ARTICLE pubs.acs.org/JPCC
Fullerene Contribution to Photocurrent Generation in Organic Photovoltaic Cells Nicolas C. Nicolaidis, Ben S. Routley, John L. Holdsworth, Warwick J. Belcher, Xiaojing Zhou, and Paul C. Dastoor* Centre for Organic Electronics, University of Newcastle, Callaghan, NSW 2308 Australia. ABSTRACT: Organic photovoltaic devices based on blends of organic donors and acceptors are a promising new energy generation technology. In the case of binary blends based on mixtures of conjugated polymer donors and fullerene acceptors, the conventional view is that light harvesting occurs in the conjugated polymer component, and following primary charge separation at a donoracceptor interface, the photogenerated charges are then transported by the respective donor and acceptor networks. By comparing the results of detailed optical modeling with device performance, we demonstrate that for 1:1 blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenylC61-butyric acid methyl ester (PCBM) this conventional view is incomplete and that there is a significant fullerene photocurrent contribution. Analysis of the photocurrent generation in these devices yields the result that ca. 13% of device short circuit current under air mass (AM) 1.5 illumination conditions arises from the contribution of the fullerene component.
1. INTRODUCTION Organic photovoltaic cells are widely predicted to play a significant part of any future energy economy, due to their inherent ability to provide low cost, flexible solutions for sustainable energy generation that can be readily integrated into variety of products.1 Typically, the best cells reported exhibit power conversion efficiencies of upward of 5% and contain active layers that comprise 1:1 blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).2 More recently, devices utilizing modified polythiophene derivatives have led to device efficiencies in excess of 7.4%.3 The conventional explanation of the operation of these devices is that light absorption by the polymer generates an exciton4 which then diffuses for approximately 10 nm whereupon if it encounters a donoracceptor interface the bound pair will split or else recombine.5 The fullerene’s role as an electron acceptor and in providing a continuous network for electron transport in polymer blend devices is well established but its function in light harvesting and subsequent photocurrent generation is less well recognized.6 Photocurrent generation from fullerenes was first discussed in the context of bilayer polymer/fullerene devices by Pettersson et al. who showed that their photocurrent action spectra could only be accurately modeled by including a photocurrent contribution from light absorbed by the C60 layer.7 However, subsequent discussion of the nature of fullerenepolymer photovoltaic cells in bilayer and bulk heterojunction architectures focused only on primary charge separation following light absorption solely by the polymer.812 More recently, Dastoor et al. showed that for poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] r 2011 American Chemical Society
(MEH-PPV):fullerene blends there was a significant fullerene photocurrent contribution. For MEH-PPV and related polyphenylenevinylene polymers, the optimal polymer:fullerene ratio is known8,9 to be of the order of 1:4, and it was shown that for these devices the fullerene component contributed over half of the total generated photocurrent.5 However, despite this work, even the most recent reviews of device function have continued to focus only on the generation of excitons by light absorbed by the polymer component.6,13,14 In the case of the P3HT:PCBM system, the role of the fullerene component in the photocurrent generation process is readily observed by the presence of a peak in the external quantum efficiency (EQE) spectrum at around 340350 nm. In addition, the effect of photocurrent generation by the fullerene component is commonly exploited by the use of C70-based fullerenes to enhance spectral sensitivity, since the change from spherical symmetry to cylindrical symmetry splits formerly degenerate energy levels leading to increased absorption in comparison with C60-based fullerenes.2 However, again despite these observations, the role of charge generated by the fullerene component is almost universally neglected in discussions regarding the operation of polymer-fullerene devices. In this work, we present optical modeling and device measurements for 1:1 blend P3HT:PCBM devices with two different active layer thicknesses. We show that for both the thick and the thin structures, the fullerene component contributes almost 13% Received: January 25, 2011 Revised: March 4, 2011 Published: March 28, 2011 7801
dx.doi.org/10.1021/jp2007683 | J. Phys. Chem. C 2011, 115, 7801–7805
The Journal of Physical Chemistry C
ARTICLE
of the total photocurrent generated by the device. The results are discussed in the context of understanding and improving the performance of polymerfullerene devices.
