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Surface Micro/Nano-Textured Hybrid PEDOT:PSSSilicon Photovoltaic Cells Employing Kirigami Graphene Chi-Hsien Huang, Zih-Yang Chen, Chi-Ling Chiu, Tzu-Ting Huang, Hsin-Fei Meng, and Peichen Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08366 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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ACS Applied Materials & Interfaces

Surface

Micro/Nano-Textured

Hybrid

PEDOT:PSS-Silicon

Photovoltaic Cells Employing Kirigami Graphene

Chi-Hsien Huang†*, Zih-Yang Chen ‡, Chi-Ling Chiu‡, Tzu-Ting Huang†, Hsin-Fei Meng§, and Peichen Yu ‡*

†Department

of Materials Engineering, Ming Chi University of Technology, New Taipei

City 24301, Taiwan ‡Department

of Photonics, College of Electrical and Computer Engineering, National

Chiao Tung University, Hsinchu 30010, Taiwan §Institute

of Physics, National Chiao Tung University, Hsinchu 30010, Taiwan

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

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Abstract Kirigami graphene allows 2D material to transform into a 3D structure, which constitutes an effective transparent electrode candidate for photovoltaic cells having a surface texture. The surface texture of an inverted pyramid was fabricated on a Si substrate using photolithography and wet etching, followed by metal-assisted chemical etching to obtain silicon nanowires (SiNWs) on the surface of the inverted pyramid. Kirigami graphene with a cross-pattern array was prepared using photolithography and plasma etching on copper foil. Then, kirigami graphene was transferred onto hybrid heterojunction photovoltaic cells with poly(ethylene terephthalate)/silicone film. These cells

consisted

of

poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)

(PEDOT:PSS) as the p-type semiconductor, Si (100) as the inorganic n-type semiconductor, and a silver comb electrode on top of the PEDOT:PSS. The conductivity of the PEDOT:PSS was greatly improved. This improvement was significantly higher than that achieved by the continuous graphene sheet without a pattern. TEM and Raman results revealed that the greater improvement with kirigami graphene was due to the larger contact area between PEDOT:PSS and graphene. By using 2-layer graphene having a kirigami pattern, power conversion efficiency, under simulated AM1.5G illumination conditions, was significantly augmented by up to 9.8% (from 10.03% to 11.01%). 2 ACS Paragon Plus Environment

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Keywords: kirigami graphene; hybrid photovoltaic cell; PEDOT:PSS; surface texture; 3-dimensional.



1. INTRODUCTION In the past decade, hybrid photovoltaic (PV) cells based on p-type poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)(PEDOT:PSS) directly coated on planar or textured n-type silicon surfaces have attracted increased attention due to low-cost solution-processable fabrication and potential relatively high power conversion efficiencies (PCE).1-5 In the case of textured silicon surfaces, such as the random upright pyramid, inverted pyramid array, nanowire, etc.6-15, the hybrid solar cell achieves high PCE, which is ascribed to its high light harvesting resulting from low light reflectivity of the surfaces.16 For light harvesting and carrier collection, transparent electrode plays a very important role in PV cells.17-19 However, with the presence of 3-dimensional (3D) micro or nanosized surface texture, it is difficult to form conformal coverage for the PEDOT:PSS layer and transparent anode.20,21 Several solutions to achieve conformal contact between PEDOT:PSS and surface texture have been attempted. They primarily focused on the additive into PEDOT:PSS, post-treating the PEDOT:PSS thin film, and pre-treating the Si surface.1,21-27 On the other hand, a silver grid is widely employed as a front electrode deposited onto the PEDOS:PSS layer through a shadow mask by an evaporation or sputtering process.28-30 Unfortunately, the contact area between the silver electrode and the PEDOT:PSS thin film is very limited (approximately 10-20% coverage) due to the opacity of the silver thin film. Although ITO thin film is sputtered onto the 4 ACS Paragon Plus Environment

