Ultrafast and slow charge recombination dynamics of diketopyrrolopyrrole–NiO dye sensitized solar cells. Issue 27 (24th June 2016)

• Record Type: Journal Article
• Authors: Zhang, Lei; Favereau, Ludovic; Farré, Yoann; Mijangos, Edgar; Pellegrin, Yann; Blart, Errol; Odobel, Fabrice; Hammarström, Leif
• Is Part Of: Physical chemistry chemical physics. Volume 18:Issue 27(2016)
• Page Start: 18515
• Page End: 18527
• Publication Date: 2016-06-24
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c6cp01762b
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : Ultrafast hole injection from photoexcited diketopyrrolopyrrole-based dyes into mesoporous NiO films was followed by charge recombination that spanned time scales from pico- to microseconds, which was reflected in large variations in dye regeneration and device solar energy conversion efficiencies. Abstract : In a photophysical study, two diketopyrrolopyrrole (DPP)-based sensitizers functionalized with 4-thiophenecarboxylic acid as an anchoring group and a bromo (DPPBr) or dicyanovinyl (DPPCN2 ) group, and a dyad consisting of a DPP unit linked to a naphthalenediimide group (DPP–NDI), were investigated both in solution and grafted on mesoporous NiO films. Femtosecond transient absorption measurements indicate that ultrafast hole injection occurred predominantly on a timescale of ∼200 fs, whereas the subsequent charge recombination occurred on a surprisingly wide range of timescales, from tens of ps to tens of μs; this kinetic heterogeneity is much greater than is typically observed for dye-sensitized TiO2 or ZnO. Also, in contrast to what is typically observed for dye-sensitized TiO2, there was no significant dependence on the excitation power of the recombination kinetics, which can be explained by the hole density being comparatively higher near the valence band of NiO before excitation. The additional acceptor group in DPP–NDI provided a rapid electron shift and stabilized charge separation up to the μs timescale. This enabled efficient (∼95%) regeneration of NDI by a Co III (dtb)3 electrolyte (dtb = 4, 4′-di- tert -butyl-2, 2′-bipyridine), according to transient absorption measurements. The regeneration of DPPBr and DPPCN2 by Co III (dtb)3 was instead inefficient, as most recombination for these dyes occurred on the sub-ns timescale. The transient spectroscopy data thus corroborated the trend of the published photovoltaic properties of dye-sensitized solar cells (DSSCs) based on these dyes on mesoporous NiO, and show the potential of a design strategy with a secondary acceptor bound to the dye. The study identifies rapid initial recombination between the dye and NiO as the main obstacle to obtaining high efficiencies in NiO-based DSSCs; these recombination components may be overlooked when studies are conducted using only methods with ns resolution or slower.