This document provides an overview of organic photovoltaic devices. It discusses the five key steps in the charge generation process in these devices: light absorption, exciton diffusion, exciton separation, charge transport, and charge collection. The excitonic nature of organic materials means that excitons must reach the donor-acceptor interface to separate into charges. The document also introduces the basic characterization of OPV devices using a current-voltage curve to determine parameters like short-circuit current, open-circuit voltage, fill factor, and power conversion efficiency.

1. Organic Electronic Devices
Week 4: Organic Photovoltaic Devices
Lecture 4.1: Overview of Organic Photovoltaic Devices
Bryan W. Boudouris
Chemical Engineering
Purdue University
1

2. Lecture Overview and Learning Objectives
• Concepts to be Covered in this Lecture Segment
• Introduction to the State-of-the-Art Power Conversion Efficiency of
Organic Photovoltaic (OPV) Devices
• Mechanism of Charge Generation in OPV Devices and Benefits and
Limitations Associated with the Materials
• Examination of an OPV Device Performance Curve and How to
Calculate OPV Parameters
• Learning Objectives
By the Conclusion of this Presentation, You Should be Able to:
1. Explain the five steps associated with charge generation in organic
photovoltaic devices.
2. Justify why organic solar cells are excitonic in nature and why this is
different than typical inorganic solar cells.
3. Calculate the short-circuit current density, open-circuit voltage, and fill
factor of an OPV device given a performance curve.

3. Rapid Efficiency Increases in Laboratory-scale OPV Devices
As compiled by the National Renewable Energy Laboratory (NREL)

4. +
–
Charge Generation in Organic Photovoltaic (OPV) Devices
-15
-5
5
15
-0.2 0.2 0.6
light
dark

5. Light Absorption in OPV Devices
• This step is similar to that in inorganic photovoltaic devices. And,
absorption can occur either in the electron donor (p-type) material or the
electron acceptor (n-type) material.
• However, organic materials have much higher absorption coefficients.
• This means that the device thicknesses can be on the order of ~100 nm, as
opposed to ~10 – 100 µm.
• The spectral response (and, thus, bandgap) will be defined by the molecular
absorption modes.

6. Exciton Diffusion in OPV Devices
• An exciton is bound electron-hole pair that has a lifetime of ~300 ps in
common organic semiconductors. This means that it is able to move in
space for ~10 nm prior to recombination.
• Because the exciton is charge neutral, it does not respond to any electric
fields present in the device, and explore space in a manner similar to that of
random walk diffusion.
• Generally, it requires > 0.3 eV of energy to separate the exciton into two free
charge carriers. Therefore, reaching the donor-acceptor interface can be
crucial.

7. Exciton Diffusion in OPV Devices
• The excitonic nature of organic solar cells make them unique relative to
inorganic solar cells. The nature of the exciton is related directly to the
dielectric constant of the material.
• Recall from Coulomb’s Law that the force (F) between two charges (qi)
separated by (r) in a medium with dielectric constant (ε)can be written as the
following.
2
0
21 1
4 r
qq
F
εεπ
=
Because the dielectric constant of organic
semiconductors are ~4x smaller than inorganic
semiconductors, the binding force between the
electron and hole is greater.

8. Exciton Separation in OPV Devices
• If the exciton reaches a location in the device where charge transfer will
lower the energy of the system, it will transfer the charge.
• This charge transfer occurs most usefully at the p-type/n-type interface. That
is way the p-type material is called the “electron donor” and the n-type
material is called the “electron acceptor”.
• In the schematic above, the hole will remain in the electron donor phase and
transfer the electron to the electron acceptor phase. This is because the
electron wishes to move farther from free vacuum and the hole wishes to
move closer to free vacuum.

9. Charge Transport in OPV Devices
• Charges will move through the device due to a combination of a drift (i.e.,
due to the electric field within the OPV) and diffusion (i.e., because of
concentration gradients in the device) currents.
• Here, we wish to move the hole and the electron through the device without
having the charges recombine. There are two classes of recombination.
1. Geminate recombination is where the hole and electron that formed the
original exciton recombine after splitting.
2. Non-geminate recombination is where an electron or hole recombines
with entities that are not the opposite charge that formed the exciton.

10. Charge Transport in OPV Devices
• Non-geminate recombination can occur for a variety of reasons. For example,
the following could occur.
1. A charge could recombine with the large amount of electrons or holes at
either of the electrodes.
2. An electron could recombine with a hole that was not part of its
excitonic pair.
3. A hole could recombine with an electron that was in a deep level trap.

11. Charge Collection in OPV Devices
• If there is a large energy barrier to overcome between the transport level of
the semiconducting phase and the work function of the metal contact, there
will be a high series resistance in the device.
• Therefore, we would prefer if the work function energy level of the anode
matched the HOMO energy level of the p-type material and the work function
energy level of the cathode matched the LUMO energy level of the n-type
material.
• Sometimes interfacial modifying layers are added to make these junctions
more level with respect to energy.

12. +
–
Simple Characterization of Organic Photovoltaic Devices
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5
15
-0.2 0.2 0.6
Jmax
Vmax
Short circuit current
(Jsc)
max power point
Open circuit voltage
(Voc)
light
dark
OCSC
MPPMPP
VJ
VJ
FF
×
×
=
Define the Fill Factor (FF) Define the Efficiency (η)
in
OCSC
in
MPPMPP
P
FFVJ
P
VJ
××
=
×
=
η
η

13. Summary and Preview of the Next Lecture
Next Time: Characterization of OPV Devices
In the simplest manifesting of a relatively high-performance cell,
an OPV device will be composed of four distinct layer. These
are the anode, an electron-donating (hole-transporting) layer, an
electron-accepting (electron-transporting) layer, and a cathode.
The transport levels (i.e., work function energy levels, HOMO
energy levels, and LUMO energy levels) will dictate which
material is which in the OPV device. While light absorption is
quite high in organic photovoltaic devices, charge generation and
transport can be limiting.
There are five key steps in the charge generation and collection mechanism of OPV devices.
In different materials, different steps will be the limiting process, but all of these steps do occur
in the highest-performing OPV devices.