Active Layer Materials for Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 18

the key photovoltaic materials for high performance PSCs, and therefore, what are
ideal properties and how to get photovoltaic materials with ideal properties are of
great importance to photovoltaic materials design.
Bulk heterojunction (BHJ) structure is the most successful structure invented by
Yu et al. [ 1 ], in which a blend of donor and acceptor with a bicontinual phase
separation can be formed. When the sunlight getting through the transparent
electrode is absorbed by the semiconducting donor and acceptor materials in the
photoactive layer, excitons (bounded electron-hole pairs) are formed, and then the
excitons diffuse to the interfaces of the donor/acceptor where the excitons disso-
ciate into electrons on the LUMO level of the acceptor and holes on the HOMO
level of the donor. The dissociated electrons and holes are driven by build-in
electric field and then moved to negative and positive electrode, respectively, and
then collected by the electrodes to realize the photon-to-electron conversion.
Figure 2.1 shows the electronic energy levels of the donor and acceptor in a
P3HT/PCBM blend system. The absorption band of P3HT/PCBM covers the range
from 380 to 670 nm, which means that the photons with energy between 2.0 and
3.3 eV can be absorbed by the active layer, and the excitons will be formed. In
order to make better utilization of the sunlight, active layer materials with broad
absorption band is required, and for this purpose, more and more low band gap
(LBG) materials have been developed and great successes have been made in the
past decade. Since, the LUMO and the HOMO of P3HT is higher than that of
PCBM, the excitons will separate into positive and negative charges at the
interface of the P3HT phase and PCBM phase. The negative charge will transport
through the LUMO of PCBM and the positive charge will transport through the
HOMO of P3HT, and then the charges can be collected by the electrodes. In order
Fig. 2.1 Electronic energy
level of P3HT and PC 60 BM