Understanding
Device-Structure-Induced Variations in Open-Circuit Voltage for Organic
Photovoltaics
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Abstract
We
investigate the structural influences on the device performance, especially
on open-circuit voltage (<i>V</i><sub>OC</sub>) in squaraine
(SQ)/fullerene (C<sub>60</sub>) bilayer cells. Simply changing the
SQ thickness could lead to 40% variation in <i>V</i><sub>OC</sub> from 0.62 to 0.86 V. The ionization potential (IP) of SQ
films and recombination at the anode surface as well as donor/acceptor
(D/A) interface sensitively vary with film thicknesses, which account
for the shifts in <i>V</i><sub>OC</sub>. The anode recombination
can be effectively suppressed by preventing direct contact between
C<sub>60</sub> and the anode with a buffer layer, delivering an elevated <i>V</i><sub>OC</sub>. Through polarized infrared–multiple-angle
incidence resolution spectroscopy measurement, the molecular structure
of SQ films is found to gradually evolve from lying-down on indium–tin
oxide substrates with noncentrosymmetric orientation at low thicknesses
to random structure at high thicknesses. The different molecular orientation
may yield different strengths of electronic coupling, which affects
the charge-carrier recombination and thus <i>V</i><sub>OC</sub>. Moreover, the oriented SQ films would spontaneously compose aligned
dipole moments at the D/A interface because of the strong dipolar
effects in SQ molecules identified by density functional theory calculations,
whereas no aligned interfacial dipole moment exists in the random
structure. The resulting interfacial dipole moments would form an
electric field at the D/A interface, leading to variations in the
IP and thus impacting <i>V</i><sub>OC</sub>. Our findings
demonstrate that <i>V</i><sub>OC</sub> in organic photovoltaic
cells is critically associated with the molecular orientation that
affects the charge-carrier recombination and interfacial dipole alignment,
which should be seriously taken into consideration for the design
of organic molecules and optimization of the cell efficiency