Following the TRMC Trail: Optimization of Photovoltaic Efficiency and

Subscriber access provided by West Virginia University | Libraries

Article

Following the TRMC Trail: Optimization of photovoltaic efficiency and structure-property correlation of thiophene oligomers Tanwistha Ghosh, Anesh Gopal, Shinji Nagasawa, Nila Mohan, Akinori Saeki, and Vijayakumar C Nair ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07508 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016

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.

ACS Applied Materials & Interfaces 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 32

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

ACS Applied Materials & Interfaces

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure-Property Correlation of Thiophene Oligomers Tanwistha Ghosh,†,‡ Anesh Gopal,§ Shinji Nagasawa,§ Nila Mohan,† Akinori Saeki,*,§ Vijayakumar C. Nair*,†,‡ †

Photosciences and Photonics Section, CSIR-National Institute for Interdisciplinary Science and

Technology (NIIST), Trivandrum 695019, Kerala, India. ‡

Academy of Scientific and Innovative Research (AcSIR), New Delhi 110 001, India.

§

Department of Applied Chemistry, Graduate School of Engineering, Osaka

University, 2-1

Yamadaoka, Suita, Osaka 565-0871, Japan. E-mail: [email protected] E-mail: [email protected]

ACS Paragon Plus Environment

1


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 32

ABSTRACT: Semiconducting conjugated oligomers having same end group (N-ethylrhodanine) but different central core (thiophene: OT-T, bithiophene: OT-BT, thienothiophene: OT-TT) connected through thiophene pi-linker (alkylated terthiophene) were synthesized for solution processable bulk-heterojunction solar cells. The effect of the incorporation of an extra thiophene to the central thiophene unit either through C-C bond linkage to form bithiophene or by fusing two thiophenes together to form thienothiophene on the optoelectronic properties and photovoltaic performances of the oligomers were studied in detail. Flash photolysis timeresolved microwave conductivity (FP-TRMC) technique shows OT-TT has significantly higher photoconductivity than OT-T and OT-BT implying that the former can outperform the latter two derivatives by a wide margin under identical conditions in a bulk-heterojunction solar cell device. However, the initial photovoltaic devices fabricated from all three oligomers (with PC71BM as the acceptor) gave power conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in the PCE of OT-T and OT-BT. On the other hand, five-fold enhancement in the device efficiency was achieved in OT-TT (PCE: 3.52%, VOC: 0.80 V, JSC: 8.74 mA cm-2, FF: 0.50), which was in correlation with the TRMC results. The structure-property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were deduced through UV-vis absorption,

atomic

force

microscopy,

bright-field

transmission

electron

microscopy,

photoluminescence quenching analysis and two dimensional grazing incidence X-ray diffraction studies.

KEYWORDS: Photovoltaics, Thiophene oligomers, Donor-acceptor systems, Time-resolved microwave conductivity, Structure-property correlation


2

Page 3 of 32

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


INTRODUCTION Solution-processed organic solar cells (OSCs) consisting of polymers or oligomers as active layers have been considered as a promising candidate for the next-generation renewable source of energy as they offer a good alternative to the conventional silicon-based solar cells.1-3 The OSCs possess advantages like low cost, light weight, and easy fabrication on a variety of substrates (including flexible) via various printing and solution techniques. With rapid progress in recent years, power conversion efficiency of 10.1% have been achieved for bulkheterojunction polymer-based organic solar cells.4 On the other hand, solution processed small molecule based OSCs have also made great advances recently,5-8 and achieved a PCE of 10.08%7 with bulk heterojunction (BHJ) architecture while over 10.1% in tandem devices.8 Besides this, they gained researchers’ interest due to the relative ease of synthesis in high purity without any batch-to-batch variations, well-defined structures and exact molecular weights over their polymer counterparts.2,9 Though significant progress have been made in the field of small molecule based OSCs, relatively low PCEs when compared to silicon based solar cells are still a major concern. This could be mainly attributed to complicated device physics,10,11 which necessitates to have a good balance between the design of the molecules, blend morphology,12-15 interfacial layer characteristics,16-18 and device architectures19,20 for obtaining the best PCEs. Organic solar cell devices mostly consist of a bulk heterojunction (BHJ) active layer having an electron donor and electron acceptor, processed from a common organic solvent. Fullerene derivatives such as PC61BM and PC71BM are generally used as electron acceptors, the latter of which is preferred over the former due to its better photoabsorption in a broad wavelength region. As the solvent evaporates, donor and acceptor phase-separate to form nanoscale domains throughout the photoactive layer. The best solar cells are those composed of domains that are


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 32

about ten nanometres in size.21,22 But both PC61BM and PC71BM would tend to aggregate and form large domains with size of ~100-200 nm.23-25 In order to prevent the formation of large domains, solution engineering through varying the solvent and by incorporating solvent additives was found to be an efficient strategy.26,27 This results in remarkable improvement in the performance of the solar cells. Apart from solution engineering, proper design of the donor molecules is also very crucial. Ideally, the donor molecules should have good solubility, broad absorption over the entire UVvisible region, suitable HOMO-LUMO energy levels with respect to acceptor molecules, good redox properties and appreciable hole mobility. Oligothiophenes are among the most investigated donor material, and have been widely used in OSCs. In addition to the above mentioned properties, relatively simple synthesis, easy access, high stability and the ability to self-assemble make oligothiophenes one of the best donor material for OSCs.28,29 Bringing together electrondeficient terminal units (acceptor) and an electron-rich central unit (donor) to form an acceptordonor-acceptor (A-D-A) structure is one of the most successful strategies to tune the absorption as well as energy levels to achieve high power conversion efficiency in oligothiophenes.30-33 Among a variety of acceptor dye units, N-ethylrhodanine is used extensively to develop donoracceptor (D-A) and acceptor-donor-acceptor (A-D-A) type semiconducting small molecules and oligomers for photovoltaic applications.7,34-37 Due to its electron withdrawing abilities, Nethylrhodanine is capable of reducing the optical band gap of associated molecules resulting in better absorption in the visible region. Moreover, it can contribute significantly towards the overall absorption of the molecules. Very recently, Chen et al. have reported an A-D-A type small molecule comprised of N-ethylrhodanine derivative as the end group, yielding a certified PCE of 10.08%.7


4

Page 5 of 32

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 the present work, a series of A-D-A type oligomers, having same end group (Nethylrhodanine) and similar pi-linker (alkylated terthiophene) but different central core (thiophene: OT-T,38 bithiophene: OT-BT, thienothiophene: OT-TT, respectively) were synthesized (Scheme 1). We have investigated how the incorporation of an extra thiophene to the central thiophene group either through C-C bond linkage (bithiophene) or by fusing two thiophenes (thienothiophene) affect the optoelectronic properties and photovoltaic performances of the oligomers. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) was used for the initial screening to find the outperformer among the three, and the device performances were optimized through solution engineering. UV-vis absorption, atomic force microscopy (AFM), bright-field transmission electron microscopy (BF-TEM), photoluminescence quenching analysis and two dimensional grazing incidence X-ray diffraction (2D-GIXRD) studies were carried out in detail to correlate the structure-property-device performance relationship.

