Page 263 - Organic Electronics in Sensors and Biotechnology

240 Cha pte r S i x

afford only limited opportunities for tuning. Organic materials can
be designed to absorb strongly over a wide spectral range to create
broadband photodetectors or over a more restrictive range to create
wavelength-selective devices. The most widely studied OPV system
is the P3HT:PCBM donor/acceptor combination, which has a rela-
tively narrow spectral range from about 300 to 650 nm (Fig. 6.13). In
recent years—and driven primarily by the need to improve solar cell
efficiencies—much research has focused on developing lower energy-
gap materials that can harvest a wider part of the solar spectrum. 5, 56–59
In the context of photodiodes, these same materials systems are of
interest for their wider spectral response. The donor-acceptor combi-
nation in Fig. 6.24a, reported by Wang and coworkers, provides one
of the widest spectral ranges reported to date: both the donor poly-
mer APFO-Green1 and the C -based acceptor molecule BTPF70 have
70
5
absorption spectra that extend well beyond 1 μm. The active range of
optimized bulk heterojunction devices based on these materials spans
330 to 1000 nm (Fig. 6.24b), with the lower wavelength limit being
due to absorption by the glass substrate. This is only marginally less
than the spectral range of Si photodiodes whose long-wavelength
sensitivity extends to about 1100 nm.

S
N
N
S S
n
N
N 30
APFO-Green1
25
20
EQE (%) 15
F 3 C APFO-Green1:BTPF70
NO 2
10
N
N
F 3 C 5
0
300 400 500 600 700 800 900 1000
BTPF70 Wavelength (nm)
(b)

(Mayor isomer)
(a)

FIGURE 6.24 (a) Chemical structures and (b) spectral response curve for low band-
gap materials developed by Wang and coworkers. Photodiodes based on this
material combination have a similar spectral range to Si devices. (Reprinted with
permission from Ref. 5. Copyright 2004, American Institute of Physics.)