Correlation between the Open Circuit Voltage and the Energetics of

Letter pubs.acs.org/JPCL

Correlation between the Open Circuit Voltage and the Energetics of Organic Bulk Heterojunction Solar Cells Ilja Lange,† Juliane Kniepert,† Patrick Pingel,† Ines Dumsch,‡ Sybille Allard,‡ Silvia Janietz,§ Ullrich Scherf,‡ and Dieter Neher*,† †

Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany Macromolecular Chemistry and Institute for Polymer Technology, Bergische Universität Wuppertal, Gauss-Strasse 20, D-42097 Wuppertal, Germany § Frauenhofer Institut für Angewandte Polymerforschung, Geiselbergstrasse 69, 14476 Potsdam, Germany ‡

S Supporting Information *

ABSTRACT: A detailed investigation of the open circuit voltage (VOC) of organic bulk heterojunction solar cells comprising three different donor polymers and two different fullerene-based acceptors is presented. Bias amplified charge extraction (BACE) is combined with Kelvin Probe measurements to derive information on the relevant energetics in the blend. On the example of P3HT:PC70BM the influence of composition and preparation conditions on the relevant transport levels will be shown. Moderate upward shifts of the P3HT HOMO depending on crystallinity are observed, but contrarily to common believe, the dependence of VOC on blend composition and thermal history is found to be largely determined by the change in the PCBM LUMO energy. Following this approach, we quantified the energetic contribution to the VOC in blends with fluorinated polymers or higher adduct fullerenes. SECTION: Energy Conversion and Storage; Energy and Charge Transport

O

rganic solar cells (SC) have shown their potential to be the next generation photovoltaic devices. Best progress has been made by blending low band gap donor polymers with soluble fullerenes. Meanwhile, internal quantum efficiencies approaching 100% and power conversion efficiencies above 9% have been achieved.1−3 One remaining issue to resolve is the rather low open circuit voltage (VOC) of these devices, which remains well below the limiting value as given by the energy gap between the relevant charge transport states. Unfortunately, despite extensive experimental and theoretical studies in recent years, there is still no conclusive understanding of the processes limiting VOC. In an illuminated two terminal solar cell, VOC is given by the difference of the Fermi levels (EF) of the two electrodes at zero external current conditions. Under the assumption that the electron and the hole current are individually zero at each point in the active layer, the quasi-Fermi levels (QFLs) EF,h and EF,e for holes and electrons, respectively, in the active material are both constant throughout the device and are equal to the respective electrode EF. In this case, the VOC is determined by the splitting of EF,h and EF,e in the bulk under illumination (Figure 1). This approach is fundamental to inorganic crystalline SC but, because of its generality, it is also applicable to organic SCs.4 Unfortunately, a direct determination of the energies EF,e and EF,h in the bulk is not possible. For given density of transport state distributions (DOS), EF,e and EF,h are related to the © 2013 American Chemical Society

Figure 1. Scheme of the overall energetics at open circuit conditions. Here, EF,e and EF,h denote quasi-Fermi level energies for electrons and holes, respectively. With the assumption of constant QFLs and ideal Fermi-level pinning at the electrodes, VOC is determined by the QFL splitting in the bulk of the active layer.

thermalized steady state concentrations of electrons and holes, n and p, via the Fermi-Dirac statistics. Therefore, a unique relation exists between the open circuit voltage and the carrier density for every individual donor−acceptor (DA) combinaReceived: September 13, 2013 Accepted: October 29, 2013 Published: October 29, 2013 3865

