Understanding StructureâProperty Relationships in All-Small

Subscriber access provided by Kaohsiung Medical University

Energy, Environmental, and Catalysis Applications

Understanding Structure-Property Relationships in All-Small-Molecule Solar Cells Incorporating a Fullerene or Nonfullerene Acceptor Jisu Hong, Min Jae Sung, Hyojung Cha, Chan Eon Park, James R. Durrant, Tae Kyu An, Yun-Hi Kim, and Soon-Ki Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14020 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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 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 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.

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 48 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

Understanding Structure-Property Relationships in All-SmallMolecule Solar Cells Incorporating a Fullerene or Nonfullerene Acceptor

Jisu Honga†, Min Jae Sungb†, Hyojung Chac†, Chan Eon Parka, James R. Durrantc, Tae Kyu And*, Yun-Hi Kime*, and Soon-Ki Kwonb*

a

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang

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

Department of Materials Engineering and Convergence Technology and ERI, Gyeongsang

National University, Jinju 660-701, Republic of Korea c

Centre for Plastic Electronics, Department of Chemistry, Imperial College London, London

SW7 2AZ, United Kingdom d

Department of Polymer Science & Engineering and Department of IT Convergence, Korea

National University of Transportation, Chungju, 380-702, Republic of Korea e

Department of Chemistry and RINS, Gyeongsang National University, Jinju 660-701,

Republic of Korea

KEYWORDS: all-small-molecule solar cell, nonfullerene solar cell, DTBDT-based small molecule, intermolecular interaction, charge carrier dynamics

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

ABSTRACT: To investigate the influence of donor molecule crystallinity on photovoltaic performance in all-small-molecule solar cells, two DTBDT-based small molecules, denoted as DTBDT-Rho and DTBDT-S-Rho and incorporating different side chains, are synthesized and characterized. The photovoltaic properties of solar cells made of these DTBDT-based donor molecules are systemically studied with the PC71BM fullerene acceptor and the OIDTBR nonfullerene acceptor to study the aggregation behavior and crystallinity of the donor molecules in both blends. Morphological analyses and a charge carrier dynamics study are carried out simultaneously to derive structure-property relationships and address the requirements of all-small-molecule solar cells. This study reveals exciton decay loss driven by large-scale phase separation of the DTBDT molecules to be a crucial factor limiting photocurrent generation in the all-small-molecule solar cells incorporating O-IDTBR. In the all-small-molecule blends, DTBDT domains with dimensions greater than 100 nm limit the exciton migration to the donor/acceptor interface, while blends with PC71BM exhibit homogeneous phase separation with smaller domains than in the O-IDTBR blends. The significant energy losses in nonfullerene-based devices lead to decreased Jsc and FF values and unusual decrease in Voc values. These results indicate the modulation of phase separation to be important for improving the photovoltaic performances of all-small-molecule blends. In addition, the enhanced molecular aggregation of DTBDT-S-Rho with the alkylthio side chain leads to higher degrees of phase separation and unfavorable charge transfer, which are mainly responsible for the relatively low photocurrent when using DTBDT-S-Rho compared to that when using DTBDT-Rho. On the other hand, this enhanced molecular aggregation improves the crystallinity of DTBDT-S-Rho and results in its increased hole mobility.


Page 2 of 48

Page 3 of 48 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 Organic solar cells (OSCs) have been extensively studied as one of the promising technologies for clean and sustainable energy due to the potential for light weight, flexibility, large-scale production, and low production cost.1-4 Recently, the effectiveness of smallmolecule semiconductors in OSCs has shown rapid progress. Compared to conjugated polymers, small-molecule semiconductors show better-defined molecular structures and less batch-to-batch variation.5-8 Their syntheses also involve a simpler work-up and purification process and are hence less expensive to carry out.5,7 However, the photovoltaic performances of OSCs composed completely of small molecules are still lagging behind those of polymerbased OSCs,9-13 for which power conversion efficiencies (PCEs) of over 13% have been achieved as a result of the rapid development of molecular design and the emergence of small-molecule nonfullerene acceptors.14-16 Although the introduction of small-molecule donor blends incorporating small-molecule nonfullerene acceptors might be expected to boost PCEs, they have not yet shown enhanced device performance — mainly due to the difficulty in controlling the blend morphology, leading to inefficient photocurrent generation, low charge carrier mobility and unbalanced charge-transport ability with poorer interpenetrating networks compared to those of conjugated polymer:nonfullerene acceptor blends.9,10,17 There is still a lack of guidelines for the molecular design of small-molecule semiconductors that accomplish appropriate combinations of donor and acceptor materials, nanomorphology of blend films, and photovoltaic properties.18,19 Herein, we report the use of two dithieno[2,3-d:2’,3’-d’]-benzo[1,2-b:4,5-b’]dithiophene (DTBDT)-based electron-donating small molecules to study the effect of small-molecule donor crystallinity in blends on all-small-molecule solar cell performance. Because benzodithiophene (BDT)-based small-molecule donors have been mainly used in all-small-



molecule solar cells, we synthesized DTBDT-based small-molecule donors that have not yet been investigated in all-small-molecule systems.9-11,18 To enhance intermolecular interactions and molecular ordering of DTBDT-based small molecule, DTBDT-Rho, which was previously reported by our group,20 we have newly synthesized DTBDT-S-Rho by introducing the alkylthio group in thiophene-substituted DTBDT units. Side chain engineering is important in that the side chain affects solubility, molecular packing, energy levels, absorption and charge transport properties.21,22 Cui et al. and Huo et al. have demonstrated the inclusion of the alkylthio side chain to lead to lower (highest occupied molecule orbital (HOMO) energy levels and consequently increased open circuit voltage (Voc) values as well as to red-shifted light absorption and consequently increased short-circuit current density (Jsc) levels.23,24 Introducing the alkylthio group into the DTBDT unit, hence forming the new small molecule DTBDT-S-Rho, was observed to decrease the HOMO and lowest unoccupied molecular orbital (LUMO) energy levels from -5.11 to -5.17 eV and from -3.43 to -3.49 eV, respectively, and to enhance the aggregation and order of the molecules in thin films. To study the impact of donor molecule crystallinity in donor:acceptor blend films and address the requirements for material design and morphology modulation in all-small-molecule blends, we compared the photovoltaic properties of DTBDT-Rho and DTBDT-S-Rho in fullerene acceptor-based and nonfullerene acceptor-based solar cells. [6,6]-phenyl-C71butyric acid methyl ester (PC71BM) and O-IDTBR were incorporated as a fullerene acceptor and a nonfullerene acceptor, respectively. Analysis of their nanoscale blend morphologies, exciton generation, and charge carrier dynamics revealed that the large-scale phase separation found in the DTBDT-based all-small-molecule systems led to poorer photovoltaic performances in the solar cells with O-IDTBR. The relatively large sizes (> 100 nm) of the domains in the DTBDT-based small-molecule:O-IDTBR blends limited their exciton ACS Paragon Plus Environment

Page 4 of 48

Page 5 of 48 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


diffusion and charge transfer and led to lower current density, indicating that modulating phase separation can be a key factor for improving the photovoltaic performance of the DTBDT-based all-small-molecule system. The enhanced molecular packing and crystallinity of DTBDT-S-Rho intensified large-scale phase separation and unfavorable charge transfer, thus lowering the PCE, while its improved crystallinity resulted in increased hole mobility.

