High Quality Metal Oxide Charge Transfer Layer Towards Highly Efficient and Stable Perovskite Solar Cells


Student thesis: Doctoral Thesis

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Awarding Institution
Award date1 Sept 2020


During the past several years, hybrid organic-inorganic perovskite solar cells has unprecedentedly rapid progress in device efficiency and stability, marking this new class of materials competitive for the development of future photovoltaic technology. Since the first solar cell with perovskite materials as the sensitizers was reported in 2009 with the power conversion efficiency (PCE) of 3.8%, extremely rapid improvements in efficiency have been achieved during the last decade reaching a certified value of 25.2% in 2019. Perovskite solar cells (PSCs) also have great advantages for large scale production compatible with the high-throughput and low-cost manufacturing processes. Despite of the encouraging results in device efficiency, the stability is still a concern which limits the commercialization of this technology. One of the degradation pathways in PSCs is at the perovskite/charge-transporting-layer (CTL) interface. Solution-processed metal oxide CTLs, featuring intrinsic stability, low-cost and ease of fabrication, have recently demonstrated with potential for achieving high efficiency and long-term stability. However, several metal oxide CTLs based PSCs still have stability issues resulted from light soaking, surface defects and poor charge transporting properties. Those are still needed to be further addressed for the future industrial production.

This dissertation focuses on the investigation and development of several engineering approaches on solution-processed metal oxides towards highly efficient and stable PSCs. The corresponding research works can be mainly divided into three parts: (1) Investigation on the underlying mechanism of light soaking phenomenon in inverted planar PSCs with sol-gel derived NiOx hole transport layer (HTL).(Chapter 3) (2) Molecular passivation on sol-gel NiOx with n-butylamine to eliminate light soaking effect. (Chapter 4) (3) Improving the charge extraction of photogenerated carriers via dual oxides (NiOx/GO) to realize over 80% fill factor in inverted planar PSCs. (Chapter 5)

The light soaking effect has been widely observed in PSCs with metal oxides as the CTLs. However, the underlying mechanism is still not clear. In the chapter 3, we have found that the light soaking effect is caused by the defect states at the NiOx/perovskite interface, as evident by photothermal spectroscopy (PDS). Taking the sol-gel NiOx inverted planar PSCs as an example, it is found that all the photovoltaic parameters increase after light soaking with UV illumination while the performance almost remains the same by filtering the UV light. It is proposed that the attached ions (CH3NH3+, I, or Pb2+) by the strong dipole moment on the surface of NiOx migrate back to the perovskite crystal with the aid of UV light, which heals the defects near the NiOx/perovskite interface and contributes to the performance improvement under light soaking.

In chapter 4, in order to eliminate the light soaking effect, we passivate sol-gel NiOx with n-Butylamine, which has similar dipole moment to the surface of NiOx. With n-Butylamine passivation, the device performance has been improved with a high PCE of 18.9% due to the increased Voc and Jsc. The passivation effect has been confirmed by the X-ray photoemission spectroscopy (XPS). Moreover, the light soaking induced instability can also be eliminated with n-Butylamine passivation. By integrating the PDS spectra over the sub-gap range, it is found that the trap states at the NiOx/perovskite interface have been largely reduced from 8.0×1017 to 2.1×1017 cm−3.

One of intrinsic issues of sol-gel NiOx is its poor charge transport property. In chapter 5, dual oxides with NiOx/rGO structure have been developed as the HTL in inverted planar PSCs. The NiOx provides well-matching energy level for hole extraction from perovskite, while the rGO increases the conductivity of the film as evident by the impedance spectroscopy. As a result, a large improvement in fill-factor (FF) from 73% to 81% has been obtained in inverted planar PSCs, which is among the highest values reported in literatures. Moreover, the improved charge extraction reduces charge accumulation at the HTL/perovskite interface which improves the device stability with negligible degradation after 70 days storage.