Final Report Summary - NANOCIS (Development of a new generation of CIGS-based solar cells)
The NanoCIS project (IRSES 269279, Development of a new generation of CIGS-based solar cells) was aimed at the establishment of a cooperative partnership between research organizations through a joint program of exchange of researchers for developing a new generation of photovoltaic (PV) solar cells. This new generation of PV solar cells is based on different approaches involving the use of new materials with high conversion efficiencies and low-cost fabrication techniques. The broad aim includes the theoretical and experimental design, synthesis and characterization of new advanced materials (chalcopyrites absorbers and nanophosphors) and the test of new concepts in photovoltaic conversion related to the improvement of photon harvesting.
The concepts investigated in this project have been new absorber materials with intermediate absorption bands and new phosphors materials for matching the sun spectrum to the characteristics of absorbers.
The prospective for new intermediate band materials with enhanced efficiency has been done through quantum modelling. By means of density functional theory (DFT) and beyond advanced hybrid functional, including self-consistently the spin-orbit relativistic effects we have previously calculated the geometric and electronic structure of materials which can be used in solar cells to obtain better efficiency than standard cells using the intermediate band concept. This material has been theoretically study and an intermediate band has been found when Gallium atoms are substituted by Cr o Ti transition metals in chalcopyrite materials. Quantum computing has also allowed the calculation the optical absorption and dielectric properties of Ti- and Cr- substituted CuGaS2. Further, using DFT techniques other materials like In2S3 and SnS2 have also been identified as suitable for hosting an intermediate when doped with V.
A thin film solar cell requires the stack of several layers: Back metal contact on a substrate, the absorber layer, the buffer layer, the window layer and the front contact.
Electrodeposition (ED) is the technique chosen for developing such approaches. ED is essentially a non-vacuum approach to fabricate high quality thin-film materials for PV modules that could lower the manufacturing costs by over 50% and increase the PV module efficiencies. The ED technique offers the most attractive range of benefits leading to the low cost fabrication of PV cells, such as high rate of deposition, high resolution, high shape fidelity, self purification, scalability and good compatibility with existing processes. ED adds another cost effective step in low-cost solar cell because the transparent conducting oxide layers (TCO) can be deposited by the same method. The use of inline processing through an exclusively non-vacuum technique will further contribute to the improvement of device performance. Other low-cost techniques like Spray pyrolysis and Chemical Bath Deposition have been also tested to prepare some parts of the PV device, in particular the buffer land the window layers.
During the project the electrodeposition of CuInSe2 thin films and other related materials such CuInGaSe2, CuGaSe2 and CuGaS2 to be used as absorbers in solar cells were established by the consortium partners.
CIS thin films were also deposited on Mo thin foil substrates by electrodeposition using a buffered aqueous electrolyte with the deposition of subsequent layers performed by spray pyrolysis. In addition, the buffer layer CdS was replaced with a wider bandgap ZnS (3.7 eV) and analysis undertaken of the fabrication pathway, morphological and compositional changes resulting from the different precursor route.
The synthesis of buffer layers as In2S3 and ZnS, alternative to the standard CdS, has been addressed in the project. The work deposition of In2S3 and ZnS by ED or spray pyrolysis was also carried out successfully in parallel with standard chemical bath deposition of CdS. The deposition and electrical and structural characterization of In2S3 and CdS thin films were also reported extensively.
The material chosen for the window layer is ZnO:Cl synthetized by ED. Reproducibility of the electrodeposition of ZnO and subsequent doping has been well established. The main requirements for optical window layers are high transparency and high conductivity. High transparency requires very smooth surfaces to avoid scattering in grain borders and conductivity is related to doping. Zinc oxide is widely used as the transparent conducting window layer in CIGS thin film solar cells. ZnO is a good substitute for transparent conducting oxide (TCO) layers in solar cells due to its high stability. In the project we have established the optimal conditions to reach very high transmittance. To synthetize ZnO electrochemically the electrolyte can be prepared with water or with an organic solvent. The main result is that when ZnO thin films are synthetized in aqueous electrolytes nanostructured films are obtained. The transparency of these nanostructured films is not high enough for PV application, because light is scattered by these nanostructures and as a result the transparency slows. However, when the electrolyte is prepared with an organic solvent like dimethilsulfoxide (DMSO) the films’ transparency rises dramatically. This effect can still be boosted if Zn precursors come from perchlorates (Zn(ClO4)) instead of chlorides (ZnCl2). Doping with Cl improves the conductivity of the ZnO layers, which is beneficial for window layers. Electrochemical doping of ZnO with Cl has also been established.
