Coevaporated Bisquaraine
Inverted Solar Cells: Enhancement
Due to Energy Transfer and Open Circuit Voltage Control
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Abstract
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-[(<i>N</i>,<i>N</i>-diisobutylamino)-2,6-dihydroxyphenyl]
squaraine), and an asymmetrical squaraine (ASSQ, 2,4-bis-[(<i>N</i>,<i>N</i>-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio)
squaraine). ASSQ absorbs blue light (λ<sub>max</sub> 540 nm)
and emits from 550 nm to the near-infrared region, which overlaps
with SQ absorption (λ<sub>max</sub> 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 (<i>V</i><sub>OC</sub>) is proportional to the fraction of the two
donors in the blend, allowing us to predict the <i>V</i><sub>OC</sub> 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