Hybrid multi-junction solar cells based on a monolithic integration of a wide-bandgap halide perovskite and low-gap organic polymer sub-cells – (MUJUPO2)

• Applicants: Privatdozentin Dr. Selina Olthof; Professor Dr. Thomas Riedl
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 415231147

Tandem solar cells based on a serial connection of wide-gap and narrow-gap sub-cells allow to minimize thermalization losses and thereby unlock improved efficiencies compared to single junctions. During the first 2-year phase of this project we explored wide-gap perovskite materials (Eg > 1.8eV), such as FA0.8Cs0.2Pb(I0.5Br0.5)3, regarding their fundamental properties and their potential for application in solar cells. By an in-depth analysis of limits inferred by the choice of charge transport layers, we were able to minimize losses and thereby realize wide gap perovskite cells (Eg = 1.85 eV) with a high open circuit voltage of 1.34 V and a concomitantly high fill factor of 80%. For the narrow-gap cell we selected a ternary organic photo-active system based on a combination of fullerene and non-fullerene acceptors, that showed an efficiency up to 17.5%. Unexpectedly, we evidenced an outstanding operational stability of the organic sub-cell in maximum power point tracking (more than 1,000h without notable degradation) under illumination conditions, where excitons are created exclusively on the NFA, which resembles the situation of the narrow-gap sub-cell in a tandem architecture. Most prominently, we developed a novel interconnect based on an ultra-thin (1.5 nanometers) metal-like indium oxide layer grown by atomic layer deposition, which afforded unprecedented low optical/electrical losses. The resulting perovskite/organic tandem cells showed an efficiency of 23.5% which sets a new milestone for perovskite/organic tandem devices and outperforms the best p-i-n perovskite single junctions. In this second phase of the project, we will explore wide-gap perovskites, such as the all inorganic compounds CsPbxSn1-xBr3 or CsPb(IxBr1-x)3, in order to further improve on the long-term stability of the perovskite sub-cell. We will identify optimal charge extraction layers that afford minimal losses in Voc and improved stability. The overreaching goal is to unravel the interactions taking place at these interfaces in order to understand their effects on phase segregation and quasi Fermi level splitting observed during the first funding phase. Complementing the device characteristics, the careful analysis of the electronic structure by photoelectron spectroscopy will be of paramount importance to gain these key insights. Regarding the organic small-gap sub cell, we will investigate the origin of the unexpected operational stability of our non-fullerene cells and explore if this is a general property that likewise applies for other NFAs, ideally with even further reduced energy gap. At the end of the second phase we aim to realize perovskite/organic multi-junction cells that outperform the best current perovskite/perovskite tandem cells.

• DFG Programme: Research Grants