dx.doi.org/10.1021/jz401971e | J. Phys. Chem. Lett. 2013, 4, 3865−3871

The Journal of Physical Chemistry Letters

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energies and the energetics of the individual components forming the heterojunction was less clear. Here, we present a detailed study of VOC as a function of n for different polymer:fullerene blends. The investigated polymers are P3HT, PCPDTBT, and F-PCPDTBT and PC70BM or ICBA (for chemical structures and full names see section A in the Supporting Information (SI)) was used as the acceptor. Comparable trends to PC70BM with slight variations in absolute values have been observed for PC60BM as well. By analyzing the experimental VOC(n) data in the framework of Fermi-level-splitting as described above, we are able to refer values for Eg and energetic disorder for all blends. These studies are then complemented by Kelvin Probe (KP) measurements of the blend layers on different electrodes. Hole or electron transfer from high or low WF electrodes, respectively, into the organic layer causes typical energetic shifts as function of distance from the electrode, which is determined by the underlying distribution of relevant states.16 Therefore, this technique enables us to acquire information on the energetics of both the HOMO and the LUMO. Moderate shifts or broadenings of the P3HT HOMO depending on crystallinity are observed and quantified in agreement with earlier publications. Contrarily to recent assumptions, we observe an even more significant variation in PCBM LUMO depending on preparation conditions and the nature of the donor polymer. We measured the carrier density n under steady state illumination at open circuit conditions by applying a new technique: bias amplified charge extraction (BACE). Here, the device is illuminated by white light of an array of LEDs with the external bias equivalent to the VOC at the respective light intensity. The value of VOC is measured in advance in the same setup under equivalent light conditions with a high resistor in series with the device. The LEDs are then switched off and the charge carriers are extracted. In classic CE, extraction is under short circuit conditions, with conceivable losses due to trapping or nongeminate recombination. In BACE, extraction is accelerated by applying a reverse bias Vrb, thereby reducing the losses stated above. In order to keep the RC-time as short as possible, BACE was performed on small 1 mm2 pixels. Note that the switch-off time of the used LEDs is ∼200 ns, which is much longer than the Vrb ramping time. To prevent nongeminate recombination losses from reducing the extractable charge density before application of the collection bias, we applied this voltage with only a few nanoseconds delay after initiating the switch-off of the LED. Although some additional charge will be photogenerated during the 200 ns LED switchoff time, we estimated this to be insignificant compared to the charge that is already present in the device under the open circuit conditions applied before (for more detailed information, see SI). The photocurrent transients were recorded via a 50 Ω resistor. Under the assumptions that the carrier distribution inside the device is homogeneous and that both carrier types are extracted, the integral of the photocurrent transient divided by e and the layer volume yields the carrier density at the respective light intensity or VOC. Figure 2 shows the VOC as a function of the extracted carrier density for as-prepared and annealed P3HT:PCBM blends of different composition. For p = n, eq 1 or 2 predict a slope of 2kBT/e when VOC is plotted as a function of n in a semilogarithmic plot, meaning that VOC rises by 115 mV at room temperature when the carrier density increases by a factor of 10. This is exactly seen for all as-prepared P3HT:PCBM devices. This indicates either a very low disorder or a Gaussian-

tion, and this relation is depending on the energetics in the particular system under study. According to classical semiconductor theory, the transport states for electrons and holes are separated by a well-defined band gap. For this case, VOC is related to n and p via the wellknown equation eVoc = Eg + kBT ln

np NeNh

(1)

where Nh and Ne are the effective density of states for holes and electrons (in the donor−acceptor blend) respectively, kB is Boltzmann’s constant, T is the temperature, and e is the elementary charge. For all-organic solar cells, Eg in eq 1 is the nominal band gap given by the difference in the onset energies of the lowest unoccupied molecular orbital (LUMO) and of the highest occupied molecular orbital (HOMO), ELUMO and EHOMO, respectively. Experimental and theoretical work has shown that the density of states distribution in organic semiconductors extends far into the nominal band gap, and the distribution of tail states has been described by either a Gaussian or an exponential function.5−8 Blakesley and Neher pointed out that this broadening will affect the dependence of VOC on n in characteristic ways.9 For a Gaussian distribution of states with the width σ, introduction of energetic disorder alters eq 1 to: eVoc = Eg* + kBT ln

np NeNh

(2)

where the Eg* is now an effective band gap given by Eg* = (ELUMO − EHUMO) − σ2/kBT. For a typical broadening of σ = 80 meV, Gaussian disorder will reduce VOC by ca. 250 mV at room temperature. In case of an exponential distribution of trap states with the characteristic trap energy Et > kBT eq 1 becomes: eVoc = Eg + mkBT ln

np NeNh

(3)

with an additional prefactor m = Et/kBT > 1. Therefore, Gaussian and exponential DOS distributions may be distinguished by their characteristic dependences of VOC on the carrier density n. For a device under steady state illumination and zero external bias, n and p are determined by the rate for free charge generation in competition to free carrier recombination, and this balance is, again, unique for each DA system. Recently, Durrant and co-workers have successfully predicted VOC via a detailed analysis of the generation and recombination dynamics.10,11 Their analysis combines knowledge on the steady-state carrier density, measured with charge extraction (CE), and of the carrier recombination lifetime measured by transient photovoltage (TPV). This work provided important information on the carrier dynamics in different donor− acceptor blends, and how this affects VOC at comparable illumination intensities. On the other hand, little is known about the energetics in different blend samples, which actually determines VOC at a given carrier concentration. Vandewal and others related the VOC to the particular energetics at the donor−acceptor interface expressed by the charge transfer energy ECT.12−15 Thereby, they were able to relate changes in VOC for different composition or upon postdeposition treatments to variations in ECT, but the relation between these CT 3866



