Scholarly article on topic 'A transition solvent strategy to print polymer:fullerene films using halogen-free solvents for solar cell applications'

A transition solvent strategy to print polymer:fullerene films using halogen-free solvents for solar cell applications Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Guan-Hui Lim, Jing-Mei Zhuo, Loke-Yuen Wong, Soo-Jin Chua, Lay-Lay Chua, et al.

Abstract Inkjet printing is a mask-less non-contact deposition technique that is potentially suited for prototyping and manufacturing of thin-film polymer organic semiconductor devices from digital images. However new strategies are needed to achieve films with good macromorphology (i.e., high-fidelity footprint and uniform cross-section) and nanomorphology on unstructured substrates using a conventional ink-jet. Here we report a new transition solvent strategy to provide the desired film macromorphology and ultrafine nanomorphology in regioregular poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) model films, without using chlorinated solvents. This strategy employs a good volatile solvent in combination with a miscible poor solvent that is much less volatile, which is the reverse of the usual low−high boiling-point solvent method. The good solvent suppresses premature aggregation in the ink head. Its removal by evaporation on the substrate leaves the poor solvent that triggers early π-stacking ordering and/or gelation of the polymer matrix that immobilizes the printed fluid on the substrate, suppressing both contact-line depinning and evaporation-induced solvent flow effects. The resultant donor–acceptor nanomorphology is further improved by vacuum drying at an optimal rate that avoids bubble formation. We have systematically characterized P3HT:PCBM films deposited with different solvents and platen temperatures to identify key macro- and nano-morphology determining processes. High-performance printed P3HT:PCBM solar cells were realized. These findings are applicable also to other printing and coating techniques based on low-viscosity inks.

Academic research paper on topic "A transition solvent strategy to print polymer:fullerene films using halogen-free solvents for solar cell applications"

Organic Electronics 15 (2014) 449-460

ELSEVIER Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel 1 m H Organic 1 Electronics 1 S

A transition solvent strategy to print polymerifullerene films using halogen-free solvents for solar cell applications q

Guan-Hui Lima,c, Jing-Mei Zhuoa,b, Loke-Yuen Wonga, Soo-Jin Chuac, Lay-Lay Chuaa,b, Peter K.H. Hoa,d'*

a Department of Physics, National University of Singapore, Lower Kent Ridge Road, S117551, Singapore b Department of Chemistry, National University of Singapore, Lower Kent Ridge Road, S117543, Singapore

c Department of Electrical and Computer Engineering, National University of Singapore, Lower Kent Ridge Road, S117576, Singapore d Solar Energy Research Institute of Singapore, National University of Singapore, Engineering Drive 1, S117574, Singapore

ARTICLE INFO ABSTRACT

Inkjet printing is a mask-less non-contact deposition technique that is potentially suited for prototyping and manufacturing of thin-film polymer organic semiconductor devices from digital images. However new strategies are needed to achieve films with good macromor-phology (i.e., high-fidelity footprint and uniform cross-section) and nanomorphology on unstructured substrates using a conventional ink-jet. Here we report a new transition solvent strategy to provide the desired film macromorphology and ultrafine nanomorphology in reg-ioregular poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) model films, without using chlorinated solvents. This strategy employs a good volatile solvent in combination with a miscible poor solvent that is much less volatile, which is the reverse of the usual low-high boiling-point solvent method. The good solvent suppresses premature aggregation in the ink head. Its removal by evaporation on the substrate leaves the poor solvent that triggers early p-stacking ordering and/or gelation of the polymer matrix that immobilizes the printed fluid on the substrate, suppressing both contact-line depinning and evaporation-induced solvent flow effects. The resultant donor-acceptor nanomorphology is further improved by vacuum drying at an optimal rate that avoids bubble formation. We have systematically characterized P3HT:PCBM films deposited with different solvents and platen temperatures to identify key macro- and nano-morphology determining processes. High-performance printed P3HT:PCBM solar cells were realized. These findings are applicable also to other printing and coating techniques based on low-viscosity inks.

© 2013 The Authors. Published by Elsevier B.V. All rights reserved.

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Article history:

Received 7 August 2013

Received in revised form 20 October 2013

Accepted 21 October 2013

Available online 2 December 2013

Keywords: Organic solar cells Ink-jet printing Morphology Solvent effects Processing

1. Introduction

A key attractive feature of polymer organic semiconductor (OSC) devices is their processability over large and/or

q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author at: Department of Physics, National University of Singapore, Lower Kent Ridge Road, S117551, Singapore. Tel.: +65 65168781.

E-mail address: phyhop@nus.edu.sg (P.K.H. Ho).

flexible substrates using additive deposition methods, such as various printing and coating techniques [1]. Inkjet printing (ijp) offers some important advantages due to its non-contact operation, and ability to give patterned films with lateral resolution of better than 10 im directly from digital image files at potentially very high speeds [2-8]. While the inkjet-printing of polymer OSC light-emitting layers in pre-formed confinement wells has reached advanced development, [9] the printing of field-effect transistors and solar cells is still in infancy and faces a number of challenges related to the drying-induced distortion of films that are not deposited into confinement wells [10-14].

