Interface Engineering for High Performance Bulk-Heterojunction Polymeric Solar Cells - Organic Solar Cells: Materials and Device Physics - page 45

In principle, ideal interfacial materials should first function as charge extraction
or transporting layer to minimize the barrier height in organic active layer/elec-
trode interfaces, which will help to form Ohmic contact in interface to reduce
recombination loss and consequently obtain enhanced short circuit current density
(J sc ). Additionally, this layer should also function as exciton blocking layer to
selectively transport only one type of carrier and block the other, i.e., an electron
extraction/transporting layer with hole blocking property, and a hole extraction/
transporting layer with electron-blocking ability. Moreover, the interfacial layer
usually functions as buffer layer to prevent the chemical reaction between organic
active layer and metal electrode, and the diffusion of metal ions into organic layer.
Furthermore, it was found that some interfacial materials can also function as
optical spacers to modulate the incident light distribution in BHJ-PSCs, so that the
maximum incident light intensity located around the center of the active layer to
ensure as much as incident light harvested by the device and consequently to
obtain maximum J sc [ 13 , 15 ].
In this chapter, we will review interfacial materials applied at the interfaces
between organic active layer and electrodes, including both anode and cathode,
and their influence on the photovoltaic performance of BHJ-PSC devices. The
design of interfacial materials and their applications will be divided into several
sections by the device structure, including conventional device, inverted device,
and tandem device. For single junction devices (including conventional and
inverted devices), the interfacial materials applied at both cathode and anode will
be separately reviewed in the categories of inorganic materials (including metals,
salts, semiconducting oxides, etc.), self-assembled monolayers (SAMs) or SAMs
modified functional layers, as well as organic/polymeric materials. For tandem
devices, we will focus on the functional materials used as intermediate connectors.
3.2 Interface Engineering for Conventional Solar Cell Devices
3.2.1 Cathode Interlayer
For a conventional BHJ-PSC, the efficient electron extraction at cathode is of
critical importance for the high J sc and the overall photovoltaic performance.
Aluminum (Al) is a most widely used electrode materials for BHJ-PSCs because
of its abundance and proper work function of 4.3 eV, which is very close to the
lowest unoccupied molecular orbital (LUMO) energy levels of acceptor materials
such as (6,6)-phenyl-C 61 -butylric acid methyl ester (PC 61 BM) and (6,6)-phenyl-
C 71 -butylric acid methyl ester (PC 71 BM) (see Scheme 3.1 ). However, the organic
optoelectronic devices with Al cathode usually suffer from poor stability, because
the reactive hot Al atoms generated at the thermal evaporation process can diffuse
into the organic layer and result in chemical reaction at the metal/organic interface
[ 17 , 18 ]. Moreover, it was reasoned that Al-C bond formation at the Al/organic