EPFL

Researchers from China's Xi’an Jiaotong University, Huazhong University of Science and Technology, Fudan University, ULVAC-PHI Instruments, National University of Singapore (NUS), Sweden's Uppsala University and EPFL have developed a self-assembled bilayer (SAB) that can be used as a hole contact material that grants improved adhesive contact with the perovskite film. 

A schematic illustration of the inverted PSCs. Image from: Nature Energy

The team went on to fabricate an inverted perovskite solar cell that utilizes the self-assembled bilayer (SAB) as a hole-selective molecular contact. The cell was made with a substrate made of glass and transparent conductive oxides (TCOs), the proposed bilayer, the perovskite absorber, an ETL based on buckminsterfullerene (C60), a bathocuproine (BCP) buffer layer, and a silver (Ag) metal contact. 

Researchers from EPFL and CSEM recently fabricated efficient (>20 %) and stable (T80 ∼ 720 h) planar FAPbI₃-based perovskite (1.54 eV) solar cells via a hybrid evaporation-spin coating process. 

FAPbI₃-based perovskite films were fabricated via a hybrid two-step evaporation-spin coating method in an inverted (p-i-n) configuration, and the effects of optimized parameters on the film growth and devices’ performances were investigated. Transferring these films into tandem devices atop single-side textured silicon heterojunction bottom cells, the team obtained an efficiency of >24 % under AM1.5 G illumination for monofacial devices with an active area of 1.21 cm². Furthermore, the bifacial devices generated >27 mW cm⁻² power output with 15 % rear illumination fraction.

Monolithic all-perovskite tandem solar cells present a promising approach for exceeding the efficiency limit of single-junction solar cells. However, the substantial open-circuit voltage loss in the wide-bandgap perovskite subcell hinders further improvements in power-conversion efficiency. Now, researchers from China's Nanjing University, Renshine Solar (Suzhou) and Ecole Polytechnique Fédérale de Lausanne (EPFL) have developed wide-bandgap perovskite films with improved crystal orientation that suppress non-radiative recombination. 

The team showed that using two-dimensional perovskite as an intermediate phase on the film surface promotes heterogeneous nucleation along the three-dimensional perovskite facets during crystallization. Preferred orientations can be realized by augmenting the quantity of two-dimensional phases through surface composition engineering, without the need for excessive two-dimensional ligands that otherwise impede carrier transport. 

Researchers from Ecole Polytechnique Fédérale de Lausanne (EPFL), North China Electric Power University, Westlake University, Lomonosov Moscow State University and others have described the addition of N,N-dimethylmethyleneiminium chloride ([Dmei]Cl) into perovskite precursor solutions to produce two cations in situ—namely 3-methyl-2,3,4,5-tetrahydro-1,3,5-triazin-1-ium ([MTTZ]⁺) and dimethylammonium ([DMA]⁺) cations - that enhanced the photovoltaic
performance and stability of perovskite solar modules.  

A schematic of the roles of [MTTZ]⁺ and [DMA]⁺ in the 3D perovskite matrix. Image from: Science

The team explained that the in situ formation of [MTTZ]⁺ cation increased the formation energy of iodine vacancies and enhanced the migration energy barrier of iodide and cesium ions, which suppressed nonradiative recombination, thermal decomposition, and phase segregation processes. 

Researchers from Helmholtz Zentrum Berlin (HZB) and École Polytechnique Fédérale de Lausanne (EPFL) have presented optimization strategies for top cell processing and integration into silicon heterojunction (SHJ) bottom cells based on industrial Czochralski (Cz)-Si wafers of 140 μm thickness. 

Schematic illustration of the perovskite/silicon tandem solar cell based on 140 μm Cz-Si. Image credit: ACS Applied Materials & Interfaces

The team showed that combining the self-assembled monolayer [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) with an additional phosphonic acid (PA) with different functional groups, can improve film formation when used as a hole transport layer improving wettability, minimizing shunt fraction and reducing nonradiative losses at the buried interface. 

