Synthesis and photovoltaic properties of three different types of terpolymers

Abstract

Three types of photoactive terpolymers (Random, Regular and Block) were synthesized by incorporating two electron-deficient moieties and one electron-rich moiety based on two parent donor polymers, poly[(2,5-bis(2-decyltetradecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT_DT) and poly[(2,5-bis(2-decyltetradecyloxy)phenylene)-alt-(5,6-dicyano-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2CNBT). All three terpolymers showed broad light absorption in the range of 300–850 nm covering the absorption of two parent polymers. Their optical, electrochemical, morphological and photovoltaic properties were compared and investigated in detail. Power conversion efficiencies (PCEs) of 5.63, 4.45 and 3.13% were obtained for PPDT2FBT_DT-random-PPDT2CNBT, PPDT2FBT_DT-regular-PPDT2CNBT and PPDT2FBT_DT-block-PPDT2CNBT based photovoltaic devices, respectively. Through intensive investigation on charge carrier mobility, photocurrent and light intensity dependence of J_SC, the random structure was proved to be the most effective molecular design in this series of terpolymers, showing weaker charge recombination and enhanced charge transport/extraction with well-distributed blend morphology. The fine modulation of different arrangements and compositions of the three monomers in a main chain is crucial for optimizing the terpolymeric structures as efficient photoactive materials.

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Introduction

Recently, polymer solar cells (PSCs) have been attracting growing attention, demonstrating successfully their great potential as portable and flexible solar cells with several advantages such as room temperature solution processability on a plastic substrate and chemical versatility to fine-tune the electrical and optoelectronic properties.^1–5 Typical bulk heterojunction (BHJ) PSCs consist of conjugated polymers as electron donor materials which function as major light harvesters while the fullerene derivatives (e.g., [6,6]-phenyl-C₇₀ butyric acid methyl ester, PC₇₀BM) act as electron acceptors.^6–9 Molecular design via donor–acceptor (D–A) type alternating copolymers using intramolecular charge transfer (ICT) interactions is a straightforward and effective design strategy to tune the band gap of the resulting photovoltaic materials. Most of the highly efficient PSC devices have been achieved using D–A copolymer donor materials in conjunction with PC₇₀BM derivatives.^10–23 Unfortunately, most of the reported D–A type copolymers absorb a limited range of light and cover only a small portion of the solar spectrum, limiting photon harvesting. To resolve this issue, ternary blend devices where multiple donors (or acceptors) are used to harvest a broad range of UV-visible-near infrared light have been applied.^24–31 A few successful examples have been reported to realize ternary systems. You et al. employed two donors in the ternary blend to form a polymer alloy and obtained a PCE of 10.5%.³² Recently, our group also reported ~10% PCE by incorporating a ternary component of poly[(2,5-bis(2-decyltetradecyloxy)phenylene)-alt-(5,6-dicyano-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2CNBT) with complementary absorption and energy levels and structural similarity into a binary blend of poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT):PC₇₀BM.³³ However, the control of blend morphology of ternary systems is very difficult where the addition of a third component often disrupts optimized BHJ morphologies of the parent binary system and is likely to introduce charge recombination sites. Therefore, the increase in the short-circuit current density (J_SC) and the fill factor (FF) is often far less than expected.

