Role of chloride in the morphological evolution of organo-lead halide perovskite thin films

Cl alloying improves thermal stability and increases luminescence in iodine-rich inorganic perovskites

The inorganic perovskite CsPbI3 shows promising photophysical properties for a range of potential optoelectronic applications but is metastable at room temperature. To address this, Br can be alloyed into the X-site to create compositions such as CsPbI2Br that are stable at room temperature but have bandgaps >1.9 eV - severely limiting solar applications. Herein, in an effort to achieve phase stable films with bandgaps <1.85 eV, we investigate alloying chlorine into iodine-rich triple-halide CsPb(I0.8Br0.2-x Cl x )3 with 0 < x < 0.1. We show that partial substitution of iodine with bromine and chlorine provides a path to maintain broadband terrestrial absorption while improving upon the perovskite phase stability due to chlorine's smaller size and larger ionization potential than bromine. At moderate Cl loading up to ≈5%, X-ray diffraction reveals an increasingly smaller orthorhombic unit cell, suggesting chlorine incorporation into the lattice. Most notably, this Cl incorporation is accompanied by a significant enhancement over Cl-free controls in the duration of black-phase stability of up to 7× at elevated temperatures. Additionally, we observe up to 5× increased steady state photoluminescence intensity (PL), along with a small blue-shift. In contrast, at high loading (≈10%), Cl accumulates in a second phase that is visible at grain boundaries via synchrotron fluorescence microscopy and negatively impacts the perovskite phase stability. Thus, replacing small fractions of bromine for chlorine in the iodine-rich inorganic perovskite lattice results in distinct improvement thermal stability and optoelectronic quality while minimally impacting the bandgap.

A universal ligand for lead coordination and tailored crystal growth in perovskite solar cells

Chemical environment and precursor-coordinating molecular interactions within a perovskite precursor solution can lead to important implications in structural defects and crystallization kinetics of a perovskite film. Thus, the opto-electronic quality of such films can be boosted by carefully fine-tuning the coordination chemistry of perovskite precursors via controllable introduction of additives, capable of forming intermediate complexes. In this work, we employed a new type of ligand, namely 1-phenylguanidine (PGua), which coordinates strongly with the PbI2 complexes in the perovskite precursor, forming new intermediate species. These strong interactions effectively retard the perovskite crystallization process and form homogeneous films with enlarged grain sizes and reduced density of defects. In combination with an interfacial treatment, the resulted champion devices exhibit a 24.6% efficiency with outstanding operational stability. Unprecedently, PGua can be applied in various PSCs with different perovskite compositions and even in both configurations: n-i-p and p-i-n, highlighting the universality of this ligand.

Influence of an Organic Salt-Based Stabilizing Additive on Charge Carrier Dynamics in Triple Cation Perovskite Solar Cells

Besides further improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSC), their long-term stability must also be ensured. Additives such as organic cations with halide counter anions are considered promising candidates to address this challenge, conferring both higher performance and increased stability to perovskite-based devices. Here, a stabilizing additive (N,N-dimethylmethyleneiminium chloride, [Dmmim]Cl) is identified, and its effect on charge carrier mobility and lifetime under thermal stress in triple cation perovskite (Cs0.05 MA0.05 FA0.90 PbI3 ) thin films is investigated. To explore the fundamental mechanisms limiting charge carrier mobility, temperature-dependent microwave conductivity measurements are performed. Different mobility behaviors across two temperature regions are revealed, following the power law Tm , indicating two different dominant scattering mechanisms. The low-temperature region is assigned to charge carrier scattering with polar optical phonons, while a strong decrease in mobility at high temperatures is due to dynamic disorder. The results obtained rationalize the improved stability of the [Dmmim]Cl-doped films and devices compared to the undoped reference samples, by limiting temperature-activated mobile ions and retarding degradation of the perovskite film.

Effect of Chloride Incorporation on the Intermediate Phase and Film Morphology of Methylammonium Lead Halide Perovskites

The influence of chloride integration on perovskite film deposition, encompassing both the structures of intermediate phases and the properties of the final films, was explored. Our methodology involved the fabrication of perovskite intermediate-phase films with varying concentrations of methylammonium chloride (MACl). Subsequently, we conducted an analysis employing X-ray diffraction and Rietveld refinement, incorporating the March-Dollase correction, to gain insights into how chloride-induced intermediate phases impact film morphology. Remarkably, a distinct preferred orientation was observed in the (020) lattice plane perpendicular to the substrate surface, and this orientation was found to be directly correlated to the MACl concentration. This distinctive arrangement of chloride-induced intermediate-phase complexes facilitated controlled crystallization, leading to highly oriented crystals and an improved film morphology. As a consequence, perovskite solar cell devices incorporating chloride-containing methylammonium lead iodide achieved a power conversion efficiency exceeding 20%. These findings suggest a crucial link between the preferred orientation observed in the final chlorine-derived perovskite films and the intermediate-phase structure formed during the initial stages of perovskite formation. These results suggest a profound impact of intermediate phase compositions and crystal structures on perovskite formation, emphasizing the importance of a comprehensive understanding of these factors to enable precise control over ideal structures and the subsequent transformation into high-quality perovskite films.

Deciphering the Roles of MA-Based Volatile Additives for α-FAPbI3 to Enable Efficient Inverted Perovskite Solar Cells

Functional additives that can interact with the perovskite precursors to form the intermediate phase have been proven essential in obtaining uniform and stable α-FAPbI₃ films. Among them, Cl-based volatile additives are the most prevalent in the literature. However, their exact role is still unclear, especially in inverted perovskite solar cells (PSCs). In this work, we have systematically studied the functions of Cl-based volatile additives and MA-based additives in formamidinium lead iodide (FAPbI₃)-based inverted PSCs. Using in situ photoluminescence, we provide clear evidence to unravel the different roles of volatile additives (NH₄Cl, FACl, and MACl) and MA-based additives (MACl, MABr, and MAI) in the nucleation, crystallization, and phase transition of FAPbI₃. Three different kinds of crystallization routes are proposed based on the above additives. The non-MA volatile additives (NH₄Cl and FACl) were found to promote crystallization and lower the phase-transition temperatures. The MA-based additives could quickly induce MA-rich nuclei to form pure α-phase FAPbI₃ and dramatically reduce phase-transition temperatures. Furthermore, volatile MACl provides a unique effect on promoting the growth of secondary crystallization during annealing. The optimized solar cells with MACl can achieve an efficiency of 23.1%, which is the highest in inverted FAPbI₃-based PSCs.

