Chen Weijie

The carbon-based hole-transporting layer (HTL)-free CsPbBr₃ perovskite solar cells (PSCs) achieve a maximum power conversion efficiency (PCE) of 10.08% with a high V _OC of 1.605 V and an excellent stability through passivating defects and enhancing charge extraction by interface modification with hexamethylenetetramine (HMTA).

The passivation of defects at perovskite films, surfaces and the promotion of charge extraction across perovskite/carbon back interface are of vital importance to develop the power conversion efficiency (PCE) and stability of carbon-based perovskite solar cells (PSCs) free of hole-transporting layers (HTLs). Herein, an electron donor material with polyamino groups, hexamethylenetetramine (HMTA), is used as an efficient surface modifier for tribrominated all-inorganic perovskite films. The modification with HMTA not only eliminates the defects by forming a bond between N atoms and positively charged ions but also optimizes the energy level structure of the perovskite film and back interface contact. Therefore, CsPbBr₃ films with decreased trap states, extended carrier lifetimes, and enhanced hole mobility are gained, which significantly restrains charge recombination and energy loss as well as facilitates charge extraction and transfer at the perovskite/carbon interface. Finally, an improvement of PCE from 6.75% to 10.08% is obtained for carbon-based HTL-free CsPbBr₃ PSCs without and with HMTA modification, respectively. Furthermore, the HMTA-modified device without encapsulation presents an enhanced long-term moisture and heat stability after being stored in the atmospheric environment with 80% relative humidity (RH) at 25 °C and 20% RH at 85 °C, respectively, due to the reduced defects and the improved hydrophobicity of perovskite film.

Inverted wide-bandgap (Eg = 1.67 eV) perovskite solar cells (PSCs) with 2,3,5,6 tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) incorporation not only reduce the trap states at the band edges and the nonradiative recombination, but also deliver a power conversion efficiency (PCE) of 20%. The unencapsulated devices maintain 88% of their peak PCE under continuous illumination in a nitrogen atmosphere after 840 h.

Trap-induced nonradiative recombination and decomposition are the major limiting factors that hinder the development of mixed-halide wide-bandgap perovskite solar cells. Specifically, the incorporation of formamidinium (FA⁺) and bromide in wide-bandgap (WBG) perovskite materials leads to shallow-energy-level traps and inferior light stability. Herein, the electron-withdrawing molecule 2,3,5,6 tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is used as an effective passivator and stabilizer, which has molecular interaction with the FA⁺ cation for reducing trap densities and enhancing the stability of WBG perovskite solar cells. It is found that the extended π aromatic system in F4-TCNQ can enhance the binding energy and stabilize the FA⁺ cation in perovskite films. Furthermore, the 1.67 eV bandgap inverted perovskite solar cells with a small amount of F4-TCNQ have shallower defect levels, reduced trap density, and decreased nonradiative recombination, therefore giving a remarkably improved power conversion efficiency (PCE) of 20.0%. Most importantly, the unencapsulated devices with F4-TCNQ additive have greatly enhanced stability, maintaining 88% of their peak PCEs under continuous illumination after 840 h, whereas the control devices only retain 48% of their PCEs after 500 h.

The synergistic effect of Li:Cu codoping in various NiO _x hole transporting layers and perovskite photovoltaics were systematically investigated. Codoped NiO _x films exhibit enhanced optical and electrical properties. Codoped NiO _x layers induce high-quality perovskite films, minimizing recombination losses in solar cells. In consequence, the Li:Cu(8:2)-based device exhibited a superior power conversion efficiency of 19.46% in comparison with the pristine device.

