Chen Weijie

Molecular additive engineering using chlorine‐based compounds such as formamidinium chloride reduces the bulk and surface carrier recombination and improves the crystallinity of the perovskite film, resulting in solar cell devices with high efficiency exceeding 21% and great stability.

Abstract

The presence of surface and grain boundary defects in organic–inorganic halide perovskite films can be detrimental to both the performance and operational stability of perovskite solar cells (PSCs). Here, the effect of chloride additives is studied on the bulk and surface defects of the mixed cation and halide PSCs. It is found that using an antisolvent technique, the perovskite film is divided into two layers, i.e., a bottom layer with large grains and a thin capping layer with small grains. The addition of formamidinium chloride (FACl) into the precursor solution removes the small‐grained perovskite capping layer and suppresses the formation of bulk and surface defects, providing a perovskite film with enhanced crystallinity and large grain size of over 1 µm. Time‐resolved photoluminescence measurements show longer lifetimes for perovskite films modified by FACl and subsequently passivated by 1‐adamantylamine hydrochloride as compared to the reference sample. Impedance spectroscopy measurements show that these treatments reduce the recombination in the PSCs, leading to a champion device with power conversion efficiency (PCE) of 21.2%, an open circuit voltage of 1152 mV and negligible hysteresis. The Cl treated PSC also shows improved operational stability with only 12% PCE loss after 700 h under continuous illumination.

It has been discovered that the existence of a potassium‐rich phase on the surface of 3D perovskite crystals passivates the grain boundaries, which can dramatically suppress the trap states as well as enhance dielectric confinement.

Recently, considerable researches have reported that the incorporation of potassium cation (K⁺) in lead halide perovskites obviously improves the performance of perovskite solar cells. The recent finding indicates that the interstitial occupancy position of K⁺ in perovskite lattice can increase ion‐migration barrier, which dramatically suppresses the hysteresis of the device. However, the enhancement of photoluminescence (PL) as well as bandgap variation cannot be interpreted by K⁺ interstitial state in perovskite. Considering this discrepancy, it has been found out that potassium‐rich phase grows in three dimensional (3D) perovskite crystal grain boundary through both experiments and theoretical simulations, which can efficiently passivate the grain boundaries on the 3D crystal surface and decrease trap states. Meanwhile, the dielectric confinement effect between potassium‐rich phase and 3D perovskite crystal, contributing to improvement of radiative recombination in perovskite absorber, can further support the enhancement and red‐shift of photoluminescence (PL) spectrum. Therefore, a power conversion efficiency of 20.4% has been achieved in K⁺ doped halide perovskite solar cell, and still maintains 90% initial efficiency after stored in 30% humidity at room temperature for 1000 h. These findings offer a new path for the structural manipulation by incorporating multi cations in perovskite materials.

Publication date: May 2019

Source: Nano Energy, Volume 59

Author(s): Zhao Wang, Ming Feng, Hao Sun, Gaoran Li, Qiang Fu, Haibo Li, Jia Liu, Liqun Sun, Alain Mauger, Christian M. Julien, Haiming Xie, Zhongwei Chen

Abstract

Graphical abstract

Owing to the facile method of Al addition, the conductivity, electron mobility, and band alignment at the SnO₂/perovskite interface have been dramatically improved. Besides, better surface coverage of SnO₂ films and lower defect density of the perovskite film are obtained. Benefiting from these advantages, the devices based on the Al: SnO₂ film exhibit a significant improvement in power conversion efficiency.

