A spiro-bifluorene based 3 D electron acceptor with dicyanovinylene substitution for solution processed non-fullerene organic solar cells

A novel electron acceptor, namely 2,2’-(12H,12'H-10,10'-spirobi[indeno[2,1-b] fluorene]-12,12'diylidene) dimalononitrile (4CN-spiro), exhibiting three-dimensional molecular structure was synthesized and its thermal, photophysical, electrochemical, crystal, and photovoltaic properties were investigated. The novel acceptor exhibits excellent thermal stability with a decomposition temperature of 460 C, an absorption extending to 600 nm, and a LUMO level of -3.72 eV. Solution processed bulkheterojunction (BHJ) organic solar cells were fabricated using 4CN-spiro as acceptor and polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) as donor polymer. The effect of the donor-toacceptor ratio and processing conditions on device performance was investigated. A device processed from tetrachloroethane with a donor to acceptor weight ratio of 1:1 yielded a power conversion efficiency (PCE) of 0.80 %.


Introduction
Organic photovoltaic (OPV) devices that can be fabricated by different techniques, such as vacuum deposition, 1-3 solution processing and roll-to-roll printing, 4,5 have received a great deal of attention. Remarkable progress has been made on solutionprocessed, bulk-heterojunction (BHJ) devices, [6][7][8][9][10][11][12][13] and the reported power conversion efficiency (PCE) has recently exceeded 10 % in single junction cells. 14 Undoubtedly, fullerene-based electron acceptors have played a vital role in achieving high PCEs owing to their high electron mobility, strong electron affinity and threedimensional (3D) electron transport. [15][16][17][18] However, fullerene acceptors have a few obvious disadvantages, such as low absorption in the visible region, limited energy-level variability, as well as tough synthesis and purification, which limit further improvements of fullerene-based OPV devices and their commercial application.

Optical and electrochemical properties
Fig. 1b shows the absorption spectra of 2O-spiro and 4CN-spiro in dichloromethane solution and as thin solid film. The absorption bands of 2O-spiro and 4CN-spiro in the thin film spectra are only slightly red-shifted compared to those in solution. This is different from the previously reported 3D star-shaped materials 34,37 , whose absorption bands are significantly red-shifted in solid state, which might be due to the different electron donating and electron withdrawing groups (push-pull effect) The optical gap (E g ) estimated from the absorption edge of the solution spectrum is 2.56 eV for 2O-spiro and 2.08 eV for 4CN-spiro, respectively.
The electrochemical properties of 2O-spiro and 4CN-spiro were investigated by cyclic voltammetry (CV) in dichloromethane solution with 0.1 M nBu 4 NPF 6 as supporting electrolyte. As shown in Fig. 1c, both 2O-spiro and 4CN-spiro exhibit reversible reduction waves. No oxidation waves could be observed in the measured potential range. The half-wave potential of 2O-spiro is -1.29 V. Upon dicyanovinylene functionalization, the reduction potentials of 4CN-spiro shift to more positive values with the E 1/2 potentials at -0.72, -1.31 and -1.44 V, respectively. The overlapping second and third reduction waves of 4CN-spiro with a lower associated current are similar to those of dicyanovinylene-functionalized(bis)indenofluorenes. 43

Crystallography
Single crystals of the new compounds 2O-spiro and 4CN-spiro were grown by slow evaporation of the solvent mixture dichloromethane/hexane, and pure dichloromethane, respectively. The crystal structure determined by X-ray diffraction is presented in Fig.  2. The functionalized 9,9-spiro-bifluorene consists of two identical fluorene π-systems, which are perpendicular to each other via a common sp 3 -hybridized carbon atom. Crystal packing of the two molecules is dominated by π-π stacking interactions. The π-stacking of 2O-spiro obviously extends into three dimensions in the single crystal, as it can clearly be seen from the packing structure (Fig. 2a). Three π-stacking axes are almost perpendicular to each other. To the best of our knowledge, this is the first example of an organic semiconductor that can adopt a 3D isotropic π-stacking. In case of the acceptor 4CN-spiro π-stacking with interplanar distances of ca. 3.48 and 3.40 Å along the a axis and b axis (Fig. 2b) was observed. Therefore, the packing geometry of the two compounds indicates the possibility of π-π interaction, implying that isotropic electron transport pathways as in fullerene derivatives could potentially be formed in donor-acceptor BHJ solar cells.

