Continuous in-line processing of CdS/CdTe devices: Process control using XRF and efficient heating

Progress in Photovoltaics

2015

A major source of loss in cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells results from light absorbed in the CdS window layer, which is not converted to electrical current. This film can be made more transparent by oxygen incorporation during sputter deposition at ambient temperature. Prior to this work, this material has not produced high-efficiency devices on tin oxide-coated soda-lime-glass substrates used industrially. Numerous devices were fabricated over a variety of process conditions to produce an optimized device. Although the material does not show a consistent increase in band gap with oxygenation, absorption in this layer can be virtually eliminated over the relevant spectrum, leading to an increase in short-circuit current. Meanwhile, fill factor is maintained, and open-circuit voltage increases relative to baseline devices with sublimated CdS. The trend of device parameters with oxygenation and thickness is consistent with an increasing conduction band offset at the window/CdTe interface. Optimization considering both initial efficiency and stability resulted in a National Renewable Energy Laboratory verified 15.2%-efficient cell on 3.2-mm soda-lime glass. This window material was shown to be compatible with SnO 2 -based transparent conducting oxide and high resistance transparent coated substrates using in-line compatible processes.

Section: Methodsmentioning

confidence: 99%

Optimization of CdTe thin‐film solar cell efficiency using a sputtered, oxygenated CdS window layer

Kephart

Geisthardt

Progress in Photovoltaics

2015

“…CdS films of thicknesses ranging from 30nm to 200nm were deposited by thermal sublimation on TEC10 glass substrates using the Advanced R&D System [18] at the PV Materials Engineering Lab at Colorado State University. These film samples were then studied for pinholes presence, identifying sources of pinholes; identifying pinholes from fully covered films, and show elemental composition of pinholes and fully covered films.…”

Section: Methodsmentioning

confidence: 99%

Morphology of CdS thin films: Pinholes and their effect on open circuit voltage in CdS/CdTe solar cells

Tashkandi

2011 37th IEEE Photovoltaic Specialists Conference

2011

CdTe solar cells are lagging other similar types of solar cell devices with similar band gap in terms of performance primarily due to lower open circuit voltage VOC (voltage deficit). Cadmium Sulfide (CdS) films affect VOC. When CdS thickness is high (more than 200nm), it absorbs some of the photons and thus generated photocurrent is reduced but the open circuit voltage is maintained. When its thickness is reduced below 100nm, photocurrent is increased but the voltage drops. This voltage loss might be due to formation of localized weak (CdTe/TCO) diodes or it might be due to some electronic effects. CdS films of different thicknesses were produced using the Advanced R& D Deposition System and were studied. The presence of pinholes in CdS film thicknesses between 30 to 200nm was investigated using Blue Light Transmission Microscopy (BLTM). SEM images and EDS analysis were conducted to confirm results of BLTM, differentiate CdS film and pinholes, to check surface morphology and chemical composition of different film areas. There are two main categories of pinholes: natural and artificial. Natural sources include lack of grain coalescence and TCO surface roughness. Artificial sources as observed in experiments include scratches, scuffing marks, dust or particulates and cleaning issues. Results thus far indicate that the primary causes of pinholes in CdS films are dust and/or particulates and cleaning issues.

“…In general, the standard HPD process is performed in a low vacuum environment where the substrate is first introduced into a CdS depositing source [8]- [9]. For this study, the plasma cleaning system was integrated into an empty HPD graphite source and placed in-line with the process.…”

Section: Introductionmentioning

confidence: 99%

Plasma cleaning of TCO surfaces prior to CdS/CdTe deposition

Swanson

Lutze

2012 38th IEEE Photovoltaic Specialists Conference

et al. 2012

Self Cite

We describe a hollow cathode plasma formed within a heated pocket deposition (HPD) source covered with a transparent conductive oxide (TCO)-coated substrate. Superhydrophilic behavior and little change in TCO transmission of TCO substrates exposed to the plasma are observed. The "plasma cleaner" source was placed in-line with the CSU advanced deposition system (ARDS), and CdS and CdTe films were deposited on both plasma and conventionally cleaned substrates. Plasma-cleaned substrates result in improved cadmium sulfide films without pinholes after plasma exposure times as short as 5 sec. Cells formed with plasma cleaned substrates display improved V OC , fill factor, and efficiency.