Chemical incorporation of copper into indium selenide thin films

Indium selenide thin-films have been treated in a copper-containing chemical bath with the goal of forming a precursor layer capable of being converted into copper indium diselenide. The conversion process was carried out by annealing the layers in a tube furnace in the presence of selenium vapour. The phase content of the layers as a function of composition and annealing temperature has been investigated by Raman spectroscopy. It is concluded that copper selenide is formed during the chemical bath treatment and that during annealing the copper selenide reacts first with elemental selenium vapour and then with the indium selenide to form chalcopyrite CuInSe2. Secondary phases of CuIn3Se5 and Cu-Au ordered CuInSe2 have been detected in annealed copper-poor layers

Alkali incorporation into solution processed CIGS precursor layers

Solution based ion-exchange reactions offer a simple, non-vacuum route for adding Cu into In- Ga-Se precursor layers as a step in a low-cost process for the preparation of Cu(In, Ga)Se2 (CIGS) solar cells. The chemically treated precursor layers may be converted into CIGS by annealing with Se vapour. Structural and compositional characterisation has shown that the converted layers have good composition, microstructure and crystalline phase content. Nevertheless, photovoltaic cells processed from these layers have failed to produce energy conversion efficiencies greater than ~4% under standard test conditions. The chemical bath used for the incorporation of Cu into the precursor layers includes a complexant for stability and this complexant contains alkali atoms, which are known to strongly influence the properties of CIGS. Low alkali content is highly desirable in CIGS layers but excessive inclusion may be detrimental. This paper reports the results of an investigation into the potential incorporation of excess alkali atoms from the solution into the precursor layers. Whilst no evidence of alkali incorporation is detected by energy dispersive X-ray analysis, clear evidence is seen in time-of-flight secondary ion mass spectrometry measurements. The implications of this are discussed in terms of reported effects on device performance

Characterisation of a filter-based external quantum efficiency measurement system

Accurate assessment of external quantum efficiency provides information useful for understanding where losses in energy conversion efficiency occur in solar cells. These systems are typically designed to measure small areas only, which makes it impossible to measure the quantum efficiency of monolithically integrated modules and thus any effects due to interconnection cannot be assessed. A system for measuring spectral response has been designed and recently commissioned at CREST with a view to making measurements on larger areas. The external quantum efficiency of solar cells is calculated based around a series of narrow-band interference filters and homogenising optical elements that are able to provide a large-area, homogeneous, monochromatic illumination. In this paper the initial characterisation of the system will be presented. It is the intention to further improve the functionality of the system over the coming year and the planned enhancements will be discussed in light of their effects on measurement accuracy, in particular for devices such as dye cells and multi-junction cells which have more complicated electro-optical characteristics than basic silicon wafer cells

Incorporation of copper into indium gallium selenide layers from solution

A chemical method for the incorporation of copper into indium gallium selenide (IGS) layers has been developed. The resulting copper-containing precursor layers have been annealed in the presence of selenium vapour with the goal of forming Cu(In, Ga)Se2 (CIGS) layers. It is found that copper ions in solution are incorporated into IGS layers during immersion, resulting in the formation of a precursor layer containing both copper selenides and IGS. When aqueous solutions are used for this process, corrosion of the molybdenum back contact occurs by reduction of copper ions in the solution. Use of an ethylene glycol solution prevents corrosion of the Mo and allows higher process temperatures, corresponding to higher reaction rates. During annealing, the precursor layers are converted into CIGS and the morphology of these layers is strongly affected by the availability of selenium whilst the substrate temperature is ramped up

Chemical incorporation of copper into indium selenide thin-films for processing of CuInSe2 solar cells

A chemical method of incorporating copper into indium selenide thin-films has been investigated, with the goal of creating a precursor structure for conversion into CuInSe2 layers suitable for solar cell processing. The precursor and converted layers have been investigated with scanning electron microscopy, x-ray diffraction, Raman spectroscopy and x-ray photoelectron spectroscopy. From these measurements, the incorporation of copper into the indium selenide layers is concluded to proceed by an ion-exchange reaction. This reaction results in the formation of a precursor layer with a graded compositional depth-profile containing the crystalline phases In2Se3 and Cu2-xSe. Selenization of the precursor layer homogenises the composition and forms chalcopyrite CuInSe2. These CuInSe2 layers exhibit a dense microstructure with rough surface morphology, which is ascribed to a non-optimal selenization process. Solar cells with the structure ZnO:Al/i-ZnO/CdS/CuInSe2/Mo/Glass have been processed from the selenized layers and have exhibited efficiencies of up to 4% under simulated AM1.5 illumination

Influence of spectral irradiance measurements on accuracy of performance ratio estimation in large scale PV systems

