Introduction

Organic electronics have attracted considerable interest over the last few decades because of their great potential for the production of flexible and large-area optoelectronic devices via solution processing technologies at low temperatures [1,2,3,4,5,6,7,8,9]. Through an in-depth understanding of chemical design and synthesis, material processing, and device performance of polymer semiconductors, unprecedented achievements have been made in organic thin-film devices, particularly in organic field-effect transistors (OFETs) and polymer solar cells (PSCs) [10,11,12,13,14,15]. In the design of organic semiconducting materials, alkyl side-chains are typically attached to molecular backbones to endow materials with solubility, allowing quality films to be fabricated via spin-coating or other solution-based processing techniques [16, 17]. A detail that has become apparent is that the regiochemistry of alkyl-substituted aromatic building blocks in a conjugated polymer backbone has a high correlation with the molecular packing and film morphology, hence significantly affecting the device performance. Poly(3-alkylthiophene), for instance, is an excellent candidate for probing the effects of the regiochemistry of alkyl thiophene [18,19,20]. During the molecular chain growth in the polymerization process, three regioisomeric linkages, i.e., head-to-head (HH), tail-to-tail (TT), and head-to-tail (HT), can be generated in the formation of 3-alkythiophene dimers (Fig. 1a). The HH coupling typically results in a highly twisted molecular backbone due to the accompanying large steric hindrance, thereby reducing the effective π-conjugation and crystallinity of the resulting polymer. Poly(3-alkylthiophene) containing a small amount of HH linkage exhibited obviously inferior device performance in both OFETs and PSCs [21,22,23]. In contrast, the HT coupling yielded a higher polymer film crystallinity with a planar backbone conformation and achieved improved optoelectronic properties, as exemplified by the regioregular HT-linked poly(3-hexylthiophene) (rr-P3HT) [2, 24, 25]. However, when the TT-linked dialkylated bithiophene is incorporated into poly(alkylthiophenes) or other copolymers, it may result in a HH linkage somewhere in the polymer backbone, thereby inducing a large torsional angle at the junction and thus yielding a reduced backbone π-conjugation and loose interchain packing [26,27,28].

Fig. 1
figure 1

Three different linkages of a alkylthiophene dimers in poly(alkylthiophene) and b cyano-functionalized alkoxythiophene dimers in this work

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In the early stage of the development of π-conjugated polymers, alkoxy-substituted polythiophenes were intensively studied because of their excellent electronic, thermal, and ionochromic properties, as exemplified by the well-known conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) [29,30,31]. The remarkable conductivity of PEDOT in the doped state is attributed to the planar polymer backbone enabled by consecutive intramolecular noncovalent (thienyl)S⋅⋅⋅(alkoxy)O coulombic interactions [32]. However, the strong electron-donating oxygen considerably elevates the energy level of the highest occupied molecular orbital (HOMO) of alkoxy-substituted polymer semiconductors [33, 34], thereby leading to a small open-circuit voltage (VOC) in PSCs or poor stability in OFETs [35,36,37]. This drawback limits the usage of alkoxythiophene as a building block for constructing high-performance polymer semiconductors. Cyano functionalization of alkoxy-substituted thiophenes is a promising method of optimizing the physicochemical properties due to the strong electron-withdrawing capability of cyano functionality, which can effectively lower the frontier molecular orbital (FMO) energy levels [38,39,40]. A polymer donor that contains cyano-functionalized TT-linked dialkoxybithiophene (BTTT, Fig. 1) and benzodithiophene had a higher VOC of 0.86 V and improved power conversion efficiency (PCE) up to 5.06% in PSCs [41]. Our group has a long-standing interest in the design and synthesis of semiconducting polymers based on HH-linked dialkoxybithiophenes [42,43,44]. We recently functionalized the HH-linked dialkoxybithiophene (BTHH, Fig. 1) with a cyano group and synthesized a series of polymer semiconductors by copolymerizing it with benzodithiophenes. The strong electron-withdrawing cyano group endows the resulting polymers with decreased FMO energy levels. As a result, a remarkably high VOC of 1.0 V and a PCE up to ca. 7% were achieved when the polymers were applied in PSCs. Therefore, the cyano group effectively optimizes the electrochemical property of semiconductors [45].

