Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance

Experimental and theoretical study of europium-doped organometal halide perovskite nanoplatelets for UV photodetection with high responsivity and fast response

Herein, we investigate the role of Eu³⁺ doping on CH₃NH₃PbBr₃ nanoplatelets (NPLs) in terms of their optoelectronic properties and photodetection application through a combined experimental and theoretical approach. The introduction of EuCl₃ in the CH₃NH₃PbBr₃ crystal structure by a facile solvothermal method enabled the tuning of the lateral and vertical dimensions of the NSs to form large-area NPLs and finally monolayer nanocrystals. The appearance of low-angle diffraction peaks with Eu doping, which are observed in layered perovskite structures, confirms the formation of quasi-2D NPLs. The bandgap of the Eu-doped mixed halide perovskite systematically increases from 2.39 eV to 2.94 eV with increasing doping concentration. Interestingly, 10 mol% EuCl₃ doping in the pure CH₃NH₃PbBr₃ crystal dramatically enhances its absorbance and photosensitivity, resulting in high-performance photodetection. Under 405 nm excitation, the CH₃NH₃Pb_0.9Eu_0.1Br_2.7Cl_0.3 photodetector exhibits self-biased behavior with an on/off ratio >10³, which is very significant. The planar device achieves a responsivity as high as 5.29 A W^- and a detectivity of 1.06 × 10¹² Jones under 405 nm with a power density of 0.14 mW cm^-2 at 5 V. In addition, the device exhibits very fast response time with a rise/fall time of 17.5/38.5 μs, which is ∼4 times faster than the pristine CH₃NH₃PbBr₃ counterpart. A linear relationship of photocurrent with light intensity in the CH₃NH₃Pb_0.9Eu_0.1Br_2.7Cl_0.3 photodetector signifies low recombination or charge trapping loss. High-performance photodetection in the Eu-doped device is ascribed to the elimination of trap states and the fast charge transfer process. To obtain better insight into the doped system, DFT analysis of the electronic structure of EuCl₃-doped CH₃NH₃PbBr₃ was performed and the results are fully consistent with the experimental findings. It was revealed that EuCl₃ doping increases the density of states near the conduction band along with a blue shift in the bandgap as compared to that of the pristine perovskite, which in turn increases the built-in potential of the fabricated device, resulting in self-biased photodetection. This work paves the way for deeper understanding of lanthanide doping in perovskites and self-biased photodetection applications of a new family of Eu-doped mixed halide perovskite nanostructures.

Efficient and Stable Perovskite Solar Cells by B-Site Compositional Engineered All-Inorganic Perovskites and Interface Passivation

Perovskite solar cells (PSCs) have emerged as a cost-effective solar technology in the past years. PSCs by three-dimensional hybrid inorganic-organic perovskites exhibited decent power conversion efficiencies (PCEs); however, their stabilities were poor. On the other hand, PSCs by all-inorganic perovskites indeed exhibited good stability, but their PCEs were low. Here, the development of novel all-inorganic perovskites CsPbI₂Br:xNd³⁺, where Pb²⁺ at the B-site is partially heterovalently substituted by Nd³⁺, is reported. The CsPbI₂Br:xNd³⁺ thin films possess enlarged crystal sizes, enhanced charge carrier mobilities, and superior crystallinity. Thus, the PSCs by the CsPbI₂Br:xNd³⁺ thin films exhibit more than 20% enhanced PCEs and dramatically boosted stability compared to those based on pristine CsPbI₂Br thin films. To further boost the device performance of PSCs, solution-processed 4-lithium styrenesulfonic acid/styrene copolymer (LiSPS) is utilized to passivate the surface defect and suppress surface charge carrier recombination. The PSCs based on the CsPbI₂Br:xNd³⁺/LiSPS bilayer thin film possess reduced charge extraction lifetime and suppressed charge carrier recombination, resulting in 14% enhanced PCEs and significantly boosted stability compared to those without incorporation of the LiSPS interface passivation layer. All these results indicate that we developed a facile way to approach high-performance PSCs by all-inorganic perovskites.

Seed-Assisted Growth of Methylammonium-Free Perovskite for Efficient Inverted Perovskite Solar Cells

The traditional way to stabilize α-phase formamidinium lead triiodide (FAPbI₃ ) perovskite often involves considerable additions of methylammonium (MA) and bromide into the perovskite lattice, leading to an enlarged bandgap and reduced thermal stability. This work shows a seed-assisted growth strategy to induce a bottom-up crystallization of MA-free perovskite, by introducing a small amount of α-CsPbBr₃ /DMSO (5%) as seeds into the pristine FAPbI₃ system. During the initial crystalization period, the typical hexagonal α-FAPbI₃ crystals (containing α-CsPbBr₃ seeds) are directly formed even at ambient temperature, as observed by laser scanning confocal microscopy. It indicates that these seeds can promote the formation and stabilization of α-FAPbI₃ below the thermodynamic phase-transition temperature. After annealing not beyond 100 °C, CsPbBr₃ seeds homogeneously diffused into the entire perovskite layer via an ions exchange process. This work demonstrates an efficiency of 22% with hysteresis-free inverted perovskite solar cells (PSCs), one of the highest performances for MA-free inverted PSCs. Despite absented passivation processes, open-circuit voltage is improved by 100 millivolts compared to the control devices with the same stoichiometry, and long-term operational stability retained 92% under continuous full sun illumination. Going MA-free and low-temperature processes are a new insight for compatibility with tandems or flexible PSCs.

