Breaking 10% Efficiency in Semitransparent Solar Cells with Fused

Dec 10, 2017 - A fused-undecacyclic electron acceptor IUIC has been designed, synthesized and applied in organic solar cells (OSCs) and semitransparen...
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Cite This: Chem. Mater. 2018, 30, 239−245

Breaking 10% Efficiency in Semitransparent Solar Cells with FusedUndecacyclic Electron Acceptor Boyu Jia,† Shuixing Dai,† Zhifan Ke,‡ Cenqi Yan,† Wei Ma,*,‡ and Xiaowei Zhan*,† †

Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China ‡ State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China S Supporting Information *

ABSTRACT: A fused-undecacyclic electron acceptor IUIC has been designed, synthesized and applied in organic solar cells (OSCs) and semitransparent organic solar cells (ST-OSCs). In comparison with its counterpart, fusedheptacyclic ITIC4, IUIC with a larger π-conjugation and a stronger electrondonating core exhibits a higher LUMO level (IUIC: −3. 87 eV vs ITIC4: −3.97 eV), 82 nm red-shifted absorption with larger extinction coefficient and smaller optical bandgap, and higher electron mobility. Thus, IUIC-based OSCs show higher values in open-circuit voltage, short-circuit current density, and thereby much higher power conversion efficiency (PCE) than those of the ITIC4-based counterpart. The as-cast OSCs based on PTB7-Th: IUIC without any extra treatment yield PCEs of up to 11.2%, higher than that of the control devices based on PTB7-Th: ITIC4 (8.18%). The as-cast ST-OSCs based on PTB7-Th: IUIC without any extra treatment afford PCEs of up to 10.2% with an average visible transmittance (AVT) of 31%, higher than those of the control devices based on PTB7-Th: ITIC4 (PCE = 6.42%, AVT = 28%).

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INTRODUCTION

absorption in near-infrared (NIR) region (generally absorption edge 900 nm).36−38 Semitransparent OSCs (ST-OSCs) have great potential for building integrated photovoltaic application and powergenerating windows.39 Most of ST-OSCs are based on polymer donors and fullerene acceptors, and exhibit relatively low PCEs, due to weak absorption of fullerene acceptors; the best PCEs reported in literature are 4−6% for single-junction and 7−8% for tandem fullerene-based devices.39−52 There have been only couple examples of nonfullerene ST-OSCs, in which active layer consisted of a narrow-bandgap polymer donor PTB7-Th and a NIR-absorbing FREA; the PCEs are 7.74%38 and 9.77%,36 respectively. In this work, we designed and synthesized a new fusedundecacyclic electron acceptor, IUIC, based on a fused-11-ring core IU, coupled with strong electron-withdrawing 2-(5,6difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile (2FIC) unit (Scheme 1). IUIC is the largest FREA and exhibits strong NIR-absorbing property. We chose IU because it possesses highly planarity, large π-conjugation and strong electron-donating ability, and has been used for constructing ptype polymer semiconductors that have exhibited promising performance in organic field-effect transistors (hole mobility as high as 0.024 cm2 V−1 s−1) and as donors in OSCs (PCE as

Organic solar cells (OSCs) have attracted much interest because of their merits, such as low cost, light weight, flexibilility, and semitransparency.1−5 Traditional OSCs are based on blends of donor materials and fullerene acceptors that form bulk heterojunctions (BHJs) in devices. However, the development of this field has recently shifted to organic nonfullerene acceptors. Fullerene derivatives suffer from some drawbacks, such as limited energy level tunability, weak absorption in the visible region, and morphology instability, which constrain the further development of OSCs.6 In contrast to the widely used fullerene acceptors, the optical properties and energy levels of nonfullerene acceptors can be easily adjusted.7−14 In 2015, we reported the first fused-ring electron acceptors (FREAs) with an acceptor−donor−acceptor (A−D−A) structure based on fused aromatic cores with strong electronwithdrawing end groups, exemplified by ITIC15 and IEIC.16 Relative to fullerene acceptors, A−D−A type FREAs exhibit much stronger absorption in the visible region. FREA-based OSCs can achieve higher power conversion efficiencies (PCEs), greater thermal and photochemical stability and longer device lifetime than their fullerene-based counterparts.17−32 To date, most of FREAs reported in literature are based on fused-5-ring to fused 10-ring cores.33−35 Most of these cores have relatively weak electron-donating property, leading to limited intramolecular charge transfer (ICT) and therefore limited © 2017 American Chemical Society