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prepared and annealed samples. We conclude that the energetics in the DA mixture is not simply determined by the intrinsic properties of the individual components forming the blend but that it is strongly affected by the layer composition and the thermal history. Credgington and Durrant already pointed out that the VOC strongly depends on the resulting microstructure of the device.11 We showed recently that Kelvin-probe (KP) measurements of the work function of organic semiconductors on various high or low work function (WF) electrodes yield information about the density of tail states for electrons and holes in pristine layers of semiconducting polymers.16 Under KP measurement conditions, the organic material is in thermal equilibrium with the substrate and energy level alignment (ELA) occurs via thermally induced exchange of charges between the electrode and the respective DOS.21 For our blends on low WF electrodes like Al, ELA is a result of the injection of electrons from the electrode into the deepest accepting states of the system, which is the PCBM LUMO. Alternatively, high WF electrodes like PEDOT:PSS inject holes into the polymer HOMO. Accumulation of charge in the organic material will lead to band-bending and move the common Fermi-level into the band gap with increasing thickness of the film. The extent of the band bending is thereby correlated to the broadening of the DOS. Therefore, the simulation of the thickness dependent WF, particularly at low thickness, allows for an estimation of position and broadening of the underlying distribution of states. Figure 3 shows exemplarily the measured thickness dependence

Figure 2. VOC versus carrier densities n for P3HT:PCBM with different blend ratios (blue 2:3, red 1:1, green 3:2 ratio by weight), as prepared (closed symbols) and annealed (open symbols), as measured by BACE. Stars show VOC(n) values corresponding to AM 1.5G illumination. The solid lines are least-squares fits using eq 1 or eq 3. The extracted values for Eg and Et are shown in the inset. The gray bar indicates the range of bulk carrier densities ∼ (1015 ± 50%) cm−3 predicted from Kelvin-simulations on thick samples for different density of states distributions (see Figure S5 in the SI). Colored numbers inside the gray bar shows the corresponding pinning level splittings from Figure 3 and Figure S3 in meV.

type DOS of the relevant transport states. On the other hand, all annealed P3HT:PCBM blends reveal a larger slope, pointing to an exponential-type disorder with n vs VOC expressed by eq 3. Application of eqs 1 or 3 to the data (solid lines in Figure 2) yields values for the characteristic energies Eg and Et. These values are listed in the insets in Figure 2. Data were analyzed by using either a rectangular DOS (slope of 2kBT/e) or an exponential trap distribution (for a slope larger than 2kBT/e). Effective density of states Ne,h were chosen such that they correspond to the same maximum density of states G0 = 1021 cm−3 eV−1 (see SI). Noticeably, the fit of eq 3 to the annealed 1:1 P3HT:PCBM device curve yields Et = 44 meV, which is in remarkable good agreement to the trap energies (40−48 meV) deduced from the analysis of transient photovoltage and photocurrent techniques.17 An exponential type DOS in that particular blend has also been reported by Street with Et changing from 35 to 65 meV when going deeper into the band gap.18 The extracted band gap of ∼1.2 eV is in agreement to reported values, which vary between 0.9 and 1.4 eV.12,17,19,20 Here we note that the exact value of Eg depends on the choice of Go, whose independent determination requires measurements over a wide range of temperatures. However, Go is not expected to vary significantly between the different blend samples, meaning that relative changes in Eg when comparing different preparation conditions or the composition shall be quite reliable. Also shown in Figure 2 are values of n corresponding to the open circuit voltage under simulated AM1.5G conditions (stars). Thermal annealing of the 1:1 P3HT:PCBM blend reduces the VOC at one sun illumination only slightly, by ∼35 mV. One is, therefore, seduced to postulate an only weak change of the energetic landscape of the blend upon annealing. Comparing values for VOC at equal carrier densities, however, reveals a dramatic decrease of the QFL-splitting of several hundred meV upon annealing. Similar trends are seen for P3HT:PCBM blends of other composition. We also find that VOC of these blends depends on composition, and that the correlation between VOC and composition is different for the as-

Figure 3. KP measurements of the WF as a function of active layer thickness of P3HT:PCBM (1:1) as prepared (closed symbols) and annealed (open symbols) on Al (∼3.2 eV) and PEDOT:PSS (∼5.4 eV). The solid lines are guides for the eyes. For a layer thickness larger than ca. 50 nm, the band bending becomes weak, and the electrode EF becomes essentially pinned by filling of tail states in the bulk of the semiconductor. The difference in the pinning levels of hole and electrons at a thickness of ca. 100 nm is indicated by arrows, with an uncertainty of ±25 meV.

