Large Perovskite Grain Growth in Low-Temperature Solution-Processed Planar p-i-n Solar Cells by Sodium Addition

Enhancing the Efficiency and Stability of Tin-Lead Perovskite Solar Cells via Sodium Hydroxide Dedoping of PEDOT:PSS

Tin-lead (Sn-Pb) perovskite solar cells (PSCs) have gained interest as candidates for the bottom cell of all-perovskite tandem solar cells due to their broad absorption of the solar spectrum. A notable challenge arises from the prevalent use of the hole transport layer, PEDOT:PSS, known for its inherently high doping level. This high doping level can lead to interfacial recombination, imposing a significant limitation on efficiency. Herein, NaOH is used to dedope PEDOT:PSS, with the aim of enhancing the efficiency of Sn-Pb PSCs. Secondary ion mass spectrometer profiles indicate that sodium ions diffuse into the perovskite layer, improving its crystallinity and enlarging its grains. Comprehensive evaluations, including photoluminescence and nanosecond transient absorption spectroscopy, confirm that dedoping significantly reduces interfacial recombination, resulting in an open-circuit voltage as high as 0.90 V. Additionally, dedoping PEDOT:PSS leads to increased shunt resistance and high fill factor up to 0.81. As a result of these improvements, the power conversion efficiency is enhanced from 19.7% to 22.6%. Utilizing NaOH to dedope PEDOT:PSS also transitions its nature from acidic to basic, enhancing stability and exhibiting less than a 7% power conversion efficiency loss after 1176 h of storage in N2 atmosphere.

Identification and Mitigation of Transient Phenomena That Complicate the Characterization of Halide Perovskite Photodetectors

Halide perovskites have shown promise to advance the field of light detection in next-generation photodetectors, offering performance and functionality beyond what is currently possible with traditional inorganic semiconductors. Despite a relatively high density of defects in perovskite thin films, long carrier diffusion lengths and lifetimes suggest that many defects are benign. However, perovskite photodetectors show detection behavior that varies with time, creating inconsistent device performance and difficulties in accurate characterization. Here, we link the changing behavior to mobile defects that migrate through perovskites, leading to detector currents that drift on the time scale of seconds. These effects not only complicate reproducible device performance but also introduce characterization challenges. We demonstrate that such transient phenomena generate measurement artifacts that mean the value of specific detectivity measured can vary by up to 2 orders of magnitude even in the same device. The presence of defects can lead to photoconductive gain in photodetectors, and we show batch-to-batch processing variations in perovskite devices gives varying degrees of charge carrier injection and photocurrent amplification under low light intensities. We utilize the passivating effect of aging to reduce the impact of defects, minimizing current drifts and eliminating the gain. This work highlights the potential issues arising from mobile defects, which lead to inconsistent photodetector operation, and identifies the potential for defects to tune photodetection behavior in perovskite photodetectors.

Recent Advances of Doped SnO2 as Electron Transport Layer for High-Performance Perovskite Solar Cells

Perovskite solar cells (PSCs) have garnered considerable attention over the past decade owing to their low cost and proven high power conversion efficiency of over 25%. In the planar heterojunction PSC structure, tin oxide was utilized as a substitute material for the TiO2 electron transport layer (ETL) owing to its similar physical properties and high mobility, which is suitable for electron mining. Nevertheless, the defects and morphology significantly changed the performance of SnO2 according to the different deposition techniques, resulting in the poor performance of PSCs. In this review, we provide a comprehensive insight into the factors that specifically influence the ETL in PSC. The properties of the SnO2 materials are briefly introduced. In particular, the general operating principles, as well as the suitability level of doping in SnO2, are elucidated along with the details of the obtained results. Subsequently, the potential for doping is evaluated from the obtained results to achieve better results in PSCs. This review aims to provide a systematic and comprehensive understanding of the effects of different types of doping on the performance of ETL SnO2 and potentially instigate further development of PSCs with an extension to SnO2-based PSCs.

