Stable High-Performance Perovskite Solar Cells via Grain Boundary Passivation

Interfacial engineering of CuSCN-based perovskite solar cells via PMMA interlayer toward enhanced efficiency and stability

We report a new interfacial engineering strategy to improve the photovoltaic performance of CuSCN-based perovskite solar cells.

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.

Understanding the Mechanism between Antisolvent Dripping and Additive Doping Strategies on the Passivation Effects in Perovskite Solar Cells

Perovskite polycrystalline films contain numerous intrinsic and interfacial defects ascribed to the solution preparation process, which are harmful to both the photovoltaic performance and the stability of perovskite solar cells (PVSCs). Although various passivators have been proved to be promising materials for passivating perovskite films, there is still a lack of deeper understanding of the effectiveness of the different passivation methods. Here, the mechanism between antisolvent dripping and additive doping strategies on the passivation effects in PVSCs is systematically investigated with a nonfullerene small molecule (F8IC). Such a passivated effect of F8IC is realized via coordination interactions between the carbonyl (C═O) and nitrile (C-N) groups of F8IC with Pb²⁺ ion of MAPbI₃. Interestingly, F8IC antisolvent dripping can effectively passivate the surface defects and thus inhibit the nonradiative charge recombination on the upper part of the perovskite layer, whereas F8IC additive doping significantly reduces the surface and bulk defects and produces a compact perovskite film with denser crystal grains, thus facilitating charge transmission and extraction. Therefore, these benefits are translated into significant improvements in the short-circuit current density (J_sc) to 21.86 mA cm^-2 and a champion power conversion efficiency of 18.40%. The selection of an optimal passivation strategy should also be considered according to the energy level matching between the passivators and the perovskite. The large energetic disparity is unsuitable for additive doping, whereas it is expected in antisolvent dripping.

Inorganic material passivation of defects toward efficient perovskite solar cells

Surface passivation with organic materials is one of the most effective and popular strategies to improve the stability and efficiency of perovskite solar cells (PSCs). However, the secondary bonding formed between organic molecules and perovskite layers is still not strong enough to protect the perovskite absorber from degradation initialized by oxygen and water attacking at defects. Recently, passivation with inorganic materials has gradually been favored by researchers due to the effectiveness of chemical and mechanical passivation. Lead-containing substances, alkali metal halides, transition elements, oxides, hydrophobic substances, etc. have already been applied to the surface and interfacial passivation of PSCs. These inorganic substances mainly manipulate the nucleation and crystallization process of perovskite absorbers by chemically passivating defects along grain boundaries and surface or forming a mechanically protective layer simultaneously to prevent the penetration of moisture and oxygen, thereby improving the stability and efficiency of the PSCs. Herein, we mainly summarize inorganic passivating materials and their individual passivation principles and methods. Finally, this review offers a personal perspective for future research trends in the development of passivation strategies through inorganic materials.

Traps in metal halide perovskites: characterization and passivation

Metal halide perovskites (MHPs) have become a research focus in the field of optoelectronics due to their excellent optoelectronic properties and simple and cost-effective thin film manufacturing processes. In particular, the power conversion efficiency (PCE) of solar cells (SCs) and external quantum efficiency (EQE) of light-emitting diodes (LEDs) based on perovskite materials have reached 25.2% and 21.6%, respectively, in a short period, making perovskites especially promising for fabricating next-generation optoelectronic devices. Despite these inspiring results, obtaining high-performance, high-stability MHP-based devices still faces many challenges, among which the defects and the consequent traps in MHPs are key factors. Defect-induced traps can trap charge carriers or even act as non-radiative recombination centers, seriously degrading the device performance, causing hysteresis and deteriorating the stability of MHP-based devices. Thus, understanding the chemical/physical nature of traps and adopting appropriate strategies to passivate traps are important to enhance the device performance and stability. Herein we present a review in which the knowledge and understanding of traps in MHPs are considered and discussed. Moreover, the latest efforts on passivating traps in MHPs for improving device performance are summarized, with the hope of providing guidance to future development of high-performance and high-stability MHP-based devices.

