Bis-Silicon-Bridged Stilbene: A New Core for Small-Molecule Electron

Jan 10, 2018 - Silole, a five-membered silacyclic with fascinating photoelectric properties originating from sp3-hybrid silicon, is widely utilized fo...
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Bis-Silicon-Bridged Stilbene: A New Core for Small-Molecule Electron Acceptor for High-Performance Organic Solar Cells Zhongbo Zhang, and Xiaozhang Zhu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04930 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Chemistry of Materials

Bis-Silicon-Bridged Stilbene: A New Core for Small-Molecule Electron Acceptor for High-Performance Organic Solar Cells Zhongbo Zhang,†,‡ and Xiaozhang Zhu†,‡,* †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: Silole, a five-membered silacyclic with fascinating photoelectric properties originating from sp3-hybrid silicon, is widely utilized for organic π-functional materials. In this work, a new small-molecule non-fullerene acceptor NSTI was designed and synthesized by using bis-silicon-bridged stilbene as the core unit. NSTI exhibited a low optical bandgap of 1.58 eV and a broadened absorption spectrum (550–800 nm). By blending with a polymer donor PBDB-T that has a complementary absorption and well-matched energy levels with NSTI, we achieved a high power conversion efficiency (PCE) of 10.33%. To the best of our knowledge, this is so far the best performance reported for OPV devices utilizing silole-based organic photovoltaic materials.

Recently, the research on non-fullerene electron acceptors has provided a new opportunity to promote the rapid development of high-performance organic photovoltaics (OPVs).1–2 Compared with fullerene acceptors, non-fullerene acceptors (NFAs) possess significant advantages of tunable electronic structure and the potential of lowering production cost. Over the past decade, considerable efforts have been dedicated to developing NFAs. An important kind of small-molecule NFAs based on an electron-rich π-extended fused rings as presented by indacenodithiophene3–9 and indacenodithieno[3,2b]thiophene10–16 with an acceptor-donor-acceptor (A-D-A) framework show very promising photovoltaic performance and have received widespread attentions. Such A-D-A architecture ensures good conjugated planarity and facilitates πelectron delocalization, which benefits the decrease of energy bandgap and the extension of light absorption wavelength. Inspiringly, fullerene-free organic photovoltaics using ITIC and its analogues as electron acceptor and conjugated polymers as electron donor exhibited impressive power conversion efficiencies (PCEs) of above 12%,10,15–19 which are even higher than those using PC71BM acceptor. Current molecular design for ideal small-molecule NFAs with an A-D-A structure mainly focused on the adjustment of bulky π-conjugated heteroaromatic cores,4,20–21 π-bridges,3,5–6,9 and terminal electrondeficient moieties.10,14–15,22 Among these, developing new polycyclic arene and heteroarenes as the central building blocks is considered to be an effective idea to pursue attractive smallmolecule NFAs. Very recently, new rigid ladder-type fused moieties based on benzo[1,2-b:4,5-b’] dithiophene and two cyclopenta[2,1-b;3,4-b’]dithiophene have been reported and PCEs over 10%.23–25 Silole is a five-membered silacyclic that possesses σ*–π* conjugation arising from the interaction between the σ* orbital of two exocyclic σ–bonds on the silicon atom and the π* orbital of the butadiene moiety, enduing silole with fascinating optoelectronic properties such as strong fluorescence in the solid state and high electron-accepting properties.26–27 Fused

