Enhancing the Performance of Perovskite Solar Cells by Hybridizing SnS Quantum Dots with CH3NH3PbI3

The combination of perovskite solar cells and quantum dot solar cells has significant potential due to the complementary nature of the two constituent materials. In this study, solar cells (SCs) with a hybrid CH₃NH₃PbI₃/SnS quantum dots (QDs) absorber layer are fabricated by a facile and universal in situ crystallization method, enabling easy embedding of the QDs in perovskite layer. Compared with SCs based on CH₃NH₃PbI₃, SCs using CH₃NH₃PbI₃/SnS QDs hybrid films as absorber achieves a 25% enhancement in efficiency, giving rise to an efficiency of 16.8%. The performance improvement can be attributed to the improved crystallinity of the absorber, enhanced photo‐induced carriers' separation and transport within the absorber layer, and improved incident light utilization. The generality of the methods used in this work paves a universal pathway for preparing other perovskite/QDs hybrid materials and the synthesis of entire nontoxic perovskite/QDs hybrid structure.     

A Two‐Stage Annealing Strategy for Crystallization Control of CH3NH3PbI3 Films toward Highly Reproducible Perovskite Solar Cells

The solvent‐engineering method is widely used to fabricate top‐performing perovskite solar cells, which, however, usually exhibit inferior reproducibility. Herein, a two‐stage annealing (TSA) strategy is demonstrated for processing of perovskite films, namely, annealing the intermediate phase at 60 °C for the first stage then at 100 °C for the second stage. Compared to conventional direct annealing temperature (DHA) at 100 °C, using this strategy, MAPbI₃ films become more controllable, leading to superior film uniformity and device reproducibility with the champion device efficiency reaching 19.8%. More specifically, the coefficient of variation of efficiency for 49 cells is reduced to 5.9%, compared to 9.8% for that using DHA. The TSA process is carefully studied using Fourier transform infrared spectroscopy, X‐ray diffraction, and UV–vis absorption spectroscopy. It is found that in comparison with DHA the formation of hydrogen bonding and crystallization of perovskite are much slower and can be better controlled when using TSA. The improvements in film uniformity and device reproducibility are attributed to: 1) controllable MAPbI₃ crystal growth stemming from the progressive formation of hydrogen bonding between methylammonium and halide; 2) suppression of intermediate phase film dewetting, which is believed to be due to its decreased mobility at the initial low‐temperature annealing stage.     

Ligand-Modulated Excess PbI2 Nanosheets for Highly Efficient and Stable Perovskite Solar Cells

Excess lead iodide (PbI₂), as a defect passivation material in perovskite films, contributes to the longer carrier lifetime and reduced halide vacancies for high-efficiency perovskite solar cells. However, the random distribution of excess PbI₂ also leads to accelerated degradation of the perovskite layer. Inspired by nanocrystal synthesis, here, a universal ligand-modulation technology is developed to modulate the shape and distribution of excess PbI₂ in perovskite films. By adding certain ligands, perovskite films with vertically distributed PbI₂ nanosheets between the grain boundaries are successfully achieved, which reduces the nonradiative recombination and trap density of the perovskite layer. Thus, the power conversion efficiency of the modulated device increases from 20% to 22% compared to the control device. In addition, benefiting from the vertical distribution of excess PbI₂ and the hydrophobic nature of the surface ligands, the modulated devices exhibit much longer stability, retaining 72% of their initial efficiency after 360 h constant illumination under maximum power point tracking measurement.     

Highly Reproducible Large‐Area Perovskite Solar Cell Fabrication via Continuous Megasonic Spray Coating of CH3NH3PbI3

A simple, low‐cost, large area, and continuous scalable coating method is proposed for the fabrication of hybrid organic–inorganic perovskite solar cells. A megasonic spray‐coating method utilizing a 1.7 MHz megasonic nebulizer that could fabricate reproducible large‐area planar efficient perovskite films is developed. The coating method fabricates uniform large‐area perovskite film with large‐sized grain since smaller and narrower sized mist droplets than those generated by existing ultrasonic spray methods could be generated by megasonic spraying. The volume flow rate of the CH₃NH₃PbI₃ precursor solution and the reaction temperature are controlled, to obtain a high quality perovskite active layer. The devices reach a maximum efficiency of 16.9%, with an average efficiency of 16.4% from 21 samples. The applicability of megasonic spray coating to the fabrication of large‐area solar cells (1 cm²), with a power conversion efficiency of 14.2%, is also demonstrated. This is a record high efficiency for large‐area perovskite solar cells fabricated by continuous spray coating.     

Cs4PbI6-Mediated Synthesis of Thermodynamically Stable FA0.15Cs0.85PbI3 Perovskite Solar Cells

The stability issue is still one of the main limitations of the commercialization of perovskite photovoltaics. The mixed cation FA_xCs_1−xPbI₃ has shown great promise owing to its improved thermal and moisture stability. However, the study of FA_xCs_1−xPbI₃ is concentrated on formamidine (FA)-rich perovskite, whereas cesium (Cs)-rich FA_xCs_1−xPbI₃ perovskites are barely studied due to the inevitable phase separation when Cs > 30 mol%. Here, a Cs₄PbI₆-mediated method is developed to synthesize Cs-rich FA_xCs_1−xPbI₃ perovskites. It is demonstrated that Cs₄PbI₆ intermediate phase has a low Cs cation diffusion barrier and therefore offers a fast ion exchange with the preformed FA-rich perovskite phase to finally form the Cs-rich FA_xCs_1−xPbI₃ perovskite. The results indicate that ≈15% alloying with organic FA cations can sufficiently stabilize the perovskite phase with excellent phase and UV-irradiation stability. The FA_0.15Cs_0.85PbI₃ perovskite solar cells achieve a champion power conversion efficiency of 17.5%, showing the great potential of Cs-based perovskites for efficient and stable solar cells.     

