This invention generally relates to a coating for optical surfaces, to improve energy conversion. More particularly, the invention relates to a coating for improving energy conversion in optoelectronic devices. The invention also relates to optoelectronic devices using such coatings.
One of the main focuses in the field of optoelectronic devices is the improvement of energy conversion efficiency (from electromagnetic energy to electric energy or vice versa). The devices often suffer reduced performance due to loss of light. Therefore, research in optical designs of these devices includes light collection and trapping, spectrally matched absorption and up/down light energy conversion.
A potential method of light collection is the reduction of light reflection by having a reflection-free surface. To this end, anti-reflection coatings of suitable refractive indices are commonly used. However, the availability of such materials having a low refractive index, e.g., between 1.0 (air) and 1.49 (glass), is very limited.
It is seen from current research that nanostructured optical thin films with controllable porosity often exhibit a very low refractive index as compared to dense materials. For example, SiO2 nanostructured porous films often have a refractive index of about 1.08, which is much lower than the value of 1.46 for an SiO2 thin film. These single layer anti-reflection coatings reduce reflectivity only in a limited spectral range, and for normal incidence.
Furthermore, the devices suffer loss of efficiency due to a thermalization mechanism in which carriers generated by high-energy photons are lost as phonons in the crystal. The absorption of incident photons with energies greater than the threshold energy for the absorption leads to the generation of only one electron-hole pair per absorbed photon, regardless of the photon energy. The excess energy of an incident photon above the threshold energy is wasted during the thermalization of the generated electron-hole pairs. Certain cell designs, employing a heterojunction window layer, lose high-energy photons due to parasitic absorption in the window layer. It is therefore desirable to convert these high-energy photons (short wavelength) to lower energy photons (long wavelength) that can be effectively absorbed in an absorber layer, and converted to collectable charge carriers.
One well-known method to overcome loss of light and related loss mechanisms involves “down-conversion” of high electromagnetic energy from a shorter wavelength to a longer wavelength. Because the absorption of high-energy photons in an undesired regions/layers of optoelectronic devices must be avoided, a down-converting layer may be disposed on a surface of the device, exposed to electromagnetic radiation.
Usually, deposition of anti-reflection and down-converting layers includes multiple processing steps. Thus, it would be desirable to produce improved coating systems having both anti-reflecting and down-converting properties, in order to meet various performance requirements for optoelectronic devices. Moreover, the new coating system can provide the advantage of easy fabrication. It would also be very desirable to develop an improved optoelectronic device including such coating.