CIGS-based solar cells fabricated by depositing CdS thin film (~80 nm) on etched CIGS surface followed by ZnO/ZnO:Cl double layer. The project produced a lab scale, 1 cm2 solar cell with Voc 455 mV, Isc 16 mA, FF 59.8%, and efficiency of 4.3% under AM 1.5 1 sun conditions, to date the flexible CIS solar cells have produced only about 1% conversion efficiency. It was found that the limiting factor is related to the annealing temperature used in the selenization post-treatment. The annealing temperature is limited to 400ºC when using flexible polyamide substrates and the quality of the absorber layer reduces the conversion efficiency. CIGS samples deposited on glass allows an annealing temperature up to 550ºC, being 500ºC the optimal temperature.
One challenge of the project has been the synthesis of materials for the manufacture of an intermediate band (IB) CIGS solar cell. Theoretical calculations carried out by the consortium members revealed that the best candidate for hosting an IB is CuGsS2, which has a bandgap of about 2.4 eV and transition metals like Ti and Cr are suitable candidates for forming the IB
CuGaS2:Cr thin films with sub band absorption due to the intermediate band associated to partially Cr levels have been demonstrated experimentally in the project. The approach consisted in the initial synthesis of CuGaSe2 form an aqueous electrolyte containing Cu, Ga, Se and Cr ions and subsequent annealing in presence of molecular sulphur.
A process has been developed to produce UV and nIR luminescent up and down converting rare-earth doped nanocrystalline phosphor materials by a novel combustion method. The nanophosphor materials (lanthanides) have been synthesized and extensively characterized in order to further develop the solar cell. A huge number of phosphors materials has been synthetized and characterized in the project. One of the best phosphors checked is SrAl2O4:Eu2+ which possess all the essential requirements of a long persistence phosphor. The formation of homogeneous single phase of monoclinic SrAl2O4 has been confirmed by Raman analysis. The single step process to produce nanocrystalline long persistence phosphor in fluffy and voluminous form without agglomeration, which can be easily converted to fine and uniform powder, is an added advantage of this process. Further optimization of the process parameters can easily overrule the slight deviations observed in the decay times. This is a process that is easily adopted for producing phosphors in nanophase (nanophosphors).
Finally, the behavior of photovoltaic devices based on intermediate band absorbers has been numerically simulated to understand the behavior of such devices and design the best architecture for such devices.
Additional information regarding the project can be found in the link: http://www.nano.institutoidf.com/
The concepts investigated in this project have been new absorber materials with intermediate absorption bands and new phosphors materials for matching the sun spectrum to the characteristics of absorbers.
The prospective for new intermediate band materials with enhanced efficiency has been done through quantum modelling. By means of density functional theory (DFT) and beyond advanced hybrid functional, including self-consistently the spin-orbit relativistic effects we have previously calculated the geometric and electronic structure of materials which can be used in solar cells to obtain better efficiency than standard cells using the intermediate band concept. This material has been theoretically study and an intermediate band has been found when Gallium atoms are substituted by Cr o Ti transition metals in chalcopyrite materials. Quantum computing has also allowed the calculation the optical absorption and dielectric properties of Ti- and Cr- substituted CuGaS2. Further, using DFT techniques other materials like In2S3 and SnS2 have also been identified as suitable for hosting an intermediate when doped with V.
A thin film solar cell requires the stack of several layers: Back metal contact on a substrate, the absorber layer, the buffer layer, the window layer and the front contact.
Electrodeposition (ED) is the technique chosen for developing such approaches. ED is essentially a non-vacuum approach to fabricate high quality thin-film materials for PV modules that could lower the manufacturing costs by over 50% and increase the PV module efficiencies. The ED technique offers the most attractive range of benefits leading to the low cost fabrication of PV cells, such as high rate of deposition, high resolution, high shape fidelity, self purification, scalability and good compatibility with existing processes. ED adds another cost effective step in low-cost solar cell because the transparent conducting oxide layers (TCO) can be deposited by the same method. The use of inline processing through an exclusively non-vacuum technique will further contribute to the improvement of device performance. Other low-cost techniques like Spray pyrolysis and Chemical Bath Deposition have been also tested to prepare some parts of the PV device, in particular the buffer land the window layers.
During the project the electrodeposition of CuInSe2 thin films and other related materials such CuInGaSe2, CuGaSe2 and CuGaS2 to be used as absorbers in solar cells were established by the consortium partners.
