Hybrid Si nanocones/PEDOT:PSS solar cell
© Wang et al.; licensee Springer. 2015
Received: 29 January 2015
Accepted: 7 April 2015
Published: 21 April 2015
Periodic silicon nanocones (SiNCs) with different periodicities are fabricated by dry etching of a Si substrate patterned using monolayer polystyrene (PS) nanospheres as a mask. Hybrid Si/PEDOT:PSS solar cells based on the SiNCs are then fabricated and characterized in terms of their optical, electrical, and photovoltaic properties. The optical properties of the SiNCs are also investigated using theoretical simulation based on the finite element method. The SiNCs reveal excellent light trapping ability as compared to a planar Si substrate. It is found that the power conversion efficiency (PCE) of the hybrid cells decreases with increasing periodicity of the SiNCs. The highest PCE of 7.1% is achieved for the SiNC hybrid cell with a 400-nm periodicity, due to the strong light trapping near the peak of the solar spectrum and better current collection efficiency.
PACS: 81.07.-b; 81.16.-c; 88.40.hj
Despite the significant progress achieved for silicon (Si) solar cell in the past several decades, its wide application is still impeded by its high cost. Since the cost of the Si material constitutes about 50% of the total cost of the solar cell, it is imperative that thin Si layer instead of bulk Si be adopted in solar cell to reduce the usage of Si material and lower the cost . However, reducing the thickness of the Si layer requires improved light absorption to ensure that the amount of sunlight absorbed is not compromised. In recent years, this has been achieved by incorporating Si solar cells with nanostructures such as nanowires [2-4], nanoholes , and nanocones [1,6] to trap sunlight effectively and result in improved light absorption. Besides the issue of material cost, simplification of the fabrication processes of Si solar cell presents another possible way to address the cost concern. In this aspect, the hybrid Si/organic solar cells that have emerged in recent years provide a potential low-cost alternative to the conventional Si solar cell [7-10]. Unlike the conventional Si solar cell that requires an expensive and high-temperature process to form the p-n junction, the junctions in the hybrid cells are formed by a low-temperature solution-based process that leverages on the advantages of organic materials, which thus greatly simplifies the fabrication process. The convergence of the above two ideas on cost reduction suggests hybrid Si/organic solar cells based on thin-film Si incorporated with nanostructure as a promising approach to lower the cost of solar cells without compromising on their efficiency .
Currently, Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the type of hybrid cell that is most commonly investigated. In the past few years, Si/PEDOT:PSS hybrid cells incorporated with Si nanostructures, in particular Si nanowires (SiNWs), have been extensively studied, and their power conversion efficiencies (PCE) have greatly improved to 13% [11,12]. To date, most of the hybrid SiNW solar cells are typically based on SiNWs fabricated using the metal-catalyzed electroless etching (MCEE) technique [4,13]. Such SiNWs are not periodic, and thus it is difficult to study and understand the detailed effects of the nanostructural parameters on the performance of the cells. Besides, there is limited control on the dimensions of the SiNWs, such as diameter and periodicity, to allow optimization of the cell performance. In addition, longer SiNWs fabricated from the solution-based MCEE technique suffers from agglomeration due to the effect of surface tension . To better understand the effects of the structural parameters on the cell performance and to optimize the cell efficiency, it is imperative that hybrid solar cells based on periodic nanostructures with controllable dimensions be studied. Many top-down approaches have been developed to fabricate periodic nanostructures, which include deep-UV lithography , electronic beam lithography, focused ion beam (FIB) writing, and nanoimprinting [16,17]. Bottom-up approaches such as the vapor-liquid-solid (VLS) growth of periodic nanostructures have also been extensively studied . However, these techniques either require the use of expensive equipment or involve complicated and time-consuming processes. Moreover, methods such as the e-beam lithography, FIB, and VLS growth also suffer from low throughput. Compared to these methods, nanosphere lithography (NSL) is a promising low-cost approach that is capable of fabricating periodic and homogenous nanostructures of various sizes at the wafer scale .
In this study, we demonstrate hybrid Si/PEDOT:PSS solar cells based on periodic Si nanocones (SiNCs) with different periodicities that range from 400 to 800 nm. The SiNCs are fabricated by dry etching of a Si substrate using assembled monolayer polystyrene (PS) nanospheres as a mask. Compared with SiNWs, SiNCs are mechanically more robust due to the larger base. Besides, their structure presents a more gradual change in the effective refractive index and thus is expected to possess better antireflective property . The hybrid SiNC/PEDOT:PSS solar cell also exhibits improved optical properties and short-circuit current density (J sc) as compared to planar Si hybrid cells . Besides experimental investigation of the SiNC hybrid solar cells, their optical properties are also studied theoretically using a simulation based on the finite element method. We found that the SiNC/PEDOT:PSS solar cell with a periodicity of 400 nm presents the highest PCE of 7.1% due to its strong light trapping around the peak of the solar spectrum and better current collection efficiency.
