Spectroscopic study of Cu2+/Cu+ doubly doped and highly transmitting glasses for solar spectral transformation

This work investigates the formation of photoluminescence (PL) centres in high-transmission glasses (HTG) doped with Cu2O and their capability to transform the solar spectrum by absorption/emission via upconversion, downconversion and Stokes-shifted PL into a more efficient spectrum for photovoltaic applications. Both the green PL Cu+ and the non-PL Cu2+ centres are formed in HTG, although their relative concentration depends on the thermal treatment and the presence of other codopants. Given that the absorption spectrum of Cu+ lies around the HTG band gap, measurement of the absorption coefficient α(λ) for these absorption bands is not easy due to corrections between the reflection coefficient and chromatic dispersion. We present a procedure, named two-thickness method, to extract the actual absorption coefficient for the spectrum of each formed centre. In addition it provides the relative Cu+/Cu2+ concentration as well as their absolute values. Analysis of the spectra also provides information on the absorption cross section, transition energy and bandwidth of each band, the knowledge of which is essential to check the suitability of such centres for photovoltaic applications in silicon solar cells.