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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 16 — Aug. 3, 2009
  • pp: 13830–13840

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Photovoltaic characteristics of polymer solar cells fabricated by pre-metered coating process

Byoungchoo Park and Mi-young Han  »View Author Affiliations


Optics Express, Vol. 17, Issue 16, pp. 13830-13840 (2009)
http://dx.doi.org/10.1364/OE.17.013830


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Abstract

We present the results of a study of flat and uniform poly(3-hexylthiophene):methanofullerene bulk-heterojunction photovoltaic (PV) layers that were produced by a simple pre-metered horizontal-dipping process for the fabrication of polymer solar cells (PSCs). It is shown that this process can produce high quality and thin films by utilizing the downstream meniscus of the solution, which can be controlled by adjusting experimental parameters of the gap height and the carrying speed. It is also shown that the produced PV film exhibits high power conversion efficiency of ca. 4.2% with a large active area. It was demonstrated that this pre-metered process for solution coating may be promising for achieving highly efficient, reliable, and large-area PSCs.

© 2009 OSA

1. Introduction

Since the pioneering works on polymer solar cells (PSCs), several important studies have been conducted on the development of organic materials and device structures for realizing cost-efficient, lightweight, flexible, and large-area PSC devices [1

N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, “Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene,” Science 258(5087), 1474–1476 (1992). [CrossRef] [PubMed]

–8

J. Peet, M. L. Senatore, A. J. Heeger, and G. C. Bazan, “The Role of Processing in the Fabrication and Optimization of Plastic Solar Cells,” Adv. Mater. 21(14–15), 1521–1527 (2009). [CrossRef]

]. Major important scientific and technological issues regarding the performance of organic devices pertain to their efficiency, stability, and simplicity in device fabrication. For example, with respect to device efficiency, the power conversion efficiency (PCE) of PSCs has been improved significantly by incorporating polymer bulk heterojunction (BHJ) structures, in which an electron donor and an acceptor materials form interpenetrating networks with increased interfacial area and efficient photo-induced charge separation, into the photovoltaic (PV) layer [2

G. Yu, J. Gao, J. C. Hummelen, F. Heeger, and A. J. Wudl, “Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions,” Science 270(5243), 1789–1791 (1995). [CrossRef]

–5

J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, and A. J. Heeger, “Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing,” Science 317(5835), 222–225 (2007). [CrossRef] [PubMed]

]. Using the BHJ structure of composite PV layers of poly(3-hexylthiophene) (P3HT) and phenyl C61-butyric acid methyl ester (PCBM) together with a pre- or post-heat treatment for stabilizing interpenetrating networks and crystalline order, PSCs with a highest PCE of up to ca. 3 ~4% were demonstrated under AM 1.5 G with 80 ~100 mW/cm2 illumination [9

G. Li, V. Shrotriya, Y. Yao, and Y. Yang, “Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene),” J. Appl. Phys. 98(4), 043704–043708 (2005). [CrossRef]

,10

F. Padinger, R. S. Rittberger, and N. S. Sariciftci, “Effects of Postproduction Treatment on Plastic Solar Cells,” Adv. Funct. Mater. 13(1), 85–88 (2003). [CrossRef]

]. On the other hand, relatively insufficient progress has been made with respect to a reliable and simple fabrication process, although ensuring that the formation of a flat and uniform PV layer over a large area is particularly important for achieving highly efficient and reliable device performance in PSC devices. For the fabrication of PSCs, the PV layers have typically been prepared by wet solution-coating processes such as spin-coating, which has been the most popular method for solution-processed devices. This method is convenient, but has several disadvantages, such as stress caused by spinning motion, poor uniformity of edges on large areas, and a large amount of wasted solution. These factors make spin-coating unsuitable for coating large active areas. An alternative method for depositing solution is to use printing, such as screen printing [11

S. E. Shaheen, R. Radspinner, N. Peyghambarian, and G. E. Jabbour, “Fabrication of bulk heterojunction plastic solar cells by screen printing,” Appl. Phys. Lett. 79(18), 2996–2998 (2001). [CrossRef]

], ink-jet printing [12

C. N. Hoth, S. A. Choulis, P. Schilinsky, and C. J. Brabec, “High Photovoltaic Performance of Inkjet Printed Polymer:Fullerene Blends,” Adv. Mater. 19(22), 3973–3978 (2007). [CrossRef]

,13

T. Aernouts, T. Aleksandrov, C. Girotto, J. Genoe, and J. Poortmans, “Polymer based organic solar cells using ink-jet printed active layers,” Appl. Phys. Lett. 92(3), 033306 (2008). [CrossRef]

], or brush painting [14

S.-S. Kim, S.-I. Na, J. Jo, G. Tae, and D.-Y. Kim, “Efficient Polymer Solar Cells Fabricated by Simple Brush Painting,” Adv. Mater. 19(24), 4410–4415 (2007). [CrossRef]

]. By using these printing techniques, organic or polymeric layers can be formed on substrates in a controlled fashion. However, limitations with respect to coating speed and inhomogeneous morphology of the printed film may prevent these methods from being used for high-throughput manufacturing. Alternative methods suggested for depositing the solution are doctor-blade coating and wire-bar coating [15

