Porphyrin Antenna-Enriched BODIPYâThiophene Copolymer for

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Porphyrin Antenna-Enriched BODIPY−Thiophene Copolymer for Efficient Solar Cells Léo Bucher,†,‡ Nicolas Desbois,† Pierre D. Harvey,*,‡ Claude P. Gros,*,† and Ganesh D. Sharma*,§ †

ICMUB (UMR CNRS 6302), Université de Bourgogne Franche-Comté9, Avenue Alain SavaryBP 47870, 21078 Dijon Cedex, France ‡ Department of Chemistry, Université de Sherbrooke, 2500, Bd de l’Université, J1K 2R1 Sherbrooke, Québec, Canada § Department of Physics, LNM Institute of Information Technology, Rupa ki Nagal, Jamdoli, Jaipur 302031, Rajasthan, India S Supporting Information *

ABSTRACT: Low bandgap A−π−D copolymer, P(BdP-DEHT), consisting of alternating BOronDIPYrromethene (BODIPY) and thiophene units bridged by ethynyl linkers, and its porphyrin-enriched analogue, P(BdP/ Por-DEHT), were prepared, and their optical and electrochemical properties were studied. P(BdP-DEHT) exhibits strong absorption in the 500−800 nm range with an optical bandgap of 1.74 eV. On the basis of cyclic voltammetry, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are evaluated to be −5.40 and −3.66 eV, respectively. After the anchoring of zinc(II) porphyrin on the BODIPY unit, P(BdP/Por-DEHT) displays broadened absorption, thanks to porphyrins, and the optical bandgap decreases to 1.59 eV because of extension of BODIPY conjugation. The resulting estimated HOMO and LUMO energy levels, respectively, move to −5.32 and −3.73 eV. After optimization of the P(BdP-DEHT) or P(BdP/Por-DEHT) to PC71BM weight ratio to 1:2 in dichlorobenzene solution, the bulk heterojunction polymer solar cells show overall power conversion efficiencies (PCEs) of 3.03 and 3.86%, respectively. After solvent vapor annealing (SVA) treatment in CH2Cl2 for 40 s, the PCEs increased to 7.40% [Voc of 0.95 V, Jsc of 12.77 mA/cm2, and fill factor (FF) of 0.61 with energy loss of 0.79 eV] and 8.79% (Voc of 0.92 V, Jsc of 14.48 mA/ cm2, and FF of 0.66 with energy loss of 0.67 eV). The increase in the PCE for P(BdP/Por-DEHT)-based devices is mainly attributed to the enhancement in Jsc and FF, which may be related to the broader absorption spectra, lower band gap, and better charge transport of P(BdP/Por-DEHT) compared to P(BdP-DEHT). This could also be related to the optimized nanoscale morphology of the active layer for both efficient exciton dissociation and charge transport toward the electrodes and a balanced charge transport in the device, induced by the SVA treatment of the active layer. KEYWORDS: A−π−D copolymer, porphyrin substitution, polymer solar cells, solvent vapor annealing, power conversion efficiency morphology of the active layer used.7,8 One of the strategies to design a low bandgap copolymer is D−A type approach, often called the push−pull polymer, in which electron-rich (D) units and electron-deficient moieties (A) are integrated into a single conjugated polymeric chain in an alternating fashion. This approach increases the highest occupied molecular orbital (HOMO) energy level and decreases the lowest unoccupied molecular orbital (LUMO) energy level simultaneously and results in a low bandgap material through a desired red shift of the absorption spectrum of the polymer, thus permitting the harvesting of more photons of the solar radiation. BOronDIPYrromethene (BODIPY) exhibits strong absorption in the visible region, a π-conjugated system, as well as high photochemical and thermal stabilities.9,10 Moreover, the simple

1. INTRODUCTION Polymer solar cells (PSCs) based on solution-processed bulk heterojunction (BHJ) active layers consist of a blend of a conjugated polymer electron donor and a fullerene derivative such as PC71BM (phenyl-C61-butyric acid methyl ester) and have become very attractive candidates in photovoltaic (PV) technology because of their simple preparation, light weight, low cost, and large flexible area.1−3 Power conversion efficiency (PCE) of these PSCs have exceeded 13% on the laboratory scale through a lot of efforts in the design of low bandgapconjugated polymers, optimization of the morphology of active layers, and device physics.4−6 However, a major obstacle for the commercialization of these high-efficiency materials is their synthetic complexity and consequently their cost to PCE ratios. Therefore, high-performance polymer donor materials must exhibit a simple synthetic route and good processability along with other factors, such as broad absorption properties, low optical bandgap, high charge-carrier mobility, and nanoscale © XXXX American Chemical Society

Received: October 25, 2017 Accepted: December 7, 2017

A

DOI: 10.1021/acsami.7b16112 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces chemical synthesis and structural robustness have enabled the fine tuning of optical properties of BODIPY-based materials via systematic structural variations to shift the absorption toward the near-infrared (NIR). Moreover, BODIPY derivatives also exhibit variable redox chemistry, and they have been used as both electron donors and acceptors.11,12 Owing to the large molar extinction coefficients, intense absorption spectra that extend into the red region of the solar spectrum, and high hole mobility, small-molecule BODIPY derivatives have recently been used as donor materials in conjunction with PCBM in BHJ solar cells.13−16 BODIPY-based conjugated polymers have been used for different applications such as NIR emitters, nonlinear optics, light harvesting, and electrochromic devices.17−19 Nevertheless, the use of BODIPY-containing polymers as donors in PSCs is very limited, and the overall low PCE may be attributed to lower charge-carrier mobility,20−22 despite the fact that BODIPY has already been used to elaborate quite efficient BODIPY-based semiconductors with high charge-transport properties for use in organic electronics.23−25 The charge-carrier transport properties in polymers can be improved by the development of molecular geometries with favorable oriented polymer chains and close π−π stacking.26 The π-linkage unit can have profound effects on the molecular conformation and electronic distribution of D−A-type conjugated polymers.27 Therefore, the incorporation of the ethynyl moiety as a π-linker into the conjugated D−π−A conjugated polymers appears beneficial. The sp hybridization of carbon can lead to relatively deep HOMO energy levels because the cylindrical-like π-electron density can improve molecular planarity and facilitate carrier mobility.28−31 Moreover, ethynyl groups also enable polymers to be synthesized via Sonagashira coupling reactions, thus avoiding the need for either toxic stannyl intermediates or unreliable lithiation reactions, and also permit large-scale synthesis.32,33 Literature shows some trends about the use of porphyrin and BODIPY as donor materials in BHJ solar cells. Indeed so far, PV performances are noted when porphyrins are incorporated in the main conjugated chain of the donor polymer,34−36 except for one of our recently reported copolymer built upon a porphyrin−diketopyrrolopyrrole (DPP) push−pull pair.37 However, the most successful results have been obtained for porphyrin-based materials in the SM form. Indeed, Peng’s DPP−porphyrin−DPP small molecules (SMs) used as figureheads reached PCEs as high as 9%.38,39 Using porphyrin chromophores as side chains of a polymer as complementary light-harvesting unit (LHU) also revealed to be a powerful strategy to improve the PCEs.40−42 The assessment for BODIPY-containing materials is very similar. If BODIPY is included in a polymer used as an electron donor in organic solar cells (OSCs), the PCEs barely reached 2%,20−22 whereas the SMs give better results.13−16 Indeed, Bulut et al. recently described triazatruxene−BODIPY conjugates, which exhibited an impressive PCE of 5.8% because of the favorable morphology of the active layer at the solid state driven by efficient π-stacking between the SMs.14 We now report new A−π−D copolymers, P(BdP-DEHT) and P(BdP/Por-DEHT), as electron donors in BHJ OSCs (Chart 1). Their structures consist of alternating electron-rich BODIPY and electron-poor thiophene units bridged by ethynyl units. P(BdP/Por-DEHT) is directly obtained from P(BdPDEHT) after the addition of porphyrins via the random formation of styryl arms on the BODIPY core (Figure 1). Therefore, P(BdP/Por-DEHT) can be considered as “por-

