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Effects on photovoltaic performance of dialkyloxybenzothiadiazole copolymers by varying the thienoacene donor Gururaj P. Kini, Sora Oh, Zaheer Abbas, Shafket Rasool, Muhammad Jahandar, Chang Eun Song, Sang Kyu Lee, Won Suk Shin, Won-Wook So, and Jong-Cheol Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12670 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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ACS Applied Materials & Interfaces

Effects on photovoltaic performance of dialkyloxybenzothiadiazole copolymers by varying the thienoacene donor

Gururaj P. Kini,a,b Sora Oh,a,b Zaheer Abbas,a,b Shafket Rasool,a,b Muhammad Jahandar,b,c Chang Eun Song,*,b,c Sang Kyu Lee,a,b Won Suk Shin,a,b Won-Wook So,a,b and Jong-Cheol Lee*,a,b

a

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT),

Daejeon, 305-600, Republic of Korea. b

Advanced Materials and Chemical Engineering, University of Science and Technology (UST),

Daejeon, 305-350, Republic of Korea. c

Center for Solar Energy Materials, Korea Research Institute of Chemical Technology (KRICT),

Daejeon, 305-600, Republic of Korea.

KEYWORDS: Thienoacene, conjugated polymer, polymer solar cells (PSCs), dithieno[3,2b:2',3'-d]thiophene (DTT), dialkyloxy-benzothiadiazole (ROBT).

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ABSTRACT A series of four donor-acceptor alternating copolymers based on dialkyloxy-benzothiadiazole (ROBT) as an acceptor and thienoacenes as a donor units were synthesized and tested for polymer solar cells (PSCs). These new polymers had different donor units with varied electrondonating

ability

(thieno[3,2-b]thiophene

(TT),

dithieno[3,2-b:2',3'-d]thiophene

(DTT),

benzo[1,2-b:4,5-b']dithiophene (BDT) and naphtha[1,2-b:5,6-b']dithiophene (NDT)) in the polymer backbone. To understand the effect of these thienoacenes on the optoelectronic and photovoltaic properties of the copolymers, we systematically analyzed and compared energy levels, crystallinity, morphology, charge recombination and charge carrier mobility in the resulting polymers. In this series, optimized photovoltaic cells yielded power conversion efficiency (PCE) values of 6.25% (TT), 9.02% (DTT), 6.34% (BDT) and 2.29% (NDT) with different thienoacene donors. The introduction of DTT into the thienoacene-ROBT polymer enabled the generation of well-ordered molecular packings with a π-π stacking distance of 3.72 Å, high charge mobilities and an interconnected nanofibrillar morphology in blend films. As a result, the PSC employing polymer with DTT exhibited the highest PCE of 9.02%. Thus, our structure–property relationship studies of thienoacene-ROBT-based polymers emphasize that the molecular design of the polymers must be carefully optimized to develop high efficienct PSCs. These findings will help us to understand the impact of the donor thienoacene on the optoelectronic and photovoltaic performance of polymers.

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INTRODUCTION Polymer solar cells (PSCs) are a promising technology for renewable energy production due to their low cost, good mechanical flexibility and solution processability.1-4 In the past decade, PSCs have been investigated extensively, improving our understanding of the relation between molecular structure and performance.5-9 Currently, a power conversion efficiency (PCE) over 10% has been achieved for PSCs by exploring novel donor materials and by optimizing their device conditions.10-12 However, a major hurdle for the commercialization of these high efficiency donor materials is their synthetic complexity and difficult device processing requiring high temperature conditions. Therefore, high-performance donor materials must also possess a simple synthesis and good processability along with other factors such as broad absorption, high charge carrier mobility, nanoscale morphology and high efficiency. Among the polymers with a simple molecular design, polymers containing acene donor have proved to be a reliable strategy to design semiconducting copolymer.13-20,23-27 In organic semiconductor research, extensive efforts have been devoted to the design and synthesis of various small molecules and polymers with π-conjugated aromatic and heteroaromatic units. Acenes are planar and rigid molecules that enable stronger - interactions and good crystallinity; hence, acenes are helpful for achieving increased charge carrier mobility.13 As a result, acene-based polymers have been employed to fabricate efficient organic field-effect transistors.13-20 In particular, ladder-type and fused heteroaromatic thienoacenes such as thieno[3,2-b]thiophene (TT), dithieno[3,2-b:2',3'-d] thiophene (DTT), benzo[1,2-b:4,5-b']dithiophene (BDT) and naphthodithiophene (NDT), have attracted attention due to their advantages, such as decreased rotational disorder, closely packed structures with cofacial π-π-stacking and effective electron delocalization in the polymer backbone.21-27 However, unsubstituted thienoacene-based polymers (without alkyl substitution) 3



