Light Makes Right: Laser Polishing for Surface Modification of Perovskite Solar Cells

Interface engineering is a common strategy for passivating surface defects to attain open circuit voltages (<i>V</i>_oc) in perovskite solar cells (PSCs). Although a large volume of reports highlights the potential of chemical passivation, an additional solvent-based processing step increases the challenges of upscaling and reliability. Moreover, as the library of chemical passivators consistently expands, there is no consensus on the result risking that each perovskite composition needs its tailor-made solution. Hence, a solvent-free, up-scalable, and one-size-fits-all approach is desirable.<br/><br/>In this work, we introduce the concept of polishing a perovskite thin-film surface using a nanosecond (ns) pulsed ultraviolet laser to reduce surface defects, such as dangling bonds, undesirable phases, and suboptimal stoichiometry. Careful control of laser energy and scanning speed improves the photophysical properties of the surface without compromising the thickness. A detailed and complementary analysis of the perovskite top layer and the full device using photoluminescence imaging and Kelvin Probe Force Microscopy reveal the removal of <i>dark spots</i> that act as traps for photogenerated carriers, resulting in surface homogeneity and narrowing of the band gap. Using laser polishing, a record <i>V</i>_oc of 1.21 V is achieved for planar PSCs with a triple cation composition (E_g ~ 1.61 eV), showing an improved perovskite/hole transport interface by mitigating surface recombination losses. We measure an efficiency boost from 18.0% to 19.3% with an improved stability of up to 1000 h. The polishing effect on the MAPbI₃ perovskite is also studied to explore the versatility of this method.<br/><br/>Furthermore, the same laser is employed to fabricate ultra-thin perovskite films up to ∼ 50 nm achieving a <i>V</i>_oc of 1.15 V, which is one of the highest reported so far for such semi-transparent PSCs. These results open the door to a new class of surface modification without any structural damage using lasers for interface passivation and semi-transparency in well-controllable, automated, scalable, and solvent-free surface treatments.