Supporting Information

Transcription

1 Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, Supporting Information for Adv. Energy Mater., DOI: /aenm Effect of Processing Additives on the Solidification of Blade- Coated Polymer/Fullerene Blend Films via In-Situ Structure Measurements Nayool Shin, Lee J. Richter,* Andrew A. Herzing, R. Joseph Kline, and Dean M. DeLongchamp*

2 Supporting Information Solubilities of P3HT and PCBM in various solvents Table S1. Room temperature solubilities of P3HT and PCBM in various solvents including primary solvent (CB) and processing additives (DIO, ODT, and CN) Solvent P3HT Solubility (mg/ml) PCBM Chlorobenzene (CB) ,8-diidooctane (DIO) Additives 1,8-octanedithiol (ODT) chloronaphthalene (CN) To measure solubilities of P3HT and PCBM, supersaturated P3HT and PCBM solutions were first prepared in the solvents listed in the Table S1. The solutions were kept at 80 ºC overnight to dissolve as much P3HT and PCBM as possible and then cooled down to room temperature. The solutions were filtered using 1 µm pore size syringe filters and the filtered solutions were then considered saturated solutions. The concentration in the saturated solutions was determined by UV-Vis absorption spectroscopy, referenced to dilute solutions (0.1 mg/ml for P3HT and 1 mg/ml for PCBM). Except in the case of P3HT solution in DIO, all the saturated solutions were diluted by known scales (from 4 to 200 times) to apply the Beer-Lambert law. To account for the observed solvatochromism, the maximum absorption intensities at around 465 nm and 500 nm were used to calculate P3HT and PCBM concentration, respectively. The optical absorption spectra were taken by a Perkin Elmer Lambda 950 UV/VIS spectrometer.

3 OPV performance in spin-coating system Figure S1. OPV performance of spin-coated BHJ films processed with 2 % of ODT, DIO, and CN. Table S2. The effect of processing additive on spin-coated P3HT:PCBM BHJ OPV device Additive V OC J SC FF PCE (V) (ma/cm 2 ) (%) (%) Neat 0.63 ± ± ± ± 0.27 ODT 2% 0.51 ± ± ± ± DIO 2% 0.50 ± ± ± ± CN 2% 0.57 ± ± ± ± OPV devices with the structure glass / ITO / PEDOT:PSS (40 nm) / P3HT:PCBM (1:1 by mass, ca. 100 nm) / Al (100 nm), employing P3HT:PCBM BHJ films, were fabricated by spin-coating 30 mg/ml P3HT:PCBM solutions in o-dichlorobenzene at 2000(2π) rad/min (i.e., 2000 rpm). To investigate the effect of processing additives, 2 % (by volume with respect to o-dichlorobenzene) of 1,8- diiodooctane (DIO), 1,8-octanedithiol (ODT), and 1-chloronaphthalene (CN) were used, and all films were used as cast without post annealing procedure.

4 Evolution of film composition obtained from in-situ SE measurements To estimate the evolution of film composition, a model equation of solvent evaporation in a solvent mixture was fit to the thickness change data obtained from in-situ SE measurements. Neglecting excess volume in the mixture (i.e. assuming simple additive swelling) the volume fraction of each component can be simply scaled by a thickness fraction in one dimension. Based on the assumption that the contents of P3HT:PCBM once deposited into film are constant, the total thickness of wet film is the sum of the thicknesses of chlorobenzene (CB), solvent additive, and P3HT:PCBM. As shown in Figure 5b in main text, the film thickness linearly decreases over the drying period, implying that the solvents have constant evaporation rates. Considering that the change in the amount of each solvent depends on the volume fraction of each solvent for a solvent mixture, the change in the thickness of CB and additive as a function of time can be described by following equations, respectively: (1) (2) where and are the thicknesses of CB and additive, is the time of film drying, and and are the constant evaporation rate of CB and additive. With the initial thicknesses of CB and additive, the total film thickness data are fit to model thickness calculated from the above equations using iteration process of changing the evaporation rates of CB and additive, and, as fitting parameters. For the neat film, only equation (1) need to be used and the volume fraction of CB is equal to 1. As shown in Figure S2, this simple model well describes the film evolution. For the additive free film, the simple model slightly overestimates the drying rate of final few nm. This may be due to constraints in the diffusion of the last bit of CB, as discussed by Wang et al. In the case of 2 % ODT, the model well describes the evolution of the film thickness at the knee. This is consistent with the absence of diffusion constraints in the still-swollen-by-additive film.

