| Dye-sensitized solar cells(DSSCs) are one of the most promising solutions to the energy crysis with low cost, nontoxicity and long-term stability. However, standard dyes of DSSC(e.g. N719) can only absorb UV and visible photons of sunlight(λ<700 nm), and the absorbances in high-energy blue-purple region are not high enough. Besides, under UV light illumination, there would be plenty of excitons in Ti O2 network, resulting in many recombinations. None absorption in near-infraed region(43% energy of solar spectrum) is another important bottleneck that limits the performance improvement of DSSC. Thus, the utilization of infrared light(especial near-infrared light) and the retard of recombination are the promising solutions to improve the performance of DSSC further.There are still arguments about the working mechanism of upconversion materials in DSSCs. In this paper, easy-prepared 2%Er, 50%Yb-YF3 is utilized in DSSC as an example. Surface photovoltage spectra, upconversion photoluminescence spectra(UCPL), incident photon to current conversion efficiency(IPCE) are used to study the working mechanism of the upconversion materials in DSSCs. Fr?ster resonance energy transfer(FRET) could be found with much higher energy transfer efficiency than luminescence-media energy transfer(LET, low efficiency due to low quantum efficiency of UCPL) from the upconversion materials to the dye N719. Besides, after being sintered, upconversion materials can share the interface with Ti O2 and an FRET-like process is found between upconversion material and Ti O2. As a result, upconversion materials can directly inject excited electrons into conduction band of Ti O2. The efficiency of DSSC with the modification of the upconversion materials is increased to 6.76%, which is 43.5% higher than that of the reference DSSC. Then the influences of matrix material and emitting ions on the FRET process and the performance of DSSC are studied. The results indicate that Na YF4(working as matrix material) shows stronger emission than YF3 and RE-Na YF4 shows better performance when assembled in DSSC. Ho3+(working as emitting ion) shows excellent brightness and monochchromaticity, which makes the light could focus in the green region(FRET process is much more efficient in green region). In summary, the sequence of performance of DSSCs is: DSSC(2%Ho,18%Yb-Na YF4)> DSSC(2%Ho,18%Yb-YF3)> DSSC(2%Er,18%Yb-Na YF4)> DSSC(2%Er,18%Yb-YF3).Polyoxometallate(Isoplyacid: decatungstate acid; Heteropolyacid: Si W11Co) is utilized to modify Ti O2 and improve the performance of DSSCs. The working mechanism of POM in DSSC is studied through surface photovoltage spectra(SPS), UV-visible absorption spectra, Mott-Schottky plot and open circuit voltage decay. The results show that POM can enhance the energy absorption in the visible region, especially in the blue-purple high-energy region, which is similar to the application of the cosensitization. POM can also absorb the UV light and retard the recombination, leading to improvement of the performance of DSSC. Moreover, decatungstate acid can improve the efficiency of DSSC from 7.59% to 8.37%. And Si W11Co-modified DSSC shows significant increase of photocurrent density(11.01 m A cm-2 â†' 18.05 m A cm-2), leading to an increase of efficiency from 5.2% to 6.0%.UC-FTO nanoparticles are synthesized by introducing rare earth elements into conductive powder of FTO(F-doped Sn O2), and it is utilized as counter electrode material to assemble DSSC. UC-FTO can be used to substitute the high-cost counter electrode material of Pt, in traditional standard DSSC, leading to significant reduction in the cost of DSSC(the cost of UC-FTO is about 1/20 th that of Pt). DSSC with UC-FTO as counter electrode shows improved efficiency as high as 7.30%, which is 9.12% higher than that of DSSC with Pt as counter electrode(6.69%). And the strategy that introduce upconversion into counter electrode in DSSC is proved to be possible. Furthermore, the performance of DSSC could be improved and the cost of DSSC could be reduced greatly. |
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