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have been employed less frequently in the PSCs compared to substituted thienoacene-based polymers (with alkyl substitution), due to their poor solubility.23-28 On the other hand, thienoacenes without any alkyl substitution groups have several advantages such as high charge carrier mobility,13-20 good photostability30,37 and relatively deeper highest occupied molecular orbital (HOMO) energy level.24,58 Recently, several highly efficient polymer donors with PCE values over 8% have been reported employing these thienoacenes.31,58-60 Therefore, polymer structures involving thienoacenes will open the door for the synthesis of efficient PSCs through fine-tailored optical and electronic properties via the selection of the appropriate thienoacene. Among the various acceptors units, dialkyloxy-benzothiadiazole (ROBT) was selected as the acceptor in our donor-acceptor (D-A) molecular design due to its unique properties, such as simple synthesis, good planarity and the possibility of grafting different alkyl chains.34,61 Moreover, the presence of oxygen on the polymer backbone helps to enhance the molecular planarity via intra- and intermolecular non-covalent coulombic interactions, resulting in a minimized torsion angle in the polymer backbone.33-35 However, different research groups have reported ROBT polymers that contain various thienoacenes, such as TT (PTTBT14),34 DTT (PDTTDABT or PDTTBT14)37 and BDT (PBDTBT14),38 which exhibited poor performance (PCE= 2.6%, 2.2% and 0.43% for the polymers PTTBT14, PDTTBT14 and PBDTBT14, respectively).,The major cause for the poor PCEs of these thienoacenes-ROBT polymers is the un-optimized molecular design, which hinder the miscibility between the conjugated polymers and the fullerene based acceptors due to excessive aggregation.34 Therefore, we improved the solubility of these polymers by drafting higher alkyl chains to the ROBT moiety and systematically optimized the molecular design by varying donor thienoacenes with different electron-donating abilities. 4


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In this study, we synthesized simple structured thienoacene-ROBT polymers involving alternating thienoacene (TT, DTT, BDT and zig-zag NDT) and ROBT units (namely, poly-{5,6bis((2-butyloctyl)oxy)-4-(thieno[3,2-b]thiophen-2-yl)benzo[c][1,2,5]thiadiazole} (P1), poly{5,6-bis((2-butyloctyl)oxy)-4-(dithieno[3,2-b:2',3'-d]thiophen-2-yl)benzo[c][1,2,5]thiadiazole} (P2), poly-{4-(benzo[1,2-b:4,5-b']dithiophen-2-yl)-5,6-bis((2hexyldecyl)oxy)benzo[c][1,2,5]thiadiazole} (P3) and poly-{ (5,6-bis((2-hexyldecyl)oxy)-4(naphtho[1,2-b:5,6-b']dithiophen-2-yl)benzo[c][1,2,5]thiadiazole} (P4)) and studied the effect of the donor thienoacene on the photovoltaic performance of the thienoacene-ROBT copolymers. In particular, we studied the correlations between the type of thienoacene and photovoltaic performance by analyzing the optical bandgap, molecular orientation, charge generation, transport mechanism and film morphology of the resulting polymers.

RESULTS AND DISCUSSION Synthesis, Characterization and Thermal properties. Scheme 1 presents the synthesis of the monomers and polymers. The ROBT monomer (2) was synthesized in two steps through a simplified route using 5,6-difluorobenzo[c][1,2,5]thiadiazole (2FBT). The functionalization of 2FBT was achieved by carrying out a reaction of 2FBT with the corresponding alcohol via sodium hydride (1), followed by bromination using bromine and hydrobromic acid to yield the final ROBT monomers 4,7-Dibromo-5,6-bis((2butyloctyl)oxy)benzo[c][1,2,5]thiadiazole (M1) and 4,7-Dibromo-5,6-bis((2hexyldecyl)oxy)benzo[c][1,2,5]thiadiazole (M2). The stannylated thienoacene monomers 2,5bis(trimethylstannyl)thieno[3,2-b]thiophene (M3),27 2,6-Bis (trimethyltin)dithieno[3,2-b:2',3'd]thiophene (M4),23 2,6-bis(trimethylstannyl)benzo[1,2-b:4,5-b']dithiophene (M5)38 and 2,75



