Surface Of Perovskite Layer Research Articles

Two-Step Perovskite Solar Cells with > 25% Efficiency: Unveiling the Hidden Bottom Surface of Perovskite Layer.

While significant efforts in surface engineering have been devoted to the conversion process of lead iodide (PbI2) into perovskite and top surface engineering of perovskite layer with remarkable progress, the exploration of residual PbI2 clusters and the hidden bottom surface on perovskite layer have been limited. In this work, a new strategy involving 1-butyl-3-methylimidazolium acetate (BMIMAc) ionic liquid (IL) additives is developed and it is found that both the cations and the anions in ILs can interact with the perovskite components, thereby regulating the crystallization process and diminishing the residue PbI2 clusters as well as filling vacancies. The introduction of BMIMAc ILs induces the formation of a uniform porous PbI2 film, facilitating better penetration of the second-step organic salt and fostering a more extensive interaction between PbI2 and the organic salt. Surprisingly, the oversized residual PbI2 clusters at the bottom surface of the perovskite layer completely diminish. In addition, advanced depth analysis techniques including depth-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) and bottom thinning technology are employed for a comprehensive understanding of the reduction in residual PbI2. Leveraging effective PbI2 management and regulation of the perovskite crystallization process, the champion devices achieve a power conversion efficiency (PCE) of 25.06% with long-term stability.

Probing the stability of perovskite solar cell under working condition through an ultra-thin silver electrode: Beyond the halide ion diffusion and metal diffusion

Perovskite solar cells (PSCs) with an excellent optoelectronic performance have intrigued mushrooming research interests, and the undesirable intrinsic stability of halide perovskite materials still remains a severe constraint for their practical application. Fortunately, the ambiguous and complicated incentives for the degradation process of PSCs under working condition can be directly reflected on the corrosion of silver electrode by halogens. Here, a new perspective for evaluating the long-term stability of PSCs is presented by the time-dependent transverse resistance of the ultrathin silver electrode under different working conditions. Being anode or cathode, the stability of ultra-thin silver layer has been systematically investigated through adjusting the external operating conditions of devices, i.e. light illumination and bias voltage. Experimental results indicate that the gradual resistance increases of silver film can be attributed to the oxidation of I− existing on the top surface of perovskite layer by non-equilibrium holes generated from light illumination or electrical injection for producing corrosive I2 gas, which will diffuse through the carrier transporting layer and attack the thin silver layer. The interaction probability of I− and non-equilibrium holes at the interface plays a critical role on the generation of I2 gas and resistance variation of the top thin silver electrode. The stability study of electrode indicator will shed light on the ambiguous degradation mechanisms of PSC under working condition, paving a path for conquering the fatal problems of the practical application.

Fabrication and Characterization of SnCl2- and CuBr-Added Perovskite Photovoltaic Devices

Perovskite photovoltaic devices added with tin (Sn) dichloride and copper (Cu) bromide were fabricated and characterized. The thin film devices were prepared by an ordinary spin-coating technique using an air blowing method in ambient air. A decaphenylcyclopentasilane layer was coated at the surface of perovskite layer and annealed at a high temperature of 190 °C. Conversion efficiencies and short-circuit current densities were improved for devices added with Sn and Cu compared with the standard devices. The energy gap of the perovskite crystal decreased through the Sn addition, which was also confirmed by first-principles calculations.

Passivation of perovskite layer surface states with pyridine in flexible and printed perovskite solar cells

Perovskite solar cells (PSCs), prepared by using solution-processed printing techniques, gained much attention over the past few years and a considerable progress has been achieved in improving the power conversion efficiencies of these devices. Nevertheless, there are still some advancements that can be implemented, especially in terms of passivation of surface defects in the perovskite photoactive layer. Passivation can afford considerable reduction in surface recombination of charge carriers in the photoactive layer and help to obtain devices with better performance. In this work, poly(3-hexylthiophene-2,5-diyl)-based inks with small amount of pyridine as an additive are used to deposit the hole transport layer and simultaneously passivate the surface defects of the perovskite layer in flexible and printed PSCs. The devices are fabricated on flexible conductive plastic substrates using a slot-die coating method. It is found that 2.5 wt.% pyridine-containing inks for preparing hole transport layer have a positive effect on the performance of resulting PSCs. On average, around 13% improvement in the power conversion efficiency is observed for the devices with passivation as opposed to the reference devices without passivation. The effect of pyridine passivation on the structural and electronic properties of the perovskite layer on a flexible substrate is studied using experimental and analytical techniques, whereas the computer simulation methods are employed to rule out the possible mechanisms for the performance improvements in the devices with passivation. The approach presented here can be useful for developing simplified protocols for printing of flexible PSCs with the passivated perovskite layer and improved device efficiency.

