SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION DOI: /NMAT3502 Hot Exciton Dissociation in Polymer Solar Cells G. Grancini 1, M. Maiuri 2, D. Fazzi 1, A. Petrozza 1, H-J. Egelhaaf 3, D. Brida 2, G. Cerullo 2 and G. Lanzani 1,2* 1 Center for Nano Science and Technology@Polimi, Istituto Italiano di Tecnologia, via Pascoli 70/ Milano, Italy. 2 IFN-CNR Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci, 32, Milano, Italy. 3 Konarka Technologies GmbH, Landgrabenstrasse 94, Nürnberg, Germany * guglielmo.lanzani@iit.it S1. Morphological Characterization Figure S1 shows the Atomic Force Microscopy images of the PCPDTBT:PC 60 BM (1:4) blend. The image clearly shows that phase separation occurs on a very fine length scale of few nm. The morphological characterization shows that the PCPDTBT and the PC 60 BM are intimately mixed in a very finely dispersed morphology, leading to a high interfacial area, in agreement with [1, 2]. NATURE MATERIALS 1

2 Figure S1 Atomic Force Microscopy map (4x4 μm 2 ) of PCPDTBT/PC 60 BM blends (1:4) Scale bar, 1μm. S2. Ultrafast spectroscopy on PCPDTBT: PC 60 BM blend processed with diiodooctane additives Our system can be considered the ideal case to observe interfacial processes, as exciton diffusion does not play a role. For sake of consistency, we show here the ultra-fast TA dynamics upon gap excitation in the PCPDTBT: PC 60 BM blend processed with diiodooctane additives, which induce a larger phase separation with domains sizes back to the nm scale [3]. In this case SE from the polymer exciton dominates the early dynamics; the SE is then quenched in 2 ps timescale upon exciton diffusion. This result strongly supports our hypothesis of a finely dispersed morphology in the blend without additives, with the dynamics dominated by interfacial charge transfer.

3 Figure S2 Dependence of Ultrafast Charge dynamics on blend PCPDTBT/PC 60 BM (1:4) processed with additive. a. ΔT/T map and b. dynamics at selected probe wavelength upon S 1 excitation. The rate equation model extracts a time constant for exciton dissociation of around 1.2 ps. S3. Pump-probe measurements in the visible spectral window Figure S3 shows the pump-probe map and selected dynamics at specific probe wavelength on the PCPDTBT/PC 60 BM blend, upon 510 nm excitation probed in the VIS spectral region [500 nm nm]. The results clearly show a positive signal (red in figure S3a) due to PB of the PCPDTBT that appears instantaneously upon photoexcitation. Figure S3 Transient absorption signal on PCPDTBT/PC 60 BM (1:4) in the VIS region. a. 2D map and b. dynamics at selected probe wavelength upon Sn excitation. At 650 nm the positive signal represents the PB of the PCPDTBT; while the 520 nm probe a negative signal related to high energetic charge transfer state formation.

4 At shorter wavelengths we detect a negative band (light blue in the map) that shows a rapid formation followed by a plateau in the first 400 fs (see Figure S3 b). This signal has been assigned, according also to QC calculations, to the CT band formed in the VIS region [3]. At the very initial time scale the PCPDTBT PB signal, probed at 650 nm, is instantaneously formed within 15 fs and does not increase, thus ruling out the possibility of an ultrafast hole transfer from the PC 60 BM. Moreover, varying the PC 60 BM content (see Figure S4) in the blend no differences in the PB dynamics are detected, thus strongly supporting the conclusion that the direct PC 60 BM excitation plays a negligible role in the very initial time window explored. Hole transfer form PC 60 BM can happen, but on a much longer timescale within tens of ps [2]. Figure S4 CTS and PB dynamics on different blend concentrations: PCPDTBT:PC 60BM (1:4) and (1:1) ΔT/T dynamics upon 510 nm excitation, monitoring the PB PCPDTBT (green dots) at 650nm probe wavelength and the CT PA at 520 nm probe wavelength (blue squares). S4. Measurements at different pump fluences The excitation densities have been deliberately kept at the very low level of about 5 μj/cm 2 in order to avoid any non linear higher order processes such as exciton-exciton annihilation, together with thermal effects or possible sample degradation. For the measurements shown in the main text we kept the pump intensity well below the onset of these processes. Those effects can be observed at higher fluencies, giving typically accelerated exciton dynamics. Figure S5 show the intensity-dependent measurements

