Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Organic photovoltaics – chosen aspects
Justyna Szostak, Ryszard Signerski, Kamila Żelechowska, Jan Godlewski
Gdańsk University of Technology, Department of Physics of Electronic Phenomena, Faculty of Applied Physics and Mathematics
Preparation of inorganic solar cells usually requires very costly
and complicated high temperature vacuum deposition techniques.
Furthermore, materials used for fabrication of these cells are
expensive and often dangerous for the environment, not to mention
the hazardous and toxic substances used during manufacturing
process and its by-products [1]. Hence, organic solar cells
(OSCs), that can be fabricated from relatively cheap materials,
using simple solution coating techniques, i.e. ink-jet printing or
spin and dip coating, which do not require high temperatures nor
the usage of hazardous substances, seem to be a promising alternative
for the inorganic photovoltaics. Moreover, the number
of organic materials is much greater than the number of inorganic
ones, so the potential of applications of organic materials in
photovoltaics is greater as well. However, relatively low efficiency
and fast degradation of organic solar cells [2, 3] are serious drawbacks
that hinder commercialization of organic photovoltaic diodes.
Designing organic solar cells with higher energy conversion
efficiency and stability is impossible without extensive knowledge
on all physical and chemical processes taking place inside organic
photovoltaic devices in the dark and under illumination. Even
though there are many experimental works showing the performance
of OSCs, theoretical model for the photovoltaic effect taking
place in these devices has not been developed yet. Significant
differences in structure of organic and inorganic materials
result in different types of interactions between molecules. Thus,
even though the mechanisms of photovoltaic effect in inorganic
systems are well known and described in literature, processes
that affect photovoltaic phenomenon in organic systems still need
further understanding. Thus, research we run in our laboratory is
focused on analysis of the above mentioned processes.
Photovoltaic phenomenon in organic systems
In organic semiconductors, absorption of light leads to creation of
columbically bound electron-hole pairs – excitons [4]. Low dielectric
constants of organic materials result in high exciton binding energies
(reaching 1.0 eV), so electron and hole cannot escape each
other and they remain bound. Such pairs may diffuse to dissociation
sites (e.g. interfaces of two different materials) where charge
transfer (CT) states are created and subsequently dissociation
of excitons into free charge carriers may occur. To collect these
charge carriers a built in electric field, that will drive them towards
respective electrodes, is necessary. The performance of organic
Fig. 1. Scheme of charge carrier photogeneration at the junction
of two organic materials. E F
– Fermi level, E g
– energy gap; HOMOhighest
occupied molecular orbital, LUMO – Lowest Unoccupied Molecular
Orbital; I in
– incident light intensity; 1 – excitation of material
II, 2 – exciton diffusion towards the junction of two organic materials,
3 – exciton dissociation into free charge carriers
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photovoltaic cells is limited by the efficiency of many processes
that constitute the photovoltaic phenomenon. Among others, limitations
are inefficient exciton dissociation and poor charge carrier
transport resulting from high rates of exciton recombination (short
exciton lifetimes) and low charge carrier mobilities. Also potential
barriers created in the near-electrode regions stand a serious problem
that limits the efficiency of charge carrier collection. Probability
of exciton dissociation into free charge carriers is quite high when
it reaches an interface with another material of suitable energetic
structure – there must be an offset between HOMO (the Highest
Occupied Molecular Orbital) of one material and LUMO (the Lowest
Unoccupied Molecular Orbital) of the other material (Fig. 1).
This is why efficient organic solar cells comprise two layers of different
materials and the larger their interface is the higher is the
number of photogenerated charge carriers. Molecule that donates
an electron in such system is called a donor (D) while the one that
accepts this charge carrier is called an acceptor (A). Energy difference
between LUMO of the acceptor and HOMO of the donor
determines the maximum open – circuit voltage (U oc
) [5].
Results and scope of research
Due to high probability of exciton dissociation at the junctions of
two different materials future of organic solar cells lays in devices
with active layer comprising two or more thin films, blends of organic
materials or hybrid systems (based on organic and inorganic
materials). Processes that play a significant role in the photovoltaic
phenomenon strongly depend on the type of organic materials
forming an active layer, this is why a thorough investigation of
performance of OSCs comprising various groups of organic materials
is inevitable.
A particularly interesting new generation of OPV systems utilizes
semiconducting conjugated polymers. This type of cells is referred
to as ‘polymer solar cells’ or ’plastic solar cells’. The principles
of operation of these solar cells have been a topic of various
articles, because plastic solar cells are expected to offer several
advantages over silicon-based ones due to the fact, that polymers
are mechanically flexible, semi-transparent and soluble in common
organic solvents. The latter property is a crucial advantage
of polymers since it allows deposition of these materials by simple
solution processing and printing technologies. Particular attention
in the field of polymer solar cells is paid to bulk heterojunction
solar cells (BHSC). In this type of cells donor and acceptor materials
are mixed, so the active interface exist within the whole
bulk of the active layer. However, it is much easier to determine
the influence of different factors on the performance of a cell and
to analyse the course of observed processes when structure of
a cell is less complicated. That is why in the course of our investigations
double-layer devices instead of bulk ones were taken into
consideration. We investigated the role of anodic and cathodic
buffer layers on the performance of polymer solar cells. Buffer
layers are thin films put between an anode or cathode and an
active layer. They are introduced to organic solar cells to improve
the efficiency, but the role they play in various systems is not fully
understood. The active layer of our cells comprised MEH-PPV
(poly[2-methoxy-5-(2-ethylhexyloxy-p-phenylenevinylen)])) – soluble
polymer used commonly as a donor material and F 16
ZnPc
(hexadecafluorozincphthalocyanine) – small molecule that is an
acceptor in this system. As buffer layers MoO 3
and BCP (bathocuproine)
were used. All the investigated systems were sandwiched
between ITO (indium tin oxide) and Ag electrodes. There
are many theories on the effect of MoO 3
and BCP interlayers, for
example, thin films of MoO deposited on the top of ITO electro-
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Elektronika 6/2012