Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 88

Nanoscale considerations have been also important implications in the devel-
opment of low cost photovoltaic devices, from the point of view of the funda-
mental principles. Conventional Si solar cells rely on high quality materials since
the carriers generated in the device after photon absorption remain in the same
material until they are extracted at the selective contacts [ 4 ]. This involves the use
of sophisticated technologies with high production cost in order to avoid carrier
recombination before their extraction. Conversely, nanoscale absorbers can
quickly separate the photogenerated carriers into two different media which allows
for a less-demanding materials quality and therefore cost reduction, as transport of
electrons and holes is occurring in different materials and recombination is
reduced. These are the fundamentals of the new paradigm introduced to the
photovoltaic conversion by the sensitized solar cells that have received a major
attention since the seminal paper of O'Regan and Grätzel at the beginning of the
1990s [ 5 ]. Most of the work carried out in sensitized solar cells has been made
using organic and/or metalorganic molecular dyes acting as light absorbing
materials. The use of inorganic semiconductors as nanoscale light harvesters has
been minority. But the undoubted photovoltaic properties of these materials have
pushed the research in this field with a continuous growing interest in the last few
years. This interest has resulted in a constant growth of the reported efficiencies.
This fact with the easy preparation and its low cost make of the inorganic semi-
conductor sensitized solar cells one of the fields of photovoltaics that is experi-
enced higher growth. This chapter is devoted to the study of these systems
focusing mainly in two aspects the low cost preparation and the role of nanoscale
design and the advanced structures for solar cell preparation
This chapter is structured in nine sections. The first one is this brief introduction.
In the second, we analyze the fundamentals of sensitized solar cells, explaining the
specificities of dye, semiconductor, and quantum dot solar cells, and reviewing the
state of the art. In Sect. 3 , the experimental techniques used for semiconductor light
absorbing material deposition are revised, stressing the fact of their low cost and
easy industrial implementation. In the fourth part, new promising substrate struc-
tures in the case of inorganic semiconductor absorber are revised, in order to
highlight the role of nanoscale and the possibilities of design. In Sect. 5 , the hole
transporting material is revised for both liquid electrolytes of all solid devices. In
Sect. 6 , the counter electrode material is determined by the hole transporting
material and proper choice has to be done as we analyze in this section. In the
seventh, the effect of carrier recombination before being collected at the output
terminals, the role of surface stats in the light absorbing materials has overviewed. In
the penultimate part, a new class of light absorbing materials ''halide perovskites''
has arisen and made revolution in this field. Despite there are several proves indi-
cating that this kind of devices are not fully behaving as a sensitized device, it has
been the intense work in sensitized solar cells that have allowed to achieve this
breakthrough. Therefore, we consider that it is important to include these systems in
this chapter. This new hot topic in photovoltaics is also taking benefit of the
knowledge and characterization techniques developed in the last decades for sen-
sitized devices. Finally, very briefly conclusions are highlighted.