By Prof. Ahmed Ennaoui, Chairman of the Scientific Council, Institut de Recherche en Energie Solaire et Energies Nouvelles (IRESEN), Rabat, Morocco.
Former head of physical Chemistry group at Helmholtz Zentrum Berlin für Materialien und Energie, Lise-Meitner-Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
One of the most promising classes of materials for thin-film photovoltaic devices is the Cupper chalcopyrite quaternary chalcogenides known with the general formula Cu(InxGa1-x)(SySe1-y)2 or CIGSS. The best solar devices are fabricated with cadmium sulfide (CdS) buffer layer between the p-doped CIGSS absorber and i-ZnO/n+-ZnO window bilayer prepared by solution growth technique known as chemical bath deposition. Solar cells with record efficiency of 22.6% using lab-sized (0.5 square centimetre) was obtained, however to avoid Cadmium as a heavy metal compound in the final modules, other buffer layers have been tested as alternative to CdS such as Zn(S,O) and similar record efficiency up to 22.9% on a 1cm² cell was published recently . The solution chemistry of the CBD process plays a crucial role. For the preparation of Zn(S,O), the reaction paths involving aqueous solution containing zinc sulfate, ZnSO4, ammonia NH3, and thiourea [(NH2)2CS)] depend critically on the equilibrium concentration of free hydrated Zn2+ versus that of Zn2+ complexes with the complexing agents present in the solution, namely, OH–, NH3, and thiourea. Upon heating, thiourea decomposes to release S2- ions, which can combine with free Zn2+ at the surface of the absorber to form ZnS through heterogeneous process. However, due to the high concentration of OH– in the alkaline solution, the formation of turbid Zn(OH)2 from Zn2+ and OH– is a competing reaction, so that the final layer can best be described as a mixture of ZnS and Zn(OH)2. Furthermore, due to the predominant presence of Zn complexes rather than free Zn2+, formation rate of ZnO is low even in alkaline solution, which is a prerequisite for ZnS formation. We observed the best Zn(S,O) layers in terms of device efficiency by first adding thiourea to an aqueous solution of ZnSO4, heating this mixture up to 80°C, and then adding ammonia (referred as here as CBD-T). The standard process obtained by combining ZnSO4 with ammonia first and then adding thiourea (referred here as CBD-N) leads to inferior devices. This is surprising, because one would expect the product of reversible complex formation between Zn2+ and the ligands ammonia and thiourea to depend only on the heat of formation of the respective complex and not on the order of adding the chemicals. For profound understanding, on the role of the two complexes CBD-T and CBD-N, we performed real time soft X-ray absorption spectroscopy (XAS) to probe chemical reactions by measuring the L-edge of Zn species in solution under ambient conditions in situ during the CBD and annealing processes . We showed that CBD-T and CBD-N processes lead to completely different zinc precursor complexes in solution, despite identical concentrations of starting materials. Obviously the Zn-S=C complex which forms in the CBD-T exhibits ligand-to-metal electron transfer and metal-to ligand back-donation. This back-donation is missing in the CBD-N process, in which the more stable zinc–ammonia complex is formed and no Zn–S interaction takes place. Based on this new type of complexation which results in homogeneous and compact nanocrystalline bilayers ZnS/Zn(S,O), the process is patented . To reduce high density of acceptor-like defects at CIGS/Zn(S,O) interfaces a post annealing step is required to passivates the defects via interdiffusion and thus improves the cell performance. In this talk we review our work on Cd-free CIGSSe solar cells performed at Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in joint effort with several industry partners. Solar cell efficiency above 16% was obtained with Zn(S,O)/CIGSSe based solar cell after post annealing in air for lab scale. We successfully up-scaled the CBD-Zn(S,O) process from laboratory scale (0.5 cm² single cell devices) to large area Cd-free solar modules (60cm x 120cm) with efficiency closed to 13%. We will highlight the fundamental understanding of CBD-Zn(S,O) on Cu(In,Ga)(S,Se)2 (CIGSSe) and Cu(In,Ga)Se2 (CIGSe) to form high performance solar cells and modules .