Chart 1. Structures of the Investigated BODIPY− Thiophene-Based Polymers

Figure 1. Porphyrin enrichment strategy.

phyrin-enriched P(BdP-DEHT).” The addition of zinc(II) porphyrin as a complementary LHU allows for the increase of the short-circuit current density (Jsc) of the devices without affecting the open-circuit voltage (Voc) by increasing the amount of photons collected by this material. We recently showed that indeed the addition of styryl arms to the BODIPY chain was beneficial to prolong the charge-separated state when the polymer was in blend with the PCBM derivative.43 This “random approach” leads to a mixture of differently extended BODIPY units (i.e., non-, mono-, and bisextended BODIPY), thus broadening the absorption bands and red-shifting them to the 700−750 nm range where the photon flux in the solar spectrum is at a maximum,44 simultaneously. Alkyl-thiophene was chosen as the comonomer to bring solubility to the polymers while keeping efficient conjugation along the polymer chain. In parallel, it minimizes the presence of solubilizing chains directly placed on light-harvesting chromophores, which are known to deactivate their excited states via fast nonradiative relaxation processes.

2. EXPERIMENTAL SECTION 2.1. Syntheses. The procedures for the syntheses of 1 and its precursors have previously been reported.43,45,46 3,4-Di(2′-ethylhexyl)thiophene,47 5-mesityldipyrromethane,48 and 2-(5′,5′-dimethyl-[1′,3′]dioxan-2′-yl)benzaldehyde were prepared from published procedures.49 3,4-Dibromo-thiophene and mesitaldehyde were purchased from Alpha Aesar. 2.1.1. 3,4-Di(2′-ethylhexyl)-2,5-diiodothiophene (2). 3,4-Di(2′ethylhexyl)thiophene (200 mg, 0.65 mmol), N-iodosuccinimide (377.0 mg, 1.69 mmol), and 10 mL of a 1:1 mixture CHCl3/pure B


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ACS Applied Materials & Interfaces acetic acid were introduced in a round-bottom flask under nitrogen. The reaction was stirred at room temperature (rt) for 12 h and quenched with NaHCO3 afterward. The organic phase was washed with water, dried with MgSO4, and then evaporated under vacuum. The crude product was purified by silica column chromatography (hexanes), and 131 mg of the pure compound was obtained in 36% yield as a yellow oil. 1H NMR (300 MHz, CDCl3, ppm): δ 2.53 (d, 4H), 1.60 (m, 2H), 1.29 (m, 16H), 0.88 (m, 12H). 13C NMR (75 MHz, CDCl3, ppm): δ 145.87 (Cq), 79.02 (Cq), 40.10 (CH), 35.83 (CH2), 32.69 (CH2), 29.07 (CH2), 25.91 (CH2), 23.24 (CH2), 14.25 (CH3), 11.35 (CH3). ESI+ infusion MS (m/z) for C20H34I2S: found [M + H]+, 561.0535; calcd, 561.0543. 2.1.2. 5-(4′-Benzaldehyde)-10,15,20-trimesitylporphyrin Zn(II) (3). 4-(5′,5′-Dimethyl-[1′,3′]dioxan-2′-yl)benzaldehyde (1.79 g, 8.14 mmol), 5-mesityldipyrromethane (4.28 g, 16.28 mmol), mesitaldehyde (1.20 mL, 8.14 mmol), and degassed CHCl3 (2.2 L) were introduced in a round-bottom flask under an inert atmosphere. The solution was stirred for 15 min at 0 °C. A catalytic amount of boron trifluoride diethyl etherate (46.5%, 320.00 μL, 1.22 mmol) was added dropwise, and the mixture was stirred for 3 h at rt. p-Chloranil (2.99 g, 8.14 mmol) was added, and the reaction was stirred for 1 more hour. The volume of the reaction solvent was reduced under vacuum to 100 mL, before being washed with water and dried over MgSO4 and evaporated. The insoluble materials and byproducts were removed with a silica plug [dichloromethane (DCM)/hexanes 5:5 to pure DCM]. The red/purple crude product was used without further purification and introduced in a round-bottom flask with DCM (30 mL) and a 1:1 mixture of H2O/trifluoroacetic acid (30 mL). The mixture was vigorously stirred overnight at rt. The organic layer was washed with aqueous NaHCO3 saturated solution and a large amount of water. The solution was dried over MgSO4 and concentrated to dryness afterward. To the solid residue were added zinc acetate dihydrate (8.90 g, 40.70 mmol), sodium acetate (6.70 g, 81.40 mmol), and a 1:2 mixture of MeOH/CHCl3 (240 mL). The reaction was heated to reflux for 4 h, and the solvent was removed under vacuum. The crude product was dissolved in DCM and washed with water. The solution was dried over MgSO4 and evaporated. The product was purified by column chromatography (silica; pure DCM) to yield 11% (741 mg) of the purple powder. 1H NMR (300 MHz, DMSO-d6, ppm): δ 10.37 (s, 1H), 8.66 (d, 3J = 4.6 Hz, 2H), 8.56 (d, 3J = 4.6 Hz, 2H), 8.53 (s, 4H), 8.40 (d, 3J = 8.1 Hz, 2H), 8.30 (d, 3J = 8.2 Hz, 2H), 7.31 (s, 6H), 2.57 (s, 9H), 1.78 (s, 18H). 13C NMR (75 MHz, DMSOd6, ppm): δ 193.34 (CH), 149.08 (Cq), 149.03 (Cq), 148.84 (Cq), 148.41 (Cq), 139.03 (Cq), 138.94 (Cq), 138.40 (Cq), 138.36 (Cq), 136.87 (Cq), 136.85 (Cq), 135.07 (Cq), 135.04 (Cq), 134.89 (CH), 131.36 (CH), 130.60 (CH), 130.53 (CH), 130.30 (CH), 130.27 (CH), 127.63 (CH), 127.55 (CH), 117.93 (Cq), 117.86 (Cq), 117.73 (Cq), 21.68 (CH3), 21.48 (CH3), 21.02 (CH3), 19.99 (CH3). UV−vis (THF) λmax (ε × 103 L·mol−1·cm−1): 597 (7.1), 558 (23.7), 425 (603.9). ΦF (THF) = 4.1% (ZnTPP as reference). MALDI-TOF MS (m/z) for C54H46N4OZn: found [M]+•, 830.2972; calcd, 830.2963. 2.1.3. P(BdP-DEHT). 3,4-Di(2′-ethylhexyl)-2,5-diiodothiophene 2 (92.2 mg, 0.17 mmol), copper(I) iodide (18.8 mg, 0.10 mmol), and tetrakis(triphenylphosphine)palladium(0) (57.2 mg, 0.05 mmol) were introduced in a round-bottom flask under an argon atmosphere. After three cycles of vacuum/argon, distilled tetrahydrofuran (THF, 14 mL) and triethylamine (456.6 μL, 3.30 mmol) were added, and the mixture was heated to 60 °C. A solution of 2,6-diethynyl-8-mesityl-1,3,5,7tetramethyl-BODIPY 1 (68.2 mg, 0.17 mmol) in THF (14 mL) was added dropwise. The reaction was stirred overnight. The solvent was removed under vacuum. The residue was dissolved in CHCl3 and passed through a Celite plug with CHCl3 as the solvent to remove the insoluble material. The product was purified by gel permeation chromatography (GPC) with BioBeads S-X3 and S-X1 afterward (using pure CHCl3) to finally yield 85% (101 mg) of the polymer as a dark blue solid film. 1H NMR (300 MHz, CDCl3, ppm): δ 6.98 (s, 2H), 2.67 (d, 6H), 2.50 (m, 4H), 2.36 (s, 3H), 2.07 (m, 6H), 1.65 (m, 2H), 1.48 (m, 6H), 1.23 (m, 16H), 0.83 (m, 12H). UV−vis (THF) λmax (ε × 103 L·mol−1·cm−1): 635 (64.5), 415 (17.5). ΦF (THF) =

