A properly tailored tail boosts solar-cell efficiency

With the help of structural insights from the Advanced Light Source (ALS), researchers optimized the fit between organic and inorganic ions in a perovskite solar-cell material.

The work increased the material’s power-conversion efficiency and stability and opens up a new avenue for improving the current-carrier dynamics of a promising class of materials.

A photovoltaic rising star

To address the effects of global climate change, it’s essential that we capitalize on energy from the sun. However, although solar energy is freely available, it needs to be converted into usable electricity in a way that’s efficient, cost-effective, and commercially scalable.

Perovskites are high-performance inorganic semiconductors recognized as some of the most promising photovoltaic materials of the future. Perovskite films—thin, lightweight, and flexible—can be produced using low-cost solution-processing techniques, and their power-conversion efficiencies (PCEs) have rapidly risen to the brink of 30% in just 15 years, surpassing conventional silicon panels.

A structure with room to tinker

The most intriguing perovskite materials today are organic–inorganic hybrids. They have the general formula ABX3, in which the inorganic B and X ions form a framework of octahedral cages, and the organic A ions are located in the spaces between the cages.

Previously, it was thought that perovskite electronic performance mainly depended on the B and X electronic orbitals, and that A merely served a structural function. In this work, researchers showed that A-site organic ions with specially designed characteristics can increase charge-carrier mobility and power conversion efficiency while also improving device stability.

Read more on the ALS website

Image: Left: The basic structure of perovskite, a promising solar-cell material, has three types of sites, A (blue), B (gray), and X (purple). Right: By attaching organic tails to the interstitial “A” sites (and testing different linker lengths), researchers improved the material’s photovoltaic response.

Tiny Chip-Based Device Performs Ultrafast Manipulation of X-Rays

Researchers from the U.S. Department of Energy’s Advanced Photon Source (APS) and Center for Nanoscale Materials at Argonne National Laboratory have developed and demonstrated new x-ray optics that can be used to harness extremely fast pulses in a package that is significantly smaller and lighter than conventional devices used to manipulate x-rays. The new optics are based on microscopic chip-based devices known as microelectromechanical systems (MEMS).

“Our new ultrafast optics-on-a-chip is poised to enable x-ray research and applications that could have a broad impact on understanding fast-evolving chemical, material and biological processes,” said research team leader Jin Wang from the X-ray Science Division Time Resolved Research (TRR) Group at the APS. “This could aid in the development of more efficient solar cells and batteries, advanced computer storage materials and devices, and more effective drugs for fighting diseases.”

In new results published in The Optical Society OSA) journal Optics Express, the researchers demonstrated their new x-ray optics-on-a-chip device (Fig. 1), which measures about 250 micrometers and weighs just 3 micrograms, using the TRR Group’s 7-ID-C x-ray beamline at the APS. The tiny device performed 100 to 1,000 times faster than conventional x-ray optics, which that tend to be bulky.

Read more on the APS website

Image: Fig. 1. The photograph shows two MEMS elements on a single chip (A), with the active elements of 250 µm × 250 µm, and the micrograph (B) highlighting the size of the diffractive element, as compared to a section of human hair (C).

Perfect recipe for efficient perovskite solar cells

A long-cherished dream of materials researchers is a solar cell that converts sunlight into electrical energy as efficiently as silicon, but that can be easily and inexpensively fabricated from abundant materials. Scientists at the Helmholtz-Zentrum Berlin have now come a step closer to achieving this. They have improved a process for vertically depositing a solution made from an inexpensive perovskite solute onto a moving substrate below. Not only have they discovered the crucial role played by one of the solvents used, but they have also taken a closer look at the aging and storage properties of the solution.

Solar cells made of crystalline silicon still account for the lion’s share of roof installations and solar farms. But other technologies have long since become established as well – such as those that convert sunlight into electrical energy through use of extremely thin layers of solar-cell material deposited upon a substrate. The perovskite solar cells that Prof. Eva Unger and her team at the Helmholtz-Zentrum Berlin (HZB) are researching belong to this group. “These are the best solar cells to date that can be made using a 2D ink”, the researcher explains. “And now their efficiencies are approaching those for cells made of crystalline silicon.”

Read more on the HZB website

Image: The liquid solution of perovskite precursor, solvent, and additive flows from a slit-shaped nozzle onto the glass substrate being conveyed below.

Credit: © Jinzhao Li / HZB

Towards industrial-scale manufacturing of perovskite solar cells

For the production of high-quality metal-halide perovskite thin-films for large area photovoltaic modules often optimized inks are used which contain a mixture of solvents. An HZB team at BESSY II has now analysed the crystallisation processes within such mixtures. A model has also been developed to assess the kinetics of the crystallisation processes for different solvent mixtures. The results are of high importance for the further development of perovskite inks for industrial-scale deposition processes of these semiconductors.

