Hopping conduction is widely considered the dominant charge transport mechanism in disordered organic semiconductors. Although theories of hopping transport have been developed in detail for applications to inorganic amorphous materials, these theories are often out of scope for the community working with organic amorphous systems. Theoretical research on charge transport in organic systems is overwhelmed by phenomenological fittings of numerical results by equations, which often make little physical sense. The aim of the current review is to bring analytical theoretical methods to the attention of the community working with disordered organic semiconductors.

The decisive feature of any material designed for photovoltaic applications is the dissociation efficiency of photogenerated excitons. This efficiency is essentially governed by the Coulomb attraction between electrons and holes. Because the dielectric constant in organic materials is rather low (ε r ≈ 3), the exciton binding energy is much larger than the thermal energy at room temperature, so that thermally governed dissociation seems improbable. Experiments show that while the dissociation probability for electron−hole pairs is indeed very low in the bulk samples, it becomes close to unity at intrinsic interfaces between two organic materials, an electron donor (usually a conjugated polymer) and an electron acceptor (usually a fullerene derivative). The driving force for this dissociation is still a matter of controversy. This Perspective provides a theoretical analysis of possible mechanisms for the efficient dissociation of electron−hole pairs at internal organic interfaces despite the strong Coulomb attraction between the charges. I t is one of the main challenges in the research on organic materials to reveal the mechanism responsible for the efficient dissociation of electron−hole pairs (EHPs) created by light. The interest of researchers in the dissociation problem is caused mainly by perspectives of photovoltaic applications of organic semiconductors. Such materials usually exhibit very high light absorption coefficients, which, combined with lowcost processing, makes them promising for applications in solar cells. 1,2 However, it is not the efficient light absorption itself but rather the combination of the efficient absorption with the efficient photocurrent generation that makes a material favorable for photovoltaic applications. The decisive step in photocurrent generation is the dissociation of EHPs created by light. In the dissociation process, the electron and hole must overcome their mutual Coulomb attraction, determined by the energywhere e is the elementary charge, ε r is the relative dielectric constant of the surrounding media, ε 0 is the permittivity of vacuum, and r is the electron−hole separation. While in inorganic semiconductors with ε r > 10 the binding energy of excitons is on the order of 10 meV, in organic semiconductors, in which ε r is typically between 2 and 4, 3 the binding energy of excitons is on the order of 1 eV. 4 The puzzling question arises then regarding the mechanism that could provide an efficient dissociation of EHPs with such a huge binding energy as compared to the thermal energy at room temperature, kT ≃ 0.025 eV.Numerous experimental and theoretical studies have been dedicated to the dissociation problem of EHPs in organic semiconductors. There is no chance to review all of them in the current report. Interested readers can find a comprehensive description of the research field in recent review articles. 1,3 Here, we only briefly describe the state of the research field.The first organic solar cells tested in the 1970s used a single material, sandwiched...

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