1566-1199/$ - see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.10.025

This problem is a consequence of the low viscosity in the "inks" printed by conventional ijp. As the solvent evaporates after deposition of the fluid puddle, the contact-line of the puddle retracts, which causes the footprint and position of the remaining fluid puddle to change. As the solvent evaporates further, materials deposition occurs and the contact line becomes pinned to the substrate. The puddle footprint now becomes fixed, but an evaporation-induced fluid flow towards the edge continues which piles up material to give a coffee-stain rim. These effects have been extensively studied for single printed droplets [15-20]. Other related phenomena, such as monolith formation [21] and buckling instability, [22] have also been documented in these films. When the droplets are arranged to give lines, new effects emerge, including droplet-bunching and line break-up [23-28]. When the droplets are arranged to give extended films, one can expect new complications from materials and solvent diffusion and from dissolution-recrystallization effects, [29] depending on how the droplets are arrayed. These latter phenomena have been much less studied. Even the basic features of how ijp processing parameters (temperature, solvent and drying conditions) affect the macromorphology of extended films are not well understood, despite their obvious importance to the free-form printing of transistor and solar cell films.

We report here a systematic study of ijp processing parameters on the macromorphology of the printed films using P3HT:PCBM as the materials model. We developed a transition solvent strategy that together with optimal vacuum drying enables "fixing" of the footprint and shape of the drying fluid puddle to give thin flat films with near uniform cross-section. The strategy employs a good volatile solvent in combination with a large fraction of a poor (not a bad!) but much less volatile solvent. This concept is opposite to the well-known strategy of combining a volatile solvent with a less volatile solvent which is also a good solvent for the OSC [11,12]. In that strategy, a rapid initial drying of the printed puddle is achieved to give a concentrated solution in a good solvent that then dries slowly to give the final film. Unfortunately this calls for the use of chlorinated aromatic solvents, usually a dichlorobenzene, for P3HT:PCBM and related materials systems, because only these solvents appear to have the required solvent power and high boiling points.

If however a volatile good solvent is combined with a much less volatile poor one, it should be possible to trigger early p-stacking and/or "gelation" of the polymer matrix on the deposition substrate by evaporation-induced quenching of the solvent quality rapidly through the borderline regime. Borderline solvents are known to induce formation of p-stacks of extended OSC polymer chains in solution to give well-ordered lamellae in neat films [30]. We show here that this strategy can simultaneously meet the twin goals of obtaining sufficient inkjet latency and dwell times (because of inclusion of the volatile good component), and surprisingly good macromorphology and nanomorphology of the printed films suitable for solar cell applications, without using chlorinated solvents. This approach is different also from the well-known use of

low-volatility solvent additives, usually a diidoalkane or dithiol, at the few % level to prolong solvent annealing process of polymer:fullerene films [31].

We investigated P3HT:PCBM because this is an important model of OSC photoactive layer systems which comprise a semicrystalline p-stacking donor polymer in an intimate blend with a fullerene acceptor [32-36]. A mixture of 4:6 vol/vol toluene:n-butylbenzene was developed as the transition solvent, where toluene is a reasonably good solvent and n-butylbenzene is the poor solvent. This system produces films with better macro- and nano-mor-phologies than previously possible using a single-component non-chlorinated aromatic solvent, such as tetralene [11,12]. The critical challenge for a single-component non-chlorinated solvent is that it needs to simultaneously meet both the vapor-pressure and solvent-power requirements, which together constitute a formidable challenge for a semicrystalline polymer. For example, tetralene has a suitable vapor pressure for ijp, but is a poor solvent for P3HT. As a result, it severely depresses the inkjet latency and dwell times, and produces very rough films with coarse phase separations due to extensive p-stacking aggregation, even for a low regioregularity P3HT material [11,12].

The present strategy of incorporating a volatile good solvent component provides a new degree-of-freedom to overcome this challenge. We were thus able to inkjet print higher quality films with an improved power conversion efficiency, 2.2% vs 1.3% reported for tetralene. Although this is still poorer than those obtained from chlorinated aromatics (often by spin-casting), which is typically 3% at same composition and thickness, the difference may be narrowed by further optimization of the solvent mixture. Crucially the strategy here can avoid the use of haloge-nated solvents. Halogenated solvents are potential environmental concerns, and pose severe compatibility issues with printing tools, e.g., chlorinated aromatics are very harsh on the seals and plastic materials in the ink head and fluid delivery system. Amorphous polymers do not face as severe constraints [37]. This provides an important step towards the manufacture of polymer solar cell foils, and possibly also transistor films.

2. Experimental section

2.1. Substrate preparation

PEDT:PSSH (Baytron P, Leverkussen, Germany) was reformulated to give a 1:16 weight/weight (w/w) PEDT:PSSH material by dilution with PSSH [38,39]. 50-nm thick films were then spin-cast on glass or indium-tin oxide (ITO)-glass substrates. These films were annealed at 110 °C for 10 min (hotplate, N2 glovebox) to remove residual moisture before P3HT:PCBM deposition.