Hybrid organic perovskite semiconductors are a class of materials for solar cells, which promise high efficiencies at low costs. They can be processed from precursor solutions that upon evaporation on a substrate form a polycrystalline thin film. Simple manufacturing processes, such as spin coating a precursor solution, often only lead to good results on a laboratory scale, i.e. for very small samples.

Read more on the HZB website

Image: Schematic illustration: the solvants (ink) are used to produce a thin film of polycrystalline perovskite. 

Credit: © HZB

Hybrid photoactive perovskites imaged with atomic resolution for the first-time

A huge step towards better performing solar cells – a collaboration identified information previously invisible using Diamond’s ePSIC microscopes of Oxford University’s Departments of Materials and Physics

A new technique has been developed allowing reliable atomic-resolution images to be taken, for the first time, of hybrid photoactive perovskite thin films.- highly favourable materials for efficient photovoltaic and optoelectronic applications. These images have significant implications for improving the performance of solar cell materials and have unlocked the next level of ability to understand these technologically important materials. The breakthrough was achieved by a joint team from the University of Oxford and Diamond who have just released a new paper published in Science.

Using the ePSIC (the Electron Physical Science Imaging Centre) E02 microscope and the ARM200 microscope in at the Department of Materials, University of Oxford, the team developed a new technique which allowed them to image the hybrid photoactive perovskites thin films with atomic resolution. This gave them unprecedented insights into their atomic makeup and provided them with information that is invisible to every other technique.

Read more on the Diamond website

Image: An example of one of the images obtained using the new protocol, which illustrates several of the phenomena that the team has been able to describe for the first time, including a range of grain boundaries, extended planar defects, stacking faults, and local inclusions of non-perovskite material.

Order in the disorder: density fluctuations in amorphous silicon discovered

For the first time, a team at HZB has identified the atomic substructure of amorphous silicon with a resolution of 0.8 nanometres using X-ray and neutron scattering at BESSY II and BER II. Such a-Si:H thin films have been used for decades in solar cells, TFT displays, and detectors. The results show that three different phases form within the amorphous matrix, which dramatically influences the quality and lifetime of the semiconductor layer. The study was selected for the cover of the actual issue of Physical Review Letters.

Silicon does not have to be crystalline, but can also be produced as an amorphous thin film. In such amorphous films, the atomic structure is disordered like in a liquid or glass. If additional hydrogen is incorporated during the production of these thin layers, so-called a-Si:H layers are formed. “Such a-Si:H thin films have been known for decades and are used for various applications, for example as contact layers in world record tandem solar cells made of perovskite and silicon, recently developed by HZB” explains Prof. Klaus Lips from HZB. “With this study, we show that the a-Si:H is by no means a homogeneously amorphous material. The amorphous matrix is interspersed with nanometre-sized areas of varying local density, from cavities to areas of extremely high order,” the physicist comments.

Read more on the BESSY II website

Image: Structural model of highly porous a-Si:H, which was deposited very quickly, calculated based on measurement data. Densely ordered domains (DOD) are drawn in blue and cavities in red. The grey layer represents the disordered a-Si:H matrix. The round sections show the nanostructures enlarged to atomic resolution (below, Si atoms: grey, Si atoms on the surfaces of the voids: red; H: white) © Eike Gericke/HZB

A probe of light-harvesting efficiency at the nanoscale

SCIENTIFIC ACHIEVEMENT

Using time-resolved experiments at the Advanced Light Source (ALS), researchers found a way to count electrons moving back and forth across a model interface for photoelectrochemical cells.

SIGNIFICANCE AND IMPACT

The findings provide real-time, nanoscale insight into the efficiency of nanomaterial catalysts that help turn sunlight and water into fuel through artificial photosynthesis.

Solar-fuel tech goes for gold

In the search for clean-energy alternatives to fossil fuels, one promising solution relies on photoelectrochemical (PEC) cells: water-splitting, artificial-photosynthesis devices that turn sunlight and water into solar fuels such as hydrogen. In just a decade, researchers have achieved great progress in the development of PEC systems made of light-absorbing gold nanoparticles (NPs) attached to a semiconductor film of titanium dioxide (TiO2).

Read more on the Advanced Light Source website

Image: Laser pulses were used to excite electrons in gold nanoparticles (AuNPs) on a titanium dioxide (TiO2) substrate. X-ray pulses were used to count the electrons moving between the nanoparticles and the substrate. (Credit: Oliver Gessner/Berkeley Lab)

Significant progress on ultraflexible solar cells

Research from Monash University, the University of Tokyo and RIKEN, partly undertaken at the Australian Synchrotron, has produced an ultra-flexible ultra-thin organic solar cell that delivered a world-leading performance under significant stretching and strain.

