Design and synthesis. The p-DSSCs in this study are based on the well documented P1 dye (Fig. 1e), and as such, the design of the molecular machinery started with this molecular scaffold. The dye PSTATION is an analogue of P1 where the terminal (dicyano)vinyl electron acceptors are replaced with cyanoacrylate esters to facilitate introduction of a glycol-tethered 1,5-dioxynaphthalene (DNP). The DNP unit acts as binding station for electron-deficient molecular rings through the formation of pseudorotaxane suprastructures. The naphthalene diimide-based macrocycle 3-NDI-ring (Fig. 1e) binds to the DNP recognition sites of PSTATION, and was designed in an analogous fashion to NDI-based macrocycles previously reported to form pseudorotaxanes with DNP recognition sites at the surface–liquid interface.27,28 As the 3-NDI-ring functions as redox mediator in the envisioned p-DSSC, its redox properties are of key importance, and these compare favorable to those of the typically used I−/I3− (vide infra). Thus the proposed 3-NDI-ring:PSTATION pseudorotaxane photosensitizer (Fig. 1e) is anticipated to improve the PCE of the DSSC device in two ways.14 Firstly, the 3-NDI-ring as redox mediator is preorganized close to the dye by the DNP recognition sites of PSTATION at the surface–electrolyte interface, favoring charge propagation (Fig. 1b, Step 3) over recombination (Fig. 1c, Pathway 6). Secondly, upon photoexcitation (Fig. 1b, Step 1) and subsequent hole injection into NiO (Fig. 1b, Step 2), the resulting PSTATION*−:3-NDI-ring species transfers an electron to the 3-NDI-ring within the pseudorotaxane (Fig. 1b, Step 3), yielding PSTATION:3-NDI-ring•−. Upon reduction the 3-NDI-ring•− loses its affinity for the thread of PSTATION and is replaced by a neutral 3-NDI-ring from the bulk electrolyte. The reduced 3-NDI-ring•− is thus actively repelled from the NiO–dye interface (launching effect (2), Fig. 1b, Step 4), preventing charge recombination (Fig. 1c, Pathway 5). The launched 3-NDI-ring●− is regenerated at counter electrode leading to photocurrent (Fig. 1b, Step 5–6). It is therefore anticipated that the creation of unidirectional charge propagation at a molecular level should translate to macroscopic charge rectification in the device, which should inhibit both recombination pathways (Fig. 1c, Pathway 5–6) resulting in enhanced VOC, JSC and therefore improve PCE.
The P1 dye29 and the PSTATION dye14 were synthesized according to literature. The absorption maximum of the PSTATION (λmax = 455 nm) experiences a blue shift compared to P1 (λmax = 472 nm) (Fig. 2a). This particular absorption, derived from an intramolecular charge transfer (ICT) in the dyes highlights the decrease in respective acceptor strength (i.e., cyanoacrylate in PSTATION vs (dicyano)vinyl in P1) between molecules. The 3-NDI-ring was synthesized in two steps, employing Mitsunobu coupling to effect ring closure between pyromellitic diimide and the 3-NDI fragment in 31% isolated yield (Supplementary Sect. 1.2).
Binding of 3-NDI-ring to the DNP recognition site within PSTATION were prohibited by limited solubility of the dye, therefore the recognition site moiety DNP-thread (Fig. 2d) was used to analyze pseudorotaxane formation by 1H NMR titration. A typical upfield shift (0.5 ppm) in the 1H NMR spectra for the aromatic protons of the 3-NDI-ring in CD2Cl2 was observed (Supplementary Fig. 6).27 Fitting the titration curve to a model for 1:1 binding revealed an association constant (Ka) of 210 M− 1. Pseudorotaxane formation between the electron rich and deficient components typically leads to charge transfer (CT) band at visible wavelengths. UV–Vis spectrophotometry in a valeronitrile/MeCN (15:85) solution of DNP-thread:3-NDI-ring (10:1) indeed revealed the characteristic CT band evolving at 460 nm in line with pseudorotaxane formation (Fig. 2b).30 The Ka ascertained from 1H NMR was complemented by spectrophotometry by probing the formation of the DNP-thread:3-NDI-ring complex by UV–Vis titration (Fig. 2d). The spectral overlap of the PSTATION ICT absorption (λ = 455 nm) precluded observation of the CT evolving from pseudorotaxane formation (λ = 460 nm) over the course of the titration.