Quantifying Efficiency Limitations in All-Inorganic Halide Perovskite Solar Cells

While halide perovskites have excellent optoelectronic properties, their poor stability is a major obstacle toward commercialization. There is a strong interest to move away from organic A-site cations such as methylammonium and formamidinium toward Cs with the aim of improving thermal stability of the perovskite layers. While the optoelectronic properties and the device performance of Cs-based all-inorganic lead-halide perovskites are very good, they are still trailing behind those of perovskites that use organic cations. Here, the state-of-the-art of all-inorganic perovskites for photovoltaic applications is reviewed by performing detailed meta-analyses of key performance parameters on the cell and material level. Key material properties such as carrier mobilities, external photoluminescence quantum efficiency, and photoluminescence lifetime are discussed and what is known about defect tolerance in all-inorganic is compared relative to hybrid (organic-inorganic) perovskites. Subsequently, a unified approach is adopted for analyzing performance losses in perovskite solar cells based on breaking down the losses into several figures of merit representing recombination losses, resistive losses, and optical losses. Based on this detailed loss analysis, guidelines are eventually developed for future performance improvement of all-inorganic perovskite solar cells.

Hydrogen Bonds in Precursor Solution: The Origin of the Anomalous J–V Curves in Perovskite Solar Cells

Perovskite Solar Cells are a promising solar energy harvesting technology due to their low cost and high-power conversion efficiency. A high-quality perovskite layer is fundamental for a highly efficient perovskite Solar Cell. Utilizing a gas quenching process (GQP) can eliminate the need for toxic, flammable, and expensive anti-solvents in the preparation of perovskite layers. It is a promising candidate technology for large scale preparation of perovskite layers, as it can be easily integrated in a production line by coupling up-scalable techniques. The GQP removes the need for polar solvents in the precursor solution layer by using nitrogen flow, rather than extracting them with non-polar solvents. The crystallization dynamics in this process can be significantly different. In this study, we found that the quality of perovskite crystal from GQP is much more sensitive to Lewis base molecules (LBMs) in the precursor solution than it is in anti-solvents technology. Thus, the processing parameters of the LBMs in anti-solvents technology cannot be directly transferred to the GQP. An XRD and 1H NMR study explains the origin of the S-shaped J–V curves and how these LBMs hinder the reaction between PbI2 and monovelent cations.

Analysis of the Photovoltaic Waste-Recycling Process in Polish Conditions—A Short Review

The rapid development of the photovoltaic (PV) industry is determined by subsequent legal documents and directives, which indicate the need to use renewable energy sources in order to counteract climate pollution and strive to increase energy efficiency. The development of the photovoltaic industry in the near future will result in an increase in the amount of electrical and electronic waste from used photovoltaic panels. The total installed capacity of photovoltaic sources in Poland at the end of 2019 was almost 1500 MW, and in May 2020, it exceeded 1950 MW, and the weight of the installation was approximately 120,000 tons. The aim of the present work is to present the types of materials used in the construction of photovoltaic panels, with particular emphasis on the possibility of recycling or utilization of individual elements. Additionally, the aim of the work is to describe the most important requirements addressed to the members of the European Union, which were formulated in the provisions of Directive 2012/19/EU. Taking into account the number of photovoltaic panels produced in Poland, the possibility of recycling individual materials from PV assembly was analyzed. The author presents the problem of recycling in the combination of legal and material aspects, which will soon become the share of Poland as a member of the European Union.

Quadruple-Cation Wide-Bandgap Perovskite Solar Cells with Enhanced Thermal Stability Enabled by Vacuum Deposition

Vacuum processing of multicomponent perovskites is not straightforward, because the number of precursors is in principle limited by the number of available thermal sources. Herein, we present a process which allows increasing the complexity of the formulation of vacuum-deposited lead halide perovskite films by multisource deposition and premixing both inorganic and organic components. We apply it to the preparation of wide-bandgap CsMAFA triple-cation perovskite solar cells, which are found to be efficient but not thermally stable. With the aim of stabilizing the perovskite phase, we add guanidinium (GA⁺) to the material formulation and obtained CsMAFAGA quadruple-cation perovskite films with enhanced thermal stability, as observed by X-ray diffraction and rationalized by microstructural analysis. The corresponding solar cells showed similar performance with improved thermal stability. This work paves the way toward the vacuum processing of complex perovskite formulations, with important implications not only for photovoltaics but also for other fields of application.

Physical Fields Manipulation for High-Performance Perovskite Photovoltaics

With the efforts of researchers from all over the world, metal halide perovskite solar cells (PSCs) have been booming rapidly in recent years. Generally, perovskite films are sensitive to surrounding conditions and will be changed under the action of physical fields, resulting in lattice distortion, degradation, ion migration, and so on. In this review, the progress of physical fields manipulation in PSCs, including the electric field, magnetic field, light field, stress field, and thermal field are reviewed. On this basis, the influences of these fields on PSCs are summarized and prospected. Finally, challenges and prospective research directions on how to make better use of external-fields while minimizing the unnecessary and disruptive impacts on commercial PSCs with high-efficiency and steady output are proposed.

Manipulating Crystallization Kinetics in High-Performance Blade-Coated Perovskite Solar Cells via Cosolvent-Assisted Phase Transition

Manipulating the perovskite solidification process, including nucleation and crystal growth, plays a critical role in controlling film morphology and thus affects the resultant device performance. In this work, a facile and effective ethyl alcohol (EtOH) cosolvent strategy is demonstrated with the incorporation of EtOH into perovskite ink for high-performance room-temperature blade-coated perovskite solar cells (PSCs) and modules. Systematic real-time perovskite crystallization studies uncover the delicate perovskite structural evolutions and phase-transition pathway. Time-resolved X-ray diffraction and density functional theory calculations both demonstrate that EtOH in the mixed-solvent system significantly promotes the formation of an FA-based precursor solvate (FA₂ PbBr₄ ·DMSO) during the trace-solvent-assisted transition process, which finely regulates the balance between nucleation and crystal growth to guarantee high-quality perovskite films. This strategy efficiently suppresses nonradiative recombination and improves efficiencies in both 1.54 (23.19%) and 1.60 eV (22.51%) perovskite systems, which represents one of the highest records for blade-coated PSCs in both small-area devices and minimodules. An excellent V_OC deficit as low as 335 mV in the 1.54 eV perovskite system, coincident with the measured nonradiative recombination loss of only 77 mV, is achieved. More importantly, significantly enhanced device stability is another signature of this approach.