2.2. Polymer:fullerene solution preparation

Regioregular P3HT (>97% regioregular, Mn = 15-45 k; Plexcore OS 1100, Plextronics) and PCBM (Nano-C) were dissolved at the ratio of 1.5:1 weight-to-weight (w/w) into

the selected solvent to give a total solids content of 2.5 mg mL-1 (sometimes 4.5mgmL-1), by heating at 100 °C for 10 min in the glovebox just before use. Typically, the solutions were cooled for 10 min immediately before loading into 3-mL printer cartridges (Dimatix DMC11610).

2.3. Inkjet-printing protocol

The P3HT:PCBM films were printed using a 16-jet shear-mode piezo print head (Dimatix DMC11610) mounted on a Dimatix platform (Dimatix DMP2831). The P3HT:PCBM solutions were de-gassed (at 100 mbar) before transferring to the ink reservoir. The excitation voltage amplitude Vo was adjusted to achieve uniform droplet speed uo of ca. 6 ms-1 (no satellites) aided by the Ohnesorge number (Oh)-Vo-uo plot [40]. 5 x 5-mm square-shaped films were printed using 14-16 jets. The print pattern was a square lattice with pre-selected pitch. Since the span of the jets is far smaller than 5 mm, the entire film was printed sequentially with a number of print lanes that depends on the pitch. For a pitch of 12 im, the film was printed in 26 lanes. The platen temperature was regulated using the built-in temperature controller for P28 °C, and a home-built Peltier-cooler for <28 °C. The films were then dried according to the protocols described in the text, and annealed at 140 °C for 10 min (hotplate, N2 glovebox) to drive the ordering transition of the P3HT polymer chains, [33] before further characterization or evaporation of the metal electrodes.

2.4. Film-thickness profile measurements

We recorded the optical transmission images of the films using a digital camera (10-megapixel CMOS sensor array) with uniform white backlight illumination. The P3HT absorbs primarily in the green channel. Therefore the signal in this channel was extracted to give the optical-density image. The optical density is given by -log(I/Io), where I is the local transmitted light intensity and Io is the transmitted intensity without the film. This image was then calibrated to give the physical thickness using profilometry to provide a rapid method to estimate film thickness (and uniformity) with lateral resolution better than a few micrometers.

2.5. Other film characterizations

UV-Vis absorption spectra of the films were collected in the N2 glovebox using a diode array spectrograph (DW1024, Ocean Optics). Viscometry measurements were performed with a moving-piston viscometer (Cambridge Viscosity Viscolab 450). 0-20 XRD diffractograms were collected on printed thin films on glass substrates using monochromatized 1.541-A Cu Ka radiation (Bruker-AXS D8 Powder X-ray Diffractometer) with a step size of 0.02° and collection time of 16 s per step.

2.6. Solar cell fabrication and characterization

10 x 10-mm square films of 1.5:1 w/w P3HT:PCBM were printed over a 50-nm-thick PEDT:PSSH film on

ITO-glass in the dark to avoid a possible photo-oxidation and photo-doping of the films [41,42]. The films were then subjected to a drying protocol (see text), and annealed at 140 °C for 10 min (hotplate, N2 glovebox). A 100-nm-thick Al film was then evaporated through a shadow mask at a base pressure of 3 x 10-7 Torr to give the electron-collecting electrodes for eight 4.3-mm2 pixels on each substrate. The current-voltage characteristics of the cells were then measured in a home-built vacuum chamber under 1.0-sun irradiance (spectral mismatch corrected) using a commercial AM1.5 solar simulator (Newport). The simulator was cross-calibrated with a AAA-class simulator at the Solar Energy Research Institute of Singapore (SERIS).

3. Results and discussion

3.1. Preliminary considerations: solvent volatility and solvent power

These considerations are well-known qualitatively. We will outline some quantitative aspects here. The required solvent volatility has often been described using the boiling point. However a more precise notion is the solvent vapor pressure. There is a limited vapor pressure pvap range for ijp. In our experience, the practical room-temperature solvent pvap range for conventional ijp using pL droplets with both ink head and substrate at room temperature is 0.2-50 mbar. This corresponds to normal boiling points between 100 °C and 210 °C for solvents which obey the standard pressure-temperature nomograph. If pvap > 50 mbar, we found stable jetting becomes nearly impossible, presumably due to fluid cavitation during piezo actuation. If pvap < 0.2 mbar, the drying becomes too slow. Already for pvap ~ 1 mbar, a fluid puddle that is ca. 70-p.m thick takes more than 1 h to dry under ambient conditions, although this can be shortened to a few minutes under optimal vacuum-drying conditions (vide infra).

In addition, the solvent quality needs to be sufficiently good that the "ink" material remains dissolved in the fluid delivery system and ink head to give long ink latency time. This is the time over which the ink remains viable in the printing tool. For semicrystalline OSC polymers, there is a simple way to estimate this from the time taken for the solvatochromic spectral shift to occur. This corresponds to a disorder-to-order transition that results from p-stack-ing of the polymer chains in solution [43,44]. We call this the time-to-aggregation tag, which we estimated visually for the P3HT:PCBM solutions from the time taken to turn from bright orange to dark red. This tag depends on concentration and polymer regioregularity. For chlorobenzene (CB) and o-dichlorobenzene (DCB), both of which are good solvents for P3HT, [30] tag exceeds 1 month at typical ijp concentrations (2-5mgmL-1) for the >97% regioregular polymer used, and is recorded as 1 here. Toluene (TOL) is a good-to-borderline solvent which gives a tag of several hours. It is the best one that we have found in a limited search to illustrate our solvent strategy. It will be more ideal to find an even better one for the volatile solvent component. Both mesitylene (MS) and butylbenzene (BB)

are poor solvents, with tag of a few minutes, although these still appear to be better than tetralene.

