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High Performance Near-Infrared Absorbing nType Porphyrin Acceptor for Organic Solar Cells Wisnu Tantyo Hadmojo, Un-Hak Lee, Dajeong Yim, Hyun Woo Kim, WooDong Jang, Sung Cheol Yoon, In Hwan Jung, and Sung-Yeon Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14577 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 3, 2018
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ABSTRACT: While the outstanding charge transport and sunlight harvesting properties of porphyrin molecules are highly attractive as active materials for organic photovoltaic devices (OPVs), the development of n-type porphyrin-based electron acceptors has been challenging. In this work, we developed a high-performance porphyrin-based electron acceptor for OPVs by substitution of four naphthalene diimide (NDI) units at the perimeter of a Zn-porphyrin (PZn) core using ethyne linkage. Effective π-conjugation between four NDI wings and the PZn core significantly broadened Q-band absorption to the near infrared region, thereby achieving the narrow bandgap of 1.33 eV. Employing a windmill structured tetra-NDI substituted PZn-based acceptor (PZn-TNDI) and mid-bandgap polymer donor (PTB7-Th), the bulk heterojunction OPV devices achieved a power conversion efficiency (PCE) of 8.15% with an energy loss of 0.61 eV. The PCE of our PZn-TNDI based device was the highest among the reported OPVs using porphyrin-based acceptors. Notably, the amorphous characteristic of PZn-TNDI enabled optimization of the device performance without using any additive, which should make industrial fabrication simpler and cheaper.
INTRODUCTION Porphyrin is a nature-selected light harvesting chromophore and its derivatives are involved in essential biological systems for light-to-charge transformation processes.1-3 For example, chlorophylls in the plants and algae operate by the porphyrin or chlorine derivatives, in which the strong absorption of solar flux and efficient charge separation/transport occurs to complete photosynthesis. The central role of porphyrin derivatives in the process of photosynthesis provided fundamental guidance for employing them in artificial light harvesting systems.4 Grätzel and coworkers employed porphyrin-based sensitizers in dye sensitized solar cells (DSSCs) to achieve power conversion efficiency (PCE) of >13 %.5-6 Recently, some porphyrin
derivatives were employed as photoactive materials for thin film organic photovoltaics (OPVs)7-8 and demonstrated the promising PCE of >9 %.9-11 However, these efforts in developing artificial porphyrin-based photoactive materials for OPVs were mostly focused on p-type donors, and porphyrin derivatives for use as n-type acceptors remain relatively unexplored. There have been a few recent reports on porphyrin-based n-type acceptors for OPVs that demonstrated promising device PCEs of 5.3–7.4 %.12-13 In previous work on acceptors, a perylene diimide (PDI) moiety was substituted onto a porphyrin unit to benefit from its strong electron withdrawing characteristic. However, the bulky backbone of PDI twisted the positions of sites needed for bonding with the porphyrin core, obstructing efficient π-electron delocalization throughout the molecules. As a result, the narrowing of the bandgap was limited.12 Moreover, the optimized device required an extra-process using additives such as pyridine and 1,8-diiodooctane (DIO). Thus, development of porphyrin-based acceptors with narrower bandgaps and simpler fabrication would be beneficial for improvement in the photonto-current conversion of near infrared (NIR) light. Naphthalene diimide (NDI) is an effective electron withdrawing moiety that is also less bulky than PDI. Due to its strong n-type characteristics, NDI has often been used as the active material for thin film organic transistors.14-17 NDI has also been employed to develop polymeric non-fullerene acceptors for OPVs; however, its strong stacking characteristic has often hampered construction of favorable bicontinuous bulk heterojunction (BHJ) nanomorphology. The research on those acceptors has mainly been focused on preventing excessive aggregation of the NDI moieties. Incorporation of PDI units (30%) in the NDI based polymer acceptors effectively reduced such aggregation in the BHJ blends, resulting in PCE improvement from 1.23 to 5.10 %.18 Constructing twisted star-shaped acceptors, in which three NDIs are flanked on the sides of triphenylamine, effectively prevented aggregation and resulted in a