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bis(trimethylstannyl) naphtha [1,2-b:5,6-b']dithiophene (M6)26 were synthesized according to the previously reported procedures. All of the polymers in this series were synthesized via the Stille reaction using Pd(dba)3 and tri(o-tolyl)phosphine in a microwave reactor using chlorobenzene (CB), with yields of approximately 60 - 90%. The ROBT monomer M1 was reacted with M3 and M4 to obtain P1 and P2, respectively. Similarly, the M2 monomer was reacted with M5 and M6 to obtain P3 and P4, respectively. The longer hexyldecyl alkyl chains (HD) in the P3 and P4 were used to improve the molecular weights and solubility. The number average molecular weight (Mn) of the polymers was determined via gel permeation chromatography at room temperature, using chloroform (CF) as the eluent and polystyrenes as calibration standards. These results are summarized in Table 1. All polymers showed good solubility in organic solvents such as CF, CB and 1,2-dichlorobenzene (ODCB). A detailed procedure for the synthesis of the monomers and polymers is presented in the supporting information. The thermal stability of these newly synthesized polymers was measured by using thermogravimetric analysis (TGA) (Figure S1). All the polymers displayed good thermal stability with decomposition temperature (Td) at 309, 313, 316 and 318oC, respectively for polymers P1, P2, P3 and P4 with 5% weight loss under a nitrogen atmosphere.

Photophysical and Electrochemical Properties. The optical properties of the polymers were studied by measuring the UV-vis absorption spectra both in solution and in the film state, which are presented in Figures 1(a) and 1(b), respectively. The detailed optical properties of the polymers are summarized in Table 1. The absorption spectra of all polymers in a thin film were approximately 30 - 50 nm red-shifted compared to the polymers in solution due to increased planarity and intermolecular π-π interactions in the solid 6


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state. As the electron-donating ability of the donor thienoacene in the polymer backbone was increased from NDT, BDT, TT to DTT,13 the corresponding absorption maxima in the thin film state of the polymer was shifted bathochromically (556, 575, 619 to 645 nm for P1, P2, P3 and P4, respectively), and their solutions also followed a similar trend. Among the polymers, the absorption spectra of P1 and P2 with TT and DTT donor units showed red-shifted compared to the P3 and P4 with BDT and NDT in the film state; this shift is caused by the stronger intermolecular interaction that is promoted by sulfur atoms in thiophenes compared to that of carbon atom in benzene.13,25,36 Moreover, P2 showed a shoulder peak at 645 nm in the film state due to its good inter-chain packing/aggregation. However, the absorption spectra of P3 and P4 were blue-shifted due to the presence of benzene units in the donor BDT and NDT compared to the P1 and P2. The optical bandgap calculated from the absorption onsets of the films state can be given as 1.75, 1.75, 1.83 and 2.02 eV for polymers P1, P2, P3 and P4, respectively. Cyclic voltammetry (CV) was used to understand how the variation thienoacene donor units influenced energy levels of the resultant polymers. In the measurement of CV, platinum (Pt) wire was used as the counter electrode and a silver chloride electrode (Ag/AgCl) was used as the reference electrode. The polymer thin film was coated onto a Pt electrode (working electrode), and the CV was measured at a scan rate of 50 mV s-1 at 25°C using 0.1 M tetrabutyl ammonium hexafluorophosphate in acetonitrile as a supporting electrolyte. The energy levels of the reference Ag/AgCl were calibrated against a ferrocene/ferrocenium (Fc/Fc+) system, which has known absolute energy level of - 4.8 eV relative to vacuum.62-63 The HOMO levels of the polymers were calculated from the onset of oxidation potential in a CV graph using the equation HOMO = - (Eox + 4.8) (eV). The detailed electrochemical properties of the polymers are summarized in Table 1, and the corresponding CV data and schematic energy levels of the 7



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polymers are shown in Figure 1c and Figure 1d, respectively. The HOMO energy levels of the polymers were - 6.02, - 5.77, - 5.52 and - 5.47 eV for P4, P3, P1 and P2, respectively. Notably, as the electron-donating ability of the donor unit decreased in the order: NDT > BDT > TT > DTT, the HOMO energy levels were down-shifted and the polymer bandgap was increased. The corresponding lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were calculated from the HOMO and the optical band gap of the polymers using the equation LUMO = Eg opt – HOMO eV, where Eg opt represents the optical bandgap of the polymers in the film state. The corresponding LUMO energy levels were - 4.00, - 3.94, - 3.77 and - 3.72 eV for P4, P3, P1 and P2, respectively. Thus, the incorporation of a weak donor can generate deep HOMO energy levels in the polymers, which is beneficial for enhancing the open-circuit voltage (VOC) in the photovoltaic device.24,58

