Spectroscopic studies of intentionally introduced defects and p dopants in ZnO nanoparticles and nanowires for photovoltaic applications
Résumé
Third generation solar cells are based on low cost fabrication concepts which should potentially permit to overcome the Shockley-Queisser limit, estimated at 33.7 % efficiency for a p-n junction made of a material having a bandgap of 1.1 eV (e.g. Si). Nevertheless, current single crystalline silicon SCs deliver a conversion efficiency of only 27.6 %. This is limited by the optical absorption, surface reflection, carrier transport and carrier collection. One of the most promising concepts for 3rd generation solar cells is based on the application of low-dimensional structures. It permits an easy bandgap engineering of the photoactive material and addressing the issue of the low efficiency of solar cells in the UV spectral domain. For example, the UV-wavelength response of an Si-based solar cell is low due to the front surface recombination of the hot photogenerated carriers. This can be improved by the application of a photoluminescent spectral converter which converts photons of inefficient frequencies into longer wavelength photons better matching the solar cell’s active layer absorption spectrum, via a so called energy down-shifting (DS) process. The ideal down-shifting material must possess a large Stokes shift and have a high luminescence quantum yield. In the present communication, we propose the application of the semiconducting nanoparticles (NPs) as down-shifters for the enhancement of the SC’s spectral response. Nanoparticles permit to achieve a high concentration of the intermediate energy levels in the bandgap, necessary for an efficient DS and at the same time, they can act as an antireflective layer. Moreover, ZnO specifically turns out be a very attractive material for DS applications, as it is non-harmful and abundant, benefits from cheap and easy fabrication methods, has a wide bandgap (3.4 eV) and a higher absorption coefficient than other wide bandgap materials such as GaN. We first show that the synthesis conditions (hydrolysis or co-precipitation), can be used to control and enhance the visible luminescence of ZnO NPs and even achieve high quantum yield. Our study particularly emphasizes the role of specific elements (Li) or ligands (polyacrylic acid coating in the core-shell structure) on the control of the quantum efficiency. The subsequent use of these nanoparticles as down-shifting material for photovoltaic cells is demonstrated. Second, taking advantage of the high surface-to-volume ratio of nanowires, we propose a new strategy for achieving p-doping in ZnO nanostructures. It relies on the dopant diffusion from a surrounding matrix into MOCVD grown nanowires. The p-dopants are identified by temperature dependent photoluminescence and cathodoluminescence and are shown to be homogeneously distributed and stable among the nanowires. This paves the way to the fabrication of ZnO p-n homojunctions either in the radial or axial geometry.