Kinetic-Controlled Crystallization of α-FAPbI3 Inducing Preferred Crystallographic Orientation Enhances Photovoltaic Performance

Crystallization kinetic controls the crystallographic orientation, inducing anisotropic properties of the materials. As a result, preferential orientation with advanced optoelectronic properties can enhance the photovoltaic devices' performance. Although incorporation of additives is one of the most studied methods to stabilize the photoactive α-phase of formamidinium lead tri-iodide (α-FAPbI₃ ), no studies focus on how the additives affect the crystallization kinetics. Along with the role of methylammonium chloride (MACl) as a "stabilizer" in the formation of α-FAPbI₃ , herein, the additional role as a "controller" in the crystallization kinetics is pointed out. With microscopic observations, for example, electron backscatter diffraction and selected area electron diffraction, it is examined that higher concentration of MACl induces slower crystallization kinetics, resulting in larger grain size and [100] preferred orientation. Optoelectronic properties of [100] preferentially oriented grains with less non-radiative recombination, a longer lifetime of charge carriers, and lower photocurrent deviations in between each grain induce higher short-circuit current density (J_sc ) and fill factor. Resulting MACl40 mol% attains the highest power conversion efficiency (PCE) of 24.1%. The results provide observations of a direct correlation between the crystallographic orientation and device performance as it highlights the importance of crystallization kinetics resulting in desirable microstructures for device engineering.

Facile NaF Treatment Achieves 20% Efficient ETL-Free Perovskite Solar Cells

Electron transporting layer (ETL)-free perovskite solar cells (PSCs) exhibit promising progress in photovoltaic devices due to the elimination of the complex and energy-/time-consuming preparation route of ETLs. However, the performance of ETL-free devices still lags behind conventional devices because of mismatched energy levels and undesired interfacial charge recombination. In this study, we introduce sodium fluoride (NaF) as an interface layer in ETL-free PSCs to align the energy level between the perovskite and the FTO electrode. KPFM measurements clearly show that the NaF layer covers the surface of rough underlying FTO very well. This interface modification reduces the work function of FTO by forming an interfacial dipole layer, leading to band bending at the FTO/perovskite interface, which facilitates an effective electron carrier collection. Besides, the part of Na⁺ ions is found to be able to migrate into the absorber layer, facilitating enlarged grains and spontaneous passivation of the perovskite layer. As a result, the efficiency of the NaF-treated cell reaches 20%, comparable to those of state-of-the-art ETL-based cells. Moreover, this strategy effectively enhances the operational stability of devices by preserving 94% of the initial efficiency after storage for 500 h under continuous light soaking at 55 °C. Overall, these improvements in photovoltaic properties are clear indicators of enhanced interface passivation by NaF-based interface engineering.

Untying the Cesium "Not": Cesium-Iodoplumbate Complexation in Perovskite Solution-Processing Inks Has Implications for Crystallization

We illustrate the critical importance of the energetics of cation-solvent versus cation-iodoplumbate interactions in determining the stability of ABX₃ perovskite precursors in a dimethylformamide (DMF) solvent medium. We have shown, through a complementary suite of nuclear magnetic resonance (NMR) and computational studies, that Cs⁺ exhibits significantly different solvent vs iodoplumbate interactions compared to organic A⁺-site cations such as CH₃NH₃⁺ (MA⁺). Two NMR studies were conducted: ¹³³Cs NMR analysis shows that Cs⁺ and MA⁺ compete for coordination with PbI₃^- in DMF. ²⁰⁷Pb NMR studies of PbI₂ with cationic iodides show that perovskite-forming Cs⁺ (and, somewhat, Rb⁺) do not comport with the ²⁰⁷Pb chemical shift trend found for Li⁺, Na⁺, and K⁺. Three independent computational approaches (density functional theory (DFT), ab initio Molecular Dynamics (AIMD), and a polarizable force field within Molecular Dynamics) yielded strikingly similar results: Cs⁺ interacts more strongly with the PbI₃^- iodoplumbate than does MA⁺ in a polar solvent environment like DMF. The stronger energy preference for PbI₃^- coordination of Cs⁺ vs MA⁺ in DMF demonstrates that Cs⁺ is not simply a postcrystallization cation "fit" for the perovskite A⁺-site. Instead, it may facilitate preorganization of the framework precursor that eventually transforms into the crystalline perovskite structure.

