Abstract
The focus of this work was mainly on the efficiency enhancement in hydrogenatedamorphous silicon (a-Si:H ) thin film and polycrystalline silicon (poly-Si) thin film solar cellsand to investigate and analyze a technologically useful innovative single solar cell designby allowing the full photovoltaic (PV) potential of a-Si:H and poly-Si thin film, which canoffer the realistic possibility of achieving an efficiency of more than 15 %. Numericalmodelling and simulation helped us to understand device properties and to design a newsolar cell heterostructure combining a-Si and poly-Si for achieving the goal. Devicemodelling and simulation tool MEDICITM was used to analyze and optimize the new deviceheterojunction thin film silicon solar cell. At the outset, two-dimensional device modellingfor a-Si:H p+-n-n+ solar cell was carried out by using MEDICI? device simulator and theinfluence of absorber layer thickness, doping concentration, and dangling bond density ofstates in absorber layer on PV parameters were investigated. A strong correlationbetween n-type doping and dangling bond density in the absorber layer relative to thestability of the a-Si:H solar cell was observed. An increased stabilized efficiency wasobtained when n-type dopant concentration in the absorber layer was higher than theoptimum value for higher initial efficiency. The window layer (p+ layer) of the device wasdesigned 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 circuitvoltage (Voc) and fill factor (FF) and hence higher efficiency () of the cell. The efficiencyof 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 andheterojunction emitter was also analyzed by using MEDICI?. The simulation resultsshowed that the PV parameters considerably depend on the grain size and passivation atthe grain boundary. The absorber layer thickness for optimum efficiency of the cell wasfound to depend on the grain size and on the passivation at the grain boundary. The polyVSi p+-n-n+ cell with a thin p+ emitter layer of a-Si showed much higher Voc than that for thehomojunction cell. The poly-Si cell with heterojunction emitter was found to be moresuitable for highly efficient thin film poly-Si solar cells. A thin layer of microcrystallinesilicon (μc-Si) at the interface of a-Si and poly-Si layers are found to be suitable for betterperformance of the poly-Si thin film solar cell with the heterojunction emitter. The highestefficiency 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, itwas possible to obtain higher short circuit current (Jsc) than the conventional a-Si homojunction cell. The new cell design having a higher Voc and FF, together with higher Jscattained a higher efficiency of 15.42 %. When a properly designed three-layered windowlayer structure was incorporated into this new heterostructure thin film single cell, theefficiency was enhanced to 16.23 %. Further enhancement in efficiency for this solar cellstructure was achieved by introducing a thin layer of μc-Si at the interface of a-Si andpoly-Si, and an efficiency of 17.04 % was obtained.For the validation of the simulated results, we carried out experiments for finding outdevice quality Si films with appropriate doping concentrations by using high densityplasma chemical vapor deposition (HDPCVD) equipment with inductively coupled plasma(ICP) source. An anomalous effect of decrease in the crystallinity of ICPCVD deposited-Sifilms with increasing hydrogen (H) dilution ratio was observed. Device quality p-dopedand n-doped a-Si and poly-Si films were obtained by ICPCVD.