Development of n-Type Porphyrin Acceptors for Panchromatic Light-Harvesting Fullerene-Free Organic Solar Cells

The development of n-type porphyrin acceptors is challenging in organic solar cells. In this work, we synthesized a novel n-type porphyrin acceptor, PZn-TNI, via the introduction of the electron withdrawing naphthalene imide (NI) moiety at the meso position of zinc porphyrin (PZn). PZn-TNI has excellent thermal stability and unique bimodal absorption with a strong Soret band (300–600 nm) and weak Q-band (600–800 nm). The weak long-wavelength absorption of PZn-TNI was completely covered by combining the low bandgap polymer donor, PTB7-Th, which realized the well-balanced panchromatic photon-to-current conversion in the range of 300–800 nm. Notably, the one-step reaction of the NI moiety from a commercially available source leads to the cheap and simple n-type porphyrin synthesis. The substitution of four NIs in PZn ring induced sufficient n-type characteristics with proper HOMO and LUMO energy levels for efficient charge transport with PTB7-Th. Fullerene-free organic solar cells based-on PTB7-Th:PZn-TNI were investigated and showed a promising PCE of 5.07% without any additive treatment. To the best of our knowledge, this is the highest PCE in the porphyrin-based acceptors without utilization of the perylene diimide accepting unit.


INTRODUCTION
For decades, organic solar cells (OSCs) have been studied as a portable and low-cost power generator due to their unique advantages: light-weight, use of earth-abundant organic materials, solution-processability and flexibility. The power conversion efficiency (PCE) of OSCs, an important parameter to determine the performance of the OSCs, has been continuously improved by optimization of light-harvesting in active layers and hole/electron charge transport pathways (Zhan et al., 2015;Zhao J. et al., 2016;Lin et al., 2017;Che et al., 2018;Hou et al., 2018). In the 2000s, low-bandgap polymer donors and n-type fullerene acceptors blended OSCs were developed to make broad absorption in the active layer. Polymer donors showing a strong absorption coefficient were responsible for light-harvesting and exciton generation, while the n-type fullerenes effectively separate and transfer the electrons to the electrode (Kim et al., 2006;Liang et al., 2010;Li et al., 2012;Liao et al., 2013). However, this was not enough to cover all the visible band by blending polymer donors and fullerene acceptors. In the 2010s, new types of OSCs replaced the fullerenes with strong light-harvesting organic nonfullerene acceptors (NFAs). The active layer was composed of organic donors and NFAs, called fullerene-free OSCs (Cheng et al., 2017;Lin et al., 2017;Tang et al., 2018;Yan et al., 2018). Due to the strong absorption of both NFAs and organic donors in the visible area, their complementary absorption is important to achieve panchromatic photon-to-current conversion in the active layer. There are several successful strategies for panchromatic absorption in the visible and near infra-red area. The most common approach is mixing wide-bandgap donors and lowbandgap small-molecule NFAs such as ITIC (Zhao W. et al., , 2017Lin et al., 2017;Yao et al., 2017;Zhao F. et al., 2017). Another approach is mixing low-bandgap donors and wide-bandgap NFAs (e.g., blending of PTB7-Th and perylenediimide (PDI)-based acceptor) (Wu et al., 2016;Duan et al., 2017;Liang et al., 2017;Eom et al., 2018). The red-dye PDIs enable a strong absorption in the short-wavelength area of 400-600 nm, while showing effective n-type characteristics.
As a new approach for panchromatic absorption in an active layer, utilization of a nature-inspired porphyrin dye has recently emerged in OSCs. (Gao et al., 2015;Li et al., 2016;Hadmojo et al., 2018) The porphyrin dyes have peculiar bimodal absorption characteristics composed of Soret and Q bands; strong transition from ground state (S0) to second excited state (S2) yields the Soret band, while the weak transition from S0 to first excited state (S1) provides the Q-band. Thus, the strong Soret absorption of porphyrin dyes enables efficient short-wavelength absorption in the 400-600 nm, which can be blended with low-bandgap donors having a dominant absorption in the 600-800 nm for panchromatic absorption. In addition, the long-wavelength absorption of the Q-band intensifies the light harvesting in the low bandgap area where abundant solar flux exists. However, most of the developed artificial porphyrin derivatives are p-type materials and only a few porphyrin derivatives currently show ntype characteristics with a promising PCEs over 5% (Hadmojo et al., 2017;Zhang et al., 2017). Exploring new structures for n-type porphyrin materials is challenging in fullerene-free OSCs.
