Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
Abstract- Conventional solar cells were
invented in the 1950s, and since then, there
has been rapid growth in the reliability and
efficiency of the devices. Even so, the cost of
solar electricity is not yet competitive with
the price of electricity from the electrical
grid. As researchers invest in alternative
ways to produce less expensive solar cells,
solar cells based on conjugated polymers and
organic dyes have gained recent interest. In
this paper, an overview of organic
photovoltaics is given, contrasting them with
inorganic solar cells. Furthermore, the three
main types of organic photovoltaic cells are
discussed, as well as methods being explored
to continually optimize their power
conversion efficiency.
I. Introduction
Even though the first solar cell was made
from selenium wafers back in 1883, it was not
until 1954 that researchers at Bell Labs created
a silicon solar cell with 6% efficiency. By 1959,
9% efficient photovoltaic devices were
developed, and the first photovoltaic-powered
satellite was launched. In 1985, the 20%
efficiency barrier was broken for silicon solar
cells, and in 1994, an inorganic solar cell was
developed with over a 30% conversion
efficiency. Meanwhile, breakthroughs in organic
photovoltaics were happening in the late ‘80’s
and early 90’s with conversion efficiencies of up
to 10% with dye-sensitized solar cells.
The emergence of photovoltaics, the
conversion of sunlight into electrical energy, is
important for energy technology for multiple
reasons. From an environmental perspective, it
produces no air pollution or hazardous waste. As
its energy source comes from the sun, it
contributes to a nation’s energy security, and
creates jobs. If photovoltaic devices can be made
economically competitive with fossil fuels and
other renewable energy technologies, these
devices will offer a sustainable energy source
that can help meet our energy needs.
When comparing solar energy with other
energy sources, the intermittence of wind rules
out many locations for wind turbines, which
limit the amount of energy they can produce. On
the other hand, from the 1.7e5 TW of solar
energy that hits the earth’s surface, a practical
solar potential value is estimated to be 600TW.10
Therefore, 60TW of power could be supplied
using 10% efficient solar farms. From this stat
alone, it is apparent that solar energy is a huge
resource to be tapped, for global energy
consumption is projected to be only 30TW in
2050. 10
Coal, though more economically feasible
than any other alternative energy source, yet it
pollutes the environment, and as we are
stewards of the planet, it is important to
research cleaner ways to create energy. This
paper details certain aspects of photovoltaics,
with different factors discussed contrasting
what limits the achievable photovoltage in
organic photovoltaic cells in respect to inorganic
photovoltaic cells.
II.
Solar Spectrum
The sun emits light with wavelengths that
range from ultraviolet and visible to infrared.
Yet, the light is influenced by atmospheric
absorption, as well as the position of the sun.
Ultraviolet light is filtered by the ozone layer,
and CO2 and water absorb mainly the infrared
light.10 These absorptions cause dips in the solar
spectrum. When there are clear skies, the
maximum radiation hits the earth when the sun
is directly overhead, since the light has the
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
shortest path length through the atmosphere.
This path length is called the AM (air mass). The
accepted solar spectrum used today for
efficiency measurements of solar cells is the AM
1.5 Global. The spectrum is normalized so that
the amount of radiant energy received from the
sun per unit area per unit time is 1000 W m-2.10
The AM 1.5 G is shown in Figure 1 below, and will
be referenced later in this paper.
These conjugated polymers are semiconductors
that react with light. Conjugated polymer EV
band gaps are usually 1.5-3eV, higher than the
1.1eV of commonly used silicon for inorganic
solar cells.
The length of the polymer has important
implications when determining the nature of the
produced excited states (excitons) when
photons are absorbed. In Figure 2, there are two
important conclusions to take away. First, since
the band profile does not vary, it must be
independent of the number of coupled
monomers, and a property of the monomer
itself. Secondly, it is shown that the higher
number of monomers in a polymer, the lower
the frequency.11
Figure 1: Photon Flux of AM 1.5 G spectrum at 1000 W m-2
with calculated photocurrent.
The energy received by an exciton is
proportional to the energy of the absorbed photon.
The energy of a photon is dependent only on its
frequency (𝑣), or inversely, by its wavelength (𝜆).
