An azafullerene acceptor for organic solar cells

Abstract

A novel azafullerene derivative, OQThC₅₉N, was prepared and used as the acceptor material in solution-processed bulk heterojunction organic solar cells. Possessing a relatively high LUMO level, OQThC₅₉N gives a high open-circuit voltage of 0.78 V and a power conversion efficiency of 4.09% in fullerene:P3HT solar cells.

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Azafullerenes are novel carbon clusters with one or more fullerene cage carbons replaced by nitrogen atoms.¹ Wudl et al. reported the first bulk preparation of structurally defined azafullerenes, (C₅₉N)₂ and RC₅₉N, via a cage-opened fullerene ketolactam.² Hirsch et al. also succeeded in the synthesis of (C₅₉N)₂ and RC₅₉N using a bisazafulleroid as the starting material.³ Gan et al. prepared azafullerene derivatives with peroxide addends, R₄XC₅₉N (R = ^tBuOO, X = H, OH, or Br).⁴ Fullerenes and their derivatives have been used as acceptor materials in organic solar cells (OSCs) due to their high electron affinity, unique sphericity, and excellent electron-transporting properties.⁵ The power conversion efficiency of bulk heterojunction (BHJ) OSCs containing a conjugated polymer donor and a fullerene acceptor has surpassed 9%.⁶ Since nitrogen contains one more electron than carbon does, replacing C by N in fullerene can be viewed as a further n-doping to fullerene.⁷ Solar cells using azafullerenes have not been reported yet. In this work, an azafullerene derivative (OQThC₅₉N) was synthesized and used as the acceptor material in OSCs. OQThC₅₉N possesses a lower optical bandgap and a higher LUMO level than [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). OQThC₅₉N shows better performance than PC₆₁BM in fullerene:P3HT solar cells.

We first prepared mono-arylated azafullerene ThC₅₉N following Hirsch's one-pot method.⁸ Heating fullerene ketolactam 1 in the presence of air and excessive p-toluenesulfonic acid (p-TsOH) generated azafullerenium carbocation C₅₉N⁺, which was then trapped by electron-rich thiophene to give green ThC₅₉N in 22% yield (Scheme 1). ¹H NMR spectrum showed three peaks at 8.32, 7.80, and 7.50 ppm, respectively, corresponding to three aromatic H of thiophene. ¹³C NMR spectrum showed 34 peaks for sp² carbons ranging from 123.66 to 153.49 ppm and one peak at 99.36 ppm for one sp³ carbon on fullerene, which is consistent with the C_s symmetry of ThC₅₉N.⁹ To obtain solution-processable acceptors, we further modified ThC₅₉N with o-quinodimethane diene through a Diels–Alder reaction.¹⁰ OQThC₅₉N was obtained in 36% yield. OQThC₅₉N consists of regioisomers as indicated by NMR. On ¹H NMR spectrum of OQThC₅₉N, the integral ratio between aromatic H (7.34–8.46 ppm) and aliphatic H (3.73–4.98 ppm) is 7 : 4, indicating that only 1 equiv. diene was added to ThC₅₉N. High resolution ESI mass spectrum for OQThC₅₉N showed the expected molecular ion peak (M + H⁺) at 910.0704 m/z. Compared with ThC₅₉N, OQThC₅₉N shows a good solubility in organic solvents, e.g. 37 mg mL⁻¹ in o-dichlorobenzene (ODCB).

Scheme 1 Synthesis of OQThC₅₉N.

Optical and electrochemical properties of azafullerenes ThC₅₉N and OQThC₅₉N were studied and compared with that of PC₆₁BM. Azafullerenes showed broader absorption extending to ∼850 nm (Fig. 1). Optical bandgaps estimated from absorption onsets are 1.45 and 1.47 eV for ThC₅₉N and OQThC₅₉N, respectively, which are much smaller than that of PC₆₁BM (1.71 eV). ThC₅₉N showed four characteristic absorption peaks at 319, 443, 715, and 799 nm, respectively, which are typical for mono-substituted azafullerenes.⁹ OQThC₅₉N showed lower absorbance in UV region but higher absorbance in both visible and NIR regions than PC₆₁BM, suggesting that OQThC₅₉N could produce higher photocurrent in OSCs.¹¹ The LUMO levels of ThC₅₉N, OQThC₅₉N, and PC₆₁BM were estimated from their first reduction potentials (E₁) by using the empirical equation, LUMO = −(E₁ + 4.8) eV (Fig. 2). Compared with PC₆₁BM's LUMO (−3.67 eV), the LUMO level for ThC₅₉N (−3.72 eV) slightly drops, while that for OQThC₅₉N (−3.56 eV) hoists. 0.16 eV hoist in LUMO level from ThC₅₉N to OQThC₅₉N results from the shrinkage of fullerene π system.^10c High LUMO level favors to enhance V_oc.¹²

Fig. 1 Absorption spectra for PC₆₁BM, ThC₅₉N and OQThC₅₉N in CHCl₃ (10⁻⁵ mol L⁻¹).

