Design and fabrication of excitonic solar cells.

Abstract

The excitonic solar cells (XSCs), including organic solar cells (OSCs) and dyesensitized

solar cells (DSSCs), have attracted a great interest due to their huge potential of

low cost technology compared to conventional silicon solar cells. Although the

technologies of XSCs have advanced significantly, XSCs still need to be improved in

various aspects to become tangible in energy market. The important criteria of a solar cell

are efficiency, cost and life time. Hence, the research in this dissertation focuses on the

design of XSCs with better choice of materials and device architecture for either

enhancement in stability and efficiency or reduction of cost.

In spite of over 7% power conversion efficiency, the OSC based on bulkheterojunction

concept has limitation in device stability due to diffusion of oxygen into

the organic layer through pinholes and grain boundaries in Al cathode and the degradation

of transparent conductive oxide (TCO) electrode, which is etched by poly (3,4-ethylene

dioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS) buffer layer. To overcome this

problem, an inverted structure was implemented. The reverse polarity of charge collection

in an inverted structure allows the usage of air-stable high-work-function metal as top

electrode and gets rid of TCO/PEDOT:PSS interface. In our design, TCO is modified with

sol-gel derived zinc oxide (ZnO) to exclusively collect electrons from active layer and

block holes. A thermal-evaporated molybdenum oxide (MoO3), which is inserted between

active layer and top electrode, increases the fill factor of the device due to exciton/electron

blocking property. It was observed that the efficiency of an inverted structure OSC can be

further improved by manipulating the resistivity, energy level and optical property of ZnO

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layer with appropriate amount of indium doping. We also verified that the stability of

device in air is significantly improved by inverted structure.

DSSC, another type of XSC, is also a promising alternative to silicon photovoltaic

technology. However, it is estimated that conducting glass is the most expensive part of

DSSC and it incurs 60% of total cost. Therefore, we designed top-illuminated structure

which can be fabricated on inexpensive opaque substrates such as metals or plastic foils

with metal coating. Although the efficiency of the top-illuminated cell is about 20% lower

than the traditional bottom-illuminated cell, it reduces the cost of DSSC tremendously by

eliminating the usage of expensive TCO. Ti is more suitable to be used as electrode in

top-illuminated DSSC than other metals because of minimum catalytic activity on redox

reaction and high resistance to corrosion. Another approach to eliminate TCO is replacing

with transparent carbon nanotube (CNT) electrode. However, the catalytic activity to

redox reaction limits its application as working electrode in DSSC. Therefore, the

implementation of DSSC with CNT electrode was realized by modifying CNT with

titanium-sub-oxide (TiOx) which inhibits the charge-transfer kinetic at CNT/redox

solution interface and facilitates the unidirectional flow of electrons in the cell. To our

best knowledge, this is the first demonstration of CNT as working electrode for liquidtype

DSSC. Based on this finding, we also realized DSSC with all carbon electrodes.