5 Figure S2. Comparison of the drying model to SE thickness for neat solutions and 2 % ODT additive.

6 Identification of ordering onset and completion Figure S3. The spectra in the gray box show the evolution of the extinction coefficient (k) in a P3HT:PCBM film. The evolution of the ratio k at 605 nm to k at 462 nm is shown for the film processed

7 (a) without additive, (b-d) with 0.5 %, 2 %, and 4% of ODT, and (e-g) 0.5 %, 2 %, and 4% of CN, respectively. The insets of (f) and (g) are the blow-up at early times. Shown in the gray box of Figure S3 is typical data for the imaginary part of the index of refraction (k) for the drying of a solution with 2% ODT additive. The change in k reflects both changes in the polymer order: planarization of the P3HT backbone and interchain coupling and changes in the polymer volume fraction due to solvent evaporation. To highlight the changes attributed to ordering, we construct a figureof-merit based on the ratio of k at 605 nm (the 0-0 vibronic feature of planarized and extended chains) to k at 462 nm (the peak of the solution spectrum). To remove sensitivity of the analysis arising from noise due the rapid sampling, the figure-of-merit was fit to a sigmoidal model, described by the following equation; where y i and y f are the equilibrium values of k 605 nm /k 462 nm before and after the transition, x 0 is the time at which k 605 nm /k 462 nm is halfway between minimum and maximum (y i and y f ), and dx is the time width which describes the steepness of the curve. The k 605 nm /k 462 nm ratio and resultant fits are shown in Figure S3(a-g). In the case of ODT 4 %, CN 2 %, and CN 4 %, the film evolution clearly involves two characteristic transitions and thus two sigmoid models were used for the early and later times. Shown in Figure S3(f), the change of k 605 nm /k 462 nm is somewhat ambiguous after 30 s. However, the absorption spectra obtained by in-situ transmission measurements confirm that the evolution of film order keeps occurring in the CN 2 % film after CB completely disappears. There is a difference in the absorption spectrum shape comparing at 40 s, 90 s, and 130 s. The onset of aggregation and completion of film formation were defined as the time points where the value of k 605 nm /k 462 nm is 2 % and 98 % point of the maximum equilibrium value, respectively. In Figure S3, the blue lines are the sigmoid fit, and the orange/red marks are the onset/end point of the P3HT solidification.

8 In-situ transmission data Figure S4. Real-time absorption spectra via in-situ transmission measurements of P3HT:PCBM BHJ films with various processing additives and various amounts of additive. Data acquisition speed is about 1.5 s per data point. To account for variations in the total amount of material deposited, the absorption intensities were scaled by a normalized thickness. The illustration in the top right corner shows in-situ transmission experimental setup during film deposition. A film is created on the transparent substrate lying horizontally on the temperature regulated substrate. A ca. 24 mm size-hole enables the p-polarized UV-Vis beam to penetrate the films at the incidence angle of 45º and the transmitted beam is reflected by the mirror aligned on the beam path and travels toward the detector.

9 Direct measurements of the p-polarized absorbance of the films were performed during film drying. Due to slow data acquisition speed ( 1.5 s) of our instrument, however, the spectra snap from wet to dry state in the BHJ films processed with neat CB and with DIO and ODT as the P3HT solidification in those samples takes place within 2 or 3 s according to the in-situ SE results. Only the films processed with 2 % and 4 % of CN show a gradual spectrum shift, very consistent with the in-situ SE results, indicating that the addition of CN prolongs P3HT solidification. Moreover, narrower gaps between the spectra after ca. 20 s compared to those in first 20 s reflects that the solidification rate after CB evaporation becomes slower, as shown with the thickness changes as a function of time in Figure 7(e-g) of main text. Figure S4 displays the data recorded for only ca. 140 s since film deposition. Previously in-situ SE results have shown that 2 % and 4 % of CN films take about 150 s and 450 s to finish P3HT solidification. So the spectra corresponding to 140 s in 2 % and 4 % of CN films are not the absorption spectra of final dry films. Out-of-plane traces of GIXD patterns Figure S5. Out-of-plane intensities were integrated along q z axis in 2D GIXS patterns and scaled by the film thickness and incident beam intensity. Note that the out-of-plane axis, q z, is not exactly parallel to substrate normal and is tilted by the scattering angle due to the geometry of grazing incidence measurements. The relative intensities of (h00) and (010) peaks vary depending on the amounts of (a) ODT and (b) CN. The out-of-plane (010) peaks in ODT and CN films are shown at higher magnification in the insets.

10 Full pole figures of (100) reflection Figure S6. Full pole figures of the (100) reflection as a function of polar angle, ω, for the BHJ films processed with varying amounts of (a) ODT and (b) CN which are constructed by combining the diffraction intensities of (100) peak measured in grazing incidence geometry and specular geometry. Figure 4(b) is prepared by multiplying sin ω to these full pole figures.