Scientists demonstrate how to improve ultrathin CIGSe solar cells by nanoparticles
The SiO2 nanoparticles (black) have been imprinted directly on the Molybdenum substrate (purple) which corresponds to the back contact of the solar cell. On top of this structured substrate the ultrathin CIGSe layer (red) was grown at HZB, and subsequently all the other layers and contacts needed for the solar cell. Since all layers are extremely thin, even the top layer is showing deformations according to the pattern of the nanoparticles. © G.Yin / HZB
CIGSe solar cells are made of a thin chalcopyrite layer consisting of Copper, Indium, Gallium and Selenium and can reach high efficiencies. Since Indium is becoming scarce and expensive, it is interesting to reduce the active CIGSe layer, which however decreases the efficiency quite strongly. Now, scientists at Helmholtz-Zentrum Berlin have produced high quality ultrathin CIGSe layers and increased their efficiency by an array of tiny nanoparticles between the back contact and the active layer.
Nanoparticles with sizes the order of a wavelength interact with light in specific ways. A young investigator group at Helmholtz-Zentrum Berlin, led by Professor Martina Schmid, is inquiring how to use arrangements of such nanoparticles to improve solar cells and other opto-electronic devices. Now the scientists report in the Journal of the American Chemical Society ACS Nano a considerable success with ultrathin CIGSe solar cells.
Problems add up below 1 micrometer
CIGSe solar cells have proven high efficiencies and are established thin film devices with active layers of a few micrometers thickness. But since Indium is a rare element, the active layer should be as thin as possible. This reduces the efficiency, since less light is absorbed. And if the active layer is thinner than one micrometer, an additional problem arises: more and more charge carriers meet and recombine at the back contact, getting “lost”.
Ultrathin CIGSe cell with efficiencies of 11.1%
“It took me more than one year to be able to produce ultrathin layers of only 0.46 micrometer or 460 nanometers which still reach reasonable efficiencies up to 11.1 %,” Guanchao Yin says about his PhD project. He then started to enquire how to implement nanoparticles between different layers of the solar cell. His supervisor Martina Schmid discussed this with Prof. Albert Polman, one of the pioneers in the field of nanophotonics, at the Center for Nanooptics, Amsterdam, with whom she was in contact for a while already. They proposed to produce arrays of dielectric nanoparticles by nanoimprinting technologies.
No big effect by nanoparticles on top
In a first step, the colleagues in Amsterdam implemented a pattern of dielectric TiO2-nanoparticles on top of Yin’s ultrathin solar cells; the idea was that they would act as light traps and increase absorption in the CIGSe layer. But this did not increase the efficiency as much as proved in Si-based solar cells. Yin then continued testing and ultimately found out what worked best: a nanoparticle array not on top but at the back contact of the cell!
Nanoparticles at the back contact: effiency increases to 12.3%
The colleagues from Amsterdam produced an array of SiO2 nanoparticles, directly on the Molybdenum substrate which corresponds to the back contact of the solar cell. On top of this structured substrate the ultrathin CIGSe layer was grown by Yin, and subsequently all the other layers and contacts needed for the solar cell. With this configuration, the efficiency increased from 11.1 % to 12.3 %, and the short circuit current density of the ultrathin CIGSe cells increased by more than 2 mA/cm2. With additional anti-reflective nanoparticles at the front efficiencies raised even to 13.1%.
Light trapping and prevention of charge carrier loss
“This leads to efficient light trapping and does not deteriorate the cell,” Yin explains. Further studies indicate that the nanoarray of dielectric SiO2 nanoparticles at the back side could also increase efficiency by reducing chances for charge carrier recombination. “This work is just a start, we have now new ideas for further designs to enhance absorption and reduce recombination, thus increasing efficiencies by making use of optical and electrical benefits of the nanoparticles,” Martina Schmid says.
to the paper:
M.-C. van Lare*, G. Yin*, A. Polman, M. Schmid “Light coupling and trapping in ultra-thin Cu(In,Ga)Se2 solar cells using dielectric scattering patterns” ACS Nano
DOI: 10.1021/acsnano.5b04091 (2015), *equal contribution
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.
- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.