In this study, we synthesized a novel porphyrin acceptor, P Zn -TNI, via Sonogashira coupling of 5,10,15,20-tetrakis-ethynyl porphyrin Zinc (II) (P Zn ) and 4-bromo-N-(2-ethylhexyl)-1,8-naphthalimide (NI). NI is easily synthesized from the commercially available 4-bromo-1,8-naphthalic anhydride. This one-step reaction is beneficial in terms of time and cost for synthesizing the star-shape molecules that require an excess of NIs. Since the NI has n-type characteristics, the substitution of NIs to the four meso positions of P Zn enables the excellent ntype properties as an electron acceptor. The ethyne π-bridge unit is incorporated between NI and P Zn to increase the backbone planarity. The synthesized P Zn -TNI showed excellent thermal stability with 5% weight loss temperature of 412 • C and showed unique bimodal absorption behavior with maximum peaks at 479 nm and 719 nm. The uncovered UV-Vis absorption spectrum from P Zn -TNI is completely covered by the blending of a polymer donor, PTB7-Th, which resulted in the panchromatic photon-to-current conversion from 300 to 800 nm in OSCs. The planar backbone structure of P Zn -TNI assists the sizable face-on orientation in the PTB7-Th:P Zn -TNI blend film without additive treatment, which resulted in the highest PCE of 5.07% (V OC = 0.72 V, J SC = 13.84 mA cm −2 , and fill factor = 0.51) in the additive-free OSCs. The excessive ordering of PTB7-Th:P Zn -TNI film via pyridine additive rather reduced the photovoltaic performances. Our successful utilization of NI moiety in the P Zn core will broaden and diversify the synthetic approaches for developing high-efficiency porphyrin acceptors.

RESULTS AND DISCUSSION
The synthetic procedure of P Zn -TNI was recorded in Scheme 1 and in the Supporting Information (SI) in detail. The porphyrin ring 1 was synthesized from pyrrole and 3-(trimethylsilyl)propiolaldehyde in the presence of BF 3 ·Et 2 O followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ). The zinc porphyrin (P Zn ) compound 2 was obtained using Zn(OAc) 2 . The deprotection of TMS group by tetra-n-butylammonium fluoride (TBAF) was performed immediately before synthesizing the final acceptor, P Zn -TNI. 4-Bromo-1,8-naphthalic anhydride purchased from Sigma-Aldrich was alkylated with 2-hexyldexylamine to give a compound 4. This one-step reaction to prepare the electron withdrawing NI unit is highly beneficial in terms of time and cost for achieving the n-type porphyrins. The final porphyrin acceptor, P Zn -TNI, was achieved via Sonogashira coupling with P Zn compound 3 and excess of NI compound 4, which was identified by 1 H-NMR and matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) (Figures S1-S4). The synthesized P Zn -TNI showed excellent solubility in common organic solvents such as tetrahydrofuran (THF), dichloromethane (DCM), and chloroform (CF). In addition, it has excellent thermal stability, which was determined by thermal gravimetric analysis (TGA), with a 5% weight loss temperature (T 5d ) of 412 • C under an N 2 atmosphere ( Figure S5).
Absorption spectra of P Zn -TNI were measured in solution and the film state, as shown in Figure 1. P Zn -TNI exhibited clear bimodal absorption composed of the Soret band (300 -600 nm) and Q-bands (600-800 nm); the maximum absorption peaks of P Zn -TNI were 479 and 713 nm in solution, and 478 and 719 nm in the film. The absorption of P Zn -TNI was complementary to that of the low-bandgap donor polymer, PTB7-Th, which induced a well-balanced short-and longwavelength absorption in the entire wavelength of 300-800 nm ( Figure 2B). Notably, the film of P Zn -TNI showed broadened and red-shifted Q-band absorption spectra compared to that in solution, indicating the enhanced intermolecular π-π stacking in the film state. Since the planar NIs and P Zn are connected by an sp-hybridized ethyne π-bridge, P Zn -TNI possesses a highly planar conjugated backbone for efficient intermolecular stacking. The optical bandgap (E •pt g ) of P Zn -TNI was 1.63 eV, which was calculated from the absorption onset wavelength of 761 nm in the film. We previously reported the pyridine additive effect on the molecular ordering of porphyrin derivatives; pyridine enhances the intermolecular ordering of porphyrin derivatives via coordination to the zinc (Hadmojo et al., 2017). As shown in Figure 1B, the absorption of P Zn -TNI was significantly broadened and red-shifted compared to that without pyridine treatment, resulting in the bathochromic shift of 42 nm in the Soret band and 15 nm in the Q-band. This supports our previous hypothesis for the pyridine effect on the molecular ordering and confirms that pyridine enhances the intermolecular ordering of P Zn -TNI in the film states.
