Chen Weijie

Despite the conspicuous achievements in perovskite solar cells (PSCs), the further improvement of power conversion efficiencie (PCE) is hindered by substantially detrimental carrier recombination resulyted from the high interfacial charge defect density and inferior charge transport kinetics. Herein, we develop an interface engineering strategy to introduce Lewis base thiophene or thiazole modified C₃N₄ layer at electron transfer layer (ETL)/perovskite interface to constitute a stepwise energy band alignment and passivate defects at interfaces of perovskite film. Attributed to its well‐matched energy level with TiO₂ and perovskite, the charge extraction efficiency and charge transfer dynamics can be remakbly promoted, greatly inhibittng charge recombination at interface. Besides, thiophene and thiazole can donate the lone pair electrons in S or N atoms to under‐coordinated Pb²⁺, which effectively passivates the electronic trap states caused by halogen vacancies, thereby greatly minimizing trap‐assisted non‐radiative recombination in the PSC devices. Eventually, thiazole‐C₃N₄/perovskite based devices acquire an outstanding efficiency of 19.23%, supported by an enhanced open‐circuit voltage (V_OC) of 1.11 V with improved moisture stability. This work provides an avenue for the interfacial energy level modulation and defect passivation strategies on the rational interface microstructure design for meliorating the performance of PSCs.

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The power conversion efficiency of the market‐dominating silicon photovoltaics approaches its theoretical limit. Bifacial solar operation with harvesting additional light impinging on the module back and the perovskite/silicon tandem device architecture are among the most promising approaches for further increasing the energy yield from a limited area. Here, the energy output of perovskite/silicon tandem solar cells in monofacial and bifacial operation is calculated, for the first time considering luminescent coupling between two sub‐cells. For energy yield calculations idealized solar cells are studied at both, standard testing as well as realistic weather conditions in combination with a detailed illumination model for periodic solar panel arrays. Typical experimental photoluminescent quantum yield values reveal that more than 50% of excess electron‐hole pairs in the perovskite top cell can be utilized by the silicon bottom cell by means of luminescent coupling. As a result, luminescent coupling strongly relaxes the constraints on the top‐cell bandgap in monolithic tandem devices. In combination with bifacial operation, the optimum perovskite bandgap shifts from 1.71 eV to the range 1.60‐1.65 eV where already high‐quality perovskite materials exist. The results can hence change a paradigm in developing the optimum perovskite material for tandem solar cells.

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Although the research on perovskite solar cells (PSCs) has achieved rapid progress, its efficiency and stability still need to be further improved to meet the industrial requirements. The defects located inside the cells, on the surfaces, interfaces, or grain boundaries, will primarily affect carrier transportation through the formation of non‐radiative recombination centers and hinder the further enhancement of the power conversion efficiency (PCE). In this work, we developed a straightforward and simple defect passivation method to increase the PCE and stability of PSCs. In the device, the N‐type semiconductor AgBiS₂ was introduced by thermal evaporation as a modified layer between the perovskite films and electron transport layer, which could improve the charge transport characteristic and band gap optimization of PSCs. Simultaneously, Dimethyl sulfoxide (DMSO) solvent mixed polyethylene glycol (PEG) was employed for solvent annealing treatment, which could further improve the quality of perovskite film and reduce the trap density by increasing grain size and enhancing the crystallinity. As a result, the PSCs with dual‐interfacial modification exhibited a remarkable improvement of PCE from 18.58% to 21.19% with exceptional long‐term and moisture stability. This work provides an innovative insight for fabricating the stable and efficient PSCs toward the industrialization.

