High Performance of PEDOT:PSS/n-Si Solar Cells Based on Textured Surface with AgNWs Electrodes
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Hybrid heterojunction solar cells (HHSCs) have gained extensive research and attention due to simple device structure and low-cost technological processes. Here, HHSCs are presented based on a highly transparent conductive polymer poly(3,4ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) directly spin-coated on an n-type crystalline silicon with microscale surface textures, which are prepared by traditional chemical etching. We have studied interface properties between PEDOT:PSS and textured n-Si by varying coating conditions. Final power conversion efficiency (PCE) could arrive at 8.54% by these simple solution-based fabrication processes. The high conversion efficiency is attributed to the fully conformal contact between PEDOT:PSS film and textured silicon. Furthermore, the reflectance of the PEDOT:PSS layer on textured surface is analyzed by changing film thickness. In order to improve the performance of the device, silver nanowires were employed as electrodes because of its better optical transmittance and electrical conductivity. The highest PCE of 11.07% was achieved which displayed a 29.6% enhancement compared with traditional silver electrodes. These findings imply that the combination of PEDOT:PSS film and silver nanowire transparent electrodes pave a promising way for realizing high-efficiency and low-cost solar cells.
KeywordsSilver nanowires PEDOT:PSS N-Si solar cells Drop-casting
Approximately 90% of global photovoltaic market is occupied by crystalline silicon solar cells for performing well on both cost and efficiency [1, 2, 3, 4]. Using n-crystalline silicon and poly(3,4ethylenedioxythiophene):poly(stylenesulfonate)(PEDOT:PSS) manufactured hybrid heterojunction solar cells (HHSCs) are favored by researchers . The properties of dopant-free, vacuum-free, low-temperature, and solution-proceeded fabrication procedures determine that PEDOT:PSS/n-Si heterojunction solar cells have a series of superiorities on the cost [6, 7]. The highest reported power conversion efficiency (PCE) of HHSCs is 16.2% created by Jian He et al. . The efficiency gap between HHSCs and conventional silicon cells is gradually narrowing.
In HHSCs, crystalline silicon, having high mobility and long minority carrier lifetime, is an active absorber for collecting photons to produce photo-generated carriers and transporting electrons. On the other hand, the PEDOT:PSS layer, with high transmittance(85% for 100 nm thickness) and high conductivity(1000 S/cm for Clevios PH1000) , works as a transparent conducting hole-transporting layer and optical window . Therefore, the HHSCs have potentials to achieve higher PCE. However, the PCE of HHSCs is greatly restricted to inferior junction quality at the PEDOT:PSS/n-Si interface.
Interface engineering is essential for the PEDOT:PSS/n-Si solar cells because it optimizes the carriers transmission and separation and reduces the interface recombination velocity . Several common methods are used to improve the PCE of PEDOT:PSS/n-Si heterojunction solar cells: reducing the thickness of crystalline silicon by depositing film crystalline silicon, applying colloidal quantum dot, texturing silicon surface into nanostructures, introducing the back surface field(BSF), and applying silicon nitride or silicon oxide as a passivation layer [5, 6, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. However, the contact properties of PEDOT:PSS with textured substrate has seldom been considered, which raises the Jsc and efficiency of PEDOT:PSS/n-Si hybrid solar cells from the perspective of interface engineering.
Our works are carried on the Si surface textured by traditional alkaline solution process . The uniformity of PEDOT:PSS film thickness is more difficult on textured Si than that on the plane ones. Unlike traditional electrodes, the silver nanowires (AgNWs) electrodes have superiority on optical transmittance. To our knowledge, the diluent of silver nanowires were difficult to coat on textured polymer film. The coating methods such as rod-coating or spin-coating cause the presence of nonuniformity and damage. In this paper, PEDOT:PSS/n-Si solar cells were fabricated with silver nanowire electrodes by means of drop-casting. The novel electrodes application on the cells provides a feasible, low-cost, and high-efficiency metallizing process.
