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Systematic Investigation of BenzodithiopheneBenzothiadiazole Isomers for Organic Photovoltaics Jia Du, Andria Fortney, Katherine E. Washington, Chandima Bulumulla, Peishen Huang, Dushanthi Dissanayake, Michael C. Biewer, Tomasz Kowalewski, and Mihaela C. Stefan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11806 • Publication Date (Web): 09 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016

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ACS Applied Materials & Interfaces

Systematic Investigation of BenzodithiopheneBenzothiadiazole Isomers for Organic Photovoltaics Jia Du,† Andria Fortney,§ Katherine E. Washington,† Chandima Bulumulla,† Peishen Huang,†∥ Dushanthi Dissanayake,† Michael C. Biewer,† Tomasz Kowalewski,§* Mihaela C. Stefan†* †

Department of Chemistry and Biochemistry, University of Texas at Dallas, Richardson, Texas

75080, United States §

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United

States KEYWORDS:

organic solar cells, benzodithiophene, benzothiadiazole, donor-acceptor

molecules, morphology

ABSTRACT: Two new donor-acceptor small molecules based on benzo[1,2-b:4,5-b']dithiophene (BDT) and benzo[c][1,2,5]thiadiazole (BT) were designed and synthesized. Small molecules 4,4'-[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(2,2'-bith iophene)-5,5'-diyl]bis(benzo[c][1,2,5]thiadiazole)

(BDT-TT-BT),

and

4,4'-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis[7-(2,2'-bi thiophene-5-yl)benzo[c][1,2,5]thiadiazole] (BDT-BT-TT) are structural isomers with 2,2bithiophene unit placed either between the BDT and BT units or at the end of the BT units. This work is targeted toward finding the effect of structural variation on optoelectronic properties,

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morphology and photovoltaic performance. Based on theoretical calculations, the molecular geometry and energy levels are different for these two molecules when the position of the 2,2bithiophene unit is changed.

Optical and electrochemical properties of these two small

molecules were characterized using UV-vis and cyclic voltammetry. The results showed that BDT-BT-TT has broader absorption and an elevated HOMO energy level when compared with BDT-TT-BT. The performance of these two isomers in solar cell devices was tested by blending with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Power conversion efficiencies as high as 3.22% and 3.71% were obtained in conventional solar cell structures for BDT-TT-BT and BDT-BT-TT, respectively. The morphology was studied using grazing incident wide angle x-ray scattering and transmission electron microscopy, which revealed different phase separation of these two molecules when blended with PC71BM.

1. Introduction Solar energy is becoming a potential solution to the energy and environmental crisis arising from the increasing population and high demand for energy. Aside from inorganic solar cells, such as single crystal silicon and gallium arsenide, organic solar cells (OSCs) are becoming a promising candidate due to their easy processing and potential structural modification.1-9 An encouraging power conversion efficiency (PCE) over 10% has been achieved for polymer based OSCs in both single bulk heterojunction (BHJ) and tandem devices with fullerene derivatives as the electron acceptors.10-16 Though the OSCs made from polymers are very promising, the difference in molecular weight and polydispersity of polymers made from different batches can result in very different solar cell performance.17-18 Compared with their polymer analogues, small molecules have several advantages such as well-defined chemical structures, less batch to batch variation, and easier purification processes. In addition, the relationship between the chemical


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structure and the photovoltaic performance can be better correlated with the unique chemical structure, thereby providing in-depth understanding of structure design.18-22 Tremendous progress has been made in developing small molecules for applications in solar cells over the last five years.23-29 So far, a record efficiency above 10% has been achieved in Dr. Chen’s group by designing and testing a series of oligothiophene-based small molecules.30 The photovoltaic performance of benzo[1,2-b:4,5-b']dithiophene (BDT) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (dFBT) based small molecules have outperformed their polymer counterparts after optimization using additives, shown by Dr. Wei’s group.31 The relative high crystallinity of these reported small molecules is beneficial for charge carrier transport in the blend films. This indicates a bright future of small molecules in solar cells after solving some limitations, such as poor film quality and unbalanced charge transport.7 Substantial progress of polymers with donor-acceptor (D-A) structures has been made in OSCs because of the tunable energy levels and broad light absorption.32 Inspired by the D-A polymers, D-A alternating small molecules featuring different units are one in a broad area of small molecules being designed and tested in BHJ solar cell devices.21, 33 Among all designed donor moieties, BDT is one of the most used donor moieties in either polymer or small molecule solar cells due to its symmetric planar structure and extended π conjugation. Molecules containing the BDT moiety possessed lower HOMO energy levels, which could result in higher open circuit voltage in BHJ solar cells.34 Benzo[c][1,2,5]thiadiazole (BT), used as an acceptor moiety, is recognized for its easy synthesis and strong electron-withdrawing properties.35 Several small molecules consisting of BDT and BT have been reported with relatively low efficiency achieved.36-40 A dramatic decrease of PCE was observed when a thiophene unit was placed between the BDT and BT units, which indicates that the sequence of the units plays a major role