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Unraveling the charge generation dynamics at the donor­acceptor interfaces is crucial for improving the photovoltaic performances of nonfullerene acceptor based organic solar cells. Here, we have employed TDDFT­based nonadiabatic dynamics simulations to explore the ultrafast photoinduced dynamics at a nonfullerene donor­acceptor PTB7@PDI interface. Based on our results, we find that such interface exhibits distinct charge generation processes upon excitation with different wavelengths. The excitation at ∽591 nm mainly results in the local exciton |PTB7 ^∗ > while the charge transfer exciton |PTB7 ⁺ PDI ⁻> also has minor contribution. Later on, the electron transfer from PTB7 to PDI, i.e. channel I charge generation process, happens in the following 1 ps. The situations are much more complex when the excitation is conducted using a ∽487 nm light. The initial populated excitons include local excitons |PDI ^∗ >, |PTB7 ^∗ >, and charge transfer exciton |PTB7 ⁺ PDI ⁻ >, after which both channel I and channel II charge generation take place ultrafastly. However, in both situations, the charge generation processes occur within a few picoseconds, which is consistent with previous experimental work. Such ultrafast charge generation processes in a wide range of solar spectrum could be one of the reasons which is responsible for the excellent photovoltaic properties of such organic solar cells.

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A thin film AlGaInP/AlGaAs/InGaAs/InGaAs inverted metamorphic multi‐junction solar cell with the bandgap of 1.96/1.53/1.16/0.83 eV has been fabricated. The photoelectric conversion efficiency has reached 34.89% with an open‐circuit voltage of 3.54 V under AM1.5G spectrum. The analysis of individual subcells is the key to evaluating the performance of multi‐junction solar cells. The current density versus voltage characteristics of four subcells are calculated using optoelectronic reciprocity relation between the external quantum efficiency and the different injection current‐densities electroluminescence. The analysis of the performance characteristics of four subcells concluded that the key to limiting the overall efficiency improvement is the deep‐level recombination of the AlGaInP top subcell and the bulk recombination of 0.83 eV InGaAs bottom subcell. Targeted optimization of the top subcell and the bottom subcell is expected to significantly improve efficiency.

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The dimensionality, optoelectronic properties, and stability of lead halide perovskite films can be fine‐tuned via surface passivation with bulky fluoroaromatic cations. This study demonstrates the practical use of fluorinated phenethylammonium iodide (FPEAI) for simultaneous efficiency and stability improvement in large‐area perovskite solar cell devices and modules.

State‐of‐the‐art perovskite solar cells (PSCs) based on three‐dimensional (3D) films have achieved high power conversion efficiencies (PCEs), but are relatively fragile in high‐temperature and humid environments. This shortcoming must be addressed before PSCs can be fully commercialized. Herein, the use of a fluorinated aromatic organic spacer cation, 4‐fluoro‐phenethylammonium iodide (FPEAI), to fine‐tune the dimensionality and surface morphology of perovskite films is demonstrated. Surface treatment with FPEAI can lead to in situ formation of a two‐dimensional (2D) (FPEA)₂PbI₄ perovskite capping layer atop a 3D perovskite film, producing novel 3D/2D interface in perovskite films. Simultaneously, FPEAI treatment can induce a novel grain‐boundary passivation effect on the film surface, which helps to suppress undesirable charge recombination. After FPEAI treatment, standard (0.09 cm²) and large‐area (2.00 cm²) PSCs achieve PCEs of 20.53% and 16.82%, respectively. The FPEAI‐treated PSCs also demonstrate superior air‐ and photo‐stability due to the hydrophobic (FPEA)₂PbI₄ capping layer that reduces moisture ingress into perovskite structures. Furthermore, a 11.2 cm² large FPEAI‐treated PSC module with a PCE of 13.66% are successfully fabricated. FPEAI passivation is a facile strategy to produce 3D/2D multi‐dimensional PSCs with superior performance and stability.

