Introduction to Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 11

produce efficient tandem solar cells [ 63 ]. Apart from connecting the subcells in
series, it has been demonstrated that connecting the cells in parallel (a 3-terminal
structure) can also result in improved efficiency. When the cells are connected in
parallel, the short-circuit current is ideally the sum of the current outputs from the
two subcells. Sista et al. have demonstrated a large short-circuit current of
15.1 mA/cm 2 and a power conversion efficiency of 4.8 % using a 3-terminal
configuration and also showed that both the common-anode and common-cathode
configurations are possible by using PEDOT:PSS/Au/V 2 O 5 and TiO 2 :Cs/Al/Au as
the intermediate layer respectively [ 64 ].
Despite the advantages, tandem solar cells suffer from difficulties in matching
subcells. As carriers from the top and bottom subcells recombine at the interlayer,
a good interlayer has to be chosen to allow efficient recombination and has to be
transparent to reduce optical losses. Also, current matching between subcells has
to be achieved in order to prevent charge accumulation on one of the subcells
which deteriorates in efficiency. During fabrication, it is also important that newly
fabricated layers will not damage the layers below as more layers are fabricated.
These factors present engineering challenges for tandem solar cell design.
1.5 Characterization of Organic Solar Cells
1.5.1 J-V Characteristics
OSCs are typically characterized under 1000 W/m 2 light of AM 1.5 solar spectrum
[ 65 ]. The operation of a solar cell at difference bases is illustrated in Fig. 1.8 .
At (i) reverse bias, the applied bias reinforces the built-in electric field,
enhancing exciton dissociation and charge transport and results in a large photo-
current. Drift current is dominant due to the presence of a strong electric field.
When (ii) the applied bias is close to zero, mainly the built-in field exists in the
device and the built-in field drives the carriers to the corresponding electrodes for
collection. When the applied bias is increased in positive direction, the positive
bias opposes the built-in field. As the resultant field inside the device reduces, drift
current becomes smaller and the magnitude of current decreases. Eventually the
field reaches a point where (iii) the applied field is equal to the built-in field.
Around this point, diffusion current dominates the current, as the electric field is
very small inside the device. When (iv) the external bias is further increased, the
applied field is larger than the built-in field and the potential gradient in the device
is reversed. As the barrier is now triangular, carrier injection occurs through the
tunneling mechanism and positive current results.
When the applied bias and current are opposite in direction, power is outputted
from the solar cell. The point where the magnitude of the product of J and V is
maximum is the maximum power output point. A number of the parameters that
are commonly used to evaluate solar cell performance are described below.