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Low-pressure assisted coating method to improve interface between PEDOT:PSS and silicon nanotips for high efficiency organic/inorganic hybrid solar cells via solution process Thiyagu Subramani, Hong-Jhang Syu, Chien-Ting Liu, Chen-Chih Hsueh, Song-Ting Yang, and Ching Fuh Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11692 • Publication Date (Web): 30 Dec 2015 Downloaded from http://pubs.acs.org on January 1, 2016

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ACS Applied Materials & Interfaces

Low-pressure assisted coating method to improve interface between PEDOT:PSS and silicon nanotips for high efficiency organic/inorganic hybrid solar cells via solution process Thiyagu Subramania, Hong-Jhang Syua, Chien-Ting Liua, Chen-Chih Hsueha, Song-Ting Yangb, Ching-Fuh Lina, b, c, d*

a

Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei

10617, Taiwan b

Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 10617,

Taiwan c

Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan

d

Innovative Photonics Advanced Research Center, National Taiwan University, Taipei 10617,

Taiwan

Keywords: Silicon nanotips; PEDOT:PSS; hybrid solar cells; low-pressure assisted coating; chemical polishing etching.

Abstract Nanostructured silicon hybrid solar cells are promising candidates for new generation photovoltaics due to their light trapping abilities and solution processes. However, the performance of hybrid organic/Si nanostructure solar cells is hindered due to carrier recombination

at

surface

and

poor

coverage

of

organic

material

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on nanostructures. Here we demonstrate low-pressure assisted coating method of PEDOT:PSS on

ACS Paragon Plus Environment


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surface-modified silicon nanotips with broadband light-trapping characteristics to improve interface property and to achieve high-efficiency hybrid solar cells through a solution process. The approach enhances the effective minority-carrier lifetime and the coverage of PEDOT:PSS on the surface of nanostructures. Hybrid solar cells fabricated with surface-modified nanotips exhibit a high fill factor of 70.94%, short-circuit current density of 35.36 mA/cm2, open-circuit voltage of 0.528 V, and power conversion efficiency of 13.36%. The high efficiency and the high fill factor are achieved due to conformal coating of PEDOT:PSS via a low-pressure assisted coating process, excellent light harvesting without sacrificing the minority-carrier lifetime, and efficient charge separation/collection of photo-generated carriers.

Introduction Recently, many efforts have been made to develop environment-friendly and renewable sources of energy due to concerns with global warming and depletion of fossil fuels. Solar energy is one of the most abundant forms of renewable energies. Among the current solar cell technologies, crystalline silicon (c-Si) solar cells are the most popular candidates, dominating the market over 80% owing to its high efficiency, long-term stability, abundant material resources, and well established manufacturing techniques.1, 2 However, the high vacuum and high temperature lead to a high cost of commercial Si-based solar cells and limit their use. Moreover, the rather long energy payback time of Si solar cells leads to a major motivation for organic/inorganic hybrids solar cells3. Therefore, the interest of developing low-cost Si-based photovoltaic devices with the simple fabrication process has been growing.4-6 Organic/Si hybrid solar cells are one of the most promising candidates. They have received considerable attention in recent years due to low-cost processing of organic materials and high carrier mobility of Si.7-8 In recent years, PEDOT:PSS/Si heterojunction hybrid solar cells with higher


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efficiency have been reported9-12 The organic layer, PEDOT:PSS, plays a major role as a hole transporting path as well as for formation of heterojunction with Si. It also serves as a passivation layer and antireflection coating.13 Over the past few years, to improve the hybrid solar cell efficiency, silicon nanostructures, such as silicon nanowires (SiNWs), silicon nanoholes (SiNHs), silicon nanocones (SiNCs), and silicon nanotubes (SiNTs), have been used for enhancing the light harvest in devices.14-18 Moreover, the large PEDOT:PSS/Si junction area enhances charge collection by shortening the paths travelled by minority carriers.19-21 Therefore, in recent years, silicon nanostructures have been exploited as a promising candidate for photovoltaic devices. However, despite strong optical absorption enhancement, nanostructures lead to the difficulty of filling conductive polymer PEDOT:PSS into the gaps inside the Si nanostructures.22-24 If PEDOT:PSS do not infiltrate into gaps, the photo-exited carriers are easily trapped by high-density surface defects owing to the large non-passivated surface area of nanostructures before being collected by electrodes. Therefore, short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) are restricted, limiting the power conversion efficiency (PCE). Consequently, significant interest has been focused on the optimization of surface morphology, mainly including silicon nanostructure density, multiscale texture, and novel silicon nanostructures with low surface area enhancement to reduce surface defects, difficulty of PEDOT:PSS infiltration, and hence surface recombination.25, 26 To overcome the problems of PEDOT:PSS/Si nanostructure solar cells, in this work, we developed a novel method of low-pressure assisted coating (LPAC) to provide better PEDOT:PSS polymer coverage on the silicon nanostructure surface. In the past, some coating processes are used to form hybrid solar cells. Those include spin-coating materials on silicon nanostructures directly and spin-coating materials on alien substrate followed by attaching silicon nanostructures on coating materials covered substrate. However, they suffer incomplete infiltration of coating materials into nanostructures. Here we develop a modified



