Diindeno[1,2-g:1′,2′-s]rubicene: all-carbon non-fullerene electron acceptor for efficient bulk-heterojunction organic solar cells with high open-circuit voltage

Abstract

An all carbon non-fullerene electron acceptor material based on diindeno[1,2-g:1′,2′-s]rubicene (DIR) was readily synthesized and processed for bulk-heterojunction organic solar cells. High PCE up to 3.05% with a relatively high V_OC of 1.22 V has been achieved with P3HT electron donor material by the variation of donor–acceptor blending ratio.

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Extensive efforts have been directed toward the research of solution processed bulk heterojunction (BHJ) organic solar cells (OSCs) due to its potential for low-cost energy harvesting and large area fabrication. Power conversion efficiencies (PCEs) have already exceeded 10% for BHJ OSCs employing the π-conjugated polymer¹ or small molecule² as electron donors when fullerene derivatives were utilized as the electron acceptor. Large electron affinity, high electron mobility through three-dimensional charge transport and desirable phase separation in the BHJ active layer are major advantages of fullerene derivatives as dominant electron acceptor materials for highly efficient BHJ OSCs.³ However, some obvious drawbacks of fullerene acceptors limited their practical usages in BHJ OSCs, such as a non-trivial matter of synthesis and purification,⁴ insufficient solubility in processing solvent. Moreover, the poor absorption in the visible wavelength and relatively deep-lying lowest unoccupied molecular orbital (LUMO) energy level of fullerene acceptors resulting in photon energy losses and low open-circuit voltage (V_OC) of the photovoltaic device. In light of these drawbacks, non-fullerene electron acceptors based on small molecules^5,6 and polymers⁷ that are readily synthesized and HOMO/LUMO energy levels tunable, as well as showing broad absorption in the visible wavelength have attracted increasing research efforts in materials development to improve the performance of BHJ OSCs.

A popular strategy for the design of non-fullerene electron acceptors is to utilize planar electron-deficient building units, such as perylene diimide (PDI),^8–13 fluoranthene-fused imide (FFI),¹⁴ diketopyrrolopyrrole (DPP),¹⁵ benzothiadiazole (BT),¹⁶ and benzothiadiazole/imide.¹⁷ Recently, the highest PCEs of BHJ OSCs using the small molecular non-fullerene acceptor based on the conventional polymer donor P3HT have already exceeded 4% by using an azadipyrromethene-based Zn complex as the acceptor.¹⁸ Moreover, lots of effort have been directed to the development of perylene diimide (PDI)-based acceptors^8–12 recently and the highest PCE exceeded 6% has been obtained when the low-band gap polymer PBDTTT-C-T was utilized as the donor material.¹³ In terms of molecular structure, fullerenes are unique because they do not contain electron-deficient building units. The electron-accepting ability of fullerenes is rationalized by recognizing that fullerenes contain cyclopentadiene rings that have the driving force to aromatize by accepting electrons through aromatic 4n + 2 stabilizations.^19,20 Within the context, cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) contain cyclopentadiene rings and behave similarly as fullerenes in exhibiting high electron affinities. Several CP-PAHs have been shown to function as n-type or ambipolar semiconductors in organic field-effect transistors (OFETs), such as indenofluorenes,²¹ cyclopenta[hi]aceantrylenes,¹⁹ dibenzopentalenes,²² and emeraldicenes.²³ However, only few CP-PAHs have utilized as electron acceptors in BHJ OSCs.^24–27 Recently, Plunkett et al. demonstrated the potential of cyclepenta[hi]aceanthrylene (CPA) small molecules²⁸ and polymers²⁹ as electron acceptors through the fluorescence quenching study of P3HT. Miao et al. also utilized a CPA derivative as the cascade material that exhibits electron-accepting properties to enhance the PCE of BHJ OSCs based on a ternary blend system.³⁰ We were therefore encouraged by their discoveries and interested in developing electron acceptors based on CPA structure.

