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Energy Conversion and Storage; Plasmonics and Optoelectronics 60

Femtosecond Dynamics of Photoexcited C Films Martina Causa', Ivan R Ramirez, Josue F. Martinez Hardigree, Moritz Riede, and Natalie Banerji J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00520 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 25, 2018

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The Journal of Physical Chemistry Letters

Femtosecond Dynamics of Photoexcited C60 Films Martina Causa’1†, Ivan Ramirez2†, Josué Martinez Hardigree2, Moritz Riede2*, Natalie Banerji1* †

1

These authors contributed equally

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland.

2

Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, OX1 3PU, Oxford, United Kingdom

AUTHOR INFORMATION Corresponding Authors *[email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The well-known organic semiconductor C60 is attracting renewed attention due to its centimetrelong electron diffusion length and high performance of solar cells containing 95% fullerene. Yet, its photophysical properties remain poorly understood. Here, we elucidate the dynamics of Frenkel and intermolecular (inter- C60) charge transfer (CT) excitons in neat and diluted C60 films from high quality femtosecond transient absorption (TA) measurements, performed at low fluences and free from oxygen or pump-induced photo-dimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield depend strongly on the C60 aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C60 anion.

TOC GRAPHIC

KEYWORDS Fullerene films, transient absorption spectroscopy, Frenkel and Charge Transfer excitons, electro-absorption, field-assisted charge dissociation.


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C60 still represents, even after decades of studies, an intriguing and relatively poorly understood organic semiconductor. It is not clear, for example, why ‘dilute’ organic solar cells containing 95% molar C60 and only 5% donor perform often better than prototypical bulk heterojunction (BHJ) solar cells.1-2 With 95% C70, power conversion even can exceeds 8%.3 Equally impressive, C60 electron diffusion lengths of centimetres have recently been reported.4 Yet, a distinctive and in-depth understanding of the photophysics of thin film C60 is still absent to date and many discrepancies persist in literature. Because transient absorption (TA) spectra are complex and studies have typically been performed at high fluences (bimolecular and degradation artefacts), with limited spectral windows or at un-representative conditions, there are conflicting theories as to the excited-state dynamics in the films.5-8 The decay path of excitons has either been argued to occur via trapped Frenkel states, triplets or triplets with charge-transfer (CT) character, and the role of intermolecular CT singlet excitons between C60 molecules has not been un-ambiguously elucidated. As a result, device physicists often rely on rates and yields obtained in C60 solution,9-11 which are however invalid in the solid state.12-16 In this contribution, we de-convolute the dynamics of Frenkel and intermolecular CT excitons in thermally evaporated C60 films from careful TA measurements, in which either species is preferentially excited. To shed light on the role of aggregates and grain boundaries on the lifetime of Frenkel excitons and triplet yield, the coupling between molecules and clusters of C60 is varied by dilution in the small molecule insulator NBPhen and by comparison to non-annealed neat PCBM films. Moreover, the field-assisted dissociation of CT excitons in C60 is directly observed from TA measurements performed on full devices. Finally, we show the influence of delocalisation on the spectral signature of the C60 anion, by adding different concentrations of a small molecule donor.


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Figure 1 (A) shows the normalised absorption spectra obtained in an integrating sphere for films of neat C60, C60 co-evaporated with 50% volume insulating NBPhen (which does not absorb in the shown wavelength range), and neat PCBM. The C60 absorption in the solid state has been previously elucidated.17-19 The peak at 355 nm corresponds to the first allowed optical transition of isolated C60 molecules, while the intramolecular S0 à S1 absorption at 650 nm is symmetry forbidden and consequently weakly observed. The visible spectrum is thus dominated by intermolecular excitations, which do not have counterparts in solution.20 Below 540 nm, transitions with charge transfer (CT) character are mixed with more localized Frenkel transitions,21 while only the latter are present at longer wavelengths. At 50% NBPhen loading, the intermolecular interactions are suppressed, as evidenced by the weaker CT absorption in the 400-600 nm region. In non-annealed PCBM films, the functional tail means that the CT absorption is also weaker and blue-shifted (to around 440 nm), reflecting a disruption of the intermolecular coupling between the fullerene moieties.22 The intermolecular CT excitons in C60 are precursors of photocurrent in organic photovoltaic (OPV) devices,21 explaining the reasonable device performances obtained in the absence of a donor.23-24 Figure 1 (B) compares the external quantum efficiency (EQE, 0 V external bias) of a neat C60 device and the C60 absorption probability within the device, obtained from transfer matrix modelling. A clear mismatch is observed, with peaks occurring at different wavelengths and photogeneration starting only below 540 nm.25 We thus attribute the EQE spectrum directly to the absorption of the intermolecular CT states. In the inset of Figure 1 (B), the electro-absorption (EA) spectrum measured in a neat C60 device under reverse bias is depicted. Consistent with literature,26 it shows a negative and positive peak centred at 505 nm and 545 nm, respectively. This is in the region of the intermolecular absorption and matches well with the EQE onset, showing that predominantly