2. EXPERIMENTAL SECTION a. Materials. PCBM was supplied by the Hummelen group at the University of Groningen. Rieke Metals Inc. provided the electronic grade regioregular (9095%) P3HT. PEDOT:PSS (Baytron P) was supplied by Bayer AG. b. Device Preparation. PEDOT:PSS (Baytron P) films were spin-coated (5000 rpm) on precleaned, patterned indium tin oxide (ITO) coated glass slides and were annealed at 120 °C for 30 min to eliminate water in the films before transfer into a dry nitrogen atmosphere glovebox. Blended films of P3HT and PCBM were spun under nitrogen from chloroform at a concentration of 25 mg/mL for the 200 nm devices and 18 mg/mL for the 85 nm devices. A weight ratio of 1:1 for all P3HT:PCBM blends was used. Film thicknesses were measured by a KLA Tencor profilometer and typically had an error equivalent to 10% of the measured thickness. Finally, the top metal electrode of total thickness ∼150 nm was deposited by thermal evaporation of aluminum. Due to the patterned ITO substrate, six individually addressable devices were fabricated from each prepared film, with the active area of each device (determined by the overlap between the ITO and the metal) being approximately 5 mm2. After the evaporation, fabricated devices were annealed at 140 °C on a hot plate (temperature variation (2 °C) for 4 min. c. Film and Device Characterization. For UVvis characterization, the relevant films were spin coated on quartz slides following the procedures mentioned above. An ultravioletvisible absorption spectrophotometer (UVvis, Varian Cary 6000i) was used to measure the reflection and transmission of the relevant component layers, before and after annealing. The photocurrent densityvoltage (JV) measurements were conducted using a Class A solar simulator with an AM 1.5 spectrum filter (Newport Corp.) to illuminate the full cells. The light intensity was measured to be 90 mW cm2 by a silicon reference solar cell (FHG-ISE) and the JV data were recorded by a Keithley 2400 source meter. The external quantum efficiency (EQE) measurements were conducted using a chopped arc lamp source coupled through an Oriel Cornerstone scanning monochromator. This 2 nm bandwidth monochromatic light is then coupled via a quartz fiber bundle into the glovebox where it is then focused to illuminate the cell area with no overfilling. The illuminated device output electrodes are connected to a Stanford Research Systems SR830 lock-in amplifier. A reference diode measuring a reflection off a glass wedge is used to monitor any source intensity fluctuations. The monitor data is acquired by a separate lock-in amplifier (Ithaco Dynatrac) with absolute spectral corrections via a lookup table generated from a reference scan performed prior to the measurement of the EQE. Output signals from both phase-locked loop measurements are recorded on a PC as is the illumination wavelength. d. Modeling. Modeling of oscillators in the PCBM/P3HT was performed in the majority by Kim oscillators as per previous studies.15 The asymmetric nature of the ππ* transition in P3HT was modeled by a CampiCorriasso model which accurately described the transmission and reflection of the both the pure and blended films.16 As described in detail elsewhere,5 prior to calculating the electric field inside the device structures the optical constants of the individual layers were first determined
Figure 1. Dielectric function of P3HT:PCBM as determined by transmission, reflection measurements and subsequent modeling. The upper (dash-dot) and lower (solid) lines show the real (ε1) and imaginary (ε2) parts respectively of the dielectric function for the P3HT:PCBM 1:1 blend film. The imaginary part of the P3HT dielectric function (dotted line) was obtained by removing the oscillators corresponding to just the PCBM transitions from the total set of oscillators used to model the blend dielectric function. Similarly, the imaginary part of the PCBM dielectric function (dashed line) was obtained by removing the oscillators corresponding to just the P3HT transitions.
from transmission and reflection measurements using techniques similar to those used in previous investigations.17 Briefly, the complex dielectric function was fitted to the experimentally obtained reflectance and transmission spectra using the SCOUT2 software package (M.Theiss, Aachen, Germany)18 for the PEDOT: PSS and P3HT:PCBM layers.
3. RESULTS AND DISCUSSION In order to deconvolve the contributions of the polymer and fullerene components to the generated photocurrent, the optical absorption characteristics of the multilayered functional devices were modeled to obtain the true absorption spectra for the active layer inside the device. In this study, the reflectance and transmission spectra of pure films of P3HT and PCBM were also measured and used to obtain the dielectric function of the polymer and fullerene components separately as a sum of model oscillator functions. This approach allowed us to identify the characteristic oscillators for the P3HT and PCBM components separately. The dielectric function for the PCBM component in the blend could then be obtained by simply removing the subset of P3HT oscillators from the complete set of oscillators used to model the P3HT:PCBM dielectric function. The resulting PCBM dielectric function was then compared with that of the pure layer and the oscillator distribution adjusted iteratively to ensure a good match between the two. This process was then repeated to obtain the dielectric function for the P3HT component in the blend by removing the subset of PCBM oscillators. Figure 1 shows the optical constants of the P3HT:PCBM blend obtained from the model software together with the imaginary parts of the P3HT and PCBM dielectric functions obtained via the method of oscillator removal. The pure film dielectric functions obtained in this manner typically exhibited root mean squared deviations of less than 0.01 for the real and imaginary component of the dielectric function when compared to those presented previously in the literature.15,19 Using the 7802
dx.doi.org/10.1021/jp2007683 |J. Phys. Chem. C 2011, 115, 7801–7805
The Journal of Physical Chemistry C
ARTICLE
Figure 2. Maps of total absorption as a function of device thickness and incident wavelength for an 85 nm active layer device. (A) PCBM absorption map, (B) P3HT absorption map, (C) combined PCBM and P3HT absorption map,and (D) layer structure and thickness.
