ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 454–458

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Exploring spray coating as a deposition technique for the fabrication of solution-processed solar cells Claudio Girotto a,b,, Barry P. Rand a, Jan Genoe a, Paul Heremans a,b a b

IMEC vzw, Polymer and Molecular Electronics, Kapeldreef 75, B-3001 Leuven, Belgium ESAT, Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium

a r t i c l e in f o

a b s t r a c t

Article history: Received 20 October 2008 Received in revised form 27 November 2008 Accepted 28 November 2008 Available online 13 January 2009

We investigate the characteristics of airbrush spray coated polymer solar cells based on a mixture of poly(3-hexyl thiophene) and (6,6)-phenyl C61 -butyric acid methyl ester, deposited with different settings. We show smooth films yielding spray coated solar cells with power conversion efficiencies of 2.8%. In addition, we find that the spray coating technique allows the realization of polymer solar cells with a structural gradient in the vertical direction: the femtoliter scale droplets, distinctive of spray coating, dry very rapidly and subsequent layers can be deposited from the same solvent, without completely dissolving the underlying layers. & 2008 Elsevier B.V. All rights reserved.

Keywords: Bulk heterojunction Organic solar cells Spray coating Solution processing Structural control

1. Introduction Solar cells based on organic semiconductors (OSCs) have the potential to provide low-cost energy production on lightweight and flexible substrates. These devices are either produced by solution processing of polymers or by evaporation of small molecules [1,2]. During the last decade, the P3HT:PCBM system has attracted the attention in the polymer solar cells field, reaching efficiencies above 5% thanks to the optimization of the processes and the understanding of the underlying physics. Since this materials combination is probably nearing optimal device performance, research activities now are focused in the development of new materials with lower band gap to enhance efficiencies even further [3–5]. The application of polymer solar cells as alternative energy source, nevertheless, is still limited by the short operation lifetimes of this devices [6] and by the techniques used to deposit the materials, difficult to scale to large production. For solution-processed cells, spin coating is considered the most reliable and reproducible deposition method, but it is not scalable to roll-to-roll production. To realize large-area coverage, various deposition techniques, such as inkjet [7,8] and screen printing [9,10] or doctor blading [11], have been demonstrated.

 Corresponding author at: IMEC vzw, Polymer and Molecular Electronics, Kapeldreef 75, B-3001 Leuven, Belgium. Tel.: +32 16 28 8757; fax: +32 16 28 1097. E-mail address: [email protected] (C. Girotto).

0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.11.052

The spray coating technique is well established in graphic arts, industrial coatings, and painting. This high-throughput large-area deposition technique ensures ideal coatings on a variety of surfaces with different morphologies and is often used for inline production. Moreover, the fluid waste is reduced to minimal quantities, and the deposition can be easily patterned by simple shadow masking. Also, the spray coating technique is able to access a broad spectrum of fluids with different rheologies, offering the opportunity to tune the system to deposit virtually any kind of solution and obtain the desired film properties. However, the usage of spray coating in the production of OSCs has not been given much attention, probably due to issues concerning the control of film thickness and roughness. A few recent publications introduced conventional spray coating as a deposition method for OSCs [12,13], and in particular focused on optimizing both solvent choice as well as device thickness. Despite the use of handheld airbrushes, these works confirm that the technique has considerable promise as a scalable deposition technique for large-area devices. Furthermore, they demonstrate that OSCs with performance approaching that of spin coating can indeed be produced with simple equipment. The results obtained from these investigations can then be easily transferred to a system with better control of the deposition parameters over a large area, such as automated and computer controlled spray coaters. As compared to the previous works, where the roughness of the samples seems to be a critical and inherent characteristic of spray coating, we extend those initial investigations in order to better understand the potential of this technique. We focus our

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attention to the influence of the airbrush settings on the film topography, finally relating the latter to the photovoltaic performance of OSCs. We show how this technique can be adapted to obtain either smooth or rough layers, or even to stack thin layers on top of each other from a common solvent.

