Effect of thermal treatment on ZnO:Tb3+ nano-crystalline thin films and application for spectral conversion in inverted organic solar cells

Down conversion has been applied to minimize thermalization losses in photovoltaic devices. In this study, terbium-doped ZnO (ZnO:Tb³⁺) thin films were deposited on ITO-coated glass, quartz and silicon substrates using the RF magnetron sputtering technique fitted with a high-purity (99.99%) Tb³⁺-doped ZnO target (97% ZnO, 3% Tb) for use in organic solar cells as a bi-functional layer. A systematic study of the film crystallization dynamics was carried out through elevated temperature annealing in Ar ambient. The films were characterized using grazing incidence (XRD), Rutherford backscattering spectrometry (RBS), atomic force microscopy, and UV-visible transmittance and photoluminescence measurements at an excitation wavelength of 244 nm. The tunability of size and bandgap of ZnO:Tb³⁺ nanocrystals with annealing exhibited quantum confinement effects, which enabled the control of emission characteristics in ZnO:Tb³⁺. Energy transfer of ZnO → Tb³⁺ (⁵D₃–⁷F₅) was also observed from the photoluminescence (PL) spectra. At an inter-band resonance excitation of around 300–400 nm, a typical emission band from Tb³⁺ was obtained. The ZnO:Tb³⁺ materials grown on ITO-coated glass were then used as bi-functional layers in an organic solar cell based on P3HT:PCBM blend, serving as active layers in an inverted device structure. Energy transfer through down conversion between ZnO and Tb³⁺ led to enhanced absorption in P3HT:PCBM in the 300–400 nm range and subsequently augmented J_sc of a Tb³⁺-based device by 17%.

Thermal annealing of Tb doped ZnO thin films was undertaken and as proof of concept, pristine films were used as a bi-functional in inverted solar cell devices.