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the CT transitions are affected by the electric field. The EA amplitude shows a quadratic dependence on the electric-field strength (SI fig. 2), while its spectral shape resembles the second derivative of the C60 absorption (inset of Figure 1 (B)). This implies that electronic transitions accompanied by a large change in dipole moment between the ground and excited state are responsible for the observed Stark effect, as expected for intermolecular CT transitions.21, 26-28

Figure 1. (A) Steady-state absorption spectra of evaporated neat C60 (50 nm) and NBPhen:C60 50:50 (by volume, 100 nm) as well as solution-processed neat PCBM (100 nm) thin films. The blue and black vertical lines represent the TA excitation wavelengths (at 450 nm and 610 nm, respectively). (B) The external quantum efficiency (EQE) of a C60 device (left axis, ITO/MoOx(2nm)/C60(50nm)/BPhen(6nm)/Al(100nm)) and the absorption probability of the C60 active layer in the device (right axis), simulated from transfer matrix modeling (TMM) are depicted. The inset shows the electro-absorption (EA) spectrum of C60 (right axis) measured in a device with an applied field of 1.2 MV/cm in reverse bias. The EA spectrum is compared to the second and the first derivatives (violet and blue curves) of the C60 absorption spectrum (red curve, left axis).


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The TA data for the neat C60 film excited either at 610 nm (where Frenkel excitons are predominant) or at 450 nm (where both Frenkel and intermolecular CT excitons are generated with an approximate 55:45 ratio, as deduced from reported IQE values),25 are shown in Figure 2. Any pump-induced photodegradation is excluded by comparison to the spectra obtained for a deliberately photo-dimerized sample (SI fig. 3).29-30 All of our data were recorded in a lowfluence regime, where the dynamics are independent on the excitation density (SI fig. 4), and the samples where never exposed to oxygen. In Figure 2 (A) we observe, at 0.2 ps following 450 nm excitation, the EA signature of C60 at 505 nm (negative peak) and 550 nm (positive peak), in agreement with the steady-state EA spectrum measured in the device. The presence of an EA signature in TA data of C60 films and other organic semiconductors is typically assigned to local electric fields induced by delocalized excited states or photogenerated charges.6, 8, 31-35 Here, we attribute the pronounced EA feature to the direct population of intermolecular CT states with 450 nm excitation, since the signature is negligible with 610 nm excitation of Frenkel excitons (Figure 2 (B)). The delocalization of the CT excitons over several molecules leads to strong associated electric dipoles, which perturb the CT transitions of the surrounding C60 aggregates more strongly than the more localized Frenkel excitons. We also exclude free charges as the cause of the EA, since these are not expected to form significantly in the absence of a field across the active layer (corresponding to VOC in a device).23-24 In addition to the EA signal, the early TA spectra of C60 in Figure 2 (A) show a broad photoinduced absorption (PIA) band throughout the 550-1250 nm region, as well as the weak tail of the ground state bleaching (GSB) of the CT band below 540 nm. The latter is strongly masked by the overlapping negative lobe of the EA, but can be revealed by subtracting the EA signature from the TA spectrum (SI fig. 5).


