A group of scientists from the University of Wuppertal, the University of TÃ¼bingen, the University of Potsdam, HZB, Max Planck Institute and the University of Cologne in Germany recently developed a perovskite-organic tandem solar cell with optimized charge extraction, a high open-circuit voltage and a thickness of just 1 Âµm.
The tandem configuration includes a narrow-bandgap organic subcell with a p-i-n-type architecture based on the polymer PM6 and molybdenum oxide (MoOx) as the hole extraction layer (HEL). The cell has a power conversion efficiency of 17.5%, an open-circuit voltage of 0.87 V, a short-circuit current of 26.7 mA cm'2, and a fill factor of 75%. The wide-bandgap perovskite subcell was built with a perovskite known as FA0.8Cs0.2Pb(I0.5Br0.5)3, with an efficiency of 16.8%, an open-circuit voltage of 1.34 V, a short-circuit current of 15.6 mA cm'2, and a fill factor of 81%.
'We leveraged the unique property of ALD to provide utmost control of layer thickness even at the level of Ã¥ngstrÃ¶ms, which is impossible with conventional deposition techniques,' they said.
The ultra-thin interconnect is claimed not to introduce notable optical losses and to boost the external quantum efficiency (EQE) of the organic back-cell and the overall short-circuit current of the tandem device by about 1.5 mA cm '2 compared to a reference cell with an interconnect based on 1 nm of silver, which results in an efficiency of only about 20%.
By contrast, the tandem solar cell achieved a power conversion efficiency of 24.0%, an open-circuit voltage of 2.15 V, a short-circuit current of 14.0 mA cm'2, and a fill factor of 80%.
'The high EQE values (>95%) in the wavelength region between 400 nm and 500 nm can be explained by optical effects associated with the high refractive index of the perovskite material, which affords an efficient in-coupling of light into the active medium in this spectral range,' the research group said. 'The high open-circuit voltage of 2.15 V of the tandem cell results from an almost ideal addition of the J-V characteristics of the subcells without any loss in open-circuit voltage.'
'At the time we started this project, perovskite/organic tandems were just around 20% efficient. We are now at 24% and simulations show that our concept should allow us to reach efficiencies even beyond 30%,' the team concluded.