Precise Composition Tailoring of Mixed-Cation Hybrid Perovskites for Efficient Solar Cells by Mixture Design Methods

Toward stable lead halide perovskite solar cells: A knob on the A/X sites components

Hybrid lead halide ABX3 perovskite solar cells (PSCs) have emerged as a strong competitor to the traditional solar cells with a certified power conversion efficiency beyond 25% and other remarkable features such as light weight, solution processability, and low manufacturing cost. Further development on the efficiency and stability brings forth increasing attention in the component regulation, such as partial or entire substitution of A/B/X sites by alternative elements with similar size. However, the relationships between composition, property, and performance are poorly understood. Here, the instability of PSCs from the photon-, moisture-, thermal-, and mechanical-induced degradation was first summarized and discussed. In addition, the component regulation from the A/X sites is highlighted from the aspects of band level alignment, charge-carrier dynamics, ion migration, crystallization behavior, residual strain, stoichiometry, and dimensionality control. Finally, the perspectives and future outlooks are highlighted to guide the rational design and practical application of PSCs.

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.

Morphology and surface analyses for CH3NH3PbI3 perovskite thin films treated with versatile solvent-antisolvent vapors

Organometal halide perovskite (CH3NH3PbI3) semiconductors have been promising candidates as a photoactive layer for photovoltaics. Especially for high performance devices, the crystal structure and morphology of this perovskite layer should be optimized. In this experiment, by employing solvent-antisolvent vapor techniques during a modified sequential deposition of PbI2-CH3NH3I layers, the morphology engineering was carried out as a function of antisolvent species such as: chloroform, chlorobenzene, dichlorobenzene, toluene, and diethyl ether. Then, the optical, morphological, structural, and surface properties were characterized. When dimethyl sulfoxide (DMSO, solvent) and diethyl ether (antisolvent) vapors were employed, the CH3NH3PbI3 layer exhibited relatively desirable crystal structures and morphologies, resulting in an optical bandgap (E g) of 1.61 eV, crystallite size (t) of 89.5 nm, and high photoluminescence (PL) intensity. Finally, the stability of perovskite films toward water was found to be dependent on the morphologies with defects such as grain boundaries, which was evaluated through contact angle measurement.

Using design of experiment to obtain a systematic understanding of the effect of synthesis parameters on the properties of perovskite nanocrystals

Design of experiments was used to systematically investigate the synthesis of MAPbI₃ nanoparticles in a flow reactor. By controlling the solvents and the ligands, we were able to tune the MAPbI₃ photoluminescence peak between 614 and 737 nm.

Efficient and stable perovskite solar cells based on a quasi-point-contact and rear-reflection structure with 22.5% efficiency

A point-contact and rear-reflection (PCRR) structure is developed for efficient and stable perovskite solar cells with an efficiency of 22.5%.

Ink Engineering of Inkjet Printing Perovskite

Inkjet printing method is one of the most effective ways for fabricating large-area perovskite solar cells (PSCs). However, because ink crystallizes rapidly during printing, the printed perovskite film is discontinuous with increasing defects. It severely restricts the application of the inkjet printing technology to the fabrication of perovskite photovoltaic devices. Here, we designed a new mixed-cation perovskite ink system that can controllably retard the crystallization rate of perovskite. In this new ink system, the printing solvent is composed of n-methyl pyrrolidone (NMP) and dimethyl formamide (DMF), and PbX₂ is replaced by PbX₂-DMSO (X = Br, I) complex as a printing precursor to create a high-quality perovskite layer. Accordingly, the printed Cs_0.05MA_0.14FA_0.81PbI_2.55Br_0.45 perovskite film exhibited high homogeneity with a large grain size (over 500 nm). Besides, the printed perovskite film possessed lower defects with improved carrier lifetime compared to the control sample. Combining these advantages, the printed PSC delivers decent power conversion efficiencies (PCEs) of 19.6% (0.04 cm²) and 17.9% (1.01 cm²). The large-area device can still retain its original efficiency of 89% when stored in air with humidity less than 20% for 1000 h.

