Non-fullerene organic solar cells (NFOSCs) that use conjugated materials to replace fullerene derivatives as electron acceptor in bulk-heterojunction solar cells have made great progress in recent years [1-4]. The power conversion efficiencies (PCEs) of NFOSCs have rapidly increased from 3% [5] to 13% [6] in five years, surpassing the performance of fullerene-based solar cells [7]. NFOSCs also show good stability under high temperature and bendable condition, indicating their promising application in flexible large-area devices [8].

In contrast to the limited fullerene derivatives, many kinds of non-fullerene electron acceptors have been developed in order to tune the absorption spectra, energy levels and crystalline properties [9]. Among them, conjugated molecules with nearinfrared (NIR) absorption spectra and deep frontier energy levels represent the highest efficient electron acceptors, such as an electron acceptor named ITIC that was developed by Zhan et al. [10]. When combining with wide band gap conjugated polymers as electron donor, NFOSCs based on the NIR acceptors perform broad photo-response, extending to 1000 nm [11]. Consequently, high photocurrent above 20 mA/cm could be obtained and meanwhile the PCEs were above 10%. Therefore, it will be important to explore electron acceptors with NIR acceptors toward high performance organic solar cells.

Porphyrin is a promising building block to construct conjugated molecules due to its strong electron-donating ability and large π-conjugated systems to provide good charge transport [12]. Porphyrin-based conjugated materials have also been widely applied in dye-sensitized solar cells [13, 14], field-effect transistors [15] and organic solar cells as electron donor [16]. However, porphyrin-based molecules have been rarely reported as electron acceptors in NFOSCs, which is due to their high-lying energy levels and strong aggregation tendency [17]. In our previous work, we have successfully developed a new star-shaped porphyrin-based molecule PBI-Por, in which the four strong electron-deficient perylene bisimide were introduced into the meso-position of porphyrin core. This strategy could significantly lower the energy levels of the molecule and meanwhile prevent the aggregation of porphyrin core. Hence, NFOSCs based on PBI-Por as electron acceptor have achieved a high PCE of 7.4%, indicating that porphyrin has great potential application in NFOSCs [18].

In this work, we design and synthesis a new star-shaped porphyrin-based molecule PhBT-Por as electron acceptor for NFOSCs (Scheme 1). The design motivation is: Although the absorption spectra of PBI-Por can extend to 800 nm, the intensity in this region is still low due to the weak intramolecular charge transfer between perylene bisimide and porphyrin. Herein, we select another end group of rhodanine (Rh) and benzothiadiazole (BT) attached to porphyrin core [19]. The molecule was found to show strong NIR absorption spectra, aligned energy levels and then was applied in NFOSCs, in which an initial PCE of 1.9% could be obtained.

Detailed synthetic procedures of the molecule RhBT-Por were present in the Supporting information. The star-shaped structure was construct by the precursor tetra-ethynyl porphyrin [18] and Br-RhBT with a high yield of 51%. We select the long branched side chain 2'-hexyldecyl attached to Rh unit in order to ensure the solubility of RhBT-Por, such as in CHCl₃ and chlorobenzene. The molecule performs strong aggregation tendency in solution and hence it showed broad peak in ¹H NMR spectra that is difficult to determine the chemical structure. Therefore, the molecule was further characterized by using high resolution MALDI-TOF and HPLC as shown in Supporting information to confirm the chemical structure and purity.

Absorption spectra of the molecule RhBT-Por in CHCl₃ and thin film were shown in Fig. 1a. Three distinct regions could be observed, in which the region of 400–500 nm was from Soret-band of porphyrin and the region of 500–600 nm was due to the RhBT end groups. Q-bands region of porphyrin originated from S0 to S1 transition disappears, while the new absorption band at 650– 850 nm is observed owing to the intramolecular charge transfer between RhBT and Por. RhBT-Por shows red-shifted absorption in thin film with enhanced intensity in NIR region, indicating that the molecule has better molecular arrangement or π-π stackingin film. The optical band gap (E_g) of the molecule can be calculated as 1.45 eV, which is similar to that of PBI-Por.

