Characterization and modeling of CdCl2 treated CdTe/CdS thin-film solar cells

CdTe photovoltaic technology has the potential to become a leading energy producer in the coming decades. Its physical properties are well suited for photovoltaic energy conversion. A key processing step in the production of high efficiency CdTe/CdS solar cells is a post-CdTe deposition heat treatment with CdCl2, which can improve performance by promoting CdTe recrystallization, QE response, defect passivation and others. Understanding the effects of the CdCl2 treatment is crucial in order to optimize processing conditions and improve performance. This study investigates the effects of variations of CdCl2 treatment duration on CdTe/CdS solar cells manufactured at Colorado State University. In order to investigate the optimal time of CdCl2 treatment, sample solar cells were tested for microstructural and performance properties. Device microstructure was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Device performance was analyzed using current density-voltage (J-V) measurements, time-resolved photoluminescence (TRPL), quantum efficiency (QE), and laser beam induced current (LBIC) measurements. Little change in microstructure was observed with extended CdCl2 treatment and is attributed to the high CdTe deposition temperatures used by heat pocket deposition (HPD). This deposition technique allows for large initial grains to be formed with low lattice strain energy which prevents recrystallization and grain growth that is often seen with other deposition techniques. The CdCl2 treatment initially improves performance significantly, but it was shown to that extending the CdCl2 treatment can reduce performance. Overall performance was reduced despite an increase in minority carrier lifetime values. The mechanism of reduced performance is suggested to be the formation of a low bandgap CdTe layer resulting from sulfur diffusion from the CdS layer. Sulfur diffusion primarily occurs during the CdCl2 treatment and also leads to thinning of the CdS layer. Solar cell modeling was employed to investigate possible mechanisms for performance degradation. Modeling was done with AMPS and SCAPS modeling software. Models were created to investigate the effects of minority carrier lifetime, CdS thickness, and a low bandgap CdTe layer. Modeling results showed that the formation of a low bandgap CdTe layer combined with CdS thinning reduces device performance. Further research is needed using a statistically significant number of samples to investigate other possible degradation mechanisms associated with extended CdCl2 treatment.

Name thesis:degree_name

Master of Science (M.S.)

Level thesis:degree_level

Masters

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor dc:publisher

Colorado State University. Libraries

Year dc:date.issued

2010

Authors dc:creator

• Maxwell, Graham Lane, author
• Manivannan, Venkatesan, advisor
• Sampath, W. S., committee member
• Sites, James R., committee member

• CdCl2
• CdS
• CdTe
• characterization
• modeling
• sulfur

Statement dc:rights

• Copyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.

Language dc:language.iso

eng, English

Identifier URI

https://doi.org/10.25675/3.018634

Identifier

ETDF2010400366MCEN

OAI identifier oai:identifier

oai:mountainscholar.org:10217/68386