demonstrated PCE of 2.8 %.19 Copolymerization of NDI with monomers of lower crystallinity has been a strategy used to reduce excessive aggregation, and has achieved PCEs of 4.9– 5.3 %.20-21 In this work, we developed a high performance Zn porphyrin (PZn)-based narrow bandgap small-molecule organic acceptor (PZn-TNDI) by incorporating four NDI wings at the perimeter of the PZn core via ethyne linkage. The porphyrin core mitigated the strong intermolecular aggregation among NDI moieties by forming a slightly twisted windmill structure, while the ethyne linkage allowed effective π-conjugation between the porphyrin core and NDIs. The PZnTNDI showed effective π-electron delocalization throughout the backbone, resulting in broad, strong Q-band absorption up to the NIR region (~930 nm). Using a mid-bandgap polymer donor (PTB7-Th), which has absorption complementary with that of PZn-TNDI, a BHJstructured OPV device with PCE of 8.15 % was achieved. To the best of our knowledge, this is the highest PCE reported among devices using porphyrin-based acceptors. Notably, the ascasted BHJ active layers demonstrated optimum performance without using any high boilingpoint additive or post-treatments (solvent or thermal annealing) because of the amorphous characteristic of PZn-TNDI. The combination of an NDI moiety with a porphyrin unit offered synergetic effects for the development of NIR absorbing small-molecule electron acceptors.
RESULTS AND DISCUSSION The synthetic route of PZn-TNDI is provided in Scheme 1 and Supporting information (SI) in detail. Trimethylsilylacetylene funtionalized porphyrin was synthesized from 3(trimethylsilyl)propiolaldehyde and pyrrole by the Lindsey method using BF3·Et2O and 2,3dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in sequence. After metalation of the porphyrin with Zn, the trimethylsilyl (TMS) group was deprotected using tetra-n-butylammonium fluoride (TBAF). The synthesized compound 3 was immediately converted to PZn-TNDI via
Sonogashira coupling with mono-brominated NDI compound 4. The structure of the final compound (PZn-TNDI) was identified using 1H-NMR, elemental analysis (EA) and MALDITOF mass spectrometry. The optical properties of PZn-TNDI are shown in Figure 1a. The film of PZn-TNDI exhibited efficient light harvesting across the entire visible region with four sizable maximum absorption peaks at 360, 448, 642, and 800 nm. The optical bandgap of PZn-TNDI was 1.33 eV, which was determined from the absorption onset wavelength in the film states. Notably, the bandgap of PZn-TNDI was narrower than that of the current-state-of-the-art porphyrin acceptor, PBI-Por, (1.48 eV)12 due to the enhanced π-conjugation and intramolecular charge transfer (ICT) interaction between the PZn core and NDI wings. This narrower bandgap of PZn-TNDI is beneficial for extended photon-to-current-conversion, which can improve the short-circuit current (JSC) of devices. Considering that the Q-band of PZn core has much weaker oscillator strength than that of Soret-bands22, the strong Q-band absorption of PZn-TNDI confirms efficient π-conjugation and ICT interaction between the PZn core and NDI wings. In particular, the broadened and intensified Q-band absorption of PZn-TNDI is highly beneficial for NIR light harvesting. Figure 2 shows the three-dimensional (3D) geometrical structures and frontier molecular orbitals (HOMO and LUMO) computed according to the previous our paper.13 The NDI wings are twisted along the PZn core with a dihedral angle of ~10°, which mitigates the severe intermolecular aggregation of PZn-TNDI and improve the π-electron delocalization over the ring. The electron density distribution in PZn-TNDI is shown in Figure 2b. The electron density is distributed throughout the entire PZn-TNDI in HOMO. In LUMO, it appears similar to that of HOMO, but the electrons were withdrawn further toward the NDI wings. This means that PZn-TNDI possesses a well-delocalized π-electron in PZn-TNDI as well as ICT interaction between PZn and NDI wings, resulting in a narrow bandgap of 1.33 eV. This is a substantially
