Since conventional TiO<sub>2</sub> electron transporting layer (ETL) can accelerate the degradation of organic lead halide perovskite solar cells, alternative ETL is required for PSCs with extended lifetime. In this work, SnO<sub>2</sub> nanoparticles, a potential ETL material, were synthesized via facile sol-gel method. Different dopants were introduced in an attempt to enhance conductivity and improve electronic properties of pristine SnO<sub>2</sub>. The prepared nanoparticles were identified to be rutile SnO<sub>2</sub> phase with crystalline size less than 4 nm. The presence of the dopant in the doped samples was confirmed by energy dispersive X-ray spectroscopy, and it is also evident from the colour change of the doped samples compared to undoped one. While the dopant incorporation has been successful, nanoparticles exhibited pronounced aggregation in water dispersions, which prevented preparation of sufficiently smooth films for application in planar perovskite solar cells. Further optimization of the nanoparticle surfaces is needed to obtain dispersions suitable for spin-coating uniform thin films of doped SnO<sub>2</sub>.
Perovskite solar cells are emerging photovoltaic technology with potential for low cost, high efficiency devices. Currently, flexible devices efficiencies over 15% have been achieved. Flexible devices are of significant interest for achieving very low production cost via roll-to-roll processing. However, the stability of perovskite devices remains a significant challenge. Unlike glass substrate which has negligible water vapor transmission rate (WVTR), polymeric flexible film substrates suffer from high moisture permeability. As PET and PEN flexible substrates exhibit higher water permeability then glass, transparent flexible backside encapsulation should be used to maximize light harvesting in perovskite layer while WVTR should be low enough. Wide band gap materials are transparent in the visible spectral range low temperature processable and can be a moisture barrier. For flexible substrates, approaches like atomic layer deposition (ALD) and low temperature solution processing could be used for metal oxide deposition. In this work, ALD SnO<sub>2</sub>, TiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub> and solution processed spin-on-glass was used as the barrier layer on the polymeric side of indium tin oxide (ITO) coated PEN substrates. The UV-Vis transmission spectra of the prepared substrates were investigated. Perovskite solar cells will be fabricated and stability of the devices were encapsulated with copolymer films on the top side and tested under standard ISOS-L-1 protocol and then compared to the commercial unmodified ITO/PET or ITO/PEN substrates. In addition, devices with copolymer films laminated on both sides successfully surviving more than 300 hours upon continuous AM1.5G illumination were demonstrated.
ZnO is considered a potential alternative of TiO<sub>2</sub> for electron transport materials of perovskite solar cells due to its relatively high electron mobility and the ease of low temperature solution-processing. Nevertheless, ZnO-based perovskite devices usually exhibit inferior device performance and stability compared to TiO<sub>2</sub> based devices due to the defect states at ZnO/perovskite interface. In this study, an ultrathin TiO<sub>2</sub> layer by ALD is applied to ZnO nanostructures, and its effect on device performance is investigated. The results indicate that TiO<sub>2</sub> ultrathin layer can effectively passivate the surface of ZnO nanostructures, resulting in enhanced device performance.