11.3% (Nile blue as reference). GPC (THF, polystyrene standard): Mn = 32.1 kDa; Mw = 57.5 kDa; PDI: 1.8. 2.1.4. P(BdP/Por-DEHT). Polymer P(BdP-DEHT) (60.1 mg, 0.083 mmol of monomer), porphyrin 3 (138.8 mg, 0.167 mmol), dry toluene (8 mL), piperidine (131.3 μL, 1.328 mmol), and recrystallized ptoluenesulfonic acid (1 crystal) were introduced in an evaporating flask. The mixture was heated up to 85 °C at 300 mbar on a rotary evaporator for 30 min. The solvent was fully removed. Dry toluene (8 mL) and piperidine (131.3 μL, 1.328 mmol) were added to the flask again, and the reaction was heated under the same conditions as previously mentioned. This last step was repeated five more times until no more change was visible by thin-layer chromatography (TLC) or ultraviolet−visible (UV−vis) monitoring. The residue was dissolved in CHCl3, and the crude product was passed through a Celite plug with CHCl3 as the solvent to remove the insoluble material. The compound was purified by GPC with BioBeads S-X3 and S-X1 afterward (using pure CHCl3) to finally yield 47% (92.0 mg) of the polymer as a dark green solid film. 1H NMR (300 MHz, CD2Cl2, ppm): δ 8.88−8.55 (m, CHβ‑porphyrin, CHalkene), 8.24−7.92 (m, CHphenyl, CHalkene), 7.29−7.17 (m, CHmesityl,porphyrin), 7.02−6.86 (m, CHmesityl,BODIPY), 2.67−2.48 (CH3 porphyrin, CH2 thiophene, CH3 BODIPY), 2.35 (m, CH3 BODIPY), 2.04 (m, CH3 BODIPY), 1.88−1.73 (m, CH3 porphyrin), 1.67−1.52 (m, CH 3 BODIPY , CH thiophene ), 1.24 (m, CH 2 thiophene ), 0.84 (m, CH3 thiophene). UV−vis (THF) λmax: 712, 639, 598, 558, 425. ΦF (THF) = 1.5% (Nile blue as reference). GPC (THF, polystyrene standard): Mn = 12.2 kDa; Mw = 23.3 kDa; PDI: 1.9. 2.2. Device Fabrication and Characterization. The PSCs were built using the conventional structure ITO/PEDOT:PSS/donor:PC71BM/PFN/Al. The indium tin oxide (ITO)-coated glass substrates were first cleaned inside an ultrasonicated bath, beginning with acetone, followed by deionized water and isopropanol, and finally dried in an oven at 40 °C for 2 h. After drying, a 40 nm-thin PEDOT:PSS (Clevios P AI4083) anode buffer layer was spin-cast onto the ITO substrate and dried by baking in a vacuum oven at 100 °C overnight. The active layers, that is, donor:PC71BM in different weight ratios, were deposited onto the top of the PEDOT:PSS layer by casting from the chlorobenzene solution and dried under ambient conditions. The solvent vapor annealing (SVA) treatment was performed as follows: the optimized donor:PC71BM (1:2) active layer was placed in a glass Petri dish containing 1 mL of CH2Cl2 for 40 s. Then, the poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) solution in methanol, containing a small trace of acetic acid, was spin-coated on the top of the active layer to form a thin interlayer of 5 nm. Finally, a thin aluminum (Al) layer with a thickness of 100 nm was thermally evaporated with a shadow mask at a low pressure of ∼10−6 torr. The overlapping area between the cathode and anode defined the active area, which was measured to be 16 mm2. The fabrication of the devices was performed at rt. A Keithley SourceMeter was used to measure the current−voltage (J−V) characteristics of the solar cells under ambient conditions and under AM1.5G (100 mW/cm2) provided by a solar simulator. The incident photon-to-current efficiency (IPCE) of the devices was measured by illuminating the devices through a monochromated light source (wavelength range 300−850 nm). The resulting current was measured using a Keithley electrometer under short-circuit conditions. To measure the hole and electron mobilities in the active layers, a “hole and electron” only device (i.e., ITO/ PEDOT:PSS/active layer/Au and ITO/Al/active layer/Al, respectively) was built, and the current−voltage characteristics in the dark were measured.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. BODIPY derivative 1 was prepared according to known procedures.43,45,46 Then, thiophene 2 was obtained starting from the commercially available 3,4-dibromothiophene (Figure S1). The first step consisted of the introduction of solubilizing 2-ethylhexyl chains, which is widely used and often leads to suitable active layer morphologies.39,47,50 This Kumada functionalization involved C


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Figure 2. Synthesis of P(BdP-DEHT) and P(BdP/Por-DEHT).