Four solvent systems were investigated: CB, DCB, 3:3:4 vol/vol (v/v) BB:MS:CB, and 8:2 v/v BB:TOL. Table 1 summarizes the physical properties of these solvent components. The first two have often been used to print P3HT:PCBM [11-13] and other polymer:fullerene [14] films. The latter two solvent systems are mixtures that we have developed to illustrate our solvent strategy to print P3HT:PCB, with tag > 1 h.

3.2. Effect of solvent on film macromorphology

Fig. 1 shows the optical-density images and selected film-thickness profiles of P3HT:PCBM films printed at 28 °C for a target average thickness of 250 nm, on a 50-nm-thick spin-cast PEDT:PSSH layer on glass substrates. The PEDT:PSSH layer provides the usual hole-collection contact in organic solar cells. The P3HT:PCBM films printed from CB and DCB replicate the square outline of the print pattern rather well. However their central region is only 100-200-nm thick and flanked by a rim that is several 100 im wide and nearly 1 im tall. This is a characteristic feature of the coffee-stain effect caused by materials buildup along the puddle edge due to an evaporation-induced outward-directed capillary flow of the solvent [45]. In contrast, films printed from the 3:3:4 v/v BB:MS:CB and 8:2 v/v BB:TOL systems are free from this coffee-stain effect. This improvement is not simply due to a change in solvent volatility of the highest-boiling component, as DCB, MS and BB all have nearly identical Pvap (1-2mbar) at room temperature. The improvement is due to a different mechanism, which we will show below to be related to quenching of the solvent quality from good to poor, when the more volatile good solvent component is lost by evaporation. Nevertheless some non-idealities due to contact-line retraction and other fluid-flow effects are still present, which can be mitigated as shown later.

3.3. The role of solvent quality quenching

The suppression of the coffee-stain effect in the solvent mixtures above is related to immobilization of the fluid puddle due to quenching of the solvent quality. We simulated the drying of the solvent puddle under quiescent condition. Fig. 2a shows the computed dependence of solvent

h_JUL il

Fig. 1. Optical-density images of P3HT:PCBM (1.5:1 w/w) films printed on PEDT:PSSH films at 28 "C from various solvent systems. (a) 4.5 mgmL-1 CB, (b) 4.5 mgmL-1 DCB, (c) 4.5 mgmL-1 3:3:4 BB:MS:CB, (d) 2.5mgmL-1 8:2 BB:TOL. A representative height profile extracted along the dotted lines is shown below each image. A depression occurs in the films in (c and d). Film size, 5 x 5 mm2. Height markers in nm.

composition on fractional drying for the 8:2 v/v BB:TOL mixture. The evaporation rate E of each component (in nm s-1) was computed taking care of diffusion and convection-flow of the air/vapor boundary layer, using the following equation [46]:

M • Pvap • Dm • z R • T • q • heff

where Dm is the solvent molecular diffusivity in air, R is the gas constant, T is the thermodynamic temperature, M is the solvent molecular weight, q is the solvent liquid density, z is the air counter-diffusion correction factor («1.0), and heff is the effective vapor thickness. The typical value of heff is 1.8 mm for an evaporating organic solvent on a large flat surface [46]. For an ideal mixture, the pvap contributed by each component is given by Raoult's law. To solve this differential equation, the volume loss of each component at each time step was computed assuming that composition equilibrium was maintained throughout the fluid film

Table 1

Physical properties of the solvents used in this study.

Solvent Notation Boiling point ("C) Surface tension (mJ m 2) Pvap (mbar)a 20 "C 60 "C tb 'ag

Chlorobenzene CB 131 33 9 66 1

1,2-Dichlorobenzene DCB 181 37 1 11 1

Toluene TOL 111 26 22 139 Few h

Mesitylene MS 165 26 2 18 6 min

Butylbenzene BB 183 28 0.7 9 4 min

a pvap Is vapor pressure.

b tag Is the time taken for p-stacking to occur in P3HT after having been brought to a good state (100 "C, 10 min) and cooled to room temperature for 2.5 mgmL-1 solution (total solids) of P3HT:PCBM (1.5:1 w/w).

500 550 600 650 700 750 800 Wavelength (nm)

Fig. 2. Characteristics of P3HT:PCBM (1.5:1 w/w) solutions in the transition solvent (8:2 BB:TOL) vs non-transition good solvents. (a) Computed dependence of solvent composition on evaporation loss fraction for a 8:2 BB:TOL mixture at 20 "C (solid line) and 60 "C (dotted line). (b) Concentration dependence of the viscosity of P3HT:PCBM solutions. The viscosity of the BB:TOL solution exceeds 100 cP at 15 mg mL-1. (c) Transmission UV-Vis spectra of P3HT:PCBM solutions, measured after cool down to room temperature (10 min), through a 2.0-mm path length cell. The absorption onset at 725 nm arises from PCBM.

