Development of a low field electrically detected magnetic resonance system to observe performance limiting defects in CdTe/CdS solar cells

Open Access
Author:
Shaw, Erik Thomas
Area of Honors:
Engineering Science
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Patrick M Lenahan, Thesis Supervisor
  • Barbara Shaw, Honors Advisor
  • Judith A Todd, Faculty Reader
Keywords:
  • EDMR
  • EPR
  • CdTe
  • CdS
  • solar cell
  • PV
  • resonance
  • low field
Abstract:
Electrically detected magnetic resonance (EDMR) has proved to be a valuable analytic tool for the characterization of defects in semiconductor devices. By measuring small changes in current in the device, EDMR has much higher defect sensitivity than conventional electron paramagnetic resonance (EPR). Low-field EDMR is a new technique that has shown promise in defect characterization. Conventional EPR has very low sensitivity at low magnetic fields, but this is not a problem with EDMR. In fact, making EDMR measurements at low magnetic field may have benefits in signal strength. We developed an EDMR system from the ground up. This system consists of several subsystems, which will be discussed in depth. The completed system is capable of producing a reproducible and well calibrated magnetic field ranging from -150 to +150 Gauss, a variable modulating field, and an applied RF power. The goal was to successfully show that the low-field EDMR system was operational, and then use it to observe spin dependent recombination (SDR) in various devices, most notably CdTe/CdS solar cells from the National Renewable Energy Lab (NREL). We will first discuss the theory of operation of the system and some background on CdTe/CdS solar cells, EDMR, and EPR of CdTe. Then, we will give a detailed description of all the equipment used in the EDMR set up and how the subsystems function. We were able to successfully achieve a zero field response for a well characterized 4H SiC DMOSFET, and then possibly achieved a zero field spectrum for CdTe/CdS under forward bias. The results will be analyzed and then further areas of research will be discussed.