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Pyrene−Fullerene Interaction and Its Effect on the Behavior of Photovoltaic Blends Claudio Carati,† Nicola Gasparini,‡ Sara Righi,§ Francesca Tinti,‡ Valeria Fattori,‡ Alberto Savoini,∥ Alessandra Cominetti,∥ Riccardo Po,∥,‡ Lucia Bonoldi,*,† and Nadia Camaioni*,‡ †

Downstream R&D, Eni S.p.A., via F. Maritano 26, I-20097 San Donato Milanese, Italy Istituto per la Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche, via P. Gobetti 101, I-40129 Bologna, Italy § Laboratorio di Micro e Submicro Tecnologie Abilitanti per l’Emilia Romagna (MIST E-R S.C.R.L.), via P. Gobetti 101, I-40129 Bologna, Italy ∥ Istituto Donegani, Renewable Energies & Environmental R&D, Eni S.p.A., via G. Fauser 4, I-28100 Novara, Italy ‡

S Supporting Information *

ABSTRACT: Improving the dispersion properties of unfunctionalized fullerenes in photovoltaic blends is a key factor for replacing the commonly employed butyric acid methyl ester derivatives (PBCM and PC71BM), with economical and light harvesting advantages. We consider here the effects of a pyrene derivative (PyBB) as dispersant of neat fullerenes (C60 or C70) in photovoltaic blends prepared with a low energy gap conjugated polymer (PTB7) as electron donor. The morphological and spectroscopic properties in the presence and absence of PyBB were evaluated by AFM, emission fluorescence, and light induced EPR, and the deep trap density of states was calculated in the two cases. The electrical properties of the devices prepared with or without PyBB were investigated and compared. The intimate interaction of fullerene/PyBB aromatic species revealed by the spectroscopic analysis is in agreement with the enhanced dispersion of the three-component blends. At the same time the deep trap density of states is varied, and the mobility of negative charge carriers is reduced in films prepared with PyBB dispersant, frustrating the beneficial effect of PyBB on the blend morphology.

1. INTRODUCTION Allotropes of carbon (fullerenes, graphene, carbon nanotubes) have gained an increasing attention over the past years for their extraordinary mechanical and electronic proprieties.1,2 These unique properties translate into potential applications such as electronic devices, including light-emitting diodes, and photovoltaics. However, one of the main drawbacks of these materials is their poor solubility in the common solvents used in the aforementioned applications. Several attempts have been carried out in recent years, including synthetic and supramolecular approaches,3 to overcome this problem. Recently the use of dispersing agents such as pyrene derivatives has gained attention due to the well-known noncovalent interactions with carbon nanotubes1,4,5 so that they are used to favor nanotube dispersion in different solvents. Noncovalent functionalization of graphene6,7 or fullerenes3,8 with pyrene derivatives has seldom been addressed in the literature but nevertheless demonstrated in some cases6 to be an effective technique to obtain stable dispersions. The same approach can be followed for the preparation of organic photovoltaic donor:acceptor blends, allowing the use of cheaper unfunctionalized fullerenes instead of the more © XXXX American Chemical Society

expensive [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). In addition to the lower cost, unfunctionalized fullerenes have the advantage of a better light harvesting capability, compared to PCBM or PC71BM.9 Recently, Cominetti et al.10 have demonstrated that the use of a pyrene derivative (1pyrenebutyric acid butyl ester, PyBB, Figure 1) is effective in preventing the aggregation of fullerene in photovoltaic blends made of regioregular poly(3-hexylthiophene) (P3HT), as electron donor, and unfunctionalized C60 or C70, as electron acceptors. In particular, it has been shown that the π−π interaction between pyrene and fullerene molecules promotes the dispersion of the acceptor in the polymer matrix. As a consequence, the resulting three-component P3HT:fullerene:PyBB blends showed a largely improved morphology with respect to those prepared without pyrene. The PyBB−fullerene interaction not only improves the blend mixing at the molecular level but can also favor electronic processes, as suggested by the Received: November 24, 2015 Revised: March 10, 2016