Sodium Incorporation for Enhanced Performance of Two-Dimensional Sn-Based Perovskite Transistors

Two-dimensional metal halide perovskites (2D MHPs) are promising candidates for transistor channel materials because of their high mobility in the lateral direction; however, Sn-based 2D MHPs exhibit poor film quality and oxidation stability. Here, we report a simple method to improve the performance and stability of 2D MHP transistors by incorporating sodium iodide (NaI) additives. By adding 1 vol % NaI (Na1), the transistors with phenethylammonium tin iodide (PEA₂SnI₄) exhibited reduced dual-sweep hysteresis, robust bias stability, and larger hole mobility (2.13 cm² V^-1 s^-1) than that of a pristine device (0.39 cm² V^-1 s^-1). Improvements in the film quality, such as increased grain size, crystallinity, and better film coverage, were observed in the PEA₂SnI₄:NaI film. In addition, NaI effectively passivated the iodine vacancies at the grain boundaries, thereby suppressing the defects.

A Sodium Chloride Modification of SnO2 Electron Transport Layers to Enhance the Performance of Perovskite Solar Cells

A sodium chloride modification was applied where different amounts of sodium chloride was physically blended in a tin oxide colloid solution to passivate the interface between the electron transport layer (ETL) and perovskite layer and improve the performance of perovskite solar cells. Sodium chloride-modified tin oxide was utilized as the electron transport material to fabricate perovskite solar cells. It was found that sodium chloride-modified tin oxide as an ETL could considerably enhance the performance of the device compared to pristine tin oxide. The power conversion efficiency of the perovskite solar cell displayed 8.8% remarkable improvement from 18.7 ± 0.4% to 20.3 ± 0.3% on average and 9.5% improvement from 18.9 to 20.7% in champion devices because of the considerable enhancement of the fill factor when 25 mM sodium chloride-modified tin oxide as the ETL was used in comparison with pristine tin oxide.

Efficient and Stable Perovskite Solar Cells Using Bathocuproine Bilateral-Modified Perovskite Layers

Surface modification engineering is an effective method to improve the crystallinity and passivate the perovskite interface and grain boundary, which can improve the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). The typical interface modification method is usually introduced at the interface of the perovskite/hole transport layer (HTL) or perovskite/electron transport layer (ETL) through coordination of the groups in the material with the perovskite. In this work, the n-type semiconductor bathocuproine (BCP) including the pyridine nitrogen bond was modified at the interfaces of perovskite/HTL or perovskite/ETL to improve perovskite crystallinity and interface contact properties. The better crystallinity and superior interface contact properties are obtained using BCP unilateral modification, which obviously increases the PCEs of PSCs. The BCP bilateral modification at both perovskite/ETL and perovskite/HTL interfaces can further improve the crystallinity with fewer defects and superior contact properties, which show the largest V_oc (1.14 V) and fill factors (FF 77.1%) compared to PSCs with BCP unilateral modification. PSCs with BCP bilateral modification obtained 20.6% PCEs, which is greatly higher than that (17.5%) of the original PSCs. The stability of PSCs with BCP bilateral modification can be greatly improved due to the better crystal quality and hydrophobic property of the interfaces. The results demonstrated that the n-type BCP material can efficiently modify both perovskite/HTL and perovskite/ETL interfaces beyond its semiconductor type, which can greatly improve the PCEs and stability of PSCs because BCP modification can passivate interfaces, improve interface contact and hydrophobic properties, promote crystallinity of the perovskite layer with fewer defects, and block carrier recombination at both interfaces.

Hot-Casting Large-Grain Perovskite Film for Efficient Solar Cells: Film Formation and Device Performance

Organic-inorganic metal halide perovskite solar cells (PSCs) have recently been considered as one of the most competitive contenders to commercial silicon solar cells in the photovoltaic field. The deposition process of a perovskite film is one of the most critical factors affecting the quality of the film formation and the photovoltaic performance. A hot-casting technique has been widely implemented to deposit high-quality perovskite films with large grain size, uniform thickness, and preferred crystalline orientation. In this review, we first review the classical nucleation and crystal growth theory and discuss those factors affecting the hot-casted perovskite film formation. Meanwhile, the effects of the deposition parameters such as temperature, thermal annealing, precursor chemistry, and atmosphere on the preparation of high-quality perovskite films and high-efficiency PSC devices are comprehensively discussed. The excellent stability of hot-casted perovskite films and integration with scalable deposition technology are conducive to the commercialization of PSCs. Finally, some open questions and future perspectives on the maturity of this technology toward the upscaling deposition of perovskite film for related optoelectronic devices are presented.

