Retarding Thermal Degradation in Hybrid Perovskites by Ionic Liquid Additives

Recent years have witnessed considerable progress in the development of solar cells based on lead halide perovskite materials. However, their intrinsic instability remains a limitation. In this context, the interplay between the thermal degradation and the hydrophobicity of perovskite materials is investigated. To this end, the salt 1-(4-ethenylbenzyl)-3-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylimidazolium iodide (ETI), is employed as an additive in hybrid perovskites, endowing the photoactive materials with high thermal stability and hydrophobicity. The ETI additive inhibits methylammonium (MA) permeation in methylammonium lead triiodide (MAPbI(3)) occurring due to intrinsic thermal degradation, by inhibiting out-diffusion of the MA(+) cation, preserving the pristine material and preventing decomposition. With this simple approach, high efficiency solar cells based on the unstable MAPbI(3) perovskite are markedly stabilized under maximum power point tracking, leading to greater than twice the preserved efficiency after 700 h of continuous light illumination and heating (60 degrees C). These results suggest a strategy to tackle the intrinsic thermal decomposition of MAI, an essential component in all state-of-the-art perovskite compositions.

Bi-functional interfaces by poly(ionic liquid) treatment in efficient pin and nip perovskite solar cells

Antonio Abate, Pietro Caprioglio, Filippo de Angelis, Giulia Grancini, Mohammad Khaja Nazeeruddin, Albertus Adrian Sutanto, Christian Michael Wolff

Approaches to boost the efficiency and stability of perovskite solar cells often address one singular problem in a specific device configuration. In this work, we utilize a poly(ionic liquid) (PIL) to introduce a multi-functional interlayer to improve the device efficiency and stability for different perovskite compositions and architectures. The presence of the PIL at the perovskite surface reduces the non-radiative losses down to 60 meV already in the neat material, indicating effective surface trap passivation, thereby pushing the external photoluminescence quantum yield up to 7%. In devices, the PIL treatment induces a bi-functionality of the surface where insulating areas act as a blocking layer reducing interfacial charge recombination and increasing the V-OC, whereas, at the same time, the passivated neighbouring regions provide more efficient charge extraction, increasing the FF. As a result, these solar cells exhibit outstanding V-OC and FF values of 1.17 V and 83% respectively, with the best devices reaching conversion efficiencies up to 21.4%. The PIL-treated devices additionally show enhanced stability during maximum power point tracking (>700 h) and unchanged efficiencies after 10 months of shelf storage. By applying the PIL to small and wide bandgap perovskites, and to nip cells, we corroborate the generality of this methodology to improve the efficiency in various cell architectures and perovskite compositions.

ROYAL SOC CHEMISTRY2021

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

Essa Awadh R Alharbi, Masaud Hassan S Almalki, Thomas Paul Baumeler, Felix Thomas Eickemeyer, Ulf Anders Hagfeldt, Anurag Krishna, Mounir Driss Mensi, Olivier Ouellette, Linfeng Pan, Zaiwei Wang, Bowen Yang, Shaik Mohammed Zakeeruddin

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.

WILEY-V C H VERLAG GMBH2021

Formation and stabilization of all-inorganic perovskites for photovoltaics

Zaiwei Wang

EPFL2020