Technical / research - Page 46

University of Cambridge scientists have developed perovskite-based floating ‘artificial leaves’ that generate clean fuels from sunlight and water. The team expects these could eventually operate on a large scale at sea. The ultra-thin flexible devices take their inspiration from photosynthesis. Since the low-cost, autonomous devices are light enough to float, they could be used to generate a sustainable alternative to gasoline without taking up space on land.

A floating artificial leaf which can generate clean fuel from sunlight and water – on the River Cam near King’s College Chapel in Cambridge, UK. Image credit: Virgil Andrei, from: Scitechdaily

Outdoor tests of the lightweight leaves on the River Cam showed that they can convert sunlight into fuels as efficiently as plant leaves. River Cam is the main river flowing through Cambridge in eastern England, and the testing occurred near iconic Cambridge sites including the Bridge of Sighs, the Wren Library, and King’s College Chapel.

Researchers from Sungkyunkwan University (SKKU), Korea University, Seoul National University and Korea's Frontier Energy Solution have demonstrated the feasibility of using the van der Waals stacking (vdWS) strategy to overcome the inefficiencies of flexible perovskite solar cells (f-PSCs).

The team explained that using halide perovskites can significantly increase the power conversion efficiency (PCE) of f-PSCs due to the low-temperature processability, ultrathin dimensions, low weight, and excellent optoelectronic properties of such cells. Several efforts were also made to develop the charge transporting layer (CTL), interface engineering between CTLs and perovskite, and highly-crystalline perovskite films on a flexible substrate, which further increased the f-PSC PCE to 22.44%. However, the efficiency is only 87% of the conventional glass-based PSCs. The f-PSC efficiency is affected by the physical process limitations caused by the flexible substrates' flexibility. Polymer substrates, such as polyethylene naphthalate (PEN) or polyethylene terephthalate (PET), or tin-doped indium oxide (ITO) are typically used as flexible substrates for f-PSCs.

Researchers from the Indian Institute of Technology Roorkee and the University of Massachusetts Amherst have developed perovskite-based photo-rechargeable supercapacitors. 

The team explained in the study that current approaches for off-grid power separate the processes for energy conversion from energy storage. However, with the right balance between the electronic and ionic conductivity and a semiconductor that can absorb light in the solar spectrum, it is possible to combine energy harvesting with storage into a single photoelectrochemical energy storage device. In their new work, the researchers reported such a device - a halide perovskite-based photorechargeable supercapacitor that simultaneously harvests and stores energy.

Researchers from the University of Pennsylvania’s Andrew M. Rappe's group and Princeton University's Yueh-Lin Loo's group recently examined how the molecular make up of certain perovskites might affect their efficiency and offered a path forward to better solar cells using a simple metric.

“The world currently needs more efficient and cost-effective photovoltaic cells, and 3D hybrid perovskite PVs have taken the world by storm,” says Rappe, a professor in Penn’s Department of Chemistry who also co-directs Penn’s VIPER program. “But they are irreversibly damaged by water, which is a showstopper for practical applications. Inserting organic molecular planes between 2D hybrid perovskite planes is a promising scheme to provide efficient, low-cost, and robust solar cells.”

Researchers from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) and Chinese Academy of Sciences (CAS) have developed a method for processing formamidinium-based perovskite films, using a unique ammonia treatment to remove pore structures formed during the processing.

Methods and solvents used in processing perovskite films have been extensively studied, but these still require additional processing steps or materials to make perovskite films work. For instance, the extremely promising methylamine gas healing method is only suitable for methylammonium-based perovskite, but is not feasible for the more efficient formamidinium-based perovskite. In order to find solutions to the gas healing of the formamidinium-based perovskite films, the researchers first studied the underlying reactions responsible for the challenges. “We have shown that the degradation of formamidinium-containing perovskites is caused by a reaction between the formamidinium cation and aliphatic amines, producing ammonia,” said WANG Xiao from CAS, the second author of the study.

Scientists at the University of California San Diego, Lawrence Berkeley National Laboratory, Stanford University, Los Alamos National Laboratory, Korea's Yonsei University and Daegu Gyeongbuk Institute of Science and Technology have developed a novel perovskite solar cell, made of a lead-free low-dimensional perovskite material with a superlattice crystal structure. The material exhibits efficient carrier dynamics in three dimensions, and its device orientation can be perpendicular to the electrodes. Materials in this particular class of perovskites have so far only exhibited such dynamics in two dimensions—a perpendicularly orientated solar cell has never been reported.