As a molecular fragment of C₇₀, rubicence (Scheme 1) has a low LUMO energy level of ∼3.4 eV and it is a structural analogue of CPA with the potential to have fullerene-like electron affinity due to aromatic 4n + 2 stabilizations mentioned above.³¹ Herein, we report an all-carbon non-fullerene electron acceptor adopting rubicene as the core structure, which is known as diindeno[1,2-g:1′,2′-s]rubicene (DIR) as shown in Scheme 1.³²

Scheme 1 Chemical structures of rubicene, DIR-2EH and the benzocyclopentadienyl anion stabilization of DIR-2EH. Inset shows the photograph of sublimed crystalline solids of DIR-2EH.

Synthetically, DIR-2EH is readily anchored with branched 2-ethylhexyl (2EH) chains in order to improve the solubility for the fabrication of solution-processed BHJ OSCs. DIR-2EH has no electronegative heteroatom but only carbon and hydrogen atoms. The synthesis of DIR-2EH was modified according to the synthetic route reported by Skidmore et al.³² DIR-2EH can be easily obtained within three synthetic steps with reasonably good to high yields from the commercially available starting material 1,5-dichloroanthraquinone as shown in Scheme S1 (ESI†). To ensure the material purity required for BHJ OSCs, DIR-2EH could be readily purified by vacuum gradient sublimation after column chromatography. The sublimed crystalline solid of DIR-2EH shows a bright red colour and exhibits excellent solubility in common organic solvents like 1,2-dichloromethane, chloroform, chlorobenzene and 1,2-dichlorobenzene at room temperature.

Fig. 1 (top) shows the UV-visible absorption spectra of DIR-2EH dissolved in 1,2-dichlorobenzene and as thin film. Owing to the four bulky 2EH chains (which prevents π–π stacking among arc-shaped structure of rubicene), comparing those in solution, absorption spectra of DIR-2EH are slightly red-shifted in thin film states. DIR-2EH shows an intense absorption band around 350 nm and a weaker but much broader absorption band with fine structures ranging from 400 to 600 nm. The absorption band of DIR-2EH in the visible wavelength corresponds to the π–π* transition of the conjugated diindeno[1,2-g:1′,2′-s]rubicene structure, which has more extended π-conjugation than the parent rubicene and hence its absorption wavelength is red-shifted.³³ Based on the thin film absorption spectrum, the optical energy gap (E_g) of DIR-2EH was calculated to be 1.97 eV. Fig. 1 (bottom) also shows the absorption spectra of P3HT:DIR-2EH blend films, in which the material blending ratio is 1 : 1, 1 : 2 and 1 : 4. Whereas PC₆₁BM has a strong absorption ∼330 nm and a series of weak absorption bands extended beyond 550 nm, the absorption band around 350 nm for the P3HT:DIR-2EH blend films is attributed to DIR-2EH. The increasing absorption intensities at 550 nm and 600 nm, which are synchronously along with increasing blending ratios, are mainly attributed to the absorption of DIR-2EH.

Fig. 1 Absorption spectra of DIR-2EH in 1,2-dichlorobenzene and DIR-2EH and P3HT as thin film (top); P3HT:DIR-2EH (1 : 1, 1 : 2, 1 : 4) and P3HT:PC₆₁BM (1 : 1) as thin film (bottom).

The HOMO/LUMO energy levels of DIR-2EH were investigated by cyclic voltammetry (CV) method. The classical electron acceptor PC₆₁BM was also measured in the same condition for comparison. The detail of CV measurement is provided in ESI†. As shown in Fig. 2, DIR-2EH exhibits two quasi-reversible reduction signals that are ascribed to the formation of two stabilized aromatic benzocyclipentadienyl anions in the rubicene core structure of DIR-2EH.²⁸ It can be seen that the first and second reduction potentials all shifted negatively when compared to PC₆₁BM. The LUMO energy level of DIR-2EH was estimated from the onset of the first reduction potential and calculated according to the onset oxidation potential of ferrocene, which is 4.8 eV below vacuum level.²⁷ The LUMO of DIR-2EH was estimated to be −3.30 eV, which is 0.5 eV higher than −3.80 eV of PC₆₁BM. It has been known that the upper limit of V_OC is determined by the difference of the LUMO energy level of the electron acceptor and the HOMO energy level of the electron donor. Therefore, the much higher LUMO level of DIR-2EH than PC₆₁BM should theoretically lead to a much higher V_OC in BHJ OSCs.

