Reduced Bimolecular Recombination in Blade-Coated, High-Efficiency, Small-Molecule Solar Cells
Sebastian Engmann, Hyun W. Ro, Andrew A. Herzing, Dean M. DeLongchamp, Chad R. Snyder, Lee J. Richter, Adam J. Barito, David J. Gundlach
The full benefit of emerging solution deposited photovoltaic devices requires development of processes compatible with high speed manufacturing. Extensive studies of the lab-to-fab challenge have been performed on polymer donor based organic photovoltaics (OPVs); similar studies of small molecule donor systems are rare. We report the device performance and morphology of blade-coated bulk heterojunction devices based on the small molecule donor p- DTS(FBTTh2)2 and phenyl-C71-butyric acid methyl ester. Blade-coating is an established model for high speed slot-die deposition. We find post-deposition solvent vapor annealing (SVA) with THF results in significantly better device performance than processing with the common solvent additive (SA), 1,8-diiodooctane (DIO). This is in contrast to spin-coating, where both SA and SVA achieve similar device performance. Electron microscopy and both grazing-incidence, wide- and small-angle X-ray scattering experiments are used to characterize the film morphology. While both SA and SVA produce highly crystalline donor domains, SVA films are characterized by a radically smaller domain structure compared to both blade and spun-coat DIO processed films. We attribute the different behavior for SA and SVA processing between spun and blade-coat films to variation in the initial nucleation density and relative ability of SVA and SA to control subsequent crystal growth. The optimized SVA de-vices achieve power conversion efficiencies, PCE, over 8 %. Importantly, the SVA blade-coated devices maintain high PCE for films up to 250 nm thickness. The ability to maintain high PCE in thick films is critical to solution based scale manufacturing. Extensive device analysis via impedance spectroscopy indicates that the SVA treated films exhibit highly suppressed bimolecular recombination compared to Langevin expectation. The distinct morphology of the SVA films place constraints on the necessary structural keys to suppressed bimolecular recombination.
, Ro, H.
, Herzing, A.
, DeLongchamp, D.
, Snyder, C.
, Richter, L.
, Barito, A.
and Gundlach, D.
Reduced Bimolecular Recombination in Blade-Coated, High-Efficiency, Small-Molecule Solar Cells, Journal of Materials Chemistry A, [online], https://doi.org/10.1039/C7TA00635G
(Accessed December 5, 2023)