In the perovskite solar cell the perovskite absorber is sandwiched in between selective contacts, the hole transport layer (HTL) and the electron transport layer (ETL). In a CIGS/pero tandem cell the HTL forms the p-type part of the recombination junction that connects top and bottom cell electrically and mechanically. Requirements for this hole-conducting material in this application are: - A formation temperature of below 200°C, because it is deposited onto the CIGS device, which degrades at higher temperature - A band gap exceeding 1.7 eV to make it transparent for long wavelength light - Sufficient carrier density to form a low-loss recombination junction together with the n-type window material of the bottom cell - Chemical compatibility with both perovskite and bottom cell window - Mitigation of the roughness of the bottom cell for reliable perovskite growth and avoidance of shunts - Long-term stability under the conditions of outdoor use of the PV device Currently the preferred approach is a combination of Nickel oxide (sputtered or from a solution containing nano-particles) and a self assembled mono-layer (SAM). Research on this topic forms part of the Solmates project.