Additive Engineering by Bifunctional Guanidine Sulfamate for Highly Efficient and Stable Perovskites Solar Cells

High efficiency and good stability are the challenges for perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high defect density and internal nonradiative recombination of perovskite (PVK) limit its development. In this work, a facile additive strategy is devised by introducing bifunctional guanidine sulfamate (GuaSM; CH₆ N₃ ⁺ , Gua⁺ ; H₂ N-SO₃ ^- , SM^- ) into PVK. The size of Gua⁺ ion is suitable with Pb(BrI)₂ cavity relatively, so it can participate in the formation of low-dimensional PVK when mixed with Pb(BrI)₂ . The O and N atoms of SM^- can coordinate with Pb²⁺ . The synergistic effect of the anions and cations effectively reduces the trap density and the recombination in PVK, so that it can improve the efficiency and stability of PSCs. At an optimal concentration of GuaSM (2 mol%), the PSC presents a champion power conversion efficiency of 21.66% and a remarkably improved stability and hysteresis. The results provide a novel strategy for highly efficient and stable PSCs by bifunctional additive.

A Dendrite-Structured RbX (X=Br, I) Interlayer for CsPbI2 Br Perovskite Solar Cells with Over 15 % Stabilized Efficiency

Interface engineering has shown great potential to improve the photovoltaic performance and long-term stability of perovskite solar cells. Herein, RbX (X=Br, I) materials were developed as the interfacial modifiers for CsPbI₂ Br solar cells that realized power conversion efficiencies (PCE) over 15 % with a high open-circuit voltage (V_OC ) of 1.27 V. The RbX interlayer having a dendrite-shaped structure could optimize the surface wetting behavior of TiO₂ films and thus enabled formation of high-quality perovskite films. More importantly, RbX could better align the devices' energy levels and passivate surface defects because of its large bandgap. The PCE of a CsPbI₂ Br device with RbX interlayer remained 87 % of its initial efficiency for 800 h in ambient atmosphere.

Poly(Ethylene Glycol) Diacrylate as the Passivation Layer for High-Performance Perovskite Solar Cells

In the past decade, greatest effect has been paid on organic-inorganic halide perovskites for approaching high-performance perovskite solar cells (PSCs). It was found that severe surface-defect within the perovskite active layer restricted further boosting device performance of PSCs. Here, we report high-performance PSCs by utilization of an ultrathin solution-processed poly(ethylene glycol) diacrylate (PEGDA) layer to passivate the surface-defect within the perovskite thin film. Systematical studies demonstrate that the PEGDA-passivated perovskite thin film exhibit suppressed nonradiative recombination and trap density, as well as superior film morphology with a smoother surface, larger crystal size, and better crystallinity. Moreover, PSCs by the PEGDA-passivated perovskite thin film exhibit suppressed charge carrier recombination, reduced charge-transfer resistance, shorter charge carrier extraction time, and enlarged built-in potential. As a result, PSCs by the PEGDA-passivated perovskite thin film show a power conversion efficiency of over 21% and a photocurrent hysteresis index of 0.037. Moreover, unencapsulated PSCs by the PEGDA-passivated perovskite thin film possess over 10 day operational stability. All these results indicate that our approach provided a facile way to boost device performance of PSCs.

Realizing Reduced Imperfections via Quantum Dots Interdiffusion in High Efficiency Perovskite Solar Cells

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic-inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid-state interdiffusion process using multi-cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid-state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs⁺ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs_0.05 (MA_0.17 FA_0.83 )_0.95 PbBr₃ (abbreviated as QDs-Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.