Recently, terpolymers have emerged as an alternative molecular design strategy for photoactive polymers which comprise three components with two different donors and one acceptor (or one donor and two different acceptors) on their conjugated polymer backbones.^34–49 Therefore, terpolymers allow broad light absorption via two different donor–acceptor ICT interactions with a distinct electronic structure. Several previous studies reported that random and regular terpolymers perform differently in BHJ PSC devices by forming different BHJ morphologies.^35,44 Lee and coworkers synthesized a D₁–A–D₂–A terpolymer (PDTSTTBDT) incorporating dithieno[3,2-b:2′,3′-d]silole (DTS, D₁), benzo[1,2-b:4,5-b]dithiophene (BDT, D₂) and thieno[3,4-b]thiophene (TT, A) in a perfectly controlled manner and reported a remarkable enhancement of PCE in the regio-regular PDTSTTBDT-based PSCs due to the improved light absorption, effective polymer ordering, and high charge carrier mobility.³⁵ Janssen et al. demonstrated successfully the potential of a random terpolymer of two electron deficient moieties and one electron rich unit (thienyl-substituted BDT), showing a very impressive PCE of ∼8%, higher than that of its binary parent polymers.³⁶ Luo et al. also prepared a random terpolymer based on phenothiazine (PZT) and BDT cores in conjugation with an electron withdrawing benzo[c][1,2,5]thiadiazole (BT) unit and showed that the optimized terpolymer exhibits a longer photoluminescence (PL) lifetime (or exciton lifetime) and prominent charge transfer ability.³⁷ You and coworkers synthesized regularly structured alternating and random terpolymers and these two terpolymers broadened the absorption and significantly reduced the aggregation signature compared to the parent D–A polymers.⁴⁴ Although there have been several reports on terpolymeric PSCs, there have been no systematic previous approaches for the synthesis and characterization of different types of terpolymers (Random, Regular and Block) as active photoactive materials. Different arrangements of the donor–acceptor in a terpolymer backbone significantly influence their photophysical, electronic, morphological and resulting photovoltaic characteristics. A clear understanding of the molecular structure–property characteristics is necessary to further optimize the terpolymer-based PSC systems.

In this contribution, we report the design and synthesis of three types (Random, Regular and Block) of photovoltaic terpolymers based on the same constituting monomeric units of the two parent donor polymers, poly[(2,5-bis(2-decyltetradecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT_DT) and PPDT2CNBT. The three terpolymers showed broader light absorption than the two corresponding parent copolymers and their optical, electrochemical, morphological and photovoltaic properties were compared and investigated in detail.

Experimental section

Instruments and characterization

¹H and ¹³C NMR spectra were recorded on a JEOL (JNM-AL300) FT NMR system operating at 300 MHz and 75 MHz, respectively. UV-Vis spectra were obtained using a JASCO V-630 spectrophotometer. The number- and weight-average molecular weights of polymers were determined by gel permeation chromatography (GPC) with o-dichlorobenzene as an eluent on an Agilent GPC 1200 series, relative to polystyrene standard. Cyclic voltammetry (CV) experiments were performed using a Versa STAT 3 analyzer. All CV measurements were carried out in 0.1 M tetrabutylammoniumtetrafluoroborate (Bu₄NBF₄) in acetonitrile with a conventional three-electrode configuration employing a platinum wire as a counter electrode, a platinum electrode coated with a thin polymer film as a working electrode, and a Ag/Ag⁺ electrode as a reference electrode (scan rate: 50 mV s⁻¹). A dimension microscope (Veeco, USA) running with a Nanoscope V controller was used to obtain atomic force microscopy (AFM) surface images of polymer thin films. AFM images were measured in a high resolution tapping mode under ambient conditions. Silicon cantilevers (Bruker) with a resonant frequency of 300 kHz were used with a rotated tip to provide more symmetric representation of features over 200 nm. Grazing incidence wide angle X-ray scattering (GIWAXS) measurements were conducted at the PLS-II 9A U-SAXS beamline of Pohang Accelerator Laboratory, Pohang, Korea. Samples for GIWAXS measurements were prepared by spin-coating pristine polymers and polymer:PC₇₀BM blend solutions on top of a silicon wafer. The interchain orientation was compared by measuring the (010) scattering intensity ratio (I_z/I_xy) in the z and xy directions for pristine and blend films.