5 upon 700 nm excitation, where the optical absorption of the polymer is maximum, for four different pump fluences from 5μJ/cm 2 (the one we used for all the experiments) to 200 μj/cm 2. Figure S5 a-d) show the ΔT/T(λ,τ) maps for the pristine PCPDTBT upon S 1 excitation. Figure S5 e-f) monitor the dependence of the PB dynamics on the pump fluence. Note that the pump-probe signal is linear with the fluence up to 20 μj/cm 2, and high order process start at higher pump Intensity (as evident by exciting with 200 μj/cm 2 ) where the PB decays become much quicker and the typical accelerated bimolecular dynamics are observed. To avoid any high order effect and keep all the experiments in a perturbative regime for all pump wavelengths, all our measurements have been performed with 5μJ/cm 2 fluence. Figure S5 Transient absorption signal on PCPDTBT:PC 60BM (1:4) in the VIS region. a-d. dynamics at 840nm selected probe wavelength upon S n excitation for four different pump fluence, as indicated in the map. e,f ΔT/T(τ) dynamics monitoring the PB of the PCPDTBT for the different fluence. f. Normalized dynamics depicted in e. The arrow indicates the increasing of the pump fluence.

6 Note that our measurements monitoring the build-up of both interfacial CT states and polarons clearly demonstrate that the dynamics are not affected by any high order process that might instead occur at much higher pump fluence. S6. Pristine PCPDTBT dynamics probed in the NIR spectral range (1000 nm-1600 nm) Figure S6 shows the comparison of the ΔT/T(λ) dynamics at λ probe= 870 nm, 930 nm and 1600 nm upon band gap excitation (pump wavelength=710 nm). At shorter wavelength, as shown in Figure 2a of the manuscript, the positive signal is assigned to the build-up of the SE [4]. On the other hand, in the IR region, the PCPDTBT shows a well defined negative band peaked at 1600 nm (see Figure S6b), assigned to PA from singlet states [2, 5], matching the SE dynamics. a b Figure S6 a. ΔT/T(λ) dynamics at λ probe= 870 nm, 930nm and 1600nm; b. ΔT/T(τ) spectrum in the IR probe region at τ= 30 fs pump probe delay, upon gap excitation (pump wavelength=710 nm). S7. ΔT/T(τ) spectra and ΔT/T(λ) dynamics of the PCPDTBT:PC 60 BM blend Figure S7 summarizes the photophysical scenario presented, combining the pump probe spectra obtained for the three different excitation energies in the [ nm] probe spectral region.

7 Figure S7 ΔT/T(τ) spectral evolution for pump probe delays from 30 fs to 1 ps, as indicated by the legend, in the [840 nm- 1600nm] probe region, upon a. gap excitation (pump wavelength=710 nm); b. S 2 excitation (pump wavelength=640 nm) and c. S n excitation (pump wavelength=510 nm). In Figure S7 three main spectral features are detected: i) the SE band in the spectral range nm that is rapidly quenched by the formation of the relaxed first interfacial excited state (CT 1 ) PA; ii) the PA peaked at 1250 nm assigned to free polaron absorption, that grows in the first ps and iii) the PA band peaking at 1600 nm assigned to singlet PA, that shows a decay that matches both CT 1 and polaron PA rise, with a time constant of 45 fs that mirrors their formation (see dynamics in Figure 4a of the manuscript). Figure S7b shows the pump probe spectra upon S 2 excitation. It reveals the presence of i) the SE band [4] that is quenched by the formation of a well defined negative signal peaked at 960 nm, which is assigned to PA of more energetic CT 2 states; ii) the free polarons PA band at 1250 nm and iii) the singlet PA band at 1600 nm. Again the singlet decay matches with the CT 2 and polarons formation with a time constant of 38 fs. Finally, Figure S7 c shows the pump-probe spectrum upon high energy photoexcitation. The scenario depicted is now different: no singlet PA is detected at longer wavelength side of the spectrum, but a broad PA band is instantaneously formed peaking around 1300 nm with a shoulder a 960 nm. The latter is assigned to the hot CT band. At shorter wavelengths (around 860 nm) note that a third PA band appears. It has a delayed formation and a decay with a time constant matching the slow rise of the main polaronic band. We suggest that it belongs to energetic CT states (note that the DOS calculated is