The development paves the way forward for a new class of stretchable and bendable solar cells in wearable devices, such as fitness and health trackers, and smart watches with complex curved surfaces.

The advance, which was published in Joule, was made possible by designing an ultra-thin material based on a blend of polymer, fullerene and non-fullerene molecules with the desired mechanical properties and power efficiency, according to Dr Wenchao Huang, a Research Fellow at Monash University and the article’s first author.
The thickness of the solar cell film is only three micrometres, which is ten times smaller than the width of a human hair.

Dr Huang, who completed his PhD in the lab of Prof Chris McNeill at Monash on flexible organic solar cells, received the Australian Synchrotron’s Stephen Wilkins Medal in 2016 for his exceptional doctoral thesis that made use of the synchrotron-based research capabilities at the facility.

>Read more no the Autralian Lightsource at ANSTO website

Image: Schematic of ultraflexible solar cell

Enhanced tandem solar cells set new standard in converting light into electricity

A collaboration between U of T Engineering and King Abdullah University of Science and Technology has created two-layered solar cells that successfully combine traditional silicon with new perovskite technology .

Researchers from University of Toronto Engineering and King Abdullah University of Science and Technology (KAUST) have overcome a key obstacle in combining the emerging solar-harvesting technology of perovskites with the commercial gold standard — silicon solar cells. The result is a highly efficient and stable tandem solar cell, one of the best-performing reported to date.
“Today, silicon solar cells are more efficient and less costly than ever before,” says Professor Ted Sargent (ECE), senior author on a new paper published today in Science. “But there are limits to how efficient silicon can be on its own. We’re focused on overcoming these limits using a tandem (two-layer) approach.”

>Read more on the Canadian Light Source website

Picture: Left to right: Postdoctoral fellows Erkan Aydin (KAUST), Yi Hou (University of Toronto) and Michele De Bastiani (KAUST) are part of an international team that has designed a new type of tandem solar cell. The device combines industry standard silicon manufacturing with new perovskite technology.
Credit: Andrea Bachofen-Echt / KAUST

New method to get stable perovskite-based material for more efficient solar cells

Perovskites materials are promising candidates for next generation solar cells. However, their use is still limited by their instability within ambient conditions. Instead of absorbing all visible light and appearing black, some of these super materials preferentially form another structure which is yellow. Since only the black form is optically active, the current challenge is achieving stable black perovskites thin films suitable for real world optoelectronic devices. An international team of scientists, led by a group from KU Leuven in Belgium, have shone a light on this problem developing a new method to stabilize the black form introducing strain into the perovskite thin film using the glass substrate on which it sits. Synchrotron-based techniques at the ALBA Synchrotron and the European Synchrotron Radiation Facility were crucial for obtaining these results, published today in Science.

>Read more on the ALBA website

Unleashing perovskites’ potential for solar cells

Perovskites — a broad category of compounds that share a certain crystal structure — have attracted a great deal of attention as potential new solar-cell materials because of their low cost, flexibility, and relatively easy manufacturing process. But much remains unknown about the details of their structure and the effects of substituting different metals or other elements within the material. Now, researchers using the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS) have been able to decipher a key aspect of the behavior of perovskites made with different formulations: With certain additives there is a kind of “sweet spot” where sufficient amounts will enhance performance and beyond which further amounts begin to degrade it. The findings were detailed in the journal Science.
Conventional solar cells made of silicon must be processed at temperatures above 1,400 degrees Celsius, using expensive equipment that limits their potential for production scale-up. In contrast, perovskites can be processed in a liquid solution at temperatures as low as 100 degrees, using inexpensive equipment. What’s more, perovskites can be deposited on a variety of substrates, including flexible plastics, enabling a variety of new uses that would be impossible with thicker, stiffer silicon wafers.

>Read more on the Advanced Photon Source (APS) website

Image: Perovskite-based solar cells are flexible, lightweight, can be produced cheaply, and could someday bring down the cost of solar energy. Shown here is the type of perovskite solar cell measured at the CNM/XSD Hard X-ray Nanoprobe at the APS.
Credit: Rob Felt

Boosting the efficiency of silicon solar cells

The efficiency of a solar cell is one of its most important parameters.

It indicates what percentage of the solar energy radiated into the cell is converted into electrical energy. The theoretical limit for silicon solar cells is 29.3 percent due to physical material properties. In the journal Materials Horizons, researchers from Helmholtz-Zentrum Berlin (HZB) and international colleagues describe how this limit can be abolished. The trick: they incorporate layers of organic molecules into the solar cell. These layers utilise a quantum mechanical process known as singlet exciton fission to split certain energetic light (green and blue photons) in such a way that the electrical current of the solar cell can double in that energy range.

