Absence of photodarkening in bulk, glassy ${\mathrm{As}}_2$${\mathrm{S}}_3$ and ${\mathrm{As}}_2$${\mathrm{Se}}_3$ alloyed with copper

We report the first observation of the absence of the photodarkening (PD) effect in bulk chalcogenide glasses. In the prototype systems ${\mathrm{Cu}}_{\mathrm{x}}$(${\mathrm{As}}_{0.4}$${\mathrm{S}}_{0.6}$${)}_{1\mathrm{-}\mathrm{x}}$ and ${\mathrm{Cu}}_{\mathrm{x}}$(${\mathrm{As}}_{0.4}$${\mathrm{Se}}_{0.6}$${)}_{1\mathrm{-}\mathrm{x}}$ the PD effect is essentially destroyed for x≥0.01 and 0.05 for the S and Se systems, respectively. A structural model for Cu-As-Se and Cu-As-S systems is proposed, which is based upon the assumptions of covalent bonding and the absence of doping. According to this model, the absence of the PD effect indicates that medium-range interactions among the lone-pair p electrons may be important for the effect to occur. This structural model can be generalized to many other amorphous semiconducting systems.