Energy levels, charge injection, charge recombination and dye regeneration dynamics for donor–acceptor π-conjugated organic dyes in mesoscopic TiO2 sensitized solar cells

Abstract

Two new D-p-A type organic sensitizers, MP124 and MP-I-50, were synthesized and their electrochemical and spectroscopic properties studied. Efficiencies of DSSC devices utilizing these dyes were also investigated, where sensitization solvent, sensitization time and additive concentration were all varied. Under standard AM 1.5G simulated solar radiation, optimized MP124 devices show an efficiency of 7.45% (V oc ¼ 0.73 V; J sc ¼ 14.44 mA cm À2 ; FF ¼ 70%) while optimized MP-I-50 devices show an efficiency of 5.66% (V oc ¼ 0.68 V; J sc ¼ 12.06 mA cm À2 ; FF ¼ 69%). Transient absorption spectroscopy studies show that regeneration of dye cations by the red-ox electrolyte was more efficient in MP124 cells which is attributed to its higher HOMO energy leading to greater driving force for the regeneration reaction. Transient photovoltage studies showed that electron lifetimes were longer lived in MP124 explaining the higher V oc for these cells compared to MP-I-50 cells. DFT and MP2 calculations indicate that this is due to the greater tendency of MP-I-50 to form charge-transfer complexes with the I 2 species in the electrolyte, due to the presence of an additional EDOT in its structure compared to MP124. This work highlights the effect that small changes to the sensitizer structure can have on the interfacial charge transfer reactions and ultimately on the device efficiency.

Organic Dyes Incorporating the Dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c]furazan Moiety for Dye-Sensitized Solar Cells

“…The DSSCs of JH dyes exhibited good power conversion efficiencies (5.27−6.18%) except for JH-2 (1.42%). Though JH-2 has higher dye density (Table 2) (2) JH-2 has high-lying HOMO level, which slowdowns dye regeneration and leads to more facile charge recombination with the oxidized dye; 52 (3) the cell of JH-2 has slower electron transport (see electrochemical impedance spectroscopy (EIS) under illumination, vide infra) and/or slower electron injection.…”

Evidence for Interfacial Halogen Bonding

“…These dye molecules were anchored to mesoporous TiO 2 thin films commonly used in dye-sensitized solar cells,w ith typical surface coverages of 410 À8 mol cm À2 . [28][29][30][31][32][33] Nanosecond transient absorption and computational studies of the Dye-X series outlined herein reveal evidence for halogen bonding interactions between iodide and the immobilized photo-oxidized dyes,T iO 2 (e À )/Dye-X + ,where Xisaheavy halogen atom. Immobilization also offers control over the dipole orientation of the dye,a llowing the halogen bond donor access to the nucleophilic species in solution.…”

Rapid Synthesis of Thiophene‐Based, Organic Dyes for Dye‐Sensitized Solar Cells (DSSCs) by a One‐Pot, Four‐Component Coupling Approach

“…To examine the scope of our developed synthetic methodology, seven protected dyes, 9 – 15 , containing different donor moieties with/without an EDOT moiety10h, 15 were rapidly synthesized, as shown in Figure 1. All the dyes were obtained in satisfactory to good yields (for the synthetic details of the building blocks, see the Supporting Information).…”