The principle of a solar cell is simple: per incident light particle (photon) a pair of charge carriers (exciton) consisting of a negative and a positive charge carrier (electron and hole) is generated. These two opposite charges can move freely in the semiconductor. When they reach the charge-selective electrical contacts, one only allows positive charges to pass through, the other only negative charges. A direct electrical current is therefore generated, which can be used by an external consumer.

>Read more on the BESSY II at Helmholtz-Zentrum Berlin website

Picture: Darstellung des Prinzips einer Silizium-Multiplikatorsolarzelle mit organischen Kristallen
Credit: M. Künsting/HZB

Stable solvent for solution-based electrical doping…

… of semiconducting polymer films and its application to organic solar cells.

Controlled and stable electrical doping of organic semiconductors is desirable for the realization of efficient organic photovoltaic (OPV) devices. Thus, progress has been made to understand the fundamental doping mechanisms.1-3 In 2016, Aizawa et al. reported the use of 12-molybdophosphoric acid hydrate (PMA) to induce p-type doping and crosslinking of neat films of poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)](PCDTBT).4 Later on, a more general approach of sequential solution-based doping was presented, by post-process immersion of donor-like polymer films in PMA-nitromethane solutions.5 However, critical to the method is the use of nitromethane, a highly unstable solvent, to dissolve PMA and thus limited the applicability to large-scale fabrication of organic solar cells.

A collaboration between a team of researchers from the Kippelen Research Group at Georgia Tech and the Toney Research Group at SSRL developed a solution-based doping method using the highly stable solvent, acetonitrile. Figure 1a displays the chemicals used in this work. In Figure 1b, the optical properties of poly(3-hexylthiophene-2,5-diyl)(P3HT) films immersed for 30 min in a 0.5 M solution of PMA in acetonitrile (PMA-im-P3HT) were studied by comparing their transmittance spectra against pristine P3HT and P3HT immersed similarly in a 0.5 M solution of PMA in nitromethane. The normalized change of transmittance ΔT T-1 as a function of wavelength (inset of Fig.1b) reveals the same spectral signatures reported for PMA-im-P3HT films when PMA was dissolved in nitromethane. That is, changes in the region where ΔT T-1< 0 correlate with the P3HT polaron bands, and deviations in the region where ΔT T-1> 0 correlate to the bleaching of the main π-π* absorption bands.6 The data suggests electrical p-doping into the depth of the organic film. Figure 1c shows that the performance of PMA-doped OPV devices using PMA in acetonitrile is comparable to that of OPVs made using PMA in nitromethane or MoO3, under simulated AM 1.5G solar illumination. Furthermore, if the light soaking mechanism is used before each measurement, OPVs made using PMA in nitromethane or acetonitrile remain stable for up to 524 h in the air, retaining 80% of their initial power conversion efficiency (PCE).

>Read more on the Standfort Synchrotron Radiation Lightsource website

Figure: (extract) of GIWAXS data as measured on pristine and PMA doped P3HT, when using various solvents to dissolve the PMA. a, Two-dimensional GIWAXS data converted to q-space for pristine P3HT and P3HT immersed in PMA solutions in nitromethane, acetonitrile or ethanol for 60 seconds. b, One-dimensional scattering profiles (out-of-plane and in-plane profiles), obtained from the two-dimensional GIWAXS data.

Rational optimization of organic solar-cell materials

Phase diagram leads the way to tailored metamaterial responses

Modifications to novel non-fullerene small molecule acceptor in organic thin film

… for solar cells demonstrates improved power conversion efficiency.

Scientists from the Imperial College London, Monash University, CSIRO, and King Abdullah University of Science and Technology have reported an organic thin film for solar cells with a non-fullerene small molecule acceptor that achieved a power conversion efficiency of just over 13 per cent.

By replacing phenylalkyl side chains in indacenodithieno[3,2-b]thiophene-based non-fullerene acceptor (ITIC) with simple linear chains to form C8-ITIC, they improved the photovoltaic performance of the material.

C8-ITIC was blended with a fluorinated analog of the donor polymer PBDB-T to form bulk-heterojunction thin films.

The research was recently published in Advanced Materials.

Dr Xuechen Jiao of McNeill Research Group at Monash University carried out grazing incidence wide angle X-ray scattering (GIWAXS) measurements at the Australian Synchrotron to gain morphological information on pure and blended thin films.

“By changing the chemical structure of the organic compound, a promising boost in efficiency was successfully achieved in an already high-performing organic solar cells” said Jiao.

>Read more on the Australian Synchrotron website