Scheme 1. Chemical structures of OT-T, OT-BT and OT-TT molecules.


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

Page 6 of 32

RESULTS AND DISCUSSION Synthesis and photophysical characterization: The oligomers were synthesized using methods reported in literature (Scheme S1, SI) and characterized by various analytical techniques such as 1H NMR,

13

C NMR, IR and mass spectrometry. All the three derivatives

exhibited good thermal stability (Figure S1, SI) and excellent solubility in common organic solvents. Due to the presence of a push-pull molecular structure, they exhibited intense and broad absorption from 350 to 600 nm in solution, and the most intense band was assigned to the π-π* transitions. The charge transfer nature of the oligomers was evident from the red shift in emission maximum and quenching in emission intensity with increasing solvent polarity (See Figure S2-4 and Table S1 in Supporting Information for the details of optical properties in solution). Cyclic voltammetry was employed to determine the HOMO-LUMO energy levels of the oligomers (Figure S5, SI). In the film state, the HOMO energy levels of OT-T, OT-BT and OT-TT were found to be -5.37, -5.46 and -5.38 eV, respectively and the LUMO levels were 3.60, -3.69 and -3.59 eV, respectively (details are summarized in Table S2 and Figure S6, SI). Comparison of the HOMO-LUMO of the oligomers with that of PC71BM (HOMO: -6.09 eV and LUMO: -4.30 eV)39 showed that oligomers may function as donors in BHJ solar cell when blended with PC71BM acceptor. The energy offset between the LUMO levels of the molecules with that of PC71BM were good enough for efficient photoinduced intermolecular electron transfer, and subsequent charge separation at the donor-acceptor interface. Photoconductivity analysis: Prior to the fabrication of the organic solar cells, we have surveyed the potential of the BHJ active layer by a direct electrode-less, non-contact technique, viz. flash-photolysis time-resolved microwave conductivity (FP-TRMC).40-42 This technique has emerged as a robust and speedy screening method for a wide variety of semiconducting organic


6

Page 7 of 32

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


materials, which gives information about the short-range (nanometre scale) intrinsic charge carrier transport properties.43-49 Here, microwave absorption of the transient charge carriers produced by the laser excitation or pseudo-sunlight white light pulse from a Xe-flash lamp was observed, and the observation could be quantified in terms of ∆σ (photoinduced transient conductivity). Photoconductivity transients obtained from TRMC usually reflect the molecular as well as supramolecular properties, and hence are helpful for correlating the structure, property, and device performance relationships.50-52 Recently, this technique was established as a unique tool for screening materials (p-type or n-type), for optimizing factors such as donor-acceptor blend ratio, solvents, solvent additives, annealing temperatures, etc. for photovoltaic device applications.53-56 In the present work, Xe-flash lamp (pseudo solar spectrum with pulse width of 10 µs) was used as the excitation source for the TRMC experiment. Details of the experimental set up and data analysis were reported in our previous papers.49,53 The photoconductivity transient signals obtained for the oligomers and the corresponding blends with PC71BM in the film state are shown in Figure 1. Transient signal obtained for PC71BM alone is also showed for comparison. In the pristine state, all three molecules exhibited comparable values of ∆σmax (OT-T: 9.2 × 10-10 S cm-1, OT-BT: 13 × 10-10 S cm-1, OT-TT: 9.7 × 10-10 S cm-1). When blended with PC71BM (1:1 w/w), OT-T and OT-BT showed only marginal enhancement in the ∆σmax values (1.9 × 10-9 S cm-1, 2.3 × 10-9 S cm-1, respectively). Interestingly, ∆σmax of OT-TT showed about 12-fold enhancement (1.2 × 10-8 S cm-1) in the presence of PC71BM under identical conditions. This indicates that OT-TT is the best donor material among the three derivatives in combination with PC71BM for solar cell applications because our previous studies have proved that the intensity of the photoconductivity signal of donor/acceptor blends has a direct correlation with that of their


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

Page 8 of 32

photoconversion efficiency when measured under identical conditions.53,56,57 This is also in accordance with some of the literature reports which suggests that introduction of fused aromatic rings like thienothiophene are beneficial for improving optoelectronic properties of semiconducting organic materials.58,59

Figure 1. Xe-flash TRMC transients of (a) OT-T, (b) OT-BT and (c) OT-TT in the film state drop cast from chlorobenzene solution; oligomer alone (black), PC71BM alone (blue), 1:1 w/w blends of oligomers with PC71BM (red). (Pulse width of the Xe-flash lamp: 10 µs) Photovoltaic properties: BHJ based solar cells have been fabricated using the oligomers as the donor and PC71BM as the acceptor with an inverted device structure60,61 consisting of ITO/ZnO/BHJ/MoO3/Ag. The active layers consisting of donor and acceptor in 1:1 weight ratio spin-coated from chlorobenzene containing 1,8-diiodooctane (DIO) as additive. DIO is a commonly used solvent additive during the film preparation for organic solar cell devices. It is proved that the presence of DIO helps to control the agglomerate formation of fullerene aggregates in the inter-mixed donor/acceptor film, resulting in an overall enhancement in donor/acceptor intermixing. This improves the generation of free charge carriers at the donor/acceptor interphase as well as the extraction of photogenerated charges at the electrodes.26,27,39 Among the various vol% tried, the highest efficiencies were obtained from


8

Page 9 of 32

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


those devices cast from solutions containing 3 vol% DIO. OT-T:PC71BM device showed PCE of 0.69 (0.56±0.11)% (short circuit current density: JSC = 3.42 mA cm-2, open circuit voltage: VOC = 0.50 V, fill factor: FF = 0.40). The OT-BT:PC71BM device showed a PCE of 0.72 (0.68±0.13)% (JSC = 4.08 mA cm-2, VOC = 0.43 V, FF = 0.41). Similarly, OT-TT:PC71BM device exhibited a PCE of 0.72 (0.57±0.13)% (JSC = 5.94 mA cm-2, VOC = 0.34 V, FF = 0.36). J-V curves of the three devices are shown in Figure 2 and the optimization data is summarized in Table S3, SI. The external quantum efficiency (EQE) spectra of the devices were found to be consistent with their variation of short circuit current density (Figure S9a, SI). The observation of comparable PCEs for all the three oligomers was counterintuitive considering the TRMC observation which implied a significantly better performance for OT-TT.