Photovoltaic Properties. To investigate the effect of the different thienoacenes unit on the photovoltaic properties of the polymers, PSCs with a conventional device architecture were fabricated (glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/polymer: [6,6]-phenyl C71 butyric acid methyl ester (PC71BM)/Ca/Al)). The performance of all of the polymers was measured under AM 1.5 G irradiation (100 mW/cm2). For all thienoacene-ROBT polymers, the PSC devices fabricated with a polymer:PC71BM blend ratio of 1:1.5 (w/w) using CF and 1,8-diiodooctane (DIO) (97:3 v/v) yielded the best performance. Table 2 shows the detailed photovoltaic parameters of the optimized devices, and Figure 2 (a) and (b) shows the corresponding J-V characteristics and external quantum efficiency spectra, respectively. The polymers P1, P2, P3 and P4 showed the best PCE values of 6.25%, 9.02%, 6.34% and 2.29%, 8


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respectively, with an active layer thickness of ~ 95-110 nm. The PCE values of all polymers processed with CF/DIO improved remarkably due to enhanced short-circuit current density (JSC) and fill factor (FF) values compared to that processed with CF solvent (Table S1 and Figure S2). In general, DIO solvent additives are known to enhance PCEs by improving the morphology and crystalline phases;39-41 hence, we utilized the DIO additive to improve the photovoltaic performances in this study. Among the polymers, P2 with DTT in the polymer backbone yielded an average PCE of 8.89% for over 20 independent devices (active area of 9 mm2, measured using a shadow mask), and champion devices showed the highest PCE value of 9.02%, with corresponding photovoltaic parameters of JSC = 15.04 mA/cm2, VOC = 0.81 V and FF = 74%. The optimal device was obtained by varying the active layer blend ratio (Table S2 and Figure S3) and the thickness (Table S3 and Figure S4). Generally, the photo-active layers that exhibit thickness tolerance are preferred for real commercialization for using large-scale R2R processing techniques.32 Under optimized device conditions, P2:PC71BM exhibited sufficient thickness tolerance with PCE values over 8% at all active layer thickness values ranging from 75 nm to 180 nm (Table S3 and Figure S4). However, at higher thickness values (~230 nm), there was a slight drop in the PCE (7.78%), which is ascribed primarily due to a reduction of FF values. Meanwhile, the VOC values of the PSCs were well matched with the corresponding HOMO energy levels of the polymers except for P4. It should be noted that the VOC value of the P4 device was similar to the P3 device, despite its deeper HOMO energy levels. As the VOC is also dependent on factors such as charge recombination and morphology,42,57 the analysis of the charge recombination and morphology was also performed to understand this result (discussed below).

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External quantum efficiency (EQE) measurements of all PSCs were performed to understand the difference of JSC values depending on varied thienoacene moeity. The EQE profiles of the PSCs nearly imitated the absorption properties of the polymers and the PSCs fabricated without the DIO exhibited much lower EQE values than the devices fabricated with DIO (Figure 2(b) versus Figure S2(b)). Among all polymer:PC71BM casting from CF/DIO, P2 displayed broad EQE spectra over a wavelength region ranging from 450 to 700 nm (EQE values over 70%), and the P2 device showed the highest EQE value of 80% at 470 nm. These higher EQE values for P2 helped to achieve the highest efficiency of 9.02% with JSC =15.04mA/cm2. All JSC values calculated from the EQE curves were well matched with the J-V measurements within 5% error. Even though the absorption spectra of P1 and P2 were similar, P1 exhibited a much lower EQE value than P2. To understand the reason for this discrepancy, the absorption co-efficient of polymer:PC71BM blends were measured under the optimized device fabrication conditions. As shown in Figure 2(c), the higher absorption co-efficient of P2 helped to absorb more light generate more excitons; hence, the P2 device yielded higher JSC values than P1. Meanwhile, the lower JSC values of the P3 and P4 device (8.97 and 4.35 mA/cm2, respectively) were attributed to their reduced absorption ranges. Therefore, the presence of the benzene ring in P3 and P4 negatively affected the polymer absorption range, which hampered JSC in the devices. In addition to efficient light absorption, the charge carrier mobility, the charge recombination dynamics and the formation of optimal nanoscale morphology also play crucial roles in determining the JSC values of the PSCs.31 To address these issues, we analyzed molecular ordering, carrier mobility, carrier recombination and film morphology.

