Hybrid Solar Cells with Polymer and Inorganic Nanocrystals - Organic Solar Cells: Materials and Device Physics - page 243

Fig. 9.1 Schematic
illustration of the energy
levels alignment and
photocurrent generation
mechanism in hybrid solar
cells. Reproduced with
permission from Ref. [ 36 ]

0.2-0.5 eV. Therefore, the exciton generation efficiency (g g ), which is
regarded as the possibility of generating one exciton by one photon, is critical
to the device performance. In addition, the excitons have a good chance to
recombine [ 38 ].
(2) Exciton diffusion. Before separating into free charges, the excitons have to
diffuse to the D/A interfaces. The exciton diffusion efficiency (g diff ) depends
on how much the excitons could successfully diffuse to the D/A interface
before recombination takes place. Noted that the exciton diffusion length is
4-20 nm for most conjugated polymers [ 36 - 38 ], the D/A domains in HSCs
should also be in this range for high exciton diffusion efficiency.
(3) Exciton dissociation. Once reaching the D/A interface, excitons could be
dissociated into free electrons and holes if the energy offset between the
LUMO of the polymer and the conduction band (CB) of the inorganic nano-
crystals overcomes the binding energy of the excitons; therefore, the relative
positions of the LUMO of the Donors and the CB level of the inorganic
acceptors determine the exciton dissociation efficiency (g d ).
(4) Charge transfer and collection. After the exciton dissociation, the free elec-
trons and holes need to transfer through the inorganic nanocrystals and con-
jugated polymers until they are collected at the electrodes. The charge transfer
efficiency (g tr ) is related to the intrinsic properties of the materials, e.g., the
carrier mobility, the crystallinity and the purity, and so on. Besides, continuous
pathways are also needed for efficient charge transfer. While the charge
collection efficiency (g cc ) mainly depends on the energy level alignments of
the active layer and the electrodes as well as the contact between them.

Therefore, the external quantum efficiency (EQE) of a HSC could be calculated
through the following equation [ 38 ]:

EQE ¼ g LHE g g g diff g d g tr g cc ð 9 : 1 Þ