2. Hetero-junction thin film solar cells
2.1. CdTe thin films
The research on CdTe thin film solar cell started since 1950's,
and the current research efforts are devoted for improving effi-
ciency of the CdTe thin film solar cells. Since CdTe has an optimal
band gap of 1.49 eV for single-junction devices, efficiencies above
20% should be achievable in the commercial CdTe solar cells [27].
For example, in August 2014, First Solar reported a device with
21.0% conversion efficiency [8]. The efficiency of the CdTe/CdS thin
film solar cells was reported to be 22% [28]. However, the stability
of efficiency could be a potential problem for the CdTe based solar
cells due to existence of defects in grain boundaries and intragrain
dislocations. It is presumed that the carriers recombine, and
reduce the average life time of minority carriers [29]. The photovoltaic
performance of the CdTe solar cells depends not only on
efficiency but also on many other factors such as open circuit
voltage Voc, fill factor (FF), choice of substrate, close circuit current
Jsc and area of deposition. The configuration of the solar cell also
influences the performance of the solar cell for example, the
superstrate solar cell has been applied in order to improve the
absorption capability of the solar cell [30]. The maximum effi-
ciency values of the laboratory and commercial scale, and the
associated solar cell parameters with respect to different preparation
methods are listed in Table 1 [30–44].
The values of Voc and FF for the optimized deposition and
fabrication technologies of the CdTe solar cells are around
1000 mV and 85% respectively. These optimized values of fill factor
(FF) and open circuit voltage (Voc) along with the short circuit
current density (Jsc) 27 mA cm2 can result in 2170.5% efficient
laboratory scale CdTe solar cell [8]. It is possible to increase Voc by
increasing the built-in voltage and maximizing the net acceptor
density in the absorber region of the CdTe thin film materials. It
was observed that a higher value of Voc can be obtained by
increasing the doping level (Cu dopant), but with the increase in
Voc the value of FF was reduced which affected the overall performance
of the solar cell [8]. The increase in the acceptor density
will decrease the width of the space charge region. Effect of
compensating acceptors was also observed due to the probability
of Cu involvement into the window layer [46]. These effects cause
the reduction in space charge width which increases the probability
of light absorbance in the undepleted region [47].
Kim et al. [48] studied the environmental issues of CdTe thin
film solar cell. Carbon emission from the CdTe device is 62.5%
lower than a-Si PV system and 83.5% lower than a single crystal
silicon photovoltaic panel [49]. For each gram of CO2 emission in
the energy production from the grid, 0.03 μg arsenic, 0.01 μg of
cadmium, 0.09 μg of chromium, 0.1 μg of lead (Pb) and 0.01 μg of
mercury (Hg) are emitted. Such emissions can be reduced by