Reexamining the Post-Treatment Effects on Perovskite Solar Cells: Passivation and Chloride Redistribution

Post-treatment is an essential passivation step for the state-of-the-art perovskite solar cells (PSCs) but the additional role is not yet exploited. In this work, perovskite film is fabricated under ambient air with wide humidity window and identify that chloride redistribution induced by post-treatment plays an important role in high performance. The chlorine/iodine ratio on the perovskite surface increases from 0.037 to 0.439 after cyclohexylmethylammonium iodide (CHMAI) treatment and the PSCs deliver a champion power conversion efficiency (PCE) of 24.42% (certificated 23.60%). The maximum external quantum efficiency of electroluminescence (EQE_EL ) reaches to 10.84% with a radiance of 170 W sr^-1  m^-2 , forming the reciprocity relation between EQE_EL and nonradiative open-circuit voltage loss (86.0 mV). After thermal annealing, 2D component of perovskite will increase while chloride decline, leading to improved photovoltage but reduced fill factor. Hence, it distinguishes that chloride enrichment can improve charge transport/recombination simultaneously and 2D passivation can suppress the nonradiative recombination. Moreover, CHMAI can leverage their roles in charge transport/recombination for better performance than phenylethylammonium iodide (Cl/I = 0.114, PCE = 23.32%), due to the stronger binding energy of Cl^- . This work provides the insight that the chloride fixation can improve the photovoltaic performance.

Probing the Genuine Carrier Dynamics of Semiconducting Perovskites under Sunlight

Understanding the nature of photogenerated carriers and their subsequent dynamics in semiconducting perovskites is important for the development of solar cell materials and devices. However, most ultrafast dynamic measurements on perovskite materials were conducted under high carrier densities, which likely obscures the genuine dynamics under low carrier densities in solar illumination conditions. In this study, we presented a detailed experimental study of the carrier density-dependent dynamics in hybrid lead iodide perovskites from femtosecond to microsecond using a highly sensitive transient absorption (TA) spectrometer. From the dynamic curves with low carrier density in the linear response range, we observed two fast trapping processes that occurred in less than 1 ps and tens of picoseconds, attributed to the shallow traps, and two slow decays with lifetimes of hundreds of nanoseconds and longer than 1 μs, related to the trap-assisted recombination and trapping at deep traps. Further TA measurements clearly show that PbCl₂ passivation can effectively reduce both shallow and deep trap densities. These results provide insights into the intrinsic photophysics of semiconducting perovskites with direct implications for photovoltaic and optoelectronic applications under sunlight.

Role of solution concentration in formation kinetics of bromide perovskite thin films during spin-coating monitored by optical in situ metrology

Optoelectronic devices based on metal halide perovskites continue to show a improved performance, and solution-based coating techniques pave the way for large-area applications. However, not all parameters influencing the thin film formation process of metal halide perovskites are identified and entirely rationalised over their full compositional range, thus hampering optimised thin film fabrication. Furthermore, while the perovskite deposition via spin-coating and annealing is an easily accessible technique, more profound insights into the chemical formation process are still lacking. Varying the precursor solution concentration is commonly used to vary the resulting thin film thickness. This study shows that varying the precursor solution concentration also affects the thin film morphology and optoelectronic quality. Hence, we herein investigate the influence of the precursor solution concentration on the formation process of a pure bromide-based triple cation perovskite (Cs0.05MA0.10FA0.85PbBr3) by fiber-based optical in situ measurement. During the spin-coating process, in situ UV-vis and PL measurements reveal formation kinetics are strongly dependent on the concentration. Furthermore, we identify delayed nucleation and retarded growth kinetics for more concentrated precursor solutions. In addition, we quantify the shifting chemical equilibrium of colloidal pre-coordination in the precursor solution depending on concentration. Namely, colloids are pre-organised to a higher degree and higher-coordination lead-bromide complexes tend to form in more concentrated precursor solutions. Thus, the modified solution chemistry rationalises retarded perovskite formation kinetics and highlights the precursor concentration as an influential and optimisable parameter for solution-based thin film deposition.

Stabilized Low-Dimensional Species for Deep-Blue Perovskite Light-Emitting Diodes with EQE Approaching 3.4

Despite recent encouraging developments, achieving efficient blue perovskite light-emitting diodes (PeLEDs) have been widely considered a critical challenge. The efficiency breakthrough only occurred in the sky-blue region, and the device performance of pure-blue and deep-blue PeLEDs lags far behind those of their sky-blue counterparts. To avoid the negative effects associated with dimensionality reduction and excess chloride typically needed to achieve deep-blue emission, here we demonstrate guanidine (GA⁺)-induced deep-blue (∼457 nm) perovskite emitters enabling spectrally stable PeLEDs with a record external quantum efficiency (EQE) over 3.41% through a combination of quasi-2D perovskites and halide engineering. Owing to the presence of GA⁺, even a small inclusion of chloride ions is sufficient for generating deep-blue electroluminescence (EL), in clear contrast to the previously reported deep-blue PeLEDs with significant chloride inclusion that negatively affects spectral stability. Based on the carrier dynamics analysis and theoretical calculation, GA⁺ is found to stabilize the low-dimensional species during annealing, retarding the cascade energy transfer and facilitating the deep-blue EL. Our findings open a potential third route to achieve deep-blue PeLEDs beyond the conventional methods of dimensionality reduction and excessive chloride incorporation.

