Poly(3-butylthiophene) Inducing Crystallization of Small Molecule

Subscriber access provided by NEW YORK MED COLL

Article

Poly(3-butylthiophene) Inducing Crystallization of Small Molecule Donor for Enhanced Photovoltaic Performance Lin Zhang, Meilin Pu, Weihua Zhou, Xiaotian Hu, Yong Zhang, Yuanpeng Xie, Baoqing Liu, and Yiwang Chen J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 Sep 2015 Downloaded from http://pubs.acs.org on September 26, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Poly(3-butylthiophene) Inducing Crystallization of Small Molecule Donor for Enhanced Photovoltaic Performance

Lin Zhanga, Meilin Pua, Weihua Zhou*a,b, Xiaotian Hua, Yong Zhangb, Yuanpeng Xiea, Baoqing Liua, Yiwang Chen*b,c

a

School of Material Science and Engineering, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

b

College of Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

c

Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

ABSTRACT Morphological control over the bulk heterojunction (BHJ) microstructure of 7’-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b’]dithiophene-2,6-diyl)bis(6-fluoro-4-( 5’-hexyl-[2,2’-bithiophen]-5-yl)benzo[c][1,2,5](thiadiazole) (p-DTS(FBTTh2)2) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) system was demonstrated by introducing a small amount of poly(3-butylthiophene) (P3BT) into the active layer. The P3BT could serve as a heterogeneous nucleating agent, inducing the crystallization of p-DTS(FBTTh2)2 to form interconnected nanofibers throughout the whole film. Moreover, the phase separation sizes of the active layer increased after incorporation of P3BT, accompanied by the enhanced surface roughness of the films. Therefore, the power conversion efficiency of the devices increased from 3.4% to 5.0% as the P3BT content reached to 10 wt%, due to the enhanced light absorption of the active layer, as well as the higher and more balanced hole and electron mobility.

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

INTRODUCTION Solution-processable small molecule bulk heterojunction (BHJ) solar cells exhibiting comparable power conversion efficiency (PCE) value of near 10% to the polymers with high potency under simple and optimized processing conditions has been the research emphasis in the organic photovoltaics (OPV) field.1-5 Comparing to the polymer solar cells, small molecule solar cells have many remarkable advantages, that is, (1) unified and definite molecular structures; (2) easily tuned energy levels and light absorptions resulting from a special chemical structure design; (3) a generally essential higher carrier mobility and open circuit voltage (Voc). These elementary reasons and recent significant developments indicate that small molecule-OPV devices could play a much bigger role in OPV field and are possible to acquire the same or even better performance than polymer-OPV devices.6-9

However, the major limits for high performance of small molecule BHJ solar cells are the low crystallinity of donor and the inappropriate phase separation size, as well as the domain purity and poor film-forming property.7-15 In order to solve these problems, scholars all over the world adopt some conventional methods, such as solvent annealing, thermal annealing, high boiling solvent annealing and so on.12,

16-23

Thermal annealing is demonstrated to be a significant means to drive the donor crystallization and promote phase separation of donor and acceptor in the solution-processed small molecule BHJ organic solar cells.12, 16-18 Solvent annealing possesses the same effect as thermal annealing on the small molecule solar cells during the process of film drying.2, 19 High boiling solvent, such as 1, 2-diiodooctane (DIO), is the most common solvent additives in the organic solar cell. It can selectively dissolve [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) molecules and facilitate the aggregation of PC71BM. And then, the donor molecules aggregate into crystals so as to improve the purity of domains.20-22

It is worth noting that introducing polymer into small molecule system is widely used

2


Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in many fields to optimize the morphology of active layer. Bazan et al22 found that adding

high

molecular

weight

polystyrene

(PS)

into

7’-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b’]dithiophene-2,6-diyl)bis(6-fluoro-4-( 5’-hexyl-[2,2’-bithiophen]-5-yl)benzo[c][1,2,5](thiadiazole), p-DTS(FBTTh2)2 and PC71BM could increase the solution viscosity, control over the dewetting and uniformity of the film, and induce small molecules to form nanofibers. Moreover, the nanofibers become more prominent with adding a trace of DIO solvent into the main solvent. Renolds et al23 found that polydimethylsiloxane (PDMS) played the role of nucleating agent, resulting in an increase in the nucleation density of small molecules. Therefore, the crystal size decreased and the crystallinity increased. However, Chen et al8 reported that a slight addition of PDMS into small molecule solar cells with a benzo[1,2-b:4,5-b’]dithiophene (BDT) unit as the central building block had no effect on the crystallization of small molecules, but it reduced surface roughness of film and slightly increased domain size. What’s more, Pace et al24 found that introducing poly(3-hexylthiophene) (P3HT) into the blends of p-DTS(FBTTh2)2 and PC71BM could inhibit the crystallization of p-DTS(FBTTh2)2 molecules, and adjusted the viscosity of ternary blends so as to inkjet printing. Wei et al25 reported a ternary system containing one acceptor and two donors including small molecule and polymer could reach to a maximal PCE value of 8.4 % at polymer weight ratio of 60 wt% and small molecule weight ratio of 40 wt%. It is noted that the PCE value of the devices showed of no obvious change at polymer weight ratio below 30 wt%.

Based on the above results, the role of polymer played in the small molecule BHJ seems to be unclear, and there are many different explanations about it. The effect of non-conjugated polymers such as PDMS and aPS on the crystallization of small molecule donors is controversial. Moreover, the conjugated polymer of P3HT may restrict the crystallization of p-DTS(FBTTh2)2. In all, the influence of polymers on the crystallization, phase separation and properties of the small molecule BHJ needs further exploration. Compared to the non-conjugated polymers, the conjugated polymers are conductive and photo-active,26-36 which may be beneficial to the 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

absorption, exciton separation and hole transportation for the small molecule BHJ. Up to now, the incorporation of conjugated polymers into the small molecule BHJ to significantly improve the photovoltaic properties has not been reported.

