Dithienobenzodithiophene-Based Small Molecule Organic Solar Cells

Article

Dithienobenzodithiophene–based small molecule organic solar cells with over 7% efficiency toward processing additive- and thermal annealing-free Hyeng Gun Song, Yu Jin Kim, Ji Sang Lee, Yun-Hi Kim, Chan Eon Park, and Soon-Ki Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11297 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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ACS Applied Materials & Interfaces

Dithienobenzodithiophene–based small molecule organic solar cells with over 7% efficiency toward processing additive- and thermal annealing-free

Hyeng Gun Song1‡, Yu Jin Kim2‡, Ji Sang Lee,1 Yun-Hi Kim3*, Chan Eon Park2* and

Soon-

Ki Kwon1*

1

Department of Materials Engineering and Convergence Technology, Gyeongsang National

University, Jinju, 528-28, Republic of Korea 2

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang

University of Science and Technology, Pohang, 790-784, Republic of Korea 3

Department of Chemistry and RIGET, Gyeongsang National University, Jinju, 528-28,

Republic of Korea

‡

These authors are equally contributed.

ACS Paragon Plus Environment


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ABSTRACT In here, we introduce a novel small molecule based on dithienobenzodithiophene and rhodanine, DTBDT-Rho, developed to study the effect of rhodanine substitutuent on small molecule bulk heterojunction (BHJ) solar cells. DTBDT-Rho possesses distinct crystalline characteristics, sufficient solubility in chlorinated solvents, and broad absorption properties. Therefore, solution-processed BHJ photovoltaic cells made up with DTBDT-Rho:PC71BM blends showed an extremely high power conversion efficiency (PCE; 7.10%), notably, this PCE value was obtained without the use of additives or thermal treatments. To our knowledge, the PCE over 7% is a significantly powerful value among rhodanine-based small molecule BHJ solar cells without additives or thermal treatments.

KEYWORDS Dithienobenzodithiophene, Rhodanine, Small molecule solar cell, Power conversion efficiency, Without additives or thermal treatments


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INTRODUCTION Solution-processed small molecule organic bulk heterojunction (BHJ) solar cells have been attracted because of their advantages comparable to that of conjugated polymers.1-7 However, published reports of small molecule OPVs over 9% PCE are still rare.

8-9

Only a few

molecular donors have been reported in devices which reached this benchmark.7, 10-11 Most small molecule organic solar cells (SMOSC) showed high efficiency with additional treatment of the active layer, for example, the use of additives, solvent vapor treatment, and/or thermal annealing to optimize the morpholology and donor/acceptor interpenetrating network.12-15 However, these additional treatments limit the ability to fabricate large area flexible SMOSCs.16-18 Recently, we reported on a dithienobenzodithiophene (DTBDT)-based SMOSC and polymer solar cell. Large heteroacenes are promising moieties for high-performance OSCs because of their enhanced charge carrier mobility and narrow energy band gap. The expanded effective conjugation of heteroacenes reduces the reorganization energy in the moieties, and facilitates exciton separation into free charge carriers; furthermore, their high crystallinity can facilitate well-percolated phase separation, and efficient charge transport.19-20 The introduction of a two-dimensional conjugated DTBDT with dialkylthiophene substituents as the central unit is promising for photovoltaic optimization.21-23 Recently, a novel electron withdrawing unit of rhodanine was widely reported that demonstrated a broad absorption spectra and strong electron affinity. As such, many research groups have used it in molecules, in particular, at the end position in compounds due to its superior small molecule properties and potential application to photovoltaic diodes.24-27 In this study, a new donor material, DTBDT-Rho, with an a dithienobenzpo[1,2-b:4,5b']dithiophene (DTBDT) core as a large heteroacene, alkyl terthiophene as the linker groups and rhodanine peripheral units as acceptor, was synthesized and used in OPV devices.



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Solution-processed small molecule organic photovoltaics cells made up with DTBDTRho:PC71BM blends showed extremely high PCE, exceeding 7% without the use of additives or thermal treatments.

