Chen Weijie

A large‐grained CsPbBr₃ perovskite film with improved energy‐level alignment and hole mobility is fabricated by compositional engineering of Cl ion doping, which suppresses charge recombination thus affording a champion power conversion efficiency (PCE) as high as 9.73% for carbon‐based all‐inorganic CsPbBr_2.98Cl_0.02 PSC free of encapsulation with excellent operational stability.

Carbon‐based CsPbBr₃ perovskite solar cells (PSCs) without hole‐transporting layers (HTLs) have aroused extensive attention due to their low manufacturing cost and prominent ambient stability. However, the defects of perovskite film and the poor charge extraction within PSCs result in severe charge recombination, which restricts the further enhancement of device efficiency. In view of this critical point, a compositional engineering of CsPbBr₃ perovskite via doping with Cl⁻ ions is presented herein to decrease the trap states and enhance the charge extraction. It is revealed that the doping of Cl⁻ ions not only enlarges the grain size and thereby reduces the trap‐state density, but also optimizes the energy‐level alignment and improves the hole mobility of the perovskite film, leading to an evidently suppressed charge recombination and improved charge extraction and transportation. As a result, a champion power conversion efficiency (PCE) of 9.73% is achieved for carbon‐based HTL‐free CsPbBr_2.98Cl_0.02 PSC, yielding a marked enhancement in comparison with 6.69% efficiency for the control. Meanwhile, the thermal and moisture stabilities of unencapsulated CsPbBr_2.98Cl_0.02 PSC are improved, maintaining 93% and 95% of the initial PCE after expose to air atmosphere with 80% relative humidity (RH) and at 80 °C over 60 days, respectively.

The defect in perovskite film is one of the most non‐negligible factors that can attenuate the performances of perovskite solar cell. This work fabricates defects‐reduced perovskite film by using the lead indicator (dithizone) as an additive of perovskite functional layer. The dithizone can retard the crystallization rate of perovskite films, passivate the defects, and enhance the structure stability of perovskite by coordinating with lead atoms. As a result, the device doped with dithizone yields outstanding power conversion efficiency and stability.

Publication date: Available online 12 August 2020

Source: Nano Energy

Author(s): George Syrrokostas, Alexandros Dokouzis, Spyros N. Yannopoulos, George Leftheriotis

Nature Communications, Published online: 11 August 2020; doi:10.1038/s41467-020-17868-0

A multi‐layer front contact system is investigated for efficient perovskite solar cells, which allows for realizing improved light incoupling by reducing the optical losses. The front contact consists of a thin nickel‐oxide (electron‐beam vapor physical deposition‐grown), and a thick pyramidal textured zinc‐oxide (metal‐organic chemical vapor deposition‐grown) layers. Optics and electrical characteristics of solar cells are investigated by 3D electromagnetic simulations.

Abstract

Efficient hole transport layer (HTL) is crucial for realizing efficient perovskite solar cells (PSCs). In this study, nickel‐oxide (NiO_X) thin‐films are investigated as a potential HTL for PSCs. The NiO_X films are prepared by electron‐beam physical vapor deposition at low temperatures. The crystalline properties and the work function are determined by X‐ray diffraction and photoelectric yield spectroscopy. The transmission and the complex refractive index of the films are determined by optical spectroscopy and ellipsometry. Furthermore, PSCs are fabricated and characterized. The short‐circuit current density (J _sc) of the PSC is limited by the optical loss due to the NiOx front contact. The optical losses of the front contact are quantified by optical simulations using finite‐difference time‐domain simulations, and a solar cell structure with improved light incoupling is designed. Furthermore, the electrical characteristics of the solar cell are simulated by finite element method simulations. As a result, it is found that the optical losses can be reduced by 70%, and the light incoupling can be improved so that the J _SC can be increased by up to 12%, allowing for the realization of PSCs with an energy conversion efficiency of 22%. Findings from the numerical simulations are compared with experimentally realized results.

The photoactive phase of CsPbI₃ stabilized with various metal dopants. The doping of Sr²⁺, Ba²⁺, or Sn²⁺ stabilizes the α‐phase, while that of Sb³⁺ or Bi³⁺ stabilizes the β‐phase. Sn²⁺ and Ba²⁺ doped thin films exhibit poor air stability compared to other metal dopants, while Sb³⁺ or Bi³⁺ doping stabilizes CsPbI₃ thin films, making them metastable.