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PEDOT:PSS layer to obtain large continuous conductive film31, it remains challenging to have conformal contact with 3-D surface texture because of the inherent disadvantage of physical deposition. Unfortunately, the conformal coverage of electrodes on surface texture of PV cells was rarely investigated until now. Very recently, kirigami and origami techniques have attracted extensive attention from researchers because these Japanese art techniques can aesthetically transform paper-like 2-D material into complex 3-D structures through cutting, folding, and bending.32-34 Graphene, the thinnest paper-like 2-D material, possesses extraordinary electrical, optical, thermal and mechanical properties, and has been investigated for numerous future nanoelectronics.35,36 Moreover, due to the flexibility of graphene, it can experience various mechanical deformations, such as folding and bending.37,38 Therefore, graphene has been investigated to replace conventional transparent conductive film, i.e., indium tin oxide, owing to its high flexibility, transparency, and conductivity. Consequently, graphene is very suitable for kirigami-like cutting and origami-like folding and offers great potential to be a 3-D transparent electrode for conformal coverage on the surface texture of PV cells. In this study, we proposed to fabricate multilayer kirigami graphene with a crosspattern array on copper foil utilizing photolithography and plasma etching. Kirigami graphene was transferred onto a surface textured hybrid PV cell, Si as n-type inorganic 5 ACS Paragon Plus Environment


substrate, and PEDOT:PSS as p-type organic emitter to form a heterojunction, using a poly(ethylene terephthalate) (PET) substrate. An inverted pyramid array, prepared by photolithography and tetramethylammonium hydroxide (TMAH) etching, was used as a surface texture. Compared with a random pyramid, an inverted pyramid array is more efficient with single side light harvesting geometry, and especially more suitable for attaching kirigami graphene when the pitches of both arrays are the same.10 We systematically investigated the optimization of an inverted pyramid array for PV cells with high power conversion efficiency (PCE) and the number of kirigami graphene layers on PV characteristics. The conformal coverage of kirigami graphene on the surface texture of the inverted pyramid array was also confirmed by transmission electron microscope and Raman spectroscopy.

2. EXPERIMENTAL SECTION 2.1. Preparation of the inverted pyramid structure Commercial n-type solar-grade monocrystalline silicon wafers (n c-Si) with a thickness of 200 μm and a resistivity of 1-5 Ω-cm were used for this work. First, the wafers were cut into 2.8 × 2.8 cm chips and then immersed in a mixture of sodium hydroxide solution (NaOH: H2O = 1: 1 in volume) at 80 ℃ for 40 min. to remove any saw damage. The array of the inverted pyramid structure preparation started with the as-cleaned wafers as 6 ACS Paragon Plus Environment

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follows. 100 nm-thick silicon dioxide (SiO2) was deposited onto both sides of the wafers using plasma-enhanced chemical vapor deposition (PECVD). The back side of the wafer could be protected from potassium hydroxide through the following procedure of inverted pyramid formation. An array of square holes was patterned by photolithography with negative photoresist of AZ5214E, followed by wet etching using buffered oxide etcher (BOE, ammonium fluoride: hydrofluoric acid = 6: 1 in volume). In order to prevent the back side of SiO2 from etching caused by BOE, the AZ5214E was spincoated onto the back side of the sample prior to wet etching. After the AZ5214E was removed by hot acetone, followed by thorough isopropyl and deionized water rinses, an array of square holes with a hole size of 20 μm and a spacing of 5 μm between holes was obtained as a hard mask in the following process of inverted pyramid formation. After preparation of an array of square holes as a hard mask, the samples were textured by anisotropic etching of silicon in 5 wt% tetramethylammonium hydroxide (TMAH) at 80 ℃ for 15, 20, 25, and 30 min to form an inverted pyramid array. A magnetic stirrer with rotation speed of 240 rpm was used during the etching to enhance etching uniformity. Finally, the SiO2 layer on both sides of samples was removed by BOE, and then rinsed in deionized (DI) water at room temperature and dried by N2 gas. The procedures of the array of the inverted pyramid structure preparation are schematically illustrated in Fig. S1. 7 ACS Paragon Plus Environment


