Supporting Information

Transcription

1 Copyright WILEY VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2011 Supporting Information for Adv. Funct. Mater., DOI: /adfm Bulk Heterojunction Organic Photovoltaics Based on Carboxylated Polythiophenes and PCBM on Glass and Plastic Substrates Brian J. Worfolk, David A. Rider, Anastasia L. Elias, Michael Thomas, Kenneth D. Harris,* and Jillian M. Buriak*

2 SUPPORTING INFORMATION DOI: /adfm Bulk Heterojunction Organic Photovoltaics Based on Carboxylated Polythiophenes and PCBM on Glass and Plastic Substrates By Brian J. Worfolk, David A. Rider, Anastasia L. Elias, Michael Thomas, Kenneth D. Harris *, and Jillian M. Buriak * [*] Prof. J.M. Buriak, B.J. Worfolk, D.A. Rider Department of Chemistry University of Alberta Edmonton, Alberta T6G 2G2 (Canada) jburiak@ualberta.ca [*] B.J. Worfolk, D.A. Rider, M. Thomas, K.D. Harris, Prof. J.M. Buriak NRC-National Institute for Nanotechnology Edmonton, Alberta T6G 2M9 (Canada) ken.harris@nrc-cnrc.gc.ca Prof. D.A. Rider (current address) Departments of Chemistry and Engineering Technology Western Washington University Bellingham, WA (United States) Prof. A.L. Elias Department of Chemical and Materials Engineering University of Alberta Edmonton, Alberta T6G 2V4 (Canada) M. Thomas Department of Electrical and Computer Engineering University of Alberta Edmonton, Alberta T6G 2V4 (Canada) 1

3 The optical images of the pyridine solutions of the polymer series containing - (CH 2 ) x COOH carboxyalkyl sidechains for x ranging from 3 to 6 is seen in Figure 1A. The series consists of regioregular poly[3-(carboxyalkyl)thiophene-2,5-diyl] (P3CATs), including poly[3-(3-carboxypropyl)thiophene-2,5-diyl] (P3CProT), poly[3-(4-carboxybutyl)thiophene- 2,5-diyl] (), poly[3-(5-carboxypentyl)thiophene-2,5-diyl] (P3CPenT) and poly[3-(6- carboxyhexyl)thiophene-2,5-diyl] (). Figure 1B depicts the solutions under UV irradiation at 365 nm and Figure 1C shows an optical image of cast films on glass substrates. A) B) C) Figure 1. A) Picture of P3CProT,, P3CPenT and (from left to right for all images) pyridine solutions. B) Picture of P3CAT solution fluorescence illuminated with a UV lamp. C) Films of P3CAT spin-cast from pyridine solutions. Thermal gravimetric analysis (TGA) was performed with a TA Instruments Q500 analyzer under nitrogen atmosphere at a heating rate of 10 C/min. Figure 2A depicts the TGA of the four P3CATs. Weight loss for the P3CATs occurs in two stages with an initial onset occurring at ~260 C. The relatively high decomposition temperature confirms sufficient thermal stability for integration into roll-to-roll fabrication. Figure 2B shows the relationship between char yield and the alkyl chain length of the polythiophene series. 2

4 A) 100 B) Weight Percent/ % P3CProT P3CPenT Temperature/ o C Char Yield at 530 o C/ % Alkyl Chain Length (x) in P3CAT Figure 2. Thermal gravimetric analysis of the P3CAT series performed in nitrogen at a heating rate of 10 C/min and relationship between char yield and alkyl chain length. Figure 3 shows the cyclic voltammetry scan of the polymer series. The onset of the oxidation peak was used for calculation of the highest occupied molecule orbital energy level. Normalized Current (a.u.) P3CProT P3CPenT Voltage (V) Figure 3. Cyclic voltammetry scans of P3CProT,, P3CPenT and films drop-cast onto a Pt working electrode. Figure 4 shows the solid state UV-vis spectra for the series of polymers mixed 1:1 (by weight) with PCBM forming a bulk heterojunction. Compared to the solution UV-vis spectra, P3CPenT is the only polymer not significantly blue-shifted with respect to the rest of the polymers. Table 1 depicts this trend by comparing the solid state absorbance λ max of the polymers only and when mixed with PCBM. The P3CProT, and polymers are significantly blue-shifted when combined with PCBM. 3

