1. Introduction

Polythiophene (PT) and its derivatives have been extensively utilized for the fabrication of polymer solar cells (PSCs) with moderate power conversion efficiency (PCE) in the past decade. The most famous PT derivative, namely poly(3-hexylthiophene-2, 5-diyl) (P3HT), was firstly synthesized by McCullough et al. with high regioregularity > 95% by metathesis polymerization ^[1,2]. P3HT has high tendency to form close packing of main chains due to strong π–π interaction, which favors transport and migration of carriers. By applying thermal annealing process, the PSC using the blend of P3HT and [6,6] phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as the active layer reached a PCE value of 4.4% ^[3]. Apart from P3HT, miscellaneous main-chain and side-chain types of PTs have widely been developed and reported in the literature ^{[4,5,6,7,8,9]}. A novel donor–acceptor PT bearing the C₆₀ fullerene pendant has also been synthesized as the photoactive layer ^[10,11,12]. Most of those PTs belong to alternating copolymers, requiring two monomers tethering with different functional groups to react for copolymerization, such as dibromothiophenes with diboronic esters via the Suzuki-coupling reaction, or dibromo-monomers with bis(trimethyltin)-monomers via the Stille-coupling reaction. These monomers require well-designed transformation of functional groups and careful handling of stoichiometric balance to achieve high molecular weights and good photovoltaic performance of final polymers.

In addition to linear architecture, bridging PTs have also been synthesized by introducing bridging moieties between polymer main chains to study their thermal and optical properties as well as device performance. A terthiophene-bridged P3HT derivative, namely PT-VTThV, was proposed by Li et al. by incorporating divinyl-terthiophene moiety between polymer main chains ^[13]. The conjugated bridging structure provides an alternative pathway for charge carriers to transport between polymer chains, and the enhancement in hole mobility of the polymer was verified. A PCE value of 1.72% for the solar device using PT-VTThV:PC₆₁BM as the active layer was obtained. Another P3HT derivative B-P3HT containing bridging 3, 3'-dithiophene moiety was reported by Tu et al. ^[14]. They found that the hole mobility of B-P3HT was decreased compared with linear P3HT, possibly due to chain distortion brought by the small and rigid 3, 3'-dithiophene moiety. The solar devices using B-P3HT:PC₆₁BM blends as the active layer showed PCE values of 0.13–2%. Two other hyperbranched conjugated polymers, namely p-3T-TCM and p-4T-TCM, were synthesized by Mangold et al. ^[15], using 2, 3-dithienylthiophene and 2, 3, 5-trithienylthiophene as bridging cores, respectively. The optimized PSC using p-3T-TCM:PC₆₁BM blend as the active layer revealed an open-circuit voltage (V_OC) of 714 mV which was significantly higher than that of P3HT:PC₆₁BM blend. The obtained efficiency of 0.58–0.61% and extra high V_OC of hyperbranched polymers show potential application in PSCs. In 2014, our group reported novel hyperbranched PT derivatives containing chlorinated perylene bisimide (PBI) or soft alkyl spacer as bridging moieties ^[16]. The synthesized polymers showed increased molecular weights and improved thermal stabilities compared with normal P3HT. Photovoltaic devices based on those hyperbranched polymers blended with PC₆₁BM showed PCE values of 0.45–0.84% and high V_OC values up to 0.70–0.72 V. To be honest, the device performance of those hyperbranched PT derivatives is still low and far beyond practical application, which requires further improvement through chemical modification.

Here we propose new linear and hyperbranched PT derivatives containing diketopyrrolopyrrole (DPP) as linking moieties. DPP possesses highly electron-withdrawing nature and are commonly used for the synthesis of alternating donor–acceptor polymers with low band-gaps ^[17,18,19]. To synthesize alternating PTs, dibromo-monomers accompanying with diboronic ester derivatives or bis(trimethyltin)-monomers must be prepared before carrying out the polymerization. In this study, new P3HT derivative were obtained by simply introducing DPP-containing dibromothiophene or two-headed monomers during polymerization of P3HT via the Universal Grignard metathesis polymerization. Normal P3HT was also synthesized according to the same polymerization condition to examine the effect of introducing DPP linking moieties. The thermal, optical, and electrochemical behaviors of the synthesized polymers were investigated via different characterizing techniques. Finally, inverted PSCs based on polymer:PC₆₁BM blends were fabricated and characterized to evaluate the performance of those polymers.

