Investigations on High Efficiency Thin Film Silicon Solar Cells

Abstract

The focus of this work was mainly on the efficiency enhancement in hydrogenated amorphous silicon (a-Si:H ) thin film and polycrystalline silicon (poly-Si) thin film solar cells and to investigate and analyze a technologically useful innovative single solar cell design by allowing the full photovoltaic (PV) potential of a-Si:H and poly-Si thin film, which can offer the realistic possibility of achieving an efficiency of more than 15 %. Numerical modelling and simulation helped us to understand device properties and to design a new solar cell heterostructure combining a-Si and poly-Si for achieving the goal. Device modelling and simulation tool MEDICITM was used to analyze and optimize the new device heterojunction thin film silicon solar cell. At the outset, two-dimensional device modelling for a-Si:H p+-n-n+ solar cell was carried out by using MEDICI™ device simulator and the influence of absorber layer thickness, doping concentration, and dangling bond density of states in absorber layer on PV parameters were investigated. A strong correlation between n-type doping and dangling bond density in the absorber layer relative to the stability of the a-Si:H solar cell was observed. An increased stabilized efficiency was obtained when n-type dopant concentration in the absorber layer was higher than the optimum value for higher initial efficiency. The window layer (p+ layer) of the device was designed with a three layered structure of graded doping for higher device performance. This window layer structure in the a-Si:H p+-n-n+ cell resulted in higher open circuit voltage (Voc) and fill factor (FF) and hence higher efficiency () of the cell. The efficiency of the modified a-Si:H solar cell structure was found to be 12.85 %. The performance of poly-Si p+-n-n+ thin film solar cell with homojunction and heterojunction emitter was also analyzed by using MEDICI™. The simulation results showed that the PV parameters considerably depend on the grain size and passivation at the grain boundary. The absorber layer thickness for optimum efficiency of the cell was found to depend on the grain size and on the passivation at the grain boundary. The polyV Si p+-n-n+ cell with a thin p+ emitter layer of a-Si showed much higher Voc than that for the homojunction cell. The poly-Si cell with heterojunction emitter was found to be more suitable for highly efficient thin film poly-Si solar cells. A thin layer of microcrystalline silicon (μc-Si) at the interface of a-Si and poly-Si layers are found to be suitable for better performance of the poly-Si thin film solar cell with the heterojunction emitter. The highest efficiency of 12.66 % was obtained for this modified cell structure with 10 μm grain size. With the newly designed a-Si/poly-Si heterojunction thin film solar cell structure, it was possible to obtain higher short circuit current (Jsc) than the conventional a-Si homo junction cell. The new cell design having a higher Voc and FF, together with higher Jsc attained a higher efficiency of 15.42 %. When a properly designed three-layered window layer structure was incorporated into this new heterostructure thin film single cell, the efficiency was enhanced to 16.23 %. Further enhancement in efficiency for this solar cell structure was achieved by introducing a thin layer of μc-Si at the interface of a-Si and poly-Si, and an efficiency of 17.04 % was obtained. For the validation of the simulated results, we carried out experiments for finding out device quality Si films with appropriate doping concentrations by using high density plasma chemical vapor deposition (HDPCVD) equipment with inductively coupled plasma (ICP) source. An anomalous effect of decrease in the crystallinity of ICPCVD deposited-Si films with increasing hydrogen (H) dilution ratio was observed. Device quality p-doped and n-doped a-Si and poly-Si films were obtained by ICPCVD.