In-situ absorption measurements for solar cell current determination during thin-film silicon PECVD

Process control is very important in the fabrication of high quality thin-film silicon solar cells. Solar cell parameters like film thickness, crystalline volume fraction or conductivity are usually measured in the back end of an industrial production line using ex-situ techniques. At the back end of solar module production the most of the money has been spent already and detrimental effects on the system performance like process drifts during the fabrication have not been detected online. Measuring in-situ, during the deposition of the thin silicon layers, utilizes the advantage that the investments in the front end of the process are still at low level. Additionally, the possibility of an active process control and hence the optimization of the solar cell is given, by monitoring process parameters in real time. In recent studies we performed transmission measurements during silicon deposition, in which the plasma emission was used as light source. It was shown, that deposition rate and the crystalline volume fraction of microcrystalline silicon layers and the roughness of ZnO:Al substrates can be detected with high accuracy using only a single optical setup. Additionally, the setup convinces with its simplicity for the use as process control which makes it interesting for the industrial mass production. In this paper we show that using the transmission measurements the absorption characteristic of the growing silicon thin film can be estimated. It can be seen that a direct correlation between the measured absorption of intrinsic absorber layers and the resulting solar cell current is possible.