Spontaneous assembly of bilayer perovskite crystals for built-in p-n homojunction

Developing sufficient dynamics for charge separation and transfer is critical to improve photovoltaic performance. Normally, dynamics can be modulated by band bending via surface engineering, but this has typically been negligible in the p-n homojunction for boosted perovskite solar cells. Herein, we adopt carbon dots as an addition to induce a bilayer p-n perovskite homojunction to improve interfacial charge transfer within perovskite solar cells. Notably, the inclusion of carbon dots induces the spontaneous assembly of a bilayer structure, where the upper layer is enriched with dots and the underlying layer remains pristine perovskite. This unique arrangement creates a p-n homojunction, accompanied by an internal electric field, which allows for modulating energy levels and facilitating charge-carrier separation and transfer. Benefiting from the p-n homojunction, the power conversion efficiency of the perovskite solar cells increases from 18.57% to 21.09%. Additionally, the hydrophobic nature of the carbon dots considerably enhances the humidity stability of the solar cells.