Unraveling the Role of Chloride in Vertical Growth of Low-Dimensional Ruddlesden-Popper Perovskites for Efficient Perovskite Solar Cells

Recently, low-dimensional Ruddlesden-Popper (LDRP) perovskite-based solar cells (PSCs) have been extensively studied because of their robust stability. However, because of the poor conductivity of the organic spacer, the charge transport across the spacers in the LDRP perovskite is considerably poor, and thus regulation of the growth orientation of LDRP cells is of primary importance. So far, the key role of organic cations in controlling the growth orientation of LDRP films has been widely studied, but the impact of halogens has not been sufficiently investigated. Herein, we demonstrate the important role of halogens in determining the characteristics of benzylamine (BZA)-based LDRP perovskite films, where different BZAX salts (X = Cl, Br, I) are adopted. Compared to Br and I, Cl is shown to prominently enlarge the grain size, promote the vertical orientation, reduce the trap state density, and prolong the carrier lifetime of LDRP film, and all these merits effectively accelerate the carrier transport within the film. As a result, a PSC device based on BZACl delivers a champion PCE of 17.25% with much improved device stability. This work unravels the vital role of Cl in regulating the crystallization process of LDRP films, which provides a facile approach for boosting the performance of LDRP-based PSCs.

Introduction of cadmium chloride additive to improve the performance and stability of perovskite solar cells

With the increase in the importance of using green energy sources to meet the world's energy demands, attempts have been made to push perovskite solar cell technology toward industrialization all around the world. Improving the properties of perovskite materials as the heart of PSCs is one of the methods to fabricate favorable photovoltaic (PV) solar cells based on perovskites. Here, cadmium chloride (CdCl2) was used as an additive source for the perovskite precursor to improve its PV properties. Results indicated CdCl2 improves the perovskite growth and tailors its crystalline properties, suggesting boosted charge transport processes in the bulk and interfaces of the perovskite layer with electron-hole transport layers. Overall, by incorporation of 1.0% into the MAPbI3 layer, a maximum power conversion efficiency of 15.28% was recorded for perovskite-based solar cells, higher than the 12.17% for the control devices. The developed method not only improved the PV performance of devices but also boosted the stability behavior of solar cells due to the passivated domain boundaries and enhanced hydrophobicity in the CdCl2-based devices.

Terbium-Doped and Dual-Passivated γ-CsPb(I1- x Brx )3 Inorganic Perovskite Solar Cells with Improved Air Thermal Stability and High Efficiency

Realizing photoactive and thermodynamically stable all-inorganic perovskite solar cells (PSCs) remains a challenging task within halide perovskite photovoltaic (PV) research. Here, a dual strategy for realizing efficient inorganic mixed halide perovskite PV devices based on a terbium-doped solar absorber, that is, CsPb_{1- x} Tb_x I₂ Br, is reported, which undertakes a bulk and surface passivation treatment in the form of CsPb_{1- x} Tb_x I₂ Br quantum dots, to maintain a photoactive γ-phase under ambient conditions and with significantly improved operational stability. Devices fabricated from these air-processed perovskite thin films exhibit an air-stable power conversion efficiency (PCE) that reaches 17.51% (small-area devices) with negligible hysteresis and maintains >90% of the initial efficiency when operating for 600 h under harsh environmental conditions, stemming from the combined effects of the dual-protection strategy. This approach is further examined within large-area PSC modules (19.8 cm² active area) to realize 10.94% PCE and >30 days ambient stability, as well as within low-bandgap γ-CsPb_0.95 Tb_0.05 I_2.5 Br_0.5 (E_g  = 1.73 eV) materials, yielding 19.01% (18.43% certified) PCE.

Multifunction Sandwich Structure Based on Diffusible 2-Chloroethylamine for High-Efficiency and Stable Tin-Lead Mixed Perovskite Solar Cells

Low-bandgap tin-lead mixed perovskites (PVKs) are necessary for all-perovskite tandem solar cells. This work proposes a multifunctional sandwich structure with 2-chloroethylamine (CEA) as the top and bottom interface layer and perovskite as the core layer. The sandwich structured CEA allows large ClCH₂CH₂NH₃⁺ and small Cl^- to diffuse into the crystal lattice and grain boundaries of perovskites, endowing an excellent antioxidation property by forming Sn(0), coordinating with SnI₂, and controlling the perovskite crystallization process. Moreover, the energy level alignment at the interface of the perovskite and transport layer becomes more matched. As a result, the CEA-modified champion device acquires a power conversion efficiency of 18.13% with an open-circuit voltage of 0.82 V and a short-circuit current density of 30.06 mA cm^-2. Meanwhile, the environmental stability of CEA-modified devices is substantially enhanced. This work introduces a new strategy for improving the performance and stability of tin-lead mixed perovskite solar cells.

Over 21% Efficiency Stable 2D Perovskite Solar Cells

Owing to their insufficient light absorption and charge transport, 2D Ruddlesden-Popper (RP) perovskites show relatively low efficiency. In this work, methylammonium (MA), formamidinum (FA), and FA/MA mixed 2D perovskite solar cells (PSCs) are fabricated. Incorporating FA cations extends the absorption range and enhances the light absorption. Optical spectroscopy shows that FA cations substantially increase the portion of 3D-like phase to 2D phases, and X-ray diffraction (XRD) studies reveal that FA-based 2D perovskite possesses an oblique crystal orientation. Nevertheless, the ultrafast interphase charge transfer results in an extremely long carrier-diffusion length (≈1.98 µm). Also, chloride additives effectively suppress the yellow δ-phase formation of pure FA-based 2D PSCs. As a result, both FA/MA mixed and pure FA-based 2D PSCs exhibit a greatly enhanced power conversion efficiency (PCE) over 20%. Specifically, the pure FA-based 2D PSCs achieve a record PCE of 21.07% (certified at 20%), which is the highest efficiency for low-dimensional PSCs (n ≤ 10) reported to date. Importantly, the FA-based 2D PSCs retain 97% of their initial efficiency at 85 °C persistent heating after 1500 h. The results unambiguously demonstrate that pure-FA-based 2D PSCs are promising for achieving comparable efficiency to 3D perovskites, along with a better device stability.

