19 October 2015

Helmholtz-Zentrum Berlin boosts ultrathin CIGS solar cell efficiency using nanoparticle array

CIGSe solar cells  (made of a thin chalcopyrite layer consisting of copper, indium, gallium and selenium) can reach high efficiencies but, since indium is becoming scarce and expensive, there is an aim to reduce the active CIGSe layer, which strongly decreases the efficiency. Now, Helmholtz-Zentrum Berlin has produced high-quality ultrathin CIGSe layers and increased their efficiency by incorporating an array of nanoparticles between the back contact and the active layer.

Nanoparticles with sizes the order of a wavelength interact with light in specific ways. A group at Helmholtz-Zentrum Berlin led by professor Martina Schmid is investigating how to use arrangements of such nanoparticles to improve solar cells and other optoelectronic devices. The team has now reported success with ultrathin CIGSe solar cells (M.-C. van Lare et al‚ 'Light coupling and trapping in ultra-thin Cu(In,Ga)Se2 solar cells using dielectric scattering patterns', ACS Nano; DOI: 10.1021/acsnano.5b04091).

CIGSe solar cells have proven high efficiencies and are established thin film devices with active layers a few microns thick. 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. Also, if the active layer is thinner than 1 micron, an additional problem arises: more charge carriers meet and recombine at the back contact, getting 'lost'.

"It took me more than one year to be able to produce ultrathin layers of only 0.46 microns (460nm) which still reach reasonable efficiencies up to 11.1%," says co-author Guanchao Yin about his PhD project. He then started to  enquire how to implement nanoparticles between different layers of the solar cell. His supervisor Schmid discussed this with professor Albert Polman (a pioneer in nanophotonic) of the Center for Nanooptics (FOM Institute AMOLF) in Amsterdam. They proposed to  produce arrays of dielectric nanoparticles by nanoimprinting technologies.

Picture: 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, followed by all other layers and contacts. Since all layers are extremely thin, even the top layer shows deformations according to the pattern of the nanoparticles. Credit: G.Yin / HZB.

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. However, this did not increase the efficiency as much as proved in silicon-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.

The colleagues in 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, followed subsequently all the other layers and contacts needed for the solar cell. With this configuration, efficiency increased from 11.1% to 12.3%, and the short-circuit current density of the ultrathin CIGSe cells rose by more than 2mA/cm2. With additional anti-reflective nanoparticles, at the front efficiencies raised even to 13.1%.

"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," Schmid says.

Tags: Helmholtz CIGS PV

Visit: http://pubs.acs.org/doi/abs/10.1021/acsnano.5b04091

Visit: www.helmholtz-berlin.de