First-principles investigation of organic photovoltaic materialsC60, C70, [C60]PCBM, and bis-

Xiaofeng Qian

¹

,

Paolo Umari

²

,

Nicola Marzari

³

Abstract

We present a first-principles investigation of the excited-state properties of electron acceptors in organic photovoltaics including C60, C70, [6,6]-phenyl-C61-butyric-acid-methyl-ester ([C60]PCBM), and bis-[C60]PCBM using many-body perturbation theory within the Hedin's G0W0 approximation and an efficient Lanczos approach. Calculated vertical ionization potentials (VIP) and vertical electron affinities (VEA) of C60 and C70 agree very well with experimental values measured in gas phase. The density of states of all three molecules is also compared to photoemission and inverse photoemission spectra measured on thin-films, exhibiting a close agreement -a rigid energy-gap renormalization owing to intermolecular interactions in the thin-films. In addition, it is shown that the low-lying unoccupied states of [C60]PCBM are all derived from the highest-occupied molecular orbitals and the lowest-unoccupied molecular orbitals of fullerene C60. The functional side group in [C60]PCBM introduces a slight electron transfer to the fullerene cage, resulting in small decreases of both VIP and VEA. This small change of VEA provides a solid justification for the increase of open-circuit voltage when replacing fullerene C60 with [C60]PCBM as the electron acceptor in bulk heterojunction polymer solar cells.

Large-scale GW -BSE calculations with N3 scaling: Excitonic effects in dye-sensitized solar cells

“…We take the atomic coordinates from Ref. 24 and choose the PBE exchange and correlation functional 25 for the starting DFT calculation together with a norm-conserving pseudo-potential 26 . The cubic simulation cell has an edge of 40 Bohr and the Coulomb interaction is truncated at 20 Bohr.…”

“…In Ref. 24 we investigated the GW quasi-particle energy levels finding a self-consistent HOMO-LUMO (electronic) gap of 4.94 eV (1.78 eV at the PBE level) and we consider the corresponding scissor operator in the BSE calculation. A basis of 2000 elements is used for representing the static polarisability operator 18,19 .…”

Ionization Energies, Electron Affinities, and Polarization Energies of Organic Molecular Crystals: Quantitative Estimations from a Polarizable Continuum Model (PCM)-Tuned Range-Separated Density Functional Approach

“…71 To allow for a direct comparison with the GW calculations from Refs. 17,18,23 , the molecular geometries (see Figure 1) were optimized using the PBE functional 72 and 6-31G(d,p) basis set. The optimization of the rangeseparation parameter ω was performed based on the LC-ωPBE 46, 73 functional and using the efficient "golden-ratio" algorithm, as described in our previous work 48,57 (see Supporting…”

“…20 This method has been shown to 5 provide quantitative predictions for the quasi-particle (QP) spectrum of organic and inorganic systems. 18,[21][22][23][24] However, not only is GW computationally much more demanding than DFT, the fact that it is typically employed non-self-consistently also leads to a significant starting-point dependence. [25][26][27][28][29][30][31] For the case of the pentacene molecule, for example, it has been found that nonself-consistent G 0 W 0 calculations based on a (semi-)local DFT starting point underestimate the fundamental gap E g in the gas phase by as much as 0.7 eV, while E g calculated at the same level of theory for the pentacene crystal are found in good agreement with the experimentally determined solid-state E g , likely due to a fortuitous cancelation of errors.…”

Low Band Gap of Novel Compounds Having Triphenylamine and Oligothiophenes Based Donor-acceptor Organic Dyes for Photovoltaic Applications: A DFT-B3LYP Calculation

“…(3), (4), (5), (7) and (8) and summarized in Table 5: The calculated values of IP and EA for all molecules range from 5.497 eV to 6.224 eV and 1.710 eV to 2.191 eV respectively (see table 5). It was found from previous research studies that raising the LUMO energy level of such systems causes the EA to increase and leads to a higher value of Voc [56]. So, we found that the presence of Thiophene and Benzothiadiazole substituents on the systems can increase the EA compared with the other compounds.…”