2. Experimental We prepared thin films and solar cells based on a 1:1 mixture of poly(3-hexyl thiophene) (P3HT) and (6,6)-phenyl C61 -butyric acid methyl ester (PCBM) dissolved in dichlorobenzene (DCB) using both spray coating and spin coating and compared the experimental results. Indium-tin-oxide (ITO) coated glass substrates, purchased from Merck Display Technologies with 20 O=&, were first patterned and then cleaned thoroughly with a sequence of detergent, de-ionized water, acetone and isopropanol, each step for 10 min in an ultra sonic bath. The cleaned substrates were purified further by oxygen plasma treatment with an oxygen pressure of 0.26 Torr and a power of 100 W for 10 min. The substrates were then spin coated with a 0:45 mm filtered poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution, purchased from HC Starck, at 3000 rpm for 30 s to produce a 30 nm thick layer. The substrates were subsequently heated on a hotplate in air at 120 1C for 10 min to remove excess water. The further steps of the production process and the current–voltage characterization occurred in a glove box under a controlled nitrogen environment. The active layer containing a mixture of P3HT (Rieke Materials) and PCBM (Solenne bv) was prepared with a concentration of 30 mg/ml under N2 environment and stirred on a hotplate at 50 1C for at least 24 h. Due to the roughness of the samples, the control of the thickness of a thin deposition over the complete sample would have been problematic. For this reason we decided to replace the typical Lithium Fluoride (0.6 nm)/Aluminum (100 nm) cathode by a bulk deposition of a low work function metal: Ytterbium (100 nm) top electrodes were deposited by thermal evaporation in ultra high vacuum ð108 TorrÞ through a shadow mask to define eight separate cells on each substrate. The active areas were measured using an optical microscope, resulting in a range between 3.1 and 3:4 mm2 . The photovoltaic characteristics were measured under nitrogen atmosphere using an Agilent 4156C parameter analyzer under 100 mW=cm2 AM1.5 simulated illumination using a LOT-Oriel Group Europe solar simulator with a 1000 W Xenon arc lamp, filtered by a Newport OD 0.8 neutral density filter. UV–visible absorption spectra were obtained using a Shimadzu UV-1601PC UV–visible spectrophotometer. Thin polymer films were deposited onto previously cleaned quartz slides with the same settings used for the production of solar cells. Atomic force microscopy (AFM) images were recorded on a Picoscan PicoSPM LE scanning probe microscope in tapping mode. The airbrush, a commercially available Badger 200 NH, was powered by N2 gas at 20 psi, a relatively low pressure that ensures a fine atomization while preventing blowing off the droplets already deposited on the substrate. The distance of the airbrush from the substrate was varied from 3 to 10 cm. Additional control of the deposition conditions was achieved by varying the solution flux and the speed of the airbrush relative to the substrate.

coating. The first is labeled single pass technique, in which the active layer is realized as a single layer with a high solution flow rate. Distinctly different is the multiple pass technique, where we build up the active layer as a superposition of several ultra-thin sublayers created with a minimum solution flow rate. The single pass technique (sample 1) allows the deposition of a single and uniform wet layer that dries after the complete deposition in the same time span as a reference layer (sample 2) produced by spin coating. A dense spray of droplets is ejected from the head of the airbrush and merges on the substrate into a single layer before drying. Moving the airbrush in parallel lines over the desired area results in a thin deposition resembling that of a spin coated layer, with uniformity extending along the edges of the substrate, otherwise difficult to achieve with spin coating. For this deposition method we set a flow rate of 0:8 ml=min and kept the airbrush at a distance of 3 cm from the substrate. The multiple pass technique is characterized by a sparse deposition of droplets onto the substrate. The airbrush is kept at a distance of 10 cm from the substrate, and the flow rate is reduced to its minimum value of approximately 20 ml=min. The airbrush is moved across the substrate with intervals of a few seconds between each pass. The droplets then dry independently, before the following sparse deposition arrives. In order to achieve a complete film with an average thickness comparable to the one achieved by spin coating, several passes over the sample are required (sample 3). 3.1. Characterization of the deposition The AFM images in Fig. 1 show the surface of films made with spin coating and the single pass spray technique, and they reveal a close similarity in both the morphology and film quality. The peak to valley (P–V) and root mean square (RMS) roughness values were 78.4 and 11.94 nm, respectively, for spin coating, and 75.5 and 12.91 nm for single pass spray coating. Due to the increased roughness of samples produced with the multiple pass technique, the same measurements could not be performed on samples deposited with this method. Layers produced with the single pass technique and with the multiple pass technique were characterized by UV–visible absorption spectra and compared with a spin coated layer. Fig. 2 shows that the single pass deposition has a profile that is almost overlapping the spin coated one (both with slow drying method [14]). On the other hand the multiple pass shows an absorption profile similar to a blend with amorphous morphology. The quality of P3HT:PCBM layers has been shown to depend on the solvent evaporation speed [15,16], where faster evaporation rate of the solvent induces finer phase separation and amorphous morphology. In this case, thermal annealing can be used to

75.5nm

78.4nm

3. Results and discussion

1µm 0.00nm

By varying the above-mentioned settings of the airbrush we developed two distinct methods to deposit materials by spray

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1µm 0.00nm

Fig. 1. (Color online) AFM of a spin coated (a) and of a single-pass spray coated deposition (b).

