Inkjet printing of conjugated polymer fullerene solar cell films

INKJET PRINTING OF CONJUGATED POLYMER:FULLERENE SOLAR CELL FILMS LIM GUAN HUI (LIN YUANHUI) (B.Eng.(Hons.), NUS; (M.Eng.), NTU A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY NUS GRADUATE SCHOOL FOR INTEGRATIVE SCIENCES AND ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 ii To Mum and Dad To my Lerv iii iv Declaration The work in this thesis is the original work of LIM GUAN HUI, performed independently under the supervision of Prof Chua Soo Jin and Assoc Prof Peter Ho (in ONDL), Physics Department, National University of Singapore I have duly acknowledged all the sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously LIM GUAN HUI (LIN YUANHUI) 24 AUGUST 2012 v vi Acknowledgements This work would not be possible without the unwavering support of my thesis advisors Prof Chua Soo Jin and Prof Peter Ho I also wish to thank Prof Sam Li for kindly agreeing to be the chair of my thesis advisory committee and providing me with fresh ideas for my work I am very thankful for the NUS Graduate School of Integrative Science and Engineering scholarship support for this PhD I wish to thank Prof Peter Ho and Prof Chua lay-lay for giving me the opportunity to carry out my PhD research at NUS ONDL with their guidance and support I am also very grateful for the generous help Dr Zhuo Jingmei, my mentor, had rendered me in completing my thesis I also wish to thank members of ONDL (Loke Yuen, Lihong, Ruiqi, Zhili, Liu Bo, Guo Han, Songjie, Dagmawi, Hu Chen, Kendra and Kim Kian), who have accompanied me during this PhD journey, for their friendship, assistance and insightful advice Big thanks to the love, care and tolerance, my family had shown me during the course of my challenging PhD journey vii viii Table of contents Declaration v  Acknowledgements vii  Table of contents ix  Abstract xiii  List of Figures xvii  List of Acronyms xxv  Chapter Introduction 1  1.1 Organic semiconductor 1  1.2 Low cost, large area, flexible, printed organic optoelectronics 5  1.3 Organic solar cell 9  1.4 Thesis motivation and outline 16  1.5 References .17  Chapter Inkjet printing for printed organic optoelectronics 21  2.1 Introduction .22  2.2 Inkjet technology .24  2.3 Piezo-based inkjet printer - FUJI-DIMAITX DMP-2831 27  2.3.1 Printer system 27  2.3.2 Jetting mechanism and control 30  ix 2.3.3 Solvent system for ink formulation 35  2.3.4 Ink and substrate interaction for inkjet film printing 39  2.3.5 Printer enhancements for inkjet printed organic optoelectronics .43  2.4 Summary 47  2.5 References .48  Chapter Jettability space of piezo-based inkjet printing 53  3.1 Introduction .54  3.2 Design of model fluids 57  3.3 Satellite-free inkjet printing .61  3.4 Control of droplet speed 68  3.5 Summary 73  3.6 References .74  Chapter Halogenated solvent-free inkjet printing of P3HT:PCBM films 79  4.1 Introduction .80  4.2 Ink formulation 83  4.2.1 Practical solvent vapour pressure range 83  4.2.2 Suppressing the coffee-stain effect .86  4.3 Platen temperature 94  4.4 Summary 98  4.5 References .99  x Therefore the use of a good–poor solvent mixture comprising a less volatile poor solvent and a more volatile good solvent, and the optimal dynamic drying protocol can produce P3HT: PCBM blend films with the ideal flat top profile Since the underlying mechanisms are size independent, we expect that good quality films from mm to bigger than cm sizes can be produced this way The use of non-halogenated aromatic hydrocarbons here is also advantageous for environmental and processing reasons However there is a question of how this new deposition protocol affects the -stacking of the P3HT chains in the blend film To investigate this, we measured the UV-Vis spectra of the pristine films printed from a 2.5 mg mL–1 solution in CB, DCB and BB: TOL (8: v/v) (Figure 5-3) The results show that the inkjet-printed films printed from all the three solvent systems are highly ordered as deposited even before annealing, as shown by the clearly resolved vibronic peaks at 605 nm, 560 nm and 520 nm, in contrast to the usual spin-cast blend films whereby the relative intensity of 0-0 (605nm) transition peak are lower [1, 15] due to more disorderd P3HT domains Therefore the inkjet printed films are considerably more ordered than spin-cast