narrower bandgap than that of the reported PDI based porphyrin acceptors (PBI-Por, 1.48 eV). In the latter, the bandgap was determined by the charge transfer interaction between the localized electron in the porphyrin ring and the PDI backbone.12 The HOMO and LUMO energy levels of PZn-TNDI were determined using cyclic voltammetry (CV). As shown in Figure 1b, the oxidation and reduction onset potential were 0.71 and -0.74 V, respectively, corresponding to HOMO and LUMO level of −5.37 and -3.92 eV, respectively. The optical LUMO energy level was also calculated as −4.04 eV from the HOMO level and optical bandgap. While the LUMO level of PZn-TNDI is comparable to that of conventional fullerene acceptors, the HOMO level of −5.37 eV is appropriate to transfer holes to conventional donor polymers. The energy diagram of the PTB7-Th:PZn-TNDI device is shown in Figure 1c. Photovoltaic properties were studied using the BHJ active layer composed of PTB7-Th and PZn-TNDI. We selected PTB7-Th as the polymer donor because it exhibited absorption complementary to that of PZn-TNDI. This enabled panchromatic absorption by the active layers (Figure 3b). The energy levels of PTB7-Th were suitable for electron/hole transfer to/from PZn-TNDI. In particular, the difference in the HOMO levels of PTB7-Th (−5.12 eV)23 and PZnTNDI was only ~0.25 eV, which is favorable for hole transfer with minimum energy loss. The structure of the OPV device (ITO/ZnO/PTB7-Th:PZn-TNDI (1:1 w/w)/MoOx/Ag) is illustrated in the inset of Figure 3a. Figure 3a shows the current density (J)-voltage (V) properties of the OPVs under AM 1.5 G. The optimized device achieved PCE of 8.15 % with a JSC of 17.34 mA cm-2, a VOC of 0.72 V and a FF of 0.65. To evaluate the effects of additives, we fabricated OPVs with a conventional additive (DIO); however the PCEs of the devices gradually decreased with greater addition of DIO (Figure S3). It is notable that the PTB7-Th:PZn-TNDI device achieved its optimum performance without using any additional additives and post-treatments (thermal and solvent) (Figure S4 and S5). This should enhance the reproducibility of such devices. To the best of our knowledge, the PCE of 8.15 % is the highest among the OPV devices reported
to use porphyrin-based acceptors. This result confirms the remarkable synergetic effects provided by the combination of PZn units with NDI moiety. The JSC of the PTB7-Th:PZn-TNDI device was as high as 17.34 mA cm-2 owing to its broad photon-to-current conversion, covering the range from 300 to 900 nm, as recorded in external quantum efficiency (EQE) graph (Figure 3b). The EQE graph resembled the absorption of the PTB7-Th:PZn-TNDI (1:1 w/w) blend film (Figure S6). This indicates that hole transport from PZn-TNDI to PTB7-Th and electron transport from PTB7-Th to PZn-TNDI are efficient in the OPV devices, whereas the lower EQE at 750-900 nm is attributed to the relatively weak Q-band absorption of PZn-TNDI. The complimentary absorption of PTB7-Th and PZn-TNDI enabled panchromatic absorption, and the EQE at 680 nm reached 72 %. Compared to the device using the state of the art porphyrin-based acceptor (PBI-Por),12 the JSC of the PTB7-Th:PZn-TNDI device was ~20 % higher due to the extended EQE of the NIR regime. Moreover, the energy loss (Eloss, defined as Eloss = bandgap − eVOC) of our PTB7-Th:PZn-TNDI device was significantly lower (only 0.61 eV), which was also smaller than that of the PBI-Por based OPVs (0.70 eV)12. To understand further the efficient panchromatic photon-to-current-conversion of the PTB7Th:PZn-TNDI device, the charge generation/extraction properties were investigated. We first measured the photoluminescence (PL) emission of PTB7-Th:PZn-TNDI blend films to figure out the difference in the charge transfer characteristics of PTB7-Th and PZn-TNDI. Considering the absorption properties of PTB7-Th and PZn-TNDI, the PL quenching by photo-excitation at 650 nm results from both electron-transfer from PTB7-Th to PZn-TNDI and hole-transfer from PZn-TNDI to PTB7-Th. On the other hand, the PL quenching by photo-excitation at 810 nm mostly represents hole-transfer from PZn-TNDI to PTB7-Th. As shown in Figure 3c, the PL emission intensity of PTB7-Th:PZn-TNDI film without DIO was significantly lower than that with DIO (2 %) at both excitations. This revealed that the charge transfer in the BHJ blend without DIO is more efficient, possibly due to more favorable nanomorphology.
