Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

Introduction
Polymer solar cells convert sunlight directly into electricity via a complex sequence of events (cf. figure
1), starting with the absorption of light (1), followed by creation of an exciton (2), dissociation of the
exciton (3), transport (4,5) and collection of charges. The expectation that lightweight, flexible, and
large area polymer solar cells can be produced at low cost, in combination with high energy efficiencies
spurs a worldwide fast growing interest in this area. Not all of these attractive properties have materialized
and although evidence is building up that polymer solar cells may live up to this appealing
scenario in the future, new inventions have to be made. Presently, state-of-the-art polymer solar cells
reach power conversion efficiencies of ~5% [1]. Projected efficiencies of 8–10% seem within reach
and expectations for the future are even higher.
Figure 1: Standard device setup of a polymer bulk heterjunction solar cell. The active layer is the
P3HT:PCBM layer.
Most polymer solar cells rely on a photoinduced charge transfer reaction at the interface of an acceptor
and a donor type organic semiconductor which are combined into a bulk heterojunction to generate
charges in a process that mimics natural photosynthesis. Following this event, charges must escape
from recombination, separate spatially, migrate to the appropriate electrode and finally be collected.
Each of these processes poses intriguing scientific questions and exciting challenges to materials design
to make the overall conversion both quantum and energy efficient. With the continuing increase in
power conversion efficiency, it is clear that the field of polymer solar cells has progressed in the last
five years from a scientific curiosity to a stage that it is now on the brink of a breakthrough technology
for the future. Yet, the transfer from test type devices that are typically 5-100 mm 2 in size to real large
areas (1 m 2 ) does require new concepts in cell design and large area processing.
The power conversion efficiency of any polymer solar cell depends critically on the quantum efficiency
of photon to electron conversion that determines the current and the potential energy efficiency that
describes how much of the initial photon energy (eV) is preserved at the operating voltage of the cell.
If one critically analyzes the best polymer solar cells made today, they often have either a high current
or a high voltage, even when they have the same optical band gap. If one –in an optimistic mood–
would combine the best parameters of these two materials already a 7.1% cell would result.
The past two years we have witnessed a strong innovation in the development of polymers that have a
small band gap and are able to absorb up to the near infrared. Within two years the maximum efficiency
of these small band gap polymers has increased to 5.5%, which is among the highest efficiencies
reported. Despite their high and promising efficiencies, the maximum EQE of these cells is still
lower compared to the cells that absorb light in the visible region only. If for these low band gap cells
the quantum efficiency could be improved, cells with 10% efficiency or more are within reach.
APOLLO, B. Ruhstaller, ICP ZHAW
136/290
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