RESULTS AND DISCUSSION Synthesis and properties of the materials. 2,7-Bis(3,3''-dioctyl-[2,2':5',2''-terthiophene]-5(3’-ethylrhodanine)-5,10-bis(4,5-didecylthiophen-2-yl) b]thiophene

(DTBDT-Rho)

and

benzo[1,2-b:4,5-b’]dithieno[3,2

2,7-bis(3,3''-dioctyl-[2,2':5',2''-terthiophene]-5-(3'-

ethylrhodanine)-5,10-bis(2-((2-ethylhexyl)thio)

thiophene-2-yl)benzo[1,2-b:4,5-

b']dithieno[3,2 b]thiophene (DTBDT-S-Rho) shown in Figure 1a were prepared by carrying out nucleophilic addition and reduction, bromination, Stille coupling, and the Knoevenagel reaction. The synthetic scheme and experimental method are described in Scheme S1. The obtained intermediate and product were characterized by performing various types of spectroscopy such as 1H-NMR,

13

C-NMR, and mass spectroscopy (MS). (Figure S1-S15).

The obtained thermogravimetric analysis (TGA) thermogram of DTBDT-S-Rho under a nitrogen atmosphere showed a 5% weight loss at about 362 °C (Figure S16a), similar to that of DTBDT-Rho (at about 383 °C),16 revealing DTBDT-S-Rho to have sufficiently high thermal stability. The obtained differential scanning calorimetry (DSC) plot of DTBDT-SRho, shown in Figure S16b, showed a melting point (Tm) at 250 °C and crystallization point (Tc) at 234 °C, while DTBDT-Rho showed a Tm of 245 °C and Tc of 22 7°C for the heating and cooling processes, respectively. These results suggested DTBDT-S-Rho and DTBDT-Rho to have similar crystalline characters; moreover, the particularly high Tm of DTBDT-S-Rho



was indicative of its strong intermolecular interactions.25 The optical properties of DTBDT-SRho were studied with UV−visible−NIR absorption spectroscopy. UV-visible-NIR absorption spectra of DTBDT-S-Rho in a chloroform solution and as a thin film are shown in Figure 2a. The absorption maxima for solution and thin film forms were observed at 507 nm and at 587 and 635 nm, respectively, with an onset of 740 nm. The bathochromic shift of 80 nm was indicative of effective intermolecular π-π stacking in the solid state.26,27 The electrochemical properties of DTBDT-S-Rho were also determined by performing cyclic voltammetry (Figure 2b). The oxidation onset and reduction onset were observed at 0.79 V and -1.24 V, respectively. The HOMO energy level of DTBDT-S-Rho estimated from the oxidation onset was -5.17 eV. The LUMO of DTBDT-S-Rho, estimated from its oxidation onset and optical bandgap Eg opt of 1.68 eV, was -3.49 eV. The optical absorption characteristics of DTBDT-SRho were similar to those of DTBDT-Rho while the HOMO and LUMO levels of DTBDT-SRho were slightly lower than those of DTBDT-Rho.22 The slightly lower energy levels may have been due to the slightly greater electron-withdrawing strengths of ethylhexylthio thiophene side chains than of the didecylthiophene side chains.28,29 The electrochemical and optical properties of the two investigated DTBDT-based small molecules are compared in Table 1.

Photovoltaic properties. To demonstrate the potential of DTBDT-based small-molecule donors in all-small-molecule solar cells, the photovoltaic properties of small-molecule donors based on DTBDT unit were evaluated in conventional solar cells with a glass/indium tin oxide (ITO)/molybdenum (IV) oxide (MoO3)/active layer/LiF/Al device structure. PC71BM and O-IDTBR were used as a fullerene acceptor and nonfullerene acceptor, respectively. OIDTBR was selected as a small molecule nonfullerene acceptor due to its light absorption


Page 6 of 48

Page 7 of 48 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


range up to 800 nm for complementary absorption, suitable HOMO and LUMO levels which matches well with those of DTBDT-based small-molecule donors, and high crystallinity for efficient charge transport in photovoltaic devices. The molecular structures of the donor and acceptor materials incorporated in this study are shown in Figure 1. A donor:acceptor blend ratio of 1:0.8 was found to be optimum for the DTBDT-based small-molecule:PC71BM devices, while the DTBDT-based small-molecule:O-IDTBR devices showed the highest efficiency values when a blend ratio of 1:1.2 was used. The current density-voltage (J-V) and external quantum efficiency (EQE) curves of the devices are shown in Figure 3, and the photovoltaic performances are summarized in Table 2. The DTBDT-Rho:PC71BM device gave an average PCE of 7.40%, with a Voc of 0.88 V, Jsc of 15.20 mA cm-2, and fill factor (FF) of 0.55. The DTBDT-S-Rho:PC71BM device showed a decreased PCE of 5.62%, with Voc of 0.88 V, Jsc of 12.05 mA cm-2, and FF of 0.53. The lower Jsc and hence lower PCE of the device with DTBDT-S-Rho which having the alkylthio group in the side chain, seemed to have been due to its bulk heterojunction morphology. When DTBDT-based small molecules were blended with an nonfullerene acceptor, namely O-IDTBR, the PCE dropped significantly. For DTBDT-Rho, the average PCE decreased to 4.20%, with a Voc of 0.76 V, Jsc of 13.28 mA cm-2, and FF of 0.41. Much lower PCEs were obtained from the device employing DTBDT-S-Rho and O-IDTBR. This device gave a PCE of 2.72%, with Voc, Jsc and FF values of 0.78 V, 8.07 mA cm-2, and 0.43, respectively. For the DTBDT-based smallmolecule solar cells incorporating the nonfullerene small molecule, the decreased Jsc and significantly decreased Voc and FF values were responsible for the decreased PCEs. When considering the light absorption spectra of small-molecule donors and acceptors and considering the difference between the HOMO levels of the DTBDT-based small molecules and the LUMO levels of PC71BM or O-IDTBR, we would have expected the nonfullerene acceptor-based solar cell devices to have higher Jsc and Voc values.30-32 The lower Jsc and Voc