Inverted perovskite solar cells (PSCs), which feature an attractive structure for diverse applications such as tandem SCs or flexible devices, continue to be rapidly developed. Among various hole-transporting layer (HTL) materials, nickel oxide (NiO _x ) is widely used as a stable and superior HTL even though it exhibits poor conductivity. Although various methods have been proposed to overcome the low conductivity of NiO _x films, codoping methods have not been extensively studied and remain poorly understood. Herein, Li:Cu:NiO _x is systematically investigated to explore the synergistic effect of codoping in various NiO _x HTLs and PSCs. The optical, chemical, and morphological properties of the films are characterized and the dependence of these properties on the codoping ratio are investigated. A gradual improvement of the electrical properties and a tunable Fermi energy level resulting from the Li:Cu codopants is subsequently demonstrated. Furthermore, the structure of the perovskite films on HTLs and the synergistic effect on the preferred crystal growth behavior are elucidated. An inverted PSC with high efficiency was attained as a result of the enhanced electrical properties of the NiO _x HTLs and the high quality of the perovskite film, which were attributed to the synergistic effect of the Li:Cu codoping method.

Methylamine gas treatment is an easy, fast, and universal method for defect healing in CH₃NH₃PbBr₃ perovskites. The recrystallized film is further evaluated during the fabrication of a semitransparent PSC device reaching a power conversion efficiency of 7.8%, an average visible transmittance of 52%, and a T80 greater than 300 h under maximum power point tracking and continuous light exposure.

High bandgap semitransparent solar cells based on CH₃NH₃PbBr₃ perovskites are attractive for building integration, tandem cells, and electrochemical applications. The lack of control of the CH₃NH₃PbBr₃ perovskite growth limit the exploitation of CH₃NH₃PbBr₃-based perovskite solar cells. Herein, a post-treatment is carried out after the initial CH₃NH₃PbBr₃ crystallization based on methylamine gas that drastically enhances the perovskite quality leading to a highly crystalline film with improved average visible transmittance (AVT) close to 56%. Opaque devices showed outstanding results in terms of open-circuit voltage and power conversion efficiency (PCE) reaching 1.54 V and 9.2%, respectively. These achievements are ascribed to a film with reduced morphological defects and better interface quality and reduced nonradiative pathways. For the first time, the fabrication of semitransparent CH₃NH₃PbBr₃-based solar cells is demonstrated reaching a maximum PCE equal to 7.6%, an AVT of the full stack device of 52%, and an excellent light stability at maximum-power point tracking.

The hydrophobic 4-chlorobenzoic acid (CBA) is used to effectively enhance the hole extraction capacity of the anode and protect the perovskite from water vapor. Ultimately, the optimal device achieves an improved power conversion efficiency (PCE) of 14.32% and the unencapsulated device can maintain more than 95% of the initial PCE in air at 20% relative humidity for 500 h.

Inorganic perovskite solar cells (PSCs) have attracted a lot of attention due to their thermal stability. Among them, CsPbI₂Br are more widely studied due to their reasonable bandgap and phase stability at room temperature. However, poor moisture resistance is an urgent issue to conquer. Herein, solution-processed 4-chlorobenzoic acid (CBA) is used to increase the work function of a Ag electrode, enabling effective hole extraction. More importantly, CBA can effectively avoid the phase conversion of the CsPbI₂Br perovskite from the cubic phase to the undesired nonperovskite δ-phase due to the hygroscopicity of lithium bis (trifluoromethanesulphony)imide (Li-TFSI) in 2,2,7,7-tetrakis(N,N-di-pmethoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD), enhancing the water stability of PSCs. It can also reduce the fabrication cost by replacing the expensive Au electrode and saving the evaporation energy consumption of the commonly used MoO₃ anode buffer layer. Consequently, the champion device achieves a power conversion efficiency (PCE) of 14.3%, with a high open-circuit voltage of 1.21 V. After storing in air at 20% relative humidity (RH) for 500 h, the unencapsulated device maintains more than 95% of the initial PCE.