Interpretation of Rubidium-Based Perovskite Recipes toward Electronic Passivation and Ion-Diffusion Mitigation

Rubidium cation (Rb⁺ ) addition is witnessed to play a pivotal role in boosting the comprehensive performance of organic-inorganic hybrid perovskite solar cells. However, the origin of such success derived from irreplaceable superiorities brought by Rb⁺ remains ambiguous. Herein, grain-boundary-including atomic models are adopted for the accurate theoretical analysis of practical Rb⁺ distribution in perovskite structures. The spatial distribution, covering both the grain interiors and boundaries, is thoroughly identified by virtue of synchrotron-based grazing-incidence X-ray diffraction. On this basis, the prominent elevation of the halogen vacancy formation energy, improved charge-carrier dynamics, and the electronic passivation mechanism in the grain interior are expounded. As evidenced by the increased energy barrier and suppressed microcurrent, the critical role of Rb⁺ addition in blocking the diffusion pathway along grain boundaries, inhibiting halide phase segregation, and eventually enhancing intrinsic stability is elucidated. Hence, the linkage avalanche effect of occupied location dominated by subtle changes in Rb⁺ concentration on electronic defects, ion migration, and phase stability is completely investigated in detail, shedding a new light on the advancement of high-efficiency cascade-incorporating strategies and perovskite compositional engineering.

Simple-Structured Low-Cost Dopant-Free Hole-Transporting Polymers for High-Stability CsPbI2Br Perovskite Solar Cells

Among the solution-processed devices, perovskite solar cells (PSCs) exhibit the highest power conversion efficiency (PCE) of over 25%; tremendous efforts are being undertaken to improve their stability. Recently, all-inorganic CsPbI₂Br-based PSCs were reported to exhibit a significantly improved device stability, with a promising PCE of up to 16.79%. In this study, we report stable all-inorganic PSCs by incorporating novel dopant-free hole-transporting materials (HTMs). The synthesis strategy of the newly synthesized polymeric HTMs was similar to that of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD), with the exception that they were designed to exhibit dopant-free characteristics. In particular, their polymeric backbone structure was significantly simpler than that of spiro-OMeTADs, and they were easily synthesized in two steps from commercially available chemicals, with an overall yield of ∼50%. The cost of synthesis at the laboratory scale was calculated to be at least 2.4 times cheaper than that of spiro-OMeTADs. The PCE of dopant-free HTM-based PSCs was 11.01%, which is 1.5 times higher than that of the dopant-free spiro-OMeTAD-based devices (7.52%) and comparable to that of the doped spiro-OMeTAD-based devices (12.22%). Notably, the stability of the device based on our dopant-free HTM to atmospheric oxygen and moisture as well as heat and light irradiation was superior to that of devices based on doped and dopant-free spiro-OMeTAD HTMs. On consideration of the synthesis cost, device efficiency, and device stability, our dopant-free HTM is highly promising for all-inorganic PSCs.

Unravelling Structure and Formation Mechanisms of Ruddlesden-Popper-Phase-like Nanodomains in Inorganic Lead Halide Perovskites

Ultrastable CsPbBr₃ nanoplates against electron beam irradiations are fabricated and nanodomains with anomalous high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) contrasts are observed within CsPbBr₃ nanoplates. Atomic resolution energy dispersive X-ray spectroscopy (EDS) mapping, which requires even higher beam currents and may cause significant damages on electron beam sensitive materials, are obtained without any detectable damages or decomposition. Combining HAADF-STEM images, atomic resolution EDS mapping, and image simulations has revealed detailed structure and chemistry of the nanodomains to be induced by Ruddlesden-Popper faults (RP faults) rather than any chemical intermixing or formation of new phases. A formation mechanism is also proposed on the basis of the atomic structure of the nanodomains. This result promotes an atomic-level understanding of inorganic lead halide perovskites and may help to reveal their structure-property relationship.

Highly Efficient and Stable CsPbTh3 (Th = I, Br, Cl) Perovskite Solar Cells by Combinational Passivation Strategy

The distorted lead iodide octahedra of all-inorganic perovskite based on triple halide-mixed CsPb(I_2.85 Br_0.149 Cl_0.001 ) framework have made a tremendous breakthrough in its black phase stability and photovoltaic efficiency. However, their performance still suffers from severe ion migration, trap-induced nonradiative recombination, and black phase instability due to lower tolerance factor and high total energy. Here, a combinational passivation strategy to suppress ion migration and reduce traps both on the surface and in the bulk of the CsPhTh₃ perovskite film is developed, resulting in improved power conversion efficiency (PCE) to as high as 19.37%. The involvement of guanidinium (GA) into the CsPhTh₃ perovskite bulk film and glycocyamine (GCA) passivation on the perovskite surface and grain boundary synergistically enlarge the tolerance factor and suppress the trap state density. In addition, the acetate anion as a nucleating agent significantly improves the thermodynamic stability of GA-doped CsPbTh₃ film through the slight distortion of PbI₆ octahedra. The decreased nonradiative recombination loss translates to a high fill factor of 82.1% and open-circuit voltage (V_OC ) of 1.17 V. Furthermore, bare CsPbTh₃ perovskite solar cells without any encapsulation retain 80% of its initial PCE value after being stored for one month under ambient conditions.