Received: October 9, 2017 Revised: December 4, 2017 Published: December 10, 2017 239

DOI: 10.1021/acs.chemmater.7b04251 Chem. Mater. 2018, 30, 239−245

Article

Chemistry of Materials Scheme 1. Chemical Structures of IUIC, ITIC4, and PTB7-Th and the Synthetic Route of IUIC

Figure 1. (a) Absorption spectra of IUIC and ITIC4 in chloroform solution and thin film; (b) cyclic voltammograms for IUIC and ITIC4 in CH3CN/0.1 M [Bu4N]+[PF6]− at 100 mV s−1, the horizontal scale refers to an Ag/AgCl electrode as a reference electrode.

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high as 6.46%).53 Difluorination of the end groups in the case of 2FIC can extend the absorption due to enhanced ICT between IU and 2FIC, and can improve electron mobility due to noncovalent F−S and F−H bonding, as we previously reported.27 For comparison, we also synthesized a fused heptacyclic electron acceptor ITIC4 (Scheme 1) with a fused7-ring core.18 Relative to ITIC4 with a smaller core, IUIC with a larger core exhibits (a) higher energy levels, (b) red-shifted absorption spectra with larger extinction coefficient, and (c) higher electron mobility, which are beneficial to (a) increasing open-circuit voltage (VOC), and (b) short-circuit current density (JSC). Indeed, as-cast OSCs based on IUIC: PTB7Th54 (Scheme 1) without any additional treatment yield PCEs of up to 11.2%, which is much higher than that of the control devices based on ITIC4: PTB7-Th (8.18%). Furthermore, the as-cast ST-OSCs based on IUIC: PTB7-Th without any additional treatment yield PCEs of up to 10.2% with an average visible transmittance (AVT) of 31%, which is much higher than that of the control devices based on ITIC4: PTB7Th (PCE = 6.42%, AVT = 28%). This is the first example of ST-OSCs with PCEs breaking 10% (the reported best was 9.77%36).

RESULTS AND DISCUSSION Synthesis and Characterization. IU was lithiated by nbutyllithium and quenched with dry dimethylformamide (DMF) to afford aldehyde IU-CHO. Subsequent Knoevenagel condensation between IU-CHO and 2FIC yielded the final product IUIC (Scheme 1). All new compounds were fully characterized by mass spectrometry, 1H NMR, 13C NMR, 19F NMR, and elemental analysis (see the Supporting Information). IUIC exhibits excellent thermal stability with decomposition temperature (5% weight loss) of 343 °C in nitrogen atmosphere by thermogravimetric analysis (Figure S1). The normalized spectra of optical absorption of IUIC and ITIC4 in chloroform solution (10−6 M) and in solid film are shown in Figure 1a. ITIC4 shows an absorption maximum at 690 nm with an extinction coefficient of 1.9 × 105 M−1 cm−1 in solution, while IUIC shows a red-shifted maximum at 772 nm with a higher extinction coefficient of 3.2 × 105 M−1 cm−1 in solution. Relative to those in solution, ITIC4 and IUIC in thin film exhibit red-shifted absorption spectra with a maximum of 730 and 788 nm, respectively. The optical bandgap of IUIC estimated from the absorption edge of the thin film is 1.41 eV, narrower than that for ITIC4 (1.52 eV). The larger π240

DOI: 10.1021/acs.chemmater.7b04251 Chem. Mater. 2018, 30, 239−245

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Chemistry of Materials Table 1. Basic Properties of IUIC and ITIC4 λmax (nm) compd

Td (°C)

solution

film

Eg (eV)

ε (M−1 cm−1)

HOMO (eV)

LUMO (eV)

μe (cm2 V−1 s−1)

IUIC ITIC4

343 332

772 690

788 730

1.41 1.52

3.2 × 105 1.9 × 105

−5.45 −5.77

−3.87 −3.97

1.1 × 10−3 8.9 × 10−4

Table 2. Performance of the optimized OSC and ST-OSC as-cast devices based on PTB7-Th:acceptor (1:1.5, w/w) VOC (V)a

acceptor IUIC IUICb ITIC4 ITIC4b

0.792 0.789 0.685 0.682

± ± ± ±

0.004 0.004 0.003 0.003

(0.796) (0.794) (0.687) (0.680)

