Preparation Of Rare-earth Conversion Luminescent Materials And Their Application In Dye-sensitized Solar Cells

Dye-sensitized solar cells （DSSC） are attractive because they are made fromcheap materials that do not need to be highly purified and can be printed at low cost.DSSC are unique compared with almost all other kinds of solar cells in that electrontransport, light absorption and hole transport are each handled by different materialsin the cell. Through building a chloroplast-like porous nanocrystalline film as aphotoanode, DSSC can absorb the visible portion of sunlight and producephotocurrent. Dye （N719） can inject electron into the conduction band of the TiO₂film, which is promoted into an electronically excited state. Thus, the performance ofinjecting electron makes a most important effect for the efficiency of whole cell.There is always a quest for TiO₂anode or sensitizers that achieve effective harnessingof the infrared and near-infrared （NIR） part of sunlight, converting these photons toelectricity better than the currently used generation of TiO₂-dye couple, i.e.,increasing the photocurrent density. However, even the most conventionally used dyein current DSSC （N-719） is allowed to absorb light only up to700nm. An alternativeapproach is spectral conversion aimed at modification of the solar spectrum toachieve a better match with the wavelength dependent conversion efficiency of thesolar cell. Different types of spectral conversion can be distinguished: two low energy（subband gap） photons are combined to one high energy photon （upconversion） orone high energy photon is transformed into one （downshifting） or two（downconversion or quantum cutting） lower energy photons.In this paper, upconversion and downconversion phosphors were synthesized,and La₂O₂S,Gd₂O₃and NaYF₄were used as host materials, which were doped intoTiO₂luminescence layer in the DSSC. The influence of the phosphors on thephotoelectronic properties of the cells was investigated by XRD, SEM, I-V curve ofthe solar cell and so on. The phosphors were introduced into the luminescence layerin the DSSC to improve its photocurrent, photovoltage and photovoltaic conversion efficiency as luminescence and p-type dopant.1) La₂O₂S:Eu³⁺downconversion phosphor was prepared by sol-gel method, whichwas introduced into the luminescence layer in the DSSC. As a luminescencemedium, La₂O₂S:Eu³⁺phosphor improved the ultraviolet light harvesting viadown-conversion luminescence process and increased the efficiency of thecells.The highest photovoltaic conversion efficiency of the DSSC reached7.21%,increasing by25%compared to the DSSC without phosphor, when the dopingamount of La₂O₂S:Eu³⁺in the luminescence layer is optimized as3wt.%. Underthe ultraviolet irradiation of24mW·cm-2, the light-to-electric conversionefficiency of the DSSC increased from0.943%to1.25%.2) Gd₂O₃:Er³⁺upconversion nanotupes phosphor was prepared by precipitationtechnique followed with a dehydration process, and then was introduced into theluminescence layer in the DSSC. The XRD, SEM, EDS, excitation spectrum, andemission spectrum of the Gd₂O₃:Er³⁺nanotupes were presented in the paper.Thephotovoltaic conversion efficiency of the DSSC with5wt.%Gd₂O₃:Er³⁺nanotupes bifunction player reached7.55%, which enhances27%compared tothat of a DSSC without phosphor doping. Under the infrared irraditation of33mW·cm-2, the DSSC doped5wt.%Gd₂O₃:Er³⁺nanotupes achieves conversionefficiency of0.229%, and the DSSC without phosphor got only0.0943%.3) NaYF₄:Yb³⁺,Er³⁺upconversion nanophosphor was synthesized hydro-thermalmethod followed with a dehydration process. The XRD, SEM, EDS, excitationspectrum, and emission spectrum of the NaYF₄:Yb³⁺,Er³⁺phosphor werepresented in the paper. When NaYF₄:Yb³⁺,Er³⁺phosphor doped into theluminescence layer in the DSSC with4wt.%, the photovoltaic conversionefficiency of the DSSC reached7.73%, which enhances30%compared to that ofthe DSSC without phosphor. Under the infrared irraditation of33mW·cm-2, theDSSC doped4wt.%NaYF₄:Yb³⁺,Er³⁺nanotupes achieves conversion efficiencyof0.235%, and the DSSC without phosphor got only0.0943%.