Environmental Engineering Reference
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to get efficient charge separation, HOMO and LUMO of the donor material should
be 0.2-0.3 eV higher than that of the acceptor material, respectively. If the offset is
too small, it would be hard to get efficient charge separation; if the offset is too big,
much energy loss would be happened. As known, open-circuit voltage (V
oc
)of
BHJ OPV devices are directly proportional to the gap between HOMO of the
donor and LUMO of the acceptor [
2
]. Although, the energy of the photon that can
be utilized by the P3HT/PCBM system is higher than 2.0 eV, V
oc
of P3HT/PCBM-
based OPV device is typically around 0.6 eV, meaning that more than 70 %
energy loss is taking place during the photoelectric conversion process. Therefore,
to minimize the energy loss, HOMO and LUMO levels of the donors and the
acceptors should be tuned carefully.
Furthermore, mobility of the donor and the acceptor materials is also an
important issue for organic photovoltaic materials. In comparison with inorganic
semiconductors, organic semiconducting materials exhibit much lower mobility,
and therefore, how to improve hole or electron mobility of organic photovoltaic
materials becomes one of the critical objectives of molecular design of materials.
For an organic semiconducting material, both inter- and intra-molecular charge
transfer properties are very important. The relationship between intra-molecular
charge transfer property and molecular structure is still unclear. To enhance inter-
molecular stacking properties has been proven to be an effective way, to improve
inter-molecular charge transportation. For examples, the hole mobility of regio-
regular P3HT is 2-3 orders higher than regioregular P3HT due to the stronger pi-
pi stacking property of the former [
3
]; to reduce the steric hindrance caused by
non-conjugated side chains is also an useful approach to improve the inter-chain
pi-pi stacking property of organic semiconducting materials and hence higher
mobility can be realized [
4
].
Besides absorption band, molecular energy levels (HOMO and LUMO), and
mobility, there are still many other issues like solubility in different solvents and
chemical stability, should be considered in molecular design of organic photo-
voltaic materials. Therefore, how to balance these properties is the key to get an
organic photovoltaic material with ideal properties. Herein, several broadly used
material systems will be introduced in this chapter to provide a general profile of
molecular structure design of organic photovoltaic materials.
Poly(phenylene vinylene)s (PPV) and Polythiophenes (PTs) are two kinds of
classic conjugated polymers which are broadly used in photovoltaic cells and light
emitting diodes. MEH-PPV [
5
] and MDMO-PPV [
6
], as shown in Scheme
2.1
are
two representatives of PPV-based materials. These two PPVs can be used as
electron donor materials in solar cells, and they exhibited much similar photo-
voltaic properties. PCE of *2 % has been recorded by using MEH-PPV/PCBM-
based solar cells. However, the absorption edges of MEH-PPV and MDMO-PPV
are at about 550 nm, corresponding to a band gap of ca. 2.3 eV, and output current
density of the solar cells based on them was limited due to the big mismatch
between their absorption spectra and the solar irradiation spectrum. In comparison
with MEH-PPV and MDMO-PPV, P3HT, one of the derivative of poly(3-alkyl-
thiophene) exhibits lower band gap, broader absorption band and also better hole
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