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enhance the phase separation and the crystallinity of the layer [17]. Nevertheless, even after an annealing step of 5 min at 140 1C on a hot plate it was not possible to shift the spectrum to overlap with that of the spin coated one, suggesting that the morphology of multiple pass depositions cannot be easily optimized via thermal treatment. In order to understand the phenomena occurring during the deposition, we investigated the drying process of single droplets. During the drying time, we observed that droplets formed a deposit ring, known as a coffee ring, along the initial contact line of the droplet [18]. Therefore, we could deduce the initial radius of the base of the droplet by measuring the radius of the dried deposit after drying. From a sparse deposition on SiO2 (cleaned thoroughly with a sequence of detergent, de-ionized water, acetone and isopropanol, each step for 10 min in an ultra sonic bath), obtained with the multiple pass settings, we could then perform a statistical analysis of the distribution of the radii of the sessile droplets, shown in Fig. 3a (bottom axis). The results show that the distribution of the radii is only slightly affected by the solute concentration, and is limited to 2 mmoro20 mm. Mean values, obtained from a Poisson fit, are between 6:8 and 7:2 mm, values which are one order of magnitude smaller than a typical inkjet printed droplet [7,19]. The droplets on the substrate were deposited in a spherical cap shape, since their radius was smaller than the capillary length [20]. The volume of the droplet is then obtained using V¼

pr

3

3 sin3 a

ð2  3 cos a þ cos3 aÞ

(1)

where r is the radius of the base and a is the measured contact angle of the droplet. By assuming a constant contact angle for the droplet size distribution considered here ð10 1Þ, we translated the measured radii to estimated volumes of the sessile droplets.

Absorbance [a.u.]

1.0 0.8 0.6 0.4

Spin coated Single pass sprayed Multiple pass sprayed Multiple pass sprayed & annealed

0.2 0.0 350

400

450

500 550 Wavelength [nm]

600

650

700

Fig. 2. UV–visible absorption spectra of layers produced with the different techniques described in this work.

Fig. 3(a) (top axis) shows that the volume of each single droplet is distributed between 1 fl and 1 pl, peaking in the range between 30 and 50 fl. The evaporation process of a sessile single droplet is diffusion limited, and controlled by the equilibrium between the liquid phase and the vapor phase surrounding the droplet [21]. In a sparse deposition, droplets are separated and dry independently, but the drying time is determined by several effects. First, each droplet is randomly surrounded by other droplets, at different distances and with various volumes, that are in any stage of their drying process: the environment around the droplet is then dynamically influenced by the amount of solvent vapor generated by the surrounding droplets. Moreover, each droplet is under a varying flux of carrier gas coming from the airbrush that speeds the drying process, since it removes the saturated environment surrounding the drying droplet under steady state conditions. Consequently, the drying process of these droplets cannot be easily observed or calculated. To try to estimate the drying time, we can assume, in general, that the evaporation of droplets is controlled by the stationary diffusion of the liquid molecules in the gas phase, so the evaporation rate is then proportional to the inverse of the droplet radius, r [22]. For sessile droplets with constant small radius, the same results are expected, so dependency of the drying time as a function of the base radius satisfies the relation t drying / r 2

(2)

confirmed by direct measurements [21]. According to this relation, a rough estimation of the drying time of the droplets deposited by spray coating gives values in the ms range. Analyzing a multiple pass deposition (sample 3) under the microscope, we observe that droplets are deposited one on top of another (Fig. 3(b)). This is a direct consequence of the fast drying time of the constituent droplets’ small volumes, which allows each droplet to dry before redissolving underlying ones. An increased size of the base radius of the droplet is also clear, demonstrating a decreased contact angle between the solution and the previously deposited material. Both the composition and the roughness of the surface, as compared to the SiO2 sample used for the contact angle measurement, might contribute to this reduction. AFM performed on the same sample confirms this view. Fig. 3(c) shows the profile of each droplet placed and dried on top of the previously deposited ones. This visual consideration probably does not exclude that the solvent dissolves a small part of the underlying material, since the deposition is performed with a common solvent. Nevertheless, the fact that individual dried drops can be distinguished indicates that only a limited quantity of the previous layers is redissolved and mixed into the top droplet.