Fig. 2 Left: cyclic voltammograms of DIR-2EH and PC₆₁BM in CH₂Cl₂; Right: energy levels of P3HT, PC₆₁BM and DIR-2EH; the HOMO and LUMO levels of P3HT was taken from the literature, which was obtained from cyclic voltammetry.³⁴

The photoluminescence (PL) quenching of the electron donor in the presence of the electron acceptor is a typical characteristic of electron transfer from donor material to acceptor material. We performed PL quenching study of P3HT in solution (chloroform) containing DIR-2EH with varying concentrations (see Fig. S1 in ESI†). The PL intensity of P3HT solution decreased dramatically with increasing concentrations of DIR-2EH. We further utilized the Stern–Volmer quenching plot to investigate the quenching efficiency of DIR-2EH (Fig. S1†). From the plot, a quenching constant (K_SV) of 2.5 × 10⁴ M⁻¹ is acquired from the linear concentration dependency and this K_SV is similar to that of PC₆₁BM (K_SV = 1.6 × 10⁴ M⁻¹).²⁸ Accordingly, we demonstrate electron-transfer from P3HT to DIR-2EH is as efficient as that to PC₆₁BM in solution.

Using DIR-2EH as an electron acceptor in BHJ OSCs was tested with classical electron donor polymer P3HT in the fabricated device, ITO/PEDOT:PSS/P3HT:DIR-2EH/Ca/Al. Details of device fabrication and characterization are provided in ESI†. P3HT was blended with DIR-2EH in three different D/A ratios, 1 : 1, 1 : 2, and 1 : 4, to optimize condition for BHJ OSCs. The P3HT device using PC₆₁BM as the electron acceptor material with 1 : 1 blending ratio was also fabricated as the reference device. As shown in Table 1 and Fig. 3, all devices based on DIR-2EH exhibit relatively high V_OC larger than 1.2 V, which is more than twice of the P3HT device using PC₆₁BM (V_OC = 0.6 V) as the electron acceptor material. As demonstrated by the CV data, the higher V_OC can be mainly attributed to the much higher LUMO level of DIR-2EH than PC₆₁BM. This high V_OC value obtained from DIR-2EH is among the highest values reported for non-tandem BHJ OSCs.^24,26 Whereas V_OC and fill factors (FFs) remain mostly unchanged, the PCEs of P3HT:DIR-2EH devices are susceptible to J_SC, which is closely related to the P3HT:DIR-2EH blending ratio.

Table 1 Device performance of P3HT:DIR-2EH BHJ OSCs

Acceptor	D/A ratio	VOC [V]	JSC [mA cm^−2]	FF [%]	PCE [%]	RS [Ω cm^2]
DIR-2EH	1 : 1	1.24	2.64	59.3	1.94	481.2
	1 : 2	1.23	3.81	59.9	2.81	363.3
	1 : 4	1.22	4.29	58.2	3.05	275.9
PC61BM	1 : 1	0.60	8.47	68.4	3.48	3.0

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Fig. 3 J–V curves (top) and EQE spectra (bottom) of P3HT:DIR-2EH and P3HT:PC₆₁BM BHJ OSCs with different D/A ratios.

The external quantum efficiency (EQE) tracks the origin of J_SC in devices. From the EQE spectra of P3HT:DIR-2EH devices, the magnitude of EQE steadily enhanced with increasing DIR-2EH ratios and similar trend was observed for the corresponding J_SC. Moreover, all DIR-2EH based devices exhibit EQE peaked around 420–470 nm that is different from ∼510 nm of the device based on PC₆₁BM electron acceptor. Considering the second reduction potential of CV, the rising EQE around 420–470 nm probably can be due to the absorption energy of charge transporting dianion state of DIR-2EH. Such EQE results indicate that DIR-2EH molecules photogenerate current not only absorb photons in BHJ OSCs.

