Bright and stable near-infrared perovskite light emitters supported by multifunctional molecule design strategy

Efficient Homojunction Tin Perovskite Solar Cells Enabled by Gradient Germanium Doping

P-type self-doping is known to hamper tin-based perovskites for developing high-performance solar cells by increasing the background current density and carrier recombination processes. In this work, we propose a gradient homojunction structure with germanium doping that generates an internal electric field across the perovskite film to deplete the charge carriers. This structure reduces the dark current density of perovskite by over 2 orders of magnitude and trap density by an order of magnitude. The resultant tin-based perovskite solar cells exhibit a higher power conversion efficiency of 13.3% and excellent stability, maintaining 95% and 85% of their initial efficiencies after 250 min of continuous illumination and 3800 h of storage, respectively. We reveal the homojunction formation mechanism using density functional theory calculations and molecular level characterizations. Our work provides a reliable strategy for controlling the spatial energy levels in tin perovskite films and offers insights into designing intriguing lead-free perovskite optoelectronics.

Nucleation Regulation and Mesoscopic Dielectric Screening in α-FAPbI3

While significant advancements in power conversion efficiencies (PCEs) of α-FAPbI3 perovskite solar cells (PSCs) have been made, attaining controllable perovskite crystallization is still a considerable hurdle. This challenge stems from the initial formation of δ-FAPbI3 , a more energetically stable phase than the desired black α-phase, during film deposition. This disrupts the heterogeneous nucleation of α-FAPbI3 , causing the formation of mixed phases and defects. To this end, polarity engineering using molecular additives, specifically ((methyl-sulfonyl)phenyl)ethylamines (MSPEs) are introduced. The findings reveal that the interaction of PbI2 -MSPEs-FAI intermediates is enhanced with the increased polarity of MSPEs, which in turn expedites the nucleation of α-FAPbI3 . This leads to the development of high-quality α-FAPbI3 films, characterized by vertical crystal orientation and reduced residual stresses. Additionally, the increased dipole moment of MSPE at perovskite grain boundaries attenuates Coulomb attractions among charged defects and screens carrier capture process, thereby diminishing non-radiative recombination. Utilizing these mechanisms, PSCs treated with highly polar 2-(4-MSPE) achieve an impressive PCE of 25.2% in small-area devices and 20.5% in large-area perovskite solar modules (PSMs) with an active area of 70 cm2 . These results demonstrate the effectiveness of this strategy in achieving controllable crystallization of α-FAPbI3 , paving the way for scalable-production of high-efficiency PSMs.