the macrocycle, with an overall yield of 11%. This compound was already described by Sazanovich et al., but was prepared according to a different synthesis procedure.62 Both P(BdPDEHT) and porphyrin 3 were then introduced together in a flask with piperidine, a catalytic amount of p-toluenesulfonic acid, and dry toluene. The reaction mixture was slowly evaporated under a partial vacuum (300 mbar) at 85 °C for 30 min. Then, piperidine and dry toluene were again added, and the reaction was left for 30 more min under the same synthesis conditions. This last cycle was repeated until the reaction was no longer progressing. This method allows for the efficient removal of water as it is formed and requires a simpler setup compared to the Dean−Stark apparatus and permits the use of milder conditions for Knoevenagel condensation. The reaction was monitored by TLC and UV−vis spectrophotometry. Figure 3 shows the UV−vis monitoring of the

the reaction between 3,4-dibromothiophene and 2(ethylhexyl)magnesium bromide (1 M in Et2O), as described in the literature.47 This dialkylthiophene product was then reacted with an excess of N-iodosuccinimide to result in 3,4di(2′-ethylhexyl)-2,5-diiodothiophene 2. This reaction was run in a chloroform/acetic acid mixture (1:1) as the solvent, halogenation conditions which have already been reported in the literature51 and turned out to greatly speed up the reaction in this case. Surprisingly, this iodothiophene 2 had not been reported in the literature, contrary to its bromo analogue.47 The synthesis of P(BdP-DEHT) was proceeded using the Sonogashira cross-coupling polymerization involving BODIPY 1 and thiophene 2 (Figure 2). Despite the fact that a number of BODIPY−thiophene copolymers have previously been reported in the literature,52,53 we used Sonogashira reaction conditions that were previously optimized by us.43 The key point was the very slow addition (about 15 drops per minute) of bisethynyl-BODIPY 1 directly to the heated reaction mixture to minimize homocoupling side reactions as much as possible. The reaction was monitored by TLC following the disappearance of the reactants. The solution rapidly turned from its initial bright orange color, due to BODIPY 1, to dark blue corresponding to the final polymer (Figure S2), resulting in P(BdP-DEHT) with a good 85% yield after purification. The final step was the incorporation of zinc(II) porphyrin, which is based on the well-known BODIPY chemistry reaction: the Knoevenagel condensation.54,55 This reaction has been extensively used since mid 2000’s to extend the conjugation pathway of the original BODIPY core and shift absorption/emission transitions toward NIR.56 This feature has been particularly useful to reach the therapeutic window for medical and bioimaging,57 for photodynamic therapy,58 or even in PV applications.59 This reaction relies on the acidic character of methyl groups at the 3- and 5-positions (also called αpositions), which are able to react with an aldehyde in the presence of a weak amine base and a catalytic amount of acid to form the vinylic bond. Noteworthy, depending on the specific functionalization around the BODIPY ring, the condensations at the 1- and 7-positions have also been reported.60,61 We first prepared formylporphyrin 3, which is the target aldehyde involved in Knoevenagel condensation (Figure S3). Its synthesis consisted in a MacDonald-type condensation between two equivalents of mesityldipyrromethane,48 one of acetalprotected aldehyde49 and one of mesitaldehyde. The porphyrin cyclization and oxidation steps were followed by subsequent deprotection of the aldehyde group and the zinc metalation of

Figure 3. UV−vis monitoring of P(BdP/Por-DEHT) synthesis.

reaction, where an absorption spectrum was recorded between each cycle. At the beginning of the reaction, one can notice the presence of bands at ∼600 and 650 nm because of the second porphyrin Q band and the absorption of P(BdP-DEHT), respectively. The reaction is followed by the decrease of the P(BdP-DEHT) band and the rise of a shoulder around 720 nm, attributed to the formation of the π-extended BODIPY species. Seven cycles were needed before the reaction slowed down and stopped. After purification, P(BdP/Por-DEHT) was obtained as a dark green polymer. 1H NMR titration was the most reliable tool to measure the porphyrin/BODIPY ratio (Figure S4). By comparing the proton integrations in the aromatic region of the newly added porphyrins with those of BODIPY, a 0.45 porphyrin/BODIPY ratio was determined. This result means that the mixture of BODIPY species D


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BODIPY-containing material we previously reported.43 Noteworthy, the very good overall spectral match with the sum of the absorption spectra of the isolated chromophores indicates that porphyrin and BODIPY cores are not conjugated in P(BdP/Por-DEHT). The absorption spectra of P(BdP-DEHT) and P(BdP/PorDEHT) as thin films display a slight red shift and a broader profile compared to solution (Figure 5). This is clearly pronounced for the Soret band of porphyrins in P(BdP/PorDEHT) (Figure 5b), where a shift from 425 to 437 nm is noticed, with full width at half-maximum changing from 611 cm−1 in solution to 1717 cm−1 in film. These phenomena are presumably indicative of some ordered structure at the solid state and could be due to π−π interactions, which would be beneficial for exciton dissociation and charge generation in the resulting PSCs.63 The absorption onsets of P(BdP-DEHT) and P(BdP/Por-DEHT) as thin films are located at 713 and 780 nm, which correspond to low optical bandgaps of 1.74 and 1.59 eV, respectively (Table 2). Cyclic voltammetry was used to determine the HOMO and LUMO energy levels of P(BdP-DEHT) and P(BdP/PorDEHT) (Figure 6, Table 2). At first glance, we can notice for the P(BdP-DEHT) polymer, one oxidation and one reduction wave, both being irreversible processes and corresponding to the BODIPY core. There are obvious appearances of new electrochemical processes for P(BdP/Por-DEHT), which are attributed to porphyrins that have been added to the polymer. Reversible oxidations are observed at +0.33 V and +0.67 V, as well as one reduction wave at −1.85 V, corresponding to classic electrochemical behavior of the porphyrin core. The HOMO/LUMO energy levels are usually estimated from the oxidation and reduction peak potentials, respectively. Herein, if each process is fully visible for the P(BdP-DEHT) material, it is not the case for P(BdP/Por-DEHT). Indeed, the oxidation and reduction potentials of the main chain of the polymer (i.e., the BODIPY−thiophene chain) are needed to find out the frontier orbitals of the material. Unluckily, the oxidation processes of side chain porphyrins are overlapped with the oxidation wave of BODIPY, making the corresponding electrochemical potential impossible to determine. Thus, the reduction potential and the associated LUMO energy level were found at first; then, the optical bandgap was used to determine the HOMO energy level (=ECorr HOMO); see legend of Table 2 for equations. It is worth mentioning that the same strategy was used for energy level estimation of P(BdP-DEHT) to be consistent. Finally, the HOMO and LUMO energy levels were found to be −5.40 and −3.66 eV for P(BdP-DEHT) and −5.32 and −3.73 eV for P(BdP/Por-DEHT), respectively. These copolymers show deep HOMO energy levels, which is generally desired for good air stability and high Voc in PSCs. Moreover, the energy gaps of the LUMOD − LUMOA (=ΔLUMO, “D” means donor and “A” acceptor) and HOMOA − HOMOD (=ΔHOMO) for P(BdP-DEHT):PC71BM and P(BdP/Por-DEHT):PC71BM blends are ∼0.44 and 0.60 eV and ∼0.37 and 0.68 eV, respectively, which are larger than the 0.3 eV considered as the threshold for efficient exciton dissociation and electron transfer from the donor to the acceptor and vice versa.64 3.3. PV Properties. BHJ PSCs using PC71BM as the electron acceptor from solution-processed devices were fabricated using these polymers as electron donors with the device structure of ITO/PEDOT:PSS (poly(3,4-