Poly(3-alkylthiophene), as an important p-type semiconducting conjugated polymer, has been widely studied.37-41 Among the P3AT polymers, poly(3-butylthiophene) (P3BT) was less studied than P3HT, due to its relatively low PCE value. Nevertheless, because of its shorter alkyl substitute on the main backbone, its crystallizability should be higher than other polythiophenes with longer side chains, showing of good electrical conductivity and charge carrier mobility.42-43 In this article, we are aimed to incorporate P3BT to the p-DTS(FBTTh2)2 system by the simple solution blending method. The influence of P3BT weight ratio on the crystallization of the donor molecules, the morphology of active layer and the photovoltaic properties of corresponding devices was investigated. It is observed that the P3BT could serve as the heterogeneous nucleating agent, inducing the crystallization of p-DTS(FBTTh2)2 with higher crystallinity. Moreover, the p-DTS(FBTTh2)2 tended to form the nanofiber structures upon incorporation of P3BT, leading to the maximal PCE value of

5.0%

at

P3BT

weight

ratio

of

10

wt%

in

p-DTS(FBTTh2)2:P71BM system of 3.4%.

4


contrast

to

pristine

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EXPERIMENTAL SECTION Materials p-DTS(FBTTh2)2 was purchased from 1-Material Chemscitech Inc. PC71BM was purchased from Nano-C Inc. P3BT (Mw=15000 g mol-1, head-to-tail regioregularity of 97%) used in this study was purchased from Sigma Aldrich. Indium tin oxide (ITO) glass was purchased from Delta Technologies Limited, whereas chlorobenzene (CB) was purchased from Aldrich. All the reagents were used directly as received without further purification. Fabrication of inverted devices The desired geometric configuration of ITO-coated glass substrates was patterned by etching. The substrates were cleaned by detergent, deionized water, and isopropyl alcohol sequentially with ultrasound treatment and dried under nitrogen flow. After UV treatment for 20 min, the cleaned ITO were covered by spin coating with the ZnO precursor to form a electron transport layer, thermal annealing at 200 °C for 1 h, and transferring

into

nitrogen-filled

glove-box.

The

photoactive

layer

of

p-DTS(FBTTh2)2:P3BT:PC71BM ternary solution were heated at 90 °C for 15 min, and then were spin coated on the substrates at 1750 rpm/s for 45 s to obtain the BHJ films that was about 120 nm thicknesses (as determined by a surface profiler). Films were allowed to dry for 30 min then heated to 70 °C for 10 min in the nitrogen-filled glove-box to drive off residual solvent. Finally, 7 nm MoO3 and 90 nm Ag were deposited on top of the active layer by thermal evaporation with a mask at a pressure of approximately 10-6 Torr as an anode. Characterization The ultraviolet-visible (UV) spectra of the specimens were recorded by a PerkinElmer Lambda 750 spectrophotometer. Fluorescence measurement was carried out on a Hitachi F-7000 PC spectrofluorophotometer. The grazing-incidence X-ray diffraction (GIXRD) profiles were obtained by using a Bruker D8 Discover reflector. To study the phase-transition temperatures and the crystallization behavior, differential scanning calorimetry (DSC) was measured by TA DSC Q2000 differential scanning

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

calorimeter with a invariable heating/cooling rate of 10 ºC/min. X-ray diffraction (XRD) study were tested by Bruker D8 Focus X-ray diffractometer with a copper target (λ=1.54 Å). Photocurrent/voltage (J/V) curves were recorded using a Keithley 2400 Source Meter under 100 mW/cm2 simulated AM 1.5 G irradiation (Abet Solar Simulator Sun 2000) and in the dark. The current-voltage characterization was recorded by using a Keithley 2400 Source Meter. The atomic force microscopy (AFM) images were gauged by nanoscope III A scanning probe microscope. The morphology was conducted by employing transmission electron microscopy (TEM) (JEM-2010 HR).

6


Page 6 of 31

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RESULTS AND DISCUSSION The molecular structure of p-DTS(FBTTh2)2, PC71BM and P3BT are shown in Figure 1a. The P3BT at different weight ratios of 5 wt%, 10 wt% and 20 wt% (in donor) was incorporated into the p-DTS(FBTTh2)2 and p-DTS(FBTTh2)2:PC71BM systems by solution blending method (Table S1). To verify the effect of P3BT on the absorption behaviors of p-DTS(FBTTh2)2, the corresponding UV-vis absorption were measured. As shown in Figure 1b, the pristine p-DTS(FBTTh2)2 specimen shows strong absorption between 450 nm and 750 nm, with two prominent absorption peaks at 622 nm and 685 nm. Thin film absorption shows of vibronic structure, typical of ordered thin films.44-46 The intensity of peak at 685 nm is contributed to the vibronic structure of the p-DTS(FBTTh2)2, suggesting the molecular ordering of the π-π stacking.11 Upon the incorporation of P3BT, the intensity of the absorption peak at 685 nm continuously increases except for the specimen containing 20 wt% P3BT. Moreover, the enhanced absorption between 450 nm and 600 nm is attributed to the existence of more content of P3BT. It is believed that the P3BT has a strong effect on the π-π stacking of p-DTS(FBTTh2)2 molecules at P3BT weight ratio below 10 wt%. Similarly, as shown in Figure 1c, the intensity of the absorption peaks at around 400 nm, 622 nm and 673 nm gradually increases in the ternary blends. The normalized absorption of the binary films and ternary films are shown in Figure S1a and Figure S1b, and the results further demonstrate that the light absorption is enhanced and the molecular ordering increased by the vibronic structure of p-DTS(FBTTh2)2 within the film. It is believed that the addition of P3BT could enhance the light absorption and improve the crystallization of p-DTS(FBTTh2)2 molecules.

The crystalline structure of the films was characterized by the GIXRD and XRD. The GIXRD and XRD patterns obtained from the films with different weight ratios of P3BT are shown in Figure 2 and Figure S2, respectively. The (100) peak at 2θ ≈ 4° is attributing to p-DTS(FBTTh2)2 as observed from the out-of-plane pattern in GIXRD (Figure 2a) and XRD spectra (Figure S2).47 Additionally, the (010) peak at 2θ ≈

