Probing the Enhanced Stability Against Oxygen Induced Photodegradation by Selection of Transport Layer and Defect Passivation
Chieh-Ting Lin a b, Jinhyun Kim a, Sebastian Pont a, Francesca De Rossi c, Jenny Baker c, Jonathan Ngiam b, Trystan Watson c, Martyn McLachlan b, James Durrant a c
a Department of Chemistry and Centre for Plastic Electronics, Imperial College London, South Kensington Campus, London, United Kingdom
b Department of Materials and Centre for Plastic Electronics, Imperial College London., Exhibition Road, London, United Kingdom
c SPECIFIC IKC, College of Engineering, Swansea University, Bay Campus, Fabian Way, Swansea SA1 8EN, U.K.
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference on Perovskite and Organic Photovoltaics and Optoelectronics (IPEROP19)
Kyōto-shi, Japan, 2019 January 27th - 29th
Organizers: Hideo Ohkita, Atsushi Wakamiya and Mohammad Nazeeruddin
Poster, Chieh-Ting Lin, 057
Publication date: 23rd October 2018

Perovskite solar cell (PSC) power conversion efficiencies (PCE) have exceeded 22%, due to their high absorption coefficients, long diffusion lengths, and tunable band gaps. For perovskite photovoltaic cells to be commercially viable, their instability needs to be overcome. This instability is attributed to factors such as heat, moisture, oxygen, and light. Specifically, the combined action of oxygen and light can limit the operational lifetime of conventional structure unencapsulated devices (FTO/cp-TiO2/mp-TiO2/MAPbI3/PTAA/Au) in ambient air.[1] Herein, we found that under oxygen-induced photodegradation conditions, inverted structure devices (ITO/PTAA/MAPbI3/PCBM/BCP/Cu) exhibit twenty-fold longer lifetimes than conventional structure devices. We also observed slower photobleaching in MAPbI3/PCBM bilayers compared to MAPbI3/Spiro-OMeTAD. This enhanced stability against oxygen-induced photodegradation is shown to be strongly related to the LUMO level of the fullerene acceptor; a lower LUMO level results in increased thin-film stability. Degradation of perovskite have been suggested to be caused by superoxide.[2] Thus, we propose the following mechanism to explain enhanced stability in these device structures: the fullerene acceptor which has a deeper LUMO level accepts electrons more efficiently from superoxide, inhibiting its formation and functions as a superoxide quencher.  Furthermore, such instability due to oxygen and light can be worsened by surface defects on perovskite. Thus, by passivating surface defects, both device and thin film lifetimes under the combination of oxygen and light, can be enhanced.

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