Molecular Packing. 10


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Charge carrier mobility in photovoltaic devices is dependent on the crystallinity and molecular ordering of photo-active films. Hence, 2D-grazing incidence wide angle X-ray scattering (GIWAXS) measurements of neat polymers and polymer:PC71BM blends were measured to determine the molecular ordering and packing.63 Figure 3 shows the in-plane (qxy) and out-ofplane (qz) cut profiles of GIWAXS images (Figure S5), and Table S4 presents the corresponding packing parameters. The GIWAXS profiles of pristine P1, P3 and P4 films showed Braggs reflection peaks (100) in both in-plane and out-of-plane directions with corresponding lamellar distances of 18.43, 20.14, 20.60 Å and 17.31 Å,19.45 Å, and 19.33 Å, respectively (Figure 3(a)(b) and Figure S5 (a), (c) and (d)) indicating a mixed orientation with face-on and edge-on. Additionally, these polymers also showed intense (010) diffraction peaks in the out-of-plane direction, which correspond to π-π stacking distance of 3.95 Å, 3.98 Å and 4.01 Å, respectively (Figure 3(a) and Figure S5(a), (c) and (d)). In contrast, P2 showed a (100) diffraction peak only in-plane direction (lamellar distance of 19.12 Å) and (010) out-of-plane π-π stacking peak at 1.64 Å-1 (d-spacing distance of 3.84 Å), indicating typical face-on molecular arrangements (Figure 3(a) and 3(b) and Figure S5(b)). Among the polymers, P2 showed shorter π-π stacking distance, signifying stronger π-interactions between the polymer chains.33 For polymer:PC71BM blends casting from CF/DIO solvent, the intensity of the π-π stacking peaks was reduced considerably (Figure S5 (e) to (h)). In the P1 and P4, the (010) peak in out-of-plane directions vanished completely, while (100) peaks were retained in both in-plane and out-of-plane directions (Figure 3(c) and 3(d) and Figure S5(e) and 5(h)). The corresponding lamellar distances were found to be 17.70 Å, 20.60 Å and 16.15 Å, 18.98 Å, respectively. These results showed the retention of mixed orientation (edge-on and face-on) in P1 and P4 with PC71BM. While, P3:PC71BM and P4:PC71BM films demonstrated intense higher ordered lamellar packing (100) and (200) peaks in 11



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out-of-plane direction and these lamellar spacings were further reduced than pristine polymers (Table S4), signifying the formation of highly aggregated polymers in photo-active layers, which proved to have negative effect on miscibility with fullerene, exciton dissociation and charge transport in PSCs (Figure 3(c) and Figure S5 (g) and (h)).34 These results are consistent with morphology of photo-active films using the atomic force microscopy (AFM) and transmission electron microscopy (TEM) analysis (discussed later in Nanoscale Film Morphology). While the P2:PC71BM film exhibited an out-of-plane (100) diffraction peak at 0.36 Å-1(lamellar distance = 17.51Å) and an (100) in-plane diffraction peak at 0.33 Å-1 (lamellar distance= 18.82 Å) (Figure 3(c) and 3(d) and Figure S5(f)). Notably, P2 displayed a retention of the (010) peak at 1.69 Å-1 in the out-of-plane direction (π-π stacking distance = 3.72 Å, which suggests a preferential face-on molecular orientation. Interestingly, the π-π stacking distance in the blend film of P2 (3.72Å) was further reduced from pristine films (3.84 Å) (Figure 3(c) and 3(d) and Figure S5(f)). A preferential face-on orientation combined with a reduced π-π stacking distance is favorable for vertical charge-transport and charge carrier mobility.43-45 We further analyzed the azimuthal scan profiles of all photo-active films extracted from the lamellar diffraction (100) in the pole figures of the GIWAXS images (Figure S6). The P2:PC71BM exhibited a smaller population of face-on orientation compared to the P1:PC71BM and P3:PC71BM films, however, π-π stacking distance in the out-of-plane direction strongly affects charge mobilities and photovoltaic properties in this series polymer.

Charge Transport Properties. To elucidate the effect of molecular packing on the charge carrier mobilities of the polymer:PC71BM blend films, we fabricated hole only (ITO/PEDOT:PSS/active layer/Au) and 12


