High Excitation Transfer Efficiency from Energy Relay Dyes in Dye-Sensitized Solar Cells

Abstract

The energy relay dye, 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), was used with a nearinfrared sensitizing dye, TT1, to increase the overall power conversion efficiency of a dye-sensitized solar cell (DSC) from 3.5% to 4.5%. The unattached DCM dyes exhibit an average excitation transfer efficiency (ĒTE) of 96% inside TT1-covered, mesostructured TiO 2 films. Further performance increases were limited by the solubility of DCM in an acetonitrile based electrolyte. This demonstration shows that energy relay dyes can be efficiently implemented in optimized dye-sensitized solar cells, but also highlights the need to design highly soluble energy relay dyes with high molar extinction coefficients.KEYWORDS Solar cell, energy transfer, dye-sensitized dolar cell, energy relay dye, titania L ong range energy transfer has recently been used to increase light harvesting 1-7 inside of dye-sensitized solar cells (DSCs). [8][9][10][11] In one architecture, energy relay dyes (ERDs) absorb high-energy photons and transfer energy via Förster resonant energy transfer to the sensitizing dyes. [3][4][5]12 ERDs can both increase and broaden light absorption for the same film thickness in DSCs by increasing the overall dye loading. Because ERDs have a fundamentally different function and design rules than the sensitizing dyes, this architecture greatly expands the range of dyes that can be used in DSCs. In order for energy relay dyes (ERDs) to be used in state-of-the-art dye-sensitized solar cells, the excited ERDs must be able to efficiently transfer energy to the sensitizing dyes. Conventional DSCs are already efficient at absorbing visible light and collecting charges and can achieve external quantum efficiencies (EQE) of 85% at peak absorption. 13 The EQE contribution from the sensitizing dye is determined by the fraction of light absorbed by the sensitizing dye and the internal quantum efficiency (IQE). DSCs have high internal quantum efficiencies (>90%) [14][15][16] and in portions of the visible spectrum can absorb >90% of the light.When photons are absorbed by the energy relay dye in ERD DSCs, they must undergo an additional energy transfer step before contributing to photocurrent; the EQE contribution from the relay dye (EQE ERD ) is thus defined by eq 1, where η ABS,ERD is the fraction of light absorbed by the ERD inside of the titania film and ETE is the average excitation transfer efficiency, or the average probability that an excited relay dye transfers its energy to a sensitizing dye. In order for ERDs to be viable in DSCs, excitation transfer efficiencies of over 90% are required to achieve a peak EQE of 85%.In our first report, only a minimum bound ETE of 46% could be estimated because of the uncertainty in determining η ABS,ERD due to light scattering caused by large TiO 2 nanoparticles and the inability to accurately measure the concentration of the dye because of rapid evaporation of the chloroform electrolyte during the electrolyte filling process. 4 In this report, the ETE is quan...

Energy and Hole Transfer between Dyes Attached to Titania in Cosensitized Dye-Sensitized Solar Cells

“…20 Showa Denko 17 nm diameter TiO 2 particles were deposited on Scheme 2. Jablonski Plot of AS02 þ C106 DSC System a a The scheme is not geometrically correct (i.e., both dyes should be on the same TiO 2 surface); processes that result in photocurrent generation are labeled in black, while processes that do not contribute to photocurrent are labeled in grey; dashed lines represent intermolecular processes.…”

“…The η ABS,ERD was determined to be 50.8% when correcting for light losses related to reflection (4%) and FTO light absorption (11%) at 780 nm. 20 Light absorption by C106 at 780 nm was considered negligible. The estimated ETE was determined to be 26%; it should be noted that the measured IQE (72.1%) is an average value of all C106 dyes, but the IQE is most likely lower for C106 dyes that are in close proximity to AS02, which have a higher probability of transferring holes to the AS02 before dye regeneration.…”

“…Energy relay dyes (ERDs) have been used previously to increase light harvesting in the blue portion of the solar spectrum. 18,19 Blue ERDs, which absorb high energy photons and undergo FRET to sensitizing dyes, can efficiently transfer energy when placed inside the electrolyte 18,20 or cosensitized 21 on nanocrystalline TiO 2 . Grimes et al recently demonstrated that ERDs unattached to the titania and slightly red-shifted relative to the sensitizing dye peak absorption were able to undergo FRET to the SD.…”

New generation solar cells: concepts, trends and perspectives

“…As well, they do not require attachment on the TiO 2 surface and thus, do not affect the dye loading, and also, the use of multiple ERDs to expand the overall spectral coverage is possible. 121 Along these lines, several analogues have been described, such as perylene derivative, PTCDI, 120 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) 122 and recently benzonitrile soluble BL302 and BL315, 123 which in TT1-sensitized solar cells (Fig. 12) gave overall efficiencies of a maximum of 4.5%.…”

An Organic Nitrile Dye with Strong Donor and Acceptor Groups for Dye-Sensitized Solar Cells

“…The best of our knowledge, this is the first report of an organic nitrile (DCM) dye applied directly to TiO 2 electrode, despite the recent report about DCM that has been employed as energy relay dyes for light harvesting and photocurrent generation of DSSC via Förster resonant energy transfer. [21][22][23] …”