Interfacial Charge Transfer and Electronic Structure Modulation in Ultrathin Graphene P3HT Hybrid Heterostructures

TL;DR

This study uses ab initio density functional theory to unravel how P3HT structural variations influence interfacial charge transfer with graphene in ultrathin graphene/P3HT heterostructures. By modeling molecular P3HT, layered films, and graphene-supported configurations across regular and random orders, it demonstrates that extended conjugation, increased film thickness, and fully periodic ordering enhance interfacial charge transfer and reduce the band gap, while disorder weakens electronic coupling. Charge-density analyses reveal electron accumulation on graphene and hole localization in P3HT, with stronger effects for thicker and more ordered layers. The findings provide actionable guidelines for maximizing performance in graphene-based polymer photovoltaics and related optoelectronic devices, emphasizing control over chain length, periodicity, and termination to optimize interfacial coupling and carrier separation.

Abstract

Ultrathin polymer-graphene heterostructures are promising materials for next generation optoelectronic and photovoltaic technologies, while the influence of the polymer's structural variation on interfacial charge transfer remains unclear. Here, using ab initio quantum mechanical calculations we show how different forms of Poly(3-hexylthiophene) (P3HT), a widely used organic semiconductor, interact with graphene. We analyze the effects of molecular chain length, end-group termination, periodicity, and the distinction between ordered and random P3HT arrangements. For isolated P3HT, the band gap decreases with increasing chain length and layer thickness, while structural disorder leads to slightly larger gaps due to reduced electronic coupling. When P3HT is deposited on graphene, all configurations exhibit spontaneous charge transfer, with electrons accumulating on graphene and holes remaining in the polymer. This effect is significantly enhanced in ordered and fully periodic structures and is noticeably weaker in disordered ones. Charge density analyses further show that thicker and more ordered P3HT layers improve electron hole separation across the interface. Our results reveal how molecular structure governs charge transfer in P3HT-graphene heterojunctions and provide practical guidelines for designing high efficiency polymer-graphene photovoltaic devices.

Figures (8)

• Figure 1: (a-c) Molecular structures and density of states (DOS) of finite P3HT molecules with 2, 3, and 4 thiophene rings. The H-termination of the thiophene ring is indicated with the circle in (a). (d) Regularly aligned periodic P3HT chain and its band structure . The Fermi level is aligned to zero.
• Figure 2: (a) The isolated unit cell of regular P3HT. The backbone and attached side chains are indicated. (b) Top and side views of the regular and (c-d) two selected random P3HT structures.
• Figure 3: The geometric structures (left) and electronic band diagrams (right) of periodic regio-random P3HT for (a) RRLL and (b) RLRL configurations. The Fermi level is set to zero in all cases.
• Figure 4: The structures (left) and corresponding electronic band diagrams (right) for (a) monolayer, (b) bilayer, and (c) trilayer periodic regio-regular P3HT. The Fermi level is set to zero. Brown, yellow, and pink spheres represent the C, S, and H atoms, respectively. The rectangular Brillouin zone (BZ) with high-symmetry points is shown.
• Figure 5: (a) Top (above) and side (below) views of the optimized structure of a monolayer regio-regular P3HT chain deposited on graphene. (b) Projected electronic band structure of the monolayer P3HT/graphene heterostructure, highlighting contributions from atoms in P3HT (green) and graphene (red) layers. The Fermi level is set to zero. (c) Side views of the charge density difference isosurfaces for monolayer, bilayer, and trilayer P3HT chains on graphene, where yellow and cyan indicate electron accumulation and depletion regions, respectively. (d) Plane-averaged charge density difference profiles along the z-direction corresponding to each configuration.