Unravelling the Potential for Dithienopyrrole Sensitizers in Dye-Sensitized Solar Cells

Abstract

Two D−π−A dyes based on the dithieno[3,2-b:2′,3′-d]pyrrole π-bridge (DTP) were synthesized, characterized using UV−vis absorption spectroscopy and electrochemistry, modeled using quantum chemical calculations, and used as sensitizers in dye-sensitized solar cells (DSCs). The photoelectrochemical properties and DSC performance are thoroughly compared with their cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT) analogues. The use of DTP results in a small increase in the zero−zero transition energy reflecting the higher lying lowest unoccupied molecular orbital that is commonly reported for DTP relative to CPDT systems. This increased optical gap manifests in slightly blue-shifted incident photon-to-collected electron conversion efficiency (IPCE) responses; however, increased opencircuit photovoltage values and improved charge-transfer kinetics relative to the CPDT systems result in comparable power conversion efficiencies. The present report highlights the potential of DTP for the development of tailored sensitizers employing stronger acceptors. KEYWORDS: dye-sensitized solar cells, D−π−A, dithienopyrrole ■ INTRODUCTIONOver the past two decades, considerable research efforts have been devoted to dye-sensitized solar cells (DSCs) due to their potential as a low-cost photovoltaic technology. 1 The basic working principle, which relies on a delicate energetic balance between the anode, sensitizer, and electrolyte, is comprehensively described in a review by Hagfeldt et al. 2 The most efficient cells are based on relatively expensive or synthetically challenging ruthenium(II) and porphyrin sensitizers that have provided efficiencies up to 11.7% and 11.9%, respectively. 3,4 More accessible organic dyes take the form of a D−π−A structure, which is reminiscent of push−pull chromophores used in nonlinear optics. 5 In this context, oligothiophenes and their fused derivatives are intensively exploited as π-bridges. 6 The two highest performing organic dyes, Y123 and JF419 (Chart 1), incorporate cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT), reaching power conversion efficiencies of up to 10.3%. 7,8 The success of CPDT is attributed to (i) the advantageous steric effect of the alkyl groups placed in the 4,4-position, which helps to prevent aggregation of the dyes on the surface, and (ii) the effective electronic communication between the donor (D) and acceptor (A), which enables efficient charge transfer.Contrary to the CPDT bridge, which is synthetically costly (35% yield over five steps), 9,10 dithieno[3,2-b:2′,3′-d]pyrrole (DTP) is more accessible (up to 80% yield in one step) as showcased by Rasmussen et al. 11,12 Notably, N-substituted DTP possesses similar electronic properties with respect to CPDT 13 but also has the potential to incorporate a variety of N-substituents. As a result, this position offers greater versatility toward the control of dye aggregation, which makes it particularly attractive for DSC applications. 14,15 The only direct comparison between CPDT and DTP π-bridges in sensitizers for DSCs concluded...

Probing the regeneration process of triphenylamine-based organic dyes in dye-sensitized solar cells

“…Although our predicted electron transfer rate for C218 is a little smaller than that for C241 which is against the above experimental results, they are very close and with the same order of magnitude (10 11 ). Likewise, dithienopyrrole (DTP) exhibits slightly promoted regeneration kinetics compared with cyclopentadithiophene (CPDT) [60]. Moreover, the rate of 4.87 Â 10 11 s À1 for C218 is very close to the rate of 4.63 Â 10 11 s À1 for T3, which is in line with their almost equal halfreaction time [18,20].…”

Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems

“…Two dyes based on dithieno[3,2-b:2′,3′-d]pyrrole (DTP), LP225 and LP227, were synthesized, characterized, and compared with CPDT, a well-known analogue having a high synthetic cost and less versatility 25 . In both cases, the characteristics of the first excited state in terms of single particles orbitals can be characterized as HOMO-LUMO transition corresponding to an intramolecular charge transfer.…”

Synthesis and photovoltaic properties of organic dyes containing N-fluoren-2-yl dithieno[3,2-b:2′,3′-d]pyrrole and different donors

“…The DTP-based dyes exhibited better photovoltaic performance than the reference bithiophene sensitizers [40]. In recent years, several other DTP-based dyes were also reported as sensitizers for application in DSSCs [41][42][43][44][45][46][47].…”