Investigation of n- and p-type tin sulfide for use in a homojunction photovoltaic device

Open Access
- Author:
- Cordell, Jacob J
- Area of Honors:
- Materials Science and Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Dr. Jeffrey Brownson, Thesis Supervisor
Dr. Robert Allen Kimel, Thesis Honors Advisor
Mark William Horn, Faculty Reader - Keywords:
- tin sulfide
SnS
photovoltaics
hot probe
homojunction - Abstract:
- Tin Sulfide is a promising material for photovoltaic applications, with a demonstrated absorption coefficient greater than 104 cm-1, carrier concentration greater than 1015 cm-3 and a band gap near 1.3 eV. SnS has been studied as a p-type absorber layer largely in conjunction with CdS to form a heterojunction in other laboratories. These studies have, however, largely been restricted to SnS while a number of SnSx stoichiometries where 1<x<2 have been less researched or disregarded for not conveying desired properties. This investigation proposes the study of a SnSx homojunction and seeks to isolate both p-type and n-type SnSx synthesis routes via manipulation of deposition and post-deposition heat treatment parameters. Films for this study were deposited via RF magnetron sputtering and annealed in the same chamber in vacuum at between 200 and 400°C for 20 to 60 minutes. Both p-type and n-type films were obtained and replicated with varying success in achieving absorption onset in the visible light spectrum and suitable morphologies for efficient carrier passage. Phases in the films were identified using x-ray diffraction and scanning electron microscopy was used to view the morphology of the films. Ultraviolet/visible light spectrophotometry was used to measure reflection and transmission through films on glass substrates which was subsequently used to calculate absorption coefficient with respect to incident energy and band gap of the material. Thickness measured via transmission line method was also used in these calculations. Band gaps measured for both n- and p-type films fell in the range from 1.6-2.1 eV, higher than the theoretical bandgap of SnS of 1.3 eV. Thicknesses of the films ranged from 200 to 450 nm with the average around 250 nm. Sheet resistance and resistivity were calculated using Ohm’s Law with voltage and current data from 4 point probe measurements. The conductivity of the samples was measured using the hot point probe technique. Several samples showed strong p-type character while only one was strongly n-type on both silicon and glass substrates, however, many samples were identified as weakly p- or n-type noted by a voltage response during hot point probe testing between 0.1 mV and 1 mV. These n-type samples are good candidates for homojunction construction with the solidly researched p-type SnS phase and further study may optimize this device construction.