Unlike the well-studied dialkylbithiophenes, the structure–property correlations of dialkoxybithiophenes with different linkages in polymer semiconductors have not yet been studied. In this work, we synthesized three dialkoxybithiophene building blocks based on 3-dodecyloxy-4-cyanothiophene: HH-linked BTHH, HT-linked BTHT, and TT-linked BTTT (Fig. 1b). By copolymerizing them with a benzodithiophene comonomer, we prepared three polymer semiconductors to study their structure–property correlations and investigate their device performance in PSCs. According to cyclic voltammetry (CV) characterization, all polymers displayed a comparable HOMO energy level (− 5.60 eV), whereas the lowest unoccupied molecular orbital (LUMO) energy levels showed a gradual decrease from PBDTBTHH (− 2.5 eV) to PBDTBTHT (− 2.62 eV) and to PBDTBTTT (− 2.82 eV). UV–Vis absorption and film morphology of these polymers largely depended on the linkage fashions of the 3-dodecyloxy-4-cyanothiophene. When the polymers were applied in OFET, the hole mobility of polymer PBDTBTHH (4.4 × 10−3 cm2/(V·s)) was one order of magnitude higher than that of the other two polymers (3.3 × 10−4–4.0 × 10−4 cm2/(V·s)). The PSCs based on these polymer donors and PC71BM acceptor exhibited greatly different device performances, with the PCEs in the range of 2.94% to 7.06%. Various characterization techniques combined with theoretical calculation were applied to examine the relationship between the molecular structure regioisomerism of cyano-functionalized dialkoxybithiophene-based polymers and their optoelectronic properties. The results provide guidance for the development of alkoxythiophene-based organic semiconductors.

Experimental Section

Material Synthesis

Figure 2 presents the chemical structures of three regioisomeric polymers based on cyano-functionalized dialkoxybithiophenes, while the Supporting Information contains the synthetic details.

Fig. 2
figure 2

Chemical structures of three regioisomeric polymers

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Fabrication of Field-Effect Transistors and Photovoltaic Devices

OFETs that have a top-gate/bottom-contact structure were fabricated to determine the charge transport characteristics of the polymer semiconductors [46, 47]. Source and drain electrodes (3 nm-thick Cr and 30-nm-thick Au) were patterned on borosilicate glass by performing typical photolithography. The active layers were then spin-coated from 5 mg/mL o-dichlorobenzene (o-DCB) solutions and then subjected to thermal annealing at 190 °C for 15 min. Afterward, the dielectric layer was cast from a dilute CYTOP solution (CTL-809 M/CT-SOLV180 = 2:1 (v/v), Asahi Glass Co., Ltd.) and annealed at 100 °C for 10 min. Al (50 nm thick) was thermally evaporated under vacuum as the gate electrode to complete the devices. The OFETs were characterized in a nitrogen-filled glove box using a semiconductor characterization system (Keithley 4200-SCS). The PSCs were fabricated with a conventional device architecture of glass/indium tin oxide (ITO)/MoO3/polymer:PC71BM/Ca/Al [48, 49]. The ITO was cleaned via sonication in deionized water, acetone, and isopropanol, followed by UV-ozone treatment for 15 min. MoO3 (10 nm) was deposited as the hole transport layer onto the ITO via thermal evaporation. The active layer was systematically optimized; findings show that 90-nm-thick film that was spin-coated from the o-DCB solution containing polymer and PC71BM (w/w = 1/2, 10 mg/mL for polymer) facilitated the optimal device performance. After being dried, the devices were loaded into an evaporator, where Ca (15 nm thick) and Al (100 nm thick) were deposited sequentially as cathode via thermal evaporation. The effective area of the solar cells was ~ 0.045 cm2. Device characterization was conducted in a nitrogen-filled glove box using a xenon lamp-based solar simulator (Newport, Oriel AM 1.5 G, 100 mW/cm2), which was calibrated with a National Renewable Energy Laboratory-certified standard silicon cell and recorded using a Keithley 2400 digital source meter.