8 quite broad) that while relaxation in the manifold, but before thermalization in CT 1, provide a further reservoir for charge generation. S8. ΔT/T(τ) dynamics at 1600nm: further analysis Note that the signal PA at 1600 nm, assigned to the singlet band is not completely decaying in the entire temporal window under investigation that is actually overlapping with the tail of the PA of polarons, visible both for S 1 and S 2 excitation. To disentangle these two contributions we retrieved the pure singlet dynamics by subtracting from the dynamics at 1550 nm the dynamics of polarons, after normalizing them at 1 ps. Figures S8a and S8b show the resulted dynamics as black open dots for S 1 and S 2 excitation, respectively. Note indeed that upon disentangling the two contributions the pure singlet decay dynamics is retrieved.

9 Figure S8 ΔT/T(λ) dynamics at probe wavelength indicated in the legend, upon a. gap excitation (pump wavelength=710 nm); b. S 2 excitation (pump wavelength=640 nm). S9. Spectral Response Measurement Figure S9 shows the comparison between the measured External Quantum Efficiency (EQE) on the PCPDTBT:PC 60 BM device and the film absorption. Figure S9 Spectra comparison between the external quantum efficiency (black line) and the blend absorption (red line). The dashed lines highlighted the three different excitation energy used to monitor the blend photophysics. The external quantum efficiency (EQE) is the fraction of incident photons that yield collected charges at short circuit. The spectral response is known as the action spectrum. The EQE may be broken up into three further figures of merit, to better identify the different limiting processes in an organic solar cell:

10 EQE(λ)=η abs(λ)x η sep (λ)x η coll(λ) where η abs is the light absorption efficiency: the fraction of incident photons that are absorbed to produce excitons; η sep is the fraction of photogenerated charges from exciton splitting and η coll is the carrier collection efficiency: the fraction of electron-hole pairs generated at the heterojunction that are collected at opposite electrodes to yield photocurrent. The enhancement is attributed to the faster and more efficient interfacial charge separation process for high-energy photons. Now, if we normalize the EQE(λ) by ηabs(λ) we come out with the Internal Quantum Efficiency (IQE (λ)) represented in Figure 5b, that includes the fraction of photogenerated charges from exciton splitting and the carrier collection efficiency. Since the carrier collection does not have a spectral dependence the IQE reflects the fraction of electron-hole pairs generated at the heterojunction that yield to photocurrent. Figure S9 clearly shows that the EQE(λ) has a higher contribution at shorter wavelength side of the spectrum, leading to an enhancement in the photocurrent. In particular, note that the IQE values have been derived by normalizing the measured EQE spectrum to the device absorption, thus including the PC 60 BM contribution. This enhancement is therefore attributed to the faster and more efficient interfacial charge separation process for high-energy photons thanks to hot dissociation mechanism. Note that micro-cavity effects inducing light interference in the device absorption has been considered, but they give a negligible contribution in the IQE shape. S10. Quantum chemical calculations S10.1 Excited States calculation for PCPDTBT, PC 60 BM and PCPDTBT/PC 60 BM interface. The quantum chemical investigation of the PCPDTBT/PC 60 BM blend has been carried out in two steps. At first we considered the polymer and the fullerene derivatives as non

