of graphene with P3HT and consequently a higher carrier separation and collection efficiency. This also contributes to the large JSC for SiNH array-based device. It worth mentioning that the PCE of 10.34% represents the best value achieved for the P3HT/Si hybrid solar cells up to now. This result ambiguously demonstrates the great potential of graphene as transparent electrode in high-efficiency Si-based hybrid solar cells.

3.6. Comparison of Graphene with Metallic Electrodes. The advantages of graphene over conventional metallic electrodes were evaluated by further detecting the photovoltaic performances of hybrid devices based on 13 nm Cu, 13 nm Au, and HNO3-doped 5L-Gr, respectively, as shown in Figure 7a. Significantly, the graphene-based device exhibits much enhanced performance as compared to the device counterparts with Cu and Au electrodes. From the electric output characteristics of the hybrid devices listed in Table 2, we note Table 2. Electric Output Characteristics of the Hybrid Devices with Different Anode Electrodes device structures Cu/P3HT(10 nm)/CH3−SiNWs Au/P3HT(10 nm)/CH3−SiNWs 5L-Gr/P3HT(10 nm)/CH3− SiNWs

VOC (V)

JSC (mA/cm2)

FF

PCE (%)

0.43 0.40 0.48

30.9 34.5 35.1

0.40 0.57 0.59

5.30 7.80 9.94

that all of the device parameters are improved by using graphene anode electrode. PCEs of Cu and Au films-based devices are 5.30% and 7.80%, repectively, while the PCE increases remarkably to 9.94% for the 5L-Gr-based device. This result can be understood from the transmittance spectra of the various electrodes (Figure 7b); the metallic films are transparent only in a narrow wavelength range with a low transmittance