2.2. Preparation of Si nanowire arrays on inverted pyramid surface After inverted pyramid preparation, the sample was further textured using metalassisted chemical etching (MACE).2,39 An aqueous hydrofluoric (HF) solution (8.6 M) containing silver nitrate (AgNO3, 0.04 M) at 20 ℃ was used to etch the structure of vertically aligned Si nanowire (SiNW) arrays on the surfaces of the inverted pyramid array (inverted pyramid/SiNWs). As-prepared SiNW samples were dipped in nitric acid solution, rinsed in DI water at room temperature, and dried by nitrogen gun. In order to prevent etching on the back side of the sample, a photoresist of PMMA was coated on it prior to texturing of SiNWs and then baked at 180 ℃ for 2 min. A secondary etching was performed using the same etchant with a lower concentration of AgNO3 (8.3 × 10-4 M) at 20 ℃ for desired periods to change the nanowire densities. The PMMA on the back side was removed with acetone and rinsed in isopropyl (5 min) and DI water (5 min), and then dried by N2 gas. 2.3. Kirigami graphene preparation The kirigami graphene preparation began with the growth of large-area single-layer graphene film on copper foil (G/Cu-foil) by chemical vapor deposition (CVD) in a tubular quartz furnace. Details of graphene growth were described elsewhere.40 Afterwards, thermal release tape (TRT) was used to adhere to graphene/Cu-foil41, and then immersed in a Fe(NO3)3·9H2O solution to etch away the Cu-foil. After removing 8 ACS Paragon Plus Environment

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the Cu-foil, the graphene and TRT combination (TRT/G) samples were intensively rinsed with DI water and then dried using N2 gas. The TRT/G sample was then placed onto the G/Cu-foil and then heated in an oven at 135 ℃ for 100 s to eliminate adhesion between graphene and TRT. After this, TRT could be easily separated from graphene to obtain two layers of graphene on Cu-foil. By repeating the TRT process, we could obtain three layers of graphene on Cu-foil. After preparation of multilayer graphene on Cu-foil, a kirigami pattern with cross arrays (cross width: 5 μm; cross length: 20 μm) was fabricated using photolithography, followed by reactive ion etching with argon gas. The array pitch was consistent with the inverted pyramid structure (25 μm). The schematic of kirigami pattern with cross arrays is presented in Fig. S2. After patterning, single or multilayer kirigami graphene was transferred from the Cu-foil to a poly(ethylene terephthalate) (PET) substrate using a roller press. The PET substrate contains a thin film of silicone layer acting as an adhesive layer to attach the kirigami graphene to it.42,43 The kirigami graphene was then separated from the Cu-foil using Fe(NO3)3·9H2O solution to obtain kirigami graphene and PET combination (OG/PET). The samples were then intensively rinsed with DI water and dried using N2 gas. 2.4. Hybrid solar cell preparation The textured samples were subsequently processed into hybrid photovoltaic cells. The samples were loaded into a thermal evaporator system for depositing a 100-nm9 ACS Paragon Plus Environment


thick Al layer as the back electrode immediately after removal of native oxide using BOE solution. Then, a highly conductive aqueous PEDOT:PSS containing carbon nanotube (TC-200, XinNano Materials, Inc.) with a conductance of 10 S/cm was dropped onto the textured samples for 60 s to penetrate the micro/nano structure, and then spun for 100 s. The coated organic layer was annealed at 150 ℃ for 15 min to introduce the surface dipole moment, leading to a higher work function for efficient hole transportation.44 Afterwards, a 100-nm-thick Ag comb electrode was thermally evaporated on the samples. Finally, OG/PETs were placed onto the samples. PET substrates were then carefully removed from the samples to leave the kirigami graphene on the samples. 2.5. Characterization of textured samples and PV cells After texturing the samples, reflectance (R) spectra were measured by a UV-VisNIR spectrophotometer (Hitachi U-4100), and the surface structures were observed by scanning electron microscope (SEM, JEOL JSM-7800F Prime). The Raman spectra of transferred graphene were collected in a LabRAM HR, Horiba Raman spectrometer with a laser wavelength of 488 nm. The sheet resistances were obtained by conducting 4probe measurements. Hall measurements were also conducted to acquire the carrier mobility of PEDOT: PSS with and without graphene on a planar SiO2 substrate at room temperature. 10 ACS Paragon Plus Environment

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PV characteristics of the hybrid PV cells were measured using a solar simulator under AM 1.5G (Air Mass 1.5, Globe) at an illumination intensity of 100 mW/cm2 calibrated by a PV-measurement (PVM-154) monocrystalline Si solar cell and a Keithley 2400 source meter. The PV cells under measurement were shielded using a black stainless metal mask with an opening area of 1 cm2, and the temperature was controlled at 25 ± 0.5 ℃.40