To evaluate the energy levels of P Zn -TNI, the highest occupied molecular orbital (HOMO) energy levels and the lowest unoccupied molecular orbital (LUMO) energy levels were measured using cyclic voltammetry. The oxidation and reduction onset potentials of P Zn -TNI were 1.07 and −0.80 V, respectively, which corresponds to HOMO levels (E HOMO,CV ) and LUMO levels (E LUMO,CV ) of −5.50 and −3.62 eV, respectively. The optical LUMO energy level (E LUMO,UV ) was calculated to be −3.87 eV from E HOMO,CV and E •pt g of P Zn -TNI. The energy diagram of the PTB7-Th and P Zn -TNI was shown in Figure 1D, and the E HOMO,CV , E LUMO,CV , and E LUMO,UV of polymer donor (PTB7-Th) were taken from our previous measurement Hadmojo et al., 2016). The LUMO energy level of P Zn -TNI is suitable for electron transport from PTB7-Th to P Zn -TNI, while the HOMO of P Zn -TNI is appropriate for hole transport from P Zn -TNI to PTB7-Th (Marcus, 1963;Clarke and Durrant, 2010). The optical and electrochemical properties of P Zn -TNI are summarized in Table 1.
Porphyrin acceptor-based fullerene-free OSCs were fabricated by blending PTB7-Th and P Zn -TNI (Figure 2A). The current density-voltage (J-V) characteristic of PTB7-Th:P Zn -TNI devices was investigated via changing the weight ratio between PTB7-Th and P Zn -TNI, and the photovoltaic performance was optimized at the weight ratio of 1:1.5 w/w. The photovoltaic properties are summarized in Figure 2C, Figure S6, and Table 2. The best PCE of 5.07% was achieved with a V OC of 0.72 V, a J SC of 13.84 mA cm −2 , and FF of 0.51 ( Figure 2C). As shown in Figure 2D, the external quantum efficiency (EQE) spectra of PTB7-Th:PZn-TNI devices cover the entire visible area of 300-800 nm and FIGURE 2 | (A) Device structure of PTB7-Th:P Zn -TNI based OSCs, (B) absorption spectra of PTB7-Th:P Zn -TNI active layer, (C) J-V characteristics of additive-free OSCs depending on the weight ratio between PTB7-Th and P Zn -TNI, and the J-V curve of pyridine-treated OSCs at the weight ratio of PTB7-Th:PZn-TNI = 1:1.5 w/w. (D) the EQE spectra of PTB7-Th:P Zn -TNI devices.
showed the panchromatic photon-to-current conversion due to the complementary solar flux absorption between PTB7-Th and PZn-TNI. Notably, the additive-free film-formation process provided superior photovoltaic performance compared to the pyridine-assisted one as shown in Figure 2C. As shown in Figure 2D, the EQE was increased in the entire wavelength, which indicates that hole/electron transport properties of both PTB7-Th and P Zn -TNI are improved in the additive-free devices.