J. Ouyang, T.-F. Guo, Y. Yang, H. Higuchi, M. Yoshioka, and T. Nagatsuka, “High-Performance, Flexible Polymer Light-Emitting Diodes Fabricated by a Continuous Polymer Coating Process,” Adv. Mater. 14(12), 915–918 (2002). [CrossRef]

–17

S.-R. Tseng, H.-F. Meng, K.-C. Lee, and S.-F. Horng, “Multilayer polymer light-emitting diodes by blade coating method,” Appl. Phys. Lett. 93(15), 153308 (2008). [CrossRef]

]. Usually, the organic film made by the wire-bar coating method is more homogeneous than that made by doctor-blade coating [16

C.-C. Kuo, M. M. Payne, J. E. Anthony, and T. N. Jackson, “TES Anthradithiophene Solution-Processed OTFTs with 1 cm2/V-s Mobility,” 2004 International Electron Device Meeting Technical Digest , 373–376 (2004).

]. However, wire-bar coating may be not suitable for organic devices because of the direct contact of the wires with the underlying layer(s) on the substrate. Therefore, despite the recent developments of such solution-processed devices, it is nevertheless necessary to find an alternative solution-coating process, because of the continuing difficulties of controlling the uniformity of organic layers made by conventional coating methods that have so far been proposed. Hence, work related to solution deposition was initiated to achieve simple and reliable fabrication of PSCs.

In this paper, we present a novel pre-metered solution-process for PSC devices. The advantage of the pre-metered coating is that the coating thickness is pre-determined, in contrast to the typical metered methods such as fixed-gap blade, knife, or wire-bar coatings. By employing this process, we demonstrate solution-processed PSCs with high efficiency and performance.

Fig. 1. Photograph (upper) with its schematic illustration (lower) of the studied pre-metered horizontal-dip (H-dip) coating process: a cylindrical coating barrier (SUS steel) with a diameter R, a gap height h0 , and a carrying speed U.

It is known that coating flows can be divided into two general categories, metered and pre-metered, according to whether the thickness of the coated film is determined by the process or imposed externally: the thickness of the film coated by the metered process is independent on capillary number, while that produced by pre-metered process increases with increasing capillary number (Ca =(µU/σ)), where µ and σ represent viscosity and surface tension of coating solution, respectively, and U is carrying (coating) speed. Representative examples of pre-metered coating flow are meniscus and dip coatings. We now introduce a simple pre-metered solution-coating process that we studied. A photographic image with its schematic illustration of the pre-metered solution-coating process that we used in our study is shown in Fig. 1. As shown in the figure, in this coating process, a cylindrical coating barrier hangs continuously at a specific height (h0 ) above a rigid or flexible substrate that is lain upon on a carrying stage that transports the substrate horizontally. The coating process proceeds in the following four major steps. (1) A substrate is attached to the carrying stage and the coating barrier is placed at the front edge of the substrate. (2) Then a bended organic solution is introduced into the empty space between the barrier and the substrate by capillary action, so that a uniform downstream meniscus of the solution will be formed on the substrate with attraction (surface tension) to the barrier. (3) The substrate is transported horizontally at a given constant velocity while maintaining the shape of the downstream meniscus. A thin solution layer of the downstream meniscus is then spread and formed evenly on the substrate. While the substrate is being transported, the blended solution may be supplied into the gap space at an appropriate rate of injection. (4) Then the wet film that has been spread on the substrate is dried; a heater may be used to assist the evaporation of the residual solvent in the wet film on the substrate. When this process is carried out, it is possible to obtain a substrate that is coated with solid organic film of uniform thickness. The transport of the substrate through the meniscus of the solution is similar to what occurs in the typical dip-coating method [18

L. D. Landau and V. G. Levich, “Dragging of a liquid by a moving plate,” Acta Physicochimica URSS. 17, 42–54 (1942).

,19

J. W. Krozel, A. N. Palazoglu, and R. L. Powell, “Experimental observation of dip-coating phenomena and the prospect of using motion control to minimize fluid retention,” Chem. Eng. Sci. 55(18), 3639–3650 (2000). [CrossRef]

]. In that method, a substrate is immersed into the coating solution and then a wet layer is formed by withdrawing the substrate vertically through the meniscus of the coating solution. However, in the proposed coating process, differently from the dip-coating method, the wet film is formed by withdrawing the substrate horizontally. That being so, we call the proposed process horizontal-dipping (H-dipping).