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in photovoltaic performance.41 Also, the poor film morphology when blended with PCBM is one of the main reasons that achieving high efficiency has been limited. So far, several high performing photovoltaic cells were fabricated from small molecules that had either an acceptor such as rhodamine at the molecular end,18, 26 or had an acceptor moiety placed beside the donor unit like p-DTS(FBTTh2)2.24 Different design strategies were employed in these two cases, however it cannot be concluded whether placing an acceptor moiety at the molecular end or having the acceptor unit directly attached to the donor unit leads to better performance. Herein, two donor-acceptor isomers shown in Figure 1 with BDT at the center were synthesized and studied. The position of the bithiophene unit and BT unit is interchanged in these two isomers (BDT-TT-BT and BDT-BT-TT). The position of these two units in small molecules has been shown to affect the solid state and optoelectronic properties. The morphology of the neat and blended films was studied by grazing incident wide angle x-ray scattering (GIWAXS) and transmission electron microscopy (TEM). The testing of these two molecules in BHJ showed that BDT-BT-TT gave the highest PCE of 3.71% when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) while BDT-TT-BT gave PCE of 3.22% with a high open circuit voltage of 0.98 V.


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Figure 1. Molecular Structure of BDT-TT-BT and BDT-BT-TT 2. Experimental Section Materials

and

2,6-Bis(trimethylstannyl)-4,8-bis(5-(2-ethylhexyl)thiophene-2-

Methods.

yl)benzo[1,2-b:4,5-b']dithiophene 5-tributylstannyl-2,2'-bithiophene44

(M1),42

4-bromobenzo[c][1,2,5]thiadiazole43

and

were synthesized according to the literature previously

reported. The synthetic procedure for M2, M3, BDT-TT-BT and BDT-BT-TT can be found in the supporting information (SI). The commercial chemicals were purchased from Sigma-Aldrich and Fisher Scientific with no further purification and were used as received unless mentioned otherwise. All reactions were conducted under nitrogen. Tetrahydrofuran (THF) and toluene were freshly distilled over sodium/benzophenone ketyl under nitrogen. 1

H and

13

C NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer. GC/MS

was recorded on an Agilent 6890-5973 GC-MS workstation. The GC column used was a Hewlett-Packard fused silica capillary column cross-linked with 5% phenylmethylsiloxane and Helium was used as carrier gas with 1 mL/min flow rate. MALDI-TOF spectra was obtained with Shimadzu Biotech Axima Confidence in reflection_HiRes mode with 2,2':5',2''-terthiophene


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used as the matrix. Thermogravimetric Analyses (TGA) and Differential Scanning Calorimetry (DSC) were recorded on a Mettler Toledo TGA/DSC-1 system with a heating rate of 10 oC/min and cooling rate of -10 oC/min under nitrogen flow. The UV-vis absorption (UV-vis) was recorded with an Agilent 8453 UV-VIS spectrometer. Cyclic voltammetry (CV) experiments were performed with a BAS CV-50W voltammetric analyzer. The measurement was carried out with three electrode system: platinum inert working electrode, platinum wire auxiliary electrode and Ag/Ag+ reference electrode. The electrolytic solution consisted of 0.1 M of electrochemical grade

tetrabutylammonium

hexafluorophosphate

in

freshly

distilled

acetonitrile.

Ferrocene/ferrocenium (Fc/Fc+) was used as internal reference for CV calibration, and the E1/2 of the redox couple was located at 0.09V vs. Ag/Ag+. Grazing incident wide angle X-ray scattering (GIWAXS) measurements were taken at the Cornell High Energy Synchrotron Source (CHESS) D1 (λ = 1.162 nm) station. A wide bandpass (1.47%) double bounce multilayer monochromator supplied an intense beam of 10.1 keV photons. GIWAXS intensities were recorded with an area detector (Medoptics) with a resolution of 46.9 µm per pixel and a total area of about 50 mm by 50 mm at a distance of 10.62 cm from the thin film sample. The sample was mounted on a sample goniometer, in order to control the incident angle and the sample azimuth. An accurate calibration of the incident angle was performed in situ by measuring the X-ray reflectivity from the sample using an ion chamber. The measurements were calibrated using a ceria standard. The intense scattering close to the direct beam was blocked with a 1.5-mm-wide tantalum rod. Exposures times under these conditions ranged from 0.1 sec to 3 sec depending on the contrast and sample quality. Theoretical calculations were carried out at B3LYP/6-31G* level using Spartan 06. The as-prepared films were imaged with a Bruker Dimension V hybrid AFM in