Preparation of Textured Si Substrates for HHSCs
N-Si(100) Czochralski (CZ) wafers (thickness 210 μm, 1–3 Ω cm) were used as substrates. Samples were cleaned using standard cleaning solution (SC1 and SC2) and then polished in a high concentration of KOH solution at 75 °C for 2–3 min to remove the damaged layer. After standard cleaning process, the substrates were textured into a double-sided random pyramids structure by immersing in the mixed solution of KOH (2 wt.%) and isopropanol (2 wt.%) at 75 °C for 15–20 min. The height of random pyramids on textured silicon surface is about 1 μm. Followed with another RCA cleaning process, the textured samples were immersed in diluted HF solution for 0.5–1 min to obtain clean oxide-free silicon surfaces.
Fabrication of Si/PEDOT:PSS Hybrid Solar Cells
The reflectance spectra measurements were carried out with an integrating sphere. The scanning electron microscope (SEM) photos were obtained using S4800 Hitachi. The J−V characteristics of the cells were performed by an Oriel solar simulator (94063A, Newport Corporation), 450 W Xe lamp, simulated air mass AM 1.5 solar spectrum irradiation source at 100 mW/cm2, mono-crystalline reference cell, and Keithley 2400 source meter. The absorption spectral lines were measured using an ultraviolet spectrophotometer (UV-8000 s Shanghai Precision instruments Co. Ltd). The transmittance measurements of the PEDOT:PSS film were gained by QEX10 (PV Measurements, Inc.). The square resistance was carried out by employing a four-probe sheet resistance tester (SDY-4, Guangzhou Semiconductor Materials Research Institute).
Results and Discussion
J-V characterization of the textured HHSCs with different PEDOT:PSS coating rates from 1000 to 5000 rpm and 8000 rpm at AM1.5
Spin-coating rate (rpm)
Fill factor (%)
In practical application on textured substrates, PEDOT:PSS film thickness could not be adjusted without considering contact properties. The spin-coating process simultaneously restricts film thickness and contact quality . It is known that a relatively high coating rate is very necessary for efficiency improvements. The enhanced heterojunction areas contribute to the separation of holes and electrons and an increase on Jsc. The high-quality interface contact leads to a falling of interface recombination velocity and a significant current boost [11, 18]. Such a fact can be found from Figs. 4 and 5, that there is no massive conductive organic material stacking over the valleys at 5000 rpm. For the reduction of PEDOT:PSS film thickness, textured silicon surface traps more light . The decreased parasitic absorption loss of the thinner PEDOT:PSS layer leads to an enhancement of photons absorbing of the silicon surface, improving photocurrent and cell efficiency. However, when the spin-coating rate reaches 8000 rpm, the open circuit voltage reduces to 0.49 V because PEDOT:PSS film may be too thin to cover the whole Si surface and the heterojunction probably shortens. A thinner film would cause the direct connection between metal electrodes and top of pyramids. Meanwhile, due to the decreased film thickness, the decreased length of P-N junction has an effect on the device performance . And, the nonuniformity of film thickness at 8000 rpm may be especially important on influencing device efficiency. Therefore, the highest performance of PEDOT:PSS/n-Si solar cells occurs at 5000 rpm.
In summary, the mixed PEDOT:PSS solution of DMSO and FS31 achieves higher conductivity and smaller contact angle on the textured hydrophobic surface. The short wavelength reflectivity of PEDOT:PSS layer on the textured surface is influenced by the combined effect of absorption coefficient and flatness of substrate surface. With better contact quality, proper film thickness, and larger contact junction area at optimized coating rate, the performance of the HHSCs gets enhanced. The application of silver nanowire electrodes demonstrated a simple promising fabrication process for getting higher PCE.
This research was supported by Shanghai Sailing Program (15YF1413200), and the CAS/SAFEA International Partnership Program for Creative Research Teams.
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The authors declare that materials, data, and associated protocols are promptly available to readers without undue qualifications in material transfer agreements.
All data generated or analyzed during this study are included in this article.
XJ and XF designed the experiments and analyzed the data. XJ and PZ prepared the manuscript. JZ prepared the textured silicon substrate. All authors discussed the experimental results and commented on the manuscript. All authors read and approved final manuscript.
The authors declare that they have no competing interests.
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