Ambient Engineering for High-Performance Organic-Inorganic Perovskite Hybrid Solar Cells

Liquid medium annealing for fabricating durable perovskite solar cells with improved reproducibility

Solution processing of semiconductors is highly promising for the high-throughput production of cost-effective electronics and optoelectronics. Although hybrid perovskites have potential in various device applications, challenges remain in the development of high-quality materials with simultaneously improved processing reproducibility and scalability. Here, we report a liquid medium annealing (LMA) technology that creates a robust chemical environment and constant heating field to modulate crystal growth over the entire film. Our method produces films with high crystallinity, fewer defects, desired stoichiometry, and overall film homogeneity. The resulting perovskite solar cells (PSCs) yield a stabilized power output of 24.04% (certified 23.7%, 0.08 cm²) and maintain 95% of their initial power conversion efficiency (PCE) after 2000 hours of operation. In addition, the 1-cm² PSCs exhibit a stabilized power output of 23.15% (certified PCE 22.3%) and keep 90% of their initial PCE after 1120 hours of operation, which illustrates their feasibility for scalable fabrication. LMA is less climate dependent and produces devices in-house with negligible performance variance year round. This method thus opens a new and effective avenue to improving the quality of perovskite films and photovoltaic devices in a scalable and reproducible manner.

Printing High-Efficiency Perovskite Solar Cells in High-Humidity Ambient Environment-An In Situ Guided Investigation

Extensive studies are conducted on perovskite solar cells (PSCs) with significant performance advances (mainly spin coating techniques), which have encouraged recent efforts on scalable coating techniques for the manufacture of PSCs. However, devices fabricated by blade coating techniques are inferior to state-of-the-art spin-coated devices because the power conversion efficiency (PCE) is highly dependent on the morphology and crystallization kinetics in the controlled environment and the delicate solvent system engineering. In this study, based on the widely studied perovskite solution system dimethylformamide-dimethyl sulfoxide, air-knife-assisted ambient fabrication of PSCs at a high relative humidity of 55 ± 5% is reported. In-depth time-resolved UV-vis spectrometry is carried out to investigate the impact of solvent removal and crystallization rate, which are critical factors influencing the crystallization kinetics and morphology because of adventitious moisture. UV-vis spectrometry enables accurate determination of the thickness of the wet precursor film. Anti-solvent-free, high-humidity ambient coatings of hysteresis-free PSCs with PCEs of 21.1% and 18.0% are demonstrated for 0.06 and 1 cm2 devices, respectively. These PSCs exhibit comparable stability to those fabricated in a glovebox, thus demonstrating their high potential.

Enhanced hole extraction by electron-rich alloys in all-inorganic CsPbBr3 perovskite solar cells

We present the fabrication of electron-rich Pt3M alloy-tailored carbon electrodes to maximize hole extraction. Through optimizing the doses and alloy species systematically, the best all-inorganic CsPbBr3 perovskite solar cell achieved a power conversion efficiency of 9.08% and showed excellent long-term stability at 80% RH over 20 days.

An Air Knife-Assisted Recrystallization Method for Ambient-Process Planar Perovskite Solar Cells and Its Dim-Light Harvesting

The photovoltaic performance of perovskite solar cells is highly dependent on the control of morphology and crystallization of perovskite film, which usually requires a controlled atmosphere. Therefore, fully ambient fabrication is a desired technology for the development of perovskite solar cells toward real production. Here, an air-knife assisted recrystallization method is reported, based on a simple bath-immersion to prepare high-quality perovskite absorbers. The resulted film shows a strong crystallinity with pure domains and low trap-state density, which contribute to the device performance and stability. The proposed method can operate in a wide process window, such as variable relative humidity and bath-immersion conditions, demonstrating a power conversion efficiency over 19% and 27% under 1 sun and 500-2000 lux dim-light illumination respectively, which is among the highest performance of ambient-process perovskite solar cells.