CIS thin films were also deposited on Mo thin foil substrates by electrodeposition using a buffered aqueous electrolyte with the deposition of subsequent layers performed by spray pyrolysis. In addition, the buffer layer CdS was replaced with a wider bandgap ZnS (3.7 eV) and analysis undertaken of the fabrication pathway, morphological and compositional changes resulting from the different precursor route.
The synthesis of buffer layers as In2S3 and ZnS, alternative to the standard CdS, has been addressed in the project. The work deposition of In2S3 and ZnS by ED or spray pyrolysis was also carried out successfully in parallel with standard chemical bath deposition of CdS. The deposition and electrical and structural characterization of In2S3 and CdS thin films were also reported extensively.
The material chosen for the window layer is ZnO:Cl synthetized by ED. Reproducibility of the electrodeposition of ZnO and subsequent doping has been well established. The main requirements for optical window layers are high transparency and high conductivity. High transparency requires very smooth surfaces to avoid scattering in grain borders and conductivity is related to doping. Zinc oxide is widely used as the transparent conducting window layer in CIGS thin film solar cells. ZnO is a good substitute for transparent conducting oxide (TCO) layers in solar cells due to its high stability. In the project we have established the optimal conditions to reach very high transmittance. To synthetize ZnO electrochemically the electrolyte can be prepared with water or with an organic solvent. The main result is that when ZnO thin films are synthetized in aqueous electrolytes nanostructured films are obtained. The transparency of these nanostructured films is not high enough for PV application, because light is scattered by these nanostructures and as a result the transparency slows. However, when the electrolyte is prepared with an organic solvent like dimethilsulfoxide (DMSO) the films’ transparency rises dramatically. This effect can still be boosted if Zn precursors come from perchlorates (Zn(ClO4)) instead of chlorides (ZnCl2). Doping with Cl improves the conductivity of the ZnO layers, which is beneficial for window layers. Electrochemical doping of ZnO with Cl has also been established.
CIGS-based solar cells fabricated by depositing CdS thin film (~80 nm) on etched CIGS surface followed by ZnO/ZnO:Cl double layer. The project produced a lab scale, 1 cm2 solar cell with Voc 455 mV, Isc 16 mA, FF 59.8%, and efficiency of 4.3% under AM 1.5 1 sun conditions, to date the flexible CIS solar cells have produced only about 1% conversion efficiency. It was found that the limiting factor is related to the annealing temperature used in the selenization post-treatment. The annealing temperature is limited to 400ºC when using flexible polyamide substrates and the quality of the absorber layer reduces the conversion efficiency. CIGS samples deposited on glass allows an annealing temperature up to 550ºC, being 500ºC the optimal temperature.
One challenge of the project has been the synthesis of materials for the manufacture of an intermediate band (IB) CIGS solar cell. Theoretical calculations carried out by the consortium members revealed that the best candidate for hosting an IB is CuGsS2, which has a bandgap of about 2.4 eV and transition metals like Ti and Cr are suitable candidates for forming the IB
CuGaS2:Cr thin films with sub band absorption due to the intermediate band associated to partially Cr levels have been demonstrated experimentally in the project. The approach consisted in the initial synthesis of CuGaSe2 form an aqueous electrolyte containing Cu, Ga, Se and Cr ions and subsequent annealing in presence of molecular sulphur.
A process has been developed to produce UV and nIR luminescent up and down converting rare-earth doped nanocrystalline phosphor materials by a novel combustion method. The nanophosphor materials (lanthanides) have been synthesized and extensively characterized in order to further develop the solar cell. A huge number of phosphors materials has been synthetized and characterized in the project. One of the best phosphors checked is SrAl2O4:Eu2+ which possess all the essential requirements of a long persistence phosphor. The formation of homogeneous single phase of monoclinic SrAl2O4 has been confirmed by Raman analysis. The single step process to produce nanocrystalline long persistence phosphor in fluffy and voluminous form without agglomeration, which can be easily converted to fine and uniform powder, is an added advantage of this process. Further optimization of the process parameters can easily overrule the slight deviations observed in the decay times. This is a process that is easily adopted for producing phosphors in nanophase (nanophosphors).
Finally, the behavior of photovoltaic devices based on intermediate band absorbers has been numerically simulated to understand the behavior of such devices and design the best architecture for such devices.
Additional information regarding the project can be found in the link: http://www.nano.institutoidf.com/