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DOI: 10.1021/acs.jpcc.5b11468 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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of 1 × 10−3 M solutions of PyBB (in C6H12), PyBB:C60, and PyBB:C70 (in C6H12/CHCl3 1:1 molar). The same amount of solution was poured and evaporated on quartz windows hollowed out at 1 mm depth in order to obtain films of approximately the same thickness. Light-Induced Electron Paramagnetic Resonance Spectroscopy. Blend films for EPR analysis were drop-casted in 3 mm i.d. EPR quartz tubes with rotaflow stopcocks, by slow evaporation at 1 mbar pressure of 1 × 10−3 w/w 1,2-odichlorobenzene solutions of PTB7:C70 1:2 w/w or PTB7:C60 1:3 w/w. The same solution was used to prepare the ternary blends by adding PyBB (PyBB:fullerene in 1:1 molar ratio). PTB7:C70 and PTB7:C60 solutions were stirred at 70 and 80 °C overnight, respectively. To prepare the ternary blends, a PTB7 and a PyBB:C70 solution were previously sonicated and stirred separately overnight at 40 and 70 °C, respectively; then the two solutions were mixed and stirred for more than 2 h at 40 °C before casting. PTB7:C60 and PTB7:PyBB:C60 solutions were sonicated and stirred overnight at 80 and 70 °C, respectively. The estimated film thickness after deposition was about 3 μm. The films were kept under dynamic vacuum (10−5 mbar) for 2 h before LEPR measurements, performed in the temperature range 20−80 K with a Bruker Elexys-500 continuous-wave X-band (9 GHz) spectrometer equipped with a helium flow cryostat (Oxford 900). Photoexcitation of the sample was provided by a SpectraPhysics 177-G01 Ar−Ne laser (488 and 514 nm wavelengths) tuned at 50 mW power. After smoothing, the spectra were simulated using the SimEPR32 software,16 and the g-values were determined with an error of ±5 × 10−4. The intensity of the signal, that is registered in derivative form, was obtained by double integration. The value of the oscillating magnetic field at the sample H1 (gauss) employed for the determination of the longitudinal relaxation times was estimated by the relationship H1 = Λ√PMW, where PMW is the incident microwave power expressed in watts and Λ is 2.2 G/√W in our experimental equipment (Bruker SHQ cavity). Density of States (DOS) Calculation. The LEPR measurements of recombination kinetics at different temperatures were used to estimate the deep trap density of states.15 Since noise in LEPR data may generate strong numerical fluctuations in the time derivative of the decay (used for DOS reconstruction15), digital filtering was applied to the normalized recombination data. In our case, filtering is a combination of simple exponential (ES) and smooth transition exponential smoothing (STES),17 designed to avoid data distortion at short acquisition times (details of the computational procedure and an example of application are provided as Supporting Information). The smoothing procedure does not affect the DOS shape, since the latter depends on a large number of experimental decay points taken at different temperatures. On the contrary, noise reduction, locally dependent on only a few adjacent data points, improves the determination of the DOS: on the one hand, data smoothing facilitates fitting the decays by a trial function, whose numerical time derivative does not suffer from fluctuations and is used in the DOS reconstruction; on the other hand, noise reduction helps identifying the (possibly Gaussian or exponential) DOS features. Appropriate experimental temperatures must be chosen in order to obtain a satisfactory signal-to-noise ratio, measure the recombination kinetics at sufficiently slow decay rates, and evidence the effect of PyBB addition to the blends. In this case

Figure 1. Chemical structures and energy levels of materials used in this study.

photoinduced electron transfer in C60-pyrene films11 or by the energy transfer in pyrene appended C60 and C70 derivatives.12 In this work, we investigate the interactions between PyBB and unfunctionalized C60 or C70 blended with poly({4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7, Figure 1) as electron donor. PTB7 is a low energy gap conjugated polymer, with outstanding electronic properties for photovoltaic applications. Similarly to the case of P3HT-based blends, a highly segregated morphology has been already reported for PTB7:C70 blends, with the formation of large domains of hundreds of nanometers, clearly detrimental for the performance of the related solar cells.13 The relevant energy levels of the materials used in this work are shown in Figure 1. A perfect ladder pattern of energy levels (with the frontier molecular orbitals of the third component intermediate between the corresponding levels of the donor and the acceptor) would undoubtedly be preferred to avoid possible effects due to energy misalignments; however, it has been shown that this is not always necessary. For instance, Chang et al.14 demonstrated that the addition of 4,4′,4″-tris(Ncarbazolyl)triphenylamine to the P3HT:PCBM system leads to an efficiency increase of 17% for the related solar cells, although the lowest unoccupied molecular orbital (LUMO) of the third component (−2.6 eV) is shallower than those of P3HT and PCBM. We were therefore encouraged to use PyBB as a dispersant in PTB7:neat fullerene blends. The consequences of PyBB addition were spectroscopically investigated by means of light-induced electron paramagnetic resonance (LEPR) and emission fluorescence spectroscopy. The density of trap states in two- and three-components blends was calculated according to a previously published method,15 and the electrical properties of the devices made of PTB7:C60 or PTB7:C70 blends prepared with or without PyBB were evaluated. Both the spectroscopic and the electrical investigation indicated that PyBB addition has important consequences on the morphology of the blends as well as on their charge transport properties.

2. EXPERIMENTAL SECTION Materials. PTB7 was purchased from Luminescence Technology Corp.; C60 and C70 were from Sigma-Aldrich. The materials were used as received. PyBB was prepared as reported elsewhere.10 Emission Spectroscopy. Fluorescence spectra were recorded at room temperature with a PerkinElmer LS-50B spectrofluorometer, in front face configuration for drop-casted films and at right angle configuration for solutions. Films of PyBB, PyBB:C60, and PyBB:C70 were obtained by evaporation B

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The Journal of Physical Chemistry C best results were obtained in the intervals 60−80 K for PTB7:C70 and 40−20 K for PTB7:C60. We verified that the recombination kinetics of negative and positive polarons were substantially coincident for each sample, which assures that no other species significantly contributed to the charge balance. Device Fabrication. Solar cells were fabricated onto patterned ITO-coated glass substrates (Kintec, sheet resistance of 20 Ω/□) that were first cleaned with detergent and water and then ultrasonicated in acetone and isopropyl alcohol for 15 min. The substrates were UV-ozone-treated before the spincoating (4000 rpm) of a 40 nm thick layer of poly(3,4ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS, Clevios P VP AI 4083). The ITO/PEDOT:PSS substrates were baked in an oven at 140 °C for 10 min, before the spin-coating of the blend in air. The blend solutions were prepared in 1,2dichlorobenzene (28−30 g L−1). For a better comparison, the active layers were prepared with a comparable thickness of around 100 nm, measured with a Tencor Alphastep 200 profilometer. The samples were transferred to an Ar-filled glovebox, where the device structure was completed with the thermal evaporation of a Ca/Al (20/80 nm) top electrode at the base pressure of 3 × 10−6 mbar. The device active area, defined by the shadow mask used for the cathode deposition, was 8 mm2. Hole-only and electron-only devices were prepared to measure charge carrier mobility by means of the spacecharge-limited-current technique. For hole-only devices ITO/ PEDOT:PSS and Au were the bottom and top contact materials, respectively, while Al and LiF/Al were used as bottom and top contacts for electron-only devices. Electrical Characterization. The electrical characterization of the devices was carried out in a glovebox at room temperature. The cells were illuminated by a solar simulator (SUN 2000 Abet Technologies, AM1.5G), and the light intensity was calibrated using a certified silicon solar cells. For light-intensity-dependent measurements, a set of quartz neutral filters was used to vary the incident light power. The current− voltage curves were taken with a Keithley 2400 source-measure unit. Impedance spectroscopy measurements were conducted using an Agilent 4294A impedance analyzer. The impedance measurements were done in |Z|−θ mode, with the frequency ranging between 40 Hz and 1 MHz and with an amplitude of the harmonic voltage modulation of 20 mV. A constant dc bias, equivalent to the open-circuit voltage of solar cells, was superimposed to the ac signal. The experimental data were analyzed with the EIS Spectrum Analyzer program.18 Atomic Force Microscopy. The surface morphology was examined by using tapping-mode atomic force microscopy (Veeco, Multimode IIID Microscope). The blends were deposited onto ITO/PEDOT:PSS substrates following the above-described procedure for the preparation of solar cells.

