Suppressing Residual Lead Iodide and Defects in Sequential-Deposited Perovskite Solar Cell via Bidentate Potassium Dichloroacetate Ligand

In sequential-deposited polycrystalline perovskite solar cells, the unreacted lead iodide due to incomplete conversion of lead iodide to perovskite phase, can contribute to ionic defects, such as residual lead ions (Pb²⁺ ). At present, passivation of interfacial and grain boundary defects has become an effective strategy to suppress charge recombination. Here, we introduced potassium acetate (KAc) and potassium dichloroacetate (KAcCl₂ ) as additives in the sequential deposition of polycrystalline perovskite thin films and found that acetate ions (Ac^- ) can effectively reduce the residual lead iodide. Compared with acetate (Ac), dichloroacetate (AcCl₂ ) can form Pb-Cl and Pb-O bonding as "dual anchoring" bonds with residual Pb²⁺ , resulting in strong binding force and effective passivation of residual Pb²⁺ defects. Furthermore, K⁺ can enlarge grain size and restrain ion migration at the grain boundaries. Consequently, perovskite solar cells with KAcCl₂ additive show power conversion efficiencies (PCE) from 19.67 % to 22.12 %, with the open-circuit voltage increasing from 1.06 V to 1.14 V. The unencapsulated device can maintain 82 % of the initial PCE under a humidity of 30±5 % for 1200 h. This work provides a new approach for the regulation of ionic defects and grain boundaries at the same time to develop high-performance planar perovskite solar cells.

Localized incorporation of cesium ions to improve formamidinium lead iodide layers in perovskite solar cells

For the perovskite solar cells with formamidinium lead iodide (FAPbI₃) as a light harvester, cesium ions (Cs⁺) can be used to stabilize the perovskite crystal structure of FAPbI₃. However, the incorporation of Cs⁺ ions usually reduces the grain size and degrades the crystallization of FAPbI₃ layers, and this is harmful to the photovoltaic performance of solar cells. In this work, we incorporate Cs⁺ ions into FAPbI₃ layers using the interfacial doping method, which is different from the mixed solution doping method in previous reports. Elemental analysis indicates that Cs⁺ dopants cannot be detected at the outer surfaces of perovskite layers, and the majority of Cs⁺ dopants should be localized in the vicinity of TiO₂/perovskite interfaces, which is remarkably different from the distribution of Cs⁺ dopants in the perovskite layers prepared using the mixed solution doping method. It is found that interfacial doping method can avoid the blue shift of the light absorption edge and can improve the crystallization of FAPbI₃ layers. For the optimized conditions, Cs_xFA_1−xPbI₃ solar cells prepared using the interfacial doping method achieve a power conversion efficiency (PCE) of 17.1%, which is better than the PCE of Cs_xFA_1−xPbI₃ devices prepared using the mixed solution doping method.

An interfacial doping method leads to a localized profile of dopants at interfaces, which results in improved photovoltaic performance.

Growth of Compact CH3NH3PbI3 Thin Films Governed by the Crystallization in PbI2 Matrix for Efficient Planar Perovskite Solar Cells

As a convenient preparation technique, a two-step method, which is normally done by spin-coating CH₃NH₃I onto PbI₂ film followed by a thermal annealing, is generally used to prepare solution-processed CH₃NH₃PbI₃ films for planar perovskite solar cells. Here, we prepare the compact CH₃NH₃PbI₃ thin films by the two-step method at a low temperature (<80 °C) and investigate the effects of PbI₂ crystallization on the structure-property correlation in the CH₃NH₃PbI₃ films. It is found that the importance of the crystallization in PbI₂ matrix lies in governing the transition from the (001) plane of trigonal PbI₂ to the (002) plane of tetragonal CH₃NH₃PbI₃ in the rapid reaction process for atoms to coordinate into perovskite during spin-coating, which actually determines the morphology and the type of vacancy defects in resulting perovskite; a better crystallized PbI₂ film has a much stronger ability to react with CH₃NH₃I solution and produces larger CH₃NH₃PbI₃ grains with a higher crystallinity. The CH₃NH₃PbI₃/TiO₂ planar solar cell derived from a better crystallized PbI₂ film exhibits significantly improved performance and stability as the result of the higher crystallinity inside the perovskite film. Moreover, it is demonstrated that the crystalline PbI₂ film matrix subjected to the annealing after a slow heating process prior to contacting CH₃NH₃I solution is more effective for CH₃NH₃PbI₃ formation than that with a direct annealing history. The results in this paper provide a guide for preparing high-quality CH₃NH₃PbI₃ thin films for efficient perovskite solar cells and CH₃NH₃PbI₃ interfacial films over the layers susceptible to temperature.

Poly(4-Vinylpyridine)-Based Interfacial Passivation to Enhance Voltage and Moisture Stability of Lead Halide Perovskite Solar Cells

It is well known that the surface trap states and electronic disorders in the solution-processed CH₃ NH₃ PbI₃ perovskite film affect the solar cell performance significantly and moisture sensitivity of photoactive perovskite material limits its practical applications. Herein, we show the surface modification of a perovskite film with a solution-processable hydrophobic polymer (poly(4-vinylpyridine), PVP), which passivates the undercoordinated lead (Pb) atoms (on the surface of perovskite) by its pyridine Lewis base side chains and thereby eliminates surface-trap states and non-radiative recombination. Moreover, it acts as an electron barrier between the perovskite and hole-transport layer (HTL) to reduce interfacial charge recombination, which led to improvement in open-circuit voltage (V_oc ) by 120 to 160 mV whereas the standard cell fabricated in same conditions showed V_oc as low as 0.9 V owing to dominating interfacial recombination processes. Consequently, the power conversion efficiency (PCE) increased by 3 to 5 % in the polymer-modified devices (PCE=15 %) with V_oc more than 1.05 V and hysteresis-less J-V curves. Advantageously, hydrophobicity of the polymer chain was found to protect the perovskite surface from moisture and improved stability of the non-encapsulated cells, which retained their device performance up to 30 days of exposure to open atmosphere (50 % humidity).

Controlled Deposition and Performance Optimization of Perovskite Solar Cells Using Ultrasonic Spray-Coating of Photoactive Layers

This study investigated a new film-deposition technique, ultrasonic spray-coating, for use in the production of a photoactive layer of perovskite solar cells. Stable atomization and facile fabrication of perovskite thin films by ultrasonic spray-coating were achieved in a one-step method through manipulating the ink formulation (e.g., solution concentration, precursor composition, and mixing solvent ratio) and the drying kinetics (e.g., post-annealing temperature). The performance of the perovskite solar cells was mainly influenced by the intrinsic film morphology and crystalline orientation of the deposited perovskite layer. By suitable optimization of the spreading and drying conditions of the ink, ultrasonic spray-coated perovskite photovoltaic devices were obtained with a maximum power conversion efficiency of 11.30 %, a fill factor of 73.6 %, a short-circuit current of 19.7 mA cm^-1 , and an open-circuit voltage of 0.78 V, respectively. Notably, the average power efficiency reached above 10 %, attributed to the large flower-like perovskite crystal with orientation along the (1 1 2)/(2 0 0) and (2 2 4)/(4 0 0) directions. Thus, the ultrasonic spray-coating method for perovskite photoactive layers, combining advantages of good photovoltaic performance results and benefits from cost and processing, has the potential for large-scale commercial production.

Proposition and computational analysis of a kesterite/kesterite tandem solar cell with enhanced efficiency

An all-kesterite tandem junction solar cell has been proposed and an efficiency ≥20% has been estimated from computational analysis.