Realization of Moisture-Resistive Perovskite Films for Highly Efficient Solar Cells Using Molecule Incorporation

The development of highly crystalline perovskite films with large crystal grains and few surface defects is attractive to obtain high-performance perovskite solar cells (PSCs) with good device stability. Herein, we simultaneously improve the power conversion efficiency (PCE) and humid stability of the CH₃NH₃PbI₃ (CH₃NH₃ = MA) device by incorporating small organic molecule IT-4F into the perovskite film and using a buffer layer of PFN-Br. The presence of IT-4F in the perovskite film can successfully improve crystallinity and enhance the grain size, leading to reduced trap states and longer lifetime of the charge carrier, and make the perovskite film hydrophobic. Meanwhile, as a buffer layer, PFN-Br can accelerate the separation of excitons and promote the transfer process of electrons from the active layer to the cathode. As a consequence, the PSCs exhibit a remarkably improved PCE of 20.55% with reduced device hysteresis. Moreover, the moisture-resistive film-based devices retain about 80% of their initial efficiency after 30 days of storage in relative humidity of 10-30% without encapsulation.

Unraveling the Crystallization Kinetics of 2D Perovskites with Sandwich-Type Structure for High-Performance Photovoltaics

2D perovskite solar cells with high stability and high efficiency have attracted significant attention. A systematical static and dynamic structure investigation is carried out to show the details of 2D morphology evolution. A dual additive approach is used, where the synergy between an alkali metal cation and a polar solvent leads to high-quality 2D perovskite films with sandwich-type structures and vertical phase segregation. Such novel structure can induce high-quality 2D slab growth and reduce internal and surface defects, resulting in a high device efficiency of 16.48% with enhanced continuous illumination stability and improved moisture (55-60%) and thermal (85 °C) tolerances. Transient absorption spectra reveal the carrier migration from low n to high n species with different kinetics. An [PbI₆ ]^4- octagon coalescence transformation mechanism coupled with metal and organic cations wrapped is proposed. By solvent vapor annealing, a recrystallization and reorientation of the 2D perovskite slabs occurs to form an ideal structure with improved device performance and stability.

Suppressing Defects-Induced Nonradiative Recombination for Efficient Perovskite Solar Cells through Green Antisolvent Engineering

Organic-inorganic hybrid perovskites have attracted considerable attention due to their superior optoelectronic properties. Traditional one-step solution-processed perovskites often suffer from defects-induced nonradiative recombination, which significantly hinders the improvement of device performance. Herein, treatment with green antisolvents for achieving high-quality perovskite films is reported. Compared to defects-filled ones, perovskite films by antisolvent treatment using methylamine bromide (MABr) in ethanol (MABr-Eth) not only enhances the resultant perovskite crystallinity with large grain size, but also passivates the surface defects. In this case, the engineering of MABr-Eth-treated perovskites suppressing defects-induced nonradiative recombination in perovskite solar cells (PSCs) is demonstrated. As a result, the fabricated inverted planar heterojunction device of ITO/PTAA/Cs_0.15 FA_0.85 PbI₃ /PC₆₁ BM/Phen-NADPO/Ag exhibits the best power conversion efficiency of 21.53%. Furthermore, the corresponding PSCs possess a better storage and light-soaking stability.

An Efficient Trap Passivator for Perovskite Solar Cells: Poly(propylene glycol) bis(2-aminopropyl ether)

Perovskite solar cells (PSCs) are regarded as promising candidates for future renewable energy production. High-density defects in the perovskite films, however, lead to unsatisfactory device performances. Here, poly(propylene glycol) bis(2-aminopropyl ether) (PEA) additive is utilized to passivate the trap states in perovskite. The PEA molecules chemically interact with lead ions in perovskite, considerably passivate surface and bulk defects, which is in favor of charge transfer and extraction. Furthermore, the PEA additive can efficiently block moisture and oxygen to prolong the device lifetime. As a result, PEA-treated MAPbI3 (MA: CH3NH3) solar cells show increased power conversion efficiency (PCE) (from 17.18 to 18.87%) and good long-term stability. When PEA is introduced to (FAPbI3)1-x(MAPbBr3)x (FA: HC(NH2)2) solar cells, the PCE is enhanced from 19.66 to 21.60%. For both perovskites, their severe device hysteresis is efficiently relieved by PEA.