Owing to its splendid electrical and optical properties, tin oxide (SnO₂) has been proven to be an effective electron transport layer (ETL) material for high‐efficiency perovskite solar cells (PSCs). However, the surface coverage, conductivity, and energy loss at the SnO₂/perovskite interface still have room for improvement. Herein, a facile method by mixing a SnO₂ QD solution with an aluminum (Al) chloride precursor solution at room temperature to achieve the addition of Al into the SnO₂ QD (Al: SnO₂) precursor is proposed. Based on this strategy, conductivity, electron mobility, and band alignment with the perovskite layer have been significantly improved. Besides, the introduction of Al also increases the coverage of the SnO₂ film, consequently contribute to improving the capability to block the charge transfer from FTO to the ETL. Furthermore, fewer defect states are also demonstrated for the perovskite films deposited on Al: SnO₂ films than the control samples. With the optimized addition ratio of 5%, the devices exhibit an average efficiency (PCE) of 17.01%, which is superior to that of the control device of 15.80%. The champion device using Al: SnO₂ ETL delivers an impressive PCE of 18.20%. This research indicates that the low‐temperature solution‐processed Al: SnO₂ is a promising ETL for high‐efficiency PSCs.

Unveiling the operation mechanism of layered perovskite solar cells

Unveiling the operation mechanism of layered perovskite solar cells, Published online: 01 March 2019; doi:10.1038/s41467-019-08958-9

Continuous wave amplified spontaneous emission in phase-stable lead halide perovskites

Continuous wave amplified spontaneous emission in phase-stable lead halide perovskites, Published online: 28 February 2019; doi:10.1038/s41467-019-08929-0

The series resistance of perovskite solar cells at their maximum power point is measured using the J_sc–V_oc technique. This technique also probes the limiting fill factor of the perovskite solar cell if the device was resistance‐free. Further investigation reveals that PTAA is an effective hole‐selective contact (capable of sustaining a high V_oc ) but is resistive to the holes it collects.

The fill factor (FF) of perovskite solar cells is considerably lower than that of gallium arsenide and silicon cells, though they have similar open‐circuit voltage deficits. To probe the FF loss, which mainly comes from series resistance, the J_sc –V_oc characterization technique is applied to perovskite solar cells. A continuous‐lamp solar simulator with an array of neutral density filters is used instead of the quasi‐steady‐state photoconductance technique commonly employed for silicon cells, which allows us to tune sweep parameters to accommodate the complex behavior of perovskites such as hysteresis. It is found that, for Cs_0.25FA_0.75Pb(Br_0.2I_0.8)₃ (CsFA) perovskite cells, sweeping from positive to negative voltage yields the same series resistance regardless of sweep speed, whereas this is not the case if the sweep is reversed. However, for CH₃NH₃PbI₃ perovskite cells, the series resistance is independent of the sweep speed in both sweep directions. It is also found that, for a poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA) hole contact, increasing the PTAA thickness barely changes the recombination‐limited pseudo‐FF, but reduces the FF due to increased series resistance. A maximum FF of 80.9% was achieved with the PTAA hole contact on a CsFA perovskite absorber.

A novel phenyl substituted benzo(1,2‐b:4,5‐b’)dithiophene (BDT) derivative containing both fluorine and sulfur atoms is designed and synthesized. A power conversion efficiency of 13.91% is achieved with a high open‐circuit voltage of 1.01 V, and a large short‐circuit current density of 18.51 mA cm⁻². The result demonstrates that PBTA‐PSF is a promising candidate for high‐performance donor polymers.

A novel phenyl substituted benzo(1,2‐b:4,5‐b’)dithiophene (BDT) derivative containing both fluorine and sulfur atoms is designed and synthesized. Furthermore, a wide bandgap polymer PBTA‐PSF based on the derivative shows a low highest occupied molecular orbital energy level and slightly reduces the donor materials’ optical bandgap, which has a complementary absorption with a narrow‐bandgap n‐type small molecule ITIC. As a result, a power conversion efficiency of 13.91% is achieved with a high open‐circuit voltage of 1.01 V, and a large short‐circuit current density of 18.51 mA cm⁻². The result demonstrates that PBTA‐PSF is a promising candidate for high‐performance donor and phenyl‐containing BDT derivatives and has potential in the design of high‐performance polymers for organic photovoltaics.

