Hybrid Perovskite Solar Cells: Semitransparent Design and Interface Optimization

Over the last 10 years, the emergence of a new thin-film PV family – the perovskite solar cells (PSCs) – has stunned the scientific community with its extraordinary performance and rapid progress in a very short time. An astonishing increase in power conversion efficiency (PCE) has been achieved from 3.8% in 2009 to 24.2% in 2019, thanks to the unique optoelectronic properties of hybrid perovskite materials and the intense research efforts devoted to optimizing film deposition methods, interfacial materials and device structures. In addition to the high performances, PSCs are easy to produce via low-cost thin-film deposition techniques and display a great potential for unconventional PV applications, such as building-integrated photovoltaics (BIPVs) and tandem PV devices, allowing color and transparency. Whilst many advances have been made in the field of PSCs, there is still much work to be done in order to enable a large-scale commercialization. This thesis aims to assist in moving towards this objective by investigating ways to overcome some of the key challenges concerning the operation, composition, cost, and stability of the devices. More specifically, the research work presented in this thesis explores new strategies for the fabrication of cost-effective, semitransparent (ST) PSCs in the prospect of BIPV applications, as well as alternative charge-transport materials, solvent additives and interface engineering approaches for effective defect passivation and performance enhancement, both in terms of efficiency and stability. First, a novel multilayer dielectric/metal/dielectric (DMD) transparent electrode based on nonprecious copper (Cu) and molybdenum suboxide (MoOx) is developed and incorporated as a top anode in planar n-i-p ST-PSCs. The formation of a continuous and percolative 9.5 nm thick copper film on top of an ultrathin Au seed layer is confirmed by various in-depth investigations. The gold seed layer is also proved to act as an effective Cu diffusion barrier. Whilst silver and gold are typically used in such DMD structures, their replacement with copper allows for a substantial cost reduction without sacrificing the device performance. Through this strategy, PCEs as high as 12.5%, along with acceptable transparency levels, are successfully achieved. Then, the attention is moved towards the effect of the perovskite layer morphology and crystallinity on the device performance and stability by using different solution-based deposition approaches. It is demonstrated that the incorporation of a small amount of α-terpineol, a non-toxic and easily accessible monoterpene alcohol, into the one-step perovskite precursor solution as a solvent additive is capable of producing more uniform and highly crystalline perovskite films with fewer defects and trap states. Through this approach, the PCE of PSCs is boosted from 13.9% to 15.4% with reduced hysteresis and improved stability. Lastly, this thesis investigates the role of selective charge-transport materials on the performance of planar and mesoscopic n-i-p PSCs. The potential of three different conjugated copolymers as alternative dopant-free hole-transport layers is explored for the first time. Improved PCEs are successfully demonstrated for one of the three copolymers compared to traditional solution-processed hole-transport layers, which typically require the use of stability-adverse dopants to reach appreciable conductivity. Preliminary studies on the development and optimization of mesoporous titania (TiO2) electron-transport layers are also presented. Specifically, Ti3+-containing blue-titania nanoparticulate (NP) films are successfully prepared and used as mesoporous scaffolds in mesoscopic n-i-p PSCs. Furthermore, a 10% PCE enhancement is demonstrated by doping the blue-titania with nitrogen element. In parallel, one-dimensional (1D) TiO2 nanorod (NR) arrays are grown by hydrothermal method and tested for the same role. The autoclaving time is opportunely optimized to achieve the best PCE.