preferentially consists of non- and monoextended BODIPY rather than bisextended ones. The limited amount of porphyrins in the final material is most likely explained by the increasing overall steric hindrance within a polymer chain induced by each addition of porphyrin. P(BdP-DEHT) and P(BdP/Por-DEHT) exhibit decomposition temperatures of 321 and 310 °C (at 5% weight loss; thermogravimetric analysis (TGA); Figure S5), respectively, which are suitable for PV applications. GPC measurements were performed to determine the molecular weights of the polymers; Table 1 summarizes the number-average molecular Table 1. Thermal Properties and GPC Data of Polymers polymer P(BdP-DEHT) P(BdP/Por-DEHT)

TDeca °C

Mnb g·mol−1

Mwb g·mol−1

PDIb

321 310

32.1 × 10 12.2 × 103

57.5 × 10 23.3 × 103

1.8 1.9

3

3

TGA at 5% weight loss under argon at heating rate of 10 °C·min−1. GPC was performed in THF at 45 °C using polystyrene as the standard.

a b

weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI). The decrease in the apparent molecular weights before and after porphyrin enrichment is clearly illogical. This is most likely due to the difference in solubility in THF, where P(BdP/Por-DEHT) is suspected to be less soluble. Note that the polymer-containing solutions are filtered prior to adding them to the GPC apparatus. 3.2. Optical and Electrochemical Properties. The absorption spectrum of P(BdP/Por-DEHT) is to be a linear combination of absorptions of the different chromophores included in the polymer in the 250−650 nm region. Indeed, Figure 4 shows the absorption spectra of P(BdP/Por-DEHT)

Figure 4. Absorption of porphyrin 3 (red trace), P(BdP-DEHT) (blue trace), and P(BdP/Por-DEHT) (black trace) in 2-MeTHF at 298 K.

in solution superimposed with those for porphyrin 3 and P(BdP-DEHT), where the sharp Soret band at 425 nm and the two Q bands at 558 and 598 nm of P(BdP/Por-DEHT) are depicted. Concurrently, a broad band located at 639 nm in the P(BdP/Por-DEHT) spectrum strongly overlapping with the Q bands is also clearly identified as being the S0 → S1 transition corresponding to the BODIPY chromophore in P(BdPDEHT). The new band at 712 nm corresponds to the extended BODIPY species, that is, to porphyrin-styryl-bearing BODIPY. The 73 nm bathochromic shift going from nonextended to extended BODIPY attests this styryl-functionalization. A clear evidence is provided in Figure S6 by comparison with the absorption spectrum of a styrylE


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Figure 5. Normalized absorption spectra of (a) P(BdP-DEHT) and (b) P(BdP/Por-DEHT) in solution in 2-MeTHF at 298 K (black) and thin film cast from chlorobenzene (CB) (red).

Table 2. Optical and Electrochemical Data of P(BdP-DEHT) and P(BdP/Por-DEHT) polymer

λabs onset (nm)

a Eopt (eV) g

Eox onset (V)

Ered onset (V)

EHOMOb (eV)

ELUMOb (eV)

c Eelec (eV) g

d ECorr HOMO (V)

P(BdP-DEHT) P(BdP/Por-DEHT)

713 780

1.74 1.59

+0.68

−1.14 −1.07

−5.48

−3.66 −3.73

1.82

−5.40 −5.32

red Optical band gap, determined from the absorption onset in thefilm. bDetermined with EHOMO = −(Eox onset + 4.8) eV and ELUMO = −(Eonset + 4.8) eV. opt Electrochemical band gap. dDetermined with ECorr HOMO = ELUMO − Eg .

a c

The current−voltage (J−V) characteristics under illumination (AM1.5G, 100 mW/cm2) of the optimized PSC are shown in Figure 7a, and the corresponding PV parameters are compiled in Table 3. The PSCs based on the as-cast P(BdPDEHT):PC71BM and P(BdP/Por-DEHT):PC71BM active layers exhibit a PCE of 3.03% [Voc of 0.99 V, Jsc of 7.86 mA/ cm2, and fill factor (FF) of 0.39] and 3.86% (Jsc = 9.68 mA/ cm2, Voc = 0.95 V, and FF = 0.42). The Voc of these devices is quite high, which is consistent with the stabilized HOMO levels of both the copolymers. The low PCE of the PSCs is mainly due to Jsc and FF, which may be attributed to the unoptimized nanoscale morphology and active layers, which limits the exciton dissociation and charge transport in the active layer. To improve the PCE values, SVA was used. Indeed, this treatment provides better control of the crystallinity and nanoscale morphology of the active layer. The current−voltage characteristics of the PSCs (Figure 7a) show that after SVA treatment, the PCE is enhanced up to 7.40% (Voc = 0.95 V, Jsc = 12.77 mA/cm2, and FF = 0.61) and 8.79% (Voc = 0.92 V, Jsc = 14.48 mA/cm2, and FF = 0.66) for P(BdP-DEHT):PC71BM (1:2) and P(BdP/Por-DEHT):PC71BM, respectively. The

Figure 6. Cyclic voltammograms of P(BdP-DEHT) (black curve) and P(BdP/Por-DEHT) (red curve) as thin films with 0.1 M TBAPF6 in MeCN solution (scan rate = 50 mV·s−1).

ethylenedioxythiophene):poly(styrenesulphonate))/polymer:PC71BM/PFN/Al, where polymer = P(Bdp-DEHT) or P(BdP/Por-DEHT) and PFN is used as the efficient cathode interlayer. After device optimization [i.e., donor to acceptor weight ratios in dichlorobenzene (DCB)], the best weight ratio was found to be 1:2, and the active layer thickness was ∼95 nm.

Figure 7. (a) Current−voltage (J−V) characteristics under illumination (1.5 AMG, 100 mW/cm2) and (b) IPCE spectra for PSCs based on the ascast and SVA-treated P(BdP-DEHT):PC71BM (1:2) and P(BdP/Por-DEHT):PC71BM (1:2) active layers; the legend is the same for both the graphs. F


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Table 3. PV Parameters of PSCs Based on As-Cast and SVA-Treated P(BdP-DEHT):PC71BM (1:2) and P(BdP/PorDEHT):PC71BM (1:2) Active Layers (1.5 AMG, 100 mW/cm2) active layer

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Rs (Ω·cm2)

Rsh (Ω·cm2)

P(BdP-DEHT):PC71BM as-cast P(BdP-DEHT):PC71BM SVA P(BdP/Por-DEHT):PC71BM as-cast P(BdP/Por-DEHT):PC71BM SVA

7.86 12.77 9.68 14.48

0.99 0.95 0.95 0.92

0.39 0.61 0.42 0.66

3.03 7.40 3.86 8.79

13.21 8.43 10.21 7.12

832 1136 976 1201

Figure 8. Normalized absorption spectra of as-cast (black trace) and SVA-treated (red trace) (a) P(BdP-DEHT):PC71BM (1:2) and (b) P(BdP/ Por-DEHT):PC71BM (1:2) blend thin films; the legend is identical for both the graphs.

