Photovoltaic Performance Of TiO₂Based Quantum Dot Sensitized Solar Cells

Solar power is becoming an ideal choice to tackle the current energy crisis and environmental problems since it is an unfailing and green clean energy resource. As a new type of solar cell possessing great potential and broad prospects, quantum dot-sensitized solar cell (QDSC) is at a stage of rapid development. Based on the material synthesis and device fabrication, we studied the photovoltaic performance of TiO2based QDSCs in this thesis. The critical factors that affect the cell performance have been explored aiming to optimize the cell fabrication process and further improve the power conversion efficiency of QDSC. The main contents and results on QDSC are presented in Chapter3,4and5. In addition, near-infrared downconversion materials have good application prospects in boosting the efficiency of silicon solar cell by minimizing the spectrum mismatch between the solar spectrum and the response curve of silicon semiconductor. We developed two novel near-infrared downconversion materials which express potential application in silicon solar cell. The relevant work is discussed in Chapter6.Chapter1is the introduction to the study and primarily introduces the related research background and basic knowledge. Firstly, the solar cell development is summarized; Secondly, the unique advantages, working principle, device structure and development progress of QDSC are emphasized and reviewed, respectively; Thirdly, the critical problems that QDSC faced and the corresponding possible solutions are discussed.In Chapter2, the details of fabrication process and characterization methods for QDSC are given. The cell-making process mainly includes the preparation of photoanode, electrolyte and counter electrode, and the assembly of them into a typical "sandwich" cell structure. Moreover, the fabrication of photoanode involves the making of TiO2paste, photoanode film and quantum dot (QD). The characterization covers morphology and structure characterization, optical property characterization, and electrochemical performance characterization.In Chapter3, the influence of cationic precursors on CdS QDSC prepared by successive ionin layer adsorption and reaction (SILAR) has been carefully studied. It is well known that SILAR is the most extensively used method for in situ growth of QDs onto porous oxide films for QDSC application. The present work demonstrates that cationic precursors have noticeable influences on the assembly of QDs on photoanode films by SILAR, and furthermore the final QDSC performance. A careful comparison of two cationic precursors, cadmium nitrate (Cd(NO3)2) and cadmium acetate (Cd(CH3COO)2), for the preparation of CdS QDSCs by SILAR showed that, compared to the commonly used Cd(NO3)2, Cd(CH3COO)2provided a significantly higher deposition rate of CdS QDs on TiO2films. A QDSC fabricated using Cd(CH3COO)2as the Cd2+precursor exhibited a power conversion efficiency as high as2.15%, achieving nearly40%enhancement compared to that obtained using Cd(NO3)2, under the same number (i.e.,12) of SILAR cycles. Further studies revealed that the pH value of the precursor solution was likely to determine the deposition rate and, consequently, to affect the amount of QDs loaded on the photoanode film; a higher pH value made the TiO2surface more negatively charged for the film dipped in the precursor and, thus, led to a higher driving force for the adsorption of Cd2+ions and a higher QD deposition rate. In addition, an increased amount of QDs loaded on the TiO2film was found to be accompanied by an increasing degree of red shift of the absorption edge. Such an apparent anomalous red shift phenomenon, not explicitly mentioned in most of the literatures, was observed for CdS QD-deposied TiO2films in our study. It expanded the optical absorption and photocurrent spectra of QDSC and benefited the enhancement of the cell performance. The present work in this chapter will be of guiding significance to the selection of precursor for the prepation of QDs and the improvement of the cell performance through the effective loading of QDs on the photoanode films.In Chapter4, systematic and in-depth study on mesoporous TiO2beads for CdS/CdSe QDSC application has been carried out. The photoanode is a very important component of QDSC, and its structure feature directly impacts the cell performance. The submicrometer-sized anatase TiO2beads were prepared from a combined precipitation and solvothermal process. Such mesoporous beads consist of large amount of packed nano-sized TiO2nanocrystallites, and possess excellent light scattering ability to enhance the light harvesting and high