Extrinsic Ion Distribution Induced Field Effect in CsPbIBr2 Perovskite Solar Cells

Excellent power conversion efficiency (PCE) and stability are the primary forces that propel the all-inorganic cesium-based halide perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high density of trap state and internal nonradiative recombination of CsPbIBr₂ perovskite film are the barriers that limit its development. In the present study, a facile additive strategy is introduced to fabricate highly efficient CsPbIBr₂ PSCs by incorporating sulfamic acid sodium salt (SAS) into the perovskite layer. The additive can control the crystallization behaviors and optimize morphology, as well as effectively passivate defects in the bulk perovskite film, thereby resulting in a high-quality perovskite. In addition, SAS in perovskite has possibly introduced an additional internal electric field effect that favors electron transport and injection due to inhomogeneous ion distribution. A champion PCE of 10.57% (steady-output efficiency is 9.99%) is achieved under 1 Sun illumination, which surpasses that of the contrast sample by 16.84%. The modified perovskite film also exhibits improved moisture stability. The unencapsulated device maintains over 80% initial PCE after aging for 198 h in air. The results provide a suitable additive for inorganic perovskite and introduce a new conjecture to explain the function of additives in PSCs more rationally.

Metal Cations in Efficient Perovskite Solar Cells: Progress and Perspective

Metal halide perovskite solar cells (PVSCs) have revolutionized photovoltaics since the first prototype in 2009, and up to now the highest efficiency has soared to 24.2%, which is on par with commercial thin film cells and not far from monocrystalline silicon solar cells. Optimizing device performance and improving stability have always been the research highlight of PVSCs. Metal cations are introduced into perovskites to further optimize the quality, and this strategy is showing a vigorous development trend. Here, the progress of research into metal cations for PVSCs is discussed by focusing on the position of the cations in perovskites, the modulation of the film quality, and the influence on the photovoltaic performance. Metal cations are considered in the order of alkali cations, alkaline earth cations, then metal cations in the ds and d regions, and ultimately trivalent cations (p- and f-block metal cations) according to the periodic table of elements. Finally, this work is summarized and some relevant issues are discussed.

Electronic Traps and Their Correlations to Perovskite Solar Cell Performance via Compositional and Thermal Annealing Controls

Herein, underlying factors for enabling efficient and stable performance of perovskite solar cells are studied through nanostructural controls of organic-inorganic halide perovskites. Namely, MAPbI₃, (FA_0.83MA_0.17)Pb(I_0.83Br_0.17)₃, and (Cs_0.10FA_0.75MA_0.15)Pb(I_0.85Br_0.15)₃ perovskites (abbreviated as MA, FAMA, and CsFAMA, respectively) are examined with a grain growth control through thermal annealing. FAMA- and CsFAMA-based cells result in stable photovoltaic performance, while MA cells are sensitively dependent on the perovskite grain size dominated by annealing time. Micro-/nanoscopic features are comprehensively analyzed to unravel the origin that is directly correlated to the cell performance with the applications of electronic-trap characterizations such as photoconductive noise microscopy and capacitance analyses. It is revealed that CsFAMA has a lower trap density compared to MA and FAMA through the analyses of 1/ f noises and trapping/detrapping capacitances. Also, an open-circuit voltage ( V_oc) change is correlated to the variation of trap states during the shelf-life test: FAMA and CsFAMA cells with the negligible change of V_oc over weeks exhibit trap states shifting toward the band edge, although the power-conversion efficiencies are clearly reduced. The origins that critically affect the solar cell performance through the characterizations of shallow/deep traps with additional mobile defects in the perovskite and interfaces are discussed.