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

24.5° is observed from the in-plane pattern (Figure 2b). The calculated parameters of (100) and (010) peaks, such as d-spacing determined by Bragg equation, the crystalline correlation length (CCL) determined by Scherrer's equation and the peak area as determined from GIXRD and XRD spectra are shown in Table S2 and Table S3. It is noted that the d-spacing of (100) peak slowly increases from 2.20 nm to 2.24 nm and the CCL value decreases from 14.60 nm to 13.36 nm continuously with the increase of P3BT content. In addition, the height and area of (100) peak for the specimen containing 10 wt% P3BT reach to maximal value indicating of enhanced crystallinity. However, the d-spacing of (010) peak in the in-plane pattern of GIXRD spectra remains the same as the P3BT content increasing, and the CCL value of specimen containing 5 wt% P3BT reaches to the maximum of 12.56 nm. Similarly, the height and area of (010) peak increase as the P3BT weight ratio increasing, illustrating that P3BT facilitates the crystallization of p-DTS(FBTTh2)2 to form the edge-on structure.48-49 As the pristine P3BT is concerned, the diffraction peak at 2θ ≈ 6.8° could be observed in the out-of-plane pattern. However, the diffraction peak of P3BT could not be obviously observed in the specimens containing 10 wt% and 20 wt% P3BT. The weak peaks at the same position might contribute to P3BT, indicating that P3BT crystallize even worse in the ternary blend films than that in the pristine P3BT:PC71BM blend films. In a word, the presence of P3BT could induce more disordered p-DTS(FBTTh2)2 molecules to form crystals and improve the crystallinity of p-DTS(FBTTh2)2 as revealed by the change of d-spacing, CCL and peak area of (100) and (010) peaks.

The interactions between p-DTS(FBTTh2)2 and P3BT are further researched by DSC analysis. As shown in Figure 3a, the P3BT exhibits a round melting peak and the melting temperature is 284.8 °C. The pristine p-DTS(FBTTh2)2 shows of a relatively sharp melting peak at 210.7 °C with the melting enthalpy of 71.5 J/g. The p-DTS(FBTTh2)2:P3BT (9:1 in weight ratio) blend exhibits double melting peaks ascribing to p-DTS(FBTTh2)2 and P3BT, respectively. It is noted that the melting temperature of p-DTS(FBTTh2)2 is 209.0 °C, which is slightly lower than that of 8


Page 8 of 31

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pristine p-DTS(FBTTh2)2. In addition, the melting enthalpy of p-DTS(FBTTh2)2 in the blend is 71.9 J/g, which is comparable to the value of pristine p-DTS(FBTTh2)2. It is worth noting that the p-DTS(FBTTh2)2 content in the p-DTS(FBTTh2)2:P3BT blend is only 90 wt%, and the theoretical melting enthalpy is calculated to be about 79.4 J/g for p-DTS(FBTTh2)2 when considering the factor of actual weight. The melting enthalpy value could indirectly represent crystallinity, and the enhanced melting enthalpy value for the p-DTS(FBTTh2)2 component in the blends reveals that the P3BT do induce the crystallization of p-DTS(FBTTh2)2. Furthermore, the melting temperature of P3BT in the blend is 257.0 °C, which is much lower than that of pristine P3BT. According to the above result, it is revealed that there exists strong interactions between P3BT and p-DTS(FBTTh2)2 that may be caused by the π-π interactions. As shown in Figure 3b, P3BT was also found to significantly influence the crystallization behavior of p-DTS(FBTTh2)2. It is observed that the pristine p-DTS(FBTTh2)2 exhibit several crystallization peaks with the temperature of about 174.1 °C and the crystallization enthalpy of 61.4 J/g. It is believed that some of the molten p-DTS(FBTTh2)2 molecules first crystallize during the cooling process, which serve as the nucleating agents for the residual molten p-DTS(FBTTh2)2 molecules, leading to the appearance of multiple crystallization peaks. After blending with P3BT, the crystallization temperature increases to 174.8 °C with crystallization enthalpy of 66.8 J/g. The p-DTS(FBTTh2)2 weight ratio in the blend is only 90 wt%, but the crystallization enthalpy of p-DTS(FBTTh2)2 in the blend (66.8 J/g) is much higher than that of pristine p-DTS(FBTTh2)2 (61.4 J/g). Moreover, the minor exothermic peak at 189.5 °C should correspond to the crystallization of P3BT in the blends, although the temperature is much lower than that of pristine P3BT. It is revealed that the π-π interaction between p-DTS(FBTTh2)2 and P3BT seriously restricted the crystallization of P3BT from the melting state. As cooling from the melting state, the P3BT molecular chains tend to segregate and crystallize firstly, serving as the heterogeneous nucleating points to induce the crystallization of p-DTS(FBTTh2)2 at relatively higher temperature with much higher crystallization enthalpy value. Similarly, we believed that the P3BT molecules crystallized firstly during the solvent 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

evaporation process and film-forming process, and then P3BT crystallites played the role of heterogeneous nucleating points to induce the crystallization of p-DTS(FBTTh2)2.

In order to explore internal morphology of the thin films and to understand the origin of the observed changes that was described previously, TEM images were presented. The TEM images of p-DTS(FBTTh2)2:P3BT binary films are shown in Figure S3 and the TEM images of the ternary system are shown in Figure 4. In Figure 4a, the pristine p-DTS(FBTTh2)2:PC71BM film shows of almost no discernable structure, attributing to a largely homogenous morphology. The p-DTS(FBTTh2)2 and PC71BM seem to mix well without obvious phase separation after casting from chlorobenzene. In contrast, the specimen containing 5 wt% P3BT shows wire-like structures that breed throughout the whole film with widths of about 15-30 nm and lengths of hundreds of nanometers as shown in Figure 4b. These domains are considered as p-DTS(FBTTh2)2 regions in the film.11,

22

Additionally, the mixed domains still

dominate the whole film. At P3BT weight ratio of 10 wt%, the film exhibits of a different morphology as shown in Figure 4c, showing of smaller p-DTS(FBTTh2)2 domains. The width of the nanofibers is ranging from 10 to 20 nm, and the density of the nanofibers obviously increases in contrast to the specimen containing 5 wt% P3BT. It seems that the nanofibers form a continuous network throughout the whole film and distribute in the matrix homogeneously. The nanofibers throughout the whole film provide good connectivity so as to improve the charge transport efficiency.13 Upon the incorporation of 20 wt% P3BT into the p-DTS(FBTTh2)2:PC71BM system, the morphology of the film shows a significant change as illustrated in Figure 4d. The p-DTS(FBTTh2)2 nanofibers with a much smaller size below 10 nm dominate the whole image. In addition, the scale of phase separation is much bigger. Some big and black regions might be attributing to the PC71BM aggregation. In a word, it indicates that the P3BT could induce p-DTS(FBTTh2)2 molecules to form nanofibers from the perspective of internal morphology. The existence of P3BT plays the role of nucleating agent, resulting in an increase of the nucleation density for 10


Page 10 of 31

Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


p-DTS(FBTTh2)2. Therefore, the crystallinity of p-DTS(FBTTh2)2 increases as more disordered p-DTS(FBTTh2)2 molecules crystallized, which should be beneficial to enhance the photovoltaic properties.

