Coevaporated bisquaraine inverted solar cells: Enhancement due to energy transfer and open circuit voltage control

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 Förster 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 (λ_max 540 nm) and emits from 550 nm to the near-infrared region, which overlaps with SQ absorption (λ_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 ∼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.