In the solar energy arena, there is a considerable market for portable, lightweight, and flexible solutions for solar energy harvesting, in which organic and perovskite photovoltaics are two promising candidates. Cathode-bottom device architecture is commonly used in organic and perovskite photovoltaics due to its long-term stability. However, in this structure the electron transport layer (ETL) adjacent to the cathode commonly requires high temperature annealing, which limits the use of flexible substrates. In this thesis, low temperature processed ZnO thin films are applied as ETLs in organic and perovskite solar cells with two fabrication methods employed: i) sol-gel chemistry and ii) aqueous ammonia approach.
In terms of ZnO films by two methods, the relationship between the film composition and the film properties (optical, electrical, surface morphological) is elucidated. The low temperature processed ZnO by sol-gel chemistry, which was previously commonly considered as amorphous ZnO, has been confirmed consisting of ZnO nanoparticles embedded in an extended amorphous organic matrix in our study. The aqueous processed counterpart has been confirmed containing only ZnO nanoparticles. The distinct compositional difference of two low temperature processed films results in some interesting comparisons, not only in the film properties but also when applied in the organic and perovskite solar cells. In addition, the influence of film processing temperature, in a range from as-cast to 450 ˚C high temperature, is investigated.
We have proved that sol-gel and aqueous processed ZnO films both are effective ETLs in the application of OPVs. Specially, the work function of the ETLs shows significant impact on the device performance. In the application of perovskite solar cells, despite the intrinsic nature of the thermal instability of perovskite when fabricated on ZnO, the ZnO/PCBM bilayer ETLs with two ZnO processing routes both enable all-low- temperature, thermal stable, low hysteresis and high performance devices.