The PCEs of P3HT:DIR-2EH BHJ OSCs improved with increasing DIR-2EH ratios. The optimum D/A ratio was 1 : 4 and achieved the best PCE of 3.05% with the highest short-circuit current density (J_SC) of 4.29 mA cm⁻², although such J_SC was just about half of that of P3HT:PC₆₁BM device. We believe that the much lower J_SC is unlikely due to the insufficient electron transfer or insufficient LUMO–LUMO offset between P3HT and DIR-2EH. First, the LUMO–LUMO offset is 0.3 eV (see Fig. 2 right), which is sufficiently large for the charge separation of P3HT exciton.³⁵ Second, a high PCE all polymer solar cell with very small LUMO–LUMO offset (<0.1 eV) between donor and acceptor has been reported recently.³⁶ On the other hand, the lower J_SC could be attributed to the poor carrier selectivity at outer contacts of devices (e.g., highly donor-rich phase attaching the cathode surface), which could also result both lower FF and higher R_S in BHJ OSC devices.³⁷ As known or demonstrated for P3HT:PC₆₁BM BHJ OSCs, PC₆₁BM has a preferential tendency of bottom distribution,³⁸ which is probably no such tendency in P3HT:DIR-2EH BHJ OSC.

Similar to the reverse trend of J_SC, the R_S steadily decreased with increasing D/A ratios, manifesting better charge transport is attainable with higher DIR-2EH content in P3HT thin film. Accordingly, we use the space-charged-limit-current (SCLC) method (details are in ESI†) to assess the charge transport property of P3HT:DIR-2EH blended thin film. The 1 : 4 blended thin film of P3HT:DIR-2EH showed an electron mobility of 1.26 × 10⁻⁸ m² V⁻¹ s⁻¹, which is substantially higher than 6.89 × 10⁻⁹ m² V⁻¹ s⁻¹ of 1 : 1 blended thin film. However, the electron mobility of P3HT:DIR-2EH (1 : 4) blend is more than one order lower than 1.99 × 10⁻⁷ m² V⁻¹ s⁻¹ of P3HT:PC₆₁BM (1 : 1) blend based on our SCLC measurements. Inferior electron mobility may be one of the reasons for the much lower J_SC of P3HT:DIR-2EH than P3HT:PC₆₁BM devices.

Regarding the higher DIR-2EH content shows the better PCE, it is evident from morphology study. Fig. 4 shows the AFM images of P3HT:DIR-2EH blend thin film with three different DIR-2EH ratios and the blend thin film of P3HT:PC₆₁BM (1 : 1). The P3HT:DIR-2EH images exhibit less small-size agglomerates with increasing DIR-2EH content. It is interesting to note that the morphology of P3HT:DIR-2EH blend thin film with higher DIR-2EH ratios is getting closer to the morphology of P3HT:PC₆₁BM blend thin film. A larger size of agglomerates may afford a longer bicontinuous charge transport channels and a less grain boundary that is obstacle to charge transport. The morphology observation is consistent with the electron mobility estimated by SCLC and EQE spectra, J_SC and R_S of fabricated BHJ OSCs.

Fig. 4 AFM Images (10 × 10 μm) of blend thin films based on P3HT:DIR-2EH (1 : 1, 1 : 2, 1 : 4) or P3HT:PC₆₁BM (1 : 1).

In conclusion, we report a novel all-carbon non-fullerene electron acceptor material based on diindeno[1,2-g:1′,2′-s]rubicene (DIR) for the application of BHJ OSCs. This novel electron acceptor material exhibits a high LUMO level, which results a relatively high V_OC larger than 1.2 V in P3HT-based BHJ OSCs. Device's electrical properties of J_SC and R_S, electron mobility of blended thin film, and the thin film morphology all have been demonstrated susceptible to the blend ratios of P3HT:DIR-2EH. The device with the highest ratio of 1 : 4 yielded the best PCE of 3.05%, which is one of few P3HT-based non-fullerene BHJ OSCs having PCE over 3%.^18,39 Our results have demonstrated that easily synthesized and processed rubicene is a promising core structure for the development of high performance non-fullerene electron acceptor materials used in BHJ OSCs.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details of syntheses and characterizations. See DOI: 10.1039/c4ra12505c

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