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tapping mode. Transmission Electron Microscopy (TEM) was performed on model FEI Tecnai G2 Spirit Biotwin with accelerating voltage of 120 kV. Solar Cell Devices fabrication and Characterization. The organic solar cell devices were fabricated

with

conventional

configuration

of

ITO/PEDOT:PSS

(poly(3,4-

ethylenedioxythiophene)-poly(styrenesulfonate))/active layer/Ca/Al. Indium tin oxide (ITO) coated glass substrates were purchased from Luminescence Technology Corp. and cleaned with ultrasonic bath for 15 min each in deionized water, acetone, hexane and isopropanol. The cleaned ITO glasses were treated under UV/ozone for 20 min before using. PEDOT:PSS (Heraeus Clevios PVP AI 4083) was spin coated onto ITO at a spin rate of 3000 rpm for 60 sec to obtain ~ 20 nm thickness, which was annealed at 150 oC for 10 min in the glove box. BDTTT-BT or BDT-BT-TT/PC71BM blends were dissolved in chlorobenzene or chloroform and stirred at 60 oC overnight before use. All the solutions were filtered through 0.2 µm polytetrafluoroethylene filter before spin coating onto the PEDOT:PSS layer. The spin-coated active layer was annealed at 60 oC for 10 min to remove the solvent. Ca (10 nm) and Al (100 nm) were deposited through shadow mask by thermal evaporation under high vacuum (10-6 torr). The active area of all the devices was 10 mm2. More than four devices were counted to calculate the average. The thickness of the active layers was measured using AMBIOS XP1 Stylus Profiler. Current density – voltage (J-V) characteristics of solar cell devices were recorded in the glove box with Keithley 2400 source meter interfaced with Labview software. The measurements were carried out under simulated AM 1.5G condition (250 W xenon lamp). The intensity of the incident light was calibrated to 100 mW/cm2 by NREL certified Hamamatsu silicon photodiode. External Quantum Efficiency (EQE) of devices was measured with QEX10 system manufactured


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by PV Measurement Inc. The intensity of the incident light was calibrated by NREL certified Hamamatsu silicon photodiode in the wavelength range of 300 nm to 1100 nm. Hole Mobility Measurement. The hole mobility of these two molecules was measured by using hole-only devices with the configuration of ITO/PEDOT:PSS/BDT-TT-BT or BDT-BTTT/Al. The devices were fabricated under the same conditions as the solar cell devices. The measurements of space charge limited current (SCLC) were carried out on cascade probe station equipped with Keithley 4200 systems. Each device has an active area of 10 mm2 with film thickness checked with XP1 Stylus Profiler. The mobility was calculated according to the MottGurney law: J = 9εoεrµhV2/8L3, where J is current density, εo is vacuum permittivity and equals to 8.854 × 10-12 F/m, εr is relative permittivity of semiconducting materials, which is used as 3.5 here, µh is hole mobility, V is applied voltage and L is the thickness. R

R

S

S Pd(PPh3) 4, Toluene

S (H3 C)3Sn

Sn(CH3) 3

Br

S S

S

S

S S

S

S S

S

R=2-ethylhexyl

R

R M1

Yield: 57%

M2 R

R

S

S S

S

S

1) n-BuLi

S S

S

S

2) Sn(CH3 )3 Cl

S

S

(H3C) 3Sn

S S

S

S Sn(CH3) 3

S

S

R

R M2

N M3

+

S

Yield: 84%

M3

N

Pd(PPh3 )4 , Toluene BD T-TT-BT

Br

Yield: 58%

N Br

S

N

1.eq Br

S S

N

Sn(C4H9) 3

Pd(PPh3 )4 , Toluene

S

N

M1

Br S

S

Pd(PPh3 )4 , Toluene

BDT-BT-TT Yield: 39%

M4 Yield: 32%

Scheme 1. Synthetic routes toward BDT-TT-BT and BDT-BT-TT


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3. Results and Discussion Synthesis and Thermal Properties. The synthetic procedure of BDT-TT-BT and BDT-BTTT is shown in Scheme 1. 2,6-Bis(trimethylstannyl)-4,8-bis(5-(2-ethylhexyl)thiophene-2yl)benzo[1,2-b:4,5-b']dithiophene (M1) was first reacted with 5-bromo-2,2'-bithiophene via Stille coupling to afford compound M2. M2 was then reacted with n-BuLi and Sn(CH3)3Cl were added to compound M2 in sequence to give product M3. Subsequently, the bis(trimethyl)tinfunctionalized compound M3 coupled with 4-bromobenzo[c][1,2,5]thiadiazole to form conjugated small molecule BDT-TT-BT. In the synthetic procedure of small molecular BDTBT-TT, compound M4 was synthesized from Stille coupling reaction by adding 4, 7dibromobenzo[c][1,2,5]thiadiazole and 5-tributylstannyl-2,2'-bithiophene in 1 to 1 ratio. BDTBT-TT was finally obtained by reacting compound M4 with 2,6-Bis(trimethylstannyl)-4,8-bis(5(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b']dithiophene (M1). All the intermediates and two final products were characterized by 1H and

