Search

In current days, the research in the photovoltaic field aims to find and investigate cheap thin-film materials based on earth-abundant element suitable for terawatt era. To this purpose Cu2ZnSnS4 (CZTS) has been widely studied in the last decade, being structurally analogous to the well-documented Cu(In,Ga)Se2 but featuring inexpensive and abundant components. Recently we have reported a simple and straightforward sol-gel process to produce CZTS-based solar cells demonstrating how it is possible to provide a working device with an easily transferable on an industrial scale and low cost process. In this work we report important improvements to the described above methodology by growing CZTS thin films on a transparent substrate aiming to produce bifacial solar cells suitable for an incorporation into a tandem solar cell architecture. The direct droplet deposition of the precursor ink onto the FTO substrate, leads to the kesterite phase. Gelation and annealing steps, without the addition of further sulphur sources make this procedure suitable for industrial scale-up saving a considerable amount of raw materials. UV–Vis absorption, μ-Raman, Photoluminescence, X-ray diffraction and Energy Dispersive Spectroscopy measurements have been performed on the so-produced thin films. The morphology of the samples has been investigated by scanning electron microscopy, proving the extremely good quality of the material. PV devices with a FTO/Mo/CZTS/CdS/ZnO/AZO architecture were obtained showing η = 1.11% in a bifacial set up. A lower η = 0.99% has been measured by illuminating the front side of the cell only, demonstrating that the bifacial setup allows a +11% efficiency enhancement.

The outdoor performance of n-type bifacial Si photovoltaic (PV) modules and string systems was evaluated for two different albedo (ground reflection) conditions, i.e., 21% and 79%. Both monofacial and bifacial silicon PV modules were prepared using n-type bifacial Si passivated emitter rear totally diffused cells with multi-wire busbar incorporated with a white and transparent back-sheet, respectively. In the first set of tests, the power production of the bifacial PV string system was compared with the monofacial PV string system installed on a grey concrete floor with an albedo of ~21% for approximately one year (June 2016&ndash, May 2017). In the second test, the gain of the bifacial PV string system installed on the white membrane floor with an albedo of ~79% was evaluated for approximately ten months (November 2016&ndash, August 2017). During the second test, the power production by an equivalent monofacial module installed on a horizontal solar tracker was also monitored. The gain was estimated by comparing the energy yield of the bifacial PV module with that of the monofacial module. For the 1.5 kW PV string systems with a 30&deg, tilt angle to the south and 21% ground albedo, the year-wide average bifacial gain was determined to be 10.5%. An increase of the ground albedo to 79% improved the bifacial gain to 33.3%. During the same period, the horizontal single-axis tracker yielded an energy gain of 15.8%.

This work evaluates the potential of selective epitaxy to mitigate the recombination losses at the p-type contact regions of plated bifacial n-type PERT cells. Following the growth of a 500 nm, 2.5 10(19) cm(-3) boron doped epitaxial layer at the emitter contact regions defined by laser ablation of the passivating dielectrics, both an increase in V-oc (+6 mV) and FF (+ 1% absolute) are measured in the final cells compared to the reference with a homogeneous diffused emitter. These results come with an average efficiency gain of 0.3 and 0.5% absolute for front and rear side illumination, respectively. If the diffused, blanket emitter in the passivated regions is replaced by a thicker, lowly doped epitaxial profile (3 mu m, 5 10(18) cm(-3)) to further reduce recombination, an additional rise in implied V-oc after metallization of 10 mV is estimated. This increase would be the result of a reduction in J(0,pass, emitter) (down to 6 fA/cm(2)) and J(0,plated, emitter) (down to 1967 fA/cm(2)).

Thermal processing of as-made fluorozirconate glasses which were additionally doped with neodymium and chlorine ions leads to enhanced upconversion fluorescence intensities. The samples were annealed between 240 degrees C and 290 degrees C and the optimum value of upconversion intensity was found for the 270 degrees C sample. The development of glasses doped with Er, whose luminescence characteristics make it a better choice for application as an upconversion layer on silicon solar cells, is in progress. In addition, we present external quantum efficiency measurements of a silicon solar cell with a NaYF4:Er3+ upconverter. We also show a theoretical analysis where erbium transitions can be enhanced selectively to increase upconversion efficiency.

An alternative process to the aluminium full back side is necessary to realize high efficiency solar cell on thin wafers. Even if metallization paste manufacturers deliver paste that minimize the wafer bowing to 1.5 mm for wafer lower than 200 μm, it is more and more critical to process wafers thinner than 150 μm with full aluminium back side. The objective of this work is to demonstrate on monocristalline silicon wafer that boron back surface field (BSF), diffused from BCl3 gas source or from oxide doped layer like Boron Silicon Glass (BSG), is as efficient as full back side aluminium BSF. In addition this work deal with integration of different passivation layer optimised to passivate high boron doped layer at the backside of a bifacial solar cell. The best efficiency obtained on monocristalline wafers shows an increase in efficiency of 0.9% absolute compare to our standard process with full aluminium back side. According to the process optimised for monocristalline wafers, the process was transferred on multicristalline wafer. Even if the gain in current, for bifacial solar cell, compared to standard process is confirmed, the high temperature process for boron diffusion, activate electrically lot of intra-grain defects and decrease the solar cell performance. A correlation between Light Beam Induced Current (LBIC) and electroluminescence highlight local shunt and explain why the solar cell performance with boron BSF diffused at the back side is impacted. An alternative solution is also proposed to reduce the thermal budget of boron diffusion and improve the solar cell efficiency.