Perovskite-based solar cells fabricated from TiO2 nanoparticles hybridized with biomaterials from mollusc and diatoms

Perovskite solar cells (PVSCs) convert solar energy into electrical energy. Current study employs fabrication of PVSCs using calcium titanate (CaTiO₃) prepared by co-precipitation of TiO₂ nanoparticle (NP) and CaCO₃ NP with later synthesized from mollusc shell. Furthermore, frustules of diatom, Nitzschia palea were used to prepare silica doped CaTiO₃ (Si-CaTiO₃) nanocomposite. CaTiO₃ NP and Si-CaTiO₃ nanocomposites film were made on fluorine doped tin oxide (FTO) glass plate using spin coater separately for two different kinds of PVSCs tested at different intensities of light. The perovskite materials were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray (EDX) spectroscopy. Thickness of the film was measured by profilometer. The maximum power density (PD_max) of CaTiO₃ made PVSCs was 0.235 mW/m² under white LED light and 0.041 mW/m² in broad spectrum light. Whereas, PD_max of PVSCs with Si-CaTiO₃ was higher about 0.0083 mW/m² in broad spectrum light and was 0.0039 mW/m² in white LED light. This is due to the fact that CaTiO₃ allowed blue and red light in broad spectrum to pass through it without being absorbed compared to white LED light which gets reflected. On the offset, in PVSC made of Si-CaTiO₃ since diatoms frustules are made up of nanoporous architecture it increases the overall porosity of PVSC making them potentially more efficient in broad spectrum of light compared to white LED light.

Additive-Free, Low-Temperature Crystallization of Stable α-FAPbI3 Perovskite

Formamidinium lead triiodide (FAPbI₃ ) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure α-FAPbI₃ conventionally requires high-temperature thermal annealing at 150 °C whilst the obtained α-FAPbI₃ is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow δ-FAPbI₃ into black α-FAPbI₃ at only 100 °C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained α-FAPbI₃ exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable α-FAPbI₃ . This overcomes the strain-induced lattice expansion that is known to cause the metastability of α-FAPbI₃ . Accordingly, pure FAPbI₃ p-i-n solar cells are reported, facilitated by the low-temperature (≤100 °C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.

Rethinking the A cation in halide perovskites

The A cation in ABX₃ organic-inorganic lead halide perovskites (OLHPs) was conventionally believed to hardly affect their optoelectronic properties. However, more recent developments have unraveled the critical role of the A cation in the regulation of the physicochemical and optoelectronic properties of OLHPs. We review the important breakthroughs enabled by the versatility of the A cation and highlight potential opportunities and unanswered questions related to the A cation in OLHPs.

Phase-Pure α-FAPbI3 for Perovskite Solar Cells

Because of the narrow bandgap and superior thermal stability, FAPbI₃ is considered the most promising perovskite material for high-performance single-junction PSCs. Nevertheless, the metastable properties of the photoactive α-FAPbI₃ becomes a primary obstacle for the development of FA-based PSCs. The main reasons for the instability of α-FAPbI₃ are the rotation disorder of the FA cation and large anisotropic lattice strain, which lead to the high formation energy of α-FAPbI₃. In this Perspective, we review various strategies for preparing phase-pure α-FAPbI₃, such as engineering, intermediate phase engineering, and dimensionality engineering. These strategies can stabilize α-FAPbI₃ by reducing the system energy, regulating the phase transition process and energy barrier, reinforcing the lattice structure, and passivating film defects. In addition, we investigate fundamental challenges of α-FAPbI₃ PSCs and propose our perspective on preparing high-quality and high-purity α-FAPbI₃.

Development of formamidinium lead iodide-based perovskite solar cells: efficiency and stability

Perovskite materials have been particularly eye-catching by virtue of their excellent properties such as high light absorption coefficient, long carrier lifetime, low exciton binding energy and ambipolar transmission (perovskites have the characteristics of transporting both electrons and holes). Limited by the wider band gap (1.55 eV), worse thermal stability and more defect states, the first widely used methylammonium lead iodide has been gradually replaced by formamidinium lead iodide (FAPbI3) with a narrower band gap of 1.48 eV and better thermal stability. However, FAPbI3 is stabilized as the yellow non-perovskite active phase at low temperatures, and the required black phase (α-FAPbI3) can only be obtained at high temperatures. In this perspective, we summarize the current efforts to stabilize α-FAPbI3, and propose that pure α-FAPbI3 is an ideal material for single-junction cells, and a triple-layer mesoporous architecture could help to stabilize pure α-FAPbI3. Furthermore, reducing the band gap and using tandem solar cells may ulteriorly approach the Shockley-Queisser limit efficiency. We also make a prospect that the enhancement of industrial applications as well as the lifetime of devices may help achieve commercialization of PSCs in the future.

Omnibearing Interpretation of External Ions Passivated Ion Migration in Mixed Halide Perovskites

Fundamental understanding of ion migration inside perovskites is of vital importance for commercial advancements of photovoltaics. However, the mechanism for external ions incorporation and its effect on ion migration remains elusive. Herein, taking K⁺ and Cs⁺ co-incorporated mixed halide perovskites as a model, the impact of external ions on ion migration behavior has been interpreted via multiple dimensional characterization aspects. The space-effect on phase segregation inhibition has been revealed by the photoluminescence evolution and in situ dynamic cathodoluminescence behaviors. The plane-effect on current suppression along grain boundary has been evidenced via visualized surface current mapping, local current hysteresis, and time-resolved current decay. And the point-effect on activation energy incremental for individual ions has been also probed by cryogenic electronic quantification. All these results sufficiently demonstrate the passivated ion migration results in the eventually improved phase stability of perovskite, of which the origin lies in various ion migration energy barriers.

Self-Assembled Donor-Acceptor Dyad Molecules Stabilize the Heterojunction of Inverted Perovskite Solar Cells and Modules

The heterointerface between a semiconducting metal oxide and a perovskite critically impacts on the overall performance of perovskite solar cells (PVSCs). Herein, we report a feasible yet effective strategy to suppress the interfacial reaction between nickel oxide and the perovskite via chemical passivation with self-assembled dyad molecules, which leads to the simultaneous improvement of the power conversion efficiencies (PCEs) and operational lifetimes of inverted PVSCs. As a result, inverted PVSCs consisting of simple methylammonium iodide perovskites have achieved an excellent PCE of 20.94% and decent photostability with 93% of the initial value after 600 h of 1 sun equivalent illumination. Moreover, this strategy can be readily translated into slot-die fabrication of perovskite modules, achieving a high PCE of 14.90% with an area of 19.16 cm² (no shade in the interconnecting area) and a geometrical fill factor of 93%. Overall, this work provides an effective strategy to stabilize the vulnerable heterointerface in PVSCs.

