Major Processes in Organic Solar Cells

Edmond Becquerel (1820-1891) was first to discover the photovoltaic effect. But it took quite a long time before there was a report on an efficient cell and that came from Chapin, Fuller and Pearson (1954) in Bell Labs. They reported a silicon cell with 6% efficiency.

Semiconductors compared

LEDs and photovolataics are opposite processes.

This slide gives a comparison of the processes that are placed in LED in a solar cell and in a field-effect transistor. The first step of the process required charge injections in the electrodes, holes injected from the anode, and electrons injected from the cathode. Those holes and electrons then migrate to diffuse in the presence of an external electric field due to the applied voltage across the organic layer or layers. Then the hole and the electron have to find one another to recombine. That leads to the formation of an excited site, an exciton. During its lifetime, the exciton can also hop from chain segment to chain segment or molecule to molecule. This is called an energy transfer. If all goes well, a decay to the ground state will occur radiatively and a photon will be released. This step is necessary; photons that are produced within the device must be released to see any visual information. A photovoltaic cell works in reverse to a light-emitting diode. While in a light emitting diode an electrical current is injected into the material and transformed into photons, a photovoltaic cell requires an absorption of the photons to generate an electrical current. The photovoltaic cell first absorbs photons, which leads to a formation of excitons in an excited state. These excitons move to the interface between the donor component and the accepter component in the organic solar cell. At the interface of the donor and accepter, the exciton disassociates so that the hole and the electron become free from one another. The separated hole and electron will be diffused within the material and get collected by the electrodes. The major processes occur in reverse order but there is one major difference; any organic material, or combination of materials that’s good for luminescence and thus good for light emitting diode will be bad for organic photovoltaic cells. The reason is that in a light-emitting diode, an electron and the hole approaching each other is favorable. This will form the exciton that will decay and lead to photon emission. On the other hand in the case of a photovoltaic cell, separation of the electron and hole is favorable so they can migrate to the opposing electrodes. These two steps are entirely different in the two types of systems.

Field effect transistors

When you look at the field-effect transistors there are three major processes. The first one is the injection of charges into your semi-conductor. In the case of light-emitting diode and photovoltaic cell there are only two electrodes. However, in a field-effect transistor, the charge injection is modulated through a third electrode called a gate. Now all is needed are the electrons and the holes. Let’s suppose you inject electrons. Those electrons must migrate and be collected. The first electrode will be referred to as a gate. Then you have a thin insulator called a dielectric. You have two other electrodes referred to as the source and the drain. Then you have your organic semi-conductor. This is one of the configurations possible. When you are given a voltage difference between the source and drain, the amount of charges that will be injected into your semi-conductor will be modulated by the voltage at the gate. The gate will modulate the injection and produce a switching effect. For a given voltage between the source and drain, the voltage of the gate can either be decreased such that there is a small injection or current or it can be increased to have a very large injection of charges into your semi-conductor and a large current. These are the components that make a transistor, which is also called a three terminal devices because you have 3 electrodes. So once you have an injection of charges into the organic semi-conductor, those charges will travel and be collected at the other electrodes. These are the main steps in an organic semi-conductor; charge injection, charge transport, and charge collection.