Page 263 - A Comprehensive Guide to Solar Energy Systems

266 A COmPreHenSIVe GuIde TO SOlAr enerGy SySTemS

FIGURE 12.9 Diagrams (left) and current-voltage characteristics (right) showing how a module of multijunction OPV
cells reported by Sullivan et al. [39] maintains a high operating voltage even in partial shade. Diagram (left) credited to
Dr. Paul Sullivan.

reasonable to expect that this would translate to a 10%–12% module efficiency, which
would be sufficient for a number of the aforementioned application areas. Importantly,
fundamental descriptions indicate that the upper efficiency limit for a single junction OPV
may be as high as 20%–24% [36,42] and so multijunction OPVs may yet achieve a module
efficiency above 15%, although this will require a much better understanding of the ideal
heterojunction morphology as well as the development of materials with improved optical
and electrical properties. Structural and chemical impurities in organic semiconductors
limit the efficiency of today’s high performance OPVs by serving as electron-hole recom-
bination centers and impeding the transport of free charges to the electrodes, which con-
strains the thickness of the organic semiconductor that can be used. The density of such
impurity states in organic semiconductors is typically many orders of magnitude higher
than the impurity level in PV grade inorganic semiconductors, and so it is likely that con-
siderable improvements in the OPV performance can be achieved simply by reducing the
density of these structural and chemical impurities.
For commercial scale OPVs based on vacuum processable organic semiconductors,
simple small molecules are favored because they are available in high volume, have no in-
sulating parts, and form highly crystalline phases. There are also many examples of small
molecule semiconductors that strongly absorb near infra-red light, in addition to those
that absorb strongly in the ultraviolet and visible parts of the solar spectrum. examples
of small molecule organic semiconductors that have proved particularly useful for OPVs
are metal phthalocyanines (particularly zinc and copper phthalocyanine) and the cage
fullerenes (particularly C 60 and C 70 ), some examples of which are given in Fig. 12.5. The
well-defined size, rigidity, and high symmetry of small molecule organic semiconductors