JSC (mA cm−2)a 21.51 18.12 16.47 13.05

± ± ± ±

0.33 0.23 0.31 0.22

(21.74) (18.31) (16.66) (13.20)

FFa 0.647 0.690 0.705 0.711

± ± ± ±

0.008 0.009 0.006 0.004

PCE (%)a (0.649) (0.703) (0.715) (0.716)

11.0 9.91 7.91 6.31

± ± ± ±

0.2 (11.2) 0.10 (10.2) 0.20 (8.18) 0.15 (6.42)

Calc JSC (mA cm−2) 20.69 18.26 16.84 13.47

a Average values with standard deviation were obtained from 20 devices, the values in parentheses are the parameters of the best device. bST-OSC devices.

Figure 2. (a) J−V curves and (b) EQE spectra of optimized OSC and ST-OSC as-cast devices based on PTB7-Th:ITIC4 and PTB7-Th:IUIC under the illumination of AM 1.5G, 100 mW cm−2. (c) Jph versus Veff characteristics and (d) JSC versus light intensity of optimized as-cast devices based on PTB7-Th:ITIC4 and PTB7-Th:IUIC.

eV, LUMO = −3.97 eV) (Table 1), due to the larger πconjugation and stronger electron-donating ability of IU core in IUIC. The electron mobilities of IUIC and ITIC4 were measured using the space charge limited current (SCLC) method in electron-only devices with a structure of Al/IUIC or ITIC4/Al (Figure S2). The electron mobility of IUIC is 1.1 × 10−3 cm2 V−1 s−1, higher than that of ITIC4 (8.9 × 10−4 cm2 V−1 s−1) (Table 1). Photovoltaic Properties. To demonstrate potential application of IUIC in OSCs, we chose PTB7-Th as a donor based on the following considerations. (a) The widely used narrow-bandgap polymer PTB7-Th exhibits strong absorption

conjugation and stronger electron-donating ability of IU core in IUIC is responsible for its red-shifted absorption and narrower bandgap. Cyclic voltammetry was employed to investigate the electrochemical properties of ITIC4 and IUIC (Figure 1b). IUIC and ITIC4 exhibit irreversible reduction and oxidation waves. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are estimated from the onset oxidation and reduction potentials, respectively, versus ferrocenium/ferrocene (FeCp2+/0), assuming the absolute energy level of FeCp2+/0 is 4.8 eV below vacuum. IUIC shows higher HOMO (−5.45 eV) and LUMO (−3.87 eV) energy levels relative to ITIC4 (HOMO = −5.77 241