Results and Discussion

Synthesis of Materials

The synthetic routes to the three building blocks (BTHH, BTHT, and BTTT) and polymers are illustrated in Scheme S1. PBDTBTHH and PBDTBTTT were synthesized by following previously reported procedures [50,51,52]. The synthesis of PBDTBTHT is a straightforward process that uses 4-(dodecyloxy)thiophene-3-carbonitrile as the starting material. As a result of the different acidity of two α-protons in 4-(dodecyloxy)thiophene-3-carbonitrile, the proton next to cyano in the presence of n-BuLi could be selectively deprotonated, and subsequent quenching with chlorotrimethyltin afforded 4-(dodecyloxy)-2-(trimethylstannyl)thiophene-3-carbonitrile. The key intermediate BTHT was readily obtained via Stille coupling between 4-(dodecyloxy)-2-(trimethylstannyl)thiophene-3-carbonitrile and 5-bromo-4-(dodecyloxy)thiophene-3-carbonitrile in a 45% yield, and the sequential treatment with n-BuLi and Br2 gave the monomer 5,5'-dibromo-3',4-bis(dodecyloxy)-[2,2'-bithiophene]-3,4'-dicarbonitrile for synthesizing polymer PBDTBTHT [53]. The 1H NMR spectra of BTHH, BTHT, and BTTT were compared to investigate the electron distribution of the aromatic protons (Fig. S1). The chemical shifts of the aromatic protons of the symmetrical BTHH and BTTT are 7.79 ppm and 6.36 ppm, respectively. For the asymmetrical BTHT, two aromatic protons were observed to have a chemical shift of 7.85 ppm and 6.22 ppm, respectively. With regard to the symmetric BTHH, two aromatic protons showed a single down-shifted peak at 7.79 ppm because both α-protons are next to the highly electron-withdrawing cyano group. We thus predict that the monomer based on BTHH may yield a polymer with higher molecular weight by facilitating its oxidative addition to the active catalytic sites during the Stille coupling process [54]. All the monomers were purified by recrystallization from a solvent mixture of methanol and dichloromethane. The molecular structures and purity of compounds were confirmed by 1H NMR, 13C NMR, and elemental analyses. After polymerization, PBDTBTs were subjected to the standard Soxhlet extraction to remove low molecular weight fractions and impurities. All polymers displayed good solubility in chlorobenzene and o-DCB and comparable number-average molecular weights (Mn). The Mn values were 24 kDa, 24 kDa, and 26 kDa with a dispersity index of 3.4, 2.0, and 2.3 for PBDTBTHH, PBDTBTHT, and PBDTBTTT, respectively.

Optoelectronic Properties of Building Blocks

Figure 3a shows the UV–Vis absorption spectra of the regioisomeric building blocks (BTHH, BTHT, and BTTT) in tetrahydrofuran (ca. 10−4 mol/L). Interestingly, the fine structures were clearly different, although their absorption was measured in the same wavelength range. The UV–Vis absorption spectra are in good agreement with the time-dependent density functional theory (TDDFT) calculation, as shown in Fig. 3b. According to the calculation, the lowest energy transition for these building blocks is associated with the transition of HOMO → LUMO. With regard to BTHT and BTTT, a higher energy transition was measured and could be assigned by TDDFT as follows: HOMO − 2 → LUMO at 264 nm for BTHT and HOMO − 1 → LUMO at 269 nm for BTTT. The energy minimum geometries and electronic structures of BTHH, BTHT, and BTTT were calculated by DFT at the B3LYP/6-31G level using Gaussian 03 W. The results are shown in Fig. 4a–c. The alkyl groups in the calculated structures were truncated with methyl groups to reduce the computational burden.

