Abstract
Cyano substitution is vital to the molecular design of polymer semiconductors toward highly efficient organic solar cells. However, how regioselectivity impacts relevant optoelectronic properties in cyano-substituted bithiophene systems remain poorly understood. Three regioisomeric cyano-functionalized dialkoxybithiophenes BTHH, BTHT, and BTTT with head-to-head, head-to-tail, and tail-to-tail linkage, respectively, were synthesized and characterized in this work. The resulting polymer semiconductors (PBDTBTs) based on these building blocks were prepared accordingly. The regiochemistry and property relationships of PBDTBTs were investigated in detail. The BTHH moiety has a higher torsional barrier than the analogs BTHT and BTTT, and the regiochemistry of dialkoxybithiophenes leads to fine modulation in the optoelectronic properties of these polymers, such as optical absorption, band gap, and energy levels of frontier molecular orbitals. Organic field-effect transistors based on PBDTBTHH had higher hole mobility (4.4 × 10−3 cm2/(V·s)) than those (ca. 10−4 cm2/(V·s)) of the other two polymer analogs. Significantly different short-circuit current densities and fill factors were obtained in polymer solar cells using PBDTBTs as the electron donors. Such difference was probed in greater detail by performing space-charge-limited current mobility, thin-film morphology, and transient photocurrent/photovoltage characterizations. The findings highlight that the BTHH unit is a promising building block for the construction of polymer donors for high-performance organic photovoltaic cells.
Graphical abstract
![](http://media.springernature.com/lw685/springer-static/image/art%3A10.1007%2Fs12209-022-00323-0/MediaObjects/12209_2022_323_Figa_HTML.png)
Similar content being viewed by others
Explore related subjects
Find the latest articles, discoveries, and news in related topics.Avoid common mistakes on your manuscript.
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].
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.
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.
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).
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.
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.
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.
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].
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.
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.
References
Feng K, Guo H, Sun HL et al (2021) N-type organic and polymeric semiconductors based on bithiophene imide derivatives. Acc Chem Res 54(20):3804–3817
Luo ZH, Liu T, Ma RJ et al (2020) Precisely controlling the position of bromine on the end group enables well-regular polymer acceptors for all-polymer solar cells with efficiencies over 15. Adv Mater 32(48):e2005942
Sun HL, Guo XG, Facchetti A (2020) High-performance n-type polymer semiconductors: applications, recent development, and challenges. Chem 6(6):1310–1326
Guo H, Yang CY, Zhang XH et al (2021) Transition metal-catalysed molecular n-doping of organic semiconductors. Nature 599(7883):67–73
Li YW, Meng L, Yang YM et al (2016) High-efficiency robust perovskite solar cells on ultrathin flexible substrates. Nat Commun 7(1):10214
Chen HY, Zhang R, Chen XB et al (2021) A guest-assisted molecular-organization approach for >17% efficiency organic solar cells using environmentally friendly solvents. Nat Energy 6(11):1045–1053
Liu BQ, Xu YH, Xia DD et al (2020) Semitransparent organic solar cells based on non-fullerene electron acceptors. Acta Phys Chim Sin 37(3):2009056
Zhang Z, Han FM, Fang J et al (2022) An organic–inorganic hybrid material based on benzo[ghi]perylenetri-imide and cyclic titanium-oxo cluster for efficient perovskite and organic solar cells. CCS Chem 4(3):880–888
Liu YH, Liu BW, Ma CQ et al (2022) Recent progress in organic solar cells (Part I material science). Sci China Chem 65(2):224–268
Ma RJ, Zhou KK, Sun YN et al (2022) Achieving high efficiency and well-kept ductility in ternary all-polymer organic photovoltaic blends thanks to two well miscible donors. Matter 5(2):725–734
