Organic Photovoltaics

Basic principles

Smilar to conventional (inorganic) semiconductors, organic semiconductors can be used for devices such as organic photovoltaics (OPV). While the charge carrier mobility and the photovoltaic efficiency is not assumed to rival that of, silicon-based photovoltaics, the idea for OPV is to produce OPV cells at lower cost and in particular lower energy consumption than for silicon. Another important issue is that organic devices may be made mechanically flexible and thinner than silicon (thanks to their higher optical absorption, i.e. stronger transitions), which potentially opens up completely new applications not possible with silicon technology, such as OPV on clothes, e.g. In order to optimise and use these systems in applications, research is required to understand the fundamental mechanisms underlying OPV. This is the subject of a focus program funded by the DFG, in which are participating.
Organic solar cells basically consist of an organic semiconductor in contact with two metal electrodes, which collect the charge carriers produced in the semiconductor upon light irradiation. In order to illuminate the semiconductor material at least one of the two electrodes has to be transparent. Glass or plastic substrates coated with ITO (indium tin oxide, a conductive oxide with low resistivity, high transparency in the visible and a relatively high work function [1]) are commonly used for this purpose.
On top of the ITO a buffer layer may be deposited. The most common material used is the polymer blend PEDOT:PSS, an organic conducting material with relatively good electrochemical, ambient and thermal stability. Moreover, the electrical properties of PEDOT:PSS can be varied by changing the mixing ratio of the blend [2]. The reported benefits of PEDOT:PSS films between the ITO and the photovoltaic active layer are:

  • preventing indium diffusion in the semiconductors
  • improving the compatibility between the energy levels of the organic semiconductors with the work function of the electrode [3].

In order to collect the photo-generated charges the second metallic contact (e.g. Al, Ag) is deposited on top of the active material. The photo-active layer usually consists of a hetero-junction formed by two different semiconductors, i.e. an electron-donor (D) and an electron-acceptor (A) material. Donor molecules exhibit a low ionization potential (high HOMO energy), while acceptor molecules have a high electron affinity (low LUMO energy). Visible light may excite electrons from the HOMO to the LUMO level, therefore leaving a hole in the HOMO level. The relatively low dielectric constant in organic materials leads to the formation of a neutral bound electron-hole pair, known as exciton. The organic semiconductors have to be chosen properly for the hetero-junction, so that the electronic band structure of the donor and acceptor materials is matched. The different alignment of the HOMO and LUMO levels allows for exciton dissociation at the interface. Electrons are then transferred through the acceptor and collected by the electrode (Al), while the respective holes diffuse in the acceptor and then in the ITO electrode. To determine the electric behaviour of a solar cell the current-vs-voltage characteristic is measured. One may distinguish different regimes:

  • If no external voltage is applied, no current flows through the junction.
  • If an external voltage is applied under reverse bias, a current flux is possible.
  • If an external voltage is applied in forward bias condition, current flow is allowed.

Upon illumination, the device will act as a current generator, therefore the I-V curve shifts downward, as illustrated in the figure below.

Organic solar cell production and characterization

In order to produce such devices on a conducting transparent substrate, the photo-active materials have to be deposited. Two main types of techniques are available:

  1. solution processing (in which the semiconductor molecules are diluted in a solution with a volatile solvent)
  2. organic molecular beam deposition (i.e. thermal evaporation of the molecules under high vacuum conditions)

Since the presence of oxygen may result in a degradation of the electric properties, vacuum conditions are preferable for the characterization of solar cells. Therefore, we decided to develop a complete UHV system both for production and characterization of the devices. With the same UHV system it is possible to deposit the organic semiconductor films and the metallic contact. Electrical contacts inside the system allow for electrical characterization in situ under high vacuum conditions.

Research project

The electric properties of organic devices depend strongly on the structure and the physical characteristics of the molecules involved in the photo-process. On the other hand the electric properties depend on the morphology of the semiconductor film, which itself depends on the structure of the single molecules. Our work in this area therefore focuses on the understanding of how these different aspects influence with each other.


[1] S.S. Sun and N.S. Sariciftci. Organic photovoltaics: mechanism, materials, and devices, CRC, 2005.
[2] H. Gerhard and J. Friedrich. Poly(alkylenedioxythiophene)s-new, very stable conducting polymers, Advanced Materials, 4(2):116-118, 1992.
[3] W. Brütting. Physics of Organic Semiconductors, Wiley-VCH (2005)

For our recent work on the optical properties of organic thin films, see list of publications.