Technical / research - Page 53

It was recently reported that Chinese researchers from the ECNU and the Ningbo Institute of Materials Technology and Engineering under the Chinese Academy of Sciences were successful in developing a type of perovskite solar cell (PSC) with high power conversion efficiency.

PSCs can be generally classified into two categories, n-i-p devices and inverted p-i-n devices. The p-i-n PSCs can be produced at low temperature with good stability, and are compatible with crystal silicon cell to achieve the development of laminated cell, said Fang Junfeng, professor at the East China Normal University (ECNU). At present, the efficiency of n-i-p perovskite cells has reached 25%, while the maximum efficiency of inverted p-i-n devices remains at 22%-23%.

Researchers from King Abdullah University of Science and Technology (KAUST) and National Yang Ming Chiao Tung University have reported what they say is the first-ever successful photovoltaic (PV) damp-heat test of perovskite solar cells.

The damp-heat test is an accelerated and rigorous environmental aging test aimed at determining the ability of solar panels to withstand prolonged exposure to high humidity penetration and elevated temperatures. The test is run for 1,000 hours under a controlled environment of 85% humidity and 85 degrees Celsius. It is meant to replicate multiple years of outdoor exposure and evaluate factors such as corrosion and delamination.

Researchers from the University of Amsterdam and NWO-Institute AMOLF have examined the efficiency gain offered by perovskite/silicon tandem solar cells containing several semiconductors with diverse energy gaps, with a spectrum splitter added between the top and bottom terminals.

This design allows the tandem solar cells to be responsive to a wider region of the sunlight's spectrum. However, such cells usually deal with ineffective light trapping and management due to parasitic light absorption in inactive layers and reflection between layers. Various studies have looked into these issues, yet the idea of spreading sunlight in the tandem subcells with controlled spectral splitting was not adequately investigated.

Researchers from Korea and the USA have used an imaging technique to observe structural changes at the atomic level suggesting strategies to reduce perovskite solar cell degradation.

Perovskite solar cells (PSCs) tend to degrade quickly. When they are exposed to sunlight, freely moving ion vacancies form in the structure and migrate towards the electrodes. In dark conditions, the effect is reversed, and the ions are once again redistributed in the perovskite structure. Repeated cycles of this ion transport during the operation of the solar cell permanently degrade the cell and result in short lifetimes. However, degradation at the atomic level due to ion migration has not been directly observed.

Scientists at the Karlsruhe Institute of Technology (KIT) and SUNOVATION Produktion have investigated the loss of efficiency caused by coloring the glass substrate, as part of their research on perovskite cells printed on glass.

PSCs and modules colored with inkjet-printed pigments in various bright colors and color patterns. Image from RRL Solar

The color was applied by inkjet printing, with first a white layer and then another color layer being deposited on top. The method is not new and has been used for years (by module manufacturer Sunovation Produktion GmbH of Elsenfeld, Germany, that was part of this research, and also by others) for colored modules with silicon cells. However, silicon cells absorb light in a different wavelength range than perovskite cells, which is why the same color application results in different final efficiencies.

Los Alamos National Laboratory researchers and their collaborators have examined the performance properties of two-dimensional perovskites in conditions representative of a device structure, finding those structures can be as efficient as their three-dimensional counterparts.

'We discovered that intrinsic to this material, there are some shallow defects or 'trap states' that can help the charge transport over a long distance,' said Wanyi Nie, researcher with the Center for Integrated Nanotechnologies group at Los Alamos National Laboratory. 'The travel distance is slightly lower than three-dimensional perovskites, but is much greater than what people believed in a typical 2D quantum-confined system. So this is a critical finding that two-dimensional perovskites can be efficient as three-dimensional perovskites.'

Scientists from the Xi'an Jiaotong University, Chinese Academy of Science and City University of Hong Kong have reported the fabrication of a highly stable perovskite solar cell by capping the photoactive layer with low-dimensional (LD) perovskitoids.

Atomic structure of (A) (p-PBA)Pb2I6 (1D) and (B) (m-PBA)2PbI6 (0D). Image from study

'Perovskitoids have the potential to effectively modify 3D perovskite due to its diverse PbI₆ connection styles and high stability,' the researchers stated, referring to the properties of the 1D and 0D capping layer materials used for 3D perovskite (m-PBA)₂PbI₆. 'Both 1D and 0D perovskitoids have intrinsically low defect densities and can withstand relatively high lattice strains; thus, they can serve as blocking channels for undesired Shockley-Read-Hall recombination and material degradation.'

Tin (Sn)-based metal halide perovskites (MHPs) could be an environmentally benign alternative to lead-based ones, which are toxic. However, some critical issues need to be resolved before Sn-based MHPs can be leveraged in planar semiconductor devices. When arranged into a 2D structure (or quasi-2D structure with a few layers), defects in the crystal structure of Sn-based MHPs called 'grain boundaries' hamper the mobility of charge carriers throughout the material. If used in a TFT, this phenomenon results in a large series resistance that ultimately degrades performance. In addition, a TFT made using an Sn-based MHP arranged into a 3D structure faces a problem of extremely high carrier density of the 3D material, that causes the transistor to be permanently ON unless very high voltages are applied.

Scientists from Tokyo Tech, National Institute for Materials Science and Silvaco Japan have proposed a novel concept based on a hybrid structure for Sn-based metal halide perovskites (MHPs), called the '2D/3D core'shell structure.' In this structure, 3D MHP cores are fully isolated from one another and connected only through short 2D MHP strips (or 'shells'). This alternating arrangement manages to address both these issues, according to the team.

Researchers from the Australian National University (ANU), China's Sun Yat-sen University and Chinese Academy of Sciences (CAS) have reportedly set a new record conversion efficiency for perovskite-based solar cells, improving on a record they already held.

The recent research details how a solar conversion efficiency of 22.6% for a one square centimeter cell was achieved through improvements on previous perovskite solar cells.

Researchers from KAUST and the University of Bologna have examined the root causes of harmful top-contact delamination in p-i-n perovskite/silicon tandem solar cells. Their findings aim to improve the stability of tandem modules, and prompt a search for new interfacial linking strategies to enable mechanically strong perovskite-based solar cells, as required for commercialization.

In their work, by combining macroscopic and microscopic analyses, the team identified the interface between the fullerene electron transport layer and the tin oxide buffer layer at the origin of such delamination. Specifically, they found that the perovskite morphology and its roughness play a significant role in the microscopic adhesion of the top layers, as well as the film processing conditions, particularly the deposition temperature and the sputtering power.