Page 8 of 48

values obtained from the nonfullerene-based solar cells were indicative of an unfavorable morphology in DTBDT-based small-molecule:O-IDTBR blends.33-35 Decreased Voc values, indicative of energy losses in nonfullerene-based devices, seem to be caused by significant charge recombination losses in the active layer.36,37 Therefore, to better understand the photovoltaic properties of our devices, their morphologies need to be analyzed. We acquired EQE curves for each system to comprehend the differences between the Jsc values of the systems by characterizing the photocurrents according to the wavelength. The fullerene-based

and

nonfullerene-based

devices

showed

different

photoconversion

characteristics as shown in Figure 3b. The EQE spectra of the DTBDT-Rho:PC71BM and DTBDT-S-Rho:PC71BM devices showed maximum EQE values of 75% at 420 nm and 59% at 410 nm, respectively, while the DTBDT-Rho:O-IDTBR and DTBDT-S-Rho:O-IDTBR devices showed maximum efficiency values of 59% and 36%, respectively, each at 680 nm. The EQE curves of nonfullerene-based solar cells indicated a broad photoresponse up to a wavelength of nearly 800 nm while those of the fullerene-based solar cells showed a photoresponse up to 700 nm. This result indicated the enhanced light absorption in nonfullerene-based devices due to the complementary absorption at the long wavelength of O-IDTBR.38 The UV-visible absorption spectrum of O-IDTBR thin film is shown in Figure S17. The significantly decreased photocurrent implied that the charge generation, transfer, and transport in the nonfullerene-based solar cells were limited.39,40 Therefore, they should be characterized to understand the unsatisfactory photovoltaic properties of these solar cells. The EQE curves of the DTBDT-Rho and DTBDT-S-Rho solar cells exhibited similar shapes but different intensities, indicating that the light absorption characteristics as a function of the wavelength were similar for the solar cell devices employing the two different DTBDT-based small molecules, but the charge carrier dynamics were different due to differences in their intermolecular interactions. ACS Paragon Plus Environment

Page 9 of 48 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


Bulk heterojunction blend morphology. Together with exploring the different charge carrier dynamics in the fullerene-based and nonfullerene-based solar cells with the two DTBDT-based small-molecule donors, we analyzed their morphologies to derive structureproperty relationships for these samples and to better understand the relatively poor performances of the nonfullerene-based devices. First, we characterized the surface morphologies of the fullerene-based and nonfullerene-based blend films. The atomic force microscopy (AFM) images shown in Figure 4 revealed the degrees of molecular aggregation in the blend systems, which affected their exciton dissociation properties. The blend films with nonfullerene acceptor showed higher degrees of aggregation than did those with the fullerene acceptor. The AFM surface image of the DTBDT-Rho:PC71BM blend film exhibited a smooth and homogeneous morphology with a root-mean-square (RMS) roughness of 0.45 nm. The DTBDT-S-Rho blend film exhibited a more aggregated morphology, with an increased RMS roughness of 0.89 nm; we attributed this increased roughness to the alkylthio group, which was introduced to enhance intermolecular interactions.24,29,41,42 The AFM images of the DTBDT-based small-molecule:O-IDTBR all-small-molecule blends showed considerably increased roughness, with RMS roughness values of 1.47 and 2.62 nm for the DTBDT-Rho:O-IDTBR and DTBDT-S-Rho:O-IDTBR blend films, respectively. The enhanced molecular aggregation, which resulted in larger-scale phase separation, can explain the limited exciton diffusion to the donor/acceptor interface and the lower photovoltaic performances of the devices using O-IDTBR.33,43,44 The bulk aggregation and phase separation behaviors for each blend system were characterized using transmission electron microscopy (TEM); these TEM images directly showed the degrees of phase separation (Figure 5). The DTBDT-Rho:PC71BM blend film



showed excellent miscibility, with domain dimensions of about 10 nm, appropriate for exciton dissociation.33,35 The DTBDT-S-Rho:PC71BM blend film showed more aggregation; larger domains, unfavorable for exciton dissociation, were observed in the TEM images of the DTBDT-S-Rho:PC71BM blend film, in agreement with the AFM results and decreased Jsc values.44,45 The DTBDT-Rho:O-IDTBR and DTBDT-S-Rho:O-IDTBR blend films exhibited phase separation on a very large scale, with domains larger than 100 nm; this observation was consistent with the AFM surface morphology and photovoltaic performances, in that the larger-scale phase separation was expected to increase the probability of exciton decay prior to the exciton dissociation at the donor/acceptor interface and limit the formation of groundstate interfacial charge-transfer states.46,47 These results suggested the severe aggregation of small-molecule donors to be a problematic issue in all-small-molecule solar cells with DTBDT-based donors and O-IDTBR acceptors. The degrees of phase separation and molecular aggregation in each blend system were also probed by taking UV-visible absorption and photoluminescence (PL) measurements. Figure 6a and Figure S18 shows UV-visible absorption spectra of the blend films. The absorption spectra of DTBDT-based small-molecule:O-IDTBR blend films exhibited extended ranges of light absorption compared to those of DTBDT-based small-molecule:PC71BM blend films, as we expected from the long-wavelength absorption of O-IDTBR and EQE measurement results. However, the absorption spectra of the blend films containing DTBDT-S-Rho exhibited strong and slightly bathochromic-shifted absorption bands compared to those of the blend films having DTBDT-Rho; this observation was attributed to the enhanced intermolecular interactions of DTBDT-S-Rho.24,29 The absorption spectrum of DTBDTRho:PC71BM showed a maximum value at a wavelength of 576 nm, while DTBDT-SRho:PC71BM blend film showed a maximum absorption at 582 nm and a strong shoulder at 640 nm. The maximum absorption peaks of the DTBDT-based small-molecule:O-IDTBR ACS Paragon Plus Environment

Page 10 of 48

Page 11 of 48 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


films were ascribed to the shoulder of the absorption spectra of DTBDT-based small molecule and O-IDTBR;20,38 and the DTBDT-S-Rho:O-IDTBR blend film also showed a bathochromic shift compared to DTBDT-Rho:O-IDTBR film. This result is in agreement with AFM and grazing-incidence wide-angle X-ray scattering (GIWAXS) results discussed below, which showed enhanced molecular aggregation and crystallinity in blend films employing DTBDT-S-Rho. PL measurements were taken to investigate the exciton dissociation behavior in blend films with fullerene and nonfullerene acceptors and to confirm that the exciton dissociations in these systems are affected by the nanoscale morphologies described above. The photoluminescence quenching efficiency (PLQE) levels of the blend films are shown in Figure 6b. While the DTBDT-based small molecule:PC71BM blend films yielded a low PL intensity with high PLQE over 94%, the DTBDT-based small-molecule:O-IDTBR blend films showed a remarkably decreased PLQE. The PL spectra of the DTBDT-based smallmolecule:O-IDTBR blends showed peaks behind 750 nm, which appeared to have been due to the emission of O-IDTBR. The reduced overall area of the interfaces between the DTBDTbased donors and O-IDTBR acceptors, due to the large-scale phase separation shown in the TEM images, limited the number of excitons diffusing to these interfaces. The film of DTBDT-S-Rho blended with PC71BM as well as that blended with O-IDTBR showed lower quenching efficiencies compared to that of DTBDT-Rho. Along with charge-generation and charge-transfer properties, charge transport is also an important consideration to achieve better photovoltaic performances.35,48 Therefore, the molecular orientation and crystallinity of the small molecules were investigated with GIWAXS. First, we characterized the crystalline structures of three neat films, specifically of the two DTBDT-based small-molecule donors and of the O-IDTBR small-molecule acceptor,



