18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

Open AccessCCS ChemistryRESEARCH ARTICLE30 May 202218.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10 Xiaolei Kong, Jinyuan Zhang, Lei Meng, Chenkai Sun, Shucheng Qin, Can Zhu, Jianqi Zhang, Jing Li, Zhixiang Wei and Yongfang Li Xiaolei Kong Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Jinyuan Zhang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Lei Meng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Chenkai Sun College of Chemistry, Green Catalysis Center, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Shucheng Qin Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Can Zhu Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Jianqi Zhang CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in NanPSCience, National Center for NanPSCience and Technology, 100190 Beijing Google Scholar More articles by this author , Jing Li Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 Google Scholar More articles by this author , Zhixiang Wei CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in NanPSCience, National Center for NanPSCience and Technology, 100190 Beijing Google Scholar More articles by this author and Yongfang Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049 Laboratory of Advanced Optoelectronic Materials, Suzhou Key Laboratory of Novel Semiconductor Materials and Devices, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202056 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The development of A-DA′D-A type small molecule acceptors (SMAs) has promoted the rapid progress of polymer solar cells (PSCs) in recent years. The outer side chains on the terminal thiophene ring and inner side chains on nitrogen atoms of the pyrrole ring of the DA′D fused ring play important roles in the photovoltaic performance of the SMAs. Here, we synthesized two new SMAs, 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-bis(4-(2-ethylhexyl)thiophen-2-yl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (T2EH) and 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-bis(3-(2-ethylhexyl)phenyl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene) (P2EH), with 2-ethylhexyl β-substituted thienyl or phenyl as the outer side chains, respectively, to improve the photovoltaic properties of the SMAs. Compared with P2EH, T2EH exhibits closer π−π stacking, slightly red-shifted absorption, and higher electron mobility. Moreover, the active layer of T2EH blended with the low-cost polymer donor poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)] (PTQ10) possesses higher mobilities, a longer lifetime, and less recombination of the charge carriers in comparison with that of the PTQ10:P2EH active layer. Eventually, the PTQ10:T2EH-based PSCs showed an outstanding power conversion efficiency (PCE) of 18.55%, while the PSC based on PTQ10:P2EH displayed a PCE of 17.50%. Importantly, 18.55% is the highest PCE in the PTQ10-based binary PSCs so far. The results indicate that T2EH is one of the best SMAs for the PTQ10-based PSCs and is a promising SMA for the application of PSCs. Download figure Download PowerPoint Introduction Polymer solar cells (PSCs) have drawn great attention in recent years due to their advantages of simple device structure, low-cost solution processing, light weight, and mechanical flexibility.1–6 The active layer of the PSCs is composed of a p-type conjugated polymer as donor and an n-type organic semiconductor [small molecule acceptor (SMA) or conjugated polymer acceptor]. Benefitting from the research progress in efficient photovoltaic materials,7–20 interface buffer layer materials,21–24 and device engineering,18,25–31 especially the emergence of the A-DA′D-A type SMAs (such as Y613), PSCs have developed rapidly and their power conversion efficiency (PCE) has exceeded 18% recently.12,16–19 For the Y6-type SMAs, the outer side chains on the terminal thiophene ring and inner side chains on the nitrogen atoms of the pyrrole ring of the DA′D-fused ring play important roles in the photovoltaic performance of the SMAs. In addition, low-cost conjugated polymer donors are pursued for realizing commercial application of the PSCs. In 2018, Sun et al. synthesized a simple structured DA