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Abstract
Triplet generation by quantum dots (QDs)-sensitized molecules emerges great potential in many applications. However, the mechanism of triplet energy transfer (TET) is still fuzzy especially due to the complicated energy level alignment of QDs and molecules or trap states in QDs. Here, CdSe QDs and 5-tetracene carboxylic acid (TCA) molecules are selected as the triplet donor and acceptor, respectively, to form a TET system. By tuning the band gap of CdSe, the CdSe-TCA complex is exactly designed to present a Type-II like alignment of relative energetics. Coupling the transient absorption and time-resolved fluorescence spectra, all carrier dynamics is distinctly elucidated. Quantitative analysis demonstrates that hole transfer persisting for ∼ 2 ps outcompetes all other carrier dynamics such as electron trapping (∼100 ps level), charge recombination (∼ 5 ns) and the so-called “back transfer charge recombination” (∼50 ns), and thus leads to a hole-transfer-mediated TET process. The low TET yield (∼34.0%) ascribed to electron behavior can be further improved if electron trapping and charge recombination are efficiently suppressed. The observation on distinguishable carrier dynamics attributed to legitimate design of energy level alignment facilitates a better understanding of the TET mechanism from QDs to molecules as well as further development of photoelectronic devices based on such TET systems.
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1. Introduction
The triplet state of molecules generally sustains for the time scale from microseconds to seconds due to the spin-forbidden transition, which survives and thus facilitates various photochemical transformations such as photoredox catalysis, photocycloaddition and photoisomerization [1–5]. Besides, triplet is also deeply involved in the development of some photophysical processes, like singlet fission [6–8] and triplet-triplet annihilation for photon upconversion (TTA-UC) [9–11], which is crucial to promoting photovoltaic applications. Early utilization of the triplet through the singlet excited state of the reagents followed by intersystem crossing (ISC) typically required ultraviolet illumination and limited the selectivity and lightfastness of the triplet reactions [12,13]. An available route to addressing this issue is exploiting the triplet energy transfer (TET) from a triplet donor (so-called “triplet sensitizer”) that possesses fine stability with respect to the interested reaction. Traditionally, TET systems with organic chromophores and metal complexes as triplet sensitizers have indeed enabled a series of photochemical transformations that are usually not accessible [14–17]. However, the drawbacks of such TET systems are also obvious, including poor energy efficiency, low triplet energy, broad photoluminescence line width and difficult recycle, thus limiting their practical applications [2].
Recently, great attentions have been paid to the TET systems with colloidal semiconductor quantum dots (QDs) as the triplet donors (or sensitizers) because of several distinct advantages [1,2,18,19] vital for high-efficiency photovoltaic conversion. To be specific, the large absorption coefficients of QDs require lower input photon flux [20]; the tunable absorption and emission through size and composition control extends their available spectra from visible light to ultraviolet (UV) as well as infrared (IR) [21,22]; the weak splitting between bright and dark states enables negligible energy loss during ISC [23,24], which benefits TTA-UC [9]. The superiorities mentioned above may not be exhaustive, but still enough to demonstrate the great potential of QDs as triplet sensitizer for efficient TET systems. However, knowing the merits and potential does not deservedly lead to a TET system with expected performance, because the mechanism of real TET process in QD-molecule complexes is generally more intricate than the traditional direct Dexter type [1,19,25]. Actually, there exist at least three mechanisms still under debate regarding the TET from QDs to molecules. Castellano’s group first revealed a direct observation of the cooperative Dexter-like TET with two-electron exchange in CdSe-ACA complexes [26]. Later, several groups proposed that TET can be mediated by either electron transfer or hole transfer [27–29]. Besides, trap-state-mediated TET was also evidenced by spectral analysis [30,31]. In addition, TET competes with other ultrafast carrier dynamics such as charge transfer, charge recombination and carrier trapping [1,2,18,19] that typically exists in actual QD-molecule complexes due to the complicated energy level alignment of QDs and molecules or trap states in QDs, which makes it more sophisticated to figure out the mechanism. Furthermore, there even exists backwards transfer from molecules to QDs in such TET complexes, when the energetics are favorable for singlet exciton fission [32,33]. Fortunately, despite all of these issues for TET, basically all carrier dynamics profoundly associated with the electronic energy level alignment between QDs and molecules, which mainly depends on the size and composition of QDs with high controllability and easy synthesis, makes it goal-oriented to study and optimize the TET performance.