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electron only (ITO/ZnO/active layer/Ca/Al) devices and measured the hole (µh) and electron (µe) mobilities by employing the space-charge limited current model (Figure S7). In this series, polymer blends cast from a CF solution showed poor hole and electron mobility values compared to devices processed with CF/DIO (Table S1 and Table 2). After the addition of DIO, the electron mobility in polymer blends was significantly enhanced (Table 2 and Figure S7(e)). These observations substantiated that the addition of DIO helped to generate well dispersed PC71BM aggregation and facilitated the formation of more crystalline phases in polymer:PC71BM films, as reported previously.35-36 A decreasing trend of the µe values was observed in the order 1.41 × 10-4cm2/Vs (P2) > 3.15 × 10-5 cm2/Vs (P3) > 1.21 × 10-5 cm2/Vs (P4) and > 4.99 × 10-6 cm2/Vs (P1). And the hole mobility values of the P2, P1, P3 and P4 blends processed with CF/DIO are 2.31 × 10-4 cm2/V s, 2.06 × 10-5, 9.71 × 10-6 and 1.44 × 10-6 cm2/V s, respectively, for P2, P1, P3 and P4 (Table 2). Notably, the P2:PC71BM film showed the highest hole and electron mobility among the photo-active layers. This enhancement of charge mobility in P2 correlates well with its well-ordered packings with a reduced π-π stacking distance, which aids efficient vertical charge transport. In contrast, the P3 and P4 photo-active layers exhibited hole mobility ~1 and 2 order lower than P1 and P2, respectively. These results were consistent with morphology of photo-active films (Figure 5), where photo-active layer with P3 and P4 showed severe aggregation of polymers compared to the P1 and P2, resulting in disconnected bicontinuous charge pathway. The high hole mobility values have proved to decrease the charge recombination,31 while the lower hole mobility hinders efficient charge transport through high bimolecular and Shockley-Read-Hall recombination.46 Furthermore, a good charge balance between hole and electron mobilities will help to decrease build in space charges, which in turn help to enhance FF in the PSCs. Therefore, the low and unbalanced charge mobilities of P1, P3 13



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and P4 significantly reduced their PCE values compared to the PSC with P2. Thus, the high JSC of the P2 device is attributed to several positive factors such as extended absorption, a high absorption co-efficient, a high charge mobility and a good charge balance between hole/electron (µh/ µe= 1.63), which lead to enhanced performance in the PSC.

Charge Generation, Charge Transport and Recombination Study. To understand the observed variation of device performance based on the type of thienoacene, charge carrier generation, charge transport and charge recombination studies of the PSCs were performed. The maximum exciton generation rate (Gmax) in the photo-active layers was determined from a plot of photocurrent density (Jph) (defined as JPh = JL - JD, where JL and JD are the current density under illumination and dark, respectively) versus effective voltage (Veff) (defined as Veff = V0 - Va, where Vo is the corresponding voltage when Jph = 0 and Va is the applied bias voltage) (Figure 4(a)).47-48 Generally, the photo-generated charge carrier moves towards the respective electrodes with minimum recombination at high Veff values. Thus, the corresponding Gmax was calculated from the equation Jsat = q•L•Gmax, where Jsat = saturated photocurrent density, q = electronic charge and L = the thickness of active layer.47-48 P1, P2 and P3 devices displayed photocurrent saturation at Veff = ~ 1 V, and the corresponding Gmax values of the polymers were 8.83 x 1027 m-3s-1 (P2), 7.07 x 1027 m-3s-1 (P1) and 4.86 x 1027 m-3s-1 (P3). The calculated Gmax value of P2 was 24% and 81% higher than those of P1 and P3, respectively, implying higher exciton dissociation in P2. However, a drop in the JPh was observed in P4. This phenomenon was probably caused by a failed device. To obtain a clear insight into the charge recombination kinetics of the photo-active layers, the dependence of photovoltaic parameters (i.e., JSC, VOC and FF) on the illumination intensity was 14


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measured.49 Figure S8 represents photocurrent density versus applied voltage curves for the optimized PSCs at illuminated light intensity values ranging from 3.2 to 100 mW/cm2. Among PSCs, the devices employing P2 showed good photocurrent saturation along with a high charge sweep out rate at all intensities, which emphasizing efficient charge transport with the least recombination (Figure S8(b)). In contrast, the devices of P1 and P3 displayed efficient photocurrent saturations only at lower light intensity values (50.1 mW/cm2). Whereas, at higher light intensity values (> 50.1 mW/cm2), slow photocurrent saturations were observed. These results suggested possible charge recombination in P1 and P3 devices at higher light intensities, probably due to the accumulation of charges in devices. Compared to other devices, the P4 device showed poor photocurrent saturation at all light intensities, indicating pronounced charge recombination (Figure S8(d)). The JSC versus illuminated light intensity have been used to elucidate the presence of bimolecular recombination in PSCs.35,49-50 As shown in Figure 4(b), double logarithmic plots of JSC versus illuminated light intensity were plotted to determine the extent of bimolecular recombination in the PSCs. The correlation between JSC and light intensity was assessed from the equation JSC ∝ Pα, where P represents the incident light intensity and α is the exponential constant that indicates the extent of bimolecular recombination. A value of α = 1 indicates minimum bimolecular recombination in PSCs.49-50 The extracted α values were 0.95, 0.97, 0.95 and 0.93, respectively, for P1, P2, P3 and P4. Thus, the PSC employing P2 demonstrated efficient charge sweep-out with suppressed bimolecular recombination over other PSCs. To further investigate the extent of monomolecular (trap assisted) and bimolecular recombination in the PSCs, we analyzed the variation of VOC as a function of the illumination light intensity (Figure 4(c)).49 The correlation between the illumination light intensity and the 15



