Chen Weijie

This review summarizes the latest progress in defect passivation in the CsPbX₃ (X represents halogen) perovskite solar cells field. Starting from the effect of non-radiative recombination on open circuit voltage losses, the defect fundamentals, techniques to identify defects, passivation mechanisms, and strategies are discussed. Finally, directions for future research about defect manipulation that will push the field to progress forward are outlined.

Abstract

The rapid development of all inorganic metal perovskite (CsPbX₃, X represents halogen) materials holds great promise for top-cells in tandem junctions due to their glorious thermal stability and continuous adjustable band gap in a wide range. Due to the presence of defects, the power conversion efficiency (PCE) of CsPbX₃ perovskite solar cells (PSCs) is still substantially below the Shockley-Queisser (SQ) limit. Therefore, it is imperative to have an in-depth understanding of the defects in PSCs, thus to evaluate their impact on device performances and to develop corresponding strategies to manipulate defects in PSCs for further promoting their photoelectric properties. In this review, the latest progress in defect passivation in the CsPbX₃ PSCs field is summarized. Starting from the effect of non-radiative recombination on open circuit voltage (V _oc) losses, the defect physics, tolerance, self-healing, and the effect of defects on the photovoltaic properties are discussed. Some techniques to identify defects are compared based on quantitative and qualitative analysis. Then, passivation manipulation is discussed in detail, the defect passivation mechanisms are proposed, and the passivation agents in CsPbX₃ thin films are classified. Finally, directions for future research about defect manipulation that will push the field to progress forward are outlined.

A power conversion efficiency of 12.91% and an average visible transmittance of 22.49% are achieved in semitransparent organic photovoltaics (OPVs) with D18:N3 (0.7:1.6, wt/wt) as active layers. This work demonstrates that the wide bandgap polymer donor with narrow photon harvesting in the visible light range has great potential in preparing efficient semitransparent OPVs.

Abstract

Wide bandgap polymer D18 with narrow photon harvesting in visible light range and small molecule N3 with near-infrared photon harvesting are adopted for building semitransparent organic photovoltaics (OPVs). To find out the optimal D18:N3 weight ratio for semitransparent OPVs, series of opaque OPVs are built with a varied D18:N3 weight ratio. The power conversion efficiency (PCE) and fill factor can be maintained over 16% and 77% in the D18:N3 (0.7:1.6, wt/wt) based opaque OPVs, respectively. The average visible transmittance (AVT) of the corresponding blend films can be achieved over 50%, demonstrating the great potential in fabricating efficient semitransparent OPVs. The semitransparent OPVs based on D18:N3 (0.7:1.6, wt/wt) are fabricated by using 1 nm Au/(10, 15, 20 nm) Ag as cathode. The thickness of Ag layers is varied to balance the optical properties and electrical properties of semitransparent top electrode. The semitransparent OPVs with 10 nm Ag achieve the highest light utilization efficiency of 2.90% with a PCE of 12.91% and an AVT of 22.49%, which should be among the best performance of reported semitransparent OPVs. This work demonstrates that the wide bandgap polymer donor with narrow photon harvesting in visible light range has great potential in preparing efficient semitransparent OPVs.

Near-infrared (NIR) photoactive semiconductor quantum dots (QDs) play a critical role for designing efficient wide-spectrum solar cells. This review provides a comprehensive analysis of the latest achievements of NIR QDs used for solar cells, including the classification of QDs and their photovoltaic performance, various strategies for performance improvements, and the challenges and perspectives for the future advances.

Abstract

Semiconductor quantum dots (QDs) are nanocrystals whose excitons are bound in 3D space. Owning to their remarkable quantum confinement effect, QDs exhibit a discontinuous electronic energy level structure similar to that of atoms, leading to novel physical, optical, and electrical properties for various optoelectronic device applications including solar cells. Near-infrared photoactive narrow bandgap (NBG) QDs can maximize the use of solar energy through the quantum size effect, offering a good opportunity for designing highly efficient wide-spectrum responsive solar cells. This review analyzes the recent research progress of NBG QDs as light absorbing materials in solar cells. The critical elaboration of the latest achievements both in material design and device optimization for NBG QD-based solar cells (QDSCs), including QD synthesis and film fabrication, design of device configuration, classification of NBG QDs and their photovoltaic performance, strategies for performance improvements is focused upon. The current challenges and perspectives for the further advance of NBG QDSCs are also discussed.

Gradual perovskite phase based on 2D cyclohexylmethylammonium iodide as the order of n and funnel-like energy level alignment during surface treatment with a simple solution process facilitates efficient charge transport electrically and improves power conversion efficiency from 20.41% to 23.91%.