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The temporal evolution of the TA spectra with 450 nm excitation in Figure 2 (A) shows strong decay of the EA feature within the first 5 ps. Concomitantly, the remaining EA signal blue-shifts, the associated negative peak almost vanishes (leaving some GSB signature), and the positive peak becomes broader, as emphasized in the inset of Figure 2 (A), where the visible range of the TA spectra is normalized. On this 5 ps time scale, there is no decay of the PIA signal in the nearIR range (above 850 nm), only a slight flattening of the weakly pronounced band around 950 nm. From the global analysis (Figure 2 (C)), the shape of the amplitude spectrum associated with a time constant of τ1 = 0.18 ps is clearly due to the decay of the EA signature. We attribute this fast decay of the EA to relaxation of the CT excitons, leading to a reduction of the local electric dipoles. The CT states initially delocalized over several C60 molecules relax to lower-lying CT states, localize to Frenkel excitons or recombine to the ground state. Direct CT photoluminescence to the ground state also occurs as shown in SI fig. 6, but given the short lifetime of the CT excitons, it is weak compared to the Frenkel emission at 720-800 nm. Mainly, Frenkel excitons are left in the TA spectra after 5 ps, which are characterized by the weak C60 GSB below 500 nm (strongly masked by overlapping positive bands) and a broad PIA band from 500-1200 nm, with an EA peak around 550 nm and a weak indent at 650-850 nm due to stimulated emission (SE) matching the Frenkel emission (SI fig. 6). The weak EA peak is present because the delocalization in the Frenkel excitons is sufficient to induce some Stark effect in surrounding C60 aggregates. When the C60 film is excited at 610 nm (Figure 2 (B)), the strong EA signal at 0.2 ps due to photoexcited CT states is quasi absent and replaced by only the Frenkel exciton signature, similar as identified at longer time delays with 450 nm excitation. Unlike with 450 nm excitation, the intensity and shape of the TA spectra below 600 nm does not


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significantly change over the first 5 ps, also when normalized (inset of Figure 2 (B)), and the 0.18 ps component due to CT state recombination is absent. While the important decay of the EA with 0.18 ps is solely assigned to the relaxation and recombination of CT excitons populated at 450 nm, a weaker decay of the EA with τ2 = 5 ps occurs with 450 nm and, to a lesser extent, with 610 nm excitation (Figure 2 (C) (D)). This possibly has a contribution due to CT state dynamics (with 450 nm excitation), but is mainly due to relaxation of Frenkel excitons (localization after excess energy excitation, migration to grain boundaries), which are directly excited or indirectly populated via the CT states. The 5 ps amplitude spectrum thus shows a weak dip around 720 nm, assigned to Frenkel SE and a small peak at 950 nm, related to the spectral changes in the near-IR region. The relaxed Frenkel excitons then decay with a time constant of τ3 = 150 ps, independently of the excitation wavelength (Figure 2 (C) (D)). The 150 ps amplitude spectrum corresponds to the signature of the relaxed Frenkel excitons and it is seen that the EA peak is blue-shifted and broader compared to the EA at early time-delays. This points to a different electronic environment surrounding the Frenkel excitons,36 as would be expected at grain boundaries between C60 aggregates. As further discussed below together with the results of the C60:NBPhen films, we assign the Frenkel exciton decay predominantly to recombination at grain boundaries. At the longest time delays, only a weak offset remains in the TA spectra at both excitation wavelengths, with an enhanced positive intensity in the 650-900 nm region (Figure 2). By comparison to C60 in solution,37 this signal centred around 700 nm is identified as the triplet state of C60, populated by intersystem crossing (ISC) from singlet Frenkel excitons (within ∼1 ns). The slow rise of the triplets is visible in the normalized TA spectra in the insets of Figure 2 (A) (B). The triplet yield in the film is drastically reduced compared to C60 in solution, since the close packing of the C60 molecules


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induces competing intermolecular decay paths,11, 38 such as the observed exciton recombination at grain boundaries. To better compare the amplitude spectra associated with similar time constants for the two excitation wavelengths, they are shown together in Figure 2 (E), after normalization relative to the 150 ps component. Overall, the shape of the amplitude spectra is the same. Thus, the excess energy needed to populate the delocalized CT excitons affects the photophysics of C60 only at ultrafast times, while at longer times, the Frenkel exciton dynamics and triplet yield are similar at 450 nm and 610 nm excitation.