(justified because this is typically <100 im thick), and the loss was integrated forward in time to obtain the composition evolution.

When the solvent components differ widely in pvap, e.g. have pvap ratio P10, the more volatile component will practically evaporate away completely first. For example, by the time the 8:2 TOL:BB fluid film is 30% evaporated, its composition is practically pure BB (Fig. 2a). This result is little dependent on small variations in temperature. If the less volatile component is a poor solvent, the solvent power can quickly change from good to poor, which causes the viscosity to rise tremendously. Fig. 2b illustrates this effect. While the viscosities of CB and DCB solutions of P3HT:PCBM increase slowly with concentration up to 20 mg mL-1, the viscosity of the BB:TOL solution increases abruptly beyond 10 mg mL-1. The increase is even larger for BB alone. This strategy is thus reminescent of the use of phase change, [47] such as the sol ? gel transition [48] and the solidification transition, [49] to immobilize the fluid. Fig. 2c shows that incipient p-stacks already begin to occur in pristine 8:2 v/v BB:TOL solution, as evidenced by redshift in the absorption edge of the P3HT chains to 650 nm (1.9 eV) [50]. The random-coil state of P3HT in CB and DCB has an absorption edge at 585 nm (2.1 eV) [43,44].

3.4. Dependence of film macromorphology on platen temperature

The macromorphology of the printed film depends strongly on the platen (substrate) temperature Ts. Fig. 3 shows the otpical-density images of films printed at various Ts from 20 °C to 60 °C for 8:2 v/v BB:TOL, compared

with CB, DCB and 9:1 v/v CB:DCB. As Ts increases, the coffee-stain effect generally becomes more pronounced and ripples appear in the print lane direction. These have been noted earlier, [26,51] but here we can see the transitions typically occur over surprisingly narrow temperature ranges. Therefore control of Ts to 1 °C or so is needed for repeatability.

For example, the CB-printed film shows the usual circular coffee-stain morphology at Ts = 20 °C, but this becomes asymmetric for Ts = 25-40 °C, when a thick buildup occurs over the earlier-deposited lanes. We call this the "hillside" morphology, and attribute it to an evaporation-driven materials transfer perpendicularly across the print lanes from the wet to the semi-dried regions of the fluid puddle. For Ts p 45 °C, the printed film shows print-lane bunching, i.e., adjacent print lanes dry together to give a strip-like appearance in the print direction.

On the other hand, the DCB-printed film does not show the hillside morphology, but contact-line retraction for Ts p 40 °C. For both these single-component good solvents, the coffee-stain effect occurs at all Ts. In contrast the mixed 9:1 CB:DCB-printed film shows a good footprint for Ts = 20-25 °C with a mild coffee-stain effect, but contact-line retraction for Ts p 30 °C, hillside morphology for Ts p 35 °C, and print-lane bunching for Ts p 45 °C.

To derive an approximate condition for the hillside morphology to occur, we consider that the postulated differential drying effects should become significant when: t' < t'd < tp, where te is the time to print a lane, ted is the time to dry the lane, and tp is the time to print the last lane (nlth), or the neth lane which still allows solvent flow to the first lane, whichever is smaller (i.e., tp = min(nl, ne)*t'). Experimental results suggest that for all the films here,

20 C 25 C 30 C 35 C 40 C 45 C 50 C 55 C 60 C

Fig. 3. Effect of platen temperature on macro-morphology of inkjet-printed films. Optical-density images from: (a) CB, (b) DCB, (c) 9:1 CB:DCB, and (d) 8:2 BB:TOL solutions at increasing platen temperature and quiescent-dried at the same temperature. Solution composition, P3HT:PCBM 1.5:1 w/w; concentration, 2.5 mg mL_1; droplet spacing, 12 im; film size, 5 x 5 mm2. Dotted lines in row (b) indicate the initial (design) boundary of the printed fluid puddle. Print direction is horizontal.

ne > nl = 28. For a nozzle pitch d = 12 im, the ijp parameters here give te« 8 s and tp « 3.7 min for all the films. The CB-film gives ted « 2 min, and the DCB-film 20 min, at 28 "C. Thus the first-printed lanes from CB, but not DCB, are partially dried by the time the final lanes are printed. This suggests that the hillside morphology should emerge in CB-printed film (and also the CB:DCB-printed film at elevated Ts), but not DCB-printed film at all Ts here, which is in agreement with experiment. To avoid this problem, one thus needs to operate where ted > tp to deposit a completely wet film, or ted < tp to produce fully dried lanes. Differential drying can cause severe complications. Previously we have observed that differential drying can lead to severe macroscopic roughness and unusual recrys-tallization in neat P3HT films not found in uniform dried ones [29].