DOI: 10.1021/acs.chemmater.7b04251 Chem. Mater. 2018, 30, 239−245

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Chemistry of Materials

PTB7-Th:ITIC4 are over 95%, indicating excellent charge extraction. The charge carrier recombination in the active layers of the optimized devices was investigated by measuring JSC under different light intensities (Plight) (Figure 2d). The correlation between JSC and Plight is expressed by JSC ∝ Plightα, where α value close to 1 indicates negligible bimolecular recombination in the devices. The values of α in PTB7-Th:IUIC and PTB7Th:ITIC4 blends are 0.97 and 0.99, respectively, which suggests very weak bimolecular recombination. The charge transport properties of active layers were investigated by SCLC method with the structure of PEDOT:PSS/active layer/Au for hole mobility and Al/active layer/Al for electron mobility (Figure S6). The blend films based on PTB7-Th:IUIC exhibit a higher hole mobility (μh) of 7.7 × 10−4 cm2 V−1 s−1 and a higher electron mobility (μe) of 5.5 × 10−4 cm2 V−1 s−1 relitive to the PTB7-Th:ITIC4 blends respectively, which is favorable for higher JSC, whereas the PTB7-Th:ITIC4 blends exhibit μh of 4.9 × 10−4 cm2 V−1 s−1 and μe of 4.2 × 10−4 cm2 V−1 s−1 (Table 2). We investigated the preliminary light and thermal stability of the OSC devices based on IUIC and ITIC4. Under continuous illumination with AM 1.5G simulator at 100 mW cm−2 for 180 min, the PCEs of devices based on IUIC and ITIC4 retained ca. 75 and 62% of their original values, respectively (Figure S7a). Under heating at 100 °C for 180 min, the PCEs of devices based on IUIC and ITIC4 retained ca. 57 and 36% of their original values, respectively (Figure S7b). Morphology Characterization. The morphology of the active layers of PTB7-Th:IUIC and PTB7-Th:ITIC4 was studied by atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS) and resonant soft X-ray scattering (R-SoXS). According to the AFM images, the blended film of PTB7-Th:IUIC is slightly smoother with the root-mean-square roughness (Rq) value of 1.0 nm relative to that of PTB7-Th:ITIC4 (Rq = 1.2 nm) (Figure S8). The molecular packing of PTB7-Th:IUIC and PTB7-Th:ITIC4 blended films was investigated by GIWAXS.58 Based on the informaiton of peaks provided in the neat films (Figure S9), the crystallinity of both donor and acceptor in the blended films is obtained. In the PTB7-Th:IUIC blended film, IUIC displays weak (100) diffraction peak (q ≈ 0.35 Å-1) and (010) π−π stacking peak (q ≈ 1.42 Å−1), whereas PTB7-Th exhibits much stronger crystallinity with sharper (100) diffraction peak (q ≈ 0.26 Å−1) and (010) π−π stacking peak (q ≈ 1.60 Å−1). ITIC4 hardly crystallizes in the blended films as it does not show visible diffracttion peaks in the scattering profiles (Figure 3). Both (100) scattering peak (q ≈ 0.28 Å−1) and (010) π−π stacking peak (q ≈ 1.65 Å−1) in the PTB7-Th:ITIC4 blended film belong to PTB7-Th. The location of PTB7-Th π−π stacking peak shifts to higher q, indicating that the molecular packing of PTB7-Th is more compact when blended with ITIC4. By calculating via the Scherrer equation,59 PTB7-Th exhibits similar π−π stacking coherence length of ∼1.2 nm in PTB7-Th:IUIC and PTB7-Th:ITIC4 blended films. R-SoXS is further utilized to characterize the mode length (domain size) and average composition variation.60,61 Figure 4 shows the R-SoXS profiles of blend films of PTB7-Th:IUIC and PTB7-Th:ITIC4. To obtain enhanced polymer/small molecule contrast and avoid too high absorption, the energy of 285.2 eV is selected. The domain size is half of characteristic mode length (domain spacing, ξ). The domain size of PTB7-Th: IUIC and PTB7-Th:ITIC4 is calculated to be ∼19 nm and ∼25

in 550−750 nm, complemented the absorption of IUIC (Figure S3). (b) The energy levels of PTB7-Th (HOMO = −5.20 eV, LUMO = −3.59 eV) match well with those of IUIC, which is favorable for efficient exciton dissociation. (c) PTB7-Th exhibited a good hole mobility of 2.8 × 10−3 cm2 V−1 s−1 measured by SCLC method,55 which is similar to the electron mobility of IUIC, ensuring balanced charge transport in the active layer. Thus, we fabricated regular devices with a structure of ITO/ZnO/PTB7-Th:IUIC/MoOx/Ag(90 nm) and semitransparent devices with a structure of ITO/ZnO/PTB7Th:IUIC/MoOx/Au(1 nm)/Ag(15 nm), and compared with the control devices based on PTB7-Th: ITIC4. The ultrathin Au (1 nm) forms dense nucleation centers, which reduce percolation of Ag film and enhance the Ag film uniformity, leading to optimal transmittance and low electrical resistance.56 We optimized the donor: acceptor weight ratio (D/A), and found the best performance was obtained at D/A = 1:1.5 (Table S1). Table 2 summarizes VOC, JSC, fill factor (FF), and PCE of the optimized OSC and ST-OSC devices. The optimized as-cast devices based on PTB7-Th:IUIC exhibit VOC of 0.796 V, JSC of 21.74 mA cm−2, FF of 0.649, and PCE of 11.2% without any extra treatment, whereas the optimized ascast devices based on PTB7-Th:ITIC4 show VOC of 0.687 V, JSC of 16.66 mA cm−2, FF of 0.715, and PCE of 8.18% (Figure 2a). The higher values in VOC and JSC of IUIC-based devices are attributed to the higher LUMO energy level, red-shifted and stronger absorption and higher electron mobility of IUIC. When the thickness of the Ag electrode decreases from 90 to 15 nm, the ST-OSCs based on PTB7-Th:IUIC exhibit VOC of 0.794 V, JSC of 18.31 mA cm−2, FF of 0.703, and PCE of 10.2% (Table 2), whereas the ST-OSCs based on PTB7-Th:ITIC4 exhibit VOC of 0.680 V, JSC of 13.20 mA cm−2, FF of 0.716 and PCE of 6.42%. The transmission spectra of the optimized STOSC devices in the visible region (370−740 nm) were measured to calculate the AVT of the ST-OSCs (Figure S4). The AVT of glass/ITO/ZnO and glass/ITO/ZnO/MoOx/ Au(1 nm)/Ag(15 nm) is 88% and 66%, respectively, while the AVT of glass/ITO/ZnO/PTB7-Th:IUIC/MoOx/Au(1 nm)/ Ag(15 nm) and glass/ITO/ZnO/PTB7-Th: ITIC4/MoOx/ Au(1 nm)/Ag(15 nm) is 31% and 28%, respectively. The transmittance is relatively high at 370−500 nm (>40%), but low after 650 nm ( 2.2 V), Jph reaches saturation, meaning that almost all the photogenerated excitons are dissociated and free charge carriers are completely collected by the electrodes. The ratio of Jph to the saturation photocurrent density (Jsat) of the devices based on PTB7-Th:IUIC and 242