Figure 3. Maps of total absorption as a function of device thickness and incident wavelength for a 200 nm active layer device. (A) PCBM absorption map, (B) P3HT absorption map, (C) combined PCBM and P3HT absorption map, and (D) layer structure and thickness.
electric field distribution (|Ez|2) obtained from the model for the P3HT:PCBM device structure, together with the imaginary parts ) and PCBM of the dielectric functions for the P3HT (εP3HT 2 ) components allows the absorption distributions to be (εPCBM 2 calculated for the individual components since the absorption (Qi) of the ith component is simply given by
Figure 4, panels A and B, shows the light absorption as a function of wavelength for the 85 and 200 nm active layer thickness devices respectively. The absorption spectra were obtained by integrating the absorption maps shown in Figures 2 and 3 with respect to device thickness across the active layer. The peak observed at an incident wavelength of around 420 nm in the modeled absorption spectrum of the 200 nm thick active layer device (Figure 4B) is not attributable to either a physical transition in P3HT or PCBM but is observed to shift as the modeled thickness is changed. Furthermore, in the modeled absorption spectrum of devices with thinner active layers (Figure 4A), this peak is no longer observed indicating that it arises from interference effects. The fractional absorption of each of the two components is shown in Figure 4C and demonstrates that, as expected, the bulk of the absorption between 370 and 670 nm is due to the polymer component, whereas the PCBM absorption dominates at wavelengths below 370 nm and above 670 nm. Figure 5, panels A and B, shows the external quantum efficiencies (EQEs) under AM1.5 radiation for the 85 and 200 nm devices, respectively. The EQEs shown in Figure 5, panels A and B, are both significantly enhanced in the region below 420 nm compared what should be expected if P3HT was the sole generator of charge. The data shown in Figure 5 shows that this enhancement can only be modeled by the PCBM component contributing directly to the photocurrent in this wavelength regime. The PCBM absorption exhibits a maximum at an incident wavelength of 330 nm, which should also be present in the EQE spectra. Unfortunately, the EQE measurements of the P3HT:PCBM blend devices presented here were limited to wavelengths above 375 nm due to the use of non quartz based elements in the measurement system and only hint at the development of a peak at lower wavelengths. However, EQE measurements demonstrating a photocurrent peak at 330 nm are widely available in the literature,8,20 and while not commented upon in the papers themselves, the absorption models presented here demonstrate that this peak can only arise from photocurrent generated by the PCBM component. In addition, recent results from devices made
Qi ¼
1 ωε0 εi2 jEz 2 j 2
ð1Þ
where ε0 is the permittivity of free space, εi2 is the imaginary parts of the dielectric function for the ith component, and ω is the angular frequency of the light. Figures 2 and 3 show the calculated two-dimensional distribution of light absorbed by the P3HT and PCBM components as a function of depth in the device and wavelength of the incident beam for active layer thicknesses of 85 and 200 nm respectively. The absorption distributions shown in Figures 2 and 3 highlight that there are two mode structures that occur over most of the observed wavelengths. First, there is an incident surface mode that occurs within the first 50 nm from the PEDOT:PSS interface, which is dominated by P3HT absorption and occurs primarily in the 450600 nm wavelength range. Absorption occurring closer to the aluminum electrode is significantly weaker due to the requirement that the electric field must reach zero at the metal interface and which is further exacerbated by large extinction coefficients of the polymer and fullerene components. Second, there is an incident bulk mode that occurs at depths greater than the first 50 nm from the PEDOT:PSS interface, which is dominated by PCBM absorption and occurs primarily in the 300350 nm wavelength range. A comparison of Figures 2 and 3 also reveals the interference effects that occur upon increasing the thickness of the active layer from 85 to 200 nm. In particular, these interference effects produce an absorption minimum in the 200 nm thick film, which covers the entire wavelength range and which lies at about one-third of the depth from the PEDOT:PSS interface.