Figure 2. J–V characteristics of the photovoltaic devices fabricated from a 1:1 w/w blend solutions of (a) OT-T:PC71BM, (b) OT-BT:PC71BM and (c) OT-TT:PC71BM in chlorobenzene + 3 vol% DIO (device structure: glass/ITO/ZnO/BHJ/MoO3/Ag). In order to get a rational explanation for the results shown in the above section, detailed atomic force microscopy (AFM) analysis and UV-vis absorption studies were carried out. Analysis of the BHJ active layer morphologies with AFM revealed that the active layers consist of large aggregates (Figure 3a-c, corresponding height images are shown in Figure S7a-c, SI).


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

Page 10 of 32

Such large aggregates are known to increase interfacial resistance resulting in poor charge collection at the electrode interface leading to low PCEs. Subsequently, UV-vis absorption properties of the oligomers were measured in solution and film states (pristine and blends; Figure 3d-f). The chlorobenzene solutions of OT-T, OT-BT and OT-TT exhibited an absorption maximum at 501, 503 and 502 nm, respectively. The corresponding spin-coated films showed relatively broad absorption with red-shifted maximum when compared to that in the solution. The absorption maximum was shifted to 596, 588 and 602 nm for OT-T, OT-BT and OT-TT, respectively. In addition to that, well-resolved shoulder peaks corresponding to the absorption of the aggregates was also observed in the red-region (OT-T: 648 nm, OT-BT: 643 nm, OT-TT: 662 nm). Observation of the well-resolved aggregate shoulders implies the ordered stacking/selfassembly of the oligomers in the film state. However, this ordered packing of the donor assembly was lost on addition of the spheroidal shaped PC71BM as evident from the blue shifting of the absorption maxima (OT-T: 475 nm, OT-BT: 470 nm, OT-TT: 477 nm) as well as the significant weakening of the aggregate absorption shoulder. The absorption features remained more or less same even in the presence of 3 vol% DIO indicating that the additive was unable to restore the donor assembly. In short, the AFM and UV-vis absorption studies revealed that blend films obtained from chlorobenzene with 3 vol% DIO consist of not only larger aggregates but also poor assembly of donor oligomers.


10

Page 11 of 32

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 3. AFM phase images of the 1:1 blend films of (a) OT-T (b) OT-BT and (c) OT-TT with PC71BM obtained from chlorobenzene + 3 vol% DIO (scale bar corresponds to 500 nm). Normalized absorption spectra of (d) OT-T (e) OT-BT and (f) OT-TT under different conditions (― chlorobenzene solution, ― pristine film from chlorobenzene, ― donor:PC71BM 1:1 blend film from chlorobenzene, ― donor:PC71BM 1:1 blend film from chlorobenzene + 3 vol% DIO). We assumed that, in order to get optimum efficiency from the blend films, it is important to reduce the aggregate size and improve donor self-assembly through solution engineering. Since chlorobenzene is a high boiling solvent (131.7 °C), it is obvious that the molecules get more time to aggregate resulting in the formation of larger aggregates. In this context, chloroform is a better solvent as it is more volatile (boiling point = 61.2 °C) and has better ability to solubilize the oligomers. As observed in the case of chlorobenzene, the oligomers were molecularly dispersed


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

Page 12 of 32

in chloroform (Figure 4a-c; λmax of OT-T = 504, OT-BT = 505 and OT-TT = 503 nm). Similarly, the films spin-coated from chloroform showed a red-shifted maximum (OT-T: 594 nm, OT-BT: 595 nm, OT-TT: 592 nm) with shoulder corresponding to the aggregate absorption (OT-T: 646 nm, OT-BT: 642 nm, OT-TT: 641 nm). However, unlike chlorobenzene, only partial breaking in the self-assembly happened in films cast from chloroform on addition of PC71BM. This was evident from less blue-shift in the absorption maximum (OT-T: 553 nm, OTBT: 557 nm, OT-TT: 567 nm) as well as the presence of shoulder band corresponding to the aggregate absorption. Nevertheless, the intensity of the shoulder bands was weak compared to that of the pristine film. To improve this aspect, various volume percentage of DIO was added to the solution, and it was found that 0.5 vol% of DIO gave the best results. In this condition, the absorption maxima of the oligomers were found to be 576 (OT-T), 595 (OT-BT) and 588 nm (OT-TT), with well-defined shoulder bands (OT-T: 655 nm, OT-BT: 646 nm, OT-TT: 654 nm) in the red-region. Interestingly, the AFM images revealed that the aggregate sizes also decreased significantly (Figure 4d-f; corresponding height images are shown in Figure S8a-c, SI). This indicates that the films drop-cast from chloroform with 0.5 vol% of DIO not only preserve the ordered self-assembly of the oligomers but also keep the aggregate sizes smaller thereby providing an ideal condition for getting the optimum PCE.