copolymer donor poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)] (PTQ10) (Figure 1a) by the copolymerization of a thiophene D-unit and a difluoro-quinoxaline A-unit. PTQ10 was synthesized with low cost in only two steps from cheap raw materials and with a high yield of 87.4%.10 And PTQ10 as a polymer donor shows high photovoltaic performance in the PSCs with the A-DA′D-A type SMA.17–19,32,33 For instance, in 2021, Yan et al. synthesized three isomeric A-DA′D-A type SMAs, o-BTP-PhC6, m-BTP-PhC6, and p-BTP-PhC6, by using phenyl side chains with a hexyl substituent at different positions on the phenyl ring as the outer side chains of the SMAs.17 Among the three SMAs, m-BTP-PhC6 (with hexyl substitution at the meta-position of the phenyl side chains) shows more ordered intermolecular packing and possesses higher electron mobility. The PSC with m-BTP-PhC6 as acceptor and PTQ10 as donor demonstrates a high PCE of 17.7%.17 Then, Cui et al. further enhanced the PCE of the PSCs to a high level of 18.89%18 by adding PC71BM as the third component and controlling the morphology by using Dithieno[3,2-b:2′,3′-d]thiophene (DTT) as a solid additive. Recently, Peng et al. synthesized a new SMA BTP-FTh by introducing fluorinated alkylthiophene outer side chains. Benefitting from the suppressed charge recombination, a record high PCE of 19.05% was realized for a ternary PSC based on the PTQ10:BTP-FTh host blend with 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(methaneylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) as the second acceptor (the third component) and DTT as the volatilizable solid additive.19 The above results indicate that the SMAs with bulky, conjugated outer side chains are favorable for the PTQ10-based PSCs. Figure 1 | (a) Molecular structures of T2EH, P2EH, and PTQ10. (b) Normalized absorption spectra of the donor and acceptors films. (c) Energy level diagram of the photovoltaic materials. (d) Device structure of the PSCs. Download figure Download PowerPoint However, the high-performance PTQ10-based PSCs with PCEs over 18% mentioned above used a ternary blend strategy with a complicated solid-additive treatment,18,19 which increases the complexity and cost of the device fabrication processes. Hence it is crucial to develop new SMAs to further improve the photovoltaic performance of the binary PTQ10-based PSCs. Recently, we designed and synthesized two isomeric A-DA′D-A SMAs, o-TEH and m-TEH, by introducing thienyl outer side chains with a 2-ethylhexyl substituent at its α- or β-position respectively.34 We found that the substitution position on the thienyl outer side chains significantly influences the molecular geometry, molecular aggregation, and photovoltaic performance of the SMAs. Compared with o-TEH (α-substitution), β-substituted m-TEH shows closer π−π stacking, stronger intermolecular interaction, and higher electron mobility. The m-TEH-based PSCs with poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen2-yl)-benzo [1,2b:4,5-b′]dithiophene))-alt-(5,5-(2′,3′-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)-6′,7′-difluoro-5′,8′-di(thiophen-2-yl)quinoxaline)] (PBQ6)11 as polymer donor achieved a higher PCE of 18.51%, which is significantly higher than the PCE (16.22%) of the PBQ6:o-TEH based PSCs. Inspired by the high-performance SMAs of m-TEH with the alkyl β-substitution on the thienyl outer side chains34 and m-BTP-PhC6 with alkyl substitution on the meta-position of the phenyl outer side chains,17 in this work, we designed and synthesized two new Y6 derivative SMAs by introducing 2-ethylhexyl β-substituted thienyl or phenyl conjugated outer side chains (named as T2EH and P2EH, respectively (T2EH = 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-bis(4-(2-ethylhexyl)thiophen-2-yl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile; P2EH = 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-bis(3-(2-ethylhexyl)phenyl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)), Figure 1a) to further improve the photovoltaic performance of the SMAs. The inner side chains of T2EH are 2-ethylhexyl which is shorter than that (2-tetra-octyl) of m-TEH, and the substituent of the phenyl outer side chains in P2EH is the branched 2-ethylhexyl