Importance of Disordered Polymer Segments to Microstructure

ARTICLE pubs.acs.org/JPCC

Importance of Disordered Polymer Segments to Microstructure-Dependent Photovoltaic Properties of PolymerFullerene Bulk Heterojunction Solar Cells Fu-Chiao Wu,† Ying-Chou Huang,† Horng-Long Cheng,*,† Wei-Yang Chou,† and Fu-Ching Tang‡ †

Institute of Electro-Optical Science and Engineering, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan, R.O.C. ‡ Department of Physics, National Cheng Kung University, Tainan 701, Taiwan, R.O.C. ABSTRACT: While regulation of the nanoscale microstructure of the active layers in organic bulk heterojunction (BHJ) solar cells, particularly for conjugated polymerfullerene blend systems, has been shown to be highly important when maximizing power conversion efficiency, little is known about the role of disordered polymer chains in the photovoltaic (PV) behaviors and electrochemical potential drops of polymerfullerene interfaces. In this study, the microstructural-dependent PV properties of a series of poly(3-hexylthiophene) (P3HT):fullerene (i.e., [6,6]-phenyl-C61butyric acid methyl ester, or PCBM) blending films with different compositions have been investigated using several experiments (i.e., absorption spectroscopy, Raman spectroscopy, X-ray diffraction, and atomic force microscopy) and theoretical methods (i.e., spectroscopic simulation and quantum mechanical calculations). A strong correlation exists between amorphous P3HT chain properties, characterized by degree of conjugation (Leff), and PV parameters. The impact of Leff of amorphous P3HT on exciton dissociation is addressed, thus providing an ideal structural model for organic BHJ solar cells. Although bigger P3HT and PCBM domains favor carrier transport, the control of disordered P3HT segments and PCBM contact is crucial to exciton dissociation, which can consequently optimize PV performance.

’ INTRODUCTION Organic bulk heterojunction (BHJ) solar cells are promising candidates for the new generation of renewable energy technology because of their flexibility and easy, low-temperature processes. Today, the power conversion efficiency (PCE) of organic BHJ solar cells has been increased to more than 8%.1 The power conversion processes, including solar light absorption, exciton generation, exciton dissociation, and charge carrier transport, all occur in the active layers. The solar light absorption and exciton generation processes correlate with the nature of materials, for example, absorption range and extinction coefficient. The exciton dissociation and charge carrier transport processes are relevant to the morphology and microstructure in active layers.2,3 In addition to selecting suitable materials, optimizing the exciton dissociation and charge carrier transport in active layers is another way to extract more charge carriers from active layers. Therefore, understanding the morphology and microstructure of active layers, and thus the photovoltaic mechanism, is important for improving the PCE of organic BHJ solar cells. The most often used technique for fabricating active layers of organic BHJ solar cells is blending donor and acceptor through a solution process. Solution processable conjugated polymer fullerene blends based photovoltaic devices are one of the most widely studied classes of organic BHJ solar cells, so-called polymer solar cells (PSCs). In a solution process, solvent type,46 donor and acceptor compositions,4,79 solution r 2011 American Chemical Society

concentration,10,11 incorporation of additives,2,12,13 post-treatment of active layers,1416 and regioregularity of conjugated polymers17 are considered to develop various morphologies and microstructures of active layers. In general, the morphology of active layers with appropriate phase separation between donor and acceptor18 and good crystallinity of polymers are favorable for high-efficiency solar cells.4,19 The above-mentioned techniques are also regularly adopted to improve the crystallinity of polymers and phase separation between donor and acceptor. As active layers, polymers often possess crystalline and amorphous portions. Changes in the crystalline or amorphous portions can give rise to a different morphology and microstructure of active layers. However, as of now, many research works focus on the crystalline portion of polymers and attempt to increase the crystallinity of polymers to enhance absorption range and promote carrier transport.2,4,20,21 In contrast, the reduction of crystallinity in the donor polymer which results in a higher photovoltaic performance of PSCs has been observed.17,22 To date, the optimal microstructure of the active layer in PSCs is not yet fully understood. To the best of our knowledge, no systematic study involving the roles of the amorphous portion of polymers on the photovoltaic properties in PSCs has been undertaken. To Received: March 11, 2011 Revised: June 30, 2011 Published: June 30, 2011 15057

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The Journal of Physical Chemistry C obtain the appropriate phase separation between donor and acceptor, in addition to the crystalline portion of polymers, the amorphous portion of polymers must also be well controlled. Consequently, the functions of both crystalline and amorphous portions of polymers in PSCs need to be identified first to further improve the performance of PSCs. In this study, we adopt regioregular poly(3-hexylthiophene) (P3HT) as the electron donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor and change the PCBM loading in P3HT:PCBM blends, as the active layers, to fabricate various PSCs. With the change in the PCBM composition, the crystalline and amorphous forms of the P3HT component in blending films can be altered. In addition to the role of crystalline P3HT, we also investigate the impact of amorphous P3HT on the electrical parameters of the PSC devices by joint experimental and theoretical studies. Absorption spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM) measurements allow us to explore the crystalline and amorphous structures of P3HT in blending films, respectively. With the aid of quantum chemical calculations, we highlight the importance of the interaction between amorphous P3HT and PCBM on the photovoltaic performance and thus propose an ideal structural model for PSC devices.

