Polymer Solar Cell - Active Layer Deposition and Annealing Process

Active Layer Deposition and Annealing Process

Since its active layer largely determines device efficiency, the morphology of this component has received much attention.

If one material is more soluble in the solvent than the other then it will deposit first on top of the substrate, causing a gradient in concentration along the film. This has been demonstrated to be the case for poly-3-hexyl thiophene (P3HT), phenyl-C₆₁-butyric acid methyl ester (PCBM) devices where the PCBM tends to accumulate towards the bottom of the device upon spin coating from ODCB solutions. This effect is seen because the more soluble component tends to migrate towards the “solvent rich” phase during the spin coating procedure, generating an accumulation of the more soluble component towards the bottom part of the film which is where the solvent dries last. It is also worth noting that the thickness of the generated film also affects the phases segregation because the dynamics of crystallization and precipitation are different for more concentrated solutions or faster evaporation rates (either one is needed to build thicker devices). Enrichment of crystalline P3HT closer to the hole collecting electrode can only be achieved for relatively thin (100 nm) P3HT/PCBM layers.

The gradients in the initial morphology are then mainly generated by the solvent evaporation rate and the differences in solubility between the donor and acceptor inside the blend. This dependence on solubility has been clearly demonstrated using fullerene derivatives and P3HT by Troshin’s group. When using solvents which evaporate at a slower rate (as chlorobenzene (CB) or dichlorobenzene (DCB)) you can get larger degrees of vertical separation or aggregation while with solvents that evaporate quicker you can get a much less effective vertical separation. In a similar manner larger solubility gradients should lead to more effective vertical separation while smaller gradients should lead to more homogeneous films. These two effects have been studied extensively and have been verified on P3HT:PCBM solar cells.

The evaporation speed of the solvent as well as posterior solvent vapor or thermal annealing procedures have also been the subject of additional studies. Some blends like P3HT:PCBM seem to greatly benefit from thermal annealing procedures while other blends like PTB7:PCBM seem to show no benefit from the application of such a procedure. In the case of P3HT the benefit seems to come from an increase of crystallinity of the P3HT phase which is generated through an expulsion of PCBM molecules from within these domains. This has been demonstrated through studies of PCBM miscibility in P3HT as well as the changes in compositions of domains as a function of annealing times.

The above hypothesis based on miscibility does not fully explain the efficiency of the devices as solely pure amorphous phases of either donor or acceptor materials never exist within bulk heterojunction devices. A 2010 paper suggests that current models which assume pure phases and discrete interfaces might run into problems as pure amorphous regions never exist within the devices. Since current models assume phase separation at interfaces without any consideration for phase purity the models might need to be changed to account for these important aspects inherent to real devices.

The thermal annealing procedure is also different depending on precisely when it is applied. Since the vertical migration of species is determined in part by the surface tension between the active layer and either air or another layer, it is different to anneal cells before or after the deposition of additional layers (most often the metal cathode). In the case of P3HT:PCBM solar cells there is a clear difference in vertical migration when cells are annealed before or after the deposition of the metal cathode with better results attained for the post-annealing treatment.

Donor or acceptor accumulation next to the adjacent layers might be beneficial as these accumulations can lead to hole or electron blocking effects which might benefit device performance. As Schwartz et al. showed in 2009, the difference in vertical distribution on P3HT:PCBM solar cells can cause problems with electron mobility which ends up with the yielding of very poor device efficiencies. In their work, they effectively demonstrate that simple changes to device architecture – spin coating a thin layer of PCBM on top of the P3HT – can greatly enhance cell reproducibility by providing reproducible vertical separation between the device components. Since higher contact between the PCBM and the cathode is required for better efficiencies, this largely increases device reproducibility.

According to neutron scattering analysis, P3HT:PCBM blends have been described as “rivers" (P3HT" interrupted by “streams” (PCBM regions).