films, as has also recently been established for inkjet printed P3HT films [1] Furthermore, the BB: TOL system results in even better order of the P3HT domains than the good solvent systems, CB and DCB [16] 111 (a) (b) d=10m 470 nm d=12m 350 nm (c) d=12m d=12m d=10m 260 nm (e) (d) 275 nm 350nm Figure 5-2: Optical image of films printed from BB:TOL (8:2) solutions at 20°C The height profiles measured along the dotted lines are shown below each image (a) and (b) are films printed with d=10 and 12 µm and left to dry under quiescent conditions on the platen (c) and (d) are films printed with d=10 and 12 µm and were subjected to dynamic drying (e) shows an example of the film dried under a non-optimal pressure-time profile leading to undesirable film morphology 112 Normalised Absorbance P3HT:PCBM (1.5:1) 2.5 mg/mL 0.8 DCB 0.6 0.4 BB:TOL dynamic drying CB 0.2 400 450 500 550 600 Wavelength (nm) 650 700 Figure 5-3: UV-Vis spectra of pristine films printed from 2.5 mg/mL solutions in CB, DCB and BB: TOL (8:2 v/v) 113 5.4 Inkjet printed P3HT:PCBM OPV performance Since the order and phase dimensions of the P3HT domains are critical parameters for solar cell performance, and these have been affected by the optimal inkjet printing protocol developed here, it is important to check the solar cell performance of devices fabricated by this protocol All device P3HT: PCBM films were inkjet printed in the dark to avoid photo-oxidation [12] These films were printed with a variety of d values between 10 and 14 μm, to obtain different final film thicknesses, on PEDT: PSSH coated ITO–glass substrates and dried under quiescent or dynamic conditions The rest of the fabrication details are found in section 5.2 An image of the typical inkjet printed OPV sample shown in Figure 5-4, with eight devices on it Figure 5-4: Typical inkjet printed OPV sample with eight OPV devices on it 114 The current–voltage characteristics of these devices were measured under an AM 1.5 simulated solar light source The typical current–voltage curves of the devices are shown in Figure 5-5 (b) (a) -3 -4 -5 -6 -7 -8 0.1 (c) Current density (mAcm ) -2 -2 Quiescent drying -2 0.2 0.3 Voltage (V) 0.4 77 nm -2 Dynamic drying -4 -6 -8 -100 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Voltage (V) -1 95 nm Quiescent drying -2 -3 -4 -5 -6 -7 -8 (d) Current density (mAcm ) -1 Current density (mAcm ) 77 nm -2 -2 Current density (mAcm ) 0.1 0.2 0.3 Voltage (V) 0.4 101 nm -2 Dynamic drying -4 -6 -8 -10 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Voltage (V) Figure 5-5: Current-voltage characteristics of inkjet-printed P3HT: PCBM (1.5:1 wt %) devices from BB: TOL (8:2 v/v) solutions at 20°C The devices were measured against a calibrated reference under 1-sun illumination The films in (a) and (b) were dried under ambient conditions while the films in (c) and (d) were subjected to dynamic drying 115 The solar cell performance parameters are summarized in Table 5-1 and it is shown that the open-circuit voltage Voc of the dynamically-dried devices are 0.60 V, higher than those of the quiescent-dried devices which are 0.45 V The short-circuit current density jsc of the dynamicallydried devices (8.3 mA cm–2) are also higher than those of the quiescent-dried devices (7.4 mA cm– 2) As for the fill factor, the inkjet printed devices for both dynamic and quiescent drying are pretty similar ~44% As a result, the dynamic-dried inkjet printed OPV devices had power conversion efficiencies (PCE) that are higher than that of the of the quiescent-dried OPV devices at comparable film thicknesses Table 5-1: Device performance for P3HT: PCBM (1.5: wt %) films of printed from BB: TOL (8: v/v) solutions at 20°C The thicknesses of the films were controlled by varying the drop spacing The films were subjected to dry under quiescent conditions and dynamic drying respectively Drying condition Thickness (nm) Jsc (mA) Voc (V) FF (%) PCE (%) Dynamic 101 8.33 0.60 45.0 2.2 Dynamic 77 8.30 0.60 43.3 2.1 Quiescent 95 7.42 0.45 43.0 1.4 Quiescent 77 7.25 0.45 44.0 1.4 This is presumably due to a longer drying time where phase coarsening of the polymer: fullerene blend may occur in a quiescent drying process as compared to a dynamic drying process Figure 5-6 shows the atomic force microscopy images of the films prepared by ijp from BB:TOL (8:2) dried