Excimer laser crystallised polysilicon solar cells.

Crystalline silicon (c-Si) has been the main material for solar cells as a renewable energy source. The main advantage has been the abundance of this material. However, due to the high costs, thin film alternatives to c-Si in the form of hydrogenated amorphous silicon (a- Si:H) has shown promising progress. The popularity of a-Si:H increased due to the lower pay back times although initial efficiencies were lower for a-Si:H. Higher efficiency of c-Si is primarily due to the absorption of broader spectrum of radiation from the sun compared to a-Si:H. Although cheaper to produce, a-Si:H suffers from light induced degradation resulting in lower efficiencies. Thinner absorber layers of a-Si:H have shown better stability in this regard. In this thesis, attempts were made to fabricate a-Si:H/c-Si hybrid solar cell structures by the use of a laser crystallisation process using excimer laser. An excimer laser, KrF 248nm, was used to crystallise a-Si:H as excimer laser readily absorb into a-Si:H within a few tens of nanometres enabling melting and solidification creating c-Si. An a-Si;H p/i structure deposited on textured ITO is crystallised in partial melting regime and is used to make solar cells. Initially, a suitable metal for back contact was investigated. Al was selected as the preferred back contact compared to Au due to the better efficiencies observed. Further, different methods were attempted to obtain an n-layer to complete a-Si:H p-i-n solar cell. Devices were fabricated for different intrinsic layer thicknesses using ion implanted phosphorus and diffused phosphorus as dopants in the n+-layer. Ion implanted phosphorus devices showed poor efficiencies due to implantation damage even after laser annealing. Phosphorus diffused and thermal annealed devices showed lower efficiencies than the reference devices, where dopant profile showed possible activation of dopants. Excimer laser was utilised to obtain a crystallised structure in two different ways as single shot and multiple shot. Both methods resulted in poor device performances with increasing laser energy density. However, reversible increment of current density was observed for these devices due to the creation of high band gap silicon nanocrystals. Finally, broadband absorbing hybrid devices were fabricated by incorporating PbS nanocrytals/BCP with a-Si:H and laser crystallised silicon. Although these devices show small contributions in the IR region compared to a-Si:H only cells, the overall device efficiencies were observed to be very low