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Enhanced photovoltaic performance of organic/silicon nanowire hybrid solar cells by solution-evacuated method Wei-Li Wang, Xian-Shao Zou, Bin Zhang, Jun Dong, Qiao-Li Niu, Yi-An Yin, and Yong Zhang* Laboratory of Nanophotonic Functional Materials and Devices, Institute of Optoelectronic Materials and Technology, South China Normal University, Guangzhou 510631, China *Corresponding author: [email protected] Received March 19, 2014; revised April 14, 2014; accepted April 15, 2014; posted April 16, 2014 (Doc. ID 208554); published May 26, 2014 A method has been developed to fabricate organic-inorganic hybrid heterojunction solar cells based on n-type silicon nanowire (SiNW) and poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) hybrid structures by evacuating the PEDOT:PSS solution with dip-dropping on the top of SiNWs before spin-coating (solution-evacuating). The coverage and contact interface between PEDOT:PSS and SiNW arrays can be dramatically enhanced by optimizing the solution-evacuated time. The maximum power conversion efficiency (PCE) reaches 9.22% for a solution-evacuated time of 2 min compared with 5.17% for the untreated pristine device. The improvement photovoltaic performance is mainly attributed to better organic coverage and contact with an n-type SiNW surface. © 2014 Optical Society of America OCIS codes: (350.6050) Solar energy; (160.5470) Polymers; (230.4000) Microstructure fabrication. http://dx.doi.org/10.1364/OL.39.003219

In recent years, photovoltaics have become a clean and renewable energy source to transfer sunlight into electricity. Crystalline silicon (c-Si) has dominated the photovoltaic markets for years because of its abundant material supply, nontoxicity, high efficiency, and longterm stability [1]. However, the expensive fabrication processes and materials for c-Si solar cells limit the wide application of photovoltaics [2,3]. Hence it is of interest to manufacture solar cells with a simple and low-cost approach. Therefore, an organic/inorganic hybrid solar cell concept has been proposed to take advantage of the solution-based processes for simple and low-cost production and the wide absorption spectrum of silicon [4]. Si nanostructures such as silicon nanowires (SiNWs) in the design of those hybrid solar cells have attracted much attention because of their excellent light-trapping characteristic and carrier transport path [5–10]. Recently, the solution-processed high conducting conjugated polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), has been widely incorporated with the Si nanostructures to fabricate the organic/inorganic hybrid devices [11–14]. PEDOT:PSS is transparent, conductive, and can produce a Schottky junction with Si [15,16]. Illuminative light is absorbed in the n-type Si, and a hole transport layer in PEDOT:PSS can extract a hole generated in the Si substrate out of the device. Thus the efficiency of the hybrid PEDOT:PSS/Si solar cell is comparable with a conventional Si p-n junction solar cell, in principle [14]. Several attempts have been made to improve the performance of PEDOT:PSS/SiNWs hybrid solar cells, such as special junction structures [17,18], tuning density and length of SiNWs [19,20], surface passivation [21,22], and processing the PEDOT:PSS with some additives [23,24], leading to power conversion efficiency (PCE) of this type of hybrid solar cell at greater than 10%. Recently, Yu et al. has reported the PCE of PEDOT: PSS/SiNWs hybrid solar cell reached a record of 13% by using interface engineering techniques to mitigate the 0146-9592/14/113219-04$15.00/0

interface oxidation reaction [25]. Therefore the interface interaction between PEDOT:PSS and SiNWs could have played a critical role in determining the performance of PEDOT:PSS/SiNWs hybrid solar cells. For the fabrication of the hybrid cell active layer, PEDOT:PSS solution is generally dropped on the prepared SiNW arrays and then spin-coated at the high speed to form the active layer of hybrid cells, resulting in poor coverage and contact of PEDOT:PSS with SiNWs due to surface-tension effects at a three-phase (PEDOT:PSS-SiNWs-air) composite interface configured on the top of SiNW arrays. Moiz et al. showed a significant increase in the photovoltaic efficiency by stamping an PEDOT:PSS active thin layer onto the top of SiNWs [18]. In this Letter, we propose a novel method by evacuating the PEDOT:PSS solution with dipdropping on the top of SiNW arrays before spin-coating (solution-evacuated) to enhance organic coverage and contact with SiNW surface. A maximum PCE of 9.22% has been obtained. The n-type Si (100) wafer with a resistivity of 1–5 Ω∕ cm was sequentially cleaned in acetone, ethanol, and deionized water and then dipped into a concentrated H2 SO4 :H2 O2 solution. The SiNW arrays were fabricated by immersing the silicon substrate into an aqueous solution of 4.8 M hydrofluoric acid (HF) and 0.02 M silver nitride (AgNO3 ) at room temperature according to the previous method [26]. The substrate was then dipped into diluted HNO3 and HF to remove residual silver and silicon oxide, respectively. 100 nm thickness aluminum was deposited onto the backside of the Si wafer by thermal evaporation to form the rear contact. High-conductive PEDOT:PSS (CLEVIOS PH1000) (1 wt. % in water) was mixed with 5 wt. % dimethyl sulfoxide (DMSO) as a secondary dopant to increase the conductivity. Meanwhile, 0.2 wt. % Triton X-100 was added as a surfactant to increase the adhesion between PEDOT:PSS and SiNW arrays. The PEDOT:PSS with the additives was then dropped onto the top of SiNW arrays and quickly transferred into the vacuum chamber to evacuate at a © 2014 Optical Society of America