Herein, defect inhibition in two-step solution-processed (FAPbI₃)_1−x (MAPbBr₃) _x films via a zwitterionic ionic liquid (ZIL) with 4-fluoro-phenylammonium (4FB⁺) as cations and tetrafluoroborate (BF₄ ⁻) as anions is demonstrated. The rationally designed ZIL with 4-fluoro-phenylammonium as doping cation and tetrafluoroborate as pseudohalogen anion could achieve effective defect suppression, enabling perovskite solar cells to exceed 22% efficiency with superior stability.

Defect passivation has been a promising route for enhancing the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Herein, defect inhibition in two-step solution-processed (FAPbI₃)_1−x (MAPbBr₃) _x films via a rationally designed zwitterionic ionic liquid (ZIL) with 4-fluoro-phenylammonium (4FB⁺) as cations and tetrafluoroborate (BF₄ ⁻) as anions is demonstrated. First, 4FB⁺ and BF₄ ⁻ can effectively fill vacancy defects caused by the migrated organic A-site cation and halogen anion, confirmed by X-ray photoelectron spectroscopy. Second, the 4FB⁺ with π conjugated benzene ring can donate electrons for carrier extraction, whereas its fluorination of the phenyl ammonium could enhance moisture blocking through the molecular packing of CF bond. The electrical characterization, including space charge limited current and Mott–Schottky measurement, proves the enhanced carrier extraction and photovoltaic performance. Third, the pseudohalogen anion BF₄ ⁻ with high ionic conductivity could significantly enhance the carrier lifetime and reduce the V _OC loss. As a result, the ZIL-modified PSCs can achieve a high PCE of 22.5% with excellent long-term stability maintaining more than 80% of the initial efficiency after storing in an ambient condition for 2000 h. Herein, a new paradigm toward accelerating the development of efficient and stable PSCs is opened up.

This work demonstrates that the Sn-Ti isostructural substitution effect results in the formation of rutile rather than anatase TiO₂ on the fluorine-doped SnO₂ substrate (FTO). Based on the key growth conditions of the rutile TiO₂ electron transport layer, the dominant distribution of the power conversion efficiency for hole transport layer-free carbon-based planar perovskite solar cells is illustrated and discussed.

Rutile TiO₂ (R-TiO₂) produced by chemical bath deposition (CBD) is widely considered as the desired electron transport layer (ETL) for perovskite solar cells (PSCs). However, the understanding of the growth mechanism of R-TiO₂ ETL and its general regular pattern affecting power conversion efficiency (PCE) is underappreciated. Herein, the growth mechanism of TiO₂ on fluorine-doped SnO₂ substrate (FTO) is demonstrated and it is revealed that pure R-TiO₂, rather than a rutile/anatase mixed crystal, is formed under an Sn–Ti isostructural substitution effect. The similarity of lattice parameters and phase structure between FTO and R-TiO₂ can reduce interface misfit and nucleation barrier, thus boosting heterogeneous nucleation and growth of R-TiO₂ simultaneously. Based on the key growth conditions of the R-TiO₂ ETL, the dominant distribution of PCE for hole transport layer (HTL)-free carbon-based planar perovskite solar cells is illustrated and discussed, and a champion efficiency of 14.0% is achieved.

Modifying the interface between SnO₂ and perovskite by the insertion of a MXene/nanotubes interfacial layer resulted in a remarkable power conversion efficiency of up to 21.42%, which paves the way for employing MXene/nanotubes hybrid structures in next-generation electronic devices.

Abstract

Incorporation of 2D MXenes into the electron transporting layer (ETL) of perovskite solar cells (PSCs) has been shown to deliver high-efficiency photovoltaic (PV) devices. However, the ambient fabrication of the ETLs leads to unavoidable deterioration in the electrical properties of MXene due to oxidation. Herein, sorted metallic single-walled carbon nanotubes (m-SWCNTs) are employed to prepare MXene/SWCNTs composites to improve the PV performance of PSCs. With the optimized composition, a power conversion efficiency of over 21% is achieved. The improved photoluminescence and reduced charge transfer resistance revealed by electrochemical impedance spectroscopy demonstrated low trap density and improved charge extraction and transport characteristics due to the improved conductivity originating from the presence of nanotubes as well as the reduced defects associated with oxygen vacancies on the surface of the SnO₂. The MXene/SWCNTs strategy reported here provides a new avenue for realizing high-performance PSCs.