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2. Materials and Method

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2.1. Characterization Methods

¹H-and ¹³C-NMR spectra were performed on a Bruker Avance 600 MHz NMR spectrometer. Mass spectra were recorded on a Micromass TRIO-2000 GC-MS instrument, using electron impact (EI) or fast atom bombardment (FAB) as ionization source. Gel permeation chromatography (GPC) assembled from a Viscotek VE3850 RI detector and three columns in series were used to measure molecular weights and polydispersity index (PDI) values of polymers relative to polystyrene standards at 32 ℃. Tetrahydrofuran (THF) was used as the eluent to carry out GPC experiments. Fourier-transform infrared (FTIR) spectra were measured on a Nicolet iS-10 spectrometer. Thermogravimetric analysis (TGA) was undertaken on a Seiko TG/DTA 7200 instrument at a heating rate of 20 ℃/min. Differential scanning calorimetry (DSC) was performed on a Seiko DSC 6200 instrument at a heating rate of 10 ℃/min. UV-vis absorption and photoluminescence (PL) spectra were obtained with a Princeton Instruments Acton 2150 spectrophotometer equipped with an 150 W Xenon lamp (USHIO UXL-150S). Cyclic voltammetric measurements of materials were performed on an AUTOLAB PGSTAT30 electrochemical instrument. Indium-tin oxide (ITO) electrodes were used as both the working and counter electrodes, and silver/silver ions (Ag in 0.1 M AgNO₃ solution, from Bioanalytical Systems, Inc.) was used as the reference electrode. The polymers were deposited on ITO electrodes and placed in acetonitrile solution containing 0.1 M tetrabutylammonium tetrafluoroborate as the supporting electrolyte with a scan rate of 50 mV/s. Ferrocene was used as an internal standard, and the potential values were obtained and converted to vs Fc/Fc⁺ (ferrocene reference electrode). Atomic force microscopy (AFM) experiments were performed on a Bruker Innova AFM instrument to investigate surface morphologies of polymer:PC₆₁BM blend films. The current density-voltage (J-V) characteristics of photovoltaic devices were taken using a Keithley 2400 source measurement unit under an AM 1.5G exposure from a Yamashito Denso YSS-150AA solar simulator in ambient environment. The external quantum efficiency (EQE) measurements were performed on an assembled apparatus in the laboratory, comprising a solar simulator (Oriel 9600,150W), the monochromator (Cornerstone^TM 130) and the optical power meter (818-UV/DB).

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2.2. Synthesis of Monomers

The synthetic routes to monomers M1 and M2 are shown in Scheme 1. The preparation of intermediates (1) and (2) as well as monomers M1 and 2, 5-dibromo-3-hexylthiophene (M3) was referred to the previous literature ^[20,21]. The detailed synthetic procedure of M2 is described as follows.

1-(6'-Bromohexyloxy)-4-methoxybenzene (3). A mixture of 4-methoxyphenol (10.0 g, 85.58 mmol), 1, 6-dibromohexane (60.0 g, 245.9 mmol), potassium hydroxide (6.0 g, 106.95 mmol) and dimethyl sulfoxide (DMSO) (100 mL) was stirred at room temperature for 6 hr. The mixture was then extracted with dichloromethane (DCM) and water, and the organic phase was dried with anhydrous magnesium sulfate (MgSO₄). The crude product was concentrated in vacuo and purified by column chromatography (silica gel, using DCM:hexane = 2/1 in volume ratio as the eluent) to give a white solid (15.0 g, 65%). ¹H-NMR (CDCl₃, ppm): 1.46–1.51 (m, 4H, –OCH₂CH₂(CH₂)₂–CH₂CH₂Br), 1.76–1.81 (m, 2H, –CH₂CH₂Br), 1.87–1.91 (m, 2H, –OCH₂CH₂–), 3.40–3.44 (t, J = 6.9 Hz, 2H, –CH₂Br), 3.77 (s, 3H, –OCH₃), 3.90–3.93 (t, J = 6.3 Hz, 2H, –OCH₂–), 6.83 (s, 4H, aromatic protons). ¹³C-NMR (CDCl₃, ppm): 25.32, 27.94, 29.21, 32.70, 33.78, 55.75, 68.41,114.65,115.46,153.22,153.75. MASS (EI): m/z 287.