respectively (Figure S19). The DTBDT-based small molecules adopted predominantly edgeon orientations, according to the (h00) reflection peaks observed along the out-of-plane direction and (010) π-π stacking peaks along the in-plane direction in their GIWAXS images. The (h00) peaks of DTBDT-S-Rho at qz = 0.34, 0.67, and 0.99 Å-1 corresponded to a dspacing of 1.85 nm, less than the 2.17 nm value for DTBDT-Rho. Compared to the GIWAXS pattern of DTBDT-Rho, that of DTBDT-S-Rho exhibited a stronger (010) peak at qxy = 1.71 Å-1 but the same π-π stacking distance of 3.67 Å. The enhanced crystallinity of DTBDT-SRho was consistent with the introduction of the alkylthio group in the side chain having increased the intermolecular interaction strength as we intended. The GIWAXS pattern of OIDTBR showed that the O-IDTBR molecules predominantly adopted a face-on orientation, as previously reported.18 We also characterized the crystallinity levels of the small-molecule donor and acceptors in the four different blend systems, which as described above showed different phase separation behaviors of the small-molecule donors and the fullerene or nonfullerene acceptors. The four blend films exhibited distinctly different GIWAXS patterns. The predominantly edge-on orientation of the DTBDT-based small molecules was maintained in the blend films, but the different blends showed different crystallinity features for the small molecules. We acquired two-dimensional (2D) GIWAXS images and line-cut profiles in the out-of-plane direction for the four blend films, as shown in Figure 7 and Figure S20. Amorphous halos were observed in the 2D-GIWAXS patterns of the DTBDT-Rho:PC71BM and DTBDT-S-Rho:PC71BM blend films, shown in Figure 7a and 7b, and were attributed to the PC71BM domains;49,50 and π-π stacking peaks appeared in the out-of-plane direction and were from O-IDTBR.14 The (h00) peaks resulting from DTBDT-Rho in the DTBDT-Rho:PC71BM blend film pattern were found at qz = 0.29, 0.58, and 0.87 Å-1, corresponding to a d-spacing of 2.17 nm. Those of DTBDT-S-Rho in the blend with PC71BM were observed at qz = 0.35, 0.70 and 1.03 Å-1, ACS Paragon Plus Environment

Page 12 of 48

Page 13 of 48 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


giving a d-spacing of 1.80 nm. The DTBDT-Rho and DTBDT-S-Rho small-molecule: PC71BM blend films also yielded peaks at qxy = 1.67 and 1.70 Å-1, respectively, corresponding to π-π stackings with distances of 3.76 and 3.70 Å for DTBDT-Rho and DTBDT-S-Rho, respectively; these distances for the blend films were greater than those from films of neat small-molecule donors. However, DTBDT-S-Rho showed shorter π-π stacking distances than did DTBDT-Rho, which we attributed to the stronger intermolecular interactions formed by DTBDT-S-Rho. The (h00) and π-π stacking peaks of DTBDT-S-Rho were more intense than those of DTBDT-Rho in the GIWAXS patterns of blend films as well as in the GIWAXS patterns of neat small-molecule films, indicative of the greater crystallinity of DTBDT-S-Rho. The GIWAXS pattern of the DTBDT-based small-molecule:O-IDTBR blend film showed stronger scattering peaks from the DTBDT-based small molecules compared to that of small molecule-donor:PC71BM blends, which was attributed to the greater crystallinity in OIDTBR blend films and which was in agreement with the morphology characterizations described above. The GIWAXS pattern of the DTBDT-Rho:O-IDTBR blend showed (h00) peaks at qz = 0.30, 0.60, and 0.89 Å-1, corresponding to a d-spacing of 2.09 nm, and attributed to DTBDT-Rho. The GIWAXS pattern of the DTBDT-S-Rho:O-IDTBR blend showed (h00) reflection peaks at qz = 0.35, 0.70, and 1.03 Å-1, corresponding to a d-spacing of 1.79 nm, and attributed to DTBDT-S-Rho. These two patterns also showed peaks at qxy = 1.69 and 1.72 Å-1, respectively; and these peaks were attributed to π-π stacking of DTBDT-Rho and DTBDT-SRho with stacking distances of 3.72 and 3.65 Å, respectively. These results showed the π-π stacking distances of DTBDT-based small molecules in blend films with O-IDTBR to be shorter than those in blend films with PC71BM. And, as a result of stronger intermolecular interactions, DTBDT-S-Rho showed a shorter π-π stacking distance than did DTBDT-Rho in the blend with O-IDTBR as well as in the blend with PC71BM described above. In addition, ACS Paragon Plus Environment


the difference between the intensities of the O-IDTBR π-π stacking peaks of the two DTBDTbased small-molecule:O-IDTBR GIWAXS patterns indicated that the O-IDTBR showed strong molecular aggregation when blended with specifically DTBDT-S-Rho. We here confirmed the high crystallinity of DTBDT-based small-molecule donors in blend films with nonfullerene acceptor, which resulted in phase separation between the small-molecule donor and nonfullerene acceptor.

Study of charge-carrier dynamics. To investigate the exciton generation and charge carrier dynamics in the neat DTBDT-based electron donating small-molecule films and blend films incorporating fullerene or nonfullerene electron acceptor, ultrafast transient absorption spectroscopy (TAS) measurements were taken and plotted, as shown in Figure S21-23. These measurements were taken in the visible (500-800 nm) and NIR (900–1300 nm) regions after excitation of DTBDT with light of a wavelength of 480 nm. The transient absorption spectra of the neat small-molecule donor films each exhibited a broad absorption peak at the NIR region and this absorption was ascribed to singlet exciton generation.51,52 The transient absorption spectra of the blend films with PC71BM showed the absorption of long-lived bound polaron pairs at 900-1000 nm. The long-lived bound polaron pair signals acquired from the blend films were attributed to electron transfer from the DTBDT-based small molecule to PC71BM. In the absorption spectra of the DTBDT-based small-molecule:OIDTBR blend films, absorption peaks attributed to excitons from O-IDTBR appeared at the initial delay time, but the polaron peaks did not appear compared to fullerene-based blends, revealing inefficient exciton dissociation into bound polarons in the O-IDTBR-based blend. The high crystallinity and large domain size of the DTBDT-Rho and DTBDT-S-Rho small molecules, in blend films with O-IDTBR, were consistent with the inefficient electron


