“Supersaturated” Self-Assembled Charge-Selective Interfacial Layers for Organic Solar Cells

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Abstract

To achieve densely packed charge-selective organosilane-based interfacial layers (IFLs) on the tin-doped indium oxide (ITO) anodes of organic photovoltaic (OPV) cells, a series of Ar2N-(CH2)n-SiCl3 precursors with Ar = 3,4-difluorophenyl, n = 3, 6, 10, and 18, was synthesized, characterized, and chemisorbed on OPV anodes to serve as IFLs. To minimize lateral nonbonded -NAr2···Ar2N- repulsions which likely limit IFL packing densities in the resulting self-assembled monolayers (SAMs), precursor mixtures having both small and large n values are simultaneously deposited. These "heterogeneous" SAMs are characterized by a battery of techniques: contact angle measurements, X-ray reflectivity, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), cyclic voltammetry, and DFT computation. It is found that the headgroup densities of these "supersaturated" heterogeneous SAMs (SHSAMs) are enhanced by as much as 17% versus their homogeneous counterparts. Supersaturation significantly modifies the IFL properties including the work function (as much as 16%) and areal dipole moment (as much as 49%). Bulk-heterojunction OPV devices are fabricated with these SHSAMs: ITO/IFL/poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][2-[[(2-ethylhexyl)oxy]carbonyl]-3-fluorothieno[3,4-b]thiophenediyl]]:phenyl-C71-butyric acid methyl ester (PTB7:PC71BM)/LiF/Al. OPVs having SHSAM IFLs exhibit significantly enhanced performance (PCE by 54%; Voc by 35%) due to enhanced charge selectivity and collection, with the PCE rivaling or exceeding that of PEDOT:PSS IFL devices -7.62%. The mechanism underlying the enhanced performance involves modified hole collection and selectivity efficiency inferred from the UPS data. The ITO/SAM/SHSAM surface potential imposed by the dipolar SAMs causes band bending and favorably alters the Schottky barrier height. Thus, interfacial charge selectivity and collection are enhanced as evident in the greater OPV Voc.

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