Sandwich Evaporation-Solvent Annealing Fabrication of Highly Crystalline MAPbIxCl3-x Perovskite Solar Cells

Perovskites doped with chlorine (Cl^-), which are usually fabricated using the solution process, can effectively improve the stability and carrier mobility. Compared with the low tolerance of the solution process that relies mostly on personal skill, thermal evaporation is an important method for large-scale production of perovskite solar cells but the production cost is high. In this study, the sandwich evaporation-solvent annealing (SE-SA) method is proposed. Using sandwich evaporation with a low-cost chamber of the sandwich evaporation technique (SET) made in the laboratory and with the help of DMSO steam-assisted crystallization, we have successfully produced chlorine-containing perovskite solar cells with a high crystallinity and a high efficiency of 15.1% with V_oc = 0.98 V, J_sc = 21.94 mA/cm², FF = 74.29%, and R_s = 3.66 Ω·cm², which can greatly reduce the production cost. It is worth mentioning that all the processes are carried out outside a glove box, which makes it possible for large-scale production of chlorine-containing perovskite solar cells by evaporation.

Oriented Halide Perovskite Nanostructures and Thin Films for Optoelectronics

Oriented semiconductor nanostructures and thin films exhibit many advantageous properties, such as directional exciton transport, efficient charge transfer and separation, and optical anisotropy, and hence these nanostructures are highly promising for use in optoelectronics and photonics. The controlled growth of these structures can facilitate device integration to improve optoelectronic performance and benefit in-depth fundamental studies of the physical properties of these materials. Halide perovskites have emerged as a new family of promising and cost-effective semiconductor materials for next-generation high-power conversion efficiency photovoltaics and for versatile high-performance optoelectronics, such as light-emitting diodes, lasers, photodetectors, and high-energy radiation imaging and detectors. In this Review, we summarize the advances in the fabrication of halide perovskite nanostructures and thin films with controlled dimensionality and crystallographic orientation, along with their applications and performance characteristics in optoelectronics. We examine the growth methods, mechanisms, and fabrication strategies for several technologically relevant structures, including nanowires, nanoplates, nanostructure arrays, single-crystal thin films, and highly oriented thin films. We highlight and discuss the advantageous photophysical properties and remarkable performance characteristics of oriented nanostructures and thin films for optoelectronics. Finally, we survey the remaining challenges and provide a perspective regarding the opportunities for further progress in this field.

Origin, Influence, and Countermeasures of Defects in Perovskite Solar Cells

Defects are considered to be one of the most significant factors that compromise the power conversion efficiencies and long-term stability of perovskite solar cells. Therefore, it is urgent to have a profound understanding of their formation and influence mechanism, so as to take corresponding measures to suppress or even completely eliminate their adverse effects on device performance. Herein, the possible origins of the defects in metal halide perovskite films and their impacts on the device performance are analyzed, and then various methods to reduce defect density are introduced in detail. Starting from the internal and interfacial aspects of the metal halide perovskite films, several ways to improve device performance and long-term stability including additive engineering, surface passivation, and other physical treatments (annealing engineering), etc., are further elaborated. Finally, the further understanding of defects and the development trend of passivation strategies are prospected.

Polymer Additive Assisted Fabrication of Compact and Ultra-Smooth Perovskite Thin Films with Fast Lamp Annealing

Perovskite solar cells (PVSC) have drawn increasing attention due to their high photovoltaic performance and low-cost fabrication with solution processability. A variety of methods have been developed to make uniform and dense perovskite thin films, which play a critical role on device performance. Herein, we demonstrate a polymer additive assisted approach with Polyamidoamine (PAMAM) dendrimers to facilitate the growth of uniform, dense, and ultra-smooth perovskite thin films. Furthermore, a lamp annealing approach has been developed to rapidly anneal perovskite films using an incandescent lamp, resulting in comparable or even better device performance compared to the control hotplate annealing. The facile polymer additive assisted method and the rapid lamp annealing technique offer a clue for the large-scale fabrication of efficient PVSCs.

Organic Ammonium Halide Modulators as Effective Strategy for Enhanced Perovskite Photovoltaic Performance

Despite rapid improvements in efficiency, long-term stability remains a challenge limiting the future up-scaling of perovskite solar cells (PSCs). Although several approaches have been developed to improve the stability of PSCs, applying ammonium passivation materials in bilayer configuration PSCs has drawn intensive research interest due to the potential of simultaneously improving long-term stability and boosting power conversion efficiency (PCE). This review focuses on the recent advances of improving n-i-p PSCs photovoltaic performance by employing ammonium halide-based molecular modulators. The first section briefly summarizes the challenges of perovskite materials by introducing the degradation mechanisms associated with the hygroscopic nature and ion migration issues. Then, recent reports regarding the roles of overlayers formed from ammonium-based passivation agents are discussed on the basis of ligand and halide effects. This includes both the formation of 2D perovskite films as well as purely organic passivating layers. Finally, the last section provides future perspectives on the use of organic ammonium halides within bilayer-architecture PSCs to improve the photovoltaic performances. Overall, this review provides a roadmap on current demands and future research directions of molecular modulators to address the critical limitations of PSCs, to mitigate the major barriers on the pathway toward future up-scaling applications.

CsPbBrI2 perovskites with low energy loss for high-performance indoor and outdoor photovoltaics

Over the years, the efficiency of inorganic perovskite solar cells (PSCs) has increased at an unprecedented pace. However, energy loss in the device has limited a further increase in efficiency and commercialization. In this work, we used (NH₄)₂C₂O₄·H₂O to treat CsPbBrI₂ perovskite film during spin-coating. The CsPbBrI₂ underwent secondary crystallization to form high quality films with micrometer-scale and low trap density. (NH₄)₂C₂O₄·H₂O treatment promoted charge transfer capacity and reduced the ideal factor. It also dropped the energy loss from 0.80 to 0.64 eV. The resulting device delivered a power conversion efficiency (PCE) of 16.55% with an open-circuit voltage (V_oc) of 1.24 V, which are largely improved compared with the reference device which exhibited a PCE of 13.27% and a V_oc of 1.10 V. In addition, the optimized treated device presented a record indoor PCE of 28.48% under a fluorescent lamp of 1000 lux, better than that of the reference device (19.05%).