To further investigate the morphology evolution of blend films after blending with P3BT, the AFM images were presented in Figure S4, Figure S5 and Figure 5. The AFM images of binary blend films (Figure S5) illustrate that the existence of 5 wt%~10 wt% P3BT could reduce the surface roughness to a low root-mean-square (RMS) value of about 2.5 nm. However, the images of ternary blend films (Figure 5) are different from the binary blend films. The pristine p-DTS(FBTTh2)2:PC71BM film (Figure 5a) shows of a considerable smooth surface with RMS roughness value of about 0.64 nm. Moreover, the phase separation of the film is not obvious. In contrast, the specimen containing 5 wt% P3BT presents a RMS value of 1.70 nm as illustrated in Figure 5b. The scale of phase separation for the film increases as compared to the pristine p-DTS(FBTTh2)2:PC71BM film, and some minor crystals contributing to p-DTS(FBTTh2)2 could also be discerned in the AFM image. As P3BT content increases to 10 wt%, more crystals could be observed in Figure 5c, and the RMS value reaches to 2.78 nm. As P3BT content increases to 20 wt%, the RMS value increases to 5.43 nm, showing of phase separation in a larger scale. Three-dimensional AFM images of the ternary blend films that are shown in Figure S6 also indicate the scale of phase separation for the film monotonic increase as the P3BT content increased. The results indicate that the incorporation of small amount P3BT into p-DTS(FBTTh2)2:PC71BM could increased the surface roughness, enlarged the phase domain sizes, and induced more p-DTS(FBTTh2)2 molecules to crystallize, showing of a more optimized morphology at 10 wt% P3BT.

In order to evaluate the apparent charge carrier mobility in the active layer, J0.5-V characteristics of single charge carrier devices were measured using the space charge-limited-current (SCLC) model according to the Mott–Gurney equation:

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

J = (9 / 8)µε0εr(V 2 / L3 ) where J is the current density, µ is the charge carrier mobility，ε0 is the permittivity of free space (8.85×10−12 F m−1), εr is the dielectric constant of p-DTS(FBTTh2)2 or PC71BM (assumed to 3), µ is carrier mobility, V is the internal voltage in the device and V = Vappl - Vr - Vbi, where Vappl is the applied voltage to the device, Vr is the voltage drop due to contact resistance and series resistance across the electrodes, and Vbi is the built-in voltage due to the relative work function difference of the two electrodes.50-52 L is the thickness of the BHJ blend for SCLC measurement that is about 120 nm. The J0.5-V characteristics of hole-only and electron-only devices are plotted in Figure 6a and 6b, respectively. Summary of the carrier mobility that was fitted with the SCLC model is shown in Table 1. The apparent hole mobility reaches to the highest value of 1.69×10-4 cm2V−1s−1 at P3BT content of 10 wt%, in contrast to 3.70×10-5 cm2V−1s−1 for the pristine p-DTS(FBTTh2)2:PC71BM. The enhancement of the hole mobility is mainly contributed to the improved crystallization of p-DTS(FBTTh2)2 induced by P3BT and the optimized phase separation size. If the P3BT content reaches to 20 wt%, the hole mobility of its film decreases to 1.02×10-4 cm2V−1s−1 due to the aggregation of P3BT and the apparent phase separation of active layer as shown in TEM images (Figure 4). At the same time, the electron mobility has the same trend of the hole mobility when the P3BT was added into the active layer. The results indicate that the addition of P3BT into the active layer can achieve an increase of both the hole mobility and electron mobility, reaching to the maximal values at P3BT weight ratio of 10 wt%. The enhanced hole mobility and electron mobility should be related to the formation of nanofibers and the continuous network throughout the whole film (Figure 4c).

The bulk heterojunction solar cells based on p-DTS(FBTTh2)2:PC71BM films containing different weight ratios of P3BT were fabricated. Device structure of ITO/ZnO/p-DTS(FBTTh2)2:P3BT:PC71BM/MoO3/Ag is shown in Figure 7a and energy-level diagram of the inverted structure is shown in Figure 7b. It seems that the

12


Page 12 of 31

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lowest unoccupied molecular orbital (LUMO) of P3BT is too high for electronic transmission in the energy-level diagram (determined by the cyclic voltammetry measurements as shown in Figure S7). But the photoluminescence (PL) spectra (Figure

S8)

shows

that

the

intensity

of

PL

emission

peak

for

the

p-DTS(FBTTh2)2:P3BT:PC71BM specimen containing 10 wt% P3BT is the lowest. It indicates that the radiative decay to the ground state decreases and the exciton dissociation efficiency increases. In addition, a small quantity of mixed domains that still exist in the ternary blend film (Figure 4c) lead to lower the valence bands (VBs) of small molecule donor. It provides driving force for holes transport created by the energetic offsets in the VBs.53 Therefore, the p-DTS(FBTTh2)2:P3BT:PC71BM specimen containing 10 wt% P3BT presents the least recombination that is consistent with the PL spectra.

Figure 7c presents the current-voltage (J-V) characteristics of the small molecule BHJ solar cell based on the p-DTS(FBTTh2)2:P3BT:PC71BM blends at different weight ratios of P3BT that were measured under AM1.5G (100 mW cm-2) light intensity illumination. The device based on p-DTS(FBTTh2)2:PC71BM exhibits a PCE value of 3.4%, with a short-circuit current (Jsc) of 8.86 mA cm-2, an open circuit voltage (Voc) of 0.789 V, and a fill factor (FF) of 49.2%, respectively. When the 5 wt% P3BT was added into the p-DTS(FBTTh2)2:PC71BM system, the Jsc and FF increased to 9.87 mA cm-2 and 52.6%, showing of a PCE of 4.1%. By increase of P3BT content to 10 wt%, the PCE reaches the highest value of 5.0% for the corresponding solar cell device. Both the Jsc and FF values reach to the maximal value of 11.28 mA cm-2 and 55.3%, respectively. As the P3BT content increases to 20 wt%, the Jsc and FF values of the corresponding device reduce substantially, showing of the values of 8.55 mA cm-2 and 44.9%, respectively. The device of P3BT:PC71BM blend shows an undesirable PCE value of 1.8%, with Jsc of 6.50 mA cm-2 and FF of 51.1%. It can be concluded that adding a small amount of P3BT into p-DTS(FBTTh2)2:PC71BM could improve the Jsc and FF values of the devices, resulting in a higher photovoltaic performance. The increased Jsc and PCE have been determined by the external quantum efficiency 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