in comparison with the linear hexyl substituent in m-BTP-PhC6. These side chains were chosen to tune the solubility and aggregation properties of the acceptor to match the PTQ10 polymer donor. The binary PSCs based on PTQ10:T2EH demonstrated a high PCE of 18.55%, while the PCE of the P2EH-based PSCs was 17.50%. The PCE of 18.55% is the highest PCE in the PTQ10-based binary PSCs reported so far. The results imply that T2EH is one of the best SMAs for the PTQ10-based PSCs and is a promising acceptor in future applications of PSCs. Experimental Methods All chemicals are commercially available and used without further treatment unless otherwise stated. The experimental details are provided in the Supporting Information, including the synthetic processes and structural characterization of the SMAs, the measurements of absorption spectra and cyclic voltammetry (CV), fabrication and characterization of the PSCs, the measurements of space-charge-limited current (SCLC) mobility, atomic force microscopy (AFM), transmission electron microscopy (TEM), and transient absorption spectroscopy (TA). Results and discussion Synthesis and physicochemical properties of the SMAs Figure 1a shows the molecular structures of the two SMAs T2EH and P2EH and the polymer donor PTQ10, and the synthetic routes of the two SMAs are described in Supporting Information Schemes S1 and S2. Both T2EH and P2EH exhibit good solubility in common processing solvents such as chloroform, toluene, and chlorobenzene. Figure 1b displays the normalized UV–vis absorption spectra of PTQ10, T2EH, and P2EH films, and Supporting Information Figure S1 shows their solution absorption spectra. Table 1 lists the optical property data of the two SMAs. T2EH and P2EH solutions show similar absorption bands in the wavelength range of 300–900 nm with maximum molar extinction coefficients of 1.12 × 105 and 1.01 ×105 M−1 cm−1 at 733 and 729 nm, respectively (see Supporting Information Figure S2a). For the SMA films, the maximum absorption peaks of T2EH and P2EH red-shifted to 805 and 802 nm with absorption coefficients of 1.14 × 105 and 1.07 × 105 cm−1, respectively (see Supporting Information Figure S2b). The large bathochromic shifts of the film absorption reveal stronger π–π interactions of the molecular backbone in the films, and the higher absorption coefficients of T2EH mean that it can absorb photons more efficiently. Compared with P2EH, the absorption of the T2EH film is red-shifted and broadened to some extent, which may be related to its molecular geometry and aggregation properties. The optical bandgaps of T2EH and P2EH were 1.39 and 1.41 eV, respectively. Furthermore, the two SMAs show complementary absorption with the PTQ10 polymer donor in the visible to near-infrared region because the PTQ10 film absorption covers the range of 450 to 620 nm, which benefits light harvesting to get a higher short circuit current density (Jsc) of the PSCs. The number average molecular weight (Mn) of PTQ10 was 36.9 kDa with an appropriate polydispersity index (PDI) of 2.27 by high-temperature gel-permeation chromatography, as shown in Supporting Information Figure S3. Table 1 | Optical Properties, Electronic Energy Levels, and Electron Mobilities of T2EH and P2EH Acceptor λmax λonset εmax Egopta EHOMO/ELUMO μe Solution (nm) Film (nm) Film (nm) Solution (M−1 cm−1) Film (cm−1) (eV) (eV) (cm2 V−1 s−1) T2EH 733 805 891 1.12 × 105 1.14 × 105 1.39 −5.77/−3.91 6.83 × 10−4 P2EH 729 802 878 1.01 × 105 1.07 × 105 1.41 −5.75/−3.88 5.87 × 10−4 aCalculated from the onset absorption wavelength (λonset) of thin films: Egopt = 1240/ λonset. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels (EHOMO/ELUMO) of the two SMAs and the polymer donor PTQ10 were measured by CV. Supporting Information Figure S4 displays the cyclic voltammograms of the three molecular films. The details of the CV measurement and EHOMO/ELUMO calculations are described in the Supporting Information. The EHOMO/ELUMO values of PTQ10, T2EH, and P2EH were −5.70/−2.96 eV, −5.77/−3.91 eV, and −5.75/−3.88 eV, respectively (as shown in Table 1 and Figure 1c). Photovoltaic properties To study the photovoltaic performance of the SMAs, the SMA-based PSCs were fabricated with PTQ10 as the polymer donor, with a device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS)/PTQ10:SMA/aliphatic amine-functionalized perylene-diimide (PDINN)22/Ag (see Figure 1d). To optimize the photovoltaic performance of the two SMAs, various device fabrication conditions were carefully screened, and the experimental results are summarized in Supporting Information Table S1. Figure 2a shows the current density–voltage (J–V) characteristics of the optimized PSCs with a D:A weight ratio of 1:1.2, 0.7% 1-CN as a solvent additive, and thermal annealing at 90 °C for 5 min. Table 2 lists the detailed photovoltaic performance parameters of the optimized PSCs. The PSC based on PTQ10:P2EH shows a PCE of 17.50%, with an open circuit voltage (Voc) of 0.892 V, a Jsc of 24.91 mA cm−2, and a fill factor (FF) of 78.75%. Impressively, the PCE of the PSCs based on PTQ10:T2EH reached a high value of 18.55%, with a Voc of 0.872 V, a higher Jsc of 26.57 mA cm−2, and a high FF of 80.05%, which is the highest PCE in the PTQ10-based binary PSCs reported to date. The improved Jsc and higher FF result in the high PCE of the PSCs based on PTQ10:T2EH. The PCE of the PSC based on PTQ10:T2EH was also certified to 18.1% by the National Institute of Metrology (NIM) of China (see the test report of NIM in Supporting Information Figure S5). Figure 2 | (a) Current density–voltage (J–V) curves of the optimized PSCs based on PTQ10:SMA (1∶1.2, w/w) with 0.7% 1-CN as solvent additive and thermal annealing at 90 °C for 5 min, under the illumination of AM1.5G 100 mW cm−2. (b) EQE spectra of the optimized PSCs. (c) Jph versus Veff plots of the PSCs based on PTQ10:SMA. (d) Plots of logJsc versus logPlight of the PSCs. (e) Photo-CELIV plots of the PSCs. (f) Charge carrier lifetime versus light intensity plots of the PSCs from TPV measurement. Download figure Download PowerPoint Table 2 | Photovoltaic Performance Parameters of the Optimized PSCs Based on PTQ10:SMA (1∶1.2, w/w) with 0.7% 1-CN as Solvent Additive and Thermal Annealing at 90 °C for 5 min, under the Illumination of AM 1.5G (100 mW cm−2) Active Layers Voc (V) Jsc (mA cm−2) FF (%) PCE (%) PTQ10:T2EH 0.872 (0.869±0.004) 26.57 (26.45±0.15) 80.05 (79.68±0.40) 18.55 (18.31±0.25)a PTQ10:P2EH 0.892 (0.890±0.003) 24.91 (24.69±0.24) 78.75 (78.36±0.53) 17.50 (17.21±0.33) aAverage values obtained from more than 10 devices. Figure 2b shows the external quantum efficiency (EQE) spectra of the optimized PSCs based on PTQ10:SMA (T2EH or P2EH). Both PSCs show high photoresponse EQE values, and the PSC based on PTQ10:T2EH displays an even stronger photoresponse with a maximum EQE value >87%. Coupled with the wider absorption range, the calculated Jsc values (Jcal) from the EQE spectra are 25.98 mA cm−2 for the PSC based on PTQ10:T2EH and 24.36 mA cm−2 for the PSC based on PTQ10:P2EH, which agrees with the Jsc values obtained from the J–V curves. For understanding the effect of the conjugated outer side chains on the charge transporting properties of the SMAs, we measured the hole (μh) and electron (μe) mobilities by the SCLC method for the pure SMAs and PTQ10 donor films and for their blend films. Supporting Information Figure S6 shows the plots of the mobility measurements, and Supporting Information Table S2 lists the μe values of the SMAs and the μh and μe values of the blend films. The μe value of T2EH (6.83 × 10−4 cm2 V−1 s−1) is higher than that of P2EH (5.87 ×10−4 cm2 V−1 s−1), and the PTQ10:T2EH blend film shows higher and more balanced μh and μe values (6.73 × 10−4/7.11 × 10−4 cm2 V−1 s−1), with a μh/μe ratio of 0.95, than that of the PTQ10:P2EH blend film (6.19 × 10−4/6.98 × 10−4 cm2 V−1 s−1) with a μh/μe ratio of 0.89. The higher and more balanced μh and μe values of the PTQ10:T2EH blend film could lead to higher FF in the PTQ10:T2EH-based PSCs. Moreover, we measured the photocurrent density (Jph) under different effective voltage (Veff) for the PSCs, to reveal the effect of the conjugated outer side chains of the SMAs on the exciton dissociation of the blend films of the SMAs with PTQ10 polymer donor. Figure 2c shows the plots of logJph versus logVeff.34 The Jph reaches saturation (Jsat) at Veff higher than 2 V, and the exciton dissociation probabilities (Pdiss) and charge