MAPbI3 Quasi-Single-Crystal Films Composed of Large-Sized Grains with Deep Boundary Fusion for Sensitive Vis-NIR Photodetectors

Perovskite single-crystal (SC) or quasi-single-crystal (QSC) films are promising candidates for excellent performance of photoelectric devices. However, it is still a great challenge to fabricate large-area continuous SC or QSC films with proper thickness. Herein, we propose a pressure-assisted high-temperature solvent-engineer (PTS) strategy to grow large-area continuous MAPbI₃ QSC films with uniformly thin thickness and orientation. Dramatic grain growth (∼100 μm in the lateral dimension) and adequate boundary fusion are realized in them, vastly eliminating the grain boundaries. Thus, remarkable diminution of the trap density (n_trap: 7.43 × 10¹¹ cm^-3) determines a long carrier lifetime (τ₂: 1.7 μs) and superior photoelectric performance of MAPbI₃-based lateral photodetectors; for instance, an ultrahigh on/off ratio (>2.4 × 10⁶, 2 V), great stability, fast response (283/306 μs), and high detectivity (1.41 × 10¹³) are achieved. The combination properties and performance of the QSC films surpass most of the reported MAPbI₃. This effective approach in growing perovskite QSC films points out a novel way for perovskite-based optoelectronic devices with superior performance.

Effective Surface Treatment for High-Performance Inverted CsPbI2Br Perovskite Solar Cells with Efficiency of 15.92

A simple and multifunctional surface treatment strategy is proposed to address the inferior-performance inverted CsPbI₂Br perovskite solar cells (PSCs). The induced-ions exchange can align energy levels, passivate both GBs and surface, and gift the solid protection from external erosions. The inverted CsPbI₂Br PSCs reveal a champion efficiency of 15.92% and superior stability after moisture, operational, and thermal ages. Developing high-efficiency and stable inverted CsPbI₂Br perovskite solar cells is vitally urgent for their unique advantages of removing adverse dopants and compatible process with tandem cells in comparison with the regular. However, relatively low opening circuit voltage (V_oc) and limited moisture stability have lagged their progress far from the regular. Here, we propose an effective surface treatment strategy with high-temperature FABr treatment to address these issues. The induced ions exchange can not only adjust energy level, but also gift effective passivation. Meanwhile, the gradient distribution of FA⁺ can accelerate the carriers transport to further suppress bulk recombination. Besides, the Br-rich surface and FA⁺ substitution can isolate moisture erosions. As a result, the optimized devices show champion efficiency of 15.92% with V_oc of 1.223 V. In addition, the tolerance of humidity and operation get significant promotion: maintaining 91.7% efficiency after aged at RH 20% ambient condition for 1300 h and 81.8% via maximum power point tracking at 45 °C for 500 h in N₂. Furthermore, the unpackaged devices realize the rare reported air operational stability and, respectively, remain almost efficiency (98.9%) after operated under RH 35% for 600 min and 91.2% under RH 50% for 300 min.

China's progress of perovskite solar cells in 2019

Perovskite solar cells (PSCs) have attracted worldwide attention due to their high efficiency and low manufacturing cost. As the largest supplier of photovoltaic modules, China has made huge endeavors in the research on PSCs. In 2019, Chinese research groups were still holding the top position for paper publications in the world. Both the efficiency and the stability of the device have been steadily increasing, pushing forward the commercialization of PSCs step by step. This review summarizes the highlights of China's PSC research progress in 2019 and briefly introduces the development of PSC modules in industry.

Organic-Salt-Assisted Crystal Growth and Orientation of Quasi-2D Ruddlesden-Popper Perovskites for Solar Cells with Efficiency over 19

Quasi-2D Ruddlesden-Popper (RP) perovskite solar cells (PSCs) have drawn significant attention due to their appealing environmental stability compared to their 3D counterparts. However, the relatively low power conversion efficiency (PCE) greatly limits their applications. Here, high photovoltaic performance is demonstrated for quasi-2D RP PSCs using 2-thiophenemethylammonium as spacer with nominal n-value of 5, which is based on the stoichiometry of the precursors. The incorporation of formamidinium (FA) in quasi-2D RP perovskites reduces the bandgap and improves the light absorption ability, resulting in enlarged photocurrent and an increased PCE of 16.18%, which is higher than that of reported analogous methylammonium (MA)-based quasi-2D PSC (≈15%). A record high PCE of 19.06% is further demonstrated by using an organic salt, namely, 4-(trifluoromethyl)benzylammonium iodide, assisted crystal growth (OACG) technique, which can induce the crystal growth and orientation, tune the surface energy levels, and suppress the charge recombination losses. More importantly, the devices based on OACG-processed quasi-2D RP perovskites show remarkable environmental stability and thermal stability, for example, the PCE retaining ≈96% of its initial value after storage at 80 °C for 576 h, while only ≈37% of the original efficiency left for FAPbI₃ -based 3D PSCs.

