Investigation of charge transport in nanofiber based solar cells

• Main Author: Sabba Dharani
• Other Authors: Subodh Gautam Mhaisalkar
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Nanotechnology; DRNTU::Engineering::Materials::Nanostructured materials; DRNTU::Engineering::Materials::Material testing and characterization
• Online Access: https://hdl.handle.net/10356/62945
• Institution: Nanyang Technological University

Dye sensitized solar cells (DSCs) have garnered a lot of attention owing to their ease of fabrication, low manufacturing costs and good device stability. One of the crucial components of high efficient dye sensitized solar cell is the mesoporous metal oxide nanocrystalline photoanode. High surface area and mesoporous photoanodes are essential for greater adsorption of sensitizer monolayer leading to improved optical density especially for low absorption coefficient sensitizers like Ru based complexes (N719). In order to enhance light harvesting efficiency which is instrumental in boosting the device performance, either thicker photoanode materials or dyes with higher absorption coefficients need to be engaged. Nevertheless utilizing thicker photoanode films leads to poor charge collection. Therefore one dimensional nanostructures are being explored as a replacement for nanoparticles with the aim of improving charge transport by providing a directional path for charge percolation and by reducing the surface traps and density of grain bound- aries associated with nanoparticles. One of the simplest yet versatile method to synthesize such 1D nanostuctures is by electrospinning. Photoanodes made of electrospun TiO 2 nanostructures with good inter connectivity and high surface area play a critical role in determining the conversion efficiency of dye-sensitized solar cells. High molecular weight polyvinylpyrrolidone polymer when mixed in to the titanium sol-gel precursor solution results in the formation of electrospun fibers with nanofibrillar morphology. This work focuses on the integration of as-spun nanofibrous films with intact fiber morphology as photoanodes in dye-sensitized/perovskite solar cells. One fundamental observation deduced from the photovoltaic measurements is that the nanofibrous based photoanodes always exhibited higher open circuit voltage than the nanoparticle systems. Electrochemical impedance spectroscopy (EIS) measurements indicated that the resultant photoanodes had better charge collection as a consequence of improved charge transport and recombination dynamics compared to previous reports of photoanodes prepared by grinding electrospun nanofibers or by using nanoparticles. This characteristic of the nanofibers is of paramount interest in the solid-state DSCs (ssDSCs) where the charge recombination kinetics is faster than the kinetics observed in liquid DSCs. In this work, the EIS measurements validated that the nanofibers demonstrated better charge dynamics even in ssDSCs thus showcasing them as potential candidates for highly effective solar cells. Regardless of the better charge dynamics exhibited by the nanofibrous DSCs, the electrochemical conversion efficiencies are observed to be lower than the nano-particle DSCs. The prime contributing factor for the poor performance of the 1D solar cells is associated with their lower surface area for dye adsorption. This resulted in meager photon harvest thereby yielding appreciable current densities. Consequently this work also focuses on modulating the morphology of electrospun nanofibers to hierarchical nanostructures as well as to sensitize the nanofibrous photoanodes with one or more highly efficient sensitizing materials with the sole purpose of enhancing the light harvesting efficiency. So initially by solvo thermal treatment the nanofibers’ morphology was adapted to a hierarchical structure comprising of anatase nanofiber backbone with single crystalline rutile nanorods branching out. The aspect ratio and density of the nanorods was meticulously modulated and investigated by varying the reaction time. The UV-Vis absorption and incident photon to electric conversion efficiency (IPCE) measurements demonstrated that the hierarchical structure enabled higher loading of dye molecules which is essential for improving the current density. Also a high open circuit voltage ensued by the compact packing of dye molecules on the hierarchical nanostructure. It also concurs well with its higher resistance to recombination between the TiO 2 conduction band electrons and the oxidizing species of hole transporting medium measured from EIS. A similar phenomenon of en- hancement in current densities and open circuit voltage was observed when an alternative approach of sensitizing the nanofibers with 2 structurally different organic dyes with complementary absorption profiles was introduced. Initially a triphenylamine D35 chromophore with bulky butoxyl groups was employed to sensitize the nanofibers followed by sensitizing them again with a smaller indoline dye called D131. This semi-tandem like co-sensitization process concomitantly improved the light harvesting efficiency and passivated the nanofiber surface by virtue of the condensed packing of both the dye molecules which suppressed the charge recombination with the tri-iodide species of the electrolyte. Having demonstrated the better electron percolation and suppressed recombination and possible routes of improving the current densities marginally, the final work of this thesis focuses on sensitizing nanofibers with high absorption coefficient hybrid organic-inorganic materials like methyl ammonium lead iodide perovskite for dramatic improvement in photon harvesting. On account of the perovskite crystal quality affecting the solar cell performance, significance of porous network of the nanofibers on the percolation of perovskite was demonstrated with respect to a thin perovskite layer. Thus in this thesis, significance of as-spun nanofibrous photoanodes in DSCs as well as in emerging novel photovoltaic systems was investigated in terms of processability, light harvesting efficiency and electron dynamics. The as-spun nanofibers have exhibited easier processability, better infiltration of hole transporting material (HTM)/perovskite absorber material, better charge transport and higher charge recombination resistance leading to higher charge collection efficiency even in the ssDSCs in contrast to the screen printed or spincoated nanoparticle films.