To understand the TET mechanism more comprehensively, we design a QD-molecule complex with CdSe and 5-tetracene carboxylic acid (TCA) as the triplet donor and acceptor, respectively. By precisely tuning the size of CdSe, the energy levels of CdSe and TCA present a Type-II like alignment analogous to the core-shell QDs. With combined analysis on the time-resolved absorption and emission spectra, the results show evident hole-transfer-mediated TET consistent with the designed energy level alignment, and hard evidence for the associated carrier dynamics. The time scales of corresponding carrier dynamic explicitly revealed by quantitative analysis also comply well with reported values.
2. Experimental methods
2.1. Spectroscopy and morphology
To generate and detect the ultrafast transient absorption (TA) spectra, the main instruments used in this work are regeneratively amplified Ti:sapphire laser system (Coherent Legend, 800 nm, 85 fs, 7 mJ/pulse, 1 kHz repetition rate), optical parameter amplifier (Coherent Topas), Helios spectrometer (Ultrafast Systems LLC) and EOS (Ultrafast Systems LLC). Usage of both Helios and EOS spectrometers covering a time domain from ∼0.1 ps up to ∼400 µs makes it possible to realize the detection on not only ultrafast carrier transfer across the interface between CdSe QDs and TCA, but also relatively slow triplet generation and decay or charge recombination process. In detail, for the femtosecond pump-probe TA, the 800 nm pulses output from the regenerative amplifier are initially split into two beams, one for probe and the other for excitation. After this, the beam for probe is aligned to the optical path through which a sapphire window is ready for the white probe beam generation, while the beam for pumping generates after passing through the optical parameter amplifier with tunable wavelength for selective excitation. Power adjustment of all beams is realized via various optical filters for linear excitation and convenient detection. With suitable power, the pump beam is focused by a lens and incident into the sample with a beam waist of 300 µm. To avoid the noise from laser intensity fluctuation, the white probe beam is again split into two parts, with one part used as probe and the other as reference. Both of the two beams are collected by a fiber optics coupled multichannel spectrometer, and then detected by a complementary metal oxide semiconductor sensor at 1 kHz frequency. The instrument response function (IRF) of this system is determined to be ∼120 fs by measuring solvent responses under the same experiment conditions.
Nanosecond pump-probe TA is performed by the EOS spectrometer (Ultrafast Systems LLC). The pump beam is generated in the same way as the femtosecond TA experiment described above. A different white light continuum (380-1700nm, 0.5 ns pulse width, 2 kHz repetition rate) is used, which is generated by focusing a Nd:YAG laser into a photonic crystal fiber. The delay time between the pump and probe beam is controlled by a digital delay generator (CNT-90, Pendulum Instruments). By comparing the probed power intensities under different conditions, with the sample pumped and unpumped, the pump induced absorbance change (ΔA) can be calculated. The sample solution is filled in a 1 mm light path length quartz cuvette and stirred constantly by a magnetic stirrer during spectral measurements to refrain from photo degradation. Note that the samples are oxygen-free when the spectral measurements are conducted.
Besides, the UV-Vis and photoluminescence (PL) spectra are obtained for comprehensive analysis of carrier dynamics of the TET system. The optical absorption of different samples is measured over the wavelength range of interest using a UV-Vis-NIR spectrometer (SolidSpec-3700, Shimadzu). The photoluminescence (PL) spectrum and PL decay are measured via a fluorescence spectrometer (FLS 980, Edinburgh Instruments Ltd, IRF: ∼200 ps) with a 350 W Xenon lamp and a picosecond pulsed diode laser (pulse width: 48.9 ps) as the excitation light source, respectively. For morphology characterization, TEM and EDS measurements are implemented using JEM-2100 (JEOL). The energy level alignment of CdSe QDs and TCA are performed using an electrochemical workstation (Autolab, PGSTAT302N) via conducting cyclic voltammogram method.