3.5. Low-volatility component: choose good or poor solvent?

Aside from the incorporation of a less volatile good solvent into the mixture as discussed above, [11,12] which is known to work generally for both amorphous [16] and semicrystalline polymers, [17,51] there are other related strategies to suppress the coffee-stain effect. These include lowering the evaporation rate to promote counter-diffusion, [26] and using mixtures of solvents with different volatilities and surface tensions to induce Marangoni flows [52]. Nevertheless all these approaches call for the use of a chlorinated aromatic solvent as the high-boiling good solvent for P3HT:PCBM.

However when a poor solvent is employed as the high-boiling solvent, the printed film surprisingly exhibits an even better macromorphology. For example, the 8:2 BB:TOL-printed film shows good footprint, good thickness uniformity and no coffee stain at Ts = 20 "C. We attribute this to improved pinning of the contact line and immobilization of the fluid film due to evaporation-induced quenching of the solvent power. Nevertheless film

uniformity degrades as Ts increases. The coffee-stain effect sets in for Ts p 35 "C, and the hillside morphology eventually emerges, presumably because solvent power improves with temperature.

3.6. Optimal vacuum-drying

It is thus clear that Ts has to be managed carefully to avoid strong differential drying of one part of the film while a neighboring part is still wet, and to regulate solvent power. P3HT:PCBM films printed from the BB:TOL take more than 1 h to dry in the ambient, which is impractical for manufacturing. The strong temperature sensitivity of film macromorphology rules out the use of elevated Ts as a general approach to speed up film drying. Therefore an alternative method has to be developed. We first attempted to impose a laminar cross flow to increase the rate of solvent transfer into air. However this was not effective even at the highest wind speed that can be applied (2 ms-1) without dragging the fluid puddle.

We then considered vacuum drying, which is a practical method that can be implemented for both batch and roll-to-roll processing using differential vacuum pumping. The question is what determines the optimal depressuriza-tion-time profile. To obtain a simple insight, we considered the following analysis. As the pressure p in the vacuum chamber decreases, Dm (which scales as p-1) increases and so E increases. This suggests p should be set as low as possible. However p cannot be set below pvap, otherwise "boiling" of the fluid occurs. In practice, p cannot be set below a higher threshold, because of air bubble out-gassing. The typical solubility of air (N2 and O2) in aromatic organic solvents is 0.5 x 10-3 mol/ mol [53]. This means the volume of air at standard temperature and pressure dissolved in the fluid is of the same order of magnitude as the fluid volume itself. The out-gassing of this dissolved air as bubbles severely disrupts the integrity of the film. Its

detailed modeling requires consideration of both nucle-ation and growth, [54] which appears to be difficult. We employed the following scaling arguments to find the maximum - dp that can be tolerated. We assumed that the diffusion of air through the fluid film is faster than fluid evaporation and can be treated as one dimensional. The mean diffusion flux of air is then given by: DCj—-, where ct (and cb) is the air concentration at the top (bottom) of the fluid layer, D is the air diffusion coefficient (^10-5 cm2 s-1), and z is the fluid thickness. If the concentration of the dissolved air at the top is in equilibrium with the applied vacuum, ct is related to p by Henry's law, i.e., p = KH ct, where KH is the Henry coefficient. We further

assumed that the concentration profile has reached steady state, and does not change with time: dfj1 = djf = DdZf. Together these give: - dp = Dpbz-p, where pb is the equilibrium pressure of air that corresponds to its concentration at the bottom of the fluid layer. We consider that (pb-pt) needs to be smaller than the Laplace pressure 2 R to avoid bubble growth, where c is the surface tension of the fluid (^30 mJ m-2) and R is a critical bubble radius. This suggests that there is an upper limit to the allowable rate of depressurization. If we assume that R is 5 im (due to par-ticulate comtaminants), pb-ps needs to be smaller than 100 mbar, which means - dp <10 mbar s-1 for z « 100 im.

Time (s)

(b) Quiescent drying (Protocol I):

d=10nm cf=12um

470-■

360" ■

Vacuum drying Protocol

Fig. 4. Effect of depressurization profile on macromorphology of inkjet-printed films. (a) Pressure-time protocols. (b) Optical-density images of films printed from 8:2 BB:TOL at 20 "C, and dried under different protocols. Parameters are as given in Fig. 3.

3.7. Depressurization-time profiles

To test the prediction that there exists an upper limit to the allowable depressurization rate, we investigated the effects of two depressurization-time profiles II and III, as shown in Fig. 4a, on 5 x 5 mm2 printed P3HT:PCBM fluid films with 8:2 BB:TOL as solvent, by throttling the vacuum line with feedback from a Pirani pressure gauge. These profiles comprise a fast depressurization to ca. 100 mbar, so that p is always larger than pvap; and a slow depressuriza-tion to the base pressure of 0.3 mbar to complete the final drying. Protocol II has an initial depressurization speed of ca. 5 mbar s-1, and III ca. 12 mbar s-1. For comparison, quiescent drying of the printed film (protocol I) in ambient was also studied.

