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

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.

💡 Research Summary

This work investigates how atomic vacancy defects at the interfaces of a TiO₂/MAPI/Cu₂O perovskite solar cell (PSC) influence band alignment and overall device performance. Using density‑functional theory (DFT) calculations, the authors first benchmark the bulk electronic structures of anatase TiO₂, cuprous oxide (Cu₂O), and methylammonium lead iodide (MAPI). TiO₂ is modeled with the local‑density approximation (LDA), Cu₂O with an LDA+U correction (U = 8 eV for Cu‑3d, U = 12 eV for O‑2p), and MAPI without additional corrections, yielding band gaps of 2.17 eV, 1.5 eV, and ≈1.5 eV respectively—values that are in reasonable agreement with experiment given the known DFT limitations.

Two realistic interface models are constructed: (I₁) TiO₂/MAPI and (I₂) MAPI/Cu₂O. To mimic the inevitable lattice mismatch, supercells are built with mismatches below 1.1 % (I₁) and 5 % (I₂). For each interface, twenty random defect configurations are generated for five vacancy types: V_Ti, V_O, V_Pb, V_I, and V_Cu. Projected density of states (PDOS) is extracted for each configuration and averaged to assess systematic trends.

The DFT analysis reveals clear defect‑induced doping behavior. In TiO₂, oxygen vacancies (V_O) act as donors, pushing the material toward p‑type conductivity, whereas Ti vacancies (V_Ti) behave as acceptors, counteracting this effect. In Cu₂O, copper vacancies (V_Cu) generate n‑type character, while oxygen vacancies again act as donors (p‑type). Within the perovskite, iodine vacancies (V_I) introduce n‑type carriers, whereas lead vacancies (V_Pb) create p‑type carriers. These dopings shift the relative positions of the conduction‑band minimum (CBM) and valence‑band maximum (VBM) at the interfaces, altering the built‑in electric fields that drive charge separation.

To translate these microscopic changes into macroscopic device metrics, the authors employ the one‑dimensional semiconductor simulation tool SCAPS. The reference PSC consists of ITO/TiO₂ (100 nm)/MAPI (330 nm)/Cu₂O (100 nm)/Au, with TiO₂ and Cu₂O slightly doped n‑ and p‑type respectively, and an intrinsic perovskite layer. Interface defect layers (IDLs) of 10 nm thickness are introduced at each interface, with defect concentrations varied from 0 to 5 × 10¹⁸ cm⁻³ and with either n‑ or p‑type character matching the DFT‑predicted doping.

Simulation results show that V_O‑induced p‑type doping of TiO₂ dramatically reduces the open‑circuit voltage (V_oc) from ~0.9 V to below 0.6 V and lowers the short‑circuit current density (J_sc) by up to 20 %. The fill factor (FF) also drops by 10–15 % due to increased recombination at the distorted interface. Analogously, V_Cu‑induced n‑type doping of Cu₂O leads to a similar degradation of V_oc, J_sc, and FF, confirming that both interfaces are highly sensitive to the sign of the defect‑induced doping. Defects within the perovskite (V_I, V_Pb) affect primarily one interface and have a comparatively smaller impact on overall performance.

The study concludes that interface vacancies are not merely trap centers; they act as effective dopants that reshape band offsets and internal electric fields, thereby controlling charge extraction efficiency. Consequently, controlling the concentration of oxygen and metal vacancies during the deposition of TiO₂ and Cu₂O is crucial for achieving high‑efficiency, stable PSCs. The authors suggest practical routes such as low‑temperature solution processing with post‑annealing, plasma oxygen treatments, or alternative deposition techniques to suppress undesirable vacancies. Future work is proposed to combine experimental defect spectroscopy (e.g., photoluminescence, deep‑level transient spectroscopy) with the presented computational framework, and to extend the methodology to other ETL/HTL material combinations for broader design guidelines.