Crystallization kinetics and morphology relations on thermally annealed bulk heterojunction solar cell blends studied by rapid heat cool calorimetry (RHC)

Abstract

Optimizing the post-production annealing conditions of polymer:fullerene bulk heterojunction solar cells is vitally important, not only for fine-tuning the morphology -thus increasing the efficiency -but also for retaining the desired morphology during long-term operation. However, optimal conditions for annealing temperatures and times can only be chosen, once thermal transition temperatures and annealing kinetics of the blends are well-known. For instance, for systems with glass transition temperatures (T g ) lower than the maximum device operation temperature of 80°C, the mobility needed for morphology coarsening is present, leading to efficiencies decreasing in the course of time. Using advanced fast-scanning thermal analysis techniques, the formation of nuclei and growth of crystals during heating or cooling can be reduced or avoided, and thus, the fast crystallization processes occurring during annealing of the polymer:fullerene blends can be followed. In this study, non-isothermal and isothermal crystallization kinetics of the P3HT:PCBM (poly(3-hexyl thiophene: [6,6] -phenyl C 61 -butyric acid methyl ester) and P3HT:bis-PCBM blends are investigated and compared by using Rapid Heating Cooling Calorimetry (RHC).

Development of non-halogenated binary solvent systems for high performance bulk-heterojunction organic solar cells

“…3b where the external quantum efficiency (EQE) increases from a maximum of 63% at 500 nm for the device from p-XL up to 70% for the p-XL-BA solvent mixture. The improved device performance of the p-XL-BA mixed solvent systems can be attributed to morphological change of the P3HT/PCBM thin film blend caused by the slower evaporation rate of the high boiling point component (BA) upon spin coating and drying process, because the morphology of the P3HT:PCBM is largely dependent on the solution processing conditions [1][2][3]6,10,17]. Table 2 summarizes the performances of the devices from the solvent systems used in this work: pure MS, p-XL, 80-20 vol.% MS-AP, 80-20 vol.% p-XL-AP, 80-20 vol.% p-XL-BA and pure DCB.…”

“…In order to improve device performances by modifying the morphology of the organic active layers [1], solvent annealing [2][3][4], thermal annealing [4][5][6], and solvent additives [7,8] have been utilized for the fabrication of solution-processed bulk-heterojunction solar cells. The most commonly used solvents in the fabrication of solution processable solar cells tend to be halogenated solvents due to their superior performance [9].…”