Page 14 of 48

Page 15 of 48 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


transfer of the DTBDT-based small-molecule:O-IDTBR blend films, as also indicated from the low PLQE as determined from the morphological studies. We investigated the exciton decay kinetics of the DTBDT-based small molecules, as shown in Figure 8. The neat DTBDT-Rho and DTBDT-S-Rho films showed exciton lifetimes of 170 ps and 150 ps, respectively, determined by exciting them at a wavelength of 480 nm and measuring their absorbance at 1100 nm. In the blend films with PC71BM, the exciton lifetimes of DTBDT-Rho and DTBDT-S-Rho were 20 ps and 15 ps, respectively. These reduced exciton lifetimes led to efficient exciton dissociation at the interface between donor and acceptor.53 However, the exciton lifetimes of the small-molecule donors in the blend films with O-IDTBR were 105 ps and 150 ps, showing inefficient exciton dissociation in the systems consisting entirely of small molecules. These results indicated that the exciton migration and/or charge carrier dynamics in the DTBDT-based small-molecule blend films significantly depended on the morphology of the DTBDT-based small molecule in blends. We also examined the charge-transport properties in the four blend systems by taking spacecharge-limited current (SCLC) measurements. These measurements were specifically taken to investigate the effect of blend morphology, especially the crystallinity of the DTBDTbased small-molecule donor. The hole and electron mobilities for each system were measured by fabricating hole-only and electron only devices with glass/ITO/MoO3/active layer/Au and glass/ITO/Al/active layer/Al device configurations, respectively, acquiring from them dark J−V curves, and then fitting these curves to the SCLC model. The dark J−V curves and charge carrier mobility values are shown in Figure 9 and Table 3, respectively. The hole mobility of the DTBDT-based small-molecule donor in blend films with fullerene and nonfullerene acceptors were similar and the electron mobility of fullerene and nonfullerene acceptors blended with DTBDT-Rho or DTBDT-S-Rho didn’t show considerable difference



when the same donor materials were used, reflecting that the different photovoltaic performances of fullerene-based and nonfullerene-based solar cells were mainly influenced by charge-transfer dynamic rather than charge-transport properties. The DTBDTRho:PC71BM and DTBDT-S-Rho:PC71BM devices gave hole mobilities of 4.56×10-4 and 8.02×10-4 cm2V-1s-1, respectively. The greater hole mobility afforded by DTBDT-S-Rho was a result of its greater crystallinity, which was in turn due to its stronger intermolecular interactions.33,54 We expected the greater crystallinity in the small molecule:O-IDTBR blends to yield a higher mobility, but similar mobilities were obtained, which was probably due to the large but disconnected domains.33,

55

The nonfullerene-based device showed hole

mobilities of 2.26×10-4 and 2.97×10-4 cm2V-1s-1, respectively, for the DTBDT-Rho:O-IDTBR and DTBDT-S-Rho:O-IDTBR devices. The fullerene-based electron-only devices with DTBDT-Rho and DTBDT-S-Rho gave electron mobilities of 8.65×10-5 and 1.83×10-4 cm2V1 -1

s , respectively. The increased electron mobility in a DTBDT-S-Rho:PC71BM blend might

result from the moderated domain size in this blend. The electron mobilities obtained from the DTBDT-Rho:O-IDTBR and DTBDT-S-Rho:O-IDTBR electron-only devices were 6.39×10-5 and 8.41×10-5 cm2V-1s-1, respectively. Although O-IDTBR have crystalline structure within the blends and PC71BM is amorphous, the electron mobilities measured from the fullerene- and nonfullerene-based electron only devices were similar. These results revealed that modulating the phase separation between the DTBDT-based donor and nonfullerene acceptor is more important than improving crystallinity for improving the photovoltaic performances of the all-small-molecule solar cells, in that the blends having DTBDT-based small molecule-donors and fullerene or nonfullerene acceptors gave similar charge carrier mobilities.


Page 16 of 48

Page 17 of 48 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


CONCLUSIONS A series of DTBDT-based small-molecule donors were incorporated in small-molecule solar cells, some with a fullerene acceptor and others with a nonfullerene acceptor, to study the aggregation behavior and crystallinity of the donor molecules in both blends. DTBDT-Rho and DTBDT-S-Rho, DTBDT-based small molecules with different side chains, were used to modulate the intermolecular interactions and investigate the influence of the strength of the intermolecular interactions on the photovoltaic performance. A solar cell device with DTBDT-Rho as an electron donor and PC71BM as a fullerene electron acceptor exhibited a maximum PCE of 7.78%. A DTBDT-S-Rho:PC71BM solar cell gave a PCE of 5.89%. When O-IDTBR was employed as a nonfullerene acceptor, the DTBDT-Rho:O-IDTBR and DTBDT-S-Rho:O-IDTBR all-small-molecule solar cells showed decreased PCEs, specifically with maximum values of 4.37% and 2.96%, respectively. To understand the structureproperty relationships and address the limitations and requirements of all-small-molecule solar cells incorporating DTBDT-based small-molecule donors and nonfullerene acceptor, we performed a morphological analysis and charge carrier dynamics study for each blend. The study of morphology and charge carrier dynamics revealed that the large-scale phase separation in all-small-molecule solar cells with nonfullerene acceptor limited exciton diffusion. Because of the unfavorable morphologies in nonfullerene-based devices, decreased Jsc and FF were observed and the Voc also decreased unusaully indicating significant energy losses. In addition, the enhanced intermolecular interactions and aggregation of DTBDT-SRho with the alkylthio group led to unfavorable charge transfer due to the larger-scale phase separation, while DTBDT-S-Rho did show improved crystallinity.

ASSOCIATED CONTENT



Supporting Information The Supporting Information is available free of charge on the ACS Publications website at ~. Experimental procedures for material synthesis, general measurement and characterization, photovoltaic device fabrication, and TAS and charge carrier mobility measurement, 1H-NMR, 13

C-NMR, mass spectra, TGA and DSC plots, GIWAXS patterns of neat DTBDT-based small

molecules and O-IDTBR films, GIWAXS line-cut profiles of the blend films, and femtosecond transient absorption spectra of neat DTBDT-based donor films and DTBDTbased donor:acceptor blend films.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (T.K.A.). *E-mail: [email protected] (Y.-H.K.). *E-mail: [email protected] (S.-K.K.). Author Contributions †

J.H., M.J.S., and H.C contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


Page 18 of 48

Page 19 of 48 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


This research was financially supported by the National Research Foundation of Korea (NRF) funded

by

the

Korean

government

(MSIP)

(2015R1A2A1A10055620

and

2017R1C1B2002888) and MOTIE & KETIE (20173010013000). This work was also supported by the Center for Advanced Soft Electronics under the Global Frontier Research Program (2013M3A6A5073175), and by Basic Science Research Program through the NRF funded by the Ministry of Education (2018R1A6A1A03023788).

REFERENCES (1) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-Based Organic Solar Cells. Chem. Rev. 2007, 107, 1324-1338. (2) Cheng, Y.J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868-5923. (3) Su, Y.-W.; Lan, S.-C.; Wei, K.-H. Organic Photovoltaics. Materials Today 2012, 15, 554-562. (4) Su, Y.-W.; Lin, Y.-C.; Wei, K.-H. Evolving Molecular Architectures of Donor– Acceptor Conjugated Polymers for Photovoltaic Applications: From One-Dimensional to Branched to Two-Dimensional Structures. J. Mater. Chem. A 2017, 5, 24051-24075. (5) Collins, S. D.; Ran, N. A.; Heiber, M. C.; Nguyen, T.–Q. Small is Powerful: Recent Progress in Solution-Processed Small Molecule Solar Cells. Adv. Energy Mater. 2017, 7, 1602242. (6) 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.