DOI: 10.1021/acs.chemmater.7b04251 Chem. Mater. 2018, 30, 239−245

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Chemistry of Materials

energy levels, red-shifted and stronger absorption in 600−900 nm, narrower bandgap, and higher electron mobility. As a result, blended with the PTB7-Th polymer donor that has matched energy levels and complementary absorption spectrum, the IUIC-based OSCs show higher values in VOC, JSC, and finally much higher PCE than the ITIC4-based OSCs. The ascast OSCs based on PTB7-Th:IUIC without any additional treatment afford PCEs of up to 11.2%, much higher than that of the control devices based on PTB7-Th:ITIC4 (8.18%). The ascast ST-OSCs based on PTB7-Th:IUIC exhibit a champion PCE of 10.2%, than that of the control devices based on PTB7Th:ITIC4 (6.42%). The 10.2% PCE is a new record for any STOSCs. These results demonstrate the great potential of the fused-11-ring unit IU for constructing NIR-absorbing nonfullerene acceptors used for high-performance ST-OSCs.

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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/acs.chemmater.7b04251. Experimental details, TGA curves, SCLC data, absorption spectra and energy levels, the representation of color coordinate of the semitransparent device on CIE 1931 xyY chromaticity diagram, visible transmission spectra, AFM images, GIWAXS of neat films, and the optimization and stability test of the OSC devices (PDF)

Figure 3. (a) 2D GIWAXS patterns and (b) scattering profiles of inplane and out-of-plane for PTB7-Th: IUIC and PTB7-Th: ITIC4 blended films.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected] (W.M.). ORCID

Wei Ma: 0000-0002-7239-2010 Xiaowei Zhan: 0000-0002-1006-3342 Notes

The authors declare no competing financial interest.

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Figure 4. R-SoXS profiles in log scale for PTB7-Th:IUIC and PTB7Th:ITIC4 blended films.

ACKNOWLEDGMENTS X.Z. thanks the NSFC (91433114 and 21734001) for support. W.M. thanks from Ministry of Science and Technology (2016YFA0200700) and NSFC (21504066, 21534003) for the support. X-ray data were acquired at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy, under Contract DE-AC0205CH11231. The authors thank Chenhui Zhu at beamline 7.3.3 and Cheng Wang at beamline 11.0.1.2 for assistance with data acquisition

nm, respectively. Because of the limited exciton diffusion length (ca. 10−20 nm), the appropriate domain size increases interfacial area between donor and acceptor, which facilitates exciton dissociation and reduces geminate recombination. Additionally, via integrating of total scattering intensity (TSI), R-SoXS can probe the average composition variation (relative domain purity). The higher the total scattering intensity (integration of the scattering profiles over q), the purer the average domain. The relative domain purity of PTB7-Th:IUIC and PTB7-Th:ITIC4 is calculated to be 1 and 0.87, respectively. The higher domain purity would minimize the possibility of bimolecular recombination and promotes charge transport. Thus, the as-cast devices based on PTB7-Th:IUIC exhibit higher EQE, JSC, and PCE than that of PTB7-Th:ITIC4.

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REFERENCES

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CONCLUSION We designed and synthesized an fused-undecacyclic electron acceptor IUIC, which is the largest FREA so far. In comparison with ITIC4, IUIC has a larger extended π-conjugation with a stronger electron-donating ability. Thus, IUIC exhibits higher 243

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