From the perspective of the device structure of perovskite solar cells (PSCs), the electron transport layer is one of the essential components and plays a significant role in suppressing carrier recombination. Furthermore, its decisiveness is related to the quality of perovskite film, the rapid interface carrier extraction, and the bandgap alignment. However, the deficiency of the semiconductor oxides based electron transport materials, especially for most studied TiO₂, is that their carrier mobility is one to three orders of magnitude lower than the most commonly used hole transport materials, leading to an imbalanced carrier flux and unpredicted hysteresis. Doping new ions are the most effective ways to improve electron mobility and tune the bandgap, while the fundamental mechanism of doping in the majority of cases are still lacking. Here we review and emphasize the doping effect on semiconductor oxide‐based electron transport materials by classifying the doping ions according to the main family of elements from the critical factors of lattice optimization, a carrier transporting improvement, and interface modification. This review is the first systematic summary of the ion doping characteristics in oxide electron transport layers of PSCs. Finally, we briefly discuss the implementation of doping ions in electron transport materials for further enhancing the photovoltaic performance of PSCs.

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Herein, a small amount of the ionic liquid methylammonium difluoroacetate is introduced to anchor the organic cations via hydrogen bonding and to enhance the Pb–O interaction in perovskite precursors for efficient and stable solar cells.

The instability of organic cations in lead halide perovskite materials is a major obstacle for the commercial breakthrough of perovskite photovoltaics due to desorption of organic cations during the thermal annealing and device operation. Herein, a novel strategy is reported to improve the performance and stability of organic halide perovskite solar cells containing organic cations by adding a small amount of the ionic liquid methylammonium difluoroacetate (MA⁺DFA⁻). Nuclear magnetic resonance and Fourier‐transform infrared spectroscopy measurements show that MA⁺DFA⁻ can anchor the organic cations via hydrogen bonding and enhance the Pb–O interaction in perovskite precursors, leading to the retardation of the perovskite crystallization and improved stability of the perovskite precursor solution. Dynamic light scattering and scanning electron microscopy verify the defect‐passivation effect of MA⁺DFA⁻ on the perovskite precursors and films. The passivated perovskite film shows superior photo carrier dynamics as investigated by time‐resolved photoluminescence and transient absorption spectra. Moreover, the hydrogen bonding of the perovskite with MA⁺DFA⁻ imparts excellent ambient and thermal stability to the film as revealed by X‐ray diffraction measurements. As a result, devices with a high efficiency of 21.46% and excellent stability over 180 days in nitrogen atmosphere at room temperature are achieved with the ionic liquid.

Half‐mixed Pb‐Sn perovskite solar cells with significantly improved performance and stability are prepared by introducing an ionic imidazolium tetrafluoroborate additive. The synergistic effects of IM cation and tetrafluoroborate anion enable efficient defect passivation at grain boundaries, reducing leakage current, and enlargement in grain size with relaxed lattice strain simultaneously, thereby exerting a remarkable impact on device performance and stability.

Abstract

Narrow‐bandgap mixed Pb‐Sn perovskite solar cells (PSCs) have great feasibility for constructing efficient all‐perovskite tandem solar cells, in combination with wide‐bandgap lead halide PSCs. However, the power conversion efficiency of mixed Pb‐Sn PSCs still lags behind lead‐based counterparts. Here, additive engineering using ionic imidazolium tetrafluoroborate (IMBF₄) is proposed, where the imidazolium (IM) cation and tetrafluoroborate (BF₄) anion efficiently passivate defects at grain boundaries and improve crystallinity, simultaneously relaxing lattice strain, respectively. Defect passivation is achieved by the chemical interaction between the IM cation and the positively charged under‐coordinated Pb²⁺ or Sn²⁺ ions, and lattice strain relaxation is realized by lattice expansion with the intercalation of BF₄ anions into the perovskite lattice. As a result, the synergistic effects of the cation and anion in the IMBF₄ additive greatly enhance the optoelectronic performance of half‐mixed Pb‐Sn perovskites, leading to much longer carrier lifetimes. The best‐performing half‐mixed Pb‐Sn PSC shows an efficiency above 19% with negligible hysteresis, while retaining over 90% of its initial efficiency after 1000 h in a nitrogen‐filled glovebox and showing a lifetime to 80% degradation of 53.5 h under continuous illumination.