Printable CsPbI3 Perovskite Solar Cells with PCE of 19% via an Additive Strategy

All-inorganic CsPbI₃ holds promise for efficient tandem solar cells, but reported fabrication techniques are not transferrable to scalable manufacturing methods. Herein, printable CsPbI₃ solar cells are reported, in which the charge transporting layers and photoactive layer are deposited by fast blade-coating at a low temperature (≤100 °C) in ambient conditions. High-quality CsPbI₃ films are grown via introducing a low concentration of the multifunctional molecular additive Zn(C₆F₅)₂, which reconciles the conflict between air-flow-assisted fast drying and low-quality film including energy misalignment and trap formation. Material analysis reveals a preferential accumulation of the additive close to the perovskite/SnO₂ interface and strong chemisorption on the perovskite surface, which leads to the formation of energy gradients and suppressed trap formation within the perovskite film, as well as a 150 meV improvement of the energetic alignment at the perovskite/SnO₂ interface. The combined benefits translate into significant enhancement of the power conversion efficiency to 19% for printable solar cells. The devices without encapsulation degrade only by ≈2% after 700 h in air conditions.     

Methylammonium Chloride as a Double-Edged Sword for Efficient and Stable Perovskite Solar Cells

The additive engineering strategy promotes the efficiency of solution-processed perovskite solar cells (PSCs) over 25%. However, compositional heterogeneity and structural disorders occur in perovskite films with the addition of specific additives, making it imperative to understand the detrimental impact of additives on film quality and device performance. In this work, the double-edged sword effects of the methylammonium chloride (MACl) additive on the properties of methylammonium lead mixed-halide perovskite (MAPbI_3-xCl_x ) films and PSCs are demonstrated. MAPbI_3-xCl_x films suffer from undesirable morphology transition during annealing, and its impacts on the film quality including morphology, optical properties, structure, and defect evolution are systematically investigated, as well as the power conversion efficiency (PCE) evolution for related PSCs. The FAX (FA = formamidinium, X = I, Br, and Ac) post-treatment strategy is developed to inhibit the morphology transition and suppress defects by compensating for the loss of the organic components, a champion PCE of 21.49% with an impressive open-circuit voltage of 1.17 V is obtained, and remains over 95% of the initial efficiency after storing over 1200 hours. This study elucidates that understanding the additive-induced detrimental effects in halide perovskites is critical to achieve the efficient and stable PSCs.     

Goethite Quantum Dots as Multifunctional Additives for Highly Efficient and Stable Perovskite Solar Cells

Minimization of defects and ion migration in organic–inorganic lead halide perovskite films is desirable for obtaining photovoltaic devices with high power conversion efficiency (PCE) and long‐term stability. However, achieving this target is still a challenge due to the lack of efficient multifunctional passivators. Herein, to address this issue, n‐type goethite (FeOOH) quantum dots (QDs) are introduced into the perovskite light‐absorption layer for achieving efficient and stable perovskite solar cells (PSCs). It is found that the iron, oxygen, and hydroxyl of FeOOH QDs can interact with iodine, lead, and methylamine, respectively. As a result, the crystallization kinetics process can be retarded, thereby resulting in high quality perovskite films with large grain size. Meanwhile, the trap states of perovskite can be effectively passivated via interaction with the under‐coordinated metal (Pb) cations, halide (I) anions on the perovskite crystal surface. Consequently, the PSCs with FeOOH QDs achieve a high efficiency close to 20% with negligible hysteresis. Most strikingly, the long‐term stability of PSCs is significantly enhanced. Furthermore, compared with the CH₃NH₃PbI₃‐based device, a higher PCE of 21.0% is achieved for the device assembled with a Cs_0.05FA_0.81MA_0.14PbBr_0.45I_2.55 perovskite layer.     

Efficient Perovskite Solar Cells Fabricated Through CsCl‐Enhanced PbI2 Precursor via Sequential Deposition

The fabrication of high‐quality perovskite film highly relies on chemical composition and the synthesis method of perovskite. So far, sequentially deposited MA_0.03FA_0.97Pb(I_0.97Br_0.03)₃ polycrystalline film is adopted to produce high‐performance perovskite solar cells with record power conversion efficiency (PCE). Fewer grain boundaries and incorporation of inorganic cation (e.g., cesium) would further increase device performance via sequential deposition. Here, cesium chloride (CsCl) is introduced into lead iodide (PbI₂) precursor solution that beneficially modulates the property of PbI₂ film, leading to larger grains with cesium incorporation in the resulting perovskite film. The enlarged crystal grains originate from a slower nucleation process for CsCl‐containing PbI₂ film when reacting with formamidine iodide, confirmed by in situ confocal photoluminescence imaging. Photovoltaic devices based on CsCl‐containing PbI₂ film demonstrate a higher averaging efficiency of 21.3% than 20.3% of the devices without CsCl additives for reverse scan. More importantly, the device stability is improved by CsCl additives that retain over 90% of their initial PCE value after 4000 min tracking at maximum power point under 1‐sun illumination. This work paves a way to further improve the photovoltaic performance of mixed‐cation‐halide perovskite solar cells via a sequential deposition method.