Colloidal quantum dots photovoltaics with low-dimensional carbon nanomaterials
Keywords: Graphene
Photovoltaic
Quantum Dot
Carbon Nanotubes

Colloidal quantum dot (CQD)-based photovoltaics are an emerging low-cost solar cell technology with power conversion efficiencies exceeding 14%, i.e., high enough to be interesting for commercialization. Well-controlled and understood charge carrier transport through the device stack is required to make the next step in efficiency improvements.
In chapter 5, polymer-wrapped single-walled carbon nanotube (SWNT) films embedded in an insulating poly(methyl methacrylate) (PMMA) matrix and capped by a thermally evaporated Au electrode are investigated as a composite hole transport layer and optical spacer. Employing transient absorption spectroscopy this chapter shows that the SWNTs enhance the charge transfer rate from CQD to CQD, ZnO, or SWNT. In order to pinpoint the underlying mechanism for the improvement, the energetics of the junction was investigated by measuring the relative alignment of the band edges, using Kelvin probe and cyclic voltammetry. The measurements of the external quantum efficiency and absorption gave a perspective that the improvement is not mainly from electronic improvements but from enhanced absorption of the CQD absorber. The hypothesis that the transparent PMMA matrix acts as an optical spacer which enhances absorption in the absorber layer, was proven experimentally and theoretically. With these electronic and optical enhancements, the efficiency of the PbS CQD solar cells improved from 4.0% to 6.0%.
In chapter 6, a SWNT-P3HT composite was utilized in a bulk heterojunction structure with a PbS EDT layer. A bulk heterojunction structure is the architectural approach to have better charge transport while thickening the PbS CQD photoactive layer of the photovoltaic devices. Devices with a bulk heterojunction layer of SWNT-P3HT composite and PbS CQD with 1.2- Ethanedithiol (EDT) were compared with control devices which have planar heterojunction structure between PbS-EDT and SWNT-P3HT with varying the thickness of PbS-EDT layer from 60 nm to 240 nm. Among all devices, bulk heterojunction devices with 120 nm PbS-EDT thickness recorded the highest power conversion efficiency of 11.5%. In general, bulk heterojunction devices exhibited better carrier transfer with less monomolecular carrier recombination. As a result, devices with bulk heterojunction structure have demonstrated better fill factor values. By thickening the PbS-EDT layer from 60 nm to 240 nm, both bulk heterojunction devices and planar heterojunction devices showed increases in absorption. The bulk heterojunction devices could increase Jsc while the thickness of PbS-EDT layers were thickened up to 240 nm. Contrary to the bulk heterojunction devices, the planar heterojunction devices had a decrease in Jsc after the peak of power conversion efficiency with 120 nm thick PbS-EDT layer. The difference in Jsc with thicker PbS-EDT layer was examined by IQE analysis and electronic impedance spectroscopy measurements.
In chapter 7, the optimized method to transfer the graphene layer to a PbS CQD thin film has been studied, and the device performance of the PbS CQD solar cells using graphene and the control PbS CQD solar cells were compared. First the effects of annealing and an acetone bath treatment to both graphene stacking and the quality of PbS CQD thin film have been explored. Although the annealing effect did not seem to have a significant impact on the stacking of the graphene layer onto PbS CQD thin films, annealing had a detrimental impact to performances of PbS CQD devices. To understand the detrimental effect of those two treatments, FTIR and XPS measurements were done. Both measurements confirmed the removal of ligands coordinating to PbS nanocrystals and the oxidation of PbS nanocrystals which degrade the quality of PbS CQD thin films. The aim of annealing graphene devices is to make the graphene layer stick well to the PbS CQD thin film. However, Raman microscope mapping revealed that the graphene layer without annealing is stuck as well as the layer with annealing treatment with the same defect level. In the latter part of the chapter, side-by-side comparisons of the graphene device and the control device have been made. The graphene devices performed better over control devices in absorption and Jsc. Moreover, intensity-dependent Voc measurements and quantum efficiency measurements confirmed that the graphene device has less monomolecular carrier recombination and better IQE which results in a better Voc and FF. The results of electronic impedance spectroscopy further confirmed the better charge transfer with a graphene monolayer in PbS CQD PV devices. Thus the graphene layer has been proven to be a good hole transport layer.