Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 44

Fig. 3.1 Comparison of conventional device structure (left) and inverted device structure (right)
for BHJ-PSCs
solution processing [ 1 - 3 ]. In the last two decades, great progress in the field of
BHJ-PSCs has been achieved with the power conversion efficiency (PCE)
increasing from around 1 % to more than 9 % [ 4 ]. Such great advancement was
driven by the efforts in various aspects including the development of new low-
band-gap conjugated polymers and new fullerene-based acceptor materials [ 5 - 7 ],
the usage of new device processing methods [ 8 - 10 ], the invention of new device
structures [ 11 , 12 ], the application of innovative interface modification materials
[ 13 - 15 ], and so forth. The simplest structure of BHJ-PSC is the so-called sand-
wich device structure, where the blend of conjugated polymer donor and fullerene
derivative acceptor was sandwiched between a transparent metal oxide, such as
indium tin oxide (ITO), and a metal electrode [ 16 ]. For conventional devices, ITO
and low work-function metal are used as anode and cathode, respectively. While in
an inverted device, the modified ITO and high work-function metal are cathode
and anode, respectively. The configurations of conventional and inverted device
for BHJ-PSCs are demonstrated in Fig. 3.1 . It is well-known that the nature of
electrical contact between the organic active layer and the electrodes is critical to
the overall device performance, for both the conventional and inverted devices
[ 13 - 15 ]. The ideal Ohmic contact in the organic blend layer/electrode interface
with an energy barrier height as low as possible is thus desired for efficient charge
carrier extraction and transport, and thereby reduce the recombination loss caused
by the accumulation of charge in the interface. Indeed, a barrier height of several
tens of mV may lead to severe charge accumulation. Therefore, the modification of
organic active layer/electrode interface is critical to achieve high efficiency and
long-term stability for BHJ-PSCs. As a result, in recent years, a lot of efforts from
research groups all around the world are devoted to the development of new
interfacial materials with different functions and their applications. The aim is to
achieve high PCE and long-term stability for BHJ-PSC devices [ 13 - 15 ].