silole derivatives such as dibenzo[b,d]silole (DBS), dithieno[3,2-b:2’,3’-d]silole (DTS), and bis-silicon-bridged stilbene (BSS) are attractive building blocks for the design of polymer and small-molecule electronic and optoelectronic materials that are widely applied in thin-film transistors (OTFTs),28 light-emitting diodes (OLEDs),29 and OPVs,28,30–31 attributing to their electron-rich nature, planar molecular structure, and electronic tunability through structural modifications. Heeger and co-workers reported a small molecular DTS(PTTh2)2 based on DTS exhibiting a high saturation hole mobility of 0.12 cm2 V–1 s–1 and Ion/Ioff ~107,28 and they fabricated solution-processable small-molecule solar cells with a designed small-molecule donor p-DTS(FBTTh2)2 and achieved an encouraging PCE over 8% as well.32 However, the photovoltaic performance of small-molecule NFAs based on silole derivatives was far from satisfactory, a DBS-based small-molecule NFA only delivered a PCE of 2.76%.33 Yamaguchi and co-workers reported a series of π-conjugated polymers based on BSS, which exhibit an intense blue to greenishblue emission.34 The easy tuning of absorption and emission scope and extended coplanar skeleton of BSS make it a valuable building block for the design of new organic π-functional materials. By incorporating strongly electron-deficient terminal groups onto BSS, new electron-accepting materials are likely to be promising for OPV applications. In our previous work, we introduced thieno[3,4-b]thiophene (TbT) as a bridge unit between strong donor-acceptor pairs utilizing its quninodal resonance35 to enhancing the π– conjugation along the molecular backbone.5,36–38 This effective strategy, “enhancing the quinoidal resonance of D-A system”, has been successfully extended to the design of donor and acceptor materials, both of which achieved excellent OPV performance. Herein, a new small-molecule acceptor NSTI with a BSS core, 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden1-ylidene)malononitrile (INCN-2F) terminals, and 2-alkylsubstituted TbT linkers was designed and synthesized (Scheme 1). Our molecular design principles are as follows: (i)

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Scheme 1. Synthesis and Chemical Structure of NSTI.a EH F

C 8H 17

Si

C8 H 17

C 8 H17

F a,b

I

I

S

OHC

Si

S

C 8H 17

Si C 8H 17

C 8H17

S

C 8H 17 2

1 F

S

Si

F

C8H17

CHO

F

HE

F

EH NC c

NC

O

F

C 8H 17

Si

Figure 1. (a) Normalized UV-vis absorption spectra of NSTI in chloroform (blue line), solid state (red line), PBDB-T in the solid state (black line). (b) Energy diagram relative to vacuum level.

S

C8 H17

S CN

S S

Si C 8H 17 C8 H17

F

O

CN

HE NSTI F

F

a

Reagents and conditions: (a) tributyl(2-(2-ethylhexyl)thieno[3,4b]thiophen-6-yl)stannane, Pd(PPh3)4, toluene/DMF, 100 °C; (b) POCl3, DMF, CH2Cl2, rt; (c) INCN-2F, pyridine, chloroform, reflux.

BSS possesses rigid and coplanar structure which is beneficial for charge transport and the four alkyl chains affiliated to silicon atoms are out of the BSS plane that can restrict excessive aggregation in blend films; (ii) INCN-2F has strong electronaccepting ability that can down shifts LUMO (the lowest unoccupied molecular orbital) energy levels; (iii) The introduction of TbT narrows the optical bandgap by quinoid-resonance effect, which is beneficial for light harvesting and thus achieving high short-circuit current (Jsc). We found that NSTI exhibited a narrow optical bandgap of 1.58 eV, inducing a strong and broad absorption that covers the spectrum range of 550– 800 nm, and appropriate energy levels as an electron acceptor. By utilizing a wide bandgap polymer donor, poly[(2,6-(4,8cbis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))] (PBDB-T) (The characteristics of PBDB-T can be found in Table S5 Supporting information) and NSTI acceptor, OPVs achieved the highest PCE value of 10.33% with an opencircuit voltage (Voc) of 0.832 V, a standout fill factor (FF) of 75.2%, and Jsc of 16.47 mA cm–2 after device optimization, which is so far the best reported for OPVs using silole-based organic photovoltaic materials.33,39–42 The synthetic route of compound NSTI is shown in Scheme 1. The central unit bis-silicon-bridged stilbene 1 was synthesized according to the literature.34 After the Stille coupling reaction of compound 1 and tributyl(2-(2ethylhexyl)thieno[3,4-b]thiophen-6-yl)stannane38 followed by Vilsmeier-Haack reaction, dialdehyde 2 was synthesized as a yellow solid in 61% yield. Subsequently, by Knoevenagel condensation of compound 2 with INCN-2F, we synthesized the target compound NSTI in 82% yield as a dark brown solid, which exhibits good solubility in common organic solvents. Thermogravimetric analysis indicates good thermal stability of NSTI with a 5% weight loss at 334 °C under a N2 atmosphere (see Figure S1 in Supporting Information). The UV-vis absorption spectrum of NSTI was examined in both dilute chloroform solution and thin film (Figure 1a). The chloroform solution of NSTI exhibited broad absorption in the 350–750 nm region. The absorption peak of NSTI solution was located at around 668 nm with a maximum molar extinction coefficient of 1.89 × 105 M–1 cm–1. The NSTI film exhibited a broadened absorption spectrum (550–800 nm) with