In-Situ Nano-Auger Probe of Chloride-Ions during CH3NH3PbI3-xClx Perovskite Formation

Organo-halide perovskite solar cells (PSCs) have emerged as next-generation photovoltaics, owing to their high power-conversion efficiency (PCE), lower production cost, and high flexibility. ABX₃-structured methylammonium lead triiodide (CH₃NH₃PbI₃ or MAPbI₃) perovskite is a widely studied light-absorbing material in PSCs. Interestingly, a small amount of chlorine incorporation into MAPbI₃ increases charge carrier diffusion lengths (from 129 nm to 1069 nm), which enables planar structured PSCs with high PCEs. However, existence of chloride ions in the final perovskite film is still under debate. Contrastingly, few studies reported a negligible amount or absence of chloride ions in the final film, while others reported detection of chloride ions in the final film. Herein, we observed the microstructure and chlorine content of MAPbI_3−xCl_x thin films with increasing temperature via an in-situ nano-Auger spectroscopy and in-situ scanning electron microscopic analysis. The relative precipitation of MAPbI_3−xCl_x films occur at lower temperature and MAPbI_3−xCl_x grains grow faster than those of MAPbI₃ grains. Local concentrations of chlorine at intragrain and the vicinity of grain boundary were analyzed to understand the behavior and role of the chloride ions during the microstructural evolution of the MAPbI_3−xCl_x films.

Electrochemically induced iodine migration in mixed halide perovskites: suppression through chloride insertion

The role of chloride in improving the stability of mixed halide perovskites (MAPbCl_xBr_0.5(1-x)I_0.5(1-x))₃ is probed using spectroelectrochemistry. The injection of holes into mixed halide perovskite films through applied anodic bias results in the selective migration of iodine with ultimate expulsion into the electrolyte. Increasing the Cl content (x = 0 to 0.1) in the mixed halide perovskite suppresses the iodine mobility and thus decreases the rate of its expulsion into the solution. Implications of iodine mobility induced by hole accumulation and its impact on overall stability is discussed.

Ambient Fabrication of Organic-Inorganic Hybrid Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted significant attention in recent years due to their high-power conversion efficiency, simple fabrication, and low material cost. However, due to their high sensitivity to moisture and oxygen, high efficiency PSCs are mainly constructed in an inert environment. This has led to significant concerns associated with the long-term stability and manufacturing costs, which are some of the major limitations for the commercialization of this cutting-edge technology. Over the past few years, excellent progress in fabricating PSCs in ambient conditions has been made. These advancements have drawn considerable research interest in the photovoltaic community and shown great promise for the successful commercialization of efficient and stable PSCs. In this review, after providing an overview to the influence of an ambient fabrication environment on perovskite films, recent advances in fabricating efficient and stable PSCs in ambient conditions are discussed. Along with discussing the underlying challenges and limitations, the most appropriate strategies to fabricate efficient PSCs under ambient conditions are summarized along with multiple roadmaps to assist in the future development of this technology.

Stable High-Efficiency Two-Dimensional Perovskite Solar Cells Via Bromine Incorporation

Two-dimensional (2D) organic-inorganic perovskites as one of the most important photovoltaic material used in solar cells have attracted remarkable attention. These 2D perovskites exhibit superior environmental stability and wide tunability of their optoelectronic properties. However, their photovoltaic performance is far behind those of traditional three-dimensional (3D) perovskites. In this work, we demonstrate the power conversion efficiency (PCE) of 2D perovskite solar cells (PVSCs) is greatly improved from 3.01% for initial to 12.19% by the incorporation of PbBr₂. The enhanced efficiency is attributed to superior surface quality, enhanced crystallinity, and the resulting reduced trap-state density. Furthermore, PbBr₂ incorporated devices without encapsulation show excellent humidity stability, illumination stability, and thermal stability. This work provides a universal and viable avenue toward efficient and stable 2D PVSCs.

Photoinduced Phase Segregation Leading to Evident Open-Circuit Voltage Loss in Efficient Inorganic CsPbIBr2 Solar Cells

The photoinduced phase segregation (PIPS) of mixed-halide perovskites (MHPs), due to halogen migration, has reaped considerable attention for its retroaction on film photostability and photovoltaic output. Nevertheless, the original mechanism is still unclear. Herein, taking the representative CsPbIBr₂ material as an example, a confocal laser scanning microscope (CLSM) technique was adopted to track the PIPS and dark recovery procedures. Besides the aggregation of iodide-rich (I-rich) domains at grain boundaries (GBs), some sporadic iodide "islands" with a swifter light response also appear throughout the polycrystalline films. It illustrates again that GBs are not essential for iodide aggregation. Furthermore, the iodide "islands" have substantial influence on a device's open-circuit voltage (V_oc), resulting in an obvious plunge in the first tens of seconds. Results reveal the internal reason for the failure to reach the larger V_oc outputs expected from wide-bandgap perovskites. Importantly, this finding can help promote the exploration of an efficient means to stabilize MHPs.

