Interfacial Engineering: The Key to Boost Perovskite Solar Cells Performance and Stability

Perovskite solar cells (PSCs) based on metal halide materials have become the rising star in the photovoltaic (PV) field since the first discovery in 2009. A remarkable advance in this technology has been demonstrated by reaching a certified power conversion efficiency (PCE) of over 25%, closing the gap with the well-established silicon-based PV. However, the instability under the operation condition hinders the commercialization of PSCs. In order to overcome this issue, interface engineering has been proposed as a solution not only to solve the stability issue but also to improve the PV performance. Thus, the works presented in this thesis are mainly focused on the interfacial design of PSCs and the investigation of the processes therein which could control the device lifetime as well as the PV performance.

A two-dimensional (2D)/three-dimensional (3D) interface has been demonstrated as an effective strategy to realize stable and efficient PSCs. Nevertheless, the fundamental understanding of the 2D/3D interface in determining the stability and performance of PSCs is still vague. Hence, I designed 2D/3D perovskite interfaces based on the thiophenealkylammonium halide cations as the building block for the 2D perovskite. The evolution of the 2D to quasi-2D perovskite phase was observed during the aging process. Only 2D/3D interface incorporating 2-thiopheneethylammonium iodide (2-TEAI) maintained the initial 2D phase. Further, in-situ grazing incidence wide-angle X-ray scattering (GIWAXS) experiments were performed to probe the structural evolution of the 2D/3D interface under thermal stress. This study confirmed that regardless of the 2D phase evolution, the 2D layer serves as a protecting layer for the 3D perovskite underneath.

Following up this work, three analogous 2-thiophenemethylammonium cations with different halides (2-TMAX, X: Cl, Br, I) were incorporated in the 2D/3D interface. 2-TMAI and 2-TMABr 2D/3D interfaces showed a transition to the p-n junction, while 2-TMACl 2D/3D exhibited a slight energetic mismatch. The favorable energy level alignment, especially in the case of 2-TMABr 2D/3D, is beneficial for hole extraction and electron blocking. Thus, eliminating the interfacial recombination by yielding zero interfacial loss and high open-circuit voltage with a PCE approaching 21%.

In addition, a Lewis base containing a phosphine oxide group, tris(5-((tetrahydro-2H-pyran-2-yl)oxy)pentyl)phosphine oxide (THPPO), has been employed at the interface. THPPO effectively passivated the undercoordinated Pb2+ defect by forming a coordinate bond, enhancing the photoluminescence (PL) intensity and carrier lifetime. Therefore, improved PCEs of 13.31% and 20.70% were achieved in hole-transporting layer (HTL)-free and n-i-p structured PSCs, respectively.

Finally, new hole-transporting materials (HTMs) have been explored due to their important roles in preventing charge accumulation and recombination at the interface as well as in maintaining devic