1-(6'-Bromohexyloxy)-2, 5-dibromo-4-methoxybenzene (4). A mixture of compound (3) (5.0 g, 17.41 mmol), a small amount of iron powder and chloroform (100 mL) was stirred at 0 ℃ in an ice-water bath. Bromine (8.0 g, 50.0 mmol) was then slowly added to the mixture and stirred at room temperature for 12 hr. The reaction mixture was then extracted with ethyl acetate (EA) and sodium thiosulfate aqueous solution for three times, and the organic phase was dried with anhydrous MgSO₄. The crude product was concentrated in vacuo and purified by column chromatography (silica gel, using EA/hexane = 1/14 in volume ratio as the eluent) to yield a white solid (5.0 g, 65%). ¹H-NMR (CDCl₃, ppm): 1.52–1.54 (m, 4H, –OCH₂–CH₂(CH₂)₂CH₂CH₂Br), 1.81–1.83 (m, 2H, –CH₂CH₂Br), 1.88–1.91 (m, 2H, –OCH₂CH₂–), 3.41–3.44 (t, J = 6.9 Hz, 2H, –CH₂Br), 3.84 (s, 3H, –OCH₃), 3.95–3.97 (t, J = 6.3 Hz, 2H, –OCH₂–), 7.09 (s, 2H, aromatic protons). ¹³C-NMR (CDCl₃, ppm): 25.19, 27.81, 28.92, 32.64, 33.74, 56.99, 70.06,110.41,111.27,117.01,118.65,150.04,150.55. MASS (EI): m/z 445.

2, 5-Bis[6-(2, 5-dibromo-4-methoxyphenyl)hexyloxy]-3, 6-dithiophen-2-yl-pyrrolo[3, 4-c]pyrrole-1, 4-dione (M2). A mixture of compounds (1) (1.0 g, 3.33 mmol), (4) (3.3 g, 7.47 mmol), K₂CO₃ (3.83 g, 27.71 mmol), tetrabutylammonium bromide (TBABr) (0.13 g, 0.4 mmol), 18-crown-6 (0.1 g, 0.38 mmol) and N, N-dimethylformamide (50 mL) was refluxed at 145 ℃ for 24 hr under nitrogen atmosphere. After cooling to room temperature, the mixture was poured into 400 mL of water and filtered. The filter solid was washed with water for several times and then purified by column chromatography (silica gel, using DCM as the eluent) to give a dark red solid (1.0 g, 29%). ¹H-NMR (CDCl₃, ppm): 1.5 (m, 4H, –NCH₂CH₂CH₂CH₂CH₂CH₂O–), 1.56 (m, 4H, –NCH₂CH₂CH₂CH₂–CH₂CH₂O–), 1.7–1.9 (m, 8H, –NCH₂CH₂CH₂CH₂CH₂CH₂O–), 3.83 (s, 6H, –OCH₃), 3.94 (m, 4H, –NCH₂CH₂CH₂CH₂CH₂CH₂O–), 4.09 (m, 4H, –NCH₂CH₂CH₂CH₂–CH₂CH₂O–), 7.07 (s, 4H, aromatic protons), 7.28 (s, 2H, aromatic protons), 7.63 (s, 2H, aromatic protons), 8.91 (s, 2H, aromatic protons). ¹³C-NMR (CDCl₃, ppm): 25.63, 26.52, 28.96, 29.85, 42.04, 56.98, 70.11,107.68,110.37,111.28,116.99,118.65,128.64,129.68,130.72,135.30,139.99,150.06,150.51,161.35. MASS (FAB): m/z 1029.

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2.3. Synthesis of Polymers

The syntheses of linear polymer P1 and hyperbranched polymer P2 are depicted in Scheme 2, which were polymerized via the Universal Grignard metathesis polymerization ^[22]. The molar ratio of isopropylmagnesium chloride–lithium chloride (i-PrMgCl·LiCl) complex to monomer was controlled to 1.3:1. The molar ratio of the catalyst 1, 2-bis(diphenyl-phosphinoethane)nickel(Ⅱ) chloride (Ni(dppp)Cl₂) to monomer was controlled to 0.005:1. The feed ratio of monomers M1 (or M2) to M3 was selected to 0.05:1. The detailed synthetic procedures of the polymer P1 were given as an example.