3. RESULTS AND DISCUSSION 3.1 Surface texture optimization The main goal of surface texture is to obtain low reflectivity of the Si wafer. The reflectance spectrum of the surface textured Si wafer was measured, and the weighted reflectivity (< R >) ranging from 300 to 1100 nm was calculated as follows: 1100

  R 

300

R( ) I AM 1.5G ( )d

1100



300

I AM 1.5G ( )d

(1)

Figure 1(a) shows the reflectivity spectrum after TMAH etching with different etching times. The bare Si wafer exhibits high reflectivity through the measured wavelength range. After TMAH etching for different etching times, reflectivities greatly decreased. The inset shows the weighted reflectivity as a function of etching time, and it is seen



Figure 1. (a) Reflectivity spectrum of Si wafer after TMAH etching for different etching times. Inset shows the weighted reflectivity. (b) SEM images of the inverted pyramid array under an etching time of 25 min.

that the lowest weighted reflectivity of 18.1% was obtained as the etching time was 25 min. Under the optimized condition for reflectivity, the SEM image of the inverted pyramid for kirigami graphene contact was approximately 11.3 μm. With this were almost the same as the designed photolithography pattern. The side length of the inverted pyramid structure, the PV cell was fabricated (see Fig. S3(a)). Power conversion efficiency (PCE) was only 7.52% (PV characteristics are shown in Fig. S3(b)). Although the inverted pyramid structure is suitable for kirigami graphene coverage, the PV 12 ACS Paragon Plus Environment

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Figure 2. (a) Reflectivity spectrum of inverted pyramid array after MACE etching for different etching times. Inset shows the weighted reflectivity. (b) SEM images of SiNWs on the sidewall of the inverted pyramid for different MACE etching times. characteristics are not acceptable due to relatively high reflectivity. In order to improve conversion efficiency, we aimed to further decrease reflectivity using MACE to fabricate the nanostructure of SiNWs on the surface of the inverted pyramid with a NaOH etching time of 25 min. Figure 2(a) displays the reflectivity spectrum after MACE for various etching times. The spectrum of 0 min is the inverted pyramid array without MACE. After MACE for various times, the reflectivities were 13 ACS Paragon Plus Environment


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significantly further decreased. The inset plots the weighted reflectivity variation and the length of SiNWs as the MACE time changes. As shown in Fig. 2(b), when the MACE time increases for 2 and 3 min, the length of SiNWs reaches 155 nm, and 189 nm, respectively, while the weighted reflectivity is suppressed to 12.6%

and 6.4%. The

reason arises from the trapping of the excitation light in the SiNWs due to strong light scattering.45 As the etching time further increases to 5 min., the weighted reflectivity, however, slightly increases to 9.5% even though the length of SiNWs still increases (~270 nm). The increase of the optical reflectivity at the longer etching time indicates that the vertically aligned and densely packed SiNWs on the inverted pyramidal surface behaves as an effective thin film, where the surface reflectivity exhibits a dependence on the film thickness. The result is supported by the uniform length distribution of SiNWs across the inverted pyramidal surface. Nevertheless, previous studies of SiNWs using MACE have shown that the reflectivity is decreased with the increased length of SiNWs fabricated on planar silicon substrates until the tips of SiNWs exhibits the agglomeration effect.46-48 Although the reflectivity is optimized ( = 6.4% at 3 min.), the PN junction formation between PEDOT: PSS and silicon nanostructures is another key point because the morphology of densely packed SiNWs makes it difficult for PEDOT: PSS to penetrate in between the nanowires. Moreover, although the optimized spin speed is 6000 rpm for conversion efficiency (8.30%, see Fig. S4(a)), only a slight 14 ACS Paragon Plus Environment

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improvement was obtained, as compared with the PV cell without the SiNWs array. An SEM image (see Fig. S4(b)) demonstrated that the PEDOT: PSS did not successfully penetrate through the dense SiNWs array to form a sufficient PN junction area. To solve this issue, we performed a secondary etching. The SiNWs array was re-etched using MACE with a lower concentration of AgNO3 to slightly etch the SiNWs. Therefore, more spacings between nanowires allow PEDOT:PSS to penetrate through and then form a larger PN junction area. Figure 3 shows the representative current density-voltage (J-V) curves of the PV cells with various re-etching times. The PV characteristics were summarized, as shown in Table 1. The conversion efficiencies of PV cells with the reetching process were all greatly improved. The optimized condition of the re-etching process for conversion efficiency (10.03%) was 5 min. Although we found that the weighted reflectivity of inverted pyramid/SiNWs after the optimized re-etching process

