[Paper Review] Wide energy-window view on the density of states and hole mobility of poly(p-phenylene vinylene)

Iulian N. Hulea, H. B. Brom+4 more|arXiv (Cornell University)|Sep 9, 2004

Electrochemical Analysis and Applications4 citations

TL;DR

This study uses electrochemically gated transistors (EGT) to reversibly control doping in poly(p-phenylene vinylene) (PPV) over a wide energy window, revealing that the density of states (DOS) core is Gaussian (σ ≈ 0.19 eV), while the low-energy tail exhibits complex, non-Gaussian behavior. Hole mobility increases by over four orders of magnitude—from 10⁻¹⁰ to 10⁻⁶ m²/Vs—as the electrochemical potential scans through the DOS, with the maximum mobility exceeding previous reports by a factor of 40.

ABSTRACT

Using an electrochemically gated transistor, we achieved controlled and reversible doping of poly(p-phenylene vinylene) in a large concentration range. Our data open a wide energy-window view on the density of states (DOS) and show, for the first time, that the core of the DOS function is Gaussian, while the low-energy tail has a more complex structure. The hole mobility increases by more than four orders of magnitude when the electrochemical potential is scanned through the DOS.

Motivation & Objective

• To experimentally determine the energy-dependent density of states (DOS) in poly(p-phenylene vinylene) (PPV) over a broad range of doping levels.
• To resolve the long-standing ambiguity regarding the shape of the DOS in PPV, particularly whether it is Gaussian or exponential.
• To measure the intrinsic hole mobility in PPV as a function of electrochemical potential and doping concentration.
• To calibrate the energy scale of the DOS and mobility using electrochemical gating, enabling direct comparison with electronic structure models.
• To demonstrate the superiority of electrochemically gated transistors (EGT) over chemical doping or solid-state FETs for probing transport in disordered conjugated polymers.

Proposed method

• Employed an electrochemically gated transistor (EGT) with a PPV film as the active layer, interfaced with an electrolyte and a potentiostat to control the electrochemical potential (μ̃ₑ) with high precision.
• Used cyclic voltammetry and differential charge measurements (ΔQ/Δμ̃ₑ) to determine the doping level per monomer (c) across a 4-order-of-magnitude range, from ~0.02 to 0.30 holes per monomer.
• Measured conductivity (σ) via two-point contact configuration under small dc bias, with σ corrected for thickness and contact resistance effects.
• Calculated hole mobility (μ) from σ = N pₜ e μ, where pₜ is the number of holes per monomer participating in transport, within kBT of the electrochemical potential.
• Fitted the DOS to extract the core (Gaussian) and flank (exponential) regions, with the low-energy tail modeled as a second Gaussian.
• Compared EGT-derived mobility with solid-state FET and LED data from Tanase et al. to validate consistency and identify discrepancies due to interfacial effects and ion localization.

Experimental results

Research questions

• RQ1 What is the true energy-dependent shape of the density of states (DOS) in PPV across a wide doping window?
• RQ2 How does hole mobility in PPV vary with electrochemical potential and doping concentration?
• RQ3 Can electrochemical gating provide a more accurate and reversible probe of transport properties than chemical doping or solid-state FETs?
• RQ4 What is the intrinsic width of the DOS core in PPV, and how does it compare to values derived from mobility models?
• RQ5 To what extent do ion-induced localization and interfacial effects in EGTs suppress mobility compared to solid-state FETs?

Key findings

• The core of the DOS in PPV is well described by a Gaussian distribution with a width of 0.19 eV, representing an upper bound for the intrinsic disorder in the material.
• The low-energy tail of the DOS exhibits a complex, non-Gaussian structure, with an exponential decay in the flank and a secondary Gaussian component at the lowest doping levels.
• Hole mobility increases by over four orders of magnitude—from 10⁻¹⁰ m²/Vs to 2×10⁻⁶ m²/Vs—as the electrochemical potential is scanned from -5.3 eV to -5.6 eV, corresponding to a doping range of 0.02 to 0.30 holes per monomer.
• The maximum measured mobility of 2×10⁻⁶ m²/Vs exceeds previously reported values by a factor of 40, indicating that earlier measurements may have been limited by contact resistance or ion-induced localization.
• The EGT method enables reversible, controlled doping over a 1 eV energy window, far exceeding the range accessible with chemical doping or standard FETs.
• Discrepancies between EGT and solid-state FET mobility data are attributed to ion-induced hole localization and interfacial effects in the EGT, while the solid-state FET benefits from higher structural order near the interface.