P1: to a mixture of M1 (0.104 g, 0.15 mmol) and M3 (1.0 g, 3.07 mmol) in anhydrous THF (40 mL) was added 1 M i-PrMgCl·LiCl (4 mL, 4.0 mmol) using syringe under nitrogen atmosphere. The reaction mixture was heated to 80 ℃ and stirred for 2 hr. A dispersion of Ni(dppp)Cl₂ (9.0 mg, 1.66 × 10⁻² mmol) in anhydrous THF (20 mL) was injected into the reaction mixture and stirred at 80 ℃ for 3 hr. The resulting solution was then poured into methanol (250 mL) and stirred for 1 hr. The crude product was collected by filtration and re-precipitated in hexane for several times to give a purple-black solid (0.33 g, 63%). ¹H-NMR (CDCl₃, ppm): 0.9–2.0 (m, alkyl–H), 2.56–2.8 (t, thiophene–CH₂–), 4.16 (t, imide–CH₂–), 6.9–7.1 (br, thiophene–H). FTIR (cm⁻¹): 3055,2955,2922,2853,1635,1509,1455,819.

P2: by following the synthetic procedure of P1 and using M2 (0.158 g, 0.152 mmol), M3 (1.0 g, 3.07 mmol), i-PrMgCl·LiCl (4 mL, 4.0 mmol) and Ni(dppp)Cl₂ (9.0 mg, 1.66 × 10⁻² mmol) as starting materials, P2 was obtained as a purple-black solid (0.16 g, 31%). ¹H-NMR (CDCl₃, ppm): 0.9–2.0 (m, alkyl–H), 2.58–2.8 (t, thiophene–CH₂–), 3.78 (s, –O–CH₃), 3.82–3.84 (m, –O–CH₂– and imide–CH₂–), 6.9–7.1 (br, aromatic–H). FTIR (cm⁻¹): 3053,2954,2922,2851,1638,1508,1451,820.

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2.4. Device Fabrication

P3HT, P1, and P2 were blended with PC₆₁BM (1:1 in weight ratio) individually in o-dichlorobenzene (o-DCB) at a total concentration of 40 mg/mL. 30 μL of 1, 8-diiodooctane (DIO) was added in the above solutions to improve morphology of resulting blend films. The solutions were treated with ultrasonication at 70 ℃ for 30 min for better dissolution. The solutions were then filtered with 0.22 μm PTFE filters before use. Inverted photovoltaic devices with the configuration of ITO/ZnO nanorod arrays/ionic PF/polymer:PC₆₁BM/PEDOT:PSS/WO₃/Au were fabricated. The ZnO nanorod arrays were prepared according to the previous report and used as the electron transporting layer (ETL) ^[23]. An ultra-thin ionic PF layer was deposited on top of ZnO nanorod arrays from its dilute solution (0.1 wt% in acetonitrile) before depositing active layers. The ionic PF was prepared according to the previous report ^[24]. The active layers of polymer:PC₆₁BM (120 nm) were prepared by spin-coating from their solutions at 1000 rpm for 30 sec, followed by solvent annealing in a covered glass petri dish at 60 ℃ for 5 min and dried in a vacuum oven at 90 ℃ for 30 min. Poly(3, 4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (CleviosTM P VP AI 4083) and tungsten trioxide (WO₃) layers were sequentially deposited on top of active layers. The WO₃ layer was obtained from spin-coating tungsten(Ⅴ) ethoxide solution (0.1 wt% in anhydrous ethanol) into thin film, followed by hydrolysis in air for 15 min. Finally, 100 nm of gold electrodes were deposited as the anode by thermal evaporation at a pressure of 8 × 10⁻⁶ torr. The area of active layers is 4 mm².

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3. Results and Discussion

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3.1. Synthesis and Characterization of Polymers