2. Experimental methods

For the experiments, an ITO layer (80 nm, 30 ohm/square, RMS roughness ~1.98 nm, UID Co. Ltd.) on glass substrate was used as an anode. After routine cleaning of the substrate using ultraviolet-ozone treatment, a blended PV solution was used to coat the ITO layer, which was precoated with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) buffer layers. Both the PEDOT:PSS and the organic PV layers were deposited successively by H-dipping on an ITO-coated glass substrate. The PEDOT:PSS solution that we used was a mixture of 1% PEDOT:PSS solution (CLEVIOS P VP AI 4083, H.C. Starck) and isopropyl alcohol with a weight ratio of 2: 1. The viscosity of the mixed PEDOT:PSS solution, measured by viscometer (RVDVII +, Brookfield Inc.), was about 11.6 cp. For the blended PV solution, we used P3HT (92% regioregularity, Rieke Metals, Inc.) and PCBM (99.5%, Rieke Metals, Inc.) without further purification. We added P3HT and PCBM in a 1.0:0.8 weight ratio to a mixed solvent (4.0 wt%) of chloroform and then added ethanol solvent at various concentrations. The viscosities of the blended P3HT:PCBM solutions were about 2.65, 2.65, 2.-70, and 4.83 cp for the concentrations of added ethanol of 0, 1.0, 3.0, and 5.0%, respectively, at a temperature of 25°C. The apparatus that we used for H-dipping had a maximum work space of 8×10 cm2 and a maximum carrying speed of 2 cm/s, a maximum gap height of 3 mm, and a maximum diameter of 20 mm. A small volume of the solution (~6.3 µl) per unit coating area (1×1 cm2) was fed into the gap between the cylindrical barrier (SUS steel, R=6.35 mm) and the glass substrate by a syringe pump (Pump Systems Inc. NE-1000). The height of the gap, h0 was adjusted vertically using two micrometer positioners and the carrying speed, U was controlled by a computer-controlled translation stage (SGSP26-200, Sigma Koki Co., Ltd). After a meniscus had formed on the solution, the substrate was transported horizontally, so that the barrier spread the solution on the transporting substrate. The transporting speed U was 1.5 cm/s. It took 2 s to prepare a complete film on a substrate with a size of 1.8×2.0 cm2. The H-dip-coated PEDOT:PSS layer and PV layer were then dried using a heating plate at 110°C for 60 min and at 90°C for 5 min, respectively, to remove the remaining solvents. The thickness of the fabricated PEDOT:PSS and the PV layers was about 40 nm and 300 nm, respectively. For comparison, conventional devices were fabricated by spin coating the blend solutions. Sequentially, 1 nm Al:Li alloy (Li: 0.1 wt%) and 60 nm of Al were evaporated on the PV layer via thermal deposition (0.5 nm/s) at a base pressure below 2.7×10-4 Pa. Note that after fabricating the PSC, no post-thermal treatment was applied. Fabrication and characterization of the device were carried out at room temperature under ambient conditions, without encapsulation. The optical properties of the solution-coated PV layers were investigated via UV-vis spectrometry at room temperature with a Cary 1E (Varian) UV-vis spectrometer. The structure of the active layers was also investigated by SEM (JEOL JSM 7001F). The performance of the PSCs was measured under an illumination intensity of 100 mW/cm generated by an AM1.5 light source (Newport, 96000 Solar Simulator). The photocurrent-voltage (J-V) characteristics were measured with a source meter (Keithley 2400) and calibrated by using a reference solar cell (Bunkoh-keiki, BS-520).

Fig. 2. (a) Photograph of the H-dip-coated film (left) and the spin-coated film (right, at 2000 rpm) on 2” substrates. (b) Coated film thickness data of PEDOT:PSS as functions of carrying speed for two gap heights: 0.9 and 0.8 mm. (c) Coated PV film thickness data of P3HT:PCBM as functions of carrying speed for two gap heights: 0.9 and 0.8 mm. The thickness was measured at the edges and center positions of the same film. Solid curves show theoretical predictions of Landau & Levich equation.

3. Results and discussion

At first, we observed spin- and H-dip-coated fluorescent organic films on 2-inch glass substrates with the naked eye, as shown in Fig. 2(a). It may be seen from the figure that the thickness of the spin-coated film varies near all edges of the substrate. This variation is due to the Bernoulli effect [20

G. A. Luurtsema, “Spin coating for rectangular substrates,” U. California, Berkeley, [Online]. Available: http://bcam.berkeley.edu/ARCHIVE/theses/gluurtsMS.pdf. (1997)

]. Of course, one may produce a fully smooth spin-coated film by adjusting the speed of rotation and increasing the amount of spreading solution, but the variation near the edge is certain to increase as the size of the substrate increases. By contrast, it may be seen that the H-dip-coated film is very smooth and uniform. Variation in the thickness of the film was observed only at the very rear edge of the substrate. In order to investigate the surface morphologies of the fabricated films further, the variation in the surface roughness of the film was monitored by atomic force microscope (AFM, Nanosurf easyscan2 FlexAFM, Nanosurf AG Switzerland Inc.). During the measurements, a contact mode was used with a cantilever (CONTR-10 point probe-silicon, Nanoworld, Inc.). The investigation clearly showed that the topography is fairly uniform; the root mean square roughness for the H-dip-coated film was only ca. 0.9 nm, which is comparable to that (ca. 1.0 nm) of the spin-coated films. Moreover, the surface roughness was identical for the H-dip-coated films at different positions. This uniformity was achieved because the conditions under which the film was formed did not include the application of any external centrifugal force. Thus, the result of the H-dipping process is a highly uniform and flat layer over a large area, in contrast to the result of the spin-coating process. Using the AFM, we also investigated the dependences of the film thickness, h of the H-dip-coated layer of hole-injecting poly(3,4-ethylene dioxy thiophene): poly(styrene sulfonate) (PEDOT:PSS) on the transporting speed U and the gap height h0 . The measured results are shown in Fig. 2(b). As shown in the figure, for a given gap height, h0 of 0.8 mm, the thickness of the H-dip-coated PEDOT:PSS layer increases continuously as the speed U increases in the observed region (red circles). Moreover, when h0 was increased from 0.8 mm to 0.9 mm, the thickness of the H-dip-coated PEDOT:PSS layer also increased at a given carrying speed U. These results may be explained by the description of the associated drag-out problem, suggested by Landau and Levich [18