Monitoring the absorption at λ = 460 nm UV–vis at 460 nm and fitting to a 1:1 binding model afforded a Ka = 160 M− 1 for the DNP-thread:3-NDI-ring pseudorotaxane (Fig. 2c, Table 1). The differences between Ka derived from NMR and UV–Vis were rationalized by the difference in solvent polarity (CD2Cl2 vs. valeronitrile/MeCN (15:85) respectively). Immobilization of PSTATION onto NiO electrodes (vide infra, Supplementary Fig. 13) and immersion into a 3-NDI-ring solution (20 µM in MeCN) led to a decrease in 3-NDI-ring absorption intensity at λ = 378 nm. Given that the control experiment with P1 in place of PSTATION (Supplementary Fig. 14) experienced no absorption drop at 378 nm, we could ascribe the absorption decreases to the binding of 3-NDI-ring to the DNP recognition sites of the PSTATION-NiO.
Cyclic voltammetry (CV, Supplementary Fig. 8) of 3-NDI-ring revealed four reductions, attributed to two independent reduction events at the NDI and two at the pyromellitic moieties of the 3-NDI-ring. The redox events were fully reversible, demonstrating electrochemical stability, which is an important requirement for redox mediators in DSSCs. The first reduction potential of the 3-NDI-ring (-0.35 V vs. NHE) is 0.55 V lower than that of PSTATION*− (Fig. 1d), facilitating exergonic electron transfer from PSTATION*− to 3-NDI-ring. CV of the DNP-thread:3-NDI-ring complex shows that binding of the DNP-thread to the 3-NDI-ring has a small effect on the reduction potential (40 mV). Importantly, scan rate dependent CV experiments demonstrate that reduction of 3-NDI-ring in the model DNP-thread:3-NDI-ring prompts a loss of affinity and unbinding from the DNP-thread (Supplementary Fig. 9).31 This “ring launching” effect reflected by the 40 mV reduction potential decrease, is only observed for the first reduction event of the 3-NDI-ring when bound to the DNP-thread (-0.39 V vs. NHE compared to -0.35 V vs. NHE for the free 3-NDI-ring (Supplementary Table 2). The absence of this typical shift in the three subsequent reduction events show that the mono-reduced ring 3-NDI-ring•− unbinds DNP-thread after the first reduction.
Table 1
Summary of the optical and electrochemical properties of P1 and PSTATION (0.5 mM) in MeCN.14 The redox potentials (vs. NHE) are determined in 0.1 M TBAPF6 with a glassy carbon working electrode, a leakless Ag/AgCl reference electrode and a Pt wire counter electrode.
Molecule
|
λmax (nm)
|
ε (× 104 M− 1 cm− 1)
|
E0 − 0 (eV)
|
ED*/D
|
ED/D-
|
ΔG°
|
P1
|
472
|
5.8
|
2.27
|
1.30
|
-0.77
|
-0.42
|
PSTATION
|
455
|
6.4
|
3.30
|
1.32
|
-0.98
|
-0.63
|
Photovoltaic performance. To explore if the pseudorotaxane strategy leads to increased photovoltaic performance, p-DSSCs were prepared using NiO photocathodes (3.5 µm, active area 0.196 cm− 2) functionalized with P1 or PSTATION by sensitizing in MeCN solution for 16 hours. Measurement of dye loading by uptake experiments revealed that the surface coverage of P1 (Γ=7.03× 10− 8 mol cm− 2) is approximately 50% higher than that of PSTATION (Γ= PSTATION 4.50 × 10− 8 mol cm− 2), rationalized by the larger molecular size of PSTATION as it is equipped with the DNP recognition site (rH = 0.68 nm for PSTATION versus rH = 0.39 nm for P1, Supplementary Table 1). The devices were assembled with poly(3,4-ethylenedioxythiophene) (PEDOT) counter electrodes using a 60 µm thermoplast frame (Meltonix polymer 1170-60) and loaded with the 3-NDI-ring electrolyte (25 mM 3-NDI-ring/3-NDI-ring•−, 1:1 in 1 M LiTFSI valeronitrile/MeCN (15:85)) inside a nitrogen filled glovebox. The photovoltaic performances of DSSCs based on PSTATION and P1 were evaluated under AM1.5G illumination (100 mW cm− 2) and are represented in Fig. 3 and summarized in Table 2. The DSSCs based on the pseudorotaxane strategy, the PSTATION:3-NDI-ring system outperforms the P1 reference system, in all photovoltaic parameters, with 123 mV enhancement in open circuit voltage (VOC) (208 mV in P1 vs. 331 mV in PSTATION) and a tripling of short circuit current density (JSC) (0.143 mA cm2 in P1 vs. 0.388 mA cm2 in PSTATION,), leading to a fivefold increase in PCE (0.009% in P1 vs. 0.048% in PSTATION). The photocurrent action spectrum of PSTATION reveals a higher incident photon-to-current conversion efficiency (IPCE) across the spectrum (Fig. 3b) with a maximum of 5.3% at 475 nm versus the P1 system (IPCEmax = 2.6% at 500 nm). The improved fill factor (FF) in the PSTATION:3-NDI-ring based cells compared to the P1 reference system in combination with the higher VOC, suggests reduced charge recombination as a result of preorganization of the redox mediator to the dye via pseudorotaxane formation.