This can be seen below, with h being the Planck
constant, a physical constant reflecting the
proportionality between the momentum and
quantum wavelength of the photon.
𝐸=
III.
ℎ𝑐
= ℎ𝑣
𝜆
Organic Photovoltaics
Organic photovoltaic material is made up of
semiconducting polymers. The large majority of
polymers have similar electrical and optical
properties, being insulators and colorless.11 Yet,
there is a special type of polymer with
conjugated double bonds in the main chain.
Figure 2: Optical absorption spectra of two-five ringed
oligomers of phenylene vinylene.
The potential of organic semiconducting
material to transport electric current and absorb
light is due to the sp2 hybridization of carbon
atoms, as seen in Figure 3 below. The π bonds
bonding the electrons together are of a
delocalized nature, resulting in electronic
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
polarizability.6 The highest occupied molecular π
orbital is referred to as HOMO, and the lowest
unoccupied molecular π orbital is called LUMO.
As stated above, the gap in energy between
these two orbital states is usually between 1.5eV
and 3eV.11 The consequences of the gap energy
will be discussed in greater detail later.
Figure 3: Orbital structure of ethene, showing sigma and pi
bonds.
Conjugated polymers are polycrystalline
systems, made up of crystalline and amorphous
regions, with the crystallites often containing
large amounts of defects. Because of the many
defects, the range of motion of an exciton is
limited to polymers with sizes in the nm range.11
Furthermore, it is important to recognize
that the mobility of injected charge carriers is
not symmetric in conjugated polymers. The
electron mobility is usually lower than the hole
mobility, which plays a role in optimizing solar
cell performances of organic devices.2
From a practical perspective, the lifetime of
an organic solar cell is important, not just the
power conversion efficiency. In OPV devices, the
stability of the solar cells is affected the most by
the photodegradation of active materials.
Organic materials are susceptible to reactions
with oxygen and water, and oxidation is
regarded as one of the most common forms of
photodegradation.7
IV.
Organic vs. inorganic
There are multiple well-known differences
between the photon conversion mechanisms of
organic photovoltaic cells and inorganic
photovoltaic cells. Most importantly, the light
absorption in organic photovoltaic (OPV) cells
leads to the creation of excitons, whereas in
inorganic photovoltaic (IPV) cells the
absorption of photons leads to the production
of free electron-hole pairs.4 This phenomenon
happens for two main reasons. First, as seen in
Figure 10, since the dielectric constant in
organic solar cells is comparably lower when
contrasted with inorganic semiconductors, the
Coulomb potential well surrounding the
electron-hole pair has a much larger impact, for
much higher energy is needed to dissociate the
excitons. Because of this low dielectric constant
(usually 2-4 compared to 10+ in inorganic
material), the electron-hole pair are bound
tightly, and will circle each other without being
impacted by an electric field. Secondly, the noncovalent electronic interactions between
organic molecules are weak (because of their
narrow band width) compared with the strong
electronic interactions of covalently bonded
inorganic material. This spatially restricts the
electrons wave form, which allows it to become
localized.4 The lack of mobility in organic
material will be seen as a major limiting factor
in developing more efficient OPV cells, as will
be discussed in a later section.
An exciton is a mobile excited state, which
is, a bound state of an electron and hole, that
are attracted to each other by an electrostatic
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
Coulomb force (the Coulomb barrier is the
energy barrier brought about from electrostatic
interaction between two electrically charged
particles).6 An exciton, since it has no net
charge, has the ability to transport energy
without transporting electric charge. The
difference of photons releasing an exciton
versus the release of free electron-hole charges
leads to fundamental consequences for efforts
towards optimizing the performance of OPV
cells.4 The process of producing a substantial
photovoltaic effect in OPV materials will be
discussed more in detail later in this paper.
Figure 4: Figure Illustrating the difference in charge-carrier
generation between inorganic and organic solar cells.
As seen in Figure 4 above, it is clear that
electrons and holes are photogenerated
wherever the light is absorbed, and the
chemical-potential-energy gradient (Δμhν),
which is represented by arrows, drives the
carriers in the same direction. Yet in OPV cells,
it is shown in the figure that electrons and holes
are both photogenereated in different phases
due to exciton dissociation at the interface.