of the WF for as-prepared and annealed 1:1 P3HT:PCBM on either Al or PEDOT:PSS (for all other data, see Figure S3 in the SI). The data display a continuous decrease of the WF with thickness for all samples on PEDOT:PSS, equivalent to the formation of a positive space charge in the organic layer. Accordingly, the WF increases with thickness on Al substrates due to electron injection. However, we note that in some samples the WF is essentially independent of the layer thickness. We propose that this is due to an inhomogeneous morphology. For example, we see more pronounced peaks in the absorbance spectra of thin P3HT:PCBM layers which we assign to a higher polymer 3867



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crystallinity near the electrode. Consequently, also deviations in the energetic structure in the different layer thicknesses have to be supposed. In this case, an accurate analysis of the thicknessdependent data with the procedure described in ref 16 is not possible. However, for all samples investigated here, WF(d) flattens considerably for a layer thickness larger than ca. 50 nm. The WF of these thicker samples measured by KP is determined by the Fermi-level position in the organic semiconductor under the thermal equilibrium that is established by the occupation of tail states in the bulk of the film (we denote this energy as “pinning-level” in the following). To estimate the corresponding carrier densities, we simulated profiles of the electrical potential and carrier density for different DOS distributions (see Figure S5 in the SI) but with the same value of Go (1021 cm−3 eV−1). We find that different DOS distributions lead to different pinning energies, but that approximately the same carrier density of about 1015 cm−3 is established within the bulk of thick layers at the distance d ≅ 100 nm from the electrode, irrespective of the exact shape of the DOS. This is a consequence of the very same boundary conditions in all Kelvin simulations, namely that the charge in the semiconducting layer exactly compensates the charge on the bottom electrode and that the drift-diffusion current is zero at any point in the layer. Under the promise that no dipolar layer forms at the upper surface, the WF determined on a thick organic layer on Al (PEDOT:PSS), therefore, measures the Fermi-level position for a bulk electron (hole) density of about 1015 cm−3. This position must then be equivalent to the energy EF,e (EF,h) of the QFL when this charge density is photogenerated under steady state illumination. Correspondingly, the difference between the pinning levels of thick blend layers on PEDOT:PSS and Al shall be equal to the splitting of the QFLs in the illuminated blend, and with that to eVOC, at a photogenerated carrier concentration of n ≅ 1015 cm−3 (gray bars in Figure 2a). Indeed, we find that VOC’s extrapolated to this carrier density coincide very well, within the experimental error, to the corresponding pinning level differences (taken from Figure 3 and Figure S3; see colored numbers in Figure 2). The agreement is striking, given that these values were derived from two very different techniques: charge extraction under illumination versus ELA in the dark. Having established that the pinning levels in the Kelvinprobe measurements provide a rather accurate measure of the energetics that determines the open circuit voltage, we now turn to a more detailed discussion of the strong effect annealing has on the VOC of our P3HT:PCBM blends. The two lower curves in Figure 3 show thickness-dependent WF data for the as-prepared and annealed 1:1 P3HT:PCBM blend on PEDOT:PSS. Upon annealing, the HOMO pinning level shifts further into the band gap. This finding is in agreement to other studies reporting, e.g., a shift of the HOMO of pure P3HT after annealing by using ultraviolet photoemission spectroscopy,22 a decrease of the oxidation potential of P3HT:PCBM by cyclic voltammetry,23 or the detection of a new DOS above the P3HT HOMO by impedance spectroscopy.24 Theoretical calculations by McMahon et al. claim a shift of the P3HT HOMO onset by ∼150 meV going from a distorted chain at the edge of a crystalline P3HT phase to a highly planar one in the ordered center.25 Indeed, we can correlate the HOMO shift to the degree of crystallinity of the P3HT phase. In Figure 4a the absorbance spectra of P3HT:PCBM, as-prepared and annealed, as well as

Figure 4. (a) Absorbance spectra of as-prepared and annealed 1:1 P3HT:PCBM blends and of an annealed layer of pure P3HT. (b) Corresponding KP measurements on PEDOT:PSS.

