Exploring the electrochemical properties of hole transport materials with spiro-cores for efficient perovskite solar cells from first-principles

Abstract

Perovskite solar cells (PSCs) with organic small molecules as hole transport materials (HTMs) have attracted considerable attention due to their power conversion efficiencies as high as 20%. In the present work, three new spiro-type hole transport materials with spiro-cores, i.e. Spiro-F1, Spiro-F2 and Spiro-F3, are investigated by using density functional theory combined with the Marcus theory and Einstein relation. Based on the calculated and experimental highest occupied molecular orbital (HOMO) levels of 30 reference molecules, an empirical equation, which can predict the HOMO levels of hole transport materials accurately, is proposed. Moreover, a simplified method, in which the hole transport pathways are simplified to be one-dimensional, is presented and adopted to qualitatively compare the molecular hole mobilities. The calculated results show that the perovskite solar cells with the new hole transport materials can have higher open-circuit voltages due to the lower HOMO levels of Spiro-F1 (-5.31 eV), Spiro-F2 (-5.42 eV) and Spiro-F3 (-5.10 eV) compared with that of Spiro-OMeTAD (-5.09 eV). Furthermore, the hole mobilities of Spiro-F1 (1.75 × 10(-2) cm(2) V(-1) s(-1)) and Spiro-F3 (7.59 × 10(-3) cm(2) V(-1) s(-1)) are 3.1 and 1.4 times that of Spiro-OMeTAD (5.65 × 10(-3) cm(2) V(-1) s(-1)) respectively, due to small reorganization energies and large transfer integrals. Interestingly, the stability properties of Spiro-F1 and Spiro-F2 are shown to be comparable to that of Spiro-OMeTAD, and the dimers of Spiro-F2 and Spiro-F3 possess better stability than that of Spiro-OMeTAD. Taking into consideration the appropriate HOMO level, improved hole mobility and enhanced stability, Spiro-F1 and Spiro-F3 may become the most promising alternatives to Spiro-OMeTAD. The present work offers a new design strategy and reliable calculation methods towards the development of excellent organic small molecules as HTMs for highly efficient and stable PSCs.

Star‐Shaped Molecules as Dopant‐Free Hole Transporting Materials for Efficient Perovskite Solar Cells: Multiscale Simulation

“…where X is the calculated HOMO level and Y is the fitting HOMO levels. The fitting HOMO energy level is more close to the corresponding experimental result, which has been testified in Chi's literature . The fitting HOMO energy of TCP‐OH is −5.57 eV, which is in good agreement with the corresponding experimental value as compared with the calculated HOMO level.…”

“…Due to the approximate exchange‐correlation functionals employed in DFT, the calculated HOMO energy of TCP‐OH (−4.93 eV) is far away from the experimental measured value (−5.47 eV) reported by Xue et al . According to the equation reported by Chi et al., …”

Characterization of intrinsic hole transport in single-crystal spiro-OMeTAD

“…We note that this value of λ reorg is much larger than those reported in previous studies based on the B3LYP functional (e.g., 131 meV in ref. 21 and 160 meV in ref. 22).…”

“…[20][21][22][23] While these investigations provide useful information on hole-mobility predictions in both amorphous and crystalline spiro-OMeTAD, a thorough characterization of the intrinsic hole-transport mechanism in single-crystal spiro-OMeTAD is still missing. In this work, our aim is to provide insight into the intrinsic hole-transport properties of spiro-OMeTAD on the basis of the X-ray single-crystal data.…”

Revealing the Unique Properties of Platinum(II) Complexes with Bidentate Bis(o‐carborane) Ligands

“…Importantly, different crystal stacking structures are induced by the various numbers of carborane ligands, so the crystal stacking structures of complexes 4 and 5 are different from those of the others (as shown in Figure S4). Moreover, the intermolecular interaction energy (Δ E ), which is an essential parameter to estimate the stability of dimers, was calculated by the expression Δ E = E (dimer) – 2 E (monomer) . Apparently, a small Δ E leads to a stable dimer, which is to the disadvantage of monomer emission.…”