04-10-2012, 05:26 PM
Zn-Phthalocyanine / C60 Solar Cells
Zn-Phthalocyanine.pdf (Size: 1.33 MB / Downloads: 418)
ABSTRACT
Organic bilayer heterojunction solar cells using zinc - phthalocyanine as
p-type and fullerene C60 as n-type material were investigated.
AFM measurements show clusters at the surface of the organic layers. The
cluster size and therefore the film roughness of the organic layers depend on
the growth rate. Photoluminescence measurements indicate that only a
small region around the interface contributes to the photocurrent.
Nevertheless the devices with 50 nm ZnPc and 75 nm C60 have the highest
efficiencies. Current-voltage measurements reflect that under illumination
strong photoconductivity (secondary photocurrent) has to be taken into
account and many devices show back diodes.
Voltage dependent photocurrent measurements show a square root
dependence of the photocurrent signal which can be attributed to the
primary photocurrent with applied voltage. This agrees with the Gärtner
model and supports the existence of a space-charge-region at the interface.
INTRODUCTION
World Energy Demand and Solar Cells
The world energy demand is steadily increasing. Fossil fuels and nuclear
energy are at the moment the main energy sources for the world. All
these resources are limited. The peak of depletion of mineral oil might be
as soon as 2006 [1]. A big problem is that the CO2 emissions of burned
fossil fuels cause global warming. Uranium does not cause CO2 emissions
but has always been under intensive public discussions because of the
imminent danger of nuclear power stations and the following problems
with the radioactive waste.
All these problems would be solved by using renewable energies. Solar
energy is an unlimited energy source that can be used all over the world.
The European Union plans to increase the contribution of renewable
energies from 14 to 23.5 % until the year 2015. By then photovoltaics
installed capacity shall be increased by a factor of 100 [2].
At the moment the main part of installed photovoltaic modules are silicon
based. However production of crystalline silicon solar cells is extremely
energy and therefore cost intensive. One way to reduce the costs are
organic thin film solar cells. Organic dyes fulfil many requirements to be
used in solar cells: They have high absorption coefficients and many of
them show p- or n-type semiconductor behaviour. Various cheap coating
technologies are available: spin coating, doctor blading, thermal
evaporation, ink jet printing, etc. Organic solar cells are light weight and
have high environmental sustainability. Another advantage is the
possibility to change the colour of an organic solar cell for architectural
purposes [3].
Aim of this thesis
Since Tang reported on organic solar cells using copper-phthalocyanine
and a perylene derivative reaching 1 % efficiency [5] enormous efforts
have been made to improve organic solar cells. One type of organic solar
cells are those prepared from small molecules: In this thesis solar cells
consisting of zinc-phthalocyanine (ZnPc) and Buckminster fullerene (C60)
are investigated. In previous investigations very often CuPc and ZnPc
have been used. Efficiencies obtained for CuPc and ZnPc are higher than
those using other central atoms in the phthalocyanine. ZnPc was chosen
as p-type material here. In previous investigations [6, 7, 8] MPP [9] and
ZnPc were used. Because of recently reported [10] high efficiencies using
C60 as electron acceptor n-type material it was used in this work
Theory
Workfunctions
The nature of a contact between a semiconductor and a metal or between
two semiconductors is determined by the workfunctions: the energetic
differences between the Fermi levels and the vacuum levels. The relative
values determine whether the contact is either an ohmic or a Schottky
contact (see 1.4.2).
Figure 1 shows the different workfunctions (EF) for ITO, PEDOTSS and Al
together with the conduction band (Ec) and valence band (Ev) level of
ZnPc and HOMO (highest occupied molecular orbital) and LUMO (lowest
unoccupied molecular orbital) of C60. For the influence of LiF on the Al
contact see 2.1.3. The workfunction of ITO heavily depends on the
preparation technique and various surface treatments, therefore a
maximum and a minimum value found in literature are shown.
Excitons
Another approach to describe the basic mechanism of charge carrier
generation is via excitons [11, 12]:
The absorbed photon leads to an excited state, the exciton. In this exciton
the hole and the electron are bound to each other, therefore they can be
described as one non-charged particle. This exciton can diffuse. Pettersson
et al. give a number for the average exciton diffusion length in C60:
7.7 nm [38]. If the exciton reaches a hetero-contact, the electron and the
hole are separated immediately. Afterwards charges may reach the
electrodes. If the electric field of a space charge region is high enough
(e.g. larger than 105 V / cm), excitons might also be split here.