Laser-Generated Supranano Liquid Metal as Efficient Electron Mediator in Hybrid Perovskite Solar Cells

Creating colloids of liquid metal with tailored dimensions has been of technical significance in nano-electronics while a challenge remains for generating supranano (<10 nm) liquid metal to unravel the mystery of their unconventional functionalities. Present study pioneers the technology of pulsed laser irradiation in liquid from a solid target to liquid, and yields liquid ternary nano-alloys that are laborious to obtain via wet-chemistry synthesis. Herein, the significant role of the supranano liquid metal on mediating the electrons at the grain boundaries of perovskite films, which are of significance to influence the carriers recombination and hysteresis in perovskite solar cells, is revealed. Such embedding of supranano liquid metal in perovskite films leads to a cesium-based ternary perovskite solar cell with stabilized power output of 21.32% at maximum power point tracing. This study can pave a new way of synthesizing multinary supranano alloys for advanced optoelectronic applications.

Boosting perovskite nanomorphology and charge transport properties via a functional D-π-A organic layer at the absorber/hole transporter interface

The photovoltaic efficiency and stability challenges encountered in perovskite solar cells (PSCs) were addressed by an innovative interface engineering approach involving the utilization of the organic chromophore (E)-3-(5-(4-(bis(2',4'-dibutoxy-[1,1'-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid (D35) as an interlayer between the perovskite absorber and the hole transporter (HTM) of mesoporous PSCs. The organic D-π-A interlayer primarily improves the perovskite's crystallinity and creates a smoother perovskite/HTM interface, while reducing the grain boundary defects and inducing an energy level alignment with the adjacent layers. Champion power conversion efficiencies (PCE) as high as 18.5% were obtained, clearly outperforming the reference devices. Interestingly, the D35-based solar cells present superior stability since they preserved 83% of their initial efficiency after 37 days of storage under dark and open circuit (OC) conditions. The obtained results consolidate the multifunctional role of organic D-π-A molecules as perovskite interface modifiers towards performance enhancement and scale-up fabrication of robust PSCs.

A dual function-enabled novel zwitterion to stabilize a Pb-I framework and passivate defects for highly efficient inverted planar perovskite solar cells

A novel zwitterion named bethanechol chloride (BTCC) was introduced to simultaneously stabilize a Pb-I framework and passivate defects for efficient inverted perovskite solar cells. The BTCC-assisted device yielded an elevated power conversion efficiency of 20.45% with an open-circuit voltage of 1.14 V.

Green Solution-Bathing Process for Efficient Large-Area Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) toward practical application relies on high efficiency, long lifetime, low toxicity, and device up-scaling. To realize large-area PSCs, a green solution-bathing strategy is delivered to prepare high-performance PSCs. By utilizing 2-pentanol as a green solvent and formamidinium chloride (FACl) as a solute in the green solution-bathing process, perovskite films with enlarged grain sizes, improved crystallinity, and alleviated defect state density were obtained, resulting in the enhancement in the power conversion efficiency of PSCs. Coupled with 2-pentanol and FACl, both a champion efficiency of 21.03% for small cells (0.103 cm²) and an efficiency of over 18% for large size (1.00 cm²) were obtained based on the GSB process, which can outperform its counterpart made via the commonly used antisolvent-dropping method. In addition, a large perovskite film (5 cm × 5 cm) with obvious mirror effect was successfully prepared. Our innovative approach paves the way to promote device up-scaling of PSCs via an environmentally friendly technique.