RESULTS AND DISCUSSION Synthesis and Characterization of DTBDT-Rho The synthesis scheme of DTBDT-Rho are displayed in Scheme 1. More detailed information on DTBDT-Rho is described in the Experimental section. The final target DRBDT-Rho compound was characterized by proton nuclear magnetic resonance spectroscopy (1H-NMR), carbon-13 NMR (13C-NMR), and mass spectroscopy; these results are presented in the Experimental section. DTBDT-Rho has sufficient solubility in various organic solvents including chloroform (CF), chlorobenzene, and 1,2-dichlorobenzene; high-quality films of DTBDT-Rho can be obtained through spin-coating from solutions, demonstrating their suitability for solution processing.28 Thermogravimetric anlaysis (TGA) showed that the synthesized small molecule possessed sufficient thermal stability, having a 5% weight loss over 380°C under nitrogen atmosphere (see Figure 1a). In addition, differential scanning calorimetry (DSC) (Figure 1b) analysis for DTBDT-Rho revealed a clear melting point (Tm, 227°C) during the heating process and recrystallization (Tc, 245°C) during the cooling process, implying that the DTBDT-Rho has a tendency to crystallize.29

Opto-physical properties of DTBDT-Rho Figure 1c represents UV-vis absorption spectra of DTBDT-Rho in CF solution and the thin film. The corresponding opto-physical data were summarized in Table 1. Generally, the small molecule, composed of an A-D-A type molecular structure, exhibits a strong, broad


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absorption in 300 - 700 nm.30 In CF solution, the UV-Vis band had a maximum absorption at 510 nm. In the solution absorption spectra showed two clear absorption points with a maximum peak at approximately 345 nm, attributed to the π-π* transition.9 For thin-film states, the absorption spectrum of the neat film is broadened considerably and 70 nm redshifted compared to its solution, which indicates ordered structure and strong π-π stacking effect by enhanced intermolecular interaction.

1, 31

The optical bandgap of 1.67 eV was

calculated from the absorption edge (749 nm) of the DTBDT-Rho compound. Cyclic voltammetry (CV) was used to study the electrochemical properties of DTBDT-Rho. The oxidation and reduction onset values of DTBDT-Rho were measured to be 0.76 and -1.21 V, respectively, as shown in Figure 1d and Table 1. The highest occupied molecular orbital (HOMO) energy level of our small molecule as -5.11 eV; the lowest unoccupied molecular orbital (LUMO) level was -3.14 eV, giving an electrochemical bandgap of 1.97 eV.32 This small difference between electrochemical and the optical bandgaps may be caused by the presence of an energy barrier at the interface between the electrode surface and the small molecule film.1, 33

DTBDT-Rho based photovoltaic performances The solution processed BHJ small molecule organic solar cells were fabricated using the conventional

ITO/PEDOT:PSS/DTBDT-Rho:PC71BM/LiF/Al

structure

to

study

the

photovoltaic characteristics. The detailed device fabrication represented in the Experimental section. To find the optimized conditions for high-performance hetero photoactive layers, we first investigated the DTBDT-Rho:PC71BM solar cell properties as a function of blend solution times (Figure 2a and Table 2). The blend solution ratio was determined to 1:0.8 donor:acceptor w/w, as most small molecule solar cells have demonstrated high performance with the 1:0.8 blend composition.34-36 We also investigated DTBDT-Rho:PC71BM



