LETTER pubs.acs.org/NanoLett
Sequential Processing: Control of Nanomorphology in Bulk Heterojunction Solar Cells Dong Hwan Wang,†,‡,|| Ji Sun Moon,†,|| Jason Seifter,† Jang Jo,† Jong Hyeok Park,*,§ O Ok Park,*,‡ and Alan J. Heeger*,† †
Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106-5090, United States Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (BK 21 Graduate Program), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea § School of Chemical Engineering and SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡
bS Supporting Information ABSTRACT: Bulk heterojunction organic photovoltaic devices based on poly[N-900 -heptadecanyl-2,7-carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole) (PCDTBT)/[6,6]phenyl C70 butyric acid methyl ester (PC70BM) can be successfully fabricated by a sequential solution deposition process. When the top layer is deposited from an appropriate cosolvent, the PC70BM penetrates a predeposited bottom layer of PCDTBT during the spin-casting process, resulting in an interdiffused structure with a layer-evolved bulk heterojunction (LE-BHJ) nanomorphology. The PCDTBT:PC70BM LE-BHJ solar cells prepared with an optimized cosolvent ratio have comparable power conversion efficiency to the conventional BHJ solar cells. The nanomorphology of the optimized PCDTBT:PC70BM LE-BHJ mixture was found to have better vertical connectivity than the conventional BHJ material. KEYWORDS: Organic photovoltaic device, bulk heterojuction, bilayer mixture, layer-evolved bulk heterojunction, nanoscale morphology, interdiffused structure, cosolvent effect
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ecently, organic photovoltaic (OPV) devices based on an active layer composed of a phase separated mixture of donor and acceptor have been of broad interest because of their many advantages, including roll-to-roll fabrication by solution processing, the potential for generation of large-area solar panels, and the flexibility required for use in a variety of future industrial applications with low cost.110 OPV devices can be fabricated with either of two representative architectures: a bilayer device or a bulk heterojunction (BHJ) device. In 1987, Tang reported that when OPV devices comprise separated electron donors and acceptors in the form of a bilayer, the power conversion efficiency (PCE) performance of the cell is ∼1%.11 Since the acceptor top layer is deposited on the donor bottom layer, the device has a small interfacial contact area between the donor and the acceptor. This arrangement, which photogenerates charge carriers with low efficiency, yields poor short-circuit current (Jsc) in OPV devices. Because of the fundamental problem associated with the bilayer architecture, bulk heterojunction (BHJ) polymer solar cells with nanoscale bicontinuous networks (both components percolated) were invented.2 The standard fabrication process involves spin-casting from a mixed solution of a conjugated polymer and a fullerene derivative. Thus, efficient charge generation (even at high illumination levels) can occur through the increased donoracceptor interfacial contact area within the BHJ active layer. Optimizing the nanoscale BHJ morphology is r 2011 American Chemical Society
an important means of enhancing the device performance.12 BHJ solar cells with PCEs of >8% have recently been reported.1316 According to recent reports, poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61-butyric acid methyl ester (PCBM) “bilayer” solar cells have been fabricated. The fabrication process involves the interdiffusion from the PCBM top layer into the P3HT bottom layer via solvent effects or through thermal annealing.1721 Transmission electron microscopy (TEM) has been used to investigate the nanomorphology of the interdiffused “bilayer” structure of a P3HT/PCBM sample fabricated by means of sequentially deposited layers.22 These papers demonstrate that for P3HT:PCBM the sequential deposition process yields a nanoscale morphology that is indistinguishable from the standard phase separated BHJ material. Moreover, the sequential deposition process can produce comparable or higher efficiency than a conventional BHJ. In this study, we compare two representative fabrication processes based on poly[N-900 -hepta-decanyl-2,7-carbazole-alt5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole) (PCDTBT) as the donor material and [6,6]-phenyl C70 butyric acid methyl ester (PC70BM) as the acceptor material; namely, a conventional BHJ and a layer-evolved BHJ (LE-BHJ) with interdiffusion controlled through the use of cosolvents for the PC70BM. The Received: April 23, 2011 Published: July 13, 2011 3163
dx.doi.org/10.1021/nl202320r | Nano Lett. 2011, 11, 3163–3168
Nano Letters
Figure 1. (a) Molecular structures of PCDTBT and PC70BM. (b) Energy level diagram of the component materials, PCDTBT and PC70BM. (c) Proposed device sequential structures of PCDTBT and PC70BM bilayer/LE-BHJ solar cells fabricated by interpenetration of the PCBM top layer (dissolved with various ratios of a DCB/CB cosolvent in a DCM solvent) to the PCDTBT bottom layer.
