Surface trap–mediated nonradiative charge recombination is a major limit to achieving high-efficiency metal-halide perovskite photovoltaics. The ionic character of perovskite lattice has enabled molecular defect passivation approaches through interaction between functional groups and defects. However, a lack of in-depth understanding of how the molecular configuration influences the passivation effectiveness is a challenge to rational molecule design. Here, the chemical environment of a functional group that is activated for defect passivation was systematically investigated with theophylline, caffeine, and theobromine. When N-H and C=O were in an optimal configuration in the molecule, hydrogen-bond formation between N-H and I (iodine) assisted the primary C=O binding with the antisite Pb (lead) defect to maximize surface-defect binding. A stabilized power conversion efficiency of 22.6% of photovoltaic device was demonstrated with theophylline treatment.

Effective management of the insulating ligands is prerequisite for achieving good electrical coupling between colloidal quantum dots (CQDs) and, thus, highperformance solar cells. Here, we developed a rationally designed post-synthetic process for effective control of ligand density on organic-inorganic hybrid formamidinium lead triiodide (FAPbI 3) perovskite CQDs. The resulting FAPbI 3 CQD solar cells demonstrated power-conversion efficiency of 8.38% with stability superior to that of bulk FAPbI 3 devices.

No abstract

Surface effects usually become negligible on the micrometer or sub-micrometer scale due to lower surface-to-bulk ratio compared to nanomaterials. In lead halide perovskites, however, their “soft” nature renders them highly responsive to the external field, allowing for extended depth scale affected by the surface. Herein, by taking advantage of this unique feature of perovskites we demonstrate a methodology for property manipulation of perovskite thin films based on secondary grain growth, where tuning of the surface induces the internal property evolution of the entire perovskite film. While in conventional microelectronic techniques secondary grain growth generally involves harsh conditions such as high temperature and straining, it is easily triggered in a perovskite thin film by a simple surface post-treatment, producing enlarged grain sizes of up to 4 μm. The resulting photovoltaic devices exhibit significantly enhanced power conversion efficiency and operational stability over a course of 1000 h and an ambient shelf stability of over 4000 h while maintaining over 90% of its original efficiency.

The operational instability of perovskite solar cells (PSCs) is known to mainly originate from the migration of ionic species (or charged defects) under a potential gradient. Compositional engineering of the “A” site cation of the ABX3 perovskite structure has been shown to be an effective route to improve the stability of PSCs. Here, the effect of size‐mismatch‐induced lattice distortions on the ion migration energetics and operational stability of PSCs is investigated. It is observed that the size mismatch of the mixed “A” site composition films and devices leads to a steric effect to impede the migration pathways of ions to increase the activation energy of ion migration, which is demonstrated through multiple theoretical and experimental evidence. Consequently, the mixed composition devices exhibit significantly improved thermal stability under continuous heating at 85 °C and operational stability under continuous 1 sun illumination, with an extrapolated lifetime of 2011 h, compared to the 222 h of the reference device.

also the multiple exciton generation phenomenon, specific to CQDs, opens an avenue to make full utilization of solar radiation. [5][6][7] Recently, halide perovskite CQDs have emerged as a new class of CQDs for photovoltaics, offering compelling combination of the advantages of traditional CQDs and exceptional properties of halide perovskite materials, such as the desirable bandgap, high absorption coefficient, and defect tolerance. [8][9][10] Furthermore, the perovskite CQDs, such as CsPbI 3 and formamidinium lead iodide (FAPbI 3 ), were reported to show superior phase stability over their bulk form owing to the size-induced lattice strain and enhanced contribution from the surface energy. [11,12] Accordingly, the CQD solar cells based on the perovskite material have demonstrated a great potential with a record power conversion efficiency (PCE) as high as 13% along with excellent operational stability. [12,13] Although perovskite materials display superior optoelectronic properties to the conventional semiconductors, the low charge carrier separation efficiency, which is one of the critical obstacles toward higher performance of traditional CQD devices, still remains in the perovskite CQD solar cells. [14] While exciton binding energies (E b ) of bulk perovskite materials were measured to be as low as few millielectronvolts, facilitating spontaneous generation of free carriers at room temperature, [15] the E b of the perovskite CQD was estimated to be significantly higher (up to ten times) than that of the bulk counterparts. [9,16] As a result, the photogenerated charge carriers in the CQD films are subjected to be lost through recombination before being extracted to selective contact layers. [17] Therefore, promoting effective charge separation to reduce recombination becomes a crucial factor for boosting the PCE of the perovskite CQD solar cells.In order to facilitate such charge separation process and thus reduce the charge recombination in conventional CQD solar cells, a lot of efforts have been developed, such as coreshell structural design, surface ligand modification, and device structure engineering. [18][19][20][21][22][23] These available strategies employed in the traditional CQDs devices, however, are still not compatible with perovskite CQD system owing to their vulnerable structural stability as well as absence of well-established Halide perovskite colloidal quantum dots (CQDs) have recently emerged as a promising candidate for CQD photovoltaics due to their superior optoelectronic properties to conventional chalcogenides CQDs. However, the low charge separation efficiency due to quantum confinement still remains a critical obstacle toward higher-performance perovskite CQD photovoltaics. Available strategies employed in the conventional CQD devices to enhance the carrier separation, such as the design of type-II core-shell structure and versatile surface modification to tune the electronic properties, are still not applicable to the perovskite CQD system owing to the difficulty in modulating surfac...

The immobilization of molecular switches within materials can give rise to new bulk properties that are useful for a variety of applications.

Bromide inclusion altered the formation dynamics and defect physics of wide-bandgap perovskite compositions.