Solution evaporation processed high quality perovskite films

Organic-inorganic hybrid perovskite solar cells (PSCs), due to their high power conversion efficiency (PCE) and low cost, have been considered as one of the most promising photovoltaic devices. Well-distributed large-grain perovskite crystals play an important role for light adsorption and charge transfer. While, high quality perovskite crystals closely relate to the preparation method. Generally, the preparation of perovskite films requires a nitrogen atmosphere and toxic anti-solvents, which hinder their practical applications. Here, we reported a novel and simple solution evaporation process followed with a methylammonium iodide (MAI) solution immersion to prepare high quality perovskite films. Porous lead iodide (PbI₂) films were firstly formed by evaporation of PbI₂ precursor solution. The obtained porous PbI₂ films were completely transformed into well-distributed large-grain perovskite films after a quick MAI solution immersion. As a result, the corresponding PCE was enhanced by 30% compared to that prepared with a regular sequential deposition. This solution evaporation method provides a convenient and practical way for preparing high-quality perovskite films.

Improved photovoltaic performance of perovskite solar cells based on three-dimensional rutile TiO2 nanodendrite array film

In order to explore high performance and stable perovskite solar cells (PSCs), the design and optimization of electron transport layer (ETL) have been paid more and more attention. Vertically oriented, one-dimensional (1D) TiO2 nanostructured array films are considered superior ETLs because of their rapid electron transporting property and open pore architectures. In this study, a three-dimensional (3D) rutile TiO2 nanodendrite array (RTNDA) film containing 1D trunks and branches was fabricated through second hydrothermal treatment of 1D rutile TiO2 nanorod array (RTNRA) film hydrothermally grown on a fluorine tin oxide (FTO) conductive glass. The resulting 3D-RTNDA film not only facilitates close contact with mixed-ion perovskite (Cs0.05(FA0.83MA0.17)0.95Pb(I0.9Br0.1)3) film, but also promotes the formation of a perovskite layer with larger crystal grain sizes. Both can efficiently retard the interface charge recombination, and thus result in a significantly improved power conversion efficiency (PCE) of 18.0%, improved by 20% as compared to that (15.0%) of the device fabricated with the 1D-RTNRA film. Spectroscopic, electrochemical and photoelectrochemical measurements indicate that the improved photovolatic performance can be mainly ascribed to the largely suppressed hysteresis effect, the increased open-circuit voltage and fill factor stemming from the more effective hole blocking and electron transport. The results presented here demonstrate that 3D-RTNDA film with 3D rutile TiO2 hierarchical nanoarchitecture is a promising ETL selection in designing high-performance PSCs.

Modulating Surface Morphology Related to Crystallization Speed of Perovskite Grain and Optical Semiconductor and Crystallization Properties of the Absorber Layer Under Controlled Doping of Potassium Ions for Solar Cells

Perovskite thin films with excellent optical semiconductor and crystallization properties and superior surface morphology are normally considered to be vital to perovskite solar cells (PSCs). In this paper, we systematically survey the process of modulating surface morphology and optical semiconductor and crystallization properties of methylammonium lead iodide film by controlling doping of K⁺ for PSC prepared in air and propose the mechanism of large K⁺-doped perovskite grain formation related to crystallization speed. The increase in the crystallization speed leads to the production of large grains without localized-solvent-vapor (LSV) pores via moderate doping of K⁺, and the exorbitant crystallization speed induces super large grains with LSV pores via excessive doping of K⁺. Furthermore, the semiconductor properties (absorption band edge wavelength, PL emission peak wavelength, energy band gap) of perovskite film can be significantly tuned by controlled doping of K⁺. The investigation of the detailed process of modulating surface morphology and semiconductor properties of perovskite thin film by controlled doping of K⁺ may provide guidance and pave the way for superior component design of absorption materials for cost-efficient PSCs.

