Scaling Up Polymer Solar Cells
Scaling up the synthesis and processing of plastic nanomaterials is the greatest obstacle to their commercialization. Thin film technologies, like solar cells and transistors, are extremely sensitive to the rate at which the film is deposited and dries. Techniques that yield highly efficient solar cells in a laboratory, like spin coating, are often not scalable to the level of industrial manufacturing. In contrast, scaled-up film deposition techniques, like gravure printing, often yield lower efficiency solar cells. Alternative processing is needed to realize the potential of plastic solar cells.
The greatest disparity between small-scale spin coating and large-scale gravure printing is the drying time of the film. In both cases, a liquid film is deposited from a solution of chlorinated solvent, polymer, and fullerene. As the solvent evaporates, it leaves behind the solid polymer and fullerene. The most efficient solar cells are produced by spin coating, with a drying time of several seconds. Gravure printed films dry over the course of several minutes, and generally exhibit lower efficiencies. This difference underscores the importance of kinetics in determining the final structure of the thin film. The rapid drying associated with spin coating causes a higher degree of polymer crystallization than in the slow dried case, directly improving the performance of the device. The initial degree of polymer crystallinity can be increased via thermal annealing, however this often leads to the growth of fullerene crystals that are detrimental to the efficiency of the device. A scalable processing technique is needed with short film-drying times.
Comparison of fast and slow drying times using small-angle neutron scattering
We investigated this problem on a bulk scale in order to use small-angle neutron scattering to characterize the nanomaterials1. The benchmark polymer solar cell materials were used: poly-3-hexylthiophene (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM), and chlorobenzene was the solvent. One sample was prepared by dropcasting, analogous to gravure-printing, in which the solution was deposited dropwise onto a substrate and allowed to dry at room temperature. A second sample was prepared by rapid precipitation, analogous to spin coating. In this case, the solution was quenched into cold methanol, a nonsolvent for both the polymer and fullerene, causing the solids to precipitate within seconds (see Fig. 1). Small-angle neutron scattering spectra for the two samples are shown in Figure 2.
Our study found that the slowly dried sample comprised only two phases: a small population of polymer crystals and a homogenous fullerene/polymer mixed phase. In contrast, the rapidly precipitated sample comprised three phases: a large population of polymer crystals, mixed phase, and a pure, amorphous fullerene phase. Before any thermal processing, the rapidly precipitated sample was 150% more crystalline than the slowly dried sample. More surprising, after thermally annealing the samples (140 ºC for 1 hour) large fullerene crystals formed in the slowly-dried sample, but not in the rapidly-precipitated sample.
We conclude that the rapid phase transition, associated with both spin-coating and rapid precipitation, drives phase separation of the polymer and fullerene. On the other hand, long drying times yield a metastable blend that appears homogeneous, but is within the two phase region. This hypothesis explains why both pure phases are present in the rapid precipitation sample, but not in the slowly dried sample. The growth of microscopic fullerene crystals in the slowly dried sample indicates a relatively low nucleation density compared to the rapidly precipitated sample. Rapid precipitation appears to nucleate a large enough population of fullerene pure phases that no single phase grows into a large crystal. The benefits of rapid quenching, viz. higher polymer crystallinity, purer phases, and smaller fullerene crystals, clearly indicate that processing research for solar cells must pursue rapid quench techniques.
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