Rotaxane-Functionalized Dyes for Charge-Rectification in p-Type Photoelectrochemical Devices

Abstract

A supramolecular photovoltaic strategy is applied to enhance power conversion efficiencies (PCE) of photoelectrochemical devices by suppressing electron-hole recombination after photoinduced electron transfer (PET). Here, the author exploit supramolecular localization of the redox mediator-in close proximity to the dye-through a rotaxane topology, reducing electron-hole recombination in p-type dye-sensitized solar cells (p-DSSCs). Dye P_Rotaxane features 1,5-dioxynaphthalene recognition sites (DNP-arms) with a mechanically-interlocked macrocyclic redox mediator naphthalene diimide macrocycle (3-NDI-ring), stoppering synthetically via click chemistry. The control molecule P_Stopper has stoppered DNP-arms, preventing rotaxane formation with the 3-NDI-ring. Transient absorption and time-resolved fluorescence spectroscopy studies show ultrafast (211 ± 7 fs and 2.92 ± 0.05 ps) PET from the dye-moiety of P_Rotaxane to its mechanically interlocked 3-NDI-ring-acceptor, slowing down the electron-hole recombination on NiO surfaces compared to the analogue . p-DSSCs employing P_Rotaxane (PCE = 0.07%) demonstrate a 30% PCE increase compared to P_Stopper (PCE = 0.05%) devices, combining enhancements in both open-circuit voltages (V_OC = 0.43 vs 0.36 V) and short-circuit photocurrent density (J_SC = -0.39 vs -0.34 mA cm^-2 ). Electrochemical impedance spectroscopy shows that P_Rotaxane devices exhibit hole lifetimes (τ_h ) approaching 1 s, a 16-fold improvement compared to traditional I^- /I₃ ^- -based systems (τ_h = 50 ms), demonstrating the benefits obtained upon nanoengineering of interfacial dye-regeneration at the photocathode.

Keywords: femtosecond transient absorption; interfacial photoelectrochemistry; p-type dye-sensitized solar cell; rotaxanes; supramolecular electronics.

Figure 1

a) Schematic representation of forward electron propagation steps (1–4) (blue arrows) and recombination pathways (5, 6) (red arrows) leading to efficiency losses within p‐DSSCs. FTO = fluorine‐doped tin oxide; band; CB = conduction band; D = dye; RC = redox couple. a) Schematic representation of the benchmark system P1 dye (no mediator interaction) and P_Rotaxane/P_Stopper dyes comprising nanoengineered dye‐mediator interactions. P_Rotaxane contains a permanently bound mediator as a built‐in regeneration system and P_Stopper is designed as a control to study the influence of a permanently bound mediator. The 3‐NDI‐ring is represented as a purple ring in the figure.

Figure 2

Molecular structures of the benchmark dye P1, the macrocyclic redox mediator 3‐NDI‐ring, P_rotaxane, and P_Stopper.

Figure 3

UV–vis spectra in MeCN of P_Rotaxane (solid violet line), P_Stopper (solid orange line), and compound 3‐NDI‐ring (red solid line) along with the normalized fluorescence spectrum upon excitation at λ _max (P_Rotaxane: violet dashed line, P_Stopper: orange dashed line, 3‐NDI‐ring: red dashed line). Fluorescence spectra are normalized with respect to the fluorescence intensity of P_Stopper at 295 nm.

Figure 4

Cyclic voltammograms (0.1 M TBAPF₆ in DCM, 0.1 V s⁻¹) of P_Stopper (orange line), P_Rotaxane (violet solid line) (0.2 mM each), and 3‐NDI‐ring (red line) (0.5 mM). The arrow indicates the scanning direction. Note: The last reduction of 3‐NDI‐ring of P_Rotaxane cannot be discerned through CV but is visible through differential pulse voltammetry (DPV, Figure S23, Supporting Information).

Figure 5

Decay of the normalized fluorescence signal at 615 nm (λ _exc. = 532 nm) and fits for both ZrO₂|P_Stopper and ZrO₂|P_Rotaxane. Note that the fluorescence of the latter is quenched, negatively affecting the signal‐to‐noise ratio and quality of the fit.

Figure 6

Transient absorption (TA, λ _exc. = 480 nm) data and fits (see Section S2, Supporting Information for details) of P_Rotaxane on NiO in supporting electrolyte (1.5 mL, 1 M LiTFSI valeronitrile/MeCN, 15:85) in absence and presence of the 3‐NDI‐ring (5.9 mM). a) TA spectra at given time delays in the absence of the 3‐NDI‐ring. The photoinduced absorbance of NiO|P_Rotaxane around 575 nm is indicated with a black dashed line and indicative of a combination of P_Rotaxane ^* and P_Rotaxane ^{• −} ; b) TA spectra at given time delays in the presence of the 3‐NDI‐ring _; c) Spectroelectrochemistry of the 3‐NDI‐ring (dotted green line), which shows an absorption band around 608 nm when the 3‐NDI‐ring^{• ‐} is formed (solid red line); d) Kinetic traces at 575 nm with (red) and without 3‐NDI‐ring present in the electrolyte (black); e) Kinetic traces at 620 nm with (red) and without 3‐NDI‐ring present in the electrolyte (black); f) Schematic representation of charge transfer processes following excitation of the P_Rotaxane dye in absence of free 3‐NDI‐ring.

Figure 7

a) Schematic representation of charge propagation of the rotaxane‐based p‐DSSC. b) Schematic energy diagram for the p‐DSSC based on the P_Rotaxane/P_Stopper dyes. Upon excitation of the P_Rotaxane hole injection takes place (process 1a) and simultaneously the mechanically bound 3‐NDI‐ring@P_Rotaxane is reduced (process 1b). Then, the reduced 3‐NDI‐ring^•−@P_Rotaxane is able to reduce the 3‐NDI‐ring present in the bulk solution (process 2). Energy levels are represented in V versus NHE.

Figure 8

Photovoltaic performances of the devices based on the P_stopper (orange line) and the P_Rotaxane dye (violet line) with the 3‐NDI‐ring as redox mediator (25 mM 3‐NDI‐ring/3‐NDI‐ring^{• −} 1:1 in 1 M LiTFSI valeronitrile/MeCN, 15:85). a) J–V curves of the on the P_stopper (orange line) and the P_Rotaxane dye (violet line). b) Photocurrent action spectrum. c) Chopped light amperometry at different light flux varying from 0.05–1.3 W cm⁻² with on/off cycles of 10 s. d) Zoom of chopped light amperometry at 80, 90, and 100 mW cm⁻² clearly showing tailing behavior indicative of mass transfer limitation.