Fig. 4b shows the macromorphologies of the films obtained. These were strongly dependent on drying conditions. Protocol II (td, 4-5 min) gave the best film-thickness profile, characterized by a wide flat top albeit with a sizeable transition width along its parameter. In contrast the more rapid protocol III gave craters and cracks in the film, which indicates air out-gassing through bubble expansion and collapse. Quiescent drying I on the other hand took much longer time (td, 1 h) to dry, and gave stronger material deposition at the film center, depending on the initial fluid thickness zi: for d = 12 im i.e., zi« 70 im, the film had a plateau appearance, but for d = 10 im, zi « 100 im, the film had a volcano profile.

These results suggest that vacuum drying of printed films is feasible and can advantageously improve film uniformity. This is very useful since fast drying was often thought to promote the coffee-stain effect.

3.8. Film nanomorphology

The nanomorphology of semicrystalline polymer OSC films, such as its molecular order, orientation and packing of the polymer chains, is strongly influenced by processing conditions [55-58]. For example, we have measured pronounced nanomorphology and field-effect mobility

differences between regioregular P3HT films fabricated by spin-casting, drop-casting and inkjet-printing [29]. Here we use optical absorption, X-ray diffraction and atomic force microscopy to show that P3HT:PCBM films printed from BB:TOL exhibit stronger P3HT p-stack ordering than films printed from CB, DCB and their mixtures, even though the phase length scale remains ultrafine in the 20-nm regime. Therefore the transition solvent strategy mitigates the severe phase coarsening that is associated with the use of single-component poor solvents [11,12].

Fig. 5a shows the optical absorption spectra of P3HT:PCBM films printed from BB:TOL, compared to DCB and CB. Spin-cast films tend to show a weaker 0 ? 0 (605 nm) vibronic band relative to 0 ? 1 (560 nm) and 0 ? 2 (520 nm) bands before thermal anneal, [50] which is related to molecular disorder in the P3HT chains [33,59]. Here all the ijp films show a much higher relative intensity ratio of the 0 ? 0 band, and hence better molecular order of the P3HT chains even before thermal annealing. In particular, the film printed from BB:TOL shows the best molecular order which is broadly independent of whether it is dried in ambient or vacuum. This suggests that pre-ordering of p-stacked aggregates has occurred in the solvent [30].

Fig. 5b shows the 0-20 X-ray diffractograms of some of these films after annealing at 140 "C. The results again suggests that BB:TOL gives better P3HT molecular order than CB:DCB. A Bragg reflection at 5.25" due to the (100) reflection of P3HT (d-spacing = 17 A) [60], with a Scherrer coher-

ence length, icoh = ,

of ca. 15 nm, was found in the

' 6fwhm cos h

BB:TOL-printed film but not the CB:DCB-printed film. However the higher-order (h00) reflections with h p 2, and PCBM reflections remain missing. Therefore pure P3HT and PCBM phases have not segregated on a coarse scale.

Fig. 6 shows the atomic force microscopy images of selected films. The vacuum-dried film printed from BB:TOL is smooth, and haracterized by hillocks tens of nm wide and 2-3-nm high. This surface topography is much smoother than what has been reported using tetralene as solvent at

Wavelength (nm) Scattering angle (26)

Fig. 5. Effect of solvent and drying conditions on molecular order in P3HT:PCBM films. (a) UV-Vis spectra of unannealed films printed onto glass substrates from different solvents and dried under the specified conditions. (b) 0-26 XRD diffractograms of 60-80-nm-thick films printed onto glass substrates and dried under the specified conditions, after heat treatment at 140 "C (10 min). Diffractograms have been offset for clarity. Parameters are as given in Fig. 3. "Quiescent" refers to drying in the cleanroom ambient. ''Vacuum'' refers to drying protocol II (see text).

Rrms=0-8nm

- janm . j -

*L_=2:SrTra

.200nm

Fig. 6. Comparative tapping-mode AFM topography images of P3HT:PCBM films. (a) 9:1 CB:DCB (spin-cast), (b) 9:1 CB:DCB, inkjet-printed and quiescent-dried, (c) 8:2 BB:TOL, inkjet-printed and quiescent-dried, (d) 8:2 BB:TOL, inkjet-printed and vacuum-dried.

even lower P3HT regioregularity [11,12]. In contrast, the film dried under quiescent conditions exhibits a highly textured surface topography characterized by stacked elongated parallel phases ca. 10-nm wide and several tens of nanometers long. This is strongly reminiscent of an eutectic morphology, which suggests extensive phase segregation of the P3HT and PCBM phases although at the ultrafine length scale. Finally, the spin-cast film and quiescent-dried printed film from CB:DCB are the smoothest.

These results show that the present good-poor solvent strategy has promoted a more extensive donor-acceptor segregation in the P3HT:PCBM photoactive layer than good solvents, which is not unexpected. The remarkable observation here is the phase-separation length scale remains confined to the sub-20 nm regime despite dominance of the poor solvent at the end stage. This suggests the initial presence of the good solvent component helps suppress the deleterious polymer aggregation that occurs otherwise, such as when tetralene as single-component solvent [11,12]. Further refinement of the phase-separation length scale appears feasible by variation of the solvent composition.