Fig. 3
figure 3

a Experimental and b calculated UV–Vis absorption spectra and c experimental FMO energy levels of BTHH, BTHT, and BTTT

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Fig. 4
figure 4

Chemical structures, optimized geometries, and FMO structures of a BTHH, b BTHT, and c BTTT. d Torsional profiles related to the rotation between two thiophenes

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The calculated HOMO levels are located at the same level (ca. − 6.1 eV), and the LUMO levels of BTHH, BTHT, and BTTT were calculated as − 1.96, − 2.20, and − 2.48 eV, respectively. CV measurements determined that the LUMO energy levels are − 2.5 eV for BTHH, − 2.62 eV for BTHT, and − 2.82 eV for BTTT (Fig. 3c, Fig. S2). Both calculated and experimental LUMO values show the same trend, and the lowest LUMO energy level of BTTT suggests that it has the strongest electron-accepting ability among the three building blocks. All these building blocks adopt a highly anti-planar conformation, which was confirmed previously by single X-ray crystal data [53, 55]. Figure 4d plots torsional profiles as a function of the dihedral angle between two thiophene rings. Interestingly, BTHT and BTTT show almost the same torsional profiles, while a higher torsional energy barrier is needed to break the anti-planar conformation in BTHH. The higher torsional barrier with a conformation lock in BTHH can be attributed to the double noncovalent S⋅⋅⋅O intramolecular interactions, which may influence the interchain packing and the resulting film morphology. According to the energy minimum geometry based on their trimers (Fig. S3), the torsional angle between BDT and bithiophene was calculated as PBDTBTHH: 3.8°, PBDTBTHT: 6°, and PBDTBTTT: 9°, respectively. A more planar conformation was observed in PBDTBTHH [56].

Polymer Thermal Properties

The thermal properties of polymers were evaluated by performing thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA thermograms in Fig. S4a show that the thermal breakdown of all polymers (Td, with a loss of 5 wt%) occurs when they are heated at more than 330 °C in the presence of nitrogen. The second heating and cooling DSC traces are displayed in Fig. S4b. PBDTBTHH shows one endothermal transition (at 156 °C) and one exothermal transition (at 111 °C), which could be attributed to the side-chain melting and recrystallization. Interestingly, PBDTBTHT and PBDTBTTT show featureless DSC thermograms, although they have a similar backbone as PBDTBTHH [57]. DSC data suggest that PBDTBTHH differs from the other two polymers in terms of polymer packing, and PBDTBTHH with the HH geometry shows a higher crystallinity.

Optical and Electrochemical Properties

The UV‒Vis absorption spectra of three polymers in dilute o-DCB (10−5 mol/L) and in films cast from o-DCB solutions (5 mg/mL) are shown in Fig. 5a, and detailed optical data are compiled in Table 1. The absorption profiles in solution and film indicate that all the polymers are expected to be significantly aggregated, even in solution. In the film, the absorption onset shifted to longer wavelengths from PBDTBTHH (697 nm) to PBDTBTHT (747 nm) to PBDTBTTT (780 nm). Thus, the optical band gap derived from the optical onset of the film decreased in the order of PBDTBTHH (1.78 eV), PBDTBTHT (1.66 eV), and PBDTBTTT (1.59 eV). The electrochemical properties of polymers were investigated by conducting CV measurements relative to ferrocene as a reference, as shown in Fig. 5b. The HOMO energy levels of PBDTBTHH, PBDTBTHT, and PBDTBTTT were − 5.56, − 5.52, and − 5.58 eV, respectively (Table 1). The low-lying HOMOs, which are generated by the strong electron-withdrawing cyano group, should lead to a high VOC in PSCs and good air stability in OFETs [58, 59]. Unfortunately, the LUMO energy levels of polymers were not obtained successfully from CV measurements. Instead, the LUMO values of PBDTHH, PBDTHT, and PBDTTT were calculated as − 3.78, − 3.86, and − 3.99 eV, respectively, according to the equation LUMO = HOMO + Eg (eV).

Fig. 5
figure 5

a Normalized UV‒Vis absorption spectra of polymers in o-DCB and in film. b Cyclic voltammograms of polymer films in 0.1 mol/L tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile solution with the Fc/Fc+ redox couple as the internal standard

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Table 1 Molecular weights and optical and electrochemical properties of PBDTBTs
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Photovoltaic Properties