(7) Eftaiha A. F.; Sun, J.-P.; Hill, I. G.; Welch, G. C. Recent Advances of Non-Fullerene, Small Molecular Acceptors for Solution Processed Bulk Heterojunction Solar Cells. J. Mater. Chem. A 2014, 2, 1201-1213. (8) Mishra, A.; Bauerle, P. Small Molecule Organic Semiconductors on the Move: Promises for Future Solar Energy Technology. Angew. Chem. Int. Ed. 2012, 51, 2020-2067. (9) Qiu, B.; Xue, L.; Yang, Y.; Bin, H.; Zhang, Y.; Zhang, C.; Xiao, M.; Park, K.; Morrison, W.; Zhang, Z.-G.; Li, Y. All-Small-Molecule Nonfullerene Organic Solar Cells with High Fill Factor and High Efficiency over 10%. Chem. Mater. 2017, 29, 7543-7553. (10) Yang, L.; Zhang, S.; He, C.; Zhang, J.; Yao, H.; Yang, Y.; Zhang, Y.; Zhao, W.; Hou, J. New Wide Band Gap Donor for Efficient Fullerene-Free All-Small-Molecule Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 1958-1966. (11) Liang, R.-Z.; Babics, M.; Savikhin, V.; Zhang, W.; Corre, V. M. L.; Lopatin, S.; Kan, Z.; Firdaus, Y.; Liu, S.; McCulloch, I.; Toney, M. F.; Beaujuge, P. M. Carrier Transport and Recombination in Efficient “All-Small-Molecule” Solar Cells with the Nonfullerene Acceptor IDTBR. Adv. Energy Mater. 2018, 1800264. (12) Li, H.; Fang, J.; Zhang, J.; Zhou, R.; Wu, Q.; Deng, D.; Adil, M. A.; Lu, K.; Guo, W.; Wei, Z. A Novel Small Molecule Based on Naphtho[1,2-b:5,6-b’]Dithiophene Benefits Both Fullerene and Non-Fullerene Solar Cells Mater. Chem. Front. 2018, 2, 143-148. (13) Bin, H.; Yang, Y.; Zhang, Z.-G.; Ye, L.; Ghasemi, M.; Chen, S.; Zhang, Y.; Zhang, C.; Sun, C.; Xue, L.; Yang, C.; Ade, H.; Li, Y. 9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cells with Absorption-Complementary Donor and Acceptor. J. Am. Chem. Soc. 2017, 139, 5085−5094. (14) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular


Page 20 of 48

Page 21 of 48 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


Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017 139, 7148-7151. (15) Xu, X.; Yu, T.; Bi, Z.; Ma, W.; Li W.; Peng, Q. Realizing Over 13% Efficiency in Green-Solvent-Processed Nonfullerene Organic Solar Cells Enabled by 1,3,4-ThiadiazoleBased Wide-Bandgap Copolymers. Adv. Mater. 2018, 30, 1703973. (16) Xiao, Z.; Jia, X.; Ding, L. Ternary Organic Solar Cells Offer 14% Power Conversion Efficiency. Sci. Bull. 2017, 62, 1562-1564. (17) Feng, G.; Xu, Y.; Zhang, J.; Wang, Z.; Zhou, Y.; Li, Y.; Wei, Z.; Li, C.; Li, W. AllSmall-Molecule Organic Solar Cells Based on an Electron Donor Incorporating Binary Electron-Deficient Units. J. Mater. Chem. A 2016, 4, 6056-6063. (18) Badgujar, S.; Song, C. E.; Oh, S.; Shin, W. S.; Moon, S.-J.; Lee, J.-C.; Jung, I. H.; Lee, S. K. Highly Efficient and Thermally Stable Fullerene-Free Organic Solar Cells Based on a Small Molecule Donor and Acceptor. J. Mater. Chem. A 2016, 4, 16335-16340. (19) Sharenko, A.; Proctor, C. M.; van der Poll, T. S.; Henson, Z. B.; Nguyen, T.-Q.; Bazan, G. C. A High-Performing Solution-Processed Small Molecule:Perylene Diimide Bulk Heterojunction Solar Cell. Adv. Mater. 2013, 25, 4403-4406. (20) Song, H. G.; Kim, Y. J.; Lee, J. S.; Kim, Y.-H.; Park, C. E.; Kwon, S.-K. Dithienobenzodithiophene-Based Small Molecule Organic Solar Cells with over 7% Efficiency via Additive- and Thermal-Annealing-Free Processing. ACS Appl. Mater. Interfaces 2016, 8, 34353-34359. (21) Zhang, Z.-G.; Li, Y. Side-Chain Engineering of High-Efficiency Conjugated Polymer Photovoltaic Materials. Sci. China. Chem. 2015, 58, 192-209.



(22) Mei, J.; Bao, Z. Side Chain Engineering in Solution-Processable Conjugated Polymers. Chem. Mater. 2014, 26, 604−615. (23) Cui, C.; Wong, W.-Y.; Li, Y. Improvement of Open-Circuit Voltage and Photovoltaic Properties of 2D-Conjugated Polymers by Alkylthio Substitution. Energy Environ. Sci. 2014, 7, 2276-2284. (24) Huo, L.; Zhou, Y.; Li, Y. Alkylthio-Substituted Polythiophene: Absorption and Photovoltaic Properties. Macromol. Rapid Commun. 2009, 30, 925-931. (25) Li, X.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti, S. C.; Feeder, N.; Clegg, W.; Teat, S. J.; Friend, R. H. A Highly ð-Stacked Organic Semiconductor for Thin Film Transistors Based on Fused Thiophenes. J. Am. Chem. Soc. 1998, 120, 2206-2207. (26) 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, 84848487. (27) 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. (28) Cui, C.; He, Z.; Wu, Y.; Cheng, X.; Wu, H.; Li, Y.; Cao, Y.; Wong, W.-Y. HighPerformance Polymer Solar Cells Based on a 2D-Conjugated Polymer with an Alkylthio Side-Chain. Energy Environ. Sci. 2016, 9, 885-891. (29) Bin, H.; Zhang, Z.-G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y. NonFullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2D-Conjugated Polymers Reach 9.5% Efficiency. J. Am. Chem. Soc. 2016, 138, 4657-4664.