(EQE). The profile of EQE as shown in Figure 7d is similar to the UV-vis absorption spectra, and the device based on specimen containing 10 wt% P3BT displays significantly

enhanced

EQE

value

as

compared

with

the

pristine

p-DTS(FBTTh2)2:PC71BM specimen. The current-voltage (J-V) characteristics (Figure S9) of the devices without illumination indicate that the leakage current of the device with a small amount of P3BT is considerably restrained. When adding 10 wt% P3BT into the p-DTS(FBTTh2)2:PC71BM system, the leakage current of the device was minimum, showing of excellent diode quality.54 The recombination of carriers could be suppressed by the addition of P3BT. The summary of the photovoltaic performance parameters is given in Table 2.

14


Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CONCLUSION The incorporation of P3BT into p-DTS(FBTTh2)2:PC71BM system was demonstrated to increase the PCE value to 5.0% at P3BT weight ratio of 10 wt%, in contrast to the pristine p-DTS(FBTTh2)2:PC71BM system of 3.4%. The P3BT played the role of heterogeneous nucleating points, facilitating the crystallization of p-DTS(FBTTh2)2 to form interconnected nanofibers throughout the whole film as revealed by the DSC, GIXRD and TEM analysis. Moreover, the phase separation sizes were enlarged and the absorption of the active layer was enhanced at P3BT weight ratio of 10 wt%. Both the hole mobility and electron mobility reached to highest value as the P3BT content increased to 10 wt%, resulting in the enhancement of Jsc and FF. The conjugated polymer as an attractive additive may open a novel way to improve the properties of solar cells based on small molecules, via increasing the crystallinity of donors, improving the film-forming properties, as well as optimizing the phase separation of the films.

ASSOCIATED CONTENT Supporting Information The more detailed experiment and composition of the ternary blend solution. Normalized absorption spectra of binary blends and ternary blends. XRD spectra, three-dimensional AFM images and photoluminescence spectra of ternary blends. The parameters of GIXRD and XRD for ternary blends by fitting the (010) peak and (100) peak, respectively. TEM graphs and AFM images of binary blend films. The cyclic voltammograms of P3BT film and the dark J-V characteristics. Photoluminescence (PL) spectra of the films. This information is available free of charge via the Internet at http://pubs.acs.org.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

AUTHOR INFORMATION Corresponding Author *Tel.: +86 791 83968703; fax: +86 791 83969561. E-mail: [email protected] (Y. Chen); [email protected] (W. Zhou) Author Contributions L. Zhang and W. Zhou contributed equally to this work. Notes The authous declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Science Fund for Distinguished Young Scholars (51425304), National Natural Science Foundation of China (51273088, 51303077 and 51563016), National Basic Research Program of China (973 Program 2014CB260409), Doctoral Programs Foundation of Ministry of Education of China (Grants 20123601120010), and Fund by State Key Laboratory of Luminescent Materials and Devices (Grants 2013-skllmd-04).

