Address: CREST, Wolfson School of Mechanical, Electrical and Manufacturing Engineering, Holywell Park, Loughborough University, Loughborough, UK, LE113TU
ABSTRACT Although the cadmium chloride treatment is an essential process for high efficiency thin... more ABSTRACT Although the cadmium chloride treatment is an essential process for high efficiency thin film cadmium telluride photovoltaic devices, the precise mechanisms involved that improve the cadmium telluride layer are not well understood. In this investigation we apply advanced micro-structural characterization techniques to study the effect of varying the time of the cadmium chloride annealing treatment on the micro-structure of cadmium telluride solar cells deposited by close spaced sublimation (CSS) and relate this to cell performance. A range of techniques has been used to observe the morphological changes to the micro-structure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Techniques used include Transmission Electron Microscopy (TEM) for sub-grain analysis and XPS for composition-depth profiling. The study provides a new insight in to the mechanisms involved in the initiation and the subsequent complete re-crystallization of the cadmium telluride layer.
ABSTRACT A new laser ablation/inkjet process has been developed for the interconnect of thin film... more ABSTRACT A new laser ablation/inkjet process has been developed for the interconnect of thin film photovoltaic modules. This process involves laser ablation and inkjet printing of insulator and conductor materials carried out with high precision. Any error will compromise the device efficiency by increasing the series resistance or by causing shunting effects. Here we present a way of characterizing these interconnects using Scanning White Light Interferometry (SWLI). The SWLI technique allows the precise measurement of the laser scribe profile. We also present the use of a transmission line method (TLM) for the measurement of the contact resistance between the inkjet silver conductor and the Transparent Conducting Oxide.
ABSTRACT Monolithic series interconnection is a key advantage of thin-film PV. The standard appro... more ABSTRACT Monolithic series interconnection is a key advantage of thin-film PV. The standard approach is to alternate layer deposition and laser scribing. Here the M-Solv patented One Step Interconnect (OSI) process is introduced as an alternative to the conventional all laser interconnect. OSI interconnects TF-PV modules in a single step after all deposition is complete with considerable advantages: reduced capital expenditure; better process control; less vacuum/air interfaces, reduced line footprint and faster panel transit. OSI employs a combination of laser scribing and inkjet printing of functional materials. OSI mini-modules have been fabricated on CdTe with good electrical performance. Although the focus here has been CdTe, OSI is applicable to all thin-film technologies. The laser and inkjet processes presented are fully scalable for industrial production.
ABSTRACT It is well known that the cadmium chloride annealing treatment is an essential step in t... more ABSTRACT It is well known that the cadmium chloride annealing treatment is an essential step in the manufacture of efficient thin film cadmium telluride solar cells. It has been recognized that the combination of annealing at ∼4000C together with the addition of cadmium chloride at the surface induces re-crystallisation of the cadmium telluride layer and also affects the n-type cadmium sulfide. We have applied advanced micro-structural characterization techniques to distinguish the effect of the annealing and the cadmium chloride treatments on the properties of the cadmium telluride deposited via close space sublimation (CSS) and relate these observations to device performance. Transmission electron microscopy (TEM) has shown a variation in stacking fault density with annealing temperature and annealing time. Stacking faults observed within the cadmium telluride grains in TEM were partially removed post annealing; these findings show that temperature alone has a role in the reduction of stacking faults. However, since we have previously observed almost complete removal of stacking faults with annealing in combination with cadmium chloride, the cadmium chloride is essential to defect removal and high efficiency cells.
ABSTRACT Cupric oxide thin films were sputtered onto soda-lime glass slides from a single pre-for... more ABSTRACT Cupric oxide thin films were sputtered onto soda-lime glass slides from a single pre-formed ceramic target using a radio-frequency power supply. The effects of oxygen partial pressure and substrate temperature on the optical, electrical and structural properties of the films were studied. It was found that increasing temperature resulted in increased crystallinity and crystal size but also increased film resistivity. The most conductive films were those deposited at room temperature. Increasing oxygen partial pressure was found to reduce resistivity dramatically. This is thought to be due to higher charge carrier concentrations resulting from increased copper vacancies. Increasing oxygen partial pressure causes an increase in the optical band gap from a minimum of 0.8eV up to a maximum of 1.42eV. Oxygen-rich films display reduced crystallinity, becoming increasingly amorphous with increased oxygen content. These results show that the optical, electrical and structural properties of sputtered cupric oxide films can be controlled by alteration of the deposition environment.