Because of this, Δμhν drives the holes and
electrons in opposite directions.4
Another important difference between IPV
and OPV cells lie in the differences in chargecarrier mobility. Unfortunately, organic
materials have fairly poor charge carrier
mobility compared to inorganic material
(magnitudes lower).6 Yet, unlike inorganic solar
cells, OPV materials have extremely high optical
absorption coefficients that offer the possibility
for production of thin solar cells. 1 This helps
balance out the low mobility.
As stated above, most organic
semiconductors have an optical band gap
around 2eV, which is much higher than many
inorganic semiconducting materials, which
greatly limits the harvesting of the solar
spectrum.6 Because the absorption bands of
conjugated polymers are narrow compared
with IPV cells, only a portion of the solar
spectrum is covered. A band gap of 1.1eV
covers 77% of the AM1.5 (air mass) solar
photon flux. However, a band gap of 2eV covers
only 30% of the AM1.5 solar photon flux.1
Furthermore, as discussed later in this paper,
the low charge carrier mobility of organic
semiconductor material limits thicknesses of
OPV cells to 100nm, which results in absorption
of about 60% of the light at the absorption
maximum.1 In Figure 5, the fraction of sunlight
contributing to energy conversion in organic
materials can be seen. It is apparent that even
though the silicon absorption spectrum extends
to 1100nm, organic materials only use the blue
side of the solar spectrum. 4
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
Figure 5: Absorption coefficients of commonly used
materials.
Because of their high absorption coefficient,
conjugated polymers absorb light very
efficiently at the maximum of their absorption
spectrum. This results in requiring a layer
thickness of a few hundred nanometers to
absorb light at their maximum wavelength.1
Contrasting this to silicon, active layer
thickness of hundreds of micrometers is needed
since silicon is an indirect semiconductor.
Indirect semiconductors are not as efficient at
photon absorption because as seen in Figure 6
below, the maximum energy of the valence
band occurs at a different point in k-space than
the minimum amount of energy in the
conduction band.
Figure 6: Comparing direct vs. indirect band gaps.
Energy levels in organic material are
relatable to inorganic semiconductors. In
inorganic material, the energy needed to
release an electron from the valence band to
the conduction band is the ionization potential,
and the electron affinity is the energy gained
when the electron moves from the vacuum level
to the conduction band. In organic material,
electrons can transfer to from the HOMO
(highest occupied molecular orbital) state to
the vacuum. The energy this involves can be
estimated based on the electrochemical
oxidation potential of the material. The electron
affinity is estimated from the reduction
potential of the molecules. The optical band gap
is the difference between these two energy
levels, as seen in Figure 7 below.
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
Breaking down the formula, the fraction of
photons absorbed is determined by the
absorption coefficient, absorption spectrum,
layer thickness, and of internal multiple
reflections at metallic electrodes, or similar
reflective devices.
Figure 7: Comparison of energy levels in inorganic and
organic semiconductors.
Even though organic semiconductors have
certain drawbacks in comparison to IPV
materials, they are able to combine the
advantages of both polymers and
semiconductors. They can be lightweight,
flexible, and used as a tunable optoelectronic
device.
V.
Working Principles
There are four main steps involved in
converting light into electrical current in OPV
cells. The first of the four consecutive steps is
the i) Absorption of a photon which leads to the
formation of an exciton. Secondly, ii) Exciton
Diffusion occurs as the exciton travels to a region
where iii) Charge Separation can ensue. The
fourth step is the iv) Charge Transport of the
electrons and holes to the cathode and anode to
generate a current. 6
Therefore, the electric current is dependent
on the fraction of photons absorbed (ƞabs), the
fraction of electron-hole pairs that dissociate
(ƞdiss), and the fraction of those charges that
reach the electrode (ƞout). These three fractions
determine the overall photocurrent efficiency (ƞj).6
The equation is shown below.
Ƞj = Ƞabs x Ƞout x Ƞabs
The absorption coefficient determines how
far light of a particular wavelength can penetrate
a material before it is absorbed. Thus, the
absorption coefficient is dependent on the
wavelength of the light, and also the material the
light is entering. If the material is too thin, it will
appear transparent to certain wavelengths, or if
the material has a low absorption coefficient, the
light will be poorly absorbed.6 Once again, Figure
5 shows absorption coefficients of various solar
energy materials.
