Easily Accessible Low Band Gap Polymer for Efficient Nonfullerene

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

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Easily Accessible Low Band Gap Polymer for Efficient Nonfullerene Polymer Solar Cells with a Low Eloss of 0.55 eV Minwoo Park† and Jae Woong Jung*,‡ †

Department of Chemical and Biological Engineering, Sookmyung Women’s University, Seoul 04310, Republic of Korea Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea

‡

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S Supporting Information *

ABSTRACT: Polymer solar cells (PSCs) have been attracting attention as a promising photovoltaic technology, and power conversion efficiencies (PCEs) of over 14% have been demonstrated. However, the state-of-the-art conjugated polymers for PSCs are based on the complex molecular structure that requires several steps for synthesis and purification, which impedes low materials cost for the PSCs. Thus, it is a key challenge to develop high-performance conjugated polymers with easily accessible building blocks for applications in practical PSCs. Here, a novel low band gap polymer (PFBTBOX) which comprises readily available building blocks was designed and synthesized to be used as an electron donor for PSCs. Affirmative optoelectronic properties of PFBTBOX for photovoltaic applications resulted in high PCEs up to 10.20 and 8.11% with nonfullerene and fullerene acceptors, respectively. Moreover, PFBTBOX exhibited minimal energy loss as low as 0.55 eV with a nonfullerene acceptor, indicating that PFBTBOX is a promising electron donor polymer for practical PSCs with high efficiency exceeding 10%. KEYWORDS: polymer solar cells, nonfullerene acceptors, low cost, high efficiency

1. INTRODUCTION Polymer solar cells (PSCs) have been regarded as a promising photovoltaic technology because of their unique advantages of lightweight, flexibility, semitransparency, and solution processing.1 The innovation in design and synthesis of organic semiconductor has rapidly improved the optoelectronic properties of conjugated polymers, which has boosted the photovoltaic performance of PSCs.2−6 In recent years, the nonfullerene acceptors have emerged as an alternative of conventional fullerene derivatives that provides great opportunities to enlarge the absorption range and photovoltage generation of PSCs.7,8 With regard to design of suitable conjugated polymers for the use in nonfullerene PSCs as electron donors, several requirements in molecular design should be kept in mind: (i) appropriate absorption range for complementary absorption with the nonfullerene acceptors; (ii) low-lying energy levels which are well-matched with those of nonfullerene acceptors for high photovoltage generation; and (iii) high crystallinity for well-developed bicontinuous and interpenetrating morphology of the bulk heterojunction with nanoscale phase separation, which guarantees unanticipated charge recombination. The optimal polymer donors which © XXXX American Chemical Society

fulfill those molecular design criteria have improved opencircuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and overall power conversion efficiency (PCE) of PSCs. As a result, the PCEs of PSCs using polymer donors have reached impressive PCEs up to >11 and >14% with fullerene and nonfullerene acceptors, respectively.9,10 The rational design of conjugated polymers is typically based on the alternating structure of donor−acceptor (D−A) to reduce the energy gap of frontier orbitals, to lower the energy levels, and to mediate the π-orbital overlap in adjacent polymer chains. Thus, the optoelectronic properties of conjugated polymers for PSCs were strongly influenced by the electronic structure of each building block of D and A units.11−14 Although a variety of building blocks have been exploited by a judicious molecular engineering to improve the photon harvesting, photovoltage, and charge carrier transporting, it is an onerous duty for the synthesis of monomers that require complex synthetic protocols with several reaction steps, and Received: November 20, 2018 Accepted: January 9, 2019 Published: January 9, 2019 A