In this research, linear PT derivative P1 and hyperbranched P2 by were synthesized by incorporating DPP-containing monomers M1 and M2, respectively, during polymerization of P3HT via the Universal Grignard metathesis. Normal P3HT was also synthesized from its monomer M3 under the same reaction condition for comparison. For polymer P1, DPP moieties are randomly incorporated into P3HT main chains to improve electron transport and dissociation of carriers. For polymer P2, both heads in M2 serve as polymerizable monomers to extend polymer chains, forming hyperbranched architectures of final polymers. The feed ratio of monomers M1 (or M2) to M3 is controlled to be 0.05:1 to ensure successful incorporation of DPP moieties and sufficient solubility of final polymers in common organic solvents, such as THF, chlorobenzene, and o-DCB. The number-average molecular weight (M_n), weight-average molecular weight (M_w), and PDI values of all synthesized polymers were determined by GPC. The M_n and M_w values of P1 are 2.54 × 10⁴ and 6.23 × 10⁴ g/mol, respectively, with a PDI value of 2.45. Moreover, the M_n and M_w values of P2 are 2.14 × 10⁴ and 3.92 × 10⁴ g/mol, respectively, with a smaller PDI value of 1.83. Meanwhile, the M_n, M_w and PDI values of synthesized P3HT were also measured to be 2.68 × 10⁴, 4.87 × 10⁴ g/mol and 1.81, respectively. It is seen that the molecular weight and PDI value of P1 are somewhat larger than those of P3HT and P2. Due to the fact that linear polystyrenes are used as the standards to construct the calibration curve, we consider that the measured molecular weight of branched P2 could be overestimated. Besides, it is reported that hyperbranched polymers have more densely packed structures, resulting in smaller hydrodynamic volume compared to those linear ones ^[25]. Hence, we speculate that the difference in molecular weights of polymers is affected by the polymer architecture and experimental method that we have for GPC measurements. The spectroscopic characterizations including NMR and FT-IR techniques were carefully carried out for all synthesized polymers. Besides, these polymers possess sufficient solubility in o-ODCB and can be cast into free-standing thin films for further investigations.

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3.2. Thermal Properties

The TGA thermograms of the synthesized polymers P1, P2 and P3HT are depicted in Figure 1(a). The main decomposition temperatures (T_d) due to thermal degradation of polymer main chains are found around 470 ℃, which is consistent with the previous literature ^[16,26]. A slight first-stage weight loss around 350 ℃, possibly due to the break of alkyl side chains or spacers, is also observed and in good accordance with our previous work ^[16].Besides, we notice that this first-stage weight loss can be suppressed by introducing DPP moiety on P3HT, indicating that the incorporation of rigid DPP moieties helps to improve thermal stability of the synthesized PT derivatives.

The glass transition temperature (T_g) and melting temperature (T_m) of the synthesized polymers were determined by DSC. Several research groups have studied the thermal transition behaviors of P3HT; different T_g values of P3HT ranging from −14 to 110 ℃ ^{[27,28,29,30]}, and T_m values in the range of 200–230 ℃ were reported in the literature ^[31,32]. This is understandable since the measurement of T_g and T_m is strongly dependent on the qualities and sources of polymers as well as experimental parameters. In this research, polymerization conditions for all polymers were carefully controlled to examine the effect of DPP linking moieties on thermal transition behaviors of final polymers. The DSC thermograms of the polymers P1, P2, and P3HT are shown in Figure 1(b). The T_g of the synthesized P1, P2, and P3HT in this study are observed at 72, 67, and 55 ℃, respectively. It is seen that DPP-containing derivatives show lower T_g values compared with P3HT. This can be attributed to less close packing of PT main chains by inserting rigid and bulky DPP moieties. P2 possesses the lowest T_g value due to its smallest molecular weights among three polymers. Furthermore, the T_m of the synthesized P1, P2, and P3HT are found at 231,230, and 215 ℃, respectively. Here again, P2 shows the lowest T_m value because of its less packed polymer chains and smaller molecular weights. As for the polymer P1, although its T_m value is close to that of P3HT, the broader melting peak indicates more disordered main chains of P1 by introducing DPP moieties compared with the regular homopolymer P3HT.

