Thin Film Silicon Nanowire/PEDOT:PSS Hybrid Solar Cells with Surface Treatment
© The Author(s). 2016
Received: 16 March 2016
Accepted: 24 June 2016
Published: 29 June 2016
SiNW/PEDOT:PSS hybrid solar cells are fabricated on 10.6-μm-thick crystalline Si thin films. Cells with Si nanowires (SiNWs) of different lengths fabricated using the metal-catalyzed electroless etching (MCEE) technique have been investigated. A surface treatment process using oxygen plasma has been applied to improve the surface quality of the SiNWs, and the optimized cell with 0.7-μm-long SiNWs achieved a power conversion efficiency (PCE) of 7.83 %. The surface treatment process is found to remove surface defects and passivate the SiNWs and substantially improve the average open circuit voltage from 0.461 to 0.562 V for the optimized cell. The light harvesting capability of the SiNWs has also been investigated theoretically using optical simulation. It is found that the inherent randomness of the MCEE SiNWs, in terms of their diameter and spacing, accounts for the excellent light harvesting capability. In comparison, periodic SiNWs of comparable dimensions have been shown to exhibit much poorer trapping and absorption of light.
In recent years, extensive research has been devoted towards rendering solar energy more cost competitive to be a viable energy source. For example, Si nanostructures, such as Si nanowires (SiNWs) have been incorporated into solar cells for light trapping, so that thinner Si absorber layer can be used to lower the material cost [1–3]. SiNWs fabricated by the low cost solution-based metal-catalyzed electroless etching (MCEE) technique have also been combined with organic semiconductors to form hybrid solar cells [4, 5]. Such cells present a very cost-effective option due to their simple structure, coupled with the solution-based, low temperature and large area fabrication process. Currently, a promising power conversion efficiency (PCE) of 13.01 % has been reported for hybrid solar cells based on SiNWs and poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) . A high PCE of 17.4 %  has also been reported for bulk Si/PEDOT:PSS cell based on a backPEDOT cell structure, where light is incident on Si instead of PEDOT:PSS to reduce parasitic absorption loss in the PEDOT:PSS. Despite the outstanding efficiency achieved, it should be noted that the hybrid cells demonstrated using expensive bulk Si wafer are not a practical option for low cost applications. It is imperative that lower cost Si thin films should be explored instead. We have previously reported SiNW/PEDOT:PSS hybrid solar cells based on a 2.2-μm Si thin film that achieved a PCE of 5.6 % . The performance of the SiNW-based cells was noted to be only marginally improved compared to their planar counterpart with the same thickness of Si thin film. This has been attributed to the high recombination rate associated with the defective surface of the SiNWs prepared by the MCEE technique, as well as the lower shunt resistance of the SiNW-based cells [9, 10]. To further improve the performance of thin film SiNW/PEDOT:PSS solar cells, in this work, we fabricate such cells using a thicker Si thin film of 10.6 μm. To address the high surface recombination associated with the SiNWs, we apply a recently developed two-step surface treatment process to improve the surface quality of the SiNWs . We achieve a PCE of 7.83 % for the treated SiNW/PEDOT:PSS cells with an optimized SiNW length of 0.7 μm. The average V oc of the SiNW/PEDOT:PSS cells is improved from 0.461 to 0.562 V as compared to the untreated counterparts. The results in this study demonstrate the potential of thin film Si/PEDOT:PSS hybrid cells incorporated with SiNWs for light trapping, and the importance of surface treatment to fully realize the advantages brought about with the use of SiNWs.
The MCEE approach can readily produce SiNWs with 20–300 nm diameter by simply immersing the Si films into an aqueous HF solution which contains metal catalyst such as metal particles (silver or gold) or metal ions, and oxidizing agents such as Fe(NO3) or H2O2 . HF-AgNO3 solution has been widely used for the MCEE SiNW fabrication since 2002, as reported by Peng et al. . The MCEE SiNWs offer excellent light trapping ability as evidenced from their generally low reflectance in the visible light range. The light reflectance can be as low as 1.4 % over the wavelength (λ) range of 300–600 nm for SiNWs fabricated on bulk crystalline Si wafers . SiNW array films fabricated on glass substrates by the MCEE technique have also shown low reflectance of less than 10 % from 300 nm < λ < 800 nm and strong broadband optical absorption of more than 90 % [14, 15]. AgNO3 prepared MCEE SiNW hybrid cells exhibited low reflectance of <5 % over a broad range 300 nm < λ < 1050 nm for nanowire length greater than 1 μm . The results are interesting given that the dimensions of the MCEE SiNWs commonly etched using AgNO3/HF solution, with diameters from 30–150 nm and spacing of 20–80 nm , are not in the optimized range for effective scattering of the main solar spectrum. This leads to the question of the origin of their strong light trapping properties. Recently, interest in the effect of disorders on the optical performance of Si nanostructures has grown [17–20]. Some theoretical studies have been done on the SiNW arrays without any underlying Si thin film using the finite-difference time-domain (FDTD) method [17, 18] and transfer matrix method (TMM) . In these studies, individual structural parameters of the SiNW arrays such as diameter , position [17–19], and length  have been varied one at a time to study their effects on the optical properties of the SiNW arrays. The results have shown that the disorders in the SiNW arrays resulted in improved light absorption as compared to the periodic structure with comparable dimensions, attributed to the presence of additional resonance modes and broadening of the existing modes [17–19]. In this work, we have carried out optical simulations based on a hybrid structure of random SiNW arrays on an underlying Si thin film, and with PEDOT:PSS on top using the finite element method (FEM) . We investigate the effects of the randomness of the MCEE SiNW arrays, in terms of their diameter and spacing, on the optical properties of the hybrid SiNW/PEDOT:PSS solar cells. Instead of varying the parameters randomly one at a time over a pre-defined range, we allow both the SiNW diameter and spacing to vary concurrently, so that the simulated structure is closer to what is observed experimentally. It is found that this has resulted in enhanced scattering and absorption, as compared to the case where the parameters are varied randomly one at a time. Overall, the simulation studies reveal that the inherent randomness of the SiNWs results in a substantial decrease in the reflectance and transmittance of light. Consequently, there is a significant increase in the absorption of light as compared to periodic SiNWs of comparable dimension, but with uniform diameter and spacing. In the presence of SiNW randomness, the ultimate efficiency is boosted from 16.9 to 27.2 % for a simulated hybrid cell based on a 2.2-μm-thick Si absorber, which represents a remarkable 60.6 % improvement.
