Overcoming efficiency limits in photovoltaic devices

Photovoltaic devices have an efficiency limit of 33% set by fundamental loss processes in the semiconductor materials. The largest loss comes about as a result of charges excited far above the bandgap losing their excess energy as they cool to the bandedge, a process known as thermalisation. Carrier multiplication offers a means of reducing thermalisation losses using a system in which excess energy from one electron-hole pair is transferred to produce a second electron-hole pair. Fully harnessing carrier multiplication would raise the efficiency limit of photovoltaic devices to 44%. Typically, various challenges mean the benefits of carrier multiplication have not yet been realised. In quantum dot materials, the fundamental understanding of the carrier multiplication process is still lacking. A more thorough understanding of the cooling and recombination mechanisms is needed to fully exploit this phenomenon. In organic materials, an excited singlet exciton splits to form two triplet excitons on neighbouring molecules. The process in organics is referred to as singlet fission. Producing a device that can make use of the singlet fission process however, has proved difficult, limited in part by the instability of singlet fission materials and also by challenges brought about by the need to couple to another material. In this thesis, we will examine both quantum dot and organic systems. The initial work in this project involved the fabrication of quantum dot devices. Steps were taken to improve the efficiency and reproducibility of the devices with the aim of investigating carrier multiplication in device stacks. However, the devices work showed limited success and the decision was taken to move to ultrafast spectroscopy. In examining quantum dot films using picosecond transient absorption spectroscopy, we found clear signatures of carrier multiplication occurring in films of dots synthesis here. We also found that some of our results were at odds with explanations in the literature. To explain our results, we took data under a range of excitation conditions and compared them to the predicted results of various explanations in the literature. We also used ultrafast spectroscopy to examine singlet fission in diketopyrrolopyrrole-based molecules. Such molecules are much more stable than the polyacenes typically used in singlet fission research. We looked for the slow recombination signature of triplet excitons in films of a diketopyrrolopyrrole-based molecule using transient absorption spectroscopy. We assigned the formation mechanism of these triplet molecules using the timescale of the singlet to triplet transition. In this work, we have considered means of increasing the harvestable energy from photovoltaic devices and improved our understanding of how such efficiency increases may be possible.