L. D. Landau and V. G. Levich, “Dragging of a liquid by a moving plate,” Acta Physicochimica URSS. 17, 42–54 (1942).

]. Based on the description provided by Landau and Levich, for a small capillary number (Ca ≪1), one may obtain a useful relation of the thickness of the film emerging from a coating bead to the radius of the associated meniscus and carrying speed, U:

h=1.34 ( μUσ) 2⁄3· Rd,
(1)

where Rd represents the radius of curvature of the downstream meniscus, which may be described by:

n· Rd= ( xd2 2R+2 h0) −h.
(2)

Here R and h0 represent the radius of the cylindrical coating barrier and the minimum gap height, respectively, and the number n is 1 for a contact angle of 90° or 2 for a contact angle of 0° at the contact line of the interface between the solution and the coating barrier. In our study, n was assumed to be 2, as shown by the photograph in Fig. 1. The curve fitted results using Eq. (1) are also shown in the figure as solid curves. The observed data fitted well with the theoretical values predicted by Eq. (1), as shown in the figure. Next, we also observed the film thickness for the PV layer of the P3HT and PCBM composite. The H-dip-coated PV layer of P3HT:PCBM shows nearly the same trends as the H-dip-coated PEDOT:PSS layer. The experimental data and well-fitted theoretical curves are also shown in Fig. 2(c). These results indicate clearly that the thickness of the H-dip-coated film can be controlled easily and precisely by adjusting the external parameters of the gap-height h0 and the carrying speed U. It was noted that the thickness of the H-dip-coated film is much less than the gap height. This is the main reason that the pre-metered H-dipping process differs from the conventional metered doctor-blade (or wire-bar) coating: the doctor-blade (or wire-bar) coating process produces film thickness of the order of the gap size and the thickness is independent of the carrying speed of the substrate, whereas the pre-metered H-dipping process can allow critical control of the thickness and produces superior quality and extremely thin films at line speeds of the order of a few meters per minute. Compared to spin-coating, it is possible to achieve reliable and uniform film thickness when using H-dipping, even on a large-area substrate, because of the effective control of the undesirable flow of a free surface at the top organic solution-air interface through the surface tension between the solution and the coating barrier.

Fig. 3. Normalized UV-vis absorption spectra of PV layers (a) prepared by H-dipping and spin-coating P3HT:PCBM blended solutions with chloroform solvent without any thermal treatment, (b) prepared by H-dipping in blended solutions for three concentrations of added ethanol to chloroform solvent without any thermal treatment, and (c) prepared by H-dipping for three concentrations of added ethanol after heating treatment at 90°C.

Next, the optical characteristics of the PV layer produced by the H-dipping process were observed by UV-vis absorption spectroscopy. Figure 3(a) shows the normalized absorption spectra of spin-coated and H-dip-coated PV layers of P3HT:PCBM without any further thermal treatment. The blended film made by H-dipping shows absorption spectra with a single broad absorption peak centered at around 480 nm. The absorption in the visible region is attributed mainly to the P3HT polymer because PCBM shows strong absorption in the UV region. These spectra are quite similar to those of the film made by spin coating. This result indicates that the H-dip-coated PV layer of P3HT:PCBM shows the same optical absorption characteristics as those of the spin-coated PV layer. Generally, it is known that one may introduce ethanol into the blended P3HT:PCBM PV solution [21

S. Cook, A. Furube, and R. Katoh, “Mixed Solvents for Morphology Control of Organic Solar Cell Blend Films,” Jpn. J. Appl. Phys. 47(2), 1238–1241 (2008). [CrossRef]

] and/or anneal the P3HT:PCBM PV layer by heat treatment to improve the PV effect by increasing the crystallinity of the P3HT polymer [9

G. Li, V. Shrotriya, Y. Yao, and Y. Yang, “Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene),” J. Appl. Phys. 98(4), 043704–043708 (2005). [CrossRef]

,10

F. Padinger, R. S. Rittberger, and N. S. Sariciftci, “Effects of Postproduction Treatment on Plastic Solar Cells,” Adv. Funct. Mater. 13(1), 85–88 (2003). [CrossRef]