’ EXPERIMENTAL DETAILS Sample Preparation. The PSC fabrication procedure started with a patterned and precleaned indium tin oxide (ITO) glass substrate. Then, the ITO was treated by oxygen plasma (Diener Electronic: Femto) for 3 min at a power of 50 W. Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron AI4083) was spin-coated on ITO at 3000 rpm and then baked at 150 °C for 30 min. Regioregular P3HT (Rieke Metals) was blended with various PCBM (Solenne BV) compositions in chlorobenzene at 20 mg/mL. Next, the P3HT:PCBM blending films, as an active layer, were spin-coated on the top of PEDOT: PSS at 1000 rpm and then baked at 130 °C for 30 min. Finally, 10 nm of calcium and 100 nm of aluminum were thermally evaporated on P3HT:PCBM successively at a deposition ratio of 1 and 3 Å/s, respectively. These PSC devices were postannealed at 150 °C for 30 min. The active area was 0.06 cm2. Characterizations. The absorption spectra were produced by a GBC Cintra 202 UV/vis spectrophotometer with a scan rate of 1000 nm/min. The spectrophotometer resolution was less than 0.9 nm. At least three absorption spectra were recorded on each sample. MicroRaman spectra produced by lattice phonons were obtained using a Jobin Yvon LabRam HR spectrometer. A 532 nm solid-state laser and a 633 nm HeNe laser served as the excitation light source and were kept to less than 0.5 mW to prevent thermal damage of the P3HT:PCBM blending films. Spatial resolution of the beam spot was around 1 μm, attained using a 100 objective microscope lens. The spectrometer resolutions are within 0.4 and 0.2 cm1 for 532 and 633 nm excitation lines, respectively. The Raman spectrum was taken for an average of three spectra, and at least three spectra were measured for each sample. The thin-film structure was studied from the XRD spectra recorded using a Rigaku RINT 2000 diffractometer (wavelength of X-ray: 1.5406 Å, step size: 0.01°). A atomic force microscope (Veeco, DI Dimension 3100) was used in the noncontact mode to measure the surface topological images of the P3HT:PCBM blends films. All images were acquired with a resolution of 256 256 pixels per scan at a scan

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rate of 0.5 Hz per line and a scan size of 5 5 μm. Electrical characteristics of the PSCs were measured through a solar simulator (Newport Corp.) with an AM1.5 filter and a Labview-controlled Keithley 2400 SourceMeter. Light intensity was 100 mW/cm2. Fabrication and electrical measurement of PSCs were performed in a nitrogen-filled glovebox. Quantum Chemical Calculations. The geometry of the thiophene monomer and a PCBM molecule was first optimized using the density functional theory (DFT) at the B3LYP/ 6-31G(d) level. The thiophene monomer was used to produce the polythiophene with various numbers of repeat units (N) and was optimized using DFT at the B3LYP/6-31G(d) level of theory with periodic boundary conditions. The optimized geometry structure of polythiophene chains was used to construct the two parallel chains with the interchain ππ stacking of 3.8 Å.23 The interchain excitonic coupling (Jnm) of the parallel chains was calculated using Zerner’s intermediate neglect of differential overlap (ZINDO/S) semiempirical quantum mechanical method. In the computations of electron affinity (EA) and ionization potential (IP) of polythiophene and PCBM, an optimized PCBM molecule and an optimized polythiophene chain were placed into a system, that is, polythiophenePCBM dimer, and the separated distance between the carbon atom in the fullerene of PCBM and the mass center of polythiophene was set to be 3.5 Å.24,25 The Coulomb-attenuating method (CAM-B3LYP), a long-range corrected version of B3LYP, was used because the EA and IP calculations are correlated with the charge transfer.26 The EA and IP of PCBM and polythiophene were computed using DFT at the CAM-B3LYP/6-31G(d) level with the counterpoise method and considering the dimer-centered basis set. For comparison, the optimized geometry of the polythiophenePCBM system was also calculated through the DFT at the CAM-B3LYP/ 6-31G(d) level. All the calculations were performed using the Gaussian suite of programs.27

’ RESULTS AND DISCUSSION A. Microstructural Properties. Figure 1 presents the absorption spectra of P3HT:PCBM blending films with different PCBM weight fractions (fPCBM). The absorption of the neat PCBM film is located in a high-energy region with a maximum (λmax) at approximately 3.0 eV. Thus, we observed that the absorption of the blend films in the high-energy region rises with the increasing fPCBM. The absorption in the low-energy region is mainly attributed to P3HT. The neat P3HT film shows a λmax at approximately 2.4 eV. To date, experimental results and theoretical predictions for P3HT films have suggested that the measured absorption for energy lower than 2.3 eV is due to the chains in the crystalline region with longer effective conjugation length (Leff). It has been suggested that the intensity of a shoulder located at approximately 2.05 eV (assigned as A0 band) increases with the increasing degree of crystallinity.28,29 According to a recent theory based on an H-aggregate model, the absorption spectrum arising from the ordered crystalline structure can be modeled using a modified FrankCondon analysis28,30,31 !2 ! eS Sm WeS Aµ Gm Γðpω E00 mEP Þ 1 m! 2EP m¼0

∑

ð1Þ where A is the relative absorption intensity; m is the vibrational level; S is the HuangRhys factor assumed to be 1;28,31 W is the 15058

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The Journal of Physical Chemistry C

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Table 1. Exciton Bandwidth (W), Half-Width of Absorption Band (β), Interchain Excitonic Coupling (Jnm), and Effective Conjugation Length (Leff) of Crystalline P3HT in the Blending Films with Various PCBM Weight Fractions (fPCBM) fPCBM (wt %)

Figure 1. (a) Absorption spectra of the P3HT:PCBM blending films with various PCBM weight fractions (fPCBM). The dashed lines serve as guidelines. (b) Comparison of experimental (circles) and simulated (lines) spectra of the P3HT:PCBM blending films with fPCBM of 25 wt % and 75 wt %. The curves are normalized to the maximum value of the simulated spectrum.

