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Abstract
 

The inverted planar perovskite solar cells (PSCs) have attracted great attention due to the simple fabrication, negligible current hysteresis and excellent device stability. The hole transport layers (HTLs) play a significant role in inverted PSCs. Among different HTLs, inorganic nickel oxide (NiO_x) is promising candidate, since it is wide bandgap semiconductor with high transparency and suitable energy levels. However, the efficiency of inverted PSCs with NiO_x HTLs still lags behind those with organic counterparts and normal structure PSCs due to the low conductivity of pristine NiO_x and large interfacial recombination losses.This thesis mainly focused on the improvement of conductivities of the NiO_x HTLs to enhance the PCE and stability via various strategies.

The impact of Cesium (Cs) dopant on the optoelectronic properties of NiO_x and the photovoltaic performance for inverted planar PSCs was firstly investigated. Cs doped NiO_x films were prepared by a simple solution-based sol-gel method. The Cs doped NiO_x exhibits better hole conductivity and higher work function than the pristine NiO_x. Therefore, Cs doping results in a significant improvement in the performance of NiO_x based inverted PSCs. The best efficiency of Cs doped NiO_x devices achieved 19.35%, and those devices show high stability as well. The improved efficiency in devices with Cs:NiO_x is attributed to a significant improvement in the hole extraction and better band alignment compared to undoped NiO_x.

To lower the process temperature of NiO_x HTLs and further improve the device efficiency, the impact of copper doping on the optoelectronic properties of NiO_x and the doping mechanism were then investigated. High quality Cu doped NiO_x HTLs were prepared by spin coating the Cu:NiO_x NPs inks at RT. Theoretical calculations revealed that Cu doping results in shallower acceptor energy levels compared to gap states of nickel vacancies in undoped NiO_x. Moreover, both Cu⁺ and Cu²⁺ states coexisted in the Cu:NiO_x, and the substitution of Ni²⁺ with Cu⁺ contributes to both increased carrier concentration and carrier mobility, leading to significant increase in the conductivity and increased work function in Cu:NiO_x films. These factors resulted in an improvement of all photovoltaic performance parameters and consequently an increased efficiency of the inverted PSCs. The PCE exceeding 20% could be achieved for small area devices, while PCE values of 17.41% and 18.07% were obtained for flexible devices and large area (1 cm²) devices on rigid substrates, respectively.

Furthermore, doping of NiO_x hole transport layerusinganorganic moleculewas realizedsuccessfully by F6TCNNQ.Results from first principle calculationsconfirmed thedirect electron transfer from NiO_x to F6TCNNQ, indicating the strong doping effect of NiO_x by the organic molecule.Determined by XPSand UPS, the fermi level of NiO_x HTLswasincreased from −4.63 to −5.07 eV after F6TCNNQ doping. This resulted in significant decrease of theenergy level offset between the VBMs of NiO_x and perovskites, dramacitally improve the charge transfer efficiency from perovskite to NiO_x. The best PCE of 20.86%has beenachievedforinvertedPSCs with molecule doped NiO_x HTL. This workoffersa promising and facilemethodfor improving the conductivity of inorganic CTLs by molecular doping in optoelectronic devices.

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