A Wide-Bandgap Donor Polymer for Highly Efficient Non-fullerene

Apr 27, 2017 - Department of Chemistry, Energy Institute and Hong Kong Branch of ... Here we report a highly efficient non-fullerene organic solar cel...
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A Wide-Bandgap Donor Polymer for Highly Efficient Non-fullerene Organic Solar Cells with a Small Voltage Loss Shangshang Chen,† Yuhang Liu,† Lin Zhang,‡ Philip C. Y. Chow,† Zheng Wang,† Guangye Zhang,† Wei Ma,*,‡ and He Yan*,†,§,⊥ †

Department of Chemistry, Energy Institute and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Hong Kong University of Science and Technology, Clear Water Bay Kowloon, Hong Kong ‡ State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China § Hong Kong University of Science and Technology-Shenzhen Research Institute, No. 9 Yuexing First RD, Hi-tech Park, Nanshan Shenzhen 518057, P. R. China ⊥ Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *

are generally considered important by the non-fullerene OSC community: (i) complementary absorptions between donor and acceptor components to enhance light harvesting; (ii) matching energy levels to ensure efficient charge separation and to minimize voltage loss (Vloss, defined as Vloss = Egap/q − Voc, where Egap is the lowest optical bandgap among the donor and acceptor components, and Voc represents open-circuit voltage);7 (iii) balanced charge mobilities to avoid charge accumulation at device interfaces; and (iv) favorable morphology including high crystallinity, face-on orientation, and small domain size. Several cases of highly efficient non-fullerene OSCs have been realized by the selection of a matching donor polymer.5a,6,8 Recently, indacenodithiophene (IDT)-based SMAs, such as IEIC,5g ITIC,5b and O-IDTBR,5e have been demonstrated to yield multiple cases of efficient non-fullerene OSCs, owing to their good solubility and crystallinity, favorable charge transport properties, and strong absorption in the wavelength range of 600−800 nm. To enhance light-harvesting, it is desirable for donor polymers to possess strong absorption in the shortwavelength range, thus forming a complementary absorption to the IDT-based SMAs. Besides complementary absorption, it is also important to match the energy levels between donors and acceptors to minimize the Vloss of the OSCs. For instance, a stateof-the-art SMA OSC based on PBDB-T can achieve a Voc = 0.90 V9 when blended with ITIC, which has an optical bandgap of 1.59 eV. The Vloss in the PBDB-T:ITIC-based OSCs is estimated to be 0.69 V, which is still relatively large compared to other types of photovoltaic technologies, including c-Si, CdTe or perovskite solar cells.10 To further improve the performances of OSCs, one needs to minimize the Vloss, thus harvesting the full potential of photons. In this Communication, we report a highly efficient nonfullerene OSC that can achieve a high PCE = 11.6% and can still maintain a small Vloss = 0.55 V. Despite the low Vloss, the OSCs can achieve a maximum internal quantum efficiency (IQE) over 90% and a high external quantum efficiency (EQE) over 70%, both of which indicate that internal charge separation process is

ABSTRACT: To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) smallmolecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our nonfullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss.

O

rganic solar cells (OSCs) have attracted extensive research interests from both academia and industry due to their advantages of light weight, mechanical flexibility, and compatibility with roll-to-roll printing processes.1 Conventional OSCs are based on the bulk heterojunctions (BHJs) of p- and n-type organic semiconductors that function as electron donor and acceptor, respectively.2 Over the past two decades, fullerene derivatives have been the dominant choice of electron acceptors, and a high power conversion efficiency (PCE) up to 11.7% has been achieved in a fullerene-based device.3 Despite great advances in this field, the fullerene acceptors still suffer from several drawbacks, such as high-production cost, poor chemical and electronic tunability, and weak absorption in the visible wavelength range.4 To address these problems, considerable efforts have been dedicated to the development of non-fullerene alternatives, especially small-molecular acceptors (SMAs).5 To achieve efficient non-fullerene OSCs, it is important not only to design high-performance SMAs, but also to develop matching donor polymers, as the interactions between donor and acceptor components play a crucial role in determining photovoltaic performances of corresponding OSCs.6 Regarding the matching rules between donor polymers and SMAs, the following criteria © XXXX American Chemical Society