Dedoping of Lead Halide Perovskites Incorporating Monovalent Cations

We report significant improvements in the optoelectronic properties of lead halide perovskites with the addition of monovalent ions with ionic radii close to Pb²⁺. We investigate the chemical distribution and electronic structure of solution processed CH₃NH₃PbI₃ perovskite structures containing Na⁺, Cu⁺, and Ag⁺, which are lower valence metal ions than Pb²⁺ but have similar ionic radii. Synchrotron X-ray diffraction reveals a pronounced shift in the main perovskite peaks for the monovalent cation-based films, suggesting incorporation of these cations into the perovskite lattice as well as a preferential crystal growth in Ag⁺ containing perovskite structures. Furthermore, the synchrotron X-ray photoelectron measurements show a significant change in the valence band position for Cu- and Ag-doped films, although the perovskite bandgap remains the same, indicating a shift in the Fermi level position toward the middle of the bandgap. Such a shift infers that incorporation of these monovalent cations dedope the n-type perovskite films when formed without added cations. This dedoping effect leads to cleaner bandgaps as reflected by the lower energetic disorder in the monovalent cation-doped perovskite thin films as compared to pristine films. We also find that in contrast to Ag⁺ and Cu⁺, Na⁺ locates mainly at the grain boundaries and surfaces. Our theoretical calculations confirm the observed shifts in X-ray diffraction peaks and Fermi level as well as absence of intrabandgap states upon energetically favorable doping of perovskite lattice by the monovalent cations. We also model a significant change in the local structure, chemical bonding of metal-halide, and the electronic structure in the doped perovskites. In summary, our work highlights the local chemistry and influence of monovalent cation dopants on crystallization and the electronic structure in the doped perovskite thin films.

Alkali Metal Doping for Improved CH3NH3PbI3 Perovskite Solar Cells

Organic-inorganic hybrid halide perovskites are proven to be a promising semiconductor material as the absorber layer of solar cells. However, the perovskite films always suffer from nonuniform coverage or high trap state density due to the polycrystalline characteristics, which degrade the photoelectric properties of thin films. Herein, the alkali metal ions which are stable against oxidation and reduction are used in the perovskite precursor solution to induce the process of crystallization and nucleation, then affect the properties of the perovskite film. It is found that the addition of the alkali metal ions clearly improves the quality of perovskite film: enlarges the grain sizes, reduces the defect state density, passivates the grain boundaries, increases the built-in potential (Vbi), resulting to the enhancement in the power conversion efficiency of perovskite thin film solar cell.

Mechanism and effect of γ-butyrolactone solvent vapor post-annealing on the performance of a mesoporous perovskite solar cell

In this paper, γ-butyrolactone (GBL) solvent vapor post-annealing (SVPA) on CH3NH3PbI3 thin films is reported, aiming to improve the complete transformation of PbI2 and increase the grain size of the CH3NH3PbI3 crystal, thus boosting the performance of mesoporous CH3NH3PbI3 perovskite solar cells (PSCs). The influence of GBL SVPA on the microstructure of perovskite layers and performance of PSCs was studied. The short circuit current density (J sc) of the devices significantly increased, yielding a high efficiency of 16.58%, which was 27.05% higher than that of thermally annealed films. A model was derived to explain the effect of GBL SVPA on PSCs. The perovskite films prepared by this method present several advantages such as complete transformation of PbI2 to CH3NH3PbI3, high crystallinity, large grain size, and fewer grain boundaries than those prepared without GBL SVPA. This improvement is beneficial for charge dissociation and transport in hybrid photovoltaic devices.

Characterization of the influences of morphology on the intrinsic properties of perovskite films by temperature-dependent and time-resolved spectroscopies

The influences of morphology on the intrinsic properties of perovskite films are quantitatively characterized by temperature-dependent and time-resolved spectroscopies.

Structural effects on optoelectronic properties of halide perovskites

This review gives a perspective on different synthetic methodologies for the preparation of halide perovskites and highlights the structural effects on their optoelectronic properties.

Defects in metal triiodide perovskite materials towards high-performance solar cells: origin, impact, characterization, and engineering

The progress of defect science in metal triiodide perovskite is critically reviewed, including the origin, impacts, characterization, and engineering.

CH3NH3Pb1−xEuxI3 mixed halide perovskite for hybrid solar cells: the impact of divalent europium doping on efficiency and stability

The crucial role of the impact of divalent europium doping in perovskite solar cells is investigated in this work.