PSC fabrication and characterization

ITO coated glass substrates were washed with detergent and ultrasonicated in distilled water, acetone, and isopropyl alcohol for 10 min and dried overnight in an oven at 100 °C. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) was deposited by spin-coating at 4000 rpm for 40 s on top of the ITO substrate and thermally annealed for 10 min at 140 °C in air. The samples were then transferred to a N₂ filled glove box. The mixed solutions of polymer (1–1.5 wt%):PC₇₀BM (1 : 1.5 (w/w) in chlorobenzene (CB)) were spin-coated on top of the PEDOT:PSS film. Subsequently, Al (∼100 nm) was thermally evaporated under high vacuum (<10⁻⁶ Torr), yielding devices with an active area of 13 mm². Current density–voltage (J–V) characteristics of PSCs were measured using a Keithley 2365A source measurement unit. PSCs were characterized under AM 1.5 G conditions with an intensity of 100 mW cm⁻² which was calibrated with a standard silicon photodiode (PV Measurements, Inc.). Incident photon to current efficiency (IPCE) spectra were measured using a QEX7 quantum efficiency measurement system (PV Measurements, Inc.) under ambient conditions. Spectral mismatch factors were found to be less than 9%. For space charge limited current (SCLC) measurements, hole-only (ITO/PEDOT:PSS/polymer:PC₇₀BM/Au) and electron-only (FTO/polymer:PC₇₀BM/Al; FTO, fluorine-doped tin oxide) devices were fabricated, respectively, under the same conditions for optimized devices. The charge mobilities were calculated by fitting the J–V characteristics of the single carrier diodes according to the Mott–Gurney relationship, J_SCLC = (9/8) × (ε_rε_oμ) × (V_a²/L³), where ε_r is the dielectric constant of the material, ε_o is the vacuum permittivity, μ is the hole or electron mobility, L is the film thickness, and V_a is the applied voltage. The film thickness was measured using a surface profiler (P6, KLA Tencor, USA).

Results and discussion

Design and synthesis of materials

Two parent polymeric structures PPDT2FBT_DT and PPDT2CNBT³³ with complementary light absorption were considered to synthesize three different types of terpolymers and investigate the structure–property relationships for terpolymer based PSCs. To attach the same side-chains on both the structures, the longer alkyl chain 2-decyltetradecyl (DT) groups were incorporated into the same backbone of PPDT2FBT (with 2-hexyldecyl side chains) which was reported previously.¹³ The final polymer structures and synthetic routes to the monomer and final polymers are described in Schemes 1 and 2. The detailed synthetic procedures can be found in the ESI.†

Scheme 1 Molecular structures of polymers.

Scheme 2 Synthetic scheme.

1-Bromo-2,5-bis(2-decyltetradecyloxy)benzene (1),⁵⁰ 4,7-bis(5-trimethylstannylthiophen-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (M1),¹³ 4,7-bis(5-trimethylstannylthiophen-2-yl)-5,6-dicyano-2,1,3-benzothiadiazole (M2),³³ 1,4-dibromo-2,5-bis(2-decyltetradecyloxy)benzene (M3),³³ PPDT2FBT_DT and PPDT2CNBT³³ were synthesized by following the previous reports. Monomer M4 was synthesized by the Stille cross-coupling reaction between compound 1 and M1 and successive bromination with N-bromosuccinimide in 65% yield. The random terpolymer (PPDT2FBT_DT-random-PPDT2CNBT, Random) was prepared via the Stille cross-coupling of M1, M2 and M3 (mole ratio of 0.5 : 0.5 : 1) in chlorobenzene with tris(dibenzylideneacetone)dipalladium(0) (3 mol%) and tri(o-tolyl)phosphine (8 mol%) as catalysts using a microwave reactor. A Regular terpolymer (PPDT2FBT_DT-regular-PPDT2CNBT, Regular) was synthesized by the Stille cross-coupling reaction of M2 with M4 (molar ratio of 1 : 1). A Block copolymer (or terpolymer) (PPDT2FBT_DT-block-PPDT2CNBT, Block) was synthesized using an end-capping method where a two-step synthesis approach involving the Stille cross-coupling polymerization of M1 and M3 for the preparation of Br-terminated PPDT2FBT_DT by end capping with excessive 2-bromothiophene followed by the Stille cross-coupling polycondensation of M2 and M3 for preparation of the PPDT2CNBT block was used.⁵¹ The PPDT2FBT_DT-block-PPDT2CNBT formation was roughly estimated by measuring the increase in the molecular weight before and after incorporating the PPDT2CNBT block by GPC analysis (M_n: 20 → 28 KDa). However, there must be a chance to form the PPDT2CNBT homopolymer as well as the block copolymer. We could not exclude the possibility of formation of PPDT2CNBT during the synthesis of PPDT2FBT_DT-block-PPDT2CNBT at the present stage. The polymerization yield was 65–73% and the number-average molecular weight was measured to be 28–34 KDa by GPC (Table 1).

