Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 132

device performance based on J-V characteristics just gives us the final result of a
series of fundamental photovoltaic conversion events such as photon absorption,
exciton generation, exciton diffusion, charge separation, charge recombination,
charge dissociation, charge transport, and charge collection. These range over nine
orders of magnitude on temporal scale from 10 -14 to 10 -5 s. Here, we demon-
strated how powerful and useful the transient absorption spectroscopy is for
directly observing transient species such as excitons and polarons involved in
photovoltaic conversion events. A transient absorption study will help us under-
stand ''what'' is going on in the device and evaluate quantitatively ''how'' fast and
efficient each event is. Consequently, we can discuss in detail ''why'' such device
performance is obtained on the basis of not the final result but each fundamental
photovoltaic conversion event. Such an in-depth understanding of the mechanism
underlying polymer solar cells will lead to the development of new materials and
progress in device engineering. In particular, the key to further improvements of
polymer solar cells is to understand the origin of high charge dissociation and
collection efficiency. The high dissociation efficiency cannot be explained ratio-
nally by the classical theory, and the reduced bimolecular recombination rate is
inconsistent with the Langevin recombination. Further progress toward 15 %
efficiency requires the synergy between developments of new materials and device
structures and better understanding of the photovoltaic conversion mechanism.
Acknowledgments This work was partly supported by the Kansai Research Foundation for
Technology Promotion, the JST PRESTO program (Photoenergy and Conversion Systems and
Materials for the Next-Generation Solar Cells) and the Global COE program (International
Center for Integrated Research and Advanced Education in Materials Science) from the Ministry
of Education, Culture, Sports, Science, and Technology, Japan.
References
1. Shaheen SE, Ginley DS, Jabbour GE (2005) Organic-based photovoltaics: toward low-cost
power generation. MRS Bull 30:10-19
2. Brabec CJ, Durrant JR (2008) Solution-processed organic solar cells. MRS Bull 33:670-675
3. Krebs FC (2009) Fabrication and processing of polymer solar cells: a review of printing and
coating techniques. Sol Energy Mater Sol Cells 93:394-412
4. Padinger F, Rittberger RS, Sariciftci NS (2003) Effects of postproduction treatment on plastic
solar cells. Adv Funct Mater 13:85-88
5. Li G, Shrotriya V, Huang J, Yao Y, Moriarty T, Emery K, Yang Y (2005) High-efficiency
solution processable polymer photovoltaic cells by self-organization of polymer blends. Nat
Mater 4:864-868
6. Ma W, Yang C, Gong X, Lee K, Heeger AJ (2005) Thermally stable, efficient polymer solar
cells with nanoscale control of the interpenetrating network morphology. Adv Funct Mater
15:1617-1622
7. Kim JY, Kim SH, Lee HH, Lee K, Ma W, Gong X, Heeger AJ (2006) New architecture for
high-efficiency polymer photovoltaic cells using solution-based titanium oxide as an optical
spacer. Adv Mater 18:572-576