Transparent conductors as solar energy materials: A panoramic review

Abstract

Transparent conductors (TCs) have a multitude of applications for solar energy utilization and for energy savings, especially in buildings. The largest of these applications, in terms of area, make use of the fact that the TCs have low infrared emittance and hence can be used to improve the thermal properties of modern fenestration. Depending on whether the TCs are reflecting or not in the near infrared pertinent to solar irradiation, the TCs can serve in “solar control” or “low-emittance” windows. Other applications rely on the electrical conductivity of the TCs, which make them useful as current collectors in solar cells and for inserting and extracting electrical charge in electrochromic “smart windows” capable of combining energy efficiency and indoor comfort in buildings. This Review takes a “panoramic” view on TCs and discusses their properties from the perspective of the radiative properties in our ambience. This approach leads naturally to considerations of spectral selectivity, angular selectivity, and temporal variability of TCs, as covered in three subsequent sections. The spectrally selective materials are thin films based on metals (normally gold or titanium nitride) or wide band gap semiconductors with heavy doping (normally based on indium, tin, or zinc). Their applications to energy-efficient windows are covered in detail, experimentally as well as theoretically, and briefer discussions are given applications to solar cells and solar collectors. Photocatalytic properties and super-hydrophilicity are touched upon. Angular selective TCs, for which the angular properties are caused by inclined columnar nanostructures, are then covered. A discussion of TC-like materials with thermochromic and electrochromic properties follows in the final part. Detailed treatments are given for thermochromic materials based on vanadium dioxide and for electrochromic multi-layer structures (incorporating TCs as essential components). The reference list is extensive and aims at giving an easy entrance to the many varied aspects of TCs.

Introduction

Future energy supply and energy security will demand revolutionary advances in technology in order to maintain or forward today's (2007) general standard of living and economic prosperity [1]. Faced with high and rising energy prices, limitations in energy supply, and growing concerns about climate changes and their environmental- and health-related effects, the magnitude of the problems may seem daunting. For example, it has been stated that the warming and precipitation trends due to anthropogenic climate change during the past 30 years already claim over 150,000 human lives annually [2], [3]. These changes are also expected to be accompanied by more common and/or extreme events such as heatwaves, heavy rainfall, and storms and coastal flooding; there is also a risk that nonlinear climate responses will lead to even more and rapid climate changes such as breakdown of the ocean “conveyor belt” circulation, collapse of major ice sheets, and/or release of large quantities of methane at high latitudes thus leading to intensified global warming [4].

Furthermore, the urgently needed advances in energy technology must take place for an increasing population, whose growing concentration in mega-cities leads to “heat islands” which tend to aggravate the warming [5] and increase the urban cooling load by up to 25% compared to the case of the surrounding rural areas [6]. By 2050, there will be some ten billion people in the World. Energy must be available to them all, and it has to be clean. New technologies are urgently needed to accomplish this. Some of these technologies—mainly related to efficient solar energy utilization and to energy savings in the built environment—will be discussed in this Review.

Before proceeding one should contemplate how large and important the energy sector is for Society. Currently (2007), the total global energy consumption of oil, coal, natural gas, nuclear and hydropower, etc., is estimated to be some 10,000 million ton oil equivalents per year [7]. This corresponds to around 6700 billion USD, which in fact might be greatly underestimated since only primary sources are accounted for. To put this number in perspective, one could note that it is about ten times larger than the total value of the biotechnology market, including everything from pharmaceuticals to medicine. The economics of renewable energy and energy savings is a challenging subject, fraught with risks but also noteworthy for its new opportunities [8].

Where do the transparent conductors (TCs), which this Review is about, enter into this large scenario? How can they contribute to meeting “The Terawatt Challenge” discussed in a recent article in which the ten most serious global concerns were listed with “energy” at the head [1]? As discussed below, the TCs—oxides as well as non-oxides—can indeed play an important role both for energy generation and for energy saving.

A basic reason why TCs are of concern is that they can show transparency in a limited and well-defined range, normally encompassing visible light in the 0.4<λ<0.7 μm wavelength interval. In the infrared (IR) their metallic property leads to reflectance and at sufficiently short wavelengths, in the ultraviolet (UV), they become absorbing due to excitations across an energy gap. If the reflectance is in the range for thermal radiation, i.e., 3<λ<50 μm at normal temperature, the emission of heat is impeded. If reflectance prevails at 0.7<λ<3 μm, covering the IR part of the solar spectrum which carries about 50% of the solar energy, one can combine visible transmittance with rejection of a large part of the solar energy and at the same time have low thermal emittance. It is then obvious that TCs have a number of diverse applications in the fields of solar energy utilization and energy efficiency. Other applications of TCs emerge from their electrical conductivity, which enables their use as current collectors in solar cells and as transparent electrodes for charging and discharging of electrochromic smart windows, etc. Clearly the TCs can be viewed as “solar energy materials”, whose properties have been given a bird's eye perspective in recent articles [9], [10], [11]. Parts of this Review can be viewed as elaborations on these earlier papers.

The applications of TCs, hence, can rely on their spectral selectivity, as clear from the discussion above. Other possible uses emerge from the angular properties of the radiation that surrounds us, specifically by the fact that one can take advantage of the Sun's passage over the vault of heaven to have different performances for midday and dawn or dusk. Still other applications ensue from the fact that ambient radiation or human needs vary during the day and season, so that solar energy and/or visible light ideally should be admitted or rejected as a function of time.

This Review is organized as follows: Section 2 presents an overview of the radiation that prevails in our natural surroundings with the main object of defining the types of spectral and angular selectivity that are of interest for solar energy and energy efficiency. Also introduced are materials with variable optical properties, known as “chromogenic” materials [12], whose usefulness for energy savings in buildings is discussed. Primers on thin-film-coating technology and on the materials capable of serving as substrates for the coatings are included too. Then follow discussions of TCs with tailored spectral selectivity, angular dependence, and temporal variability in 3 Applications based on spectral selectivity, 4 Applications based on angular selectivity, 5 Applications based on temporal variability (chromogenics), respectively. Regarding spectral selectivity, most attention is devoted to architectural windows and glass façades, which require TCs for admitting and reflecting visible light, solar energy, and thermal radiation in different wavelength regions. Special attention is devoted to noble-metal-based films, which are commonly used on today's windows, and n-doped wide band gap oxide semiconductor films with particular consideration of In₂O₃:Sn. Theoretical and experimental data are given for the spectral optical properties. The possible use of TCs to avoid or modify the condensation of water on windows is also discussed, as are applications to “vaccum glazings”, photovoltaic cells, and thermal solar collectors. Angular selectivity is given a shorter presentation with focus on obliquely deposited metal-based films having inclined columnar nanostructures. Materials and devices enabling temporal variability attract much current interest, and this part of the Review covers photochromic, thermochromic, electrochromic, and gasochromic options. In-depth discussions are given for thermochromic VO₂-based films and for oxide-based electrochromics. Concerning the latter, the discussion embraces, in particular, general device design, discussions of WO₃ and NiO films, and data on foil-type devices incorporating these two materials. The Review is concluded with an outlook towards some future possible materials and applications of TCs in Section 6.

The aim is to give a “panoramic” view on TCs, including those that may be of interest in forthcoming technologies, and to give an easy entrance to recent literature. The latter ambition has led to the incorporation of an extensive and detailed reference list covering, in particular, the most recent work.