Page 22 of 48

Page 23 of 48 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


(30) Li, W.; Yao, H.; Zhang, H.; Li, S.; Hou, J. Potential of Nonfullerene Small Molecules with High Photovoltaic Performance. Chem. Asian J. 2017, 12, 2160-2171. (31) Bredas, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Molecular Understanding of Organic Solar Cells: The Challenges. Acc. Chem. Res. 2009, 42, 1691-1699. (32) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar Cells—Towards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789-794. (33) 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. (34) Hoppe, H.; Sariciftci, N. S. Morphology of Polymer/Fullerene Bulk Heterojunction Solar Cells. J. Mater. Chem. 2006, 16, 45-61. (35) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-Based Organic Solar Cells. Chem. Rev. 2007, 107, 1324-1338. (36) Elumalai, N. K.; Uddin, A. Open Circuit Voltage of Organic Solar Cells: An In-Depth Review. Energy Environ. Sci. 2016, 9, 391-410. (37) Shi, Q.; Fan, H.; Liu, Y.; Chen, J.; Ma, L.; Hu, W.; Shuai, Z.; Li, Y.; Zhan, X. Side Chain Engineering of Copolymers Based on Bithiazole and Benzodithiophene for Enhanced Photovoltaic Performance. Macromolecules 2011, 44, 4230-4240. (38) Holliday, S.; Ashraf, R. S.; Wadsworth, A.; Baran, D.; Yousaf, S. A.; Nielsen, C. B.; Tan, C. –H.; Dimitrov, S. D.; Shang, Z.; Gasparini, N.; Alamoudi, M.; Laquai, F.; Brabec, C. J.; Salleo, A.; Durrant, J. R.; McCulloch, I. High-Efficiency and Air-Stable P3HT-Based Polymer Solar Cells with a New Non-Fullerene Acceptor. Nat Mater. 2016, 7, 11585.



(39) Chen, J.-D.; Cui, C.; Li, Y.–Q.; Zhou, L.; Ou, Q.-D.; Li, C.; Li, Y.; Tang, J.-X. Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1035-1041. (40) Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.; Yang, X.; Feng, H.; Chen, Y. Solution-Processed Organic Solar Cells Based on Dialkylthiol-Substituted Benzodithiophene Unit with Efficiency near 10%. J. Am. Chem. Soc. 2014, 136, 1552915532. (41) Yao, H.; Ye, L.; Hou, J.; Jang, B.; Han, G.; Cui, Y.; Su, G. M.; Wang, C.; Gao, B.; Yu, R.; Zhang, H.; Yi, Y.; Woo, H. Y.; Ade, H.; Hou, J. Achieving Highly Efficient Nonfullerene Organic Solar Cells with Improved Intermolecular Interaction and Open-Circuit Voltage. Adv. Mater. 2017, 29, 1700254. (42) Liu, Y.; Mu, C.; Jiang, K.; Zhao, J.; Li, Y.; Zhang, L.; Li, Z.; Lai, J. Y. L.; Hu, H. Ma, T.; Hu, R.; Yu, D.; Huang, X.; Tang, B. Z.; Yan, H. A Tetraphenylethylene Core-Based 3D Structure Small Molecular Acceptor Enabling Efficient Non-Fullerene Organic Solar Cells. Adv. Mater. 2015, 27, 1015-1020. (43) Liao, H. –C.; Ho, C.-C.; Chang, C.-Y.; Jao, M.-H.; Darling, S. B.; Su, W.-F. Additives for Morphology Control in High-Efficiency Organic Solar Cells. Materials today, 2013, 16, 326-336. (44) van Franeker, J. J.; Turbiez, M.; Li, W.; Wienk, M. M.; Janssen, R. A. J. A Real-Time Study of the Benefits of Co-Solvents in Polymer Solar Cell Processing. Nat. Commun. 2015, 6, 6229. (45) Guo, S.; Wang, W.; Herzig, E. M.; Naumann, A.; Tainter, G.; Perlich, J.; MüllerBuschbaum, P. Solvent−Morphology−Property Relationship of PTB7:PC71BM Polymer


Page 24 of 48

Page 25 of 48 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


Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 3740-3748. (46) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736-6767. (47) Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo, M. A.; Manca, J. V.; Voorhis, T. V. Charge Transfer State Versus Hot Exciton Dissociation in PolymerFullerene Blended Solar Cells. J. Am. Chem. Soc. 2010, 132, 11878-11880. (48) Liu, T.; Guo, Y.; Yi, Y.; Huo, L.; Xue, X.; Sun, X.; Fu, H.; Xiong, W.; Meng, D.; Wang, Z.; Liu, F.; Russell, T. P.; Sun, Y. Ternary Organic Solar Cells Based on Two Compatible Nonfullerene Acceptors with Power Conversion Efficiency >10%. Adv. Mater. 2016, 28, 10008-10015. (49) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. (50) Deng, D.; Zhang, Y.; Zhang, J.; Wang, Z.; Zhu, L.; Fang, J.; Xia, B.; Wang, Z.; Lu, K.; Ma, W.; Wei, Z. Fluorination-Enabled Optimal Morphology Leads to Over 11% Efficiency for Inverted Small-Molecule Organic Solar Cells. Nat. Commun. 2016, 7, 13740. (51) Gélinas, S.; Rao, A.; Kumar, A.; Smith, S. L.; Chin, A. W.; Clark, J.; van der Poll, T. S.; Bazan, G. C.; Friend, R. H. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes. Science 2014, 343, 512-516. (52) Ohkita, H.; Ito, S. Transient Absorption Spectroscopy of Polymer-Based Thin-Film Solar Cells. Polymer 2011, 52, 4397-4417. (53) Hong, J.; Ha, Y. H.; Cha, H.; Kim, R.; Kim, Y. J.; Park, C. E.; Durrant, J. R.; Kwon,



S.-K.; An, T. K.; Kim, Y.-H. All-Small-Molecule Solar Cells Incorporating NDI-Based Acceptors:Synthesis and Full Characterization. ACS Appl. Mater. Interfaces 2017, 9, 44667−44677. (54) Fei, Z.; Eisner, F. D.; Jiao, X.; Azzouzi, M.; Röhr, J. A.; Han, Y.; Shahid, M.; Chesman, A. S. R.; Easton, C. D.; McNeill, C. R.; Anthopoulos, T. D.; Nelson, J.; Heeney, M. An Alkylated Indacenodithieno[3,2-b]thiophene-Based Nonfullerene Acceptor with High Crystallinity Exhibiting Single Junction Solar Cell Efficiencies Greater than 13% with Low Voltage Losses. Adv. Mater. 2018, 30, 1705209. (55) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A General Relationship Between Disorder, Aggregation and Charge Transport in Conjugated Polymers. Nat. mater. 2013, 12, 1038-1044.


Page 26 of 48

Page 27 of 48 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 1. Molecular structures of (a) DTBDT-based small-molecule electron donors and (b) electron acceptors incorporated in small-molecule solar cells.



Page 28 of 48

Figure 2. (a) Normalized UV-visible-NIR absorption spectra of DTBDT-S-Rho in a CHCl3 solution and as a thin film. (b) Cyclic voltammogram of DTBDT-S-Rho. energy levels of the components in small-molecule solar cells.


(c) Diagram of the

Page 29 of 48 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. (a) J-V characteristics and (b) EQE curves of DTBDT-based small-molecule solar cells with PC71BM or O-IDTBR electron acceptors.



Figure 4. AFM height images of (a) DTBDT-Rho:PC71BM, (b) DTBDT-S-Rho:PC71BM, (c) DTBDT-Rho:O-IDTBR and (d) DTBDT-S-Rho:O-IDTBR blend films.