An Electrically Modulated Single-Color/Dual-Color Imaging Photodetector

An electrically modulated single-/dual-color imaging photodetector with fast response speed is developed based on a small molecule (CO_i 8DFIC)/perovskite (CH₃ NH₃ PbBr₃ ) hybrid film. Owing to the type-I heterojunction, the device can facilely transform dual-color images to single-color images by applying a small bias voltage. The photodetector exhibits two distinct cut-off wavelengths at ≈544 nm (visible region) and ≈920 nm (near-infrared region), respectively, without any power supply. Its two peak responsivities are 0.16 A W^-1 at ≈525 nm and 0.041 A W^-1 at ≈860 nm with a fast response speed (≈10² ns). Under 0.6 V bias, the photodetector can operate in a single-color mode with a peak responsivity of 0.09 A W^-1 at ≈475 nm, showing a fast response speed (≈10² ns). A physical model based on band energy theory is developed to illustrate the origin of the tunable single-/dual-color photodetection. This work will stimulate new approaches for developing solution-processed multifunctional photodetectors for imaging photodetection in complex circumstances.

Intermolecular π-π Conjugation Self-Assembly to Stabilize Surface Passivation of Highly Efficient Perovskite Solar Cells

Surface passivation is an effective approach to eliminate defects and thus to achieve efficient perovskite solar cells, while the stability of the passivation effect is a new concern for device stability engineering. Herein, tribenzylphosphine oxide (TBPO) is introduced to stably passivate the perovskite surface. A high efficiency exceeding 22%, with steady-state efficiency of 21.6%, is achieved, which is among the highest performances for TiO₂ planar cells, and the hysteresis is significantly suppressed. Further density functional theory (DFT) calculation reveals that the surface molecule superstructure induced by TBPO intermolecular π-π conjugation, such as the periodic interconnected structure, results in a high stability of TBPO-perovskite coordination and passivation. The passivated cell exhibits significantly improved stability, with sustaining 92% of initial efficiency after 250 h maximum-power-point tracking. Therefore, the construction of a stabilized surface passivation in this work represents great progress in the stability engineering of perovskite solar cells.

Interface Defects Passivation and Conductivity Improvement in Planar Perovskite Solar Cells Using Na2S-Doped Compact TiO2 Electron Transport Layers

Numerous trap states and low conductivity of compact TiO₂ layers are major obstacles for achieving high power conversion efficiency and high-stability perovskite solar cells. Here we report an effective Na₂S-doped TiO₂ layer, which can improve the conductivity of TiO₂ layers, the contact of the TiO₂/perovskite interface, and the crystallinity of perovskite layers. Comprehensive investigations demonstrate that Na cations increase the conductivity of TiO₂ layers while S anions change the wettability of TiO₂ layers, thus improving the crystallinity of perovskite layers and passivate defects at the TiO₂/PVK interface. The synergetic effects of dopants lead to a champion efficiency as high as 21.25% in unencapsulated perovskite solar cells (PSCs), with much-improved stability. Our work provides new insights on anion dopants in TiO₂ layers, which is usually neglected in previous reports, and also proposes a simple approach to produce low-cost and high-performance electron transport layers for high-performance PSCs.

An efficient and stable inverted perovskite solar cell involving inorganic charge transport layers without a high temperature procedure

Despite the successful enhancement in the high-power conversion efficiency (PCE) of perovskite solar cells (PSCs), the poor stability of PSCs is one of the major issues preventing their commercialization. The attenuation of PSCs may be due to the lower heat resistance of the organic charge transport layer and the tendency to aggregate at high temperatures. Here we report cerium oxide (CeO x ) as an electron transport layer (ETL) prepared through a simple solution processed at a low temperature (∼100 °C) to replace the organic charge transport layer on top of the inverted planar PSCs. The CeO x layer has excellent charge selectivity and can provide the perovskite film with protection against moisture and metal reactions with the electrode. The solar cell with CeO x as the electron transport layer has a power conversion efficiency of 17.47%. These results may prove a prospect for practical applications.