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technique to overcome the above problem. Furthermore, chemical polishing etching (CPE) treatment is applied on SiNHs to reduce the surface defects and metal contamination of SiNHs formed by metal-catalyst chemical etching (MacEtch). The CPE solution contains both nitric acid (HNO3) and hydrofluoric acid (HF). HNO3 functions as an oxidant to oxidize Si to SiO2, and HF can etch as-generated SiO2. Moreover, the rough nanostructure surface can touch more oxidants and etchants than smooth surface does, so rough surfaces can be more quickly etched away and the surfaces are smoothed. Once the surface becomes smooth, the surface area of the nanostructure decreases so that surface defects can be reduced. Additionally, in a particular condition of CPE solution, SiNH surface can be modified into randomly distributed tapered silicon nanotips and the gaps in nanostructures can be enlarged because some parts of Si are removed by CPE treatment. An important advantage of the processes mentioned above is that they consume less energy and are less costly than conventional Si p-n junction solar cells27-29. The results show that the effective minority-carrier lifetime of PEDOT:PSS covered Si nanostructures is greatly improved in the sample prepared by the CPE and LPAC processes. In addition, the total reflectance of CPE treated Si nanostructures is still lower than 10% in the wavelength range of 300-1100 nm though gaps in nanostructures are enlarged. These concurrent improvements reflect their accomplishment in hybrid solar cells as evident with the Jsc of 35.36 mA/cm2, Voc of 0.528 V, FF of 70.94%, and PCE of 13.36%. Also, the PCE is one of the highest value of PEDOT:PSS/Si nanostructure solar cells without any involvement of other small molecules and organic materials. Moreover, there is no back contact optimization30 for this work, which indicates the present method is a good way to improve the contact between nanostructured silicon and PEDOT:PSS layer. The work provides a feasible solution to improve organic/Si nanostructured hybrid solar cells in an energy- and cost-effective manner.


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Results and discussion The work explores low-pressure assisted coating (LPAC) method to improve coverage of solution-processed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) coated on silicon nanostructures to form thorough junction of hybrid solar cells. The schematic diagram of coating process is shown in Figure 1(a). In LPAC process, the silicon nanostructure are placed on holder inside a chamber, which will be evacuated with a pump. PEDOT:PSS is also dropped on the same holder, which is first tilted to clearly separate silicon nanostructure and PEDOT:PSS. Afterwards, pump out the air in wire gaps or nanoholes, and then tilt the holder to make PEDOT:PSS flow toward silicon nanostructure. Due to air vacancy in silicon nanostructure, PEDOT:PSS can infiltrate into gaps fluently. Moreover, surface carrier recombination is directly correlated to surface morphology. The high carrier recombination mostly originates from the increased surface area in the silicon nanostructures. To enhance the performance of nanostructured hybrid silicon solar cells, recombination channels should be suppressed and the rough nanostructures should be smoothed. To further resolve this issue, we used CPE treatment to reduce the surface defects and to smooth the surface of SiNH formed by MacEtch before the LPAC step. Figure 1(b) and 1(c) shows the schematic diagram of silicon nanoholes and modified tapered silicon nanotips formed by the CPE process. Figure 2 displays the high-resolution transmission electron microscopy (HR-TEM) images of the individual silicon nanohole and silicon nanotip before and after CPE treatment. One can clearly see that the silicon nanohole surface has rough edges, as shown in Figure 2(a). The atomically rough surface induces additional surface defects and causes severe carrier recombination. Moreover, Ag nanoparticles are not completely removed by an HNO3 solution due to high density of silicon nanostructures and hydrophobicity of etched Si surfaces. The residual Ag nanoparticles found at the bottom are confirmed from the EDX spectrum shown in Figure S1(a). It has been reported that silicon nanostructures fabricated by



