From electrolytes to hole transport materials in dye-sensitised and perovskite solar cells

Dye-sensitised solar cells (DSSCs) and perovskite solar cells (PSCs) have brought new impetus to the photovoltaic research field due to their artistry such as colour and flexibility, and stable performance under low-intensity light. These solar cells adopt a common architecture of <electron transport layer / light absorber layer / electrolyte or hole transport layer>, however show distinct features owing to the different light absorber materials, i.e. a donor-acceptor type dye for the DSSC and a metal-halide perovskite for the PSC. One of the remaining challenges for the DSSC and PSC is their long-term stability. Leakage of the volatile electrolyte especially in DSSCs has pushed the field towards developing solid-state hole transport materials (HTMs). However, depositing the HTM in DSSCs has often led to poor interfaces, lowering the performance of the cell. PSCs have overcome this challenge by increasing the thickness of the perovskite layer, but have suffered from shorter shelf-lifetime than DSSCs, owing to the instability of the perovskite material against moisture and ion migration within the cell. This thesis explores the solution towards highly efficient and stable DSSCs or PSCs, by taking a broad approach for both solar cells with a focus on the <electrolyte or HTM> layer. Our first study (Chapter 3) investigates a previously reported copper bipyridyl electrolyte in respect to the standard iodide/triiodide electrolyte in DSSCs. We demonstrate that the copper bipyridyl DSSCs reach high photovoltages over 1.0 V, with co-sensitised devices attaining power conversion efficiencies (PCE) over 9% at 1 sun, 10% at 0.1 sun, and 29% at 1000 lux. Owing to the low-cost 5T dye co-sensitised with the efficient XY1 dye, the cost performance was found to be improved. These copper bipyridyl DSSCs are known for their ability to be turned into solid-state cells by slowly drying out the electrolyte, which leads to our second study (Chapter 4) on solid-state polyiodide DSSCs. Here we demonstrate that a novel solid-state polyiodide (Ply-I) DSSC can be produced from a standard iodide/triiodide electrolyte. These solid-state cells reached a stabilised maximum PCE of 5.0% compared to the corresponding liquid-state cells at maximum 5.7%. The Ply-I cell was shown to be stable over 8000 h without encapsulation. Our third study (Chapter 5) moves onto PSCs. A novel monothiatruxene-based organic HTM (TrxS-2MeOTAD) was synthesised and demonstrated to work as efficiently as the standard Spiro-MeOTAD HTM (~19%), and with higher operational stability. In our last study (Chapter 6) we investigate copper iodide complexes as dopant-free organometallic HTM candidates in PSCs. This unique class of HTMs has led to relatively low performance (maximum 6.5%) than predicted from their conductivity, apparently due to the poor interface and grain boundaries observed in the fabricated PSCs. Overall, we believe that the approach and results described in this thesis will provide useful insights in designing alternative electrolytes and HTMs to achieve highly practical DSSCs and PSCs