Perovskite Solar Cells: A Porous Graphitic Carbon based Hole Transporter/Counter Electrode Material Extracted from an Invasive Plant Species Eichhornia Crassipes

Perovskite solar cells (PSCs) composed of organic polymer-based hole-transporting materials (HTMs) are considered to be an important strategy in improving the device performance, to compete with conventional solar cells. Yet the use of such expensive and unstable HTMs, together with hygroscopic perovskite structure remains a concern - an arguable aspect for the prospect of onsite photovoltaic (PV) application. Herein, we have demonstrated the sustainable fabrication of efficient and air-stable PSCs composed of an invasive plant (Eichhornia crassipes) extracted porous graphitic carbon (EC-GC) which plays a dual role as HTM/counter electrode. The changes in annealing temperature (~450 °C, ~850 °C and ~1000 °C) while extracting the EC-GC, made a significant impact on the degree of graphitization - a remarkable criterion in determining the device performance. Hence, the fabricated champion device-1c: Glass/FTO/c-TiO2/mp-TiO2/CH3NH3PbI3-xClx/EC-GC10@CH3NH3PbI3-x Clx/EC-GC10) exhibited a PCE of 8.52%. Surprisingly, the introduced EC-GC10 encapsulated perovskite interfacial layer at the perovskite/HTM interface helps in overcoming the moisture degradation of the hygroscopic perovskite layer in which the same champion device-1c evinced better air stability retaining its efficiency ~94.40% for 1000 hours. We believe that this present work on invasive plant extracted carbon playing a dual role, together as an interfacial layer may pave the way towards a reliable perovskite photovoltaic device at low-cost.

Influence of Chloride/Iodide Ratio in MAPbI3-xClx Perovskite Solar Devices: Case of Low Temperature Processable AZO Sub-Layer

A significant current challenge for perovskite solar technology is succeeding in designing devices all by low temperature processes. This could help for both rigid devices industrialisation and flexible devices development. The depositions of nanoparticles from colloidal suspensions consequently emerge as attractive approaches, especially due to their potential for low temperature curing not only for the photoactive perovskite layer but also for charge transporting layers. Here, NIP solar cells based on aluminium doped zinc oxide (AZO) electron transport layer were fabricated using a low temperature compatible process for AZO deposition. For the extensively studied perovskites based on methylammonium lead halides (MAPbI3-xClx), the chloride/iodide equation is widely proposed to follow an optimal value corresponding to an introduced MAI:PbCl2 ratio of 3:1. However, the perovskite formulation should be considered as a key parameter for the optimization of power conversion efficiency when exploring new perovskite sub-layers. We here propose a systematic method for the structural determination of the optimal ratio. It may depend on the sublayer and results from structural changes around the optimal value. The functional properties gradually increase with the addition of chlorine as long as it remains intercalated in a single phase. Above the optimal ratio, the appearance of two phases degrades the system.

Tetraethylenepent-MAPbI3-xClx Unsymmetrical Structure-Enhanced Stability and Power Conversion Efficiency in Perovskite Solar Cells

Two-dimensional (2D) perovskite solar cell (PSC) can achieve high stability by alternating interface cations. However, its main transmissive charge is limited owing to the 2D structure. Therefore, compared with a 3D device, the 2D PSC has poor power conversion efficiency (PCE). Further enhanced performance will require an increase in the transmission dimension of 2D PSC. Here, a novel tetraethylenepent (TEPA)-MAPbI_3-xCl_x analogous 2D unsymmetrical perovskite film was developed to improve the stability and PCE of the corresponding device. Based on the interaction of the active amino linear short chain of TEPA and the halogen ion, the symmetry of the mechanical structure of ions is disrupted, and the TEPA ion is embedded in the perovskite structure to form a perovskite structure with a dimension between 3D and 2D. Noticeably, the TEPA-MAPbI_3-xCl_x devices deliver high PCEs up to 19.73% which stands as the highest for MAPbI_3-xCl_x-based PSC. The environmental, thermal, and illumination stability also showed improvements ranging between 10%-30%. The enhanced PSCs are due to the higher quality of perovskite films, stronger charge transmission, and less trap density. This approach provides a new method to improve and modify 2D PSCs.

Enhancing Device Performance in Quasi-2D Perovskite ((BA)2(MA)3Pb4I13) Solar Cells Using PbCl2 Additives

Quasi-2D Ruddlesden-Popper perovskites exhibit excellent photostability/environmental stability. However, the main drawback is their relatively low photovoltaic properties compared with three-dimensional perovskites. Herein, we demonstrated that chlorine-based additives via adjusting the proportion of PbI₂ and PbCl₂ in the precursor (BA)₂(MA)₃Pb₄I₁₃ (n = 4) solutions show an optimized device performance of over 15%, and the devices exhibit much improved humidity stability. Upon PbCl₂ addition, the quasi-2D perovskites have larger and more compact grains, which result in high quality of films. The photoluminescence gives rise to a much prolonged lifetime under the PbCl₂ additive, indicating fewer trap states to reduce the nonradiative recombination. The capacitance characteristics confirm that the PbCl₂ additive can largely decrease the trap states in quasi-2D perovskite films. The capacitance-voltage characteristics indicate that using the PbCl₂ additive decreases the charge accumulation toward increasing the charge collection in quasi-2D perovskite solar cells. Our work indicates that the addition of PbCl₂ is an effective method to improve the device performance by reducing trap states and increasing charge collection toward developing high-performance quasi-2D perovskite devices.

Anomalous inclusion of chloride ions in ethylenediammonium lead iodide turns 1D non-perovskite into a 2D perovskite structure

A 2D ethylenediammonium lead iodide perovskite structure can form just by adding some chloride ions into the solution.

Causes and Solutions of Recombination in Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley-Queisser limit efficiency (30.5%). Obviously, trap-assisted nonradiative (also called Shockley-Read-Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post-treatment are explored. The focus herein is on the recombination at perovskite/electron-transporting material and perovskite/hole-transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap-assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Oriented Attachment as the Mechanism for Microstructure Evolution in Chloride-Derived Hybrid Perovskite Thin Films

Hybrid organic-inorganic perovskites with appealing optoelectronic properties have attracted significant interest for photovoltaic application. The use of chloride (Cl^-)-containing species to induce improved perovskite thin-film microstructures and improved optoelectronic properties is well-established. However, the mechanism for the formation of perovskite films with highly textured, micron-sized grains in the presence of Cl^- is not well established. Using synchrotron-based in situ two-dimensional grazing incidence wide-angle X-ray scattering complemented by scanning electron microscopy imaging, we present an oriented attachment mechanism via mineral bridge formation for the microstructural evolution of perovskite films post-treated with methylammonium chloride. We have identified the crucial role of the chlorine-containing intermediate phase as the mineral bridge, which enables the reorientation of primary, nanoscale perovskite grains followed by fusion into uniaxial oriented quasi-single crystal grains. The resulting perovskite films exhibit micron-sized grains with preferential orientation of the tetragonal (110) direction perpendicular to the substrate, resulting in improved solar cell efficiency attributed to improved charge collection. Our findings help to understand the fundamental mechanisms of microstructure evolution via soft processing in hybrid perovskite films.