Table 1 Physical properties of polymers

Polymer	M n (Mw)^a [kg mol^−1]	λ abs (in solution) (nm)	λ abs (in film) (nm)	λ onset (nm)	E ^optg (eV)	HOMO^b (eV)	LUMO^c (eV)
a Number-average molecular weight (Mn) and weight-average molecular weight (Mw) determined by GPC with o-dichlorobenzene as the eluent at 80 °C. b HOMO level was estimated from the tangential onset of oxidation (Eox) by cyclic voltammetry. HOMO (eV) = −(Eox − E1/2(Fc/Fc^+) + 4.8). c LUMO level was estimated from the HOMO value and the corresponding optical band gap.
PPDT2FBTDT	31 (62)	638	649	714	1.74	−5.37	−3.63
Random	30 (77)	643	653	843	1.47	−5.56	−4.09
Regular	34 (79)	664	714	836	1.48	−5.60	−4.12
Block	28 (73)	630	643	860	1.44	−5.43	−3.99
PPDT2CNBT	28 (62)	685	737	838	1.48	−5.67	−4.19

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Optical and electrochemical properties

UV-Vis absorption spectra of three terpolymers and their parent D–A copolymers (PPDT2FBT_DT and PPDT2CNBT) in chloroform and in films are shown in Fig. 1(a) and (b). Random, Regular and Block show broad absorption covering a range of 350–850 nm, where a peak at ∼400 nm corresponds to π–π* transition and the longer wavelength absorption is related to ICT interaction. The absorption of the three terpolymers covers the sum of absorption of the two parent copolymers PPDT2FBT_DT and PPDT2CNBT. A comparison of the spectra in films and in solution indicates a clear red-shift in films for all polymers, suggesting enhanced intermolecular π–π interactions.^47,49 However, the detailed peak shape and structure are different for the three terpolymers. Random and Regular show similar spectra in chloroform, but the spectra in films show a clearly different absorption maximum wavelength (λ_abs). In Block, the relatively weak absorption at ∼750 nm (originating from PPDT2CNBT absorption) is probably due to the shorter chain length of the PPDT2CNBT block in Block, which is consistent with the measured GPC data before and after incorporation of the PPDT2CNBT block.

Fig. 1 Normalized UV-Vis absorption spectra in chloroform (a) and in the film (b) and cyclic voltammograms (c) of polymers.

The highest occupied molecular orbital (HOMO) energy levels of the polymers were determined by CV measurements and the lowest unoccupied molecular orbital (LUMO) energy levels were estimated from the optical band gap and the HOMO energy level. The HOMO and LUMO energy levels of polymers are summarized in Table 1, and their cyclic voltammograms are shown in Fig. 1(c). In the anodic scan, the oxidation onsets relative to an internal standard, ferrocene (Fc⁺/Fc), were determined and the corresponding HOMO levels were determined to be −5.37, −5.56, −5.60, −5.43, and −5.67 eV for PPDT2FBT_DT, Random, Regular, Block, and PPDT2CNBT, respectively. The HOMO levels of the three terpolymers are between those of PPDT2FBT_DT and PPDT2CNBT.

Photovoltaic properties

The photovoltaic devices were fabricated using a conventional architecture of ITO/PEDOT:PSS/polymer:PC₇₀BM/Al. A series of devices were tested by changing the blend ratio, thickness of the active layer, solvent and processing additives. Finally, the optimized BHJ PSCs using PPDT2FBT_DT, Random, Regular, Block and PPDT2CNBT as a donor and PC₇₀BM as an acceptor were fabricated by spin-coating the blend solutions (polymer:PC₇₀BM = 1 : 1.5 (w/w)) in chlorobenzene (CB) solvent and 3 vol% diphenyl ether (DPE) as a processing additive. J–V characteristics and the corresponding IPCE spectra of the devices for the five polymers are shown in Fig. 2(a) and (b) and the resulting photovoltaic parameters (open-circuit voltage (V_OC), J_SC, and FF) are summarized in Table 2.