the spectrum of an annealed pure P3HT layer is shown. While the P3HT phase in the as-prepared blend is rather amorphous, increasing absorbance peaks at around 550 and 600 nm indicate a significantly higher crystallinity in the latter two cases.26,27 This increase in crystallinity goes along with an overall shift of the pinning level by about 130 meV toward the band gap (Figure 4b), which matches pretty well the theoretical prediction quoted above. For the annealed blend, the absorption spectra suggest that the P3HT crystallinity remains below that of the annealed pure layer (see also ref 28). Correspondingly, the WF shift of the P3HT:PCBM blend upon annealing is quite moderate, ∼80 meV. This rather moderate change of the P3HT HOMO cannot account for the large decrease in VOC at comparable carrier densities upon annealing of the 1:1 P3HT:PCBM blend. According to our KP results, the major contribution to this decrease stems from a significant change of the PCBM LUMO energetics, as expressed by an about 170 meV lowering of the PCBM pinning level in the 1:1 blend with P3HT. Similar though weaker shifts in the PCBM LUMO upon annealing are seen for the other P3HT:PCBM blends. Recently, Durrant and co-workers reported that pure solid PCBM has a 100−200 meV higher electron affinity than PCBM diluted into polystyrene, suggesting that fullerene aggregation consistently lowers the LUMO value.29,30 We complemented this work by studying as-prepared layers of the well-known blend PCPDTBT:PCBM. This blend was reported to exhibit a homogeneous fine-intermixed morphology under the preparation conditions applied here, with no evidence for severe fullerene aggregation.31,32 Figure 5a displays the respective VOC as a function of the steady state carrier density. Again, the data can be well fitted with eq 1, pointing to a very low (or Gaussian type) disorder in that particular blend. The extrapolation of the BACE data to a carrier density of 1015 3868



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pinning level, but we also noticed a slight (ca. 50 meV) upward shift of the PCBM LUMO. We propose that the origin of that effect lies in a reduced interaction of PCBM with the fluorinated polymer backbone, owing to stronger phase separation. It has also been shown that the VOC of P3HT:PCBM or PCPDTBT:PCBM can be significantly improved when PCBM is replaced by ICBA with a higher LUMO energy.36−38 Our KP results confirm a large upward shift of the acceptor pinning level by about 370 meV when PCBM is replaced by ICBA in a blend with PCPDTBT (Figure 5b), whereas the new acceptor causes only very low shift of the PCPDTBT pinning level. Accordingly, fitting the BACE data in Figure 5a also reveals an increase of the effective band gap of about 315 meV with an appropriate improvement of the VOC at equal carrier densities. The summarized results in Figure 6 show that the position and/or broadening of the PCBM LUMO depends largely on

Figure 5. (a) VOC versus carrier densities n for PCPDTBT:PCBM, FPCPDTBT:PCBM and PCPDTBT:ICBA as measured by BACE. The star shows VOC(n) according to AM 1.5G illumination. The solid lines are the least-squares fits using eq 1 with the fit parameter Eg displayed. The gray bar indicates the range of bulk carrier densities ∼(1015 ± 50%) cm−3. Colored numbers inside the gray bar show the pinning level splitting from the corresponding KP measurements as shown in (b).

Figure 6. Generic overview of KP experiments of different blends and of as-prepared PCBM on Al. The data show a large variation of the PCBM LUMO pinning level, depending on the host material, the composition, and/or thermal treatment. Lines are guide to the eye.

cm−3 yield a Voc of 520 mV, which corresponds quite well to the QFL-splitting of about 470 meV as deduced from the Kelvin-probe data shown in Figure 5b. Interestingly, despite the significantly deeper lying PCPDTBT HOMO, the VOC is only slightly larger than in as-prepared P3HT:PCBM blends at comparable charge densities. KP measurements reveal that the WF in the pinning regime of this blend on Al is the lowest among all samples studied, meaning that the LUMO pinning level moves rather deep into the nominal band gap. Though we currently lack a conclusive interpretation of this result, it is not unlikely that the intimate contact between fullerenes and polymer chains in these highly intermixed blends affects the energy of electrons located on the dispersed PCBM molecules. Clearly, the low lying LUMO in this particular blend is the main cause of the overall low Voc reported here and in other work for PCPDTBT:PCBM blends.33−35 We have recently shown that the open circuit voltage of PCPDTBT-related blends can be increased upon fluorination of the benzothiadiazole-unit, which was mainly rationalized by the increase in ionization potential of the polymer by 150 meV.34 We also observed that fluorination enforces the formation of more extended and purer polymer-rich phases, thereby reducing the efficiency of geminate and nongeminate recombination. Reduced recombination is evidenced by the higher steady-state carrier density of F-PCPDTBT:PCBM at simulated AM1.5G illumination. Our KP results in Figure 5b confirm the ca. 150 meV downward shift of the HOMO

the choice of donor polymer and on the preparation conditions. Noticeably, the deepest lying LUMO is measured in asprepared PCPDTBT:PCBM, despite the very high degree of intermixing in this blend. We conclude that aggregation phenomena are important in determining the LUMO DOS, but that other morphological aspects, e.g., local donor− acceptor interactions, might be equally important. Unfortunately, only few publications deal with the LUMO energetics in layers of pure fullerene and in donor−acceptor blends. This is in part due to the fact that only very few techniques are able to investigate the unoccupied DOS with the required accuracy. Whereas to our knowledge there is no theoretical prediction of such tremendous variations, it has been mentioned by Cheung et al. that PCBM shows an unusual distribution of localized and delocalized states making the treatment with standard models challenging.39 In summary, we have successfully described the VOC of various donor polymers in blends with PC70BM within the framework of Quasi-Fermi-level splitting. Bias amplified charge extraction measurements in combination with Kelvin Probe experiments provided conclusive information on the relevant energetics. We find the dependence of VOC on blend composition and thermal history to be largely determined by variations of the fullerene LUMO energy. Control of the PCBM energetics, e.g., via proper tuning of the blend microstructure, may offer promise for a considerable increase in VOC. 3869



