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

Francis Otieno,David G Billing,Mildred Airo,Daniel Wamwangi,Alexander Quandt,Rudolph M Erasmus

doi:10.1039/c8ra04398a

Francis Otieno, David G Billing + Show 4 more

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https://doi.org/10.1039/c8ra04398a

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Abstract

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 Jsc of a Tb3+-based device by 17%.

Highlights

Rare-earth (RE)-doped semiconductors have attracted signi cant interest for possible applications in high-power lasers, optical manipulation as visible emitting phosphors in display devices, and other optoelectronic products.[1,2] This enormous interest for their application potential is due to stable intra-4f shell transitions of rare earth ions, which favor energy transfer from the host semiconductors to dopant RE ions.[3]
The donor and acceptors in the bulk heterojunction are mixed in a mass ratio of 1 : 1. We report an increase in efficiency of 17.2%
This is plausible since the particle size measured from atomic force microscopy (AFM) is the surface morphology of coalesced grains.[14]

Summary

Introduction

Rare-earth (RE)-doped semiconductors have attracted signi cant interest for possible applications in high-power lasers, optical manipulation as visible emitting phosphors in display devices, and other optoelectronic products.[1,2] This enormous interest for their application potential is due to stable intra-4f shell transitions of rare earth ions, which favor energy transfer from the host semiconductors to dopant RE ions.[3]. Zinc oxide (ZnO) thin lms have continued to attract widespread research interest as transparent conducting oxides (TCOs) due to their high electrical conductivity (2 Â 10À6 to 2 Â 10À4 S cmÀ1)[4] and optical transmission When doped with a rare earth metal, ZnO can be used as a sensitizer to excite rare-earth (RE) ions such as Ce3+, Er3+, Ho3+, Nd3+, Tm3+, Dy3+, Eu3+ and Tb3+ This is possible due to its large absorption cross-section and broad excitation spectrum.[10] In this manner, RE can absorb the UV-blue emission from ZnO and emit in the visible or infra-red range, enabling this material to serve as an excellent doping host for rare earth ions with an optimal spectrum modifying matrix for diverse solar cells

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