Page 30 of 48

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


Figure 5. TEM images of the (a) DTBDT-Rho:PC71BM, (b) DTBDT-S-Rho:PC71BM, (c) DTBDT-Rho:O-IDTBR and (d) DTBDT-S-Rho:O-IDTBR blend films.



(a)

(b) Photoluminescence (a. u.)

DTBDT-Rho:PCBM DTBDT-S-Rho:PCBM DTBDT-Rho:O-IDTBR DTBDT-S-Rho:O-IDTBR

0.4

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

Page 32 of 48

0.3

0.2

0.1

0.0

DTBDT-Rho DTBDT-S-Rho DTBDT-Rho:PCBM DTBDT-S-Rho:PCBM DTBDT-Rho:O-IDTBR DTBDT-S-Rho:O-IDTBR

1.0 0.8 0.6 0.4 0.2 0.0

400

500

600

700

800

Wavelength (nm)

900

650

700

750

800

850

Wavelength (nm)

Figure 6. (a) UV-visible absorption spectra of DTBDT-based small-molecule donor:fullerene or non-fullerene acceptor blend films and (b) photoluminescence spectra of neat DTBDTbased small-molecule and blend films.


Page 33 of 48 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 7. 2D-GIWAXS patterns of the (a) DTBDT-Rho:PC71BM, (b) DTBDT-SRho:PC71BM, (c) DTBDT-Rho:O-IDTBR, and (d) DTBDT-S-Rho:O-IDTBR blend films.



1.5 DTBDT-Rho DTBDT-Rho:PCBM DTBDT-Rho:O-IDTBR

1.0

0.5

λexc 480 nm λprob 1100 nm fluence 5.0 μJcm-2 0.0

(b) Normalized ∆ OD

(a) Normalized ∆ OD

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 34 of 48

1.5 DTBDT-S-Rho DTBDT-S-Rho:PCBM DTBDT-S-Rho:O-IDTBR

1.0

0.5

0.0 1

10

100

1000

10000

1

Time (ps) DTBDT τ 1/2 = 170 ps DTBDT:PCBM τ 1/2 = 20 ps DTBDT:O-IDTBR τ 1/2 = 105 ps

10

100

1000

10000

Time (ps) DTBDT-S τ 1/2 = 150 ps DTBDT-S:PCBM τ 1/2 = 15 ps DTBDT-S:O-IDTBR τ 1/2 = 150 ps

Figure 8. Transient absorption spectra of neat and blend films of (a) DTBDT-Rho and (b) DTBDT-S-Rho.


Page 35 of 48

0

(b)


LogJD (A/cm2)

(a) LogJD (A/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


-1

0

-1


-2

-3

-2

0.01

0.1

1

-4 0.01

Voltage (V)

0.1

1

Voltage (V)

Figure 9. Dark J-V curves obtained from (a) hole-only and (b) electron-only devices of DTBDT-based small-molecule donor:fullerene acceptor or DTBDT-based small-molecule donor:nonfullerene acceptor.



Page 36 of 48

Table 1. Optical and electrochemical properties of the DTBDT-based small molecules λmax (nm) solution

λmax (nm) film

λonset (nm) film

Egopt a (eV)

HOMOb (eV)

LUMOc (eV)

DTBDT-Rho

510†

585, 636†

740†

1.68†

-5.11†

-3.43†

DTBDT-SRho

507

587, 635

740

1.68

-5.17

-3.49

Small molecule

a

Estimated from the absorption edge in film (Egopt=1240/ λonset eV).

b

Estimated from the onset oxidation potential.

c

Determined by HOMO + Eg opt .

†

Data from the reference 16.


Page 37 of 48 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 2. Photovoltaic characteristics of the DTBDT-based small-molecule solar cells

Voc (V)

Jsc (mA cm-2)

FF

PCE (%)

PCEmax (%)

DTBDTRho:PC71BM

0.88 (±0.01)

15.20 (±0.29)

0.55 (±0.01)

7.40 (±0.33)

7.78

DTBDT-SRho:PC71BM

0.88 (±0.01)

12.05 (±0.30)

0.53 (±0.01)

5.62 (±0.31)

5.87

DTBDT-Rho:OIDTBR

0.76 (±0.01)

13.28 (±0.20)

0.41 (±0.01)

4.20 (±0.14)

4.38

DTBDT-S-Rho:OIDTBR

0.78 (±0.01)

8.07 (±0.13)

0.43 (±0.01)

2.72 (±0.15)

2.96

Donor:Acceptor

The average values and standard derivations in parentheses were obtained from 8 devices.



Page 38 of 48

Table 3. Charge carrier mobilities in DTBDT-based small-molecule donor:fullerene or non-fullerene acceptor blends 2

1 -1

2

1 -1

µh (cm V- s )

µe (cm V- s )

DTBDT-Rho:PC71BM

4.56 × 10-4

8.65 × 10-5

DTBDT-S-Rho:PC71BM

8.02 × 10-4

1.83 × 10-4

DTBDT-Rho:O-IDTBR

2.26 × 10-4

6.39 × 10-5

DTBDT-S-Rho:O-IDTBR

2.97 × 10-4

8.41 × 10-5

Donor:Acceptor


Page 39 of 48 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 1. Molecular structures of (a) DTBDT-based small-molecule electron donors and (b) electron acceptors incorporated in small-molecule solar cells. 231x115mm (150 x 150 DPI)


Page 40 of 48

Page 41 of 48 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 2. (a) Normalized UV-visible-NIR absorption spectra of DTBDT-S-Rho in a CHCl3 solution and as a thin film. (b) Cyclic voltammogram of DTBDT-S-Rho. (c) Diagram of the energy levels of the components in small-molecule solar cells. 225x64mm (150 x 150 DPI)



Figure 3. (a) J-V characteristics and (b) EQE curves of DTBDT-based small-molecule solar cells with PC71BM or O-IDTBR electron acceptors. 244x89mm (150 x 150 DPI)


Page 42 of 48

Page 43 of 48 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. AFM height images of (a) DTBDT-Rho:PC71BM, (b) DTBDT-S-Rho:PC71BM, (c) DTBDT-Rho:OIDTBR and (d) DTBDT-S-Rho:O-IDTBR blend films. 134x134mm (150 x 150 DPI)



Figure 5. TEM images of the (a) DTBDT-Rho:PC71BM, (b) DTBDT-S-Rho:PC71BM, (c) DTBDT-Rho:O-IDTBR and (d) DTBDT-S-Rho:O-IDTBR blend films. 132x131mm (150 x 150 DPI)


Page 44 of 48

Page 45 of 48 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


236x92mm (150 x 150 DPI)



193x153mm (150 x 150 DPI)


Page 46 of 48

Page 47 of 48 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


205x83mm (150 x 150 DPI)



205x80mm (150 x 150 DPI)


Page 48 of 48

Understanding StructureâProperty Relationships in All-Small

Recommend Documents

Understanding StructureâProperty Relationships in All-Small