The probability of an exciton becoming a
free electron-hole pair is dependent upon
whether the exciton diffused to an area where
charge separation occurs, and what the charge
separation probability is there.
Once charge separation occurs, the charge
carriers need to reach the electrodes. Therefore,
a driving force is needed, which can come in the
form of internal electric fields and what the
concentration gradients of the respective charge
carriers looks like in the material.6 The electric
field induces a drift of the carriers, while the
second creates a diffusion current, as the
carriers attempt to spread evenly through the
material.
Figure 8 below shows the effects of drift and
diffusion current. In Figure 8(a), the organic
diode operates without voltage under closed
circuit conditions. Under illumination charge
carriers drift towards the contacts, with the
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
electrons moving to lower energy states. In
Figure 8(b), there is no voltage applied as the
circuit is open. VOC stands for the open circuit
voltage, which in this case corresponds to the
difference in the metals work function. The work
function being the minimum energy needed to
free an electron from a particular surface. Figure
8(c) shows the organic diode in reverse bias,
where photogenerated charges drift in strong
electric fields. Figure 8(d) shows a forward bias
larger than VOC and the current flows.
Lastly, another factor that determines the
overall solar-to-electrical energy conversion
frequency is the fill factor. The fill factor is a
value between 0 and 1 and is defined as the ratio
of the maximum power of the solar cell divided
by the open circuit voltage and maximum short
circuit current (Jsc) as seen in Figure 9 below.10
The fill factor is dependent on the materials used
to create the solar cell.
Figure 8: Metal Insulator Metal picture of an organic diode.
Figure 9: Schematic representation of the light and dark
current voltage characteristics of a solar cell.
As stated above, to add to the
photogenerated current, the free electrons travel
to the cathode where they can be collected by
the electrode and transferred into the external
circuit. The internal electric field, which is
caused by the use of electrodes with different
work functions, is the main driving factor
pushing the electrons and holes to their
respective destinations.
In the example above, the electron flowed
towards the cathode. It is important to
remember that in a device that consumes power,
the cathode is negative, while if the device
provides the power, the cathode is positive.
FF = Pmax / (Jsc x Voc)
As can be seen in Figure 9 above, the value of
the maximum power density and its fill factor
can be as high as 85% for inorganic solar cells,
but is usually much less. The formula below is
used to determine the efficiency limit. From the
equation, it can be seen that the fill factor is one
of the three primary factors determining the
power conversion efficiency (Ƞ), where Psolar is
the incident solar radiation, limiting the internal
resistance is paramount to more efficient cells.
Ƞ=
βFF x Jsc x Voc
Psolar
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
The process of gaining a higher power
efficiency conversion is discussed further on in
the optimization section.
VI.
Disassociation of Excitons
To produce a photovoltaic effect, the
neutrally charged excitons must diffuse or be
created at an interface where they can
dissacociate into a separate electron and hole
pair.4 Unfortunatley, in organic photovoltaic
cells, the internal electric field is not strong
enough to overcome the Coulomb force between
the bonded electron-hole pair in an exciton.
Based upon typical values (organic
semiconductors usually have exciton binding
energies of .1-1eV), an electric field greater than
106 V/cm would be required to dissacociate the
excitons directly.4
bandwidth and higher Coulomb potential well
contribute to excitons. In most OPV cells, the
charge generation mechanism is the interfacial
dissociation of excitons at a heterointerface. A
free electron will separate into one material, and
its free hole will stay on the other side of the
interface as can be seen in Figure 11. Ti is shown
that excitons created by photon absorption in
the organic semiconductors 1 and 2 do not have
enough energy to break the Coulomb force and
dissociate in the bulk. Yet, the band offset
between cells 1 and 2 provide an exothermic
path for the dissociation of excitons. For this
dissociation to happen, the band offset must be
greater than the exciton band energy.4
Figure 10: Binding energy between a photogenerated hole at
the origin, and electron at the indicated distance from the
hole.
Figure 11: Energy-level diagram for an excitonic solar cell
with band offset (no band bending).