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performance with respect to the solution stirring time ranging from 6 – 12 h in 0.5 h intervals in D:A 1:0.8 w/w, total concentration of 15 mg ml-1, and CB solvent conditions. The PCE of 1.89 % was recorded for the 1:0.8 active solution, blended for 10.5 h (open-circuit voltage, Voc ; 0.69 V, short-circuit current, Jsc ; 5.81 mA cm-2 and fill factor, FF ; 47.3 %). Next, we tested the DTBDT-Rho:PC71BM device with respective to various host solvents in blend active solutions with 10.5 h stirring. Solar cells processed with CF showed higher performance reaching 3.22 % efficiency. (Figure 2b and Table 3). In blend solutions, the total concentration of the solution is quite important as well as the solution times and solvents used. For this reason, DTBDT-Rho:PC71BM blend actives were investigated using various solution concentrations (10, 15, 20, and 30 mg mL-1), as shown in Table 4 and Figure 3a. For a ratio of donor : acceptor (1:0.8) w/w, a 10.5 h stirring time, CF solvent and a solution concentration of 30 mg/mL, the PCE improved to 5.01 % ( Voc ; 0.81 V, Jsc ; 15.1 mA cm-2, FF ; 40.9 %). Adjustment of active film thickness improved the PCE slightly, 5.42% (Figure 3b and Table 5) (at that time, the active blend is CF solution dissolved to D:A 1:0.8 w/w and 30 mg ml-1 concentration with 10.5h stirring time and spin-cast to 93 nm). Through the sub-divided different conditions in the active layer, we successfully controlled the heterojunction film, and thus the solar cell efficiency significantly increased from ~1% to over 5%. When we carefully confirmed device performance parameters (i.e., Voc, Jsc and FF), the Voc value is slightly varied but photo-current and FF factors are largely changed. In particularly, the Jsc value is dramatically varied at all different conditions. In Voc factors, a small change amount is possibly due to photocurrent, Jsc, because the Voc is significantly related to follow equation37-38: ≈

∆ ln +

2

Here, is the device ideality factor, is the Boltzmann’s constant, is the absolute


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temperature, ∆ is a difference between the donor (HOMO energy) and acceptor (LUMO energy) level.15 is the reverse-bias saturation current, which is dominated by material properties, the carrier generation and recombination rate. For Jsc and FF factors, we believe that these values are associated with active nanoscale morphology, packing structure and corresponding to their charge carrier mobility.39-40 Thus, we will discuss them below. Finally, we then applied the cathode interlayer to the active layer of the DTBDT-Rho-based device via a solution-processed interlayer spin-cast method. The results are summarized in Figure 4a and Table 6. A poly[(9,9-bis(3’-(N,N-dimethlamino)propyl-2,7-fluorene)-alt-2,7(9,9-dioctylfulorene))] (PFN) /Al bilayer cathode increased the DTBDT-Rho:PC71BM device efficiency to 6.48 %. Interestingly, application of a ZnO nanoparticles/Al bilayer to the cathode, resulted in the highest performance for the device: specially, for Voc, Jsc, and FF values approaching 0.81V, 17.2 mA cm-2, and 51.0 %, respectively, a PCE of 7.10 % was obtained. Thus, in DTBDT-Rho-based solar cells, the solution-processed interlayer leads to a distinctly positive effect. Figure 4b represents the external quantum efficiency (EQE) of photovoltaic cells using DTBDT-Rho:PC71BM (1:0.8) blend ratios, 10.5 h stirring time, CF solvent, and 30 mg mL-1, and 93 nm thick active layer and a ZnO NPs/Al bilayer cathode. The EQE values for this solar cell were near 70% over the range of 400 - 700 nm wavelength.

Photo-active layer film and other device characteristics Atomic force microscopy (AFM) was measured to investigate the DTBDT-Rho:PC71BM photo-active layer film, in an attempt to better understand the surface morphology. Figure 5a, shows a homogeneous, uniform surface topography, with a root mean square (RMS) roughness of 1.38 nm; aggregated domains visible in small areas were possibly attributed to the strong intermolecular interaction of DTBDT-Rho compounds.41-42