exciton bandwidth; EP is the energy of the main vibrational mode of P3HT coupled with the electronic transition; Gm is a constant equal to Σn(6¼m)Sn/n!(n m) (n is the vibrational quantum number); Γ is a Gaussian function; ω is the vibrational frequency; and E00 is the energy of the 00 electronic transition. Thus, absorption spectra of crystalline P3HT in blending films were obtained from fitting eq 1 to experimental data, as shown in the examples in Figure 1b. The observed W and the full width at half-maximum (β) of the absorption bands are listed in Table 1. For the blending films, the W values decrease with increasing fPCBM. Particularly, when the fPCBM is more than 50 wt %, we can observe a significant decrease in the W values. The Leff of P3HT chains in the crystalline regions can be evaluated from the Jnm using the relations W ∼ 4Jnm for free excitons.28,30 The Jnm values were also listed in Table 1. Using ZINDO/S calculations, for parallel polythiophene chains, a scaling relation of Jnm ∼ Leff1.81 can be observed for Leff being 10 repeat units length above (Figure 2). The relation is consistent with Barford’s model32 for parallel conjugated polymer chains (Jnm ∼ Leff1.8). Through Jnm ∼ Leff1.81, the estimated Leff of crystalline P3HT in various blending films is also listed in Table 1.33 Specifically, when fPCBM is more than 50 wt %, the Leff of P3HT in the blending films becomes longer. Meanwhile, a redshift of absorption spectra (Figure 1b) also suggests an extended Leff of P3HT chains in blending films with higher fPCBM. Interestingly, the β values of absorption bands of crystalline P3HT in blending films were not affected significantly by adding various fPCBM. The results suggested that the structural features of the P3HT crystalline domain in blending films are fairly homogeneous and are without interference from PCBM blending. Therefore, the longer Leff of P3HT chains in the

W (meV)

β (meV)

Jnm (meV)

Leff

0

145

196

36

25

25

168

202

42

23

33

170

200

43

23

50

155

204

39

24

67

131

202

33

26

75

72

202

18

36

Figure 2. Double-logarithmic plots of interchain excitonic coupling (Jnm, in meV) of parallel polythiophene chains versus various effective conjugation lengths (Leff, defined as the number of repeat units). The distance between polythiophene chains is 3.8 Å.

presence of large amounts of PCBM can be considered due to the immiscibility between the two components. The structural features of crystalline and amorphous P3HT in the P3HT:PCBM blending films can be further studied by joint Raman spectroscopy with different excitation wavelengths and DFT vibrational analysis.29,34 Moreover, we did not detect any Raman peaks from PCBM. The Raman spectra of P3HT:PCBM blending films with various fPCBM are shown in Figure 3. First, a laser with a wavelength of 633 nm was used to explore the crystalline portion of P3HT in blending films because the excitation energy is located in the red edge of the absorption region of the P3HT. In Figure 3a, the selected vibrational modes at around 14001500 cm1 represent the most intense symmetric CRCβ stretching deformation (∼1444 cm1, denoted as v1 band) present in the aromatic thiophene ring, which coupled to the electronic transition and also reflected the homogeneity of P3HT chain distribution.29,34,35 With the increase in fPCBM in the blending films, the v1 band gradually shifts to a low wavenumber (see Table 2). Quantum chemical calculations have shown that the increase in Leff causes the v1 band to downshift and follow an exponential decay law.29,34 In other words, when Leff is gradually extended, a slight downshift of the v1 band implies the largely increased Leff. Thus, the redshift of this v1 band indicates the increase in the Leff of P3HT. The difference spectra in Figure 3a (lower panel) show a decreased peak intensity in the high-frequency region (at ∼1450 cm1, which is attributed to the segments with short Leff) and an increased intensity in the lower-frequency range (at ∼1436 cm1, which is attributed to more extended conjugated segments with longer Leff), compared 15059



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Table 2. Peak Positions (λpeak) and Corresponding HalfWidths (β) of the v1 Band of the Blending Films With Various PCBM Weight Fractions (fPCBM) Excited by Different Laser Light Sources (Standard Deviations in Parentheses)

Figure 3. Raman spectra of the P3HT:PCBM blending films, with various PCBM weight fractions (fPCBM), excited by (a) 633 nm (crystalline-selective) and (b) 532 nm (amorphous-selective) laser light sources, respectively. Lower panels: Differences between the Raman spectra were obtained by subtracting the normalized spectra of the specimens and the reference sample. Reference spectra: Specimens with fPCBM of (a) 75 wt % and (b) 50 wt %, respectively. The dashed lines serve as guidelines.

with the spectrum of the sample with fPCBM of 75 wt %. However, the β of the v1 band (see Table 2) is almost the same for these blending films with various fPCBM. This would suggest that the homogeneity of chain conformations in the crystalline portion of P3HT is almost unchanged. These results are in agreement with the results of the absorption spectroscopy. In addition to the crystalline portion, the amorphous portion of P3HT in the blending films can be investigated by Raman spectroscopy with 532 nm laser excitation, as presented in Figure 3b. With the energy of the laser being larger than 2.3 eV, it can be used to study the P3HT chains with a relatively short Leff; that is, the chains are located at a disordered or amorphous region.29 Evidently, as compared to Figure 3a, the v1 band appears to have a considerable upshift to approximately 1447 cm1 and enlarged β (Table 2). With the increase in fPCBM from 25 to 50 wt %, the v1 band shifted to a high wavenumber; however, when the fPCBM further increased to 75 wt %, a significant downshift with a peak center even lower than that of the neat P3HT specimen occurs in the v1 band (see also Table 2). The results suggest that with fPCBM of 50 wt %, the disordered P3HTs in the blending films have a relatively short Leff compared to those of other films. When fPCBM is more than 50 wt %, the degree of conjugation of disordered P3HT chains is extended; thus, the Leff increases. The above-mentioned observations can be easily recognized from the difference spectra in the lower panel of Figure 3b using the sample with fPCBM of 50 wt % as the reference spectrum. Like the peak shift trend, the β of the v1 band increases to maximum when the fPCBM is 50 wt %. When the fPCBM is more than 50 wt %, the β decreases significantly. This result indicates the worst homogeneity of chain conformations in the amorphous portion of P3HT in the films with a fPCBM of 50 wt %, thus suggesting a better mix of amorphous P3HT and PCBM. Once the blending films have an fPCBM of more than 50 wt %, the increased homogeneity of the amorphous P3HT chain conformations suggests a higher degree of phase separation between P3HT and PCBM. Kim and Frisble showed a solubility limit of 3050 wt % for PCBM in the P3HT: PCBM blends.18 Consequently, the changes in the PCBM loading