Photovoltaic properties
To demonstrate the potential application of 4CN-spiro as acceptor in photovoltaic devices, it was blended with PTB7 47 as electron donor polymer, whose absorption band is more red-shifted than that of P3HT and partially complementary to the absorption of 4CN-spiro. BHJ OPV cells of the structure ITO/PEDOT:PSS/PTB7:4CN-spiro/Ca/Al were prepared. In addition, the effects of varying processing solvents and blend composition were investigated. Table 1 summarizes the obtained open-circuit voltage (V oc ), short-circuit current density (J sc ), fill factor (FF), and PCE of the devices. The device prepared from tetrachloroethane with a donor-acceptor weight ratio of 1:2 exhibited an open-circuit voltage of 0.87 V, a shortcircuit current density of 1.13 mA cm -2 , a fill factor of 0.49, and a power conversion efficiency of 0.64 %. Encouragingly, the blend at a donor-acceptor weight ratio of 1:1 demonstrated an increased PCE of up to 0.80 % with a V oc , J sc and FF of 0.89 V, 1.41 mA cm -2 and 0.48, respectively. We note, that tetrachloroethane is a rather uncommon solvent in photovoltaic device preparation, however, TCE is also a good solvent for the acceptors used in the present study. AFM height images show a homogeneous surface structure of the blend with demixing on the length scale of exciton diffusion, which ensures efficient exciton quenching and dissociation (Fig. 4). We have also tried further  device optimization including the use of solvent additives. In fact, a small amount of 1,8-diiodooctane (DIO) in a volume ratio of 3 % was used as solvent additive to improve the photovoltaicperformance in PTB7:PCBM devices. 47 However, in the present case the device prepared with the additive exhibited no response to light at all, even though the AFM images indicated a rather uniform surface structure (Fig. S2). This implies that other processes such as geminate recombination of interfacial charge-transfer (CT) states or insufficient charge carrier transport due to a lack of charge carrier percolation pathways limit the photovoltaic performance. Furthermore, we have also investigated the device performance upon using DCB as solvent or a mixture of DCB and tetrachloroethane. However, in these cases, the AFM images indicated the formation of large crystallites as high RMS values were observed. This is detrimental for device efficiency (Fig. S2) as demonstrated also by the low PCE value. We note that even for the optimized device only a moderate short circuit current and fill factor were obtained compared to devices that use fullerene as acceptor. The latter is a consequence of the pronounced bias dependence of the current density, which does not even saturate at high negative bias, as previously also observed by us for a polymer-PDI blends. 50 To better understand the origin of the former, that is, the moderate short circuit current of the PTB7:4CN-spiro blends spun from C 2 D 2 Cl 4 , transient absorption (TA) spectroscopy was performed on the picosecond to nanosecond timescale. The transient absorption spectra shown in Fig. 5 a) are dominated by a positive feature in the spectral region from 540 to 830 nm peaking at 685 nm, which we assigned to a combination of the ground-state bleaching (GSB) of PTB7, as it coincides with the ground state absorption of the polymer and stimulated emission (SE) of PTB7 singlet excitons. The observation of SE points towards inefficient exciton quenching in these blends, in part explaining the lower device performance compared to PTB7:fullerene blends. Fig. 5 b) shows the decay dynamics of the GSB at various excitation intensities. Clearly, the signal decay is independent of the excitation intensity pointing towards geminate recombination of charges that do not manage to entirely dissociate into free charges. We note that a substantial fraction of > 80% of the initially created excited states decays on the sub-ns timescale and consequently does not contribute to the photocurrent of the device. Thus, we conclude that the devices are limited by both inefficient exciton quenching as well as pronounced geminate recombination of bound charges on the sub-ns timescale.

Conclusions
In conclusion, a novel 3D acceptor 4CN-spiro containing spiro-bifluorene as core was synthesized and fully characterized. As revealed by single-crystal analysis, the new compound presents the possibility of isotropic charge transport, which is similar to the situation in fullerene derivatives. The solar cells based on PTB7:4CN-spiro processed from tetrachloroethane yield the highest PCE of 0.80 %. Our results demonstrated for the first time that dicyanovinylene substituted 4CN-spiro could be an alternative to fullerene-based acceptors. However, we also observed that device performance is limited by incomplete exciton quenching and fast geminate recombination on the sub-ns timescale. Further experiments are required to determine whether the bias dependence of the photocurrent is caused by field-dependent charge generation or limited by a low charge carrier mobility leading to a competition of charge extraction and non-geminate recombination or a combination of both processes. Finally, future work will aim towards new acceptor structures to improve charge separation and to overcome the limits set by geminate recombination and the bias dependence observed in the present study.