Abstract

Insufficient charge extraction at the interfaces between light-absorbing perovskites and charge transporting layers is one of the drawbacks of state-of-the-art perovskite solar cells. Surface treatments and/or interface engineering are necessary to approach the Shockley–Queisser limit. In this work, novel 2D layered perovskites, such as CHA₂PbI₄ (CHAI = cyclohexylammonium iodide) and CHMA₂PbI₄ (CHMAI = cyclohexylmethylammonium iodide), are introduced in between 3D perovskites and hole transporting layers by a simple solution process and the 2D/3D perovskite heterojunction is formed and confirmed. Spontaneous photoluminescence quenching is observed by efficient hole extraction with a favorable valence band alignment. The charge extraction ability and recombination are directly measured by the transient photocurrent and photovoltage. Moreover, the interface resistance of the devices significantly is decreased to 30% as compared to devices without 2D perovskites. As a result, the devices with 2D/3D perovskite heterojunction exhibit improved power conversion efficiency (PCE) from 20.41% to 23.91% primarily because of the increased open-circuit voltage (1.079 to 1.143 V) and fill factor (78.22% to 84.25%). The results provide a detailed insight into hole extraction and high PCEs with the formation of a 2D/3D perovskite heterojunction.

Towards stable solar cells, ferrocene as a perpetual recovering agent has been developed to fix all elemental defects in ABX₃ perovskite by a chain-reaction cycle. The ferrocene cations can form a 1D perovskite structure which has suitable dissociation energy to convert back to light-harvesting 3D perovskite and reactivate its photovoltaic performance. Based on this recovering agent, perovskite solar cell can achieve >10 000 h lifetime.

Abstract

Lead halide perovskites always emerge complex interactions among different elemental ions, which lead to multiple intrinsic imperfections. Elemental defects, such as amine, Pb, and I vacancies at A-, B-, and X-sites, are main issues to deteriorate perovskite solar cells (PSCs). Unfortunately, most previous passivators can only temporarily fix partial inactive vacancies as sacrificial agents. Herein, we propose a recovery agent, ferrocene (Fc), which can form a one-dimensional perovskite with adequate steric cavities and suitable dissociation energy to recover all elemental defects back to active light-harvesting perovskites, and regenerate Fc itself meanwhile. Based on this perpetual chain-reaction cycle, corresponding PSCs maintain >10 000-hour lifetime in inert condition and >1000-hour durabilities under various extreme environments, including continuous 85 °C heating, 50 % relative humidity wetting, and 1-sun light soaking.

A poling process that simultaneously modulates the built-in field and interface barriers of the perovskite solar cells has been conducted for the perovskite doped by P(VDF-TrFE). It has been unveiled that the new devices have achieved a high power conversion effectivity of 22.1%, attributed to the piezophototronic effect that effectively enhances the performance of the perovskite solar cell.

As a candidate for next-generation solar devices, perovskite solar cells are increasingly being studied for their rapid increased power conversion efficiency (PCE). One of the possible routes to further increase PCE is the introduction of polarization in the absorption layer which functions as a method for increasing the built-in potential and reducing the interface barrier, leading to much improved carrier separation and extraction. This technique uses the principle of the piezophototronic effect utilized for obtaining enhanced optoelectronic performances. Herein, to introduce internal polarization while maintaining optical absorption performance of the perovskite, organic–inorganic hybrid perovskite composite film solar cells are fabricated by doping polarized polyvinylidenefluoride-co-trifluoroethylene (P(VDF-TrFE)) into the perovskite. The composite film is polarized with an external potential, subsequently inducing the piezophototronic effect to enhance the performances of perovskite solar cells. Experimental results show that this simple polarization method has effectively improved several key characteristics of the solar cell. The PCE has reached up to 22.1%, the short-circuit current (J _sc) increases to 24.2 mA cm⁻², and the open-circuit voltage (V _oc) increases to 1.18 V.

A low defect halide perovskite film can be fabricated using trimethyl phosphate (TMP). TMP directly forms perovskite without intermediate phase due to weak Lewis basicity. The TMP-based film reduces hysteresis in current-voltage curve of perovskite solar cell. The TMP-based device exhibits better performance (>20%) than other solvent-based ones.