Researchers from EPFL, University of Bern and HZB have introduced a new lead-halide-based Ruddlesden–Popper perovskite structure based on a visible-light-absorbing naphthalene-iminoimide cation. The team stated that the optoelectronic properties of this new material represent an important step toward enhancing light harvesting and affording the spatial separation of charge carrier transport in stable layered perovskite-based devices.

Incorporating organic semiconductor building blocks as spacer cations into layered hybrid perovskites provides an opportunity to develop new materials with novel optoelectronic properties, including nanoheterojunctions that afford spatial separation of electron and hole transport. However, identifying organics with suitable structure and electronic energy levels to selectively absorb visible light has been a challenge in the field. In their recent paper, the team introduced a new lead-halide-based Ruddlesden–Popper perovskite structure based on a visible-light-absorbing naphthalene-iminoimide cation (NDI-DAE).

Researchers from Peking University, Tsinghua University, Beijing Institute of Technology and Ecole Polytechnique Fédérale de Lausanne (EPFL) have tackled a reproducibility challenge in black-phase formamidinium lead iodide (α-FAPbI₃) perovskites. They explained that while this is the desired phase for photovoltaic applications, water can trigger formation of photoinactive impurity phases such as δ-FAPbI₃. The team found that the classic solvent system for perovskite fabrication exacerbates this reproducibility issue. 

Growth of the photoactive black phase of formamidinium lead iodide (α-FAPbI₃) usually requires dimethyl sulfoxide solvent, but the hygroscopic nature of this chemical also promotes water-induced degradation to the photoinactive phase. the scientists showed that a larger chlorinated organic molecule can form a hydrophobic capping layer that enables perovskite crystallization under humid conditions by protecting growing crystallites from water. 

Researchers at EPFL, Shanghai University and Université catholique de Louvain recently developed a method based on machine-learning to quickly and accurately search large databases, leading to the discovery of 14 new materials for solar cells.

The research project, led by EPFL's Haiyuan Wang and Alfredo Pasquarello, developed a method that combines advanced computational techniques with machine-learning to search for optimal perovskite materials for photovoltaic applications. The approach could lead to more efficient and cheaper solar panels, transforming solar industry standards.

Researchers from EPFL, CSEM and Empa have demonstrated a cell design combining additive and substrate engineering that yields consistently high power conversion efficiencies and discussed various design aspects that are important for reproducibility and performance. 

The team presented two key developments with a synergetic effect that boost the PCEs of tandem devices with front-side flat Si wafers—the use of 2,3,4,5,6-pentafluorobenzylphosphonic acid (pFBPA) in the perovskite precursor ink that suppresses recombination near the perovskite/C60 interface and the use of SiO₂ nanoparticles under the perovskite film that suppress the enhanced number of pinholes and shunts introduced by pFBPA, while also allowing reliable use of Me-4PACz as a hole transport layer. 

Researchers from EPFL, Soochow University, Chinese Academy of Sciences, Lomonosov Moscow State University, Luxembourg Institute of Science and Technology (LIST), Julius Maximilian University of Würzburg, Toin University of Yokohama, Southern University of Science and Technology, Xi’an Jiaotong University, North China Electric Power University and Toyota Motor Europe recently developed a solar panel relying on EPFL's record-breaking 25.32%-efficient 2D/3D perovskite solar cells unveiled in July 2023.

The group's research demonstrates a larger surface area of 27.22 cm², achieving an impressive efficiency of 23.3%. In the paper, the scientists explain that the module's high efficiency was achieved thanks to a synergistic dopant-additive combination strategy aimed to improve the cell absorber's uniformity and crystallinity. They used, in particular, methylammonium chloride (MACl) as a dopant and a Lewis-basic ionic liquid known as 1,3-bis(cyanomethyl)imidazolium chloride ([Bcmim]Cl) as an additive.