Investigation of charge transport in nanofiber based solar cells

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.DOCTOR OF PHILOSOPHY (MSE

A maskless synthesis of TiO2-nanofiber-based hierarchical structures for solid-state dye-sensitized solar cells with improved performance

TiO2 hierarchical nanostructures with secondary growth have been successfully synthesized on electrospun nanofibers via surfactant-free hydrothermal route. The effect of hydrothermal reaction time on the secondary nanostructures has been studied. The synthesized nanostructures comprise electrospun nanofibers which are polycrystalline with anatase phase and have single crystalline, rutile TiO2 nanorod-like structures growing on them. These secondary nanostructures have a preferential growth direction [110]. UV–vis spectroscopy measurements point to better dye loading capability and incident photon to current conversion efficiency spectra show enhanced light harvesting of the synthesized hierarchical structures. Concomitantly, the dye molecules act as spacers between the conduction band electrons of TiO2 and holes in the hole transporting medium, i.e., spiro-OMeTAD and thus enhance open circuit voltage. The charge transport and recombination effects are characterized by electrochemical impedance spectroscopy measurements. As a result of improved light harvesting, dye loading, and reduced recombination losses, the hierarchical nanofibers yield 2.14% electrochemical conversion efficiency which is 50% higher than the efficiency obtained by plain nanofibers.Published versio

Perovskite-Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting

Photoelectrochemieal water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to riptoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic inorganic halide perovskite (CH3NH3PI3) solar cell in tandem with a Fe2O3 photoanode can achieve overall, unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic dectro-optical studies were performed to investigate the performance of tandem device. It was found that the overall efficiency was limited by the hematite's photocurrent and onset potential. To understand these limitations, we have estimated the intrinsic solar to chemical conversion efficiency of the doped and undoped Fe2O3 photoanodes. The total photopotentid generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias

Flexible, low-temperature, solution processed ZnO-based perovskite solid state solar cells

A ZnO compact layer formed by electrodeposition and ZnO nanorods grown by chemical bath deposition (CBD) allow the processing of low-temperature, solution based and flexible solid state perovskite CH3NH3PbI3 solar cells. Conversion efficiencies of 8.90% were achieved on rigid substrates while the flexible ones yielded 2.62%

Facile synthesis of a hole transporting material with a silafluorene core for efficient mesoscopic CH3NH3PbI3 perovskite solar cells

A novel electron-rich small-molecule, 4,4′-(5,5-dihexyl-5H-dibenzo[b,d]silole-3,7-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (S101), containing silafluorene as the core with arylamine side groups, has been synthesized via a short efficient route. When S101 was incorporated into a CH3NH3PbI3 perovskite solar cell as a hole transporting material (HTM), a short circuit photocurrent density (Jsc) of 18.9 mA cm−2, an open circuit voltage (Voc) of 0.92 V, and a fill factor (FF) of 0.65 contributing to an overall power conversion efficiency (PCE) of ∼11% which is comparable to the PCE obtained using the current state-of-the-art HTM 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) (η = 12.3%) were obtained. S101 is thus a promising HTM with the potential to replace the expensive spiro-OMeTAD due to its comparable performance and much simpler and less expensive synthesis route.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Energy level alignment at the methylammonium lead iodide/copper phthalocyanine interface

The energy level alignment at the CH3NH3PbI3/copper phthalocyanine (CuPc) interface is investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). XPS reveal a 0.3 eV downward band bending in the CuPc film. UPS validate this finding and further reveal negligible interfacial dipole formation – verifying the viability of vacuum level alignment. The highest occupied molecular orbital of CuPc is found to be closer to the Fermi level than the valance band maximum of CH3NH3PbI3, facilitating hole transfer from CH3NH3PbI3 to CuPc. However, subsequent hole extraction from CuPc may be impeded by the downward band bending in the CuPc layer.Published versio

Novel hole transporting materials based on triptycene core for high efficiency mesoscopic perovskite solar cells

Three novel hole-conducting molecules (T101, T102 and T103) based on a triptycene core have been synthesized using short routes with high yields. The optical and electrochemical properties were tuned by modifying the functional groups, through linking the triptycene to diphenylamines via phenyl and/or thienyl groups. The mesoporous perovskite solar cells fabricated using T102 and T103 as the hole transporting material (HTM) showed a power conversion efficiency (PCE) of 12.24% and 12.38%, respectively, which is comparable to that obtained using the best performing HTM spiro-OMeTAD. The T102 based device showed higher fill factor (69.1%) and Voc (1.03 V) than the spiro-OMeTAD based device (FF = 63.4%, Voc = 0.976 V) whereas the T103 based device showed comparable Jsc (20.3 mA cm−2) and higher Voc (0.985 V) than the spiro-OMeTAD (Jsc = 20.8 mA cm−2) based cell.Published versio

Low-temperature solution-processed wavelength-tunable perovskites for lasing

Low-temperature solution-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light sources. Organic semiconductors and colloidal quantum dots are considered the main candidates for this application. However, stumbling blocks in organic lasing(1-4) include intrinsic losses from bimolecular annihilation and the conflicting requirements of high charge carrier mobility and large stimulated emission; whereas challenges pertaining to Auger losses and charge transport in quantum dots(5-7) still remain. Herein, we reveal that solution-processed organic-inorganic halide perovskites (CH3NH3PbX3 where X = Cl, Br, I), which demonstrated huge potential in photovoltaics(8-11), also have promising optical gain. Their ultra-stable amplified spontaneous emission at strikingly low thresholds stems from their large absorption coeffcients, ultralow bulk defect densities and slow Auger recombination. Straightforward visible spectral tunability (390-790 nm) is demonstrated. Importantly, in view of their balanced ambipolar charge transport characteristics(8), these materials may show electrically driven lasing