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To clearly confirm whether the aggregated domains is due to crystalline structures of DTBDT-Rho or not, we investigated the molecule’s packing structures using two dimensional-grazing incidence wide angle X-ray scattering (2D-GIWAXS). From the 2DGIWAXS pattern shown in Figure 5b, DTBDT-Rho adopt highly ordered edge-on orientation (up to (004) scattering peak, qz ≈ 0.22 Ǻ-1) with respect to substrate in the DTBDTRho:PC71BM blend. Furthermore, the crystalline domains for DTBDT-Rho compounds also appear horizontal direction with respect to substrate (Detailed peak position and intensity are shown in 1D extracted profile).43-44 This result indicates that DTBDT-Rho small molecule has efficiently ordered crystalline structures compared to previously other reported small molecules.45-47 The charge transport properties of DTBDT-Rho:PC71BM blend films were studied by estimating the single-charge carrier mobility from space charge-limited current density (SCLC) measurement. The mobility of hole and electron were investigated in a hole-only device, [ ITO/PEDOT:PSS/DTBDT-Rho:PC71BM/Au (100 nm)] and an electron-only device, [Al (110 nm)/DTBDT-Rho:PC71BM/Al (110 nm)], respectively. The SCLC was estimated by the Mott-Gurney equation, J= 9 V2/8L3, in which J is the dark current density, is the dielectric constant, is the permittivity of free space, is the hole mobility, V is the internal voltage of the device, and L is the film thickness of the active layer (93 nm).16, 48 The hole and electrom mobilities of DTBDT-Rho were 5.24 ×10-4 cm2 V-1 s-1 and 2.78 ×10-4 cm2 V-1 s-1in the hole-only and electron-only diodes, respectively as depicted in Figure 5c. The carrier mobility characteristics of our small molecule appear to be higher than that of a compound, DR3TBDTT (composed of benzodithiophene and rhodanine units, previously reported by Chen et al., owing to more rigid dithienobenzodithiophene groups.17,49 The bimolecular recombination in DTBDT-Rho:PC71BM devices was assessed by measuring the dependence of Jsc on light intensity.50 This analysis proved insight into the


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dominant recombination mechanism for devices, as exciton quenching in the photoactive layer is closely related to charge recombination kinetics. The relationship between Jsc and light intensity can be described as JSC ∝ light intensityα; α is unity when the bimolecular recombination is negligible.50-51 According to Figure 5d, the logarithmic plots of Jsc versus light intensity show a slope of 0.934 for our DTBDT-Rho:PC71BM device; an α value of 0.934 means that bimolecular recombination is significantly suppressed in the working device.52

Conclusions In summary, a new donor material, DTBDT-Rho, with a dithienobenzpo[1,2-b:4,5b']dithiophene (DTBDT) core as a large heteroacene, alkyl terthiophene as the linker groups and rhodanine peripheral units as acceptors, was synthesized and used in solution-processable OSCs. These DTBDT-Rho showed excellent thermal stability with high crystallinity, broad absorption, relatively low HOMO levels, and superior charge mobility. The solutionprocessed organic small molecule BHJ photovoltaic cells made up with DTBDTRho:PC71BM blends showed an extremely high power conversion efficiency (PCE; 7.10%), notably, this PCE value was obtained without the use of additives or thermal treatments. To our knowledge, the PCE over 7% is a significantly powerful value among rhodanine-based small molecule solar cells.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed experimental procedures for material synthesis, general measurement and device



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fabrication and characterization. Two supporting figures (1H-NMR and

13

C-NMR for

DTBDT-Rho) are also added.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This publication is based upon work supported by the National Research Foundation grant funded by MSIP (2015R1A2A1A10055620) and a New & Renewable Energy of the KETEP grant

funded

by

the

Korean

government

Ministry

of

Knowledge

Economy

(20123010010140).

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137, 2231-2234. 34. Cui, C.; Guo, X.; Mun, J.; Guo, B.; Cheng, X.; Zhang, M.; Brabec, C. J.; Li, Y. HighPerformance Organic Solar Cells Based on a Small Molecule with Alkylthio-Thienyl-Conjugated Side Chains without Extra Treatments, Adv. Mater. 2015, 27, 7469-7475.


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35. 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. 36. Wang, Z.; Xu, X.; Li, Z.; Feng, K.; Li, K.; Li, Y.; Peng, Q. Solution-Processed Organic Solar Cells with 9.8% Efficiency Based on a New Small Molecule Containing a 2D Fluorinated Benzodithiophene Central Unit, Adv. Electron. Mater. 2016, DOI:10.1002/aelm.201600061.

37. Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. Molecular and Morphological Influences on the Open Circuit Voltages of Organic Photovoltaic Devices. J. Am. Chem. Soc. 2009, 131, 9281-9286. 38. Kim, Y. J.; Park, C. E. The Impact of P(NDI2OD-T2) Crystalline Domains on the OpenCircuit Voltage of Bilayer All-Polymer Solar Cells with an Inverted Configuration. APL Mater. 2015, 3, 126105. 39. Hoppe, H.; Sariciftci, N. S. Morphology of Polymer/Fullerene Bulk Heterojunction Solar Cells. J. Mater. Chem. 2006, 16, 45-61. 40. Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chem. Rev. 2014, 114, 7006-7043. 41. Weintraub, M. T.; Xhakaj, E.; Austin, A.; Szarko, J. M. The effects of donor:acceptor intermolecular mixing and acceptor crystallization on the composition ratio of blended, spin coated organic thin films, J. Mater. Chem. C 2016, 4, 7756-7765.

42. Kim, Y. J.; Jang, W.; Ahn, S.; Park, C. E.; Wang, D. H. Dramatically Enhanced Performances and Ideally Controlled Nano-Morphology via Co-Solvent Processing in Low Bandgap Polymer Solar Cells, Org. Electron. 2016, 34, 42-49.

43. Müller-Buschbaum, P. The Active Layer Morphology of Organic Solar Cells Probed with Grazing Incidence Scattering Techniques. Adv. Mater. 2014, 26, 7692-7709.



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44. Chen, W.; Nikiforov, M. P.; Darling, S. B. Morphology Characterization in Organic and Hybrid Solar Cells. Energy Environ. Sci. 2012, 5, 8045-8074. 45. Min, J.; Jiao, X.; Ata, I.; Osvet, A.; Ameri, T.; Bäuerle, P.; Ade, H.; Brabec, C. J. TimeDependent Morphology Evolution of Solution-Processed Small Molecule Solar Cells during Solvent Vapor Annealing. Adv. Energy Mater. 2016, 6, 1502579. 46. Kim, Y. J.; Park, C. E. Following the Nanostructural Molecular Orientation Guidelines for Sulfur versus Thiophene Units in Small Molecule Photovoltaic Cells. Nanoscale 2016, 8, 7654-7662. 47. Moon, M.; Walker, B.; Lee, J.; Park, S. Y.; Ahn, H.; Kim, T.; Lee, T. H.; Heo, J.; Seo, J. H.; Shin, T. J.; Kim, J. Y.; Yang, C. Dithienogermole-Containing Small-Molecule Solar Cells with 7.3% Efficiency: In-Depth Study on the Effects of Heteroatom Substitution of Si with Ge. Adv. Energy Mater. 2015, 5, 1402044. 48. Huang, J.; Carpenter, J. H.; Li, C. –Z.; Yu, J. –S.; Ade, H.; Jen, A. K. –Y. Highly Efficient Organic Solar Cells with Improved Vertical Donor–Acceptor Compositional Gradient Via an Inverted Off-Center Spinning Method, Adv. Mater. 2016, 28, 967-974.

49. Feng, G.; Xu, Y.; Zhang, J.; Wang, Z.; Zhou, Y.; Li, Y.; Wei, Z.; Li, C.; Li, W. All-SmallMolecule Organic Solar Cells Based on an Electron Donor Incorporating Binary ElectronDeficient Units, J. Mater. Chem. A 2016, 4, 6056-6063.

50. Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Gratzel, M. Understanding the Rate-Dependent J–V Hysteresis, Slow Time Component, and Aging in CH3NH3PbI3 Perovskite Solar Cells: the Role of a Compensated Electric Field, Energy Environ.

Sci. 2015, 8, 995-1004. 51. Proctor, C. M.; Nguyen, T. –Q. Effect of Leakage Current and Shunt Resistance on the Light Intensity Dependence of Organic Solar Cells, Appl. Phys. Lett. 2015, 106, 083301.

52. Yang, Y.; Chen, W.; Dou, L.; Chang, W. –H.; Duan, H. –S.; Bob, B.; Li, G.; Yang, Y.


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High-Performance Multiple-Donor Bulk Heterojunction Solar Cells, Nat. Photonics 2015, 9,

190-198.



