Europium-Doped CsPbI2Br for Stable and Highly Efficient Inorganic Perovskite Solar Cells

All-inorganic perovskite films hold promise for improving the stability of perovskite solar cells (PSCs). However, the 3D alpha phase of narrow-bandgap inorganic perovskites is thermodynamically unstable at room temperature, limiting the development of high-performance inorganic PSCs. Here, we show that europium doping of CsPbI2Br stabilizes the alpha phase of this 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 amaximum power-conversion efficiency of 13.71% for an inorganic PSC with the CsPb0.95Eu0.05I2Br perovskite and alpha stable power output of 13.34%. Using electroluminescence we show that incorporation of europium reduces non-radiative recombination, resulting in high open-circuit voltage of 1.27 V. The devices retain 93% of the initial efficiency after 370 hr under 100 mW cm(-2) continuous white light illumination under maximum-power point-tracking measurement.

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

Benzylammonium‐Mediated Formamidinium Lead Iodide Perovskite Phase Stabilization for Photovoltaics

Anwar Qasem M Alanazi, Masaud Hassan S Almalki, Felix Thomas Eickemeyer, David Lyndon Emsley, Michael Graetzel, Ulf Anders Hagfeldt, Dominik Józef Kubicki, Jovana Milic, Aditya Mishra, Dan Ren, Zaiwei Wang, Shaik Mohammed Zakeeruddin, Hong Zhang

There is an ongoing surge of interest in the use of formamidinium (FA) lead iodide perovskites in photovoltaics due to their exceptional optoelectronic properties. However, thermodynamic instability of the desired cubic perovskite (α-FAPbI3) phase at ambient conditions leads to the formation of a yellow non-perovskite (δ-FAPbI3) phase that compromises its utility. A stable α-FAPbI3 perovskite phase is achieved by employing benzylammonium iodide (BzI) and the microscopic structure is elucidated by using solid-state NMR spectroscopy and X-ray scattering measurements. Perovskite solar cells based on the FAPbI3(BzI)0.25 composition achieve power conversion efficiencies exceeding 20%, which is accompanied by enhanced shelf-life and operational stability, maintaining 80% of the performance after one year at ambient conditions.

2021

Defect Suppression in Oriented 2D Perovskite Solar Cells with Efficiency over 18% via Rerouting Crystallization Pathway

Yong Ding, Cheng Liu, Xuehui Liu, Mohammad Khaja Nazeeruddin, Olga Syzgantseva, Yi Yang

Vertically oriented 2D perovskites exhibit promising optoelectronic properties and intrinsic stability, but their photovoltaic application is still limited by the low power conversion efficiency (PCE) compared to 3D analogs. Here, a new crystallization pathway (RCP) is reported to suppress defects in vertically oriented 2D perovskite caused by its over-rapid self-assembly behavior. By controlling the specific adsorption of an ammonium halide additive on different perovskite crystal planes, the dynamic preferred growth of (111) plane is intentionally restrained, and the minority (202) planes emerge as secondary nucleation sites to stimulate the creation of large grains. As the halogen-regulated deprotonation of ammonium proceeds, the (111) crystal plane gradually recovers its growth dominance, and a vertically oriented 2D perovskite film finally forms with high homogeneity, reduced trap density of states, and desired carrier transport/collection kinetics. Solar cells using RCP-2D films show a highly reproducible and stable PCE reaching 18.5% with a high fill factor of 83.4%. These findings provide critical missing information on simultaneously achieving highly oriented and less defective 2D perovskite films for excellent device performance.

2021

Formation and stabilization of all-inorganic perovskites for photovoltaics

Zaiwei Wang

EPFL2020