Liao Q, Kang Q, Yang Y et al (2022) Highly stable organic solar cells based on an ultraviolet-resistant cathode interfacial layer. CCS Chem 4(3):938–948
Sun HL, Liu B, Yu JW et al (2020) Reducing energy loss via tuning energy levels of polymer acceptors for efficient all-polymer solar cells. Sci China Chem 63(12):1785–1792
Kang Q, Liao Q, Yang CY et al (2022) A new PEDOT derivative for efficient organic solar cell with a fill factor of 0.80. Adv Energy Mater 12(15):2103892
Ma RJ, Yu JW, Liu T et al (2021) All-polymer solar cells with over 16% efficiency and enhanced stability enabled by compatible solvent and polymer additives. Aggregate: e58
Liu B, Wang YF, Sun HL et al (2022) Backbone configuration and electronic property tuning of imide-functionalized ladder-type heteroarenes-based polymer acceptors for efficient all-polymer solar cells. Adv Funct Materials 32(21):2200065
An QS, Wang JW, Ma XL et al (2020) Two compatible polymer donors contribute synergistically for ternary organic solar cells with 17.53% efficiency. Energy Environ Sci 13(12):5039–5047
Li XJ, Luo SW, Sun HL et al (2021) Medium band-gap non-fullerene acceptors based on a benzothiophene donor moiety enabling high-performance indoor organic photovoltaics. Energy Environ Sci 14(8):4555–4563
Liu Y, Xian KH, Gui RH et al (2022) Simple polythiophene solar cells approaching 10% efficiency via carbon chain length modulation of poly(3-alkylthiophene). Macromolecules 55(1):133–145
Yuan XY, Zhao YL, Xie DS et al (2022) Polythiophenes for organic solar cells with efficiency surpassing 17%. Joule 6(3):647–661
Yin BY, Chen ZL, Pang ST et al (2022) The renaissance of oligothiophene-based donor–acceptor polymers in organic solar cells. Adv Energy Mater 12(15):2104050
Xian KH, Liu Y, Liu JW et al (2022) Delicate crystallinity control enables high-efficiency p3ht organic photovoltaic cells. J Mater Chem A 10(7):3418–3429
Gao MY, Liu Y, Xian KH et al (2022) Thermally stable poly(3-hexylthiophene): Nonfullerene solar cells with efficiency breaking 10%. Aggregate:e190
Ye L, Ke HZ, Liu Y (2021) The renaissance of polythiophene organic solar cells. Trends Chem 3(12):1074–1087
Sun HL, Liu B, Ma YL et al (2021) Regioregular narrow-bandgap n-type polymers with high electron mobility enabling highly efficient all-polymer solar cells. Adv Mater 33(37):e2102635
Sun HL, Liu T, Yu JW et al (2019) A monothiophene unit incorporating both fluoro and ester substitution enabling high-performance donor polymers for non-fullerene solar cells with 16.4% efficiency. Energy Environ Sci 12(11):3328–3337
Liu B, Sun HL, Lee JW et al (2021) Achieving highly efficient all-polymer solar cells by green-solvent-processing under ambient atmosphere. Energy Environ Sci 14(8):4499–4507
Wang P, Zhao YX, Wang T (2020) Recent progress and prospects of integrated perovskite/organic solar cells. Appl Phys Rev 7(3):031303
Liao QG, Sun HL, Li BL et al (2019) 26 mA/cm2 JSC achieved in the integrated solar cells. Sci Bull 64(23):1747–1749
Zhang YD, Ma RJ, Wang Y et al (2021) Highly crystalline acceptor materials based on benzodithiophene with different amount of fluorine substitution on alkoxyphenyl conjugated side chains for organic photovoltaics. Mater Rep Energy 1(4):100059
Genene Z, Lee JW, Lee SW et al (2022) Polymer acceptors with flexible spacers afford efficient and mechanically robust all-polymer solar cells. Adv Mater 34(6):e2107361
Xu XP, Li Y, Peng Q (2022) Ternary blend organic solar cells: Understanding the morphology from recent progress. Adv Mater, p 2107476
Sun HL, Liu B, Koh CW et al (2019) Imide-functionalized heteroarene-based n-type terpolymers incorporating intramolecular noncovalent sulfur center dot center dot center dot oxygen interactions for additive-free all-polymer solar cells. Adv Funct Mater 29(42):1903970
Jia JC, Huang QR, Jia T et al (2022) Fine-tuning batch factors of polymer acceptors enables a binary all-polymer solar cell with high efficiency of 16.11%. Adv Energy Mater 12(3):2103193
Liao XF, Pei HQ, Zhao H et al (2022) The synergistic effects of central core size and end group engineering on performance of narrow bandgap nonfullerene acceptors. Chem Eng J 435:135020
Liu T, Zhang YD, Shao YM et al (2020) Asymmetric acceptors with fluorine and chlorine substitution for organic solar cells toward 16.83% efficiency. Adv Funct Mater 30(24):2000456
