Basics of organic solar cells
Organic solar cells are made of thin layers of organic materials with thickness in the 100 nanometer range. They were first introduced by the research group of Dr. Ching Tang at Kodak Research Laboratories in 1986. The motivation for using organic dyes is to replace the expensive silicon in conventional photovoltaics and to apply simple production techniques. Additionally, organic solar cells can be prepared on plastic foil and so they are ideal candidates for flexible and portable systems.
Organic solar cells basically comprise the following layers: first electrode, electron transport layer, photoactive layer, hole transport layer, and second electrode. In general, a solar cell absorbs light, separates the created electrons and holes from each other and delivers electrical power at the contacts. The fundamental difference between the working principles of organic and inorganic solar cells is the direct generation of free charge carries in the inorganic solar cells. In organic materials the light absorption is followed by the creation of excitons with a typical binding energy (due to coulomb-interaction) of 0.3-0.5 eV.
Since the necessary electric field (> 106 V/cm) to overcome this binding energy is not available in an organic solar cell, the excitons are usually separated a the interface between two different organic layers (heterojunction). The energy alignment of these two materials has to be optimised, so that on the one hand the excitons are efficiently separated, but on the other hand no energy might be lost in this process.
Today three different types of organic solar cells are known: the organic semiconducting material can either be comprised of so-called small molecules (SM solar cells) or polymers (polymer solar cells). The third type of organic solar cells is called dye-sensitised solar cell (or Grätzel cell) and contains a highly porous layer of titanium dioxide as electron transport layer on which dye molecules are adsorbed. Small molecule solar cells are processed in vacuum by physical vapour deposition, whereas polymer solar cells are processed by spin-coating or ink-jet printing (vacuum deposition is still necessary for metal deposition). Grätzel cells are typically processed by screen-printing of the titanium dioxide with subsequent sintering and dying. The OSOL group at the IAPP concentrates on small molecule solar cells.
The pin-concept in OSC
Most non-polymeric organic solar cells are based on a heterojunction between two highly absorbing materials. This heterojunction is needed to separate the excitons, which are rather strongly bound in organic semiconductors. Due to the small diffusion lengths in most of the organic semiconductors, the photoactive region of such cells is only a narrow layer at both sides of the heterointerface. However, the cells have to be much thicker than the active region to avoid shorts and recombination at the metallic contacts, i.e. there are strongly absorbing regions that do not contribute to the photocurrent.
Our concept is to replace these regions by transparent materials (wide-gap transport materials). The layers sequence is therefore: first electrode, transparent layer, photoactive layer(s), second transparent layer and second electrode. The benefit of this concept is that the solar cells only absorb the light in the photoactive region and achieve very high internal quantum efficiencies. The latter fact is crucial for preparing tandem cells (see below).
The second key technology of our organic p-i-n solar cell concept is the controlled doping of the wide-gap transport layers. Although the doping process is crucial for inorganic solar cells, organic solar cells are usually still prepared without controlled doping. The main reason that the advantages of doped layers have been scarcely used previously is the difficulty to produce stable and reproducible doped organic layers. We have published detailed analysis of stable and reproducible doping in organic systems and shown that the properties of organic devices are strongly improved by controlled doping. For example, using weakly acceptor-type matrix materials such as C60, conductivity values of higher than 10-5 S/cm are reached, which is already high enough to make ohmic losses in n-doped transport layers of organic solar cells negligibly small.
Figure 2 shows the energy diagram of our concept: It displays an ideal p-i-n-heterostructure for an organic solar cell. In such a cell, only the intrinsic layer absorbs visible light. This layer can be, e.g. a bilayer of two highly absorbing materials or a bulk-heterojunction (blended layer). The p- and n-layers are realized by doped wide-gap materials. The interfaces at the photoactive layers play an essential role in this concept: The transition of photogenerated charge carriers from the photoactive region to the respective transport layer must be both barrier-free and it should take place without loss in free energy. A good energy level-alignment is necessary to achive this goal including the challenge to find suitable materials. Additionally, the injection of photogenerated carriers into the transport layers as minority carriers should be suppressed by high barriers. This way, the interfaces act as membranes allowing only the “correct” type of charge carriers to pass and to leave the photoactive layer.
A major benefit of doped wide-gap materials is the freedom to optimize the cells in terms of thin-film optics: The cells do not suffer losses by parasitic absorption when the incident light passes the first organic layer towards the photoactive region; also the light reflected at the back contact can be efficiently used. The thickness of the photoactive layer can be varied without limitations due to shorts so that a high internal efficiency can easily be reached. By varying the thicknesses of the wide-gap layers, it allows to place the photoactive layer at the maximum of the optical field distribution inside the device leading to increased absorption.