λexc = 633 nm

λexc = 532 nm

(crystalline-selective)

(amorphous-selective)

fPCBM (wt %)

λpeak (cm1)

β (cm1)

λpeak (cm1)

β (cm1)

0

1444.6 (0.09)

24.9 (0.22)

1448.4 (0.17)

30.4 (0.19)

25

1444.3 (0.12)

24.9 (0.17)

1448.5 (0.05)

30.7 (0.06)

33

1444.5 (0.01)

24.6 (0.13)

1448.9 (0.05)

30.7 (0.06)

50

1444.3 (0.05)

25.1 (0.09)

1449.0 (0.08)

31.2 (0.02)

67

1444.2 (0.08)

24.8 (0.17)

1448.0 (0.05)

29.8 (0.03)

75

1443.5 (0.14)

24.8 (0.25)

1447.8 (0.03)

29.4 (0.17)

in P3HT:PCBM blending films have more obvious influences on the chain conformations on the amorphous P3HT than on the crystalline P3HT. The crystal qualities of the blending films with various fPCBM were examined by XRD measurement as presented in Figure 4a. For all the blending films, we observed that the peaks, which are (h00) planes, come from the edge-on orientation of the crystalline P3HT chains.36 The (100) peak is at approximately 5.33 ( 0.07° 2θ, confirming that P3HT backbones self-organize into a lamellar supramolecular structure36 due to the interchain ππ interactions (see Figure 4b). The estimated lattice constant a of approximately 16.56 ( 0.22 Å is the distance between P3HT backbones. This a value is consistent with that of P3HT microcrystalline found in the literature.23 For the neat P3HT films, the full width at half-maximum (B) of the (100) diffraction peak was 0.51° 2θ. As the fPCBM was less than 67 wt %, the blending films showed smaller B values of the (100) diffraction peak compared to those of the neat P3HT films. The B corresponds to a DebyeScherrer dimension of the coherently scattering crystal domains. The crystallite size (La) along the aaxis, i.e., in the vertical direction of the substrate, was rough estimated using the Scherrer equation described as follows37 La ¼

Kλ B cos θ

ð2Þ

where K is the Scherrer constant; λ is the wavelength of the X-ray; and θ is the diffraction angle of the peak. Here, K was set to be 0.9.37 The La as a function of fPCBM is plotted in Figure 4c. The observed La values are comparable with recently published data.38 The microstrain broadening effect, which may arise from alkyl side chain disorder,39 has been ignored. Thus, the calculated crystallite size is at a minimum. For the blending films, the La decreases as the fPCBM increases. Especially when the PCBM loading is more than 50 wt %, the decrease in the La of P3HT is great, and the XRD spectra have a more asymmetrical shape. This means P3HT has a better crystallinity in the vertical direction of the blending films with the fPCBM less than 50 wt %. On the basis of the absorption spectroscopy, the observed Leff values can be assumed to be the crystallite size (Lc) along the caxis (see Figure 4b). The Lc as a function of fPCBM is also plotted in Figure 4c. Thus, when fPCBM is less than 50 wt %, the crystalline P3HT appears to have a tetragonal-shaped stand on the surface as shown in the inset of Figure 4c. For the film with fPCBM of 75 wt %, the crystalline P3HT Lc is slightly larger than La, thus near-cubedshaped microcrystals. 15060



Figure 4. (a) XRD spectra of the P3HT:PCBM blending films, with different PCBM weight fractions (fPCBM). The B indicate the full-widthat-half-maximum of the (100) peak. Dashed lines serve as guidelines. (b) Simplified illustration of the P3HT crystalline domain within the blending films. Only one crystal domain is drawn. Inset: Self-assembled lamellar-like crystal orientations of P3HT. Crystallite size is shown along the a-axis (La) and c-axis (Lc), respectively. (c) La and Lc as a function of fPCBM. The shape of the crystalline domain of P3HT is also shown.

