In the past few years, perovskite solar cells (PSCs) have attracted increasing attention from researchers. Due to its unique properties, such as efficient light absorption, long carrier diffusion length, solution processability, tunable band structure, etc., PSCs show an ultra-fast development with a certified power conversion efficiency (PCE) over 22%. As a part of PSCs devices, inverted structure devices exhibit their advantages like simple process, hysteresis free, better stability, dopant free organic layer and so on. My thesis research is directed at understanding and developing inverted structure PSCs, especially on material growth control and interface design of the NiO based PSC devices.

There are four layers need to be well controlled in the fabrication of inverted struc...[ Read more ]

In the past few years, perovskite solar cells (PSCs) have attracted increasing attention from researchers. Due to its unique properties, such as efficient light absorption, long carrier diffusion length, solution processability, tunable band structure, etc., PSCs show an ultra-fast development with a certified power conversion efficiency (PCE) over 22%. As a part of PSCs devices, inverted structure devices exhibit their advantages like simple process, hysteresis free, better stability, dopant free organic layer and so on. My thesis research is directed at understanding and developing inverted structure PSCs, especially on material growth control and interface design of the NiO based PSC devices.

There are four layers need to be well controlled in the fabrication of inverted structure PSCs, the hole transporting material, perovskite absorber, electron transporting material, and the metal electrode. All these layers should have suitable properties, such as fast carrier transfer, good transmittance for the electrode, strong absorption and efficient carrier generation for perovskite absorber. In addition, the interfaces between these layers, such as the interface between hole transporting materials and perovskite, perovskite and electron transporting layer, electron transporting layer and metal electrode, are also needed to be well controlled because each of them has strong impact on the ultimate device performance. The hole transporting materials and perovskite interfaces could influence the hole extraction process, the growth of perovskite from quality, and morphology to the growth direction. The perovskite and electron transporting layer interface affects the electron extraction process, resulting from the perovskite morphology and electron transporting layer coverage. The contact between electron transporting layer and metal electrode plays a key role in the carrier collection.

In this thesis, the NiO based perovskite solar cells with electron transporting layer of PCBM have been used to demonstrate our work. The studies focused on the perovskite material growth through different deposition methods, and interface design including the NiO/perovskite and perovskite/PCBM interfaces. Furthermore, the less studied perovskite/perovskite interface, which is appeared as grain boundaries in the perovskite crystal film, was also investigated.

In detail, systematic studies of material and interface based on the inverted structure perovskite solar cells were conducted, including (1) The influences of film coverage and morphology at the perovskite/PCBM interface on the charge transfer and recombination process. Avoiding the direct contact of perovskite with metal electrode led to the reduction of recombination, which contributed to a significant enhancement on V_oc; (2) The effects of molecular modification at the NiO/perovskite interface on perovskite film growth and charge transfer. The molecule, which was used to modify the interface, formed chemical bonds with NiO and perovskite. This structure enabled a slower and moderate formation of perovskite crystal film with better interfacial contact, and thus obtained an enhanced light absorption of perovskite absorber and higher carrier injection efficiency from perovskite to NiO; (3) The studies of perovskite growth mechanism based on investigating the relationship between intermediate phase and perovskite film, and was applied for device fabrication. The composition of intermediate films showed strong impact on the formation of perovskite crystal film. The intermediate film with pure MA₂Pb₃I₈DMSO₂ phase enabled an up-growth route, which led to a high quality, monolithic perovskite crystal film with compact interfacial contact, facilitating the charge transfer within perovskite film and charge extraction at interface; (4) Developing of a new deposition strategy for depositing 3D-2D graded perovskite interface, which combined the advantages of 2D and 3D perovskite with better stability and higher efficiency. The surface energy level adjusting reduced the recombination from PCBM to perovskite, which significantly increased the V_oc of the device to a record value. The existing 2D structure not only protected the perovskite from the invasion of water and oxygen from air, but also suppressed the ion diffusion of perovskite to metal electrode, which enhanced the stability of entire device under ambient environment and thermal treatment. With all these studies and efforts, the efficiency of NiO based perovskite solar cell has been promoted from less than 9% to approximately 20% with good stability and without hysteresis. At the same time, the influences of interfaces and materials on the charge transfer/recombination, perovskite polycrystalline film growth as well as energy alignment have been well investigated and discussed.

In my thesis, different strategies have been developed to modify the interfaces and enhance the perovskite film growth. Chemical and material mechanisms associated with perovskite film growth have been systematically investigated. The combination of material and interface design leads to an outstanding performance of the NiO-based inverted structure perovskite solar cells. Overall, the study and development of NiO-based perovskite solar cell showed its potential for the further improvement.