Highly efficient CsPbI_1.5Br_1.5 perovskite solar cells (PSCs) are achieved via introducing fluorescein isothiocyanate (FITC) organic dye as passivator. FITC not only reduces the metal ion related trap states but also improves film crystallinity, resulting in an enhancement of device efficiency from 12.3% to 14.05%. In addition, it is demonstrated that CsPbI_1.5Br_1.5 perovskite shows the optimal halide composition for inorganic PSCs.

Abstract

All‐inorganic perovskite solar cells (PSCs) have recently received growing attention as a promising template to solve the thermal instability of organic–inorganic PSCs. However, the thermodynamic phase instability and relatively low device efficiency pose challenges. Herein, highly efficient and stable CsPbI_1.5Br_1.5 compositional perovskite‐based inorganic PSCs are fabricated using an organic dye, fluorescein isothiocyanate (FITC), as a passivator. The carboxyl and thiocyanate groups of FITC not only minimize the trap states by forming interactions with the under‐coordinated Pb²⁺ ions but also significantly increase the grain size and improve the crystallinity of the perovskite films during annealing. Consequently, perovskite films with superior optoelectronic properties, prolonged carrier lifetime, reduced trap density, and improved stability are obtained. The resulting device yields a champion efficiency of 14.05% with negligible hysteresis, which presents the highest reported efficiency for inorganic CsPbI_1.5Br_1.5 solar cells reported thus far. In addition, FITC can be generally adopted as attractive passivator to improve the performance of CsPbI₂Br‐ and CsPbIBr₂‐based PSCs. Furthermore, with a comprehensive comparison of mixed‐halide inorganic perovskites, it is demonstrated that CsPbI_1.5Br_1.5 compositional perovskite is a promising candidate with the optimal halide composition for high‐performance inorganic PSCs.

The mystery of the buried interface in perovskite photovoltaics is deciphered by combining advanced spectroscopy techniques with a lift‐off strategy. The findings open a new avenue to understanding performance losses and thus the design of unique passivation strategies to remove imperfections at the top surfaces and buried interfaces of perovskite photovoltaics, resulting in substantial enhancement in device performance.

Abstract

Understanding the fundamental properties of buried interfaces in perovskite photovoltaics is of paramount importance to the enhancement of device efficiency and stability. Nevertheless, accessing buried interfaces poses a sizeable challenge because of their non‐exposed feature. Herein, the mystery of the buried interface in full device stacks is deciphered by combining advanced in situ spectroscopy techniques with a facile lift‐off strategy. By establishing the microstructure–property relations, the basic losses at the contact interfaces are systematically presented, and it is found that the buried interface losses induced by both the sub‐microscale extended imperfections and lead‐halide inhomogeneities are major roadblocks toward improvement of device performance. The losses can be considerably mitigated by the use of a passivation‐molecule‐assisted microstructural reconstruction, which unlocks the full potential for improving device performance. The findings open a new avenue to understanding performance losses and thus the design of new passivation strategies to remove imperfections at the top surfaces and buried interfaces of perovskite photovoltaics, resulting in substantial enhancement in device performance.

The fundamental reasons for efficient emission of halide perovskites are investigated, which is helpful to get efficient light emission of lead‐free halide perovskites. The synthesis, crystal structure, optical and optoelectronic properties of different molecular dimensional lead‐free halide perovskites with different forms are then systematically reviewed. Finally, the applications of light‐emitting devices (phosphor‐converted LEDs and electroluminescent LEDs) are discussed.