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Page 18 of 30

C10H21 C10H21

C10H21 C10H21 S

C10H21

O

S

S

S S

S S O

S S

S S

n-BuLi / THF S

Sn

SnCl

C10H21

SnCl2

S

n-BuLi / THF

S

S

S C10H21 C10H21

C10H21 C10H21

2

1 Sn S

POCl 3 / DMF

Pd(PPh3)4 / Toluene

3

1,2-dichloroethane

C10H21 C10H21 Sn

S S

S

S

O

C8H17 C8H17

S S

S

C8H17

S

S

N

C10H21 C H 10 21 S

S

S

S

S

C8H17 S

S S C10H21 C10H21

Pd2(dba)3 / Tri-o-tolylphosphine Sn

Toluene

S C10H21

S

C10H21

C10H21 S

S S

Chloroform S

S S

C8H17

C10H21 S

O

7

Scheme 1. Synthetic routes of DTBDT-Rho by Stille coupling reaction.


S

C10H21

S S C8H17 C8H17

C8H17

2

C10H21

S

Piperidine / 3-ethylhodanine

S S

S

O

S

6

O

C8H17

Chloroform

5

C8H17 S

S C8H17 C8H17

O

C8H17 C8H17

4

O Br

S

S

C8H17 C8H17

S

NBS

S

S C8H17

Br

S

S

Br

Acetic acid

C8H17

S Sn

S

NBS

Sn

S

8

S S

S

N O

Page 19 of 30

nd

100

2 Heating Cooling st 1 Heating

o

Td,5% = 383 C Heat Flow (W/g)

Weight (%)

80 60 40 20

(a)

0

100

(b) 200

300

400

500

600

700

800

50

100

o

Temperature ( C)

150

200

250

o

Temperature ( C)

DTBDT-Rho Sol. DTBDT-Rho Film.

1.0

DTBDT-Rho

(d)

0.8 Current (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


0.6 0.4 0.2

(c) 0.0 300

400

500

600

700

800

900

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

+

Wavelength (nm)

Potential (V) vs. Ag/Ag

Figure 1. (a) TGA curve (b) DSC traces (c) UV-vis absorption spectra (normalization) in chloroform and in thin film. (d) Cyclic voltammetry curve of DTBDT-Rho.



2 0

6h 7.5 h 9h 10.5 h 12 h

0

-4 -6

(a) -0.2

0.0

(b)

2

-2

-8 -0.4

Current density (mA/cm )

4 Current density (mA/cm2)

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 20 of 30

0.2

0.4

0.6

0.8

1.0

-4 -8 TCB DCB CB CF

-12 -16 -0.4

-0.2

0.0

Voltage (V)

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 2. J-V curve characteristics of photovoltaic cells using DTBDT-Rho in various conditions: (a) solution stirring times and (b) host solvents (the blend solution has D:A 1:0.8 w/w, total concentration of 15 mg ml-1 and CB solvent conditions for Figure (a)).


Page 21 of 30

-4

10 mg/ml 15 mg/ml 20 mg/ml 30 mg/ml

0 2

-8 -12

(a)

-16 -0.4

-0.2

0.0

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


0.2

0.4

0.6

0.8

1.0

-4

75 nm 93 nm 110 nm 132 nm

-8 -12 -16 -0.4

(b) -0.2

0.0

Voltage (V)

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 3. J-V characteristics of the DTBDT-Rho donor in bulk heterojunction devices with PC71BM in various solution concentration (a) and photo-active film thickness (b).



-4

80

LiF/Al PFN/Al ZnO NPs/Al

-8 -12

40

20

-16 -20 -0.4

(b)

60

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

Page 22 of 30

(a) -0.2

0.0

0.2

0.4

0.6

Voltage (V)

0.8

1.0

ITO/PEDOT:PSS/DTBDT-Rho:PC71BM

0 300

/ZnO NPs/Al device

400

500

600

700

800

Wavelength (nm)

Figure 4. (a) J-V illuminated characteristics of DTBDT-Rho-based small molecule solar cells using different cathode interlayer and (b) EQE curve at the highest device performance (the optimized active condition is D:A 1:0.8 w/w, 30 mg ml-1 concentration, CF solvent, 93 nm thickness and 10.5h stirring time).


