Collodial quantum dot solar cells

Thesis

This thesis presents three specific strategies to enhance colloidal quantum dot (CQD) solar cells’ performances, including band alignment engineering, quantum dot passivation, and smart device architecture design.

Firstly, by inserting a PbS CQD layer with a relatively smaller band gap as the hole transport layer (HTL), the carrier extraction of the solar cells is much improved, so as the efficiency. The improvement is due to a better interfacial band alignment between the HTLs and the absorber layer, proved by Kelvin probe and cyclic voltammetry results. Small band gap PbS CQDs have deeper Fermi levels because of the easy oxidation. Coupled with a p-type inducing ligand, 1,2-ethanedithiol (EDT), a better valence band alignment is achieved to extract the holes more efficiently.

Secondly, alcohol-dissolvable CsI is used as the ligands to passivate QDs’ surface with the aim of reducing dangling bonds and defects on the surface. Compared to the commonly used ligand, tetrabutylammonium iodide (TBAI), CsI resulted in better passivation, which was proved by the full ligand exchange, a narrow photoluminescence peak, fewer oxide defects, and the existence of Cs-S bond. A high efficiency of 10% is achieved, which is attributed to that fewer defects and better passivation lead to larger depletion region pushing its optimal thickness and current output higher, as well as the efficiency.

Thirdly, a microgroove-structured flexible PbS CQD micro-module solar cell is reported for the first time with a record V_oc of 9.2 V. This device was fabricated by automatic dip coating methods, and it avoids the complex recombination layers required in monolithic tandem devices. By e-beam depositing two electrodes on the two walls of the V-shape grooves, devices were connected in series in less than 100 μm width. By using three-dimensional characterisations, the reasons for low efficiency were explained.

Authors: Zhang, N

• DOI: 10.5287/ora-8re2w5q8y
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English