12

Page 13 of 32

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. Normalized absorption spectra of (a) OT-T (b) OT-BT and (c) OT-TT under different conditions (― chloroform solution, ― pristine film from chloroform, ― donor:PC71BM 1:1 blend film from chloroform, ― donor:PC71BM 1:1 blend film from chloroform + 0.5 vol% DIO). AFM phase images of the 1:1 blend films of (d) OT-T (e) OT-BT and (f) OT-TT with PC71BM obtained from chloroform + 0.5 vol% DIO (scale bar corresponds to 500 nm). Photovoltaic devices were fabricated under the above optimized condition (chloroform as solvent and 0.5 vol% of DIO as additive). OT-TT:PC71BM gave the best photovoltaic performance of 3.52 (3.27±0.36)%. This could be attributed to the preservation of the donor selfassembly as well as the smaller aggregate sizes which lead to better bi-continuous donoracceptor network facilitating excellent charge separation, transport and collection of free charge carriers. Under the similar condition, OT-T:PC71BM based device showed marginal decrease in PCE 0.40, (0.37±0.07)%, whereas, the PCE of OT-BT:PC71BM remained same 0.76


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 32

(0.74±0.02)%. The EQE spectra of the OT-TT based device exhibited higher spectral response when compared to other two devices, which is consistent with the PCE of the oligomers (Figure S9b, SI). J–V curves obtained under the above mentioned condition are shown in Figure 5. The photovoltaic parameters obtained for the best devices from chlorobenzne and chloroform is summarized in Table 1. Details of the optimization data is given in Table S3 in the supporting information. Photovoltaic devices were also fabricated with varying weight ratios of PC71BM (0.5, 1, 2 and 3 w%) with the best performer among the three oligomers (OT-TT) using the above mentioned optimized condition (chloroform with 0.5 vol% DIO). Unfortunately, no further improvement in PCE was obtained and 1:1 w/w of OT-TT: PC71BM was found to be the best donor/acceptor ratio. The obtained photovoltaic parameters are summarized in the supporting information (Table S4, SI).

Figure 5. J–V characteristics of the photovoltaic devices fabricated from the 1:1 w/w blend solutions of (a) OT-T:PC71BM, (b) OT-BT:PC71BM and (c) OT-TT:PC71BM in chloroform + 0.5 vol% DIO (device structure: glass/ITO/ZnO/BHJ/MoO3/Ag).


14

Page 15 of 32

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 photovoltaic characteristics of BHJ solar cells with active layers of OT-T, OT-BT and OT-TT with PC71BM (1:1 w/w). The average values of the photovoltaic parameters along with the standard deviation is given in the bracket.

Active layer

Solvent

DIO (vol%)

CB

3.00

CF

0.50

1.58 0.75 0.34 (1.09±0.28) (0.75±0.02) (0.34±0.02)

0.69 (0.56±0.11) 0.40 (0.37±0.07)

CB

3.00

0.43 0.41 4.08 (0.39±0.05) (0.40±0.01) (3.59±0.24)

0.72 (0.68±0.13)

CF

0.50

2.63 0.80 0.36 (2.46±0.15) (0.79±0.03) (0.38±0.02)

0.76 (0.74±0.02)

CB

3.00

0.34 0.36 5.94 (5.28±0.44) (0.20±0.07) (0.34±0.03)

0.72 (0.57±0.13)

CF

0.50

0.80 0.50 8.74 (7.15±2.64) (0.78±0.15) (0.42±0.05)

3.52 (3.27±0.36)

OT-T: PC71BM

OT-BT: PC71BM

OT-TT: PC71BM

JSC (mA cm-2)

VOC (V)

FF

3.42 0.50 0.40 (3.75±0.20) (0.35±0.08) (0.37±0.02)

PCEmax (PCE)(%)

In literature, such difference in the PCEs obtained for oligomers/polymers were correlated with the donor-acceptor phase segregation by analyzing the nanoscale morphology. Our UV-vis analysis suggests that, all the three oligomers show good self-assembly/stacking in films prepared from chloroform with 0.5 vol% of DIO. However, this study reflects only the molecular level interactions which may or may not be extended to the hierarchical levels. AFM is also not helpful in this context as it can show only the surface morphology, and not the donor-acceptor domain formations. Bright field transmission electron microscope (BF-TEM) analysis is, on the other hand, an ideal tool for this purpose as it can clearly reveal the phase segregation through contrast difference (darker domains for PC71BM and lighter domains for thiophene oligomers).62-


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

64

Page 16 of 32

Donor-acceptor domains were partially visible in the BF-TEM images of both OT-T:PC71BM

(Figure 6a) and OT-BT:PC71BM (Figure 6b) but the distribution was found to be less uniform. On the other hand, clear donor-acceptor domains consisting of ten-nanometre scale donor fibrils were present uniformly in the case of OT-TT:PC71BM (Figure 6c). This well-defined domain formation in OT-TT:PC71BM ensure facile charge carrier separation and easier charge carrier transport to the respective electrodes leading to better efficiency.

Figure 6. BF-TEM images of (1:1 w/w) blended films (a) OT-T:PC71BM, (b) OT-BT:PC71BM and (c) OT-TT:PC71BM. PL quenching and 2D-GIXRD analysis: At this juncture, it is interesting to note that the PCEs of the oligomers obtained after solution engineering are in accordance with the TRMC results, i.e. OT-TT outperforms OT-T and OT-BT by a wide margin. It could be seen that in the case of OT-T and OT-BT blends, the best efficiencies achieved in the previous devices itself, and no further enhancement was possible through solution engineering. On the other hand, OTTT:PC71BM blend was intrinsically capable of delivering better performance, which was achieved through changing solvent and solvent additive concentration.


16

Page 17 of 32

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


For understanding the fundamental reasons for the improvements in the device efficiency in OT-TT:PC71BM blend on solution engineering, we have carried out the photoluminescence (PL) quenching and two-dimensional grazing incidence X-ray diffraction (2D-GIXRD) studies of the blend films. The former gives an idea about how efficiently the exciton dissociation occurs at the interface, whereas, the latter provides an insight into the molecular level packing of the oligomers, which correlates well with the charge transport properties. Compared to the PL of pristine film (spin coated from chlorobenzene solution), 83% quenching was observed for the blend film (cast from chlorobenzene with 3 vol% DIO; Figure 7). On the other hand, 89% quenching was obtained for blend film cast from chloroform containing 0.5 vol% DIO. The increased PL quenching in the latter case suggests that the excitons generated by the absorbed photons would be dissociated to free charge carriers (electrons and holes) more effectively when the film is prepared from chloroform with 0.5 vol% DIO.65-67 Similar studies were conducted with other two derivatives which are shown in the supporting information (Figure S10, SI). We have also carried out the PL quenching for the best performing molecule (OT-TT) in film state (spin coated from chloroform solution with 0.5 vol% DIO) with different ratios of PC71BM (0, 0.5, 1, 2 and 3 wt%; Figure S11, SI). The results indicated that more than 94% exciton is efficiently quenched in 1:1 w/w blend of OT-TT:PC71BM.