]. In order to see the effects of adding ethanol into the blended PV solution, we observed the absorption spectra of H-dip-coated P3HT:PCBM PV layers, as shown in Fig. 3(b). As the concentration of added ethanol increased from 0%, the absorption in the red wavelength region, particularly the shoulder at ca. 600 nm, was much stronger and the three vibronic absorption peaks were much more evident. These results confirm that the H-dip-coated PV layer has a higher degree of ordering of P3HT polymer as the concentration of ethanol increases. In order to see the effect of thermal annealing, we also observed the absorption spectra of the annealed H-dip-coated PV layers at 90°C for 5 min for the samples used in Fig. 3(b), as shown in Fig. 3(c). It is clear from the figure that after the heat treatment, the absorption in the red region was much stronger with a red shift of absorption peak (ca. 507 nm) and the pronounced three vibronic absorption peaks could be distinguished much more easily. The improvement in the ordering of P3HT polymer with thermal annealing is also confirmed from the absorption spectra. These results are almost identical to those of the spin-coated PV layers, which indicates that the H-dipping process may be used to produce a P3HT:PCBM PV layer that has nearly the same optical absorption characteristics as the spin-coated PV layer.

Next, PSCs were fabricated by the H-dipping process with various concentrations of ethanol added to PV blended solutions. In the devices, both a hole buffer layer of PEDOT:PSS and a PV layer of P3HT:PCBM composite were fabricated successively by H-dipping at room temperature and dried at temperatures of 110°C and 90°C, respectively. Figure 4(a) shows the dark current density-voltage (J-V) characteristics in a log-linear plot of the solar cell devices under study. The slope of the dark J-V curve between 0.2 V and 0.7 V represents the diode behavior of the solar cells. All sets of devices tested in our experiments show an excellent rectification ratio and thus good coverage for the H-dip-coated PEDOT:PSS buffer layer and the P3HT:PCBM PV layer. However, as shown in the figure, the small but clear difference in diode behavior among the H-dip-coated cells for the various concentrations of added ethanol indicates that there may be some difference in PV effects. Thus, to see the PV effect of the studied devices, the light current density-voltage (J-V) characteristics were also observed under AM 1.5 illumination with 100 mW/cm2, as shown in Fig. 4(b). For the reference case of the PSC device fabricated by H-dipping without adding ethanol, fairly good performance was observed with a short-circuit current density (JSC ) of 6.43 mA/cm2, an open-circuit voltage (VOC ) of 0.624 V, and a fill factor (FF) of 40.2%. This corresponds to a PCE (η) of 1.6%, which is comparable to that prepared by conventional spin-coating [22

M. Al-Ibrahim, O. Ambacher, S. Sensfuss, and G. Gobsch, “Effects of solvent and annealing on the improved performance of solar cells based on poly(3-hexylthiophene): Fullerene,” Appl. Phys. Lett. 86(20), 201120 (2005). [CrossRef]

]. For the sample case, it was found that the device performance depends heavily on the particular concentration of ethanol that is added: the device that was made with an ethanol concentration of 3% has a significantly higher JSC of 12.34 mA/cm2, a VOC of 0.601 V, and a FF of 58.3%. Thus, the PSC coated by H-dipping with a 3% ethanol mixture had the highest PCE of 4.2%. This highest value of efficiency among those of the tested devices can be attributed to the efficient extraction of photogenerated charges combined with a higher mobility of charges through the interpenetrating networks. It is noteworthy that the PCE of 4.2% is among the highest values reported till now for PSCs made by using chloroform solvent. By contrast, when the concentration of ethanol was increased to 5%, the PSC device showed poor efficiency with VOC =0.574 V, JSC =4.31 mA/cm2, FF=47.4%, and η=1.15%. To elucidate the reason behind this result, particularly for the low JSC , we recalled the UV-vis absorption spectra, shown in Fig. 3(c). As shown in the figure, after thermal annealing at a temperature of 90°C, the absorption spectra in the red region, particularly the shoulder with the vibronic absorption peaks in the wavelength region of 510~650 nm, were nearly identical to each other for the PV films made with 0%, 3%, and 5%, which indicates nearly the same high degrees of ordering for the H-dip-coated PV films. Although the absorption results would lead one to expect that a high JSC would lead to high efficiency, devices made with an ethanol concentration of over 5% performed very poorly. Hence, we focused on the influence of monomolecular recombination, which has a critical influence on the charge separation and transport. The losses in the JSC due to monomolecular recombination were estimated from the dependence of the current on negative voltage (Fig. 4(c)) [23

C. Waldauf, M. C. Scharber, P. Schilinsky, J. A. Hauch, and C. J. Brabec, “Physics of organic bulk heterojunction devices for photovoltaic applications,” J. Appl. Phys. 99(10), 104503 (2006). [CrossRef]