12H,12'H-10,10'-Spirobi[indeno[2,1-b]fluorene]-12,12'-dione (2O-spiro)
A mixture of 1 (1.40 g, 1.80 mmol) and palladium acetate (0.16 g, 0.72 mmol) in dry dimethylacetamide (100 mL) was heated to 130 o C overnight under argon atmosphere. The mixture was cooled to room temperature and the solvent was evaporated under vacuum . Then, water was added. The mixture was extracted with ethyl acetate, and the organic extracts were washed with 2 M HCl, dried over sodium sulfate and concentrated. The residue was purified on silica gel with dichloromethane to provide the green product in 65% yield. 1

Materials and Characterization
1 H NMR and 13 C NMR spectra were recorded in deuterated solvents such as CD 2 Cl 2 , using a Bruker DPX 500 spectrometer, with the solvent proton or carbon signal as an internal standard. FD mass spectra were performed with a VG-Instrument ZAB 2-SE-FDP. High resolution mass spectra (HRMS) were carried out by the Microanalytical Laboratory of Johannes Gutenberg-University, Mainz. UV-vis absorption spectra were recorded at room temperature using a Perkin Elmer Lambda 900 spectrophotometer. Fluorescence spectra were recorded on a SPEX-Fluorolog II (212) spectrometer. CV measurements were carried out on a computer-controlled GSTAT12 in a three-electrode cell in a DCM solution of Bu 4 NPF 6 (0.1 M) with a scan rate of 100 mV/s at room temperature, with glassy carbon discs as the working electrode and Pt wire as the counter electrode, Ag/AgCl electrode as the reference electrode. Thermogravimetric analysis (TGA) was carried out on a Mettler 500 at a heating rate of 10 º C/min under nitrogen flow. All reagents and starting materials were obtained from commercial suppliers and used without further purification. Column chromatography was performed on silica gel 60 (Macherey-Nagel, Si60) with dichloromethane, hexane, ethyl acetate or tetrahydrofuran (Sigma-Aldrich). All reported yields are isolated yields.

Solar cell preparation and measurement
Solar cells were fabricated on patterned ITO-coated glass substrates (Präzisions Glas & Optik GmbH, Germany). Cleaning included successive ultrasonication in detergent, acetone and iso-propanol. Furthermore, the samples were treated with an argon plasma before spincoating a ~40 nm thick poly (3,4- IV characteristics were obtained under illumination with a solar simulator (K.H. Steuernagel Lichttechnik GmbH, Germany) using a 575 W metal halide lamp in combination with a filter system to create a spectrum according to AM1.5G conditions. Yet, the intensity was at 70 mW cm -2 . Current-voltage curves were measured with a Keithley 236 Source Measure Unit (SMU) wihin a glovebox. The light intensity was measured with a calibrated silicon photodiode.
AFM images were taken with a Dimension Icon FS with ScanAsyst using an Olympus OMCL-AC 240TS-W2 Cantilever Type at F 0 = 70 kHz in non-contact mode. Transient absorption spectroscopy was described previously by our group. Transient absorption (TA) measurements were performed with a home-built pump-probe setup. To measure in the time range of 1-4 ns with a resolution of ~100 fs, the output of a commercial titanium:sapphire amplifier (Coherent LIBRA-HE, 3.5 mJ, 1 kHz, 100 fs) was split into two beams that pumped two independent commercial optical parametric amplifiers (Coherent OPerA Solo). One optical parametric amplifier (OPA) was used to generate the tunable excitation pulses in the visible, while the second OPA was used to generate the seed beam for white-light generation. For measurements in the spectral range between 550-1100 nm a 1300 nm seed of a few µJ was focused into a c-cut 3 mm thick sapphire window for white-light generation. The variable delay of up to 4 ns between pump and probe was introduced by a broadband retroreflector mounted on a mechanical delay stage. Mostly reflective elements were used to guide the probe beam to the sample to minimize chirp. The excitation pulse was chopped at 500 Hz, while the white-light pulses were dispersed onto a linear silicon photodiode array, which was read out at 1 kHz by home-built electronics. Adjacent diode readings corresponding to the transmission of the sample after an excitation pulse and without an excitation pulse were used to calculate ∆T/T.