LUMO levels closer to each other. In PSCs, the large energy loss (Eloss) (0.7−1.0 eV) is one of the crucial issues to reach high efficiencies. The large Eloss is associated with the larger driving force for exciton dissociation and a large nonradiative charge recombination.65 Therefore, it is imperative to minimize the energy loss to increase both Jsc and Voc simultaneously, thereby improving the overall PCEs. This goal can be achieved by designing polymeric donor materials with a low bandgap and suitable energy levels. The energy loss (Eloss) is evaluated from the expression Eloss = Eg − qVoc, where Eg is the lowest optical bandgap of the donor and the acceptor. The Eloss for the devices are 0.79 eV for the P(BdP-DEHT):PC71BM device and 0.67 eV for the P(BdP/Por-DEHT):PC71BM device and compared to those for other literature PSCs based on low bandgap polymers.66,67 To further investigate the exciton dissociation and chargetransfer kinetics in the blend active layer, photoluminescence (PL) spectra of P(BdP-DEHT) and P(BdP/Por-DEHT) in the solid state and in their blend with PC71BM were measured (Figure S7a,b). The PL spectra of neat P(BdP-DEHT) and P(BdP/Por-DEHT) (black traces) showed a PL peak, respectively, at 752 and 808 nm (at λex = 640 nm). As expected, the PL intensity is almost entirely quenched upon mixing with PC71BM, resulting in a very low emissive material.68 Interestingly, for the same sample, the PL quenching is more pronounced for the SVA-treated blended film (blue traces) compared to the as-cast blend (red traces), indicating that charge separation in this film is more efficient, most likely because of the better nanoscale morphology induced by the SVA treatment. To tentatively explain the higher performance of the SVAtreated PSCs, the absorption spectra of SVA-untreated and -treated P(BdP/Por-DEHT):PC71BM active layers are presented in Figure 8b. The SVA-treated active layer showed redshifted absorption compared to that for the as-cast counterpart. As said above, this spectral behavior is also similar to the IPCE spectrum. Moreover, the absorption intensity also increased for the film that underwent a SVA treatment, notably for the

PCE values increased significantly for both copolymers after the SVA treatment. This is mainly attributed to the enhanced values of Jsc and FF, which again may be due to the better nanoscale morphology and charge transport in the active layers after the SVA treatment. To verify the measurement accuracy, the IPCE traces using untreated and SVA treated active layers were obtained (Figure 7b). Together with the absorption spectra of the blended active layer (Figure 8), the broad IPCE traces in the 350−820 nm range suggest that both copolymer/ PC71BM blends complementarily contribute considerably to the photocurrent. The IPCE values for the PSCs with SVA treatments are much higher than those without, reaching a maximum value of 63% and with an average value of 58%. This is indicative of a highly efficient photoelectron conversion process in the SVA-treated PSCs. Evolution of IPCE from the as-cast to SVA-treated devices follows changes in the absorption profiles of both P(BdP-DEHT) and P(BdP/PorDEHT) active layers. Moreover, the calculated Jsc values extracted from the IPCE curves for the SVA-treated devices [12.63 mA/cm2 for P(BdP-DEHT):PC71BM, and 14.85 mA/ cm2 for P(BdP/Por-DEHT):PC71BM] are consistent with the values observed in the J−V characteristics under illumination. Noteworthy, the overall IPCE efficiency is clearly broadened and improved in the 440 and 600 nm regions, thanks to the porphyrins, along with the 780 to 820 nm window, where the extended BODIPY species contributes to higher IPCE values. The Voc values are quite high, again due to the more stabilized HOMO levels of both the copolymers (Table 3). Moreover, the Voc for the devices based on P(BdP-DEHT) is larger than that for the P(BdP/Por-DEHT) analogue for both as-cast and SVA-treated, which may be attributed to the more stabilized HOMO energy level of P(BdP-DEHT). This hypothesis is based on the fact that Voc of the BHJ devices is directly related to the energy difference of the LUMO of the acceptor and the HOMO of the donor component used in the BHJ active layer. This assessment was expected because the structural modification of BODIPY units on extending the conjugation lowers the bandgap and brings both HOMO and G


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photocurrent density (Jph) was monitored as a function of the effective voltage (Veff) (Figure 9), with Jph = JL − JD, where

absorption band in the 500−850 nm range associated with the P(BdP-DEHT) polymer. The absorption efficiency (ηA) of the untreated P(BdP-DEHT):PC71BM films with the same thickness is determined by eq 1

∫ (1 − 10−αd)S(λ)dλ ηA = ∫ S(λ)dλ

(1)

where S(λ) is the number of photons available (photon flux) at a particular wavelength λ with an AM1.5G solar condition, α is the absorption coefficient, and d is the thickness of the film. The ηA values are calculated from integration of the absorbance versus wavelength from 350 to 850 nm for the untreated (42.15%) and SVA-treated P(BdP/Por-DEHT):PC71BM films (48.27%). The resulting values indicate that more excitons are formed in the SVA-treated active layer than in the as-cast counterpart, which may explain, in part, the larger Jsc. A similar behavior has been observed for P(BdP-DEHT):PC71BM active layers. Moreover, the broader absorption spectra of the P(BdP/ Por-DEHT):PC71BM active layer as compared to P(BdPDEHT):PC71BM (Figure 8a,b) may also contribute to the increased Jsc, regardless of the processing conditions. The higher light-harvesting efficiency of the P(BdP/PorDEHT):PC71BM active layer as well as the increasing Jsc were expected because of the addition of a new chromophore to absorb light (the porphyrin) and also because of the broadening of the absorption spectrum caused by BODIPY units extension. Our strategy is based on the use of energy transfer mechanisms to converge light excitation from porphyrin toward the BODIPY−thiophene main chain (Figure S8). The latter would have a suitable LUMO energy level to efficiently transfer electrons to the acceptor. Indeed, a recent study from our group showed that ultrafast energy transfer from the zinc(II) porphyrin unit to BODIPY (picosecond) and ultrafast photo-induced electron from both zinc(II) porphyrin and BODIPY units to fullerene derivatives (femtosecond− picosecond) may very well take place at the donor/acceptor interfaces.69,70 Preliminary photophysical studies were carried out on P(BdP/Por-DEHT) to confirm the porphyrin → BODIPY energy transfer phenomena. Figure S9a−c shows the steady-state measurements, namely, absorption, emission, and excitation spectra of P(BdP/Por-DEHT). Emission spectra have been measured when exciting almost only the porphyrin (i.e., the energy donor) at 425 nm, where porphyrin absorption is ∼30 times stronger than BODIPY, whereas excitation spectra have been recorded when looking to BODIPY emission exclusively. At first glance, in solution at 298 K (Figure S9a), the emission coming from porphyrin (λem = 600 and 650 nm) is almost only noticed, revealing a low energy transfer yield. In solid-state solution (Figure S9b), the band at 750 nm corresponding to BODIPY emission is clearly intensified, reflecting an increase in the transfer efficiency when movements in the polymer structure are restricted. This is supported with the excellent match between excitation and absorption spectra. Interestingly, the behavior of P(BdP/Por-DEHT) changes again in the thin film (Figure S9c). The emission of porphyrin is barely observed in this case, indicating that an efficient energy transfer from porphyrin to BODIPY is at work when the material is organized at the solid state. To understand the exciton generation and their dissociation into free charge carriers and charge-transport properties in the P(BdP-DEHT):PC71BM and P(BdP/Por-DEHT):PC71BM active layers (untreated and treated), the change in the

