Interfacial Engineering of Solid-State Hybrid Solar Cells based on Organic Functional Materials

Abstract

Recent remarkable advances in dye-sensitized solar cells (DSCs) led lots of scientist to investigate the various inorganic and organic materials as well as charge transfer / transport mechanisms in DSCs. With the advantages including low production cost, their relatively high power conversion efficiencies are thought to be an alternative to conventional silicon-based solar cells. While the liquid-electrolyte based DSCs are suffering from leakage and corrosion problems, solid-state hole transporting materials (HTMs) exhibited bright future for practical applications as well as for next generation flexible photovoltaics. During the last decade since the first successful demonstration in 1998, the efficiencies are gradually increasing which reach over 15 % up to date. However, lack of charge transfer mechanism at the heterogeneous interfaces of semiconducting oxide / dye / HTM often hamper the development of new materials for sDSCs. Herein we investigated the charge transfer and developed various organic functional materials including coadsorbents, sensitizers, and HTMs. In Chapter 2, we carefully investigated the charge recombintation reaction at the interface of TiO2/dye/HTM and newly designed 3,4,5-Tris(dodecyloxy)benzoic acid (DOBA) coadsorbent in combination with Z907 dye which coadsorbed to form a light harvesting monolayer in solid-state dye-sensitized solar cell (sDSC). Coadsorption of DOBA which has three hydrocarbon chains permitted preparation of a denser monolayer of dyes and DOBA. This dense monolayer formed interlayer between TiO2 and Spiro-OMeTAD (hole conductor), effectively preventing charge recombination, while half of photocurrent was dissipated via recombination reaction when Z907 solely anchored on the surface of TiO2. Moreover, the DOBA induced less population of density-of-state (DOS) in the surface of TiO2, shifting the position of the conduction band (CB) toward negative values. This resulted in higher open-circuit voltage (VOC) than was measured for the Z907-sensitized solar cell. These surface properties were investigated using electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent / photovoltage spectroscopy (IMPS and IMVS).In Chapter 3, the interfacial properties were systematically investigated using an organic sensitizer (3-(5'-{4-[(4-tert-Butyl-phenyl)-p-tolyl-amino]-phenyl}-[2,2'] bithio-phenyl-5-yl)-2-cyano-acrylic acid (D)) and inorganic sensitizer (N719) in a liquid-state and a solid-state dye-sensitized solar cell (DSC). For liquid-DSCs, the faster charge recombination for the surface of D-sensitized TiO2 resulted in shorter diffusion length (LD) of ~ 3.9 m than that of N719 (~ 7.5 m), limiting the solar cell performance at thicker films used in liquid-DSCs. On the other hand, for solid-DSCs using thin TiO2 films (~ 2 m), D-sensitized device outperforms the N719-sensitized device in an identical fabrication condition, mainly due to less perfect wetting ability of solid hole conductor into the porous TiO2 network, inducing the dye monolayer act as an insulation layer, while liquid electrolyte is able to fully wet the surface of TiO2. Such insulation effect was attributed to the fact that the significant increase in recombination resistance (from 865 to 4,400 /cm2) but shorter electron lifetime (from 10.8 to 0.8 ms) when compared to liquid-DSCs. Higher recombination resistance for solid-DSCs induced the electron transport-limited situation, showing poor performance of N719-sensitized device which has shorter electron transport time and similar LD (2.9 m) with D-sensitized device (3.0 m).In Chapter 4, a series of organic dyes having an unsymmetrical geometry, 3-(5'-{4-[(4-tert-butyl-phenyl)-(4-fluoro-phenyl)-amino]-phenyl}-[2,2']bithio-phenyl-5-yl) -2-cyano-acrylic acid (D-F), 3-(5'-{4-[(4-tert-butyl-phenyl)-p-tolyl-amino]-phenyl}-[2,2'] bithiophenyl-5-yl)-2-cyano-acrylic acid (D-CH3), and 3-(5'-{4-[(4-tert-butyl-phenyl)-(4-methoxy-phenyl)-amino]-phenyl}-[2,2']bithiophenyl-5-yl)-2-cyano-acrylic acid (D-OCH3), were designed and synthesized for use in solid-state dye-sensitized solar cells (sDSCs). The dye regeneration energy levels and surface properties were characterized to determine the hole transfer yield from the oxidized dye to the hole conductor (spiro-OMeTAD) by measuring the degree of pore-filling by the spiro-OMeTAD and the transient absorption spectra (TAS). An electrode sensitized with D-OCH3 exhibited the highest spiro-OMeTAD filling fraction and hole transfer quantum yield (Φ) to spiro-OMeTAD, resulting in an enhanced photocurrent and a power conversion efficiency of 3.56% in the sDSC, despite a lower energy driving force for hole transfer compared to those of D-F, or D-CH3. This result illustrates the importance of the chemical compatibility between the hole conductor and the dye on the surface of TiO2.In Chapter 5, 2,5-Bis-(2-decyl-dodecyl)-3-(5-methyl-thiophen-2-yl)-6-{5'-[2-(5-methyl-thiophen-2-yl)-vinyl]-[2,2']bithio-phenyl-5-yl}-2,5-dihydro-pyrrolo[3,4-c] pyrrole-1,4-dione (PDPPDBTE) polymer was successfully incorporated as p-type hole transporting material in solid-state organic-inorganic hybrid solar cells. With the excellent optical and electrical property of organo-lead halide perovskite (CH3NH3PbI3) nanocrystals as light harvester, 9.2% of power conversion efficiency was achieved, which lies beyond the ever-best performing hole conductors 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine) 9,9'-spirobifluorene (spiro-MeOTAD) (7.6%) mainly contribute to a proper oxidation potential of the polymer (5.4 eV) and excellent charge carrier mobility. Long-term aging test over 1000 hrs also confirmed enhanced stability of the PDPPDBTE-based cells since hydrophobic nature of the polymer prevent permeation of water into porous perovskite heterojunction.