▪ Abstract Solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤5%, are presented. Preparation of monodisperse samples enables systematic characterization of the structural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it possible to manipulate nanocrystals into close-packed, glassy, and ordered nanocrystal assemblies (superlattices, colloidal crystals, supercrystals). Rigorous structural characterization is critical to understanding the electronic and optical properties of both nanocrystals and their assemblies. At inter-particle separations 5–100 A, dipole-dipole interactions lead to energy transfer between neighboring nanocrystals, and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity. At separations <5 A, exchange interactions cause otherwise insulating ass...

Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies

In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illuminati...

Small molecular weight organic thin-film photodetectors and solar cells

The lowest charge‐transfer excited state (CT1) of electron donor–acceptor crystals or polymers is demonstrated to be a plausible precursor of free charge carriers when such materials are photoexcited Rate constants for the dissociation of charge–transfer states are formulated for two approximate descriptions of CT1: classical ion pair and Wannier exciton The electric field dependence of the dissociation rate constant is postulated to be given by Onsager’s 1934 theory (O‐34) of ion pair dissociation This formulation of CT1 dissociation obviates the need to invoke electron–hole ‘‘thermalization’’ lengths of 2 to 3 nm in order to explain free charge carrier formation in donor–acceptor materials

Electric field assisted dissociation of charge transfer states as a mechanism of photocarrier production

The charge carrier transport and recombination in two types of thermally treated bulk-heterojunction solar cells is reviewed: in regioregular poly(3-hexylthiophene) (RRP3HT) mixed with 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) and in the blend of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) mixed with PCBM. The charge carrier mobility and bimolecular recombination coefficient have been comparatively studied by using various techniques including Time-of-Flight (ToF), Charge Extraction by Linearly Increasing Voltage (CELIV), Double Injection (DI) transients, Current–Voltage (I–V) technique. It was found that the carrier mobility is at least an order of magnitude higher in RRP3HT/PCBM blends compared to MDMO-PPV/PCBM. Moreover, all used techniques demonstrate a heavily reduced charge carrier recombination in RRP3HT/PCBM films compared to Langevin-type carrier bimolecular recombination in MDMO-PPV/PCBM blends. As a result of long carrier lifetimes the formation of high carrier concentration plasma in RRP3HT/PCBM blends is demonstrated and plasma extraction methods were used to directly estimate the charge carrier mobility and bimolecular recombination coefficients simultaneously. A weak dependence of bimolecular recombination coefficient on the applied electric field and temperature demonstrates that carrier recombination is not dominated by charge carrier mobility (Langevin-type recombination) in RRP3HT/PCBM blends. Furthermore, we found from CELIV techniques that electron mobility in RRP3HT/PCBM blends is independent on relaxation time in the experimental time window (approx. hundreds of microseconds to tens of milliseconds). This reduced carrier bimolecular recombination in RRP3HT/PCBM blends implies that the much longer carrier lifetimes can be reached at the same concentrations which finally results in higher photocurrent and larger power conversion efficiency of RRP3HT/PCBM solar cells. Copyright © 2007 John Wiley & Sons, Ltd.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pip.791

A review of charge transport and recombination in polymer/fullerene organic solar cells

Organic solar cells: a review

The Onsager theory of dissociation has been used to explain the electric field, excitation wavelength, and temperature dependence of photogeneration in amorphous selenium. The Onsager theory was formulated to explain the departure from Ohmic behavior in either weak electrolytes or solid dielectrics, and the analysis of charge separation was carried out using the theory of Brownian motion of one particle under the action of Coulomb attraction and the collecting field. Both the absolute magnitude and functional dependence on electric field of the photogeneration efficiency in amorphous selenium at any excitation wavelength can be unambiguously explained using a single parameter which is the initial separation between thermalized electron-hole pairs. This initial separation varies from 7.0 nm at 400-nm excitation to 0.84 nm at 620-nm excitation. The application of the theory to the measured photogeneration data also leads to the important conclusion that each absorbed photon creates a pair of thermalized carriers bound by their mutual Coulomb attraction. The low quantum efficiency measured for long-wavelength excitation is due to the smaller initial separation between oppositely charged thermalized pairs of carriers resulting in smaller dissociation efficiency. Good agreement is also obtained between the measured temperature dependence of the photogeneration efficiency and that predicted by the theory.

Onsager mechanism of photogeneration in amorphous selenium

Analysis of the temperature-dependent decay of surface voltage on an amorphous film after charging but prior to exposure (xerographic dark decay) and of residual decay after exposure can, in combination, be used to map the density of states and position the Fermi level. The procedure is illustrated for $a$-Se where both electron and hole residuals can be measured. $a$-Se is found to be characterized by a relatively discrete gap-state structure. These measurements readily discern thermostructural and photostructural effects on gap-state populations. Thus, during structural relaxation of glassy films in the As:Se alloy system, systematic variation in a number of localized states distributed throughout the mobility gap is observed. This observation is consistent with the view that native defects play a key role in photoelectronic behavior of amorphous chalcogenides.

R. C. Enck

Papers

Onsager mechanism of photogeneration in amorphous selenium

Xerographic spectroscopy of gap states in amorphous semiconductors

Photoenhanced metastable deep trapping in amorphous chalcogenides near room temperature