AFM images in Figure 5 were used to examine the surface morphology of the P3HT:PCBM blending films. The neat P3HT film shows a relatively large surface roughness compared to that of the blending films. From the XRD data, the crystalline P3HT chains adopted the edge-on orientation on the surface. Thus, we deduced that the hump peaks are a result of the development of the crystalline domains. When 25 wt % PCBM was added into the blending films, the shapes of the hump peaks become similar to those of the neat P3HT films but with smaller dimensions. Recently, it has been found that PCBM clusters appear in dark contrast of the frequency modulation mode AFM.40 Thus, the dark areas may contain more amorphous P3HT and PCBM. By performing a pixel analysis, we also found that the dark area fractions increased with increasing amount of PCBM (see Figure 5, right panels). It has been shown that PCBM is likely dissolved in the amorphous P3HT regions when the PCBM content is less than the phase separation point, i.e., ca. 3050 wt %.18 For specimens with fPCBM of 50 wt %, the hump peaks and dark areas cannot be clearly distinguished. This may provide support for a better mixed P3HT and PCBM, as suggested by Raman data. When fPCBM is more than 50 wt %, the surface clearly displays two distinct bigger phases. A possible origin for this morphology would be a serious phase separation between P3HT and PCBM due to an excess amount of PCBM. Summarizing the results of structural studies, the model of P3HT interacting with PCBM in blending films is illustrated in Figure 6. With the increasing fPCBM, the crystalline size

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perpendicular to the substrate, i.e., La, of P3HT was significantly reduced. At the same time, the crystalline size parallel to the substrate, i.e., Lc, showed opposite trends of changes, that is, slightly increased, whereas the homogeneity of chain conformations in the crystalline portion remains unchanged. Compared with the structure of crystalline P3HT, that of amorphous P3HT was sensitive to the addition of PCBM. Particularly, for amorphous P3HT, when loading is 50 wt % PCBM, the extent of conjugation is relatively short and has maximum inhomogeneous distribution. We thus deduced that the presence of an appropriate amount of PCBM would impede the P3HT chains to self-organize into crystalline domains, thus creating more amorphous domains with relatively short Leff as compared to other blending films. B. Electrical Characteristics of Solar Cells. The electrical characteristics, including short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and PCE, of the P3HT: PCBM blending film-based solar cells are shown in Figure 7. The devices with less than 50 wt % PCBM have larger Jsc of more than 11.0 mA/cm2. The observed values are consistent with the stateof-the-art P3HT:PCBM solar cells with fPCBM of approximately 40 wt %.8,41,42 However, as the PCBM loading is more than 50 wt %, the Jsc decreases significantly. Unlike the trend of Jsc changes, the Voc was increased with increasing fPCBM until 50 wt % and then saturated at near 0.6 V. The saturated Voc is similar to that found in most recent reports.8,41,42 For the FF properties, particularly, the device with 50 wt % PCBM has the highest FF value compared with the other devices. Meanwhile, the device reached the maximum PCE with an average value of ca. 3.6% (a maximum value up to near 4.5%). Obviously, when the fPCBM is less than 50 wt %, the PCE of the devices is limited by the low Voc. When the fPCBM is more than 50 wt %, the low PCEs of less than 2.0% in devices are due to the very low Jsc values. C. Correlation between Microstructural Properties and Photovoltaic Properties. The work mechanism of the organic solar cell can be simply described in the following steps: (i) absorption of a photon leading to the formation of an exciton, i.e., the bound electronhole pair; (ii) exciton diffusion to a region where exciton dissociation, i.e., charge separation, occurs; and (iii) charge transport within the organic semiconductor to the respective electrodes.43 Sunlight photons, which are absorbed in the polymer:fullerene BHJ PSC devices, excite the donor; this leads to the creation of photocarriers, i.e., excitons, in the conjugated polymer. Thus, absorption spectra analysis indicated stronger absorbance of the blending films with increasing amount of P3HT, thus benefitting the creation of a relatively large amount of excitons. These photogenerated excitons would undergo a diffusion process to a dissociation zone, which is a well-defined pn heterojunction interface with a strong built-in electric field from P3HT:PCBM complexes. In organic solar cells, the Voc is found to be proportional to the effective band gap of chargetransfer complexes, i.e., the difference of the highest occupied molecular orbital (HOMO) level of the donor and the lowest unoccupied molecular orbital (LUMO) level of the acceptor. That means Voc is affected by the interaction between P3HT and PCBM in morphology of the active layer of solar cells. When fPCBM is less than 50 wt %, we observe the relatively high absorbance (Figure 1a) and low Voc values (Figure 7b) compared to those of other samples. This would suggest the formation of a lesser amount of dissociation zone due to an excess amount and large crystalline domain of P3HT. This situation would reduce the contact probability of P3HT and PCBM, thus creating low 15061



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Figure 5. Left panels: AFM 3D images of the P3HT:PCBM blending films with various PCBM weight fractions. Root-mean-square roughness was also shown in the left bottom corner. Right panels: The relative surface height map, observed by performing a pixel analysis on a plane view AFM image, for the selected specimens. The farea was defined as the area fractions of height less than or equal to 95.

Figure 6. Simplified structural evolutions of the crystalline and amorphous P3HT chains as a function of PCBM weight fractions in blending films: La and Lc are crystallite sizes along the a-axis and c-axis, respectively; Leff is the effective conjugation length; Hcry and Hamo indicate the homogeneity of chain conformation distributions in the crystalline and amorphous regions, respectively. (Note: Hcry and Hamo have been evaluated from the half-width of the v1 Raman band using 633 and 532 nm laser lines, respectively.)

Figure 7. Electrical characteristics of the P3HT:PCBM blending films based solar cells as a function of PCBM weight fractions (fPCBM): (a) short circuit current density (Jsc); (b) open circuit voltage (Voc); (c) fill factor (FF), and (d) power conversion efficiency (PCE).