Ambient Air Condition for Room-Temperature Deposition of MAPbI3 Films in Highly Efficient Solar Cells

The power conversion efficiency of perovskite solar cells has been boosted rapidly, it has so far exceeded that of commercial polycrystalline silicon solar cells. This has prompted great interest in large-scale production and deployment of perovskite solar cells. However, state-of-the-art perovskite solar cells are fabricated inside gloveboxes and further annealing at high temperatures (typically at >100 °C for 30 min) is needed. These two required conditions are not compatible with, either in the respect to high-throughput or thermal budget, a feasible industrial production process. By eliminating the two requirements, the deposition of perovskite films both at room temperature and under ambient air condition will make the scalable roll-to-roll fabrication scheme feasible. Here, the anti-solvent (chloroform) washing is introduced to the previously developed hydrochloride-assisted method and demonstrate that the room-temperature method can be carried out under ambient air condition for MAPbI₃ film deposition. Through this new procedure, a power conversion efficiency as high as 17.72% is achieved for MAPbI₃ planar devices fabricated under a relative humidity of 30% at room temperature. Further, it is revealed that the room-temperature process MAPbI₃ films show a near monoexponential decay pathway with a long photoluminescence lifetime of >400 ns.

Bulk Heterojunction-Assisted Grain Growth for Controllable and Highly Crystalline Perovskite Films

Perovskite optoelectronic devices are being regarded as future candidates for next-generation optoelectronic devices. Device performance has been shown to be influenced by the perovskite film, which is determined by the grain size, surface roughness, and film coverage; therefore, developing controllable and highly crystalline perovskite films is pivotal for highly efficient devices. In this work, an innovative bulk heterojunction (BHJ)-assisted grain growth (BAGG) technique was developed to accurately control the quality of perovskite films. By a simple modulation of the polymer-to-PC₆₁BM ratio in the BHJ film, the transition to a complete film phase from the perovskite precursor was accurately regulated, resulting in a controllable perovskite grain growth and high-quality final perovskite film. Moreover, because the BHJ layer could seep deeply into the perovskite active layer through the grain boundaries in the BAGG process, it facilitated the interface engineering and charge transport. The perovskite solar cells containing an optimized CH₃NH₃PbI₃ film presented a high efficiency of 18.38% and fill factor of 83.71%. The perovskite light-emitting diode that contained a nanoscale and uniform CH₃NH₃PbBr₃ film with full coverage presented enhanced emission properties with a brightness value of 1600 cd/m² at 6.0 V and a luminous efficiency of 0.56 cd/A.

Perovskite Nanowire Extrusion

The defect tolerance of halide perovskite materials has led to efficient optoelectronic devices based on thin-film geometries with unprecedented speed. Moreover, it has motivated research on perovskite nanowires because surface recombination continues to be a major obstacle in realizing efficient nanowire devices. Recently, ordered vertical arrays of perovskite nanowires have been realized, which can benefit from nanophotonic design strategies allowing precise control over light propagation, absorption, and emission. An anodized aluminum oxide template is used to confine the crystallization process, either in the solution or in the vapor phase. This approach, however, results in an unavoidable drawback: only nanowires embedded inside the AAO are obtainable, since the AAO cannot be etched selectively. The requirement for a support matrix originates from the intrinsic difficulty of controlling precise placement, sizes, and shapes of free-standing nanostructures during crystallization, especially in solution. Here we introduce a method to fabricate free-standing solution-based vertical nanowires with arbitrary dimensions. Our scheme also utilizes AAO; however, in contrast to embedding the perovskite inside the matrix, we apply a pressure gradient to extrude the solution from the free-standing templates. The exit profile of the template is subsequently translated into the final semiconductor geometry. The free-standing nanowires are single crystalline and show a PLQY up to ∼29%. In principle, this rapid method is not limited to nanowires but can be extended to uniform and ordered high PLQY single crystalline perovskite nanostructures of different shapes and sizes by fabricating additional masking layers or using specifically shaped nanopore endings.