PSCs were fabricated with a conventional device architecture of glass/ITO/MoO3/polymer: PC71BM/Ca/Al to investigate the regiochemistry effect on polymers’ photovoltaic properties. The devices were optimized by varying the blend ratio of donor: acceptor, film thickness, and processing solvent. A weight ratio of donor/acceptor of 1:2, a film thickness of 130 nm, and a processing solvent of chlorobenzene could lead to the optimum photovoltaic parameters. The current density–voltage (J-V) curves under the illumination of air mass (AM) 1.5 G (100 mW/cm2) are shown in Fig. 6a, and the resulting photovoltaic parameters are summarized in Table 2. All the devices displayed a similarly high VOC of 0.9 V, which was attributed to the similar low-lying HOMOs determined by CV. For devices based on PBDTBTHH, PBDTBTHT, and PBDTBTTT, the average PCEs were 6.22%, 2.94%, and 3.83%, respectively. Both higher short-circuit current density (Jsc) and fill factor (FF) values are responsible for a higher PCE for PBDTBTHH. Several solvent additives were also tested for further device optimization [60, 61]. The addition of 2 vol% 1-chloronaphthalene (CN) resulted in clearly improved photovoltaic performance. The resulting PCEs of the PBDTBTHH-, PBDTBTHT-, and PBDTBTTT-based PSCs reached 7.06%, 3.96%, and 5.08%, respectively, which mainly originated from the increased Jsc values (Fig. 6a). The enhancement is reflected by the higher external quantum efficiency in their photo-response regions (Fig. 6b). The underlying mechanism for such enhancement in PCEs will be discussed in terms of charge mobility and blend morphology in the following sections. The impressive device performances of our polymers in fullerene solar cells also prompted us to explore their application in non-fullerene solar cells. We fabricated devices with the best polymer PBDTBTHH in this series with non-fullerene acceptor Y6 (Table S1, Fig. S9). The PBDTBTHH-based non-fullerene solar cell yielded a PCE of 1.79%. We speculate that the unsatisfactory performance is mainly due to the poor compatibility between our polymer and Y6. Atomic force microscopy (AFM) and polarizing optical microscopy (POM) are applied to investigate the morphology of PBDTBTHH: Y6 blend film, with the PM6: Y6 blend film used as a control. As shown in Fig. S10, the PBDTBTHH:Y6 blend film shows a much rougher surface with a larger aggregation than that of the PM6:Y6 blend film, as confirmed by the POM images (Fig. S11a, f). These findings suggest poor compatibility between PBDTBTHH with Y6. The AFM phase images of two blend films are shown in Fig. S11. The PBDTBTHH:Y6 blend film exhibits a more obvious state of aggregation and rough domain-like morphology (Fig. S11b–e). The root-mean-square roughness of the PBDTBTHH:Y6 blend film (Fig. S11b) is ∼8.5 nm within the 3 μm × 3 μm scanned area, which is much rougher than those (< 2 nm) of PM6:Y6 and PBDTBTHH:PC71BM blend films (Fig. S6b and Fig. S11). Such blend morphology results in a negative effect on the charge separation and transportation. Therefore, the PBDTBTHH:Y6-based device produces a poor Jsc and FF.

Fig. 6
figure 6

Photovoltaic characteristics of PSCs: a J-V curves and b corresponding EQE spectra with or without CN additive

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Table 2 Summary of photovoltaic parameters in PSCs
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Charge Carrier Mobility

