Introduction to Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 8

The efficiency of the four steps is represented by g A , g diff , g diss , and g C
respectively. The external quantum efficiency (EQE) is defined as

g eqe ¼ g A g diff g diss g C ð 1 : 1 Þ

The EQE represents the percentage of photons that are eventually converted to
charge carriers collected at the electrodes. Generally, there are two major factors
which limit the EQE. (1) Incomplete absorption of the solar spectrum, either due to
narrow absorption band or a thin active layer, leads to a reduction in g A .
(2) Recombination of excitons due to various reasons such as quenching at metal
electrodes and limited phase separation in active layer lead to a loss of photo-
generated excitons and a lower EQE. To reduce these losses, different solar cell
architectures have been proposed and they will be discussed in the next section.

1.4 Solar Cell Architectures

1.4.1 Bilayer Solar Cell

The formation of the heterojunction by Tang et al. [ 22 ] in 1986 was first dem-
onstrated in the form of a bilayer solar cell. As shown in Fig. 1.5 , the common
structure of a bilayer solar cell consists of an anode, hole collection layer, active
layer composed of donor and acceptor, electron collection layer, and cathode
fabricated sequentially. The hole collection layer and electron collection layer are
used to modify the work function of the electrodes to form an ohmic contact.
A single, well-defined interface exists between the donor and acceptor at which
excitons dissociate. With this structure, the bilayer solar cell is the simplest
structure described by the basic operating principle of the solar cell.
A significant drawback for bilayer solar cell is that the short exciton diffusion
length of organic materials limits the thickness of the donor and acceptor layers. If the
donor or acceptor layer is too thick, the excitons generated far away from the

Fig. 1.5 Structure of a
bilayer solar cell