11 interacting systems, thus calculating both ground and excited state properties of isolated molecules; secondly, once the single molecule properties have been rationalized, we considered different dimer structures made by a polymer fragment (an oligomer of length n, n=2-5) interacting with the PC 60 BM. For each dimers we calculated the excited states and the transition density matrices in order to identify and assigning the interfacial excited state character. PCPDTBT has been modelled by using an oligomeric approach thus considering n oligomer units, with n ranging from 1 to 5, as previously reported in (see [9]). We performed for both PCPDTBT and PC 60 BM, a semiempirical investigation, by using the AM1 method for the geometries optimizations and the ZINDO/S for the excited state calculations, and a density functional theory study by using different DFT XC functionals such as the range separated CAM-B3LYP, the hybrid B3LYP and meta GGA M06-2X functional [6, 7], with 6-31G* and 6-311G** basis sets. Different dimers have been built by considering various PCPDTBT/PC 60 BM intermolecular orientations and relative positions (see below). All dimers are made by the n=4 oligomer of PCPDTBT, namely CPDTBT 4 and the PC 60 BM, both geometries previously fully DFT optimized. The following three cases have been considered for the dimer study: i) a PC 60 BM hexagon-ring face set upon to the cyclopenta-dithiophene (CPDT) moiety of PCPDTBT; ii) iii) a PC 60 BM pentagon-ring face on the CPDT moiety; a PC 60 BM carbon-carbon bond on CPDT. For each case (i-iii) we varied the intermolecular distance d, from 2.7 to 4.1 Å by steps of 0.2 Å, and the lateral position (along the polymer chain, from CPDT to BT unit) to map the effect of different relative positions between PC 60 BM and PCPDTBT on the interfacial excited states. We found that effective interactions take place when a PC 60 BM hexagonring face has been set upon to the cyclopenta-dithiophene (CPDT) moiety of PCPDTBT.

12 Here below we report the CPDTBT 4 /PC 60 BM structure as optimized at the DFT (CAM- B3LYP/6-31G*) level. Atom type x/y/z coordinate C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C

13 C C C C C C C C C C C C C C O C H H H O H H H H H H C C C C C C H H H H H S N C C N C C C C H H C C C C S C C C C C

14 S C C H H H H H H H H H C C C C S C C C C C S C C H H H H H H C C C C C C H H N N S C C C C S C C C C C S C C H H H

15 H H H C C C C C C H H N N S C C C C S C C C C C S C C C C C C H H H N N S C C H H H H H H H H H H H H

16 ZINDO/S excited states calculations showing the above described effects are reported in Figures S10-S12. CPCDTBT 4 Figure S10 ZINDO/S interfacial excited states (green, blue, purple, cyan, yellow, black bars) and single molecule excited states (CPDTBT 4 red - and PC 60BM orange ) calculated on CAM-B3LYP/6-31G** optimized geometries by considering different intermolecular distances (as reported in brackets, Å). In the model dimer a PC 60BM hexagon-ring face has been set upon to the cyclopenta-dithiophene (CPDT) moiety of PCPDTBT.

17 CPCDTBT 4 Figure S11 ZINDO/S interfacial excited states (green, blue, purple, cyan bars) and single molecule excited states (CPDTBT 4 red and PC 60BM black ) calculated on CAM-B3LYP/6-31G** optimized geometries by considering different intermolecular distances (as reported in brakets, Å). In the model dimer a PC 60BM pentagon-ring face has been set upon to the cyclopenta-dithiophene (CPDT) moiety of PCPDTBT.

18 <1.0> <2.0> <3.0> <4.0> <5.0> <6.0> Figure S12 ZINDO/S interfacial excited states calculated on CAM-B3LYP/6-31G** optimized geometries by considering different translations (as reported in brackets, Å) at a fixed intermolecular distance of 3.5 Å. In the model dimer a carboncarbon PC 60BM bond between two faces has been set upon to the cyclopenta-dithiophene (CPDT) moiety of PCPDTBT. To validate our approach, we performed also a full DFT geometry optimization of a PCPDTBT/PC 60 BM dimer (with n=2 as PCPDTBT oligomer), namely CPDTBT 2 /PC 60 BM, by using the M06-2X functional with the 6-311G** basis set. On the DFT optimized dimer structure we calculated the excited states by using both TD-CAMB3LYP (the same method used before) and the TD-M06-2X/6-311G** (see Figure S13).

