Researchers at Australia's University of Sydney, University of New South Wales, Macquarie University, Germany's Forschungszentrum Jülich, China's Southern University of Science and Technology ans Slovenia's University of Ljubljana have developed a perovskite-silicon solar cell design using a top perovskite PV device with an energy bandgap of 1.67 eV and a self-assembly monolayer based on carbazole. The tandem cell achieved a higher efficiency compared to counterparts without the monolayer and passed the IEC 61215 standard thermal cycling test.
The device is intended for applications as a top cell in perovskite-silicon tandem solar devices, where the upper cells must have a high energy bandgap to achieve output current matching. These top cells, however, suffer from a higher bandgap-voltage offset, due to non-radiative recombination and energetic misalignment between the perovskite and charge-selective layers. To address this issue, the team utilized a self-assembled monolayer (SAM) based on carbazole, which acts as an effective hole-selective layer (HSL). These SAMs were previously utilized in experimental solar cells and are commonly developed through a molecular glue added during processing in order to dramatically improve adhesion between the light-absorbing perovskite layer and the electron transport layer.
“The SAM typically consists of an anchoring group, a spacer, and a terminal group,” the scientists explained, noting that their monolayer is also based on phosphonic acid (Ph-2PACz) and two benzene rings to create an extended conjugated system. “Carbazole-based SAM-HSLs have emerged due to their respectable stability and energy level alignment with perovskites, and they are ‘electron rich,' favoring hole selectivity.”
The top perovskite cell was made with an indium tin oxide (ITO) substrate, the SAM, a perovskite absorber, a buckminsterfullerene (C60) electron transport layer, a bathocuproine (BCP) buffer layer, and a copper (Cu) metal contact. It achieved a power conversion efficiency of 21.3%, an open-circuit voltage of 1.26 V, a short-circuit density of 20.5 mA/cm2, and a fill factor of 82.6%.
“The bandgap-voltage offset at 0.41 V is one of the lowest for any 1.67 eV perovskite solar cells with a record efficiency,” the team said.
The scientists used the top cell in a 1.03 cm2 monolithic perovskite-silicon tandem solar cell, and they found that the latter achieved a power conversion efficiency of 28.9% and an open-circuit voltage of 1.91 V. Some reference cells used based on other types of monolayers achieved efficiencies ranging from 25% to 26%.
“The Ph-2PACz-based tandem device showed enhanced damp-heat durability, producing negligible performance losses after 280 h of damp heat,” the group stated. “Notably, the encapsulated Ph-2PACz- based tandem device retained 98.8% of its initial efficiency after 200 thermal cycles, passing the IEC 61215 photovoltaic module standard.”