High-Energy Photon Spectroscopy Using All Solution-Processed Heterojunctioned Surface-Modified Perovskite Single Crystals

Organic-inorganic hybrid perovskites have been intensively studied for their use in optoelectronic devices due to their utilization of low-cost, earth-abundant precursors that are solution-processed at low-temperatures into high-quality devices. Despite this progress, interdevice variability and long-term stability have prevented the widespread commercial adoption of perovskite devices, especially for high-energy photon detectors. Using methylammonium lead iodide perovskite single crystals grown via inverse-temperature crystallization, we demonstrate a facile solution-based technique to coat the single-crystalline bulk with a micrometer-scale thick surface layer comprised of a wider band gap two-dimensional Ruddlesden-Popper (RP) hybrid perovskite. The resulting perovskite room-temperature γ-ray detector devices exhibit greatly improved device yield and repeatability from run-to-run and device-to-device within a given processing run. With an energy resolution of under 15% (12.0 keV) for incident 81 keV photons, this solution-based technique resolves interdevice variability concerns and could pave the way for low-cost, scalable manufacturing of optoelectronic devices based on RP hybrid perovskite films.

Influence of Defects on Excited-State Dynamics in Lead Halide Perovskites: Time-Domain ab Initio Studies

This Perspective summarizes recent research into the excited-state dynamics in lead halide perovskites that are of paramount importance for photovoltaic and photocatalytic applications. Nonadiabatic molecular dynamics combined with time-domain ab initio density functional theory allows one to mimic time-resolved spectroscopy experiments at the atomistic level of detail. The focus is placed on realistic aspects of perovskite materials, including point defects, surfaces, grain boundaries, mixed stoichiometries, dopants, and interfaces. The atomistic description of the quantum dynamics of electron and hole trapping and recombination, provided by the time-domain ab initio simulations, generates important insights into the mechanisms of charge and energy losses and guides the development of high-performance perovskite solar cell devices.

Effective Control of Chlorine Contents in MAPbI3- xCl x Perovskite Solar Cells Using a Single-Source Vapor Deposition and Anion-Exchange Technique

In this study, a new method is developed to control the Cl-to-I ratio in MAPbI_{3- x}Cl _x perovskite solar cells (PSCs) more easily and precisely using single-source vapor deposition of MAPbCl₃ thin films and a subsequent anion exchange by repeated spin-coatings of methylammonium iodide (MAI) solution. This method can overcome the problems of previous vapor-deposition techniques for PSCs such as the occurrence of morphological defects in the films and difficulty in controlling the stoichiometry of the elements. The repetitive MAI treatments gradually fill the interstitial voids in the perovskite film and increase the average grain size up to 1.2 μm, which improves the charge-transfer property of the cells. The atomic Cl content, i.e., the x value, of the MAPbI_{3- x}Cl _x film can also be simply controlled by changing the number of MAI treatments. The energy levels and resistive elements of the cells are strongly dependent on the x value of the MAPbI_{3- x}Cl _x film. A maximum power conversion efficiency of 19.1% is achieved at x = 0.005.

Controlling the Morphology of Organic-Inorganic Hybrid Perovskites through Dual Additive-Mediated Crystallization for Solar Cell Applications

To realize a high-efficiency perovskite solar cell (PSC), it is critical to optimize the morphology of the perovskite film for a uniform and smooth finish with large grain size during film formation. Using a chemical compound as an additive to the precursor solution has recently been established as a promising method to control the morphology of the perovskite film. In this study, we propose a new method to achieve an improved morphology of the methylammonium lead iodide perovskite film by simultaneous addition of dimethyl sulfoxide (DMSO) and methoxyammonium salt (MeO) (dual additives). We demonstrated that an appropriate amount of the MeO additive helps the precursors form a stable intermediated PbI₂-DMSO adduct during film formation and enlarges the perovskite grains by retarding the kinetics of conversion of the adduct to the perovskite. Furthermore, we experimentally observed that the optical band gaps and crystal structures of perovskite films are reasonably unaffected by the MeO additive because MeO is almost eliminated during annealing. By optimizing the amount of MeO, we achieved improved device performances of the PSCs with a high power conversion efficiency of 19.71% that is ∼15% higher than that obtained for the control device (17.15%).

Air-Stable and Oriented Mixed Lead Halide Perovskite (FA/MA) by the One-Step Deposition Method Using Zinc Iodide and an Alkylammonium Additive

We present a one-step method to produce air-stable, large-grain mixed cationic lead perovskite films and powders under ambient conditions. The introduction of 2.5 % of Zn(II), confirmed by X-ray diffraction (XRD), results in stable thin films which show the same absorption and crystal structure after 2 weeks of storage under ambient conditions. Next to prolonged stability, the introduction of Zn(II) affects photophysical properties, reducing the bulk defect density, enhancing the photoluminescence (PL), and extending the charge carrier lifetime. Furthermore, 3-chloropropylamine hydrochloride is applied as the film-forming agent. The presence of this amine hydrochloride additive results in highly oriented and large crystal domains showing an ulterior improvement of PL intensity and lifetime. The material can also be prepared as black precursor powder by a solid-solid reaction under ambient conditions and can be pressed into a perovskite pellet. The prolonged stability and the easy fabrication in air makes this material suitable for large-scale, low-cost processing for optoelectronic applications.