19 Figure S13 TDDFT excited states computed for n=2 oligomer (red), DFT optimized molecular model dimer (green and blue) and PC 60BM (black): TD-CAM-B3LYP/6-311G** excited states calculated for M06-2X/6-311G** optimized CPDTBT 2 - red lines, TD-M06-2X/6-311G** interfacial excited states for the fully optimized dimer - green lines -, TD- CAM-B3LYP/6-311G** interfacial excited states for the fully optimized dimer - blue lines and TD-M06-2X/6-311G** excited states for PC 60BM molecule black lines. The stabilization interaction energy, as evaluated within the basis set superposition error (BSSE), between CPDTBT 2 and PC 60BM has been evaluated of 10 kcal/mol. Here below we report the optimized M06-2X/6-311G** structure of CPDTBT 2 /PC 60 BM dimer: Atomic number / x / y / z atom coordinate

20

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22 Following the recent findings of Troisi et al. [see ref 8, 26 of the manuscript], we also fully optimized the first triplet state (T 1 ) of PCPDTBT/PC 60 BM at the UM06-2X/6-311G** level. Troisi demonstrated in fact, that the geometry, energy and configuration of the first triplet dimer state well match with the energy and geometry of the low lying charge transfer (CT) state. From the fully UDFT optimization we found a T 1 energy of 1.6 ev with respect to the dimer ground state that is lower than the vertical TDDFT energy (i.e. 2.2 ev); this confirm the same trend found by Troisi for another system as P3HT/PC 60 BM. Here below we report the Cartesian coordinates of the optimized structure of the triplet state T 1 for CPDTBT 2 /PC 60 BM at UM06-2X/6-311G** Atomic number / x / y / z Cartesian coordinate

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26 Parallel to the semiempirical level (ZINDO/S) calculations we computed the interfacial excited states (as described already above) at the TDDFT level. On the same dimer structure reported in Figure 3c of the manuscript, we have evaluated the single molecule (polymer) and dimer excited states, by calculating also the density of excited states (DOS) at the interface (molecular dimer). Here below we report TD-CAM-B3LYP/6-31G** calculations on CPDTBT 4 and CPDTBT 4 /PC 60 BM. In particular we emphasise that, despite the absolute energy values are different, for the different level of theory, the interfacial excited state picture is the same as found at the semiempirical level. The higher is the polymer excited state, the higher is the density of interfacial excited states which are resonant with it and the first low lying interfacial excited states is lower than S 1 of the polymer and it features an oscillator strength being dipole active. ** Figure S14: TD-CAM-B3LYP/6-31G** excited states for CPDTBT 4 (red lines), CPDTBT 4/PC 60BM (green lines) and PC 60BM (blue lines). TDDFT DOS of the interfacial excited states. Figure S14 is fully comparable to Figure 3a-b reported in the manuscript. Changing the level of theory, from ZINDO/S to TDDFT, energies and DOS change too but the main results in terms of interfacial excited state character (see below) and energy landscape, remain the same.

27 S10.2 Excited to excited state transitions for PCPDTBT Figure S15 reports the calculated semiempirical ZINDO/S excited-to-excited state transitions for PCPDTBT (i.e. oligomer CPDTBT 4 ) in order to support the PA band presented in the transient absorption spectra of Figure 4 of the manuscript. In particular are reported the S 2 S n transition with S 2 the second excited state of the polymer and S n a general excited state higher in energy than S 2. The onset of the energy has been set to the energy of S 2. Figure S15: ZINDO/S excited-to-excited state transitions calculated on the optimized CAM-B3LYP/6-31G** CPDTBT 4 oligomer, for S 2 S n singlet excited state transitions. The energy on set has been set to the S 2 energy. In Figure S16 are reported the S 4 S n transition with S 4 the fourth excited state of the polymer and S n a general excited state higher in energy than S 4. The onset of the energy has been set to the energy of S 4.

