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Advanced strategies for substitution of critical raw materials in photovoltaics

Periodic Reporting for period 2 - STARCELL (Advanced strategies for substitution of critical raw materials in photovoltaics)

Okres sprawozdawczy: 2018-07-01 do 2019-12-31

STARCELL aims to eliminate all materials classified as CRM from cost effective thin film PV technologies through development and use of earth abundant kesterite materials from Cu, Zn, Sn, S and Se.
The project targets the optimisation of the material processes and the device interfaces for improving conversion efficiency at cell and mini-module level by the end of the project. These figures are well in line with the efficiency target values of the SET Plan , and with the requirements for the future commercialization of the technology, bringing to the Society a new sustainable thin film PV technology for clean Energy production.
The final goal will be accomplished through 8 specific sub-objectives, undertaken by world leaders in the technology who are joining forces for the first time, in our international consortium.
The main achievements obtained in the frame of each WP in more detail are:
WP1- Understanding of the kesterite formation mechanisms, full development of Li and Ge doping strategies, demonstration of graded bandgap concepts using Ge/Sn and S/Se, demonstration that non-radiative recombination can explain the very low electrons life-time. Achievement of several world efficiency récords in these technologies
WP2-It was demonstrated that Mo is the best back contact option. Cd-free and Cd-reduced buffer layers were developed with efficiencies at the same level than CdS.
WP3- Materials Modelling: it was demonstrated that Sn-related defects are the most detrimental one and that can be hardly supressed. Devices Modelling; a 2D solar cell model was developed and impact of grain boundaries and holes analysed.
WP4- A failure map of the CRM-free kesterite solar cell based on literature review has been completed. Sucessfull upscaling towrds 5x5cm2 by IMRA and 20x30 cm2 by Midsummer. Demonstration of estability and durability of the technology including in-field test by AYESA.
WP5- Recycling activities: successful recycling process for kesterite mini-modules either onto glass and Steel substrate. Demonstration of the viability of the scaled-up technology. LCA activities: complete LCA analyisis of Midsummer technology demonstrating that kesterite perform better in terms of sustainability with respect to CIGS, in 11 of the 11 parameters under analysis.
WP6- Dissemination and communication: webpage and social media updated frequently, organization of the second press campaign, preparation of two Newsletters, organization of two dissemination Workshops, and publication of 23 research papers. Exploitation: preparation of the final plan for exploitation and dissemination, organization of two exploitation Workshops during 24M, and 30M project meetings.
WP7- Organization of 3 General Meetings and 1 Review Meeting, data monitoring on the use of resources, communication with all the WP leaders and intermediation and communication with EC Project Officer for a good project performance.
The main progresses beyond the state of the art obtained so far for each sub-objective of STARCELL are the following:
SO-1 (Improved modelling of CRM free PV absorbers and devices by identifying core problems and possible solutions): a complete model for kesterite was developed including all type of defects. It was demonstrated that Sn-related defects are most probably the main efficiency limitation, reducing the carriers life-time, increasing the non-radiative recombination, and increasing the Voc deficit.
SO-2 (Optimization of Kesterite bulk properties by modified thermal treatments and advanced absorber engineering): different absorber engineering strategies were investigated including doping, alloying and customized thermal annealing processes. The importance of Li and Ge doping and the interaction of alkalis with Sn were discovered, graded bandgap concepts using Sn/Ge and S/Se were developed, and the main kesterite formation mechanisms elucidated.
SO-3 (Re-design and optimization of the device structure): (Zn,Sn)O buffer layer was developed by CEA, and additionally IREC has developed a Cd-reduced buffer layer, where (Zn,Cd)S is the most promising candidate for the moment, achieving conversion efficiencies over 10%.
SO-4 (Study of kesterite surface modification for improved solar cell devices): EMPA has proposed Li as the most promising alkali dopant, observing a strong dependence of the optimal alkaline content on the Sn concentration. Additionally, IREC has studied surface passivation, optimizing (NH4)2S as very effective passivating agent. IREC has started a very innovative selective surface doping, based in the incorporation of alkali elements during the CdS deposition, demonstrating the effectiveness for incorporating the alkalis at the surface.
SO-5 (Back contact design to reduce recombination and efficiency loss due to non-fully ohmic rear contact): Different transition metal oxides were investigated and compared, and particularly, the use of 20 nm of TiO2 or Nb2O5 allows to improve the long wavelength response of the solar cells. Additionally, some selenides (CuGaSe2), nitrides and other metals were investigated as possible back contacts. Mo still remains as the best option.
SO-6 (Study of non-homogeneity problems at micro and macro scale): the homogeneity of state-of-the-art kesterite based devices were investigated by several techniques including synchrotron based techniques. Slight compositional fluctuations at macro-scale and some secondary phases has been identified. Nevertheless, at micro-scale, relevant compositional fluctuations have been observed.
SO-7 (Sustainability of materials (life-cycle analysis, recycling/reusing). Demonstration of the technology at TRL5): CEA has developed a complete LCA analysis for kesterite demonstrating their sustainability. Additionally, WIREC has developed a route for the complete recycling of mini-modules onto glass and Steel substrates, demonstrating their scalability and cost analysis.
SO-8 (Cost-effectiveness and commercial exploitation potential): A final plan for exploitation and dissemination was presented opening interesting perspectives for the future commercialization of kesterite.
STARCELL prototype (in-field operation) and corresponding micrograph.