Glass is desirable for numerous properties and applications, including optical clarity and overall visual appearance. For some example applications, certain optical properties (e.g., light transmission, reflection and/or absorption) are desired to be optimized. For example, in certain example instances, reduction of light reflection from the surface of a glass substrate may be desirable for storefront windows, display cases, photovoltaic devices (e.g., solar cells), picture frames, other types of windows, and so forth.
Photovoltaic devices such as solar cells (and modules therefor) are known in the art. Glass is an integral part of most common commercial photovoltaic modules, including both crystalline and thin film types. A solar cell/module may include, for example, a photoelectric transfer film made up of one or more layers located between a pair of substrates. One or more of the substrates may be of glass, and the photoelectric transfer film (typically semiconductor) is for converting solar energy to electricity. Example solar cells are disclosed in U.S. Pat. Nos. 4,510,344, 4,806,436, 6,506,622, 5,977,477, and JP 07-122764, the disclosures of which are hereby incorporated herein by reference.
Substrate(s) in a solar cell/module are sometimes made of glass. Incoming radiation passes through the incident glass substrate of the solar cell before reaching the active layer(s) (e.g., photoelectric transfer film such as a semiconductor) of the solar cell. Radiation that is reflected by the incident glass substrate does not make its way into the active layer(s) of the solar cell, thereby resulting in a less efficient solar cell. In other words, it would be desirable to decrease the amount of radiation that is reflected by the incident substrate, thereby increasing the amount of radiation that makes its way to the active layer(s) of the solar cell. In particular, the power output of a solar cell or photovoltaic (PV) module may be dependant upon the amount of light, or number of photons, within a specific range of the solar spectrum that pass through the incident glass substrate and reach the photovoltaic semiconductor.
Because the power output of the module may depend upon the amount of light within the solar spectrum that passes through the glass and reaches the PV semiconductor, certain attempts have been made in an attempt to boost overall solar transmission through the glass used in PV modules. One attempt is the use of iron-free or “clear” glass, which may increase the amount of solar light transmission when compared to regular float glass, through absorption minimization.
In certain example embodiments of this invention, an attempt to address the aforesaid problem(s) is made using an antireflective (AR) coating on a glass substrate (the AR coating may be provided on either side of the glass substrate in different embodiments of this invention). An AR coating may increase transmission of light through the light incident substrate, and thus the power of a PV module in certain example embodiments of this invention.
Conventional wet chemical methods to produce silica coatings may employ sol-gel processes involving hydrolysis and condensation reactions of silicon alkoxides to produce stable sols. Silica precursor coatings (formed from silica sols) may be cured at elevated temperatures to convert to silicon dioxide coatings. Silica coatings formed by conventional sol-gel processes may have a refractive index of about 1.45 at 550 nm. However, an example optimum refractive index for monolayer antireflective coatings may be about 1.24. While addition of colloidal silica to silicon alkoxides in sol-gel processes may facilitate lowering a coating's refractive index, it has generally been possible to achieve a coating having a refractive index of about 1.32.
Conventional methods to produce low index silica coatings may result in porous coatings that may not be mechanically robust (and hence perhaps not suitable for practical applications). It will be appreciated that there may exist a need for a method to produce low index silica coatings having improved mechanical properties.
A silica sol may be aged for several hours after preparation in order to ensure thorough hydrolysis of precursor alkoxides. The stability of silica sol may be affected by several factors including pH, water content, concentration of solids, etc., and the resultant sol may be dynamic in nature with continuing changes to the rheological characteristics of the sol. Thus producing stable sols at manufacturing volumes can be challenging. While the shelf life of silica sols may be influenced by storage and transportation conditions, the useful pot life during processing may be affected by loss of volatiles and exposure to ambient humidity, etc. In order to produce optical quality coatings that remain crack-free during high temperature densification process wet coatings of silica sols may be carefully processed during initial heating stages. Heating profiles including gradual temperature ramp rates can be employed to promote condensation and cross-linking reactions. Thus there exists a need for a method to produce stable silica precursor formulations which remain largely unaffected by variations in temperature and humidity during storage, transportation and processing. There also exists a need for a method by which silica coatings could be formed instantly without the need for initial thermal processing prior to high temperature densification.
Thus, it will be appreciated that there may exist a need for an improved AR coating, for use in PV or other applications, to reduce reflection off of glass or other substrates.