Impact of interface defects on the band alignment and performance of TiO$_2$/MAPI/Cu$_2$O perovskite solar cells

TL;DR

This work investigates how interfacial vacancy defects in a TiO2/MAPI/Cu2O perovskite solar cell stack modify band alignment and device performance. By combining ab initio DFT calculations for bulk materials and interface structures with SCAPS simulations of interfacial defect layers, the authors map vacancy-induced doping effects to photovoltaic metrics. They find that specific vacancies induce donor or acceptor behavior that can either hinder or enhance carrier collection, with iodine and lead vacancies preferring ETL-PRV interfaces and TiO2 and Cu2O vacancies significantly impacting band offsets near active interfaces. The results yield practical passivation targets and demonstrate a robust defect-engineering framework for PSCs incorporating inorganic HTLs.

Abstract

Optimizing the interfaces in perovskite solar cells (PSCs) is essential for enhancing their performance, improving their stability, and making them commercially viable for large-scale deployment in solar energy harvesting applications. Point defects, like vacancies, have a dual role, as they can inherently provide a proper doping, but they can also reduce the collected current by trap-assisted recombination. Moreover, they can play an active role in ion migration and degradation. Using {\it ab initio} density functional theory (DFT) calculations we investigate the changes in the band alignment induced by interfacial vacancy defects in a TiO$_2$/MAPI/Cu$_2$O based PSC. Depending on the type of the vacancy (Ti, Cu, O, Pb, I) in the oxide and perovskite materials, additional doping is superimposed on the already existing background. Their effect on the performance of the PSCs becomes visible, as shown by SCAPS simulations. The most significant impact is observed for $p$ type doping of TiO$_2$ and $n$ type doping of Cu$_2$O, while the effective doping of the perovskite layer affects one of the two interfaces. We discuss these results based on modifications of the band structure near the active interfaces and provide further insights concerning the optimization of electron and hole collection.

Figures (8)

• Figure 1: (a) The structure of the reference PSC: ITO as transparent conductive oxide, TiO$_2$ (ETL), MAPI perovskite absorber, Cu$_2$O (HTL), Au for the metallic back contact. The IDLs account for vacancies accumulations near the two interfaces. (b) A schematic band diagram for this structure evidences the small band offsets in the conduction band and valence band, for the ETL-PRV and PRV-HTL, respectively.
• Figure 2: Relaxed interface structures, MAPI@TiO$_2$, with Ti-O/Pb-I terminations and MAPI@Cu$_2$O, with Cu/Pb-I terminations. The supercells are periodic in the $x$-$y$ directions, while a 15 Å vacuum was considered along the $z$-direction.
• Figure 3: Band structure and PDOS for the ETL and HTM: (a) anatase-TiO$_2$, $E_g = 2.17$ eV; (b) cuprous oxide (Cu$_2$O), $E_g = 1.49$ eV, using a DFT+U correction with $U = 8$ eV and $J = 0.8$ eV on 3d Cu orbitals and $U = 12$ eV for 2p Oxygen orbitals.
• Figure 4: Band structure and PDOS for MAPI: (a) tetragonal and (b) orthorhombic structures. The energy gap is similar in both instances, $E_g =1.48$ eV and $E_g =1.52$ eV, respectively.
• Figure 5: Band alignment at MAPI@TiO$_2$ interface obtained from the PDOS of the two materials: ideal system (upper plot), followed by structures with $V_{\rm I}$, $V_{\rm Pb}$, $V_{\rm Ti}$ and $V_{\rm O}$. A number of 20 disorder realizations are depicted together with the averaged PDOS.