3.9. Solar cell characteristics

P3HT:PCBM solar cells were printed in the dark with a P3HT-to-PCBM weight ratio of 1.5:1 and a film thickness

of 80-100 nm using 8:2 BB:TOL as solvent, over PEDT:PSSH hole-collection layers spincast on ITO-glass substrates. This photoactive layer thickness was chosen to nearly

0 0.1 0.2 0.3 0.4 0.5 0.6 Voltage (V)

Fig. 7. Current-voltage characteristics of inkjet-printed P3HT:PCBM solar cells tested at AM1.5 irradiance (1-sun). Device structure: glass/ITO/50-nm PEDT:PSSH/P3HT:PCBM/Al. Photoactive layer composition: P3HT:PCBM, 1.5:1 w/w. Printing parameters: 8:2 BB:TOL, 20 "C, 14-jet 10-pL print head with variable pitch. The device notation (a-d) is given in Table 2.

Table 2

Device performance of inkjet-printed P3HT:PCBM solar cells.

Device type Thickness3 (nm) Drying condition Jscb (mAcm-2) Vocc (V) FFd (%) PCEe(%)

(d) 101 Vacuum 8.3 0.60 45 2.2

(c) 77 Vacuum 8.3 0.60 43 2.1

(b) 95 Quiescent 7.4 0.45 43 1.4

(a) 77 Quiescent 7.3 0.45 44 1.4

Device structure: glass/ITO/50-nm PEDT:PSSH/P3HT:PCBM/Al. Photoactive layer composition: 1.5:1 w/w P3HT:PCBM. Printing parameters: 8:2 BB:TOL, 20 "C, 14-jet 10-pL print head with variable pitch. ''Vacuum'' refers to drying protocol II. ''Ambient'' refers to quiescent drying. a Photoactive layer thickness. b Short-circuit photocurrent. c Open-circuit voltage. d Fill factor.

e Power conversion efficiency, all measured at simulated AM1.5 irradiance and spectral-mismatch corrected. Typically, four devices of each type were tested.

match the first optical absorption thickness optimum [61]. The films were then quiescent-dried in the ambient, or vacuum-dried following protocol II. We did not investigate CB and CB:DCB solvent systems because these degrade the materials and seals present in the fluid delivery system and print head of theprinting tool.

Fig. 7 shows the typical current-voltage characteristics measured for the cells under simulated AM 1.5 irradiance at 1.0-sun. Table 2 summarizes the performance parameters. The open-circuit voltage Voc of the vacuum-dried cells (0.60 V) is significantly higher than the ambient-dried ones (0.45 V) and those printed using tetralene (0.45 V), [11,12] but similar to the usual CB spin-cast cells [33,61]. The short-circuit current density Jsc of the vacuum-dried cells (8.3 mA cm-2) is also higher than the ambient-dried ones (7.4 mA cm-2), but similar to the CB spin-cast ones with similar photoactive layer composition and thickness [33,61]. Clearly vacuum drying has improved solar cell characteristics over quiescent ambient drying. This can be attributed to the tendency for phase discontinuity in the quiescent-dried film. It was already previously noted that films dried slowly after a short spin-cast from DCB gave poor performance [32].

The best ijp P3HT:PCBM cells here show a power conversion efficiency of 2.2%, which is about three-quarters of that found in the usual spin-cast cells from CB or DCB at the same photoactive layer composition and thickness, [33,61] and twice of that reported for cells printed from tetralene [11,12]. This indicates a significant improvement has been reached for non-halogenated solvents, although there is still scope for further refinement. The main loss is in the fill factor, which may point to a still sub-optimal donor-acceptor morphology.

4. Conclusions

In summary, we have demonstrated a new inkjet-printing solvent strategy that is capable of producing thin films of an important semicrystalline polymer organic semiconductor composite P3HT:PCBM with a good macromorphology with relatively uniform thickness free from coffee-stain and other fluid-flow artifacts, and a good nanomorphology with ordered but intermixed donor-acceptor phases at the ultrafine length scale. The key is the combination of a good solvent that is volatile

with a miscible poor solvent that is much less volatile. The presence of the good solvent suppresses premature aggregation of the polymer to ensure sufficient ink latency time. The preferential evaporation of this component on the substrate then leaves the poor solvent component which triggers the desired p-stacking and/or gelation of the polymer matrix to suppress fluid flow and the undesirable over-coarsening of the donor-acceptor morphology. This outcome is significantly improved by using a vacuum drying step with the optimal depressurization-time profile. Our approach now opens a way to inkjet print polymer organic semiconductor films without using halogenated solvents. Since a number of other printing techniques, such as gravure and flexography, and also coating techniques, such as slot die, also requires the use of non-halogenated, low-viscosity and relatively slow-drying inks, the solvent strategy developed here should also be applicable to them.

Acknowledgements

We acknowledge the preliminary studies conducted by Mr. Wei-Han Teo which seeded this work. We thank the Ministry of Education, Singapore, for financial support (MOE2010-T2-2-112 Grant: R-144-000-293-112 and R-143-000-471-112). We also thank Dr. Bo Liu and Dr. Rui-Qi Png for useful discussions. The Solar Energy Research Institute of Singapore (SERIS) is sponsored by the National University of Singapore (NUS) and the National Research Foundation (NRF) of Singapore through the Singapore Economic Development Board (EDB).

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