Effect of Photosensitizers, Electrolyte Additives, and Counter Electrode Materials on Performance Characteristics of Quantum Dot Solar Cells

Kumar, K Ramesh and Deepa, Melepurath (2019) Effect of Photosensitizers, Electrolyte Additives, and Counter Electrode Materials on Performance Characteristics of Quantum Dot Solar Cells. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Quantum dots (QDs) have aroused great scientific interest due to various attractive features, which include tunable bandgap, multiple-exciton generation, quantum confinement, high absorption coefficient, and ease of preparation. Liquid-junction quantum dot solar cells (QDSCs) are third generation solar cells, which have now reached power conversion efficiencies (PCEs) of about 13.85%, thus giving tough competition to the dye-sensitized solar cells (DSSCs). In this thesis, QDSCs were developed with novel photoanode architectures, modified sulfide-based hole transport layer or electrolyte, and novel counter electrodes (CEs) for reliable and reproducible solar cell performances. A photoanode architecture comprising of CdS QDs as the visible light harvesters, co-sensitized with PbSe QDs as NIR light absorbers, and carbon (C)-dots as electron conduits, with TiO2 as the wide bandgap semiconductor, delivered a PCE of 4.97% under 1 sun illumination with multi-walled carbon nanotubes (MWCNTs) as CE. In the next work, PbS QDs were anchored to TiO2-MWCNTs composite instead of conventional TiO2 to increase the charge transport capabilities of the photoanode. Further, the sulfide electrolyte was also modified with TiO2 nanoparticles (NPs) and poly(vinyl alcohol) (PVA), and the CE was composed of SnS nanoparticles (NPs) coated C-fabric. The resulting QDSC of TiO2- MWCNTs/PbS/ZnS-S 2- /PVA/TiO2-SnS/C-fabric showed a PCE of 6.30%. Forster resonance energy transfer (FRET) based solution processed solar cell was fabricated with CdS as the energy donor and poly[N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT) as the energy acceptor. C-dots tethered to C-fabric was used as the CE. The relative quantum yield of the CdS donor was ~0.3, the Forster radius was ~3.7 nm and an energy transfer efficiency of ~55% was confirmed between CdS and PCDTBT. The complete FRET cell: TiO2/PCDTBT/CdS/ZnSS 2- -C-dots/C-fabric produced a PCE of 7.42%, which was 23 times higher than the donor only cell (TiO2/CdS) and several orders of magnitude (2291 times) higher than the acceptor only cell (TiO2/PCDTBT). In another work, solid state QDSCs were fabricated with succinonitrile/Na2S (SN/S2- ) mixed in a 2:1 molar ratio and it delivered a PCE of 5.2% with TiO2/CdS photoanode and C-fabric as CE. In another work, QDSCs with following configurations: TiO2/QDs -S 2- /Sn 2- -CE (QDs: CdS or CuInS2 (CIS) or CIS/CdS and CE: C-fabric or Poly(3,4-ethylenedioxythiophene) (PEDOT)/C-fabric) were assembled. Electron and hole transfer mechanisms in the photoanodes were compared and analyzed, by using a redox inactive medium (0.1 M NaOH, which does not conduct holes) and a redox active medium (0.1 M Na2S, which conducts holes). The PCE(max) was 5.24%. In the last work, a photoanode was fabricated with non-toxic In2Se3 NPs anchored to TiO2, and the PCEs achieved for the cells with a Cu2ZnSnS2Se2/C-fabric CE was 4.15% with a poly(hydroxyethyl methacrylate)/sulfide gel as the electrolyte. This thesis gives a comprehensive understanding of (i) the effect of co-sensitization and FRET on QDSC efficiencies, (ii) the role of different CEs in controlling PCE of QDSCs, (iii) the effect of electrolyte modifications on QDSC performances, and (iv) a detailed account of charge transfer and transport processes in QDSCs

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