Abstract

Lead‐based halide perovskites have received great attention in light‐emitting applications due to their excellent properties, including high photoluminescence quantum yield (PLQY), tunable emission wavelength, and facile solution preparation. In spite of excellent characteristics, the presence of toxic element lead directly obstructs their further commercial development. Hence, exploiting lead‐free halide perovskite materials with superior properties is urgent and necessary. In this review, the deep‐seated reasons that benefit light emission for halide perovskites, which help to develop lead‐free halide perovskites with excellent performance, are first emphasized. Recent advances in lead‐free halide perovskite materials (single crystals, thin films, and nanocrystals with different dimensionalities) from synthesis, crystal structures, optical and optoelectronic properties to applications are then systematically summarized. In particular, phosphor‐converted LEDs and electroluminescent LEDs using lead‐free halide perovskites are fully examined. Ultimately, based on current development of lead‐free halide perovskites, the future directions of lead‐free halide perovskites in terms of materials and light‐emitting devices are discussed.

Chlorination of the β‐position of benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione can enhance the intermolecular interaction. Single‐crystal analysis demonstrates that TTO‐Cl‐β exhibits the smallest π‐π stacking distance of 3.23 Å, much smaller than that of TTO‐Cl‐α and TTO. Accordingly, PBBD‐Cl‐β based on TTO‐Cl‐β achieved an outstanding power conversion efficiency (PCE) of 16.20%, providing a new insight for the design of acceptor units.

Abstract

The position of a chlorine atom in a charge carrier of polymer solar cells (PSCs) is important to boost their photovoltaic performance. Herein, two chlorinated D‐A conjugated polymers PBBD‐Cl‐α and PBBD‐Cl‐β are synthesized based on two new building blocks (TTO‐Cl‐α and TTO‐Cl‐β) respectively by introducing the chlorine atom into α or β position of the upper thiophene of the highly electron‐deficient benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione moiety. Single‐crystal analysis demonstrates that the chlorine‐free TTO shows a π‐π stacking distance (d _π‐π) of 3.55 Å. When H atom at the α position of thiophene of TTO is replaced by Cl, both π‐π stacking distance (d _π‐π = 3.48 Å) and Cl···S distance (d _Cl‐S = 4.4 Å) are simultaneously reduced for TTO‐Cl‐α compared with TTO. TTO‐Cl‐β then showed the Cl···S non‐covalent interaction can further shorten the intermolecular π‐π stacking separation to 3.23 Å, much smaller than that of TTO‐Cl‐α and TTO. After blending with BTP‐eC9, PBBD‐Cl‐β:BTP‐eC9‐based PSCs achieved an outstanding power conversion efficiency (PCE) of 16.20%, much higher than PBBD:BTP‐eC9 (10.06%) and PBBD‐Cl‐α:BTP‐eC9 (13.35%) based devices. These results provide an effective strategy for design and synthesis of highly efficient donor polymers by precise positioning of the chlorine substitution.

Self‐assembled P3HT‐COOH is an excellent hole extraction layer to fabricate robust, high‐performance, and extremely reproducible perovskite solar cells. The well‐aligned self‐assembled P3HT‐COOH generates a dipole layer between indium tin oxide and perovskite, substantially retarding interface charge recombination and producing highly sensitive devices to dim light. The enhanced crystallinity and preferred out‐of‐plane orientation play a key role to suppress the device degradation process.

Abstract

Crystallinity and crystal orientation have a predominant impact on a materials’ semiconducting properties, thus it is essential to manipulate the microstructure arrangements for desired semiconducting device performance. Here, ultra‐uniform hole‐transporting material (HTM) by self‐assembling COOH‐functionalized P3HT (P3HT‐COOH) is fabricated, on which near single crystal quality perovskite thin film can be grown. In particular, the self‐assembly approach facilitates the P3HT‐COOH molecules to form an ordered and homogeneous monolayer on top of the indium tin oxide (ITO) electrode facilitate the perovskite crystalline film growth with high quality and preferred orientations. After detailed spectroscopy and device characterizations, it is found that the carboxylic acid anchoring groups can down‐shift the work function and passivate the ITO surface, retarding the interface carrier recombination. As a result, the device made with the self‐assembled HTM show high open‐circuit voltage over 1.10 V and extend the lifetime over 4,300 h when storing at 30% relative humidity. Moreover, the cell works efficiently under much reduced light power, making it useful as power source under dim‐light conditions. The demonstration suggests a new facile way of fabricating monolayer HTM for high efficiency perovskite devices, as well as the interconnecting layer needed for tandem cell.