Reducing Detrimental Defects for High-Performance Metal Halide Perovskite Solar Cells

In several photovoltaic (PV) technologies, the presence of electronic defects within the semiconductor band gap limit the efficiency, reproducibility, as well as lifetime. Metal halide perovskites (MHPs) have drawn great attention because of their excellent photovoltaic properties that can be achieved even without a very strict film-growth control processing. Much has been done theoretically in describing the different point defects in MHPs. Herein, we discuss the experimental challenges in thoroughly characterizing the defects in MHPs such as, experimental assignment of the type of defects, defects densities, and the energy positions within the band gap induced by these defects. The second topic of this Review is passivation strategies. Based on a literature survey, the different types of defects that are important to consider and need to be minimized are examined. A complete fundamental understanding of defect nature in MHPs is needed to further improve their optoelectronic functionalities.

Grain Enlargement and Defect Passivation with Melamine Additives for High Efficiency and Stable CsPbBr3 Perovskite Solar Cells

The preparation of high-quality perovskite films with low grain boundaries and defect states is a prerequisite for achieving high-efficiency perovskite solar cells (PSCs) with good environmental stability. An effective additive engineering strategy has been developed for simultaneous defect passivation and crystal growth of CsPbBr₃ perovskite films by introducing 1,3,5-triazine-2,4,6-triamine (melamine) into the PbBr₂ precursor solution. The resultant melamine-PbBr₂ film has a loose, large-grained structure and decreased crystallinity, which has a positive effect on the crystallization process of the perovskite as it retards the crystallization rate as a result of the interaction between melamine and lead ions. Additionally, the passivation by melamine gives a high-quality CsPbBr₃ perovskite film with fewer grain boundaries, lower defect densities, and better energy level matching is achieved by multistep liquid-phase spin-coating, which greatly suppresses the nonradiative recombination resulting from the defects and promotes charge extraction at the interface. A champion power conversion efficiency as high as 9.65 % with a promising open-circuit voltage of 1.584 V is achieved for PSCs with an architecture of fluorine-doped tin oxide/c-TiO₂ /m-TiO₂ /melamine-added CsPbBr₃ /carbon-based hole-transporting layer. Furthermore, the unencapsulated melamine-added CsPbBr₃ PSC shows superior thermal and humidity stability in ambient air at 85 °C or 85 % relative humidity over 720 h.

Tuning Magnetism and Photocurrent in Mn-Doped Organic-Inorganic Perovskites

Organic-inorganic perovskites have attracted increasing attention in recent years owing to their excellent optoelectronic properties and photovoltaic performance. In this work, the prototypical hybrid perovskite CH₃NH₃PbI₃ is turned into a ferromagnetic material by doping Mn, which enables simultaneous control of both charge and spin of electrons. The room-temperature ferromagnetism originates from the double exchange interaction between Mn²⁺-I^--Mn³⁺ ions. Furthermore, it is discovered that the magnetic field can effectively modulate the photovoltaic properties of Mn-doped perovskite films. The photocurrent of Mn-doped perovskite solar cells increases by 0.5% under a magnetic field of 1 T, whereas the photocurrent of undoped perovskite decreases by 3.3%. These findings underscore the potential of Mn-doped perovskites as novel solution-processed ferromagnetic material and promote their application in multifunctional photoelectric-magnetic devices.

Defect Control Strategy by Bifunctional Thioacetamide at Low Temperature for Highly Efficient Planar Perovskite Solar Cells

Titania (TiO₂) has wide applications in the realm of perovskite solar cells (PSCs). Because high-temperature processing severely limits the application of flexible and tandem devices, it is significant to develop a high-quality electron-transport layer (ETL) by low-temperature processing. Here, we design a new strategy by introducing a bifunctional molecule (thioacetamide, TAA) in the TiO₂ ETL. During the low-temperature annealing, the N and S atoms in TAA can bond with the Ti atom in the ETL and the Pb atom in the perovskite (PVK) layer, respectively. The formation of coordinate bonds is beneficial to increase the crystallinity and reduce the roughness of TiO₂ ETLs and PVK layers, which effectively passivate the defects. Meanwhile, the energy level matching between the ETL and PVK is optimized. The structure characterization and electrochemical measurement demonstrate the design. Compared with precursor doping, surface spin-coating is a more effective method for introducing TAA into TiO₂. Significantly, the PSC based on the surface spin-coated TAA TiO₂ ETL achieves the best power conversion efficiency (PCE) of 21.17%. Nevertheless, the PSC fabricated with the pristine TiO₂ ETL offers a PCE of 19.52% under the same conditions. The results demonstrate a novel method for optimizing the properties of PSCs.