Results and discussion
Photovoltaic parameters of the hybrid cells based on planar Si and SiNCs with different periodicities
J sc (mA/cm 2 )
V oc (V)
P 400 nm
P 500 nm
P 600 nm
P 800 nm
In terms of the photovoltaic parameters, the SiNC cells of smaller periodicities exhibit a better J sc than the planar cell. For example, the one with P = 400 nm has a J sc of 29.1 mA/cm2 in comparison to that of a planar cell of 26.1 mA/cm2. On the other hand, the V oc of the SiNC hybrid cells is only about 0.42 V, which is much lower than the V oc of 0.59 V observed for the planar cell . This is attributed to the high recombination rate of the SiNC hybrid cells as compared to that of the planar cell. The Cl2 plasma etching and the ion bombardment will introduce surface defects at the SiNCs that act as recombination sites and promote carrier recombination. This and the larger surface area of the SiNCs are responsible for the increased carrier recombination. The degradation in V oc observed for the SiNC hybrid cells is also corroborated by the increased reverse saturation current density (J 0) of the dark J-V curve shown in Figure 3d, which is up to 1 order of magnitude higher than that of the planar cell. This issue can be addressed by proper surface treatment and passivation to minimize the defects [21-23]. Figure 3c shows the external quantum efficiency (EQE) of the hybrid solar cells where it is seen that the cell with P = 400 nm exhibits the strongest spectral response, particularly at a shorter wavelength range. Overall, the EQE decreases slightly with increasing periodicity, which is consistent with the J sc trend as observed in Table 1.
Figure 4b,c depicts the simulated light absorption in the Si and PEDOT:PSS materials, respectively, whereas Figure 4d,e shows the simulated overall reflectance and transmittance spectra of the hybrid structures, respectively. At the smallest P of 200 nm, the light absorption is enhanced at a shorter wavelength with λ < 550 nm, attributed to the strong scattering resulting from the comparable size of the structural dimension and the light wavelength . As P is increased to 400 nm, the light absorption peak shifts to 550 nm and there is also a significant increase in the light absorption for long wavelength light (λ > 600 nm) compared to the P = 200 nm structure. This is due to the increase in the structural dimension which correspondingly shifts the strong scattering to longer wavelengths. When P is 600 nm, though there is a slight drop in the light absorption for a shorter wavelength range below 600 nm, the light absorption is significantly enhanced for λ > 600 nm as compared to the structures with smaller P. The strong scattering in the longer wavelength range increases the optical path length and results in a low transmittance, as can be seen in Figure 4e. This indicates that the light is strongly absorbed before it could penetrate through the structure. As P is further increased to 800 nm, the structure exhibits a slightly higher light absorption at a longer wavelength as compared to the P = 600 nm structure, but the absorption is generally lower for the λ < 800 nm range, resulting in a degradation of its overall performance. It is noted that as P increases, the light absorption in the PEDOT:PSS layer is strong for the structures with P = 600 and 800 nm, as there is a strong scattering at the longer wavelength range of λ > 800 nm where the absorption coefficients of PEDOT:PSS are higher.
where I(E) is the spectral solar intensity corresponding to the air mass 1.5 direct normal and circumsolar spectrum , E is the photon energy, Eg is the Si band gap energy of 1.1 eV, and α(E) is the absorption spectrum of the hybrid structure. This formula assumes that each photon with energy greater than the band gap will be absorbed in the solar cell to generate one electron-hole pair, which is then extracted as an electric current without loss . From Figure 4f, it is seen that the SiNC/PEDOT:PSS hybrid structure reveals the highest ultimate efficiency of 33.7% at P = 600 nm, which is attributed to the enhancement in light absorption over a broad wavelength range that includes the peak of the solar spectrum.
It is noted that the simulated result is not exactly consistent with the experimental result, as the highest efficiency for the latter is found to occur for the structure with a smaller P of 400 nm. The deviation can be attributed to the fact that our simulation was performed for a Si thin film of a 3-μm thickness, whereas the actual devices were fabricated on thick Si substrates of 575 μm. Note that due to computing resource constraints, it is challenging to simulate structures with thick Si substrates. Due to the different Si absorbing layer thicknesses, the overall absorption characteristics and hence the optimum structural parameter will differ. Indeed as can be seen from Figure 4e, the structures with smaller P of 200 and 400 nm have high transmittance at a longer wavelength of λ > 600 nm, attributed to the lower absorption coefficients at these wavelengths and the thin Si absorbing layer. However, this is not the case when thick Si substrates are used as the light will be fully absorbed. Therefore, the maximum absorption is expected to occur at larger P for the simulated Si thin film structure. Note that for low-cost practical application, the hybrid cells should be fabricated using Si thin films instead of bulk Si wafers. Thus, our simulation results are relevant and reveal how the structures should be optimized when Si thin films are employed. Another factor that accounts for the variation between the experimental and simulated results is that the fabricated structures are not perfectly periodic as noted from Figure 2f, in contrast to the simulated structures.
In summary, periodic SiNCs have been fabricated by dry etching of the Si substrate using patterned PS nanospheres as a mask. Hybrid SiNC/PEDOT:PSS solar cells with different SiNC periodicities have been fabricated and characterized. Excellent light trapping ability of the SiNCs has been demonstrated by both the experimental results and theoretical simulation. The highest PCE of 7.1% has been achieved for the hybrid cell with a periodicity of 400 nm, due to the strong light trapping near the peak of the solar spectrum and good current collected efficiency. With proper surface treatment and passivation to minimize the defects on the SiNC surface, it is expected that the performance of the cells can be further improved. The fabrication process employed in this work can also be readily applied to a Si thin film to realize low-cost and highly efficient hybrid Si/PEDOT:PSS thin film cells based on SiNCs.
We acknowledge the funding support from the Singapore Ministry of Education Academic Research Fund Tier 2, Grant No: MOE2012-T2-1-104.
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