FF values especially for cases of fPCBM of 25 wt %. In spite of this, we still observed the higher Jsc values (greater than 8 mA/cm2) 15062



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Figure 8. Plots of PCE (left axis) and FF (right axis) versus effective conjugation length (Leff) of P3HT in the amorphous region, in terms of the Raman shift of the v1 band observed at 532 nm excitation.

compared with those in samples with fPCBM more than 50 wt % (less than 5 mA/cm2). The Jsc in the solar cells can be determined by the following equation43 Jsc ¼ neμE

ð3Þ

where n is the density of charge carriers originating from photoinduced charge generation; e is the elementary charge; μ is the mobility; and E is the electric field. The higher n and μ both contribute to higher Jsc values. When fPCBM is 50 wt % or less, the larger La and stronger Jnm of the crystalline portion of P3HT are favorable for carrier transport in solar cells, which can reduce the carrier recombination during carrier transport to electrodes. This offers a reasonable explanation for higher Jsc values because of better hole transport properties of P3HT in the blending films. However, it has been reported that a balance between electron and hole mobilities is required for higher FF and PCE properties of the organic photovoltaic blend films.44 The low FF for the samples with fPCBM less than 50 wt % would reflect the low electron mobility due to the lesser amount of PCBM. Thus, when an excess amount of PCBM is added, the saturated Voc is an indicator of good contact between P3HT and PCBM. We believed that the low Jsc (less than 5 mA/cm2), low FF (less than 0.48), and poor PCE (less than 1.5%) of the devices were mainly due to a low amount of photocarriers from the low absorbance, poor carrier mobility, and imbalance of electron and hole mobilities. Furthermore, the significantly increased Leff of the P3HT crystalline portion would enable carrier transport along the P3HT chains instead of toward the electrodes. D. Impact of Amorphous P3HT Chains on the Photovoltaic Properties. The importance of the chain conformation of amorphous P3HT on the photovoltaic properties could be stressed by plotting the PCE versus the Raman shift of the v1 band observed at 532 nm excitation wavelength (Figure 8). Remember that increasing the Leff reduces the Raman shift of the v1 band,29 thus the Raman shift can be transformed into the Leff. In this figure, a good relationship between PCE and Leff of amorphous P3HT can clearly be observed. Moreover, a significantly high FF value is observed when the Leff is short (Figure 8). Thus, it would be important to study the impact of various Leff of P3HT on the interaction between P3HT and PCBM and thus on the photovoltaic properties. With the aid of quantum chemical

Figure 9. Plots of (a) electron affinity (EA), (b) ionization potential (IP), (c) differences in EA (ΔEA), and (d) built-in voltage (Vbi) as a function of the number of repeat units (N) of polythiophene (PT) chains for the PTPCBM complexes without (closed symbols) and with (open symbols) geometry optimization. The square, circle, and triangle symbols represent the PT, PCBM, and PT-PCBM complex. The dashed lines serve as guidelines.

Figure 10. Schematic illustration of the polythiophene (PT, the number of repeat units of 10)PCBM complex system before and after geometry optimization. The HOMO of PT and LUMO of PCBM are shown. The arrow indicates the dipole moment (μD) of the complex.

calculations, we constructed a system containing a PCBM molecule and polythiophene chains with varying Leff to calculate the EA and IP of the polythiophene chain and PCBM, respectively. With an increase in N, i.e., Leff, we observe that the EA of polythiophene (EAPT) decreases and its IP (IPPT) increases, as shown in Figure 9a and b, respectively. In the presence of a PCBM molecule, however, we do or do not try to optimize the systems (e.g., see Figure 10).45 This indicated a narrower energy band gap of polythiophene with increasing Leff. These calculated results are similar to the experimental results commonly observed in a conjugated polymer without PCBM.46 However, the EA and IP of PCBM (EAPCPM and IPPCBM) are not sensitive to the change in the Leff even after geometry optimization calculations (Figure 9a and b). The difference between EAPT and EAPCBM, defined as ΔEA, can be correlated with the exciton dissociation. As excitons diffuse to the donoracceptor interface, 15063



Figure 11. (a) Conventional and (b) modified ideal structure of the P3HT:PCBM BHJ solar cells.

the exciton binding energy (ΔEb) must be overcome to dissociate excitons into free electrons and holes. For overcoming the ΔEb, in general, the ΔEA should be larger than 0.4 eV.47,48 As shown in Figure 9c, with increasing Leff, the ΔEA is significantly reduced, which is unfavorable for the exciton dissociation. The difference between IPPT and EAPCBM, defined as the built-in voltage (Vbi), could be relevant to the photovoltaic parameter Voc. As the Leff of polythiophene chain lengthens, the Vbi decreases (Figure 9d), which would result in the lower Voc. These results indicate that P3HT with shorter Leff interacting with PCBM would be able to make excitons dissociate more easily and thus have higher Voc. Moreover, after interaction with PCBM, the optimized polythiophene chains form an extended conformation toward a curvature shape (Figure 10) and thus interrupt the conjugation of the π-orbitals. The optimized P3HT:PCBM system (disordered P3HT chain interacting with PCBM) has a relatively low potential energy, enhanced dipole moment, and relatively large ΔEA and Vbi compared with that of a nonoptimized system (imaging as crystalline P3HT with extended chain conformation interacting with PCBM). That means the introducing PCBM would lead to the formation of distorted segments of P3HT with short Leff. The larger ΔEA and Vbi would be favorable for excitons to dissociate and thus yield higher Voc of the solar cells. Moreover, with larger dipole moments, the charge separation is easier. The maximum photovoltaic performance from the device with fPCBM of 50 wt % could now be interpreted as the P3HT chains in

