Time evolution studies of dithieno[3,2-b:2′,3′-d]pyrrole-based A–D–A oligothiophene bulk heterojunctions during solvent vapor annealing towards optimization of photocurrent generation

Abstract

Solvent vapor annealing (SVA) is one of the main techniques to improve the morphology of bulk heterojunction solar cells using oligomeric donors. In this report, we study time evolution of nanoscale morphological changes in bulk heterojunctions based on a well-studied dithienopyrrole-based A–D–A oligothiophene (dithieno[3,2-b:2′,3′-d]pyrrole named here 1) blended with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) to increase photocurrent density by combining scanning transmission electron microscopy and low-energy-loss spectroscopy. Our results show that SVA transforms the morphology of 1 : PC₇₁BM blends by a three-stage mechanism: highly intermixed phases evolve into nanostructured bilayers that correspond to an optimal blend morphology. Additional SVA leads to completely phase-separated micrometer-sized domains. Optical spacers were used to increase light absorption inside optimized 1 : PC₇₁BM blends leading to solar cells of 7.74% efficiency but a moderate photocurrent density of 12.3 mA cm⁻². Quantum efficiency analyses reveal that photocurrent density is mainly limited by losses inside the donor phase. Indeed, optimized 1 : PC₇₁BM blends consist of large donor-enriched domains not optimal for exciton to photocurrent conversion. Shorter SVA times lead to smaller domains; however they are embedded in large mixed phases suggesting that introduction of stronger molecular packing may help us to better balance phase separation and domain size enabling more efficient bulk heterojunction solar cells.