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VOC was derived from the equation: (1 − ) oc ≈ ln

Where G is the electron and hole generation rate, which is directly proportional to the illumination intensity (I), is the Boltzmann constant, T is the temperature and q is the elementary charge.51 Typically, monomolecular recombination demonstrated a stronger dependency of VOC on light intensity, with a slope (S) of 2kT/q. In contrast, bimolecular recombination shows a slope of kT/q.49-51 The PSCs with P1, P2, P3 and P4 exhibited slope (S) values of 1.04, 1.14, 1.07 and 1.14, respectively. All of the polymers exhibited nearly identical slopes, suggesting that bimolecular recombination is the dominant mechanism and monomolecular recombination is comparatively weaker in PSCs. In Figure 4(d), PSCs with all polymers in the series displayed higher FF values at lower intensities. Under higher light intensities, the FF values of the PSCs decreased in the order P4< P1< P3 < P2. However, P2 exhibited high FF values at almost all intensities due to its high charge mobility and good charge balance, which considerably reduced the charge recombination and accumulation of buildup charges.

Nanoscale Film Morphology. To understand the effect of a different thienoacene donor unit on the surface and bulk nanoscale morphology of photo-active layers, the polymer:PC71BM films were analyzed using tapping mode AFM and TEM (Figure 5).The phase and topographic AFM images of all photo-active layers exhibited distinct morphological characteristics depending on the thienoacene unit. The polymers P1:PC71BM and P2:PC71BM showed smooth uniform morphology, whereas 16


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P3:PC71BM and P4:PC71BM blend films displayed inhomogeneous surface morphology regardless of processing solvents (Figure 5 and S9). Generally, bulk heterojunction (BHJ) structures with bicontinuous interpenetrating and nano-fibrillar networks are favorable for efficient charge separation and transport in PSCs.33,54-56 In addition to good BHJ morphology between the polymer and PC71BM, the polymer fibril width also plays a crucial role in determining efficient exciton diffusion at the D-A interface.29,55 The wider fibril length is proved to hinder excitons from reaching the fullerene interfaces, while finer fiber width are beneficial for efficient exciton dissociation.29,55 Notably, as the thienoacene donor moiety in the polymer backbone was varied from TT, DTT, BDT to NDT, the fibril width was increased in photo-active layers as shown in TEM images (Figure 5(c1) to (c4)). These increased fibril widths of P3:PC71BM and P4:PC71BM films were well correlated with higher molecular ordering and aggregations of these polymers in GIWAXS (Figure 3(c) and Figure S5 (g) and (h)). The TEM images of P1:PC71BM and P2:PC71BM blend films exhibits bicontinuous interpenetrating nanofibrillar networks (Figure 5(c1) and (c2)), whereas macro-phase separation33,52-53 was observed in P3:PC71BM and P4:PC71BM films (Figure 5 (c3) and (c4)). Among photo-active films, P2:PC71BM blend showed a well-maintained balance between miscibility and connectivity through the formation of semicrystalline nano-fibrillar structures with optimal fibril width (~ 10nm).29 As a result of well-developed D-A interfaces and charge carrier pathways, PSCs fabricated with P2 showed much higher and balanced carrier mobility, as well as superior JSC, FF and PCE values. Thus, un-optimal fibrils width in P1:PC71BM and P3:PC71BM blend negatively affected overall charge extraction mechanism and further enhancement of PCEs. In P4:PC71BM, excessive polymer aggregations and decreased bicontinuous network hindered efficient exciton dissociation and photo-generated charge transport.29,55 Thus, the inferior performance of 17



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P4:PC71BM is attributed to several factors, such as low charge carrier mobilities, high recombination and inferior film morphology, which negatively affected JSC, VOC, FF and PCE in the devices.

