CdTe solar cells
CdTe solar cells have shown an extraordinary efficiency improvement in recent years. This was obtained by a complete redesign of the device with the (1) development of a stable and efficient back contact that works as well as back reflector, (2) band gap tuning by introduction of CdSexTe1−x in the CdTe absorber, (3) substitution of CdS with MZO transparent buffer layer.
The final result is an efficiency of 22.1%, this value is in line with other technologies but with a higher scalability and the possibility of new BIPV structures.
These exceptional results have brought to a module installation of more than 17 GW globally with efficiencies on large areas up to 19% in commercial modules. How- ever open circuit voltage needs still additional improvement to reach the nominal value of 1000 mV, for this reason the research is now concentrating on the systematic doping of CdTe.
If a suitable doping would be obtained it is plausible to expect efficiencies in the range of 25% in the next years, with a further cut of the costs by the reduction of materials usage and optimization of the production processes.
Usually, conversion efficiencies of photovoltaic devices decrease with increasing temperatures, in particular open circuit voltage reduces consistently and short circuit current slightly improves. For CdTe it has been observed that the decrease in open- circuit voltage is remarkably less than for CIGS and silicon based solar cells, resulting in 20% more overall power generation in high temperature environment.
Finally, a large number of scientific articles prove the extremely low environmental impact of this technology. Even non-friendly reports show that the possibility for broken modules to have high cadmium leakage in the soil is very unlikely. As well as no emission of cadmium can occur in case of residential fires. In addition, the complete recyclability of the CdTe modules makes this great technology absolutely clean. The last delivered efficiency at LAPS for this technology was 16.1% (19.1% is the world record for evaporated devices).
Cu2(Sn,Zn)(S,Se)4 (CZTS) solar cells
Since the constituents are not rare elements, like Indium or Gallium in CIGS, whose price is going always higher and higher and whose demand is now greater than its supply, this technology will become cheaper, if compared with other PV technologies. At the same time CZT(S,Se) does not contain hazardous elements, resulting more safety from a healthily and environmentally point of view.
In order to overcome the environmental issues and most important to remove any doubt about massive production connected with material scarcity in 1996 Katagiri et al. have introduced a new device based on Cu2ZnSnS4 (CZTS) absorber layer, with a device structure similar to the CuInSe2 (CIS) or CIGS solar cells, where indium and gallium are substituted with zinc and tin, much more available on the earth crust. Spin-Coating of CZTS Precursor Solution is one of the most successful technique, best efficiency was presented first by Todorov et al. by preparing a CZTS solution based on hydrazine. The best cell efficiency was 9.66%, improved by Mitzi et al. up to 10.1%, 12,6% and finally 13.1%.
The final goal is to deliver thin film solar cells with a variety of structures for BIPV and PIPV (namely bifacial cells, cells on flexible/ultralight substrates) with efficiency above 10%, which could be the basis for the next development of higher efficiency integrated solar cell devices. The last delivered efficiency at LAPS for this technology is 7.5% (13.1 is the world record)
Sb2Se3 solar cells
Sb2Se3 (ASe) is a typical V2VI3 binary chalcogenide with single phase and fixed composition, avoiding the complexity of phase and defect control as in CZTSSe and CIGS. ASe displays a narrow band gap of (1.1÷1.3) eV which approaches the ideal Shockley Queisser value.
Antimony trisulphide, Sb2S3, has found applications in different devices as camera sensors, microwaves, switching and optoeletronic. The use of Sb2S3 and Sb2Se3 thin film in solar cells with 5,8% conversion efficiencies, has been recently reported.
In sight of the various potential applications, different deposition methods have been employed, for the preparation of Sb2S3 thin film as the chemical bath deposition (CBD), spray pyrolysis (SP), high vacuum evaporation (HVE), Sputtering deposition (SPD) and selenization of metallic layers. Often, a post-deposition heat treatment in air at high temperatures (400÷450) °C is carried out. As a result, a change in both optical and structural properties is observed. High substrate temperature delivers, typically, higher crystallinity and better quality of the material but does not allow deposition on soft material such as polymers (for flexible applications). For this reason UniVR will pursue ASe deposition by evaporation and pulsed electron deposition. A large experience on thermal evaporation of CdTe solar cells on flexible polymers has already been developed at LAPS.
The last delivered efficiency at LAPS for this technology is 3.5% (which is a world record for evaporated Sb2Se3 absorbers).
Near Field Communication antennas
Study of a preparation method of a flexible thin-film NFC tag, based on the preparation of p / n junctions deposited on flexible polymer, transferring know-how for the preparation of a flexible tag to be inserted inside the product and initialization of the tag on the software part.
SnS solar cells
Despite the big improvement in efficiency for CuInGaSe2 and CdTe based solar cells, demonstrating the high feasibility of these technologies, there is still ongoing research for new absorbers: low cost thin films made of abundant and non-toxic materials.
Among different compounds available, SnS is a promising compound due to its simple stoichiometry, its low temperature growth and the abundance of its elements. However record efficiencies are still very low and stoichiometric growth, free of secondary phases, is not easy to achieve.
It has been already reported that SnS structure and morphology strictly depends on the substrate temperature. In our case, we observe a threshold above which the material grows highly crystallized, compact and smooth. Below this temperature the layer is more porous and shows the presence of porous material on the surface.
Another possibility to increase crystal quality is to apply a post deposition treatment. Previously we have discussed the effect of CdCl2 and Freon gas on the SnS structure , whereas in this present work the recrystallization of SnS crystals is achieved by annealing the absorber layer at high temperature under a flux of molten KI or SnCl2 salts.
The last delivered efficiency at LAPS for this technology was 1.5% (3.88% is the world record).