SiNWs were fabricated on the epitaxial Si films by the MCEE technique in a solution consisted of 4.6 M HF and 0.02 M silver nitrate (AgNO3) . SiNWs with different lengths of L = 0.4, 0.7, 0.95, 1.5, and 2.7 μm were fabricated by adjusting the etch time. Following that the top surface of the SiNWs was spin coated with highly conductive PEDOT:PSS (PH1000) mixed with 5 wt% dimethyl sulfoxide (DMSO) at 2800 r/min, and then annealed at 105 °C for 10 min in atmosphere. The cells were completed by depositing electrodes that comprised a layer of Ag grid on the PEDOT:PSS layer and Ti/Pd/Ag on the backside of the Si substrate using e-beam evaporation. Each cell has a size of 0.95 cm2 and a 12 % incident light power loss due to the Ag grid shadowing. For the two-step surface treatment process presented in the dotted box in Fig. 1, instead of using ozone , we have tried a different approach of oxidizing the surface of SiNWs using oxygen plasma. The SiNWs were first treated in a RF 13.56 MHz inductively coupled oxygen plasma for 480 s to form a layer of sacrificial oxide of ~4–5 nm. The plasma was generated with an O2 gas flow of 30 sccm, RF power of 30 W and pressure of 200 mTorr. The SiNWs were then etched in 5 % dilute hydrofluoric (HF) acid for 85 s to partially remove the oxide layer, together with the embedded Ag nanoparticles, leaving behind a thin layer of residual SiOx of ~1 to 2 nm for surface passivation . Our results reveal that the treatment process using oxygen plasma is as effective as the one using ozone which we have reported previously .
Results and Discussion
As seen in Fig. 8, the performance of the cells is poor at longer SiNW length, even for the treated cells. Due to the bundling, only SiNWs that are situated at the outer perimeter of the bundles are treated by the oxygen plasma, while those that are within the bundles are not well exposed to the plasma. Thus the surface treatment has limited effect on performance improvement for the very long SiNW cells, as compared to those with shorter SiNWs. Therefore, there is increased carrier recombination loss associated with the defective SiNW surface, which is compounded by the larger surface area of the longer SiNWs.
According to the simulation results, randomness in the SiNW structure is beneficial and it accounts for the excellent light harvesting ability of the MCEE SiNWs, in spite that their structural dimensions are much smaller than the wavelengths of light in the main solar spectrum. It is noted that the simulated reflectance spectrum of the R-SiNW structure is not as low compared to the experimental results shown in Fig. 9b, especially in a longer wavelength range. The deviation can be attributed to the fact that our simulation does not reflect a truly random structure but rather a quasi-random one. Besides, there are other factors not taken into account in our simulation, which include variation in the SiNW length, non-vertical alignment of the SiNWs, the rough surface and the imperfect cylinder shape of the SiNWs, and the rough coverage of the PEDOT:PSS layer on SiNWs. In addition, due to the computing resource constraints, our simulation was performed for a thinner Si film of 2.2 μm instead of the thicker 10.6 μm Si film that was used in the experimental structure. Nevertheless, the simulation results clearly reveal the effect of the randomness of the MCEE SiNWs on enhancing the scattering and absorption of light in SiNW/PEDOT:PSS hybrid cells.
Our experimental cell structure is not optimized due to the absence of a back reflector. There is also parasitic optical loss in the PEDOT:PSS as light is incident on the top PEDOT:PSS layer. If these issues are addressed, it is expected that the performance of the thin film cells will be substantially improved. Indeed, recently, a high PCE of 10.3 % has been reported for planar multicrystalline Si/PEDOT:PSS solar cell based on the backPEDOT structure, using a Si absorber thickness of 5 μm passivated with Al2O3 . The performance of such thin film-based Si/PEDOT:PSS cells can be further improved by enhancing optical absorption in the long wavelength range, through having a good light trapping scheme such as the one demonstrated in this work based on SiNWs with the surface treatment. The promising performance, coupled with the simple cell structure fabricated using low cost and low temperature process, will render such thin film Si/PEDOT:PSS hybrid structure attractive for low cost and high efficiency solar cells applications.
We have demonstrated thin film SiNW/PEDOT:PSS solar cells using a 10.6 μm Si absorber. High efficiency of 7.83 % has been achieved for 0.7-μm-long SiNW cells with surface treatment. The light harvesting ability of the MECC SiNWs is studied both experimentally and theoretically. The inherent randomness of the low cost MCEE SiNWs is found to be beneficial for light trapping and absorption in the hybrid solar cell. The promising results obtained demonstrate the potential of realizing efficient Si/PEDOT:PSS hybrid solar cells using thin film Si.
We acknowledge the support from the Singapore Ministry of Education Academic Research Fund Tier 2, Grant No: MOE2012-T2-1-104.
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