7803
dx.doi.org/10.1021/jp2007683 |J. Phys. Chem. C 2011, 115, 7801–7805
The Journal of Physical Chemistry C
ARTICLE
Figure 4. Absorption spectra as a function of wavelength for devices with different active layer thicknesses. The spectra show the calculated contribution of the P3HT (dotted line) and PCBM (dashed line) components together with the total spectra (solid line). (A) Absorption spectra for an active layer thickness of 85 nm. (B) Absorption spectra for an active layer thickness of 200 nm. (C) Fractional absorption spectrum for the P3HT (dotted line) and PCBM (dashed line) components.
Figure 5. EQE spectra as a function of wavelength for devices with different active layer thicknesses. The spectra show the calculated contribution of the P3HT (dotted line) and PCBM (dashed line) components together with the total spectra (solid line). (A) EQE spectra for an active layer thickness of 85 nm. (B) EQE spectra for an active layer thickness of 200 nm.
purely from PCBM have an EQE spectrum that correspond very closely to the PCBM component calculated for these devices.21 Figure 6 plots the contribution of the P3HT and PCBM components to the total photocurrent generated under incident AM1.5 solar radiation for the two different active layer thicknesses. These data were calculated by convoluting the calculated EQE spectra with the AM1.5 spectrum and show that the contribution to the total photocurrent from the PCBM component is significant, especially at wavelengths below 500 nm. Integrating under the graphs in Figure 6 provides a measure of the total photocurrent that would be generated by the polymer and fullerene components under AM1.5 conditions and the results are summarized in Table 1. In agreement with previous measurements,22 the total photocurrent generated by the devices varies with active layer thickness, with the 85 nm device producing a total current density of 6.4 mA/cm2 and the 200 nm device producing a total current density of 9.4 mA/cm2. The calculated short circuit current densities are in good agreement ((5%) with the corresponding measured short circuit current densities under AM1.5 conditions, which were 6.8 and 9.8 mA/cm2 respectively.23 Additionally, it is possible to calculate the fraction of the current density that is generated by the polymer and fullerene components in the blend under AM1.5 illumination for each of the active layer thicknesses. As shown in Table 1, the photocurrent contribution from the fullerene component corresponds to around 13% of the total current density generated by the device for both thicknesses.
Figure 6. Contribution of P3HT and PCBM to the photocurrent generated under incident AM 1.5 solar radiation for different active layer thicknesses. The spectra have been smoothed by a 5 point average to remove the rapid fluctuations inherent in the AM1.5 spectrum. Calculated component current densities shown in Table 1 use the unsmoothed data. The spectra show the calculated contribution of the P3HT (dotted line) and PCBM (dashed line) components together with the total spectra (solid line). (A) Active layer thickness of 85 nm; (B) active layer thickness of 200 nm. The total integrated current under the graphs A and B correspond to 6.4 and 9.4 mA/cm2, respectively.
The observation that the fullerene makes a significant contribution to the current density is consistent with previous studies that have demonstrated PCBM generated photocurrent in both bilayer poly(3-(40 -(100 ,400 ,700 -trioxaoctyl)phenyl)thiophene) (PEOPT)/C60 thin film heterojunction7 and MEH-PPV/PCBM bulk heterojunction devices.5 It is also interesting to compare the fractional contribution of the PCBM photocurrent in different polymer systems with different polymer:fullerene blend ratios. For example, in the case of the MEH-PPV:PCBM system, the ratio is 1:4 and for this system the fullerene contributed approximately 50% of the generated photocurrent.5 For the P3HT: PCBM system discussed here, the ratio is 1:1 and the fullerene contributes approximately 13% of the generated photocurrent. Thus, it would appear that the result for the PCBM contribution in P3HT:PCBM blend systems is broadly consistent with that observed in the MEH-PPV:PCBM system, since the system here has one-quarter of the PCBM in its composition and the current contribution has been reduced by about the same corresponding fraction. Thus, it would appear that the phenomenon of photocurrent originating from light absorbed by the fullerene component is general and occurs in both bilayer and bulk heterojunction devices. Further work is currently underway to determine whether this 7804
dx.doi.org/10.1021/jp2007683 |J. Phys. Chem. C 2011, 115, 7801–7805
The Journal of Physical Chemistry C
ARTICLE
Table 1. Characteristics of P3HT:PCBM Blend Devicesa active layer thickness
open cicuit short circuit current density voltage (Voc)
(nm)
(V)
(Jsc) (mA/cm2)
PCBM current
P3HT current
fraction PCBM
fraction P3HT
fill factor
power conversion efficiency
density under AM1.5
density under AM1.5
current density under
current density under
(FF)
(PCE) (%)
(mA/cm2)
(mA/cm2)
AM1.5 (%)
AM1.5 (%)
85
0.58
6.8
0.51
2.0
0.85
5.58
13.2
86.8
200
0.58
9.8
0.52
3.0
1.27
8.12
13.5
86.5
Experimental device performance of the best cells are shown in the first four columns. The measured short circuit current density has been corrected for spectral mismatch of the AM 1.5 simulator. The modeled currents for the components are computed using the EQE and then scaled to the AM 1.5G standard and partitioned as described by eq 1.