Fig. 2 (a) J–V characteristics of optimized PSCs for five polymer blends and (b) their IPCE spectra, (c) logarithmic plot of J_SCvs. light intensity and (d) photocurrent with respect to effective voltage (V_eff).

Table 2 Summary of photovoltaic device parameters for five polymers

Polymer	J SC [mA cm^−2]	Cal. JSC [mA cm^−2]	V OC [V]	FF	PCE [%]
PPDT2FBTDT	10.40	9.92	0.79	0.67	5.45
Random	12.90	12.69	0.82	0.53	5.63
Regular	9.38	9.81	0.87	0.54	4.45
Block	6.10	6.25	0.86	0.60	3.13
PPDT2CNBT	5.99	5.50	0.97	0.48	2.80

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The observed V_OCs are 0.79, 0.82, 0.87, 0.86, and 0.97 V for the binary blends of PPDT2FBT_DT, Random, Regular, Block and PPDT2CNBT with PC₇₀BM, respectively. Replacement of fluorine (F) with nitrile (CN) groups on the benzothiadiazole (BT) units hence induces the increase in V_OC for devices of Random, Regular and Block compared to the PPDT2FBT_DT device. However, the extent of enhancement is different for each polymer, derived from the specific arrangement of two different BT units (difluoro and dicyano substituted). The resulting PCE was measured to be 5.45, 5.63, 4.45, 3.13 and 2.80% for PPDT2FBT_DT, Random, Regular, Block and PPDT2CNBT devices, respectively.

The IPCE responses to the incident irradiance cover a broad range of 300–850 nm for all polymers except for PPDT2FBT_DT, which are well matched with their UV-Vis absorption spectra. All J_SCs obtained from the J–V characterization agree well with the calculated J_SCs integrated from IPCE spectra (within 9% error). Random shows the highest quantum efficiency with a maximum of 60% near 450 nm. The IPCE spectrum of Block shows a low current generation over 700 nm. The spectral characteristic suggests the poor photovoltaic performance of PPDT2CNBT compared to PPDT2FBT_DT. In addition, the smaller chain length of the PPDT2CNBT block in Block may also contribute to the smaller IPCE over 700 nm.

Charge transport and extraction analysis

Analysis of vertical charge transport in polymer:PC₇₀BM films provides a detailed understanding of photovoltaic characteristics. The electron and hole mobilities (μ_e and μ_h) were measured using the SCLC technique.⁵² Detailed electron-only and hole-only device fabrication and measurement conditions are described in the Experimental section. J–V curves of electron-only and hole-only diodes are plotted in Fig. S1 (ESI†) and the calculated μ_e and μ_h with mobility ratios (μ_e/μ_h) are summarized in Table S1 (ESI†). The measured μ_es of polymers are in the range of 8.67 × 10⁻⁴–2.37 × 10⁻³ cm² V⁻¹ s⁻¹, where Block showed relatively lower electron mobility than the other polymers. μ_h of 3.56 × 10⁻⁵ to 1.15 × 10⁻⁴ cm² V⁻¹ s⁻¹ was measured and the resulting μ_e/μ_h values are in the range of 10.4–33.7. μ_e/μ_h for PPDT2FBT_DT is 10.4 and that for PPDT2CNBT is 33.7.