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3.3. Optical Properties

The UV-vis absorption spectra of all polymers in solution or thin film state are shown in Figure 2. o-DCB is selected as solvent for optical measurement since it is also used for the preparation of blend solutions to fabricate photovoltaic devices in this study. In Figure 2(a), the absorption band centered at 460 nm are observed for P1 and P3HT in o-DCB, corresponding to π–π* transition along conjugated main chains. Besides, P2 reveals a slightly blue-shifted absorption maximum at 454 nm. This can be explained by the bridging architecture of P2 that forms twisted polymer chains to reduce the effective conjugation length. Figure 2(b) shows the absorption bands of all three polymers ranging from 400 to 650 nm in thin film state, revealing three vibronic absorption bands located at 527,552 and 602 nm. The wavelengths at 527 and 552 nm come from π–π* transition along shorter and longer conjugated polymer chains, respectively ^[32].The shoulder band at 602 nm is related to aggregates of polymer chains in thin film state ^[33]. It is seen that the absorption behaviors of the synthesized polymers in this study are similar to the regioregular P3HT reported in the literatures ^[21,27,33]. Moreover, we notice some differences in absorption between hyperbranched P2 and linear polymers P3HT and P1. For P3HT and P1, both spectral shapes are quite similar and the intensity of longer conjugation at 552 nm is stronger than the shorter one at 527 nm, as shown in Figure 2(b). Apparently, the two polymers P3HT and P1 own close conjugation length in thin film state. On the contrary, P2 shows stronger absorption at shorter wavelength of 527 nm, and its absorption shoulder at 602 nm is weaker than other two polymers. This could be due to the insertion of bridging DPP moieties that twist polymer chains and reduce effective conjugation length. The weaker absorption shoulder band implies hindered interaction and less packing between polymer chains, which may affect the charge transport and conversion efficiency of solar devices. The optical bandgaps (E_g) of polymers are estimated to be 1.89–1.91 eV from the absorption edge of their UV-vis absorption spectra, as shown in Figure 2(b).

The original PL emission spectra of polymers were depicted in Figure 3, which were obtained by excitation of samples at 460 nm in solution and 552 nm in solid state. The main emission band of all polymers is observed at 588 nm in o-DCB that is not affected by introducing DPP moieties. The vibronic shoulder band arised from interchain interaction can also be found around 635 nm for all polymers in Figure 3(a). Turning to PL emission spectra in Figure 3(b), two clear emission maxima around 650 and 700 nm are observed for all polymers in thin film state, which are attributed to intra-and inter-chain emissive behaviors, respectively ^[33,34]. Besides, two DPP-containing polymers P1 and P2 show distinct PL decay both in solution and thin film states compared to normal P3HT. This phenomenon implies a more effective transport process for carriers instead of recombination process to emit light, which has been reported in the previous reports ^[9,16,35]. It is noted that all samples were prepared according to the same standard, i.e., the same concentration in o-DCB and thickness in film.

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3.4. Electrochemical Properties

Figure 4 shows the cyclic voltammograms of the synthesized polymers in the oxidation process. The oxidation onset (E_OX) of P3HT is found at 0.81 V, corresponding to its highest-occupied molecular orbital (HOMO) of −5.21 eV calculated from the semi-empirical equations ^[16]. The HOMO of our synthesized P3HT is close to several previous reports, including values of −5.2 ^[4], −5.1 ^{[16,36,37,38]}, and −4.9 eV ^[39]. Here again, the determination of the HOMO of a certain P3HT can be varied with different material sources and experimental parameters. In our CV experiments, we carefully controlled all parameters in the same condition, such as the supporting electrolyte, scan rate and working/counter electrodes, to verify the influence of DPP moieties on polymers. The DPP-containing polymers P1 and P2 show higher E_OX at 0.89 and 0.84 V, indicating their HOMO levels of −5.29 and −5.24 eV, respectively. The lower HOMO levels of DPP-containing polymers compared with P3HT is resulted from the incorporation of electron-withdrawing DPP groups. The electrochemical properties of P1, P2 and P3HT are summarized in Table 1. It is known that lower HOMO levels of polymers bring benefits of increasing V_OC values for solar devices, since V_OC can be determined from the energy difference between LUMO of electron acceptor PC₆₁BM (LUMO = −4.3 eV) and HOMO of electron-donor polymers ^[35,40,41]. Combining the results of PL decay and lower HOMO levels, the synthesized DPP-containing P1 and P2 in this study may show comparable or even higher photovoltaic performance compared with normal P3HT.

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3.5. Photovoltaic Properties