Figure 3. Representative current density-voltage (J-V) curves of PV cells with various re-etching times. 15 ACS Paragon Plus Environment


Table 1. PV characteristics of PV cells with various re-etching times.

became greater than that without the re-etching process (see Fig. S5(a)), the conversion efficiency could be apparently augmented because the SiNWs array became loosened (see Fig. S5(b)), and then the possibility to form a PN junction was increased owing to the penetration of PEDOT:PSS in between the SiNWs (see Fig. S5(c)). Therefore, we have determined the surface texture having inverted pyramids containing SiNWs for the kirigami graphene coverage. 3.2 Kirigami graphene preparation and characterization Figure 4(a) shows the optical images of 1-3 layers of kirigami graphene with crosspattern arrays on Cu-foil. The cross-pattern arrays with various graphene layers can be observed, and the sizes of the cross-pattern are highly consistent with the designed one. Figure 4(b) shows the Raman spectrum of the representative etched and unetched locations, as displayed in Fig. 4(a). The spectrum of A, B, and C, which are representative unetched locations of 1-layer, 2-layer, and 3-layer graphene, respectively, exhibit clear graphene featured peaks of G and 2D bands. On the other hand, no graphene featured peaks were observed in the spectrum of D, E, and F, which 16 ACS Paragon Plus Environment

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Figure 4. (a) Optical images of 1-3 layers of kirigami graphene with cross-pattern arrays on Cu-foil; (b) representative Raman spectrum of etched unetched regions (A, B, and C) and etched regions (D, E, and F) for 1-3 layers of kirigami graphene on Cu-foil.

are representative etched locations of 1-layer, 2-layer, and 3-layer graphene, respectively. This result indicates the successful preparation of kirigami pattern with multilayer graphene on Cu-foil using photolithography and plasma etching processes. After the successful kirigami pattern, 1 to 3-layer kirigami graphene and unpatterned graphene were transferred onto the inverted pyramid/SiNWs coated with PEDOT:PSS using PET substrate, as schematically shown in Fig. 5(a). After removing the PET 17 ACS Paragon Plus Environment


substrate, the sheet resistance measurement was performed to investigate conductivity enhancement. As shown in Fig. 5(b), both unpatterned and kirigami graphene reduced the sheet resistance of PEDOT:PSS. Overall, as the layer number of graphene increases, the sheet resistance decreases. It is worth noting that kirigami graphene causes larger sheet resistance reduction of PEDOT:PSS, even though the kirigami graphene is not a continuous sheet, as is an unpatterned one. We consider that kirigami graphene can cover the side of the inverted pyramid structure, leading to a larger

Figure 5. (a) Scheme of unpatterned graphene and 1-3 layers of kirigami graphene transferred to inverted pyramid/SiNWs coated with PEDOT:PSS; (b) sheet resistances of unpatterned graphene and kirigami graphene vary with the number of graphene layers. 18 ACS Paragon Plus Environment

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contact area between PEDOT: PSS and graphene. In contrast, unpatterned graphene is a continuous sheet and is suspended above the inverted pyramid, resulting in a smaller contact area between PEDOT: PSS and graphene. Therefore, kirigami graphene results in higher conductivity enhancement than that of the unpatterned one. In order to understand the sheet resistance reduction after transferring graphene onto PEDOT: PSS. The Hall measurement was performed to obtain the carrier mobility of PEDOT: PSS and graphene/PEDOT: PSS on a planar SiO2 substrate. The hole mobilities are 6.7 ×10 -1 and 1.2×10 2 cm 2 /V s respectively. The mobility of PEDOT: PSS was significantly improved after transferring graphene onto it, indicating the conductivity enhancement by using the graphene layer. TEM measurement was performed to confirm kirigami graphene covering the side of the inverted pyramid. In order to easily observe kirigami graphene coverage, five layers of kirigami graphene were prepared on Cu-foil and then transferred onto the inverted pyramid for this measurement. The observed locations were divided into three parts, including top, side, and bottom of the inverted pyramid, as schematically displayed in Fig. 6(a). The three parts were prepared using focus ion beam etching from different inverted pyramid units. Fig. 6(b) shows the representative TEM images of those three parts. At the top part, it was easy to observe the TEM images, as shown in Fig. 6(b)-1. Five-layer graphene was observed, as shown in the inset of Fig. 6(b)-1, and the profile along the green line was 19 ACS Paragon Plus Environment


