Ultrafast and slow charge recombination dynamics of diketopyrrolopyrrole–NiO dye sensitized solar cells

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.

Insights into the Mechanism of a Covalently Linked Organic Dye–Cobaloxime Catalyst System for Dye‐Sensitized Solar Fuel Devices

“…A redshift of the emission as a function of time was observed. This phenomenon could be explained by a large change in the molecular geometry after excitation, such as a structural relaxation and a significant twisting of dicyanovinyl in the dye structure . However, in solution (see Figure S2b), we cannot see the same phenomenon, and femtosecond data on the related “P1” dye suggest a much faster emission Stokes shift in both polar and less polar solvents ( τ =1–2 ps), which cannot be resolved by the streak camera measurements.…”

“…Presumably, this is followed by an even faster hole shift from the oxidized dye to NiO. Finally, we attributed the weak SAS of the τ 4 and τ 5 components to the reduced dye, which recombines with NiO holes in a typical multiexponential fashion . For PB‐2 /NiO, τ 4 is much shorter than for PB‐1 /NiO (160 ps vs. 335 ps) and the τ 5 SAS has a lower amplitude.…”

“…This phenomenon could be explained by al arge change in the molecular geometry after excitation, such as as tructural relaxation and as ignificant twistingo fd icyanovinyl in the dye structure. [53] However, in solution (see Figure S2b), we cannot see the same phenomenon, andf emtosecondd ata on the related "P1" dye suggest am uch faster emissionS tokes shift in both polar and less polar solvents (t = 1-2 ps), [54] which cannotb er esolved by the streak camera measurements. PB-1 was also dissolved in ap oly(methyl methacrylate) (PMMA) matrix, in which large-scale structural changes should be hindered, and as imilar behavior was observedt ot hat on the films (Figure S2 a).…”

A Blue Diketopyrrolopyrrole Sensitizer with High Efficiency in Nickel‐Oxide‐based Dye‐Sensitized Solar Cells

“…Currently, the highest short‐circuit photocurrent densities ( J sc ) reported for p‐DSSC with the optimal dyes are situated at near 7.5–8.2 mA cm −2 , whereas most of the dyes usually produce 3–5 mA cm −2 . Previous works devoted to diketopyrrrolopyrroles (DPPs) demonstrated the potential of this class of dyes for NiO‐based DSSCs . In an effort to increase the performances of the preceding series of systems, we replaced the phenyl unit appended on the DPP core by a thienyl moiety (Figure ).…”

Charge Transfer from Carbon Nanotubes to Silicon in Flexible Carbon Nanotube/Silicon Solar Cells

“…Its experimental trace (black dots) is fitted by a Gaussian function that gives a peak centered at 1285 ± 4 nm, the inset of Figure f is a clarification of the blueshift. Such shift is direct proof of exciton formation and transfer; it has been observed between SWNTs of different chiralities and it is a well‐known process in other hybrid systems …”