Hasitha Mahabaduge

Determination of heterojunction band offsets between CdS bulk and PbS quantum dots using photoelectron spectroscopy

Photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (ΔEV) of a CdS/PbS quantum dot (QD) heterojunction for which the PbS QD layer was deposited using solution based layer-by-layer dip coating method on top of RF magnetron sputtered CdS. A value of ΔEV = 1.73 eV was obtained using the Cd 3d and Pb 4f energy levels as references. Given the band gap energies of the CdS and PbS-QD layers, the conduction band offset ΔEC was determined to be 0.71 eV.

Sputtered HRT layers for CdTe solar cells

This work focuses on demonstrating the suitability of various high resistivity transparent (HRT) layers prepared by magnetron sputtering for sputtered CdS/CdTe cells. HRT buffer layers added between the transparent conducting oxide (TCO), and the CdS layer are important for reducing the effects of non-uniformities and shunts in large-area thin-film devices. CdS/CdTe cells were fabricated on Pilkington TEC 7 glass coated with a sputter deposited HRT layer of ZnO:Al or SnO2. In some cases O2 was added to the Ar sputter gas to increase the resistivity of the HRT buffer layers. Film properties were optimized for HRT performance by adjustments in the substrate deposition temperature, sputter gas pressure, and RF power. Best results have been obtained with reactively sputtered ZnO:Al with 2 % O2 in Ar.

Semitransparent PV windows with sputtered CdS/CdTe thin films

Photovoltaic devices have been increasingly relied upon for distributed power generation at various levels (portable power, residential, commercial and municipal). One particularly appealing option for PV devices is their incorporation as building-integrated or vehicle-integrated structures which would serve both electrical generation and aesthetic requirements. Two particular embodiments of this application are semitransparent PV windows and curtain walls for residential/commercial buildings as well as PV sunroofs for vehicles. The technology of semitransparent PV modules with good photo-conversion efficiency and electrical performance is non-trivial. This report discusses the use of ultrathin devices based upon CdS/CdTe films augmented with color shifting technologies to meet these requirements.

PbS quantum dot thin film solar cells using a CdS window layer

We describe results of our investigations of the structural, optical, and electronic properties of PbS-QD films fabricated using layer-by-layer dip coating based on 1,2-ethanedithiol as an insolubilizing agent. Our investigations extend to a study of the photovoltaic properties of heterojunction thin film solar cells fabricated by sputter-deposition of a CdS ntype thin film followed by deposition of a PbS-QD thin film. Our CdS/PbS-QD solar cells exhibit open circuit voltage in excess of previously reported PbS-QD solar cells. Under standard simulated AM1.5G illumination, we observe short circuit current density as high as 12 mA cm-2, open circuit voltage as high as 0.65 V, and a maximum efficiency of 3.3%.

CdTe/CdS thin film solar cells in the substrate configuration on a single-wall carbon nanotube back contact

CdTe and CdS thin films have been grown by vapor assisted transport in a commercial deposition process onto carbon single-wall nanotube (SWNT) thin films of various thickness at temperatures in excess of 570 °C. The structure and morphology of the films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The SWNT layers were undamaged by the high temperature deposition process, and the semiconductor layers exhibited good crystal quality and grain orientation. Solar cells were fabricated in the substrate configuration (SWNT/CdTe/CdS/TCO) and current density/voltage measurements were obtained. Devices exhibited a Voc of 540 mV and an efficiency of 4% without optimization. These results indicated the possibility of using SWNT layers as a back contact layer for CdTe/CdS devices grown in the substrate configuration.