Recent developments in organic solar cells show interesting power conversion efficiencies. However, with the use of organic semiconductors and bulk heterojunction cells, many new concepts have to be introduced to understand their characteristics. Only few models investigate these new concepts, and most of them are one-dimensional only. In this work, we present a two-dimensional model based on solving the drift-diffusion equations. The model describes the generation of excitons in the donor phase of the active layer and their diffusion towards an interface between the two separate acceptor and donor domains. Then, when the exciton reaches the interface, it forms a charge transfer state which can split into free charges due to the internal potential. Finally, these free charges are transported toward the electrodes within their respective domains (electrons in acceptor domain, holes in donor domain) before being extracted. In this model, we can follow the distribution of each species and link it to the physical processes taken into account. Using the finite element method to solve the equations of the model, we simulate the effect of the bulk heterojunction morphology on photocurrent curves. We concentrate on the morphology parameters such as the mean acceptor/donor domain sizes and the roughness of,the interface between the donor and acceptor domains. Results are discussed in relation with experimental observations.
The improvement of our model concerning a single nanocrystal that belongs to a nanocrystal floating gate of a flash memory is presented. In order to extend the gate voltage range applicability of the model, the 3D continuum of states of either metallic or semiconducting electrodes is discretized into 2D subbands. Such an approach gives precise information about the mechanisms behind the charging or release processes of the nanocrystal. Then, the self-energy and screening effects of an electron within the nanocrystal are evaluated and introduced in the model. This enables a better determination of the operating point of the nanocrystal memory. The impact of those improvements on the charging or release time of the nanocrystal is discussed.
We propose a theoretical study for charging the floating gate composed of Si nanocrystals (NCs), in a non-volatile flash memory. Only a few electrons tunnel from the channel of a metal-oxide-semiconductor transistor into the two-dimensional array of nanocrystals.Our model is based on the geometrical and physical properties of the device, in order to take the dispersion of the relevant parameters into account: NC radii, inter-NC distances, tunnel oxide and gate oxide thicknesses. The energy subbands of the channel are explicitly included, together with the doping density.This three-dimensional model of electron tunneling into a NC is numerically solved through a two-dimensional finite element approach, which allows extensive numerical experimentations.The tunneling times to charge a single NC or the whole NC floating gate are evaluated in a finer detail, and the influence of the dispersion of the relevant parameters is discussed.Such a study may help the experimentalists to build efficient quantum flash memories.
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