marked red shift (ca. 50 nm) and displayed a strong shoulder peak at 656 nm, indicating that NSTI forms ordered packing structure during the film forming process. The absorption onset of the NSTI thin film was located at 786 nm, which corresponds to a narrow optical bandgap of 1.58 eV that is 0.01 eV smaller than that of ITIC (Table S5 in Supporting information). The absorption spectrum of PBDB-T in thin film was also plotted in Figure 1a for comparison. Apparently, the complementary absorption of PBDB-T and NSTI is desirable for enhancing light harvest so as to increase Jsc of the solar cells. The electrochemical property of NSTI was examined by cyclic voltammetry measurement and shown in Figure S2 (Supporting Information). The potentials were internally calibrated using the ferrocene/ferrocenium (Fc/Fc+) redox couple (4.8 eV below the vacuum level) and the HOMO (highest occupied molecular orbital) and LUMO energy levels were estimated based on the onsets of the oxidation and reduction curves. The HOMO and LUMO energy levels of NSTI are – 5.54 eV and –3.87 eV, respectively, which are slightly deeper than those of ITIC (HOMO/LUMO: –5.48/–3.83) (Table S5 in Supporting Information). The energy diagram relative to the vacuum level is shown in Figure 1b. The LUMO energy offset between PBDB-T and NSTI is 0.71 eV, which may lead to efficient electron transfer from PBDB-T to NSTI. On the other hand, since the acceptor absorption also contributes to the overall photocurrent, the hole transfer from the acceptor to the donor need to be considered. The efficient hole transfer from NSTI to PBDB-T is confirmed by the high EQE values of the device in the spectral range of 700–800 nm, where the light absorption of the PBDB-T is negligible.43 To evaluate the photovoltaic performance of NSTI, we fabricated the OPVs based on PBDB-T donor with a conventional device architecture of ITO/PEDOT:PSS/PBDB-T:NSTI/PDINO/Al were fabricated, where PDINO is an efficient cathode buffer layer.44 After screening the D:A ratio/concentration and processing solvent, the active layers were prepared by spin-coating the PBDBT:NSTI (1:1, w/w) solution with a total weight concentration of 18 mg mL−1 in

Figure 2. (a) The J–V curves of the optimized OPVs based on PBDB-T:NSTI (1:1, w/w) without (blue line) and with 1-CN (red line) additive under the illumination of AM 1.5G, 100 mW cm–2. (The inset shows the histogram of the PCE counts for 50 devices with 1-CN.) (b) EQE spectra of the corresponding devices.