Abstract

Solvent engineering by Lewis-base solvent and anti-solvent is well known for forming uniform and stable perovskite thin films. The perovskite phase crystallizes from an intermediate Lewis-adduct upon annealing-induced crystallization. Herein, it is explored the effects of trimethyl phosphate (TMP), as a novel aprotic Lewis-base solvent with a low donor number for the perovskite film formation and photovoltaic characteristics of perovskite solar cells (PSCs). As compared to dimethylsulfoxide (DMSO) or dimethylformamide (DMF), the usage of TMP directly crystallizes the perovskite phase, i.e., reduces the intermediate phase to a negligible degree, right after the spin-coating, owing to the high miscibility of TMP with the anti-solvent and weak bonding in the Lewis adduct. Interestingly, the PSCs based on methylammonium lead iodide (MAPbI₃) derived from TMP/DMF-mixed solvent exhibit a higher average power conversion efficiency of 19.68% (the best: 20.02%) with a smaller hysteresis in the current-voltage curve, compared to the PSCs that are fabricated using DMSO/DMF-mixed (19.14%) or DMF-only (18.55%) solvents. The superior photovoltaic properties are attributed to the lower defect density of the TMP/DMF-derived perovskite film. The results indicate that a high-performance PSC can be achieved by combining a weak Lewis base with a well-established solvent engineering process.

A poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS)-free organic solar cell (OSC) architecture is successfully constructed by employing a binary solvent-chlorinated indium tin oxide anode, which can simultaneously improve the device performance and operational stability of non-fullerene OSCs.

Abstract

Despite the tremendous development of different high-performing photovoltaic systems in non-fullerene polymer solar cells (PSCs), improving their performance is still highly demanding. Herein, an effective and compatible strategy, i.e., binary-solvent-chlorinated indium tin oxide (ITO) anode, is presented to improve the device performance of the state-of-the-art photoactive systems. Although both ODCB (1,2-dichlorobenzene) solvent- and ODCB:H₂O₂ (hydrogen peroxide) co-solvent-chlorinated ITO (ITO-Cl_-ODCB and ITO-Cl_-ODCB:H2O2) show similar optical transmittance, electrical conductivities, and work function values, ITO-Cl_-ODCB:H2O2 exhibits higher Cl surface coverage and more suitable surface free energy close to the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-buffered ITO anode (ITO/PEDOT:PSS). As a direct consequence, the performance of ITO-Cl_-ODCB-based PBDB-T-2F:BTP-eC9:PC₇₁BM PSCs is comparable as the bare ITO-based devices. In contrast, the performance of ITO-Cl_-ODCB:H2O2-based devices with both small and the scaled-up areas significantly surpass the ITO/PEDOT:PSS-based devices. Furthermore, detailed experimental studies are conducted linking optical property, blend morphology, and physical dynamics to find the reasons for the performance difference. By applying the ITO-Cl_-ODCB:H2O2 anode to six other photovoltaic systems, the device efficiencies are enhanced by 3.6–6.2% relative to those of the ITO/PEDOT:PSS-based control devices, which validates its great application potential of co-solvent-modified ITO anode employed into PEDOT:PSS-free PSCs.

Incorporation of the chromium-based metal–organic framework as an A-site cation allows realizing a new multiple-dimensional electronically coupled CsPbI₂Br perovskite, which is theoretically and experimentally proved to improve the carrier transport ability and stability of perovskite solar cells (PSCs). Consequently, the as-fabricated CsPbI₂Br PSCs demonstrate 17.02% power conversion efficiency while superior long-term stability.

Abstract

Inorganic CsPbI _x Br_{3− x} perovskite solar cells (PSCs) have gained enormous interest due to their excellent thermal stabilities. However, their intrinsically poor moisture stability hampers their further development. Herein, a chromium-based metal–organic framework group is intercalated inside the inorganic PbI framework, resulting in a new multiple-dimensional electronically coupled CsPbI₂Br perovskite. In this structurally and electronically coupled perovskite, the π-conjugated terpyridyl can delocalize the excited valence electrons of metal Cr³⁺ ion, enabling multi-interactive charge-carrier transport channels within CsPbI₂Br perovskites. The stability and efficiency of the produced devices are substantially enhanced in comparison to their counterparts with only a pristine CsPbI₂Br active layer. The optimized all-inorganic PSC yields a power conversion efficiency (PCE) as high as 17.02%. Remarkably, the stabilized device retains 80% of its PCE after 1000 h in the ambient atmosphere. This study provides a new paradigm toward addressing the stability challenge of the inorganic perovskite while enhancing its carrier transport ability.

A novel fullerene molecular template with a solubility enhancer arm (R1) and a π–π interaction inducer arm (R2) is deliberately proposed. This design effort delivers the highest power conversion efficiency over 20% of the device with corresponding fulleropyrrolidine electron transport material for the first time.

