Photoanode and Counter Electrode Strategies to Enhance the Power Conversion Efficiency of Quantum Dot Sensitized Solar Cells

Halder, Ganga (2019) Photoanode and Counter Electrode Strategies to Enhance the Power Conversion Efficiency of Quantum Dot Sensitized Solar Cells. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

In spite of dedicated research efforts to enhance the photoconversion efficiency (PCE) of quantum dot sensitized solar cells (QDSSCs), optimization of individual solar cell components and better understanding of the underlying science, QDSSCs have unfortunately lacked behind promise due to the occurrence of several unwanted charge recombination processes. Most of these recombination events are facilitated at the metal oxide (MO)/QD/electrolyte and counter electrode (CE)/electrolyte interfaces. The overall performance of a QDSSC is highly dependent on the rates of different recombination processes vis-à-vis rates of charge injection and charge extraction. The rate of these processes can be altered if the properties of the each component of the solar cell and their interfacial properties can be well understood. The focus of this thesis is to investigate the charge recombination processes which occur at MO/QD/electrolyte and CE/electrolyte interfaces and to tune the optoelectronic properties of the QDs, CE materials and interfacial properties of MO/QD/electrolyte and CE/electrolyte interfaces in such a way, that the rates of unwanted recombination events can be minimized without affecting the favorable processes of charge separation to enhance the PCE of QDSSCs. In chapter 1, introduction about the work is presented. It describes the prospect of QDSSCs among the third generation photovoltaic (PV) technologies along with its limitations. This chapter begins with the energy crisis and the necessities of different renewable energy resources followed by a brief discussion on utilization of solar energy and development of solar cells. The different generations of solar cells are also discussed followed by highlighting the different types of third generation solar cells e.g. organic solar cell, organic-inorganic hybrid solar cell, perovskite solar cell and dye sensitized solar cell along with their working principles. Later, the photophysical properties of QDs are elaborated which make them attractive for the next generation PV technology. Subsequently, different charge transfer and recombination pathways of QDSSCs are described along with the working principles. These charge transfer and recombination pathways can be modified by optimizing the basic components of QDSSCs e.g. MO, QD sensitizers, electrolyte and CE materials. A detailed literature review on every topic is also provided and finally, scope of the present study is discussed. In chapter 2, synthesis methodologies of colloidal QDs e.g. heating up, hot injection and aqueous methods are discussed. A brief description of several characterization tools used to characterize the materials e.g. powder X-Ray diffraction (XRD), scanning electron microscope (FESEM), transmission electron microscope (TEM), high angle annular dark field scanning transmission electron microscope (HAADF-STEM), inductively coupled plasma–mass spectrometry (ICP-MS), energy dispersive analysis of X-rays (EDAX), Fourier transform infrared spectroscopy (FTIR), electron paramagnetic resonance (EPR) spectroscopy, ultraviolet-visible spectroscopy (UV-Vis), photoluminescence spectroscopy (PL), time correlated single photon counting (TCSPC) are illustrated. Later, QD deposition techniques e.g. successive ion layer adsorption and reactions (SILAR), chemical bath deposition (CBD), electrophoretic deposition (EPD), direct deposition and molecular linker assisted deposition technique are discussed followed by fabrication of solar cells. A brief description of different characterization tools for PV devices e.g. current-voltage (J-V) measurement, electrochemical impedance spectroscopy (EIS) measurement and open circuit voltage decay (OCVD) are also provided. The third chapter deals with the application of Mn-doped CdS QDs as light harvester in QDSSCs and the type of doping responsible for increment of PCE was also investigated by EPR spectroscopy. Mn-doped CdS QDs with different at% of Mn²⁺ was synthesized by hot injection method and deposited over mesoporous TiO2 film by EPD technique. Although the in situ deposition techniques e.g. SILAR and CBD provide good surface coverage and better connectivity, the type of doping responsible for higher efficiencies could not be ascertained. The goal of this work is to investigate the distinctive Mn2+ sites and the extent of doping % in the colloidal QDs that helps in boosting the efficiencies of doped QDSSCs instead of aiming towards a highly efficient solar cell. It was observed from EPR spectroscopy that maximal substitutional doping of Mn²⁺ ions inside the lattice of CdS QDs was responsible for the highest PCE as it can create long-lived photogenerated charge carrier