Tunable relativistic quasiparticle electronic and excitonic behavior of the FAPb(I1−xBrx)3 alloy

The quasiparticle and excitonic properties of mixed FAPb(I_1−xBr_x)₃ 0 ≤ x ≤ 1 alloys are studied. We show that Br-doping provides an efficient and controllable way to tune the band gap and optical properties, beneficial for material design of high performance tandem solar cells.

Engineering Halide Perovskite Crystals through Precursor Chemistry

The composition, crystallinity, morphology, and trap-state density of halide perovskite thin films critically depend on the nature of the precursor solution. A fundamental understanding of the liquid-to-solid transformation mechanism is thus essential to the fabrication of high-quality thin films of halide perovskite crystals for applications such as high-performance photovoltaics and is the topic of this Review. The roles of additives on the evolution of coordination complex species in the precursor solutions and the resulting effect on perovskite crystallization are presented. The influence of colloid characteristics, DMF/DMSO-free solutions and the degradation of precursor solutions on the formation of perovskite crystals are also discussed. Finally, the general formation mechanism of perovskite thin films from precursor solutions is summarized and some questions for further research are provided.

A Thermodynamically Favored Crystal Orientation in Mixed Formamidinium/Methylammonium Perovskite for Efficient Solar Cells

Crystal orientation has a great impact on the properties of perovskite films and the resultant device performance. Up to now, the exquisite control of crystal orientation (the preferred crystallographic planes and the crystal stacking mode with respect to the particular planes) in mixed-cation perovskites has received limited success, and the underlying mechanism that governs device performance is still not clear. Here, a thermodynamically favored crystal orientation in formamidinium/methylammonium (FA/MA) mixed-cation perovskites is finely tuned by composition engineering. Density functional theory calculations reveal that the FA/MA ratio affects the surface energy of the mixed perovskites, leading to the variation of preferential orientation consequently. The preferable growth along the (001) crystal plane, when lying parallel to the substrates, affects their charge transportation and collection properties. Under the optimized condition, the mixed-cation perovskite (FA_{1- x} MA_x PbI_2.87 Br_0.13 (Cl)) solar cells deliver a champion power conversion efficiency over 21%, with a certified efficiency of 20.50 ± 0.50%. The present work not only provides a vital step in understanding the intrinsic properties of mixed-cation perovskites but also lays the foundation for further investigation and application in perovskite optoelectronics.

Formamidinium Incorporation into Compact Lead Iodide for Low Band Gap Perovskite Solar Cells with Open-Circuit Voltage Approaching the Radiative Limit

To bring hybrid lead halide perovskite solar cells toward the Shockley-Queisser limit requires lowering the band gap while simultaneously increasing the open-circuit voltage. This, to some extent divergent objective, may demand the use of large cations to obtain a perovskite with larger lattice parameter together with a large crystal size to minimize interface nonradiative recombination. When applying the two-step method for a better crystal control, it is rather challenging to fabricate perovskites with FA⁺ cations, given the small penetration depth of such large ions into a compact PbI₂ film. In here, to successfully incorporate such large cations, we used a high-concentration solution of the organic precursor containing small Cl^- anions achieving, via a solvent annealing-controlled dissolution-recrystallization, larger than 1 μm perovskite crystals in a solar cell. This solar cell, with a largely increased fluorescence quantum yield, exhibited an open-circuit voltage equivalent to 93% of the corresponding radiative limit one. This, together with the low band gap achieved (1.53 eV), makes the fabricated perovskite cell one of the closest to the Shockley-Queisser optimum.