Simultaneously Mitigating Anion and Cation Defects Both in Bulk and Interface for High‐Effective Perovskite Solar Cells

Perovskite layer, as the origin of optical–electrical conversion of devices, plays a very important role in perovskite solar cells (PSCs). However, lots of lead cations and halogen anions defects inevitably exit in the bulk and surface of the perovskite layer. These defects, serve as nonradiative recombination centers, degrade the performance, and damage the stability of PSCs. Herein, a strategy that anion and cation defects on the interface and in the bulk of perovskites are simultaneously passivated by doping organic molecule 4‐aminomethyl tetrahydropyran (4‐AMPR) and the surface modification with 1‐chlorobutane (1‐CB) in/on the perovskite is demonstrated. The O atoms on 4‐AMPR can coordinate with the Pb vacancy and antisite Pb defects, as well as improve perovskite morphology. The volatilization of low‐boiling 1‐CB can passivate the halogen ion defects and promote the uniform nucleation of perovskite. The PSCs jointly optimized by 4‐AMPR and 1‐CB achieve a power conversion efficiency of 22.74% and retain 90.7% of the initial efficiency after storage in air environment (RH 10 ± 5%, 25 °C) for more than 1000 h. This research demonstrates a promising strategy for simultaneously mitigating anion and cation defects both in bulk and surface of perovskite layer and thus enhancing the performance and stability of the devices.

Combined in Situ Photoluminescence and X-ray Scattering Reveals Defect Formation in Lead-Halide Perovskite Films.

Lead-halide perovskites have established a firm foothold in photovoltaics and optoelectronics due to their steadily increasing power conversion efficiencies approaching conventional inorganic single-crystal semiconductors. However, further performance improvement requires reducing defect-assisted, nonradiative recombination of charge carriers in the perovskite layers. A deeper understanding of perovskite formation and associated process control is a prerequisite for effective defect reduction. In this study, we analyze the crystallization kinetics of the lead-halide perovskite MAPbI3-xClx during thermal annealing, employing in situ photoluminescence (PL) spectroscopy complemented by lab-based grazing-incidence wide-angle X-ray scattering (GIWAXS). In situ GIWAXS measurements are used to quantify the transition from a crystalline precursor to the perovskite structure. We show that the nonmonotonous character of PL intensity development reflects the perovskite phase volume, as well as the occurrence of the defects states at the perovskite layer surface and grain boundaries. The combined characterization approach enables easy determination of defect kinetics during perovskite formation in real-time.

Dual Defect-Passivation Using Phthalocyanine for Enhanced Efficiency and Stability of Perovskite Solar Cells.

Semiconducting molecules have been employed to passivate traps extant in the perovskite film for enhancement of perovskite solar cells (PSCs) efficiency and stability. A molecular design strategy to passivate the defects both on the surface and interior of the CH3 NH3 PbI3 perovskite layer, using two phthalocyanine (Pc) molecules (NP-SC6 -ZnPc and NP-SC6 -TiOPc) is demonstrated. The presence of lone electron pairs on S, N, and O atoms of the Pc molecular structures provides the opportunity for Lewis acid-base interactions with under-coordinated Pb2+ sites, leading to efficient defect passivation of the perovskite layer. The tendency of both NP-SC6 -ZnPc and NP-SC6 -TiOPc to relax on the PbI2 terminated surface of the perovskite layer is also studied using density functional theory (DFT) calculations. The morphology of the perovskite layer is improved due to employing the Pc passivation strategy, resulting in high-quality thin films with a dense and compact structure and lower surface roughness. Using NP-SC6 -ZnPc and NP-SC6 -TiOPc as passivating agents, it is observed considerably enhanced power conversion efficiencies (PCEs), from 17.67% for the PSCs based on the pristine perovskite film to 19.39% for NP-SC6 -TiOPc passivated devices. Moreover, PSCs fabricated based on the Pc passivation method present a remarkable stability under conditions of high moisture and temperature levels.

Interfacial defect passivation by chenodeoxycholic acid for efficient and stable perovskite solar cells

Passivating the surface defects of perovskite has become one of the important strategies to suppress non-radiative recombination and improve the photovoltaic performance and stability of perovskite solar cells (PVSCs). Herein, we introduce chenodeoxycholic acid (CDCA) on the surface of perovskite layer. The infrared spectra and nuclear magnetic resonance (1H NMR) spectra prove the carboxyl (–COOH) group on CDCA combination with the formamidinium (FA+) ions on perovskite. The water-contact angle measurement indicates that the long alkyl chain of CDCA covers the surface of perovskite film. The photoelectrical characterization demonstrates that the surface defects of the perovskite layer are passivated and carrier non-radiative combinations are suppressed. Under an optimized CDCA concentration of 0.2 mg⋅ml−1, the CDCA passivated PVSC achieves an average power conversion efficiency (PCE) of 20.41%, as compared with the pristine PVSC with an average PCE of 18.18%.