Fang Y, Zhang Q, Li FY et al (2022) Exploring inorganic hole collection materials from mixed-metal Dawson-type polyoxometalates for efficient organic photovoltaic devices. Sol RRL 6(2):2100827
Duan XP, Song W, Qiao JW et al (2022) Ternary strategy enabling high-efficiency rigid and flexible organic solar cells with reduced non-radiative voltage loss. Energy Environ Sci 15(4):1563–1572
Kim GU, Sun C, Lee DC et al (2022) Effect of the selective halogenation of small molecule acceptors on the blend morphology and voltage loss of high-performance solar cells. Adv Funct Mater, p 2201150
Zhang WQ, Sun CK, Angunawela I et al (2022) 16.52% efficiency all-polymer solar cells with high tolerance of the photoactive layer thickness. Adv Mater 34(20):e2108749
Yang QG, Yu WY, Lv J et al (2022) Effects of fluorination position on all-polymer organic solar cells. Dyes Pigments 200:110180
Li ZY, Peng F, Quan HL et al (2022) A universal strategy via polymerizing non-fullerene small molecule acceptors enables efficient all-polymer solar cells with > 1 year excellent thermal stability. Chem Eng J 430:132711
Zhang J, Lu XQ, Sun YN et al (2022) Case study of metal coordination to the charge transport and thermal stability of porphyrin-based field-effect transistors. ACS Mater Lett 4(4):548–553
Chen XZ, Huang R, Han YF et al (2022) Balancing the molecular aggregation and vertical phase separation in the polymer: nonfullerene blend films enables 1309% efficiency of organic solar cells with inkjet-printed active layer. Adv Energy Mater 12(12):2200044
Wang ZJ, Zhou JW, Zhang YH et al (2022) Accessing highly efficient photothermal conversion with stable open-shell aromatic nitric acid radicals. Angew Chem 61(10):e202113653
Ma SX, Zhang H, Feng K, et al (2022) Polymer acceptors for high-performance all-polymer solar cells. Chem Eur J, p e202200222
Shao JY, Zhong YW (2021) Design of small molecular hole-transporting materials for stable and high-performance perovskite solar cells. Chem Phys Rev 2(2):021302
Shi YQ, Guo H, Qin MC et al (2018) Imide-functionalized thiazole-based polymer semiconductors: Synthesis, structure-property correlations, charge carrier polarity, and thin-film transistor performance. Chem Mater 30(21):7988–8001
Xiong Y, Ye L, Zhang C (2022) Eco-friendly solution processing of all-polymer solar cells: recent advances and future perspective. J Polym Sci 60(6):945–960
Song X, Sun P, Sun DW et al (2022) Investigation of tunable halogen-free solvent engineering on aggregation and miscibility towards high-performance organic solar cells. Nano Energy 91:106678
Li ZY, Liang YF, Qian XT et al (2022) Conquering the morphology barrier of ternary all-polymer solar cells by designing random terpolymer for constructing efficient binary all-polymer solar cells. Chem Eng J 439:135491
Liao CT, Gong YF, Xu XP et al (2022) Cost-Efficiency balanced polymer acceptors based on lowly fused Dithienopyrrolo[3, 2b]benzothiadiazole for 16.04% efficiency all-polymer solar cells. Chem Eng J 435:134862
Liu HL, Wang LQ, Liu H et al (2022) Ternary polymerization strategy to approach 12% efficiency in all-polymer solar cells processed by green solvent and additive. Chem Eng J 429:132407
Huang J, Tang YM, Gao K et al (2017) Head-to-head linkage containing dialkoxybithiophene-based polymeric semiconductors for polymer solar cells with large open-circuit voltages. Macromolecules 50(1):137–150
Jia T, Zhang JB, Tang HR et al (2022) Synchronously regulating the alkyl side-chain and regioisomer of polymerized small molecule acceptor enabling highly efficient all-polymer solar cells processed with non-halogenated solvent. Chem Eng J 433:133575
Hergué N, Mallet C, Savitha G et al (2011) Facile synthesis of 3-alkoxy-4-cyanothiophenes as new building blocks for donor-acceptor conjugated systems. Org Lett 13(7):1762–1765
Wen TJ, Xiang JL, Jain N et al (2022) Non-fused medium bandgap electron acceptors for efficient organic photovoltaics. J Energy Chem 70:576–582
Zhou BL, Zou YL, Yi YQQ et al (2022) Conjugated extension of non-fullerene acceptors enables efficient organic solar cells with optoelectronic response over 1000 nm. ACS Appl Energy Mater 5(4):4664–4672