surface area to ensure sufficient QD loading. The addition of different amount of ammonia during the solvothermal treatment process could adjust the surface area, porosity and pore size of the beads, and the optimization of the beads structure led to significant improvement of the cell performance. A power conversion efficiency up to4.05%was achieved for a CdS/CdSe QDSC based on the photoanode composed of TiO2mesoporous beads. Therefore, the as-prepared mesoporous TiO2beads were considered to be promising materials for photoanode films in sensitized solar cells. On the basis of the study on the beads, further effort has been made to optimize the photoanode configuration for QDSC through the combined use of mesoporous TiO2beads and nanoparticles. The incorporation of individually dispersed TiO2nanoparticles into the large voids between submicrometer-sized beads would maximize the accessible surface area for QD loading, and improve the charge transfer in the photoanode. The developed double-layer and mixture configurations tried to reach the requirements of high QD loading, strong light scattering, efficient electron transport and quick electrolyte diffusion for the photoanode. Photovoltaic characteristics revealed that the photoanodes of double-layer and mixture configurations really delivered further improvements in the cell performance (4.33%and4.65%), compared with the simple single-layer beads or photoanode films. The present work in this chapter has an important referential significance for the follow-up study on the photoanode configuration optimization.In Chapter5, efficient and stable QDSCs with broad spectral response covering the visible and near-infrared region have been developed based on the metal sulfide QDs with narrow bandgap (PbS and Ag2S). Based on the quantum size effect, the light absorption of PbS and Ag2S can be readily extended into near-infrared region, and the QDSC employing these kinds of QDs are hopefully to greatly boost the power conversion efficiency. Photovoltaic characteristics revealed that the strong and broad absorption of PbS and Ag2S contributed much to the photocurrent. The synergy effect of CdS QD vastly improved the power conversion efficiency and minimized the problem of unstable performance of PbS and Ag2S QDSCs with polysulfide electrolyte. Particularly, the high-quality (Pb,Cd)S photoanode prepared by the cationic precursor containing Pb2+and Cd2+delivered a power conversion efficiency up to2.66%. It is expected that the QDSC with better performance will be deveolped through further optimization of the cell-making process in succedent work based on the panchromatic sensitized solar cell design.In Chapter6, two novel near-infrared downconversion materials of CaNb2O6:Yb3+and YNbO4:Bi3+,Yb3+have been reported. Powder samples of two materials with different Yb3+doping concentration were prepared by high-temperature solid-state reaction. X-ray diffraction analyses showed that the samples presented good crystallization, and the introduction of doped ions didn’t change or destroy the crystal structure. The luminescent properties of the materials were carefully studied through the measurements of excitation spectrum, emission spectrum and decay curve. Spectroscopic analysis and lifetime calculation revealed that there exists energy transfer from [NbO6]7-and Bi3+to Yb3+in CaNb2O6:Yb3+and YNbO4:Bi3+,Yb3+, respectively, and cooperative energy transfer was proposed to rationalize the downconversion process. That is, the downconversion material absorbs a high-energy UV photon and then converts it into two low-energy near-infrared photons. CaNb2O6:Yb3+exhibited broadband UV absorption of [NbO6]7-groups in the region of250～300nm; while YNbO4:Bi3+,Yb3+extended the absorption range to250～350nm due to the introduction of Bi3+ions and achieved a higher solar energy utilization rate. As the near-infrared emission (～1000nm) of Yb3+ion matches well with the response peak of silicon solar cell, the two downconversion materials present potential application in boosting the power conversion efficiency of silicon solar cell. However, concentration quenching is a serious existing problem in downconversion materials and largely restricts the actual energy conversion efficiency, especially at high Yb3+doping concentration. For instance, at the doping concentration of16%, CaNb2O6:Yb3+and YNbO4:Bi3+,Yb3+exhibited theoretical quantum efficiencies of164%and147%, respectively, and the values dropped to142%and120%after taking the concentration quenching of Yb3+into account. The inbibition of concentration quenching is of great importance for practical application of near-infrared downconversion materials in silicon solar cell.