For its inherent advantages, such as lightweight, low cost, flexibility, and opportunity to
cover large surface areas, organic solar cells have attracted more and more attention in
both academia and industry. However, the efficiency of organic solar cell is still much
lower than silicon solar cells, but steadily rising as it now stands above 8%.
The architecture of bulk heterojunction solar cells can improve the performance of
organic solar cell a lot, but these improvements are highly dependent on the
morphology of photoactive layer. Therefore, by controlling the morphology of
photoactive layer, most commonly composed of a P3HT donor polymer and PCBM small
molecule, the performance of organic solar cells could be optimized. The use of solvent
additives in the solution formulation is particularly interesting, because it is a low cost
method of controlling the phase separation of the photoactive layer and possibly
removing the need for subsequent thermal and solvent vapor annealing. However, the
role of the solvent additive remains not well understood and much debate remains on
the mechanisms by which it impacts phase separation. In the first part of this thesis, we
investigate the role of the solvent additive on the individual components (solvent, donor
and acceptor) of the solution and the photoactive layer both in the bulk solution, during
solution-processing and in the post-processing solid state of the film. In the second part of this thesis, we investigate the role of the additive on the blended solution state and
resulting thin film phase separation. Finally, we propose a new method of controlling
phase separation based on the insight into the role of the solvent additive.
In the first part, we used an additive [octandiethiol (OT)] in the solvent to help the
aggregation of P3HT in the solution. From the UV-vis experiments, the crystallinity of
P3HT in the solutions increased while it decreased in thin films with steady increase of
additive concentration. This method could be used for one step, annealing-free
fabrication of organic solar cell with high performance. The solution can potentially be
used to prepare ink for the large scale roll-to-roll ink-jet printing of P3HT thin films.
Secondly, from the experiments it is found that differences in the evaporation rate and
solubility of the components of the photoactive layer may be part of the reason for
morphological changes. With lower evaporation rate than the host solvent, the additive
concentration in the solution keeps increasing with time during the final stages of spin coating.
In addition, the phase separation is increased with the increase of additive
concentration, as demonstrated by AFM and TEM. By controlling the additive
concentration, it is possible to control the phase separation of photoactive layer in
pristine device. It is also found that the additive can change the wetting ability of the
solvent to produce films with high surface coverage.
With this information in hand, we modified the solution process of BHJ layers. A layer of
crystals was deposited from the OT-containing solution by postponing the start of the
spin coating for several minutes (delay time) after the solution is dropped on the surface of substrate. We found this to be a very effective method of increasing the phase
separation and crystallinity of the photoactive materials. This effect was not possible
when using oDCB solvent without any additive.
|Date of Award||Jul 23 2011|
|Original language||English (US)|
- Physical Sciences and Engineering
|Supervisor||Aram Amassian (Supervisor)|
- Organic solar cell
- Photoactive layer
- Morphology control
- Solvents additives
- Pre-spin-coating deposition