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Final Report Summary - EXCITONIC SOLAR CELL (Photovoltaic Excitonic Solar Cells)

PbS QDs were used as light harvesters in conjunction with films composed of 18nm-sized TiO2 nanocrystals containing both meso-and macropores. Also, at the same time the PbS acts as a hole conductor, rendering superfluous the use of an additional p-type material for transporting positive charge carriers. The best photovoltaic performance of the PbS QDs solar cells was achieved using anatase TiO2 nanosheets. The TiO2 nanosheets has an exposed (001) facet, which is different than the normal anatase TiO2 nanoparticles (NPs) which have (101) dominant exposed facet. Researches have shown that the (001) facet has a higher surface energy than the (101) facet therefore their surface is more reactive. Thin films of those nanosheets were deposit by spin coating on the FTO glass. Figure 1 shows HR-SEM of the cross section of the device using the TiO2 nanosheets.

Two sizes of PbS QDs were tried, Energy gap (Eg) of 1.38eV and Eg of 1.24eV, in addition two sizes of TiO2 nanosheets, 30nm and 80nm were tried as well. The best results were achieved with 30nm size of nanosheets and PbS QDs with Eg of 1.38eV. The power conversion efficiency in this case was 4.73%, which is one of the highest reported in literature. Figure 1B shows the J-V curve of the best cell in comparison with cell made of (101) TiO2 NPs. The incident photon to current conversion efficiency (IPCE) of the solid state QDs (figure 2B) cell shows a good response from the visible through the near infra-red (NIR), the IPCE spectrum using the TiO2 nanosheets is reaching its maximum of 100% at 420nm (rectangles) while the IPCE spectrum using the TiO2 NPs is reaching its maximum of 90% at 400nm. Photons of these wavelengths are converted most efficiently as they are absorbed by PbS particles located close to the TiO2 interface.

According to eq. 1 it is possible to calculate the Light Harvesting Efficiency (LHE) of the QDs film from the absorbance spectra.
(1) LHE(%) =1−10−Absorbance
The absorbed photons–to–current efficiency (APCE) values taking into account the light- harvesting efficiency (LHE), or light actually absorbed by the monolayers of QDs. The APCE can be calculated according to eq.2.
(2) APCE(%) = IPCE(%)/ LHE(%)
The APCE spectra against the photon energy can be seen in figure 2A. The APCE values are over 100% for photon energies of 2.8eV - 3.1eV, which are around 2 times the QDs Eg. Those APCE values, which exceed 100%, can suggest the possibility of multiple exciton generation (MEG) effect in our PbS QDs solar cells.

This work presents a simple structure of förster resonance energy transfer (FRET) system inside a dye-sensitized solar cell. The donors are CdSe QDs, which have broad absorption spectrum in the visible regions. The acceptor is a molecularly engineered squaraine sensitizer labelled as VG1-C10, this dye has an additional carboxylic acid group and two long carbon chains compared to the standard squaraine dye. The presence of two carboxylic acid anchoring groups, and the hydrophobic long chains provides better dye stability and allows efficient energy transfer from the high energy QD’s to squaraine sensitizer. The use of cobalt complex (Co+2/Co+3) as electrolyte in these cells permits direct contact between the QDs and the electrolyte. Moreover there is no need to change the original ligands of the QDs prior to deposition; the two C10 chains of the dye molecules and the oleic acid ligands coated the QDs provide the optimum distance for FRET, hence the preparation and the structure of the cell are simple. As a result of the energy transfer the cell power conversion efficiency (table1) was increased and its solar response was expanded from the visible to the near infra-red.

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