As can be seen in Figure 10, the narrower
Coulomb potential well, as well as the wider
bandwidth, and lower effective mass, lead to
greater delocalization of the carrier wave
functions, resulting in free electron-hole pairs.4
In organic semiconductors, the narrower
In most cases, with the exception being dyesensitized solar cells (DSSC), an exciton must
diffuse to the heterointerface to dissociate. DSSC
will be discussed in further detail later in this
paper. The thermodynamic requirements for
exciton dissociation deal with the exciton
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
binding energy, which, due to the disorder in
conjugated polymers prevents the binding
energy from being a well-defined material
quality. Therefore, since the thermal energy
present at room temperature (KT) is not
sufficient to dissociate the exciton, an electron
acceptor is used to create free charges.1
Excitons also have a certain lifetime in which
they need to dissociate, or else they will
recombine in a process called
photoluminescence (the emission of light). The
typical lifetime of an exciton is rather limited,
with orders in the nanoseconds for organic
material.4 In summary, the dissociation of an
exciton is limited by the constraints of exciton
lifetime and its lack of mobility.
VII.
Figure 12: Diagram showing an exciton in a polymer, and a
dissociated exciton (free electron-hole pair).
Figure 12 shows an exciton in a bulk
heterojunction solar cell, which will be discussed
later. The distance the exciton is from the
interface must be less than the exciton diffusion
length or the exciton will not disassociate. A
polaron is a charge (hole or electron), plus the
distortion of the charge’s surroundings. In
inorganic material, a free charge does not impact
the surroundings, because the rigid crystal
lattice.11 In most organic semiconducting
material this is not the truth. Putting a charge on
a certain molecular site can deform the
molecule, which has implications on the ease of
charge transport. Furthermore, this may lead to
the degeneration of the organic material, as
more charges distort the original polymer
structure.
Single Junction Cell
The first organic solar cells were thermally
evaporated molecular organic layers, stuck
between two electrodes of different work
functions. In Figure 13, it can be seen that in the
depletion region W, a band bending results from
the Schottky contact. This band bending
happens because the Fermi level of the organic
material matches that of the conductive metal.
This depletion region corresponds to an electric
field where excitons can dissociate, creating free
electron-hole pairs.6
Figure 13: Schematic of single layer device with Schottky
contact at Aluminum contact.
Yet, because the exciton diffusion length in
most organic semiconductor materials is below
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
20nm, only the excitons generated in the area
within 20nm from the contact of the two
materials will contribute to the photocurrent.
Therefore, since the absorption of a photon in
organic material produces an exciton rather
than a free electron-hole pair, and in single
junction cells, there is only a small area where
charge separation can occur, the single junction
cell has minimal power conversion efficiency. In
most cases, heavily doped conjugated materials
resulted in power conversion efficiencies up to
3%.8
VIII.
Bilayer Heterojunction Cell
In 1986, a breakthrough in organic
photovoltaics took place, with Tang introducing
a double-layer structure of p-type and n-type
materials as seen in Figure 14. The concept
behind the bilayer heterojunction OPV cell was
to sandwich two organic materials between a
transparent conducting oxide and
semitransparent metal electrode to generate
higher power conversion efficiencies than could
be reached with a single layer.6 In these devices,
the Donor/Acceptor interface is much more
efficient than a single OPV cell, with excitons
formed within the diffusion length of the
interface. If the excitons formed outside the
diffusion length of the interface, they would
yield luminescence, rather than contribute to the
photocurrent.1
Figure 14: Schematic band diagram of a bilayer device.
The dashed line represents energy level of acceptor,
full lines indicate energy level of donor.
In OPV cells, molecular materials that have
low ionization potential, and can therefore
donate an electron easily, are termed electron
donors. Molecular materials that have a high
electron affinity and can easily accept an
electron are noted as electron acceptors. The
ability of being an electron donor or acceptor is
an intrinsic property of the material itself.9
In summary, due to the molecular nature of
the heterojunction, efficient charge separation
occurs only when the exciton is near the D/A
interface. Thus, excitons created beyond the
mean diffusion length from the interface never
have a chance to create free charge carriers. So,
even though the quantum efficiency of photoinduced charge separation is near unity at the
D/A interface, the energy conversion efficiency
is severely limited by its material design.2
Certain bilayer heterojunction cells have
reached a power conversion efficiency of 4%.
IX.
