Chen Weijie

Environmentally friendly 2,2,2‐trifluoroethylamine hydrochloride (TFEACl) is used in synergy with SnF₂ to enhance the efficiency and stability of FASnI₃‐based solar cells, due to the improvements in film quality, suppression of the Sn²⁺ oxidation and more favorable energy band alignment.

The environmentally friendly additive 2,2,2‐trifluoroethylamine hydrochloride (TFEACl) is used in synergy with SnF₂ to enhance the efficiency and stability of FASnI₃‐based solar cells. Both TFEA⁺ and Cl⁻ are present in the films, but only Cl⁻ is incorporated into the crystal lattice of the perovskite. The addition of TFEACl suppresses the segregation of SnF₂, resulting in improvements in film morphology, in addition to a more favorable energy band alignment, and improved suppression of the formation of Sn⁴⁺. Consequently, reduced charge recombination and improved charge collection result in an efficiency enhancement from 3.63 to 5.30%. The stability of the devices is also significantly enhanced, with devices with TFEACl retaining over 60% of initial PCE after 350 h of light soaking in ambient, while devices without TFEACl experience failure in 120 h under the same testing condition.

Various deposition methods to satisfy the application of SnO₂ in different configuration‐based PSCs are summarized. Moreover, the efforts aimed at improving the performance of PSCs are cataloged according to different purposes and methods.

Organic‐inorganic hybrid perovskite solar cells (PSCs) have developed rapidly in recent years owing to the low cost and high power conversion efficiency achieved. The excellent performance of PSCs is attributed to the superior electrical properties of each layer, including the electron transport layer (ETL), light‐harvest layer, hole transport layer. As one of the most promising ETL materials for PSCs, SnO₂ shows excellent transmission, an appropriate energy band gap, a deep conduction band level, and high electron mobility, leading to efficient electron extraction and transport. Here, recent advancements in the PSCs with SnO₂ ETLs and endeavors aimed at improving the performance of this photovoltaic device are reviewed. Several typical configurations of SnO₂ based PSCs are discussed, including the planar structure, mesoporous structure, inverted structure and flexible PSCs. The efforts of modification and composite SnO₂ with other metal oxides are also assessed. Finally, an overview of the perspectives and challenges for the future of SnO2 based PSCs is provided.

Phase‐engineered Weyl semi‐metallic Mo_xW_1‐xTe₂ nanosheets, are exploited as a highly efficient counter electrode for dye‐sensitized solar cells, achieving higher efficiency of ≈10% over the conventional Pt. The T_d‐Mo_xW_1‐xTe₂ ‐based counter electrode can robustly promote the charge transfer during the redox reaction at the counter electrode/electrolyte interface, leading to the stable electrocatalytic performance over 200 scanning cycles in the I₃‐/I^‐ medium.

The emerging Weyl semi‐metals with robust topological surface states are very promising candidates to rationally develop new‐generation electrocatalysts for dye‐sensitized solar cells (DSSCs). In this study, a chemical vapor deposition (CVD) method to synthesize highly crystalline Weyl semi‐metallic Mo_xW_1‐xTe₂ nanocrystals, which are applied for the counter electrode (CE) of DSSCs for the first time, are employed. By controlling the temperature‐dependent phase‐engineered synthesis, the nanocrystal grown at 760 °C exhibits the mixed phases of semiconducting T_d ‐ & 2H‐Mo_0.32W_0.67Te_2.01 with charge carrier density of (1.20 ± 0.02) × 10¹⁹ cm⁻³; whereas, the nanocrystal synthesized at 820 °C shows a single phase of semi‐metallic T_d ‐Mo_0.29W_0.72Te_1.99 with much higher carrier density of (1.59 ± 0.04) × 10²⁰ cm⁻³. In the cyclic voltammetry (CV) analysis over 200 cycles, the Mo_xW_1‐xTe₂‐based electrodes show better stability in the I⁻/I₃ ⁻ electrolyte than a Pt electrode. In DSSC tests, a T_d ‐Mo_0.29W_0.72Te_1.99‐decorated CE achieves the efficiency (η) of 8.85%, better than those CEs fabricated with T_d ‐ & 2H‐Mo_0.32W_0.67Te_2.01 (7.81%) and sputtered Pt (8.01%). The electrochemical impedance spectra reveal that the T_d ‐Mo_0.29W_0.72Te_1.99 electrode possesses low charge‐transfer resistance in electrocatalytic reactions. These exceptional properties make Weyl semi‐metallic T_d ‐Mo_xW_1‐xTe₂ a potential electrode material for a wide variety of electrocatalytic applications.

