Sustainable and low-cost materials for printable solar cells

Abstract

Sustainable and low-cost materials have attracted extensive research attention for new generation solar cells. With the emergence of hybrid lead-halide perovskite solar cells, the last decade has witnessed an ever-increasing power conversion efficiency rise in the field of printable photovoltaics. However, major issues in lead-halide perovskite solar cells research concern the toxicity of lead and the long-term device stability, which potentially hampers the large-scale manufacturing process for the next stages of development. Meanwhile, tedious synthesis routes and the expensive cost of the commonly used hole transport materials (HTM) further slow the commercialisation. Therefore, alternative lead-free absorbers with long-term stability and low-cost HTMs are needed.

This thesis starts with two inorganic Bi-based absorbers, Ag3BiI6 (Chapter 3) and NaBiS2 (Chapter 4). Although the silver rudorffites (Ag-Bi-I) family has been widely reported with promising efficiency of up to 5.44% in the conventional p-i-n solar cell configurations, no previous research has been carried out using these in the fully printable triple-mesoscopic solar cells (TM-SC). In Chapter 3, the study sets out to examine the feasibility of using Ag3BiI6 as an absorber in TM-SC and to investigate the effects of pre-treatment (e.g. biPY, 4-tBP, CPDT-1 and N719) on pore-filling and overall device performance. The best as-prepared device achieved 0.33% efficiency. To improve the device performance, CuSCN was added as post-treatment to enhance the hole extraction, boosting the power conversion efficiency (PCE) to 0.74% after ageing. Another Bi-based material, NaBiS2, has not previously been studied as a light absorber, whereby most studies have focused on the photocatalytic application. Previous theoretical studies highlighted that NaBiS2 could potentially achieve better performance than the analogue AgBiS2, which has been reported with 6.3% efficiency. In Chapter 4, the overall aim of the project is to apply a straightforward xanthate decomposition method to obtain NaBiS2 thin-films. Although this study presented here confirmed a largely phase-pure and fast photo-response of NaBiS2 films, the device performance in conventional n-i-p solar cells was poor. By introducing the xanthate decomposition method in TM-SC, the as-prepared device achieved 0.01% efficiency and improved performance (0.04%) was observed after one-week ageing.

This thesis also includes a study of low-cost HTM—1,3,5-tris(2’-(N-phenothiazylo)phenyl)benzene (TPB-2-TPTZ) that has been previously reported as an alternative for commonly used Spiro-OMeTAD. Despite promising properties held by TPB(2-TPTZ), the device performance was below expectation in previous studies. In Chapter 5, the aim is to determine what limited the solar cell efficiency and whether changing the dopants (e.g. NOBF4 and CuBr2) and using polymer (e.g. PTAA and PMMA) additives could improve the film quality and thus the conductivity. It is evidently clear from the findings that the previously reported low efficiency is largely affected by the low conductivity due to poor film quality. In this study, although this was still lower than state-of-the-art HTMs used in perovskite solar cells, with the combination of 22%-CuBr2 and 14%-PMMA additives, TPB(2-TPTZ) exhibited 10 times higher conductance compared to conventional Li-TFSI doping.