Supergen SuperSolar: Photovoltaic materials and devices

Supergen SuperSolar: Photovoltaic materials and device

Copper zinc oxide: investigation into a p-type mixed metal oxide system

In photovoltaics, ZnO is widely used both as an n-type window and buffer layer and as the basis for a range of transparent conducting oxide (TCO) top contact materials. Whilst there are reports of p-type doping, there have been no successful attempts to create a p-type ZnO-based TCO. Synthesis of an effective p-type TCO could be of significant benefit, enabling further development of technologies such as bifacial and multijunction devices. This study investigates the effects of combining ZnO with CuO, an intrinsically p-type narrow band-gap metal oxide, with the aim of synthesising a range of mid to wide band-gap p-type alloys. Alloying ZnO with CuO gave a range of p-type films with varying electrical, optical and structural characteristics. XRD patterns show that as copper content was increased above 24.5% of the total metal content, crystal structure changed from a typical ZnO wurtzite structure to a CuO tenorite structure. A second change occured at 35% copper with the emergence of two further tenorite peaks. These structural changes correlate to significant local increases in band-gap, visible and infrared transmission, and resistivity. These dramatic changes correspond to relatively minor compositional variations. Through careful control of the alloy composition it is therefore theoretically possible to tailor the material properties to suit a wide range of applications, both in photovoltaics and in other fields

Cupric oxide-based p-type transparent conductors

This study examines the impact of doping on the resistivity of sputtered cupric oxide (CuO), and investigates the effects of co-sputtering CuO with tin dioxide (SnO2). It was found that films sputtered from a 2 at. % sodium-doped target have resistivities of four orders of magnitude lower than equivalent undoped films. Addition of oxygen was found to reduce the resistivity further. The best films were found to have resistivities of 4.3x10-2 Ω.cm. Co-sputtering with SnO2 was found to increase the band gap significantly, although it also caused an increase in the resistivity. All mixed oxide films were both amorphous and p-type

Optical optimization of perovskite solar cell structure for maximum current collection

High conversion efficiency has been recently demonstrated for Perovskite thin film photovoltaic devices. Perovskite thin film solar cells are multilayer opto-electrical structures in which light interference occurs. This phenomenon can be used to maximise the light transmission into the absorber material and increase the device efficiency. Fine tuning of the layer thicknesses within the stack can be used to control interference at the interfaces. Optical reflection losses can be reduced by achieving destructive interference within the structure of the cell. The light transmission to the Perovskite absorber of a thin film solar cell on a fluorine doped tin oxide transparent conductor has been modelled using the transfer matrix method. Alternative transparent conductor materials have been also investigated including AZO and ITO. The modelling showed that replacing FTO with ITO could increase the photocurrent by as much as 4.5%. The gain can be further increased to 6.5% by using AZO as the TCO material. Fine tuning of the TiO2 layer thickness can increase the current density by 0.3%. Furthermore, the current density of a Perovskite solar cell can be increased by application of a multilayer anti-reflective coating by another 3.5%. Optical optimisation of the stack design offers a significant increase in conversion efficiency

A tunable amorphous p-type ternary oxide system: the highly mismatched alloy of copper tin oxide

The approach of combining two mismatched materials to form an amorphous alloy was used to synthesise ternary oxides of CuO and SnO2. These materials were analysed across a range of compositions, and the electronic structure was modelled using density functional theory. In contrast to the gradual reduction in optical band gap, the lms show a sharp reduction in both transparency and electrical resistivity with copper contents of greater than 50 %. Simulations indicate this change is caused by a transition from a dominant Sn 5s to Cu 3d contribution to the upper valence band. A corresponding decrease in energetic disorder results in increased charge percolation pathways: a `compositional mobility edge'. Contributions from Cu(II) sub band-gap states are responsible for the reduction in optical transparency

Combinatorial study of Sn-Ti-W-O transparent conducting oxide thin films for photovoltaic applications

a combinatorial study of transparent conducting oxide thin films based on SnO2–TiO2-WO3 phase space is reported. These multinary oxide films were fabricated by magnetron reactive co-sputtering of tin monoxide (SnO), titanium (Ti) and tungsten (W) targets. SnO2–TiO2-WO3 film compositions with Ti/Sn ratio (0.02 – 0.12) and W/(Ti+Sn) ratio (0.02 – 0.25) were explored. The effect of oxygen partial pressure on composition, structure and optical properties was evaluated. High optical transparency above 80% across the visible spectrum was obtained for sputtered ternary SnO2-TiO2 oxide films for oxygen partial pressure >19.4%. A positive correlation between optical bandgap and Ti/Sn ratio was observed. However, optical properties deteriorated as Ti-content increased in the as-deposited SnO2-TiO2-WO3 films. All studied as-deposited SnO2-TiO2-WO3 thin films were found to be highly resistive. X-ray diffraction data indicated no long-range structural order

