Preparation and optical characterization of Cu2ZnGeSe4 thin films

Cu2ZnGeSe4 (CZGSe) films have been fabricated by ion beam sputtering onto glass substrates at a substrate temperature of 300 and 420 K. CZGSe films were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy, scanning electron microscopy and by the method of normal incidence transmittance and reflectance. XRD studies reveal an improved crystallinity of the polycrystalline CZGSe films with tetragonal structure when the substrate temperature was increased. The refraction index and extinction coefficient were extracted from the optical measurements. Spectral dependence of the absorption coefficient and the energy band gaps values of CZGSe films were also determinedFinancial supports from IRSES PVICOKEST 269167, MICINN projects (KEST-PV; ENE2010- 21541-C03-01/-02/-03) and FRCFB 13.820.05.11/BF projects are acknowledged. RC also acknowledges financial support from Spanish MINECO within the program Ramón y Cajal (RYC-2011-08521

Фотокаталітичні властивості нанокомпозитів анатаз/брукіт

Роботу присвячено дослідженню фотокаталітичних властивостей синтезованих гідротермальною методою ультрадисперсних систем на основі анатазу та брукіту. Проаналізовано перебіг фотодеструкції метилену блакитного та формальдегіду. Встановлено зв’язки фотокаталітичної активности одержаних матеріялів із їхніми фазовим складом, структурно-морфологічними й оптичними характеристиками. Виявлено, що найвищою фотокаталітичною активністю при деструкції метилену блакитного володіє матеріял з максимальним вмістом брукіту (93 мас.%) з частинками складної морфології, злами та ребра яких слугують центрами перебігу реакцій окиснення. У випадку деструкції формальдегіду найбільшою фотокаталітичною активністю володіє композит анатаз/брукіт (масове співвідношення фаз — 3:1), що, ймовірно, пов’язане зі збільшенням ефективности розділення фотоґенерованих носіїв зарядів і підвищенням квантового виходу реакції за умови існування межі поділу фаз між поліморфами діоксиду титану в межах однієї частинки матеріялу.Работа посвящена исследованию фотокаталитических свойств синтезированных гидротермальным методом ультрадисперсных композитов анатаз/брукит. Анализировалась фотодеструкция метилена синего и формальдегида. Установлены связи фотокаталитической активности полученных материалов и их фазового состава, структурно-морфологических и оптических характеристик. Максимальной фотокаталитической активностью при деструкции метилена синего обладает материал з максимальным содержанием брукита (93 масс.%) с частицами сложной морфологии, изломы и ребра которых служат центрами реакций окисления. При деструкции формальдегида максимальная скорость окисления зафиксирована при использовании композита анатаз/брукит (массовое соотношение фаз — 3:1), что, по всей вероятности, связано с увеличением эффективности разделения фотогенерированных носителей зарядов и повышением квантового выхода реакции при условии существования границ раздела фаз между полиморфами диоксида титана в пределах одной частицы материала.The photocatalytic properties of hydrothermally obtained anatase/brookite nanocomposite are studied. The influence of phase composition, structure, morphology, and optical characteristics of synthesized materials on their photocatalytic characteristics is investigated. Methylene blue and formaldehyde photooxidation reactions are analysed. The maximal photocatalytic activity in the case of methylene blue oxidation is detected for material with the highest brookite content (93 mass.%) with particles of complex morphology. The anatase/brookite composite (with the phase mass ratio 3:1) has the highest activity for formaldehyde aqueous-solution destruction as a result of both the separation-efficiency increase for photogenerated charge carriers and the quantum-yield enhancement as a consequence of the anatase/brookite phase boundaries’ presence within the bulk of the one particle

Reverse-bias behaviour of thin-film solar cells: effects of measurement-induced heating

When a solar cell is subjected to a negative voltage bias, it locally heats up due to the deposited electrical power. Therefore, every investigation of cell characteristics in the negative voltage regime faces the challenge that the measurement itself changes the state of the cell in a way that is difficult to quantify: On the one hand, the reverse breakdown is known to be strongly temperature dependent. On the other hand, negative voltages lead to metastable device changes which are also very sensitive to temperature. In the current study, we introduce a new approach to suppress this measurement-induced heating by inserting time delays between individual voltage pulses when measuring. As a sample system we use thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) absorber layers. First we verify that with this approach the measurement-induced heating is largely reduced. This allows us to then analyse the impact of the heating on two characteristics of the cells: (i) the reverse breakdown behaviour and (ii) reverse-bias-induced metastable device changes. The results show that minimising the measurement-induced heating leads to a significant increase of the breakdown voltage and effectively slows down the metastable dynamics. Regarding the reverse breakdown, the fundamental tunneling mechanisms that are believed to drive the breakdown remain qualitatively unchanged, but the heating affects the quantitative values extracted for the associated energy barriers. Regarding the reverse-bias metastability, the experimental data reveal that there are two responsible mechanisms that react differently to the heating: Apart from a charge redistribution at the front interface due to the amphoteric (VSe–VCu) divacancy complex, the modification of a transport barrier is observed which might be caused by ion migration towards the back interface. The findings in this study demonstrate that local sample heating due to reverse-bias measurements can have a notable impact on device behaviour which needs to be kept in mind when developing models of the underlying physical processes

Limiting efficiencies of novel solar cell concepts in space

In this work we study the limiting efficiency in space environmental conditions of three novel solar cell concepts (hot carrier solar cells, multiple exciton generation solar cells and intermediate band solar cells) and how this limiting efficiency is impacted by the temperature and degradation due to radiation. Comparisons are made with state of the art triplejunction solar cells whose performance is taken as reference. In the last section of the work we briefly review the status related to the experimental achievements of these cells to dat

Investigation of the effect of different thicknesses and thermal annealing on the optical properties of GaAs0.1P0.89N0.01 alloys grown on GaP substrates

This work investigates the effect of the thickness of the epitaxial layer (100 nm and 1 μm) on the optical properties of quaternary GaAs0.1P0.89N0.01 alloys. Furthermore, the effect of rapid thermal annealing (RTA) on their properties has been studied using the Photoluminescence (PL) technique. Increasing the thickness of the epilayer led to an enhancement of the PL intensity as well as the energy bandgap, which was shifted to higher energy (from 1.82 eV in 100 nm to 1.94 eV in 1 μm layer). However, the 1.94 eV bandgap energy is not ideal for solar cells based materials grown on GaP substrates. Post-growth thermal annealing by rapid thermal annealing (RTA) for both samples resulted in an enhancement in the optical properties as observed by a decrease of the Full Width at Half Maximum (FWHM) and an increase of the PL intensity. Therefore, all results obtained in this study indicate that GaAs0.1P0.89N0.01 with 100 nm epilayer thick is better choice to fabricate good efficiency solar cells based materials on GaP substrates as compared to 1 μm sample