collection efficiency (Pcoll) of the PSCs can be calculated by the ratios of Jph/Jsat at the short-circuit and maximum power output conditions, respectively.35 The calculated Pdiss and Pcoll values of the PSCs based on PTQ10:T2EH were 95.5% and 84.8% respectively, which are higher than those of the PSCs based on PTQ10:P2EH (95.1% and 80.9%, respectively). The results imply that the PSCs based on PTQ10:T2EH possess more efficient exciton dissociation and charge collection than the PSCs based on PTQ10:P2EH, which could contribute to the higher Jsc and high FF of the PTQ10:T2EH-based PSCs. We also measured the dependence of Jsc and Voc on the irradiated light intensity (Plight) of the PSCs to reveal the charge carrier recombination mechanism of the devices.36–39 The relationship of Jsc and Plight can be described as Jsc∝Plightα, where α is the power law exponent.40 Figure 2d shows the plots of logJsc versus logPlight, and the α values obtained from the slope of logJsc versus log Plight are 0.990 for the PSCs based on PTQ10:T2EH and 0.974 for the PSCs based on PTQ10:P2EH. The α value of 0.990 (close to 1) for the T2EH-based PSC implies less bimolecular recombination40 in the PSCs based on PTQ10:T2EH. Supporting Information Figure S7a shows the plots of Voc versus lnPlight for the PSCs, where the slope of the linear section should be nkT/q (1 < n < 2), and a larger n value (close to 2) implies more trap-assisted recombination.41 The calculated slopes for the T2EH and P2EH-based devices are 1.22 kT/q and 1.31 kT/q, respectively, which indicates that the major charge recombination mechanism for both PSCs should be bimolecular recombination, with the T2EH-based PSC having less trap-assisted recombination. To study the charge extraction behavior of the PSCs, we measured the transient photocurrent (TPC) of the PSCs, as shown in Supporting Information Figure S7b. The T2EH-based PSC displays a faster turn-off dynamic than the P2EH-based PSC, which implies that there are rapid charge extraction and less trapped charge in the T2EH-based PSC. The above results imply that the PSC based on PTQ10:T2EH has more efficient exciton dissociation, higher charge carrier mobility, and less charge carrier recombination, which benefits obtaining a higher Jsc and FF in the PSCs. For investigating the charge carrier recombination and transport behavior of the PSCs under light illumination, the charge carrier mobilities were measured by the photon-induced charge-carrier extraction in linearly increasing voltage (photo-CELIV) and the charge carrier lifetime was measured by the transient photovoltage (TPV) for the PSCs. The photo-CELIV provides information on the PSCs under light irradiation. Under the illumination, the increment speed of the current density is determined by the conductivity of the device active layer, and the time spent to reach the maximum extraction current density is used to estimate the drift mobility of the photogenerated charge carriers. Figure 2e shows the transient signal of photo-CELIV in the PSCs. The carrier extraction mobilities obtained from the photo-CELIV measurement are 3.73 × 10−4 cm2 V−1 s−1 and 3.14 × 10−4 cm2 V−1 s−1 for the PSCs based on PTQ10:T2EH and PTQ10:P2EH, respectively. Figure 2f illustrates the charge carrier lifetime curves obtained from the TPV measurement under the illumination with different light intensity. The charge carrier lifetime of the T2EH-based PSC is 1.201 μs (Figure 2f), while the lifetime of the P2EH-based PSC is only 0.604 μs, under the illumination of 100 mW cm−2. The longer charge carrier lifetime in the PSCs based on PTQ10:T2EH is from less charge carrier recombination and more efficient exciton dissociation in the T2EH-based PSCs. The longer charge carrier lifetime and the higher photo-charge carrier mobility should result in the higher Jsc and FF of the T2EH-based PSCs.42–44 Morphology analysis To further explore the effect of conjugated outer side chain on the molecular aggregation and morphology of the SMAs in the active layer of the SMAs-based PSCs, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were carried out for the pristine SMAs and the blended films of the SMAs with PTQ10 polymer donor, and the results are shown in Figure 3 and