Received: February 15, 2017

A

DOI: 10.1021/jacs.7b01606 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

1,2,4-trimethylbenzene (TMB). The UV/vis absorption spectra of PvBDTTAZ, O-IDTBR, and PvBDTTAZ:O-IDTBR films are shown in Figure 1c. The absorption of PvBDTTAZ film mainly lies in the short-wavelength range with a wide optical bandgap of 2.05 eV (determined from the absorption onset of the film, 605 nm). The relatively wide bandgap of PvBDTTAZ enables complementary absorption to O-IDTBR, which has been demonstrated to be an effective strategy to broaden the absorption range, thus facilitating the light harvesting of nonfullerene OSCs.11 More importantly, the PvBDTTAZ polymer exhibits a large bathochromic shift of ∼80 nm from the hot solution to film state (Figure S1). There is also an absorption shoulder appearing at ∼550 nm for the film, indicating strong aggregation and effective stacking of the polymer chains in the solid state. This is the main property difference between PvBDTTAZ and a previously reported BDT-based analogue polymer named J51, which did not show such bathochromic shift.12 The aggregation property of PvBDTTAZ originates from the 2-decyltetradecyl alkyl chains sitting between thiophenes along the polymer backbone, a key structural feature enabling strong aggregation as described before.1c The aggregation behavior of PvBDTTAZ led to well-controlled aggregation of the polymer during the film drying process, resulting in highly crystalline yet small domains, which will be discussed in the morphology part. In contrast, as the orientation of the thiopheneBDT-thiophene unit is rotated by 90° for the J51 polymer, the second position branched alkyl chains are no longer sitting between two thiophenes, which is the reason why the J51 polymer does not exhibit the aggregation property of PvBDTTAZ. To further investigate the backbone conformation of PvBDTTAZ, density functional theory (DFT) calculations at the B3LYP/6-31G* level were performed on a vBDTTAZ trimer. The DFT results show a relatively twisted backbone of vBDTTAZ trimer, and a dihedral angle of 56° between vBDT and nearby thiophenes was observed (Figure S2). The twisted backbone is favorable for downshifting the highest occupied molecular orbital (HOMO) of PvBDTTAZ.6 The energy levels of PvBDTTAZ were investigated by cyclic voltammetry (CV, Figures 1d and S3), and PvBDTTAZ exhibits a slightly higher HOMO than that of O-IDTBR (−5.47 vs −5.51 eV).5e To rule out the large HOMO shift of PvBDTTAZ upon the introduction of O-IDTBR, ultraviolet photoelectron spectroscopy (UPS) was carried out to investigate the HOMO levels of both PvBDTTAZ neat and blend films (Figure S10), and only a small HOMO shift (∼20 meV) was observed. Even through an energy offset of ∼0.3 eV was regarded necessary for charge separation in early reports on organic photovoltaics,13 recent studies have proved that this is not an essential requirement, and efficient charge separation can occur despite an energy offset less than 0.3 eV.6,8c,11 Photoluminescence (PL) measurement was then carried out to investigate the exciton dissociation in the blend film. As presented in Figure S4a, the PL of PvBDTTAZ decreases dramatically upon the introduction of O-IDTBR with a quenching efficiency up to 97%, indicating the highly efficient electron transfer from PvBDTTAZ to O-IDTBR. Most importantly, despite the relatively small HOMO−HOMO offset (40 meV estimated from CV) between two components, the hole transfer from O-IDTBR to PvBDTTAZ is also efficient with a PL quenching efficiency of ∼90% (Figure S4b). The highly efficient electron/hole transfer between PvBDTTAZ and O-IDTBR provides the basis for effective charge separation and current generation in the corresponding OSCs.

Figure 1. Chemical structures of (a) PvBDTTAZ and (b) O-IDTBR. (c) UV/vis absorption spectra of PvBDTTAZ, O-IDTBR, and PvBDTTAZ:O-IDTBR films. (d) Energy level diagrams of PvBDTTAZ and O-IDTBR. The HOMO level was measured by CV, while the LUMO level was calculated based on the HOMO level and the optical bandgap.