Microstructural Evolution of Hybrid Perovskites Promoted by Chlorine and its Impact on the Performance of Solar Cell

The role of Cl in halide hybrid perovskites CH₃NH₃PbI₃(Cl) (MAPbI₃(Cl)) on the augmentation of grain size is still unclear although many reports have referred to these phenomena. Herein, we synthesized MAPbI₃(Cl) perovskite films by using excess MACl-containing precursors, which exhibited approximately an order of magnitude larger grain size with higher <110>-preferred orientation compared with that from stoichiometric precursors. Comprehensive mechanisms for the large grain evolution by Cl incorporation were elucidated in detail by correlating the changes in grain orientation, distribution of grain size, and the remaining Cl in the perovskite during thermal annealing. In the presence of Cl, <110>- and <001>-oriented grains grew faster than other grains at the initial stage of annealing. Further annealing led to the dissipation of Cl, resulting in the shrinkage of <001> grains while <110> grains continuously grew, as analyzed by x-ray rocking curve and diffraction. As a result of reduced grain boundaries and enhanced <110> texture, the trap density of perovskite solar cells diminished by ~10% by incorporating MACl in the precursor, resulting in a fill factor more than 80%.

Charge carrier recombination dynamics in a bi-cationic perovskite solar cell

The compositional engineering is of great importance to tune the electrical and optical properties of perovskite and improve the photovoltaic performance of perovskite solar cells. The exploration of the corresponding photoelectric conversion processes, especially the carrier recombination dynamics, will contribute to the optimization of the devices. In this work, perovskite with mixed methylammonium (MA) and formamidinium (FA) as organic cations, MA0.4FA0.6PbI3, is fabricated to study the influence of the bi-cation on the charge carrier recombination dynamics. X-ray diffraction analysis indicates the existence of the MAPbI3-FAPbI3 phase segregation in the bi-cationic perovskite crystal. The time-resolved photoluminescence dynamics presents a relatively fast carrier recombination process ascribed to the charge transfer from MAPbI3 to FAPbI3 in the bi-cationic perovskite film. The carrier recombination dynamics investigated by transient photovoltage measurements reveals a biphasic trap-assisted carrier recombination mechanism in the bi-cationic device, which involves carrier recombination in the MAPbI3 phase and FAPbI3 phase, respectively. The ultimate presentation of the carrier recombination process is closely related to the charge transfer between the two perovskite phases.

Perovskite Photovoltaics: The Significant Role of Ligands in Film Formation, Passivation, and Stability

Due to their outstanding optoelectronic properties, metal halide perovskites have been intensively studied in recent years. The latest certificated efficiency of 23.3% recently achieved in perovskite solar cells (PVSCs) enables them to be used as a very promising candidate for next-generation photovoltaics. The morphology, defect density, and water resistance of perovskite films have an enormous impact on the performance and stability of PVSCs. Ligands, with coordinating capability, have been widely developed to improve the quality and stability of perovskite materials significantly. In the first section of this review, the role of ligands in fabricating perovskite films by different methods (one-step, two-step, and postdeposition treatment) is discussed. In the second section, the progress on ligand-passivated perovskites via post-treatment, in situ passivation during perovskite formation, and modifying the substrates before perovskite formation is reviewed. In the third section, a discussion of ligand-stabilized perovskite films from the perspectives of crystal crosslinking, dimensionality engineering, and interfacial modification is presented. Finally, a summary and an outlook are given.

Efficient Planar Heterojunction FA1- xCs xPbI3 Perovskite Solar Cells with Suppressed Carrier Recombination and Enhanced Open Circuit Voltage via Anion-Exchange Process

Introduction of Cs into FAPbI₃ displayed great potential to stabilize the black perovskite phase by forming FA_{1- x}Cs _xPbI₃, which has been investigated widely based on solution process. During solution processing, the over-rapid intercalating reaction rate between PbI₂ and A cations (FA⁺ and Cs⁺) can bring some undesirable structural transitions. However, in vapor-assisted solution process (VASP), the over-rapid intercalating reaction rate can be reduced effectively. In addition, the formation process can be regulated significantly by the intermediate perovskite phase. In this study, FACl was employed together with FAI to improve the FA_0.9Cs_0.1PbI₃ films by VASP. In the vapor deposition process, the FACl and FAI vapor coreacted with the PbI₂ solid films, preferentially forming the intermediate perovskite phase FA_0.9Cs_0.1PbI _xCl _y. The intermediate perovskite phase FA_0.9Cs_0.1PbI _xCl _y supplied a plenty of seeds for rapid nucleation of perovskite, which prolonged the crystallization time of FA_0.9Cs_0.1PbI₃, and thus, a smooth FA_0.9Cs_0.1PbI₃ film with suppressed nonradiative recombination, prolonged carrier lifetime and decreased trap state density was acquired. Corresponding planar heterojunction perovskite solar cells achieved a champion power conversion efficiency (PCE) of 16.39% with a V_oc of 0.99 V, J_sc of 22.87 mA/cm², and fill factor of 74.82% under reverse scanning. Meanwhile, a hysteresis index of the FACl-10 device was decreased to 0.024 compared with 0.075 of the control device. Moreover, under the condition of nitrogen atmosphere, the normalized PCE of FACl-10 device diminished only 4.9% which was more stable comparing with 31.88% diminishing of the control device after 30 days.

Kinetics of Ion-Exchange Reactions in Hybrid Organic-Inorganic Perovskite Thin Films Studied by In Situ Real-Time X-ray Scattering

The exchange of ions in hybrid organic-inorganic perovskites with the general formula APbX₃ (A = MA, FA; X = I, Cl, Br) is studied in five different systems using in situ real-time grazing incident X-ray diffraction (GIXD). In systems where the organic cation is exchanged, we find a continuous shift of the lattice parameter. The relative shift compared to the pure materials is used to quantify the exchange. Whether or not a conversion is possible, as well as the amount of exchanged cations, depends on the halide used. In the case of the interconversion of MAPbI₃ and MAPbCl₃, we observe a decay of the diffraction peaks of the original perovskite and the emergence of new peaks corresponding to the structure with the alternative halide. Moreover, we determined the relevant time scales of the growth and decay of the perovskite structures.