Bulk Heterojunction Cell
The idea behind bulk heterojunction cells is as
follows: by blending two polymers that have
different electron affinities and ionization
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
potentials, the potential difference may be
enough to induce exciton dissociation. The
electron would be accepted by the material with
greater electron affinity, and the hole accepted
by the material with the lower ionization
potential.1
Figure 16: Bulk-Heterojunction Concept.
Figure 15: Schematic band diagram of a bulk
heterojunction. Dashed line represents energy level of
acceptor, full lines indicate energy level of donor.
As stated above, because the exciton
diffusion length in conjugated polymers (1020nm) is much smaller than the photon
absorption length (100nm), the efficiency of a
bi-layer heterojunction cell is dependent upon
the percent of photons absorbed within the
exciton diffusion length at the electron acceptor
interface.1 Therefore, to solve this problem, bulk
heterojunction OPV cells blend the polymer with
a soluble electron acceptor. Ideally, this allows
every exciton formed to be within the diffusion
length from an electron donor/acceptor
interface. This blend between donor and
acceptor material is shown in Figure 16 below.
It is important to note however, that the
idealized schematic shown in Figure 15 is not
realistic to actual composite samples. It is well
known that polymer blends often split into
separate phases rather than form a uniform
medium. Therefore, the D/A interface looks like
a jumble of islands between the two materials as
seen in Figure 16.2 The important optimizing step
is to decrease the island size, therefore
increasing the effective contact region between
the two materials.
While optimizing the bulk heterojunction
nanomorphology, certain issues arise. As stated
above, to achieve high quantum efficiency, all
photogenerated excitons have to diffuse and
dissociate into free charges at a D/A interface,
with those charges reaching their respective
electrodes. Yet, if too intimate of mixing of
between the donor and acceptor materials take
place, the result is too small of mean free paths.6
This will produce poor charge-carrier transport
and enhance recombination. To date, bulk
heterojunction cells can achieve conversion
efficiencies of 6% under the AM 1.5 G standard,
but improvements are still needed if they are to
become commercially viable.
X.
Dye-Sensitized Solar Cells
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
In contrast to conventional p-n junction
photovoltaic devices, dye-sensitized solar cells
(DSSC) do not use the semi-conductor to both
absorb light and transport the charge carrier. In
DSSC devices, light is absorbed by a sensitizer,
and charge separation takes place at the
interface of the conduction band, and it is
transported in the conduction band to the
charge collector.5
As can be seen in Figure 17, an organic dye
absorbed at the surface of an inorganic wideband gap semiconductor is used for the
absorption of light. From this, it is responsible
for the injection of the photoexcited electron
into the conduction band of the semiconductor.
The figure shows that after the absorption of
light by the ruthenium dye, the photogenerated
electron is transferred to the conduction band of
TiO2. Next, the dye is reduced by a redox
electrolyte, which is contained in the organic
solvent. The positive charge is then transported
to the metal electrode via liquid electrolyte. The
electron in TiO2 travels to the TCO electrode
while this happens.9
X.I
Redox Agent
When the dye is ‘reduced’ by a redox agent,
this is explaining the recombination of the free
hole with an electron in an electrolyte layer that
sits between the semiconductor and a second
electrode. This is a very important process,
because it contributes to maintaining a stable
dye. The instability of a dye presents severe
practical drawbacks, because its degradation
leads to a lower quantum yield.
Figure 17: Dye-sensitized solar cell.
X.II
Semiconductor Surface
Dye sensitized cells gained momentum in 1991
when Grätzel greatly improved the interfacial
area between the organic donor and inorganic
acceptor. This was done by using nanoporous
titanium dioxide (TiO2). Traditionally, one of the
problems dye-sensitized solar cells faced was
that of poor light harvesting. One a smooth
surface, a monomolecular layer of sensitizer
absorbs less than 1% of the incident
monochromatic light. On the other hand, as seen
in Figure 17 below, the rough surface of TiO2
leads to an enlargement of the contact area
between the semiconductor and the dye
created.8 This is because the nanometer-sized
semiconductor crystals provide multiple spaces
for the dye molecules to bond. This sponge-like
approach to bonding the dye molecules
multiplies the surface area available to the dye
immensely. After the breakthrough in 1991, the
area of dye molecules in direct contact with the
redox electrolyte was over 1000 times previous
levels used by scientists.8
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
Figure 18: Electron microscope picture of a nanocrystalline
TiO2 film used in a dye-sensitized solar cell.
