Formation of High-Performance Multi-Cation Halide Perovskites Photovoltaics by delta-CsPbI3/delta-RbPbI3 Seed-Assisted Heterogeneous Nucleation

The performance of perovskite solar cells is highly dependent on the fabrication method; thus, controlling the growth mechanism of perovskite crystals is a promising way towards increasing their efficiency and stability. Herein, a multi-cation halide composition of perovskite solar cells is engineered via the two-step sequential deposition method. Strikingly, it is found that adding mixtures of 1D polymorphs of orthorhombic delta-RbPbI3 and delta-CsPbI3 to the PbI2 precursor solution induces the formation of porous mesostructured hexagonal films. This porosity greatly facilitates the heterogeneous nucleation and the penetration of FA (formamidinium)/MA (methylammonium) cations within the PbI2 film. Thus, the subsequent conversion of PbI2 into the desired multication cubic alpha-structure by exposing it to a solution of formamidinium methylammonium halides is greatly enhanced. During the conversion step, the delta-CsPbI3 also is fully integrated into the 3D mixed cation perovskite lattice, which exhibits high crystallinity and superior optoelectronic properties. The champion device shows a power conversion efficiency (PCE) over 22%. Furthermore, these devices exhibit enhanced operational stability, with the best device retaining more than 90% of its initial value of PCE under 1 Sun illumination with maximum power point tracking for 400 h.

Atomic-level passivation mechanism of ammonium salts enabling highly efficient perovskite solar cells

Anwar Qasem M Alanazi, Essa Awadh R Alharbi, Andrés Burgos Caminal, David Lyndon Emsley, Fabrizio Giordano, Dominik Józef Kubicki, Jingshan Luo, Jacques-Edouard Moser, Alexander Roland Uhl, Brennan James Walder, Shaik Mohammed Zakeeruddin

The high conversion efficiency has made metal halide perovskite solar cells a real break-through in thin film photovoltaic technology in recent years. Here, we introduce a straight-forward strategy to reduce the level of electronic defects present at the interface between the perovskite film and the hole transport layer by treating the perovskite surface with different types of ammonium salts, namely ethylammonium, imidazolium and guanidinium iodide. We use a triple cation perovskite formulation containing primarily formamidinium and small amounts of cesium and methylammonium. We find that this treatment boosts the power conversion efficiency from 20.5% for the control to 22.3%, 22.1%, and 21.0% for the devices treated with ethylammonium, imidazolium and guanidinium iodide, respectively. Best performing devices showed a loss in efficiency of only 5% under full sunlight intensity with maximum power tracking for 550 h. We apply 2D-solid-state NMR to unravel the atomic-level mechanism of this passivation effect.

2019

Atomic-Level Microstructure of Efficient Formamidinium-Based Perovskite Solar Cells Stabilized by 5-Ammonium Valeric Acid Iodide Revealed by Multinuclear and Two-Dimensional Solid-State NMR

Anwar Qasem M Alanazi, Essa Awadh R Alharbi, Marine Eva Fedora Bouduban, David Lyndon Emsley, Fabrizio Giordano, Michael Graetzel, Farzaneh Jahanbakhshi, Dominik Józef Kubicki, Jovana Milic, Marko Mladenovic, Jacques-Edouard Moser, Daniel Prochowicz, Dan Ren, Ursula Röthlisberger, Shaik Mohammed Zakeeruddin

Chemical doping of inorganic–organic hybrid perovskites is an effective way of improving the performance and operational stability of perovskite solar cells (PSCs). Here we use 5-ammonium valeric acid iodide (AVAI) to chemically stabilize the structure of α-FAPbI3. Using solid-state MAS NMR, we demonstrate the atomic-level interaction between the molecular modulator and the perovskite lattice and propose a structural model of the stabilized three-dimensional structure, further aided by density functional theory (DFT) calculations. We find that one-step deposition of the perovskite in the presence of AVAI produces highly crystalline films with large, micrometer-sized grains and enhanced charge-carrier lifetimes, as probed by transient absorption spectroscopy. As a result, we achieve greatly enhanced solar cell performance for the optimized AVA-based devices with a maximum power conversion efficiency (PCE) of 18.94%. The devices retain 90% of the initial efficiency after 300 h under continuous white light illumination and maximum-power point-tracking measurement.

2019

Formation and stabilization of all-inorganic perovskites for photovoltaics

Zaiwei Wang

All-inorganic perovskite films hold promise for improving stability of perovskite solar cells. However, the desirable narrow-bandgap inorganic perovskites (such as CsPbI3 and CsPbI2Br) are suffering from the thermodynamic phase instability, as the photoactive black phase (Î±, Î², or Î³ phase) spontaneously transforms into a more thermodynamically stable yellow phase (ÎŽ-phase) in ambient condition, which is limiting the further development of high-performance inorganic PSCs. The stabilization of black phase and the formation of high-quality perovskite films are promoting the remarkable progress for the performance and stability of inorganic perovskite solar cells. Firstly, we show that europium doping of CsPbI2Br stabilizes the black phase of narrow band gap inorganic perovskite at room temperature. We rationalize it by using solid-state nuclear magnetic resonance and high-angle annular dark-field scanning transmission electron microscopy, which show that europium is incorporated into the perovskite lattice. We demonstrate a power-conversion efficiency (PCE) of 13.71% for an inorganic PSC with the composition CsPb0.95Eu0.05I2Br perovskite. Stability tests show that the devices retain 93% of the initial efficiency after 370 hours under maximum power point tracking measurement. Secondly, we demonstrate a function of dopants by adding barium in CsPbI2Br. We find that barium is not incorporated into the perovskite lattice but induces phase segregation, resulting in a change in the iodide/bromide ratio compared to the precursor stoichiometry and consequently a reduction in the band gap energy of the perovskite phase. The device with 20 mol% barium shows a high PCE of 14.0% with a high open-circuit voltage (Voc) of 1.33 V. Then we propose an intermediate-phase engineering strategy to improve the inorganic perovskite/metal oxide interface by utilizing volatile salts. The introduction of organic cations (such as methylammonium and formamidinium), which can be doped into the perovskite lattice, leads to the formation of an organic-inorganic hybrid perovskite intermediate phase, promoting a robust interfacial contact through hydrogen bonding. A champion CsPb(I0.75Br0.25)3-based device with a PCE of 17.0% and a Voc of 1.34 V was realized. Last, the intermediate phase engineering strategy is reported to promote the pure ÃÂ³-phase CsPbI3 perovskite film formation. According to the investigation of the phase transformation processes of pure CsPbI3 and that with different additives, including NH4I, BAI, MAI, FAI and DMAI, we found that the formation of a 3D organic-inorganic hybrid perovskite intermediate phase is essential to avoid the generation of undesired yellow phase and promote the formation of pure black-phase CsPbI3 film. A champion PCE of 17.70% with a stabilized power output of 17.58% based on the optimized CsPbI3-DMAI device is achieved. Overall, we investigate two possible scenarios for element doping: (1) the dopants are incorporated into the crystal lattice for the improvement of stability and efficiency of inorganic PSCs; (2) the dopants form separate phases to induce the halide phase segregation and suppress non-radiative recombination of perovskite films. Besides, based on the formation of 3D organic-inorganic perovskite intermediate phase, we not only promote the formation of pure black phase CsPbI3, but also promotes a robust interfacial contact of inorganic perovskite and metal oxide transport layers.

EPFL2020

Formation and stabilization of all-inorganic perovskites for photovoltaics

Zaiwei Wang

EPFL2020