Figure 2. Transient absorption (TA) spectra of neat C60 film at selected time delays (0.2 ps, 0.5 ps, 5 ps, 100 ps, 300 ps and 1.5 ns) and the associated amplitude spectra following 450 nm (A and C) or 610 nm (B and D) pump excitation. The corresponding normalized TA spectra (until 300 ps) are depicted in the insets of (A) and (B). The dotted black curve in (A) corresponds to


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the steady-state EA spectrum obtained for the C60 device at 6 V applied reverse bias. (C) and (D) display the amplitude spectra obtained by global analysis of the corresponding TA data depicted in (A) and (B), respectively with a four- (C) or a three- (D) exponential function. In (E), the amplitude spectra associated with similar time constants (5 ps, 150 ps and offset) recorded at 450 nm (solid lines) nm and 610 nm (dashed lines) excitation are compared (with normalization relative to the 150 ps component). To evaluate the role of C60 aggregation on the excited-state dynamics, we diluted the C60 film with 50% insulating NBPhen. As evidenced by the suppressed CT state absorption at 400-540 nm (Figure 1 (A)), intermolecular coupling is reduced, since either isolated C60 molecules or small isolated C60 clusters are formed. The TA features with 450 nm excitation and their evolution are very different than in neat C60 (Figure 3 (A)). The pronounced EA signal due to CT excitons and its 0.18 ps decay are not evident at early times, in agreement with reduced intermolecular interactions and predominant formation of Frenkel excitons even at 450 nm. These Frenkel excitons show two broad PIA bands around 530 nm and 1000 nm. Compared to neat C60 (at 50 ps, Figure 3 (B)), the 530 nm band is broader, less GSB around 500 nm is seen (as there is less steady-state CT absorption in this region), there is no sharp EA peak at 550 nm (as the Frenkel excitons are not surrounded by large C60 aggregates), and the near-IR band at 1000 nm is more pronounced. Weak spectral dynamics occur in the 50% NBPhen blend with a time constant of τ1 = 1.4 ps (Figure 3 (A) and inset), including a decay and blue shift of the 530 nm band and a slight red shift of the near-IR band. This is either due to relaxation of the Frenkel excitons, or due to residual CT states in small C60 clusters, which localize to Frenkel excitons.


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Figure 3. (A) TA data for the NBPhen:C60 50:50 blend upon 450 nm excitation. The inset displays the amplitude spectra associated with the time constants (shown in the legend) obtained by multi-exponential global analysis. (B) represents the normalized TA spectra at 50 ps for neat C60, NBPhen:C60 50:50 and PCBM films following 450 nm (and for C60 also 610 nm) excitation. Importantly, the decay of the relaxed Frenkel excitons occurs slowly, with a time constant of τ2 = 532 ps (Figure 3 (A)) compared to 150 ps in neat C60 films. We attribute the increased lifetime to the suppression of low-lying states at grain boundaries at which Frenkel excitons recombine, since the C60 molecules and small clusters are isolated by NBPhen. ISC can now compete better with the Frenkel exciton recombination, so that the triplet yield is enhanced. Indeed, unlike for neat C60, there is still considerable TA signal left at the longest measured time delay of 1500 ps. This offset consists of both remaining Frenkel excitons, given their slow decay, as well as of


