Functionalized dihydronaphthyl-C60 derivatives as acceptors for efficient polymer solar cells with tunable photovoltaic properties
Graphical Abstract
Highlights
► The methoxy substituted dihydronaphthyl-C60 bisadduct shows high PCE of 4.58%. ► Facile synthesis of five alkoxy substituted dihydronaphthyl-C60 derivatives. ► Tunable photovoltaic property depending on substituent group of dihydronaphthyl-C60.
Introduction
Polymer solar cells have emerged as a promising alternative for producing clean and renewable energy because of their advantages of simple fabrication in large scale with lightweight flexible substrates by solution processing at low cost [1], [2], [3], [4], [5], [6], [7]. As a closed-cage molecule, fullerene is an excellent electron acceptor for polymer bulk heterojunction (BHJ) solar cells. [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), originally reported for photovoltaic application as early as 1995 [8], remains one of the most widely used electron acceptors. Poly (3-hexylthiophene) (P3HT): PCBM is currently the most widely investigated photovoltaic system, showing power conversion efficiency (PCE) up to 4% [9], [10]. In the past decade, numerous PCBM-like fullerene derivatives similar to or modified from PCBM have been studied as electron acceptors [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. These comparable experiments [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], with tunable photovoltaic performances depending on the functional groups in PCBM analogs, corroborate the understanding of some important requirements (e.g., high LUMO level, strong visible absorption, high electron mobility, and good solubility) for a promising fullerene-based electron acceptor [22], [23].
One of the inherent factors to suppress PCE of PCBM-related solar cells is the low-lying LUMO of PCBM, which results in lower open-circuit voltage (Voc) and, in turn, limited PCE because Voc is proportional to the difference between the LUMO level of the acceptor and the HOMO level of the donor [13], [24], [25], [26], [27]. To further improve the PCE of fullerene-based solar cells, efforts have been paid for functionalization of fullerenes to increase the LUMO levels. (1) Altering fullerene core can change the LUMO level of fullerene acceptor intrinsically. Ross et al. [28], [29] demonstrated that using endohedral fullerene derivative of Lu3N@C80-PCBH as acceptor in combination with P3HT as donor could add 260 mV to the Voc of the P3HT: PCBM reference device. (2) Adding an electron-donating group onto fullerene cage has been proposed to raise LUMO levels of some fullerene acceptors [13], [15], [18], [30], [31]. Due to the lack of resonance between electron-donating groups and fullerene cores, however, by this strategy the LUMO levels of fullerenes can only be modified in a slight range. (3) Modifying fullerene acceptor from mono- to multi-addition has recently been demonstrated to effectively increase the LUMO level of fullerene [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45]. The bis-adduct of PCBM (bisPCBM), for example, has a LUMO level ∼0.1 eV higher than that of PCBM, and the P3HT:bisPCBM device shows higher Voc than P3HT: PCBM device [32].
In light of these guidelines, it is highly desired to design a fullerene acceptor with higher LUMO than PCBM. Among several fullerene derivatives having high-lying LUMO levels, Li et al. reported a remarkable fullerene bisadduct, indene-C60 bis-adduct (ICBA) [33]. Polymer solar cells based on P3HT:ICBA showed PCE of 5.44% with Voc of 0.84 V (the PCE can be up to 6.48% [34] with optimization). Starting from this work, a series of fullerene bis-adducts similar to the indene-based or dihydronaphthyl-based ICBA-like fullerene derivatives have been used as acceptors with high-lying LUMO levels and high photovoltaic performances for polymer solar cells [33], [34], [35], [43], [44], [45]. However, the influence of functional groups on the photovoltaic performance as well as the LUMO level and visible absorption of ICBA-like fullerene acceptor remains unknown. Here we report a series of alkoxyl-functionalized dihydronaphthyl-C60 derivatives, including 2-methoxy-(5,8)-dihydronaphthyl-(6,7)-[6,6]–C60 mono- and bis-adduct (MDNC and Bis-MDNC), 2-benzyloxy-(5,8)-dihydronaphthyl-(6,7)-[6,6]–C60 (BDNC), 2- benzoyloxy-(5,8)-dihydronaphthyl-(6,7)-[6,6]–C60 (BoDNC), 2-hexyloxy-(5,8)-dihydronaphthyl-(6,7)-[6,6]–C60 (HDNC) and 2-ethoxycarbonylpropyloxy-(5,8)-dihydronaphthyl-(6,7)-[6,6]–C60 (EDNC) (as shown in Scheme 1). Photovoltaic performances alterable depending on substituent groups (methyloxy, benzyloxy, benzoyloxy, hexyloxy, and 3-ethoxycarbonylpropyloxy) have been revealed for corresponding P3HT: alkoxyl-dihydronaphthyl-C60 polymer solar cells. Among them, Bis-MDNC stands out because of its high-lying LUMO energy level (0.18 eV higher than that of PCBM) and high photovoltaic performance.