An infra-red reflecting optical coating for solar cover glass

A major problem with silicon solar cells is that they lose efficiency with increased operating temperature, at a rate of about 0.5% per 1◦C increase. This causes a significant reduction in power output, particularly in hot climates. A solution in the form of an optical coating is presented, which reflects infrared (IR) radiation to limit the module temperature increase. The optical coating is also anti-reflecting (AR) in the visible wavelength range, increasing the amount of light reaching the cell absorber. Modelling results show that the weighted average reflection (WAR) is reduced to 1.22% in the wavelength range associated with the band gap of silicon. The optical coating then reflects up to 70% of the infra-red. Although the model presented is based on silicon, the coating design can be modified to work with other photovoltaic technologies. The coating design uses only 4 layers and can be deposited using conventional high throughput magnetron sputtering systems already familiar to glass manufacturers. Preliminary work on optimising the coating deposition parameters is also presented here alongside modelling results. Deployment of the infra-red reflecting optical coating on solar cover glass represents a potential breakthrough in solar technology and will result in a significant increase in the power output of photovoltaic modules.<br

Comparison of DC and RF sputtered aluminium-doped zinc oxide for photovoltaic applications

AZO was sputter deposited using both RF and DC power supplies from a pre-formed ceramic AZO target with a 2 wt % aluminium oxide dopant concentration. The target was formed by hot pressing pre-doped nanoparticles produced using an emulsion detonation synthesis technique. AZO films were found to display good optical and electrical properties regardless of deposition technique. Increased temperatures above 200 °C were found to have a negative impact on the electrical properties, with a corresponding increase in infrared transmission. Visible transmission was found to be good (above 79 %) for all films except those deposited using DC power at room temperature. The DC sputtering process was stable and was found to approximately double the deposition rate with no corresponding drop in film quality for films deposited at moderate temperatures. This makes DC deposition more industrially attractive when process temperature is not a concern. For room temperature depositions, RF was found to produce superior transmission and electrical characteristics

Assessment and improvement of thermoelectric pyranometer measurements

This work evaluates the variability of thermoelectric pyranometer calibration values seen when using different calibration methods and practices. The pyranometer calibration ISO 9847:1992 standard leaves many procedural details to the user’s discretion. The variability resulting from different interpretations influences PV system performance monitoring and energy yield modelling. Improved methods and more robust standardisation are therefore needed to reduce uncertainty in field-deployed thermoelectric pyranometers and consequently reduce risk in PV system energy yield assessment. This paper investigates the variability induced by relaxed calibration procedures defined in the standard Furthermore, it proposes indoor procedures for the characterisation of pyranometer response to incidence angle and temperature which have not yet been defined in the standards. Uncertainty of calibration factors including under high angles of incidence and a few cloudy data series from outdoor methods were found to be up to 2.08%, compared with 1.4% stated by the manufacturer. Uncertainty increases up to 4.73% when reference and test sensors are of different types. Results of indoor calibration procedures agreed to within 1.21% even when calibrating multiple sensors at the same time. The instability of the irradiance source contributed more to the overall uncertainty than the selection of the procedure. The angular response of the devices tested was close to the prescribed limits [1]

Development of ZnTe as a back contact material for thin film cadmium telluride solar cells

Cadmium telluride (CdTe) is high-efficiency commercialised thin film photovoltaic technology. However, developing a stable low-resistivity back contact to the CdTe solar cells is still an issue. High work function and low level of doping of this material don't allow to create an ohmic contact with metals directly. Copper is commonly used to lower the back contact barrier in CdTe solar cells, but an excessive amount of copper diffusing through the cell is harmful for the device performance and stability. In this work a copper-doped ZnTe (ZnTe:Cu) buffer layer was incorporated in between CdTe and gold metal contact by high-rate pulsed DC magnetron sputtering. The back contact was then activated by rapid thermal processing (RTP) resulting in spectacular improvement in key device performance indicators, open circuit voltage (VOC) and fill factor (FF)