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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - STARCELL (Advanced strategies for substitution of critical raw materials in photovoltaics)

Teaser

Summary

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 to achieve a challenging solar cell efficiency of 18% (16% for a 10x10 cm2 area mini-module) by the end of the project. These figures are well in line with the efficiency target values of the SET Plan (14-20% for 2020), 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.

Work performed

The main achievements obtained in the frame of each WP in more detail are:
WP1- Optimization of the different annealing procedure for kesterite formation, first attempts of synthesis of Zn-substituted absorbers, investigation of the effect of alkaline dopants and optimization of the Li and Ge incorporation, and preliminary investigations of processes leading to graded bandgap absorbers.
WP2- A TiN sputtered and ALD HfO2 diffusion barrier has been used as well between absorber and back electrode to suppress MoSe2 growth, two materials have been tested as Cd-free buffers: ZnO1-xSx and (Zn,Mg)O, absorbers and improved interfaces have been finely characterized with EDS in a deep TEM analysis.
WP3- Materials Modelling: effect of extended defects on the electronic structure, donor and acceptor levels of native defects in CZTS and CZTSe, electron capture by sulphur vacancy, SnZn as a source of electron trapping. Devices Modelling; the effect of voids on photovoltaic performance.
WP4- A failure map of the CRM-free kesterite solar cell based on literature review has been developed, development at Midsummer of a prototype semi-industrial in-line sputtering process for upscaling purpose, upscaling process to the size 5x5cm2 by IMRA, with their colloidal ink spray route obtaining relative variation of the cell performance parameters lower than 5%.
WP5- Recycling activities: successful separation of the kesterite absorber from the cells is achieved by chemical treatment with hydrochloric acid; synthetic kesterite is thermally oxidized and the condensate of selenium dioxide is reduced with ascorbic acid; metal oxides are solubilized in HCl solution, except SnO2 which is recovered by filtration and reduced with active carbon to elemental Sn. LCA activities: definition of the LCA scope and the collection of data, definition of a reference technology which will be compared to the final product presented at the end of STARCELL project.
WP6- Dissemination and communication: availability of webpage and social media, organization of the first press campaign, preparation of a project leaflet and two Newsletters, organization of four dissemination Workshops, participation in two meetings of the European Photovoltaic Cluster, and publication of thirteen research papers. Exploitation: preparation of the first plan for exploitation and dissemination, organization of three exploitation Workshops during 6M, 12M and 18M project meetings, in order to get a first approach to Exploitable Results (ER), define their exploitation claims and their intellectual property rights (IPR).
WP7- Organization of 3 General Meetings and 1 Review Meeting, submission of two amendments, 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.

Final results

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 reliable ab initio study of the deep-level carrier non-radiative recombination rate in CZTS and CZTSe has been performed. Non-radiative recombination centres have been identified for CZTS (S vacancies and Sn anti-sites) and CZTSe (mainly Sn anti-sites). Additionally, the formation energy of extended defects like anti-site domain boundaries (ADB), two types of stacking faults (SFs), and Î£3 (114) grain boundary in CZTS was obtained, and correlated with the electron extraction efficiency.
SO-2 (Optimization of Kesterite bulk properties by modified thermal treatments and advanced absorber engineering): a Round Robin experiment was organized exchanging absorbers and solar cells and the starting point in conversion efficiency was fixed at 10%. EMPA has optimized the alkali doping achieving record conversion efficiency of 12.3% in active area. IREC has developed a doping process using Ge to obtain high efficiency solar cell devices achieving an 11.8% record efficiency. AIST has worked in the alloying with Ge, substituting up to 40% of Sn, obtaining a record efficiency of 13.1% in active area.
SO-3 (Re-design and optimization of the device structure): Zn(O,S) and (Zn,Mg)S have revealed as non-optimal buffer layers mainly due to the non-well suited band alignment with kesterite. Similar study is being conducted by CEA in (Zn,Sn)O, and additionally IREC is studying the possibility to implement alternative hybrid buffer layers with the objective to reduce the Cd content, where (Zn,Cd)S is the most promising candidate for the moment, achieving conversion efficiencies of 9.5%.
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): At IREC, 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.
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. 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 first LCA analysis of the reference technology, performing a benchmarking with Cu(In,Ga)Se2. Additionally, WIREC has developed a route for the recovering of Se from kesterite with a global yield of 75.6% and a purity of 97.5%, and currently is developing routes for the recovering of metals (Cu, Zn and Sn) with very successful results.
SO-8 (Cost-effectiveness and commercial exploitation potential): A complete first plan for exploitation and dissemination was presented in month 6 (Deliverable D6.1), and was updated in month 18 (Deliverable D6.4).