Methylamine Gas Treatment Affords Improving Semitransparency, Efficiency, and Stability of CH3NH3PbBr3-Based Perovskite Solar Cells

High bandgap semitransparent solar cells based on CH3NH3PbBr3 perovskites are attractive for building integration, tandem cells, and electrochemical applications. The lack of control of the CH3NH3PbBr3 perovskite growth limit the exploitation of CH3NH3PbBr3-based perovskite solar cells. Herein, a post-treatment is carried out after the initial CH3NH3PbBr3 crystallization based on methylamine gas that drastically enhances the perovskite quality leading to a highly crystalline film with improved average visible transmittance (AVT) close to 56%. Opaque devices showed outstanding results in terms of open-circuit voltage and power conversion efficiency (PCE) reaching 1.54 V and 9.2%, respectively. These achievements are ascribed to a film with reduced morphological defects and better interface quality and reduced nonradiative pathways. For the first time, the fabrication of semitransparent CH3NH3PbBr3-based solar cells is demonstrated reaching a maximum PCE equal to 7.6%, an AVT of the full stack device of 52%, and an excellent light stability at maximum-power point tracking.

Isomer-Pure Bis-PCBM-Assisted Crystal Engineering of Perovskite Solar Cells Showing Excellent Efficiency and Stability

Dongqin Bi, Michael Graetzel, Xiong Li, Jingshan Luo, Norman Pellet, Chenyi Yi, Shaik Mohammed Zakeeruddin, Fei Zhang

A fullerene derivative (alpha-bis-PCBM) is purified from an as-produced bis-phenyl-C-61-butyric acid methyl ester (bis-[60]PCBM) isomer mixture by preparative peak-recycling, high-performance liquid chromatography, and is employed as a templating agent for solution processing of metal halide perovskite films via an antisolvent method. The resulting alpha-bis-PCBM-containing perovskite solar cells achieve better stability, efficiency, and reproducibility when compared with analogous cells containing PCBM. alpha-bis-PCBM fills the vacancies and grain boundaries of the perovskite film, enhancing the crystallization of perovskites and addressing the issue of slow electron extraction. In addition, alpha-bis-PCBM resists the ingression of moisture and passivates voids or pinholes generated in the hole-transporting layer. As a result, a power conversion efficiency (PCE) of 20.8% is obtained, compared with 19.9% by PCBM, and is accompanied by excellent stability under heat and simulated sunlight. The PCE of unsealed devices dropped by less than 10% in ambient air (40% RH) after 44 d at 65 degrees C, and by 4% after 600 h under continuous full-sun illumination and maximum power point tracking, respectively.

Wiley-Blackwell2017

Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

Perovskite photovoltaics is one of the fastest growing research fields in materials science. Apart from photovoltaics, a wide range of energy-related research fields have gained substan-tial new momentum owing to the fantastic optoelectronic properties of hybrid lead halide per-ovskites. Starting with solar cells showing a mere 3% power conversion efficiency in 2009, researchers have been able to increase this value to over 22% in 2017, comparable to the best monocrystalline silicon solar cells. These improvements are chiefly due to the compositional tuning and mixing of the ABX3 perovskite structure. However, this strategy increases not only the efficiency but also in the same time the complexity of the perovskite formulation. Differ-ent phases, from the nano- to macroscale, can potentially form. In order to rationalize the effi-ciency progress, one needs to understand and characterize the complex perovskite composi-tion and crystal structure in great detail. The main focus of this thesis lies in the in-depth compositional and phase analysis of perovskite thin films as well as full perovskite solar cells, trying to rationalize the consequences of A cation and X anion tuning and mixing. In the first part, it is shown that fabricating solar cells from a non-stoichiometric perovskite precursor composition, namely one with PbI2 excess, leads to increased grain size, enhanced cristallinity, reduced recombination as well as an improved TiO2/perovskite interface. As op-posed to lead iodide phases resulting from film degradation, which is hampering device per-formance, unreacted excess PbI2 phases originating from an excess of PbI2 in the precursor solution are very beneficial for the final solar cell efficiency, reproducibility and stability. The next scientific challenge adressed in this thesis is related to the nano- and microscale composition of the record-breaking mixed cation/mixed anion perovskite composition. Mac-roscale investigations have previously suggested that this formulation results in a single ho-mogenous phase. However, using nanoscale elemental and charge carrier distribution mapping as well as microscale structural and optical film analysis, it is here found that in high efficient solar cells partial phase segregation does take place at the micro- and nanoscale. Moreover, it is shown that mixed cation/anion formulations allow the formation of never re-ported 3D hexagonal lead halide perovskite polytypes, namely 4H and 6H. These polytypes are shown to play a key role during the crystallization process, which is also revealed for the first time here. Indeed, mixed cation/mixed anion perovskite films are fabricated via the crys-tallization sequence 2Hï 4Hï 6Hï 3R(3C). It is demonstrated that the complex crystalliza-tion via these defect-prone hexagonal intermediates can be by-passed by the incorporaton of low amounts of Cs+ cations into the structure. This can explain the improved stability as well as the increased power conversion efficiency and device reproducibility. It gives for the first time a rational explanation as to why the halide perovskite community rapidly adopted the incorporation of Cs+ in mixed perovskites. The last results presented in this thesis are related to the hole-transporting materials (HTM) used in perovskite solar cells. It is shown that carbazole-based small molecule HTMs are a cheap and efficient alternative to the much costlier commercial spiro-OMeTAD.

EPFL2018

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

Compositional Characterization of Organo-Lead tri-Halide Perovskite Solar Cells

Paul Gratia

EPFL2018