Rand BP, Burk DP, Forrest SR (2007) Offset energies at organic semiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells. Phys Rev B 75(11):115327
Luo ZH, Ma RJ, Yu JW et al (2022) Heteroheptacene-based acceptors with thieno[3, 2-b]pyrrole yield high-performance polymer solar cells. Natl Sci Rev. https://doi.org/10.1093/nsr/nwac076
Wang D, Zhou GQ, Li YH et al (2022) High-performance organic solar cells from non-halogenated solvents. Adv Funct Materials 32(4):2107827
Li Y, Song JL, Dong YC et al (2022) Polymerized small molecular acceptor with branched side chains for all polymer solar cells with efficiency over 16.7. Adv Mater 34(14):e2110155
Cho Y, Park J, Jeong S et al (2022) Stretchable N-type high-performance polymers based on asymmetric thienylvinyl-1, 1-dicyanomethylene-3-indanone for plastic electronics. Chem Mater 34(4):1554–1566
Li BB, Zhang XY, Wu ZA et al (2022) Over 16% efficiency all-polymer solar cells by sequential deposition. Sci China Chem 65(6):1157–1163
Yan LH, Liang ZZ, Si JH et al (2022) Ultrafast kinetics of chlorinated polymer donors: a faster excitonic dissociation path. ACS Appl Mater Interfaces 14(5):6945–6957
Li WW, Hendriks KH, Furlan A et al (2014) Effect of the fibrillar microstructure on the efficiency of high molecular weight diketopyrrolopyrrole-based polymer solar cells. Adv Mater 26(10):1565–1570
Liu T, Huo LJ, Chandrabose S et al (2018) Optimized fibril network morphology by precise side-chain engineering to achieve high-performance bulk-heterojunction organic solar cells. Adv Mater 30(26):e1707353
Fu HT, Li YX, Wu ZA et al (2022) Side-chain substituents on benzotriazole-based polymer acceptors affecting the performance of all-polymer solar cells. Macromol Rapid Commun, p 2200062
Can A, Choi GS, Ozdemir R et al (2022) Meso-π-extended/deficient Boipys and low-band-gap donor-acceptor copolymers for organic optoelectronics. ACS Appl Polym Mater 4(3):1991–2005
Cowan SR, Street RA, Cho S et al (2011) Transient photoconductivity in polymer bulk heterojunction solar cells: competition between sweep-out and recombination. Phys Rev B 83(3):035205
Wu Q, Wang W, Wu Y et al (2021) Tailoring polymer acceptors by electron linkers for achieving efficient and stable all-polymer solar cells. Natl Sci Rev 9(2):nwab151
Acknowledgements
This work is supported by the National Natural Science Foundation of China (Nos. 52173172, 52173171, and 21801124), the Natural Science Foundation for Distinguished Young Scholars of Guangdong Province (2021B1515020027), Guangdong Basic and Applied Basic Research Foundation (2021A1515110892), China Postdoctoral Science Foundation (2021M700062), the Shenzhen Science and Technology Innovation Commission (JCYJ202103243104813035 and JCYJ20180504165709042), and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology). X.G. is thankful for the financial support from the Songshan Lake Materials Laboratory (2021SLABFK03). HYW is grateful for the financial support from the NRF of Korea (2016M1A2A2940911, 2017K2A9A2A12000315). Our work was also supported by the Center for Computational Science and Engineering of Southern University of Science and Technology.
Author information
Authors and Affiliations
Contributions
HS LN and XG proposed and supervised the project. QB performed device fabrication and characterization. JH and HS synthesized these monomers and polymers. HYW conducted 2D-GIWAXS measurements. MS and BL performed TRPL measurements. HG and QB carried out OTFT tests and analyses. JY and YK performed CV and GPC measurements. QB, JH, HS, LN, and XG co-wrote the manuscript. All authors discussed the results and participated in revising the manuscript.
Corresponding authors
Ethics declarations
Conflict of Interest
The authors declare that there is no conflict of interest.
Supplementary Information
Below is the link to the electronic supplementary material.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Bai, Q., Huang, J., Guo, H. et al. Regioisomeric Polymer Semiconductors Based on Cyano-Functionalized Dialkoxybithiophenes: Structure–Property Relationship and Photovoltaic Performance. Trans. Tianjin Univ. 28, 385–397 (2022). https://doi.org/10.1007/s12209-022-00323-0
Received:
Revised:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s12209-022-00323-0