Figure 9. Variation of the photocurrent density (Jph) with the effective voltage (Veff) for the PSCs with the as-cast (i.e., untreated) and SVAtreated active layers [donor:PC71BM (1:2)].

JL and JD are the current densities under illumination and in the dark, respectively, and Veff = Vo − Va, where Va is the applied voltage and Vo is the voltage at which Jph is zero.71 Veff determines the electric field in the BHJ active layer that affects the exciton dissociation and charge transport processes. The Jph values of PSCs based on untreated active layers could not reach saturation even at the highest value of Veff, demonstrating that the exciton dissociation is probably low. For both PSCs based on SVA-treated active layers, Jph begins to saturate at Veff ≈ 0.63 [P(BdP-DEHT):PC71BM] and 0.42 V [P(BdP/Por-DEHT):PC71BM] and completely saturate at 1.85 V, suggesting that at high Veff, all photogenerated excitons are dissociated into free charge carriers and are collected by the electrodes with a minimized recombination loss.72 At a high Veff, the saturation photocurrent (Jsat) is only limited by the incident photon absorption. The Jsat values for the PSCs based on untreated and SVA-treated P(BdP-DEHT):PC71BM active layers are 9.13 and 13.74 mA/cm2, respectively, whereas these values are, respectively, 12.22 and 14.98 mA/cm2 for P(BdP/ Por-DEHT):PC71BM active layers. This result is consistent with enhancement in the absorption coefficient of the SVAtreated active layer compared to that of the untreated one. The exciton dissociation and charge collection probability P(E,T) could be estimated using the expression P(E,T) = Jph/Jsat. Under short circuit/maximum power output conditions, the values of P(E,T) are 0.86/0.73 and 0.89/0.77 for the untreated P(BdP-DEHT):PC71BM and P(BdP/Por-DEHT):PC71BM layers, respectively, and 0.93/0.79 and 0.80/0.96 for the SVAtreated P(BdP-DEHT):PC 7 1 BM and P(BdP/PorDEHT):PC71BM layers, respectively. The increased P(E,T) values indicate that the PSCs based on the SVA-treated active layer exhibit higher exciton dissociation and more charge collection efficiency compared to the untreated ones.73 To compare the exciton dissociation and bimolecular recombination in both PSCs, the Gmax values (charge generation characteristics; Gmax = Jsat/qL, where q is the electronic charge and L is the thickness of the active layer) was estimated, assuming that all photogenerated excitons dissociate into free charge carriers and contributed to photocurrent generation in the saturation region. The Gmax values for the SVA-treated devices, respectively, are 9.01 × 10 27 for P(BdPDEHT):PC71BM and 1.06 × 1028 m−3 s−1 for P(BdP/PorDEHT):PC71BM and are larger than those for the untreated H


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ACS Applied Materials & Interfaces counterparts (5.80 × 1027 and 9.34 × 1027 m−3 s−1, respectively). These data are also consistent with their corresponding absorption efficiencies. The larger Gmax value for the P(BdP/Por-DEHT):PC71BM active layer than that for P(BdP-DEHT):PC71BM is consistent with the more broader absorption bands of the former. The effect of SVA treatment on the charge mobility of the devices exhibiting the ITO/PEDOT:PSS/P(BdPDEHT):PC71BM/Au and ITO/Al/P(BdP-DEHT):PC71BM/ Al structures was studied. The hole (μh)/electron mobilities (μe) estimated from the dark J−V characteristics (Figure 10)

These enhancements in the Jsc and FF values are also consistent with the change in the series resistance (Rs) and shunt resistance (Rsh) (Table 3), estimated from the slopes of J−V plots when the devices are under illumination, at Voc and Jsc, respectively. The decrease in Rs from 13.21 to 8.43 Ω·cm2 and the increase in Rsh from 832 to 1136 Ω·cm2 for the active layers that were SVA-treated are in line with the increased charge collection probability, which originates from the larger and more balanced electron/hole mobilities.76,77 The light intensity (Pin) dependence of Jsc and Voc were monitored to investigate the recombination kinetics in the devices based on untreated and SVA-treated active layers (Figure 11a,b, respectively). The line Jsc versus Pin follows the relationship Jsc ∝ Pγin (Figure 11a),77 where γ is the exponential factor that is close to unity when the bimolecular recombination is weak. The values of γ estimated from the Jsc−Pin plots are 0.88 and 0.94 for PSCs prepared with untreated and SVAtreated active layers, respectively. The larger γ value for the devices with the SVA-treated active layers indicates that the bimolecular recombination is more suppressed than that for the untreated devices. The variation of Voc with Pin (Figure 11b) shows light intensity dependence with a slope of 1.48 and 1.25kT/q for the devices based on untreated and SVA-treated active layers, respectively. The device prepared with the SVAtreated active layer exhibits less charge-trapped recombination, which is consistent with the larger Jsc and FF values. These results suggest that the bimolecular recombination and space charge effect were more suppressed for the device based on the SVA-treated active layer. The effect of the pendent porphyrins on the relative molecular ordering in the solid state of the microstructures of the films were characterized by X-ray diffraction (XRD) patterns (Figure 12). The untreated P(BdP-DEHT) layer

Figure 10. Dark J−V characteristics of the ITO/PEDOT:PSS/ polymer:PC71BM/Au cells [SVA-treated and untreated (as-cast); polymer = P(BdP-DEHT) and P(BdP/Por-DEHT)].

and the fit of these curves, using a space charge limited current model,74 are approximately 4.31 × 10−5/2.65 × 10−4 and 7.11 × 10−4/2.71 × 10−4, respectively, (all units in cm2/V·s) for the untreated P(BdP-DEHT):PC 71 BM and P(BdP/PorDEHT):PC71BM active layers. The ratios of electron to hole mobility (μe/μh) are ∼6.19 and 3.81, indicating an imbalanced charge transport responsible for the lower Jsc, FF, and PCE values. However, the μh/μe values increase to 9.92 × 10−4/2.79 × 10−4 and 1.48 × 10−4/2.82 × 10−4 (all units in cm2/V·s) for SVA-treated P(BdP-DEHT):PC 71 BM and P(BdP/PorDEHT):PC71BM active layers, respectively. Their respective μe/μh ratios are 2.81 and 1.90, indicating an improved balanced charge transport, and also resulting in an enhancement of the Jsc, FF, and PCE values. This behavior is again assumed to be due to the increased crystallinity and nanoscale bicontinuous (see below) interpenetrating phase separation for charge transport with reduced recombination.75

Figure 12. XRD patterns of P(BdP-DEHT) and P(BdP/Por-DEHT) as-cast thin films.

