The current disclosure relates to the field of photovoltaics, in particular to novel ultra-thin and highly transparent wafer-type plasmonic solar cells and their manufacturing methods.
Photovoltaics, the method of directly converting sunlight into electrical energy, could play an important role in fulfilling the global energy needs. However, there are unsolved issues associated with cost and feasibility that must be addressed with greater urgency. Efficient, affordable, stable, safe, and abundant photovoltaic devices will considerably contribute to renewable energy being incorporated to fulfill the present energy demand and lead to a resource proficient society.
The mainstream trend at present involves in decreasing the thickness of the wafer-based solar cells at a reduced efficiency loss so as to save costs. Attempts to replace toxic components in the compositions are another trend. If both of these enhancements are successful, it will encourage new uses and applications including photon-energy up-conversion, high-performance solar cells that are capable of harvesting a broader range of solar energy (so-called tandem solar cells), building-integrated photovoltaics, solar-powered electronic devices and sensors in homes, and wearable gadgets. In the past few years, the thickness of the wafer has been decreased from nearly 400 μm down to about 2 - 3 μm and this trend still continues.
Generally, thin-film solar cells have a thickness in the range of 1 - 2 μm, and are deposited on various substrates such as plastic, stainless steel, or glass. Specifically, they offer an efficient and economical solution as a reasonable substitute to other energy sources, being set to face particularly challenging requirements, for example, tandem cells, self-charging devices, portable and lightweight consumer applications, and so on.
A tandem cell includes at least two sub-cells that together convert additional sunlight spectrum into electricity, thus increasing the overall cell efficiency. The sub-cells are stacked on top of one another and can be fabricated from same or different solar cell materials. For instance, a tandem cell integrating perovskite as the “upper cell” with either copper indium gallium selenide (CIGS) or silicon-based “bottom” cells was expected to be capable of increasing 20% of power generation.
The recent world record in efficiency was realized in June 2016 with a CIGS thin-film solar cell that had 22.6% energy conversion efficiency, as confirmed by the Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany. The cell had a thickness of around 3 - 4 μm and was developed by ZSW (Zentrum für Sonnenenergie- und Wasserstoff-Forschung, the Center for Solar Energy and Hydrogen Research, Baden-Württemberg) in Stuttgart, Germany. Conversely, the thickness of the existing film prevents its application in colourless building integration and self-charging devices.
Alternate photovoltaics are evolving, besides the extensively commercialized semiconductor technologies based on crystalline and thin film, Si-based photovoltaic cells. There has been an increasing interest in the advancement of plasmonic solar cells, in addition to thin-film systems like CdTe, GaAs, CuInSe2, CuInGaSe2 (i.e. CIGS), as well as perovskites and organic semiconductors.
The main drawback for existing technologies is linked to the low absorbance of near-bandgap light. Metal nanoparticles strongly scatter light at wavelengths close to the plasmon resonance. This is because of a collective oscillation of the conduction electrons in the metal. At the localized surface plasmon (LSP) resonance, the cross-section of the scattering can exceed the geometrical cross-section of the particle. In such situations, to first-order, a substrate enclosed with a 10% areal density of particles could completely absorb and scatter the impinging light. For instance, it can be noted that tiny silver nanoparticles in the air have a scattering cross-section that is nearly ten times than that of the actual cross-sectional area of the particle.
Sá et al. (Energy Environ. Sci. 6 (2013) 3584) established that electron-hole pairs are produced during LSP excitation, quantified by high-resolution X-ray absorption spectroscopy at the Au L3-edge. LSP excitation resulted in an upward shift of the ionization energy threshold by approximately 1.0 eV, as well as an increase of Au d-band hole population, which is consistent with the formation of hot electrons and their excitation to high-energy states.
Theoretically, while plasmonic nanostructures can be directly used in solar cells, the light-generated electron-hole pairs are short-lived, that is, they last only a few femtoseconds. This makes it difficult to draw current from the instrument. Therefore, to boost charge separation lifetime, the charge carriers can be limited to spatially separated locations where reactions will occur, for example, by transferring them to a semiconductor (similar to dye-sensitized solar cells). LSP hot electrons have adequate energy to be introduced into a TiO2 conduction band, which, in turn, expands their lifetime from few femtoseconds to 100s of nanoseconds.
The plasmon resonance is influenced by the local dielectric environment and particle morphology. Consequently, the distance between the semiconductor and the light absorber affects the efficiency of different solar cell geometries using metal nanoparticles within the active layers of solar cells. Therefore, the overall effect of changing the distance is a trade-off between increased cross-section and reduced coupled fraction, meaning that the optimal distance is determined by the preferred particle density (which is related to the cross section), the absorption coefficient of the semiconductor layer, and the degree of Ohmic loss in the particles.
Zhang et al. (Sci. Rep. 4 (2014) 4939) revealed that Al or Ag nanoparticles (NPs) integrated Silicon ultra-thin photovoltaic cells with a wafer thickness of 5 μm can possibly achieve an efficiency of 15.3%. On the other hand, the film’s present thickness prevents its use in colorless building integration and in self-charging devices.
Islam et al. (Solar Energy 103 (2014) 263-268) studied the influence of gold nanoparticles on the performance of hydrogenated amorphous silicon (a-Si:H) solar cells, and observed that the plasmonic effect of the Au nanoparticles enables extra scattering at the surface thus decreasing the overall reflectivity. If the size of the nanoparticle is larger, then more scattering is attained along with the drop in median reflectivity from around 23% to 18%. Their outcomes revealed an increase in the short-circuit current density (Jsc) and efficiency with increasing size of the nanoparticles. The Jsc also increased from 9.34 to 10.1 mA/cm2. Likewise, the efficiency also increases from 4.28% to 5.01%.
US 2009/0188558 reveals photovoltaic devices having metal oxide electron transport layers, as well as the use of bifunctional organic compounds in the electron-transport layers. However, this disclosure does not consider plasmonic solar cells, as well as the use of metal nanoparticles as light absorbers. The nanoparticles discussed in the ‘558 document relates to the metal oxides, particularly zinc oxide.
One goal of the current disclosure is to offer an ultra-thin wafer type plasmonic solar cell, which is environmentally compatible, stable, and can be easily assembled, and which preferably is highly transparent. Such types of solar cells are appropriate for wearable electronics, colorless building integration, self-charging technologies, for powering electronic gadgets and sensors in homes, and also in photon-energy up-conversion and high-efficiency tandem solar cells, to name a few non-restricting examples.
Another goal is to offer a process for producing ultra-thin and preferably highly transparent wafer type solar cells, which are reliable, simple, and also preferably environmentally compatible.
This information has been sourced, reviewed and adapted from materials provided by Peafowl Solar Power.
For more information on this source, please visit Peafowl Solar Power.