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(BCP), that has been shown to have sufficiently high electron conductivity[11]. The
cathode metal-deposition-induced damage on BCP results in a high density of conducting
trap states. A second type of EBL is based on tris-(acetylacetonato) ruthenium (III)
[Ru(acac) 3 ] and related compounds that have a small HOMO energy [12]. In this case,
holes from the cathode are transported along the HOMO of Ru(acac) 3 and recombine
with electrons at the acceptor/EBL interface. Most recently, a third type of EBL has been
introduced based on 3,4,9,10 perylenetetracarboxylic (PTCBI) [13]. The LUMO of
PTCBI is aligned with that of the acceptor C 60 , allowing for low-resistance transport of
electrons directly from acceptor to cathode. As a result, this PTCBI layer serves as an
efficient electron conductor and forms a low energy barrier contact with the Ag cathode.
Adding an transparent wide band gap 1,4,5,8-napthalene-tetracarboxylic-dianhydride
(NTCDA) in combination with PTCBI allows for optimized optical spacing and efficient
exciton blocking [13]. Tris-(8-hydroxyquinoline) aluminum (Alq 3 ) [14] and
bathophenanthroline (BPhen) [15] are also examples of the organic materials used as
EBLs in planar solar cells. In this work, both BCP and PTCBI have been used as the
EBL for all devices studied.
An alternative approach to overcoming the short L D problem as discussed in a planar
HJ is to form a bulk HJ between the donor and acceptor materials with a large interface
area, as shown in Fig. 1.2(b). In this case, an entangled region with two constituents is
formed whereby photons can be absorbed over a long distance, creating excitons within a
diffusion length of a D/A interface where photoinduced charge transfer can occur [16].
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