Figure 2. JV curves of devices made of the PCDTBT:PC70BM C-BHJ, bilayer mixture (100 wt % DCM without cosolvent), and LEBHJ (030 wt % cosolvent).
LE-BHJ solar cells were fabricated by spin-casting a PC70BM layer that is dissolved in a mixed solvent with different proportions of chlorobenzene (CB)/dichlorobenzene (DCB) in dichloromethane (DCM) on the top of the predeposited PCDTBT bottom layer. These solution proccessible LE-BHJ solar cells yield high efficiency and more importantly offer a means to control the morphology of the final BHJ. Comparison of Conventional BHJ and LE-BHJ. The PCDTBT:PC70BM conventional BHJ solar cells were fabricated as reported previously.13 The LE-BHJ solar cells were fabricated in a several step procedure. Details are given in the Supporting Information. The following is a synopsis: A hole-transporting buffer layer made of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron PH), was spin-cast on an indium tin oxide glass substrate. The PCDTBT was dissolved in a DCB/CB cosolvent (3:1) and then spin-cast on top of the PEDOT:PSS layer. The PC70BM was dissolved in a solvent mixture of DCB/CB in DCM (with the DCB/CB proportions ranging from 0 to 30 wt.%) and then directly spin-cast onto the PCDTBT bottom layer. The LE-BHJ was generated by interpenetration of the PC70BM layer
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Figure 3. IPCE spectra of devices fabricated with the C-BHJ, and LEBHJ (20 wt % cosolvent) of PCDTBT:PC70BM as a function of the wavelength (in nanometers).
into the PCDTBT bottom layer at a rate determined by the various DCB/CB ratios. The active layer was then dried immediately for 10 min on a digital controllable hot plate set at 80 °C. A TiOx layer, which functions as an electron transporter and hole blocker, was spin-cast onto the active layer in air, and an aluminum cathode was then thermally deposited (with the thickness of approximately 100 nm) under a pressure of 4.0 106 Torr. Figure 1 shows the molecular structures of PCDTBT and PC70BM and a band diagram of the component materials. A schematic diagram of the device structure is also shown in Figure 1 of a bilayer/LE-BHJ solar cell with a partial interpenetration zone. This will be described in detail in the following section. We also fabricated conventional PCDTBT:PC70BM BHJ solar cells for comparison with the LE-BHJ devices. As shown in Figure 2, the optimized LE-BHJ devices achieve a device performance level of >6%. However, the efficiency of the LE-BHJ solar cells fabricated by spin-casting a PC70BM top layer dissolved in 100% DCM has a significantly lower efficiency because of only partial interdiffusion (see Figure 1c). Figure 3 compares the incident photon-to-current efficiency (IPCE) spectra of the conventional BHJ material and the optimized LE-BHJ. As expected from Figure 2, the data from the conventional BHJ and the LE-BHJ are very similar. The “Bilayer Mixture”. The devices fabricated with PC70BM dissolved in DCM (no cosolvents) show lower efficiency (see Figure 2). Atomic force microscopy (AFM) images show an unusual surface morphology with “disks” of aggregated PC70BM sitting on top of a BHJ layer with incomplete penetration of PC70BM; see the AFM images in Figure 4b. In Figure 2, the photocurrent versus voltage (JV) curves show that this “bilayer mixture” has a relatively low device efficiency of 4.18% with the following parameters: Voc = 0.88 V, Jsc = 8.10 mA/cm2, and FF = 0.59. In contrast, the conventional BHJ device efficiency is 6.26% with Voc = 0.87 V, Jsc = 10.60 mA/cm2, and FF = 0.68. Note that the bilayer mixture has a lower Jsc value and a lower FF value. These differences originate from the morphology of the bilayer mixture as shown in Figure 5. LE-BHJ solar cells fabricated by spin-casting a PC70BM top layer derived from a DCB/CB cosolvent increase the device performance. As shown in Figure 2, the extent of the increase depends on the concentration of the cosolvent in DCM. These 3164
dx.doi.org/10.1021/nl202320r |Nano Lett. 2011, 11, 3163–3168
Nano Letters
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Figure 4. Top-down AFM height images (2 μm by 2 μm) of (a) the C-BHJ, (b) bilayer mixture, (c) LE-BHJ (5 wt % cosolvent), and (d) LE-BHJ (20 wt % cosolvent) of PCDTBT:PC70BM.