Figure 3d represents the photocurrent density (Jph)-effective voltage (Veff) properties and the characterization method was provided in SI.24 As shown in Figure 3d, the Jph values of the devices are saturated at Veff of > 2 V. Under a sufficiently high effective-voltage regime (Veff > 2 V), it is assumed that all of the dissociated photogenerated excitons (electron-hole pairs) contribute to the current. Therefore, the saturated current density (Jsat) is proportional to the maximum charge generation rate (Gmax).25-26 The PTB7-Th:PZn-TNDI device without DIO exhibited a higher Jsat value (182.46 A m-2) than that with DIO (149.73 A m-2). The calculated Gmax of the device without DIO (1.14 × 1028 m-3s-1) was higher than that of the device with DIO (0.936 × 1028 m-3s-1), which indicated that the PTB7-Th:PZn-TNDI blend possesses higher interfacial area for exciton dissociation without DIO. The improved exciton dissociation of the PTB7-Th:PZn-TNDI device without DIO was also consistent with the analysis from the PL emission spectra in Figure 3c. The PL emission of PTB7-Th:PZn-TNDI blends was better quenched in the absence of DIO. The charge extraction probability under short circuit condition (PC = Jph/Jsat) was also determined to understand the charge transport properties in the OPVs. The device without DIO exhibited PC of 92 % under the short circuit condition, whereas that with DIO showed only 81 %. This result revealed that both charge generation and extraction were superior in the PTB7-Th:PZn-TNDI device without DIO. The molecular ordering of the PTB7-Th:PZn-TNDI blend films was investigated using twodimensional grazing incident wide-angle X-ray diffraction (2D-GIWAXS). Figure 4a-d shows 2D-GIWAXS patterns and the line-cut intensities are plotted in Figure 4e. The pristine PTB7Th film shows intermolecular π-π stacking along the Qz axis of 1.60 Å–1, corresponding to the d-spacing of 3.90 Å with the face-on ordering. The pristine PZn-TNDI exhibited no particular ordering, indicating its amorphous characteristic. In the PTB7-Th:PZn-TNDI BHJ films, the ordering of PTB7-Th was weakened, probably due to compatible mixing with PZn-TNDI. The miscibility of PTB7-Th and PZn-TNDI weakened the local π-π intermolecular interaction of
the PTB7-Th backbones. By the addition of DIO (2 %), the face-on orientation of PTB7-Th became more discernable (Figure 4d), which is consistent with the literature.27-28 To study the effects of molecular ordering on the charge transport characteristics, we determined charge mobility by measuring the space-charge-limited-current (SCLC) in the holeand electron-dominant devices. The hole and electron mobility of the PTB7-Th:PZn-TNDI blends were 1.22×10-3 and 1.8×10-4 cm- 2V-1s-1, respectively, without DIO, and 1.41 × 10-3 and 4.51 × 10-5 cm- 2V-1s-1, respectively, with DIO (Figure S8 and S9). Although there was no particular molecular ordering, the PTB7-Th:PZn-TNDI film demonstrated sufficiently high charge mobility, which is consistent with the resulted OPV performance. Notably, the electron mobility of the blend film was considerably decreased by addition of DIO, whereas the hole mobility was slightly increased (Table S5). The increased hole mobility after addition of DIO was consistent with the results for other BHJ blends containing PTB7-Th;23, 28-29 however, the decreased electron mobility of the blend caused by addition of DIO indicated that the nanomorphology of PZn-TNDI became less favorable. As a result, the hole/electron balance was harmed by the addition of DIO. The charge mobility from SCLC measurements is summarized in Table S5. We measured atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of the PTB7-Th:PZn-TNDI films (Figure 5). As shown in the topographic image (Figure 5a and 5c), the root-mean-square roughness (Rq) of the PTB7Th:PZn-TNDI film without DIO was less than that with DIO, indicating higher miscibility in the absence of DIO. The TEM images were also supportive of the AFM results. Nano-fibrillar structures were observed in both PTB7-Th:PZn-TNDI films (with and without DIO); however, larger aggregations were developed by addition of DIO (Figure 5b and 5d). Based on the 2DGIWAXS, SCLC, AFM, and TEM results, the amorphous PZn-TNDI successfully formed bicontinuous BHJ with crystalline PTB7-Th without any additive due to good miscibility, and in which the charge transport (and balance) was sufficiently high. Although the DIO treatment
enhanced the molecular ordering of PTB7-Th to facilitate hole-transport, the detrimental effects on the nanomorphology of the BHJ blends decreased electron transport (and charge balance). The formation of optimal nanomorphology without using any additive should be advantageous for device processing and reproducibility.
