Abstract
The aim of the present study is the development of doped microcrystalline silicon oxide (µc‑SiOx:H) alloys and its application in thin‑film silicon solar cells. The doped µc‑SiOx:H material was prepared from carbon dioxide (CO2), silane (SiH4), hydrogen (H2) gas mixtures using plasma enhanced chemical vapour deposition (PECVD) with process conditions
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which are fully compatible with the overall solar cell manufacturing processes. Doping was achieved by adding phosphine (PH3) or trimethyl boron B(CH3)3 to the process gas. Particular focus in the material development is to establish the relationship between the deposition process parameters and the material properties of doped µc‑SiOx:H such as optical band gap, refractive index, conductivity, and crystalline volume fraction. To understand the individual influences of the different structural phases of the composite material µc‑SiOx:H the link between the optoelectronic properties and the material structure as well as the material composition was investigated.
It is shown that doped µc‑SiOx:H material is a mixture of crystalline silicon nanoparticles (highly crystalline µc‑Si:H) and amorphous silicon oxide (a‑SiOx:H), where the a‑SiOx:H phase itself consists of a‑SiO2 and a‑Si:H. This is advantageous since an oxygen rich amorphous silicon phase (a‑SiOx:H) has a wide optical band gap, allowing to reduce optical losses in the device, while a volume fraction of the highly crystalline µc‑Si:H phase above a value as low as 30% already ensures sufficiently high electrical conductivity to reduce electrical losses. The optical properties such as optical band gap and refractive index of the doped µc‑SiOx:H films can be conveniently adjusted over a wide range by the CO2/SiH4 gas flow ratio. The crystalline volume fraction at a given CO2/SiH4 ratio can be increased by decreasing the silane concentration SC = SiH4 / (SiH4 + H2). The ideal preparation conditions to obtain optimum material were identified and the resulting doped µc‑SiOx:H layers were evaluated in single and tandem junction solar cells.
To establish a link between the µc‑SiOx:H material properties and the solar cell performance the research addresses the parasitic absorption of the doped layers and the in‑coupling of light to reduce optical losses and gain more photocurrent in the device. Additionally, the device stability against light exposure was investigated for a variety of absorber layer thicknesses. It was shown that in a‑Si:H/µc‑Si:H tandem solar cells the thickness of the absorber layer of the a‑Si:H top cell can be reduced, without impairing device efficiency, by incorporation of µc‑SiOx:H as intermediate reflector. Most importantly, such thinner cells show improved stability yielding a stabilized efficiency of 10.3% for single junction solar cells.
It is concluded that doped µc‑SiOx:H can be beneficially applied to thin‑film silicon solar cells, reducing the parasitic absorption, improving light‑incoupling, and acting as an intermediate reflector thanks to the low refractive index and the wide band gap at a sufficiently high conductivity.
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