13

C NMR, mass spectrometry, and MALDI-

TOF. The thermal properties of BDT-TT-BT and BDT-BT-TT were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Thermal stability of these two molecules was determined by TGA (Figure S1 in the supporting information, SI) and the thermal decomposition temperature (Td, 5% weight loss) was estimated to be 322 oC and 433 oC, respectively, which indicates the higher thermal stability of BDT-BTTT. According to the DSC profile, changing the position of the BT unit in the small molecules influences the thermal properties. Interestingly, a cold crystallization peak and two melting transition peaks were observed at 96 oC, 131 oC, and 161 oC, respectively, on the heating stage for BDT-TT-BT. However, there is no crystallization peak on the cooling stage. Such cold crystallization behavior is rare to see in conjugated small molecules, and this might be due to


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complex molecule rearrangement in the heating process.45-46 For BDT-BT-TT, sharp melting transition and crystallization transition can be found at 247 oC and 221 oC, respectively. The higher melting temperature of BDT-BT-TT also indicates stronger intermolecular interactions in the solid state. Those apparent sharp peaks in DSC curve also manifest higher crystallinity for BDT-BT-TT. Theoretical Analysis. In order to understand the structure and electronic properties of these two small molecules, theoretical calculations were carried out at B3LYP/6-31G* level of theory using Spartan 06 program. Ethylhexyl groups were replaced by methyl groups to simplify the calculation in the model compounds, BDT-TT-BT’ and BDT-BT-TT’. In Figure 2, both of these two molecules are shown to have almost planar backbones in the optimized geometry structure, with a minor difference of dihedral angles between each unit showing that the backbone is more twisted when the BT moiety is close to the BDT moiety. For the substituted thienyl pendant group, torsional angel around 58o was observed and this spatial hindrance was due to the large atom size of sulfur.47 The electron delocalization in the molecular structure is also shown in Figure 2. The HOMO is located on the entire molecular structure while the LUMO is mainly on the BT unit. From the calculations, both of these molecules have the same HOMO energy levels at -4.89 eV, which could be attributed to the similar electron distribution over the backbone. However, the LUMO energy level was lowered when the BT unit is beside BDT. So BDT-TTBT’ has a higher LUMO energy level at -2.66 eV in contrast to -2.81 eV for BDT-BT-TT’. Therefore, the position of the BT unit plays a major role in changing the LUMO energy level.


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Figure 2. Molecular geometries and energy levels calculated by Spartan 06 for BDT-TT-BT and BDT-BT-TT

10

1.2 BDT-TT-BT (Chloroform) BDT-BT-TT (Chloroform) BDT-TT-BT (Film) 1.0 BDT-BT-TT (Film)

9 8

-1 cm )

5

4

ε ( x10 M

6

-1

7

0.8

0.6

4 0.4

3 2

0.2

1 0 300

400

500

600

700

800

900

Normalized Absorption (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


0.0 1000

Wavelength (nm)

Figure 3. UV-vis absorption of BDT-TT-BT and BDT-BT-TT in CHCl3 (1×10-6 M) and in spincoated thin film


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Optical Properties. The absorption behavior of these two molecules was recorded in chloroform and in thin film as shown in Figure 3 and Table 1. Two absorption bands located at 333 nm and 546 nm were observed for BDT-BT-TT in chloroform. The absorption peak centered at 333 nm is attributed to π-π* transition of the conjugated backbone and the peak at 546 nm is due to the intramolecular charge transfer (ICT).48 As indicated in theoretical calculation, ICT corresponds to the transition from delocalized HOMO to BT-localized LUMO. Compared with BDT-BT-TT, BDT-TT-BT has a major absorption peak at 483 nm, which is about 80 nm shifted to the shorter wavelength region. The LUMO level has a higher energy level which is caused by less delocalization of the LUMO; this causes a blue shift to be observed for BDT-TT-BT. In addition, BDT-TT-BT has a relatively higher extinction coefficient than BDT-BT-TT as shown in Figure 3. In the solid state, the absorption of these two molecules is red-shifted compared with those in solution. Both of these molecules displayed two peaks in the solid state in the band from 450-700 nm. The vibronic peak and the red-shift observed suggest favorable molecular ordering in the solid state. Optical band gap of 2.03 eV and 1.76 eV for BDT-TT-BT and BDT-BT-TT, respectively, were estimated from the onset at 611 nm and 705 nm in the spectrum. In terms of extinction coefficient in the solid state, which was calculated at the highest absorption peak, the value of BDT-TT-BT is 9.71×104 cm-1, which is higher than the value of BDT-BT-TT (3.98×104 cm-1). Table 1. Optical and electrochemical Properties of BDT-TT-BT and BDT-BT-TT HOMOa