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REFERENCES

(1) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat Photon 2009, 3, 297−302. (2) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat Photon 2012, 6, 591−595. (3) Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Zhang, J.; Chand, S.; Bazan, G. C.; Heeger, A. J. Efficient Solution-Processed SmallMolecule Solar Cells with Inverted Structure. Adv. Mater. 2013, 25, 2397−2402. (4) Koster, L. J. A.; Mihailetchi, V. D.; Ramaker, R.; Blom, P. W. M. Light Intensity Dependence of Open-Circuit Voltage of Polymer: Fullerene Solar Cells. Appl. Phys. Lett. 2005, 86, 123509−3. (5) Bässler, H. Charge Transport in Disordered Organic Photoconductors a Monte Carlo Simulation Study. Phys. Status Solidi B 1993, 175, 15−56. (6) Scholes, G. D.; Rumbles, G. Excitons in Nanoscale Systems. Nat. Mater. 2006, 5, 683−696. (7) Sueyoshi, T.; Fukagawa, H.; Ono, M.; Kera, S.; Ueno, N. LowDensity Band-Gap States in Pentacene Thin Films Probed with Ultrahigh-Sensitivity Ultraviolet Photoelectron Spectroscopy. Appl. Phys. Lett. 2009, 95, 183303−3. (8) Hulea, I. N.; Brom, H. B.; Houtepen, A. J.; Vanmaekelbergh, D.; Kelly, J. J.; Meulenkamp, E. A. Wide Energy-Window View on the Density of States and Hole Mobility in Poly(p-phenylene vinylene). Phys. Rev. Lett. 2004, 93, 166601. (9) Blakesley, J. C.; Neher, D. Relationship between Energetic Disorder and Open-Circuit Voltage in Bulk Heterojunction Organic Solar Cells. Phys. Rev. B 2011, 84, 075210. (10) Maurano, A.; Hamilton, R.; Shuttle, C. G.; Ballantyne, A. M.; Nelson, J.; O’Regan, B.; Zhang, W.; McCulloch, I.; Azimi, H.; Morana, M.; Brabec, C. J.; Durrant, J. R. Recombination Dynamics as a Key Determinant of Open Circuit Voltage in Organic Bulk Heterojunction Solar Cells: A Comparison of Four Different Donor Polymers. Adv. Mater. 2010, 22, 4987−4992. (11) Credgington, D.; Durrant, J. R. Insights from Transient Optoelectronic Analyses on the Open-Circuit Voltage of Organic Solar Cells. J. Phys. Chem. Lett. 2012, 3, 1465−1478. (12) Vandewal, K.; Gadisa, A.; Oosterbaan, W. D.; Bertho, S.; Banishoeib, F.; Van Severen, I.; Lutsen, L.; Cleij, T. J.; Vanderzande, D.; Manca, J. V. The Relation between Open-Circuit Voltage and the Onset of Photocurrent Generation by Charge-Transfer Absorption in Polymer: Fullerene Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2008, 18, 2064−2070. (13) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V. On the Origin of the Open-Circuit Voltage of Polymer-Fullerene Solar Cells. Nat. Mater. 2009, 8, 904−909. (14) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca, J. V. Relating the Open-Circuit Voltage to Interface Molecular Properties of Donor:Acceptor Bulk Heterojunction Solar Cells. Phys. Rev. B 2010, 81, 125204. (15) Piersimoni, F.; Chambon, S.; Vandewal, K.; Mens, R.; Boonen, T.; Gadisa, A.; Izquierdo, M.; Filippone, S.; Ruttens, B.; D’Haen, J.; Martin, N.; Lutsen, L.; Vanderzande, D.; Adriaensens, P.; Manca, J. V. Influence of Fullerene Ordering on the Energy of the Charge-Transfer State and Open-Circuit Voltage in Polymer:Fullerene Solar Cells. J. Phys. Chem. C 2011, 115, 10873−10880. (16) Lange, I.; Blakesley, J. C.; Frisch, J.; Vollmer, A.; Koch, N.; Neher, D. Band Bending in Conjugated Polymer Layers. Phys. Rev. Lett. 2011, 106, 216402. (17) Foertig, A.; Rauh, J.; Dyakonov, V.; Deibel, C. Shockley Equation Parameters of P3HT:PCBM Solar Cells Determined by Transient Techniques. Phys. Rev. B 2012, 86, 115302. (18) Street, R. A. Localized State Distribution and Its Effect on Recombination in Organic Solar Cells. Phys. Rev. B 2011, 84, 075208.