SnO₂ QDs and CsMBr₃ (M = Sn, Bi, Cu) QDs are applied as ETMs and HTMs for all‐inorganic CsPbBr₃ PSCs, respectively. Arising from high optical transmittance and electron mobility of SnO₂ QDs ETL as well as hole extraction of CsMBr₃ QD HTL, the device achieves a good PCE of 10.60% and improved stability.

The power conversion efficiency (PCE) of state‐of‐the‐art perovskite solar cells (PSCs) with mesoscopic titanium dioxide (TiO₂) has rushed to 23.7% in recent years. However, photodegradation of perovskites under illumination (including ultraviolet light), assisted by TiO₂, significantly reduces the long‐term stability of the corresponding device, which in turn limits the commercialization of PSCs. Owing to the advantages of high electron mobility, wide bandgap, high transparency, and good photostability, nanostructured tin oxide (SnO₂) is demonstrated to be a more promising electron‐transporting material for planar PSCs. Herein, low‐temperature solution‐processed SnO₂ quantum dots (QDs) are employed as the electron transport layer (ETL) for all‐inorganic cesium lead bromide (CsPbBr₃) PSC applications. Through optimizing the aging time of SnO₂ QDs and adding a hole transport layer (HTL) of CsMBr₃ (M = Sn, Bi, Cu) QDs between the CsPbBr₃ layer and carbon electrode, the all‐inorganic PSC with a structure of FTO/SnO₂/CsPbBr₃/CsMBr₃/carbon achieves a good PCE of 10.60% with an ultrahigh open‐circuit voltage up to 1.610 V. These optimized devices, free of encapsulation, present excellent stability in 80% humidity or temperature of 80 °C. The maximized PCE report to date and improved environmental‐tolerance for all‐inorganic CsPbBr₃ solar cells provide new opportunities to dramatically promote the commercialization of PSC platforms.

Electrical characterizations reveal that CdTe surface etching using diluted hydrogen iodide removes native oxides and/or chlorides produced during the CdCl₂ treatment, and creates a Te‐rich surface layer, which reduces the back barrier height and improves the performance of the resulting CdTe solar cells.

An appropriate electrical back contact in CdTe solar cells is crucial to achieving high power conversion efficiency. In this work, a facile back surface treatment method for CdTe solar cells using hydroiodic acid (HI) is developed, and the effects of HI etching on the CdTe surfaces investigated. The electrical properties of the CdTe absorber and interfaces are characterized by current–voltage, capacitance–voltage, admittance spectroscopy, and complex capacitance spectroscopy measurements. The HI‐etched devices show slightly higher apparent carrier concentrations than the control devices, suggesting an increased copper doping in the CdTe absorber. The potential barrier height of the back contact is reduced from 0.430 to 0.368 eV after the HI‐treatment, accompanied by reduced contact resistance and carrier recombination. Additionally, the HI‐treatment eliminates a defect signature at 0.409 eV. The HI‐treatment effects lead to improved power conversion efficiency through enhancement of the fill factor, the short circuit current, and open circuit voltage.

The origins of high device performance and degradation in air of mixed perovskite cells are investigated by monitoring defect states and compositional changes of the perovskite layer over time. The results reveal that a defect formed by Br atoms exists at deep levels of the perovskite, and its defect state shifts when the film is aged.

Abstract

The origins of the high device performance and degradation in the air are the greatest issues for commercialization of perovskite solar cells. Here this study investigates the possible origins of the mixed perovskite cells by monitoring defect states and compositional changes of the perovskite layer over the time. The results of deep‐level transient spectroscopy analysis reveal that a newly identified defect formed by Br atoms exists at deep levels of the mixed perovskite film, and its defect state shifts when the film is aged in the air. The change of the defect state is originated from loss of the methylammonium molecules of the perovskite layer, which results in decreased J _SC, deterioration of the power conversion efficiency and long‐term stability of perovskite solar cells. The results provide a powerful strategy to diagnose and manage the efficiency and stability of perovskite solar cells.

Polymethyl methacrylate is coated on a perovskite grain boundary, blocking moisture penetration. The distributed C60 clusters create a dipole‐like electric field inside the perovskite layer, which favors exciton dissociation, and improves conversion efficiency of perovskite solar cells.

Abstract

Lead halide perovskite solar cells (PSCs) have demonstrated great potential for realizing low‐cost and easily fabricated photovoltaics. At this juncture, power conversion efficiency and long‐term stability are two important factors limiting their transition. PSCs exhibit rapid environmental degradation since the perovskite layer is very sensitive to factors such as humidity, temperature, and ultraviolet light. Here, a novel successful approach is demonstrated that simultaneously improves the efficiency and stability of PSCs. This approach relies on incorporation of a dual‐functional polymethyl methacrylate (PMMA)–fullerene complex into the perovskite layer. The fullerene within perovskite layer forms a localized dipole‐like electric field that favors electron–hole separation, resulting in significant improvement in current density and fill factor with conversion efficiency reaching 18.4%. The molecular‐scale coating of hydrophobic PMMA on the perovskite grain boundary effectively blocks moisture penetration into the perovskite, thereby, significantly improving the stability against moisture, heat, and light. The PSCs with PMMA–fullerene complex showed no photovoltaic performance degradation for 250 d and exhibited 60 times higher stability compared to the state‐of‐the‐art devices under continuous 1 sun illumination in ambient air.

