Thin Film Cadmium Telluride Solar Cells

Abstract

This chapter discusses a number of deposition techniques used to produce polycrystalline CdTe solar cells, including progress of photovoltaic (PV) performances in recent years. Focus is on the CdTe absorber and the effects from impurities, which are dependent on the process conditions used, but also due to self-compensating nature of the material itself influencing the acceptor levels in the layer. Impurities can introduce deep donor/acceptor levels that act as traps for both majority and minority carriers. This leads to greater recombination and reduced carrier lifetimes, causing a loss in the level of generated photocurrent and overall performance of the PV cell. Impurities are typically concentrated at the CdTe grain boundaries, making grain size an important parameter for defect density control. Post-growth treatment using CdCl2 and annealing improves PV performances in several ways: grain re-crystallisation and growth; inter-diffusion at the CdS–CdTe interface removing defects related to the lattice mismatch between the two layers; and passivation of deep acceptor states through complex formation with the ClTe+ shallow donor. High p-type doping is necessary for the formation of a back contact with good ohmic properties without a Schottky barrier restricting conduction of majority carriers. Stable back contacts are also required, with the Sb2Te3–Mo system possibly offering the best solution. Finally MOCVD is presented as a prospective technique for large-scale industrial production of CdTe solar modules, with discussion of the beneficial impact in reducing CdTe absorber thickness and the processing challenges associated with it.

Metastable CdTe@HgTe Core@Shell Nanostructures Obtained by Partial Cation Exchange Evolve into Sintered CdTe Films Upon Annealing

“…At the same time, the ICP analysis revealed that the amount of Hg decreased from 1% at 200 °C to below the detection limit at 350 °C. Overall, these results were quite remarkable, considering that to sinter a film of colloidal CdTe NCs, one would typically require annealing temperatures above 350 °C. − ,− Indeed, as a control, we observed that a non-exchanged CdTe NC sample annealed at 200 °C did not undergo any appreciable sintering (Table and Figure S5).…”

“…Spherical CdTe NCs, with a mean diameter of 4.5 ± 1 nm, were synthesized following the procedure reported by Zhang et al (see Figure a). The elemental analysis, conducted via ICP-OES, revealed that the composition of the NCs was nearly stoichiometric (i.e., CdTe), and the XRD characterization indicated the presence of a mixture of both CdTe wurtzite (WZ) and zinc blende (ZB) structures (ICSD numbers 620518 and 43712, respectively) (Figure c, blue pattern and Table ).…”

“…CdTe is a material of great relevance in thin-film photovoltaics technology, , and strategies are being developed in many groups to lower the costs of film fabrication, for example by solution approaches based on colloidal NC inks. However, to produce bulk-like films from such inks, the deposited NC films need to be annealed at temperatures above 350 °C in order to sinter the NCs together and to remove the ligands. − By leveraging on our Cd 2+ → Hg 2+ exchange protocol, we could prepare CdTe sintered films using much milder annealing conditions (200 °C).…”