Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of

Publication Date (Web): May 19, 2009 ... The physical properties of these polymers were finely tuned to optimize their photovoltaic effect. ..... Moha...
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Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties Yongye Liang,† Danqin Feng,† Yue Wu,‡ Szu-Ting Tsai,‡ Gang Li,*,‡ Claire Ray,† and Luping Yu*,† Department of Chemistry and the James Franck Institute, The UniVersity of Chicago, 929 E. 57th Street, Chicago, Illinois 60637, and Solarmer Energy Inc., 3445 Fletcher AVenue, El Monte, California 91731 Received February 27, 2009; E-mail: [email protected]; [email protected]

Abstract: This paper describes synthesis and photovoltaic studies of a series of new semiconducting polymers with alternating thieno[3,4-b]thiophene and benzodithiophene units. The physical properties of these polymers were finely tuned to optimize their photovoltaic effect. The substitution of alkoxy side chains to the less electron-donating alkyl chains or introduction of electron-withdrawing fluorine into the polymer backbone reduced the HOMO energy levels of polymers. The structural modifications optimized polymers’ spectral coverage of absorption and their hole mobility, as well as miscibility with fulleride, and enhanced polymer solar cell performances. The open circuit voltage, Voc, for polymer solar cells was increased by adjusting polymer energy levels. It was found that films with finely distributed polymer/fulleride interpenetrating network exhibited improved solar cell conversion efficiency. Efficiency over 6% has been achieved in simple solar cells based on fluorinated PTB4/PC61BM films prepared from mixed solvents. The results proved that polymer solar cells have a bright future.

Introduction

Semiconducting polymers have shown physical properties similar to those of typical inorganic semiconductors, although the underlying mechanisms are usually different from each other.1 Photovoltaic effect is one of these properties that stimulate people’s enthusiasm because solar energy conversion into electricity is the cleanest way to harvest this vast renewable energy source. So far, the most efficient architecture to build polymeric photovoltaic solar cells is the bulk heterojunction (BHJ) structure prepared by mixing electron-rich polymers and electron-deficient fullerides.2 This approach can be easily implemented and allows a quick survey of the best composition of polymeric active films. Detailed studies by many groups in the past years have identified poly(3-hexylthiophene) (P3HT) as the most attractive donor material. Power conversion efficiency (PCE) of about 5% has been achieved in solar cells based on P3HT/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) derivatives.3 Although it is significant progress in a relatively short period of research time, it is still far away from commercial viability.4 After an exhaustive research effort, it becomes increasingly apparent that the PCE of solar cells based on P3HT/PC61BM is approaching its limit. New materials exhibiting better performance are needed in order to achieve the desired performance in these types of solar cells for practical application.5 †

The University of Chicago. Solarmer Energy Inc. (1) Skotheim, T. A.; Reynolds J. Handbook of conducting polymers; CRC: London, 2007. (2) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (b) Gnes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324. ‡

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The performance of polymer solar cells is characterized by three parameters: open-circuit voltage (Voc), short-circuit current density (Jsc), and fill-factor (FF), all of which are related to the PCE by the following equation: PCE ) (Voc × Jsc × FF)/ (Ip × M), where Ip is the power density of the incident light irradiation and M is spectral mismatch factor. It has been realized that the ideal polymer in BHJ structure should exhibit a broad absorption with high coefficient in the solar spectrum, high hole mobility, suitable energy level matching to fulleride, and appropriate compatibility with fulleride to form bicontinuous interpenetrating network on a nanoscale.6 It is difficult to design a polymer to fulfill all these requirements. Current polymer solar cells often suffer from small values in some or all of these parameters due to a variety of issues related to the nature of materials and device engineering. So far, besides the P3HT system, there are very few polymer solar cell systems reported which exceed 5% in power conversion efficiency.7 (3) (a) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (b) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K. H.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617. (c) Li, G.; Shrotriya, V.; Yao, Y.; Yang, Y. J. Appl. Phys. 2005, 98, 043704. (4) Scharber, M.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. AdV. Mater. 2006, 18, 789. (5) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (6) Roncali, J. Macromol. Rapid Commun. 2007, 28, 1761. (7) (a) Muhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z. G.; Waller, D.; Gaudiana, R.; Brabec, C. AdV. Mater. 2006, 18, 2884. (b) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497. (c) Wang, E. G; Wang, L; Lan, L. F.; Luo, C.; Zhuang, W. L.; Peng, J. B.; Cao, Y. Appl. Phys. Lett. 2008, 92, 33307. (d) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2008, 130, 16144. 10.1021/ja901545q CCC: $40.75  2009 American Chemical Society