DOI: 10.1021/acsami.8b20276 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces they usually suffer from low yield with poor purity.15,16 Furthermore, these delicate monomers are usually vulnerable to oxidation in storage, which are prone to yield unoptimized conjugated polymers with low molecular weight and high dispersity. Such problems limit the large-scale synthesis of the complex conjugated polymers at low cost, and thus, they would not be competitive counterparts for inorganic photovoltaic technologies.17 On the other hand, the conjugated polymers with a relatively simple molecular structure have drawn much interest for the use in practical PSCs. In particular, a proper choice of D and A molecules that can be prepared in a few synthetic steps or are commercially available will provide an effective strategy of high-performance PSCs with low materials cost.18 In this study, an easily accessible conjugated polymer (PFBTBOX) composed of 3,3′-difluoro-2,2′-bithiophene (FBT) and 5,6-bis(octyloxy)-2,1,3-benzooxadiazole (BOX) as D and A molecules, respectively, was designed and synthesized to investigate its photovoltaic properties in PSCs. FBT is a simple but a promising electron-rich building block for highperformance conjugated polymers of efficient PSCs.19,20 2,1,3Benzooxadiazole is a well-known electron-deficient heterocycle, in which two imine nitrogens possess strong electronwithdrawing property forming a stable quinoidal structure of the resulting polymers.21,22 Both FBT and BOX can be prepared via straightforward synthesis in a few steps with high yield. What is more, they are available from several commercial sources at relatively low cost. In this contribution, we synthesized PFBTBOX which possesses low band gap, lowlying energy levels, high crystallinity, and good chargetransporting properties. The PSCs based on PFBTBOX achieved PCEs up to 10.20 and 8.11% using nonfullerene and fullerene acceptors, respectively. In addition, the nonfullerene device for PFBTBOX yielded a low energy loss of 0.55 eV, which is less than the empirical threshold of 0.6 eV for conventional PSCs. The results studied in this work suggest that PFBTBOX is a promising candidate for practical electron donors of high-efficiency PSCs.

Figure 1. (a) Molecular structure, (b) absorption spectra, and (c) energy level alignment for the materials studied in this work.

onset at 720 nm, which corresponds to an optical band gap (Eg) of 1.72 eV. Two absorption peaks of the PFBTBOX thin film were observed at 608 and 650 nm, and the intense vibronic shoulder indicates a certain degree of intermolecular packing of the polymer chains in the thin-film state. The absorption spectrum of PFBTBOX is well matched with the absorption range of electron acceptors for complementary photon harvesting in the UV−Vis−NIR region. In particular, the intense absorption in 500−800 nm of ITIC-4F would provide broad absorption band for maximized photocurrent generation. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PFBTBOX were measured by electrochemical cyclic voltammetry as shown in Figure S4a. The HOMO and LUMO energy levels are estimated to be −5.33 and −3.61 eV, respectively, using Fc/Fc + (0.55 eV) as a reference. Thus, the energy levels of PFBTBOX are well-matched to the electron acceptors (ITIC-4F and PC71BM) used in this work with the HOMO and LUMO offsets of 0.33 and 0.50 eV, respectively, for ITIC4F, and 0.77 and 0.59 eV, respectively, for PC71BM, which will be enough driving force for exciton dissociation (Figure 1c). The electronic structure of PFBTBOX was further investigated by calculating the molecular orbital distributions at the HOMO and LUMO levels using density functional theory. As shown in Figure S4b, the HOMO electrons are mainly localized on bithiophene units, whereas the LUMO electrons are delocalized along the polymer backbone, indicating a wellformed quinoidal structure (strong orbital hybridization) of PFBTBOX upon photoexcitation.26 This would be beneficial for efficient charge generation and transport. In addition, the calculated values for HOMO and LUMO were well-matched to the CV results. The photovoltaic property of PFBTBOX was investigated coupled with ITIC-4F in the inverted device structure of indium tin oxide (ITO)/ZnO/PFBTBOX:ITIC-4F/MoO3/ Ag. The cross-sectional SEM image for the representative device is shown in Figure 2a. The device performance was varied using different D−A weight ratios, different thicknesses of the photoactive layer, and different solvents for photoactive

2. RESULTS AND DISCUSSION The molecular structure of materials studied in this work is displayed in Figure 1a. PFBTBOX is an alternating structure of the D−A copolymer using FBT and BOX as D and A units, respectively. FBT is a strong electron-rich unit with fluorine substitution, which has been demonstrated as a promising D unit to lower the energy levels of the conjugated polymers.23,24 BOX can be prepared in a few step with a high yield.25 The polymer was synthesized by the Stille-coupling polymerization reaction as displayed in Figure S1 and characterized by 1H nuclear magnetic resonance (NMR) as shown in Figure S2. The monomers for PFBTBOX were readily available from commercial sources or can be prepared in a few steps. PFBTBOX was soluble in organic solvents such as toluene, chloroform, and chlorobenzene (CB), and the number-average molecular weight (Mn) and polydispersity index were 31.1 kDa and 2.02, respectively. Thermogravimetric analysis (TGA) measurement was employed to evaluate the thermal stability of the polymer (Figure S3). The decomposition temperature (Td) was observed to be 390 °C (5% weight loss), which meets the requirements of thermal stability for photovoltaic applications. The UV−vis absorption spectra of the materials were compared as shown in Figure 1b. PFBTBOX possesses intense and broad absorption in 500−700 nm with an absorption B