To investigate the charge carrier transport properties of polymers, an OFET device was first fabricated. The hole mobilities of polymer films after annealing at 190 °C for 15 min are summarized in Table 3, and the corresponding transfer curves are displayed in Fig. S5. The hole mobility (μh) of OFETs based on PBDTBTHH (4.4 × 10−3 cm2/(V·s)) was ~ 10 times higher than those of PBDTBTHT (3.3 × 10−4 cm2/(V·s)) and PBDTBTTT (4.0 × 10−4 cm2/(V·s)). Aside from the OFET mobility in a parallel direction, the vertical charge mobility was also studied by applying a space-charge-limited current (SCLC) method [62]. The hole-only and electron-only devices, which have a device architecture of glass/ITO/PEDOT:PSS/polymer:PC71BM/MoO3/Ag and a glass/ITO/ZnO/polymer:PC71BM/Ca/Al, respectively, were then prepared. The resulting J-V characteristics are plotted in Fig. 7, and the calculated data are collected in Table 3. Both the hole mobility (μh) and the electron mobility (μe) of PBDTBTHH:PC71BM film presented the highest values of μh = 2.3 × 10−4 and μe = 1.7 × 10−4 cm2/(V·s), respectively; these values are one to two orders of magnitude higher than those of PBDTBTHT:PC71BM (μh: 2.2 × 10−6 cm2/(V·s), μe: 2.6 × 10−5 cm2/(V·s)) and PBDTBTTT:PC71BM (μh: 2.4 × 10−6 cm2/(V·s), μe: 4.6 × 10−5 cm2/(V·s)). The CN addition slightly improved the mobility of PBDTBTHH:PC71BM film (μh: 2.4 × 10−4 cm2/(V·s), μe: 2.0 × 10−4 cm2/(V·s)), thus resulting in more balanced charge mobilities (μh/μe = 1.2). As a result, PBDTBTHH:PC71BM showed the best PCE with a Jsc of 12.21 mA /cm2 and an FF of 0.64. For PBDTBTHT- and PBDTBTTT-based blend films, the clearly increased hole and electron mobilities were also measured upon the addition of CN. However, the μh/μe values were almost unchanged, far from the ideal value of 1. The charge mobility has a strong relation to the interchain packing and film morphology [63, 64]. This relation will be examined through 2D-grazing incidence X-ray diffraction (2D-GIXD) and transmission electron microscopy (TEM) measurements.

Table 3 Summary of charge carrier mobilities
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Fig. 7
figure 7

J‒V characteristics of a, c hole-only devices of blend films and b, d electron-only devices of blend films with and without the CN additive, respectively

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Film Morphologies

AFM measurements were performed to understand the surface morphology of active layers in PSCs (Fig. S6). All the blend films, with or without the CN additive, displayed smooth surface topographies. For the PBDTBTHH-based blend films, CN addition affected the surface roughness noticeably, with the root-mean-square roughness decreasing sharply from 2.58 nm to 1.58 nm. Despite the positive effect of the additive on PCEs for PBDTBTHT- and PBDTBTTT-based devices, no discernible morphological changes could be observed. We also measured the blend film morphology using TEM. As shown in Fig. 8, the three blend films exhibit obvious nanofibrillar structures after the addition of CN, leading to significantly improved Jsc and thus better PCE compared with those of additive-free ones [65, 66]. PBDTBTHH-based blend film has the most uniform morphology with finer phase separation, which yields the highest Jsc (12.21 mA/cm2) and is the best PCE in this series. Interestingly, no recognizable changes in TEM morphologies were observed for the other two systems. On the basis of the AFM and TEM characterizations, we conclude that morphology transformation that results from the addition of CN is clearly observed in the PBDTBTHH-based film.

Fig. 8
figure 8

TEM images of blend films d, e, f with and a, b, c without CN additive

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The detailed interchain packing and molecular orientation in pristine films and donor:acceptor blends were investigated by 2D-GIXD. Figure 9 shows the 2D diffraction patterns, and Fig. S7 presents the corresponding line-cut profiles. The pristine PBDTBTHH film has a face-on dominant bimodal orientation, as confirmed by a strong out-of-plane (OOP) π–π stacking peak (010) and an in-plane (IP) lamellar (100) scattering, together with lamellar peaks (up to (300)) in the OOP direction (Fig. 9a). The PBDTBTHH film showed a π–π stacking distance of 0.39 nm (qz = 1.61 Å−1) and lamellar interdigitation distances of 3.1 nm and 2.7 nm in the IP and OOP directions, respectively. PBDTBTHT and PBDTBTTT showed a similar 2D-GIXD diffraction pattern, reflecting the bimodal orientation of polymer chains with a hump-shaped peak (100) (Fig. 9b, c). However, their OOP (010) scattering intensities are lower than those of the polymer PBDTBTHH. In the blend films, an isotropic broad and strong PC71BM aggregation diffraction was observed at q = 1.24‒1.34 Å−1. Blending with PC71BM resulted in all three blend films showing an edge-on dominant bimodal orientation with lamellar scatterings up to (300) in the OOP direction (Fig. 8d, e, f). In the case of PBDTBTHH:PC71BM, a clear OOP peak (010) still remains relative to the other blends. In the polymer:PC71BM blends, the lamellar interdigitation distances were 2.7‒3.1 nm for all three polymers, showing a similar lamellar d-spacing to the pristine polymer films. Upon the addition of CN, the diffraction peaks became stronger for all blends. Particularly for the PBDTBTHH:PC71BM blend film, the OOP peak (010) became clearer without any noticeable change in the lamellar interdigitation distance (Fig. 9g–i). The enhanced face-on stacking is beneficial for charge extraction in the vertical direction in photovoltaic cells, thereby improving the values of both Jsc and FF [67, 68].