Inverted solar cells with the configuration of ITO/ZnO nanorod arrays/ionic PF/polymer:PC₆₁BM/PEDOT:PSS/WO₃/Au were fabricated and evaluated. The synthesized polymers P1, P2 and P3HT were used as the electron donor, while commercial PC₆₁BM was used as the electron acceptor to form the active layer. ZnO nanorod arrays were adopted as the ETL for the fabrication of inverted solar cells ^[42]. To improve contact between inorganic ZnO nanorods and organic polymer:PC₆₁BM blend layer, a thin layer of ionic PF containing trimethylammonium hexafluorophosphate (–N⁺(CH₃)₃PF₆⁻) groups were deposited on top of ZnO nanorods, which was also used in our previous study ^[23]. Moreover, a thin layer of WO₃ was inserted between PEDOT:PSS and Au electrode to act as hole extraction layer to achieve higher conversion efficiency ^[43,44]. Apart from WO₃, one may choose other metal oxide buffer layers for efficient and stable polymer solar cells, such as nickel oxide, molybdenum trioxide, vanadium oxide and rhenium oxide ^[45]. The device parameters, including V_OC, short-circuit current density (J_SC), fill factor (FF) and PCE of PSCs based on P1, P2 and P3HT are summarized in Table 2, and corresponding current density-voltage characteristics as well as EQE spectra versus wavelength are shown in Figure 5. The optimized inverted solar device based on P1:PC₆₁BM blend showed V_OC, J_SC, FF, and PCE values of 0.57 V, 16.21 mA/cm², 40%, and 3.74%, respectively, which were significantly higher than those of the reference device using P3HT:PC₆₁BM in this study. In other words, the introduction of DPP moieties enhances the photovoltaic properties of P3HT. Besides, the device using hyperbranched P2:PC₆₁BM as the active layer showed V_OC, J_SC, FF, and PCE values of 0.58 V, 11.29 mA/cm², 37%, and 2.38%, respectively. It can be seen that the devices using P1 and P2 as the electron donor showed larger V_OC than the one of normal P3HT. This is reasonable since the deeper HOMO levels are found for polymers P1 and P2, as discussed in the previous part. The device results also show an increase in V_OC for hyperbranched PTs that is in accordance with previous literatures ^[15,16], although the increment is less significant. From EQE spectra of three solar devices in Figure 5(b), it is also seen that P1 possesses the highest EQE from 300 to 650 nm, indicating the best J_SC value among three polymers. Comparing the EQE spectra of P2 and P3HT, the intensity of P2 around 600 nm is lower and the curve drops sharply, making its final J_SC and PCE value smaller than normal P3HT. The comparison of photovoltaic properties of PSCs based on bridging polymers from the previous literature and our study are listed in Table 3. It is seen that the device based on our synthesized hyperbranched P2 shows much higher J_SC and PCE values compared with other works. To the best of our knowledge, the synthesized DPP-containing P2 in this study demonstrates the best device result among bridging polymers so far.

From Table 2 and Figure 5, it is concluded that P1 shows the best device performance among three polymers in this study, mainly due to larger J_SC, FF and EQE behavior. Although the P2 device shows lower J_SC value compared with P3HT, the higher V_OC and FF values make it comparable with P3HT. To further understand the performance trends of different polymers, we applied the AFM technique for morphological investigations. It is believed that proper phase separation between donor polymer and acceptor PC₆₁BM is the key issue to high-efficiency solar devices ^[46,47,48]. The AFM topographic images of different polymer:PC₆₁BM blend films are shown in Figure 6. The P1:PC₆₁BM blend film shows the most homogeneous dispersion of aggregations, which is beneficial for charge dissociation between electron donor and acceptor. The root-mean-square roughness (R_a) of P1:PC₆₁BM blend film is measured to be 7.25 nm that is also better than P3HT and P2 blend films, revealing R_a values of 8.24 and 11.5 nm, respectively. Combining adequate aggregate distribution and R_a value, the P1:PC₆₁BM blend film represents the best surface morphology and film-forming property among three polymer blends, and higher device performance based on this P1:PC₆₁BM blend is expected.

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4. Conclusion

In this research, two different ways for incorporating DPP linking groups to form linear or hyperbranched PT derivatives were proposed and characterized. The effects of introducing DPP groups onto P3HT were investigated by miscellaneous techniques, such as GPC, thermal analysis, optical and electrochemical measurements. The distinct PL decay of DPP-containing polymers in thin film state indicates a more efficient transport process for charge carriers. The lower HOMO levels of polymers resulted from incorporating electron-withdrawing DPP groups brings benefits in increasing V_OC values for photovoltaic application. The inverted solar device based on P1:PC₆₁BM blend shows high J_SC and PCE values of 16.21 mA/cm² and 3.74%, respectively. The solar device using P2:PC₆₁BM blend as the active layer reveals a PCE value of 2.38% that demonstrates the highest record among bridging polymers in the literature.

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Acknowledgments

The authors gratefully thank to the Ministry of Science and Technology of the Republic of China (NSC 102-2113-M-009-007) for financial support of this work.

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Conflict of Interest

All authors declare no conflicts of interest in this paper.

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