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Researchers fabricate all-perovskite tandem solar cells with improved efficiency

Researchers fabricate all-perovskite tandem solar cells with improved efficiency

A kind of solar cell having an important perovskite structured element known as Perovskite tandem solar cells (PSCs) has been fabricated by a group of scientists from Nanjing University, China and the University of Toronto, Canada. Hairen Tan, the lead researcher told that instead of making single-junction perovskite solar cells, the primary idea was to make more efficient all-perovskite tandem solar cells. The findings are reported in Nature Energy journal.

Perovskites are a group of minerals having the same crystal structure as perovskite which is yellow, black or brown mineral comprising mostly of calcium titanate. Many researchers over the past few years have been attempting to build solar cells using this material, either wide-bandgap (~1.8 eV) or narrow-bandgap (~1.2 eV) perovskites.

Merging wide and narrow bandgap perovskites together could enhance power conversion efficiency (PCEs) than that achieved by single-junction cells without any increase in fabrication costs. Scientists need to find a method to strengthen the efficiency of individual subcells, while also integrating the wide and narrow-bandgap cells synergistically for building this type of cell.

Tan said that low efficiencies (PCE~18-20 percent) and low short-circuit current densities (Jsc~28-30 mA/cm2) have been demonstrated by the mixed Pb-Sn narrow-bandgap perovskite solar cells which fall under their capacity, and under the performance of the best Pb-based single-junction perovskite cells. One of their vital components, Sn2+, readily oxidizes into Sn4+ is responsible for the weak performance in narrow-bandgap perovskite solar cells. Tan and his team wanted to determine solutions that could overcome the high trap densities and short carrier diffusion lengths exhibited by the resultant cells.

He also added that their main purpose is to extend the diffusion of narrow-bandgap perovskite solar cells thus to achieve better-performed tandem solar cells. Also, they took a perspective to stop the oxidation of Sn2+ to Sn4+ in the precursor solution to enhance charge carrier diffusion length and whose inclusion in the mixed Pb-Sn perovskites causes Sn vacancies. A new chemical method was used by Tan’s team that is based on a comproportionation reaction and leads to significant improvements in the charge carrier diffusion lengths of mixed Pb-Sn narrow-bandgap perovskites. This could eventually increase the performance of PSCs.

The team obtained an extraordinary 3 μm diffusion length that allows performance-record-breaking Pb-Sn cells and all-perovskite tandem cells unlike the earlier intended method characterized by sub-micrometer diffusion lengths, that can reduce the efficiency of the cell. He also explained that a tin-reduced precursor solution was developed to obtain this by restoring the Sn4+ (an oxidization product of Sn2+) back to Sn2+ through comproportionation reaction in the precursor solution.

The major challenge for the advancement of solar cells with a perovskite element is the oxidation of tin-containing perovskites as it adversely affects their efficiency and hampers their utilities. A substitute path for fabricating tandem solar cells using tin-containing narrow-bandgap perovskite is given by the new chemical method introduced by Tan and his co-workers making cells more stable and efficient.

Tan added that the electronic quality of tin-containing perovskites is comparable to that of lead halide perovskites that have shown efficiency similar to crystalline silicon cells. This approach will eventually provide them a way to very inexpensive and highly efficient solar devices.

The performance of monolithic all-perovskite tandem cells was tested using the chemical approach after fabrication. Remarkable independently approved PCEs of 24.8 percent for small-area devices (0.049 cm2) and 22.1 percent for large-area devices (1.05 cm2) was obtained by their cells. Additionally, after functioning for over 400 hours at their highest power point under full one sun illumination, the cells retained 90 percent of their performance. The method introduced by this team of scientists could lead to the development of more efficient and cost-effective solar-powered devices in the future.

Tan said that they are now planning to further enhance the power conversion efficiency of all-perovskite tandem solar cells above 28 percent. Minimizing the photovoltage loss in the wide-bandgap perovskite solar cell will be the primary feasible method to attain this while minimizing the optical losses in the tunneling recombination junction is another possibility.

Journal Reference: Nature Energy

‘Deforming’ solar cells could be clue to improved efficiency

‘Deforming’ solar cells could be clue to improved efficiency

  • Deformations and defects in structures of photoelectric technologies shown to improve their efficiency
  • University of Warwick physicists demonstrate that strain gradient can prevent recombination of photo-excited carriers in solar energy conversion
  • Increasingly important as devices become miniaturised

Solar panelsSolar cells and light-sensing technologies could be made more efficient by taking advantage of an unusual property due to deformations and defects in their structures.

Researchers from the University of Warwick’s Department of Physics have found that the strain gradient (i.e. inhomogeneous strain) in the solar cells, through physical force or induced during the fabrication process, can prevent photo-excited carriers from recombining, leading to an enhanced solar energy conversion efficiency. The results of their experiments have been published in Nature Communications.

The team of scientists used an epitaxial thin film of BiFeO3 grown on LaAlO3 substrate to determine the impact of inhomogenous deformation on the film’s ability to convert light into electricity by examining how its strain gradient affects its ability to separate photo-excited carriers.

Most commercial solar cells are formed of two layers creating at their boundary a junction between two kinds of semiconductors, p-type with positive charge carriers (electron vacancies) and n-type with negative charge carriers (electrons). When light is absorbed, the junction of the two semiconductors sustains an internal field splitting the photo-excited carriers in opposite directions, generating a current and voltage across the junction. Without such junctions the energy cannot be harvested and the photo-excited carriers will simply quickly recombine eliminating any electrical charge.

They found that the strain gradient can help prevent recombination by separating the light-excited electron-holes, enhancing the conversion efficiency of the solar cells. The BiFeO3/LaAlO3 film also exhibited some interesting photoelectric effects, such as persistent photoconductivity (improved electrical conductivity). It has potential applications in UV light sensors, actuators and transducers.

Dr Mingmin Yang from the University of Warwick said: “This work demonstrated the critical role of the strain gradient in mediating local photoelectric properties, which is largely overlooked previously. By engineering photoelectric technologies to take advantage of strain gradient, we may potentially increase the conversion efficiency of solar cells and enhance the sensitivity of light sensors.

“Another factor to consider is the grain boundaries in polycrystalline solar cells. Generally, defects accumulate at the grain boundaries, which would induce photo-carrier recombination, limiting efficiency. However, in some polycrystalline solar cells, such as CdTe solar cells, the grain boundaries would promote the collection of photo-carriers, where the giant strain gradient might play an important role. Therefore, we need to pay attention to the local strain gradient when we study the structure-properties relations in solar cells and light sensor materials.”

Previously, the effect of this strain on efficiency was thought to be negligible. With the increasing miniaturisation of technologies, the effect of strain gradient becomes magnified at smaller sizes. So in reducing the size of a device using one of these films, the magnitude of strain gradient increases dramatically.

Dr Yang adds: “The strain gradient induced effect, such as flexo-photovoltaic effect, ionic migration, etc, would be increasingly important at low dimensions.”

Materials provided by University of Warwick