Interface modification effect between p-type a-SiC:H and ZnO:Al in p-i-n amorphous silicon solar cells
© Baek et al; licensee Springer. 2012
Received: 9 September 2011
Accepted: 18 January 2012
Published: 18 January 2012
Aluminum-doped zinc oxide (ZnO:Al) [AZO] is a good candidate to be used as a transparent conducting oxide [TCO]. For solar cells having a hydrogenated amorphous silicon carbide [a-SiC:H] or hydrogenated amorphous silicon [a-Si:H] window layer, the use of the AZO as TCO results in a deterioration of fill factor [FF], so fluorine-doped tin oxide (Sn02:F) [FTO] is usually preferred as a TCO. In this study, interface engineering is carried out at the AZO and p-type a-SiC:H interface to obtain a better solar cell performance without loss in the FF. The abrupt potential barrier at the interface of AZO and p-type a-SiC:H is made gradual by inserting a buffer layer. A few-nanometer-thick nanocrystalline silicon buffer layer between the AZO and a-SiC:H enhances the FF from 67% to 73% and the efficiency from 7.30% to 8.18%. Further improvements in the solar cell performance are expected through optimization of cell structures and doping levels.
TCO plays an important role in a silicon-based thin-film solar cell because of good electrical conductivity as well as optical properties such as transparency and haze ratio. Additionally, the interface properties with an adjacent p-layer are also important. The FTO has been widely used as a front TCO in a single p-i-n a-Si:H solar cell. The FTO, however, has a low haze ratio in a wavelength longer than 700 nm, which is a limiting factor for tandem solar cells that incorporates a low-bandgap bottom cell. The AZO has several advantages such as good electrical conductivity, plasma robustness, light-scattering properties [1–3], etc. One drawback of AZO is that when it is used in amorphous silicon [a-Si] solar cells, having a-SiC:H or a-Si:H window layer, the FF deteriorates [4–7]. It is assumed that the work function of AZO is lower than that of FTO , causing an increased barrier potential to occur at the front interface obstructing the carrier movements and thus, lowering the FF. In this paper, we report some attempts in interface engineering to lower the effect of the barrier between the AZO and the p-type a-SiC:H in order to enhance the FF and efficiency.
Interface modification of AZO/p-type a-SiC:H is carried out by inserting a highly conductive material between the AZO and p-type a-SiC:H as a buffer layer. It is first done by profiling the flow rate of B2H6 during the deposition of p-type a-SiC:H. Initially, the flow rate of B2H6 is kept high to minimize the depletion in the p-layer, and then, the flow rate of B2H6 is decreased to reduce the light absorption. After the B2H6 profiling, the cell results in a AZO/p+ a-SiC:H/p a-SiC:H/i a-Si:H/n a-Si:H structure. In another set, a nanocrystalline silicon layer is used as the buffer layer. A few-nanometer-thick nc-Si:H is deposited on AZO before the cell fabrication. All investigated cells have a 500-nm Ag back contact that defines a 0.25-cm2 cell area. The illuminated I-V properties of solar cells are measured by an AM 1.5 G double-beam solar simulator (Wacom, Co., Ltd, Kazo-shi, Saitama, Japan), and series resistance is obtained from the I-V curves.
Results and discussion
Solar cell parameters for cells with and without the interface engineering
There are a number of factors that limit the solar cell performance, the potential barrier at the front interface being one of them. Since the material properties of nanocrystalline silicon differ from the amorphous silicon, its electrical characteristics such as electron affinity, mobility gap, electrical conductivities, etc. also differ, and some are favorable for the solar cells. That is why the solar cell performance with the nc-Si as a buffer layer is much better. In this paper, improvements of a-Si p-i-n solar cell with AZO as TCO have been obtained only by engineering the interface between the p-type a-SiC:H and AZO and shows that a large increase in the solar cell efficiency can be obtained by using nc-Si as a buffer layer. In this study, no other optimizations such as optimizing the thickness of each layer, etc. have been carried out.
In order to improve the FF of a-Si p-i-n solar cells with AZO as TCO, interface engineering has been carried out by inserting a buffer layer between the AZO and p-type a-SiC:H. It is shown that by engineering the interface between the two materials, the effect of potential barrier at the interface can be reduced, resulting in an increase in the FF and cell efficiency. The best cell performance obtained in this study is by using nc-Si as a buffer layer since it provides better optical transmission and electrical conductivity suitable for the solar cell performance. The cell efficiency improves from 7.30% to 8.18% by using a few-nanometer-thick nc-Si layer. Further improvements in the solar cell performance are possible through optimization of thickness of each layer, doping concentration, etc.
This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0020210).
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