To gain insight into the charge carrier transport and recombination characteristics of the PSC devices of the 5 different polymers, we also measured the light intensity dependent J_SC at various illumination light intensities in the range of 2.5–100 mW cm⁻², from which information regarding bimolecular recombination and space charge accumulation can be obtained. The J_SC curve can be fitted according to a power-law, eqn (1), where I is the light intensity and α indicates a fitting parameter or the slope of a logarithmic plot of J_SCversus light intensity.^53,54

J_SC ∝ I^α (1)

As the value α is close to unity, the bimolecular recombination becomes weak during charge sweep out. In Fig. 2(c), the PPDT2FBT_DT:PC₇₀BM blend device showed the highest value α (0.988), followed by Random (0.980) and Regular (0.981), respectively. In contrast to these polymers, Block and PPDT2CNBT devices displayed substantially lower α values of 0.937 and 0.933, respectively, indicating the poor charge transport characteristics with significant electron–hole recombination. The poor charge transport/extraction of PPDT2CNBT was previously reported due to the tilted polymer chain.³³

To characterize the charge generation and extraction properties, we studied the photocurrent (J_ph) versus effective voltage (V_eff) characteristics. The J_ph is defined as J_ph = J_L − J_D, where J_L and J_D are the current density under illumination (100 mW cm⁻²) and in the dark, respectively. V_eff is defined as V₀ − V_a, where V₀ is the built-in voltage, which corresponds to the voltage at J_ph = 0, and V_a is the applied bias. Fig. 2(d) displays a logarithmic relationship of J_ph with respect to V_eff for the binary blends of five polymers. The Random blend shows the highest J_ph whereas Block and PPDT2CNBT exhibited substantially low J_ph. With increasing V_eff (>1 V), the photocurrent becomes gradually saturated for PPDT2FBT_DT, Random and Regular binary blends, but Block and PPDT2CNBT blends show no saturation region, which shows a good agreement with the above mobility and light intensity dependent J_SC data. The saturated photocurrent density (J_sat) is approximately given by J_sat = qLG_max, where q is the elementary charge, L is the thickness of the active layer, and G_max is the maximum photo-induced carrier generation rate per unit volume. When the excitons are photo-generated, only a part can be dissociated into the free charge carriers, which can be estimated from charge dissociation probability (P_D). The P_D value is calculated as J_SC/J_sat, where J_SC = qLG_maxP_D under short circuit conditions.⁵⁵ The P_D values are roughly estimated to be 94.5, 97.7 and 94.0%, for PPDT2FBT_DT, Random and Regular, respectively. In addition, the charge extraction probability (P_E) indicates the extraction efficiency of the separated charge carriers at the electrodes. The P_E can be defined as J_MP/J_sat, where J_MP is the current density under maximum power conditions.⁵⁶ The higher P_E was calculated to be 81.5% for PPDT2FBT_DT and 74.9% for Random, compared to Regular (P_E of 63.5%).

According to the calculated P_D and P_E values, the terpolymers show relatively good charge separation/generation, but the charge extraction is probably hindered with significant recombination especially for the Block system. PPDT2FBT_DT and Random devices showed higher charge extraction than other terpolymers with suppressed recombination, which may induce a higher J_SC (>10 mA cm⁻²) and FF in the PSCs.

Morphological analysis

To explore intermolecular packing and orientation in pristine polymer and polymer:PC₇₀BM blend films, GIWAXS characteristics were investigated (Fig. 3). Detailed GIWAXS line-cuts and packing parameters are also summarized in Fig. S2 and Table S2 (ESI†). As shown in Fig. 3, five pristine polymers show multi-diffractions, indicating the bimodal molecular packing along both in-plane (q_xy) and out-of-plane (q_z) directions. Clear (010) π–π stacking peaks were also observed in the out-of-plane direction for all polymers with a d-spacing of 3.6–3.7 Å (Table S2, ESI†). The π–π stacking was found to be more ordered in the q_z direction for PPDT2FBT_DT and Random compared to Regular, Block and PPDT2CNBT where the hump-shaped (010) scatterings were observed due to randomly packed inter-chain structures. This pronounced face-on π–π stacking in PPDT2FBT_DT and Random supports efficient charge transport in the vertical direction in PSC devices. By blending with PC₇₀BM (Fig. 3(f–j)), the strong (010) scattering was also observed in the q_z direction for PPDT2FBT_DT and Random, but the (010) peak intensity was measured to decrease substantially for other terpolymers and PPDT2CNBT. In addition, a broad hump peak in all blend films was observed originating from PC₇₀BM aggregates at q = 1.2–1.4 Å⁻¹. Furthermore, the orientation ratio was also estimated by comparing the maximum intensity ratio (I_z/I_xy) of the π–π stacking (010) scatterings along the q_z and q_xy directions for both pristine and blend films. The PPDT2FBT_DT and Random pristine films showed the highest I_z/I_xy ratio (maximum value over ∼4). It is noted that both PPDT2FBT_DT and Random tend to arrange strongly in the face-on fashion relative to other polymers. The orientation ratio, I_z/I_xy, in the blend films was also calculated similarly: 2.7 for Random, 2.3 for PPDT2FBT_DT, and 1–1.5 for the other three polymers, respectively. A dominant face-on orientation is considered to be advantageous for efficient charge transport in a vertical direction in diode structures such as photovoltaics.^57–59 The relative inter-chain orientation in the blends is well consistent with the measured J_SC values: 12.90 mA cm⁻² and 10.40 mA cm⁻² for Random and PPDT2FBT_DT based PSCs, respectively.