28 Figure S16: ZINDO/S excited state transitions calculated on the optimized CAM-B3LYP/6-31G** CPDTBT 4 oligomer for S 4 S n singlet excited state transitions. IES n Figure S17: ZINDO/S excited state transitions calculated on the optimized CAM-B3LYP/6-31G** CPDTBT 4/PC 60BM for interfacial excited state transitions.

29 In Figure S17 are reported the IES i IES j calculated for the molecular dimer representative of the polymer/pc 60 BM interface. The computed transitions can be used to support the PA band assignment in the pump-probe spectra reported in Figure 2 and 4 of the manuscript. S10.3 Calculation of charged species properties for both PCPDTBT and PC 60 BM To further validate the experimental findings we calculated at the (U)DFT level (CAM- B3LYP/6-311G**) the charged molecular species, namely the cationic PCPDTBT species and the anionic PC 60 BM one, in order to compute i) their optimized geometries, ii) the ionization potential (IP) and electron affinity (EA) of the donor and acceptor species respectively. Extra hole on the electronic structure of PCPDTBT and extra electron on PC 60 BM We considered the oligomer CPDTBT 4 and we optimized the geometry at UCAM- B3LYP/6-311G** level. Here below we report the optimized structure: Atomic number / x / y / z Cartesian coordinate

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31 The computed Adiabatic Ionization Potential (AIP) is (CAM-BLYP/6-311G**) AIP (E(+1) E(0)) = ev With E(+1/0) total energy of the charged/neutral species.

32 In parallel we computed at the UCAM-B3LYP/6-311G** level the equilibrium structure of PC 60 BM with an extra electron. Here below the optimized structure: Atomic number / x / y / z Cartesian coordinate

33 The Adiabatic Electron Affinity (AEA) is: AEA (E(0) E(-1)) = ev

34 S11. Evaluation of the electronic coupling integrals V ij. The electronic couplings (V ij ) are reported in Table S1. They are the intermolecular charge transfer integrals defined as in the non-adiabatic transfer theory [8]. Table S1. Calculated ZINDO/S square of electronic couplings (V 2 ij ) between the singlet polymer excited states (S i ) and the nearly resonant interfacial excited states (IES j ). Coupled states V 2 ij (ev 2 ) S 1 IES S 2 IES S 2 IES S 4 IES S 4 IES S 4 IES V ij have been evaluated at the semiempirical ZINDO/S level as widely described in many papers and reviews [9, 10]. In this case we are interesting in the electron transfer from the polymer excited state S i to the interfacial excited state IES j, thus the matrix element to be evaluated is <S i H IES j >, with H ZINDO/S Hamiltonian of the system. In a first approximation, the excited state wavefunction can be reduced to the main single particle component thus expressing it in terms of molecular orbitals. In this case the above matrix element can be reduced to the coupling between the unoccupied molecular orbitals mainly describing the excited state S i and IES j. In these terms V ij can be roughly approximated as the coupling between the unoccupied molecular orbitals of the polymer and the molecular dimer, and the numerical expression used for their evaluation has been widely reported in literature [11] and used in the past by the authors in different studies regarding charge transfer processes.

35 For example, if we are interested in the evaluation of the electronic coupling between the polymer S 2 excited state and the molecular dimer IES 3 excited state, the following steps have to be carried out: -Analysis of the excited state character in terms of molecular orbital. In our specific case S 2 can be described by the HOMO LUMO+1 polymer transition, while IES 3 by the HOMO-1 LUMO+2 dimer transition. In particular the LUMO+2 dimer wavefunction is localized on the PC 60 BM molecule. In such a way S 2 can be roughly approximated in a single particle orbital picture as the LUMO+1 polymer wavefunction and IES 2 as the LUMO+2 PC 60 BM wavefunction. -Evaluation of V ij. Once characterized the excited states in terms of molecular orbital transitions, the matrix element <S 2 H IES 3 > can be evaluated by calculating the ZINDO/S Hamiltonian, H, of the whole dimer and then by projecting the molecular orbital wavefunctions on it such as <LUMO+1 a H ab LUMO+2 b >, with a and b PCPDTBT and PC 60 BM respectively. The computed V ij have been transformed in an orthogonalized basis as described in previous studies [12]. The electronic couplings above described represent an approximation, however in this study we are interesting in computing a trend rather than evaluating the absolute value of the transfer rate (k ij ). Regarding the last point, further approximations and limitations should be considered (for instance the evaluation of the external reorganization energy, the calculation of the free enthalpy, and others) resulting in a non completely reliable prediction. For these reasons we evaluated only the electronic coupling and the density of excited states at the interface, to describe from a qualitative point of view the experimental observed trends and to rationalize the observed mechanisms. S12. Calculations of transition density matrix at ZINDO/S level of theory