Figure 5. Cross-sectional TEM (a) bright-field image of location 1 and defocus phase contrast image of (c) location 1 and (d) location 2 of the PCDTBT:PC70BM bilayer mixture (∼30 μm defocused). Threedimensionally converted AFM height image of the PCDTBT:PC70BM bilayer mixture (5 μm by 5 μm, the scales of x, y, and z axes are equal).
results imply the penetration of the PC70BM molecules from the top layer into the PCDTBT bottom layer. This type of penetration can create a LE-BHJ structure through the effect of the cosolvent via swelling and interdiffusion.2325 Thus, the JV curves show that the LE-BHJ with the PC70BM derived from DCM with a 5 wt % cosolvent outperforms the bilayer mixture solar cells as evidenced by its enhanced power conversion efficiency, PCE = 6.04%, with improved values for both Jsc and FF. Note also that the PCDTBT/PC70BM LE-BHJ solar cells with the PC70BM layer derived from 20 wt % of the cosolvent in
Figure 6. Cross-sectional TEM (a) bright-field image, (b) defocus phase contrast image of the PCDTBT:PC70BM LE-BHJ (20 wt % cosolvent) ∼30 μm defocused.
DCM have the highest device performance which is comparable efficiency with the conventional BHJ solar cell, PCE = 6.34%, with Voc = 0.90 V, Jsc = 10.74 mA/cm2, and FF = 0.66. The LEBHJ solar cells fabricated from a cosolvent ratio of more than 30 wt % exhibit poorer device performance than the conventional BHJ due to lower photocurrent density and lower FF. 3165
dx.doi.org/10.1021/nl202320r |Nano Lett. 2011, 11, 3163–3168
Nano Letters
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Figure 7. (a) Crop of the PCDTBT:PC70BM LE-BHJ (20 wt % cosolvent) from the cross-sectional defocused TEM image. (b) Cross-sectional defocused TEM image (bottom, P3HT:PC70BM C-BHJ; middle, pristine PCDTBT layer; top, PCDTBT:PC70BM C-BHJ). Both images were ∼30 μm defocused. The magnification of image (a) has been adjusted to that of image (b).
Cross-Sectional TEM Measurements. We investigated the
nanomorphology of the PCDTBT:PC70BM bilayer mixture, the LE-BHJ, and the conventional PCDTBT:PC70BM BHJ with cross-sectional TEM.21,22,2629 The cross-sectional TEM image can provide important information on the nanomorphology of BHJs related to the solar cell performance because the photogenerated current flows across the cross section.30 We used the multistacked TEM structure22 in order to observe multiple layers obtained from the same conditions of focused ion beam (FIB) slicing and TEM imaging and to avoid artifacts from thickness variation that might occur during the milling process.22 The multilayers were separated from each other by the blocking layers; a thick PEDOT:PSS layer and a TiOx layer (Figures 5, 6, and 7) which provide the same chemical environment (PEDOT:PSS layer underneath and TiOx layer on the top) as experienced by the active layers in polymer solar cells.22 All the layers of the multistack TEM samples were fabricated in the same way, but a little thicker than the active layers of the optimized solar cell devices for better observation. A pristine PCDTBT layer was added into each TEM multistack sample to highlight any FIB milling effects such as Ga/ Pt deposition or surface roughness variation on the cross section.22 The TEM samples were sliced to very thin cross sections (slice thickness