Self-Organized Fullerene Interfacial Layer for Efficient and Low-Temperature Processed Planar Perovskite Solar Cells with High UV-Light Stability

In this Communication, a self-organization method of [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)-ethyl) (methyl)amino)ethyl ester (PCBDAN) interlayer in between 6,6-phenyl C61-butyric acid methyl ester (PCBM) and indium tin oxide (ITO) has been proposed to improve the performance of N-I-P perovskite solar cells (PSCs). The introduction of self-organized PCBDAN interlayer can effectively reduce the work function of ITO and therefore eliminate the interface barrier between electron transport layer and electrode. It is beneficial for enhancing the charge extraction and decreasing the recombination loss at the interface. By employing this strategy, a highest power conversion efficiency of 18.1% has been obtained with almost free hysteresis. Furthermore, the N-I-P PSCs have excellent stability under UV-light soaking, which can maintain 85% of its original highest value after 240 h accelerated UV aging. This self-organization method for the formation of interlayer can not only simplify the fabrication process of low-cost PSCs, but also be compatible with the roll-to-roll device processing on flexible substrates.

Enhanced optoelectronic quality of perovskite films with excess CH3NH3I for high-efficiency solar cells in ambient air

Solution-processed polycrystalline perovskite films contribute critically to the high photovoltaic performance of perovskite-based solar cells (PSCs). The inevitable electronic trap states at grain boundaries and intrinsic defects such as metallic lead (Pb⁰) and halide vacancies in perovskite films cause serious carrier recombination loss. Furthermore, the film can easily decompose into PbI₂ in a moist atmosphere. Here, we introduce a simple strategy, through a small increase in methylammonium iodide (CH₃NH₃I, MAI), molar proportion (5%), for perovskite fabrication in ambient air with ∼50% relative humidity. Analysis of the morphology and crystallography demonstrates that excess MAI significantly promotes grain growth without decomposition. X-ray photoemission spectroscopy shows that no metallic Pb⁰ exists in the perovskite film and the I/Pb ratio is improved. A time-resolved photoluminescence measurement indicates efficient suppression of non-radiative recombination in the perovskite layer. As a result, the device yields improved power conversion efficiency from 14.06% to 18.26% with reduced hysteresis and higher stability under AM1.5G illumination (100 mW cm^-2). This work strongly provides a feasible and low-cost way to develop highly efficient PSCs in ambient air.

Single-Crystal-like Perovskite for High-Performance Solar Cells Using the Effective Merged Annealing Method

We report a simple, low cost, and quite effective method for achieving single-crystal-like CH₃NH₃PbI₃ perovskite leading to a significant enhancement in the performance and stability of inverted planar perovskite solar cells (IPSCs). By employing a merged annealing method during the fabrication of an IPSC for preparing the perovskite CH₃NH₃PbI₃ film, we remarkably increase the crystallinity of the CH₃NH₃PbI₃ film and enhance the device performance and stability. An IPSC with the indium tin oxide/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/CH₃NH₃PbI₃ (active layer)/[6,6]-phenyl-C₆₁-butyric acid methyl ester/Al structure was fabricated using the merged annealing method and exhibited significantly enhanced performance with a high power conversion efficiency of 18.27% and a fill factor of 81.34%. Moreover, since two separate annealing processes are merged in the proposed annealing method, the fabrication step becomes much simpler and easier, leading to a reduction in fabrication costs.

Fulleropyrrolidinium Iodide As an Efficient Electron Transport Layer for Air-Stable Planar Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells have attracted great attention in recent years. But there are still a lot of unresolved issues related to the perovskite solar cells such as the phenomenon of anomalous hysteresis characteristics and long-term stability of the devices. Here, we developed a simple three-layered efficient perovskite device by replacing the commonly employed PCBM electrical transport layer with an ultrathin fulleropyrrolidinium iodide (C₆₀-bis) in an inverted p-i-n architecture. The devices with an ultrathin C₆₀-bis electronic transport layer yield an average power conversion efficiency of 13.5% and a maximum efficiency of 15.15%. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements show that the high performance is attributed to the efficient blocking of holes and high extraction efficiency of electrons by C₆₀-bis, due to a favorable energy level alignment between the CH₃NH₃PbI₃ and the Ag electrodes. The hysteresis effect and stability of our perovskite solar cells with C₆₀-bis become better under indoor humidity conditions.