P3HT (Sepiolid P200, Riecke Metals Inc.):PCBM (Solenne) blends in different weight ratios were spin-cast from chloroform with a resulting film thickness of about 200 nm. Respective annealing has been done for 15 min at 150 °C on a hot plate. Blends comprising PCPDTBT or F-PCPDTBT (synthesized according to literature34,40):PCBM or ICBA (Lumtec) were spin-cast from chlorobenzene (1:3 by weight) with about 200 nm thickness. This thickness is larger than typically employed in some highly efficient devices, but was chosen here to allow for an accurate determination of the steady state charge density from charge extraction experiments.41 All solar cells in the structure ITO/PEDOT:PSS (CLEVIOS P VP AI 4083)/active layer/Ca/Al had an active area of 1 mm2 given by the overlap of the ITO and Ca/Al electrodes. Preparation and KP measurements were performed exclusively in a N2-filled glovebox. For BACE measurements under ambient conditions the devices were encapsulated. For illumination an array of four LEDs (Lumiled Rebel) with converging lenses powered by the amplified signal of an Agilent 33220A function generator has been used. The bias for VOC conditions and charge extraction were provided by an Agilent 81150A function generator in conjunction with an amplifier. The VOC has been measured via the voltage drop at a 10 MΩ resistor, current transients at the 50 Ω input of a Yokogawa DL9140 oscilloscope. Kelvin Probe measurements have been carried out with an SKM KP 4.5 (KP Technology Ltd.) with 2 mm probe diameter and in a N2-filled glovebox at room temperature in the dark. Calibration of the tip work function was done against highly ordered pyrolytic graphite (HOPG), for which we assumed a work function of 4.6 eV.42

S Supporting Information *

Full names and structures of applied materials, additional information on BACE measurements, derivation of VOC from QFL-splitting and assumed shape of the DOS, display of all KP measurements on blends, thickness-dependent change in P3HT crystallinity by absorption measurements, and simulation of Kelvin-data for different DOS distributions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 331 9771265. Fax: +49 331 977 5615. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Alessandro Troisi and Dan Credgington for a fruitful discussion and Florian Fischer from the University of Stuttgart for the high-temperature GPC. S.J. acknowledges Eileen Katholing for help with the synthesis. This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Centre HIOS (SFB 951) and the SPP 1355, and by the German Ministry of Science and Education within “Aufbau-PVComB” (FKZ 03IS2151D). 3870



