Although organic heterojunctions can separate charges with near-unity efficiency and on a subpicosecond timescale, the full details of the charge-separation process remain unclear. In typical models, the Coulomb binding between the electron and the hole can exceed the thermal energy kBT by an order of magnitude, suggesting that it is impossible for the charges to separate before recombining. Here, we consider the entropic contribution to charge separation in the presence of disorder and find that even modest amounts of disorder have a decisive effect, reducing the charge-separation barrier to about kBT or eliminating it altogether. Therefore, the charges are usually not thermodynamically bound at all and could separate spontaneously if the kinetics otherwise allowed it. Our conclusion holds despite the worst-case assumption of localised, thermalised carriers, and is only strengthened if mechanisms like delocalisation or 'hot' states are also present.Although organic solar cells (OSCs) have the potential to become low-cost renewable-energy sources, the precise mechanisms of how they convert photoexcitations into free charge carriers are not completely understood. The efficiency of OSCs depends on the separation of chargetransfer (CT) states formed at donor-acceptor interfaces ( Fig. 1), where the Coulomb binding energy between the hole and the electron is usually assumed to bewhere r is the distance between them, e is the elementary charge, ε 0 is the permittivity of vacuum, and ε r is the dielectric constant. Organic semiconductors typically have low dielectric constants, ε r ≈ 2 − 4, meaning that a CT state with a typically assumed nearestneighbour electron-hole separation of r = 1 nm experiences a Coulomb binding of about 500 meV [1]. Because this barrier is much greater than the available thermal energy (k B T = 25 meV), one might expect that the charges could never separate, or at least not within the lifetime of the CT state. Nevertheless, CT-state dissociation approaches 100% efficiency in some OSCs [2]; explaining this has been a central question in the field. Efficient charge separation can be reproduced using several kinetic models, either kinetic Monte Carlo simulations [3-6] or analytical generalisations of Onsager's theory [7][8][9][10][11][12][13]. However, it is often difficult to distill the fundamental physics from intricate simulations. In the present case, the simulations have not set aside the widespread view that additional mechanisms are needed to explain charge-separation in OSCs [14,15]. Among the proposed mechanisms, charge delocalisation could decrease the Coulomb binding [16][17][18][19][20][21][22], while the excess energy of the initially "hot" CT state could help the charges overcome their attraction [19,20,23,24].A related conceptual difficulty is distinguishing bound charges from free ones when the binding potential (such as Eq. 1) monotonically increases [25]. It is conventionally assumed the charges are free if U (r) < k B T , i.e., if their separation exceeds the Bjerrum length ...

The performance of kesterite thin-film solar cells is limited by a low open-circuit voltage due to defect-mediated electron-hole recombination. We calculate the non-radiative carrier-capture cross sections and Shockley-Read-Hall recombination coefficients of deep-level point defects in Cu 2 ZnSnS 4 (CZTS) from first-principles. While the oxidation state of Sn is +4 in stoichiometric CZTS, inert lone pair (5s 2 ) formation lowers the oxidation state to +2. The stability of the lone pair suppresses the ionization of certain point defects, inducing charge transition levels deep in the band gap. We find large lattice distortions associated with the lone-pair defect centers due to the difference in ionic radii between Sn(II) and Sn(IV). The combination of a deep trap level and large lattice distortion facilitates efficient non-radiative carrier capture, with capture cross-sections exceeding 10 −12 cm 2 .The results highlight a connection between redox active cations and 'killer' defect centres that form giant carrier traps. This lone pair effect will be relevant to other emerging photovoltaic materials containing ns 2 cations. arXiv:1810.11259v1 [cond-mat.mtrl-sci]

The first three-dimensional model of transport in moderately disordered materials shows that a little delocalisation can dramatically enhance mobilities.

Improvement in the optoelectronic performance of halide perovskite semiconductors requires the identification and suppression of nonradiative carrier trapping processes. The iodine interstitial has been established as a deep level defect and implicated as an active recombination center. We analyze the quantum mechanics of carrier trapping. Fast and irreversible electron capture by the neutral iodine interstitial is found. The effective Huang−Rhys factor exceeds 300, indicative of the strong electron−phonon coupling that is possible in soft semiconductors. The accepting phonon mode has a frequency of 53 cm −1 and has an associated electron capture coefficient of 1 × 10 −10 cm 3 s −1 . The inverse participation ratio is used to quantify the localization of phonon modes associated with the transition. We infer that suppression of octahedral rotations is an important factor to enhance defect tolerance.