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

Page 18 of 32

Figure 7. Photoluminescence spectra of OT-TT under different conditions (― pristine film from chlorobenzene; ― 1:1 w/w blend film with PC71BM from chlorobenzene + 3 vol% DIO; ― 1:1 w/w blend film with PC71BM from chloroform + 0.5 vol% DIO; λex = 600 nm). Emission spectra of the blend films are multiplied by a factor of three for better visibility. 2D-GIXRD analysis was carried out in OT-TT:PC71BM blend films spin-coated from chlorobenzene with 3 vol% DIO (Figure 8a) and chloroform with 0.5 vol% DIO (Figure 8b). Both showed strong diffractions corresponding to the lamellar stacking (100 plane and its higher orders) in the out-of-plane direction. In the former case, lamellar stacking (100) peak was observed at 13.65 Å, whereas it was observed at 14.71 Å in the latter case. The films also exhibited reflections corresponding to the π-π stacking (010) at higher angles. Though the lamellar distance between the molecules was higher in the film cast from chloroform, the π stacking distance was lower (3.70 Å) than that of the film cast from chlorobenzene (3.78 Å). This clearly suggests that the stacking has improved in films cast from chloroform. More importantly, the peak corresponding to 010 plane was observed only in the in-plane diffraction profile of the film cast from chlorobenzene (inset of Figure 8a) indicating that the oligomers having a preferred edge-on orientation on the substrate.68 On the other hand, peak corresponding to the 010 reflection was seen in both out-of-plane as well as in-plane profiles in film cast from chloroform (inset of Figure 8b). This implies that some of the oligomers stack with a face-on orientation on the substrate.68-70 The presence of oligomer stacks with face-on orientation is known to facilitate charge transfer at the electrode interface, and hence could be attributed to the improvement in efficiency of OT-TT:PC71BM blend film cast from chloroform.


18

Page 19 of 32

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 8. 2D-GIXRD patterns of OT-TT:PC71BM 1:1 w/w blend films cast from (a) chlorobenzene + 3 vol% DIO and (b) chloroform + 0.5 vol% DIO on glass/ITO/ZnO substrate. Lower and upper panels are the 2D image and extracted spectra in the out-of-plane (red) and inplane (black) directions, respectively (inset shows the zoomed region from 1.5 to 2.0 Å-1). It must be noted that the lamellar packing mode is dominant in the out-of-plane reflection than that in the in-plane, which indicates that the fraction of the edge-on orientation is more than that of the face-on orientation.68 To improve the photovoltaic efficiencies, it is important to increase the fraction of face-on oriented molecules. This could be achieved by incorporating moieties with larger π-planes into the oligomer backbone. In addition to that, the performance can also be


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

Page 20 of 32

improved by enhancing the absorbance in the red and near infra-red regions by incorporating strong electron withdrawing groups in the N-ethylrhodanine moiety. CONCLUSIONS In summary, we have synthesized three solution processable A-D-A type small molecules consisting of N-ethylrhodanine as the acceptor group and different central cores (thiophene, bithiophene terthiophene).

and The

thienothiophene) oligomer

connected

containing

through

thiophene

thienothiophene

as

core

pi-linker exhibited

(alkylated superior

photoconductivity properties in FP-TRMC analysis compared to that of other two derivatives. By taking TRMC observation as a guiding tool, solution engineering was carried out to get the best performance of the oligomers. The improved cell fabricated from thienothiophene based oligomer exhibited a PCE of 3.52%. UV-vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence quenching and two-dimensional grazing angle X-ray diffraction studies revealed that the improvements in PCE is due to the improved molecular stacking, good morphology, optimum donor-acceptor phase separation, enhanced charge carrier dissociation and increased fraction of the donor oligomers with face-on stacking. ASSOCIATED CONTENT Supporting Information Experimental procedures and the characterization data of OT-T, OT-BT and OT-TT, Scheme S1, Figures S1-S14, Table S1-4.

AUTHOR INFORMATION


20

Page 21 of 32

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


Corresponding Authors * Akinori Saeki, Tel. +81-668-794-587, E-mail: [email protected] * Vijayakumar C. Nair, Tel. +91-471-251-5484, E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript ACKNOWLEDGEMENTS This work was financially supported by DST through the Ramanujan Fellowship (SR/S2/RJN-133/2012), CSIR network project NWP-54 (TAPSUN), and Grant-in-aid for Scientific Research B: A252880840 (to A.S.) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. T.G. is grateful to UGC for the Senior Research Fellowship. The authors thank Dr. Tomoyuki Koganezawa at JASRI, Japan and Dr. Itaru Osaka at RIKEN, Japan, for the 2D-GIXRD experiments (Proposal 2014B1915). ABBREVIATIONS OSCs, organic solar cells; BHJ, bulk-heterojunction; PCE, power conversion efficiency; A-D-A, acceptor-donor-acceptor; FP-TRMC, flash-photolysis time-resolved microwave conductivity; CF, chloroform; CB, chlorobenzene; EQE, external quantum efficiency; AFM, atomic force microscopy; BF-TEM, bright-field transmission electron microscopy; 2D-GIXRD, two dimensional grazing incidence X-ray diffraction. REFERENCES


21


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

(1)

Page 22 of 32

Heeger, A. J. Semiconducting Polymers: The Third Generation. Chem. Soc. Rev. 2010, 39, 2354–2371.

(2)

Chen, Y.; Wan, X.; Long, G. High Performance Photovoltaic Applications Using Solution-Processed Small Molecules. Acc. Chem. Res. 2013, 46, 2645–2655.

(3)

Kaltenbrunner, M.; White, M. S.; Głowacki, E. D.; Sekitani, T.; Someya, T.; Sariciftci, N. S.; Bauer, S. Ultrathin and Lightweight Organic Solar Cells with High Flexibility. Nat. Commun. 2012, 3, 770.

(4)

Chen, J.-D.; Cui, C.; Li, Y.-Q.; Zhou, L.; Ou, Q.-D.; Li, C.; Li, Y.; Tang, J.-X. SingleJunction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1035–1041.

(5)

Zhang, Q.; Kan, B.; Liu, F.; Long, G.; Wan, X.; Chen, X.; Zuo, Y.; Ni, W.; Zhang, H.; Li, M.; Hu, Z.; Huang, F.; Cao, Y.; Liang, Z.; Zhang, M.; Russell, T. P.; Chen, Y. SmallMolecule Solar Cells with Efficiency over 9%. Nat. Photonics 2015, 9, 35–41.