2.2. Synthesis of the donor-acceptor system
For reagents, cadmium oxide (CdO, 99.998%), octadecylphosphonic acid (ODPA, 97%), trioctylphosphine oxide (TOPO 99%), trioctylphosphine (TOP, 97%), selenium powder (99.999%), sulphur powder (99.999%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), ethanol (99%), toluene (99%), n-hexane (99%) and acetone (99%) are purchased from Sigma Aldrich. All chemicals are used without further purification. TCA are prepared as previously described [34,35].
CdSe QDs are prepared via a typical synthesis by crucially tuning the reaction parameters [36,37]. Briefly, cadmium oxide (60 mg, 0.71 mmol), octadecylphosphonic acid (280 mg, 0.84 mmol) and trioctylphosphine oxide (3.0 g, 7.76 mmol) are firstly mixed in a round bottom flask (50 mL). The reaction mixture is then kept under vacuum for ∼1.5 hours and the temperature is elevated to ∼130 °C. After that, the flask is inflated with nitrogen and heated beyond 350 °C to form a clear and colorless solution. At this moment, 1.0 mL trioctylphosphine (TOP) is added into the mixed solution and the temperature is raised to 375 °C. Meanwhile, Se/TOP (60 mg, 0.76 mmol Se in 0.5 mL TOP) solution is fleetly injected into the flask. The reaction temperature then drops to ∼370 °C. When the CdSe QDs grow to target size (4.64 nm diameter in this work), the reaction is ended by removing the heating mantle and immersing the flask in an ice-water mixture bath. The cooled CdSe QDs are washed repeatedly by precipitating with ethanol and redispersible in n-hexane for further experiments.
To prepare the TET system, TCA acceptors are bonded to CdSe donors by ultrasonic self-assembly. In short, TCA powders are added into the CdSe solution in n-hexane, whereby the solution is ultrasonicated for 20 min, to form CdSe-TCA complexes. Considering the insolubility of TCA in n-hexane, the solution containing CdSe-TCA complexes can be obtained by filtration of the solution after ultrasonic treatment. Based on the extinction coefficients of CdSe QDs (∼3.8 × 105 L·mol-1·cm-1 at 603 nm) and TCA (∼7400 L·mol-1·cm-1 at 482 nm) and their absorption spectra (see Section 3.2), the average number of TCA bonded to each NC is calculated to be ∼50 according to Lambert-Beer law [35,38,39].
3. Results and discussion
3.1. Design of the donor-acceptor system
It is crucial to design a proper and precise energetic properties of the donor-acceptor system since it involves variable mechanism of TET process for different energy alignments. The selection of CdSe as the donor lies in its extensive applications in optoelectronics [40–43], which facilitates the synthesis and design of corresponding TET complexes as well as spectral analysis because of the well-established preparation methods and energetic property of such QDs with different sizes. In particular, the first exciton bleach (XB) recovery of CdSe QDs purely contributed by electron transfer is explicit also enabling easier analysis on the transient absorption spectra [44–46]. We choose TCA as the acceptor because the spectral features of its ground state, ionic state (cation or anion) and triplet state is distinct and well-known [35,47]. Besides, the emission of CdSe QDs can be tuned to minimize the overlap with the absorption of TCA. This elimination of Förster resonance energy transfer (FRET) enables easier analysis on spectral information associated with TET process at the interface. From the above consideration, by preparing CdSe QDs with an average diameter of 4.64 ± 0.59 nm (Fig. 1(a) and Fig. S1 in Supplement 1) and TCA with specific molecular structure (Fig. 1(b)), we design a Type-II like energetic property of CdSe-TCA complexes as shown in Fig. 1(c), whose energy level alignment is analogous to those in common donor-acceptor systems for optoelectronic applications. The energy gap of CdSe QDs determined from cyclic voltammogram (see Fig. S2) is ∼2.1 eV, which well consists with the optical gap, ∼2.06 eV, estimated from the first exciton absorption peak at ∼603 nm (Fig. 2(a)). Under such energy level alignment, electron transfer from CdSe to TCA is energetically inhibited. Consequently, if any, either a direct or a hole-transfer-mediated TET process is expected across the interface between CdSe QD and TCA. Besides, the way of carrier trapping, either electron or hole trapping, should also be distinguished since it generally complicates the carrier dynamics in CdSe QDs [48–51].
