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.

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