ABSTRACT Although the cadmium chloride treatment is an essential process for high efficiency thin... more ABSTRACT Although the cadmium chloride treatment is an essential process for high efficiency thin film cadmium telluride photovoltaic devices, the precise mechanisms involved that improve the cadmium telluride layer are not well understood. In this investigation we apply advanced micro-structural characterization techniques to study the effect of varying the time of the cadmium chloride annealing treatment on the micro-structure of cadmium telluride solar cells deposited by close spaced sublimation (CSS) and relate this to cell performance. A range of techniques has been used to observe the morphological changes to the micro-structure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the micro-structural properties of the cells have also been undertaken. Techniques used include Transmission Electron Microscopy (TEM) for sub-grain analysis and XPS for composition-depth profiling. The study provides a new insight in to the mechanisms involved in the initiation and the subsequent complete re-crystallization of the cadmium telluride layer.
ABSTRACT A new laser ablation/inkjet process has been developed for the interconnect of thin film... more ABSTRACT A new laser ablation/inkjet process has been developed for the interconnect of thin film photovoltaic modules. This process involves laser ablation and inkjet printing of insulator and conductor materials carried out with high precision. Any error will compromise the device efficiency by increasing the series resistance or by causing shunting effects. Here we present a way of characterizing these interconnects using Scanning White Light Interferometry (SWLI). The SWLI technique allows the precise measurement of the laser scribe profile. We also present the use of a transmission line method (TLM) for the measurement of the contact resistance between the inkjet silver conductor and the Transparent Conducting Oxide.
ABSTRACT Monolithic series interconnection is a key advantage of thin-film PV. The standard appro... more ABSTRACT Monolithic series interconnection is a key advantage of thin-film PV. The standard approach is to alternate layer deposition and laser scribing. Here the M-Solv patented One Step Interconnect (OSI) process is introduced as an alternative to the conventional all laser interconnect. OSI interconnects TF-PV modules in a single step after all deposition is complete with considerable advantages: reduced capital expenditure; better process control; less vacuum/air interfaces, reduced line footprint and faster panel transit. OSI employs a combination of laser scribing and inkjet printing of functional materials. OSI mini-modules have been fabricated on CdTe with good electrical performance. Although the focus here has been CdTe, OSI is applicable to all thin-film technologies. The laser and inkjet processes presented are fully scalable for industrial production.
ABSTRACT It is well known that the cadmium chloride annealing treatment is an essential step in t... more ABSTRACT It is well known that the cadmium chloride annealing treatment is an essential step in the manufacture of efficient thin film cadmium telluride solar cells. It has been recognized that the combination of annealing at ∼4000C together with the addition of cadmium chloride at the surface induces re-crystallisation of the cadmium telluride layer and also affects the n-type cadmium sulfide. We have applied advanced micro-structural characterization techniques to distinguish the effect of the annealing and the cadmium chloride treatments on the properties of the cadmium telluride deposited via close space sublimation (CSS) and relate these observations to device performance. Transmission electron microscopy (TEM) has shown a variation in stacking fault density with annealing temperature and annealing time. Stacking faults observed within the cadmium telluride grains in TEM were partially removed post annealing; these findings show that temperature alone has a role in the reduction of stacking faults. However, since we have previously observed almost complete removal of stacking faults with annealing in combination with cadmium chloride, the cadmium chloride is essential to defect removal and high efficiency cells.
ABSTRACT Cupric oxide thin films were sputtered onto soda-lime glass slides from a single pre-for... more ABSTRACT Cupric oxide thin films were sputtered onto soda-lime glass slides from a single pre-formed ceramic target using a radio-frequency power supply. The effects of oxygen partial pressure and substrate temperature on the optical, electrical and structural properties of the films were studied. It was found that increasing temperature resulted in increased crystallinity and crystal size but also increased film resistivity. The most conductive films were those deposited at room temperature. Increasing oxygen partial pressure was found to reduce resistivity dramatically. This is thought to be due to higher charge carrier concentrations resulting from increased copper vacancies. Increasing oxygen partial pressure causes an increase in the optical band gap from a minimum of 0.8eV up to a maximum of 1.42eV. Oxygen-rich films display reduced crystallinity, becoming increasingly amorphous with increased oxygen content. These results show that the optical, electrical and structural properties of sputtered cupric oxide films can be controlled by alteration of the deposition environment.
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Papers by Jake Bowers