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Metal-based semiconductor 8

nanomaterials for thin-film

solar cells

Wenxi Guo, Zijie Xu, Teng Li
Xiamen University, Xiamen, China

8.1 Introduction

Searching for “green,” renewable, and sustainable energy resources is one of the most
urgent challenges our civilization faces, particularly as it relates to the threat of energy
crises and environmental pollution [1–6].
As one of the most promising renewable energy resources for sustainable de-
velopment of the future, thin-film solar cells, such as quantum dot solar cells
(QDSCs), dye-sensitized solar cells (DSSCs), organic photovoltaics (OPVs), per-
ovskite solar cells (PSCs) have received extensive attention because of their easy
fabrication, cost-effectiveness, and tunable optical properties that permit varied
aesthetic qualities [7–10]. A typical thin-film solar cell is usually a sandwiched
structure with two electrodes (cathode and anode) outside and a photoactive layer
inside. Fig. 8.1 shows the three thin-film solar cells discussed here, DSSC/QDSC,
PSC, and OSC. The DSSC/QDSCs’ metal-oxide semiconductor layers are usually
mesoporous and act as a scaffold for the adsorption of dye molecules and quantum
dots. The metal- oxide semiconductor layers in PSC can act as a compact layer,
avoiding short circuits; also, the porous scaffold presenting under perovskite may
help promote crystallization or increase the surface area. As shown schematically
in Fig. 8.1C, OSCs have two typical configurations, conventional and inverted, ac-
cording to the charge flow direction. When the light irradiates on the active layer of
a conventional OSC, the photogenerated excitons separate into holes and electrons
and diffuse toward the highest occupied molecular orbital (HOMO) of the donor
and the lowest unoccupied molecular orbital (LUMO) of the acceptor, respectively.
Then the separated holes and electrons are collected by the opposite electrodes
after transporting within the respective phases of the active layer. In this regard,
the energy level structure at electrode interfaces is critical to prevent carriers from
reaching the opposite electrodes [11,12]. An ideal interface layer needs good ohmic
contact with high charge selectivity and minimum resistance. Moreover, the work
functions (WFs) of the interface materials should match the energetic levels of the
donor and acceptor materials.
Two groups of metal-based semiconductor layers are categorized according to
their unique features: (i) semiconductor nanomaterials as interfacial materials for

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