  Engineering the energetics of perovskite photovoltaic devices through the
deliberate introduction of dipoles to control the built-in potential of the
devices offers the opportunity to enhance their performance without the need to
modify the active layer itself. In this work, we demonstrate how the
incorporation of molecular dipoles into the bathocuproine (BCP) hole-blocking
layer of inverted perovskite solar cells improves the device open-circuit
voltage (VOC) and consequently, its performance. We explore a series of four
thiaazulenic derivatives that exhibit increasing dipole moments and demonstrate
that these molecules can be introduced into the solution-processed BCP layer to
effectively increase the built-in potential within the device, without altering
any of the other device layers. As a result the VOC of the devices is enhanced
by up to 130 mV with larger dipoles resulting in higher VOCs. To investigate
the limitations of this approach, we employ numerical device simulations that
demonstrate that the highest dipole derivatives used in this work eliminate all
limitations on the VOC stemming from the built-in potential of the device.