Figure 2. Fluorescence spectra of PyBB dilute solution (light blue), PyBB film (red, intensity reduced by a factor 1000), PyBB:C70 film (black), and PyBB:C60 film (gray). PyBB:fullerene blends were prepared in 1:1 molar ratio. Excitation wavelength = 327 nm.

vibronic structure (with peaks at 375, 396, and ≈417 nm) as in the diluted PyBB solution. These results indicate that both C60 and C70 are strongly interacting with PyBB and that are well dispersed at the molecular level in the investigated films. Moreover, the fluorescence signal of PyBB is strongly quenched in the presence of fullerene moieties. This effect is likely due to electron transfer processes between the pyrene derivative and fullerenes,11 although an effect of the varied morphology cannot be excluded. These findings suggest that the introduction of PyBB could improve the morphology of the highly segregated PTB7:C70 photovoltaic blends,13 similarly to what previously reported for three-component blends made of P3HT, PyBB, and C70.10 The expected reciprocal dispersion of C70 and PyBB in photovoltaic blends with PTB7 as electron donor was confirmed by the investigation of the surface morphology of PTB7:PyBB:C70 films using atomic force microscopy (AFM) in tapping mode. The three-component blends were prepared with a fixed (1:1) PyBB:C70 molar ratio and different PTB7:C70 donor-toacceptor (D/A) weight ratios. The AFM images (Figure 3) confirm that PyBB is a good dispersant for the blend components, preventing the strong aggregation of C 70 molecules in the PTB7 matrix. The surface of PTB7:PyBB:C70 films (Figure 3a−f) appears smooth and with no macroscopic phase separation on the investigated length scale, while the

3. RESULTS AND DISCUSSION A relevant interaction between fullerenes and PyBB was suggested by fluorescence emission results. The photoluminescence spectra of a cyclohexane dilute PyBB solution and of a film of pure PyBB are compared in Figure 2 with those of PyBB:C70 and PyBB:C60 films (λexc = 327 nm). The emission from the pure PyBB film (λmax emission = 487 nm) is only due to excimers19 originated from the interaction of closely lying molecules. In addition to the excimeric component, the fluorescence of PyBB (isolated) monomers is also observed in PyBB:C70 and PyBB:C60 films, with the characteristic

Figure 3. AFM images (2 μm × 1 μm) of PTB7-based blends, deposited onto ITO/PEDOT:PSS substrates. PyBB:C70 1:1 mol/mol. C

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The Journal of Physical Chemistry C PTB7:C70 blend presents a highly segregated morphology, with large domains whose size is of the order of 100 nm (Figure 3g,h). The smooth morphology of PTB7: PyBB:C70 blends is consistent with the low root-mean-square roughness (Rq), evaluated on a scan area of 2 μm × 1 μm. The Rq values for PTB7:PyBB:C70 films prepared with 1:1, 1:2, and 1:3 D/A weight ratio were 0.66, 0.63, and 2.66 nm, respectively, clearly lower than the value of 5.31 nm measured for PTB7:C70. Similar smooth morphologies were also observed for PTB7:PyBB:C60 blends, as shown in Figure S4. An additional confirmation of the improved mixing of PyBBcontaining blends was provided by the comparison of the photoluminescence spectra of PTB7:C70:PyBB and PTB7:C70 films. Figure S5 shows the absorption and emission spectra of the ternary and binary blends prepared in 1:1 PTB7:C70 weight ratio. Differently from the PyBB-containing blend, for which a complete quenching of PTB7 was obtained, a residual emission from the polymer was observed for the binary blend, indicating a higher degree of intermixing upon the addition of PyBB. Further insight into the effects of pyrene addition to PTB7based photovoltaic blends is provided by LEPR. The lightinduced spin density measured by LEPR in organic photovoltaic blends is due to oppositely charged polarons, generated by photoinduced electron transfer, that reside in charge trap states whose lifetime is significantly longer than 10 μs20 (i.e., the signal integration time at 100 kHz modulation in the detection system) at the fixed experimental temperature. The signal intensity depends on a balance between generation and recombination of carriers,21 so that under laser irradiation a stationary state population of long-lived species is present and the so-called “light-on” spectrum of all trapped polarons is recorded; the “dark” spectrum, measured in the absence of light excitation, is due to permanent defects22 or trapped carriers whose binding energy is higher than the thermal energy at the temperature of preparation of sample. The LEPR spectrum is defined as the difference between the light-on and the dark spectrum. In our LEPR measurements on blend films no electrodes are present; thus, the photogenerated charges simply diffuse due to thermally activated hopping until charge recombination occurs; after laser switch-off, recombination causes the time decay of the ESR “light-off” spectrum. An inversion procedure15 applied to the decay kinetics at different temperatures allows to recover (at least partially) the density of trap states (DOS) extending within the blend HOMO−LUMO gap. In Figure 4, the LEPR spectra of PTB7:C70 and PTB7:C60 films prepared with or without PyBB are shown, together with their simulations. The LEPR spectra coincide with the light-on spectra since the dark signal (not reported) is null for all cases, provided that ambient light does not reach the sample during cooling and measurement. As for other polymer:fullerene blends,23−25 two radical species generated by photoexcited electron transfer are identified for each sample: the (spin 1/2) positive polaron P+ on the polymer (gx = 2.0040, gy = 2.0033, gz = 2.0017) and the negative polaron on the fullerenes (for C70−: gx = 2.0017, gy = 2.0015, gz = 2.0006; for C60−: gx = gy = 1.9994, gz = 1.9978). Differences in the relative signal intensities of positive and negative charges depend on the different saturation properties of the signals under microwave irradiation, as commonly observed in many photovoltaic blends.20,23 No features of radical species involving PyBB were observed. In fact, the energy of the light used for the LEPR experiment (2.54/2.41 eV) is too low to photoexcite PyBB, whose energy