Fig. 9
figure 9

2D-GIXD patterns of pristine films and blend films with and without CN additive

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Charge Transfer, Extraction, and Recombination

Photoluminescence (PL) is a simple method of examining the photo-induced charge transfer at the interface between the donor and the acceptor in PSCs (Fig. S8). Unlike that of pristine polymer films, the PL emission of blend films was strongly quenched via photo-induced electron transfer to fullerene by exciting polymers at 345 nm. The PL intensity in blends was further reduced upon the addition of CN, indicating that the exciton separation was more effective due to morphological optimization. We conducted transient photocurrent (TPC) measurement to analyze the charge carrier extraction process in the series of blends [69]. The TPC measurements were performed at zero bias, and the results are shown in Fig. 10a. The photocurrent decay time of the PBDTBTHH:PC71BM-based device was 0.32 μs, which is shorter than those of the PBDTBTHT:PC71BM and PBDTBTTT:PC71BM-based PSCs (0.41 μs and 0.50 μs). The shorter charge extraction time is attributed to the well-balanced and higher SCLC charge mobilities in PBDTBTHH:PC71BM compared with those of the other two blends, as discussed above. The decay time of the PBDTBTHH-based device dropped from 0.32 to 0.24 μs with the addition of CN, showing good agreement with the enhancement of Jsc and PCE. Transient photovoltage (TPV) measurements, which facilitate the investigation of the charge recombination dynamics [70], were also performed. As shown in Fig. 10b, the addition of CN produced a longer charge recombination time, revealing improved photovoltaic performance in PSCs. Notably, the recombination time for the PBDTBTHH-based PSCs with CN was 78.8 μs, indicating a remarkably slower charge recombination compared with that of the other two devices (26.1 and 36.6 μs for PBDTBTHT:PC71BM and PBDTBTTT:PC71BM, respectively). The results from charge carrier dynamics are well consistent with the device performances, suggesting that BTHH is a promising building block for realizing semicrystalline high-performance semiconducting polymers.

Fig. 10
figure 10

a Transient photocurrent and b transient photovoltage measurements

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Conclusions

Three regioisomeric building blocks (BTHH, BTHT, and BTTT) based on the 3-alkoxy-4-cyanothiophene were synthesized. The HOMO energy levels of dialkoxybithiophenes decreased substantially when a strong electron-withdrawing cyano substituent was incorporated. The LUMO energy level was down-shifted gradually from BTHH to BTHT and to BTTT. A higher torsional energy barrier was found in the BTHH moiety, which is likely due to the double noncovalent coulombic S⋅⋅⋅O interactions. When integrated into OFETs, PBDTBTHH exhibited 10 times higher hole mobility (4.4 × 10−3 cm2/(V·s)) compared with the other two polymers (ca. 10−4 cm2/(V·s)). When blended with PC71BM, PBDTBTHH-, PBDTBTHT-, and PBDTBTTT-based PSCs showed the best PCEs of 7.06%, 3.96%, and 5.08%, respectively. Such difference in PCEs is due to the variation of Jsc values and FF values. The data from SCLC measurements, morphological characterization, and transient photocurrent/photovoltage analyses enabled us to understand the varied electrical and photovoltaic characteristics that depend on polymer regiochemistry. The HH-linked cyano-functionalized dialkoxybithiophene has great potential for the construction of crystalline semiconducting polymers for optoelectronic applications. More importantly, our study emphasizes that the regiochemistry of alkoxy-substituted building blocks needs to be carefully considered to fine-modulate the molecular conformation and the electrochemical, morphological, and resulting electrical properties.