Supporting Information Figure S8. As shown in the two-dimensional (2D) and one-dimensional (1D) GIWAXS patterns (Figures 3a–3c), the (010) diffraction peaks of the T2EH and P2EH films in the out-of-plane (OOP) direction are at 1.687 Å−1 with d-spacing of 3.724 Å and 1.590 Å−1 with d-spacing of 3.951 Å, respectively. The crystal coherence lengths (CCLs) of the T2EH and P2EH films are 19.662 and 9.054 Å, respectively. The results imply that the thienyl outer side chains in T2EH lead to closer π–π stacking and significantly larger CCL, which could benefit the charge transport and higher FF in the PSCs. In the blend films, both donor and acceptor remain in the predominant face-on orientation (Figures 3d–3f). The CCLs of the π–π stacking are 28.805 and 25.446 Å for the PTQ10:T2EH and PTQ10:P2EH blend films, respectively ( Supporting Information Table S3). The results imply that the PTQ10:T2EH blend film has better π–π stacking vertical to the substrate, which benefits the charge transport in the PSCs. Figure 3 | 2D GIWAXS patterns of (a) neat T2EH film, (b) neat P2EH film, (d) PTQ10:T2EH blend film, and (e) PTQ10:P2EH blend film. The corresponding 1D scattering profiles of (c) the neat films and (f) the blend films along the in-plane and out-of-plane directions. Download figure Download PowerPoint Furthermore, we also characterized the morphology of the active layers of the PSCs by AFM and TEM, and the results are shown in Figures 4a–4d and Supporting Information Figure S9. The T2EH and P2EH-based blend films with PTQ10 show uniform surfaces and suitable root-mean-square (RMS) roughness values of 1.22 and 0.89 nm, respectively. Importantly, the suitable phase separation improves the hole and electron mobilities of the PSCs. Figure 4 | AFM height images (2 μm scale) of the blend films of (a) PTQ10:T2EH and (b) PTQ10:P2EH. TEM images (200 nm scale) of the blend films of (c) PTQ10:T2EH and (d) PTQ10:P2EH. Download figure Download PowerPoint To better understand the charge transfer (CT) process, we carried out the broadband femtosecond TA (fs-TA) measurement for the PTQ10:T2EH and PTQ10:P2EH blend films, as well as the T2EH and P2EH pristine films, as shown in Figure 5 and Supporting Information Figure S10. We first selectively photoexcited the acceptor T2EH in the blend at 830 nm wavelength to investigate the hole transfer behavior. Figure 5a shows the TA of PTQ10:T2EH blend film at selected delay times. The predominant transient species early on is the excited state of T2EH, which manifests a broad ground state bleach (GSB) with peaks at 650, 750, and 850 nm, and an excited state absorption (ESA) with a peak at 950 nm. Then the excited state of T2EH decays rapidly, and after 20 ps the spectrum is mainly composed of two major GSB peaks at 610 and 850 nm, corresponding to PTQ10 and T2EH GSBs, respectively. This long-lived species is identified as the CT state, and it decays very slowly over the next several nanoseconds, which implies a slow charge recombination in the blend. The long lifetime of the CT state is crucial for the charge to be transported and finally extracted at the electrodes. As shown in Figure 5b, the blend film of PTQ10:P2EH exhibits a similar spectral profile and lifetimes as that of the PTQ10:T2EH blend film, except for the different intensity and kinetics of the ESA peak at 570 nm. This ESA peak is a representative feature of the excited state for both T2EH and P2EH (Figure 5c). Figure 5d shows a lower intensity and faster decay rate of the 570 nm ESA peak in the PTQ10:T2EH blend film, indicating a faster hole transfer from the T2EH excited state to the donor PTQ10, which leads to a higher CT state yield and benefits photocurrent generation. This result agrees with the higher photovoltaic performance of the PSC based on PTQ10:T2EH than that of the device based on PTQ10:P2EH. Figure 5 | Femtosecond transient absorption spectra of (a) PTQ10:T2EH blend film, (b) PTQ10:P2EH blend film, and (c) P2EH pristine film. (d) Kinetic traces probing at 570 nm for the excited state of T2EH (black), and CT processes of PTQ10:T2EH (red) and PTQ10:P2EH (blue) blend films. Download figure Download PowerPoint Conclusions Two A-DA′D-A structured SMAs T2EH and P2EH were synthesized by using 2-