The frontier energy levels of RhBT-Por were determined by cyclic voltammetry (CV) measurement in thin films, as shown in Fig. 1b. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels determined by oxidation (E_ox) and reduction (E_red) potential were -5.48 eV and -3.55 eV. RhBT-Por shows similar LUMO level and slightly high HOMO level with those of PBI-Por [18].

The molecular configuration of RhBT-Por was studied by DFT calculations at B3LYP/6-31G, in which methyl units were used to replace branched side chains and Zn was removed in order to reduce the calculation time. HOMO and LUMO levels of RhBT-Por were delocalized on the conjugated backbone of Por and Rh-BT units, indicating the efficient intramolecular charge transfer between these two groups (Fig. 2). The calculated HOMO and LUMO levels were -5.36 eV and -3.74 eV, which was slightly highlying compared to those from CV measurement.

We then apply the electron acceptor RhBT-Por into nonfullerene solar cells by using a diketopyrrolopyrrole (DPP) based conjugated polymer, PDPP5T as electron donor (Scheme 1) [20]. PDPP5T also performed near-infrared absorption spectra with onset around 850 nm. The photoactive layers based on PDPP5T: RhBT-Por were carefully optimized, concerning the ratio of donor to acceptor, thickness and the high-boiling point solvent (Table S1, in Supporting information). The optimized J-V characteristic was shown in Fig. 3a.

PDPP5T:RhBT-Por (2:1) based solar cells were found to provide the optimized PCEs of 1.9% with a short-circuit current density (J_sc) of 7.4 mA/cm², an open-circuit voltage (V_oc) of 0.54 V and fill factor (FF) of 0.47. The J_sc was also reflected by the external quantum efficiency (EQE), as shown in Fig. 3b. The solar cells perform broad photo-response from 300 nm to 850 nm, but the maximum EQE is below 0.20. This explains the low photocurrent in solar cells. A wide band gap conjugated polymer PBDB-T [21] (Fig. S1 in Supporting information) as electron donor was also applied into RhBT-Por based non-fullerene solar cells, in which a similar PCE of 1.6% was also achieved (Table S2 in Supporting information). The photo-response from 650 nm to 850 nm can be clearly observed, contributing to the near-infrared absorption from RhBT-Por. This also confirms that RhBT-Por has great potential application in NFOSCs.

We further use atomic force microscopy (AFM) to study the morphology of blended thin films, as shown in Fig. 4. PDPP5T: RhBT-Por thin films show well-optimized phase separation with roughness of RMS = 2.1 nm, in which crystal domain can also be observed (as shown in the phase image, Fig. 4b). We also study the charge transport properties in blended thin films by using space charge limit current (SCLC) measurement, as shown in Fig. S2 and Table S3 in Supporting information. PDPP5T:RhBT-Por blend shows balanced hole and electron mobilities of 4.98 × 10^-5 cm² V^-1 s^-1 and 2.57 ×10^-5 cm² V^-1 s^-1, indicating the low charge recombination. Therefore, we infer that the poor J_scs in cells are originated from the inefficient exciton diffusion.

In conclusion, a near-infrared electron acceptor based on porphyrin as core and electron-withdrawing rhodanine and benzothiadiazole as end groups were developed for non-fullerene solar cells. The new electron acceptor shows broad absorption spectra from 300 nm to 850 nm with well-aligned frontier energy levels. Solar cells by using the new porphyrin-based acceptor and a DPP-polymer as electron donor provide an initial PCE of 1.9%. Further optimization of the chemical structures and device fabrication condition is in progress to improve the PCEs of porphyrin-based electron acceptors in our lab.

Acknowledgments

This work was supported by the Recruitment Program of Global Youth Experts of China, . The National Natural Science Foundation of China (Nos. 21574138, 51603209 and 91633301) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12030200).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.08.006.