Scheme 1. Synthetic Route of PvBDTTAZ

highly efficient for these cells. The high performance is enabled by a novel donor polymer named PvBDTTAZ, which is based on a benzodithiophene (BDT) and a difluorinated triazole unit. However, unlike conventional BDT-based polymers, in which BDT unit is linked into the polymer backbone via the α positions of the thiophene units, the BDT units in our PvBDTTAZ polymer are connected along the polymer backbone via the phenyl group of the BDT unit. The PvBDTTAZ polymer also consists of two flanking thiophenes with second position branched alkyl chains, which is the key structural feature that enabled strong aggregation properties of the donor polymers as demonstrated before.1c With these molecular designs, the PvBDTTAZ polymer paired with O-IDTBR yields a highly favorable BHJ morphology, in which both the donor and acceptor can maintain their crystallinity and form crystalline and small domains. As it has an optical bandgap of 2.05 eV, the PvBDTTAZ polymer is a good match to low-bandgap SMAs by forming complementary absorptions. These factors combined produced highly efficient non-fullerene OSCs with a PCE = 11.6%, which is the highest efficiency reported to date for OSCs with a small Vloss. The chemical structures of PvBDTTAZ and O-IDTBR are shown in Figure 1a,b. The PvBDTTAZ polymer is a typical D−A conjugated donor polymer prepared via Stille-coupling polymerization (Mn = 68.8 kDa, PDI = 2.01); details of its synthesis can be found in Scheme 1 and Supporting Information. The final donor polymer exhibits good solubility in chloroform, toluene or B

DOI: 10.1021/jacs.7b01606 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Figure 2. (a) J−V characteristics under AM 1.5G illumination (100 mW cm−2). (b) EQE spectrum of the PvBDTTAZ:O-IDTBR device.

Table 1. Photovoltaic Performances of the Solar Cells Based on PvBDTTAZ:O-IDTBR (Averages Calculated from 32 Devices) Voc (V)

Jsc (mA cm−2)

FF

PCE (%)

PCEmax (%)

1.08 ± 0.004

16.26 ± 0.18

0.636 ± 0.008

11.2 ± 0.2

11.6

The photovoltaic devices based on PvBDTTAZ:O-IDTBR were fabricated with an inverted structure of glass/ITO/ZnO/ PvBDTTAZ:O-IDTBR (12 mg mL−1:18 mg mL−1 in TMB)/ V2O5/Al, and a current-density−voltage (J−V) curve is plotted in Figure 2a, with the detailed device parameters summarized in Table 1. A high PCE up to 11.6% was achieved, with a high Voc = 1.08 V. The Vloss is estimated to be ∼0.55 V, one of the lowest values achieved for efficient OSCs. Despite the small Vloss, the system can still achieve efficient charge separation between PvBDTTAZ and O-IDTBR, as the PvBDTTAZ:O-IDTBR based devices exhibit a maximum IQE over 90% (Figure S7) and a high EQE (Figure 2b) over 70% in a wide absorption range. Specifically, the high EQE values in the absorption range of OIDTBR (600−800 nm) indicate that the photons absorbed by OIDTBR are converted to electrons efficiently despite the relatively small HOMO−HOMO offset between PvBDTTAZ and O-IDTBR (Figure 2b), which is in good agreement with the high PL quenching efficiency in Figure S4b. The wide-range light harvesting and high photon-to-current conversion efficiency contribute to a high short-circuit current density (Jsc) = 16.26 mA cm−2, which agrees well with the Jsc (15.67 mA cm−2) integrated from the EQE spectrum within 5% mismatch. The relatively high fill factor (FF) of 0.636 is partially ascribed to the high carrier mobilites estimated via space-charge-limited current (SCLC) method (Figure S8). The hole and electron mobilities of PvBDTTAZ:O-IDTBR blend were estimated to be 7.1 × 10−4 and 5.8 × 10−4 cm2 V−1 s−1, respectively. The balanced hole/ electron mobilities in the blend film are beneficial for reducing space charge accumulation and thus facilitating charge extraction.14 Grazing incidence wide-angle X-ray scattering (GIWAXS)15 was employed to investigate the microstructure of the neat and blend films. The 2D GIWAXS patterns of PvBDTTAZ, OIDTBR, and PvBDTTAZ:O-IDTBR are shown in Figure 3a−c, with the corresponding 1D profiles presented in Figure 3d. The PvBDTTAZ polymer exhibits high crystallinity as the high-order lamellar stacking peak of (200) is clearly visible in both in-plane and out-of-plane directions. Moreover, the (010) peak is located at 1.63 Å−1 in the out-of-plane direction with a coherence length (CL) of 2.2 nm (estimated via Scherrer equation16), which suggests a preferential face-on orientation for the polymer that is beneficial to charge transport in the vertical direction across the electrodes.17 Similarly, the neat O-IDTBR film possesses high

