The a-Si:H solar cells having high open-circuit voltage (Voc>950 mV) are highly desirable for the top junction in thin-film silicon based tandem and triple-junction solar cells. Except for the high Voc, the top cell should also have high spectral response (between 350-600 nm wavelength range) to allow thinner absorber layer in order to reduce the light-induced degradation. Therefore, doped layers with high transparency are required. Commonly, the a-Si:H solar cells deploy a-SiC:H as p-layer and nc-Si:H or a-Si:H as n-layer. Those doped layers will inevitably lead to high parasitic absorption losses, and thus make it difficult to achieve sufficient photocurrent with a thin absorber layer.

In this contribution, we will first discuss how to achieve high Voc by processing a-Si:H at high-pressure and high-power regime. Then deployment of highly transparent silicon oxide (SiOx) doped layers will be discussed to obtain better spectral response (or external quantum efficiency) and higher Voc than the conventional doped layers. Specifically, following key points will be presented in the conference.

1. The a-Si:H is deposited at high-pressure (>5 mbar ) and high-power (>0.1 W/cm2) regime, which results in larger bandgap than materials commonly processed at low-pressure and low-power regime. The bandgap of a-Si:H can be tuned by H2/SiH4 dilution, power and pressure. High performance device-grade a-Si:H can be obtained over wide deposition window, in contrast to the narrow window at low-pressure regime.

2. Highly transparent p-SiOx:H with sufficient conduction is investigated. Firsly, it should have good ohmic contact with front ZnO TCO. Second, the control of crystallinity of p-SiOx:H is a critical point to obtain high Voc for a particular absorber layer. Finally, a very thin layer of i-SiOx:H inserted in the p/i interface can significantly reduce the boron diffusion during the deposition of i-layer, and thus considerably improve the blue spectral response. Consequently, EQE higher than 70% at λ=400 nm is achievable.

3. Low absorption n-SiOx:H to replace absorptive n-aSi:H is necessary to achieve high spectral response over 500-700 nm wavelength range. Furthermore, the low refractive-index n-SiOx:H layer can also function as intermediate reflection layer in multijunction devices. Control of the i/n interface is crucial to achieve high FF. The insertion of an ultra-thin (<3 nm) n-aSi:H or n-aSiOx layer can significantly increase the FF of solar cells and result in FF comparable to cells with a-Si:H n-layer, without reduction of spectral response compared to single n-SiOx layer. After optimization of the p-SiOx:H and n-SiOx:H doped layers, a-Si:H solar cells with high Voc, high FF and excellent spectral response is obtained (Voc>960 mV, FF>74%, and efficiency>10%). The light-induced degradation of solar cells with SiOx:H doped layers are investigated, and will be compared to the solar cells with conventional doped layers.

Hairen Tan

Delft University of Technology

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