Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 89

2 Sensitized Solar Cells

As commented in the introduction the fundamentals of sensitized solar cells is to
decouple light absorption and charge transports making that these processes occur
in different media. When light is absorbed by molecular dyes the devices are called
Dye-Sensitized Solar Cells (DSSCs), and are the most extensively studied class of
sensitized devices [ 6 ]. But here we are going to focus our attention in the case
where inorganic semiconductor are used as light absorbing materials instead of
dyes giving place to the Semiconductor-Sensitized Solar Cells (SSSCs) [ 7 ]. When
the size of the semiconductor material is small enough to observe effects due to
quantum confinement (particle radius lower than the Bohr radius) the devices is
called Quantum Dot-Sensitized Solar Cells (QDSSCs) [ 8 - 10 ]. The line separating
SSSCs from QDSCs is fuzzy and many times the denomination QDSSCs is pre-
ferred even in cases where quantum confinement is not observed.
The working principle of QDSSCs is fundamentally similar to DSSCs and they
are represented in Fig. 1 . A wide bandgap semiconductor material is sensitized
with an semiconductor with a bandgap in the visible or near IR region. The most
extensively used wide bandgap semiconductor has been TiO 2 , but several exam-
ples of the utilization of other ones as ZnO [ 11 ] or SnO 2 [ 12 - 14 ] have been
reported. Light irradiation photo excites electron-hole pairs from the QD Valence
Band (VB) to the QD Conduction Band (CB). Photoexcited carries are injected
into two different transporting media. Electrons are injected into the CB of the
wide bandgap semiconductor, while hole are injected into a hole transporting
material (HTM). Then both carriers diffuse to their respective contacts. In order to
optimize cell performance, recombination of diffusing carriers should be avoided.
As the light absorbing layer is extremely thin effective surface area is significantly
enhanced with the use of a nanostructured electrode, increasing consequently the
light harvesting [ 5 ]. This description is perfectly valid also for DSSCs. In this
sense, in a first analysis it could be thought that semiconductor QDs are only one
more of the thousand of dyes that have been checked in DSSCs. But we want to
highlight here that it is not the case. The different nature of QDs in comparison
with molecular dyes makes that the complete design of the solar cell device have
to be rethought.
Main differences between DSSCs and QDSSCs could be divided into five
aspects:

(i) Preparation of the sensitizer.

(ii) Nanostructured electrode.

(iii) Hole Transporting Material (HTM).

(iv) Counter electrode.

(v) Recombination and surface states.

In the next five sections, we will develop each of these points reviewing the
current state of the art of QDSSCs.