 Solar Cell Efficiency Tables (Version 51), Progress in Photovoltaics: Research and Applications 2018.
 ChemPhysChem (2009), vol.10, pp.532-535
 US Patent 7.704,863 B2 Method of the application of zinc sulfide buffer layer on a semiconductor substrate
 Special issue: Prog. Photovolt: Res. Appl. vol. 18 (2010) pp. 411–433
Most of this work was achieved at HZB, “Helmholtz Zentrum Berlin für Materialien und Energie and supported partly by the European Commission within the framework of several projects (CHEETAH, ATHLET, NEBULES, CISLINE, Joules II)) and by the “Bundesministerium für Bildung und Forschung (BMBF)”. I especially acknowledge all industry partners for their supply of the samples, and I am grateful to my former Ph.D. students, Post-Doc, and all my colleagues at the department of heterogeneous material systems either for their technical assistance or for critical discussions.
Ahmed Ennaoui studied physics and chemistry and obtained his master and doctoral degree in solid-state electronics from the University of Bourgogne, France. He moved to Germany and conducted research for his habilitation (1983-1987) with Prof. Helmut Tributsch in the department “Abteilung Solare Energetik und Materialforschung” at the Hahn Meitner Institute (HMI), Berlin, Germany. After obtaining his habilitation (summa cum laude) for his research on novel earth-abundant and non-toxic materials (Iron Disulfide, Pyrite) for solar energy conversion, he served as Professor of physics at the University Mohamed V. He moved back to Germany and worked as senior scientist at the HMI, while maintaining strong contact with Moroccan universities. He was appointed head of a research group at Helmholtz-Zentrum Berlin (HZB) “the former HMI” and explored in his group novel alternative materials for solar energy conversion. He received different grant projects financed by the European commission and the German Federal Ministry of Education and conducted successful feasibility studies financed by the industry (Siemens/Shell-Solar/Avancis, Solibro, Bosch, Atotech). All his projects are targeting thin Film solar cells and new concept, from planar films to nanostructures. He served as member of Qatar Foundation (2015-2017) with a position of joint Professor at Hamad Bin Khalifa University (HBKU) and as Research Director of Solar Energy group at Qatar Environmental and Energy Research Institute (QEERI). At QEERI he was conducting research, managing research groups, recruiting top talents. He established track record, building partnerships with industry and academia at QEERI/HBKU during the period from January 2015 to May 2017 and the management of scientists at QEERI ‘s solar test facilities (STF with: ~200 kW). He contributed to the development of PV soiling solution, improving O&M of the infrastructure at the STF. At QEERI he proposed also a relevant low CAPEX manufacturing Thin Film PV technology to reduce the cost per watt peak of solar modules through building of a base line for inkjet printing as a precision deposition technology for flexible PV. He designed syllabus for HBKU and taught several seminar on Solar Photovoltaic Technology for graduate students. Actually Prof. Ahmed Ennaoui serves as president of the Scientific Council of IRESEN, the Moroccan Solar Energy Research Institute, deeply engaged at for the establishment of mechanisms to support and reinforce R&D on renewable energy. Ahmed Ennaoui has deep expertise in processing materials and devices for solar energy conversion and thin film chalcogenide solar cells. He is author and co-author of more than 200 peer-reviewed journal articles, numerous conference contributions, five special issues and granted several patents, his published work on pyrite FeS<sub>2</sub>, 2D layered materials, kesterite, cadmium free chalcopyrite solar cells, DSSC solar cells has being recognized with h-index 41. He is permanent member of the Editorial Board of Solar Energy Materials and Solar Cells, reviewer for several top scientific journals, member of International Solar Energy Society (ISES), member of IEEE and chair of the scientific-technical committee for the International Renewable and Sustainable Energy Conference (IRSEC). During his career he supervised more than 40 completed thesis and ca. 15 Postdoctoral have worked in his group.