Bulk organic–inorganic hybrid perovskite single crystals are used to effectively distinguish the circularly polarized light in the near infrared region by chirality-dependent second harmonic generation circular dichroism (SHG-CD) effect with a high anisotropy factor (g_SHG-CD=0.21).

Abstract

The introduction of chirality into organic–inorganic hybrid perovskites (OIHPs) is expected to achieve excellent photoelectric and nonlinear materials related to circular dichroism. Owing to the existence of asymmetric center and intrinsic chirality in the chiral OIHPs, the different efficiencies of second harmonic generation (SHG) signal occurs when the circularly polarized light (CPL) with different phases passes through the chiral crystal, which is defined as second harmonic generation circular dichroism (SHG-CD). Here, the SHG-CD effect is developed in bulk single crystals of chiral one-dimensional (1D) [(R/S)-3-aminopiperidine]PbI₄. It is the first time that CPL is distinguished using chirality-dependent SHG-CD effect in OIHPs bulk single crystals. Such SHG-CD technology extends the detection range to near infrared region (NIR). In this way, the anisotropy factor (g_SHG-CD) through SHG-CD signal is as high as 0.21.

A dopant-free D-π-A type HTM named CI-TTIN-2F has been developed which shows excellent charge collection properties and multisite defect-healing effects. All-inorganic CsPbI₃ PSCs with CI-TTIN-2F HTM feature high efficiencies up to 15.9 %, along with 86 % efficiency retention after 1000 h under ambient conditions. All-inorganic perovskite solar modules were also fabricated that exhibit an efficiency of 11.0 % with a record area of 27 cm².

Abstract

The emerging CsPbI₃ perovskites are highly efficient and thermally stable materials for wide-band gap perovskite solar cells (PSCs), but the doped hole transport materials (HTMs) accelerate the undesirable phase transition of CsPbI₃ in ambient. Herein, a dopant-free D-π-A type HTM named CI-TTIN-2F has been developed which overcomes this problem. The suitable optoelectronic properties and energy-level alignment endow CI-TTIN-2F with excellent charge collection properties. Moreover, CI-TTIN-2F provides multisite defect-healing effects on the defective sites of CsPbI₃ surface. Inorganic CsPbI₃ PSCs with CI-TTIN-2F HTM feature high efficiencies up to 15.9 %, along with 86 % efficiency retention after 1000 h under ambient conditions. Inorganic perovskite solar modules were also fabricated that exhibiting an efficiency of 11.0 % with a record area of 27 cm². This work confirms that using efficient dopant-free HTMs is an attractive strategy to stabilize inorganic PSCs for their future scale-up.

One of the challenges in perovskite solar cells is passivating the perovskite surface without hindering charge extraction. In this work, a conjugated ligand is introduced to the interface between perovskite and hole-transporting layer, showing efficient hole extraction with improved energy level alignment and suppressed phase segregation. Therefore, devices with high efficiency and stability are achieved.

Abstract

Surface passivation is an effective way to boost the efficiency and stability of perovskite solar cells (PSCs). However, a key challenge faced by most of the passivation strategies is reducing the interface charge recombination without imposing energy barriers to charge extraction. Here, a novel multifunctional semiconducting organic ammonium cationic interface modifier inserted between the light-harvesting perovskite film and the hole-transporting layer is reported. It is shown that the conjugated cations can directly extract holes from perovskite efficiently, and simultaneously reduce interface non-radiative recombination. Together with improved energy level alignment and the stabilized interface in the device, a triple-cation mixed-halide medium-bandgap PSC with an excellent power conversion efficiency of 22.06% (improved from 19.94%) and suppressed ion migration and halide phase segregation, which lead to a long-term operational stability, is demonstrated. This strategy provides a new practical method of interface engineering in PSCs toward improved efficiency and stability.