Figure 4. LEPR spectra of films of PTB7:fullerene (red) and PTB7:fullerene:PyBB (green). Thin lines represent the smoothed experimental spectra and thick lines the simulated spectra (simulation parameters are reported in the text). T = 70 K, power = 0.2 mW.

gap is 3.2 eV. However, the presence of PyBB modifies the LEPR signal intensity of fullerene anions, and the bandwidth of the perpendicular component is narrowed in the case of C60− (from 1.45 to 1 G). This suggests that the molecular interactions between fullerenes are modified in the threecomponent blends, possibly by adjacent PyBB molecules. In Figure 5, the peak-to-peak intensity of the LEPR is reported as a function of the square root of the incident

Figure 5. Peak-to-peak LEPR signal intensity of fullerene radical anions as a function of the square root of the microwave power for PB7:fullerene (red symbols) and PTB7:fullerene:PyBB (green symbols): (a) C70 fullerene; (b) C60 fullerene.

microwave power. In the absence of signal saturation a linear trend is expected, as shown at lower microwave power; at higher exciting power saturation sets in and the signal intensity passes through a maximum. Indeed, the signal saturation of both C60− and C70− occurs at lower microwave power in the presence of PyBB. A possible explanation of this behavior may be the hindrance of a reorientational degree of freedom, D

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The LEPR intensity (double integration of the derivative signal) measured at sufficiently low microwave power to exclude saturation effects (0.02 mW at 70 K) is also reported in Table 1 in arbitrary units. The intensity enhancement of the LEPR signal in PyBB-containing films can be due, at least partly, to a better reciprocal dispersion of the donor and the acceptor phase in the three-component blends, confirmed by the microscopy analysis and leading to a higher interfacial area favoring charge generation efficiency.29,30 However, such an increase can also be ascribed to a longer lifetime of trapped carriers; therefore, we studied the charge recombination kinetics to determine whether it is affected by PyBB addition. In Figure 6, the kinetics of EPR signal of the fullerene anions in the absence and presence of PyBB are reported, showing that the rates of trap filling (signal intensity buildup under illumination) and emptying (signal intensity decay after light switch-off) decreased, and a stronger persistent signal was detected at long times for PyBB-containing films. These observations suggest that charge trapping is enhanced in ternary blends, with a more pronounced effect in PTB7:PyBB:C70 films. Indeed, lower experimental temperatures (20−60 K against 60−90 K) were needed to observe the effect of PyBB addition on the kinetics of PTB7:PyBB:C60 samples, suggesting that the DOS is modified at shallower levels in comparison to C70-based films. The densities of trap states obtained from the calculations are shown in Figures 7 and 8, where the abscissa represents energy values relative to the transport level.29 The DOS of the binary blend PTB7:C70 (Figure 7, bottom) shows two well-defined, Gaussian-like features peaked at about 0.037 and 0.052 eV from the transport level. A possible feature is inferred at 0.021 eV, but the low number of data points in the initial part of the decay does not allow for a precise reconstruction. Also, the long tail extending to energies lower than 0.08 eV cannot be completely reconstructed, since it depends on the roughly constant persistent signal at long decay times, but a Gaussian peak may be estimated at 0.071 eV. Although only partially computed from the available LEPR data, the DOS of the ternary blend PTB7:PyBB:C70 (Figure 7, top) shows important differences with respect to the previous one. The major Gaussian-like peak is found at 0.047 eV, while a second feature is visible at 0.034 eV. The persistent LESR decay signal only allows for a partial reconstruction of a Gaussian

assuming that the magnetic relaxation processes are mobility driven.26 LEPR signals of C60− and C70− were satisfactorily simulated with Lorentzian line shape, so that the transverse (T2) and longitudinal (T1) magnetic relaxation times could be estimated by the classical analysis of amplitude saturation of homogeneously broadened lines:27 T2 =

2 3 γδ0

(1a)