DOI: 10.1021/acsami.8b20276 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

showed a maximum VOC up to 1.04 V which is responsible for the low energy loss (Eloss = Eg − e·VOC) of 0.51 eV, with regard to Eg’s of 1.72 and 1.55 eV for PFBTBOX and ITIC-4F, respectively.28 The low Eloss indicates the limited charge trap sites and suppressed nonradiative charge recombination in the photoactive layer.29 This issue will be further addressed below. The incident photon-to-current conversion efficiency (IPCE) spectra of the devices are shown in Figure 2c. According to the IPCE spectrum, the integrated Jsc of PFBTBOX:ITIC-4Fbased devices with different conditions were in reasonable agreement to the measured Jsc (Table 1). The devices cast from CB:DIO followed by TA exhibited a maximum conversion efficiency >70% at 700 nm, indicating the efficacy of TA for the improvement of photon-to-current conversion response rather than SA. The photon-to-electron conversion efficiencies for the optimal condition of PFBTBOX:ITIC-4Fbased devices were almost constant in 400 to 800 nm, which means that PFBTBOX and ITIC-4F are a promising combination for broadband photon harvesting with a complementary photon absorption in the UV−vis−NIR region. The distribution of the PCEs for the optimized PSCs based on PFBTBOX:ITIC-4F is shown in Figure 2d. To gain insights of different processing conditions of the photoactive layer on the device performance, phase separation morphologies of the blend films of PFBTBOX:ITIC-4F were studied by transmission electronic microscopy (TEM). As shown in Figure 3a, the blend film cast from CB exhibited a

Figure 2. (a) Cross-sectional image, (b) J−V curves, (c) IPCE spectra, and (d) PCE distributions for the devices based on PFBTBOX:ITIC-4F in different conditions.

layer coating, as outlined in Table S1 (Supporting Information). Further optimization of photovoltaic performance was conducted by adding solvent additives, followed by thermal and solvent treatment, as listed in Table 1, and their current density−voltage (J−V) curves are shown in Figure 2b. The blend film of PFBTBOX:ITIC-4F with a 1:1.2 weight ratio exhibited a maximum PCE of 5.48%, coupled with a VOC of 0.98 V, a JSC of 9.95 mA·cm−2, and an FF of 0.56 from CB solution. It is well-known that a small amount of solvent additives promoted the PCEs of the PSCs in both fullerene and nonfullerene-based devices.27 The addition of 1 vol % of 1,8diiodooctane (DIO) showed a marked enhancement of PCE to 8.99% with a VOC of 1.04 V, a JSC of 14.37 mA·cm−2, and an FF of 0.62. The device performance was further optimized by additional thermal or solvent treatment of the photoactive layer. Details of conditions for thermal annealing (TA) and the solvent annealing (SA) were described in Experimental Section. As TA was applied to the photoactive layer, maximum PCE of the PSCs reached to 10.20% with a VOC of 1.04 V, a JSC of 15.35 mA·cm−2, and an FF of 0.65, while SA resulted in decreased PCE of 8.48%. The PCE improvement upon the addition of DIO mainly lies on increased JSC and FF by 44 and 11%, respectively, and the subsequent TA further enhanced the JSC and FF by 7 and 5%, respectively. It is interesting that the posttreatments also led to the enhancement of FF to 0.64, which are far higher value of the pristine blend film without the additional treatments. In addition, the optimum devices

Figure 3. TEM images for the blend films of PFBTBOX:ITIC-4F from (a) CB, (b) CB:DIO, (c) CB:DIO (TA), and (d) CB:DIO (SA).

smooth and uniform surface without discernible phase separation. When DIO was used as an additive, the blend

Table 1. Photovoltaic Parameters of PSCs Based on PFBTBOX and ITIC-4F as a Donor and an Acceptor, Respectively solvent

treatment

CB CB:DIOc

N/A N/A TAd SAd

VOCa [V] 0.97 1.02 1.01 0.97

± ± ± ±

0.01 (0.98) 0.02 (1.04) 0.02 (1.04) 0.3 (0.99)