To understand the charge recombination mechanisms of PTB7-Th:P Zn -TNI devices in the presence and absence of pyridine additive, the J-V characteristics were investigated as a function of the illumination intensity. The power law dependence of J SC on the illumination intensity is generally expressed as J SC ∞ I α , where I is the light intensity and α is an exponential factor ( Figure 3A) Azmi et al., 2016). The α value of the PTB7-Th:P Zn -TNI devices was close to unity regardless of additive treatment, indicating the negligible bimolecular recombination in PTB7-Th:P Zn -TNI devices. However, the V OC vs. illumination intensity was highly affected by the pyridine treatment. Assuming there is no trap-assisted recombination under an open-circuit condition, the slope of V OC vs. the illumination intensity produces 1.00 kT/q (Mihailetchi et al., 2006). The higher value of kT/q indicates the more probability of trap-assisted recombination under an open-circuit condition (Mandoc et al., 2007;Azmi et al., 2016). As shown in Figure 3B,   PTB7-Th:P Zn -TNI devices with and without additives show the slope of 2.63 and 1.93 kT/q, respectively. This implies that PTB7-Th:P Zn -TNI devices in the absence of additives have the lowest trap-assisted recombination in anopen-circuit condition. The hole and electron transport properties of PTB7-Th:P Zn -TNI devices were measured by a space-charge-limitedcurrent (SCLC) analysis (Figures 3C,D) (Mihailetchi et al., 2005). The electron-and hole-only devices were fabricated with a structure of ITO/ZnO/PTB7-Th:P Zn -TNI/ZnO/Al and ITO/PEDOT:PSS/PTB7-Th:P Zn -TNI/MoO x /Ag, respectively. In the presence of pyridine additive, the hole and electron mobilities of PTB7-Th:P Zn -TNI were 2.4 × 10 −4 and 1.3 × 10 −6 cm 2 V −1 ·s −1 , respectively, whereas, in the absence of additives, the hole and electron mobilities were increased to 2.9 × 10 −4 and 2.5 × 10 −6 cm 2 V −1 ·s −1 , respectively. Thus, it is expected that the pyridine additive worsens the nanomorphology of PTB7-Th:P Zn -TNI devices via excessive intermolecular aggregation. The morphology of the PTB7-Th:P Zn -TNI active layer was investigated by atomic force microscopy (AFM) (Figure 4) and two-dimensional grazing incidence X-ray diffraction (2D-GIXD) analyses ( Figure 5). In AFM images, PTB7-Th:P Zn -TNI blended film possesses bicontinuous crystalline domains in the absence of additives (Figures 4A,B), whereas the addition of pyridine additive intensifies the intermolecular ordering of P Zn -TNI domains, leading to severe phase segregation between PTB7-Th and P Zn -TNI (Figures 4C,D). The 2D-GIXD results also support the AFM analysis. Additive-free PTB7-Th:P Zn -TNI film showed clear π-π stacking orientation (010) peak at ∼1.6 Å −1 along the q z axis, which indicates the face-on orientation with a dspacing of ∼3.9 Å (Figures 5B,C). However, the pyridine-treated PTB7-Th:P Zn -TNI film showed the increased π-π stacking FIGURE 5 | (A) 2D-GIXD images of (A) pristine film of PTB7-Th and P Zn -TNI, and (C) additive-free PTB7-Th:P Zn -TNI blend film and pyridine-treated one. (B) Out-of-plane line-cut of PTB7-Th, P Zn -TNI, and PTB7-Th:P Zn -TNI film.
interaction and induced the phase aggregation ( Figure 5C). The (010) peak in the blend film is assigned to the orientation of PTB7-Th domains (Figure 5A), which implies that the decrease in the photovoltaic performances in presence of pyridine is probably due to the aggregation of the PTB7-Th domains in the PTB7-Th:P Zn -TNI blend film. As a result, the P Zn -TNI having highly planar molecular structure possesses the sizable π-π intermolecular stacking and crystalline nanomorphology in the additive-free solvent system, which means that no more post-treatment is required in PTB7-Th:PZn-TNI blend system. In addition, the additive-free system can prevent undesirable morphological change and photo-oxidation degradation by additives in the active layer .

CONCLUSIONS
We have synthesized a novel porphyrin acceptor, P Zn -TNI, by incorporating four naphthalene imide (NI) units at the meso position of the P Zn core. P Zn -TNI showed unique bimodal absorption with a strong Soret band and a weak Q-band. The insufficient long-wavelength absorption of P Zn -TNI was covered by a low-bandgap donor, PTB7-Th. As a result, bulk heterojunction fullerene-free OSCs composed of P Zn -TNI and PTB7-Th showed panchromatic photon-to-current conversion covering entire area of 300-800 nm. The PTB7-Th:P Zn -TNI devices exhibited a promising PCE of 5.07%, which is the highest and the first promising PCE in the porphyrin-based acceptors except for those utilizing the PDI units. Notably, the additivefree solution process provided the best photovoltaic performance, whereas the pyridine additive had a negative effect on the nanomorphology by the excessive molecular aggregation of the PTB7-Th:P Zn -TNI film. The planar backbone structure of P Zn -TNI assists the sizable molecular ordering in the PTB7-Th:P Zn -TNI film without additive treatment, which is favorable for practical applications.

AUTHOR CONTRIBUTIONS
IJ and SY conceived the ideas and designed the PZn-TNI. U-HL synthesized all the materials and JK assisted the characterization of all the materials. S-YJ supervised all the device fabrication and optimization. WH fabricated all the OSC devices and SE assisted the device characterization.