(6)

Sun, K.; Xiao, Z.; Lu, S.; Zajaczkowski, W.; Pisula, W.; Hanssen, E.; White, J. M.; Williamson, R. M.; Subbiah, J.; Ouyang, J.; Holmes, A. B.; Wong, W. W. H.; Jones, D. J. A Molecular Nematic Liquid Crystalline Material for High-Performance Organic Photovoltaics. Nat. Commun. 2015, 6, 6013.

(7)

Kan, B.; Li, M.; Zhang, Q.; Liu, F.; Wan, X.; Wang, Y.; Ni, W.; Long, G.; Yang, X.; Feng, H.; Zuo, Y.; Zhang, M.; Huang, F.; Cao, Y.; Russell, T. P.; Chen, Y. A Series of Simple Oligomer-like Small Molecules based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency. J. Am. Chem. Soc. 2015, 137, 3886–3893.


22

Page 23 of 32

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


(8)

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.

(9)

Lin, Y.; Li, Y.; Zhan, X. Small Molecule Semiconductors for High-Efficiency Organic Photovoltaics. Chem. Soc. Rev. 2012, 41, 4245–4272.

(10)

Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Device Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells. Adv. Mater. 2007, 19, 1551–1566.

(11)

Johnson, B.; Kendrick, M. J.; Ostroverkhova, O. Charge Carrier Dynamics in Organic Semiconductors and Their Donor-Acceptor Composites: Numerical Modeling of TimeResolved Photocurrent. J. Appl. Phys. 2013, 114, 094508.

(12)

Bouthinon, B.; Clerc, R.; Vaillant, J.; Verilhac, J.-M.; Faure-Vincent, J.; Djurado, D.; Ionica, I.; Man, G.; Gras, A.; Pananakakis, G.; Gwoziecki, R.; Kahn, A. Impact of Blend Morphology on Interface State Recombination in Bulk Heterojunction Organic Solar Cells. Adv. Funct. Mater. 2015, 25, 1090–1101.

(13)

Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Influence of Blend Microstructure on Bulk Heterojunction Organic Photovoltaic Performance. Chem. Soc. Rev. 2011, 40, 1185–1199.

(14)

Lee, D.; Brownell, L. V; Yan, L.; You, W. Morphological Effects on the SmallMolecule-based Solution-Processed Organic Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 15767–15773.

(15)

Scholes, F. H.; Ehlig, T.; James, M.; Lee, K. H.; Duffy, N.; Scully, A. D.; Singh, T. B.; Winzenberg, K. N.; Kemppinen, P.; Watkins, S. E. Intraphase Microstructure-


23


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 32

Understanding the Impact on Organic Solar Cell Performance. Adv. Funct. Mater. 2013, 23, 5655–5662. (16)

Paek, S.; Cho, N.; Song, K.; Jun, M.-J.; Lee, J. K.; Ko, J. Efficient Organic Semiconductors

Containing

Fluorine-Substituted

Benzothiadiazole

for

Solution-

Processed Small Molecule Organic Solar Cells. J. Phys. Chem. C 2012, 116, 23205– 23213. (17)

Chen, L.-M.; Xu, Z.; Hong, Z.; Yang, Y. Interface Investigation and Engineering – Achieving High Performance Polymer Photovoltaic Devices. J. Mater. Chem. 2010, 20, 2575–2598.

(18)

Yip, H.-L.; Jen, A. K.-Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994–6011.

(19)

Williams, G.; Sutty, S.; Aziz, H. Interplay between Efficiency and Device Architecture for Small Molecule Organic Solar Cells. Phys. Chem. Chem. Phys. 2014, 16, 11398– 11408.

(20)

Cao, W.; Xue, J. Recent Progress in Organic Photovoltaics: Device Architecture and Optical Design. Energy Environ. Sci. 2014, 7, 2123–2144.

(21)

Chang, L.; Lademann, H. W. A.; Bonekamp, J.-B.; Meerholz, K.; Moulé, A. J. Effect of Trace Solvent on the Morphology of P3HT:PCBM Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2011, 21, 1779–1787.

(22)

He, F.; Yu, L. How Far Can Polymer Solar Cells Go? In Need of a Synergistic Approach. J. Phys. Chem. Lett. 2011, 2, 3102–3113.

(23)

Thompson, B. C.; Fréchet, J. M. J. Polymer-Fullerene Composite Solar Cells. Angew. Chem. Int. Ed. Engl. 2008, 47, 58–77.


24

Page 25 of 32

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)

Hedley, G. J.; Ward, A. J.; Alekseev, A.; Howells, C. T.; Martins, E. R.; Serrano, L. a; Cooke, G.; Ruseckas, A.; Samuel, I. D. W. Determining the Optimum Morphology in High-Performance Polymer-Fullerene Organic Photovoltaic Cells. Nat. Commun. 2013, 4, 2867.

(25)

Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future-Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, E135–E138.

(26)

Wang, D. H.; Morin, P.; Lee, C.; Ko, K. Effect of Processing Additive on Morphology and Charge Extraction in Bulk- Heterojunction Solar Cells. J. Mater. Chem. A 2014, 2, 15052–15057.

(27)

Zusan, A.; Zerson, M.; Dyakonov, V.; Magerle, R. The Effect of Diiodooctane on the Charge Carrier Generation in Organic Solar Cells based on the Copolymer PBDTTT-C. Sci. Rep. 2015, 5, 8286.

(28)

Zhang, F.; Wu, D.; Xu, Y.; Feng, X. Thiophene-based Conjugated Oligomers for Organic Solar Cells. J. Mater. Chem. 2011, 21, 17590–17600.

(29)

Fichou, D. Handbook of Oligo- and Polythiophenes; Fichou, D., Ed.; Wiley-VCH Verlag GmbH: Weinheim, Germany, 1998.

(30)

Ni, W.; Wan, X.; Li, M.; Wang, Y.; Chen, Y. A–D–A Small Molecules for SolutionProcessed Organic Photovoltaic Cells. Chem. Commun. 2015, 51, 4936–4950.

(31)

Liu, Y.; Wan, X.; Wang, F.; Zhou, J.; Long, G.; Tian, J.; Chen, Y. High-Performance Solar Cells Using a Solution-Processed Small Molecule Containing Benzodithiophene Unit. Adv. Mater. 2011, 23, 5387–5391.


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

(32)

Page 26 of 32

Fitzner, R.; Mena-Osteritz, E.; Walzer, K.; Pfeiffer, M.; Bäuerle, P. A-D-A-Type Oligothiophenes for Small Molecule Organic Solar Cells: Extending the Π-System by Introduction of Ring-Locked Double Bonds. Adv. Funct. Mater. 2015, 25, 1845–1856.

