Thin film amorphous silicon cells by inductive PECVD, with a view towards flexible substrates

Description/Abstract

A range of amorphous silicon based films and solar cells have been fabricated using an inductively coupled PECVD reactor in order to investigate how the deposition conditions influenced their properties. Complete cells were deposited onto rigid and flexible substrates, and homogeneous thin films were grown to investigate their optical and electrical properties separately from the influence of whole devices.

Initial photoconductivity measurements performed on intrinsic amorphous silicon showed that it was suitable for photovoltaic applications (10-4
-1cm-1), complete single junction pin superstrate cells were then fabricated. The deposition conditions were altered to
optimise cell efficiency, and the impact of substrate temperature, RF power, gas flow rate, system pressure, and substrate choice was commented on. The best cell produced
was grown at 250C and was 8.05% efficient, after reversal of its structure to nip an efficiency of 5.77% was observed.

The deposition temperature was lowered to 200C, and the properties of doped and intrinsic amorphous silicon films were investigated using measurements of the dark conductivity
variation with temperature and spectroscopic ellipsometry. Optimum growth conditions were determined for each type of film and an nip cell was deposited using them resulting in an efficiency of 4.59%. The requirement of a carefully selected intrinsic region thickness in order to achieve effective collection of generated carriers was considered.

Lastly the process was transfered from rigid glass to flexible polished stainless steel and Kapton substrates, requiring inversion of the cell orientation. A 1cm2 cell on a steel substrate
was found to be functional under illumination and was 0.27% efficient. A 1cm2 cell was deposited onto Kapton but was only measured under dark conditions. It was concluded that the rear contact topography and defect density is of significant importance when dealing with flexible substrates and that sputtering rather than evaporation would have been a more suitable technique to fabricate it.