Fig. 1. Design of donor-acceptor complexes of the TET system. (a) The transmission electron microscopy (TEM) image of CdSe QDs. The scalebar is 20 nm. The label of Cd:Se (1.04:1) ratio determined via EDS spectrum (Fig. S1 in Supplement 1) indicates a non-stoichiometric ratio with extra Cd element. (b) The molecular structure of TCA. (c) Schematic energy level alignment between CdSe QDs and TCA determined by cyclic voltammogram (Fig. S2). The lowest electron and hole energy level in the conduction and valence bands of CdSe QDs are Ee and Eh, respectively. Ered, Eox and Ered,T are the ground-state reduction potential energy, oxidation potential energy and triplet state reduction potential energy, respectively, of TCA. Triplet energy of TCA is labeled as ET.
Fig. 2. Optical properties of CdSe-TCA complexes. (a) Absorption of free CdSe QDs, TCA, CdSe-TCA complexes; (b) Steady photoluminescence (PL) and (c) TR-PL spectra (∼200 ps time resolution) of both free CdSe QDs and CdSe-TCA complexes under 340 nm excitation and probed at 613 nm. Gray lines are exponential fits of TR-PL spectra.
3.2. Optical properties
Figure 2(a) displays the absorption features of free CdSe QDs, TCA and CdSe-TCA. The first exciton absorption peak of the synthesized CdSe QDs locates at ∼603 nm, from which the size is determined to be ∼4.7 nm according to the depth-well EMA theory wherein the absorption peak location of QDs is regarded as the function of the diameter [41]. This estimation is well consistent with the TEM results (4.64 ± 0.59 nm) in Fig. 1(a). In addition, the optical gap estimated from the first exciton absorption peak at ∼603 nm is ∼2.06 eV, also agreeing well with the band gap (∼2.1 eV) of CdSe QDs as previously mentioned. The negligible absorption of TCA, much weaker than that of CdSe at 340 nm, enables selective excitation of the CdSe QDs during latter analysis on the carrier dynamics. Note that the absorption of TCA in CdSe-TCA complexes is red-shifted by ∼8 nm compared to TCA in ethanol, indicating successful bonding of TCA onto NC surfaces which induces a change to the local dielectric environment of the molecules [35]. The average number of TCA bonded on each CdSe QD is determined to be ∼50 according to Lamb-Beer law based on absorption properties [38,39]. Figure 2(b) reveals the emission spectra of CdSe-TCA complexes and pure CdSe QDs under 340 nm excitation. The emission of CdSe shows a sharp fluorescence peak around 613 nm implying good uniformity. This is expected since large size CdSe QD (with first exciton absorption peak at 603 nm) is generally easier to synthesize [37]. Besides, the emission spectra of CdSe (from 550 nm to 700 nm in Fig. 2(b)) and the absorption of TCA (<510 nm in Fig. 2(a)) have no overlap excluding FRET from CdSe to TCA. The spectral feature predicts the exciton quenching (by ∼90%) of CdSe QDs by anchored TCA. Similarly, in Fig. 2(c), the obviously accelerated decay of the time resolved PL (TR-PL) spectra of CdSe-TCA complexes compared to free CdSe QDs is also an indication of exciton quenching, either through direct or hole-transfer-mediated TET from CdSe to TCA considering the exclusion of electron transfer and FRET, respectively based on the designed energy alignment and spectral overlap. Actually, although ultrafast TR-PL (with picosecond order of magnitude) is not available at this stage, analyzing the fitting results (Fig. 2(c) and Table S1) reveals two slow decay components for both free CdSe QDs and CdSe-TCA complexes corresponding to charge recombination processes due to directly excited exciton (∼5 ns) [49–51] and shallow electron traps in non-stoichiometric CdSe QDs (∼ 30 ns) with extra Cd element [36,52], and a fast decay component with TCA bonded on CdSe, which indicates a very short-lived hole transfer process from CdSe to TCA (see Table S1). Clearer and more comprehensive proof of the hole-transfer-mediated TET process will be demonstrated below via TA spectra analysis.