Calculation droplet volume [fl]

10

1

Normalized distribution

1.0

100

1000

5mg/ml 10mg/ml 30mg/ml

0.8

310 nm

0.6 0.4

0 nm

50

0.2 0.0

1

2

3

4

5

6 7 8 9

10

µm

50

µm

Droplet radius [µm] Fig. 3. (Color online) (a) Distribution of the measured radii for 5, 10 and 30 mg/ml solution of P3HT:PCBM in dichlorobenzene (bottom axis) and of the calculated volume (top axis). Curves indicate the Poisson fit of the distributions. (b) Optical microscope picture of a multiple pass deposition. The bar represents 50 mm. (c) AFM scan ð50 mm  50 mmÞ of a multiple pass deposition.

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The peculiar characteristic of the multiple-pass spray coating technique that of allowing the deposition of material without completely dissolving the previous layers, suggested us to deposit an active layer with a gradient in composition (sample 4). Indeed, vertical composition gradients with increasing acceptor concentration closer to the cathode have been shown to benefit solar cell performance in the case of both polymer and small-molecule devices [23–25]. To this end, we gradually varied the composition of the spray from pure P3HT to pure PCBM (both dissolved at 30 mg/ml in DCB), by adding small amounts of PCBM solution to the P3HT directly in the reservoir of the airbrush in subsequent steps before each pass.

3.2. Characterization of polymer solar cells (PSCs)

2

10

-1 -4

5

-7 -1.0

-0.5

0.0

0.5

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V [V]

0 3 Multiple pass 4 Gradient

-5

As produced Annealed at 140°C Annealed at 180°C/140°C

-0.2

0.0

0.2

0.4

0.6

0.8

Current Density ,JSC [mA/cm2]

log(JSC) [mA/cm2]

Solar cells were produced using each of the techniques previously introduced. Results from a single run that reflects the

-10 1.0

2

10

-1 -4

5 -1.0

-0.5

0.0 V [V]

0.5

1.0

0

-5 1 Spin coated 2 Single pass sprayed

Current Density ,JSC [mA/cm2]

log(JSC) [mA/cm2]

Voltage, V [V]

-10 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage, V [V] Fig. 4. (Color online) The photovoltaic response of solar cells produced with the different techniques described in this work under 100 mW=cm2 AM1.5 simulated illumination. The inner graph shows a semi-log plot of the same devices in dark.

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general trend of several experiments are shown in Fig. 4. An overview about the significant parameters of all the devices is also reported in Table 1. The as produced multiple pass and gradient solar cells show poor characteristics: the cells are characterized by high series resistances. The poor performance is partially due to the considerable roughness of the polymer layer, yielding regions with very high and very low thickness, as shown in Fig. 3(c). The heterogeneity of the active layer could also contribute: each droplet dries creating microdomains with their own structure, resulting in an ill-defined, non-uniform distribution of P3HT and PCBM. The structure of the complete layer, composed of several droplets one on top of another, can be characterized by the repetition of these droplet-size domains, leading to a nonoptimum P3HT:PCBM phase separation along the complete thickness of the layer. Despite the poor performance, the benefits of the gradient deposition are clear comparing the dark curves of the two devices: the gradient sample shows better diode characteristic, if compared to the multiple pass one, despite the larger leakage current in reverse bias, with two order of magnitude higher values, probably due to the difference in thickness of the active layer (multiple pass thickness: 490 nm; gradient: 320 nm). Both the devices are characterized by a large open circuit voltage ðV OC Þ, above 680 mV, confirming a behavior similar to samples produce without thermal or solvent annealing [15]. An annealing step of 5 min at 140 1C, performed on these two devices after metal evaporation improved their performance, but only partially: the leakage current in dark is reduced for both the depositions, the diode characteristics improve, with rectification ratios larger than 4 orders of magnitude at 1 V=1 V in dark, and the short circuit current densities ðJ SC Þ reach 5 mA=cm2 . It is clear from the increased slope of the curves above V OC that the annealing step reduces the resistance of the active layer, probably fusing the microdomains that compose the layer into larger domains, thus improving charge transport as well as charge collection. Exposing the devices to a 5 s annealing step at 180 1C, above the glass temperature of the mixture, further increased JSC and resulted in a power conversion efficiency ðPCEÞ above 2%. In detail, the gradient technique is still limited in JSC , while showing higher fill factor (FF) and V OC than the multiple pass. Even without a good control of the mixing ratios between the two solutions during the deposition, the improvements in the FF of the gradient are ascribable to reduced shunts and possibly improved carrier collection with the presence of P3HT-rich and PCBM-rich regions at the interfaces with anode and cathode, respectively [26]. Also the higher V OC has been shown to be ascribable to the gradient structure, where the pristine P3HT and PCBM regions at the contacts create a barrier to charge injection from the electrodes into the film, enhancing the electrode selectivity [24]. The thickness and the annealing procedure for this structure might need a different optimization than for the standard solar cells, since these cells show a positive trend for the FF, while JSC reaches a saturation level.