One of the advantages of dye-based cells is
that the band gap of the semiconductor does not
have to match the wavelength of the incoming
photon. The absorption spectrum of the dye in
the sensitizer can be tuned to the spectrum of
the light, which is why TiO2 can be used, since it
has a wide band gap.
Figure 19: Photocurrent action spectra obtained with
the N3 dye (L) and black dye (L’) as a sensitizer. The
photocurrent response of a bare TiO2 film is shown for
comparison.
Figure 19 above gives an excellent visual to
explain the aforementioned concept. It is
apparent that the photon absorption to current
efficiency with a bare TiO2 film is limited to a
certain range of light wavelengths from Figure
19. Yet, when a dye sensitizer is added to TiO2, a
much larger spectrum of light waves can be
transmitted into electrical energy.
To further explain this concept, since TiO2
has a wide band gap, it is insensitive to much of
the visible spectrum of light, and would be
unable to capture those photons. In dyesensitized solar cells, the incoming photons
instead are absorbed in the dye (sensitizer) and
then injected into the conduction band of TiO2,
rendering it conductive.3
Figure 20: Schematic representation of the principle of
the dye-sensitized photovoltaic cell to indicate the
electron energy level during different phases.
S=sensitizer, S*= electronically excited sensitizer,
S+=oxidized sensitizer.
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
As understood in Figure 20 above, the cell
voltage seen under illumination corresponds to
the voltage difference between the quasi-Fermi
level of TiO2 and the electrochemical potential of
the electrolyte. The electrochemical potential of
the electrolyte is equal to the potential of the
redox agent (R/R-) used to aid charge transfer
between the electrodes.8
X.III
Present Research
Looking forward, even though the positive
charges are transported by the liquid electrolyte
efficiently, which allows for the thickness of the
photovoltaic device to be extended into the µm
range, the liquid electrolyte is a disadvantage.
Therefore, much research is being done to
replace the liquid electrolyte with a solid, wideband gap hole transport material.
ii) be able to carry attachment groups like
phosphonate to firmly graft to the
semiconductor oxide surface, iii) be able to inject
electrons into the solid with a quantum
efficiency of unity (every photon produces an
electron-hole pair), iv) be stable enough to
sustain 20 years of natural light (108 turnover
cycles), and finally, v)its redox potential should
be high enough that it can be regenerated via
electron donation from the redox electrolyte.3
Since the dye plays such a critical role, much
research is spent identifying and synthesizing
possible dyes that meet these requirements.
XI.
Optimization Processes
By lowering the band gap of organic material, it
is possible to harvest more sunlight, and
therefore increase the photocurrent. Increasing
the layer thickness is beneficial for light
absorption, but it often reduces charge
transport, which results in a lower fill factor,
which is crucial to the overall quantum
efficiency of a solar cell.9 As shown below in
Figure 22, it is practically impossible for a singlejunction cell to have more than 34% efficiency,
because it is only tuned to a certain wavelength.
Figure 21: Chemical structure of N3 ruthenium complex
used as charge transfer sensitizer in Dye-sensitized
solar cells.
Research is also being done for the ideal
sensitizer. Above is the chemical structure of
one common sensitizer, N3, as shown in Figure
21. For a single junction photovoltaic cell, the
idea sensitizer should be able to i) absorb all
light below the threshold wavelength of 920nm,
Figure 22: Structure of a multi-junction solar cell, showing
the different layers of p-n junctions.
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
Developing tandem cells, with each tuned to
certain wavelengths, as shown in Figure 22, it is
possible to capture energy that would otherwise
be lost in relaxation, while still capturing the
lower energy photons. Yet, this process is more
expensive and not as economically feasible at
this time.
One of the key solar parameters limiting the
photovoltaic power conversion efficiency is the
internal resistance of the cell. The internal
resistance is composed of the interfacial layer
resistances, electrode resistances, and contact
and interconnect resistances.10 Since the internal
resistance impacts the short circuit current
density (Jsc) and open circuit voltage (Voc) it is
clear that the fill factor is affected, as can be seen
from the fill factor equation stated earlier.
