  Herein, we report on the synthesis and investigation of two
triazino-isoquinoline tetrafluoroborate electrolytes as hole-blocking layers in
methylammonium triiodide perovskite photovoltaic devices with fullerene
electron extraction layer. We find that increasing the thickness of the dipolar
hole-blocking layer results in a gradual increase in the open-circuit voltage
suggesting that aggregation of the molecules can enhance the dipole induced by
the layer. This finding is confirmed by theoretical calculations demonstrating
that while both molecules exhibit a similar dipole moment in their isolated
state, this dipole is significantly enhanced when they aggregate. Ultra-violet
photoemission spectroscopy measurements show that both derivatives exhibit a
high ionisation potential of 7 eV, in agreement with their effective
hole-blocking nature demonstrated by the devices. However, each of the
molecules shows a different electron affinity due to the increased conjugation
of one of the derivatives. While the change in electron transport level between
the two derivatives is as high as 0.3 eV, the difference in the open-circuit
voltage of both types of devices is negligible, suggesting that the electron
transport level plays only a minor role in determining the open-circuit voltage
of the device. Numerical device simulations confirm that the increase in
built-in potential, arising from the high dipole of the electrolyte layer,
compensates for the non-ideal energetic alignment of the charge transport
levels, resulting in high VOC for a range of electron transport levels. Our
study demonstrates that the application of small molecule electrolytes as
hole-blocking layer in inverted architecture perovskite solar cells is a
powerful tool to enhance the open-circuit voltage of the devices and provides
useful guidelines for designing future generations of such compounds.
