The newly emerging photovoltaic technology based on organo-metal halide perovskite solar cells (PSC) is a competing contender with the currently dominating silicon solar cells. PSC show promising optoelectronic properties including long carrier lifetimes, high light absorption, low cost and lightweight, reaching a power conversion efficiency (PCE) of 25.5%. Further, the low temperature processing of perovskite film make it uniquely suitable for flexible PSC (f-PSC) owing to characteristic low formation energies. While the record PCE is yet to be realized for f-PSC, in particular when zinc oxide (ZnO) is used as electron transporting layer (ETL) because the Lewis basic nature of ZnO surface leads to deprotonation of perovskite layer. However, the low formation energies also cause instability for long-term operation and restrict high mechanical durability due to low toughness of the perovskite film. In view of these challenging, f-PSC is developed with ZnO-based ETL with the objective to achieve high efficiency, utilize external strain via the piezo-phototronic effect, stability, minimize lead (Pb)-leaching, and high mechanical endurance. 

First, this thesis summarizes the fundamental properties of perovskite solar cell and its application in flexible devices and highlights the benchmark issues thereof, starting from the effect of external strain on the photovoltaic properties, lead leaching, and mechanical endurance for long-term operation, which lays the foundation for designing this study.

Second, we report an interfacial approach to stabilize the ZnO-based f-PSC by converting ZnO surface to zinc sulfide (ZnS) for f-PSC. While the Lewis basic nature and the presence of surface defect on ZnO surface led to the deprotonation the perovskite layer, the ZnS interlayer plays a significant role by passivating the ZnO/perovskite interface and reducing the hydroxyl group on the ZnO surface. The ZnS strongly coordinates with the lead ion (Pb²⁺ ion) of the perovskite forming Zn-S-Pb pathway, thus adjusting the energy level and significantly improve the charge extraction from the perovskite layer. Taking advantage of the piezoelectric properties of ZnO coupled with its electrical and optical properties, the strain induced piezopotential generated in ZnO upon mechanical deformation, known as the piezo-phototronic effect, modulates the band structure at the interface and thus effecting the photovoltaic performance. The PCE remarkably improved from 12.94% (strain-free) to 14.68% under static external strain of 1.5%, which is accredited to piezopotential charges generated at the interfaces, which modulate the energy band alignment and thereby facilitating the spatial separation of the photoinduced charge carriers and reducing charge recombination probability. 

Third, an interfacial design based on zeolitic imidazole framework-8 (ZIF-8) based metal organic frameworks (MOF) was used as interlayer between ZnO-based ETL and perovskite interface for f-PSC. The modified ZnO with ZIF-8 interlayer obstructs the deprotonation of perovskite layer and passivates the ZnO surface by alleviating the surface defects, improving device stability, and significantly improving the charge extraction and transport from the perovskite layer. Together with uplifted work function, V_oc and FF increased substantially from 0.98 to 1.01 V and from 67% to 74%, respectively, resulting in a stable PCE of 16.05%, and reaching 18.44% at tensile strain of 1.77% via the piezo-phototronic effect. The ZIF-8 modified ZnO exhibits high adsorption capacity of Pb, trapping the mobile Pb²⁺ ion at the ZnO/perovskite interface, and thus preventing the negative impact of lead leaching on environmental sustainability.


Fourth, the instability and low mechanical endurance due to the high brittleness of perovskite film stemming from low formation energies are overcome by incorporating soft and sticky elastomer (s-ELA) into the perovskite film to intrinsically reinforce the grain boundaries (GBs) and connect the rigid perovskite grains. The s-ELA act as scaffold in the perovskite crystallization process, reducing the structural defect at GBs and significantly improving the charge transport properties. This soft-rigid design combined with the device engineering promote stretchability and bending stability, while simultaneously improves the mechanical endurance and preserves the morphology after 10,000 deformation cycles at narrow bending curvature of 2 mm and stretching of 20%. Further the hydrophobicity of s-ELA protects against ingress of moisture and oxygen and thus improves the device stability for long term operation.

In summary, this thesis systematically investigates the electron transporting layer with core-shell structure as passivating interlayer for reducing charge recombination, improving long-term stability and capability to adsorb Pb from damaged device with high mechanical endurance for f-PSC via interfacial and device engineering approaches. This work provides insights on the fundamental understanding of charge extraction and transfer at ETL/perovskite interface and introduces the concept for high stretchability beyond bending toward a wide range of applications in next generation flexible and wearable electronics.