Furthermore, as the fill factor is one of the three
primary factors determining the power
conversion efficiency (Ƞ), limiting the internal
resistance is paramount to more efficient cells.
The decrease of the internal resistance will
lead to an increase in the mobility of charge
carriers. Therefore, research in using different
organic materials for active layers, as well as
slowing down growth rates of the organic
material (to limit defects in the conjugated
polymers) are being pursued.
Concerning the open circuit voltage (Voc), it
has been shown it is dependent on the difference
in the electrode work functions. Also, the
dependence of the charge transport levels is
subject to the light intensity and temperature as
observed from the open source voltage.6
Therefore, it is clear that the Voc is a function of
the materials energy levels as well as dependent
upon the connections of the interfaces and
contacts. Research in different materials for
donors and acceptors are continually being
exhausted, and also different fabrication
techniques are being explored as well.
To maximize the short-circuit photocurrent
(Isc), if it dependent on the amount of absorbed
photons, an increase in layer thickness would
increase the photocurrent, as can be seen in
Figure 23 below.6 Unfortunately, the internal
workings of organic semiconductors is not fully
understood at this time, so further research is
needed to effectively optimize OPV cells.
Figure 23: Calculated photocurrent under ideal
assumptions of an internal quantum efficiency of unity.
Furthermore, when considering the organic
material, UV light waves damage polymers, since
the energy from the photon can cause the bonds
holding the monomers together to break. This
problem can be circumvented by the use of a
multi-junction photovoltaic cell, with an
inorganic material absorbing the UV light waves,
and then the organic material absorbing the
photons with longer wavelengths. Unfortunately,
by pairing organic material with inorganic
material, you lose the low cost and flexibility
that organic materials offer.
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
efficiency of OPV cells, and finding proficient
methods to dissociate the exciton will lead to
organic solar cells that could rival the efficiency
of inorganic solar cells one day.
Figure 24: Structure of a multi-junction solar cell,
showing the different layers of p-n junctions.
Therefore, as can be seen in the figure above,
by stacking different material on top of each
other, each tailored to a different wavelength, it
is possible to capture a large portion of the solar
spectrum. This drastically increases the
efficiency of the solar cell, but also greatly
increases the cost. In today’s world, where we
are not on the brink of an exhaustion of current
energy sources, it is more important to be
economically feasible than to have an extremely
large power conversion efficiency.
XII.
Conclusion
Organic photovoltaic cells have shown
tremendous leaps in power conversion
efficiencies over the last few decades. Though
there are multiple limitations holding OPV cells
from gaining efficiency levels similar to
inorganic material, as the understanding of the
conduction abilities of conjugated polymers
becomes clearer, a continuing growth in power
conversion efficiency should be shown.
Generally speaking, one of the main
differences between inorganic material and
organic solar cells is the inability for OPV cells to
generate a free electron-hole pair from the
absorption of a photon. This difference is a
limiting factor for the power conversion
In the paper, the three main types of
inorganic solar cells at this moment were
discussed, giving an overview of how they
worked, and drawbacks represented in their
designs. In bilayer heterojunction cells, it is clear
that the exciton diffusion length impedes the
efficiency of those devices, therefore research is
being done to create OPV devices that will have
D/A interfaces within the range of every photon
induced exciton. This led to the creation of the
bulk heterojunction cell, where the electron
donor and acceptor are blended in an attempt to
create a material with interfaces within the
exciton diffusion length.
In recent years dye-sensitized solar cells
have become a credible competitor to solid-state
junction devices for the conversion of sunlight
into electrical energy, and as developments in
the search for an ideal sensitizer come to
fruition, the additional benefits gained should
further their competitiveness.
As the world continues its ‘going green’
trend, organic solar cells should gain more of the
energy spotlight, along with other forms of
renewable sources. Furthermore, as increases in
technology enhance our understanding of the
intricacies of conjugated polymers, this will
allow for more efficient OPV devices.
Overall, the flexibility of OPV cells to be
integrated into multiple different products gives
it a competitive advantage over inorganic
materials, and this will guarantee a continual
Organic Photovoltaics: An Overview
Josh Mandich – Spring 2012
supply of research money into the field to keep
improving current efficiency levels.
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