Molecular Weight Dependence of the Morphology in P3HT:PCBM Solar Cells

TLDR

Optimizing active‑layer morphology is crucial for improving polymer‑based photovoltaic efficiency. The study investigates how P3HT molecular weight influences active‑layer morphology, absorption, and crystal structure. P3HT of defined MWs was synthesized by GRIM and the blends were characterized by GISAXS and resonant soft X‑ray scattering to probe morphology. Crystallinity peaked at intermediate MWs, (100) plane spacing increased with MW while crystal size decreased, nonlinear lattice expansions and enhanced π‑π stacking were observed, PCBM altered P3HT ordering and interaction parameters, and a MW of ~20 kDa produced the best device performance.

Abstract

In polymer-based photovoltaic devices, optimizing and controlling the active layer morphology is important to enhancing the device efficiency. Using poly(3-hexylthiophene) (P3HT) with well-defined molecular weights (MWs), synthesized by the Grignard metathesis (GRIM) method, we show that the morphology of the photovoltaic active layer and the absorption and crystal structure of P3HT are dependent on the MW. Differential scanning calorimetry showed that the crystallinity of P3HT reached a maximum for intermediate MWs. Grazing-incidence wide-angle X-ray diffraction showed that the spacing of the (100) planes of P3HT increased with increasing MW, while the crystal size decreased. Nonlinear crystal lattice expansions were found for both the (100) and (020) lattice planes, with an unusual π-π-stacking enhancement observed between 50 and 100 °C. The melting point depression for P3HT, when mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and, hence, the Flory-Huggins interaction parameter depended on the MW. PCBM was found to perturb the ordering of P3HT chains. In photovoltaic devices, P3HT with a MW of ∼20K showed the best device performance. The morphologies of these blends were studied by grazing-incidence small-angle X-ray scattering (GISAXS) and resonant soft X-ray scattering. In GISAXS, we observed that the low-molecular-weight P3HT more readily crystallizes, promoting a phase-separated morphology.