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the amorphous region exhibiting the shortest Leff, thus creating a better P3HTPCBM interface for exciton dissociation and, consequently, high Voc and FF. The inferior homogeneity of Leff in the amorphous region of P3HT was the result of P3HT interaction with PCBM. For devices with fPCBM of 25 wt % and 33 wt %, the longer Leff of P3HT in the amorphous portion would create a smaller value for ΔEA and Vbi of the P3HT:PCBM complexes and result in a small Voc. Meanwhile, this situation would increase the probability of exciton recombination and lower FF, although those specimens have better microstructure in the crystalline portion of P3HT. The slightly low Voc and very low FF of the device with fPCBM of 75 wt % could also be attributed to the inferior P3HT:PCBM complexes with smaller ΔEA due to longer Leff of the amorphous and crystalline P3HT. E. Ideal Structure for Polymeric BHJ Solar Cells. Previously, an ideal structure for BHJ solar cells has been proposed (Figure 11a).43,49 The phases of donor and acceptor are considered to possess the ordered heterojunction nanostructures with an interspace of within or below 1020 nm due to their short exciton diffusion length in organic materials. A well-defined pn heterojunction interface between the donor and acceptor should be established for exciton dissociation. Moreover, to ensure high mobilities of charge carriers and low recombination rates, the two phases must be separated40 and well connected to the correct electrodes for charge extraction. However, the importance of the requirement of the donoracceptor interface area and its electrochemical potential drop remains poorly understood and is not addressed in the model. For the P3HT: PCBM systems, the amorphous P3HT plays an important role in determining the photovoltaic parameters, particularly Voc, FF, and performance η, as previously discussed. The molecular properties of conjugated P3HT were significantly modified by interaction with PCBM. Consequently, a modified ideal structure for the active layer for PSCs was proposed (Figure 11b). In addition to the above-mentioned requirements, a necessary contact area from disordered P3HT chains and PCBM molecules was highlighted. The Leff of P3HT chains in the contact area should be as short as possible to gain large ΔEA and Vbi for efficient exciton dissociation. Through this, the ultimate PCE of PSCs can be achieved.

’ CONCLUSIONS The microstructure-dependent photovoltaic properties of the BHJ solar cells based on P3HT:PCBM blends was studied by varying the P3HT and PCBM compositions. When 50 wt % fPCBM was used, the cells showed higher Jsc level, saturated Voc, and a maximum FF value. Thus, maximum PCE was achieved, which was not the case with cells with different fPCBM. The absorbance, crystal quality, surface morphology of the blending films, and the structural properties of the crystalline portion of P3HT could not provide a full explanation of the photovoltaic behaviors resulting from these different blending films. In contrast, the Leff and chain conformation homogeneity of amorphous P3HT in the blending films show a strong correlation with photovoltaic performance. With the aid of quantum mechanical calculations of the electrochemical potential of the polythiophene:PCBM complexes with various Leff in the polymer chains, a shorter Leff was found to lead to higher ΔEA and Vbi and to facilitate dissociation of excitons and lowering of recombination rates. This suggests that the amorphous P3HT with shorter Leff, i.e., disordered segments, has a higher probability of contact with 15064


The Journal of Physical Chemistry C PCBM and thus forms greater pn heterojunction interfaces. Larger crystalline/domain size for the P3HT and PCBM remains a requirement for efficient carrier transport to electrodes and for balancing hole and electron mobilities. Finally, an ideal structure of the polymeric BHJ solar cells was proposed to highlight the importance of the interfacial area from the disordered P3HT chains interacting with PCBM. Nevertheless, much work is still needed to achieve a well-organized donoracceptor interdigitated nanostructure with a well-controlled pn interface from amorphous P3HT and PCBM; such structures can lead to ultrahigh photovoltaic performance.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Science Council, Taiwan, through Grant NSC 99-ET-E-006-002-ET. We are grateful to the National Center for High-performance computing of Taiwan for computer time and facilities. The authors would like to thank the Center for Micro/Nano Technology, National Cheng Kung University, for equipment access and technical support. ’ REFERENCES (1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W. Prog. Photovoltaics: Res. Appl. 2011, 19, 84–92. (2) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2008, 130, 3619–3623. (3) Pingree, L. S. C.; Reid, O. G.; Ginger, D. S. Nano Lett. 2009, 9, 2946–2952. (4) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297–303. (5) Moule, A. J.; Meerholz, K. Adv. Mater. 2008, 20, 240–245. (6) Huang, J.-H.; Ho, Z.-Y.; Kekuda, D.; Chang, Y.; Chu, C.-W.; Ho, K.-C. Nanotechnology 2009, 20, 0252021–0252029. (7) M€uller, C.; Ferenczi, T. A. M.; Campoy-Quiles, M.; Frost, J. M.; Bradley, D. D. C.; Smith, P.; Stingelin-Stutzmann, N.; Nelson, J. Adv. Mater. 2008, 20, 3510–3515. (8) Baek, W.-H.; Yoon, T.-S.; Lee, H. H.; Kim, Y.-S. Org. Electron. 2010, 11, 933–937. (9) Kim, J. Y.; Qin, Y.; Stevens, D. M.; Ugurlu, O.; Kalihari, V.; Hillmyer, M. A.; Frisbie, C. D. J. Phys. Chem. C 2009, 113, 10790–10797. (10) Barber, R. P., Jr.; Gomez, R. D.; Herman, W. N.; Romero, D. B. Org. Electron. 2006, 7, 508–513. (11) Baek, W.-H.; Yang, H.; Yoon, T.-S.; Kang, C. J.; Lee, H. H.; Kim, Y.-S. Sol. Energy Mater. Sol. Cells 2009, 93, 1263–1267. (12) Dante, M.; Garcia, A.; Nguyen, T.-Q. J. Phys. Chem. C 2009, 113, 1596–1600. (13) Johansson, E. M. J.; Yartsev, A.; Rensmo, H.; Sundstr€om, V. J. Phys. Chem. C 2009, 113, 3014–3020. (14) Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Nat. Mater. 2008, 7, 158–164. (15) Kim, H.; Shin, M.; Kim, Y. J. Phys. Chem. C 2009, 113, 1620–1623. (16) Park, J. H.; Kim, J. S.; Lee, J. H.; Lee, W. H.; Cho, K. J. Phys. Chem. C 2009, 113, 17579–17584. (17) Tajima, K.; Suzuki, Y.; Hashimoto, K. J. Phys. Chem. C 2008, 112, 8507–8510.