LUMOb

Eg(ec)c

Eg(opt)d

λmaxsol

λmaxfilm

λonsetfilm

Attenuation

(eV)

(eV)

(eV)

(eV)

(nm)

(nm)

(nm)

Coefficient (cm-1)

BDT-TT-BT

-5.31

-3.13

2.18

2.03

483

520/560

611

9.71×104

BDT-BT-TT

-5.26

-3.26

2.00

1.76

333/546

349/570/618

705

3.98×104

a

Calculated from the onset of oxidation curve in cyclic voltammetry, b Calculated from the onset of reduction curve in cyclic voltammetry, c Calculated from LUMOb- HOMOc, d Calculated from onset of film in UV-vis absorption.


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BDT-TT-BT BDT-BT-TT

Current

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-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Potential (V)

Figure 4. Cyclic voltammograms of BDT-TT-BT and BDT-BT-TT Electrochemical Properties. Cyclic voltammetry (CV) was used to investigate the electrochemical band gap and HOMO/LUMO energy levels. The onset value of oxidation and reduction were used to calculate the HOMO and LUMO of these two molecules by using Ag/Ag+ as the reference electrode and tetrabutylammonium hexafluorophosphate as the electrolyte. HOMO and LUMO were calculated by the following equations: EHOMO= -( Eox+4.8E1/2,Fc/Fc+) eV; ELUMO= -( Ered+4.8-E1/2,Fc/Fc+) eV. In Figure 4, the HOMO levels calculated for these two molecules are -5.31 eV and -5.26 eV for BDT-TT-BT and BDT-BT-TT, respectively. Compared with BDT-BT-TT, BDT-TT-BT has a difference of 0.05 eV, which indicates that a higher Voc can be potentially gained in solar cell measurements. For the LUMO levels, BDT-BTTT was 0.13 eV lower than BDT-TT-BT, implying that by placing BT electron deficient units beside the BDT unit, the LUMO energy level can be lowered. Overall, the electrochemical band gap of BDT-BT-TT is smaller than BDT-TT-BT. This is consistent with the optical bandgap calculated from the absorption spectrum. Crystallinity. To investigate crystalline properties of these two molecules, tapping mode atomic force microscopy (TMAFM) and (GIWAXS) were applied. The samples were prepared


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in low boiling point solvent chloroform (CF) and high boiling point solvent o-dichlorobenzene (DCB) with the same concentration of 1 mg/mL and then drop casted onto silicon wafer with native oxide. From the TMAFM images shown in Figure 5 (a, b), the pristine BDT-TT-BT film cast from DCB gave layered flakes stacking on each other, while amorphous morphology was observed with the film cast from CF, this is due to the fast evaporation of the low boiling point solvent. In terms of BDT-BT-TT, slow evaporation of DCB gave the molecule enough time to reorganize to form a fibril pattern as shown in Figure 5 (c, d). In contrast, no fibril structure was observed for the sample prepared from CF. In order to gain further information of aggregation and film morphology of BDT-TT-BT and BDT-BT-TT, GIWAXS data shown in Figure 6 and Figure S4 was collected at various casting conditions including drop-casting, spin-casting and blends with PC71BM in both CF and DCB. The GIWAXS pattern revealed characteristic lamellar stacking reflection peaks from interdigitated side chains and π-stacking peaks from the molecular backbone as seen in Figure 6. The average lamellar and π-stacking distances were obtained from “slices” of 2D pattern, and calculated to be 19.04Å (100) and 3.65 Å (010) for BDT-TT-BT, 16.09 Å (100) and 3.54 Å (100) for BDT-BT-TT. For drop-casted samples of both BDT-TT-BT and BDT-BT-TT in CF, lamellar peaks (100) in the qz direction with corresponding π-stacking peaks in the qxy direction indicated an edge-on orientation. While the sample prepared from DCB revealed mixed edge-on and face-on orientation to the substrate surface with π-stacking peak found in both directions. Compared to drop-casted samples, lower crystallinity was observed in Figure 6 (c, d and g, h) for both molecules, this was the result from the quick drying process that occurs during spin-casting. When both small molecules blended with PC71BM in CF, a strong apparent broad isotropic PC71BM peak was observed, which indicates phase separation of PC71BM from the small molecules. Comparing 2D GIWAXS pattern of BDT-TT-BT and BDT-


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BT-TT, the crystallinity of BDT-TT-BT was suppressed with the barely visible lamellar stacking peak, which means there is a nearly amorphous phase of the small molecules in the blend. However, BDT-BT-TT showed a distinct crystalline peak of lamellar ordering (100) with a slight π-stacking (010) peak, as well as PC71BM aggregation peak. This indicated a three phase system existed in the blend: crystalline BDT-BT-TT, crystalline PC71BM and a mixed phase. When switching the solvent from CF to DCB, no PC71BM streak was found for both blend films, suggesting it was completely amorphous and fully mixed at the molecular level for both species.