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Figure 1. Synthetic routes for polymers PTB1-PTB6 and structure of PC61BM. Scheme 1. Synthesis Routes for Monomers

Experimental Section

Recently, we have developed a new polymer, namely PTB1, based on alternating thieno[3,4-b]thiophene and benzodithiophene units (Figure 1). Simple single-layer polymer solar cells exhibited solar conversion efficiency of 4.8% based on PTB1/ PC61BM BHJ structure and 5.6% on PTB1/PC71BM structure.8 The Jsc and FF obtained from such polymer solar cells are among the highest values reported for solar cell system based on lowband-gap polymers. However, the Voc of the polymer solar cells is relatively small, just about 0.56-0.58 V. In this paper, we describe our results in the development of new polymers which exhibit higher solar cell conversion efficiencies. These polymers were developed via synthetic fine-tuning of their structural and electronic properties.

Materials. Otherwise stated, all of the chemicals are purchased from Aldrich and used as received. The 4,6-dihydrothieno[3,4-b] thiophene-2-carboxylic acid (1),9 4,6-dibromothieno[3,4-b]thiophene2-carboxylic esters,8 1,5-bis(trimethyltin)-4,8-dioctylbenzo[1,2-b: 4,5-b’]dithiophene,7 and benzo[1,2-b:4,5-b′b′]dithiophene-4,8dione10 were synthesized according to the procedures reported in the literature. Other monomers were synthesized according to Scheme 1. 3-Fluoro-4,6-dihydrothieno[3,4-b]thiophene-2-carboxylic acid (2). The 4,6-dihydrothieno[3,4-b]thiophene-2-carboxylic acid (1.46 g, 7.85 mmol) was dissolved in 60 mL of THF and cooled in an acetone/dry ice bath under nitrogen protection. Butyllithium solution ((6.9 mL, 17.3 mmol) was added dropwise with stirring. The

(8) Liang, Y. Y.; Wu, Y.; Feng, D. Q.; Tsai, S. T.; Son, H. J.; Li, G.; Yu, L. P. J. Am. Chem. Soc. 2009, 131, 56.

(9) Yao, Y.; Liang, Y. Y.; Shrotriya, V.; Xiao, S. Q.; Yu, L. P.; Yang, Y. AdV. Mater. 2007, 19, 3979. (10) Beimling, P; Koβmehl, G. Chem. Ber. 1986, 119, 3198. J. AM. CHEM. SOC.