Morphology Control of Monomer–Polymer Hybrid Electron Acceptor for Bulk-Heterojunction Solar Cell Based on P3HT and Ti-Alkoxide with Ladder Polymer

We report the morphology control of a nano-phase-separated structure in the photoactive layer (power generation layer) of organic–inorganic hybrid thin-film solar cells to develop highly functional electronic devices for societal applications. Organic and inorganic–organic hybrid bulk heterojunction solar cells offer several advantages, including low manufacturing costs, light weight, mechanical flexibility, and a potential to be recycled because they can be fabricated by coating them on substrates, such as films. In this study, by incorporating the carrier manager ladder polymer BBL as the third component in a conventional two-component power generation layer consisting of P3HT—the conventional polythiophene derivative and titanium alkoxide—we demonstrate that the phase-separated structure of bulk heterojunction solar cells can be controlled. Accordingly, we developed a discontinuous phase-separated structure suitable for charge transport, obtaining an energy conversion efficiency higher than that of the conventional two-component power generation layer. Titanium alkoxide is an electron acceptor and absorbs light with a wavelength lower than 500 nm. It is highly sensitive to LED light sources, including those used in homes and offices. A conversion efficiency of 4.02% under a 1000 lx LED light source was achieved. Hence, high-performance organic–inorganic hybrid bulk heterojunction solar cells with this three-component system can be used in indoor photovoltaic systems.

High throughput screening of promising lead-free inorganic halide double perovskites via first-principles calculations

Perovskite solar cells (PSCs) have been intensively investigated and made great progress due to their high photoelectric conversion efficiency and low production cost. However, poor stability and the toxicity of Pb limit their commercial applications. It is particularly important to search for new non-toxic, high-stability perovskite materials. In this study, 760 Cs₂B²⁺B'²⁺X₆ (X = F, Cl, Br, I) inorganic halide double perovskites are screened based on high-throughput first-principles calculations to obtain an ideal perovskite material. The band gaps of this type of double perovskite are mainly determined by the elements X and B²⁺, decreasing monotonously with the increase in the atomic number of X (from F to I). We obtain 14 optimal and unreported materials with suitable band gaps as potential alternative materials for Pb-based photovoltaic absorbers in PSCs. This theoretical investigation can provide theoretical guidance for developing novel lead-free PSC materials.

High-Performance Tin-Lead Mixed-Perovskite Solar Cells with Vertical Compositional Gradient

Sn-Pb mixed perovskites with bandgaps in the range of 1.1-1.4 eV are ideal candidates for single-junction solar cells to approach the Shockley-Queisser limit. However, the efficiency and stability of Sn-Pb mixed-perovskite solar cells (PSCs) still lag far behind those of Pb-based counterparts due to the easy oxidation of Sn²⁺ . Here, a reducing agent 4-hydrazinobenzoic acid is introduced as an additive along with SnF₂ to suppress the oxidation of Sn²⁺ . Meanwhile, a vertical Pb/Sn compositional gradient is formed spontaneously after an antisolvent treatment due to different solubility and crystallization kinetics of Sn- and Pb-based perovskites and it can be finely tuned by controlling the antisolvent temperature. Because the band structure of a perovskite is dependent on its composition, graded vertical heterojunctions are constructed in the perovskite films with a compositional gradient, which can enhance photocarrier separation and suppress carrier recombination in the resultant PSCs. Under optimal fabrication conditions, the Sn-Pb mixed PSCs show power conversion efficiency up to 22% along with excellent stability during light soaking.

Surface-Anchored Acetylcholine Regulates Band-Edge States and Suppresses Ion Migration in a 21%-Efficient Quadruple-Cation Perovskite Solar Cell

Although incorporating multiple halogen (bromine) anions and alkali (rubidium) cations can improve the open-circuit voltage (V_oc ) of perovskite solar cells (PSCs), severe voltage loss and poor stability have remained pivotal limitations to their further commercialization. In this study, acetylcholine (ACh⁺ ) is anchored to the surface of a quadruple-cation perovskite to provide additional electron states near the valence band maximum of the perovskite surface, thereby enhancing the band alignment and minimizing the V_oc loss significantly. Moreover, the quaternary ammonium and carbonyl units of ACh⁺ passivate the antisite and vacancy defects of the organic/inorganic hybrid perovskite. Because of strong interactions between ACh⁺ and the perovskite, the formation of lead clusters and the migration of halogen anions in the perovskite film are suppressed. As a result, the device prepared with ACh⁺ post-treatment delivers a power conversion efficiency (PCE) (21.56%) and a value of V_oc (1.21 V) that are much higher than those of the pristine device, along with a twofold decrease in the hysteresis index. After storage for 720 h in humid air, the device subjected to ACh⁺ treatment maintained 70% of its initial PCE. Thus, post-treatment with ACh⁺ appears to be a useful strategy for preparing efficient and stable PSCs.

Nanoscale chemical heterogeneity dominates the optoelectronic response of alloyed perovskite solar cells

Halide perovskites perform remarkably in optoelectronic devices. However, this exceptional performance is striking given that perovskites exhibit deep charge-carrier traps and spatial compositional and structural heterogeneity, all of which should be detrimental to performance. Here, we resolve this long-standing paradox by providing a global visualization of the nanoscale chemical, structural and optoelectronic landscape in halide perovskite devices, made possible through the development of a new suite of correlative, multimodal microscopy measurements combining quantitative optical spectroscopic techniques and synchrotron nanoprobe measurements. We show that compositional disorder dominates the optoelectronic response over a weaker influence of nanoscale strain variations even of large magnitude. Nanoscale compositional gradients drive carrier funnelling onto local regions associated with low electronic disorder, drawing carrier recombination away from trap clusters associated with electronic disorder and leading to high local photoluminescence quantum efficiency. These measurements reveal a global picture of the competitive nanoscale landscape, which endows enhanced defect tolerance in devices through spatial chemical disorder that outcompetes both electronic and structural disorder.

