Page 231 - Organic Electronics in Sensors and Biotechnology

208 Cha pte r S i x

(Fig. 6.7e) and poly[2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinyl-
ene] (MEH-PPV) (Fig. 6.7f). 18, 26, 34 In combination with fullerenes, PPV-
based devices have been reported to reach external quantum efficien-
cies of 66% and power conversion efficiencies of 3% under AM 1.5 solar
35
simulation. However, MDMO-PPV and MEH-PPV have relatively
low glass transition temperatures of 45 and 65°C, respectively, 36, 37
which can lead to poor thermal stability under intense illumination or
in hot ambient conditions. They also exhibit relatively low charge
−5
−4
mobilities of ~ 10 and 10 cm /(V . s), which may exacerbate recombi-
2
nation losses under low field conditions. 19, 38 For these reasons, poly-
thiophenes tend to be the donor of choice in bulk heterojunction
devices. The archetypal polythiophene, poly(3,hexylthiophene) [P3HT]
(Fig. 6.7h), has a high glass transition temperature and a high charge
2
mobility of up to 10 cm /(V . s) due to its crystalline morphology. 39
−2
The P3HT:PCBM donor/acceptor combination is the most widely
studied material system to date, and so far it has yielded the best
across-the-board device performance (for solar applications) in terms
of long-term stability and power conversion efficiencies of 4 to 5%. 39–43
The P3HT:PCBM system will be used as an exemplar of many of the
concepts discussed in this chapter. Due to its advanced state of devel-
opment, P3HT:PCBM can be considered to be a benchmark material
against which others should be judged.
The most widely used anode material is the degenerate n-type
semiconductor indium tin oxide (ITO), which combines high conduc-
tivity with good transparency in the visible part of the spectrum.
Indium tin oxide, however, has a spiked morphology that can result
in non-uniform current flow though the device and premature aging.
It is therefore usual to coat the indium tin oxide layer with a thick layer
of a conducting polymer such as poly(3,4 ethylene-dioxythiphene):
polystryrene sulfonate (PEDOT:PSS) that acts as an ameliorating
layer for the ITO, resulting in substantially improved operating
lifetimes. The cathode is usually a thermally deposited metal of low
to moderately low work function such as Ca or Al.

6.3.3 Current-Voltage Characteristics
Figure 6.8a shows a simple schematic for an OPV device that is con-
nected to a load resistance R. We consider the idealized case of a simple
single-layer bulk heterojunction solar cell, in which the blended mate-
rial can be treated as a simple composite semiconductor, in which the
HOMO level is derived from the donor and the LUMO level from the
acceptor. This approximation allows us to ignore the microscopic struc-
ture of the active layer and to analyze device operation using simple
ideas from conventional semiconductor physics. There are two extreme
situations we can consider: short-circuit and open-circuit. In short-circuit,
the load resistance is zero and so presents no obstacle to the flow of
charge. This results in the maximum possible photocurrent, known as