(33)

Demeter, D.; Rousseau, T.; Leriche, P.; Cauchy, T.; Po, R.; Roncali, J. Manipulation of the Open-Circuit Voltage of Organic Solar Cells by Desymmetrization of the Structure of Acceptor-Donor-Acceptor Molecules. Adv. Funct. Mater. 2011, 21, 4379–4387.

(34)

Holliday, S.; Ashraf, R. S.; Nielsen, C. B.; Kirkus, M.; Röhr, J. A.; Tan, C.-H.; ColladoFregoso, E.; Knall, A.-C.; Durrant, J. R.; Nelson, J.; McCulloch, I. A Rhodanine Flanked Nonfullerene Acceptor for Solution-Processed Organic Photovoltaics. J. Am. Chem. Soc. 2015, 137, 898–904.

(35)

Li, Z.; He, G.; Wan, X.; Liu, Y.; Zhou, J.; Long, G.; Zuo, Y.; Zhang, M.; Chen, Y. Solution Processable Rhodanine-based Small Molecule Organic Photovoltaic Cells with a Power Conversion Efficiency of 6.1 %. Adv. Energy Mater. 2012, 2, 74–77.

(36)

Kim, Y.; Song, C. E.; Moon, S.; Lim, E. Rhodanine Dye-based Small Molecule Acceptors for Organic Photovoltaic Cells. Chem. Commun. 2014, 50, 8235–8238.

(37)

Zhou, J.; Zuo, Y.; Wan, X.; Long, G.; Zhang, Q.; Ni, W.; Liu, Y.; Li, Z.; He, G.; Li, C.; Kan, B.; Li, M.; Chen, Y. Solution-Processed and High-Performance Organic Solar Cells Using Small Molecules with a Benzodithiophene Unit. J. Am. Chem. Soc. 2013, 135, 8484–8487.

(38)

Liu, F.; Fan, H.; Zhang, Z.; Zhu, X. Low-Bandgap Small-Molecule Donor Material Containing Thieno[3,4-b]thiophene Moiety for High-Performance Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 3661–3668.


26

Page 27 of 32

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


(39)

Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. SolutionProcessed Small-Molecule Solar Cells with 6.7% Efficiency. Nat. Mater. 2012, 11, 44– 48.

(40)

Haas, M. P. De; Warman, J. M. Photon-Induced Molecular Charge Separation Studied by Nanosecond Time-Resolved Microwave Conductivity. Chem. Phys. 1982, 73, 35–53.

(41)

Tio, P.; Intensity, L.; Martin, S. T.; Herrmannt, H.; Hoffmann, M. R. Time-Resolved Microwave Conductivity. J. Chem. Soc. Faraday Trans. 1994, 90, 3323–3330.

(42)

Saeki, A.; Seki, S.; Sunagawa, T.; Ushida, K.; Tagawa, S. Charge-Carrier Dynamics in Polythiophene Films Studied by in-situ Measurement of Flash-Photolysis Time-Resolved Microwave Conductivity (FP-TRMC) and Transient Optical Spectroscopy (TOS). Philos. Mag. 2006, 86, 1261–1276.

(43)

Acharya, A.; Seki, S.; Saeki, A.; Koizumi, Y.; Tagawa, S. Study of Transport Properties in Fullerene-Doped Polysilane Films Using Flash Photolysis Time-Resolved Microwave Technique. Chem. Phys. Lett. 2005, 404, 356–360.

(44)

Saeki, A.; Seki, S.; Koizumi, Y.; Tagawa, S. Dynamics of Photogenerated Charge Carrier and Morphology Dependence in Polythiophene Films Studied by in Situ Time-Resolved Microwave Conductivity and Transient Absorption Spectroscopy. J. Photochem. Photobiol., A 2007, 186, 158–165.

(45)

Savenije, T. J.; Ferguson, A. J.; Kopidakis, N.; Rumbles, G. Revealing the Dynamics of Charge Carriers in Polymer:Fullerene Blends Using Photoinduced Time-Resolved Microwave Conductivity. J. Phys. Chem. C 2013, 117, 24085-24103.


27


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

(46)

Page 28 of 32

Grozema, F. C.; Siebbeles, L. D. A. Charge Mobilities in Conjugated Polymers Measured by Pulse Radiolysis Time-Resolved Microwave Conductivity: From Single Chains to Solids. J. Phys. Chem. Lett. 2011, 2, 2951–2958.

(47)

Bird, M. J; Reid, O. G.; Cook, A. R.; Asaoka, S.; Shibano, Y.; H. Imahori, Rumbles, R.; Miller, J. R. Mobility of Holes in Oligo- and Polyfluorenes of Defined Lengths. J. Phys. Chem. C 2014, 118, 6100–6109.

(48)

Marchioro, A.; Teuscher, J.; Friedrich, D.; Kunst, M.; van de Krol,R.; Moehl, T.; Grätzel, M.; Moser, J.–E. Unravelling the Mechanism of Photoinduced Charge Transfer Processes in Lead Iodide Perovskite Solar Cells. Nat. Photonics 2014, 8, 250–255.

(49)

Li, H.; Earmme, T.; Ren, G.; Saeki, A.; Yoshikawa, S.; Murari, N. M.; Subramaniyan, S.; Crane, M. J.; Seki, S.; Jenekhe, S. A. Beyond Fullerenes: Design of Nonfullerene Acceptors for Efficient Organic Photovoltaics. J. Am. Chem. Soc. 2014, 136, 14589– 14597.

(50)

Ghosh, T.; Gopal, A.; Saeki, A.; Seki, S.; Nair, V. C. P/n-Polarity of Thiophene Oligomers in Photovoltaic Cells: Role of Molecular vs. Supramolecular Properties. Phys. Chem. Chem. Phys. 2015, 17, 10630–10639.

(51)

Vijayakumar, C.; Balan, B.; Saeki, A.; Tsuda, T.; Kuwabata, S.; Seki, S. Gold Nanoparticle Assisted Self-Assembly and Enhancement of Charge Carrier Mobilities of a Conjugated Polymer. J. Phys. Chem. C 2012, 116, 17343–17350.

