Design Concept for High-LUMO-level Fullerene Electron-acceptors for Organic Solar Cells

Abstract

This review article describes design concept, synthesis, and features of fullerene derivatives having high lowest unoccupied molecular orbital (LUMO) levels to achieve high open-circuit voltage in organic thin-film photovoltaic devices. Installation of organic electron-donating groups onto fullerene and decrease of the size of the fullerene ³-electron-conjugated system raise the LUMO levels, affording high-performance organic solar cells. Addition of the methano group as the smallest carbon addend to fullerene to obtain 56³-electron fullerene derivatives is likely a promising strategy for this purpose. Ç IntroductionOrganic thin-film solar cells are lightweight and flexible, and thus are expected to allow for low-cost production through a printing process. 1 The active layer that converts light into electricity contains organic semiconductors, whose development is the key to improving power conversion efficiency. Accordingly, organic chemistry has a major role to play. Up to now, the development of organic thin-film solar cells has been led by material scientists and device engineers, but there is increasing demand for collaboration with synthetic chemists who can design and synthesize organic semiconductor molecules.Although the power conversion efficiency of organic thinfilm solar cells was about 1% before 2000, a sharp increase in conversion efficiency was seen after 2000, owing to the discovery of the combination of poly(3-hexylthiophene) (P3HT; an organic electron-donor) and phenyl-C 61 -butyric acid methyl ester (PCBM; an organic electron-acceptor).2 When heated and exposed to solvent vapor, P3HT undergoes self-assembly and forms ³-stacked structures, improving its electrical properties and light absorption characteristics. Furthermore, PCBM is a fullerene derivative that is soluble in organic solvents, and its solubility allows for blend solutions containing P3HT to be prepared over a range of concentrations. Furthermore, thin films prepared through the use of a P3HT/PCBM blend solution have made it possible to develop composite films with a bulk heterojunction structure, in which an electron-donor and an electron-acceptor are suitably mixed.3 In comparison with conventional pn heterojunction devices, bulk heterojunction devices have a greater interface area between the donor and acceptor and charge separation is more efficient. In this way, around 2005, power conversion efficiency reached 45%. 4 In the last few years, the power conversion efficiency of organic thin-film solar cells has increased further to about 10%. 5This vast improvement was largely due to the development of new materials, for example, new electron-donors such as low band gap polymers, 6 which are capable of absorbing longwavelength light. These materials are alternating copolymers with electron-rich and electron-deficient parts, and for this reason they have high-lying HOMO and low-lying LUMO levels. Intermolecular charge transfer from electron-rich parts to electron-deficient parts makes such materials capable of absorbing...

Establishment of a linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed: An experimental and theoretical study

“…Recently, raising the LUMO energy level of the fullerene electron-acceptor is also attracted much current interest [16][17][18][19][20]. Several concepts have been introduced to raise the LUMO energy level of fullerenes [21], for example, the installation of organic electron-donating groups onto fullerene [22], the introduction of multi-substitution to decrease of the size of the fullerene -electron-conjugated system [23][24][25], and the placement of ortho-substitution spatial proximity to the fullerene core [26,27].…”

Tightly Bound Double‐Caged [60]Fullerene Derivatives with Enhanced Solubility: Structural Features and Application in Solar Cells

“…[4] The desired increasei no pen-circuit voltage of OSCs can be achieved by upshifting the fullereneL UMO levels by meanso fe xohedral functionalization with electron-donating groups, or with other addends of an on-acceptor nature throughb roadening of the HOMO-LUMO gap. [5][6][7][8][9][10] However,e xcessive functionalization is knownt os uppress the electron mobility.F or example, OSCs based on fullerenet ris-adducts demonstrate at wofold drop in electron mobility and disappointingly low current density relative to mono-a nd bis-adducts. [11] An alternative approacht o optimization of fullerene-based acceptor phases consists of the replacement of traditional C 60 andC 70 with higher fullerenes, [12] endohedralf ullerenes, [13] heterofullerenes, [14] or homofullerenes, [15,16] but the resulting benefit of increased opticald ensity is largely outweighed by their higher cost and limited availability.…”

Efficient bulk heterojunction photovoltaic devices based on modified PCBM

“…The V oc can be augmented by raising the lowest unoccupied molecular orbital (LUMO) level of the fullerene relative to the highest occupied molecular orbital (HOMO) level of the donor [30,31]. The V oc of a BHJ solar cell may also be increased using electron irradiation method [32].…”