Understanding the quality of irradiance measurements is an essential part of PV monitoring. For precise estimation of solar radiation all of its properties must be considered. There are two different ways to describe irradiance – broadband and spectral. Broadband irradiance measurements are by far the most commonly applied technique and can be undertaken by the use of pyranometers or calibrated reference cells. Broadband measurements give integrated power over a specified bandwidth. Spectral irradiance describes not only the integrated energy of the sunlight, but also its distribution by wavelength. PV modules are strictly wavelength selective devices. Their spectral sensitivity depends mostly on the cell material technology. Spectral effects can be observed at their most extreme for thin-film a-Si modules. Knowing the accurate intensity and spectral distribution of the sunlight may have a significant influence on accurate prediction of the available energy for different types of PV device

Voltage-dependent quantum efficiency measurements of amorphous silicon multijunction mini-modules

Multi-junction solar cells have the potential to provide higher efficiencies than single junction devices and to reduce the impact of Staebler-Wronski degradation on amorphous silicon (a-Si)devices. They could, therefore, reduce the cost of solar electricity. However, their characterization presents additional challenges over that of single junction devices. Achieving acceptable accuracy of any current-voltage calibration requires correction of the current-voltage data with external quantum efficiency measurements and spectral mismatch calculations. This paper presents voltage dependant EQE curves for both single junction and double junction a-Si solar cells, along with dispersion curves extracted from these data. In the case of single junction a-Si devices the mismatch factor is known to be voltage dependent and a similar trend is shown to apply to multi-junction devices as well. However, the error introduced into current voltage calibrations due to this bias dependence is found to be < 1% for spectral mismatch calculations

Structural properties of Cu(In,Ga)Se2 thin films prepared from chemically processed precursor layers

We have developed a chemical process for incorporating copper into indium gallium selenide layers with the goal of creating a precursor structure for the formation of copper indium gallium diselenide (CIGS) photovoltaic absorbers. Stylus profilometry, EDX, Raman spectroscopy, XRD and SIMS measurements show that when indium gallium selenide layers are immersed in a hot copper chloride solution, copper is incorporated as copper selenide with no increase in the thickness of the layers. Further measurements show that annealing this precursor structure in the presence of selenium results in the formation of CIGS and that the supply of selenium during the annealing process has a strong effect on the morphology and preferred orientation of these layers. When the supply of Se during annealing begins only once the substrate temperature reaches ≈ 400 °C, the resulting CIGS layers are smoother and have more pronounced preferred orientation than when Se is supplied throughout the entire annealing process

Effects of spectrum on the power rating of amorphous silicon photovoltaic devices

The effects of different spectra on the laboratory based performance evaluation of amorphous silicon solar cells is investigated using an opto-electrical model which was developed specifically for this purpose. The aim is to quantify uncertainties in the calibration process. Two main uncertainties arise from the differences in the test spectrum and the standard spectrum. First, the mismatch between reference cells and the measured device, which is shown to be voltage dependent in the case of amorphous silicon devices. Second, the fill factor of the device is affected by different spectra. Different cell structures and states (specifically different i-layer thickness and levels of degradation) for the different light sources are investigated in this work. These sources are different solar simulators, LED sources, Tungsten as well as the standard terrestrial AM1.5G radiation. It is shown that the performance cannot be evaluated by short circuit current alone. The voltage dependent quantum efficiency of p-i-n devices can introduce a mismatch in the PMPP of 1% for 250nm i-layer devices in as prepared state, rising to up to 4% for the 600nm i-layer devices at degraded state

Automated characterisation of multi-junction thin film silicon solar cells

Accurate measurements and calibration of amorphous silicon and micromorph multi-junction solar cells poses a major challenge. Device measurements with commonly used single-lamp solar simulators can be associated with large uncertainties, as small changes in the simulator light can lead to significant non-linear differences in the current production of the device under test. To properly characterise multi-junction solar cells, a multi-source or spectrally adjustable solar simulator is required. This way, the spectral distribution of the simulator can be changed and stacked cells can be current matched. However, the methods available are somewhat slow, as they require the measurement of a quantum efficiency, and difficult to operate when done manually. This paper presents a method for automated characterisation of multi-junction and single-junction solar cells. Its key element is a fitting method for the spectral response from device measurements at different spectra. The automated approach is underlined with a simulated measurement of a double junction amorphous silicon cell. Simulations reflect the capabilities of the LED-based solar simulator prototype developed at CREST. Results show that the method is able to deliver high accuracy without the need for additional spectral response measurement systems or closely matched reference cells. Single- and multi-junction device calibration methods are briefly reviewed and the automated approach is described in detail including its potential error sources and requirements