Energy-saving potential of thermochromic coatings in transparent building envelope components

Advances in the energy management of buildings are essential for reducing the carbon footprint in the building sector. Applying special window coatings of varying optical properties offers new chances for improved energy efficiency. Thermochromic vanadium oxide (VO2) is an important material for this development and is, therefore, one of the most investigated thermochromic materials. It changes its transmittance in the infrared spectral range in response to a changing temperature. In this study, VO2 coating was deposited on ultra-thin flexible glass in a continuous roll-to-roll sputtering process. The thermochromic layer had a thickness of 70 nm, and it was embedded between two zirconium oxide layers of 170 nm each. The luminous transmittance of the stack was 50%. A solar modulation of 9.6% was reached between the low and high-temperature states. The transition temperature between the cold infrared transparent and the warm infrared opaque state was determined to be 22°C. Different application scenarios for this material were evaluated. The modulation of the solar transmittance was calculated for the combination of VO2 with state-of-the-art low-e coatings. Our findings show that such a combination does not offer a benefit for reducing the energy demand of a building. However, a stand-alone implementation of thermochromic coatings has a high potential if the energy consumption of the building is dominated by cooling demands

Perovskite photovoltaics on roll-to-roll coated ultra-thin glass as flexible high-efficiency indoor power generators

The internet of things revolution requires efficient, easy-to-integrate energy harvesting. Here, we report indoor power generation by flexible perovskite solar cells (PSCs) manufactured on roll-to-roll indium-doped tin oxide (ITO)-coated ultra-thin flexible glass (FG) substrates with notable transmittance (>80%), sheet resistance (13 Ω/square), and bendability, surpassing 1,600 bending procedures at 20.5-mm curvature. Optimized PSCs on FG incorporate a mesoporous scaffold over SnO2 compact layers delivering efficiencies of 20.6% (16.7 μW⋅cm−2 power density) and 22.6% (35.0 μW⋅cm−2) under 200 and 400 lx LED illumination, respectively. These represent, to the best of our knowledge, the highest reported for any indoor flexible solar cell technology, surpassing by a 60%–90% margin the prior best-performing flexible PSCs. Specific powers (W/g) delivered by these lightweight cells are 40%–55% higher than their counterparts on polyethylene terephthalate (PET) films and an order of magnitude greater than those on rigid glass, highlighting the potential of flexible FG-PSCs as a key enabling technology for powering indoor electronics of the future

Nanoimprint texturing of transparent flexible substrates for improved light management in thin-film solar cells

We present a nanoimprint based approach to achieve efficient light management for solar cells on low temperature transparent polymer films. These films are particularly low-priced, though sensitive to temperature, and therefore limiting the range of deposition temperatures of subsequent solar cell layers. By using nanoimprint technology, we successfully replicated optimized light trapping textures of etched high temperature ZnO:Al on a low temperature PET film without deterioration of optical properties of the substrate. The imprint-textured PET substrates show excellent light scattering properties and lead to significantly improved incoupling and trapping of light in the solar cell, resulting in a current density of 12.9 mA/cm2, similar to that on a glass substrate. An overall efficiency of 6.9% was achieved for a flexible thin-film silicon solar cell on low cost PET substrate

Roll-to-roll deposition of permeation barrier layers using a rotatable dual magnetron sputtering system

High performance permeation barrier coatings are usually multilayer stacks consisting of inorganic and organic layers. Besides the water vapor transmission rate the optical properties of such layers have a high importance. This study focuses on the deposition of silicon oxynitride as the inorganic layer. This material attracts a widespread interest due to its varying refractive index depending on its oxygen and nitrogen content. The experiments were carried out in a roll-to-roll coating machine using a rotatable magnetron system. The layers were deposited by reactive pulsed magnetron sputtering using targets of one meter length. It was established that the ratio of oxygen and nitrogen in the layer was not only determined by the reactive gas mixture but also by the chosen set point of the closed loop control. Both the Secondary Ion Mass Spectrometry and the Glow Discharge Optical Emission Spectroscopy (GDOS) measurements revealed a vertically different composition of the SiOxNy layers. This result was attributed to the fact that the substrate passes zones of different plasma density during the deposition process. A model for the description of the optical properties was developed on the basis of these observations. A five layer system was introduced in place of the SiOxNy single layer. These five sub-layers differed in their dispersion relation in accordance with the results of the GDOS measurement. The model provided an excellent description of the measured transmittance spectra

Vacuum coating on polymer films for outdoor applications

This paper discusses functionalization of ETFE webs by vacuum roll-to-roll deposition of permeation barrier layers and highly conductive transparent electrodes for outdoor applications. A 100 nm thick gas barrier layer (zinc-tin-oxide – ZTO) and a 60nm thick transparent conductive layer stack(indium-tin-oxide – ITO 25 nm – Ag 10 nm –ITO 25 nm) achieve a water vapor transmission rate of 0.01 g/m²d (at38°C/90% r.h.) and sheet resistance of< 6 Ω/sq on a commercial grade ETFE film. However, the elastic properties of ETFE as well as its creep-behavior require specific attention during roll-to-roll processing and for design of the functional layer stacks. Inconsequence, layer thicknesses and material selection must be balanced between functional performance and the ability to survive deformation of the polymer web. This paper compares functional performance and crack formation under strain load of different layer stacks on both PET and ETFE substrates

Roll-to-Roll Sputter Deposition of Low Resistance Electrodes on Ultrathin Glass

Ultrathin glass is a new flexible material. It is distinguished from its polymer counterparts by a couple of outstanding properties. Most importantly, the permeation barrier for moisture and oxygen is virtually infinite. Thus, lifetime problems for organic electronics can effectively be avoided. Further, the possibility to exert heat on the substrate offers considerable advantages for transparent conductive oxides. The paper features the roll-to-roll sputter deposition of indium tin oxide (ITO) based transparent electrodes. The deposition technology was the dual anode sputtering from a rotatable ceramic target. The paper provides results for ITO coatings which were either deposited at substrate temperatures of up to 300°C or which were annealed at those temperatures after the deposition. The lowest resistivity of the coatings was 1.6x10-4 Ohm-cm, corresponding to a sheet resistance of less than 11 Ohm/square.This value is much better compared to ITO coatings deposited at room temperature or annealed to moderate temperaturelevels around 150 °C. In addition to the optical and electrical properties, surface roughness and mechanical stability are also crucial propertiesfor the applicability of the substrates.Various annealing routes were compared. The indium tinoxide properties achieved by room temperature and subsequent annealing were related to those obtained by the depositionon a heated substrate. Besides the transmission, reflection and resistivity, atomic force microcopy and X-ray diffraction results were also included in the discussion