The team reported that thanks to its specific structure, this new type of superlattice solar cell reaches an efficiency of 12.36%, which is the highest reported for lead-free low-dimensional perovskite solar cells (the previous record holder’s efficiency is 8.82%). The new solar cell also has an unusual open-circuit voltage of 0.967 V, which is higher than the theoretical limit of 0.802 V. Both results have been independently certified.

Perovskite LEDs can be produced quite easily and at low cost. They show great promise as they are lightweight and can offer flexibility compared to OLEDs, with color purity and tunability similar to LEDs based on III-V semiconductors. However, the poor device stability of perovskite LEDs will have to be overcome before commercial applications can emerge. Typical lifespans of perovskite LEDs are on the order of 10 to 100 hours. In contrast, the minimum lifetime required for an OLED display is 10,000 hours. It is currently challenging to reach this threshold, as halide perovskite semiconductors can be intrinsically unstable due to the ionic nature of their crystal structures—the ions can move around when voltages are applied to the LEDs, leading to material degradation.

In their recent work, a research group led by Prof. Di Dawei and Prof. Zhao Baodan at the College of Optical Science and Engineering of Zhejiang University discovered that by using a dipolar molecular stabilizer, it is possible to make efficient and stable perovskite LEDs with ultralong lifetimes, satisfying the demands of commercial applications. The research was carried out in collaboration with the research groups of Prof. Li Cheng at Xiamen University, Prof. Hong Zijian at Zhejiang University, and Prof. Li Weiwei at NUAA and formerly at Cambridge University. 

Researchers from the University of Surrey's Advanced Technology Institute (ATI), KIOS Research and Innovation Center of Excellence at the University of Cyprus, China's Zhengzhou University, and the UK's National Physical Laboratory (NPL) have demonstrated a new photo-rechargeable system, which merges zinc-ion batteries with perovskite solar cells.

(a) device configuration and (b) working principle of the integrated flexible photo-rechargeable system. Credit: Energy Storage Materials

The new system could allow wearables to charge without the need to plug in. In fact, as little as thirty seconds of sunlight could boost the battery life of future smartwatches and other wearables by tens of minutes. The new environmentally friendly, photo-rechargeable system is unique because of its elegant design between the integrated battery and solar cell, allowing it to demonstrate high energy and volume density comparable to state-of-the-art micro-batteries and supercapacitors. 

Scientists from Syracuse University, South Dakota State University and Huzhou University have examined the use of polyaniline as a material for improved perovskite solar cells. The team demonstrated a facile, low-cost fabrication route for polyaniline hole transport mechanisms that display enhanced power conversion efficiency compared to conventional PEDOT:PSS hole transport layers in perovskites. This could provide a route toward low-cost, high-efficiency perovskite solar cells.

PEDOT:PSS is a commonly used material in perovskites, used as a hole transport layer between the photoactive perovskite layer and indium tin oxide layer. Using PEDOT:PSS improves the power conversion efficiency of perovskite solar cells. However, several issues have been observed with PEDOT:PSS. One of the fundamental issues is the degradation of active layers and defect formation associated with the large particle size of this material. Moreover, the use of this material as a hole transport layer is hindered by issues with low electrical conductivity limits and cost.

University of New South Wales (UNSW) team, led by renowned PV scientist Martin Green, have shown that perovskite solar cells may be especially susceptible to damage from reverse bias, caused by uneven shading or other issues that may appear in real-world environments. Both the reverse-bias itself and resulting build up of heat can cause several of the materials commonly used in perovskite solar cells to degrade, and these issues have received only limited attention in research published thus far. 

Stability issues with perovskite solar cells linger, despite impressive research achievements in the last few years. Much of the research focused on improving stability to date has focused on the issues that arise under normal operating conditions – for example sensitivity to oxygen and moisture, which can be solved through encapsulation, or degradation under UV light, which can be solved with reflective coatings. Other issues, however, may present serious challenges to developing perovskite devices that can function in outdoor conditions for years and even decades. “…thermal degradation and reverse-bias instability are remaining issues that pose challenges even for intrinsically much more stable silicon cells, suggesting that innovative approaches may be required to satisfactorily address these for perovskite cells”, explain the authors of the new paper.