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resulting mixture was kept in a dry ice bath for 1 h. Then N-fluorobenzenesulfonimide (3.22 g, 10.2 mmol) in 20 mL of THF was added dropwise, and the solution was stirred at RT overnight. The reaction was quenched with 50 mL of water, and the organic solvent was removed by evaporation at reduced pressure. The solid residue was collected by filtration and purified by chromatography on silica with ethyl acetate. A mixture of 1.30 g containing fluorinated product and unfluorinated reactant with a 4:1 ratio was obtained. The calculated mass of fluorinated product is 1.04 g, 65%. 1 HNMR (D6-DMSO): δ 4.01-4.05 (2H, t, J ) 3 Hz), 4.20-4.24 (2H, t, J ) 3 Hz). MS (EI): Calcd, 204.0; found (M - 1)-, 202.9. Octyl 3-Fluoro-4,6-dihydrothieno[3,4-b]thiophene-2-carboxylate (3). The raw materials of 2 (1.30 g, 6.3 mmol), DCC (1.58 g), and DMAP (260 mg) were added to a 50 mL round-bottom flask with CH2Cl2 (15 mL). 1-Octanol (8.22 g, 63 mmol) was added to the flask and then stirred for 20 h under N2 protection. The reaction mixture was poured to 100 mL of water and extracted with CH2Cl2. The organic phase was dried by sodium sulfate, and the solvent was removed. Column chromatography on silica gel using hexane/ CH2Cl2 ) 1/1 yielded the title compound as an oil (1.42 g, 71%). 1 HNMR (CDCl3): δ 0.85-0.91 (3H, t, J ) 6 Hz), 1.22-1.45 (10H, m), 1.68-1.76 (2H, m), 3.99-4.02 (2H, t, J ) 3 Hz), 4.15-4.18 (2H, t, J ) 3 Hz), 4.24-4.29 (2H, t, J ) 6 Hz). MS (EI): Calcd, 316.1; found (M + 1)+, 317.0. Octyl 3-Fluorothieno[3,4-b]thiophene-2-carboxylate (5). A solution of compound 3 (1.42 g, 4.5 mmol)) in 100 mL of ethyl acetate was stirred and cooled in a dry ice bath. MCPBA (0.78 g, 4.5 mmol) in 30 mL of ethyl acetate was added dropwise to the reaction solution. The resulting mixture was stirred overnight. The solvent was removed by evaporation, and the residue contained a crude product of 4 and 3-chlorobenzoic acid. The residue was refluxed in acetic anhydride for 2.5 h. The mixture was cooled, and the solvent was removed by evaporation. The residue was purified by flash chromatography on silica gel with hexanes/ dichloromethane (2:1) to give compound 5 (0.95 g, 67%). 1HNMR (CDCl3): 0.85-0.92 (3H, t, J ) 7 Hz), 1.23-1.48 (10H, m), 1.70-1.80 (2H, m), 4.30-4.35 (2H, t, J ) 7 Hz), 7.27-7.29 (1H, d, J ) 3 Hz), 7.65-7.67 (1H, d, J ) 3 Hz). Octyl 4,6-Dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate (6). To a solution of compound 5 (0.95 g, 3.0 mmol) in 10 mL of DMF was added dropwise a solution of NBS (1.34 g, 7.56 mmol) in 10 mL of DMF under nitrogen protection in the dark. The reaction mixture was stirred at RT for 24 h. Then it was poured into saturated sodium sulfite solution in an ice-water bath and extracted with dichloromethane. The organic phase was collected and dried by sodium sulfate. Removal of the solvent and column purification on silica get using dichloromethane/hexane (1/4) yielded the target product (0.98 g, 69%) as a light yellow solid. 1HNMR (CDCl3): 0.85-0.92 (3H, t, J ) 7 Hz), 1.23-1.48 (10H, m), 1.70-1.80 (2H, m), 4.30-4.35 (2H, t, J ) 7 Hz). MS (EI): Calcd, 469.9; found (M + 1)+, 471.9. 4,8-Bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (8). The benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (1.0 g, 4.5 mmol) was mixed with zinc dust (0.65 g, 10 mmol) in a flask. Ethanol (4 mL) and NaOH solution (15 mL, 20%) were added, and the mixture was refluxed for 1 h. 2-Ethylhexyl p-toluenesulfonate (4.3 mL) was added in portions with stirring until the color changed to red. The resulting precipitate was filtered; the filtrate was diluted with 100 mL of water and extracted with chloroform (100 mL). The organic extraction was dried with anhydrous sodium sulfate and evaporated in vacuo. Column chromatography on silica gel using dichloromethane and hexanes mixed eluents yielded the compound 8 as a light yellow oil (0.80 g, 40%). 1HNMR (CDCl3): δ0.72-0.90 (12H, m), 1.10-1.38 (16H, m), 1.50-1.62 (2H, m), 3.84-3.97 (4H, m), 7.32-7.36 (2H, d, J ) 5 Hz), 7.46-7.50 (2H, d, J ) 5 Hz). MS (EI): Calcd, 446.2; found (M + 1)+, 447.2. 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′] dithiophene (9). Compound 8 (0.62 g, 1.4 mmol) was dissolved in 20 mL of anhydrous THF and cooled in an acetone/dry ice bath under 7794