3.3 Hole-transfer-mediated TET from CdSe QDs to TCA
To detail the carrier dynamics dominated by the designed energy level alignment and further illustrate the energy transfer mechanism in the TET system, TA measurement covering a wide time domain (from ∼0.1 ps to 400 µs) is implemented. The pump wavelength is 340 nm for selective excitation of CdSe QDs. For accurate analysis, the excitation power is kept low enough (28 µW with 280 µm spot diameter) to ensure a single exciton rather than multiexciton process. The sample solution is stirred constantly during measurements to minimize photo degradation. Figure 3(a) shows the TA spectra of free CdSe QDs udner 340 nm excitation at specific time delays. The peaks at ∼603 nm correspond to the first exciton bleach (XB) dominated by the state filling of conduction band electron levels [44–46]. The peaks at ∼500 nm arise due to the 2S1/2-1Se transition [53]. In Fig. 3(b), the TA spectra of CdSe-TCA complexes, measured at the same condition as free CdSe, shows faster decay at corresponding wavelengths and implies exciton quenching of CdSe by bonded TCA, involving possible direct or hole-transfer mediated TET processes. The red dotted lines in Figs. 3(a) and 3(b) respectively represent TA spectra of CdSe and CdSe-TCA complexes at long time delays. Comparing these two curves also results in the conclusion that a possible TET process emerges with TCA bonded on CdSe since no characteristic peaks remain in free CdSe while the long-lived photoinduced absorption feature of the triplet in tetracene (3TCA*) [27,35,54] at ∼482 nm persists in tens to hundreds microseconds. Besides, compared with Fig. 3(c), the photoinduced absorption feature in the near-infrared region (∼875 nm) is also observed in Fig. 3(d) after attachment of TCA to CdSe, which consists with the absorption of the cation radical of TCA (TCA+) [35,54], implying hole transfer from CdSe to TCA. The possibility for the photoinduced absorption feature referring to the anion radical of TCA (TCA-) is excluded since electron transfer from CdSe to TCA is energetically forbidden according to the energy level alignment in Fig. 1(c), although anion and cation absorption can be very similar [55]. Quantitative analysis on carrier dynamics below will further confirm the hole-transfer mediated TET process.
Fig. 3. Averaged TA spectra of free CdSe QDs and CdSe-TCA complexes at varied delay times under 340 nm excitation. The averaged spectra (red dotted lines) at long delay times (20-100 µs) are amplified by a factor of 4 for clearer presentation. The samples both contain the same concentration of CdSe QDs. All spectra measurements are conducted when the samples are oxygen-free.
