New MAterials for Tandem Solar cells

The solar cell many of us know and love - the silicon solar cell - is very efficient at absorbing visible light with low energy, such as red light. It is, however, not too efficient at transforming light with more energy, such as blue light, to electricity, with much of the energy lost to heat. The MATS project investigates a new and promising material that better converts the blue light to electricity, while still being transparent to the lower energy red light. Thus, one can simply place the new solar cell on top of the more traditional solar cell, allowing both solar cells to transform the sunlight to electricity most efficiently. Theoretically, this can increase the overall efficiency from approximately 30% (Si only) to over 40% (tandem structure). The methods used to produce the new solar cell will be easy to scale up, and the materials used are both cheap and environmentally friendly – sustainability is key. As the new solar cell is meant to be used in tandem with the existing Si solar cell, it will also not require a revolution in solar cell production, but rather an expansion in the already existing factories. It is thus more likely to be implemented fast and can more easily be part of the solution for the transition into a renewable society. In the most cost-efficient structure, the 2-terminal tandem solar cell, the two solar cells connected electrically in series. This means the two solar cells should be optimized for the same current flow, otherwise the efficiency of the tandem structure will be reduced. While such a structure reduce costs for external components, it does set strict material requirement of the top solar cell material. It is essential that it absorbs some wavelengths, but transmits others. This is to a large degree decided by the band gap, which optimally should be 1.7 eV. In addition, the material should satisfy common solar cell material requirements; high absorption above bandgap, high transmission below band gap, high lifetime, high mobility, and a possibility to fabricate a rectifying junction; the basic component of a solar cell. To achieve this, the carrier concentration in the solar cell material should be sufficiently low, preferably below 10^18 cm-3. In the MATS project, we have used magnetron sputtering to fabricate zinc oxynitride films that could fit the above criteria. It is an industrially friendly method suitable for large scale-up, as well as a powerful technique with many options for changing film quality by tuning various parameters. We have investigated the effect of deposition pressure, temperature and power in RF sputtering, DC sputtering and High Power Impulse Magnetron sputtering. In all cases we have made series with varying oxygen partial pressures to investigate the full range of the zinc oxynitride films, as the band gap can be tuned by changing the oxygen to nitrogen ratio in the films. By tuning these parameters, films with drastically different properties has been produced - with electron mobilities from below 1 up to 150 cm^2/Vs, carrier concentrations in the range 10^15 - 10^20 cm^-3, band gaps from 1.1 eV - 3.4 eV, and films that were nearly amorphous as well as strongly crystalline. For promising materials, rectifying diodes were produced to further investigate the defects in the films. The stability of such defects were investigated both as a function of oxygen to nitrogen ratio, time as well as exposed film area. The surface of the films have been shown to be highly sensitive to oxygen and hydrogen exposure, where even exposure over two days, or likely less, in a vacuum system change the band parameters obtained from UPS (Ultraviolet Photoelectron Spectroscopy), which are essential to control when fabricating a device structure. To better optimize the device structure, evaluate the suitability of the new material and make it easier to locate compatible materials in the structure, a model for an entire tandem device was developed in the program SILVACO. In the model, we have used experimental data where possible, and otherwise literature data. These show that the material system has promise, but challenges remain with obtaining a suitable p-type emitter material, which is essential to the success of the device.

The MATS (New MAterials for Tandem Solar cells) project will establish a new solar cell based on zinc oxynitride. The semiconducting material, which has a tunable band gap with nitrogen content, will be optimized towards an absorber in a top cell in a tandem structure, where the bottom cell utilizes silicon. The optimization will initially aim for a band gap of ~1.7 eV, high mobility, and a carrier concentration low enough to establish rectifying pn-junction with the depletion region within the absorber layer. Zinc oxynitride is a cheap, abundant and non-toxic material, and the deposition method magnetron sputtering is easy to scale up, making incorporation into the existing silicon solar cell industry feasible. While the final goal is to demonstrate a functioning solar cell, the focus of the project will be on developing the material and characterizing the fundamental properties of the optimized zinc oxynitride. Both thin films and pn-junction devices will be fabricated, allowing a wide array of characterization methods into the materials optical, electrical, compositional, structural and morphological properties. The defect concentrations and their energy positions will also be thoroughly investigated. The knowledge gained from these investigations will be used to improve the efficiency of the resulting solar cell. We will also use modelling to elucidate the effect of the material properties on the solar cell efficiency. Thus, MATS is an application motivated basic research project, but where a zinc oxynitride based solar cells will ultimately be demonstrated.