Hybrid Solar Cells with Polymer and Inorganic Nanocrystals - Organic Solar Cells: Materials and Device Physics - page 242

polymers and fullerene derivatives, which are all organics, therefore, such devices
are also called BHJ organic solar cells (OSCs). When the fullerene derivatives are
replaced by their inorganic counterparts such as CdSe [ 8 - 12 ], CdS [ 13 - 15 ], TiO 2
[ 16 - 22 ], ZnO [ 23 - 31 ], SnO 2 [ 32 ], CuInS 2 [ 33 ] etc., the devices are then named
hybrid solar cells (HSCs). Those inorganic materials have the advantages of high
dielectric constant, high electron mobility and affinity, and good thermal stability
and their optoelectronic properties can be tuned by changing their sizes and shapes,
which could facilitate the design of high performance HSCs [ 34 - 38 ].
HSCs have been developed for years and insights of the underlying physics
were gradually disclosed. Various inorganic nanocrystals with different mor-
phologies, such as nanoparticle, nanorods, nanowires, and vertically aligned
nanorod-, nanowire-, nanotube arrays, have been used together with many con-
jugated polymers. Both the materials and the nanofabrication techniques are
critical issues to the HSCs, which have been developed by many research groups
recently. Although the current PCEs of the HSCs are still much lower than the
OSCs and DSSCs, higher PCEs are expected in the future when the device physics
is further understood and new breakthroughs in synthesis of materials and device
fabrication are achieved.
In this chapter, an overview of device structure and operation principle will be
first introduced. Then, the developments of HSCs will be reviewed in view of the
morphologies of the inorganic nanomaterials. Finally, the interface modification
that would help to improve the device performance will be discussed. As there are
hundreds of papers have been published in this area, we will not go through all of
them but give a glance at the most representative and interesting results.
9.2 Device Physics and Device Structure
The HSCs work in the same way as OSCs, where the conjugated polymers serve as
light absorber and electron donor (D), the inorganic nanocrystals serve as electron
acceptor (A). Figure 9.1 gives the schematic illustration of the energy level
alignment and the photocurrent generation mechanism in HSCs. Upon illumina-
tion, the photocurrent is generated via the following processes [ 36 , 38 ]:
(1) Photo absorption and exciton generation. The conjugated polymers mainly
account for the light absorption in HSCs. In some cases, the inorganic nano-
crystals could also absorb light, but the majority of the light absorption are
attributed to the conjugated polymers. The light harvesting efficiency (g LHE )is
determined by the bandgap (E g ) and the absorption coefficient of the polymers.
After absorbing the incident photons, electrons will be excited from the
highest occupied molecular orbit (HOMO) to the lowest unoccupied molecular
orbit (LUMO). Due to the low dielectric constant of the polymers, the elec-
trons in LUMO and the holes in HOMO are not free charges but excitons with
strong Coulomb interaction. The exciton binding energy (E b ) is typically