a
photocurrent generation occurs through energy or charge transfer at the fullerene:polymer interface. Notwithstanding the detailed origin of the charge generation mechanism, this study demonstrates that the photocurrent generated by light that is absorbed by the fullerene component needs to be considered when evaluating the performance of OPV systems containing PCBM.
4. CONCLUSIONS Optical modeling has been used to probe the origin of charge generation in P3HT:PCBM blend organic solar cells. The modeling work presented here shows that light absorbed by the PCBM component generates a significant proportion of the total photocurrent generated by the solar cell. In the case of P3HT:PCBM 1:1 blends the PCBM component generates up to 13% of the total photocurrent produced by the active layer in the device. This result is consistent with previous results for MEH-PPV: PCBM 1:4 devices in which the PCBM component generated up to 50% of the observed photocurrent. Overall, this study indicates that PCBM plays a general role in both light harvesting and charge transport and that this behavior needs to be considered when evaluating the performance of new materials in similar blend systems. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: þ61 2 49215468. Fax: þ61 2 49216907.
(9) Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.; Hinsch, A.; Meissner, D.; Sariciftci, N. S. Adv. Funct. Mater. 2004, 14, 1005–1011. (10) Roncali, J. Chem. Soc. Rev. 2005, 34, 483–495. (11) Spanggaard, H.; Krebs, F. C. Solar Energy Mater. Solar Cells 2004, 83, 125–146. (12) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533–4542. (13) Nicholson, P. G.; Castro, F. A. Nanotechnology 2010, 21, 492001. (14) Moule, A. J. Curr. Opin. Solid State Mater. Sci. 2010, 14, 123–130. (15) Hoppe, H.; Sariciftci, N. S.; Meissner, D. Mol. Cryst. Liq. Cryst. 2002, 385, 113–119. (16) Campi, C.; Coriasso, D. J. Appl. Phys. 1988, 64, 4128–4134. (17) Hoppe, H.; Arnold, N.; Meissner, D.; Sariciftci, N. S. Thin Solid Films 2004, 451452, 589–592. (18) Theiss, W., Hard- and Software, Dr.-Bernhard-Klein-Str. 110, D-52078 Aachen, Germany (www.mtheiss.com). (19) Gurau, M. C.; Delongchamp, D. M.; Vogel, B. M.; Lin, E. K.; Fischer, D. A.; Sambasivan, S.; Richter, L. J. Langmuir 2007, 23, 834–842. (20) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. J. Mater. Sci. 2005, 40, 1371–1376. (21) Song, Q. L.; Gong, C.; Yang, H. B.; Li, C. M. Solar Energy Mater. Solar Cells 2010, 94, 1429–1439. (22) Moule, A. J.; Bonekamp, J. B.; Meerholz, K. J. Appl. Phys. 2006, 100, 094503. (23) Gilot, J.; Wienk, M. J.; Janssen, R. A. J. Adv. Funct. Mater. 2010, 20, 3904–3911.
’ ACKNOWLEDGMENT We thank the Australian Research Council for funding this work and N.N. gratefully acknowledges the University of Newcastle for the provision of a research scholarship. ’ REFERENCES (1) Dennler, G.; Sariciftci, N. S. Proc. IEEE 2005, 93, 1429–1439. (2) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222–225. (3) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, E135. (4) Winder, C.; Sariciftci, N. S. J . Mater. Chem. 2004, 14, 1077–1086. (5) Dastoor, P. C.; McNeill, C. R.; Frohne, H.; Foster, C. J.; Dean, B.; Fell, C. J.; Belcher, W. J.; Campbell, W. M.; Officer, D. L.; Blake, I. M.; Thordarson, P.; Crossley, M. J.; Hush, N. S.; Reimers, J. R. J. Phys. Chem. C 2007, 111, 15415–15426. (6) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736–6767. (7) Pettersson, L. A. A.; Roman, L. S.; Ingan€as, O. J. Appl. Phys. 1999, 86, 487–496. (8) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 1077–1086. 7805
dx.doi.org/10.1021/jp2007683 |J. Phys. Chem. C 2011, 115, 7801–7805