Abstract

Low‐temperature α‐phase stabilization using HI or zwitterions in cesium lead iodide (CsPbI₃) endures the metastable phase properties but is thermally unstable. Doping with a small amount of heterovalent metals (i.e., Bi³⁺, Sb³⁺) in CsPbI₃ has been assumed to stabilize the α‐phase, while here this assumption is challenged. It is demonstrated that heterovalent metal ion doping stabilizes β‐CsPbI₃ at low temperatures without replacing the Pb²⁺ cations, while divalent cations (i.e., Ba²⁺, Sr²⁺, and Sn²⁺) doping stabilizes the α‐CsPbI₃ by replacing the Pb²⁺ cations. This finding is demonstrated by both theoretical and experimental results. It is also found that the divalent cations stabilize α‐CsPbI₃ films, making thermally stable at high temperatures, whereas heterovalent metal‐doping stabilizes β‐CsPbI₃ films, making metastable. The doping influence on crystal grains and the chemical composition of thin films is discussed. In particular, the charge dissociation kinetics for the Sr doped thin film are much enhanced than α‐CsPbI₃ and Ba doped thin films, also the initial results of the fabricated perovskite red‐light‐emmiting diode suggests that the Sr‐doped thin films would be more suitable for the device fabrication. These findings will guide a way for further development in thermally and air‐stable optoelectronic devices.

Water is added into the precursor solution to assist crystal growths of quasi‐2D perovskite films featuring ordered phase distribution and favored crystal orientation. A champion efficiency of 18.04% is realized in (BA)₂(MA_0.8FA_0.15Cs_0.05)₄Pb₅I₁₆‐based quasi‐2D perovskite solar cells.

Abstract

Organic–inorganic hybrid quasi‐2D perovskites have shown excellent stability for perovskite solar cells (PSCs), while the poor charge transport in quasi‐2D perovskites significantly undermines their power conversion efficiency (PCE). Here, studies on water‐controlled crystal growth of quasi‐2D perovskites are presented to achieve high‐efficiency solar cells. It is demonstrated that the (BA)₂MA₄Pb₅I₁₆‐based PSCs (n = 5) processed with water‐containing precursors display an increased short‐circuit current density (J _sc) of 19.01 mA cm⁻² and PCE over 15%. The enhanced performance is attributed to synergetic growths of the 3D and 2D phase components aided by the formed hydration (MAI∙H₂O), leading to modulations on the crystal orientation and phase distribution of various n‐value components, which facilitate interphase charge transfer and charge sweepout throughout the device. The water‐assisted crystallization is further applied to triple cation‐based (BA)₂(MA_0.8FA_0.15Cs_0.05)₄Pb₅I₁₆ quasi‐2D perovskites, which generate a remarkable PCE of 18.04%. Despite the presence of water in the precursors, the devices exhibit a satisfactory thermal stability with the PCE degradation <15% under continuous thermal aging at 60 °C for over 500 h.

An ultra‐flexible and transparent biomass‐derived conductive substrate is fabricated from polylactic acid. It exhibits high mechanical durability even when subjected to 15 000 bending cycles. Perovskite solar cells based on the biomass electrodes show good mechanical stability, retaining over 86% of its original power conversion efficiency after bending 1500 times at a curvature radius of 5 mm.

Biomass substrates are urgently needed to develop green electronics. Herein, an ultra‐flexible and transparent biomass‐derived conductive substrate is originated from nature polylactic acid (PLA) with silver nanowires (AgNWs) modified by PH1000. The composite electrode exhibits low sheet resistance of 25 Ω sq⁻¹, high transmittance (over 82% in the region of 400–800 nm), and excellent mechanical durability. After bending tests of 15 000 times at a curvature radius of 3 and 5 mm, the sheet resistances of the composite electrodes only increase to 89 and 51 Ω sq⁻¹, respectively. Biomass electrode–based flexible perovskite solar cells are demonstrated with a champion power conversion efficiency (PCE) of 11.44% and high bending tolerance with preserving over 86% of the initial PCE after 1500 bending cycles at a curvature radius of 5 mm. The biomass electrode exhibits great potential for the development of green flexible devices.

Herein, allylammonium cations (CH₂CH₂CH₃NH₃ ⁺) are incorporated into tin‐based perovskite to induce a preferred crystal orientation, leading to enhanced carrier transport and reduced trap density in the lead‐free films. The photovoltaic devices with an optimized perovskite layer exhibit a high power conversion efficiency of 9.48% and improved stability.

The low power conversion efficiency (PCE) of tin‐based perovskite solar cells is mainly caused by severe Sn²⁺ oxidation and trap state formation. Herein, the reduction of Sn⁴⁺ content and trap density is realized through the formation of quasi‐2D tin‐based perovskite by incorporating allylammonium (ALA) cations into formamidinium tin iodide (FASnI₃). The composition modification substantially enhances the crystallinity and morphology of the perovskite films, leading to a preferred crystal orientation that facilitates charge carrier transport. After optimization, a high maximum PCE of 9.48% is achieved in a planar p‐i‐n photovoltaic device based on (ALA)₂(FA) _n−1Sn _n X_3n+1 (<n> = 25). In the meantime, the device also shows improved stability compared with the FASnI₃‐based one.