5 Absorbance (a.u.) P3CProT:PCBM :PCBM P3CPenT:PCBM :PCBM Wavelength (nm) Figure 4. Absorbance spectra of P3CAT:PCBM films spin-cast from the optimized pyridine:chlorobenzene solvent system on quartz substrates. Table 1. Table summarizing the absorbance λ max of the P3CAT-only and combined P3CAT:PCBM films. P3CAT-Only λ max BHJ P3CProT P3CPenT 468 nm 496 nm 505 nm 512 nm λ max 451 nm 466 nm 504 nm 487 nm Initial infrared (IR) spectroscopy studies of the P3CATs in each of the stages required for casting films, was conducted. Results were qualitatively quite similar for the series and the representative P3CPenT is described. Figure 5 illustrates the carbonyl (C=O) region for P3CPenT dispersed in (i) a KBr pellet, (ii) as a solution fixed between two KBr discs and (iii) a film cast from 1:6 pyridine:chlorobenzene on a KBr disc. The carbonyl region for all spectra consists of two peaks corresponding to free C=O stretches at lower energy (~1735 cm -1 ) and hydrogen bonded C=O vibrations at higher energy (~1700 cm -1 ). The powder P3CPenT and spin-cast film have similar degrees of hydrogen bonding in the solid-state. However, the solution P3CPenT contains a significantly lower proportion of hydrogen bonds as evident from the small shoulder at 1714 cm -1. The magnitude of the shift from lower to higher energies is correlated to the strength of the bonding interaction, and P3CPenT in the solution 4

6 state has the lowest shift. It is interesting to note that when the film dries, the hydrogen bonding is restored and the strength of the interaction is increased. Figure 6 shows the IR spectra of the polythiophene series in KBr pellets. Figure 7 shows the IR spectra of the polymer-only films spin-cast from their optimized solvent compositions onto a KBr disc. A blow-up of the carbonyl region can be seen in the paper in Figure 4. Table 2 shows the peak area of the free C=O and hydrogen bonded C=O stretches for the P3CAT-only films. P3CPenT-only films contain a larger extent of hydrogen bonding compared to the other polymers. Figure 8 shows the combined polythiophene:pcbm spin cast films on a KBr disc. Figure 5. The carbonyl (C=O) region of the IR spectra of P3CPenT (x=5) as a powder in a KBr pellet, cast as a film and as a solution with a pyridine:chlorobenzene composition of 1:6. 5

7 % Transmission P3CProT (x=3) (x=4) P3CPenT (x=5) (x=6) 65 Figure 6. The powder IR spectra of P3CProT,, P3CPenT and as KBr pellets with significant regions designated: (A) C-H stretches, broad O-H stretch; (B&C) O-H bands; (D) C=O stretches; (E) C-O-H in-plane bend; (F) thiophene ring stretch; (G) C-O stretches; (H) C-C stretches; (I) aromatic C-H stretches Transmission/ % P3CProT P3CPenT Wavenumber/ cm Figure 7. IR spectra of P3CAT films spin cast on a KBr disc from the optimized solvent compositions. Table 2. The peak areas of the free and hydrogen bonded C=O peak and the % hydrogen bonded based on peak area. P3CAT Free C=O Peak Hydrogen Bonded % Hydrogen Area C=O Peak Area Bonded C=O P3CProT % % P3CPenT % % 2000 Wavenumbers (cm -1 )