]. For added ethanol concentrations of 0, 1, 3, and 5%, the loss in the JSC were estimated at 12.3, 9.1, 7.8, and 11.6%, respectively. The estimated losses of JSC are relatively small, which indicates that little monomolecular recombination occurred in all tested devices. Thus, from the above analysis, one may conclude that there is no significant difference in recombination mechanisms among the cells fabricated when ethanol solvent had been added the PV solution. Hence, we interpret the low JSC achieved for the PSC made with 5% ethanol as being due mainly to the morphology of the active layer. Figure 5 shows the scanning electron microscopy (SEM) images of the H-dip-coated PV layers for the concentrations of ethanol that were studied. In contrast to the SEM images of the H-dip-coated layers made with an ethanol concentration below 3%, severe roughened surface morphology, which is due to the aggregation of PCBM, was observed in the SEM images for the H-dip-coated layer made with an ethanol concentration of 5%. Considering the diffusion length of excitons in the conjugated polymers, the charge separation of photo-generated excitons may be very poor because of the presence of large aggregates of PCBM. In addition, poor charge transport is also to be expected, because of the poor percolation of P3HT and PCBM. Consequently, the low JSC and efficiency may be attributed to the formation of large aggregates, which limit efficient separation and transport of the charge. In other words, even though sufficient ordering of the polymer chains could be achieved at high concentrations of ethanol, optimized nanoscale morphology for efficient PSCs was not found at a concentration of ethanol of over 5%. The observed results from the devices are summarized in Table 1. It should be noted that we could not make an homogeneous and uniform thin PV layer by spin-coating for PV solutions with a concentration of ethanol of over 3 wt% because of their high viscosities. This result implies that the H-dipping process is also suitable for fabricating films from viscous solutions.

Fig. 4. The J-V characteristic curves of polymer solar cells fabricated by the H-dipping process for various concentrations of ethanol in mixed PV solutions: (a) in the dark, and (b) under light (air mass 1.5G with an incident light intensity of 100 mW/cm2). (c) The voltage dependence of illuminated photocurrent for extended reverse bias. The solid lines represent linear fits of the J-V curves.

Finally, in order to check the processability of large-area PV devices, we also fabricated a 2.5”×3.5” flexible PSC device by using the H-dipping process on a flexible ITO-coated PET

Table 1.  Device performance of polymer solar cells made by H-dipping process for various concentrations of added ethanol to chloroform solvent used in this work.
Concentration of ethanol (%) VOS (V) JSC (mA/cm2) FF (%)H (%)ratio of η (%)
00.6246.4340.21.58100.0
10.59510.4757.83.53223.2
30.60112.3458.34.24267.9
50.5744.3147.41.1572.7

substrate. A photographic image of the fabricated device is shown in Fig. 6. As shown in the figure, it is easy to fabricate flexible large-area PSCs by using the H-dipping process. These results demonstrate that the H-dipping process promises simple fabrication with easy scaling up to a large size at lower cost than the spin-coating process. From the above results, one may know that the H-dipping process for solution coating is promising for the fabrication of flexible and large-area PSCs based on high-throughput roll-to-roll manufacturing, which would make it possible to realize fast processing and low-cost organic electronics.

Fig. 5. SEM images of the PV layers produced by H-dipping process for mixed solutions with ethanol concentrations of 0, 1.0, 3.0, and 5.0%.
Fig. 6. Photograph of a flexible polymer solar cell made by the H-dipping process on a flexible substrate (2.5”×3.5”).

5. Conclusions

In summary, a simple pre-metered H-dipping process was investigated as a promising solution-coating process for cost-efficient and large-area PSCs. It was demonstrated that uniform coating with well-defined optical and electrical properties can be achieved by H-dipping in a solution by controlling the meniscus of the solution with two important external parameters of gap-height and coating-speed. It was also shown that the efficiency of organic devices made by H-dipping in an appropriate PV solution was higher than that of conventional spin-coated devices. This novel process for depositing the solution on the substrate can be expanded to slot-die and slit-die coatings and will provide a solid foundation for extending the fabrication of flexible and large-area organic devices to include various advanced PSCs.

Acknowledgments

This work was supported by technology development project of new and renewable energies of the Ministry of Knowledge Economy of the Republic of Korea (2009). This work was supported by the Brain Korea 21 Project 2009.

References and links

1.

N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, “Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene,” Science 258(5087), 1474–1476 (1992). [CrossRef] [PubMed]

2.

G. Yu, J. Gao, J. C. Hummelen, F. Heeger, and A. J. Wudl, “Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions,” Science 270(5243), 1789–1791 (1995). [CrossRef]

3.

C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, “Plastic solar cells,” Adv. Funct. Mater. 11(1), 15–26 (2001). [CrossRef]

4.

G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, “High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends,” Nat. Mater. 4(11), 864–868 (2005). [CrossRef]

5.

J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, and A. J. Heeger, “Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing,” Science 317(5835), 222–225 (2007). [CrossRef] [PubMed]

6.

A. C. Arias, N. Corcoran, M. Banach, R. H. Friend, J. D. MacKenzie, and W. T. S. Huck, “Vertically segregated polymer-blend photovoltaic thin-film structures through surface-mediated solution processing,” Appl. Phys. Lett. 80(10), 1695–1697 (2002). [CrossRef]

7.

