Page 262 - A Comprehensive Guide to Solar Energy Systems

Chapter 12 • Organic Photovoltaics 265

FIGURE 12.8 Schematic diagram of the structure of a three heterojunction OPV, with junctions optimized to harvest the
blue, green, and red parts of the solar spectrum (left). The solar spectrum at ground level highlighting the approximate
range of wavelengths harvested by each organic heterojunction in a triple junction OPV (right).

devices [35,39]. The potential to achieve much greater output voltages than possible for
a single junction OPV is well illustrated by the work of Sullivan et al. [39] who demon-
strated that a V oc exceeding 5 V could be achieved for a device comprising five individual
heterojunctions. In that work the motivation was to achieve an output voltage sufficient
to recharge a battery for consumer electronics application, which requires a threshold
potential for recharging of 4.0–4.2 V. The advantage of that seemingly unconventional ap-
proach, over simply connecting five individual OPV cells in series to achieve the same
voltage, is that a PV module comprising high voltage multijunction OPVs connected in
parallel can continue to charge the battery even if in partial shade (Fig. 12.9), which is
particularly useful for portable consumer electronic aplications.
now that the power conversion efficiency of laboratory scale OPVs is very close to
threshold for large-scale applications, the research effort in industry and academia is in-
creasingly focusing on addressing challenges around scale-up of materials and processes
to enable low cost production of modules, as well as developing approaches to improving
module stability toward the fluctuations in light intensity and temperature experienced
in real world applications. The following sections highlight a number of areas from which
significant advances in OPV performance are likely to stem in the coming years. The power
conversion of a PV device is proportional to the product of the open-circuit voltage (V oc ),
the fill factor (FF), and the short circuit-current density ( J sc ) [3], and so in the following
part of this chapter each point is discussed in terms of the effect on one or more of these
device parameters.

12.4.1 Increasing Power Conversion Efficiency

For single junction OPVs empirical considerations suggest that a power conversion effi-
ciency of 12% is within reach [36,40], and for multijunction device architectures 15% is
technically feasible using refinements of existing device structures and materials [35,41].
A laboratory scale device efficiency of 15% is an important benchmark because it is