The effect of the molecular orientation
distribution of the first
monolayer of donor molecules at the hole-harvesting contact in an
organic photovoltaic (OPV) on device efficiency was investigated.
Two zinc phthalocyanine (ZnPc) phosphonic acids (PA) deposited on
indium tin oxide (ITO) electrodes are compared: ZnPc(PA)4 contains PA linkers in all four quadrants, and ZnPcPA contains a
PA linker in one quadrant. ZnPcPA monolayers exhibited a broad distribution
of molecular orientations whereas ZnPc(PA)4 adsorption
produced a monolayer with a narrower orientation distribution with
the molecular plane more parallel to the ITO surface. We used potential-modulated
attenuated total reflectance spectroelectrochemistry (PM-ATR) to characterize
the charge-transfer kinetics of these films and show that the highest
rate constants correspond to ZnPc subpopulations that are oriented
more parallel to the ITO surface plane. For ZnPc(PA)4,
rate constants exceeded 104 s–1 and are
among the highest ever reported for a surface-confined redox couple,
which is attributable to both its orientation and the small ZnPc–electrode
separation distance. The performance of OPVs with ITO hole-harvesting
contacts modified with ZnPc(PA)4 was comparable to that
achieved with highly activated bare ITO contacts, whereas for ZnPcPA-modified
contacts, the OPV performance was similar to that observed with (hole-blocking)
alkyl-PA modifiers. These results demonstrate the synergism between
molecular structure, energetics, and dynamics at interfaces in OPVs.
In this communication, the nanoscale through-film and lateral photo-response and conductivity of large-grained methylammonium lead iodide (MAPbI3) thin films are studied.
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