S. Cho, J. Yuen, J. Y. Kim, K. Lee, and A. J. Heeger, “Photovoltaic effects on the organic ambipolar field-effect transistors,” Appl. Phys. Lett. 90(6), 063511 (2007). [CrossRef]

8.

J. Peet, M. L. Senatore, A. J. Heeger, and G. C. Bazan, “The Role of Processing in the Fabrication and Optimization of Plastic Solar Cells,” Adv. Mater. 21(14–15), 1521–1527 (2009). [CrossRef]

9.

G. Li, V. Shrotriya, Y. Yao, and Y. Yang, “Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene),” J. Appl. Phys. 98(4), 043704–043708 (2005). [CrossRef]

10.

F. Padinger, R. S. Rittberger, and N. S. Sariciftci, “Effects of Postproduction Treatment on Plastic Solar Cells,” Adv. Funct. Mater. 13(1), 85–88 (2003). [CrossRef]

11.

S. E. Shaheen, R. Radspinner, N. Peyghambarian, and G. E. Jabbour, “Fabrication of bulk heterojunction plastic solar cells by screen printing,” Appl. Phys. Lett. 79(18), 2996–2998 (2001). [CrossRef]

12.

C. N. Hoth, S. A. Choulis, P. Schilinsky, and C. J. Brabec, “High Photovoltaic Performance of Inkjet Printed Polymer:Fullerene Blends,” Adv. Mater. 19(22), 3973–3978 (2007). [CrossRef]

13.

T. Aernouts, T. Aleksandrov, C. Girotto, J. Genoe, and J. Poortmans, “Polymer based organic solar cells using ink-jet printed active layers,” Appl. Phys. Lett. 92(3), 033306 (2008). [CrossRef]

14.

S.-S. Kim, S.-I. Na, J. Jo, G. Tae, and D.-Y. Kim, “Efficient Polymer Solar Cells Fabricated by Simple Brush Painting,” Adv. Mater. 19(24), 4410–4415 (2007). [CrossRef]

15.

J. Ouyang, T.-F. Guo, Y. Yang, H. Higuchi, M. Yoshioka, and T. Nagatsuka, “High-Performance, Flexible Polymer Light-Emitting Diodes Fabricated by a Continuous Polymer Coating Process,” Adv. Mater. 14(12), 915–918 (2002). [CrossRef]

16.

C.-C. Kuo, M. M. Payne, J. E. Anthony, and T. N. Jackson, “TES Anthradithiophene Solution-Processed OTFTs with 1 cm2/V-s Mobility,” 2004 International Electron Device Meeting Technical Digest , 373–376 (2004).

17.

S.-R. Tseng, H.-F. Meng, K.-C. Lee, and S.-F. Horng, “Multilayer polymer light-emitting diodes by blade coating method,” Appl. Phys. Lett. 93(15), 153308 (2008). [CrossRef]

18.

L. D. Landau and V. G. Levich, “Dragging of a liquid by a moving plate,” Acta Physicochimica URSS. 17, 42–54 (1942).

19.

J. W. Krozel, A. N. Palazoglu, and R. L. Powell, “Experimental observation of dip-coating phenomena and the prospect of using motion control to minimize fluid retention,” Chem. Eng. Sci. 55(18), 3639–3650 (2000). [CrossRef]

20.

G. A. Luurtsema, “Spin coating for rectangular substrates,” U. California, Berkeley, [Online]. Available: http://bcam.berkeley.edu/ARCHIVE/theses/gluurtsMS.pdf. (1997)

21.

S. Cook, A. Furube, and R. Katoh, “Mixed Solvents for Morphology Control of Organic Solar Cell Blend Films,” Jpn. J. Appl. Phys. 47(2), 1238–1241 (2008). [CrossRef]

22.

M. Al-Ibrahim, O. Ambacher, S. Sensfuss, and G. Gobsch, “Effects of solvent and annealing on the improved performance of solar cells based on poly(3-hexylthiophene): Fullerene,” Appl. Phys. Lett. 86(20), 201120 (2005). [CrossRef]

23.

C. Waldauf, M. C. Scharber, P. Schilinsky, J. A. Hauch, and C. J. Brabec, “Physics of organic bulk heterojunction devices for photovoltaic applications,” J. Appl. Phys. 99(10), 104503 (2006). [CrossRef]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(160.5470) Materials : Polymers
(310.1860) Thin films : Deposition and fabrication
(350.6050) Other areas of optics : Solar energy
(310.6845) Thin films : Thin film devices and applications

ToC Category:
Detectors

History
Original Manuscript: June 10, 2009
Revised Manuscript: July 15, 2009
Manuscript Accepted: July 15, 2009
Published: July 24, 2009

Citation
Byoungchoo Park and Mi-young Han, "Photovoltaic characteristics of polymer solar cells fabricated by pre-metered coating process," Opt. Express 17, 13830-13840 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-16-13830