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metal- assisted chemical etching have rough side wall surfaces.31 The Ag contamination creates carrier trapping centers, which reduce the minority carrier life time, leading to high carrier recombination and poor Voc and FF values. By utilizing CPE treatment after fabricating high density SiNH structure, edge roughness decreased significantly and achieved atomically smooth surface, as shown in Figure 2(b). The CPE solution is a mixture of hydrofluoric acid (HF) and nitric acid (HNO3). In the HF/HNO3 mixture, HNO3 assists in the formation of an oxide layer, while HF aids to remove the oxide layer. This technique was previously used to make thin silicon surface from bulk wafer.9 When increasing the CPE etching time, the silicon nanostructures diminish due to high oxidation and reduction processes. Even when the CPE etching time is shorter (10 sec), the high density silicon nanoholes are changed into well-spaced silicon nanotips, as shown in Figure 3(c). Optimization of the silicon nanostructure density is an additional important aspect in enhancing the performance of nanostructured silicon solar cells. Owing to CPE treatment, the density and the length of silicon nanoholes are reduced to form randomly distributed tapered silicon nanotips. In addition, HNO3 solution can easily penetrate into the bottom of a nanostructure to completely remove the residual Ag caused by MacEtch, as confirmed by the HR-TEM and EDX spectrum shown in Figure S1(b). Figure 3(a-e) shows the scanning electron microscopy (SEM) images of high-density silicon nanoholes and tapered silicon nanotips formed by the etching process. Figure 3(a) shows the 45° tilted view SEM images of high density nanoholes. The nanoholes with average diameters of 25-35 nm and high density of 1011/cm2 are homogenously formed on the Si wafer. The SiNH’s are directly fabricated through electro-less metal deposition in HF/AgNO3 solution and vertical etching step in HF/H2O2 solution. Figure 3(b) and 3(c) shows the CPE treated tapered silicon nanotips. The CPE treatment is carried out to reduce the surface defects, free metal contamination, and control the surface morphologies. During the CPE treatment, HNO3 in the HNO3/HF mixture assists in the formation of oxide layer,


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while HF helps for the removal of the oxide layer. Hence, the surface morphologies can be controlled depending on the polishing etching time. The SEM images of modified tapered silicon nanotips with different durations (5 and 10 sec) of CPE etching time are shown in Figure 3(b) and 3(c). One can clearly see that the density and the depth of the nanostructures are reduced. In addition, the gap in nanostructures is enlarged and the nanohole structures are changed into tapered silicon nanotips due to the fast etching process of the CPE solution. Figure 3(d) and 3(e) shows the cross-sectional images of silicon nanotips prepared by CPE process for 5 and 10 sec. One can see that, after subjecting to CPE treatment for just 10 sec of immersion time, density of the nanostructures is much reduced as compared to 5 sec. To obtain a superior light absorption while maintaining a low surface recombination rate, CPE treatment at 5 and 10 sec is the optimal condition, according to our investigation. Previously, HNO3 solution was used to clean the residual Ag after nanohole or nanowire formation. Then, the samples were immersed in HF solution to remove the oxide layer. However, Ag nanoparticles may not be completely removed by HNO3 cleaning due to the hydrophobicity of etched Si surfaces. Thus, they remain at the bottom of NHs32 and create carrier trapping centers due to Ag contamination and reduced minority carrier lifetimes. Thus, we applied CPE treatment to reduce metal contamination and surface defects. For comparison, we also prepared the samples through non-CPE process, by separately immersing in HNO3 and HF. After fabricating SiNH, the samples are immersed into HNO3 solution to remove silver and to form a thin oxide layer. After that, the same sample is immersed into HF to remove the oxide layer, and this process is repeated several times. Figure 3(f) shows the SEM image of samples prepared by immersing into an HNO3 solution for 2 min and then dipping in to an HF solution for 1 min (non-CPE process). The whole process is repeated 4 times. Compared to the CPE process, the non-CPE treated samples show the relatively higher nanostructure density, as shown in Figure 3(f). To utilize the useful photon management using silicon nanotips and silicon