Perovskite Bifunctional Diode with High Photovoltaic and Electroluminescent Performance by Holistic Defect Passivation

Integration of photovoltaic (PV) and electroluminescent (EL) functions and/or units in one device is attractive for new generation optoelectronic devices but it is challenging to achieve highly comprehensive efficiency. Herein, perovskite solar cells (PSCs) are fabricated, assisted by 3-sulfopropyl methacrylate potassium salt (SPM) additive to tackle this issue. SPMs not only induce large grain size during the film formation but also produce a secondary phase of 2D K₂ PbI₄ to passivate the grain boundaries (GBs). In addition, its sulfonic acid group and potassium ion can coordinate to lead ion and fill the interstitial defects, respectively. Thus, SPM reduces the defective states and suppresses nonradiative recombination loss. As a result, planar PSC delivers a power conversion efficiency of ≈22%, with a maximum open-circuit voltage (V_oc ) of 1.20 V. The V_oc is 94% of the radiative V_oc limit (1.28 V), higher than the control device (V_oc of 1.12 V). In addition, the reciprocity between PV and EL is also correlated to quantify the energy losses and understand the device physics. When operated as a light-emitting diode, the maximum EL external quantum efficiency (EQE_EL ) is up to 12.2% (EQE_EL of 10.7% under an injection current of short-circuit photocurrent), thus leading to high-performance PV/EL dual functions.

Centimetre-scale perovskite solar cells with fill factors of more than 86 per cent

Owing to rapid development in their efficiency¹ and stability², perovskite solar cells are at the forefront of emerging photovoltaic technologies. State-of-the-art cells exhibit voltage losses^3-8 approaching the theoretical minimum and near-unity internal quantum efficiency^9-13, but conversion efficiencies are limited by the fill factor (<83%, below the Shockley-Queisser limit of approximately 90%). This limitation results from non-ideal charge transport between the perovskite absorber and the cell's electrodes^5,8,13-16. Reducing the electrical series resistance of charge transport layers is therefore crucial for improving efficiency. Here we introduce a reverse-doping process to fabricate nitrogen-doped titanium oxide electron transport layers with outstanding charge transport performance. By incorporating this charge transport material into perovskite solar cells, we demonstrate 1-cm² cells with fill factors of >86%, and an average fill factor of 85.3%. We also report a certified steady-state efficiency of 22.6% for a 1-cm² cell (23.33% ± 0.58% from a reverse current-voltage scan).

Additive-Assisted Optimization in Morphology and Optoelectronic Properties of Inorganic Mixed Sn-Pb Halide Perovskites

Halide perovskite solar cells (HPSCs) are promising photovoltaic materials due to their excellent optoelectronic properties, low cost, and high efficiency. Here, we demonstrate atmospheric solution processing and stability of cesium tin-lead triiodide (CsSnPbI₃) thin films for solar cell applications. The effect of additives, such as pyrazine and guanidinium thiocyanate (GuaSCN), on bandgap, film morphology, structure, and stability is investigated. Our results indicate the formation of a wide bandgap (>2 eV) structure with a mixed phase of tin oxide (SnO₂) and Cs(Sn, Pb)I₃. The addition of pyrazine decreases the intensity of SnO₂ peaks, but the bandgap does not change much. With the addition of GuaSCN, the bandgap of the films reduces to 1.5 eV, and a dendritic structure of Cs(Sn, Pb)I₃ is observed. GuaSCN addition also reduces the oxygen content in the films. To enable uniform film crystallization, cesium chloride (CsCl) and dimethyl sulfoxide (DMSO) additives are used in the precursor. Both CsCl and DMSO suppress dendrite formation with the latter resulting in uniform polycrystalline films with a bandgap of 1.5 eV. Heat and light soaking (HLS) stability tests at 65 °C and 1 sun for 100 h show all film types are stable with temperature but result in phase segregation with light exposure.

Defect Passivation Using Trichloromelamine for Highly Efficient and Stable Perovskite Solar Cells

Nonradiative recombination losses caused by defects in the perovskite layer seriously affects the efficiency and stability of perovskite solar cells (PSCs). Hence, defect passivation is an effective way to improve the performance of PSCs. In this work, trichloromelamine (TCM) was used as a defects passivator by adding it into the perovskite precursor solution. The experimental results show that the power conversion efficiency (PCE) of PSC increased from 18.87 to 20.15% after the addition of TCM. What’s more, the environmental stability of PSCs was also improved. The working mechanism of TCM was thoroughly investigated, which can be ascribed to the interaction between the –NH– group and uncoordinated lead ions in the perovskite. This work provides a promising strategy for achieving highly efficient and stable PSCs.

Stability Improvement of Perovskite Solar Cells by the Moisture-Resistant PMMA:Spiro-OMeTAD Hole Transport Layer

Perovskite solar cells (PSCs) based on the 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) hole transport layer have exhibited leading device performance. However, the instability caused by this organic function layer is a very important limiting factor to the further development of PSCs. In this work, the spiro-OMeTAD is doped with polymethyl methacrylate (PMMA), which is further used as the hole transport layer to improve the device stability. It is shown that the PMMA can effectively improve the moisture and oxygen resistance of spiro-OMeTAD, which leads to improved device stability by separating the perovskite layer from moisture and oxygen. The device efficiency can maintain 77% of the original value for PSCs with the PMMA-doped spiro-OMeTAD hole transport layer, under a natural air environment (RH = 40%) for more than 80 days. The results show that the moisture- and oxygen-resistant PMMA:spiro-OMeTAD hole transport layer is effective at improving the device performance.

