Regulating Surface Termination for Efficient Inverted Perovskite Solar Cells with Greater Than 23% Efficiency

Passivating
surface and bulk defects of perovskite films has been
proven to be an effective way to minimize nonradiative recombination
losses in perovskite solar cells (PVSCs). The lattice interference
and perturbation of atomic periodicity at the perovskite surfaces
often significantly affect the material properties and device efficiencies.
By tailoring the terminal groups on the perovskite surface and modifying
the surface chemical environment, the defects can be reduced to enhance
the photovoltaic performance and stability of derived PVSCs. Here,
we report a rationally designed bifunctional molecule, piperazinium
iodide (PI), containing both R2NH and R2NH2+ groups on the same six-membered
ring, behaving both as an electron donor and an electron acceptor
to react with different surface-terminating ends on perovskite films.
The resulting perovskite films after defect passivation show released
surface residual stress, suppressed nonradiative recombination loss,
and more n-type characteristics for sufficient energy
transfer. Consequently, charge recombination is significantly suppressed
to result in a high open-circuit voltage (VOC) of 1.17 V and a reduced VOC loss of
0.33 V. A very high power conversion efficiency (PCE) of 23.37% (with
22.75% certified) could be achieved, which is the highest value reported
for inverted PVSCs. Our work reveals a very effective way of using
rationally designed bifunctional molecules to simultaneously enhance
the device performance and stability.