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

Abstract

Spiro-OMeTAD remains a prominent hole-transport material in perovskite and solid-state dye-sensitized solar cells. However, an understanding of its intrinsic hole-transport properties is still limited. Here, hole transport in spiro-OMeTAD is systematically characterized on the basis of the recently reported X-ray single-crystal data. An approach combining density functional theory calculations, tight-binding modeling, and kinetic Monte Carlo simulations are exploited to describe the key parameters governing hole transport and to investigate the transport mechanism and hole mobilities in the spiro-OMeTAD single crystal. The results provide insight into: (i) why an anisotropic hole-transport mechanism, with an upper range of intrinsic hole mobilities on the order of~10 −3 cm 2 /Vs, can be expected in the single crystal; and (ii) how detrimental factors, related to the presence of the spiro motif and of the 4,4′-dimethoxydiphenylamine substituents, limit the intrinsic hole mobilities of the system.

Enhanced Stability of Perovskite Solar Cells Incorporating Dopant‐Free Crystalline Spiro‐OMeTAD Layers by Vacuum Sublimation

“…[13,14] The most common issue is that ionic additives can contribute to the degradation of the perovskite film. [28] Their results are supported by theoretical calculations performed by Brédas and Houk, [29,30] who reported a very high intrinsic hole conduction of crystalline Spiro-OMeTAD layers along the π-π stacking direction of the crystal. [23,24] The issue is so severe that several authors have aimed at the development of PSCs in a hole-transport layer free configuration.…”

“…The oscillations are caused by a resonant diffuse scattering that is linked to the thickness of a layer with correlated interfaces. According to literature, both from the experimental [28] or calculation, [29,30] a closely packed π-π system provides significant electronic coupling between stacked conjugated fragments. In fact, the separation between the maxima of the oscillations far from the Yoneda peak is related to the layer thickness through the expression d = 2π/Δq z [34] that results in a value of d = 306 nm, also in agreement with the thickness observed by SEM (≈250 nm).…”

“…Despite the low intrinsic hole mobility reported for dopant‐free Spiro‐OMeTAD layers fabricated by wet approaches, the group of Bakr has recently reported significantly improved hole‐transporting properties of pristine Spiro‐OMeTAD single crystals . Their results are supported by theoretical calculations performed by Brédas and Houk, who reported a very high intrinsic hole conduction of crystalline Spiro‐OMeTAD layers along the π–π stacking direction of the crystal. Their findings demonstrated two key aspects: i) the crystalline order of the SSHC is crucial to promote new charge transport pathways and ii) the negligible effect of the lithium salts on the hole mobility of the bulk Spiro‐OMeTAD .…”

The researcher's guide to solid-state dye-sensitized solar cells

“…Unlike PEs and quasi-solid gels, in HTMs holes move across the layer through inter-molecular hopping processes, rather than relying on ion diffusion. 238,239 In this case, it is more correct to talk about electronic (or charge) diffusion. Since there is no molecular movement involved through a solid medium during charge transport, ssDSCs relying on an HTM layer have the potential to rival liquid DSCs in efficiency, while retaining the benefits of solid-state devices.…”

Parameter-Free Multiscale Simulation Realising Quantitative Prediction of Hole and Electron Mobilities in Organic Amorphous System with Multiple Frontier Orbitals

“…We envisaged that not only the HOMO or LUMO but also other molecular orbitals (MOs) may potentially contribute to charge transport. However, all previous studies simulating charge transport in organic “ amorphous ” systems do not consider MOs other than the HOMO and LUMO, except for crystalline 17 , 18 or liquid crystalline 19 , 20 systems. In this study, we consider the involvement of multiple MOs in charge transport in organic “ amorphous ” systems.…”