Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 71

recombination, rendering an exact summation of V oc (1.18 V) of the two subcells
and a high FF (61.8 %). Maximum PCE of 2.78 % is mainly due to the fact of
using identical subcells.
3.5 Summary and Outlook
The overall performance of BHJ-PSCs (for either conventional device, inverted
device, or tandem device) is determined by a series of sequential optoelectronic
processes including charge separation, transporting, and extraction, which can be
controlled by the manipulation of intrinsic properties of the photoactive materials,
the morphology of active layer and the electrical contact of interfaces between
different layers, etc. The interface problems between different thin film layers
inside a polymer solar cell device is neglected for a long time by researchers as
most of their efforts were devoted to the development of new active materials, the
optimization of active layer morphology. Thus only until recently did the main-
stream researchers realize the importance of interface engineering on the photo-
voltaic performance of devices. Great improvements in performance were thus
achieved for BHJ-PSCs field via the understanding of interface function and the
development of highly efficient interface modification materials. As discussed in
this chapter, the interlayers function in many aspects, such as improving interfacial
electrical contact, passivating charge trapping states, altering electrode work
function, controlling energy alignment, enhancing charge collection, inducing
active layer phase separation, redistributing the incident light field, and even
improving the yield and quality of device fabrication process. However, most of
the reported interfacial materials function only the above-mentioned aspects par-
tially and may result in some side effects. Moreover, some interfacial materials are
sensitive to the active layer, i.e., functional for some certain photovoltaic materials
but not for others. Therefore, the integration of all kinds of different functions into
one material is still challenging and in the near future, the development of more
efficient interfacial materials is still required. Nevertheless, the great progress in
the field of BHJ-PSCs gives the researchers opportunity and confidence to develop
more powerful interfacial materials and roll-to-roll compatible fabrication meth-
ods, and thereby pave the way for commercialization of polymer solar cells.
References
1. Günes S, Neugebauer H, Sariciftci NS (2007) Conjugated polymer-based organic solar
cells. Chem Rev 107:1324-1338
2. Thompson BC, Fréchet JMJ (2008) Polymer-fullerene composite solar cells. Angew Chem
Int Ed 47:58-77
3. Helgesen M, Sondergaard R, Krebs FC (2010) Advanced materials and processes for
polymer solar cell devices. J Mater Chem 20:36-60