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(19) Rauh, D.; Wagenpfahl, A.; Deibel, C.; Dyakonov, V. Relation of Open Circuit Voltage to Charge Carrier Density in Organic Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2011, 98, 133301−3. (20) Guan, Z.-L.; Kim, J. B.; Wang, H.; Jaye, C.; Fischer, D. A.; Loo, Y.-L.; Kahn, A. Direct Determination of the Electronic Structure of the Poly(3-Hexylthiophene):Phenyl-[6,6]-C61 Butyric Acid Methyl Ester Blend. Org. Electron. 2010, 11, 1779−1785. (21) Blakesley, J. C.; Greenham, N. C. Charge Transfer at PolymerElectrode Interfaces: The Effect of Energetic Disorder and Thermal Injection on Band Bending and Open-Circuit Voltage. J. Appl. Phys. 2009, 106, 034507−7. (22) Aarnio, H.; Sehati, P.; Braun, S.; Nyman, M.; de Jong, M. P.; Fahlman, M.; Ö sterbacka, R. Spontaneous Charge Transfer and Dipole Formation at the Interface between P3HT and PCBM. Adv. Energy Mater. 2011, 1, 792−797. (23) Clarke, T. M.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. Free Energy Control of Charge Photogeneration in Polythiophene/Fullerene Solar Cells: The Influence of Thermal Annealing on P3HT/PCBM Blends. Adv. Funct. Mater. 2008, 18, 4029−4035. (24) Boix, P. P.; Garcia-Belmonte, G.; Munecas, U.; Neophytou, M.; Waldauf, C.; Pacios, R. Determination of Gap Defect States in Organic Bulk Heterojunction Solar Cells from Capacitance Measurements. Appl. Phys. Lett. 2009, 95, 233302−3. (25) McMahon, D. P.; Cheung, D. L.; Troisi, A. Why Holes and Electrons Separate So Well in Polymer/Fullerene Photovoltaic Cells. J. Phys. Chem. Lett. 2011, 2, 2737−2741. (26) Clark, J.; Silva, C.; Friend, R. H.; Spano, F. C. Role of Intermolecular Coupling in the Photophysics of Disordered Organic Semiconductors: Aggregate Emission in Regioregular Polythiophene. Phys. Rev. Lett. 2007, 98, 206406. (27) Clark, J.; Chang, J.-F.; Spano, F. C.; Friend, R. H.; Silva, C. Determining Exciton Bandwidth and Film Microstructure in Polythiophene Films Using Linear Absorption Spectroscopy. Appl. Phys. Lett. 2009, 94, 163306−3. (28) Turner, S. T.; Pingel, P.; Steyrleuthner, R.; Crossland, E. J. W.; Ludwigs, S.; Neher, D. Quantitative Analysis of Bulk Heterojunction Films Using Linear Absorption Spectroscopy and Solar Cell Performance. Adv. Funct. Mater. 2011, 21, 4640−4652. (29) Jamieson, F. C.; Domingo, E. B.; McCarthy-Ward, T.; Heeney, M.; Stingelin, N.; Durrant, J. R. Fullerene Crystallisation as a Key Driver of Charge Separation in Polymer/Fullerene Bulk Heterojunction Solar Cells. Chem. Sci. 2012, 3, 485−492. (30) Shoaee, S.; Subramaniyan, S.; Xin, H.; Keiderling, C.; Tuladhar, P. S.; Jamieson, F.; Jenekhe, S. A.; Durrant, J. R. Charge Photogeneration for a Series of Thiazolo−Thiazole Donor Polymers Blended with the Fullerene Electron Acceptors Pcbm and ICBA. Adv. Funct. Mater. 2013, 23, 3286−3298. (31) Etzold, F.; Howard, I. A.; Forler, N.; Cho, D. M.; Meister, M.; Mangold, H.; Shu, J.; Hansen, M. R.; Müllen, K.; Laquai, F. The Effect of Solvent Additives on Morphology and Excited-State Dynamics in PCPDTBT:PCBM Photovoltaic Blends. J. Am. Chem. Soc. 2012, 134, 10569−10583. (32) Gu, Y.; Wang, C.; Russell, T. P. Multi-Length-Scale Morphologies in PCPDTBT/PCBM Bulk-Heterojunction Solar Cells. Adv. Energy Mater. 2012, 2, 683−690. (33) Dennler, G.; Scharber, M. C.; Brabec, C. J. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2009, 21, 1323−1338. (34) Albrecht, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; Neher, D. Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2012, 134, 14932−14944. (35) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright FutureBulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, E135−E138.

(36) He, Y.; Chen, H.-Y.; Hou, J.; Li, Y. Indene−C60 Bisadduct: A New Acceptor for High-Performance Polymer Solar Cells. J. Am. Chem. Soc. 2010, 132, 1377−1382. (37) Zhao, G.; He, Y.; Li, Y. 6.5% Efficiency of Polymer Solar Cells Based on Poly(3-Hexylthiophene) and Indene-C60 Bisadduct by Device Optimization. Adv. Mater. 2010, 22, 4355−4358. (38) Rao, A.; Chow, P. C. Y.; Gelinas, S.; Schlenker, C. W.; Li, C.-Z.; Yip, H.-L.; Jen, A. K. Y.; Ginger, D. S.; Friend, R. H. The Role of Spin in the Kinetic Control of Recombination in Organic Photovoltaics. Nature 2013, 500, 435−439. (39) Cheung, D. L.; Troisi, A. Theoretical Study of the Organic Photovoltaic Electron Acceptor PCBM: Morphology, Electronic Structure, and Charge Localization. J. Phys. Chem. C 2010, 114, 20479−20488. (40) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.; Scharber, M.; Brabec, C. Panchromatic Conjugated Polymers Containing Alternating Donor/Acceptor Units for Photovoltaic Applications. Macromolecules 2007, 40, 1981−1986. (41) Kirchartz, T.; Nelson, J. Meaning of Reaction Orders in Polymer: Fullerene Solar Cells. Phys. Rev. B 2012, 86, 165201. (42) Beerbom, M. M.; Lägel, B.; Cascio, A. J.; Doran, B. V.; Schlaf, R. Direct Comparison of Photoemission Spectroscopy and in Situ Kelvin Probe Work Function Measurements on Indium Tin Oxide Films. J. Electron Spectrosc. Relat. Phenom. 2006, 152, 12−17.

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