1 2T2γ 2H12

(1b)

and

T1 =

where γ is the electron gyromagnetic ratio, δ0 is the intrinsic line width, which was obtained from the simulations following the method proposed by Konkin et al.,28 and H1 is the oscillating magnetic field at the sample at the maximum of the saturation curve. The computed values are reported in Table 1. Table 1. Relaxation Times of Negative Polarons and LEPR Intensity (T = 70 K, Microwave Power = 0.02 mW) sample

T2 (10−8 s)

T1 (10−7 s)

LEPR intensity (au)

PTB7:C70 PTB7:PyBB:C70 PTB7:C60 PTB7:PyBB:C60

7 7 4 6

2.3 9.1 0.9 2.5

1.70 1.90 0.45 0.75

The errors in T2 and T1, due to the evaluation of the intrinsic line bandwidth and the exact maximum position of the saturation curve, are of the order of 1 × 10−8 s and 5 × 10−8 s, respectively. PyBB addition significantly increases T1 of both C60− and C70−, indicating a reduced interaction with the phononic environment responsible for spin−lattice relaxation. In the case of C60−, the increase of T2 in the presence of PyBB reveals a reduction of spin−spin interactions, i.e., more isolated fullerene spins. Overall, the LEPR observations can be explained by hypothesizing that PyBB intercalates between fullerenes, decreasing the phononic and/or magnetic interaction between the fullerene moieties and the polymer in the blend.

Figure 6. Kinetics of the signal of fullerene radical anions for PTB7:fullerene (red) and PTB7:fullerene:PyBB (green): (a) C70 fullerene, T = 70 K; (b) C60 fullerene, T = 40 K. The stationary state intensity in the light-on measurement is normalized, and abscissa values were shifted in order to align the light-off starting time. E

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Figure 7. Trap state distributions of PTB7:PyBB:C70 (top) and PTB7:C70 (bottom) blends. Information on possible long tails at energies lower than 0.08 eV is not available due to the persistent LESR signal. Different colors refer to different temperatures (blue: 60 K; green: 70 K; red: 80 K; cyan: 90 K; y-axis scale is arbitrary units). The Gaussian fit is shown for convenience.

Figure 8. Trap state distributions obtained for PTB7: PyBB:C60 (top) and for PTB7:C60 (bottom). Information on possible long tails at energies lower than 0.09 eV is not available due to the persistent LESR signal. Different colors refer to different temperatures (blue: 20 K; green: 30 K; red: 40 K; cyan: 60 K; y-axis scale is arbitrary units).

The addition of PyBB produces a significantly deeper DOS for C70− polarons, with a 4-fold increase in spin−lattice relaxation T1. Differently, the DOS of C60− remains confined to the same energy interval (0.02−0.07 eV), although the features change in shape and position, with a concomitant more than 2-fold increase of T1. It should be noted that the concentration of long-lived species almost doubles in PTB7:PyBB:C60 with respect to PTB7:C60, while the increase in PTB7:PyBB:C70 compared to PTB7:C70 is moderate. Nonetheless, the DOS analysis has shown that the new traps in the PTB7:PyBB:C60 blend are formed in the same energy range as in the binary blend, while in the ternary C70 blend the new states are generated at much higher binding energy. This suggests that PyBB in C70-based blends intercalates to a higher degree, generating deeper traps. The hypothesis is consistent with the behavior observed for pure binary PyBB:fullerene systems:10 the XRD spectra of the binary blend PyBB:C70 showed well-defined patterns, indicating the formation of a crystalline complex due to a noncovalent interaction between C70 and PyBB. Crystalline phases were not observed in the case of PyBB:C60, although theoretical calculations and photovoltaic device performance suggested that an interaction does also exist between PyBB and C60. We are thus led to infer that in ternary PTB7:PyBB:fullerene blends the interactions between fullerenes and PyBB is governed by the same mechanism, but with different effectiveness. The new features observed in the DOS of PyBB-containing blends could impact on the charge transport properties of the films. To verify this, hole-only and electron-only devices were

feature peaked at 0.073 eV, while no information is available on a possible long time tail. A further Gaussian feature at 0.055 eV is inferred upon completion of the multi-Gaussian fit to the DOS. It should be mentioned that the DOS measured at 70 K, although confirming the existence of a low energy feature at 0.066 eV, has not been used for the multi-Gaussian fit because the shift in the low energy (0.065 eV) feature is probably due to an experimental problem in the temperature control, affecting the long-time part of the decay. These differences suggest that PyBB addition gives rise to a distribution of deeper trap states that may negatively affect the electrical conduction properties of ternary blends, frustrating the beneficial effect of PyBB on blend mixing (i.e., charge separation efficiency). In order to overcome this problem, an additive with concomitant dispersive properties and proper energy level positioning would be requested. The DOSs of the binary and ternary C60 blends are quite different from the previous ones. Two major features are present for PTB7:C60, at 0.029 and 0.048 eV, with a long tail extending to 0.09 eV and more. In PTB7:PyBB:C60 the two major features are at 0.02 and 0.032 eV; a feature at about 0.051 eV is present, while a more pronounced feature appears around 0.065 eV. The long tail below 0.07 eV is apparently steeper. The lowest energy features are less defined, being derived by the persistent signal at long times. Summarizing, the C70 binary blend film presents a higher concentration of trapped charges (from LESR intensity) and a differently featured DOS with respect to the C60 binary blend. F

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The Journal of Physical Chemistry C prepared, and the mobility of positive and negative charge carriers was evaluated from the current−voltage (J−V) characteristics. Figure 9 shows, as an example, the J−V curves

Figure 10. Current−voltage curves of electron-only PTB7:C70 (a) and PBT:PyBB:C70 (b) devices, prepared with a PTB7:C70 weight ratio of 1:3 and a PyBB:C70 molar ratio of 1:1. The lines indicate the linear fits to the experimental data. Figure 9. Current−voltage curves of hole-only PTB7:C70 (a) and PBT:PyBB:C70 (b) devices, prepared with a PTB7:C70 weight ratio of 1:3 and a PyBB:C70 molar ratio of 1:1. The lines indicate the linear fits to the experimental data. The slopes are indicated in the figure.