JSCa [mA·cm−2] 9.65 ± 0.41 (9.95) 14.20 ± 0.26 (14.37) 15.11 ± 0.28 (15.35) 12.76 ± 0.75 (13.44)

FFa 0.54 0.61 0.63 0.62

± ± ± ±

0.02 0.02 0.02 0.03

PCEa [%] (0.56) (0.62) (0.65) (0.64)

5.28 8.65 9.95 8.08

± ± ± ±

0.36 0.31 0.24 0.44

(5.48) (8.99) (10.20) (8.45)

JSCb [mA·cm−2]

Jph/Jsat [%]

9.68 15.26 15.74 13.92

89.5 93.4 96.5 92.4

a The values in parenthesis are the best values. bIntegrated JSC values from IPCE measurements. c1 vol % DIO was added in CB solution. dTA and SA denote thermal annealing and solvent annealing, respectively.

C

DOI: 10.1021/acsami.8b20276 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces film exhibited phase-separated feature with a bicontinuous film morphology. After TA, the phase separation interpenetrated network became stronger. The degree of phase separation as a different solvent mixture with TA or SA was also confirmed by atomic force microscopy (AFM), as seen in Figure S5. These results indicate that TA is effective to develop the optimal phase separation with suitable domain size in the blend film of PFBTBOX and ITIC-4F, and it would be beneficial for effective exciton dissociation and charge transport. These excellent morphology features contribute to the high PCEs for the PSCs based on PFBTBOX and ITIC-4F. In order to correlate the influence of morphology variation of the photoactive layer on the device performance, the efficiency of exciton dissociation and charge collection of the devices were studied. Figure 4a compares the photocurrent

Figure 4. (a) Jph−Veff graphs and (b) JSC dependence on light intensity of the devices based on PFBTBOX:ITIC-4F in different conditions. Figure 5. (a) J−V curves and (b) IPCE spectra, (c−f) device parameter comparisons for PFBTBOX-based devices using ITIC-4F and PC71BM acceptors.

density (Jph, defined as Jph = JL − JD) of the devices versus the effective voltage (Veff, defined as Veff = V0 − V) on a double logarithmic scale, where JL and JD are the current density under AM 1.5G 100 mW·cm−2 illumination and in dark, respectively, and V0 is the voltage when Jph is zero and V is the applied voltage, respectively.30 The saturated current density (Jsat) mainly depends on the absorbed incident photons at high Veff as all the photogenerated excitons are dissociated into free charge carriers and collected by corresponding electrodes. Therefore, charge extraction could be characterized by the ratio of Jph/Jsat.31 Under short-circuit condition, the Jph/Jsat of the devices were 89.5, 93.4, 96.5, and 92.4 for the blend films of PFBTBOX:ITIC-4F cast from CB, CB:DIO, CB:DIO (TA), and CB:DIO (SA), respectively. This result clearly accounts for the efficacy of TA for superior charge dissociation and efficient charge collection for the devices. The charge recombination behavior of PFBTBOX:ITIC-4F-based PSCs was further investigated by measuring the JSC variation under different light densities (P). If the photogenerated charge carriers in the photoactive layer could be swept out and thus collected at corresponding electrodes prior to recombination, JSC is proportional to Pα, where α is close to unity as no charge recombination happens.32 Figure 4b shows the linear relation of JSC of the devices on different P on a double logarithmic scale. The α values of the devices CB, CB:DIO, CB:DIO (TA), and CB:DIO (SA) were 0.93, 0.97, 0.98, and 0.97, respectively, indicating very weak bimolecular recombination and effective carrier extraction in these devices, especially for the devices with additional thermal treatment. We then compared the photovoltaic performance of the nonfullerene devices of PFBTBOX to that of the fullerenebased devices. As shown in Figure 5a, PFBTBOX with a