Section snippets
Materials
C60 was purchased from Puyang Yongxin Fullerene Co., Ltd. Other reagents and chemicals were purchased from Alfa-Aesar Co. or Sigma-Aldrich Co. and used as received without further purification unless otherwise noted. P3HT was obtained from FEM Technology Co., Inc. and used as received.
Measurements
1H NMR and 13C NMR spectra were measured on a Bruker Biospin Advance III 500 MHz spectrometer. Chemical shifts were reported in ppm relative to the singlet of CDCl3 at 7.26 and 77 ppm for 1H NMR and 13C NMR,
Synthesis of fullerene derivatives
Methoxy substituted dihydronaphthyl C60 mono- and bis-adduct (MDNC and Bis-MDNC) were synthesized using a Diels-Alder reaction ([4+2] cycloaddition) (as shown in Scheme 1), modifying a procedure previously reported by Nishimura et al. [46]. By using 1.0 and 2.0 equivalent of 3,4-bis(bromomethyl)methoxybenzene to C60, the reaction would predominately produce MDNC and Bis-MDNC, respectively. Demethylation of MDNC with BBr3 afforded the molecule of hydroxy substituted dihydronaphthyl-C60, which
Conclusions
We synthesized a series of alkoxy substituted dihydronaphthyl-C60 derivatives, MDNC, Bis-MDNC, BDNC, BoDNC, HDNC, and EDNC. The photovoltaic performances of polymer solar cells using P3HT as donor and the as-synthesized alkoxyl-dihydronaphthyl-C60 as acceptor are susceptible to the substituent groups (methyloxy, benzyloxy, benzoyloxy, hexyloxy, and 3-ethoxycarbonylpropyloxy). The molecular structures and the assemblies of fullerene acceptors may influence the photo-electron conversion
Supporting materials
Acknowledgments
This work was financially supported by the National Basic Research 973 Program of China (2011CB935901) and the National Nature Science Foundation of China (21031004, 21021061).
References (47)
Fabrication and processing of polymer solar cells: a review of printing and coating techniques
Solar Energy Materials and Solar Cells
(2009)- et al.
The influence of materials work function on the open circuit voltage of plastic solar cells
Thin Solid Films
(2002) - et al.
Biindene-C60 adducts for the application as acceptor in polymer solar cells with higher open-circuit-voltage
Solar Energy Materials and Solar Cells
(2011) - et al.
Triphenylamine-substituted methanofullerene derivatives for enhanced open-circuit voltages and efficiencies in polymer solar cells
Solar Energy Materials and Solar Cells
(2011) - et al.
Plastic solar cells
Advanced Functional Materials
(2001) - et al.
Conjugated polymer-based organic solar cells
Chemical Reviews
(2007) - et al.
Polymer-fullerene composite solar cells
Angewandte Chemie International Edition
(2008) - et al.
Product integration of compact roll-to-roll processed polymer solar cell modules: methods and manufacture using flexographic printing,slot-die coating and rotary screen printing
Journal of Materials Chemistry
(2010) - et al.
Upscaling of polymer solar cell fabrication using full roll-to-roll processing
Nanoscale
(2010) - et al.
Manufacture, integration and demonstration of polymer solar cells in a lamp for the lighting Africa initiative
Energy and Environmental Science
(2010)