Influence of the Grain Size on the Properties of CH3NH3PbI3 Thin Films

Hybrid perovskites have already shown a huge success as an absorber in solar cells, resulting in the skyrocketing rise in the power conversion efficiency to more than η = 22%. Recently, it has been established that the crystal quality is one of the most important parameters to obtain devices with high efficiencies. However, the influence of the crystal quality on the material properties is not fully understood. Here, the influence of the morphology on electronic properties of CH₃NH₃PbI₃ thin films is investigated. Postannealing was used to vary the average grain size continuously from ≈150 to ≈1000 nm. Secondary grain growth is thermally activated with an activation energy of E_a = 0.16 eV. The increase in the grain size leads to an enhancement of the photoluminescence, indicating an improvement in the material quality. According to surface photovoltage measurements, the charge-carrier transport length exhibits a linear increase with increasing grain size. The charge-carrier diffusion length is limited by grain boundaries. Moreover, an improved morphology leads to a drastic increase in power conversion efficiency of the devices.

Understanding and Tailoring Grain Growth of Lead-Halide Perovskite for Solar Cell Application

The fundamental mechanism of grain growth evolution in the fabrication process from the precursor phase to the perovskite phase is not fully understood despite its importance in achieving high-quality grains in organic-inorganic hybrid perovskites, which are strongly affected by processing parameters. In this work, we investigate the fundamental conversion mechanism from the precursor phase of perovskite to the complete perovskite phase and how the intermediate phase promotes growth of the perovskite grains during the fabrication process. By monitoring the morphological evolution of the perovskite during the film fabrication process, we observed a clear rod-shaped intermediate phase in the highly crystalline perovskite and investigated the role of the nanorod intermediate phase on the growth of the grains of the perovskite film. Furthermore, on the basis of these findings, we developed a simple and effective method to tailor grain properties including the crystallinity, size, and number of grain boundaries, and then utilized the film with the tailored grains to develop perovskite solar cells.

Thermodynamically self-organized hole transport layers for high-efficiency inverted-planar perovskite solar cells

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a popular and promising hole transport material for making efficient inverted-planar perovskite solar cells (IP-PSCs). However, the mismatch between the work function of conventional PEDOT:PSS and the valence band maximum of perovskite materials is still a challenge for efficient hole extraction. Here, we report systematic studies on the work-function modification and thermodynamic morphological evolution of PEDOT:PSS films by tuning the PSS/PEDOT ratio, along with its effects on the photovoltaic responses of IP-PSCs. We found that the open-circuit voltage (V_OC) of an IP-PSC could be enhanced by controlling the work function of PEDOT:PSS. Furthermore, the optical transmittance of the PEDOT:PSS film could be maximized by controlling the morphological evolution, which will further increase the short-circuit current density (J_SC) of the IP-PSC. The V_OC and J_SC of the IP-PSC with the optimized PEDOT:PSS composition increased from 0.88 to 0.93 V and from 17.11 to 20.77 mA cm^-2, respectively, compared with an IP-PSC containing commercial PEDOT:PSS, which results in a power conversion energy that is greatly improved from 12.39 to 15.24%.

Magnetic Field-Assisted Perovskite Film Preparation for Enhanced Performance of Solar Cells

Perovskite solar cells (PSCs) are promising low-cost photovoltaic technologies with high power conversion efficiency (PCE). The crystalline quality of perovskite materials is crucial to the photovoltaic performance of the PSCs. Herein, a simple approach is introduced to prepare high-quality CH₃NH₃PbI₃ perovskite films with larger crystalline grains and longer carriers lifetime by using magnetic field to control the nucleation and crystal growth. The fabricated planar CH₃NH₃PbI₃ solar cells have an average PCE of 17.84% and the highest PCE of 18.56% using an optimized magnetic field at 80 mT. In contrast, the PSCs fabricated without the magnetic field give an average PCE of 15.52% and the highest PCE of 16.72%. The magnetic field action produces an ordered arrangement of the perovskite ions, improving the crystallinity of the perovskite films and resulting in a higher PCE.

Organic-Inorganic Hybrid Perovskite with Controlled Dopant Modification and Application in Photovoltaic Device

Organic-inorganic hybrid perovskite as a kind of promising photovoltaic material is booming due to its low-cost, high defect tolerance, and easy fabrication, which result in the huge potential in industrial production. In the pursuit of high efficiency photovoltaic devices, high-quality absorbing layer is essential. Therefore, developing organic-inorganic hybrid perovskite thin films with good coverage, improved uniformity, and crystalline in a single pass deposition is of great concern in realizing good performance of perovskite thin-film solar cell. Here, it is found that the introduction of suitable amounts of LiI plays a dramatically positive role in enlarging the grain size and reducing the grain boundaries of absorbing layer. In addition, the carrier lifetime and built-in potential of the LiI doped perovskite device are observed to increase. Thus, it leads to about 15% gain in solar cell efficiency comparing to that without the LiI doping. Meanwhile, a hysteresis reduction is observed and 18.16% power conversion efficiency is achieved in LiI doped perovskite device, as well.

