Boosting internal quantum efficiency via ultrafast triplet transfer to 2H-MoTe ₂ film

Abstract

Organic systems often allow to create two triplet spin states (triplet excitons) by converting an initially excited singlet spin state (a singlet exciton). An ideally designed organic/inorganic heterostructure could reach the photovoltaic energy harvest over the Shockley-Queisser (S-Q) limit because of the efficient conversion of triplet excitons into charge carriers. Here, we demonstrate the molybdenum ditelluride (MoTe 2 )/pentacene heterostructure to boost the carrier density via efficient triplet transfer from pentacene to MoTe 2 using ultrafast transient absorption spectroscopy. We observe carrier multiplication by nearly four times by doubling carriers in MoTe 2 via the inverse Auger process and subsequently doubling carriers via triplet extraction from pentacene. We also verify efficient energy conversion by doubling the photocurrent in the MoTe 2 /pentacene film. This puts a step forward to enhancing photovoltaic conversion efficiency beyond the S-Q limit in the organic/inorganic heterostructures.

Harnessing Hot Carriers in Two-Dimensional Materials

“…Jang et al have demonstrated efficient MEG of MoTe 2 and SEF of pentacene in MoTe 2 /pentacene heterostructure. 36 Unfortunately, although efficient triplet extraction has been achieved by ultrafast electron transfer from pentacene to MoTe 2 , MEGT from MoTe 2 to pentacene through hole transfer occurs on a slow nanosecond scale.…”

“…MEG processes in 2D semiconductors have been investigated by a few groups using ultrafast spectroscopy techniques. − By ultrafast transient absorption spectroscopy covering both visible and mid-IR probe range, Kim et al. have observed highly efficient MEG in MoTe 2 and WSe 2 thin films (∼16.4 nm) with threshold energy close to twice the bandgap and a high conversion efficiency exceeding 93% (Figure b) .…”

“…Compared to their 3D counterparts, 2D semiconductors exhibit enhanced many-body interactions due to reduced dielectric screening and quantum confinement, resulting in significant exciton binding energy (e.g., up to 500 meV for monolayer semiconductors) and ultrafast electron–electron scattering within femtoseconds. − Additionally, the 2D geometry and atomic flatness without dangling bonds guarantee high density of states and allow readily interfacing 2D semiconductors with charge extraction components to achieve ultrafast interfacial charge transfer within 100 fs. ,− The combination of ultrafast electron–electron scattering and interfacial charge transfer in 2D semiconductors suggests exciting potential to explore and realize efficient hot carrier harvesting. Indeed, with the help of powerful transient spectra, efficient hot carrier transfer from graphene and other 2D semiconductors before interband electron–electron scattering has been observed, and multiple exciton generation and transfer with high yield and low threshold have been reported in MoTe 2 and black phosphorus (BP). − In this Perspective, we highlight three primary directions: (1) hot electron harnessing from graphene; (2) hot electron transfer from 2D semiconductors; and (3) multiexciton generation in 2D semiconductors. Through these experimental examples, we underscore the importance of the unique properties of 2D semiconductors, such as high density of states, strong Coulomb interaction, and ultrafast interfacial charge transfer, making them an exceptional platform for harnessing hot carriers.…”

Developments and Challenges Involving Triplet Transfer across Organic/Inorganic Heterojunctions for Singlet Fission and Photon Upconversion

“…Jang et al. demonstrated electron transfer from the triplet states in pentacene (triplet energy 0.86 eV) to MoTe 2 (band gap 1.1 eV) doubling the photocurrent in a bilayer device . This type of bilayer solar cells are beneficial for exceeding the Shockley–Queisser limit as the MoTe 2 shows efficient carrier multiplication (QY ≈ 2 for 2 E g < E < 3 E g , where E g is the band gap).…”

“…demonstrated electron transfer from the triplet states in pentacene (triplet energy 0.86 eV) to MoTe 2 (band gap 1.1 eV) doubling the photocurrent in a bilayer device. 37 This type of bilayer solar cells are beneficial for exceeding the Shockley–Queisser limit as the MoTe 2 shows efficient carrier multiplication (QY ≈ 2 for 2 E g < E < 3 E g , where E g is the band gap). Ye et al have shown for TIPS-pentacene/MoS 2 heterostructure triplets generated by singlet fission dissociate at the interface and transfer electrons to MoS 2 enhancing the photocurrent beyond 100%.…”