16


Page 16 of 31

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES 1. Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.; Yang, X.; Feng, H.; Chen, Y. Solution-Processed Organic Solar Cells Based on Dialkylthiol-Substituted Benzodithiophene Unit with Efficiency near 10%. J. Am. Chem. Soc. 2014, 136, 15529-15532. 2. Sun, K.; Xiao, Z.; Lu, S.; Zajaczkowski, W.; Pisula, W.; Hanssen, E.; White, J. M.; Williamson, R. M.; Subbiah, J.; Ouyang, J., et al. A Molecular Nematic Liquid Crystalline Material for High-Performance Organic Photovoltaics. Nat. Commun. 2015, 6, 6013-6021. 3. Zhang, Q.; Kan, B.; Liu, F.; Long, G.; Wan, X.; Chen, X.; Zuo, Y.; Ni, W.; Zhang, H.; Li, M., et al. Small-Molecule Solar Cells with Efficiency over 9%. Nat. Photonics 2015, 9, 35-41. 4. Liu, Y.; Chen, C. C.; Hong, Z.; Gao, J.; Yang, Y. M.; Zhou, H.; Dou, L.; Li, G.; Yang, Y. Solution-Processed Small-Molecule Solar Cells: Breaking the 10% Power Conversion Efficiency. Sci. Rep. 2013, 3, 3356-3363. 5. Kyaw, A. K.; Wang, D. H.; Wynands, D.; Zhang, J.; Nguyen, T. Q.; Bazan, G. C.; Heeger, A. J. Improved Light Harvesting and Improved Efficiency by Insertion of an Optical Spacer (ZnO) in Solution-Processed Small-Molecule Solar Cells. Nano Lett. 2013, 13, 3796-3801. 6. Long, G.; Wan, X.; Kan, B.; Liu, Y.; He, G.; Li, Z.; Zhang, Y.; Zhang, Y.; Zhang, Q.; Zhang, M., et al. Investigation of Quinquethiophene Derivatives with Different End Groups for High Open Circuit Voltage Solar Cells. Adv. Energy Mater. 2013, 3, 639-646. 7. Chen, Y. S.; Wan, X. J.; Long, G. K. High Performance Photovoltaic Applications Using Solution-Processed Small Molecules. Acc. Chem. Res. 2013, 46, 2645-2655. 8. Zhou, J.; Zuo, Y.; Wan, X.; Long, G.; Zhang, Q.; Ni, W.; Liu, Y.; Li, Z.; He, G.; Li, C., et al. Solution-Processed and High-Performance Organic Solar Cells Using Small Molecules with a Benzodithiophene Unit. J. Am. Chem. Soc. 2013, 135, 8484-8487. 9. S Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency. Nat. Mater. 2012, 11, 44-48. 10. Graham, K. R.; Stalder, R.; Wieruszewski, P. M.; Patel, D. G.; Salazar, D. H.; Reynolds, J. R. Tailor-Made Additives for Morphology Control in Molecular Bulk-Heterojunction Photovoltaics. ACS Appl. Mater. Interfaces 2013, 5, 63-71. 11. Love, J. A.; Proctor, C. M.; Liu, J.; Takacs, C. J.; Sharenko, A.; van der Poll, T. S.; Heeger, A. J.; Bazan, G. C.; Nguyen, T.-Q. Film Morphology of High Efficiency Solution-Processed Small-Molecule Solar Cells. Adv. Funct. Mater. 2013, 23, 5019-5026. 12. Sharenko, A.; Kuik, M.; Toney, M. F.; Nguyen, T.-Q. Crystallization-Induced Phase Separation in Solution-Processed Small Molecule Bulk Heterojunction Organic Solar Cells. Adv. Funct. Mater. 2014, 24, 3543-3550. 13. Lyons, B. P.; Clarke, N.; Groves, C. The Relative Importance of Domain Size, Domain Purity and Domain Interfaces to the Performance of Bulk-Heterojunction 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Organic Photovoltaics. Energy Environ. Sci. 2012, 5, 7657-7663. 14. Mukherjee, S.; Proctor, C. M.; Tumbleston, J. R.; Bazan, G. C.; Nguyen, T. Q.; Ade, H., Importance of Domain Purity and Molecular Packing in Efficient Solution-Processed Small-Molecule Solar Cells. Adv. Mater. 2015, 27, 1105-1111. 15. Mukherjee, S.; Proctor, C. M.; Bazan, G. C.; Nguyen, T. Q.; Ade, H., Significance of Average Domain Purity and Mixed Domains on the Photovoltaic Performance of High-Efficiency Solution-Processed Small-Molecule BHJ Solar Cells. Adv. Energy Mater. 2015, DOI: 10.1002/aenm.201500877. 16. Peumans, P.; Uchida, S.; Forrest, S. R. Efficient Bulk Heterojunction Photovoltaic Cells Using Small-Molecular-Weight Organic Thin Films. Nature 2003, 425, 158-162. 17. Walker, B.; Tomayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T. Q. Nanoscale Phase Separation and High Photovoltaic Efficiency in Solution-Processed, Small-Molecule Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2009, 19, 3063-3069. 18. Shin, W.; Yasuda, T.; Watanabe, G.; Yang, Y. S.; Adachi, C. Self-Organizing Mesomorphic Diketopyrrolopyrrole Derivatives for Efficient Solution-Processed Organic Solar Cells. Chem. Mater. 2013, 25, 2549-2556. 19. Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R. Solvent-Annealed Crystalline Squaraine: PC70BM (1:6) Solar Cells. Adv. Energy Mater. 2011, 1, 184-187. 20. Perez, L. A.; Chou, K. W.; Love, J. A.; van der Poll, T. S.; Smilgies, D. M.; Nguyen, T. Q.; Kramer, E. J.; Amassian, A.; Bazan, G. C. Solvent Additive Effects on Small Molecule Crystallization in Bulk Heterojunction Solar Cells Probed During Spin Casting. Adv. Mater. 2013, 25, 6380-6384. 21. Seifter, J.; Sun, Y.; Heeger, A. J. Transient Photocurrent Response of Small-Molecule Bulk Heterojunction Solar Cells. Adv. Mater. 2014, 26, 2486-2493. 22. Huang, Y.; Wen, W.; Mukherjee, S.; Ade, H.; Kramer, E. J.; Bazan, G. C. High-Molecular-Weight Insulating Polymers Can Improve the Performance of Molecular Solar Cells. Adv. Mater. 2014, 26, 4168-4172. 23. Graham, K. R.; Mei, J.; Stalder, R.; Shim, J. W.; Cheun, H.; Steffy, F.; So, F.; Kippelen, B.; Reynolds, J. R. Polydimethylsiloxane as a Macromolecular Additive for Enhanced Performance of Molecular Bulk Heterojunction Organic Solar Cells. ACS Appl. Mater. Interfaces 2011, 3, 1210-1215. 24. Pace, G.; Grimoldi, A.; Natali, D.; Sampietro, M.; Coughlin, J. E.; Bazan, G. C.; Caironi, M. All-Organic and Fully-Printed Semitransparent Photodetectors Based on Narrow Bandgap Conjugated Molecules. Adv. Mater. 2014, 26, 6773-6777. 25. Zhang, Y.; Deng, D.; Lu, K.; Zhang, J.; Xia, B.; Zhao, Y.; Fang, J.; Wei, Z. Synergistic Effect of Polymer and Small Molecules for High-Performance Ternary Organic Solar Cells. Adv. Mater. 2015, 27, 1071-1076. 26. Xin, H.; Kim, F. S.; Jenekhe, S. A. Highly Efficient Solar Cells Based on Poly(3-Butylthiophene) Nanowires. J. Am. Chem. Soc. 2008, 130, 5424-5425. 27. Lee, S. H.; Park, D. H.; Kim, K.; Joo, J.; Kim, D.-C.; Kim, H.-J.; Kim, J. Confocal Microscope Photoluminescence and Electrical Characteristics of Single Poly(3-Hexylthiophene) Nanowire Strand. Appl. Phys. Lett. 2007, 91, 263102. 18