under ambient conditions or under vacuum, compared to a CB:DCB (9:1) film dried 116 under ambient conditions and one prepared from the CB:DCB solvent by spin-casting The film from BB:TOL dried under vacuum conditions is smooth, with hillocks that are tens of nm wide and 2–3 nm high, and finer nodules In contrast, the film dried under quiescent conditions exhibits a highly textured surface topography characterized by ultrafine elongated and parallel phases that are 10 nm wide and several tens of nm long (a) 10nm (b) 5nm 1nm Rrms=0.8nm (c) 200nm 25nm 5nm Rrms=2.8nm 20nm Rrms=3.8nm (d) 200nm 10nm 5nm 200nm Rrms=2.1nm 200nm Figure 5-6: Tapping mode AFM topography micrographs of 1.5:1 w/w P3HT:PCBM films formed from 2.5 mg mL−1 solutions of (a) CB:DCB {9:1}, spin-cast, 20nm-thick, (b) CB:DCB (9:1), inkjet-printed and subjected to quiescent drying, 60nm-thick, (c) BB:TOL {8:2}, inkjetprinted and subjected to quiescent drying, 77nm-thick, (d) BB:TOL {8:2} inkjet-printed and subjected to optimal vacuum drying (refer to Figure 5-1), 77nm-thick Solid lines in yellow represent line profiles of images Dotted lines in yellow indicate where the line profiles were extracted 117 This suggests a more complete phase separation occurs between P3HT and PCBM when dried very slowly (ca h), even though the separation length scale remains on the ultrafine length scale In contrast, the film from CB:DCB dried under quiescent conditions is smooth with hillocks several tens of nm wide and few nm tall Finally the spin-cast film from CB:DCB has the smoothest topography, as expected of late phase separation in the fast film formation process Overall these results show remarkably that the ijp solvent quenching method has promoted a more extensive phase separation but still on the ultrafine (sub-20-nm) length scale that should be conducive to OPV operation, provided that phase contiguity is preserved throughout the film thickness Furthermore, x-ray diffraction data reveal that the P3HT:PCBM films prepared from the good–borderline solvent system, but not the good solvent systems, shows an incipient -stacking to give ordered (planarized) P3HT chains Figure 5-7 shows the –2 X-ray diffractograms of films prepared from the BB:TOL system, compared to CB:DCB Both systems dry down finally from solvents with similarly low volatilities However the films from BB:TOL again shows a Bragg reflection at 5.25º (d-spacing = 17 Å) that is due to the (100) reflection of P3HT The coherence length in the [100] direction computed by the Scherrer formula  coh  0.9  fwhm cos  is ca 15 nm However no higher order reflections were observed Also no reflections from PCBM crystals were visible Therefore the solvent quenching here has not led to the formation of macroscopic pure phases The incipient -stacking molecular order suggested by XRD produced by the solvent quenching method may be useful for OPV applications 118 1200 1000 9:1 CB:DCB 8:2 BB:TOL quiescent vacuum Counts 800 600 400 200 10 15 20 25 30 35 Scattering angle (2) Figure 5-7: θ-2θ XRD diffractograms of films on glass substrates printed from 2.5 mg mL–1 solution in 8:2 BB:TOL compared to 9:1 CB:DCB, after anneal at 140ºC (10 min) Quiescent refers to drying in the cleanroom ambient Vacuum refers to optimal vacuum drying protocol defined in Figure 5-1 These results are remarkable as it demonstrates the possibility as well as unconventional wisdom of depositing organic functional films in a borderline solvent that can broadens the choice solvent selection thinkable previously In this context, it allows the use of non-chlorinated aromatic hydrocarbon, which is much more environmentally friendly than the typical chlorinated aromatic hydrocarbon that has practically limitation in terms of industrial usage approval as the world pushes for ‘green’ manufacturing, without making significant, if any, sacrifice in organic optoelectronics performance 119 5.5 Summary It was demonstrated that film morphology issues can be substantially overcome through the simple use of a low-volatility borderline–poor co-solvent together with an appropriate dynamic drying profile, for a model regioregular poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) donor–acceptor system suitable for photovoltaic applications These two approaches together successfully produce the ideal “bread-top” profile for these films Printed organic solar cells fabricated this way show power conversion efficiencies that closely approach those of corresponding optimized spin-cast devices, although there is a small performance gap that is related to a residual phase coarsening in the printed films This work thus reveals two new processing guidelines that can successfully overcome the most severe challenges of printing quality patterned functional polymer OSC thin films without using chlorinated solvents Since these challenges are common to other forms of printing which require the use of slow-drying solvents, the approaches here are also likely to be useful for those techniques 120 5.6 References Wong, L.-Y., Png, R.-Q., Silva, F.B.S., Chua, L.-L., Repaka, D.V.M., Shi-Chen, Gao, X.-Y., Ke, L., Chua, S.-J., Wee, A.T.S., and Ho, P.K.H.: ‘Interplay of Processing, Morphological Order, and Charge-Carrier Mobility in Polythiophene Thin Films Deposited by Different Methods: Comparison of Spin-Cast, Drop-Cast, and Inkjet-Printed Films’, Langmuir, 2010, 26, (19), pp 15494-15507 Wong, L.Y., Png, R.Q., Silva, F.B.S., Chua, L.L., Chen, S., Gao, X.Y., Ke, L., Chua, S.J., Wee, A.T.S., and Ho, P.K.H.: ‘Interplay of processing, morphological order and charge-carrier mobility in polythiophene thin films deposited by different methods: unusually high crystallinity in inkjet-printed films’, Langmuir, 2010, 26, pp 15494–15507 Deegan, R.D., Bakajin, O., Dupont, T.F., Huber, G., Nagel, S.R., and Witten, T.A.: ‘Capillary flow as the cause of ring stains from dried liquid drops’, Nature, 1997, 389, (6653), pp 827-829 Hu, H., and Larson, R.G.: ‘Maragoni effect reverses coffee-ring depositions’, J Phys Chem B, 2006, 110, pp 7090-7094 Lim, J.A., Lee, W.H., Lee, H.S., Lee, J.H., Park, J.D., and Cho, K.: ‘Self-organisation of inkjet-printed triisopropylsilylethynyl pentacene via evaporation-induced flows in a drying droplet’, Adv Func Mater., 2008, 2008, pp 229-234 Deegan, R.D., Bakajin, O., Dupont, T.F., Huber, G., Nagel, S.R., and Witten, T.A.: ‘Contact line deposits in an evaporating drop’, Phys Rev E, 2000, 62, pp 756-764 Blom, P.W.M., Mihailetchi, V.D., Koster, L.J.A., and Markov, D.E.: ‘Device Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells’, Advanced Materials, 2007, 19, (12), pp 15511566 Marsh, R.A., Hodgkiss, J.M., Albert-Seifried, S., and Friend, R.H.: ‘Effect of Annealing on P3HT:PCBM Charge Transfer and Nanoscale Morphology Probed by Ultrafast Spectroscopy’, Nano Letters, 2010, 10, (3), pp 923-930 Yang, X., Loos, J., Veenstra, S.C., Verhees, W.J.H., Wienk, M.M., Kroon, J.M., Michels, M.A.J., and Janssen, R.A.J.: ‘Nanoscale Morphology of High-Performance Polymer Solar Cells’, Nano Letters, 2005, 5, (4), pp 579-583 10 Liu, B.: ‘Very high internal quantum efficiency over wide composition phase space in polymer:fullerene solar cells based on crosslinked polymer donor networks’, Nat Commun, 2012, 3, pp 1321 121 11 Chia, P.-J., Yeo, Y.-C., Burroughes, J.H., Friend, R.H., and Ho, P.K.H.: ‘Chemical reversability of the electrical dedoping of conducting polymers: An organic chemically erasable programmable read-only memory’, Appl Phys Lett, 2008, 93, (3), pp 033314-033313 12 Zhuo, J.-M., Zhao, L.-H., Png, R.-Q., Wong, L.-Y., Chia, P.-J., Tang, J.-C., Sivaramakrishnan, S., Zhou, M., Ou, E.C.W., Chua, S.-J., Sim, W.-S., Chua, L.-L., and Ho, P.K.H.: ‘Direct Spectroscopic Evidence for a Photodoping Mechanism in Polythiophene and Poly(bithiophene-alt-thienothiophene) Organic Semiconductor Thin Films Involving Oxygen and Sorbed Moisture’, Advanced Materials, 2009, 21, (46), pp 4747-4752 13 Mihailetchi, V.D., Wildeman, J., and Blom, P.W.M.: ‘Space-Charge Limited Photocurrent’, Physical Review Letters, 2005, 94, (12), pp 126602 14 Ma, W., Yang, C., Gong, X., Lee, K., and Heeger, A.: ‘Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology’, Advanced Functional Materials, 2005, 15, (10), pp 1617-1622 15 Kim, Y., Cook, S., Tuladhar, S.M., Choulis, S.A., Nelson, J., Durrant, J.R., Bradley, D.D.C., Giles, M., McCulloch, I., Ha, C.