Journal
ACS NANO
Volume 5, Issue 6, Pages 4756-4768Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nn200744q
Keywords
organic photovoltaics; nanocomposites; conjugated polymers; neutron scattering; phase behavior
Categories
Funding
- Sustainable Energy Education Research Center
- Joint Institute for Neutron Sciences at the University of Tennessee
- National Science Foundation [DMR-1005987]
- Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy
- Division Of Materials Research
- Direct For Mathematical & Physical Scien [1005987] Funding Source: National Science Foundation
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Organic photovoltaics (OPVs) have attracted increasing Interest as a lightweight, low-cost, and easy to process replacement for inorganic solar cells. Moreover, the morphology of the OPV active layer Is crucial to its performance, where a bicontinuous, interconnected, phase-separated morphology of pure electron donor and acceptor phases is currently believed to be optimal. In this work, we use neutron scattering to investigate the morphology of a model OPV conjugated polymer bulk heterojunction, poly[3-hexylthiophene] (P3HT), and surface-functionalized fullerene 1-(3-methyloxycarbonyl) propyl(1-phenyl [6,6]) C-61 (PCBM). These results show that P3HT and PCBM form a homogeneous structure containing crystalline P3HT and an amorphous P3HT/PCBM matrix, up to ca. 20 vol % PCBM. At 50 vol % PCBM, the samples exhibit a complex structure containing at least P3HT crystals, PCBM crystals, and a homogeneous mixture of the two. The 20 vol % PCBM samples exhibit behavior consistent with the onset of phase separation after 6 h of thermal annealing at 150 degrees C, but appear to be miscible at shorter annealing times. This suggests that the miscibility limit of PCBM in P3HT Is near 20%. Moreover, for the 50 vol % PCBM sample, the interface roughens under thermal annealing possibly owing to the growth of PCBM crystals. These observations suggest a different morphology than is commonly presented in the literature for optimal bulk heterojunctions. We propose a novel rivers and streams morphology to describe this system, which is consistent with these scattering results and previously reported photovoltaic functionality of P3HT/PCBM bulk heterojunctions.
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