Herein, we focus on the principles of photoconduction in random semiconductors—the key processes being optical generation of charge carriers and their subsequent transport. This is not an overview of ...the current work in this area, but rather a highlight of elementary processes, their involvement in modern devices and a summary of recent developments and achievements. Experimental results and models are discussed briefly to visualize the mechanism of optical charge generation in pure and doped organic solids. We show current limits of models based on the Onsager theory of charge generation. After the introduction of experimental techniques to characterize charge transport, the hopping concept for transport in organic semiconductors is outlined. The peculiarities of the transport of excitons and charges in disorderd organic semiconductors are highlighted. Finally, a short discussion of ultrafast transport and single chain transport completes the review.
The principles of photoconduction in random, organic semiconductors are discussed, the key processes being optical generation of charge carriers and their subsequent transport (see picture). This Review highlights elementary processes and their involvement in modern devices, and addresses recent developments and achievements.
Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) ...bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.
The observation that in efficient organic solar cells almost all electron-hole pairs generated at the donor-acceptor interface escape from their mutual coulomb potential remains to be a conceptual ...challenge. It has been argued that it is the excess energy dissipated in the course of electron or hole transfer at the interface that assists this escape process. The current work demonstrates that this concept is unnecessary to explain the field dependence of electron-hole dissociation. It is based upon the formalism developed by Arkhipov and co-workers as well as Baranovskii and co-workers. The key idea is that the binding energy of the dissociating "cold" charge-transfer state is reduced by delocalization of the hole along the polymer chain, quantified in terms of an "effective mass", as well as the fractional strength of dipoles existent at the interface in the dark. By covering a broad parameter space, we determine the conditions for efficient electron-hole dissociation. Spectroscopy of the charge-transfer state on bilayer solar cells as well as measurements of the field dependence of the dissociation yield over a broad temperature range support the theoretical predictions.
While it has been argued that field‐dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic ...solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT‐type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p‐DTS(FBTTh2)2 layer as donor material and C60 as acceptor. The authors measure JV‐characteristics as a function of intensity and charge‐extraction‐by‐linearly‐increasing‐voltage‐type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (Lp) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p‐DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s‐shape of the JV‐curves and the concomitant additional decrease of the FF with increasing layer thickness.
Back diffusion of holes is identified to control the fill factor and thus the efficiency of bilayer organic solar cells. The competition between recombination at the donor–acceptor interface and extraction at the electrodes is studied by varying the donor layer thickness and light intensity such as to differentiate between monomolecular, i.e., geminate recombination and bimolecular, nongeminate recombination.
The high efficiencies reported for organic solar cells and an almost negligible thermal activation measured for the photogeneration of charge carriers have called into question whether photoinduced ...interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be reconciled with very low activation energies. Here, this question is addressed in a combined experimental and theoretical approach. The interfacial binding energy of a charge‐transfer state in a blend of MeLPPP:PCBM is determined by using energy resolved electrochemical impedance spectroscopy and is found to be about 0.5 eV. Temperature‐dependent photocurrent measurements on the same films, however, give an activation energy that is about one order of magnitude lower. Using analytical calculations and Monte Carlo simulation the authors illustrate how i) interfacial energetics and ii) transport topology reduce the activation energy required to separate the interfacial electron–hole pair, with about equal contributions from both effects. The activation energy, however, is not reduced by entropy, although entropy increases the overall photodissociation yield.
The coulomb binding energy of an electron and a hole on adjacent chromophores is in the order of 0.5 eV, yet for efficient solar cells, very little activation energy is required for the photodissociation of excitations. It is shown here how the combined effects of interfacial electrostatics, wave function delocalization, and disorder can account for this.
Abstract
In an endeavor to understand why the dissociation of charge‐transfer (CT) states in a PM6:Y6 solar‐cell is not a thermally activated process, measurements of energy‐resolved impedance as ...well as of intrinsic photoconduction are employed. This study determines the density of states distributions of the pertinent HOMO and LUMO states and obtains a Coulomb binding energy (
E
b
,
CT
) of ≈150 meV. This is 250 meV lower than the value expected for a pair of localized charges with 1 nm separation. The reason is that the hole is delocalized in the polymer and the electron is shared among Y6 molecules forming a J‐like aggregate. There are two key reasons why this binding energy of the CT state is not reflected in the temperature dependence of the photocurrent of PM6:Y6‐diode: i) The e–h dissociation in a disordered system is a multi‐step process whose activation energy is principally different from the binding energy of the CT state and can be substantially less than
E
b
,
CT
, and ii) since dissociation of the CT state competes with its intrinsic decay, the dissociation yield saturates once the rate of dissociation grossly exceeds the rate of intrinsic decay. This study argues that these conditions are met in a PM6:Y6‐solar cell.
In order to unravel the intricate interplay between disorder effects, molecular reorganization, and charge carrier localization, a comprehensive study was conducted on hole transport in a series of ...conjugated alternating phenanthrene indenofluorene copolymers. Each polymer in the series contained one further comonomer comprising monoamines, diamines, or amine-free structures, whose influence on the electronic, optical, and charge transport properties was studied. The series covered a wide range of highest occupied molecular orbital (HOMO) energies as determined by cyclovoltammetry. The mobility, inferred from time-of-flight (ToF) experiments as a function of temperature and electric field, was found to depend exponentially on the HOMO energy. Since possible origins for this effect include energetic disorder, polaronic effects, and wave function localization, the relevant parameters were determined using a range of methods. Disorder and molecular reorganization were established first by an analysis of absorption and emission measurements and second by an analysis of the ToF measurements. In addition, density functional theory calculations were carried out to determine how localized or delocalized holes on a polymer chain are and to compare calculated reorganization energies with those that have been inferred from optical spectra. In summary, we conclude that molecular reorganization has little effect on the hole mobility in this system while both disorder effects and hole localization in systems with low-lying HOMOs are predominant. In particular, as the energetic disorder is comparable for the copolymers, the absolute value of the hole mobility at room temperature is determined by the hole localization associated with the triarylamine moieties.
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