Cesium-Incorporated Triple Cation Perovskites Deliver Fully Reversible and Stable Nanoscale Voltage Response

Perovskite solar cells that incorporate small concentrations of Cs in their A-site have shown increased lifetime and improved device performance. Yet, the development of fully stable devices operating near the theoretical limit requires understanding how Cs influences perovskites' electrical properties at the nanoscale. Here, we determine how the chemical composition of three perovskites (MAPbBr₃, MAPbI₃, and Cs-mixed) affects their short- and long-term voltage stabilities, with <50 nm spatial resolution. We map an anomalous irreversible electrical signature on MAPbBr₃ at the mesoscale, resulting in local V _oc variations of ∼400 mV, and in entire grains with negative contribution to the V _oc. These measurements prove the necessity of high spatial resolution mapping to elucidate the fundamental limitations of this emerging material. Conversely, we capture the fully reversible voltage response of Cs-mixed perovskites, composed by Cs_0.06(MA_0.17FA_0.83)_0.94Pb(I_0.83Br_0.17)₃, demonstrating that the desired electrical output persists even at the nanoscale. The Cs-mixed material presents no spatial variation in V _oc, as ion motion is restricted. Our results show that the nanoscale electrical behavior of the perovskites is intimately connected to their chemical composition and macroscopic response.

Two Low-Cost and Efficient Hole-Transporting Materials for n-i-p Type Organic-Inorganic Hybrid Perovskite Solar Cells

The simpler the design, the better and more effective it is. Two novel conjugated triarylamine derivatives in donor-π-donor structure employing biphenyl core and pyrene core as π-bridges, which are termed as Bp-OMe and Py-OMe, have been synthesized and characterized and then applied to perovskite solar cells (PSCs) as hole-transport materials (HTMs) successfully. Using 2,2',7,7'-tetrakis(N,N-di-p-methoxy-phenylamine)-9,9'-spirobiuorene (spiro-OMeTAD) as a relative reference, Py-OMe-based PSCs showed the best power conversion efficiency (PCE) of 19.28% under AM 1.5 G illumination at 100 mW cm-2, which is comparable to that of PSCs based on spiro-OMeTAD with a best PCE of 18.57% with doping. Although Bp-OMe-based PSCs performed with relatively poor PCEs (best PCE of 15.06%) than those of Py-OMe-based PSCs, attributing to the poor planarity and hole mobility, taking the cost into consideration, Bp-OMe and Py-OMe are much more likely to be promising efficient HTMs for PSCs.

Ab initio study of the moisture stability of lead iodine perovskites

The stability of hybrid lead iodine perovskite in a humid environment has been a major obstacle to developing long-term photovoltaic devices. However, understanding the detailed degradation mechanism of lead iodine perovskite in moisture is still challenging. Herein, using first-principles calculations, we show that embedded water molecules will facilitate the decomposition of lead iodine perovskite. Alloying FAPbI₃ and CsPbI₃ to form mixed-cation lead iodine perovskites not only can optimize the tolerance factor to obtain better phase stability, but also can improve the moisture stability of them. With the accumulation of water molecules in the perovskite lattice, the optical absorption spectra show a blue-shift and decreased intensity, and the moisture stabilities of lead iodine perovskites are further lowered. The iodine vacancy in lead iodine perovskites can facilitate the water molecule migration and thus is a disadvantage in improving the moisture stability of them, which should be minimized during perovskite growth. These findings provide new insight in understanding the poor moisture stability of lead iodine perovskites, which should be helpful for the future design and optimization of stable perovskite solar cells.

Unraveling the Growth of Hierarchical Quasi-2D/3D Perovskite and Carrier Dynamics

The construction of hybrid perovskites (both two-dimensional (2D) and three-dimensional (3D)) has attracted intensive research interest recently. Here, a facile, two-step consecutive deposition method was developed for the first time to grow a hierarchical quasi-2D/3D perovskite superstructure, with an oriented quasi-2D ((BA)₂(MA)_n-1Pb_nI_3n+1) perovskite nanosheet (NS) perpendicularly aligned on 3D perovskites. The superstructures are found to be mixtures of multiple perovskite phases, with n = 2, 3, 4 and 3D perovskite; however, the n value was naturally increased from top to the bottom, which is distinct from many other works. We found that the concentration gradient, namely, the initial ratio and amount of BAI/MAI, collectively contributes to the spatially confined nucleation and growth of oriented quasi-2D superstructure perovskite on 3D perovskites. An efficient charge carrier transfer was demonstrated from small-n to large-n phases in this perovskite superstructure, indicating a different type of energy funnel from top to bottom.