CONCLUSION A series of thienoacene-ROBT copolymers based on alkyl-substituted benzothiadiazole as an electron acceptor and different thienoacenes as donor units were synthesized. The incorporation of TT and DTT in thienoacene-ROBT molecular design reduced polymers bandgap, whereas incorporation of BDT and NDT resulted in higher bandgap due to the varied electron-donating strength of the donor thienoacenes. As the electron-donating ability of the donor unit decreased in the order NDT > BDT > TT > DTT, the HOMO energy levels were down-shifted. Therefore, the incorporation of donor thienoacene units with varied electron-donating ability led to easy fine-tuning of the bandgap and energy levels of the thienoacene-ROBT copolymers. Moreover, the variation of the thienoacene donor in photo-active layers had a significant impact on the molecular ordering, charge carrier mobilities, recombination mechanism and film morphologies. The DTT-ROBT (P2) shows broad light absorption spectra and high absorption co-efficient, well-ordered packings with a reduced π-π stacking distance, high charge mobility, lower charge recombination and optimum film morphology. Thus, P2 exhibits outstanding photovoltaic performance with PCE values of over 9% in the PSCs. The other polymers with TT, BDT and NDT displayed PCE values of 6.25% (P1), 6.34% (P3) and 2.29% (P4), respectively. Consequently, our structure–property relationship studies of thienoacene-ROBT-based polymers emphasize that the molecular design of the polymers must be carefully optimized to develop high efficient PSCs. 18


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Figure 1. Absorption spectra of the polymers in (a) chloroform solution and (b) thin films, (c) cyclic voltammograms of the polymers and (d) schematic energy level diagram of the polymers.

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Figure 2. (a) The characteristic J-V curves, (b) EQE profiles and (c) absorption co-efficients of polymer blends under optimized device fabrication conditions with polymer:PC71BM (1.0:1.5, w/w) processed with CF/ DIO 3vol%.

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Figure 3. Graphs (a) and (c) represent out-of-plane line cut profiles and (b) and (d) indicate inplane line cut profiles of the GIWAXS images obtained from pristine polymers and polymer:PC71BM blends, respectively. 24


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Figure 4. (a) Photo current density (Jph), versus effective voltage for polymer:PC71BM devices; (b) the short-circuit current density (JSC), (c) open-circuit voltage (VOC) and (d) fill factor (FF) dependence on the incident light intensity.

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Figure 5. (a1 to a4 and b1 to b4 ) Tapping mode AFM phase images and height images, respectively and (c1 to c4) TEM images of polymer:PC71BM blend films processed with CF/DIO.

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Scheme 1. Chemical structures and synthetic routes of a) dialkyloxy-benzothiadiazole monomers and b) polymers.

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Table 1. Thermal, optical and electrochemical properties of the polymers. Thermal

Electrochemical properties

Optical properties Mn/PDI

properties

Polymer (kDa) o

Td ( C)

a

λmax (nm)

λmax (nm)

λonset (nm)

solution

film

film

Eg opt HOMO

LUMOb

(eV)a

(eV)

(eV)

P1

38.9/3.2

309

647

619

710

1.75

-5.52

-3.77

P2

41.9/3.0

313

651

614, 645

710

1.75

-5.47

-3.72

P3

66.5/1.8

316

573

575

675

1.83

-5.77

-3.94

P4

17.5/2.8

318

504

556

612

2.02

-6.02

-4.00

Estimated values from the UV-vis absorption edge of the thin films (Eg opt = 1240/ λonset eV).

b

Calculated from HOMO energy levels and optical bandgaps.

HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital.

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Table 2. PSCs performance of different solar cell devices (P1:PC71BM, P2:PC71BM, P3:PC71BM and P4:PC71BM) processed with CF/DIO under the illumination of AM 1.5 G, 100 mWcm-2. d

VOC

JSC

FF

PCE

µh

µe

[nm]

[V]

[mA/cm2]

[%]

[%]

[cm2/V s]c

[cm2/V s]d

P1

~95

0.90

11.37 (11.11)a

61

6.25(6.10)b

2.06 × 10-5

4.99 × 10-6

4.12

P2

~110

0.81

15.04 (14.89)a

74

9.02(8.89)b

2.31 × 10-4

1.41 × 10-4

1.63

P3

~105

1.01

8.97 (8.90)a

70

6.34(6.19)b

9.71 × 10-6

3.15 × 10-5

0.31

P4

~100

1.00

4.35 (4.26)a

53

2.29(2.15)b

1.44 × 10-6

1.21 × 10-5

0.12

Polymer

The device architecture is ITO/PEDOT:PSS/Polymer:PC71BM/Ca/Al. a

The value is calculated from the EQE spectrum.

b

c

The average PCE in the brackets is obtained from over 10 independent devices.

The hole-only device is ITO/PEDOT:PSS/Active Layer/Au.

d

The electron-only device is ITO/ZnO/Active Layer/Ca/Al.

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µh/µe

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed Experimental section, experimental procedure, additional figures as mentioned in the main text, 1H and 13C NMR spectra.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015M1A2A2056214) and the KRICT core project (KK1702-A00) funded by the National Research Council of Science & Technology.

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Effects on Photovoltaic Performance of Dialkyloxy ... - ACS Publications

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