Journal
ACS PHOTONICS
Volume 2, Issue 1, Pages 86-95Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ph500282z
Keywords
small-molecule solar cells; Forster resonance energy transfer; open circuit voltage control; thermal coevaporation; photophysics
Categories
Funding
- National Science Foundation [DMR-1410171]
- Yale Climate and Energy Institute (YCEI)
- NSF-CAREER award [CBET-0954985]
- U.S. Department of Energy, Office of Basic Energy Sciences [DE-AC02-98CH10886]
- YINQE
- NSF MRSEC (CRISP) [DMR 1119826]
- Direct For Mathematical & Physical Scien
- Division Of Materials Research [1410171] Funding Source: National Science Foundation
- Div Of Chem, Bioeng, Env, & Transp Sys
- Directorate For Engineering [0954985] Funding Source: National Science Foundation
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There is currently enormous interest in the development of small molecule organic solar cells (SMSC), as in principle, these systems offer advantages over both conventional Si photovoltaics and organic polymer solar cells. Here, we report Forster Resonance Energy Transfer (FRET) enhanced inverted SMSC fabricated by coevaporating two different squaraine donors, a symmetrical squaraine (SQ, 2,4-bis-4-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine), and an asymmetrical squaraine (ASSQ, 2,4-bis-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio) squaraine). ASSQ absorbs blue light (lambda(max) 540 nm) and emits from 550 nm to the near-infrared region, which overlaps with SQ absorption (lambda(max) 690 nm). Therefore, by utilizing a thin film containing the two squaraine donors as the active layer in a SMSC, we can both broaden the photovoltaic absorption spectrum, and reduce recombination loss as a result of FRET. This strategy has resulted in SMSC with power conversion efficiencies (PCE) which are up to 46% greater than those obtained by using a single squaraine donor. Ultrafast time-resolved photoluminescence and transient absorption spectroscopy provide clear evidence of FRET between the small molecules, with a rapid energy transfer time of similar to 1 ps. At optimal blending, which correlates to the highest PCE measured, the efficiency of energy transfer is as high as 85%. Furthermore, in the devices containing two different squaraine molecules, the open circuit voltage (V-OC) is proportional to the fraction of the two donors in the blend, allowing us to predict the V-OC as the ratio of the two donors is changed. SMSC with inverted structures also demonstrate long-term stability in ambient conditions compared to devices employing a conventional architecture.
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