Abstract

[6,6]-phenyl-C₆₁-butyric acid methyl ester remains indispensable as the electron transport material (ETM) for perovskite solar cells (PSCs), but its synthesis involves complicated multisteps with low productivity. In contrast, the potential of synthesizing simpler fulleropyrrolidine derivatives has long been overlooked, and little has been understood regarding their structure-dependent effects on photovoltaic (PV) performance. Herein, seven novel fulleropyrrolidine derivatives (F1–F7) are deliberately designed, synthesized, and comprehensively characterized in both solution and thin-film states and subsequently investigated as ETMs for PSCs. Notably, the F4 delivers the highest power conversion efficiencies over 20% of devices, which surpass all reported fulleropyrrolidine ETMs due to its optimal photoelectric property. Moreover, the structure-dependent effects of the fullerenes on PV parameters are uncovered, including solubility, intermolecular interaction, packing structure, and charge-transfer ability, which can guide the future design of high-performance and stable fullerene ETMs for PSCs.

The multi-scale defect repair strategy is developed to fabricate scalable and flexible perovskite solar cells. By inhibiting the aggregation behavior of colloidal particles to avoid pinholes and intergranular cracking in the perovskite film, along with repairing the deep defects at the interface, the target flexible devices (1.01 cm²) deliver a superior efficiency of 18.12% with improved air atmosphere stability.

Abstract

Homogeneity and stability of flexible perovskite solar cells (PSCs) are significant for the commercial feasibility in upscaling fabrication. Concretely, the mismatching between bottom interface and perovskite precursor ink can cause uncontrollable crystallization and undesired dangling bonds during the printing process. Herein, methylammonium acetate, serving as ink assistant (IAS) can effectively avoid the micron-scale defects of perovskite film. The in situ optical microscope is applied to prove the IAS can inhibit the colloidal aggregation and induce more adequate crystallization growth, thus avoiding the micron-scale defects of pinholes and intergranular cracking. Concurrently, 4-chlorobenzenesulfonic acid is introduced into the electrode surface as a passivation layer to restore the deep traps at perovskite interface in nano-scale. Finally, the target flexible devices (1.01 cm²) deliver a superior efficiency of 18.12% with improved air atmosphere stability. This multi-scale defect repair strategy provides an integrated design concept of homogeneity and stability for scalable and flexible PSCs.

In this paper, a transparent solution-processed inorganic QLED device is fabricated. The low-temperature sol-gel derived copper doped nickel oxide interlayer improves the hole injection efficiency, reducing the QLED turn-on voltage. In addition, the modeling and simulation of QLED are completed in COMSOL, where the simulation results show good accordance with the experimental results and theoretical analysis.

Abstract

Quantum dot light-emitting diodes (QLEDs) represent an exciting new technology that has many desirable attributes when compared to existing organic LEDs (OLEDs) including increased brightness, contrast, and response time. Solution-based fabrication approaches have the advantage of being able to produce large-area electronic systems at reduced costs and critical in applications such as large display fabrication and electronics on curved surfaces including low-profile augmented reality glasses. In this paper, for the first time, a fully solution-processed transparent inorganic QLED is described. Traditional QLED fabrication methodologies require the use of air-sensitive materials that make fabrication of these devices challenging and expensive. Instead of using air-sensitive organic materials, in the approach, nickel oxide (NiO) is used as the hole transport layer and is deposited using a sol-gel method. Copper doping of the NiO to reduce the turn-on voltage of the QLED device is investigated. Importantly, the post-annealing temperature of the sol-gel process is below 275 °C, which permits the fabrication of QLEDs on a wide range of substrates. The experimental results are concordant with the COMSOL simulation data and demonstrate the feasibility of fabricating fully transparent inorganic QLED devices using a solution-based process.

PTCDA is for the first time implemented as an anode active material for aqueous sodium-ion batteries, with a novel “hybrid” aqueous electrolyte based on inexpensive non-fluorinated salts. The hybrid electrolyte supresses material dissolution during cycling and enables all-organic electrodes with high capacity retention and Coulombic efficiencies.

Abstract

Aqueous sodium-ion batteries (ASIBs) are aspiring candidates for low environmental impact energy storage, especially when using organic electrodes. In this respect, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) is a promising anode active material, but it suffers from extensive dissolution in conventional aqueous electrolytes. As a remedy, we here present a novel aqueous electrolyte, which inhibits the PTCDA dissolution and enables their use as all-organic ASIB anodes with high capacity retention and Coulombic efficiencies. Furthermore, the electrolyte is based on two, hence “hybrid”, inexpensive and non-fluorinated Na/Mg-salts, it displays favourable physico-chemical properties and an electrochemical stability window >3 V without resorting to the extreme salt concentrations of water-in-salt electrolytes. Altogether, this paves the way for ASIBs with both relatively high energy densities, inexpensive total cell chemistries, long-term sustainability, and improved safety.

Nature Materials, Published online: 16 September 2021; doi:10.1038/s41563-021-01097-x