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pressure of 10 Pa (solution-evacuated) for the different times before it was spin-coated at 3000 r∕ min. The substrates with PEDOT:PSS films were then annealed on a hot plate at 120°C for 15 min in ambient. Finally, a 100 nm thickness silver grid frontal electrode was thermally evaporated through a shadow mask. Figure 1(a) shows the architecture of the PEDOT:PSS/SiNWs hybrid solar cells. The scanning electron micrograph (SEM) images of the top view and cross section of the corresponding SiNWs arrays are shown in Fig. 1(b). The essential steps of solution-evacuated have been schematically summarized in Fig. 1(c). Figures 2(a)–2(d) show the SEM images of the SiNWs array coated with PEDOT:PSS by the different solutionevacuated times. It can be seen that the coverage is much improved by solution-evacuated time. When PEDOT:PSS is dropped on the top of SiNWs, a PEDOT:PSS-SiNWs-air three-phase composite interface is configured on the tips of SiNWs due to surface tension effects, and it prevents the infiltration of PEDOT:PSS droplet further down to the bottom of SiNWs. For without a shorter solutionevacuated time, PEDOT:PSS droplet mostly stays on the top of the SiNW arrays and forms a thin and continuous canopy above the SiNWs arrays after spin-coating [Figs. 2(a) and 2(b)]. As the solution-evacuated time increases, the air of PEDOT:PSS-SiNWs-air three-phase composite runs out by bubbles [shown in the insert of Fig. 1(c)] and the pressure of the corresponding threephase composite reduces. Therefore PEDOT:PSS droplet easily infiltrates down to the bottom of SiNWs. This leads to better coverage and contact of PEDOT:PSS with the SiNW sides [Figs. 2(c) and 2(d)] after spin-coating. We further measure the Si and carbon (C) atomic concentrations variation of the corresponding cross-section top and bottom area by energy-dispersive spectrometer (EDS), as shown in Fig. 3. As the solution-evacuated time increases, Si atomic concentration of the top area of PEDOT:PSS/SiNW profile increases from 42.4% to 55.5% while that of the bottom area reduces from 73.8% to 62.4%. On the other hand, C atomic concentration of the top area of PEDOT:PSS/SiNWs profile decreases from

Fig. 1. (a) Schematic structure of the hybrid solar cells. (b) SEM images of top view and cross-section of SiNW arrays. (c) Schematic of the solution-evacuated process for the hybrid active layer.

Fig. 2. SEM cross-section of the hybrid active layers with the different solution-evacuated times. A corresponds to top area of the hybrid active layer profile while B corresponds to bottom area of the hybrid active layer profile.

53.7% to 41.6% while that of the bottom area increases from 25.1% to 36.1%. Si atomic is mainly from the SiNWs arrays and C atomic comes from PEDOT:PSS. This variation trend of Si and C atomic concentration of the top and bottom area is consistent with enhancing PEDOT: PSS infiltration down to the bottom of SiNWs arrays as solution-evacuated time increases. Figure 4 displays the current density and voltage (J-V) characteristics of the PEDOT:PSS/SiNW hybrid solar cells with the different solution-evacuated times under AM 1.5G and light intensity of 100 mW∕cm2 . The opencircuit voltage (Voc ), short-circuit current density (Jsc ), fill-factor (FF), and PCE of the fabricated devices are summarized in Table 1. It can be seen in Table 1 that Voc , Jsc , FF, and PCE of the hybrid cells are correlated with the solution-evacuated times. At without or shorter solution-evacuated times, PEDOT:PSS is mostly coated on the top of SiNW arrays, and a large portion of air void is created between the PEDOT:PSS and SiNWs arrays, resulting in a poor organic coverage and contact of the hybrid active layer. A longer solution-evacuated time leads to a better organic coverage on the SiNW surface,

Fig. 3. Variation of the relative profile atomic concentrations that were averaged from three-spots test results for the crosssection area of the corresponding Fig. 2.