Figure 11. Variation of Jsc (a) and Voc (b) with the illumination intensity (Pin) for the PSCs based on untreated and SVA-treated P(BdP/PorDEHT):PC71BM (1:2) active layers. I


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gel condensation was used as an original synthetic pathway to functionalize BODIPY units with pendant porphyrins. Their optical and electrochemical properties were investigated; the P(BdP-DEHT) copolymer shows a low bandgap of 1.74 eV and a broad absorption profile of up to 800 nm as a thin film, whereas P(BdP/Por-DEHT) exhibits absorption bands up to 850 nm with an optical bandgap of 1.59 eV. These polymers were used as electron donors, along with PC71BM as the acceptor, for the fabrication of solution-processed BHJ PSCs. After the optimization of active layer deposition conditions, the optimized 1:2 donor/acceptor weight ratio cast from DCB showed overall PCEs of 3.03 and 3.86% for P(BdP-DEHT) and P(BdP/Por-DEHT), respectively, with a slightly higher Voc of 0.99 V for P(BdP-DEHT) (vs 0.95 V), attributed to the more stable HOMO level of −5.40 eV (vs −5.32 eV). After the SVA treatment of the active layer, the resulting PCEs increased up to 7.40% (Voc = 0.95 V, Jsc = 12.77 mA/cm2, and FF = 0.61) and 8.79% (Voc = 0.92 V, Jsc = 14.48 mA/cm2, and FF = 0.66), with a low energy loss of 0.79 and 0.67 eV for P(BdP-DEHT) and P(BdP/Por-DEHT), respectively. Jsc was first clearly improved with porphyrin addition, and even better with SVA treatment, as well as FF, due to the favorable nanoscale morphology of the active layer, improved crystallinity, balanced charge transport, and reduced recombination. We used herein a ternary blend strategy, which has already proved its efficiency in the past,5,79,80 except that porphyrin is covalently linked to the BODIPY polymer. We expected this could control the distance and orientation at the solid state between the energy donor and acceptor, which are both key parameters for efficient energy transfers. We were then able to benefit from this process taking place from porphyrin to BODIPY to increase the Jsc and overall PCE in the BHJ OSC. Complete photophysical studies are currently under investigation in our labs to gain more insight about energytransfer processes from porphyrins to the different BODIPY species present all along the main chain as well as the kinetics of electron transfers, which could happen from both porphyrin and BODIPY units to the PCBM acceptor.

shows a (100) diffraction peak at 2θ = 4.93° corresponding to a lamellar spacing and a weak and broader (010) diffraction peak at 2θ ≈ 22.9° and 23.08° for P(BdP-DEHT) and P(BdP/PorDEHT) corresponding to stacking distances of ∼0.42 and 0.40 nm, respectively. Conversely, the XRD pattern of the P(BdP/ Por DEHT) layer exhibits the same (100) diffraction peak (at 2θ = 4.93°) but with a higher intensity. The broad (010) peak shifts to 2θ ≈ 22.4° corresponding to a stacking distance of ∼0.39 nm, also with an enhanced intensity. The increased intensity may be attributed to the enhancement in the crystallinity degree by the incorporation of porphyrin. This increase in the diffraction intensity indicates the presence of more crystal microdomains. The nanocrystalline size along the stacking direction was estimated using Scherrer’s equation. It is found that the size of the crystal nanodomain in the (010) direction is larger (2.45 nm) with the SVA-treated versus untreated film (2.38 nm). The increase in the crystallinity of the P(BdP-DEHT) layer may contribute to the improvement in the hole mobility, which is beneficial for efficient PSCs. The transmission electron microscopy (TEM) images of the untreated and SVA-treated layers provide information on the phase separation and bulk morphology of the films (Figure 13).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16112. Synthesis schemes; 1H and 13C NMR and mass spectra of 2 and 3; TGA traces and 1H NMR of polymers; and absorption, emission, and excitation spectra in solution (298 and 77 K) and in thin film of P(BdP/Por-DEHT) (PDF)

Figure 13. TEM images of the (a) as-cast and (b) SVA-treated P(BdP-DEHT):PC71BM (1:2) active layers and the (c) as-cast (d) SVA-treated P(BdP/Por-DEHT):PC71BM (1:2) active layers. The scale bar is 100 nm.

The bright and dark regions correspond to the donor-rich and acceptor-rich domains, respectively. These TEM images show no feature associated with a bulk morphology, indicating finely mixed P(BdP/Por-DEHT) or P(BdP-DEHT) and PC71BM on the nanometer scale. The TEM images of SVA-treated blend films showed longer and more distinct nonfibrillar domains with higher densities than those of as-cast respective blend films. The nonfibrillar structure is desirable because a bicontinuous interpenetrating network in the blend Förster’s exciton diffusion, exciton dissociation, and charge transport leads to enhanced Jsc and FF.78

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.D.H.). *E-mail: [email protected] (C.P.G.). *E-mail: [email protected] (G.D.S.). ORCID

Pierre D. Harvey: 0000-0002-6809-1629 Claude P. Gros: 0000-0002-6966-947X Ganesh D. Sharma: 0000-0002-1717-0116

4. CONCLUSIONS In summary, two BODIPY−thiophene copolymers, P(BdPDEHT) and its porphyrin-enriched analog P(BdP/PorDEHT), were rationally designed and synthesized. Knoevena-

Notes

The authors declare no competing financial interest. J


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ACKNOWLEDGMENTS This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), le “Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT)”, and the “Centre d’Etudes des Matériaux Optiques et Photoniques de l’Université de Sherbrooke (CEMOPUS)”. We thank Prof. Gessie Brissard (Univ. Sherbrooke). The “Centre National de la Recherche Scientifique” (ICMUB, UMR CNRS 6302) is gratefully thanked for the financial support. L.B. also gratefully acknowledges the French Research Ministry for a PhD fellowship. Support was provided by the CNRS, the “Université de Bourgogne Franche-Comté”, the FEDER, and the “Conseil Régional de Bourgogne” through the PARI II CDEA project. “Réalisé avec le soutien du Service de Coopération et d’Action Culturelle du Consulat Général de France à Québec” (Samuel de Champlain grant).

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