An effective strategy is developed to synthesize high‐nuclearity Cu₅₃ clusters that are surface‐capped by alkynyl and acetate ligands. These nanoclusters are unexpectedly soluble in ether, enabling the easy formation of high‐quality films. The cluster films are readily converted into high‐quality CuI thin films for applications as a hole transport layer in perovskite solar cells.

Abstract

An effective strategy is developed to synthesize high‐nuclearity Cu clusters, [Cu₅₃(RCOO)₁₀(C≡CtBu)₂₀Cl₂H₁₈]⁺ (Cu₅₃ ), which is the largest Cu^I/Cu⁰ cluster reported to date. Cu powder and Ph₂SiH₂ are employed as the reducing agents in the synthesis. As revealed by single‐crystal diffraction, Cu₅₃ is arranged as a four‐concentric‐shell Cu₃@Cu₁₀Cl₂@Cu₂₀@Cu₂₀ structure, possessing an atomic arrangement of concentric M₁₂ icosahedral and M₂₀ dodecahedral shells which popularly occurs in Au/Ag nanoclusters. Surprisingly, Cu₅₃ can be dissolved in diethyl ether and spin coated to form uniform nanoclusters film on organolead halide perovskite. The cluster film can subsequently be converted into high‐quality CuI film via in situ iodination at room temperature. The as‐fabricated CuI film is an excellent hole‐transport layer for fabricating highly stable CuI‐based perovskite solar cells (PSCs) with 14.3 % of efficiency.

An intriguing maze‐like CH₃NH₃PbI₃ film featuring a bilayer structure with a dense bottom layer and a porous top layer is judiciously designed for electron transport layer‐free perovskite solar cells (PSCs). Such maze‐like perovskite film shows high crystallinity, superior light‐harvesting capability, and enables facilitated hole extraction at the perovskite/hole transport layer interface, thus leading to a PCE of 18.5% with negligible hysteresis.

Perovskite solar cells (PSCs) without an electron transport layer (ETL) exhibit fascinating advantages such as simplified configuration, low cost, and facile fabrication process. However, the performance of ETL‐free PSCs has been hampered by severe charge carrier recombination induced either by current leakage (insufficient perovskite film coverage) or inferior charge extraction. Herein, an additive‐assisted morphological engineering strategy is used to construct an intriguing bilayer perovskite film featuring a dense bottom layer and a maze‐like top layer. Such maze‐like perovskite films enable the construction of ETL‐free PSCs with a PCE of 18.5% and negligible hysteresis, which can be attributed to the higher crystallinity and superior light‐harvesting capability of the resultant perovskite film, as well as facilitated hole extraction at the hole transport layer (HTL)/perovskite interface. This work provides a simple approach to modify the perovskite film morphology and demonstrates the correlation between facilitated charge‐carrier extraction and high‐performance ETL‐free perovskite photovoltaics.

Halide perovskites exhibit extraordinary properties of slow hot‐carrier cooling; long‐range hot‐carrier transport; and efficient hot‐carrier extraction that are capable of unlocking disruptive high‐efficiency hot‐carrier photovoltaics which will overcome the Shockley–Queisser limit. Herein, the intricate photophysical mechanisms behind the novel phenomenon are distilled; an engineering and developmental toolkit is assembled; and the challenges and opportunities in this fledging area are examined.

Abstract

Rapid hot‐carrier cooling is a major loss channel in solar cells. Thermodynamic calculations reveal a 66% solar conversion efficiency for single junction cells (under 1 sun illumination) if these hot carriers are harvested before cooling to the lattice temperature. A reduced hot‐carrier cooling rate for efficient extraction is a key enabler to this disruptive technology. Recently, halide perovskites emerge as promising candidates with favorable hot‐carrier properties: slow hot‐carrier cooling lifetimes several orders of magnitude longer than conventional solar cell absorbers, long‐range hot‐carrier transport (up to ≈600 nm), and highly efficient hot‐carrier extraction (up to ≈83%). This review presents the developmental milestones, distills the complex photophysical findings, and highlights the challenges and opportunities in this emerging field. A developmental toolbox for engineering the slow hot‐carrier cooling properties in halide perovskites and prospects for perovskite hot‐carrier solar cells are also discussed.