Table 2
Summary of the photovoltaic performance data for DSSC based on P1 and PSTATION under AM 1.5G illumination (100 mW cm− 2) with the 3-NDI-ring electrolyte (25 mM) in 1 M LiTFSI valeronitrile/MeCN (15:85). The average performance (N = 9 for P1 and PSTATION) is provided with the best performing cell in brackets.
Dye
|
VOC (mV)
|
JSC (mA cm− 2)
|
FF
|
PCE (%)
|
P1
|
208 ± 23 (210)
|
0.143 ± 0.044 (0.146)
|
0.311 ± 0.050
(0.361)
|
0.009 ± 0.006
(0.015)
|
PSTATION
|
331 ± 53 (385)
|
0.388 ± 0.112
(0.500)
|
0.359 ± 0.057
(0.412)
|
0.048 ± 0.013
(0.060)
|
Chopped light amperometry experiments were performed, where the light is switched on and off in periods of 10 seconds with an increasing illumination density starting from 5 mW cm− 2 to 50 mW cm− 2 at short current conditions. For both solar cells the JSC increases with the light intensity, as expected for these DSSCs (Fig. 3c), and the larger increase of the PSTATION:3-NDI-ring based cell is in line with the better performance. Interestingly, the shape of the photocurrent response for the P1 shows tailing behavior that increases with light intensity. This indicates mass transfer limitations of the redox mediator through the mesoporous electrode (Fig. 3c inset), which can be expected for large molecules like 3-NDI-ring at low concentrations.32,33 This tailing behavior is not observed for PSTATION based DSSCs in line with preorganization of the redox mediator and efficient replacement of reduced 3-NDI-ring•− for neutral 3-NDI-ring0, leading to high local concentrations of 3-NDI-ring at the dye–electrolyte interface even at very low (25 mM) concentrations of redox mediator.
Differences in solar cell performance originating from pseudorotaxane formation were further probed by electrochemical impedance spectroscopy (EIS). Performing EIS under varying light intensities affords insight into electron–hole recombination at the semiconductor–dye interface through determination of the hole lifetime (τh) as a function of VOC (Fig. 3d).34 At any given VOC the hole lifetime for PSTATION (624 ms at 0.1 V) is two times longer than P1 (324 ms at 0.1 V), implying that less recombination occurs in the pseudorotaxane system. This could either arise from a difference in recombination resistance (RREC) or from a change in chemical capacitance (Cµ), originating from a valence band shift.35 The Cµ (Supplementary Fig. 25) shows no dependency on the applied voltage and a minimal shift between the P1 and PSTATION DSSCs, expected given the similarity of the systems, thus cannot be the reason for VOC enhancements in PSTATION:3-NDI-ring p-DSSCs. The measured RREC for the PSTATION system (3.20 × 105 Ω cm− 2 at 0.1 V) is higher than for P1 (2.93 × 105 Ω cm− 2 at 0.1 V) meaning that the difference in hole lifetime originates from lower recombination at the semiconductor–electrolyte interface. This effect further supports the active charge rectification bestowed PSTATION:3-NDI-ring p-DSSCs by introducing molecular machinery to the influence the preorganization and replacement of the redox mediator in the solar cell.