Highly Efficient, Iodine‐Free Dye‐Sensitized Solar Cells with Solid‐State Synthesis of Conducting Polymers

Abstract

Dye-sensitized solar cells (DSSCs) are low-cost photovoltaic devices that provide a high energy conversion effi ciency of 11%. [ 1 ] High conversion effi ciency, good stability, and easy fabrication are essential for commercialization of DSSCs. [ 2 ] Although DSSCs with liquid electrolytes have been reported to exhibit high effi ciency, there has been interest in developing a solid-state DSSC (ssDSSC) due to its potential to decrease the overall weight of the cells and to provide long-term durability and fl exibility. Several methods have been developed to fabricate ssDSSCs employing quasi-solid or solid polymer electrolytes. [3][4][5][6][7][8][9][10][11] In ssDSSCs, hole transporting materials (HTMs) and the control of interfacial properties between the nanoporous TiO 2 layer and the HTM are critical to the photoconversion effi ciency of the cells. Recently, HTMs have been extensively investigated as potential replacements for conventional I 3 − /I − redox electrolyte systems using iodine (I 2 ) in DSSCs. [12][13][14][15][16][17][18] As an organic HTM, spiro-OMeTAD was used for an iodine free ssDSSC and showed higher effi ciency of 5.1%. [ 17 , 18 ] Inorganic HTMs such as CuI showed a good effi ciency of 4.7%, [ 12 ] but the crystal formation of metal oxide gradually reduced cell performance. [ 19 ] Another possibility is the utilization of p-type conducting polymers, e.g., polypyrrole, polyaniline, polydiacetylene, poly (3-octylthiopehene), and poly(3,4-ethylenedioxythiophene) (PEDOT). [20][21][22][23][24][25][26] These polymers are advantageous over other small molecules due to their low cost, good stability, simple fabrication using spin-coating, and easy preparation of designable structures. Recently, Ho et al. reported an iodine (I 2 )-free ssDSSC using polyaniline/carbon black composite with imidazolium iodide derivatives, which effectively generates I − / I 3 − redox couples without addition of iodine (I 2 ). [ 27 ] However, while conductive polymers offer many advantages, they show low energy conversion effi ciencies due to poor penetration into the nanopores of TiO 2 photoelectrodes that arises from mismatches between the molecular sizes of the polymers and the pore sizes of the TiO 2 layer. Recently, Yanagida et al. were successful at improving the power conversion effi ciency signifi cantly up to 2.85%, which was the highest effi ciency for an iodine-free ssDSSC using a conductive polymer as HTM. [28][29][30] The penetration of a conductive polymer into TiO 2 porous layers was successfully improved via the in situ photoelectrochemical polymerization of a heterocyclic monomer, e.g., 2,2'-bis(3,4-ethylenedioxythiophene) (bis-EDOT). [ 29 ] However, the maximum penetration depth of the conductive polymer into the TiO 2 layer was 4-5 μ m, above which cell effi ciency was decreased, resulting from insuffi cient interfacial properties of electrode/HTM despite increased dye adsorption. More recently, Liu et al. further improved cell efficiency up to 6.1% using indoline D149 dye as a sensitizer, [ 31 ] whe...

Efficiently cubic platinum-cobalt bimetallic nano-catalysts for use in low-cost dye-sensitized solar cells

“…The equivalent circuit model (Inset of Fig. 7) used for fitting the resultant impedance spectra has been already reported [36][37][38]. EIS measurements were repeated many times with different DSSCs, and the fitted results of the impedance data are summarized in Table 1.…”

Multi-layering of a nanopatterned TiO2 layer for highly efficient solid-state solar cells

“…Next, a solution containing 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) was Multi-layering of a Nanopatterned TiO 2 Layer J Na et al directly dropped onto the dye-adsorbed TiO 2 photoanode, followed by a solid-state polymerization 11,22,23 at 60°C to produce the conductive polymer, PEDOT as a HTM, to prepare an I 2 -free ssDSSC.…”

“…Among the various solar cells, iodine-free solid-state dye-sensitized solar cells (ssDSSCs) based on a conductive polymer as hole transporting materials have been attracting much interest recently owing to their low toxicity, light overall weight, long-term durability and processibility. 11,22,23 However, the photoconversion efficiency of ssDSSCs is low due to the wasted light that is transmitted through the photoanode. In our previous work, we introduced a nanopatterned TiO 2 photoanode, showing 40% and 33% enhancements of the short-circuit current (J sc ) and power conversion efficiency (PCE), respectively, compared with a flat TiO 2 photoanode in ssDSSCs.…”

Recent advances in small molecular, non-polymeric organic hole transporting materials for solid-state DSSC

“…Nevertheless, polymers cast from solutions must penetrate into the pores of the nanoparticles and should form a good contact with the adsorbed sensitizer. Semiconducting polymer as poly(3,4-ethylenedioxythiophene) (PEDOT) [18,19] and poly(3-hexylthiophene) (P3HT) [20] are mainly used in solid state DSSC as solid hole conductor [21]. However, the conversion efficiencies reported with conducting polymers as HTM are lower in comparison to the cells using a liquid electrolyte or small organic hole conductor molecules, mainly due to the incomplete filling of the TiO 2 nanopores by the HTM [22], which leads to poor electronic contact between the dye molecules and the hole conductor.…”

“…To overcome this drawback, in situ polymerized HTMs have been developed. The polymerization of monomers has taken place directly inside the TiO 2 networks leading to high efficient solid state DSSCs with η ranges from ∼4-7% [18,19,[23][24][25][26]. A breakthrough was recently reported in solid-state hybrid solar cells by using organolead iodide perovskite (CH 3 NH 3 PbI 3 ) nanoparticles [27] as a light absorber and spiro-MeOTAD as HTM, which shows an impressive PCE of 9.7% with superior stability over 500 h. Etgar et al [28] have described the photovoltaic device comprised of a mesoscopic CH 3 NH 3 PbI 3 /TiO 2 heterojunction with CH 3 NH 3 PbI 3 nanoparticles as both a light harvester and a HTM, which shows a high efficiency of 5.5% under 1 Sun illumination.…”