Final Report Summary - PHOTORODS (Photovoltaic cells based on nano-structured CdTe) Cadmium telluride (CdTe) is already established as a prominent solar cell semiconductor material due to its energy gap 1.5 eV with an almost perfect match to the solar spectrum. Thin-film photovoltaic (PV) technologies for CdTe-based flat-plate modules are attractive because they consume much lower amounts of expensive semiconductors and are more amenable to much higher levels of production automation than wafered silicon. Commercial thin-film CdTe manufacturing companies such as First Solar, Antec Solar Energy, Solar Fields, Primestar Solar, AVA Technologies, Canrom, Matsushita Battery, etc. have introduced CdTe-based thin-film PV panels at > 9 % efficiency, with attractive pricing and availability for large-scale utility in ground-mounted systems. As manufacturing volume increases, the cost advantage can expand even more significantly. Thus, the CdTe-based thin-film PV panel production is likely to remain an important PV market in the future. The aim of the PHOTORODS project was to explore the use of nanostructured CdTe as the absorbing layer in PV solar cells, and to evaluate the performance and industrial potential of these cells. The project progressed through three main phases / work packages (WPs) as follows. WPI: 'Technology development and characterisation of CdTe nanorods for PV cells' dealt with: (i) the development and set-up of technological facilities required (sputtering, thermal evaporator, thermal annealing, chemical bath, substrate holders, crucibles, etc.); (ii) the preparation and fabrication of starting materials such as substrates, CdTe evaporation sources, catalysts, etc.; (iii) the development of the fabrication technology of CdTe nanorods by thermal evaporation / vapour phase epitaxy (VPE) using catalyst-prompted CdTe nanogrowth approach; and (iv) the characterisation of CdTe nanostructured layers by a wide range of techniques (SEM, XRD, PL, TEM, RBS, XPS, etc.). WPII: 'Design and fabrication techniques for CdTe-based nanostructured PV cells' investigated: (i) the integrations of CdTe nanorods with other potential materials to be used for the PV cell fabrication (window layers - CdS, ZnTe, ZnO; back metal contacts; polymers); (ii) the effects of temperature, grain size, and stress on CdTe layer properties; (iii) the complementary technologies (chemical bath for CdS, close-space VPE, polymer deposition, contact fabrication, etc.); (iv) effects of local structure and additional treatments (annealing, CdCl2 treatment, etc.) on nanoCdTe-based PV cells. WPIII: 'Functional characterisations of CdTe-based nanostructured PV cells' studied: (i) the different PV cell geometries (CdTe thickness, back contact, insulating / conducting polymers, etc.); (ii) procedures of PV cell prototype fabrication; and (iii) PV cell characterisation and performance statistics. The project results in the demonstration of technological feasibility of the industrially-applicable low-temperature fabrication process of CdTe-based nanostructured PV cells. The challenging points of this process were found to be the starting vapor deposition of metal (Bi, Sn, In) catalyst dropped layer on commercial glass substrates (layer thickness 10 - 100 nm, deposition rate 0.1 - 0.5 nm / sec, temperature 150 - 3 200 degrees of Celsius) followed by the isothermal VPE catalyst-prompted CdTe nanorod layer of controllable shapes and surface filling (layer thickness 100 - 1 000 nm, deposition rate 0.01 - 0.2 nm / sec, temperature 280 - 4 000 degrees of Celsius). The fabricated PV cell prototypes demonstrated a good spatial uniformity and time-temperature stability while their PV characteristics have still to be much improved during future investigations. The major factors limiting to the PV cell parameters were found to be (i) the mixture of hexagonal and cubic structure phases into thick (= 100 nm) CdTe nanorod layers which causes the high-carrier recombination and dramatic decrease of PV efficiency and (ii) the high (=1 kOhm) resistance of CdTe-based nanostructured PV cells originated by the catalyst metal doping of CdTe nanorods. The project results are expected to contribute to the further development of new generations of PV high-efficiency cells. Additionally, the same CdTe-based PV nanostructures could be explored for their usage as efficient imaging flat-panel direct-conversion semiconductor detectors for applications in such diverse fields as nuclear medicine, homeland security, astrophysics, and environmental remediation. Related documents Final Report - PHOTORODS (Photovoltaic cells based on nano-structured CdTe)