Enhanced efficiency and thermal stability of mesoscopic perovskite solar cells by adding PC70BM acceptor

Perovskite-based solar cells (PSCs) have emerged as a very promising solar photovoltaic technology with fivefold increase observed in the device efficiency within less than 10 years of its inception. The efficiency of these solar cells is strongly affected by the quality of perovskite layer surface coverage and its grain size. Although use of antisolvents such as chlorobenzene has been used to treat methylammonium lead iodide (CH3NH3PbI3) resulting in grain size of ca. 200–300 nm, one still needs to minimize the grain boundaries to reduce their impact on recombination. In this study, we have successfully incorporated [6, 6]-phenyl-C70-butyric acid methyl ester (PC70BM) acceptor as an additive in the perovskite layer of the mesoscopic solar cell devices, which not only leads to the formation of larger grains with fewer grain boundaries resulting in reduced charge carriers recombination, the band alignment between CH3NH3PbI3 and TiO2 was also found to improve. The device having the optimum PC70BM concentration in perovskite showed enhanced power conversion efficiency (PCE) due to improvement in current density (JSC) and fill-factor (FF), correlated to the decrease in charge transfer resistance (RCT). The devices using PC70BM additive exhibit over 15% improvement in the power conversion efficiency or PCE (15.5% for PC70BM containing device and 13.3% PCE for the reference device with no acceptor additive). More importantly, the device containing PC70BM acceptor additive, retained ca. 75% of the original PCE for 250 h when kept at 85 °C without encapsulation in an ambient at 20% RH.

Surface passivation with nitrogen-doped carbon dots for improved perovskite solar cell performance

Undercoordinated lead cations and halide anions on the surface of perovskite layer can form surface trap states and cause electronic disorders which reduce the performance of perovskite solar cells. Nitrogen-doped carbon dots (NCDs) that have rich nitrogen- and oxygen-containing functional groups can effectively interact with the unsaturated metal sites and halide anions on the surface and boundaries of perovskite grains. Herein, low-cost NCDs are utilized as efficient additives to passivate the surface of a solution-processed CH3NH3PbI3 perovskite film, which remarkably reduce charge carrier recombination, as evidenced by the results of time-resolved photoluminescence and electrochemical impedance spectrum measurements. FTIR spectra indicate the formation of hydrogen bonds between the undercoordinated iodine ions on perovskite grains and hydroxyl as well as nitrogenous groups of NCDs. In addition, NCDs additives also help increase interfacial charge transfer from perovskite to electron-transporting layer, leading to an improvement in power conversion efficiency for the solar cell device from 12.12 ± 0.28% (standard cell fabricated in same conditions) to 15.93 ± 0.15%.

TiO2 Sub-microsphere Film as Scaffold Layer for Efficient Perovskite Solar Cells.

TiO2 sub-microspheres composed of anatase granular-like nanocrystallines with an average diameter ∼250 nm are synthesized using sol-gel method and employed as the scaffold layer for efficient mesocopic perovskite solar cells. Compared with mesoporous TiO2 films composed of ∼18 nm nanoparticles, the sub-microsphere films show superior light-trapping characteristics and significantly improve the light-harvesting capability of the solar cells. In addition, the charge-transport performance is also dramatically improved according to the transient photocurrent decay despite there being no significant difference in the perovskite layer surface morphology. As a result, an average power conversion efficiency of 15% with a highly uniform distribution is achieved for the solar cells with TiO2 sub-microsphere films, 12% higher than those with TiO2 nanoparticle films. The combination of light-harvesting capability and fast charge transfer make the TiO2 sub-microsphere film a good candidate as the scaffold layer for efficient perovskite solar cells.

New Scalable Cold-Roll Pressing for Post-treatment of Perovskite Microstructure in Perovskite Solar Cells

We introduce a new cold-roll pressing technique to enhance microstructure of deposited pinholed perovskite layers in perovskite solar cells. This technique is proposed as an efficient post-treatment method for deposited perovskite layers that are mainly suffering from excessive number of pinholes. In this regard, we followed a set of sequential steps of partial dissolving the perovskite layer surface and a cold-roll pressing to compress and spread the semidissolved portion of the perovskite that is exposed to slight N,N-dimethylformamide (DMF) vapor. Because of applying no heating during the compressing process, the cells showed good stability against decomposition. Transformation of perovskite layer from columnar microstructures containing a large amount of uncoated TiO2 surface to a continuous and approximately pinhole-free structure resulted in an increase in the power conversion efficiency (PCE) of the cells from 8.16 to 13.24%, showing a 62% enhancement in the cell performance. For cell performance e...

Influence of void-free perovskite capping layer on the charge recombination process in high performance CH3NH3PbI3 perovskite solar cells.

The stunning rise of methylammonium lead iodide perovskite material as a light harvesting material in recent years has drawn much attention in the photovoltaic community. Here, we investigated in detail the uniform and void-free perovskite capping layer in the mesoscopic perovskite devices and found it to play a critical role in determining device performance and charge recombination process. Compared to the rough surface with voids of the perovskite layer, surface of the perovskite capping layer obtained from sequential deposition process is much more uniform with less void formation and distribution within the TiO2 mesoscopic scaffold is more homogeneous, leading to much improved photovoltaic parameters of the devices. The impact of void free perovskite capping layer surface on the charge recombination processes within the mesoscopic perovskite solar cells is further scrutinized via charge extraction measurement. Modulation of precursor solution concentrations in order to further improve the perovskite layer surface morphology leads to higher efficiency and lower charge recombination rates. Inhibited charge recombination in these solar cells also matches with the higher charge density and slower photovoltage decay profiles measured.