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.

Synergistic Interface Energy Band Alignment Optimization and Defect Passivation toward Efficient and Simple-Structured Perovskite Solar Cell

Efficient electron transport layer-free perovskite solar cells (ETL-free PSCs) with cost-effective and simplified design can greatly promote the large area flexible application of PSCs. However, the absence of ETL usually leads to the mismatched indium tin oxide (ITO)/perovskite interface energy levels, which limits charge transfer and collection, and results in severe energy loss and poor device performance. To address this, a polar nonconjugated small-molecule modifier is introduced to lower the work function of ITO and optimize interface energy level alignment by virtue of an inherent dipole, as verified by photoemission spectroscopy and Kelvin probe force microscopy measurements. The resultant barrier-free ITO/perovskite contact favors efficient charge transfer and suppresses nonradiative recombination, endowing the device with enhanced open circuit voltage, short circuit current density, and fill factor, simultaneously. Accordingly, power conversion efficiency increases greatly from 12.81% to a record breaking 20.55%, comparable to state-of-the-art PSCs with a sophisticated ETL. Also, the stability is enhanced with decreased hysteresis effect due to interface defect passivation and inhibited interface charge accumulation. This work facilitates the further development of highly efficient, flexible, and recyclable ETL-free PSCs with simplified design and low cost by interface electronic structure engineering through facile electrode modification.

Introduction of Multifunctional Triphenylamino Derivatives at the Perovskite/HTL Interface To Promote Efficiency and Stability of Perovskite Solar Cells

Surface passivation is a widely used approach to promote the efficiency and stability of perovskite solar cells (PSCs). In the present project, a series of new organic surface passivation molecules, which contain the same triphenylamino group with the hole transfer material of PSCs, have been synthesized. These new passivation molecules are supposed to have both "carrier transfer" capability and "defect passivation" potential. We find that, by using N-((4-(N,N,N-triphenyl)phenyl)ethyl)ammonium bromide (TPA-PEABr) as a surface passivation molecule, the efficiency of the PSCs can be improved from 16.69 to 18.15%, mainly due to an increased Voc (1.09 V compared with 1.02 V in control devices). The increased Voc is due to the reduced surface defect density and a better alignment for the related energy levels after introducing the TPA-PEABr molecules. Moreover, the stability of the PSCs can be significantly improved in TPA-PEABr passivated devices due to the hydrophobic nature of TPA-PEABr. Our results successfully demonstrate that passivation of the perovskite surface with a carefully designed multifunctional small organic molecule should be a useful approach for more stable PSCs with high efficiency.

In Situ Passivation on Rear Perovskite Interface for Efficient and Stable Perovskite Solar Cells

Despite the rocketing rise in power conversion efficiencies (PCEs), the performance of perovskite solar cells (PSCs) is still limited by the carrier transfer loss at the interface between perovskite (PVSK) absorbers and charge transporting layers. Here, we propose a novel in situ passivation strategy by using [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) to improve the charge dynamics at the rear PVSK/CTL interface in the n-i-p structure device. A pre-deposited PCBM-doped PbI₂ layer is redissolved during PVSK deposition in our routine, establishing a bottom-up PCBM gradient that is facile for charge extraction. Meanwhile, the surface defects are in situ-passivated via PCBM-PVSK interaction, which substantially suppresses the trap-assisted recombination at the rear interface. Due to the synergistic effect of charge-extraction promotion and trap passivation, the fabricated PSCs deliver a champion PCE of 20.10% with attenuated hysteresis and improved long-term stability, much higher than the 18.39% of the reference devices. Our work demonstrates a promising interfacial engineering strategy for further improving the performance of PSCs.