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triplet states with absorption around 700 nm.39 Interestingly in non-annealed neat PCBM, very similar TA spectra and dynamics as for the NBphen:C60 50:50 film are obtained (Figure 3 (B), SI fig. 7), independently of excitation at 450 nm or 610 nm, since higher excitation energy is needed to significantly excite CT states in PCBM.22 Frenkel excitons are indeed populated in both cases and decay slowly in about 600 ps, yielding a significant population of triplets. This is consistent with the reduced intermolecular coupling due to the functional tails of PCBM. It also agrees with the significantly lower internal quantum efficiency (IQE) for photocurrent generation observed in PCBM compared to C60 diodes, and with the lower oscillator strength of the CT state absorption in PCBM.25 Very different results for PCBM films were recently reported (strong EA and generation of long-lived charges), which is related to excitation of CT states at 350 nm.39 Having discussed CT excitons, Frenkel excitons and triplet states, we now turn to the properties of free charges in C60. The fast decay of the EA in the TA measurements with 450 nm excitation confirms that there is no significant splitting of the CT states without an electric field.23-24 Therefore, we performed photocurrent measurements and TA spectroscopy with nearIR probing on a working C60 device under voltage bias. The extracted photogenerated charge as a function of applied bias (from integrated photocurrent transients, inset of Figure 4 (A)), agrees with the strong field-dependence of free charge generation in neat C60 solar cells,25, 40 and their EQE onset at 540 nm (Figure 1 (B)). We measure a strong increase of photocurrent with reverse bias for 450 nm excitation (where CT excitons are formed). This process is not significant with 610 nm excitation of the Frenkel excitons, although a small photocurrent is observed at the highest fields, for which the less bound Frenkel states can (probably extrinsically) dissociate.25 We thus confirm that free charges are mainly obtained by field-induced dissociation of intermolecular CT states. To spectroscopically detect the free charges, the TA spectra at long


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time delays (averaged at 1000-1500 ps) are compared with and without applied bias in Figure 4 (A)(B). The black curve shows the differential spectrum obtained by field-modulated measurements (see experimental part). Since the visible part of the TA spectrum with bias (SI fig. 8) is dominated by EA effects induced by the external electric field,41 we show here only the near-infrared region. For the Frenkel excitons (Figure 4 (A), 610 nm excitation), there is no overall difference between the TA spectrum with or without the bias. On the other hand, for the CT states (Figure 4 (B), 450 nm excitation) there is an increase of the PIA intensity in the nearIR region with the applied bias due to the formation of free positive and negative charges. The differential spectrum shows that the free charges are represented by a broad band centred around 1000 nm, as expected for overlapping C60 cation and anion signatures, absorbing at 980 nm and 1080 nm, respectively according to reported photoinduced absorption data, electrochemical measurements and theoretical calculations.11,

42-44

In Figure 4 (C), the time evolution of this

differential free charge spectrum shows a relatively slow rise of the charge absorption, with time constants of 14 ps and 200 ps, from the dynamics at 1050 nm shown in the inset. This points to relatively slow CT dissociation in the presence of the applied field, suggesting that the CT states live longer under reverse bias.


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Figure 4. Transient absorption spectra of a neat C60 device, averaged 1000-1500 ps after photoexcitation at 610 nm (A) or 450 nm (B) without applied bias (green curve) or at 6 V reverse bias (red curve). The difference with and without bias was recorded by modulating the field and is shown as the black curve. The fluence used was 9.2 µJ/cm2 (610 nm) or 2.8 µJ/cm2 (450 nm), which corresponds approximately to a similar number of absorbed photons (without including cavity effects). The inset of (A) shows the extracted photogenerated charge as a function of reverse bias, from the integrated current transients measured from the device with an oscilloscope over a 50 Ω resistance, at the two excitation wavelengths (it was normalized by the fluence for better comparison). (C) depicts the difference spectra (measured with modulated field) at a reverse bias of 8 V recorded at different time delays after photoexcitation at 450 nm with a fluence of 75 µJ/cm2 (to increase the signal-to-noise, very similar results were obtained at


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a lower fluence (SI fig. 9)). The inset in (C) shows the corresponding dynamics probed at 1050 nm. (D) displays the spectrum of the charges recorded in the neat C60 device (difference with and without bias), and recorded in thin films of C60 containing electron donor TAPC at two concentrations, 5% and 50%, where mainly the C60 anion signature peaking at 1070 nm is observed (excitation wavelength of 450 nm). We notice that the charge absorption of C60 in Figure 4(C) is very broad and featureless, which is in contrast to much sharper ion bands reported for fullerenes in literature.11, 42, 45-46 The spectral broadness can on the one hand be attributed to the overlap of the C60 anion and cation bands, but it is also related to the delocalization of the charges in C60 aggregates. To confirm this, we compare the spectral signature to the photoinduced anion band obtained when the C60 film is blended with the prototypical small molecule donor TAPC. This blend is known to yield charges in the absence of an electric field,1 and it has been shown that a small amount of TAPC (5% molar) can be accommodated in the C60 film without significantly disrupting its crystallinity.47-49 At high loading of TAPC (50% molar), the intermolecular interactions of C60 are however suppressed, as shown by the steady-state absorption spectra in SI fig. 10. Following photoexcitation of C60 at 450 nm, charges in the 5% blend are formed on the 10 ps time scale (due to exciton diffusion to TAPC through the C60 aggregates), while they are generated promptly in the 50% blend thanks to intimate mixing of the donor and acceptor (see dynamics of TAPC+ at 700 nm in SI fig. 11). We focus here on the C60- anion absorption, since the holes in the blend reside on TAPC. The shape of the C60 anion band around 1070 nm at 300 ps for the 5% and 50% TAPC blends is shown in Figure 4 (D), together with the charge spectrum obtained by inducing CT exciton splitting in the C60 device under reverse bias. We observe an important broadening of the anion band, starting from the 50% blend with TAPC, followed by the 5%