Figure 6. (a) Scheme of inverted pyramid and observed locations with TEM measurements; (b) TEM images of different locations of inverted pyramid indicated in (a); (c) profile of multilayer graphene on inverted pyramid along the green line, as shown in the inset of (b)-1.

presented in Fig. 6(c). The spacing between the graphene layers is approximately 0.4 nm, which is consistent with the literature.49 At the location of the side part, 5-layer graphene at the upper region was clearly observed, as shown in Fig. 6(b)-2. It has been reported that graphene can self-assemble to facilitate origami-like folding and kirigamilike cutting processes.50,51 As a result, the TEM results indicated that the graphene layer self-folded and lay on the side of the inverted pyramid after gently releasing the PET film during the transfer process. However, graphene was not observed at the lower region, as displayed in Fig. 6(b)-3. According to our measurements, 5-layer graphene was not observed at the bottom of the inverted pyramid, as shown in Fig. 6(b)-4. Raman spectrum of unpatterned and kirigami graphene with 1-3 layers covering the inverted pyramid Si substrate was acquired. For each layer number, the intensity ratio of 2D to G 20 ACS Paragon Plus Environment

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bands (I2D/IG) of unpatterned graphene was all higher than that of kirigami graphene, as demonstrated in Fig. S7. It is reported that the I2D/IG of suspended graphene is higher than that of supported graphene due to interaction with the substrate.52 In the case of unpatterned graphene, the 2D material covers the surface texture as a continuous thin film without deformation, which indicates that the most of the unpatterned graphene is suspended above the hole of the inverted pyramid, leading to higher I2D/IG. On the other hand, the kirigami graphene can transform into a 3D structure to cover the side of the inverted pyramid, which indicates that almost all of the kirigami graphene makes contact with the substrate, leading to lower I2D/IG resulting from interaction with the substrate. Given the TEM and Raman results, the kirigami art can efficiently transform 2D material into a 3D structure to cover the surface texture conformally. It is also highly expected that the conductivity of PEDOT:PSS could be further improved if kirigami graphene covers the side of the inverted pyramid to a greater extent. 3.3 Hybrid PV cells with kirigami graphene Hybrid PV cells with kirigami graphene with 1-3 layers were fabricated. The sample structure and energy band diagram of the hybrid PV cell are illustrated in Fig. S8(a) and (b) respectively. In order to measure the individual enhancement of PV characteristics of hybrid PV cells for each layer number, reference samples labeled as Reference 1, Reference 2, and Reference 3 without kirigami graphene were measured 21 ACS Paragon Plus Environment


first, and the kirigami graphene with 1-3 layers were transferred to Reference 1, Reference 2, and, Reference 3, respectively, for the following measurement of PV characteristics. Figure 7(a)-(c) shows the representative J-V curves of the PV cells with 1 - 3 layers of kirigami graphene. The PV characteristics, including open-circuit (Voc), short-circuit current density (J sc ), fill factor (FF), and PCE of the PV cells are summarized in Table 2. For each number of kirigami graphene, the Jsc and FF are all improved, leading to PCE enhancement. The hybrid PV cell with 1 layer of kirigami graphene performs a V oc of 0.46 V, J sc of 32.4 mA/cm 2 , and FF of 69.92%. As

Figure 7. J-V curves of hybrid PV cells of (a) Reference 1 vs. with 1 layer of kirigami graphene, (b) Reference 2 vs. with 2 layers of kirigami graphene, and (c) Reference 3 vs. with 3 layers of kirigami graphene.