J = qn0μ

V d

(2)

where q is the electron charge and d the film thickness. As voltage increases, the injected carriers start to dominate the conduction, the SCLC regime is established, and the current varies quadratically with V.31 The transition voltage (Vtr) from the Ohmic region to the SCLC region is given by32

of hole-only devices made of PTB7:C70 (D/A 1:3 w/w) and PTB7:PyBBC70 (1:2 D/A w/w), where V represents the applied voltage corrected for the built-in voltage and for the voltage drop across the series resistance. For all the investigated hole-only devices, irrespective of the presence of PyBB in the blends, a linear regime (J ∝ V) in the low voltage range is followed by a space-charge limited-current (SCLC) regime (J ∝ V2), as indicated by the slopes of the double-logarithmic plots of Figure 9. The same behavior was also observed for the J−V of electron-only devices based on PTB7:C70 (an example is shown in Figure 10a). On the other hand, all the investigated electron-only PTB7:PyBB:C70 devices showed three distinct regions in the J−V plots (Figure 10b shows a typical result): a linear trend at low voltage is followed by a narrow SCLC regime that rapidly evolves, at higher voltage, in a steep curve with a slope of about 6 in the log−log J−V plot. The same two and three regions were also exhibited by the J−V characteristics of electron-only PTB7:C60 and PTB7:PyBB:C60 devices, respectively. A reduced slope of the steep region of the J−V curve of PyBB-containg devices prepared with C 60 was observed in comparison to PTB7:PyBB:C70 (5.4−5.5 against 6, as shown in the example of Figure S6). Charge carrier mobility (μ) was calculated from the Ohmic region of the J−V curves to directly compare all the results. In the low-voltage region, the density of thermally generated free carriers (n0) is predominant over the injected charge carriers, so that Ohm’s law holds:

Vtr =

8qn0d 2 9εε0

(3)

where ε is the relative dielectric constant and ε0 the permittivity of vacuum. We derived the concentration of free charge carriers from Vtr (assuming ε = 3 in eq 3) and used its value to compute μ from eq 2. The results are collected in Table 2. The values of free carrier density, ranging between 4.5 × 1014 and 1.6 × 1016 cm−3, are typical of organic films.33 Hole mobility ranges between 2.1 × 10−5 and 5.6 × 10−5 cm2 V−1 s−1 in the donor phase of PTB7:PyBB:C70, and a similar value was calculated for PTBT:C70 (4.3 × 10−5 cm2 V−1 s−1). The mobility of negative carriers in the acceptor phase of PTB7:PyBB:C70 ranged between 3.5 × 10−7 and 8.6 × 10−7 cm2 V−1 s−1, significantly lower than the hole mobility in the polymer phase. Moreover, the addition of PyBB significantly lowered the electron mobility in the blends (3.5 × 10−7 cm2 V−1 s−1 against 4.1 × 10−4 cm2 V−1 s−1 calculated for PTBT:C70 at the same D/A ratio of 1:3). A similar drop for the electron mobility was obtained by adding PyBB to C60-based blends (Table 2). The significant reduction of mobility of negative charge carriers in PyBB-containing films can be explained by enhanced trapping effects, in agreement with LEPR results and confirmed by the steep increase of the current in the higher voltage range of electron-only devices, pointing to a distribution of trap states.31 G

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The Journal of Physical Chemistry C Table 2. Parameters Extracted from the J−V Curves of Single-Carrier Devices n0 (cm−3)

sample PTB7:C70 (1:3 w/w) PTB7:PyBB:C70 (1:1 w/w) PTB7:PyBB:C70 (1:2 w/w) PTB7:PyBB:C70 (1:3 w/w) PTB7:C60 (1:2 w/w) PTB7:C60 (1:3 w/w) PTB7:PyBB:C60 (1:2 w/w) PTB7:PyBB:C60 (1:3 w/w)

holes electrons holes electrons holes electrons holes electrons electrons electrons electrons electrons

2.4 1.6 4.5 5.3 4.5 1.4 6.8 3.9 7.7 6.0 7.5 9.0

× × × × × × × × × × × ×

15

10 1016 1015 1015 1014 1016 1015 1015 1015 1015 1015 1015

the lower amount of light-absorbing material in the threecomponent blend, although the better intermixed morphology in PTB7:PyBB:C70 blends should have beneficial effects on the dissociation of photogenerated excitons, reflecting on the generation of free carriers. The open circuit voltage, ranging between 0.67 and 0.72 V, was enhanced with respect to PTB7:C70 solar cells which were affected by higher leakage currents.13 The dependence of the photocurrent on voltage and incident light power (Pin) was investigated to shed light on the mechanisms limiting the performance of PTB7:PyBB:C70 solar cells. Figure 11 shows the variation of the net photocurrent Jph,

μ (cm2 V−1 s−1) 4.3 4.1 4.2 8.6 5.6 4.2 2.1 3.5 5.1 3.8 4.1 5.6

× × × × × × × × × × × ×

10−5 10−4 10−5 10−7 10−5 10−6 10−5 10−7 10−4 10−4 10−7 10−6

To ascertain the validity of the method used to estimate charge carrier mobility for the considered blends, mobility was also derived by applying the Mott−Gurney law for the SCLC regime31 J=