Page 23 of 30

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Figure 5. (a) AFM surface image of optimized DTBDT:PC71BM photo-active layer, (b) 2DGIWAXS patterns (left side) and 1D extracted profile (out-of-plane and in-plane direction) (right side), (c) dark J-V curves of the hole-only (red line) and electron-only (blue line) DTBDT-based devices and (d) dependence of Jsc on the light intensity of DTBDT:PC71BM solar cells.



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 30

Table 1. Thermal, optical and electrochemical characteristics of a newly synthesized small molecule, DTBDT-Rho.

Compound DTBDT-Rho

Td

λmax (nm)

(°C)

solution

383

510

ox

λmax (nm) λonset (nm) E onset film film (eV) 585,636

740


0.76

red

EHOMO

ELUMO

(eV)

(eV)

(eV)

-1.21

-5.11

-3.14

Eonset

Page 25 of 30

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Table 2. Photovoltaic performance of small molecule solar cells based on the DTBDTRho:PC71BM photoactive layer as function of solution stirring times. Active layer

DTBDT-Rho : PC71BM

Time (h)

Voc (V)

Jsc 2 (mA/cm )

FF (%)

PCE (%)

6

0.66

3.93

42.6

1.10

7.5

0.61

4.94

41.7

1.26

9

0.71

4.92

45.1

1.58

10.5

0.69

5.81

47.3

1.89

12

0.65

4.12

45.5

1.23



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Page 26 of 30

Table 3. Small molecule solar cell parameters for DTBDT-Rho:PC71BM in various solvent systems.

Active layer

DTBDT-Rho : PC71BM

Solvent*

Voc (V)

Jsc 2 (mA/cm )

FF (%)

PCE (%)

TCB

0.61

3.83

41.6

0.98

DCB

0.69

4.46

40.4

1.25

CB

0.69

5.81

47.3

1.89

CF

0.76

14.1

29.2

3.22

*TCB: trichlorobenzene, DCB: dichlorobenzene, CB: chlorobenzene, and CF: chloroform


Page 27 of 30

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Table 4. PV performance of small molecule DTBDT-Rho-based photovoltaic devices using different solution concentrations.

Active layer

DTBDT-Rho : PC71BM

Concentration (mg/ml)

Voc (V)

Jsc 2 (mA/cm )

FF (%)

PCE (%)

10

0.80

6.66

41.7

2.25

15

0.79

10.8

39.8

3.42

20

0.81

13.9

35.3

4.02

30

0.81

15.1

40.9

5.01



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Page 28 of 30

Table 5. Device performance parameters for organic solar cells using DTBDT-Rho:PC71BM bulk heterojunction as a function of active film thickness.

Active layer

DTBDT-Rho : PC71BM

Thickness (nm)

Voc (V)

Jsc 2 (mA/cm )

FF (%)

PCE (%)

75

0.82

14.9

37.7

4.60

93

0.81

15.6

42.6

5.42

110

0.81

15.1

40.9

5.01

132

0.79

11.32

37.3

3.33


Page 29 of 30

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Table 6. DTBDT-Rho:PC71BM device performance parameters with different cathode interlayer.

Active layer

DTBDT-Rho : PC71BM

Interlayer*

Voc (V)

Jsc 2 (mA/cm )

FF (%)

PCE (%)

LiF

0.81

15.6

42.6

5.42

PFN

0.82

16.5

48.0

6.48

ZnO NPs

0.81

17.2

51.0

7.10

*PFN : poly[(9,9-bis(3’-(N,N-dimethlamino)propyl-2,7-fluorene)-alt-2,7-(9,9dioctylfulorene))and ZnO NPs : ZnO nano-particles.



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For Table of Contents Only


Page 30 of 30

Dithienobenzodithiophene-Based Small Molecule Organic Solar Cells

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