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Chemistry of Materials chlorobenzene and the optimized thickness of the active layer was ∼100 nm (Table S1-S3 in Supporting Information). OPV devices based on the as-cast PBDB-T:NSTI film gave a high PCE of 7.92% with a Voc of 0.807 V, a high Jsc of 14.17 mA cm−2, and a FF of 69.2%. Processing additives played an important role in organic photovoltaic systems to optimize the active layer film morphology and further improving the device performance. Upon adding 0.5% v/v 1-chloronaphthalene (1CN) to the solvent, the Jsc of the resulting OPVs was significantly increased to 16.47 mA cm–2. At the same time, the Voc was enhanced to 0.832 V and the FF increased to 75.2%. The combined improvements lead to a PCE of 10.33%, which is the highest PCE for OPVs using silole-based small-molecule acceptors. The current density−voltage (J−V) curves of the ascast and with 1-CN as additive devices were shown in Figure 2a, and their corresponding photovoltaic parameters are summarized in Table S1. The external quantum efficiency (EQE) curves of the optimal NSTI-based devices without and with 1CN covered a wide wavelength range from 300 to 800 nm (Figure 2b), which originated from complementary absorptions of the donor and acceptor materials. The EQE values for a device with 1-CN were much higher than those for the ascast device, reaching a maximum value of 75% at 660 nm indicating rather efficient photo-electricity conversion in the range of 375−750 nm with EQE values over 50%. Comparing to the current densities obtained from the J–V measurement, the Jsc values calculated from the EQE curves under the standard solar spectrum (AM 1.5 G) were 13.56 mA cm–2 and 16.06 mA cm–2, respectively, which represented a small deviation of less than 5%. Balanced hole/electron mobility played an important role in obtaining high performance in OPVs. By using the space charge limited current (SCLC) method with the hole/electrononly devices in the optimized blend ratio, the hole and electron mobilities were investigated (see Figure S4 in Supporting Information). By fitting the J-V curves from both hole- and electron-only devices, the carrier mobilities of PBDB-T:NSTI blends were determined. For the as-cast blend film, the hole and electron mobilities were estimated as 1.59 × 10–4 and 3.00 × 10–5 cm2 V–1 s–1, respectively. After mixing of 0.5% v/v 1CN to the solvent, the hole and electron mobilities are improved to 1.97 × 10–4 and 1.06 × 10–4 cm2 V–1 s–1, respectively. In comparison, the mobilities of the blend film processing of 1-CN are relatively higher, and more balance for hole and electron transportation (Table S1 in Supporting Information). The higher and balanced charge carrier mobilities of the film processed with additives are beneficial to the efficient exciton dissociation and charge transportation, which may contribute to the higher Jsc and FF of the corresponding as-cast OPVs. By plotting the photocurrent density (Jph, defined as JL–JD, where JL and JD are the current densities under illumination and in the dark, respectively) as a function of effective voltage (Veff, defined as V0–Vbias, where V0 is the voltage at which Jph is zero and Vbias is the applied external voltage bias, we investigated the exciton dissociation and charge collection properties in the devices without and with 1-CN. As can be seen from Figure 3a, Jph is saturated (Jsat) at Veff higher than 2 V, suggesting that charge recombination is minimized at higher voltage due to the high internal electric field in the devices. The charge dissociation and charge collection probability (P(E,T)) in the devices could be estimated from the value of Jph/Jsat.45 Under their short-circuit and maximal power output conditions, the P(E,T) values are 95%, 80% for the as-cast device,

and 97%, 83% for the device with 1-CN, respectively. The increased P(E,T) values indicate that the device processed with additives exhibits higher exciton dissociation rate and a more efficient charge collection efficiency compared to those of the as-cast device.45–46 To understand the charge recombination behavior of the OPVs, the effect of light intensity (P) and short-circuit current density was then studied (Figure 3b). Generally, the relationship between Jsc and light intensity can be described by the formula of Jsc∝Pα. If all free carriers are swept out and collected at the electrodes prior to recombination, α should be equal to 1, while α < 1 indicates some extent of bimolecular recombination.45,47 The α value for the device processed with 1-CN is 0.95, which is higher than 0.92 for the as cast device. It is indicated that there is more efficient transportation of carriers and less bimolecular recombination in the device processed with additives. This reduced bimolecular recombination thus resulted in high Jsc and better device performance. Typically, charge recombination is directly related with the FF of the devices. The lower bimolecular recombination in the additive treated devices agrees well with higher FF value of the device processed with 1-CN (75.2%) relative to that as cast device (69.2%).