3.4. Carrier dynamics in free CdSe QDs and CdSe-TCA complexes
TA kinetics at several characteristic wavelengths of free CdSe QDs and CdSe-TCA complexes under 340 nm excitation are extracted as Fig. 4 and Fig. S3 and quantitatively fitted using proper exponential curves for more exact revelation of the hole-transfer mediated TET process and other carrier dynamics. Note that the characteristic kinetics for TCA+ and 3TCA* are both extracted by subtracting the kinetics of free CdSe from CdSe-TCA complexes considering spectral overlap at corresponding wavelengths. The kinetics provide clear evidences for the hole-transfer-mediated TET from CdSe to TCA, including explicit temporal information of hole transfer (HT) (or TCA+ cation generation), electron trapping (ETr), triplet formation (TF) (or electron transfer (ET)) and decay (TD). Specifically, XB kinetics of free CdSe QDs and CdSe-TCA complexes probed at ∼603 nm are illustrated in Fig. 4(a) for comparison. Since the XB of CdSe QDs is dominated by state filling of the conduction band [44–46], it only reflects the behavior of electrons rather than holes. As Fig. 4(a) shows, the XB recovery of both CdSe and CdSe-TCA follows nearly the same traces at the beginning, implying identical carrier dynamics of the electron that persists for ∼100 ps scale, after which the XB recovery of CdSe QDs is accelerated by at least one electron behavior with bonded TCA. Besides, as has been discussed in Fig. 2(c), two slow carrier dynamics associated with charge recombination exist in both CdSe and CdSe-TCA, which should also be responsible for XB recovery. Taking all these analysis into account, the XB kinetics of CdSe and CdSe-TCA at 603 nm are fitted using three- and four-exponential models (see Table S2 for details), respectively. Except for an extra ∼1.3 ns component in CdSe-TCA, other three components in both CdSe and CdSe-TCA sustain for comparable lifetimes (i.e. ∼100 ps, ∼5 ns, ∼50 ns, respectively), indicating identical electron behaviors corresponding to certain time scales in only CdSe. The ∼5 ns component is associated with charge recombination (CR) of directly excited CdSe QDs, which generally persists for several to a dozen nanoseconds [49–51]. The ∼50 ns component can be attributed to a so-called “back transfer charge recombination” (BTCR) in the case that electrons are trapped in shallow electron traps and transfer back to the conduction band for further recombination, which is normal in non-stoichiometric CdSe QDs with extra Cd element and delays CR process [36,52]. In addition, the two slow components of TR-PL results (∼5 ns and ∼30 ns respectively) in Fig. 2(c) are comparable with the ∼5 ns and ∼50 ns components in Fig. 4(a), making it more believable for the analysis on corresponding carrier dynamics. Even more, the consistent components in both XB kinetics and TR-PL (compared in Table S2) lead to the exclusion of possible hole trapping process because hole trapping should result in serious deviation between the XB and PL kinetics given that electron and hole behaviors both contribute to PL quenching while XB kinetics only depends on electron behavior rather than hole behavior in CdSe QDs [39,49–51]. On the other hand, if hole trapping occurs, there should not be a ∼100 ps component in XB kinetics of CdSe. Note that the absence of ∼100 ps component in TR-PL is due to limited temporal resolution (∼200 ps) instead of an indication of inexistence of ETr process. Consequently, with shallow electron traps existing and hole trapping excluded, the ∼100 ps component is assigned to ETr process. This assignation is also reasonable considering that ETr process generally persists for sub-100 ps to 100 ps scale in CdSe QDs [56–59]. Besides, the proportion of ETr in CdSe XB consists with that in CdSe-TCA (both ∼0.29), which is unsurprising since ETr (∼100 ps) outcompetes CR (∼5 ns) and BTCR (∼50 ns) and thus should principally depend on the concentration of electron traps in only CdSe QDs.
Fig. 4. Carrier dynamics in free CdSe QDs and CdSe-TCA complexes at several characteristic wavelengths under 340 nm excitation. (a) TA kinetics probed at the XB (∼603 nm) of CdSe QDs (black circles) and CdSe-TCA complexes (red circles). (b, c) TA kinetics probed at (b) ∼875 nm (TCA+ cation) and (c) ∼482 nm (3TCA*). Solid lines are fitting curves of corresponding kinetics signals. All fitting parameters are shown in Table S2. Insets: hexagons represent various carrier dynamics including hole transfer (HT) (or TCA+ cation generation), electron trapping (ETr), triplet formation (TF) (or electron transfer (ET)). Orange dotted lines in hexagons imply shallow electron traps in non-stoichiometric CdSe QDs with extra Cd element. Arrows represent different spin states responsible for TET process. Other symbols have been defined in Fig. 1(c).