Figure 3. 2D GIWAXS patterns for (a) PvBDTTAZ, (b) O-IDTBR, and (c) PvBDTTAZ:O-IDTBR films. (d) In-plane and out-of-plane GIWAXS profiles for (top to bottom) PvBDTTAZ, O-IDTBR, and PvBDTTAZ:O-IDTBR films. (e) R-SoXS profile for PvBDTTAZ:OIDTBR film.

crystallinity and a face-on orientation with a high-intensity lamellar stacking peak (100) and a π−π stacking peak (010) located at 0.37 and 1.82 Å−1 (CL = 5.1 nm), respectively. To achieve an optimal BHJ morphology, it is essential to maintain the crystallinity of the donor and acceptor components in the blend films. However, many reports show that conventional donor polymers and SMAs are prone to lose their crystallinity when they are introduced into the BHJ blends.5a,c,e This problem resulted in much lower hole and electron mobilities of the BHJ blends compared to those of the pure films. Unlike these previous cases, our PvBDTTAZ:O-IDTBR material system can maintain the high crystallinity of both the donor and acceptor components in the BHJ blends. As shown in Figure 3c,d, two strong and well-separated (010) peaks can be clearly observed for the PvBDTTAZ:O-IDTBR blend film, and the peak positions are in good agreement with those values obtained from the neat PvBDTTAZ and O-IDTBR films. The quantitative fitting gave CL values of 3.0 and 3.9 nm for PvBDTTAZ and O-IDTBR, respectively, which are comparable to the CL values of the pure films and supports that both PvBDTTAZ and O-IDTBR can maintain their own lamellar and π−π stackings in the blend film, resulting in crystalline and pure domains. As a result, the high hole/electron mobilities (7.1 × 10−4/5.8 × 10−4 cm2 V−1 s−1) were achieved, which are comparable to those of neat PvBDTTAZ and O-IDTBR films (7.6 × 10−4/6.3 × 10−4 cm2 V−1 s−1). Furthermore, the phase separation of the blend was studied via resonant soft X-ray scattering (R-SoXS)18 and the detailed profile is shown in Figure 3e. The domain spacing calculated from R-SoXS profile (284.2 eV) was estimated to be C

DOI: 10.1021/jacs.7b01606 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

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36 nm, indicating a relatively small domain size of 18 nm assuming a two-phase morphology. This is consistent well with the features observed from the atomic force microscopy results (Figure S9). Both high crystallinity and small domain size have been shown to be crucial for efficient charge generation and minimized recombination.17a,18a Overall, a combination of morphology characterization results suggests that PvBDTTAZ:O-IDTBR can achieve a favorable morphology for efficient charge separation and transport, thus the high Jsc and FF. In summary, we report a novel wide-bandgap donor polymer named PvBDTTAZ for highly efficient non-fullerene OSCs. The PvBDTTAZ-based cells can achieve a high PCE = 11.6% while maintaining a high Voc = 1.08 V and a low Vloss = 0.55 V. The high performance was enabled by the favorable morphology of the blends, in which both the donor polymer and SMA can maintain their high crystallinity and form small domains at the same time. The PL quenching results indicate highly efficient hole/electron transfer between PvBDTTAZ and O-IDTBR despite the small HOMO−HOMO offset. The enhanced light harvesting (due to the complementary absorption) and effective charge transfer contributed to the high EQE (>70%) of PvBDTTAZ:O-IDTBRbased cells. Our study shows the promise of non-fullerene OSCs that can achieve high PCEs and low Vloss at the same time.

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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/jacs.7b01606. Experimental procedures, characterization methods, synthesis, and additional characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

He Yan: 0000-0003-1780-8308 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.Y. thanks Hong Kong ITC for financial support through project ITS/083/15. W.M. thanks the Ministry of Science and Technology of China (No. 2016YFA0200700) and NSFC (21504066) for 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 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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REFERENCES

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DOI: 10.1021/jacs.7b01606 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX