Efficiency - Page 50

Researchers from Florida State University and Georgia Tech have been working on new ways for solar cells to absorb and use infrared light, a portion of the solar spectrum that is typically unavailable for solar cell technology.

'We're working on a process to optimize the efficiency of solar cells,' said Assistant Professor of Chemistry and Biochemistry Lea Nienhaus. 'The main drive is to optimize this process for solar applications'. The team has created a new approach for solar cells to facilitate a process called photon upconversion. In photon upconversion, two low energy photons are converted into one high-energy photon that emits visible light.

Researchers from Helmholtz-Zentrum Berlin (HZB), Eindhoven University of Technology and Technical University Berlin have combined rear junction silicon heterojunction bottom cells with p'i'n perovskite top cells into highly efficient monolithic tandem solar cells with a power conversion efficiency (PCE) of 26.0%.

Colored cross sectional SEM image of the top cell (upper panel) and back side of the bottom cell (lower panel) of a typical monolithic tandem solar cell used in this work. (b) schematic device layout of the tandem architecture utilized in this work.

Starting from a certified efficiency of 25.0%, further improvements have been reached by reducing the current mismatch of the certified device. The top contact and perovskite thickness optimization allowed increasing the JSC above 19.5 mA cm'2, enabling a remarkable tandem PCE of 26.0%, however with a slightly limited fill factor (FF).

A collaborative research team from Los Alamos National Laboratory, Rice University, Purdue University, Northwestern University, Institut FOTON CNRS UMR 6082 (France) and Argonne National Laboratory has created a number of hybrid perovskite solar cells with a FA_0.7MA_0.25Cs_0.05PBI₃ composition and measured them using a variety of techniques including grazing-incidence wide-angle x-ray scattering (GIWAXS) maps at the X-ray Science Division 8-ID-E x-ray beamline of the APS (an Office of Science user facility at Argonne).

The experimental setup (top left) and the corresponding light-induced lattice expansion effect, which leads to curing defects and relieving of lattice strain (bottom left) and as a result an increase in the open circuit voltage of a solar cell

In most of the cells, the researchers noted a substantial improvement in PCE from 18.5% to 20.5% under continuous light soaking with a 1-sun (100 mW/cm²) source as the lattice structure of the hybrid cells uniformly expanded. This expansion relieved local strains in the bulk material and better aligned the crystal planes, as evidenced by narrowing and uniform shifting of the Bragg peaks toward lower scattering values as seen by GIWAXS. The researchers explain that constant illumination generates electron-hole pairs in the perovskite material, decreasing the distortions of some bonds while elongating others, resulting in a generalized lattice expansion and relaxation. A similar phenomenon was seen with pure MAPbI₃ thin films, suggesting that such lattice expansion under light is common for hybrid perovskite materials.

Researchers at the University of Toronto have designed a method of growing a more stable electron selective layer for perovskite solar cells and tandem solar cells combining crystalline silicon with perovskite.

Perovskite raw materials can be mixed into a liquid in a kind of 'solar ink.' This solar ink could be printed onto glass, plastic or other materials with a relatively simple inkjet printing process. However, in order to generate electricity, electrons excited by solar energy from perovskite cells must be extracted from a layer of quantum dots that is held together by a passivation layer. Some types of quantum dots are known to change their 3D structure even at room temperature, making them transparent and ineffective. This passivation layer is also known to break down at temperatures above 100°C. The team's breakthrough made both quantum dots and perovskites more stable when combined than they are separated and the solar cell combining of Perovskite material and quantum dots achieved 20.1% efficiency.

Parasitic absorption in transparent electrodes is one of the main roadblocks to enabling power conversion efficiencies (PCEs) for perovskite'based tandem solar cells beyond 30%. To reduce such losses and maximize light coupling, the broadband transparency of such electrodes should be improved, especially at the front of the device.

Erkan Aydin and coworkers from KAUST Photovoltaics Laboratory have recently shown the excellent properties of Zr'doped indium oxide (IZRO) transparent electrodes for such applications, with improved near'infrared (NIR) response compared to conventional tin'doped indium oxide (ITO) electrodes. Optimized IZRO films feature very high electron mobility (up to '77 cm² V'1 s'1), enabling highly infrared transparent films with a very low sheet resistance ('18 Ω ^'1 for annealed 100 nm films). For devices, this translates to a parasitic absorption of only '5% for IZRO within the solar spectrum (250'2500 nm range), to be compared with '10% for commercial ITO.

Researchers from the University of Toledo have reported progress that may push the performance of tandem perovskite solar cells to new levels. Working in collaboration with the U.S. Department of Energy's National Renewable Energy Lab and the University of Colorado, Dr. Yanfa Yan, UToledo professor of physics, envisions that the new high efficiency tandem perovskite solar cell will be ready to debut in full-sized solar panels in the consumer market in the near future.

"We are producing higher-efficiency, lower-cost solar cells that show great promise to help solve the world energy crisis," Yan said. "The meaningful work will help protect our planet for our children and future generations. We have a problem consuming most of the fossil energies right now, and our collaborative team is focused on refining our innovative way to clean up the mess."

Researchers at the Eindhoven University of Technology in the Netherlands have found a way to address the issue of stability in perovskite solar cells by adding a small amount of fluoride during the production process, which was found to increase the stability of such cells.

Fluoride stablizes perovskite solar cells by encouraging the formation of strong hydrogen bonds and ionic bonds on the surface of the perovskite material.

The scientists found the fluoride ions form a protective layer around perovskite crystals, preventing the ill effects of light, heat and moisture. "Our work has improved the stability of perovskite solar cells considerably," said Shuxia Tao, assistant professor at Eindhoven University of Technology's Center for Computational Energy Research. "Our cells maintain 90% of their efficiency after 1,000 hours under extreme light and heat conditions. This is many times as long as traditional perovskite compounds. We achieve an efficiency of 21.3%, which is a very good starting point for further efficiency gains."

Researchers at the National Renewable Energy Laboratory (NREL) report the creation of an efficient tandem perovskite solar cell, using a new chemical formula which also improved the structural and optoelectronic properties of the solar cell.

Most of the research efforts in the field of PSCs have focused on lead-based perovskites, which have a wide bandgap. High efficiency, low bandgap perovskites would enable the fabrication of very high efficiency all-perovskite tandem solar cells where each layer absorbs only a part of the solar spectrum and is optimally configured to convert this light into electrical energy. However, low bandgap perovskites have long suffered from large energy losses and instability limiting their use in tandems.

A joint research team including scientists from the Chinese Academy of Scinces (CAS), Shijiazhuang Tiedao University in China and Chiao Tung University in Taiwan has developed a novel type of highly flexible and stable perovskite-based solar cell that could be used in wearable electronics.

The team stated that current PSCs are mainly made of a polymer substrate, which has been proven fragile, unstable and not adequately waterproof. The team built a new type of PSC based on an inorganic mica substrate, which could reduce the strain in the device even under large bending deformation. Mica is a mineral that separates easily into small flat transparent pieces of rock.

Researchers at Linköping University have developed efficient perovskite near-infrared (NIR) light-emitting diodes. The external quantum efficiency is a record 21.6%. The work was led by Linköping scientist Feng Gao, in close collaboration with colleagues in China, Italy, Singapore and Switzerland.

The external quantum efficiency (the ratio of charge carriers emitted as light over all of those fed into the materials) of light-emitting diodes based on perovskites has until now been limited by defects that arise in the material during manufacture. The defects act as traps for the charge carriers and thus cause energy losses.