CONCLUSION We developed a high performance porphyrin based electron acceptor for OPV devices. The Zn porphyrin based acceptor (PZn-TNDI) was synthesized by incorporating four NDI units at the perimeter of the PZn core. The effective π-delocalization throughout PZn-TNDI realized a significantly narrow bandgap (1.33 eV) and strong Q-band absorption extended to the NIR region (~930 nm). The efficient photon-to-current-conversion covering the visible and NIR areas was achieved by the blending of PZn-TNDI and PTB7-Th. The PTB7-Th:PZn-TNDI device achieved the promising PCE of 8.15% via the high JSC of 17.34 mA cm-2 and the low energy loss of 0.61 eV. Notably, the result was the highest PCE among those reported for OPV devices using porphyrin-based acceptors. Moreover, the as-cast PTB7-Th:PZn-TNDI blends achieved optimum performance without using any high boiling-point additive or posttreatments (solvent or thermal annealing) due to the good miscibility of amorphous PZn-TNDI and crystalline PTB7-Th. The formation of optimal nanomorphology in the as-cast active layers is advantageous for application to other commercial device processing techniques.
ASSOCIATED CONTENT Supporting Information The synthetic procedure of PZn-TNDI, and the device fabrication and analysis were described in detail. J-V and EQE properties of PTB7-Th:PZn-TNDI devices, and SCLC mobility and AFM/TEM images of PTB7-Th:PZn-TNDI films were provided.
Notes The authors declare no competing financial interest.
ACKNOWLEDGEMENT This study was supported from the National Research Foundation (NRF) of Korea (MSIP, No. 2016R1A5A1012966 and No. 2017R1A2B2009178), Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20163030013960), and the Global Scholarship Program for Foreign Graduate Students at Kookmin University in Korea.
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Figure 1. (a) Absorption spectra of PZn-TNDI in solution and film, (b) Cyclic voltammogram of PZn-TNDI, and (c) Energy diagram of the PTB7-Th:PZn-TNDI device.
Figure 2. (a) Geometrical structures, and (b) HOMO and LUMO energy levels of PZn-TNDI calculated using the B3LYP function and employing the 6-31G(d, p) basis set.
Figure 3. (a) J-V characteristics and (b) EQE spectra of the PTB7-Th:PZn-TNDI device and optical absorption of PTB7-Th and PZn-TNDI. (c) PL emission intensity of PTB7-Th:PZn-TNDI BHJ films under excitation at 650 nm (line with filled squares) and 810 nm (line with empty squares). (d) Jph – Veff characteristics of PTB7-Th:PZn-TNDI devices. The amount of DIO added was 2 %.
Figure 4. 2D-GIWAXS patterns of (a) pristine PTB7-Th film, (b) Pristine PZn-TNDI film, (c) PTB7-Th:PZn-TNDI blend film without DIO, and (d) PTB7-Th:PZn-TNDI blend film with DIO. The line-cut data of the corresponding 2D-GIWAXS patterns along (e) the out-of-plane.
Figure 5. AFM phase images of PTB7-Th:PZn-TNDI blend films (a) Without DIO (inset: height image) and (c) With DIO (inset: height image). TEM images of PTB7-Th:PZn-TNDI blend films (b) Without DIO and (d) With DIO. The amount of DIO added was 2 %.