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Table 2. PV characteristics of hybrid PV cells with various layers of kirigami graphene (KG) and their reference samples.

compared with Reference 1, the Jsc and FF were increased from 31.69 to 32.40 mA/cm2 and from 68.84 to 69.92 %, respectively. The improvement of FF is ascribed to that the higher conductivity of the graphene layer improves the carrier conduction of PEDOT:PSS through the interface between PEDOT:PSS and Si. On the other hand, the improvement of Jsc is due to that the contact area between the graphene layer and the PEDOT:PSS layer is enlarged, which significantly augments the efficiency of carrier collection.53 As the layer number increases from 1 to 2, the PCE enhancement further increases because the conductivity of the PEDOT:PSS layer was further improved, as shown in Fig. 5(b). However, as the layer number further increases to three, the PCE enhancement decreases. The reduction results from the decrease of transmittance with the three-layer graphene.40 Therefore, the greatest PCE enhancement is the PV cell with 2-layer of kirigami graphene. Jsc and FF were increased from 30.84 to 32.72 mA/cm2 and from 65.80 to 67.98%, respectively, resulting in PCE enhancement from 10.03 to 23 ACS Paragon Plus Environment


11.01%, which improves upon that of Reference 2 by 9.8%. In order to confirm the benefit of kirigami graphene on PV cells, we also fabricated the hybrid PV cell with two-layers of unpatterned graphene for comparison. The J-V curve is shown in Fig. S9 and the PV characteristics are summarized in Set 4 of Table 2. The PCE enhancement of hybrid PV cells with kirigami graphene greatly outperforms that with unpatterned graphene. The reason is ascribed to the larger contact area with PEDOT:PSS, resulting from higher conductivity improvement of PEDOT:PSS and 3D structure of kirigami graphene, as demonstrated in Fig. 5 and 6, respectively. Although the PCE of hybrid PEDOT:PSS/Si solar cells with the kirigami graphene is still not comparable to state of the art in literature, over 13% PCE2,54, the device performance can be further optimized via engineering the optical reflectivity and wettability of PEDOT: PSS on the micro/nano-textured surface template.

4. CONCLUSIONS The surface texture of an inverted pyramid was fabricated to allow the 2D material of graphene to transform into a 3D structure, obtaining a larger contact area between graphene and surface texture. The lowest weighted reflectivity of 18.1% was achieved upon TMAH etching time. However, the hybrid PV cell with an inverted pyramid only yielded a PCE of 7.52%, due to relatively high reflectivity. The MACE process was 24 ACS Paragon Plus Environment

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utilized to fabricate a SiNW array on the surface of the inverted pyramid in order to further decrease reflectivity. Although 6.4% of the reflectivity of the inverted pyramid/SiNWs was obtained, the hybrid PV cell only slightly improved from 7.52 to 8.30% due to the dense SiNW array where PEDOT:PSS could not successfully penetrate. The SiNW array was re-etched to loosen the SiNWs array to achieve more PN junction between PEDOT:PSS and Si. The optimized PCE upon re-etching time was greatly improved to 10.03%. After optimization of the surface micro/nano texture, kirigami graphene patterned on Cu foil using photolithography and plasma etching processes was transferred to hybrid PV cells with PET substrate. The TEM images demonstrated that kirigami graphene could cover the side of the inverted pyramid and improve the conductivity of PEDOT:PSS to a greater extent than unpatterned graphene. With twolayer kirigami graphene, hybrid PV cell with 9.8% of PCE enhancement was obtained and outperformed that with 2-layer unpatterned graphene. The study identifies a promising way to transform 2D material into a 3D structure to cover the surface texture of PV cells.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website. 25 ACS Paragon Plus Environment


The procedures to prepare the inverted pyramid array. The schematic of kirigami pattern with a cross array. Scheme and PV characteristics of hybrid PV cell with the optimized condition for the inverted pyramid structure. PV characteristics of hybrid PV cell with various spin speeds for PEDOT:PSS coating on the inverted pyramid structure. The SEM image of PEDOT:PSS coating on inverted pyramid structure at 6000 rpm of spin speed. The reflectance spectra and SEM images of inverted pyramid structure before and after optimized re-etching condition and PEDOT:PSS coated on it after optimized re-etching condition. The observation locations of inverted pyramid structure with five layers of kirigami graphene for TEM observation. Raman mapping of I2D/IG for kirigami and unpatterned graphene transferred onto the inverted pyramid structure. Scheme of hybrid PV cell with kirigami graphene and the energy band diagram of the device. PV characteristics of hybrid PV cell with and without 2-layer of unpatterned graphene. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. 26 ACS Paragon Plus Environment

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Acknowledgments This work was funded by the Ministry of Science and Technology of Taiwan under grant numbers 106-2221-E-009-134-MY3 and 107-2221-E-131-006-MY3.



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