Rational Unraveling of Alkali Metal Concentration-Dependent Photovoltaic Performance of Halide Perovskites: Octahedron Distortion vs Surface Reconstruction

Adding alkali metal in organic-inorganic halide perovskites effectively improves its photovoltaic performance, while excessive alkali metal incorporation would produce a detrimental effect. Through density functional theory and nonadiabatic molecular dynamics simulations, we demonstrate how and why the photogenerated carrier lifetime mutates with the increase of alkali metal concentration. A small amount of Rb doping in the lattice introduces a slight distortion of the octahedron, reducing the overlap of frontier orbitals and decreasing the nonadiabatic coupling, effectively enhancing the photogenerated carrier lifetime. In contrast, excessive Rb will introduce defect states, resulting in the low carrier lifetime by a factor of 2-3 orders of magnitude. Strikingly, the surface formamidinium (FA) cations exhibit unexpected responsibility for the carrier dynamics since its high-frequency thermal vibration strongly leads to the ultrafast hole trapping and carrier recombination. Our results provide new insight into the concentration-dependent photovoltaic performance of alkali metal cations in organic-inorganic halide perovskites.

Developments on Perovskite Solar Cells (PSCs): A Critical Review

This review provides detailed information on perovskite solar cell device background and monitors stepwise scientific efforts applied to improve device performance with time. The work reviews previous studies and the latest developments in the perovskite crystal structure, electronic structure, device architecture, fabrication methods, and challenges. Advantages, such as easy bandgap tunability, low charge recombination rates, and low fabrication cost, are among the topics discussed. Some of the most important elements highlighted in this review are concerns regarding commercialization and prototyping. Perovskite solar cells are generally still lab-based devices suffering from drawbacks such as device intrinsic and extrinsic instabilities and rising environmental concerns due to the use of the toxic inorganic lead (Pb) element in the perovskite (ABX3) light-active material. Some interesting recommendations and possible future perspectives are well articulated.

Substitutional alkaline earth metals delay nonradiative charge recombination in CH3NH3PbI3 perovskite: A time-domain study

Experiments reported that alkaline earth metal dopants greatly prolong carrier lifetime and improve the performance of perovskite solar cells. Using state-of-the-art ab initio time-domain nonadiabatic molecular dynamics (NAMD), we demonstrate that incorporation of alkaline earth metals, such as Sr and Ba, into MAPbI₃ (MA = CH₃NH₃ ⁺) lattice at the lead site is energetically favorable due to the largely negative formation energies about -7 eV. The replacement widens the bandgap and increases the open-circuit voltage by creating no trap states. More importantly, the substitution reduces the mixing of electron and hole wave functions by pushing the hole charge density away from the dopant together with no contribution of Sr and Ba to the conduction band edge state, thus decreasing the NA coupling. The high frequency phonons generated by enhanced atomic motions and symmetry breaking accelerate phonon-induced loss of coherence. The synergy of the three factors reduces the nonradiative recombination time by a factor of about 2 in the Sr- and Ba-doped systems with respect to pristine MAPbI₃, which occurs over 1 ns and agrees well with the experiment. The study highlights the importance of various factors affecting charge carrier lifetime, establishes the mechanism of reduction of nonradiative electron-hole recombination in perovskites upon alkaline earth metal doping, and provides meaningful insights into the design of high performance of perovskite solar cells and optoelectronics.

Properties of Halide Perovskite Photodetectors with Little Rubidium Incorporation

This study investigates the effects of Rb doping on the Rb-formamidinium-methylammonium-PbI₃ based perovskite photodetectors. Rb was incorporated in the perovskite films with different contents, and the corresponding photo-response properties were studied. Doping of few Rb (~2.5%) was found to greatly increase the grain size and the absorbance of the perovskite. However, when the Rb content was greater than 2.5%, clustering of the Rb-rich phases emerged, the band gap decreased, and additional absorption band edge was found. The excess Rb-rich phases were the main cause that degraded the performance of the photodetectors. By space charge limit current analyses, the Rb was found to passivate the defects in the perovskite, lowering the leakage current and reducing the trap densities of carriers. This fact was used to explain the increase in the detectivity. To clarify the effect of Rb, the photovoltaic properties were measured. Similarly, h perovskite with 2.5% Rb doping increased the short-circuit current, revealing the decline of the internal defects. The 2.5% Rb doped photodetector showed the best performance with responsivity of 0.28 AW⁻¹ and ~50% quantum efficiency. Detectivity as high as 4.6 × 10¹¹ Jones was obtained, owing to the improved crystallinity and reduced defects.

Shedding light on the energy applications of emerging 2D hybrid organic-inorganic halide perovskites

Unique performance of the hybrid organic-inorganic halide perovskites (HOIPs) has attracted great attention because of their continuous exploration and breakthrough in a multitude of energy-related applications. However, the instability and lead-induced toxicity that arise in bulk perovskites are the two major challenges that impede their future commercialization process. To find a solution, a series of two-dimensional HOIPs (2D HOIPs) are investigated to prolong the device lifetime with highly efficient photoelectric conversion and energy storage. Herein, the recent advances of 2D HOIPs and their structural derivatives for the energy realms are summarized and discussed. The basic understanding of crystal structures, physicochemical properties, and growth mechanisms is presented. In addition, the current challenges and future directions to provide a roadmap for the development of next generation 2D HOIPs are prospected

Direct Atomic-scale Imaging of a Screw Dislocation Core Structure in Inorganic Halide Perovskites

Topological defects such as dislocations in crystalline materials usually have major impacts on materials’ mechanical, chemical and physical properties. Detailed knowledge of dislocation core structures is essential to understand their...

Efficient and stable perovskite solar cells using manganese-doped nickel oxide as the hole transport layer

The device based on a Mn-doped NiOx HTL retained 70% of its initial efficiency after 35 days’ storage under a continuous halogen lamp matrix exposure.

