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

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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.

Rapid Efficiency Increases in Laboratory-scale OPV Devices

As compiled by the National Renewable Energy Laboratory (NREL)

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Charge Generation in Organic Photovoltaic (OPV) Devices

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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.

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.

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.

20

21 14 r

qqFεεπ

=

Because the dielectric constant of organic semiconductors are ~4x smaller than inorganic semiconductors, the binding force between the electron and hole is greater.

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.

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.

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.

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.

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Simple Characterization of Organic Photovoltaic Devices

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Jmax

Vmax

Short circuit current (Jsc)

max power point

Open circuit voltage (Voc)

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OCSC

MPPMPP

VJVJFF

××

=

Define the Fill Factor (FF) Define the Efficiency (η)

in

OCSC

in

MPPMPP

PFFVJ

PVJ

××=

×=

η

η

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.