As for the extra ∼1.3 ns component, it is believed to be an ET behavior associated with TET process from CdSe to TCA. Figure 4(b) displays the TA kinetics of CdSe-TCA at ∼875 nm, which generally implies the generation or decay of TCA+, viz., HT and the following ET from CdSe to TCA [35,54]. Single exponential fitting results reveals that the kinetics grows to its maximum and decay away in ∼2 ps and ∼1.7 ns, respectively. The ∼2 ps process is reasonably attributed to the HT (or TCA+ generation) process allowing for the sub-picosecond to several picoseconds time scale usually appropriate in donor-acceptor systems with CdSe or other QDs as the hole donor [35,50,60,61]. The ∼1.7 ns decay of TCA+ is coincidently comparable with the extra ∼1.3 ns recovery component in Fig. 4(a), and the ∼1.5 ns growth of the kinetics implying TF [27,35,54] at ∼482 nm in Fig. 4(c), which adequately confirms the ET neutralizing the TCA+ and producing 3TCA*. Note that the proportion of ET in the XB of CdSe-TCA is ∼0.33, which is just the total proportion decrement of CR (∼0.16) and BTCR (∼0.17) compared to that of CdSe. This is expected given that ET should mainly compete with the slower CR and BTCR rather than the faster ETr. Besides, the ratios of CR to BTCR proportion in both CdSe XB (0.34:0.37 = 1:1.09) and CdSe-TCA XB (0.18:0.20 = 1:1.11) almost equal, further confirming their occurrence in only CdSe whether with or without bonded TCA, and simultaneously eliminating possible existence of trap-state-mediated TET [30,31]. Also note that the serious mismatch between the fast HT (∼ 2ps) and the following slow ET (∼1.5 ns) owes to the high ratio of acceptor to donor (∼50:1) in CdSe-TCA complexes since HT should be positively correlated with the amount of bonded acceptor, while single TCA only accepts one electron from a probabilistic perspective considering the low excitation power [35,62]. By fitting the TD kinetics in Fig. S3, it reveals a long-lived triplet state sustaining for ∼212 µs in TCA, which is approximate to the reported value (∼124 µs) [35].
The TET yield is evaluated to be ∼32.7% based on the fitting parameters shown in Table S2. In detail, since HT (∼2 ps) outcompetes both CR (∼5 ns) and BTCR (∼50 ns), the HT yield can be approximately determined as 100% [35]. Thus the TET yield solely depends on the ensuing ET yield. Because of nonnegligible ETr and CR processes, the proportion of ET is ∼32.7% in CdSe-TCA. Supposing the spin-flip of the electron in CdSe QDs is fast enough for the electron adjusting its spin to generate a triplet in TCA, the total TET yield in this case should be equal to the ET proportion. Besides, the TET yield is also estimated through the combination of TA signal amplitudes of 3TCA* absorption and CdSe XB and their extinction coefficients at corresponding wavelengths [35]; see Eq. (S1). The calculated TET yield is ∼34.0%, close to ∼32.7% determined from the fitting parameters of the TA kinetics. Obviously, efficient ways to achieve higher TET yield in such systems must reduce trap states and prolong the charge recombination lifetime.
4. Conclusions
To sum up, for more complete understanding of the TET mechanism in QD-molecule complexes, a series of spectral measurements were implemented and analyzed focusing on the energetics design of a TET system comprising of CdSe donor and TCA acceptor and its elaborate carrier dynamics. The hole transfer persisting for ∼2 ps outcompetes all other slower carrier dynamics such as electron trapping (∼100 ps), charge recombination (∼5 ns) and the so-called back transfer charge recombination (∼50 ns), which leads to a clear hole-transfer-mediated TET from CdSe to TCA and conforms to the energetics design as expected. Besides, the time constants derived from TA signals, respectively characterizing the excited states of free CdSe QDs and CdSe-TCA complexes, TCA+ cation and 3TCA*, also coinciding with each other proves the rationality of our experiments and analysis. To realize higher TET yields of such complexes, more steps should be taken to remove the trap states in QDs and prolong the lifetime of charge recombination. This work will expedite recognizing the energetics design and the mechanism of TET process in QD-molecule complexes and its potential applications in photovoltaic conversion.
Funding
National Natural Science Foundation of China (22279031); National Key Research and Development Program of China (2019YFE0107200); Key Research and Development Plan of Hubei Province (2021BGE037); Science and technology research project of Hubei Provincial Department of Education (B2022167); Teacher Research Ability Cultivation Foundation of Hubei University of Arts and Science (2020kypytd001); Hubei Key Laboratory of Low Dimensional Optoelectronic Material and Devices (HLOM222008).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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