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nanoholes in practical hybrid solar cells applications, an organic polymer, PEDOT:PSS, is used to fabricate these structured surfaces by using a solution process. Three kinds of surfaces (non-CPE, 5 and 10 sec CPE) are used to fabricate core/shell nanostructure hybrid solar cells. The schematic diagram of hybrid solar cell is shown in Figure 4(a). The PEDOT:PSS diluted with IPA at a 1:2 ratio is used to fabricate the hybrid device. First, the LPAC process is used to infiltrate the PEDOT:PSS into the gaps of nanostructures. The non-CPE treated samples, and 5 and 10 sec CPE treated samples are placed on a holder inside a chamber, which is connected with a pump as shown in Figure 1(a). Diluted PEDOT:PSS is dropped on the same holder, but it does not contact Si. Afterwards, air is pumped out of the chamber. The air in the gaps between the nanostructures is extracted as the chamber pressure is 30 torr. After 5 min, the holder is tilted to make PEDOT:PSS cover the silicon tips and silicon nanohole arrays. During this process, PEDOT:PSS infiltrates into the gaps of nanostructures fluently. After the LPAC process, the samples are picked up for spinning out superfluous PEDOT:PSS. To complete the fabrication process, the PEDOT:PSS film is dried after LPAC, and then, PEDOT:PSS (PHCV4) solution is spin-coated on the ITO substrate. Later on, SiNH samples are fixed on the ITO/glass. Finally, the fabricated devices are annealed in air at 140 °C for 10 min. The fabricated devices are measured under AM 1.5 G illumination, as shown in the current density-voltage (J-V) characterization in Figure 4(b). Figures 4(d) show the current density vs. voltage (J-V) characteristics of best performing devices in dark conditions through LPAC method for different structure. The results are summarized in table 1. Hybrid solar cells fabricated with the nanohole structures formed through non-CPE treatment display the lowest efficiency of 10.08% with Jsc of 33.50 mA/cm2 and Voc of 0.475V due to high density surface defects, high carrier recombination rate, and highly defective surfaces, as confirmed by the minority carrier lifetime measurement results in the following discussion. Compared to the non-CPE treated sample, the hybrid cells with nanotips have higher Voc and FF due to reduced surface defect, increased minority


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carrier life time, and complete coverage of PEDOT:PSS owing to an efficient LPAC process. One can see that Voc is increased from 0.475 V to 0.528 V for 5 sec CPE treated sample, which is due to the reduced surface defects and improved carrier life time. The significant improvement in FF from 63.40% to 70.94% is due to three major reasons. First, CPE process removes the surface defects/contamination and reduces the nanostructure density to form nanotips. Second, diluting PEDOT:PSS with IPA allows more efficient infiltration of solution into the gaps of nanotips. Third, the LPAC process provides superior coverage of PEDOT:PSS down to the bottom of the nanotips. Interestingly, the samples treated with CPE process for 5 sec and the as-formed nanotips coated with PEDOT:PSS diluted at the ratio of 1:2 provide the highest PCE, which is 13.36%. As shown in Figure 4(b), the best hybrid heterojunction cell has a PCE of 13.36% with high Jsc of 35.36 mA/cm2, Voc of 0.528 V, and FF of 70.94%. However, upon further increasing the polishing etching time to 10 sec, the efficiency is reduced to 11.62% due to the reduced nanostructures and poor junction contact as compared to 5 sec CPE treatment, which is reflected in the high series resistance, low FF of 65.40% and low Jsc of 33.78 mA/cm2. The high efficiency of hybrid solar cells fabricated using 5 sec CPE treatment and with nanotip structures can be attributed to high enhancement in FF. For comparison, we also fabricated hybrid solar cells with PEDOT:PSS diluted at 1:1 and 1:3 ratio. However, their performance is worse than that prepared with a 1:2 ratio. The results are summarized in Table S1. The current density-voltage (J-V) characterization results are shown in Figure S2. Figure 4(c) displays the external quantum efficiency (EQE) of non-CPE treated sample, 5 and 10 sec CPE treated samples diluted with PEDOT:PSS at 1:2 ratio. Obviously, the improvement in the device performance can be attributed to the 5 sec CPE treatment and the LPAC process using diluted PEDOT:PSS solution. The EQE improved markedly in the visible to near infra-region (

Low-Pressure-Assisted Coating Method To ... - ACS Publications

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