Introduction to Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 6

electrodes in a solar cell. Commonly, a high work function anode and low work
function cathode are used and this difference creates a built-in electric field within
the solar cell that determines the open circuit voltage (V oc ) of the cell. When an
external bias is applied, the internal electric field is modified and the drift current
changes. The carriers drift along the resultant internal electric field of the solar cell
toward the respective electrodes for collection. Another mechanism of carrier
transport is the diffusion current, which is the diffusion of carriers along the carrier
concentration gradient within a solar cell. As the geminate pairs are generated
around the solar cell heterojunction, the concentration of electrons and holes are
commonly higher around the heterojunction. Carriers hence diffuse along the
concentration gradient away from the heterojunction, leading to the diffusion
current. The diffusion current mainly dominates when the applied bias modifies the
internal electric field to nearly zero, while drift current dominates when the
internal electric field is large.
The main limitation of carrier transport is the mobility in the active layer. As hole
and electron mobilities in organic materials are commonly low, the active layer has to
be kept relatively thin to allow carriers to reach the electrodes within their lifetimes.
The mobility difference is also a critical factor in determining charge transport
characteristics, as a difference of more than a factor of 10 will lead to space charge
limited current (SCLC) [ 26 - 28 ]. In short, SCLC arises when one type of carrier, say
electrons (electron commonly has higher mobility in OSC materials [ 29 ]) is trans-
ported much more efficiently to the cathode. As the rate of electrons reaching the
cathode is higher than that of holes to the anode, electrons may accumulate in the
active layer near the cathode interface and creates the space charge effect, which
modifies the charge transport characteristics of the active layer and creates an upper
limit for the current output of a solar cell. Hence, to achieve efficient carrier transport
in the active layer of solar cell, balanced hole and electron mobilities is desired.
1.3.4 Charge Extraction at Electrodes
After the charge carriers transport to the active layer/electrode interface, they are
extracted from the active layer to the electrodes. To achieve high efficiency in
charge extraction, the potential barrier at the active layer/electrode interfaces have
to be minimized. Thus, the work function of the anode is ideally expected to match
the donor HOMO, while the work function of the cathode is expected to match the
acceptor LUMO. When these occur, the contacts are called ohmic contacts and V oc
correlates positively with the difference between the acceptor LUMO and donor
HOMO [ 30 ]. On the other hand, if the work functions of anode and cathode
materials are not near the donor HOMO or acceptor LUMO, correspondingly an
ohmic contact cannot be formed. In this case, the carrier extraction behavior is
governed by the metal-insulator-metal (MIM) model [ 31 ].
A method to improve the work function matching at the electrodes is to utilize
different types of materials as electrodes. Commonly, indium tin oxide (ITO) is