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nitrogen protection. Butyllithium solution (1.4 mL, 3.5 mmol) was added dropwise with stirring, after the addition the mixture was kept in a dry ice bath for 30 min and at RT for 30 min. The mixture was cooled in the dry ice bath and trimethyltin chloride solution (4.2 mL, 4.2 mmol, 1 M in hexane) was added, and the mixture was stirred at RT overnight. The mixture was quenched with 50 mL of water and extracted with hexanes. The organic extraction was dried with anhydrous sodium sulfate and evaporated in vacuo. Recrystallization of the residue from isopropanol yield the compound 9 as colorless needles (0.8670 g, 80%). 1HNMR (CDCl3): δ 0.43 (18H, s), 0.90-1.10 (12H, m), 1.33-1.85 (18H, m), 1.54-1.58 (4H, m), 4.15-4.23 (4H, d, J ) 5 Hz), 7.51 (2H, s). 4,8-Dioctynbenzo[1,2-b:4,5-b′]dithiophene (10). Isopropylmagnesium chloride (2 M solution, 12 mL, 24 mmol) was added dropwise to a solution of 1-octyne (2.97 g, 27 mmol) at RT. The reaction mixture was heated up to 60 °C and stirred for 100 min. It was cooled to RT, and benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (1 g, 4.53 mmol) was added. The reaction mixture was heated up to 60 °C and kept for 60 min. It was then cooled to RT, and 7 g of SnCl2 in HCl solution (16 mL 10%) was added dropwise to the reaction mixture. The reaction mixture was heated at 65 °C for 60 min, then cooled down to room temperature, and poured into 100 mL of water. It was extracted with 50 mL of hexanes twice. The organic phase was combined and dried with anhydrous Na2SO4, and the organic solvent was removed by vacuum evaporation. The residue was purified by column chromatography on silica with hexanes/dichloromethane (3/1, volume ratio), yielding compound 10 (1.66 g, 90%). 1HNMR (CDCl3): δ 0.88-0.96 (6H, t, J ) 5 Hz), 1.32-1.42 (8H, m), 1.53-1.63 (4H, m), 1.68-1.77 (4H, m), 2.61-2.66 (4H, t, J ) 7 Hz), 7.48-7.51 (2H, d, J ) 6 Hz), 7.56-7.58 (2H, d, J ) 6 Hz). MS (EI): Calcd, 406.2; found (M + 1)+, 407.1. 4,8-Dioctylbenzo[1,2-b:4,5-b′]dithiophene (11). To the solution of compound 2 (1.66 g, 4.07 mmol) in 75 mL of THF was added Pd/C (0.45 g, 10%), and the reaction mixture was kept in a hydrogen atmosphere for 18 h at RT. The mixture was filtered with Celite, and the solvent was removed by vacuum evaporation. The residue was purified by column chromatography on silica with hexane as the eluent, yielding compound 11 (0. 95 g, 56%) as white solids. 1 HNMR (CDCl3): δ 0.85-0.92 (6H, t, J ) 7 Hz), 1.20-1.40 (16H, m), 1.40-1.50 (4H, m), 1.76-1.84 (4H, m), 3.13-3.21 (4H, t, J ) 8 Hz), 7.43-7.45 (2H, d, J ) 6 Hz), 7.46-7.48 (2H, d, J ) 6 Hz). MS (EI): Calcd, 414.2; found (M + 1)+, 415.2. 2,6-Bis(trimethyltin)-4,8-dioctylbenzo[1,2-b:4,5-b′]dithiophene (12). Compound 3 (0.95 g,2.3 mmol) was dissolved in 20 mL of anhydrous THF and cooled in an acetone/dry ice bath under nitrogen protection. Butyllithium solution (2.3 mL, 5.7 mmol) was added dropwise with stirring. The mixture was kept in a dry ice bath for 30 min and then at RT for 30 min. The mixture was cooled in the dry ice bath, and 6.5 mL (6.5 mmol) of trimethyltin chloride solution (1 M in hexane) was added and stirred at RT for overnight. The mixture was quenched with 50 mL of water and extracted with hexanes. The organic extraction was dried with anhydrous sodium sulfate and evaporated in vacuo. Recrystallization of the residue from isopropanol yields the titled compound 12 (0.50 g, 88%). 1 HNMR (CDCl3): δ 0.45 (18H, s), 0.87-0.91 (6H, t, J ) 7 Hz), 1.25 -1.42 (16H, m), 1.42-1.51 (4H, m), 1.76-1.85 (4H, m), 3.17-3.23 (4H, t, J ) 8 Hz), 7.49 (2H, s). Synthesis of Polymers. PTB4. Octyl-6-dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate (6) (236 mg, 0.50 mmol) was weighted into a 25 mL round-bottom flask. 2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (9) (386 mg, 0.50 mmol) and Pd(PPh3)4 (25 mg) were added. The flask was subjected to three successive cycles of vacuum followed by refilling with argon. Then, anhydrous DMF (2 mL) and anhydrous toluene (8 mL) were added via a syringe. The polymerization was carried out at 120 °C for 12 h under nitrogen protection. The raw product was precipitated into methanol and collected by filtration. The precipitate was dissolved in chloro-

