Simple Scalable Superstrate Cells wtih Good Thermal Stability and High Voc.
The project will focus on the development, optimisation and characterisation of printed/solution processed solar cells with carbon contacts. We will use both mesoporous carbon stacks prepared at SPECIFIC and screen-printed carbon substrates prepared in Bath. The electron/hole selective substrate will be prepared by spin coating a relevant oxide layer (we can make 15% efficient inverted perovskite cells on spin coated p-type NiO and have expertise with n-type TiO2 and SnO2). The active photo-absorbing layer will be prepared by spin-coating, spray deposition or simple infiltration with a syringe followed by annealing. A range of inorganic absorber layers will be investigated, including CsPbBr3 and binary metal sulphides such as Sb2Se3. We have worked extensively with CsPbBr3, which is a yellow material and has a non-ideal band gap above 2eV. The band gap can be reduced by introducing increasing fractions of iodide into the material. The problem with pure CsPbI3 is that is exists in two phases – a room temperature δ -phase and a high temperature α -phase. The black α -phase has a lower band gap which is desirable for solar cells, but it is not stable and usually converts to the δ -phase within hours. A number of strategies have been developed to try and solve this problem. Recently Li et al. used a polymer, poly-vinylpyrrolidone (PVP), to stabilise the CsPbI3 surface and maintain the cubic α -phase for more than 80 days . This exciting approach opens the possibility of screening a range of surface modifying agents, from small molecules to polymers to stabilise the desired phase of the material. An alternative approach is to work with mixed halides as the addition of Br to CsPbI3 helps to stabilise the desired α -phase. The main problem with this approach is that phase separation has previously been shown to occur under illumination – where the material separates into bromide-rich and iodide rich regions . Beal et al. managed to avoid this problem by working with low substitution levels . The looked at CsPb(BrxI1−x)3, where 0.2 < x < 0.4. Above x = 0.2 the cubic phase was preferred in the material, and below x = 0.4 phase separation did not occur (as evidenced by photoluminescence (PL) measurements). Li et al achieved champion cell efficiencies of just over 10% using PVP stabilized CsPbI3. Beal et al. achieved champion efficiencies of 6.7% for CsPbBrI2 cells. We have already achieved champion efficiencies of 8% for pure CsPbBr3 devices on carbon. We believe that one reason for the high efficiency is that the crystallization of the CsPbBr3 within the confined nanopores of the carbon support strongly influences the material – something we are investigating in more detail at the moment. We therefore believe there is scope to pursue a two-pronged approach. Firstly we will substitute iodide into CsPbBr3, working at low enough substitution levels to avoid phase separation. The stability and composition of these materials will be studied using PL and XPS in Swansea. The band gap and phase may not be ideal for ultra-high efficiency devices, but the benefits of good stability, high voltages and thermal processability need to be considered, especially if modules with efficiencies above 10% can be achieved. In a second approach we will test small co-ordinating molecules for their ability to stabilise the desired phase of CsPbI3. Finally metal Sulphides will be deposited using single-source metal xanthate precursors synthesised in collaboration with Andy Johnson. This is another material that has not been widely studies within carbon contacts and the potential efficiency of the devices is unknown.The aim is to make stable high voltage cells (particularly in the case of the perovskite materials) and modules (SPECIFIC) and to characterise the limits on their stability and efficiency. The student will have access to the full range of characterisation techniques in Bath (including SEM, TEM, XRD, photocurrent mapping and EQE measurements, Impedance) as well as cutting-edge facilities in Swansea.
Kaya Davies Brechley