36 To correctly assign a character to the interfacial excited states computed for the molecular dimer (i.e. molecular interface) we computed the transition density matrix (TDM) for both the low lying IES and high energy IES. TDM, deeply described in many papers and reviews [13], in our specific case can be described as block matrices; in particular, diagonal blocks refer to the electronic density variation upon excitation of the single molecule component (PCPDTBT or PC 60 BM), while out of diagonal blocks represent the polymer:pc 60 BM interaction. Here below a sketch of a TDM: TDMs have been computed for CPDTBT 4 /PC 60 BM dimer in order to study the character of the interfacial excited states and to assigned an exiton-like ( XC>) or charge transfer-like ( CT>) character. A pure exciton-like IES should present only diagonal blocks active. A pure CT-like IES should present only out of diagonal block active, while a mixed state (as usually occur) present both diagonal and out-of-diagonal blocks active. In Figure S18 we report the calculated ZINDO/S TDM for the first low lying interfacial excited states (IES 1,2,3 ) and for the high lying (hot) IESn (IES 12,13,14 ) states. From the analysis of IES 1,2,3 TDMs, we observed that these interfacial excited states reveal a mixed exciton-like ( XC>) character (diagonal blocks with non vanishing contributions) and charge transfer-like ( CT>) character with out of diagonal contribution. In particular for IES 1,2,3 the XC character prevails on the CT one. Another point supporting this attribution,

37 better specified in the next section, is that these IES has a non vanishing oscillator strength thus further validating the XC character. On the opposite high lying excited states as IES 12,13,14 reveals a completely different TDM with non vanishing out of diagonal blocks (CT character) and a strong contribution of the PC 60 BM one. Parallel to what above reported, these states has vanishing oscillator strengths thus further supporting the CT assignment. In this frame, as reported in the manuscript, we infer this peculiar excited state electronic structure to an interfacial excited state more delocalized to PC 60 BM moiety with a huge propensity to evolve toward charge separated states and then free charges. Figure S18: ZINDO/S transition density matrix as evaluated on CAM-B3LYP/6-31G** PCPDTBT:PC 60 BM dimer geometry. To complete our assignment to IES we report in table S2 the ZINDO/S IES for the dimer reported in the manuscript by reporting the energy, the main molecular orbital (MO) contribution and the oscillator strength. Table S2: ZINDO/S calculated interfacial excited state picture for the PCPDTBT 4:PC 60BM dimer considered in the manuscript.

38 Interfacial Energy (ev) Oscillator MO Character of IES: Excited State strength (a.u.) IES> = c XC XC> IES> + c CT CT> H L XC H-1 L XC H-1 L+1 XC H-2 L+1 H-2 L H-6 L+5 CT H-4 L+6 CT H-6 L+3 H-6 L H-5 L+6 CT H-5 L+4 H-5 L+5 On the basis of the analysis of the transition density matrix and oscillator strengths we assigned the character of the interfacial excited states as XC (exciton like) or CT (charge transfer like). From Table S2 we can observe that the low lying interfacial excited states have a more XC character (thus featuring not vanishing oscillator strength) while high lying (hot) excited states have a pronounced CT character. To conclude IES 1 features oscillator strength and has an exciton like character and, according to experimental data, this state could acts as a trap for charges resulting in not a net charge separation. High lying excited states, IES 14 for instance, as those in resonance