9 V2 εε0μ 3 8 d

(4)

to the J−V curves of PyBB-free blends, which showed the quadratic regime over a wide voltage range. The mobilities evaluated with the two different methods were in excellent agreement, thus validating the values reported in Table 2. As an example, the values calculated from the quadratic region of the J−V curves reported in Figures 9a and 9b were 4.4 × 10−5 and 2.5 × 10−5 cm2 V−1 s−1, respectively, to be compared with 4.3 × 10−5 and 2.1 × 10−5 cm2 V−1 s−1 (Table 2) obtained from the Ohmic region of the same curves. The mobility values of Table 2 indicate an unbalanced charge transport in PyBB-containing blends, with the electron mobility in the acceptor phase lower by roughly 2 orders of magnitude than the hole mobility in the donor phase. This could result in enhanced recombination losses in the related solar cells; therefore, charge carrier dynamics in PTB7:PyBB:C70 solar cells was investigated. The photovoltaic parameters extracted from the J−V curves under illumination, shown in Figure S7, are listed in Table 3. By changing the D/A weight ratio, a moderate variation of the photovoltaic parameters was observed. The short-circuit current (Jsc) varied between 7.12 and 8.14 mA cm−2, the opencircuit voltage (Voc) between 0.67 and 0.72 V, and the fill factor (FF) between 0.36 and 0.46, resulting in a power conversion efficiency (PCE) ranging between 1.83 and 2.43%. These values are consistently lower with respect to PTB7:C70 solar cells prepared with a D/A weight ratio of 1:3, for which FF = 0.55, Jsc = 9.08 mA cm−2 (increasing to 9.21 mA cm−2 for 1:2 D/A weight ratio), and PCE = 3.17% have been reported.13 The Jsc reduction in PyBB-containing solar cells may be explained by

Figure 11. Photocurrent as a function of the effective voltage at different light power intensities (in the range 24−100 mW cm−2) for PTB7:PyBB:C70 solar cells prepared with a PTB7:C70 weight ratio of 1:2 and with a PyBB:C70 molar ratio of 1:1. The method for obtaining the values of the saturation voltage is illustrated for the bottom plot in the figure: Vsat is the abscissa of the intercept between the linear fit to the points in the low range of the effective voltage (dashed line) and the horizontal line corresponding to the saturated photocurrent (solid line).

defined as the difference between the current under illumination and in dark conditions, as a function of the effective voltage V0−V for PTB7:PyBB:C70 solar cells (D/A 1:2 w/w). Here V is the applied voltage and V0 is the compensation voltage, that is, the voltage at which Jph is zero.34 A square-root voltage dependence of Jph was observed at low V0−V for all Pin. The transition from field-dependent to saturation regime defines the transition voltage Vsat (Figure 11). Saturation was reached at high effective voltages, for which nearly all the pairs of free charge carriers generated by exciton dissociation at the donor/acceptor interface are effectively collected at the electrodes. The same behavior of Jph was observed for PTB7:PyBB:C70 solar cells prepared with 1:1 and 1:3 D/A weight ratios. Two reasons may give rise to the square-root regime of photocurrent: the formation of space charge and a low

Table 3. Series Resistance (Rs), Shunt Resistance (Rsh), and Photovoltaic Parameters at 100 mW cm−2 AM1.5G Irradiation for Best-Performing PTB7:PyBB:C70 Solar Cells (Average Values over Eight Devices Shown in Parentheses), Prepared with Different PTB7:C70 Weight Ratios and with 1:1 Molar Ratio for PyBB:C70 D:A (w/w)

Rs (Ω cm−2)

Rsh (Ω cm−2)

Jsc (mA cm−2)

Voc (V)

FF

PCE (%)

1:1 1:2 1:3

8.1 (8.5) 3.1 (3.0) 2.5 (2.5)

4.6 × 105 (2.5 × 105) 4.9 × 105 (2.5 × 105) 4.8 × 105 (3.3 × 105)

7.12 (7.05) 8.14 (7.76) 7.16 (7.06)

0.72 (0.72) 0.67 (0.67) 0.67 (0.67)

0.36 (0.36) 0.45 (0.42) 0.46 (0.45)

1.83 (1.78) 2.43 (2.16) 2.19 (2.13)

H

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The Journal of Physical Chemistry C mobility−lifetime product (μτ). The photocurrent Jph and the saturation voltage Vsat were then measured against variations of Pin to discriminate between the two mechanisms.35 The case of of PTB7:PyBB:C70 solar cells is displayed in Figure 12. From

Figure 13. Comparison of Nyquist plots (a) and of capacitance as a function of frequency (b) for PTB7:PyBB:C70 (PyBB:C70 1:1 mol/ mol) and PTB7:C70 solar cells, prepared with a PTB7:C70 weight ratio of 1:2, under 1 sun irradiation and open-circuit conditions.

Figure 12. Photocurrent at low and high effective voltage (a) and saturation voltage (b) as a function of light power intensity for PTB7:PyBB:C70 solar cells prepared with a PTB7:C70 weight ratio of 1:2 and with a PyBB:C70 molar ratio of 1:1. The lines indicate the linear fits to the experimental data.