Table 1 Thickness ðdÞ and photovoltaic response of solar cells produced with the different techniques: spin coated (1), single pass (2), multiple pass (3), gradient (4). Sample

Description

d (nm)

V OC (V)

J SC ðmA=cm2 Þ

FF (%)

PCE (%)

1 2 3 4 3 4 3 4

Spin coated, as produced Single pass, as produced Multiple pass, as produced Gradient, as produced Multiple pass, post annealed: 300 s at 140 1C Gradient, post annealed: 300 s at 140 1C Multiple pass, post annealed: 5 s at 180 1C and 300 s at 140 1C Gradient, post annealed: 5 s at 180 1C and 300 s at 140 1C

310 380 490 320 490 320 490 320

610 610 680 690 630 640 630 660

8.3 9.2 1.8 2.1 5.2 5.4 7.5 5.9

48.7 50.8 31.8 38.5 49.9 49.8 45.3 52.6

2.47 2.84 0.40 0.55 1.64 1.73 2.16 2.04

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On the other side, as already expected from the similarities obtained from AFM and absorption, the performances of the devices made by spin coating and single pass are similar. The FF and V OC of the two devices are identical, suggesting the development of the same morphology inside the layers during the drying time, with analogous phase separation along their thickness. The enhanced JSC in the single pass one, resulting in a higher PCE, above 2.8%, can be ascribed to a thicker deposition. Further work is necessary to optimize the composition and control over the different sublayers within the active layer obtained with the multiple pass spray coating technique, to grade the composition while avoiding performance-limiting microdomains within the layer.

4. Conclusions In conclusion, this work demonstrates that spray coating is an excellent alternative to spin coating for the fabrication of polymer-based solar cells, showing power conversion efficiencies in the same range. The single pass technique developed in this work shows that a deposition with the same characteristics of spin coating can be achieved from the same solution. This is a great advantage when considering the optimization of the final devices, since the studies performed on spin coated films could be directly applied to spray coated ones. In addition, the fast drying times of femtoliter-scale droplets generate possibilities to create more complex device architectures, using solutions from a single solvent system: with the multiple pass technique we could create a gradient in the composition of the sublayers, which gives an additional degree of freedom for optimization of solution-processed bulk heterojunction solar cells. This intriguing opportunity needs further development in order to solve the problems that are limiting the results obtained in this work. A better control over the processing with more sophisticated equipment, such as an automated and computer controlled spray coater, should allow for improved performance of polymer devices.

Acknowledgments The research was (partly) performed in the framework of the IWT SBO-project 060843 ‘‘PolySpec’’ funded by the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT). References [1] B.C. Thompson, J.M.J. Fre´chet, Polymer-fullerene composite solar cells, Angew. Chem. Int. Ed. 47 (2008) 58–77. [2] B.P. Rand, J. Genoe, P. Heremans, J. Poortmans, Solar cells utilizing small molecular weight organic semiconductors, Prog. Photovoltaics 15 (2007) 659–676.