Nature Photonics, Published online: 05 July 2021; doi:10.1038/s41566-021-00829-4

Publication date: October 2021

Source: Nano Energy, Volume 88

Author(s): Yousheng Wang, Hui Ju, Tahmineh Mahmoudi, Chong Liu, Cuiling Zhang, Shaohang Wu, Yuzhao Yang, Zhen Wang, Jinlong Hu, Ye Cao, Fei Guo, Yoon-Bong Hahn, Yaohua Mai

The MA-free organic-inorganic hybrid perovskite (FAPbI₃) have drawn intense attention. The imidazole bromide functionalized graphene quantum dots is introduced to regulate the interface between SnO₂ layer and FAPbI₃ perovskite layer. The resulting reduced interface defects, better energy level alignment, and better perovskite film achieve a high efficiency of 22.37% with enhanced long-term stability.

Abstract

Organic–inorganic hybrid perovskites have reached an unprecedented high efficiency in photovoltaic applications, which makes the commercialization of perovskite solar cells (PSCs) possible. In the past several years, particular attention has been paid to the stability of PSC devices, which is a critical issue for becoming a practical photovoltaic technology. In particular, the interface-induced degradation of perovskites should be the dominant factor causing poor stability. Here, imidazole bromide functionalized graphene quantum dots (I-GQDs) are demonstrated to regulate the interface between the electron transport layer (ETL) and formamidinium lead iodide (FAPbI₃) perovskite layer. The incorporation of I-GQDs not only reduces the interface defects for achieving a better energy level alignment between ETL and perovskite, but also improves the film quality of FAPbI₃ perovskite including enlarged grain size, lower trap density, and a longer carrier lifetime. Consequently, the planar FAPbI₃ PSCs with I-GQDs regulation achieve a high efficiency of 22.37% with enhanced long-term stability.

The synergistic strategy of tuning the solution coordination and crystallization process by introducing ionic liquid is implemented to successfully fabricate pinhole-free tin perovskite films with preferential crystal orientation, which possess improved oxidation repellency for Sn(II) and enhanced hydrophobicity. As a result, the stabilization of high-efficiency lead-free tin halide perovskite solar cells is achieved.

Abstract

Tin halide perovskites attract incremental attention to deliver lead-free perovskite solar cells. Nevertheless, disordered crystal growth and low defect formation energy, related to Sn(II) oxidation to Sn(IV), limit the efficiency and stability of solar cells. Engineering the processing from perovskite precursor solution preparation to film crystallization is crucial to tackle these issues and enable the full photovoltaic potential of tin halide perovskites. Herein, the ionic liquid n-butylammonium acetate (BAAc) is used to tune the tin coordination with specific O…Sn chelating bonds and NH…X hydrogen bonds. The coordination between BAAc and tin enables modulation of the crystallization of the perovskite in a thin film. The resulting BAAc-containing perovskite films are more compact and have a preferential crystal orientation. Moreover, a lower amount of Sn(IV) and related chemical defects are found for the BAAc-containing perovskites. Tin halide perovskite solar cells processed with BAAc show a power conversion efficiency of over 10%. This value is retained after storing the devices for over 1000 h in nitrogen. This work paves the way toward a more controlled tin-based perovskite crystallization for stable and efficient lead-free perovskite photovoltaics.

Nature Materials, Published online: 01 July 2021; doi:10.1038/s41563-021-01035-x

Publication date: October 2021

Source: Nano Energy, Volume 88

Author(s): Jie Xu, Jun Xi, Hua Dong, Namyoung Ahn, Zonglong Zhu, Jinbo Chen, Peizhou Li, Xinyi Zhu, Jinfei Dai, Ziyang Hu, Bo Jiao, Xun Hou, Jingrui Li, Zhaoxin Wu