Organic semiconductors: What makes the spin relax?

Spin relaxation in organic materials is expected to be slow because of weak spin–orbit coupling. The effects of deuteration and coherent spin excitation show that the spin-relaxation time is actually limited by hyperfine fields

Nonperturbative theory of exciton-phonon resonances in semiconductor absorption

We develop a theory of exciton-phonon sidebands in the absorption spectra of semiconductors. The theorydoes not rely on an ad hoc exciton-phonon picture, but is based on a more fundamental electron-phononHamiltonian, thus avoiding a priori assumptions about excited-state properties. We derive a nonperturbativecompact solution that can be looked upon as the semiconductor version of the textbook absorption formula fora two-level system coupled to phonons. Accompanied by an illustrative numerical example, the importance andusefulness of our approach with respect to practical applications for semiconductors is demonstrated

Is there more than meets the eye?

Letter to the edito

Calculating charge-carrier mobilities in disordered semiconducting polymers: Mean field and beyond

We model charge transport in disordered semiconducting polymers by hopping of charges on a regular cubic lattice of sites. A large on-site Coulomb repulsion prohibits double occupancy of the sites. Disorder is introduced by taking random site energies from a Gaussian distribution. Recently, it was demonstrated that this model leads to a dependence of the charge-carrier mobilities on the d. of charge carriers that is in agreement with exptl. observations. The model is conveniently solved within a mean-field approxn., in which the correlation between the occupational probabilities of different sites is neglected. This approxn. becomes exact in the limit of vanishing charge-carrier densities, but needs to be checked at high densities. We perform this check by dividing the lattice in pairs of neighboring sites and taking into account the correlation between the sites within each pair explicitly. This pair approxn. is expected to account for the most important corrections to the mean-field approxn. We study the effects of varying temp., charge-carrier d., and elec. field. We demonstrate that in the parameter regime relevant for semiconducting polymers used in practical devices the corrections to the mobilities calcd. from the mean-field approxn. will not exceed a few percent, so that this approxn. can be safely used. [on SciFinder (R)

Anisotropy effects in phonon-assisted charge-carrier transport in organic molecular crystals

We present a theoretical description of polaron dc conductivities in organic molecular crystals. Our approach is based on a rigorous evaluation of the Kubo formula for electrical conductivity within a mixed Holstein-Peierls model. It generalizes the result of Holstein's local-coupling theory by treating both local and nonlocal electron-phonon interactions nonperturbatively. The general theory is supplemented by an application to a simplified model crystal in order to emphasize the essential physics. Accompanied by an illustrative numerical example, special emphasis is put on the emergence of anisotropy effects in the temperature dependence of the conductivity tensor. These anisotropy effects are shown to originate from phonon-assisted currents due to the nonlocal electron-lattice interaction which demonstrates the importance to go beyond local-coupling theories in order to describe the experimental observation

Lowest-order vertex-correction contribution to the direct gap of silicon

We have calculated the contribution of the lowest-order vertex-correction diagram to the direct gap of silicon at the Gamma -point, taking into account the dynamic screening of the electron-electron interaction. Our best calculation yields a contribution of 0.12 eV. This result supports the assumption of the GW approximation that vertex corrections can be neglected. We do not find a significant shift of the absolute energie

Accurate and efficient band gap predictions of metal halide perovskites using the DFT-1/2 method:GW accuracy with DFT expense

\u3cp\u3eThe outstanding optoelectronics and photovoltaic properties of metal halide perovskites, including high carrier motilities, low carrier recombination rates, and the tunable spectral absorption range are attributed to the unique electronic properties of these materials. While DFT provides reliable structures and stabilities of perovskites, it performs poorly in electronic structure prediction. The relativistic GW approximation has been demonstrated to be able to capture electronic structure accurately, but at an extremely high computational cost. Here we report efficient and accurate band gap calculations of halide metal perovskites by using the approximate quasiparticle DFT-1/2 method. Using AMX\u3csub\u3e3\u3c/sub\u3e (A = CH\u3csub\u3e3\u3c/sub\u3eNH\u3csub\u3e3\u3c/sub\u3e, CH\u3csub\u3e2\u3c/sub\u3eNHCH\u3csub\u3e2\u3c/sub\u3e, Cs; M = Pb, Sn, X = I, Br, Cl) as demonstration, the influence of the crystal structure (cubic, tetragonal or orthorhombic), variation of ions (different A, M and X) and relativistic effects on the electronic structure are systematically studied and compared with experimental results. Our results show that the DFT-1/2 method yields accurate band gaps with the precision of the GW method with no more computational cost than standard DFT. This opens the possibility of accurate electronic structure prediction of sophisticated halide perovskite structures and new materials design for lead-free materials.\u3c/p\u3