In situ silicon oxide based intermediate reflector for thin-film silicon micromorph solar cells

Abstract

We show that SiO-based intermediate reflectors ͑SOIRs͒ can be fabricated in the same reactor and with the same process gases as used for thin-film silicon solar cells. By varying input gas ratios, SOIR layers with a wide range of optical and electrical properties are obtained. The influence of the SOIR thickness in the micromorph cell is studied and current gain and losses are discussed. Initial micromorph cell efficiency of 12.2% ͑V oc = 1.40 V, fill factor= 71.9%, and J sc = 12.1 mA/ cm 2 ͒ is achieved with top cell, SOIR, and bottom cell thicknesses of 270, 95, and 1800 nm, respectively.A micromorph tandem solar cell consists of a high-gap amorphous ͑a-Si: H͒ top cell and a low-gap microcrystalline silicon ͑ c-Si: H͒ bottom cell stacked on top of each other. The thickness of the a-Si: H cell has to be kept reasonably thin to minimize the impact of light-induced degradation, 1 and its current, therefore, generally limits the current of the tandem device. To overcome this issue, an intermediate reflecting layer ͑IRL͒ can be introduced between the two cells to increase the current of the top cell. 2 For an intermediate layer to act as a reflector, its refractive index n must be lower ͑typically n IRL Ϸ 2͒ than that of silicon ͑n Si = 3.8 at 600 nm͒ such as to produce a refractive index step that causes the reflection of light at the material interface. The layer which serves as IRL is required to be sufficiently conductive to avoid blocking current and as transparent as possible to minimize the current losses due to absorption of light outside the active layers. In the first attempts to realize this intermediate reflector, zinc oxide ͑ZnO͒ has been used. [2][3][4] In a recent study, the top-cell current could be increased by 2.8 mA/ cm 2 , using a 110-nm-thick ZnO IRL with a 180-nm-thick top cell. 3 When considering industrialization, there are, however, two main drawbacks of ZnO-based IRL: the need for an additional ex situ deposition step and an additional laser scribe for monolithic series interconnection to avoid lateral shunting of solar module segments. 5 Another group reported in situ fabrication of a different IRL but without specifying the material used. 6 In this paper, we propose the preparation of an IRL based on a "doped silicon oxide" material fabricated by plasma enhanced chemical vapor deposition ͑PECVD͒ in the same reactor as the solar cells. We demonstrate that it is possible to produce such a SiObased intermediate reflector ͑SOIR͒, with a refractive index close to 2 and electrical properties suitable for incorporation into micromorph devices.The SiO-based layers are deposited by very-high frequency PECVD at 110 MHz, 200°C, and with a power density of 0.01-0.1 W / cm 2 . Optical and electrical characterizations are performed on ϳ100-nm-thick layers deposited on glass. Optical reflectance and transmittance are measured with a Perkin-Elmer photospectrometer, type lambda 900, within a spectral range from 320 to 2000 nm. The refractive index n and the absorption coefficient ␣ are estimated by fitti...

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