Fig. 3 2D GIWAXS images of (a–e) pristine polymer films and (f–j) polymer:PC₇₀BM blend films. The blend films were prepared under the same conditions for the optimal PSC devices.

The surface film morphology of polymer:PC₇₀BM BHJ blends on top of a glass/ITO/PEDOT:PSS substrate was also studied by tapping mode AFM. Height and phase images with root mean square (RMS) roughness are shown for binary blends of five polymers in Fig. 4. Random exhibited a smooth and homogeneous surface with the smallest RMS roughness of 2.32 nm. While PPDT2FBT_DT and Regular blends displayed intermediate roughness (3.39–3.64 nm), Block and PPDT2CNBT possessed the roughest surface with RMS roughness of 6.11 and 4.29 nm, respectively. The surface roughness was significantly influenced by the different manners of arrangements of the three monomers in the terpolymeric structures. In Fig. 4(f–j), phase images also revealed a similar trend. The detailed inter-chain ordering, surface morphology and the charge transport/recombination characteristics show good agreement with the higher photovoltaic properties (i.e., J_SC and FF) of Random and PPDT2FBT_DT polymers.

Fig. 4 AFM (a–e) height and (f–j) their corresponding phase images of polymer:PC₇₀BM blend films. Each film was prepared on a glass/ITO/PEDOT:PSS substrate. The size of all images is 3 μm × 3 μm.

Conclusions

In summary, we synthesized a series of terpolymers (Random, Regular and Block) based on dithienylphenylene, difluorobenzothiadiazole and dicyanobenzothiadiazole moieties, and studied their optical, electrochemical, morphological, electrical and photovoltaic properties. The terpolymers show a broad light absorption covering all absorption ranges of parent two copolymers (PPDT2FBT_DT and PPDT2CNBT). The fine structures in the absorption, film morphology, electronic structure and the resulting electrical properties are significantly dependent on different manners of arrangements of three moieties in a polymeric backbone. A combination of SCLC modeling, photocurrent and light intensity dependence of J_SC data made possible the quantitative analysis of the difference in charge generation, transport and extraction in different terpolymer blends. Through thin film characterization using GIWAXS and AFM, the morphological properties of each polymer were found to be closely correlated to photovoltaic properties. Through these intensive investigations, the random terpolymer (Random) was proved to be the most effective molecular design in this series of terpolymers to show a weaker charge recombination and enhanced charge transport/extraction (compared to other terpolymers), showing well-distributed blend morphology with the highest PCE of 5.63%. Further optimization of terpolymer structures (by changing the composition, etc.) may reveal the great potential of terpolymers as an efficient light harvesting material compared to D–A copolymers.

Acknowledgements

This work was supported by the National Research Foundation (NRF) of Korea (2015R1A2A1A15055605, 2015M1A2A2057506, 2015R1D1A1A09056905, and 20100020209).

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Footnotes

† Electronic supplementary information (ESI) available: Synthesis and characterization of intermediates, SCLC measurements, GIWAXS line-cut profiles and packing parameters. See DOI: 10.1039/c6qm00267f

‡ These authors contributed equally.

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