Solar‐blind deep ultraviolet light photodetectors (DUVPDs) have been receiving increasing research interest lately due to their promising application in military surveillance, target detection, and flame detection. In article number 1806006, Feng‐Xia Liang, Lin‐Bao Luo, Yu‐Cheng Wu, and co‐workers summarize the recent advances in the development of various DUVPDs based on different kinds of inorganic ultrawide bandgap semiconductors such as Ga₂O₃, Mg _x Zn_1−x O, III‐nitride compounds, and diamonds.

Prospects for low-toxicity lead-free perovskite solar cells

Prospects for low-toxicity lead-free perovskite solar cells, Published online: 27 February 2019; doi:10.1038/s41467-019-08918-3

Publication date: May 2019

Source: Nano Energy, Volume 59

Author(s): Zhen Wang, Ajay K. Baranwal, Muhammad Akmal kamarudin, chi huey Ng, Manish Pandey, Tingli Ma, Shuzi Hayase

Abstract

Graphical abstract

Xanthate-induced sulfur doped CsPbIBr₂ perovskite꞉ Cesium Xanthate (CsXth) can be decomposed to realize sulfur doped in perovskite to stabilize ɑ-phase of CsPbIBr₂. 210-fold stability improvement was achieved after sulfur doped in perovskite at humid air with 65% relative humidity (RH). phase segregation and device hysteriesis was suppressed effectively due to the existence of sulfur with strong electronegativity. We obtained a champion efficiency of 9.78% with the highest V_oc of 1.30 V compared with that of 5.16% for the reference device. devices performance shows almost no decay in 10 h under humid air with 65% RH.

The crucial step of stacking photovoltaics is the close contact of two laminated electrodes. Interfacial elastic contact can achieve it. Because the connecting electro‐des can realize convex–concave form by spraying the specific p‐type organic semiconductor and the elastic deformation of the core achieves a desirable conductive contact between the conductive films with sufficient contact area.

Abstract

Stacking perovskite solar cells (PSCs) are emerging cells with two detached electrodes during fabrication process, in which the crucial step is the intimate mechanical and electronic contact between the laminated parts. The interfacial elastic contact of stacking PSCs with four different p‐type organic semiconductors has been systematically studied. Studies confirm that poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is an elastic interface material with excellent performance. The convex–concave morphology of PEDOT:PSS surface and the interfacial elastic deformation make a contribution to a desirable conductive contact between two electrodes with sufficient contact area, thereby providing a highly reliable connection and efficient charge transport channel between the semi‐cells. Based on conducting polymer PEDOT:PSS, planar‐structured stacking PSCs are fabricated exhibiting power conversion efficiency of 13.49% with 1.0 cm² area. Connecting electrodes with excellent conductivity and elasticity are also proved to be essential to the flexible assembly and readily detachment.

Alkyl‐amine bound organolead halides are soluble in chlorobenzene. This finding prompts the authors to prepare perovskite precursor solutions using this nonpolar solvent and produce perovskite devices from conventionally layered to specially designed structures with consecutive (CH₃NH₃)PbI₃/(C₄H₉NH₃)₂PbI₄ double layers or PTAA@(CH₃NH₃)PbI₃ blend layers. This study is the first to use a nonpolar solvent for perovskite material processing, device fabrication, and architecture design.

Chlorobenzene (CB), which has been used as an anti‐solvent, is demonstrated here as a processing solvent for the fabrication of perovskite films and solar cells. This approach results from the unexpected finding that butylamine‐bound organolead iodides can be dissolved in CB to form precursor solutions. Together with previously established perovskite composition transformation methods, not only conventionally structured perovskite solar cells (PSCs) with a (CH₃NH₃)PbI₃ active layer but also nonconventionally structured devices with a consecutive (CH₃NH₃)PbI₃/(C₄H₉NH₃)₂PbI₄ double layer or a mixed poly(triaryl amine) (PTAA) and (CH₃NH₃)PbI₃ bulk heterojunction layer are successfully prepared and demonstrated with an optimal efficiency of 16.44%. This is the first time that nonpolar solvents are used for perovskite material processing, which can remove the solvent limitation and open a new avenue for designing and preparing perovskite devices with desired structures.