PTB7:C70 devices of equal surface area and thickness (Figure 13b), and the higher capacitance suggests enhanced charge trapping in PyBB-containing cells. The equivalent circuit depicted in Figure 13a provides a high quality fit of the impedance spectra of PTB7:PyBB:C70 cells. The resistor Rs accounts for the device series resistance, Rrec is the recombination resistance, related to the recombination current, while Cmu is the constant phase element representing the chemical capacitance38 due to the accumulation of charge carriers.39 The additional series combination of a resistor Rtrap and a constant phase element Ctrap accounts for charge trapping phenomena.40−42 We performed impedance measurements at several light power intensities to obtain information on the order of recombination processes in the cells. It has been shown that Rrec is related to Voc (dependent on Pin) by the expression

the double-logarithmic plot of Jph vs Pin we found a 2-fold power law dependence: the slope of the plot was SHV = 0.92 at high effective voltage (Figure 12a), while at low effective voltage the slope was SLV = 0.74, that is roughly 3/4. This result suggests that Jph is limited by the formation of space charge at low Pin. This is confirmed by the square-root dependence of the saturation voltage on light intensity36 (Figure 12b, slope Ssat = 0.52). This behavior was observed for all cells prepared at different weight ratios, with SHV ranging between 0.92 and 0.94, SLV between 0.74 and 0.77, and Ssat between 0.46 and 0.52. The addition of PyBB thus seems to increase recombination losses because of space-charge formation, reflecting in the low FF values (Table 3) and consistent with the strongly unbalanced charge transport between the donor and acceptor phase of the blend.37 Differently, for PTB7:C70 solar cells Jph has been found to be limited by a low μτ product.13 The presence of enhanced charge trapping in PyBBcontaining blends was also confirmed by impedance spectroscopy measurements of cells under illumination. A Vocequivalent dc bias was superimposed to the armonic signal. A typical impedance spectrum under 1 sun irradiation is displayed in Figure 13a in the Nyquist representation, with frequency as the implicit variable. The Nyquist plots of PTB7:PyBB:C70 devices exhibited a major arc in the low frequency range and a smaller one toward high frequency, as shown in Figure 13a where the spectrum of a PTB7:C70 cell is also included for comparison. The impedance spectra of PTB7:PyBB:C70 cells show higher resistance and capacitance with respect to

⎛ qβ ⎞ Voc⎟ R rec ∝ exp⎜ − ⎝ 2kT ⎠

(5)

where k is the Boltzmann constant, T is the absolute temperature, and the parameter β represents the recombination order.41 From the slopes of the semilogarithmic plots of Rrec vs Voc obtained for PTB7:PyBB:C70 cells prepared with 1:1, 1:2, and 1.3 D/A ratio (Figure 14), β values of 0.86, 1.02, and 1.04 were respectively computed. The roughly unitary value of β indicates the existence of first-order recombination processes, presumably between mobile and trapped charge carriers acting as sinks, and provides a further confirmation of charge trapping in PyBB-containing blends. I

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The Journal of Physical Chemistry C

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Additional information on the calculation of the density of trap states from LESR measurements, morphological characterization by AFM, and additional optical and electrical characterization (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Phone +39 02 52046678; Fax +39 02 52036347; e-mail lucia. [email protected] (L.B.). *Phone +39 051 6399779; Fax +39 051 6399844; e-mail nadia. [email protected] (N.C.). Figure 14. Recombination resistance as a function of the open-circuit voltage for PTB7:PyBB:C70 (PyBB:C70 1:1 mol/mol) solar cells prepared with a PTB7:C70 weight ratio of 1:1 (black symbols) and 1:3 (blue symbols). The lines indicate the linear fits to the experimental data.

Present Address

4. CONCLUSIONS The effects of PyBB addition on the morphological and electronic properties of photovoltaic blends made of PTB7 as electron donor and neat fullerenes C60 and C70 as electron acceptors were investigated. PyBB acts as an excellent dispersant for fullerene molecules, preventing their aggregation in the investigated blends, in agreement with the high affinity of the two aromatic species. However, the smooth morphology of PyBB-containing blends does not translate into improved photovoltaic performance because of the relevant impact of the pyrene derivative on the electronic properties of the systems. The DOS of negative polarons in PTB7:PyBB:C70 presents a distribution of deep traps at lower energies with respect to the control sample without pyrene. Also, the DOS of PTB7:PyBB:C60 shows the presence of additional features upon PyBB addition, although the density of states is confined to the same energy range as the control PTB7:C60 sample. The observed 4- and 2-fold variation of the spin−lattice relaxation times of fullerene anions upon PyBB addition suggests that their distribution of trap levels may be modified by the different microscopic surroundings, likely due to fullerene/PyBB intercalation, as confirmed by fluorescence emission of isolated PyBB in the presence of fullerenes. The differences between C60 and C70 containing blends is consistent with previous findings,10 showing that C70-PyBB interaction is stronger than the C60-PyBB one. The findings of the LEPR investigation account for the reduced mobility of negative charge carriers in PyBB-containing blends due to an enhanced electron trapping and leading to a strong unbalanced charge transport, with the mobility of positive charge carriers, unaffected by the introduction of PyBB, around 2 orders of magnitude higher. The strong interaction of pyrene with fullerene molecules leads to the hypothesis that the reduction of electron mobility is likely due to hindered hopping caused by the intercalation of fullerenes with PyBB moieties. As a result, enhanced charge carrier recombination losses, due to space-charge formation, affect the behavior of solar cells prepared with PyBB and frustrate the beneficial effect on the blend morphology.

Notes

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N.G.: Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander University of Erlangen-Nuremberg, Martensstraße 7, D-91058 Erlangen, Germany. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Andrea Pellegrino, who provided the 1pyrenebutyric acid butyl ester used in this study. N.G., F.T., and N.C. thank Eni S.p.A. for the financial support to this work (contract no. 4700007315).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11468. J

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