Figure 3. (a) Photocurrent density versus effective applied voltage (Jph-Veff) characteristics. (b) Light intensity dependence of the short circuit current of the devices.

To understand the relationship between OPV performance and the molecular packing of BHJ films, we investigated the pure and blend films by the grazing incidence X-ray diffraction (GIXD). Figure 4 shows the 2D GIXD patterns and the corresponding line cut profiles in the in-plane (IP) and out-ofplane (OOP) direction of the pure films and blend films with and without 1-CN treatment. For the NSTI pure film, it showed a strong degree of crystallinity, the π–π stacking peak was located at 1.74 Å−1 (d-spacing of 3.61 Å) in the IP direction. The PBDB-T pure film shows obvious lamellar (100) peak at 0.29 Å−1 (lamellar d-spacing of 21.8 Å) in the IP direction and strong π–π stacking (010) peak at 1.68 Å−1 (d-spacing of 3.75 Å) in the OOP direction. The as-cast blend film without additive showed an edge-on orientation, and the π–π stacking peak was located at 1.74 Å−1 (d-spacing of 3.61 Å) in the IP direction. For the blend film treated with 1-CN, it showed lamellar (100) peak at 0.29 Å−1 (lamellar d-spacing of 21.8 Å) in the IP direction and the molecule packing was changed to a face-on orientation, which is advantageous for the charge transport. The π–π stacking (010) diffraction peak in the OOP direction was located at 1.71 Å−1 (d-spacing of 3.67 Å). The coherence length deduced from the full width at half maximum of OOP (010) peaks were calculated via Scherrer equation, the coherence lengths of the PBDB-T and NSTI are increased from 2.25/2.50 nm for the as cast blend film to 2.90/5.33 nm for 1-CN treated blend film, which means that higher ordering of NSTI packing is formed after 1-CN treatment.

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Figure 4. (a) 2D GIXD patterns and (b) line cut profiles for pure films of NSTI, PBDB-T and blend films of PBDB-T:NSTI with/without 1-CN (red line: out-of-plane; black line: in-plane).

In summary, a new small-molecule non-fullerene electron acceptors NSTI was designed and synthesized by using bissilicon-bridged stilbene as the core unit, INCN-2F as ending moieties, and TbT serving as the bridge to enhance the quinoidal resonance. NSTI exhibits a broadened absorption spectrum (550–800 nm) that is complementary with the absorption of PBDB-T for achieving high Jsc. Through device optimization, we obtained the optimal efficiency of 10.33% with a Jsc of 16.47 mA cm–2, and a FF of 75.2%, which is so far the best reported for OPV devices utilizing silole-based organic photovoltaic materials. Our work indicates that the bis-silicon-bridged stilbene should be a very promising moiety for the development of high-performance fullerene-free OPVs.

ASSOCIATED CONTENT Supporting Information. The general experimental methods, thermal gravimetric analysis, cyclic voltammetry measurement, charge transport property, OPV device data, the AFM, TEM, and NMR spectra of all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

ORCID Xiaozhang Zhu: 0000-0002-6812-0856

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank National Key R&D Program of China (2017YFA0204700), National Basic Research Program of China (973 Program, 2014CB643502), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12010200), and the National Natural Science Foundation of China (21661132006, 21572234) for the financial support. We appreciate Prof. Changduk Yang at Department of Energy Engineering, School of Energy and Chemical Engineering Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798 (South Korea) for GIXD measurements.

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