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(18) Kim, J. Y.; Frisbie, C. D J. Phys. Chem. C 2008, 112, 17726–17736. (19) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, E135–E138. (20) Miller, S.; Fanchini, G.; Lin, Y.-Y.; Li, C.; Chen, C.-W.; Su, W.-F.; Chhowalla, M. J. Mater. Chem. 2008, 18, 306–312. (21) Chuang, C.-H.; Lin, Y.-Y.; Tseng, Y.-H.; Chu, T.-H.; Lin, C.-C.; Su, W.-F.; Chen, C.-W. J. Phys. Chem. C 2010, 114, 18717–18724. (22) Park, J. H.; Park, J.-I.; Kim, D. H.; Kim, J.-H.; Kim, J. S.; Lee, J. H.; Sim, M.; Lee, S. Y.; Cho, K. J. Mater. Chem. 2010, 20, 5860–5865. (23) Joshi, S.; Pingel, P.; Grigorian, S.; Panzner, T.; Pietsch, U.; Neher, D.; Forster, M.; Scherf, U. Macromolecules 2009, 42, 4651–4660. (24) Kanai, Y.; Grossman, J. C. Nano Lett. 2007, 7, 1967–1972. (25) Marchiori, C. F. N.; Koehler, M. Synth. Met. 2010, 160, 643–650. (26) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004, 393, 51–57. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, revision B.01; Gaussian, Inc.: Wallingford, CT, 2010. (28) Clark, J.; Silva, C.; Friend, R. H.; Spano, F. C. Phys. Rev. Lett. 2007, 98, 2064061–2064064. (29) Cheng, H.-L.; Lin, J.-W.; Jang, M.-F.; Wu, F.-C.; Chou, W.-Y.; Chang, M.-H.; Chao, C.-H. Macromolecules 2009, 42, 8251–8259. (30) Spano, F. C. J. Chem. Phys. 2005, 122, 2347011–23470115. Chem. Phys. 2006, 325, 22–35. (31) Clark, J.; Chang, F. C.; Spano, F. C.; Friend, R. H.; Silva, C. Appl. Phys. Lett. 2009, 94, 1633061–1633063. (32) Barford, W. J. Chem. Phys. 2007, 126, 1349051–1349056. (33) Even applying PCBM background correction to the spectra, we still observed similar trends in the Leff values of blending films with different fPCBM. (34) Cheng, H. L.; Lin, J. W.; Wu, F. C.; She, W. R.; Chou, W. Y.; Shih, W. J.; Sheu, H. S. Soft Matter 2011, 7, 351–354. (35) Yun, J.-J.; Peet, J.; Cho, N.-S.; Bazan, G. C.; Lee, S. J.; Moskovits, M. Appl. Phys. Lett. 2008, 92, 2519121–2519123. (36) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwing, P.; et al. Nature 1999, 401, 685–688. (37) (a) Langford, J. I.; Wilson, A. J. C. J. Appl. Crystallogr. 1978, 11, 102–113.(b) Kasai, N.; Kakudo, M. X-ray Diffraction by Macromolecules; Springer: NY, 2005; p 364. (38) For examples, see: (a) Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stuhn, B.; Schilinsky, P.; Waldauf, C.; Brabec, C. J. Adv. Funct. Mater. 2005, 15, 1193–1196. (b) Beal, R. M.; Stavrinadis, A.; Warner, J. H.; Smith, J. M.; Assender, H. E.; Watt, A. A. R. Macromolecules 2010, 43, 2343–2348. (c) Yin, W.; Dadmun, M. ACS Nano 2011, 5, 4756–4768. (d) Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.; Chabinyc, M. L. Adv. Energy Mater. 2011, 1, 82–89. (39) (a) Prosa, T. J.; Winokur, M. J.; Moulton, J.; Smith, P.; Heeger, A. J. Macromolecules 1992, 25, 4364–4372. (b) Chabinyc, M. L.; Toney, M. F.; Kline, R. J.; McCulloch, I.; Heeney, M. J. Am. Chem. Soc. 2007, 129, 3226–3237. (40) Spadafora, E. J.; Demadrille, R.; Ratier, B.; Grevin, B. Nano Lett. 2010, 10, 3337–3342. (41) Kim, J.-H.; Park, J. H.; Lee, J. H.; Kim, J. S.; Sim, M.; Shim, C.; Cho, K. J. Mater. Chem. 2010, 20, 7398–7405. (42) Li, S.-S.; Tu, K.-H.; Lin, C.-C.; Chen, C.-W.; Chhowalla, M. ACS Nano 2010, 4, 3169–3174. (43) (a) G€unes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338. (b) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Adv. Mater. 2007, 19, 1551–1566. (44) Kumar, A.; Liao, H.-H.; Yang, Y. Org. Electron. 2009, 10, 1615–1620. (45) Unfortunately, because of the extensive computational resources required to conduct calculations when the geometry of polythiophene PCBM systems with long-chain polythiophenes is optimized, we were unable to conduct exhaustive calculations. 15065



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(46) Bredas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. Rev. 2004, 104, 4971–5004. (47) Barth, S.; B€assler, H. Phys. Rev. Lett. 1997, 79, 4445–4448. (48) Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys. Lett. 2006, 88, 0935111–0935113. (49) Mayer, A. C.; Scully, R. S.; Hardin, B. E.; Rowell, M. W.; McGehee, M. D. Mater. Today 2007, 10, 28–33.

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Importance of Disordered Polymer Segments to Microstructure

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