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Title: Role of Crystallization in the Morphology of Polymer: Non-fullerene Acceptor Bulk Heterojunctions

Many high efficiency organic photovoltaics use fullerene-based acceptors despite their high production cost, weak optical absorption in the visible range, and limited synthetic variability of electronic and optical properties. To circumvent this deficiency, non-fullerene small-molecule acceptors have been developed that have good synthetic flexibility, allowing for precise tuning of optoelectronic properties, leading to enhanced absorption of the solar spectrum and increased open-circuit voltages ( V OC). We examined the detailed morphology of bulk heterojunctions of poly(3-hexylthiophene) and the small-molecule acceptor HPI-BT to reveal structural changes that lead to improvements in the fill factor of solar cells upon thermal annealing. The kinetics of the phase transformation process of HPI-BT during thermal annealing were investigated through in situ grazing incidence wide-angle X-ray scattering studies, atomic force microscopy, and transmission electron microscopy. The HPI-BT acceptor crystallizes during film formation to form micron-sized domains embedded within the film center and a donor rich capping layer at the cathode interface reducing efficient charge extraction. Thermal annealing changes the surface composition and improves charge extraction. In conclusion, this study reveals the need for complementary methods to investigate the morphology of BHJs.
Authors:
 [1] ;  [2] ;  [3] ;  [1] ;  [4] ;  [3] ;  [1]
  1. Univ. of California, Santa Barbara, CA (United States)
  2. Univ. of Colorado, Boulder, CO (United States); National Renewable Energy Lab. (NREL), Golden, CO (United States)
  3. Colorado School of Mines, Golden, CO (United States)
  4. Univ. of Colorado, Boulder, CO (United States)
Publication Date:
Report Number(s):
NREL/JA-5K00-68704
Journal ID: ISSN 1944-8244; TRN: US1701983
Grant/Contract Number:
AC36-08GO28308
Type:
Accepted Manuscript
Journal Name:
ACS Applied Materials and Interfaces
Additional Journal Information:
Journal Volume: 9; Journal Issue: 22; Journal ID: ISSN 1944-8244
Publisher:
American Chemical Society (ACS)
Research Org:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; crystallization; non-fullerene acceptor; organic photovoltaics; small molecules; transmission electron microscopy; x-ray scattering
OSTI Identifier:
1364166