Facile Face-Down Annealing Triggered Remarkable Texture Development in CH3NH3PbI3 Films for High-Performance Perovskite Solar Cells

Herein, we demonstrate that the facile face-down annealing route which effectively confines the evaporation of residual solvent molecules in one-step deposited precursor films can controllably enable the formation of (110) textured CH₃NH₃PbI₃ films consisting of high-crystallinity well-ordered micrometer-sized grains that span vertically the entire film thickness. Such microstructural features dramatically decrease nonradiative recombination sites as well as greatly improve the transport property of charge carries in the films compared with that of the nontextured ones obtained by the conventional annealing route. As a consequence, the planar-heterojunction perovskite solar cells with these textured CH₃NH₃PbI₃ films exhibit significantly enhanced power conversion efficiency (PCE) along with small hysteresis and excellent stability. The champion cell yields impressive PCE boosting to 18.64% and a stabilized value of around 17.22%. Particularly, it can maintain 86% of its initial value after storage for 20 days in ambient conditions with relative humidity of 10-20%. Our work suggests a facile and effective route for further boosting the efficiency and stability of low-cost perovskite solar cells.

Enhanced performance of perovskite solar cells by strengthening a self-embedded solvent annealing effect in perovskite precursor films

Smooth perovskite films with large grains are fabricated by strengthening the self-embedded solvent annealing effect in the perovskite precursor film via pre-depositing a protective layer.

Interfacial electronic structures revealed at the rubrene/CH3NH3PbI3 interface

The promising rubrene-based PSC device performance demonstrates the potential of rubrene as a suitable hole transport material in PSCs due to an optimal energy level alignment at the rubrene/CH₃NH₃PbI₃ interface.

Retarding Crystallization during Facile Single Coating of NaCl-Incorporated Precursor Solution for Efficient Large-Area Uniform Perovskite Solar Cells

We demonstrated crystallization retardation of CH₃NH₃PbI₃ thin film during single coating of precursor solution by simple addition of NaCl. NaCl was codissolved into a precursor mixture solution containing PbI₂ and methylammonium iodide (MAI). Dissolved NaCl interacted with the PbI₂ in solution and produced a stable intermediate phase, which was converted to a full-coverage uniform perovskite absorber layer via reaction with MAI during a single spin-coating. The resulting planar-structure perovskite solar cell made from NaCl-supplemented precursor solution showed a 48% improvement in power conversion efficiency (PCE) (maximum value 15.16%) over the device fabricated without the additive. Our NaCl-supplemented single coating represents an easy approach to effectively obtain highly reproducible uniform performance at an overall position in 5 cm × 5 cm sized cells (divided into 20 subcells with an active area of 0.06 cm²) with average PCEs of 12.00 ± 0.48%.

Efficiency-Enhanced Planar Perovskite Solar Cells via an Isopropanol/Ethanol Mixed Solvent Process

Solution processable perovskite solar cells traditionally employed isopropanol as the solvent of CH3NH3I in a two-step method. One of the largest issues of this technique is the uncontrollable morphology of the perovskite film. In this study, a homogeneous and dense PbI2 film was prepared by introducing DMSO as an additive into DMF and then reacting the mixture with CH3NH3I dissolved in an isopropanol/ethanol solvent to fabricate high-quality perovskite films. Results revealed that ethanol played a crucial role on morphology and components of perovskite films. When the ratio of isopropanol to ethanol was optimized, a power conversion efficiency of 15.76% was achieved, which was on average ∼50% higher than that of PSCs without DMSO and ethanol processing.

Simultaneous enhancement in stability and efficiency of low-temperature processed perovskite solar cells

Low temperature (140 °C) processed MA_0.6FA_0.4PbI₃/AZO devices exhibit 20% higher PCE and two-fold higher device stability compared to MA_0.6FA_0.4PbI₃/ZnO devices.