Charge Transport Processes in Quantum Dot Sensitized Solar Cells

Narayanan, R (2014) Charge Transport Processes in Quantum Dot Sensitized Solar Cells. PhD thesis, Indian Institute of Technology, Hyderabad.

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Abstract

Colloidal nanocrystals or quantum dots (QDs) have attained immense scientific interest as light sensitive materials due to band gap tunability by size control, ease of preparation, multiple exciton generation and low cost. In the last decade, although reasonably high power conversion efficiencies (PCEs) of 5% have been realized in quantum dot sensitized solar cells (QDSSCs), the efficiencies of QDSSCs continue to lag behind that of dye sensitized solar cells or DSSCs. To address the issue of improving overall PCE values in QDSSCs, new strategies such as Forster resonance energy transfer or FRET, plasmonic effects, and blending the photosensitizer dots with carbon nanostructures etc., were implemented in this thesis. Since photovoltaic performance parameters are closely linked to the energetics of the components which, in turn dictates electron transfer dynamics, in this work, we fabricated hitherto unexplored photoanode architectures and charge transfer and transport mechanisms were studied by detailed fluorescence quenching, and lifetime analyses. The effect of a carbon nanostructure, ionic liquid functionalized graphene oxide (FGO) and metal nanoparticles (Au) in promoting electron transport in a QDSSC with TiO2/CdSe as the photoactive electrode was studied and a PCE of ~1% (under 1 sun) was realized for this assembly relative to other configurations. Taking this further, the effect of another carbon nanostructure, C60 nanowhiskers, on a CdS QD solar cell performance was discerned by comparing the charge propagation dynamics of CdS/C60 nanowhiskers with that of an assembly containing conventional C60 clusters (i.e., CdS/C60) and the advantage of the whiskers was clearly brought out. Apart from the use of carbon nanostructures, another concept, FRET, was also exploited in this work to realize improved efficiencies in QDSSCs. An electrode tethered QD assembly of ZnS/CdS/ZnS was used as the donor and copper phthalocyanine (CuPc) molecules dissolved in the electrolyte were used as the acceptors. Luminescent and conductive carbon dots or C-dots were also included in this cell to improve electron transfer and transport. Incident photon to current conversion efficiency or IPCE measurements revealed an optimal coverage of the visible spectrum due to FRET. A quasi-solid state FRET enabled QDSSC was demonstrated. Extending this further, another FRET cell was fabricated, with reversed roles, wherein CdS/CdSe QDs served as acceptors and a Lucifer yellow dye dissolved in electrolyte functioned as the donor. Vis-à-vis FRET, an enhanced solar cell performance and a PCE of 1.8% was obtained. Further, the hole transporting ability of the conventional Sn2-/S2- layer was also improved by use of poly(3,4-ethylenedioxythiophene) microfibers. Plasmonic effects were tapped for improving solar cell responses, by integrating Au microfibers with CdS/TiO2 electrodes. This work was further extended by combining this plasmonic photoanode with an electrical double layer capacitor based on multiwalled carbon nanotubes (MWCNTs). A new design for a solar powered supercapacitor was implemented wherein the photocurrent generated by the plasmonic electrode was channelized to charge the MWCNT supercapacitor. The benefit of plasmonics in improving solar cell parameters was also demonstrated by using Au encapsulated C-dots in a ZnO based DSSC and a PCE of 4.1% was achieved. Another practical application was illustrated by using electrochromic MoO3 as a counter electrode in the ZnO/Dye/Au@C-dots based cell and photoelectrochromism was shown. Overall, in this thesis, the charge transport mechanisms that prevail in QDSSCs and how they control photovoltaic parameters is demonstrated.

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