[3] S. Gu¨nes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chem. Rev. 107 (2007) 1324–1338. [4] E. Bundgaard, F.C. Krebs, Low band gap polymers for organic photovoltaics, Sol. Energy Mater. Sol. Cells 91 (2007) 954–985. [5] R. Kroon, M. Lenes, J.C. Hummelen, P.W.M. Blom, B. de Boer, Small bandgap polymers for organic solar cells (polymer material development in the last 5 years), Polym. Rev. 48 (2008) 531–582. [6] M. Jorgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 686–714. [7] T. Aernouts, T. Aleksandrov, C. Girotto, J. Genoe, J. Poortmans, Polymer based organic solar cells using ink-jet printed active layers, Appl. Phys. Lett. 92 (2008) 033306 (pp. 1–3). [8] C.N. Hoth, S.A. Choulis, P. Schilinsky, C.J. Brabec, High photovoltaic performance of inkjet printed polymer: fullerene blends, Adv. Mater. 19 (2007) 3973–3978. [9] S.E. Shaheen, R. Radspinner, N. Peyghambarian, G.E. Jabbour, Fabrication of bulk heterojunction plastic solar cells by screen printing, Appl. Phys. Lett. 79 (2001) 2996–2998. [10] F.C. Krebs, J. Alstrup, H. Spanggaard, K. Larsen, E. Kold, Production of largearea polymer solar cells by industrial silk screen printing, lifetime considerations and lamination with polyethyleneterephthalate, Sol. Energy Mater. Sol. Cells 83 (2004) 293–300. [11] P. Schilinsky, C. Waldauf, C.J. Brabec, Performance analysis of printed bulk heterojunction solar cells, Adv. Funct. Mater. 16 (2006) 1669–1672. [12] D. Vak, S.-S. Kim, J. Jo, S.-H. Oh, S.-I. Na, J. Kim, D.-Y. Kim, Fabrication of organic bulk heterojunction solar cells by a spray deposition method for lowcost power generation, Appl. Phys. Lett. 91 (2007) 081102 (pp. 1–3). [13] R. Green, A. Morfa, A.J. Ferguson, N. Kopidakis, G. Rumbles, S.E. Shaheen, Performance of bulk heterojunction photovoltaic devices prepared by airbrush spray deposition, Appl. Phys. Lett. 92 (2008) 033301 (pp. 1–3). [14] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Highefficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nat. Mater. 4 (2005) 864–868. [15] P. Vanlaeke, A. Swinnen, I. Haeldermans, G. Vanhoyland, T. Aernouts, D. Cheyns, C. Deibel, J. D’Haen, P. Heremans, J. Poortmans, J.V. Manca, P3HT/ PCBM bulk heterojunction solar cells: relation between morphology and electro-optical characteristics, Sol. Energy Mater. Sol. Cells 90 (2006) 2150–2158. [16] G. Li, Y. Yao, H. Yang, V. Shrotriya, G. Yang, Y. Yang, ‘‘Solvent annealing’’ effect in polymer solar cells based on poly(3-hexylthiophene) and methanofullerenes, Adv. Funct. Mater. 17 (2007) 1636–1644. [17] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Effects of postproduction treatment on plastic solar cells, Adv. Funct. Mater. 13 (2003) 85–88. [18] R.D. Deegan, O. Bakajin, T.F. Dupont, G. Huber, S.R. Nagel, T.A. Witten, Capillary flow as the cause of ring stains from dried liquid drops, Nature 389 (1997) 827–829. [19] B. de Gans, P.C. Duineveld, U.S. Schubert, Inkjet printing of polymers: state of the art and future developments, Adv. Mater. 16 (2004) 203–213. [20] B. Widom, Line tension and the shape of a sessile drop, J. Phys. Chem. 99 (1995) 2803–2806. [21] G. Guena, C. Poulard, M. Voue, J. De Coninck, A.M. Cazabat, Evaporation of sessile liquid droplets, Colloid Surf. A Physicochem. Eng. Asp. 291 (2006) 191–196. [22] I. Langmuir, The evaporation of small spheres, Phys. Rev. 12 (1918) 368–370. [23] A.C. Arias, N. Corcoran, M. Banach, R.H. Friend, J.D. MacKenzie, W.T.S. Huck, Vertically segregated polymer-blend photovoltaic thin-film structures through surface-mediated solution processing, Appl. Phys. Lett. 80 (2002) 1695–1697. [24] H.J. Snaith, N.C. Greenham, R.H. Friend, The origin of collected charge and open-circuit voltage in blended polyfluorene photovoltaic devices, Adv. Mater. 16 (2004) 1640–1645. [25] J. Xue, B.P. Rand, S. Uchida, S.R. Forrest, A hybrid planar-mixed molecular heterojunction photovoltaic cell, Adv. Mater. 17 (2005) 66–71. [26] M. Campoy-Quiles, T. Ferenczi, T. Agostinelli, P.G. Etchegoin, Y. Kim, T.D. Anthopoulos, P.N. Stavrinou, D.D.C. Bradley, J. Nelson, Morphology evolution via self-organization and lateral and vertical diffusion in polymer: fullerene solar cell blends, Nat. Mater. 7 (2008) 158–164.