Figure 5. Tapping Mode AFM phase images of BDT-TT-BT (a, b) and BDT-BT-TT (c, d) prepared from chloroform (a, c) and o-dichlorobenzene (c, d)


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Figure 6. 2D GIWAXS pattern of (a) BDT-TT-BT neat film drop-casted from CF, (b) BDT-TTBT neat film drop-casted from DCB, (c) BDT-TT-BT neat film spin-casted from CF, (d) BDTTT-BT neat film spin-casted from DCB, (e) BDT-BT-TT neat film drop-casted from CF, (f) BDT-BT-TT neat film drop-casted from DCB, (g) BDT-BT-TT neat film spin-casted from CF, (h) BDT-BT-TT neat film spin-casted from DCB, (i) BDT-TT-BT and PC71BM blend film spincasted from CF, (j) BDT-TT-BT and PC71BM blend film spin-casted from DCB, (k) BDT-BTTT and PC71BM blend film spin-casted from CF, (l) BDT-BT-TT and PC71BM blend film spincasted from DCB. (All the films prepared were annealed at 90 oC for 10 minutes) Photovoltaic Properties and Mobility. The photovoltaic performance of BDT-TT-BT and BDT-BT-TT was tested using PC71BM as the electron acceptor with a conventional structure of ITO/ PEDOT:PSS/ [BDT-TT-BT or BDT-BT-TT]: PC71BM/ Ca/ Al under one sun illumination (AM 1.5G, 100mW cm-2). The light absorption layer was carefully optimized by changing the small molecule-PC71BM ratio, film thickness and solvents. The samples were prepared in chlorobenzene with various weight ratios between donor and acceptor materials and stirred for at least 12 hours at 60 oC to ensure all the materials were solubilized. As shown in Table S2, an increase of PCE was observed at first and then it dropped with increasing percentage of PC71BM, consistent with previously published data.39, 49 The best


16

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ratio was found to be 1 to 2 with a total concentration of 30 mg/mL in chlorobenzene (CB) and an average efficiency of 2.50% was obtained for BDT-TT-BT. In terms of BDT-BT-TT, the best ratio was found to be 1 to 1.5 with an average efficiency of 3.46%. After finding out the best ratio for BDT-TT-BT and BDT-BT-TT, the thickness of the active layer was varied by changing the spin rate. Table S3 and Table S4 show the photovoltaic performance of these two molecules with different active layer thicknesses. The best efficiency of 3.21% was acquired for BDT-TTBT at spin rate of 3750 rmp with a thickness of around 60 nm. The average parameters for the devices made from BDT-TT-BT are summarized in Table 2 with a Voc of 0.97 V, Jsc of 7.66 mA cm-2, FF of 0.39 and PCE of 2.87%. The best device gave the highest Voc of 0.98 V and PCE of 3.22%. As shown in Table 2 and Figure 7 (a), the best efficiency for BDT-BT-TT is 3.71% with an average of 3.50%. The BDT-BT-TT has a smaller Voc of 0.88 V when compared with BDTTT-BT. This is consistent with the HOMO levels measured from CV. However, BDT-BT-TT has a relatively larger Jsc when compared with BDT-TT-BT, which is probably due to the smaller bandgap and broader absorption of BDT-BT-TT. The solar cell devices fabricated from BDTBT-TT had relatively higher FF, which could be due to the higher hole mobility and better interpenetrated network morphology, which will be discussed later. The best photovoltaic performance of these two molecules was achieved in very thin films compared with other reported solution processed small molecules and polymers. This might be due to the high possibility of the recombination effect in the active layer.50-51 The effect of solvent was also studied by changing the solvent from CB to CF. After optimization, a decrease of PCE (2.59%) was observed for BDT-TT-BT which was due to lower Jsc (6.67 mA cm-2) and Voc (0.93 V) values. Comparing with BDT-TT-BT, the device fabricated from BDT-BT-TT in CF had similar photovoltaic performance in contrast to the device made


