Breaking 10% Efficiency in Semitransparent Solar Cells with Fused

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Breaking 10% Efficiency in Semitransparent Solar Cells with Fused-Undecacyclic Electron Acceptor Boyu Jia, Shuixing Dai, Zhifan Ke, Cenqi Yan, Wei Ma, and Xiaowei Zhan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04251 • Publication Date (Web): 10 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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Chemistry of Materials

Breaking 10% Efficiency in Semitransparent Solar Cells with Fused-Undecacyclic 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

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, fused-heptacyclic ITIC4, IUIC with a larger π-conjugation and a stronger electron-donating core exhibits a higher LUMO level (IUIC: −3. 87 eV vs. ITIC4: –3.97 eV), 82 nm-redshifted 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 STOSCs 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%).

INTRODUCTION Organic solar cells (OSCs) have attracted much interest due to 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 bulkheterojunctions (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 acceptor-donor-acceptor (A-D-A) structure based on fused aromatic cores with strong electron-accepting 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 absorption in near-infrared (NIR) region (generally absorption edge < 800 nm with couple exceptions > 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-11ring core IU, coupled with strong electron-accepting 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 p-type 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 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 non-covalent 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

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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: PTB7-Th54 (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 Supporting Information). IUIC exhibits excellent thermal stability with decomposition temperature (5% weight loss) of 343 oC 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 redshifted 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 redshifted absorption spectra with a maximum of 730 nm 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 πconjugation 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 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 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, while the optimized as-cast 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, redshifted and stronger absorption and higher electron mobility of IUIC. When the thickness of the Ag electrode decreases from 90 nm to 15 nm, the STOSCs 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), while 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 ST-OSC 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/MoO3/Au(1 nm)/Ag(15 nm) is 88% and 66%, respectively, while the AVT of glass/ITO/ZnO/PTB7-Th:IUIC/MoO3/Au(1 nm)/Ag(15 nm) and glass/ITO/ZnO/PTB7-Th: ITIC4/MoO3/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 (< 20%). The Comission Internationale de I’Eclairage (CIE) 1931 color coordinate, correlated color temperature (CCT), and color rendering index (CRI) are calculated to evaluate the color perception and color rendering property of the STOSCs.57 ST-OSCs based on IUIC and ITIC4 have CIE 1931 color coordinates of (0.2331, 0.2870) and (0.2649, 0.3204) (Figure S5), CCT of 16753 K and 10918 K and CRI of 75 and 85, respectively. The external quantum efficiency (EQE) spectra of the optimized regular devices exhibit broad photoresponse with the maxima of 80% for PTB7-Th:IUIC in 300-900 nm and 78% for PTB7-Th:ITIC4 in 300-800 nm, while the EQE maxima of the ST-OSCs decrease to 71% for PTB7-Th:IUIC and 62% for PTB7-Th:ITIC4 (Figure 2b). To gain insight into charge generation and extraction properties, the relationship of the photocurrent density (Jph)


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and the effective voltage (Veff) was measured (Figure 2c). At high applied voltage (Veff > 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 PTB7Th:IUIC and 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 PTB7Th:IUIC and PTB7-Th: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 favourable for higher JSC, while 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 oC 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 Xray 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), while 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 PTB7Th: IUIC and PTB7-Th:ITIC4 is calculated to be ~19 nm and ~25 nm, respectively. Due to 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 PTB7Th:ITIC4.

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 πconjugation with a stronger electron-donating ability. Thus, IUIC exhibits higher energy levels, redshifted 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 as-cast 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 as-cast ST-OSCs based on PTB7-Th:IUIC exhibit a champion PCE of 10.2%, than that of the control devices based on PTB7-Th:ITIC4 (6.42%). The 10.2% PCE is a new record for any ST-OSCs. These results demonstrate the great potential of the fused-11ring unit IU for constructing NIR-absorbing nonfullerene acceptors used for high-performance ST-OSCs.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. 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.

AUTHOR INFORMATION



Corresponding Author *E-mail: [email protected] (X. Zhan). *E-mail: [email protected] (W. Ma).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT X.Z. wish to thank the support from NSFC (91433114 and 21734001). W.M. thanks the support from Ministry of Science and Technology (2016YFA0200700), NSFC (21504066, 21534003). X-ray data was 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 No. DE-AC02-05CH11231. The authors thank Chenhui Zhu at beamline 7.3.3, and Cheng Wang at beamline 11.0.1.2 for assistance with data acquisition

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1.0 0.8

(b) IUIC-soln IUIC-film ITIC4-soln ITIC4-film

0/+

FeCp2

0.04 mA

Current

Normalized absorbance (a.u.)

(a)

0.6 0.4

ITIC4

0.2 0.0 500

IUIC

600

700

800

900

-0.5

0.0

0.5

1.0

1.5

2.0

Potential (V)

Wavelength (nm)

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. (b)

0 -5

80 PTB7-Th:ITIC4 PTB7-Th:IUIC PTB7-Th:ITIC4 (ST-OSC) PTB7-Th:IUIC (ST-OSC)

60

EQE (%)

-2

Current Density (mA cm )

(a)

-10 -15

40

20

-20 -25 0.00

ITIC4 IUIC ITIC4 (ST-OSC) IUIC (ST-OSC)

0.15

0.30

0.45

0.60

0 300

0.75

Voltage (V)

(c)

(d)

10

10

-2

1

400

500

600

700

800

900

Wavelength (nm)

J SC (mA cm )

Jph (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 9

PTB7-Th:IUIC PTB7-Th:ITIC4

PTB7-Th:IUIC PTB7-Th:ITIC4

1 0.1 0.1

Veff (V)

1

10

100

Light Intensity (mW cm-2)

Figure 2. (a) The J-V curves and (b) EQE spectra of optimized OSC and ST-OSC as-cast devices based on PTB7-Th:ITIC4 and PTB7Th: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.


Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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

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



Page 8 of 9

Scheme 1. Chemical structures of IUIC, ITIC4 and PTB7-Th and the synthetic route of IUIC.

Table 1. Basic properties of IUIC and ITIC4. Compound

Td

λmax (nm)

(oC)

Solution

IUIC

343

ITIC4

332

Eg

ε

HOMO

LUMO

µe

Film

(eV)

(M−1 cm−1)

(eV)

(eV)

(cm2 V−1 s−1)

772

788

1.41

3.2 × 105

-5.45

-3.87

1.1 × 10−3

690

730

1.52

1.9 × 105

-5.77

-3.97

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). Acceptor

VOC

JSC

(V)a

(mA cm-2) a

IUIC

0.792±0.004 (0.796)

21.51±0.33 (21.74)

IUICb

0.789±0.004 (0.794)

ITIC4 ITIC4b

FF a

PCE

Calc JSC

(%)a

(mA cm-2)

0.647±0.008 (0.649)

11.0±0.2 (11.2)

20.69

18.12±0.23 (18.31)

0.690±0.009 (0.703)

9.91±0.10 (10.2)

18.26

0.685±0.003 (0.687)

16.47±0.31 (16.66)

0.705±0.006 (0.715)

7.91±0.20 (8.18)

16.84

0.682±0.003 (0.680)

13.05±0.22 (13.20)

0.711±0.004 (0.716)

6.31±0.15 (6.42)

13.47

a)

Average values with standard deviation were obtained from 20 devices, the values in parentheses are the parameters of the best device. b)

ST-OSC devices.


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