Fig. 2 Cyclic voltammograms for PC₆₁BM, ThC₅₉N and OQThC₅₉N.

Solar cells with a structure of ITO/PEDOT:PSS/fullerene:P3HT/Ca/Al were studied. Donor/acceptor (D/A) ratio, film thickness, and annealing temperature were optimized for OQThC₅₉N cells (Table S1–S3†). OQThC₅₉N cells with a D/A ratio of 1 : 0.6 (w/w), an active layer thickness of ∼100 nm, being annealed at 110 °C for 10 min afforded the best results, giving a V_oc of 0.78 V, a J_sc of 7.57 mA cm⁻², a FF of 69.2%, and a PCE of 4.09%. Under the same conditions, PC₆₁BM cells afforded a lower PCE of 3.67%, with a V_oc of 0.65 V, a J_sc of 8.10 mA cm⁻², and a FF of 69.8% (Fig. 3 and Table 1). Obviously, the better performance of OQThC₅₉N cells benefits from V_oc, which is 0.13 V higher than that of PC₆₁BM cells. The increase in V_oc is consistent with higher LUMO of OQThC₅₉N. OQThC₅₉N cells showed lower external quantum efficiency (EQE) than PC₆₁BM cells in 400–650 nm region, corresponding with the lower J_sc of OQThC₅₉N cells (Fig. S5†). The expected stronger film absorption and EQE response in the long wavelength region was not observed for OQThC₅₉N cells (Fig. S6†), which might be due to small absorption coefficient of OQThC₅₉N in this region.

Fig. 3 J–V curves for OQThC₅₉N:P3HT and PC₆₁BM:P3HT solar cells.

Table 1 Performance for fullerene:P3HT solar cells under AM 1.5G illumination (100 mW cm⁻²)

Fullerene	Voc [V]	Jsc [mA cm^−2]	FF [%]	PCE^a [%]	μe [cm^2 V^−1 s^−1]
a The highest PCEs.
OQThC59N	0.78	7.57	69.2	4.09	8.9 × 10^−5
PC61BM	0.65	8.10	69.8	3.67	3.0 × 10^−4

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Electron mobility of fullerene was measured by space charge limited current (SCLC) method. The mobility was determined by fitting the dark current to the model of a single carrier SCLC.¹³ Fig. 4(a) shows J–V curves for the electron-only devices. The mobility was calculated from the slope of J^1/2–V lines. OQThC₅₉N possesses a lower electron mobility of 8.9 × 10⁻⁵ cm² V⁻¹ s⁻¹ than that of PC₆₁BM (3.0 × 10⁻⁴ cm² V⁻¹ s⁻¹). Atomic force microscope (AFM) indicates that OQThC₅₉N:P3HT blend film is coarser than PC₆₁BM:P3HT blend film (Fig. 5). The root-mean-square (RMS) roughnesses for OQThC₅₉N:P3HT and PC₆₁BM:P3HT blend films are 2.6 and 1.4 nm, respectively, suggesting that OQThC₅₉N might possess lower miscibility with P3HT than PC₆₁BM. Lower mobility of OQThC₅₉N might account for lower J_sc of OQThC₅₉N cells.

Fig. 4 J–V curves (a) and the corresponding J^1/2–V curves (b) for electron-only devices (in dark). The thicknesses for OQThC₅₉N:P3HT and PC₆₁BM:P3HT blend films are 110 nm and 101 nm, respectively.

Fig. 5 AFM images (height) for OQThC₅₉N:P3HT (left) and PC₆₁BM:P3HT (right) blend films.

In summary, for the first time we have introduced an azafullerene, OQThC₅₉N, as an acceptor used in OSCs. The 4.09% PCE and 0.78 V V_oc given by OQThC₅₉N:P3HT solar cells demonstrate the potential of azafullerenes. There is plenty of room for developing efficient azafullerene acceptors by improving the mobility and the miscibility with donor materials.

Acknowledgements

This work was supported by the “100 Talents Program” of Chinese Academy of Sciences and National Natural Science Foundation of China (21374025, 21372053 and 21102028). Z. X. appreciates the support from Beijing National Laboratory for Molecular Sciences. We thank Gang Ye and Shan Chen for technical assistance.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details including synthesis, measurements, and instruments. See DOI: 10.1039/c4ra02757d

‡ Z. Xiao and D. He contributed equally to this work.

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