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from CB. When DCB was used as the solvent, the PEDOT:PSS surface was not uniformly covered and no devices were successfully fabricated. Additionally, 1,8-diiodooctance and 1chloronaphthlene were checked as additives, and no improvement was obtained. The external quantum efficiency (EQE) of the optimal device of BDT-TT-BT and BDT-BTTT is displayed in Figure 7 (b). The photo response across the wavelength from 300 nm to 750 nm was observed when the BDT-TT-BT and BDT-BT-TT was blended with PC71BM in CB. From the EQE spectrum, BDT-TT-BT has a slightly higher efficiency in the range of 400 nm to 550 nm with highest EQE of 59% at 480 nm. However, from 550nm to 750nm, BDT-BT-TT has broader absorption which results in a higher EQE value compared with BDT-TT-BT. This is consistent with the absorption spectrum of the two molecules. For the devices fabricated from CF, lower EQE across the spectrum was found for BDT-TT-BT while BDT-BT-TT showed similar light response. This is consistent to the close Jsc value obtained from J-V measurements. Table 2. Photovoltaic characterization of BDT-TT-BT and BDT-BT-TT with different active layer thickness Compound

Solvent

Ratio

a

CB

1: 2.0

BDT-BT-TTb

CB

1: 1.5

BDT-TT-BTc

CF

1: 2.0

BDT-TT-BT

Voc (V) 0.97 ± 0.02 (0.98) 0.89 ± 0.01 (0.90) 0.93 ± 0.01 (0.93)

Jsc (mA/cm2) 7.66 ± 0.48 (8.07) 8.80 ± 0.33 (9.17) 6.67 ± 0.19 (6.82)

Jsce (mA/cm2) 7.80 9.06 6.69

FF 0.39 ± 0.02 (0.41) 0.45 ± 0.01 (0.46) 0.41 ± 0.01 (0.42)

PCE (%) 2.87 ± 0.26 (3.22) 3.50 ± 0.20 (3.71) 2.56 ± 0.04 (2.59)

Thickness(nm) 80.9 61.3 90.2

0.88 ± 0.01 8.76 ± 0.25 0.43 ± 0.01 3.34 ± 0.10 8.79 96.5 (0.89) (9.04) (0.01) (3.45) a 30 mg/mL in CB, spin-coated at 3750 rpm, b 20 mg/mL in CB, spin-coated at 1750 rpm, c 15 mg/mL in CF, spin-coated at 4000 rpm, d 10 mg/mL in CF, spin-coated at 1500 rpm, e Integrated Jsc from EQE spectrum. BDT-BT-TTd

CF

1: 1.5


18

2

14 12 10 8 6 4 2 0 -2 -4 -6 -8 -10 -12 -14 -0.2

a) BDT-TT-BT(CB) BDT-BT-TT(CB) BDT-TT-BT(CF) BDT-BT-TT(CF)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage (V)

60

b)

BDT-TT-BT(CB) BDT-BT-TT(CB) BDT-TT-BT(CF)

50

EQE (%)

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


Current Density (mA/cm )

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BDT-BT-TT(CF)

40 30 20 10 0 300

350

400

450

500

550

600

650

700

750

800

Wavelength (nm)

Figure 7. (a) Typical J-V curve of BDT-TT-BT and BDT-BT-TT fabricated from CB or CF with optimized conditions. (b) EQE spectrum of BDT-TT-BT or BDT-BT-TT and PC71BM blend from the optimized devices The hole mobilities of BDT-TT-BT and BDT-BT-TT were measured by the space charge limited current method (SCLC) using Mott-Gurney Law and curves were fitted linearly in the


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space charge region as shown in Figure S5. Synthesized small molecules were tested in the configuration of ITO/PEDOT:PSS/ [BDT-TT-BT or BDT-BT-TT]/ Al. The mobilities of 7.2×106

cm-2 V-1 s-1 and 1.5×10-5 cm-2 V-1 s-1 were obtained separately for BDT-TT-BT and BDT-BT-

TT. The mobility of BDT-BT-TT is twice that of BDT-TT-BT, which may be attributed to the relatively higher crystallinity of BDT-BT-TT.

Figure 8. TEM images of the active layers with (a) BDT-TT-BT/PC71BM prepared from CB, (b) BDT-BT-TT/PC71BM prepared from CB, (c) BDT-TT-BT/PC71BM prepared from CF, (d) BDT-BT-TT/PC71BM prepared from CF Morphology of active layers. The morphology of the blended films was examined using TEM. Figure 8 shows morphology of BDT-TT-BT/PC71BM and BDT-BT-TT/PC71BM prepared from different solvents at optimized conditions. BDT-TT-BT active layers display distinct larger scale phase separation compared to BDT-BT-TT, which is nearly featureless. This corroborates well with GIWAXS data which showed an intermixed morphology at the molecular level with less intense reflection peak from PC71BM aggregation for BDT-BT-TT blend films prepared from CF. In the case of BDT-TT-BT, slight phase separation of the PC71BM was seen for the active layers prepared from both solvents. This intimate mixing increases the interfacial area between small