Excited-state charge polarization and electronic structure of mixed-cation halide perovskites: the role of mixed inorganic–organic cations in CsFAPbI3

Mixed-cation perovskite materials have shown great potential for sunlight harvesting and have surpassed unmixed perovskite materials in solar cell efficiency and stability. The role of mixed monovalent cations in the enhanced optoelectronic properties and excited state response, however, are still elusive from a theoretical perspective. Herein, through time dependent density functional theory calculations of mixed cation perovskites, we report the electronic structure of Cs formamidinium (FA) mixed cationic lead iodide (Cs_0.17FA_0.87PbI₃) in comparison to the corresponding single monovalent cation hybrid perovskite. The results show that the Cs_0.17FA_0.87PbI₃ and FAPbI₃ had negligible differences in the optical band gap, and partial and total density of states in comparison to a single cation perovskite, while the effective mass of carriers, the local atomic density of states, the directional transport, and the structural distortions were significantly different. A lattice-distortion-induced asymmetry in the ground-state charge density is found, and originates from the co-location of caesium atoms in the lattice and signifies the effect on the charge density upon cation mixing and corresponding symmetry breaking. The excited-state charge response and induced polarizabilities are quantified, and discussed in terms of their importance for effective light absorption, charge separation, and final solar cell performance. We also quantify the impact of such polarizabilities on the dynamics of the structure of the perovskites and the implications this has for hot carrier cooling. The results shed light on the mechanism and origin of the enhanced performance in mixed-cation perovskite-based devices and their merits in comparison to single cation perovskites.

The roles of mixed monovalent cations in CsFAPbI₃ perovskites in terms of the optoelectronic properties and excited-state charge polarization are reported.

Cation Substitution Effects on the Structural, Electronic and Sun-Light Absorption Features of All-Inorganic Halide Perovskites

All-inorganic perovskites (such as CsPbI3) are emerging as new candidates for photovoltaic applications. Unfortunately, this class of materials present two important weaknesses in their way to commercialization: poor stability and...

Over 21% Efficiency Stable 2D Perovskite Solar Cells

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

Probing the Origin of the Open Circuit Voltage in Perovskite Quantum Dot Photovoltaics

Perovskite quantum dots (PQDs) have many properties that make them attractive for optoelectronic applications, including expanded compositional tunability and crystallographic stabilization. While they have not achieved the same photovoltaic (PV) efficiencies of top-performing perovskite thin films, they do reproducibly show high open circuit voltage (V_OC) in comparison. Further understanding of the V_OC attainable in PQDs as a function of surface passivation, contact layers, and PQD composition will further progress the field and may lend useful lessons for non-QD perovskite solar cells. Here, we use photoluminescence-based spectroscopic techniques to understand and identify the governing physics of the V_OC in CsPbI₃ PQDs. In particular, we probe the effect of the ligand exchange and contact interfaces on the V_OC and free charge carrier concentration. The free charge carrier concentration is orders of magnitude higher than in typical perovskite thin films and could be tunable through ligand chemistry. Tuning the PQD A-site cation composition via replacement of Cs⁺ with FA⁺ maintains the background carrier concentration but reduces the trap density by up to a factor of 40, reducing the V_OC deficit. These results dictate how to improve PQD optoelectronic properties and PV device performance and explain the reduced interfacial recombination observed by coupling PQDs with thin-film perovskites for a hybrid absorber layer.

Oxidized Spiro-OMeTAD: Investigation of Stability in Contact with Various Perovskite Compositions

The power conversion efficiency of perovskite solar cells (PSCs) has risen steadily in recent years; however, one important aspect of the puzzle remains to be solved-the long-term stability of the devices. We believe that understanding the underlying reasons for the observed instability and finding means to circumvent it is crucial for the future of this technology. Not only the perovskite itself but also other device components are susceptible to thermal degradation, including the materials comprising the hole-transporting layer. In particular, the performance-enhancing oxidized hole-transporting materials have attracted our attention as a potential weak component in the system. Therefore, we performed a series of experiments with oxidized spiro-OMeTAD to determine the stability of the material interfaced with five most popular perovskite compositions under thermal stress. It was found that oxidized spiro-OMeTAD is readily reduced to the neutral molecule upon interaction with all five perovskite compositions. Diffusion of iodide ions from the perovskite layer is the main cause for the reduction reaction which is greatly enhanced at elevated temperatures. The observed sensitivity of the oxidized spiro-OMeTAD to ion diffusion, especially at elevated temperatures, causes a decrease in the conductivity observed in the doped films of spiro-OMeTAD, and it also contributes significantly to a drop in the performance of PSCs operated under prolonged thermal stress.

Bridging Effects of Sulfur Anions at Titanium Oxide and Perovskite Interfaces on Interfacial Defect Passivation and Performance Enhancement of Perovskite Solar Cells

Interfacial defects at the electron transport layer (ETL) and perovskite (PVK) interface are critical to the power conversion efficiency (PCE) and stabilities of the perovskite solar cells (PSCs) via significantly affecting the quality of both interface contacts and PVK layers. Here, we demonstrate a simple ionic bond passivation method, employing Na₂S solution treatment of the surface of titanium dioxide (TiO₂) layers, to effectively passivate the traps at the TiO₂/Cs_0.05(MA_0.15FA_0.85)_0.95Pb(Br_0.15I_0.85)₃ PVK interface and enhance the performance of PSCs. X-ray photoelectron spectroscopy and other characterizations show that the Na₂S treatment introduced S^2- ions at the TiO₂/PVK interface, where S^2- ions effectively bridged the TiO₂ ETL and the PVK layer via forming chemical bonds with Ti atoms and with uncoordinated Pb atoms and resulted in the reduced defect density and improved the crystallinity of PVK layers. In addition, the S^2- ions can effectively enlarge the grain size of the PVK layers. The average PCE of solar cells is improved from 15.77 to 19.06% via employing the Na₂S-treated TiO₂ layers. This work demonstrates a simple and facile interface passivation method using ionic bond passivation to afford high-performance PSCs. The bridging effect of S^2- ions may inspire the further exploration of the ionic bond passivation and sulfur-based passivation materials.