Determination of the microscopic prefactor of the conductivity of a-Si:H using temperature dependent field-effect measurements

Enhanced thermoelectric performance by resonant vibrational mode-selective density-of-states distortions

“…To study the redistribution of band carriers to DOS tail states due to the mode-selective absorption of incident light, we first incorporate broadening in a model DOS to replicate the biexponential decay in the tail that is typically measured in soft organic materials 20,42,62 and disordered wide bandgap inorganic materials, 43–45 recognizing that this realistic energy dependence is particularly important for calculation of thermoelectric properties such as the Seebeck coefficient ( S ). 12,39,63,64 Using this DOS we then develop a framework for the non-thermal excitation of a particular vibrational mode given by the following model for an isolated vibrating molecule interacting with light in an optical cavity: 50–52,65 H = H 0 + H I ,where the unperturbed Hamiltonian H 0 is the Hamiltonian of the non-interacting material system and the interaction Hamiltonian ( H I = H radia.…”

“…G 0 ( E ) = ( E − H 0 ) −1 is the Greens function for the non-interacting Hamiltonian, to which we have applied a homogeneous broadening ∑′′( E ) = πλk B T /ℏ to replicate the biexponential decay of the DOS tail that is typically measured in soft organic materials and some amorphous wide bandgap inorganic materials ( λ = 0.17 is the electron–phonon coupling associated with thermal disorder in the prototypical material rubrene). 20,42–44,62 E p is energy of the new electronic ground state formed upon small polaron formation. G ( E ′) describes the renormalization of carriers in the highest-occupied molecular orbital (HOMO) band dispersion A ( k , E ) (due to the dephasing of carriers brought about by IR vibrations),where ∑′′( E ) = 1/2 τ el (defined above) is recognized as the inverse lifetime broadening or the carrier dephasing time.…”

“…For example, sharp tail features are known to appear in the class of weakly-bonded (soft) organic materials under both thermal (equilibrium) [17][18][19][20][21][22] and non-thermal (non-equilibrium) [23][24][25][26][27][28][29] conditions, and have also been routinely engineered to occur within the broader class of disordered organic and inorganic wide band gap materials [30][31][32][33][34][35][36][37][38] that exhibit Fermi level pinning in the DOS tail due to deep trap states (e.g., rubrene, pentacene, and amorphous silicon). 20,[39][40][41][42][43][44][45] However, very little is known about how to take advantage of these resonant tail features for optimal power factor enhancement, even though tail features could be particularly useful for optimizing PF, since achieving sufficient carrier concentration (p) to place the Fermi level near a resonant Dirac-delta-like DOS distortion in the tail may be possible with present doping techniques, 39,46,47 while achieving sufficient levels of p needed to place E F near a similar feature within the band may not be possible. 34,35,48,49 Here, we adopt a well-established model for the formation of Dirac-delta-like distortions in DOS tails by the optical excitation of vibrational modes [50][51]…”

“…, rubrene, pentacene, and amorphous silicon). 20,39–45 However, very little is known about how to take advantage of these resonant tail features for optimal power factor enhancement, even though tail features could be particularly useful for optimizing PF, since achieving sufficient carrier concentration ( p ) to place the Fermi level near a resonant Dirac-delta-like DOS distortion in the tail may be possible with present doping techniques, 39,46,47 while achieving sufficient levels of p needed to place E F near a similar feature within the band may not be possible. 34,35,48,49…”