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molecules and PC71BM and should result in a large portion of generated excitons reaching the interface for charge separation. The relatively good photovoltaic performance of these two molecules was readily attributed to the large interfacial area, and better charge separation resulted in higher Jsc values in devices.52-53 4. Conclusion Solution processed small molecules for OSCs from on BDT donor and BT acceptor were synthesized. The impact of the position of the BT unit revealed the importance of the sequence of each unit in designing molecules for OSCs. These two isomers, BDT-TT-BT and BDT-BT-TT, have comparable photovoltaic performance with the difference lying in the Voc and Jsc. BDT-TTBT has relatively lower lying HOMO energy levels and results in a higher Voc with an average of 0.97 V compared with the Voc of BDT-BT-TT, which is 0.89 V on average. However, the higher Jsc of BDT-BT-TT is due to the broader absorption compared with BDT-TT-BT. As a result, the highest PCE achieved for BDT-TT-BT and BDT-BT-TT are 3.22% and 3.71%, respectively. In addition, these two molecules showed different thermal properties and molecular packing in thin film. From the morphology study, different domain sizes were observed for these two molecules when blending with PC71BM. In summary, our results point out a strategy to carefully design the materials for OSCs by considering the position of the acceptor moiety.

ASSOCIATED CONTENT Detailed synthetic procedure for all the compounds, TGA, DSC, PESA, additional device characterization, SCLC curve, 1H and

13

C NMR spectrum can be found in supporting

information. This material is available free of charge via the Internet at http://pubs.acs.org


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected].

Present Addresses

∥ P.S. Huang’s present address: Department of Materials Science and Engineering, University of Wisconsin-Madison, 1500 Engineering Drive, WI53706 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by NSF (DMR-1505950), Welch Foundation (AT-1740) and NIH (DMR1505950). We also thank the NSF-MRI grant (CHE-1126177) for supporting the Bruker Avance III 500 NMR. Reference 1.

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of Small Molecules by a Combination of Linear and Branched Alkyl Side Chains. ACS Nano 2014, 8, 5988-6003. 46. Chen, S. H.; Wu, Y. H.; Su, C. H.; Jeng, U.; Hsieh, C. C.; Su, A. C.; Chen, S. A. Cold Crystallization of Poly(9,9-di-n-octyl-2,7-fluorene). Macromolecules 2007, 40, 5353-5359. 47. Huang, P.; Du, J.; Gunathilake, S. S.; Rainbolt, E. A.; Murphy, J. W.; Black, K. T.; Barrera, D.; Hsu, J. W. P.; Gnade, B. E.; Stefan, M. C.; Biewer, M. C. Benzodifuran and Benzodithiophene Donor-Acceptor Polymers for Bulk Heterojunction Solar Cells. J. Mater. Chem. A 2015, 3, 6980-6989. 48. Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Spectral Engineering in π-Conjugated Polymers with Intramolecular Donor−Acceptor Interactions. Acc.Chem. Res. 2010, 43, 13961407. 49. Ni, W.; Li, M.; Liu, F.; Wan, X.; Feng, H.; Kan, B.; Zhang, Q.; Zhang, H.; Chen, Y. Dithienosilole-Based Small-Molecule Organic Solar Cells with an Efficiency over 8%: Investigation of the Relationship between the Molecular Structure and Photovoltaic Performance. Chem. Mater. 2015, 27, 6077-6084. 50. Kirchartz, T.; Agostinelli, T.; Campoy-Quiles, M.; Gong, W.; Nelson, J. Understanding the Thickness-Dependent Performance of Organic Bulk Heterojunction Solar Cells: The Influence of Mobility, Lifetime, and Space Charge. J. Phys. Chem. Lett. 2012, 3, 3470-3475. 51. Bartesaghi, D.; Perez, I. d. C.; Kniepert, J.; Roland, S.; Turbiez, M.; Neher, D.; Koster, L. J. A. Competition Between Recombination and Extraction of Free Charges Determines the Fill Factor of Organic Solar Cells. Nat. Commun. 2015, 6, 7083.


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Abstract Graphic

2

Current Density (mA/cm )

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BDT-TT-BT

14 12 10 8 6 4 2 0 -2 -4 -6 -8 -10 -12 -14 -0.2

BDT-TT-BT(CB) BDT-BT-TT(CB) BDT-TT-BT(CF) BDT-BT-TT(CF)

0.0

0.2

BDT-BT-TT

0.4

0.6

0.8

1.0

1.2

Voltage (V)


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Systematic Investigation of Benzodithiophene ... - ACS Publications

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