8 % Transmission P3CProT P3CPenT Wavenumber cm -1 Figure 8. IR spectra of P3CAT:PCBM films cast from their optimized solvent compositions on a KBr disc cm -1 C-H stretch; cm -1 C=O stretch; 1455 cm -1 C-O-H in plane bend; 1415 cm -1 thiophene ring stretch; cm -1 C-O stretch From the x-ray diffraction (XRD) of the combined P3CAT:PCBM films, the crystallite size can be calculated for the polythiophene (100) peak using the Scherrer equation. The crystallite sizes for the polymers are presented in Table 3. P3CPenT forms the largest crystallite sizes of the series in combined films with PCBM. Table 3. The summary of the P3CAT crystallite size as calculated from the Scherrer equation of the combined polymer:pcbm films. P3CATs Crystallite Size (nm) P3CProT P3CPenT Figure 9 shows the cross-sectional SEM images of combined P3CPenT:PCBM (1:1 by weight) cast from total 34, 26, 17 and 9 mg/ml solutions with a 1:3 pyridine:chlorobenzene solvent composition. At this solvent composition large bumps or clusters can clearly be seen. Similar effects have been attributed to clustering of PCBM

9 Thickness = 460 nm Thickness = 300 nm 34 mg/ml 26 mg/ml Thickness = 170 nm 17 mg/ml 9 mg/ml Thickness = 110 nm Figure 9. Cross-sectional SEM images of P3CPenT:PCBM (1:1 by weight) BHJ films cast from 34 mg/ml, 26 mg/ml, 17 mg/ml and 9 mg/ml solutions with a solvent ratio of 1:3 pyridine:chlorobenzene or 25% pyridine. Figure 10 shows the modulus and hardness / contact depth profiles for the nanoindentation of the polymer-only and combined polymer:pcbm films. In general, the P3CAT series has a higher indentation modulus and greater hardness compared to P3HT for polymer-only and combined films. 6 P3CProT 0 A) B) 5 P3CPenT P3HT Indentation Modulus/ GPa Indentation Modulus/ GPa Indentation Depth/ nm 10 P3CPenT:PCBM 9 :PCBM 8 P3HT:PCBM Indentation Depth/ nm Hardness/ GPa C) D) Hardness/ GPa P3CProT P3CPenT P3HT Indentation Depth/ nm P3CPenT:PCBM :PCBM P3HT:PCBM Indentation Depth/ nm Figure 10. (A) Indentation modulus/contact depth and (B) hardness/contact depth profiles of P3CAT-only films. (C) Indentation modulus/contact depth and (D) hardness/contact depth profiles of the P3CAT:PCBM BHJ films. 8

10 In order to determine the optimal mixed solvent composition of pyridine and chlorobenzene, a series of photovoltaic devices with different solvent compositions were fabricated with the following architecture: ITO/PEDOT:PSS/P3CAT:PCBM/Al. The optimal solvent composition was chosen based on the maximum power conversion efficiency obtained. Optimized total pyridine:chlorobenzene solvent mixtures for the P3CAT:PCBM were 1:7, 1:8, 1:6 and 1:7 for P3CProT,, P3CPenT and respectively. Plots displaying the different PV parameters and the solvent composition are summarized in Figure The effect of annealing the device before top contact evaporation was studied and is summarized in Figure 14. The power conversion efficiency decreases with annealing temperature. PCE (%) J sc (ma/cm 2 ) Jsc PCE FF Figure 11. Photovoltaic properties of P3CProT:PCBM OPV devices in the optimization of the pyridine:chlorobenzene solvent composition. V oc (V) FF Voc 9

11 PCE (%) J sc (ma/cm 2 ) PCE Voc Jsc 0.7 FF Figure 12. Photovoltaic properties of :PCBM OPV devices in the optimization of the pyridine:chlorobenzene solvent composition. V oc (V) FF PCE 0.7 Voc 1.2 PCE (%) V oc (V) 4.0 Jsc 3.5 FF J sc (ma/cm 2 ) FF Figure 13. Photovoltaic properties of :PCBM OPV devices in the optimization of the pyridine:chlorobenzene solvent composition. 10

12 η (%) η Jsc Anneal Temp ( o C) Figure 14. The effect of annealing devices for 15 minutes after the deposition of the photoactive BHJ for P3CPenT J sc (ma/cm 2 ) 11