PC71BM exhibited a maximum PCE of 8.11% with a VOC of 0.96 V, a JSC of 13.46 mA·cm−2, and an FF of 0.63, and the PCE distribution for 14 devices is shown in Figure S6. When the IPCE was measured for both devices, the fullerene-based device showed limited photon-to-current conversion range less than 800 nm, which is responsible for much narrower absorption range of PC71BM than ITIC-4F (Figure 5b). The device parameter comparison is also shown in Figure 5c−f. It is clearly shown that VOC, JSC, and FF are inferior in fullerenebased devices than in nonfullerene devices. The inferior VOC for the device using PC71BM acceptor is mainly originated from the low-lying LUMO level as compared to that of ITIC4F. The corresponding Eloss for the fullerene-based device was 0.76 eV, which indicates that there occurs larger energy loss in the PC71BM-based device as compared to that in ITIC-4Fbased devices. In order to further study the relatively lower JSC and FF for the fullerene-based devices, the charge transport behaviors of the photovoltaic layer with different acceptors were studied by using the space charge-limited current method. As shown in Figure 6, the hole and electron mobilities were 4.22 × 10−4 and 2.86 × 10−4 cm2·V−1·s−1, respectively, for PFBTBOX:ITIC-4F, whereas the hole and electron mobilities were 1.82 × 10−4 and 5.61 × 10−5 cm2·V−1·s−1, respectively, for PFBTBOX:PC71BM-based devices. The higher hole and electron mobilities than PC71BM-based blend films suggest that the blend film with ITIC-4F possesses better charge transport, which contributes to enhanced Jsc, FF, and overall PCE of PSCs. In addition, it should be noted here that D

DOI: 10.1021/acsami.8b20276 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

The optical absorption spectra were obtained by an UV−vis spectrophotometer (Cary100, Agilent). Cyclic voltammetry was conducted on a potentiostat/galvanostat (VMP 3, Biologic) in (CH3CH2CH2CH2)4N(PF6) (0.1 M) as the supporting electrolyte in acetonitrile with a Pt working electrode, a platinum-wire counter electrode, and a Ag/AgCl reference electrode. The thin film of PFBTBOX was cast on an ITO-coated substrate for fabrication of a working electrode. The film morphologies were observed by TEM (JEM1010, JEOL) operating in 80 kV of acceleration voltage by floating the blend film onto the Cu grid after immersing the device in deionized water. Grazing incidence wide-angle X-ray scattering (GIWAXS) spectra were obtained at the Advanced Light Source (ALS) in the Lawrence Berkeley National Laboratory (LBNL). 4.2. Device Fabrication and Test. Solar cell devices were fabricated with an inverted configuration of ITO/ZnO/PFBTBOX:acceptors/MoO3/Ag. Onto the precleaned ITO substrate, ZnO precursor solution (zinc acetate dihydrate in 2-methoxyethanol and ethanolamine) was coated and then annealed at 200 °C for 1 h to yield a thickness of ∼30 nm. The active layers were then spin-coated from the blend solution of PFBTBOX and ITIC-4F (2 wt %) at 1000 rpm for 120 s to form an ∼100 nm thick film. In order to control the solvent evaporation rate, the wet film was further kept in a closed jar for 2 h before thermal evaporation of MoO3 (8 nm) and Ag (150 nm) under vacuum (400 nm Thick Active Layer. ACS Appl. Mater. Interfaces 2015, 7, 13666−13674. (30) Yoo, S.; Domercq, B.; Kippelen, B. Efficient thin-film organic solar cells based on pentacene/C60 heterojunctions. Appl. Phys. Lett. 2004, 85, 5427−5429. (31) Jung, J. W.; Russell, T. P.; Jo, W. H. A Small Molecule Composed of Dithienopyran and Diketopyrrolopyrrole as Versatile Electron Donor Compatible with Both Fullerene and Nonfullerene Electron Acceptors for High Performance Organic Solar Cells. Chem. Mater. 2015, 27, 4865−4870. (32) Jung, J. W.; Jo, W. H. Low-Bandgap Small Molecules as NonFullerene Electron Acceptors Composed of Benzothiadiazole and Diketopyrrolopyrrole for All Organic Solar Cells. Chem. Mater. 2015, 27, 6038−6043. (33) Jung, J. W.; Liu, F.; Russell, T. P.; Jo, W. H. Semi-crystalline random conjugated copolymers with panchromatic absorption for highly efficient polymer solar cells. Energy Environ. Sci. 2013, 6, 3301−3307.

the Ministry of Science, ICT & Future Planning (grant number: 2017R1C1B2009691).

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DOI: 10.1021/acsami.8b20276 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX