The first organic solar cells where based on an active layer made of a single material. By the absorption of light, strongly Coulomb-bound electron hole pairs where created, singlet excitons. As described in part zero, these have to be split in order to finally generate a photocurrent. In order to overcome the binding energy, one has to either hope on the thermal energy, or dissociate the exciton at the contacts. Unfortunately, both processes have a rather low efficiency: under normal conditions, the temperature is not high enough, and the sample thickness is much thicker than the exciton diffusion length. The consequence: excitons are mostly not dissociated, but recombine instead. This leads to luminescence, and light emitting solar cells do not belong to the most efficient… there is just not enough current output.
The introduction of a second layer was a quantum leap in terms of power conversion efficiency (though still on a low level): organic bilayer solar cells, presented in the mid eighties [Tang 1986]. The light is usually absorbed mainly in the so-called donor material, a hole conducting small molecule. The photogenerated singlet excitons now can diffuse within the donor towards the interface to the second material, the acceptor, which is usually strongly electronegative. A prominent example for an electron acceptor material is the buckminsterfullerene (C60).
The energy difference between the electron level of the donor and the corresponding acceptor level has to be larger than the exciton binding energy, in order to initiate a charge transfer from donor to acceptor material. If an exciton moves – by diffusion, as it is neutral – towards the donor-acceptor heterojunction, it is energetically favourable if the electron is transferred to the acceptor molecule. This charge transfer, or electron transfer, is reported to be very fast (can be faster than 100fs in polymer-fullerene systems) and very efficient, as the alternative loss mechanisms are much slower [Sariciftci 1992]. The hole stays on the polymer: the exciton is dissociated, the charge carriers are now spatially separated. Even though residing on two separate materials now, electron and hole are still Coulomb bound, even though the recombination rate is clearly lowered (lifetime: micro to milliseconds) as compared to the singlet exciton (lifetime: nanoseconds). Therefore, a further step is necessary for the final charge pair dissociation. Here, an electric field is needed to overcome the Coulomb attraction, and this dependence becomes manifest in the typical, strongly field dependent photocurrent of organic solar cells, also influencing fill factor and short circuit current. The basic steps from light generation / exciton generation to photocurrent are shown in the figure.
If no or just a low electric field is applied, the so-called monomolecular recombination of the charge carrier pair is very probable. As a brief sidenote, important here is not directly the externally applied field, but the internal field, which is influenced by the built-in potential due to the work function difference of the electrodes. But back to the dissociation: only if the field supported charge carrier separation is successfull, can electron and hole hop towards their respective contacts, in order to generate a photocurrent. C.W. Tang (cited above), who reported the first organic bilayer solar cell made of two conjugated small molecules, achieved a power conversion efficiency of about 1 percent. The limiting factor in this concept is that for a full absorption of the incident light, a layer thickness of the absorbing material is to be of the order of the absorption length, approx. 100nm. This is much more than the diffusion length of the excitons, about 10nm. In this example, maybe 100 percent of the incoming photons (within the absorption band) can be absorbed, but only 10 percent of these could reach the donor-acceptor interface and be dissociated to charge carrier pairs (also called polaron pairs). As mostly the exciton diffusion length is much lower than the absorption length, the potential of the bilayer solar cell is difficult to exploit. Fortunately, there are advanced concepts, e.g. the so called bulk heterojunction solar cell, which we will talk about the next time [Update 16.6.2009: link]. See you then;-)