Engineering Nanoscale Materials for Solar Cells

Abstract

The purpose of this work is to contribute towards developing high-efficiency low cost solar cells that have the potential to decrease the cost of solar energy. The focus is on novel device structures that aim to minimise losses and/or allow high throughput fabrication for antimony sulphide (Sb2S3) and the perovskite methyl ammonium lead iodide (MAPbI3). The first part of the research is on planar Sb2S3 solar cells, and led to a twofold efficiency increase through the use of a planar Sb2S3 layer with a high proportion of c axis aligned crystal planes perpendicular to the substrate. The transport of photo generated carriers along the c-axis aligned Sb2S3 crystal planes has a lower recombination rate and longer effective diffusion lengths than for other crystal planes. A completely planar top Sb2S3 surface on a textured (non planar) substrate was fabricated from a non-planar sulphur rich Sb2S3 layer. The planar top surface of the Sb2S3 layer facilitates the subsequent deposition of compact, thin and uniform layers of other materials which contributes to improve the photovoltaic performance. The second part of the research focused on fabrication of porous TiO2 layers via a flame aerosol system, applied to both Sb2S3 and MAPbI3 solar cells. The flame aerosol system is a high throughput deposition method that could rapidly coat a large area substrate as part of a continuous industrial production line. The mechanical stability of flame-made porous TiO2 layers is crucial to withstanding the subsequent material depositions processes via solution methods. Different annealing methods were used to increase the mechanical stability of flame made porous layers for solar cells. The porosity of the flame made porous TiO2 layers was easily adjusted over a wide range: from 97% to 35%. A porous TiO2 layer with a high porosity could improve the solar cell efficiency by increasing the collection efficiency through better infiltration of the other solar cell materials in the porous layer. The optimised MAPbI3 solar cell with flame made porous TiO2 layer had a comparable efficiency to the control MAPbI3 solar cell with the standard spin-coated porous TiO2 layer, demonstrating its potential with scope for further improvement. The efficiency and stability of perovskite solar cells could be also improved by using SnO2 instead of TiO2 as the former has better electronic and photo catalytic properties than the latter. For this reason, MAPbI3 perovskite solar cells with a flame-made porous SnO2 layer were also investigated. The MAPbI3 solar cell with a flame-made porous SnO2 had promising efficiencies even though the main limitation for a higher efficiency was the use of a compact TiO2 layer with the porous SnO2 layer. The work contained in this thesis provides pathways to reduce recombination losses and fabricate a high-throughput low-cost porous structure for Sb2S3 and MAPbI3 solar cells. The findings from this work could also be implemented with other materials; particularly with mixed-perovskites and sulphur based materials.