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References

  1. N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, “Photoinduced electron-transfer from a conducting polymer to buckminsterfullerene,” Science 258(5087), 1474–1476 (1992). [CrossRef] [PubMed]
  2. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, “Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions,” Science 270(5243), 1789–1791 (1995). [CrossRef]
  3. C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, “Plastic solar cells,” Adv. Funct. Mater. 11(1), 15–26 (2001). [CrossRef]
  4. G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, “High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends,” Nat. Mater. 4(11), 864–868 (2005). [CrossRef]
  5. J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, and A. J. Heeger, “Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing,” Science 13(5835), 222–225 (2007). [CrossRef] [PubMed]
  6. A. C. Arias, N. Corcoran, M. Banach, R. H. Friend, J. D. MacKenzie, and W. T. S. Huck, “Vertically segregated polymer-blend photovoltaic thin-film structures through surface-mediated solution processing,” Appl. Phys. Lett. 10(10), 1695–1697 (2002). [CrossRef]
  7. S. Cho, J. Yuen, J. Y. Kim, K. Lee, and A. J. Heeger, “Photovoltaic effects on the organic ambipolar field-effect transistors,” Appl. Phys. Lett. 90(6), 063511 (2007). [CrossRef]
  8. J. Peet, M. L. Senatore, A. J. Heeger, and G. C. Bazan, “The Role of Processing in the Fabrication and Optimization of Plastic Solar Cells,” Adv. Mater. 21(14-15), 1521–1527 (2009). [CrossRef]
  9. G. Li, V. Shrotriya, Y. Yao, and Y. Yang, “Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene),” J. Appl. Phys. 98(4), 043704–043708 (2005). [CrossRef]
  10. F. Padinger, R. S. Rittberger, and N. S. Sariciftci, “Effects of Postproduction Treatment on Plastic Solar Cells,” Adv. Funct. Mater. 13(1), 85–88 (2003). [CrossRef]
  11. S. E. Shaheen, R. Radspinner, N. Peyghambarian, and G. E. Jabbour, “Fabrication of bulk heterojunction plastic solar cells by screen printing,” Appl. Phys. Lett. 79(18), 2996–2998 (2001). [CrossRef]
  12. C. N. Hoth, S. A. Choulis, P. Schilinsky, and C. J. Brabec, “High Photovoltaic Performance of Inkjet Printed Polymer:Fullerene Blends,” Adv. Mater. 19(22), 3973–3978 (2007). [CrossRef]
  13. T. Aernouts, T. Aleksandrov, C. Girotto, J. Genoe, and J. Poortmans, “Polymer based organic solar cells using ink-jet printed active layers,” Appl. Phys. Lett. 92(3), 033306 (2008). [CrossRef]
  14. S.-S. Kim, S.-I. Na, J. Jo, G. Tae, and D.-Y. Kim, “Efficient Polymer Solar Cells Fabricated by Simple Brush Painting,” Adv. Mater. 19(24), 4410–4415 (2007). [CrossRef]
  15. J. Ouyang, T.-F. Guo, Y. Yang, H. Higuchi, M. Yoshioka, and T. Nagatsuka, “High-Performance, Flexible Polymer Light-Emitting Diodes Fabricated by a Continuous Polymer Coating Process,” Adv. Mater. 14(12), 915–918 (2002). [CrossRef]
  16. C.-C. Kuo, M. M. Payne, J. E. Anthony, and T. N. Jackson, “TES Anthradithiophene Solution-Processed OTFTs with 1 cm2/V-s Mobility,” 2004 International Electron Device Meeting Technical Digest, 373–376 (2004).
  17. S.-R. Tseng, H.-F. Meng, K.-C. Lee, and S.-F. Horng, “Multilayer polymer light-emitting diodes by blade coating method,” Appl. Phys. Lett. 93(15), 153308 (2008). [CrossRef]
  18. L. D. Landau and V. G. Levich, “Dragging of a liquid by a moving plate,” Acta Physicochimica URSS. 17, 42–54 (1942).
  19. J. W. Krozel, A. N. Palazoglu, and R. L. Powell, “Experimental observation of dip-coating phenomena and the prospect of using motion control to minimize fluid retention,” Chem. Eng. Sci. 55(18), 3639–3650 (2000). [CrossRef]
  20. G. A. Luurtsema, “Spin coating for rectangular substrates,” U. California, Berkeley, [Online]. Available: http://bcam.berkeley.edu/ ARCHIVE/theses/gluurtsMS.pdf . (1997)
  21. S. Cook, A. Furube, and R. Katoh, “Mixed Solvents for Morphology Control of Organic Solar Cell Blend Films,” Jpn. J. Appl. Phys. 47(2), 1238–1241 (2008). [CrossRef]
  22. M. Al-Ibrahim, O. Ambacher, S. Sensfuss, and G. Gobsch, “Effects of solvent and annealing on the improved performance of solar cells based on poly(3-hexylthiophene): Fullerene,” Appl. Phys. Lett. 86(20), 201120 (2005). [CrossRef]
  23. C. Waldauf, M. C. Scharber, P. Schilinsky, J. A. Hauch, and C. J. Brabec, “Physics of organic bulk heterojunction devices for photovoltaic applications,” J. Appl. Phys. 99(10), 104503 (2006). [CrossRef]

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