Page 18 of 31

Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28. Choe, M.; Hoon Lee, B.; Park, W.; Kang, J.-W.; Jeong, S.; Cho, K.; Hong, W.-K.; Hun Lee, B.; Lee, K.; Park, S.-J. et al. Characteristics of Light-Induced Electron Transport from P3ht to Zno-Nanowire Field-Effect Transistors. Appl. Phys. Lett. 2013, 103, 223305. 29. Wu, F.-C.; Huang, Y.-C.; Cheng, H.-L.; Chou, W.-Y.; Tang, F.-C. Importance of Disordered Polymer Segments to Microstructure-Dependent Photovoltaic Properties of Polymer–Fullerene Bulk Heterojunction Solar Cells. J. Phys. Chem. C 2011, 115, 15057-15066. 30. Banerji, N.; Gagnon, E.; Morgantini, P.-Y.; Valouch, S.; Mohebbi, A. R.; Seo, J.-H.; Leclerc, M.; Heeger, A. J. Breaking Down the Problem: Optical Transitions, Electronic Structure, and Photoconductivity in Conjugated Polymer Pcdtbt and in Its Separate Building Blocks. J. Phys. Chem. C 2012, 116, 11456-11469. 31. Hochbaum, A. I.; Yang, P. D. Semiconductor Nanowires for Energy Conversion. Chem. Rev. 2010, 110, 527-546. 32. Ayzner, A. L.; Wanger, D. D.; Tassone, C. J.; Tolbert, S. H.; Schwartz, B. J., Room to Improve Conjugated Polymer-Based Solar Cells: Understanding How Thermal Annealing Affects the Fullerene Component of a Bulk Heterojunction Photovoltaic Device. J Phys Chem C 2008, 112, 18711-18716. 33. Lei, C. H.; Das, A.; Elliott, M.; Macdonald, J. E. Conductivity of Macromolecular Networks Measured by Electrostatic Force Microscopy. Appl. Phys. Lett. 2003, 83, 482. 34. Mei, J.; Bao, Z. Side Chain Engineering in Solution-Processable Conjugated Polymers. Chem. Mater. 2014, 26, 604-615. 35. Hao, Y. Z.; Pei, J.; Wei, Y.; Cao, Y. H.; Jiao, S. H.; Zhu, F.; Li, J. J.; Xu, D. H. Efficient Semiconductor-Sensitized Solar Cells Based on Poly(3-Hexylthiophene)@CdSe@ZnO Core-Shell Nanorod Arrays. J. Phys. Chem. C 2010, 114, 8622-8625. 36. Devynck, M.; Rostirolla, B.; Watson, C. P.; Taylor, D. M. Photo-Response of a P3HT:PCBM Blend in Metal-Insulator-Semiconductor Capacitors. Appl. Phys. Lett. 2014, 105, 183301. 37. Chen, Y. C.; Hsu, C. Y.; Lin, R. Y.; Ho, K. C.; Lin, J. T. Materials for the Active Layer of Organic Photovoltaics: Ternary Solar Cell Approach. Chemsuschem 2013, 6, 20-35. 38. Lee, H. S.; Cho, J. H.; Cho, K.; Park, Y. D. Alkyl Side Chain Length Modulates the Electronic Structure and Electrical Characteristics of Poly(3-Alkylthiophene) Thin Films. J. Phys. Chem. C 2013, 117, 11764-11769. 39. Vandewal, K.; Oosterbaan, W. D.; Bertho, S.; Vrindts, V.; Gadisa, A.; Lutsen, L.; Vanderzande, D.; Manca, J. V. Varying Polymer Crystallinity in Nanofiber Poly(3-Alkylthiophene): PCBM Solar Cells: Influence on Charge-Transfer State Energy and Open-Circuit Voltage. Appl. Phys. Lett. 2009, 95, 123303. 40. Stevens, D. M.; Speros, J. C.; Hillmyer, M. A.; Frisbie, C. D. Relationship between Diode Saturation Current and Open Circuit Voltage in Poly(3-Alkylthiophene) Solar Cells as a Function of Device Architecture, Processing Conditions, and Alkyl Side Chain Length. J. Phys. Chem. C 2011, 115, 20806-20816. 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

41. Salammal, S. T.; Mikayelyan, E.; Grigorian, S.; Pietsch, U.; Koenen, N.; Scherf, U.; Kayunkid, N.; Brinkmann, M. Impact of Thermal Annealing on the Semicrystalline Nanomorphology of Spin-Coated Thin Films of Regioregular Poly(3-Alkylthiophene)s as Observed by High-Resolution Transmission Electron Microscopy and Grazing Incidence X-Ray Diffraction. Macromolecules 2012, 45, 5575-5585. 42. Lu, G. H.; Tang, H. W.; Qu, Y. P.; Li, L. G.; Yang, X. N. Enhanced Electrical Conductivity of Highly Crystalline Polythiophene/Insulating-Polymer Composite. Macromolecules 2007, 40, 6579-6584. 43. Lu, G. H.; Li, L. G.; Yang, X. N. Morphology and Crystalline Transition of Poly (3-Butylthiophene) Associated with Its Polymorphic Modifications. Macromolecules 2008, 41, 2062-2070. 44. Gierschner, J.; Ehni, M.; Egelhaaf, H. J.; Milian Medina, B.; Beljonne, D.; Benmansour, H.; Bazan, G. C. Solid-State Optical Properties of Linear Polyconjugated Molecules: Pi-Stack Contra Herringbone. J. Chem. Phys. 2005, 123, 144914. 45. Ostroverkhova, O.; Shcherbyna, S.; Cooke, D. G.; Egerton, R. F.; Hegmann, F. A.; Tykwinski, R. R.; Parkin, S. R.; Anthony, J. E. Optical and Transient Photoconductive Properties of Pentacene and Functionalized Pentacene Thin Films: Dependence on Film Morphology. J. Appl. Phys. 2005, 98, 033701. 46. van der Poll, T. S.; Love, J. A.; Nguyen, T.-Q.; Bazan, G. C. Non-Basic High-Performance Molecules for Solution-Processed Organic Solar Cells. Adv. Mater. 2012, 24, 3646-3649. 47. Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Leong, W. L.; Ke, L.; Bazan, G. C.; Heeger, A. J. Intensity Dependence of Current-Voltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells. ACS Nano 2013, 7, 4569-4577. 48. Kim, D. H.; Park, Y. D.; Jang, Y. S.; Yang, H. C.; Kim, Y. H.; Han, J. I.; Moon, D. G.; Park, S. J.; Chang, T. Y.; Chang, C. W. et al. Enhancement of Field-Effect Mobility Due to Surface-Mediated Molecular Ordering in Regioregular Polythiophene Thin Film Transistors. Adv. Funct. Mater. 2005, 15, 77-82. 49. Kim, D. H.; Jang, Y.; Park, Y. D.; Cho, K. Layered Molecular Ordering of Self-Organized Poly(3-Hexylthiophene) Thin Films on Hydrophobized Surfaces. Macromolecules 2006, 39, 5843-5847. 50. Heeger, A. J. 25th Anniversary Article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26, 10-28. 51. Cheng, Y.-J.; Hsieh, C.-H.; Li, P.-J.; Hsu, C.-S. Morphological Stabilization by in Situ Polymerization of Fullerene Derivatives Leading to Efficient, Thermally Stable Organic Photovoltaics. Adv. Funct. Mater. 2011, 21, 1723-1732. 52. Chen, H. C.; Lai, C. W.; Wu, I. C.; Pan, H. R.; Chen, I. W.; Peng, Y. K.; Liu, C. L.; Chen, C. H.; Chou, P. T. Enhanced Performance and Air Stability of 3.2% Hybrid Solar Cells: How the Functional Polymer and CdTe Nanostructure Boost the Solar Cell Efficiency. Adv. Mater. 2011, 23, 5451-5455. 53. Sweetnam, S.; Graham, K. R.; Ngongang Ndjawa, G. O.; Heumuller, T.; Bartelt, J. 20


Page 20 of 31

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A.; Burke, T. M.; Li, W.; You, W.; Amassian, A.; McGehee, M. D. Characterization of the Polymer Energy Landscape in Polymer:Fullerene Bulk Heterojunctions with Pure and Mixed Phases. J. Am. Chem. Soc. 2014, 136, 14078-14088. 54. Lee, B. H.; Coughlin, J.; Kim, G.; Bazan, G. C.; Lee, K. Efficient Solution-Processed Small-Molecule Solar Cells with Titanium Suboxide as an Electric Adhesive Layer. Appl. Phys. Lett. 2014, 104, 213305.