Two sequentially deposited SnO₂ layers doped with a low and a high amount of ammonium chloride, respectively, boost the open‐circuit voltage and fill factor of perovskite solar cells. The main effect of the novel electron transport layer is a change in the energy level alignment with the perovskite interface leading to decreased carrier recombination.

Abstract

Tin oxide (SnO₂) is an emerging electron transport layer (ETL) material in halide perovskite solar cells (PSCs). Among current limitations, open‐circuit voltage (V _OC) loss is one of the major factors to be addressed for further improvement. Here a bilayer ETL consisting of two SnO₂ nanoparticle layers doped with different amounts of ammonium chloride is proposed. As demonstrated by photoelectron spectroscopy and photophysical studies, the main effect of the novel ETL is to modify the energy level alignment at the SnO₂/perovskite interface, which leads to decreased carrier recombination, enhanced electron transfer, and reduced voltage loss. Moreover, X‐ray diffraction reveals reduced strain in perovskite layers grown on bilayer ETLs with respect to single‐layer ETLs, further contributing to a decrease of carrier recombination processes. Finally, the bilayer approach enables the more reproducible preparation of smooth and pinhole‐free ETLs as compared to single‐step deposition ETLs. PSCs with the doped bilayer SnO₂ ETL demonstrate strongly increased V _OC values of up to 1.21 V with a power conversion efficiency of 21.75% while showing negligible hysteresis and enhanced stability. Moreover, the SnO₂ bilayer can be processed at low temperature (70 °C), and has therefore a high potential for use in tandem devices or flexible PSCs.

Perovskite Solar Cells

Solution‐processing of the photoactive layer on top of the hole‐transporting material can be challenging.Solvent damage leads to the formation of direct contact with the electrode, and consequently to reduced performance parameters. In article number 2000597, Vytautas Getautis and co‐workers show that cross‐linking of the organic semiconductor is advantageous for application in the p‐i‐n perovskite solar cells.

The three main approaches for reducing lead content in perovskite solar cells while keeping high efficiency are reviewed: i) the partial replacement of Pb by another element with similar charge and size, ii) the partial replacement of lead and halide units by organic cations, and iii) the engineering of the cells to optimize the light harvesting by the perovskite layer.

The rise and commercialization of perovskite solar cells (PSCs) is hindered by the toxicity of lead present in the perovskites used as the solar light absorber. To counter this problem, lead (Pb) can be fully (lead‐free) or partially (lead‐less) replaced by diverse elements. The former compounds suffer from poor efficiency and poor stability, whereas the later appear more promising. Herein, a survey of the methods reported in the literature to reduce Pb content in PSCs to fabricate “lead‐less” (also called “lead‐deficient”) PSCs is offered. First, the comparison of Sn and Pb elements and the partial replacement of Pb by Sn are developed. Then, its substitution by either Ge, Sr, or other alkaline‐earth‐metals, transition metals, and elements from columns 12, 13, and 15 of the periodic table are detailed. The new families of perovskites based on the insertion of organic cations to replace lead and halogen units, namely the “lead‐deficient” and “hollow” halide perovskites are then presented and discussed. Finally, atypical ways to reduce the toxicity of PSCs are presented: perovskite layer thickness reduction via optimization of photon collection, integration of photonic structures, and usage of recycled lead. The current achievements and the outlook of those strategies are presented and discussed.

Nature Materials, Published online: 11 January 2021; doi:10.1038/s41563-020-00872-6

Nature Energy, Published online: 11 January 2021; doi:10.1038/s41560-020-00756-8