which reduces the electron-hole recombination. On the other hand, exchange coupled Mn²⁺ pairs increase the non-radiative electron-hole recombination and decreases the PCE. At Mn:Cd at% of 1.8 or below, the Mn²⁺ ions remain distributed both at the lattice sites as well as QD surface, where the latter was observed not to have any marked effect on the QDSSCs. The generation of long-lived charge carriers in lattice doped QDs and higher charge recombination resistance in the corresponding devices were investigated from Bode phase diagram and Nyquist plot; respectively. This strategy can boost the PCE of Mn-doped CdS QDSSCs by 61.5% as compared to the undoped counterpart. The fourth chapter describes the photovoltaic application of ternary Zn-diffused AgInSe₂ (ZAISE) QDs as sensitizers. AgInX₂ (X = S, Se) based QDSSCs suffer from very low PCEs due to the presence of defect states at internal atom vacancies although they have broad light harvesting range to harvest a large number of solar photons. Zn²⁺ diffusion at these vacant sites not only improves carrier mobility but also improves crystallinity of the QDs. In this work, oleylamine capped Zn²⁺-diffused AgInSe₂ QDs having different Zn:(Ag+In) at% were synthesized by hot-injection method. The QDs were made water dispersible by ligand exchange with MPA molecules to ensure maximum loading onto TiO₂ photoanode. By applying 8 SILAR cycles of ZnS as surface passivating agents on top of the QD-deposited TiO₂ photoanode, the PCE was improved by 83.4%. The PCE was further enhanced by 17% by dual inorganic passivation according to the TiO₂/ QD/ amorphous TiO₂/ ZnS/ SiO₂configuration. These inorganic surface passivation steps can reduce the recombination pathways of back electron transfer from photoexcited electrons of QDs and also from the CB of TiO₂ to the polysulfide redox electrolyte which was investigated from EIS measurements. The combined strategy of alloying with Zn²⁺ and inorganic passivation could achieve the highest ever PCE of 3.57% for environmental friendly AgInX₂ based QDSSCs. A similar strategy coupled to surface passivation approach is effective to enhance the device efficiency of other low performing QD systems in PV applications. From the fourth chapter, we have already observed that inorganic passivation onto the photoanode based on Zn-AgInSe₂ QDs could enhance PCE by passivating QD surface trap states partially. However, it cannot passivate the interfacial trap states between MO and QD. On the other hand, growing of an epitaxial inorganic shell over the core QDs can suppress charge recombination at the interfacial defect states between MO and QDs, as well as photogenerated electrons and the electrolyte. In fifth chapter, Zn-Ag-In-Se (ZAISE) core QDs with an alloyed shell of Ag-In-Zn-Cd-Se was applied as a sensitizer. The core/shell QDs were synthesized by hot injection method followed by cation exchange with Cd²⁺ ions at a moderate temperature of 120 °C. The shell thickness was controlled by changing the reaction time. The QDs were deposited onto the TiO2 film by molecular linker assisted self-assembly technique after ligand exchange with MPA molecules. PCE was enhanced from 3.01% for core ZAISE QDs to 4.71% for core/alloyed shell QDs by optimizing the shell thickness. Density of states (DOS) formalism also shows that the feasibility of charge transporation varies with the thickness of alloyed shell. QDSSCs based on the core/shell QDs show higher charge recombination resistance than those with the core QDs as obtained from EIS measurements. Applying the gradient alloyed shell around the core QDs suppress charge recombination at the interfacial defect states between metal oxide and QDs, as well as between photogenerated electrons and the electrolyte. Furthermore, a solar-to-hydrogen efficiency (ηSTH) of 2.66±0.06% was achieved when four best performing QDSSC were integrated and connected with NiFe-LDH (+) // NiMo-alloy (-) electrolyzer. In the sixth chapter, two different materials, Cu-rich CuxS nanostructures and earth abundant copper pyrite (CuFeS₂) were used as CE in QDSSCs with CdS photoanode. CuxS nanostructures with tunable chemical compositions were synthesized by microwave irradiation. The device made with slightly copper rich CE (Cu₁.₁₈S) shows better PV performance than the excess copper (Cu₁.₇S) or sulfur (Cu₀.₂₄S) containing CEs. The trend of photovoltaic performance was explained by EIS analysis, Tafel polarization measurement and CV studies. The best performing Cu₁.₁₈S CE could achieve a PCE of ~3.05% for CdS sensitized and ~4.54% for CdS/CdSe co-sensitized and ~3.48% for ZAISE/am-TiO2/8 ZnS/SiO₂ QDSSCs. Earth abundant CuFeS₂ (CFS) was also tested as a CE and the PCE was highly dependent on surface capping ligands. The CE based on CFS shows almost comparable PCE with Cu₁.₁₈S CE and better than brass/Cu₂S CE after ligand exchange with MPA molecules. The seventh chapter provides the work summary and future prospects.

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