21



(b) 0.6

0.4

(c)

π−π stacking

0 wt% 5 wt% 10 wt% 20 wt% 100 wt%

0.8

Absorbance (a.u.)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.2

0.0

0.6

Page 22 of 31

0 wt% 5 wt% 10 wt% 20 wt% 100 wt%

π−π stacking

0.4

0.2

0.0 400

500

600

700

800

400

500

Wavelength (nm)

600

700

800

Wavelength (nm)

Figure 1. (a) Molecular structure of p-DTS(FBTTh2)2, PC71BM and P3BT. UV-vis spectra

of

(b)

p-DTS(FBTTh2)2:P3BT

binary

blends

and

(c)

p-DTS(FBTTh2)2:P3BT:PC71BM ternary blends at different weight ratios of P3BT, respectively.

22


Page 23 of 31

(a)

(b)

Out-of-plane

(100)

In-plane

Intensity (a. u.)

(010) Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 wt% 5 wt% 10 wt%

0 wt% 5 wt% 10 wt%

20 wt%

20 wt%

100 wt% 2

4

6

8

10

12

100 wt%

14

5

10

2 theta (degree)

Figure

2.

(a)

Out-of-plane

15

20

25

30

35

2 theta (degree)

and

(b)

in-plane

GIXRD

profiles

p-DTS(FBTTh2)2:PC71BM films at different weight ratios of P3BT, respectively.

23


for


(a)

(b) P3BT 284.8 °C p-DTS(FBTTh2)2 210.7 °C 71.5 J/g p-DTS(FBTTh2)2:P3BT

100

209.0 °C 71.9 J/g 150

200

257.0 °C

250

174.8 °C 66.8 J/g

< Εndo Heat flow (J/g) Exo >

< Εndo Heat flow (J/g) Exo >

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

300

189.5 °C

p-DTS(FBTTh2)2 P3BT 230.0 °C

100

Temperature (°C)

p-DTS(FBTTh2)2:P3BT

174.1 °C 61.4 J/g

150

200

250

300

Temperature (°C)

Figure 3. DSC (a) heating and (b) cooling curves of p-DTS(FBTTh2)2, P3BT and p-DTS(FBTTh2)2:P3BT blend at P3BT content of 10 wt% in donor, respectively.

24


Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. TEM graphs of the p-DTS(FBTTh2)2:PC71BM films containing (a) 0 wt%, (b) 5 wt%, (c) 10 wt% and (d) 20 wt% P3BT, respectively.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. AFM topography images (5×5 µm) of p-DTS(FBTTh2)2:PC71BM films containing (a) 0 wt%, (b) 5 wt%, (c) 10 wt% and (d) 20 wt% P3BT, respectively.

26


Page 26 of 31

Page 27 of 31

(a)1.0

(b)

0 wt% 5 wt% 10 wt% 20 wt% 100 wt%

1.0

0.5

(A /cm)

0.6

0.5

J

0.5

0.4

0.5

0.5

(A /cm)

1.5

0 wt% 5 wt% 10 wt% 20 wt% 100 wt%

0.8

J

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2 0.0

0.0 0

1

2

3

4

5

0

1

Vappl-Vr-Vbi (V)

Figure 6. J

0.5

2

3

4

5

Vappl-Vr-Vbi (V)

-V characteristics of (a) hole-only and (b) electron-only devices

containing different weight ratios of P3BT.

27



(c)

(d)80

2

-5

0 wt% 5 wt% 10 wt% 20 wt% 100 wt%

0 wt% 10 wt%

60

EQE (%)

0

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

40

20

-10

0.0

0.2

0.4

0.6

0 300

0.8

Voltage (V)

400

500

600

700

800

Wavelength (nm)

Figure 7. (a) Device structure of the solution-processed small molecule solar cell. (b) energy-level diagram of the inverted structure used in ternary blend. (c) current-voltage (J-V) characteristics and (d) EQE spectra of solar cells based on p-DTS(FBTTh2)2:PC71BM blends containing different weight ratios of P3BT.

28


Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Summary of the carrier mobility of the p-DTS(FBTTh2)2:PC71BM blends containing different weight ratios of P3BT. P3BT content (wt%)

Hole mobility (cm2/V·s)a

Electron mobility (cm2/V·s)b

0

3.70×10-5

1.18×10-4

5

8.84×10-5

3.66×10-4

10

1.69×10-4

4.68×10-4

20

1.02×10-4

3.62×10-4

100

8.58×10-5

2.65×10-4

a

Hole-only device configuration: ITO/PEDOT:PSS/p-DTS(FBTTh2)2:P3BT:PC71BM/MoO3/Ag.

b

Electron-only device configuration: ITO/ZnO/ p-DTS(FBTTh2)2:P3BT:PC71BM /Al.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

Table 2. Summary of the photovoltaic performance of p-DTS(FBTTh2)2:PC71BM solar cells containing different weight ratios of P3BT under AM 1.5G solar illumination. P3BT content (wt%)

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

0

8.86±0.3

0.789±0.02

49.2±1

3.4±0.1

5

9.87±0.3

0.799±0.02

52.6±1

4.1±0.1

10

11.28±0.3

0.801±0.02

55.3±1

5.0±0.1

20

8.55±0.2

0.789±0.02

44.9±2

3.0±0.1

100

6.50±0.2

0.540±0.03

51.1±1

1.8±0.1

All values represented averages from 0.04 cm2 devices on a single chip. And all data of devices had been tested from more than five substrates (15 chips) to ensure reproducibility.

30


Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphic of TOC

31


Poly(3-butylthiophene) Inducing Crystallization of Small Molecule

Recommend Documents