An Investigation Of The I-V Characteristics And Collection Efficiency For A-Si:H Tandem Solar Cells Interconnected By Transparent Conductive Oxide
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Date
1996-10
Authors
Darkwi, Ali Yaskwi
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Abstract
Obtaining stable high efficiency solar cells is one of the basic problems in
hydrogenated amorphous silicon (a.Si:H) solar cell technology. A multijunction
tandem solar cells design has been found to be most effective in improving cell
performance. The electrical interconnection between the cells in tandem structure
is one of the factors limiting the performance of the device. The behavior of solar
cell is usually predicted through the analysis of current density voltage (J-V)
relation.
A new method has been introduced in the analysis of J-V curve for tandem
structure which shows that the output current has a bias voltage dependence. For
this analysis, the U-shape distribution of density of states in the energy gap was
used to find the charge density in the depletion region. The electric field obtained
by solving Poisson's equation shows that, the electric field is higher at the p/i and
iln interface while is low near the middle of the i-layer. Under the assumption that
the current in the i-layer is due to the minority carrier of the holes and collection of
photo-carrier in the low field region is due to diffusion, the J-V curve was found
analytically. The calculations showed that the efficiency increases by increasing
the diffusion length.
Numerical solution of the non-linear second order differential equation for
transport equations ( Poisson's equation and continuity equations ) for charge
carriers in p-i-n a-Si:H solar cell was carried out using a FORTRAN program.
Since these equations are boundary-value problems, the finite difference method
with Newton iteration process was used to solve the equations simultaneously.
The current density obtained has a bias voltage dependence. The open circuit
voltage of the device can be precisely determined from the graph rather than from
the conventional equation {Voc=nkT/q In[(Jsc/Jo)+1] } which is often used, however
it does not show voltage dependence. The simulation developed here provides'
flexibility in the choice of physical parameters so that different models can be
evaluated and compared for development of high-efficiency photovoltaic devices.
A new method has been introduced for simulation of reflectance, transmittance
and absorbance as a function of wavelength of incident photons in tandem solar
cells. In this case transparent conductive oxide (TeO) was used as
interconnection between the cells. Col/ection efficiency was calculated for different
thicknesses of titanium dioxide (Ti00. The aim of this simulation is to find the
effect of Ti02 in the spectral absorbance. For small thicknesses of Ti02• there is
no significant reduction in the total spectral absorbance of the tandem cells.
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Keywords
A multijunction tandem solar cells , Most effective in improving cell performance