-S., and Ree, M.: ‘A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells’, Nat Mater, 2006, 5, (3), pp 197-203 16 Clark, J., Silva, C., Friend, R.H., and Spano, F.C.: ‘Role of Intermolecular Coupling in the Photophysics of Disordered Organic Semiconductors: Aggregate Emission in Regioregular Polythiophene’, Physical Review Letters, 2007, 98, (20), pp 206406 122 Chapter Summary and outlook From this work, we propose that the jettable fluid space particularly for a commercial shortchannel microfabricated transducer can be usefully represented on a VoOh diagram, where Vo is the voltage pulse amplitude of the actuation transducer These results provide new insights into an operation regime of the droplet-on-demand printers, and useful scaling rules for optimizing jet formation for materials and device research (i.e organic semiconductor and organic (opto)electronics in this context) It was also found that through the use of a less volatile aromatic hydrocarbon that is a borderline-poor co-solvent as the major component together with a more volatile good solvent as the minor component, it is possible to effectively suppress the undesirable contact-line depinning and fluid migration to produce macroscopic printed thin films with the near ideal bread top profile Such a mixture provides a sufficiently high solvation power to avoid polymer choking of the print head nozzle and give a long solution jetting viability This presents a new solvent processing strategy to suppress the “coffee stain” and the other undesirable drying morphology effects Lastly, it was demonstrated that film mporophology issues can be substantially overcome through the simple use of a low-volatility borderline–poor co-solvent together with an appropriate dynamic drying profile, for a model regioregular poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) donor–acceptor system suitable for photovoltaic applications These 123 two approaches together successfully produce the ideal “bread-top” profile for these films Printed organic solar cells fabricated this way show power conversion efficiencies that closely approach those of corresponding optimized spin-cast devices, although there is a small performance gap that is related to a residual phase coarsening in the printed films This work thus reveals two new processing guidelines that can successfully overcome the most severe challenges of printing quality patterned functional polymer OSC thin films without using chlorinated solvents Since these challenges are common to other forms of printing which require the use of slow-drying solvents, the approaches here are also likely to be useful for those techniques Currently, the general strategies proposed here for printing near ideal film morphology for organic device are only demonstrated at a lab scale It will be interesting to adopt this set of guidelines at an even larger scale approaching commercial product scale to identify new challenges and findings Some possible challenges such as printing time, in-situ vacuum drying and different drying dynamics of larger film can be foreseeable for inkjet printing of organic functional film for low cost, large area and flexible organic opto-electronics 124 Appendix Publications related to work done in this thesis The jettable fluid space and jetting characteristics of a microfabricated print head L.Y Wong, G.H Lim, T Ye, F.B S Silva, J.M Zhuo, R.Q Png, S.J Chua and P.K.H Ho Journal of fluid mechanics, 2012, 713, pp 109-122 Halogenated solvent-free printing of functional thin films for polymer: fullerene solar cells G.H Lim, L.Y Wong, J.M Zhuo, S.J Chua and P.K.H Ho (In preparation) Publications (up till 2012) from work not described in this thesis Evidence for a transition adlayer interface structure that determines field-effect transport in semicrystalline polymer organic semiconductor films Jing-Mei Zhuo, Rui-Qi Png,Han Guo, Loke-Yuen Wong, Guan-Hui Lim, Li-Hong Zhao, XiaoJiang Yu, Richard H Friend, Lay-Lay Chua and Peter K.H Ho (Submitted) 125 ... Introduction .104  5.2 Inkjet printing of P3HT:PCBM solar cell films 106  5.3 Dynamic drying of inkjet printed P3HT:PCBM films .109  5.4 Inkjet printed P3HT:PCBM OPV performance... integrated into the printing technology Figure 1-6: Printing technology (a) Flexographic printing (b) Gravure printing (c) Offset printing (d) Inkjet printing (e) Screen printing (Source: OE-A... Typical I-V curve of organic solar cell under illumination where power conversion efficiency of the solar cell is acquired 14  Figure 1-13: Tandem organic solar cell (source: www.heliatek.com)