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Fig. 4. Current density and voltage (J-V) characteristics of the hybrid solar cells by the different solution-evacuated times.

Fig. 5. Reflectance and external quantum efficiency spectra of the hybrid solar cells by the different solution-evacuated times.

which facilitate the carrier separation and transport through the PEDOT:PSS layer. It is observed that as the solution-evacuated time increases from 0 to 2 min, the value of PCE improved obviously from 5.17% to 9.22%, and the FF of the hybrid solar cell followed the trend as the solution-evacuated time grew. However, the device performance begins to drop at the longer solutionevacuated time (3 min). The longer solution-evacuated time makes excess PEDOT:PSS fill into SiNW arrays; this could increase series resistance (Rs ) associated with a thicker PEDOT:PSS layer due to its poor conductivity, resulting in a poorer carrier transportation from the PEDOT:PSS layer to the adjacent nearest silver finger electrode. As the solution-evacuated time increases from 0 to 2 min, the Rs of the device decreases to a minimum value of 3.87 Ω, and then it rises to 10.92 Ω as the process time increases to 3 min, as listed in Table 1. The reflectance spectra of the SiNWs coated with PEDOT:PSS for different solution-evacuated times is measured using an integrating sphere by an Ocean Optics Maya 2000 spectrophotometer system. The results are shown in Fig. 5, while the external quantum efficiency (EQE) spectra of the corresponding hybrid sells are also shown in Fig. 5. As the solution-evacuated time increases from 0 to 2 min, the reflectance of the sample reduces. This decreased reflectance of the sample with the process time of 2 min accounts for its higher EQE (Fig. 5) and thus higher Jsc (Table 1). When the process time increases from 2 to 3 min, the reflectance of the samples continue to decrease, but the corresponding EQE becomes lower, which indicates a higher recombination rate associated with samples with a longer process time. It has been observed that the strong oxidation reaction occurs between PEDOT:PSS and silicon interface, and the oxide layer

impedes charge transport [25]. Figures 2(a)–2(d) show more PEDOT:PSS infiltrating down to the bottom of SiNWs and enhancing the coverage of PEDOT:PSS on the SiNW array surface as increasing process time. Meanwhile, the oxidation rate and defect density at PEDOT: PSS/SiNWs interface will increase for longer sample process time, leading to increased surface recombination [14,15]. Therefore the optimum solution-evacuated time should be balanced between the organic coverage and the oxidation rate. This is consistent with the Voc results, where a significant drop is observed for the device with a process time of 3 min. The degradation of Voc should be attributed to the higher recombination rate for longer solution-evacuated time. In addition, the different PEDOT: PSS concentration has an important effect on organic/ inorganic coverage and contact between PEDOT:PSS and SiNW arrays for PEDOT:PSS-SiNWs-air three-phase composite under solution-evacuated process. The relative active layer fabrications by different PEDOT:PSS concentration solution-evacuated are underway to optimize the performances of the hybrid solar cells. In conclusion, we have demonstrated the fabrication of a hybrid solar cell by evacuating the PEDOT:PSS dropped on the top of SiNW arrays before spin-coating for efficient photovoltaic application. A maximum PCE of 9.22% has been obtained by a solution-evacuated time of 2 min. The improvement performance of the hybrid cells is attributed to better organic coverage and contact between PEDOT:PSS and SiNW arrays. The device performance can be further improved by interface engineering techniques to block the oxidation reaction occurring between PEDOT:PSS and silicon. Most importantly, the fabrication process demonstrated here might provide a promising route to realize low-cost and highly efficiency polymer-nanowire hybrid solar cell for future applications.

Table 1. Summary of Photovoltaic Characteristics of the Hybrid Solar Cells by the Different Solution-Evacuated Times Solution-Evacuated Time (min)

Voc (V)

Jsc mA∕cm2 

FF (%)

PCE (%)

Rs (Ω)

0 1 2 3

0.44 0.48 0.48 0.37

31.38 32.43 32.99 21.82

37.4 48.8 58.2 33.8

5.17 7.60 9.22 2.73

7.96 5.01 3.87 10.92

This work was supported by the Nature Science Foundation of China (U1174001, 61377065), the Natural Science Foundation of Guangdong Province, China (S2011010003400), the Science and Technology Project of Guangzhou City (2012J2200023), the China Postdoctoral Science Foundation Funded Project (2012M511584), and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology).

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silicon nanowire hybrid solar cells by solution-evacuated method.

A method has been developed to fabricate organic-inorganic hybrid heterojunction solar cells based on n-type silicon nanowire (SiNW) and poly (3,4-eth...
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