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form and filtered with Celite to remove the metal catalyst. The final polymers were obtained by precipitating in hexanes and drying in vacuum for 12 h, yielding PTB4 (309 mg, 82%). 1 HNMR (CDCl2CDCl2): δ 0.80-2.40 (45H, br), 3.90-4.70 (6H, br), 7.00-7.90 (2H, br). GPC: Mw (19.3 × 103 g/mol), PDI (1.32). PTB2, PTB3, PTB5, and PTB6 are synthesized according to the same procedure as PTB4 with respective monomers. The 1 HNMR and gel permeation chromatography (GPC) data of the polymers are listed below. PTB2. 1HNMR (CDCl2CDCl2): δ 0.70-2.42 (45H, br), 3.90-4.80 (6H, br), 6.70-8.00 (3H, br). GPC: Mw (23.2 × 103 g/mol), PDI (1.38). PTB3. 1HNMR (CDCl2CDCl2): δ 0.70-2.35 (45H, br), 2.90-3.40 (4H, br), 4.20-4.70 (2H, br), 6.70-8.20 (3H, br). GPC: Mw (23.7 × 103 g/mol), PDI (1.49). PTB5. 1HNMR (CDCl2CDCl2): δ 0.90-2.40 (45H, br), 3.90-4.70 (6H, br), 7.00-7.60 (2H, br), 7.60-8.10 (1H, br). GPC: Mw (22.7 × 103 g/mol), PDI (1.41). PTB6. 1HNMR (CDCl2CDCl2): δ 0.70-2.42 (53H, br), 3.90-4.80 (6H, br), 6.70-8.00 (3H, br). GPC: Mw (25.0 × 103 g/mol), PDI (1.50). Characterization. 1H NMR spectra were recorded at 400 or 500 MHz on Bruker DRX-400 or DRX-500 spectrometers, respectively. Molecular weights and distributions of polymers were determined by using GPC with a Waters Associates liquid chromatograph equipped with a Waters 510 HPLC pump, a Waters 410 differential refractometer, and a Waters 486 tunable absorbance detector. THF was used as the eluent and polystyrene as the standard. The optical absorption spectra were taken by a Hewlett-Packard 8453 UV-vis spectrometer. Cyclic voltammetry (CV) was used to study the electrochemical properties of the polymers. For calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured under the same conditions, and it is located at 0.09 V to the Ag/Ag+ electrode. It is assumed that the redox potential of Fc/Fc+ has an absolute energy level of -4.80 eV to vacuum.11 The energy levels of the highest (HOMO) and lowest unoccupied molecular orbital (LUMO) were then calculated according to the following equations

EHOMO ) -(φox + 4.71) (eV) ELUMO ) -(φred + 4.71) (eV) where φox is the onset oxidation potential vs Ag/Ag+ and φred is the onset reduction potential vs Ag/Ag+. Hole mobility was measured according to a similar method described in the literature,12 using a diode configuration of ITO/ poly(ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS)/polymer/Al by taking current-voltage current in the range of 0-6 V and fitting the results to a space charge limited form, where the SCLC is described by

J ) 9ε0εrµV2/8L3 where ε0 is the permittivity of free space, εr is the dielectric constant of the polymer, µ is the hole mobility, V is the voltage drop across the device, L is the polymer thickness, and V ) Vappl - Vr - Vbi, where Vappl is the applied voltage to the device, Vr is the voltage drop due to contact resistance and series resistance across the electrodes, and Vbi is the built-in voltage due to the difference in work function of the two electrodes. The resistance of the device was measured using a blank configuration ITO/PEDOT:PSS/Al and (11) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bassler, H.; Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551. (12) (a) Malliaras, G. G.; Salem, J. R.; Brock, P. J.; Scott, C. Phys. ReV. B 1998, 58, 13411. (b) Goh, C.; Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Frechet, J. M. J. Appl. Phys. Lett. 2005, 86, 122110.

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was found to be about 10-20 Ω. The Vbi was deduced from the best fit of the J0.5 versus Vappl plot at voltages above 2.5 V and is found to be about 1.5 V. The dielectric constant, εr, is assumed to be 3 in our analysis, which is a typical value for conjugated polymers. The thickness of the polymer films is measured by using AFM. Device Fabrication. The polymers PTB1-PTB6 were codissolved with PC61BM in 1,2-dichlorobenzene (DCB) in the weight ratio of 1:1, respectively. PTB1, PTB2, and PTB6 concentrations are 10 mg/mL, while PTB3, PTB4, and PTB5 concentrations are 13 mg/mL. For the last three polymer solutions, we also studied mixed solvent effect with about 3% (volume) 1,8-diiodooctance, also used to further improve the final device performances. ITO-coated glass substrates (15Ω/0) were cleaned stepwise in detergent, water, acetone, and isopropyl alcohol under ultrasonication for 15 min each and subsequently dried in an oven for 5 h. A thin layer (∼30 nm) of PEDOT:PSS (Baytron P VP A1 4083) was spin-coated onto ITO surface which was pretreated by ultraviolet ozone for 15 min. Low-conductivity PEDOT:PSS was chosen to minimize measurement error from device area due to lateral conductivity of PEDOT:PSS. After being baked at 120 °C for ∼20 min, the substrates were transferred into a nitrogen-filled glovebox (