Cyclobutadiene–C₆₀ Adducts: N‐Type Materials for Organic Photovoltaic Cells with High V_OC

Abstract

We have developed new tetraalkylcyclobutadiene-C 60 adducts, via Diels-Alder cycloaddition of C 60 with in situ generated cyclobutadienes. The cofacial π-orbital interactions between the fullerene orbitals and the cyclobutene are shown to decrease the electron affinity and thereby increase the LUMO energy level of C 60 significantly (ca. 100 meV and 300 meV for mono-and bisadducts, respectively). These variations in LUMO levels of fullerene can be used to generate higher open-circuit voltages (V OC ) in bulk heterojunction polymer solar cells. The tetramethylcyclobutadiene-C 60 monoadduct displays an open-circuit voltage (0.61V) and a power conversion efficiency (2.49%) comparable to the widely used P3HT/PCBM composite (0.58V and 2.57% respectively). The role of the cofacial π-orbital interactions between C 60 and the attached cyclobutene group was probed chemically by epoxidation of the cyclobutene moiety and theoretically through density functional theory (DFT) calculations. The electrochemical, photophysical, and thermal properties of the newly synthesized fullerene derivatives support the proposed effect of functionalization on electron affinities and photovoltaic performance.

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

“…On the other hand, the fine-tuning of the LUMO level of fullerene electron-acceptor is crucial to improve the performance of polymer solar cells. 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].…”

Prediction of the structural and spectral properties and reactivity of the silicon analogs of cyclobutadiene C4-nSinH4 (n=0-4) by density functional theory computations

“…Our studies are based on density functional theory (DFT) computations. Therefore, I hope this study will be a big window to the world of the silicon analogs of [4]-annulene.…”

“…Low stability of the cyclobutadiene contributes to its high reactivity and short lifetime [3]. Various sources claimed that the pi electrons of cyclobutadiene system are not conjugated [4]. The energy of this system is higher than that of the 1,3-butadiene system.…”

V OC enhancement in polymer solar cells with isobenzofulvene–C 60 adducts

“…[11] In addition, the extent of this change can be influenced by both the structural properties of the functionalized fullerenes and the specific electronic properties of the attached functional groups. For example, we previously demonstrated with tetraalkylcyclobutadiene-C 60 adducts [12] that cofacial interactions between C 60 and π-orbital of the pendant group induce a significant upshift of the fullerene's LUMO energy level. Another class of materials, indene-C 60 adducts, also displays π-orbital interactions between the aromatic ring and the C 60 π-surface, which results in an improved V OC and PCE when blended with P3HT in solar cell devices.…”