Graded Morphologies and the Performance of PffBT4T-2OD:PC71BM Devices Using Additive Choice

• Published in: Nanomaterials (Basel, Switzerland) Vol. 11; no. 12; p. 3367
• Main Authors: Gaspar, Hugo, Parnell, Andrew J., Pérez, Gabriel E., Viana, Júlio C., King, Stephen M., Mendes, Adélio, Pereira, Luiz, Bernardo, Gabriel
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 12-12-2021; MDPI
• Subjects: Additives; bulk-heterojunction morphology; Butyric acid; Crystal structure; Crystallinity; Dichlorobenzene; Diffusion length; Domains; Efficiency; Energy conversion efficiency; Excitons; Fullerenes; Glass substrates; Hexadecane; Morphology; Neutron scattering; o-Dichlorobenzene; Organic photovoltaics; Organic semiconductors; PffBT4T-2OD; Photovoltaic cells; Photovoltaics; Polymers; Solar cells; Solvents; Variation; X-ray scattering; United Kingdom > UK
• ISSN: 2079-4991; 2079-4991
• DOI: 10.3390/nano11123367

The impact of several solvent processing additives (1-chloronaphthalene, methylnaphthalene, hexadecane, 1-phenyloctane, and p-anisaldehyde), 3% v/v in o-dichlorobenzene, on the performance and morphology of poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)-2,2′,5′,22033,5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)-based polymer solar cells was investigated. Some additives were shown to enhance the power conversion efficiency (PCE) by ~6%, while others decreased the PCE by ~17–25% and a subset of the additives tested completely eliminated any power conversion efficiency and the operation as a photovoltaic device. Grazing-Incidence Wide Angle X-ray Scattering (GIWAXS) revealed a clear stepwise variation in the crystallinity of the systems when changing the additive between the two extreme situations of maximum PCE (1-chloronaphthalene) and null PCE (hexadecane). Small-Angle Neutron Scattering (SANS) revealed that the morphology of devices with PCE ~0% was composed of large domains with correlation lengths of ~30 nm, i.e., much larger than the typical exciton diffusion length (~12 nm) in organic semiconductors. The graded variations in crystallinity and in nano-domain size observed between the two extreme situations (1-chloronaphthalene and hexadecane) were responsible for the observed graded variations in device performance.