(52)

Balan, B.; Vijayakumar, C.; Saeki, A.; Koizumi, Y.; Tsuji, M.; Seki, S. Optical and Electrical Properties of Dithienothiophene based Conjugated Polymers: Medium Donor vs. Weak, Medium, and Strong Acceptors. Polym. Chem. 2013, 4, 2293–2303.


28

Page 29 of 32

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


(53)

Saeki, A.; Yoshikawa, S.; Tsuji, M.; Koizumi, Y.; Ide, M.; Vijayakumar, C.; Seki, S. A Versatile Approach to Organic Photovoltaics Evaluation Using White Light Pulse and Microwave Conductivity. J. Am. Chem. Soc. 2012, 134, 19035–19042.

(54)

Balan, B.; Vijayakumar, C.; Saeki, A.; Koizumi, Y.; Seki, S. P/n Switching of Ambipolar Bithiazole–Benzothiadiazole-based Polymers in Photovoltaic Cells. Macromolecules 2012, 45, 2709–2719.

(55)

Schwenn, P. E.; Gui, K.; Nardes, A. M.; Krueger, K. B.; Lee, K. H.; Mutkins, K.; Rubinstein-Dunlop, H.; Shaw, P. E.; Kopidakis, N.; Burn, P. L.; Meredith, P. A Small Molecule Non-Fullerene Electron Acceptor for Organic Solar Cells. Adv. Energy Mater. 2011, 1, 73–81.

(56)

Tsuji, M.; Saeki, A.; Koizumi, Y.; Matsuyama, N.; Vijayakumar, C.; Seki, S. Benzobisthiazole as Weak Donor for Improved Photovoltaic Performance: Microwave Conductivity Technique Assisted Molecular Engineering. Adv. Funct. Mater. 2014, 24, 28–36.

(57)

Saeki, A.; Tsuji, M.; Seki, S. Direct Evaluation of Intrinsic Optoelectronic Performance of Organic Photovoltaic Cells with Minimizing Impurity and Degradation Effects. Adv. Energy Mater. 2011, 1, 661–669.

(58)

Lee, J. S.; Son, S. K.; Song, S.; Kim, H.; Lee, D. R.; Kim, K.; Ko, M. J.; Choi, D. H.; Kim, B. S.; Cho, J. H. Importance of Solubilizing Group and Backbone Planarity in Low Band Gap Polymers for High Performance Ambipolar Field-Effect Transistors. Chem. Mater. 2012, 24, 1316–1323.


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

(59)

Page 30 of 32

Zhang, Q.; Wang, Y.; Kan, B.; Wan, X.; Liu, F.; Ni, W.; Feng, H.; Russell, T. P.; Chen, Y. A Solution-Processed High Performance Organic Solar Cell Using a Small Molecule with the thieno[3,2-b]thiophene Central Unit. Chem. Commun. 2015, 51, 15268-15271.

(60)

He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat. Photonics 2012, 6, 591–595.

(61)

Intemann, J. J.; Yao, K.; Li, Y. X.; Yip, H. L.; Xu, Y. X.; Liang, P. W.; Chueh, C. C.; Ding, F. Z.; Yang, X.; Li, X.; Chen, Y.; Jen, A. K. Y. Highly Efficient Inverted Organic Solar Cells through Material and Interfacial Engineering of indacenodithieno[3,2-b] thiophene-based Polymers and Devices. Adv. Funct. Mater. 2014, 24, 1465–1473.

(62)

Sun, J.; Zhu, Y.; Xu, X.; Lan, L.; Zhang, L.; Cai, P.; Chen, J. High Efficiency and High Voc Inverted Polymer Solar Cells based on a Low-Lying HOMO Polycarbazole Donor and a Hydrophilic Polycarbazole Interlayer on ITO Cathode. J. Phys. Chem. C 2012, 116, 14188 −14198.

(63)

Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Zhang, J.; Chand, S.; Bazan, G. C.; Heeger, A. J. Efficient Solution-Processed Small-Molecule Solar Cells with Inverted Structure. Adv. Mater. 2013, 25, 2397–2402.

(64)

Rujisamphan, N.; Murray, R. E.; Deng, F.; Ni, C.; Shah, S. I. Study of the Nanoscale Morphology of Polythiophene Fibrils and a Fullerene Derivative. ACS Appl. Mater. Interfaces 2014, 6, 11965–11972.

(65)

Kumar, C. V.; Cabau, L.; Viterisi, A.; Biswas, S.; Sharma, G. D.; Palomares, E. Solvent Annealing Control of Bulk Heterojunction Organic Solar Cells with 6.6% Efficiency


30

Page 31 of 32

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


based on a Benzodithiophene Donor Core and Dicyano Acceptor Units. J. Phys. Chem. C 2015, 119, 20871–20879. (66)

Cnops, K.; Rand, B. P.; Cheyns, D.; Verreet, B.; Empl, M. A.; Heremans, P. 8.4% Efficient Fullerene-Free Organic Solar Cells Exploiting Long-Range Exciton Energy Transfer. Nat. Commun. 2014, 5, 3406.

(67)

Zusan, A.; Gieseking, B.; Zerson, M.; Dyakonov, V.; Magerle, R.; Deibel, C. The Effect of Diiodooctane on the Charge Carrier Generation in Organic Solar Cells based on the Copolymer PBDTTT-C. Sci. Rep. 2015, 5, 8286.

(68)

Liu, Y.; Shi, Q.; Ma, L.; Dong, H.; Tan, J.; Hu, W.; Zhan, X. Copolymers of benzo[1,2b:4,5-b′]dithiophene and Bithiazole for High-Performance Thin Film Phototransistors. J. Mater. Chem. C 2014, 2, 9505–9511.

(69)

Su, G. M.; Pho, T. V.; Eisenmenger, N. D.; Wang, C.; Wudl, F.; Kramer, E. J.; Chabinyc, M. L. Linking Morphology and Performance of Organic Solar Cells based on Decacyclene Triimide Acceptors. J. Mater. Chem. A 2014, 2, 1781–1789.

(70)

Ide, M.; Saeki, A.; Koizumi, Y.; Koganezawa, T.; Seki, S. Molecular Engineering of Benzothienoisoindigo Copolymers Allowing Highly Preferential Face-on Orientations. J. Mater. Chem. A 2015, 3, 21578-21585.


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

Page 32 of 32

Table of Contents

The structure-property correlation and optimization of photovoltaic efficiency in three thiophene based oligomers with different central cores (thiophene, bithiophene and thienothiophene) are described.


32

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and

Recommend Documents