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blend and finally the neat C60. We relate this to the spatial extent of the electron, which is localized on a single C60 in the 50% blend, but can delocalize in C60 aggregates in the 5% blend and even more in C60. Intermolecular delocalization leads to narrow band formation, which increases the number of available end states and homogeneously broadens transition for charges and excitons.50 This effect has been observed a number of times in organic semiconductors,50-54 and should not be confused with inhomogeneous broadening due to disorder.53 We note that electron delocalization into fullerene aggregates plays an important role in the functioning of organic solar cells,32, 55-56 highlighting the importance of evaluating this parameter via the shape of the C60 anion band. In summary, we have elucidated the dynamics of Frenkel and intermolecular CT excitons in C60 films (summarized in Scheme 1) from high quality transient absorption measurements, performed at low fluences and free from oxygen or pump-induced photo-dimerisation. We show from preferential excitation of either species, that the CT excitons give rise to a strong electroabsorption signal, but are extremely short-lived (recombination in 0.18 ps). They can dissociate to free charges in a full device under reverse bias, which we monitored by transient absorption measurements with and without applied external electric field. This is the first time to our knowledge that field-assisted CT state dissociation is directly spectroscopically observed. We demonstrate that the C60 cluster size determines the spectral signature of the C60 anion band, which becomes sharper in more localized systems. The relaxation and signature of the C60 Frenkel excitons also depends on the C60 aggregation. For isolated C60 molecules/small clusters dispersed in an insulator or for PCBM (where intermolecular interactions are weakened by the functional tails), the Frenkel lifetime increases to >500 ps compared to 150 ps in neat C60. Thus, we attribute the fast excited-state decay in neat C60 films to the presence of low-lying Frenkel


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states at grain boundaries, where non-radiative recombination efficiently competes with intersystem crossing, explaining the low triplet yield.

Scheme 1 shows the Jablonski diagram summarizing the relaxation pathways in neat C60 film (left side) and in the NBPhen:C60 50:50 blend (right side). Electronic states are depicted as solid lines, vibrational states as dashed lines, radiative transitions as straight arrows and non-radiative transitions as wavy arrows. The more important deactivation pathways of the excited states are shown as thicker arrows. S1(GB) refers to Frenkel excitons at grain boundaries.


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ASSOCIATED CONTENT Supporting

Information.

The

following

file

is

available

free

of

charge:

The experimental techniques as well as the sample preparation and data-analysis are described in the Supporting Information together with additional data, as mentioned in the main text. ACKNOWLEDGMENT NB and MC acknowledge the Swiss National Science Foundation (grant PP00P2_150536), the University of Bern and the University of Fribourg for financial support. IR gratefully acknowledges a Doctoral Training grant award from EPSRC and the COST Action StableNextSol (MP1307) for a Short Term Scientific Mission (STSM), as well as Dr. Koen Vandewal, Dr. Donato Spoltore and the Lesker team at the TU Dresden, Germany, where the EQE samples were made and characterized during IR's STSM to Dresden. MR acknowledges funding from EPSRC (grant EP/L026066/1) and from the European Union in FP 7 (grant 630864).

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Riede, M.; Salleo, A. Increased open‐circuit voltage of organic solar cells by reduced donor‐ acceptor interface area. Adv. Mater. 2014, 26, 3839-3843.


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