Hybrid photovoltaic cells are electronic devices
designed for conversion of light into the noblest form of energy,
electricity. They are cost effective alternatives to
conventional silicon solar cells. Although their power
conversion efficiency has been low so far, they can potentially surpass the
conventional silicon cells not only in the production
cost but also in performance because of their unique
architectures combining organic and inorganic materials.
Figure 1 Hybrid solar cell: a) ZnO nanowires, b) flocky ZnO nanowires;
c) architecture of a polymer infiltrated solar cell.
The project is engaged in development and
investigation of novel hybrid photovoltaic cells that will form a base for cost effective devices
with high power conversion. The design of organic/inorganic photocell structures with
implementation of new fabrication approaches and study of nanomaterial interfaces is the
essence of the project. Particularly ZnO nanostructures with different morphologies
including vertical ZnO nanowires and flocky nanorods (Figure 1a and 1b) prepared by
simple methods are investigated and used in the designed photovoltaic device structures
(Figure 1c). The ZnO nanowires, inherently n-type semiconductors,
are infiltrated by organic p-type conducting materials to provide
large-area p-n heterojunctions. The problem of the size and
functions of the heterojunction interface and the infiltration
by organic semiconductors is specifically investigated
for wettability, electronic interfacial structures, effective
dissociation of photo-induced excitons, their diffusion length
and interfacial field separation. It is foreseen that within the
designed photovoltaic cells the charge recombination
process will be suppressed considerably to provide a
high conversion efficiency of light to electricity. Optimizing
the electrode configuration and possible suppression
of charge recombination in different parts of the devices
by engineering the interfaces in hybrid photovoltaic devices will lead to considerable
improvement of the device efficiency.
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The project team from left: C. Yan (PhD student), Dr O. Kutzay, Dr S.K. Jha, Dr J. A. Zapien (Assistant Prof, Co-I), Prof I. Bello (PI), Dr W.J. Zhang (Associate Prof, Co-I), H.E. Wang (PhD student), Z.H.Chen (PhD student) and C.P. Liu (PhD student).
The project focuses on fundamental
engineering strategies for improving the conversion efficiency. The fact that only about
a third of the excitons induced in the distance of the diffusion length (2-20nm) from the
interface can reach the dissociative interface formulates the first research problem, i.e.,
optimization and control of the interspacing in nanowire
arrays. The equally important task is providing a large electrically active interface
between the p-type conjugated polymer and n-ZnO nanostructures.
The large electrically active interface between the p-type
conjugated polymer and n-ZnO nanostructures is especially important. This
provision can hardly be met with technologies used so
far, since i) large molecules of conjugated polymers
can difficult to creep into small interspacing of the
nanostructures, ii) adsorbed gases and gases present
in cavities are enclosed at interface by infiltrated polymers and iii)
poor wettability of the nanostructure to the conjugated polymers causing electrically
inactive interfaces and interfacial separation. As a result the project
introduces a novel technology to infiltrate ZnO nanostructures. The
arrays of nanowires are outgassed at elevated temperature in
vacuum and then infiltrated with solutions of conjugated polymers
in a protective environment and under ultrasonic agitations. The
proposed procedures of infiltration are anticipated to be crucial for
final device performance. Since, wettability, dissociative electric field
and open circuit voltage of final devices, which are closely related
to the nature of interfaces, as well as functionalization of
surface are investigated thoroughly too.
Prof Igor BELLO
Department of Physics and Materials Science
City University of Hong Kong
apibello@cityu.edu.hk
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