MSE+302+Problem+Set+4

# MSE+302+Problem+Set+4 - MSE 302 Problem Set 4 Due 1 You...

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MSE 302 Problem Set 4 Due May 23, 2008 1. You have the following 5 materials with the energy levels shown. The energy levels are all given in electron volts (eV): Material Work Function Ionization Potential Bandgap Metal 1 -3.8 Metal 2 -5.0 Polymer A -5.0 2.0 Polymer B -5.0 2.5 Polymer C -5.9 2.5 Make a solar cell using all of these materials in one device that optimizes the use of the energy levels. Don’t worry about how you would deposit one organic layer on another. Just assume that you can deposit the materials in any way you wish. a. Draw the energy diagrams of the work functions of the metals and HOMO and LUMO levels of the polymers, with energy on the y axis and distance on the x axis, in the correct order in which you would make the device. b. Draw the energy diagrams in the same order but AFTER you put the materials together with zero applied volts (i.e., short circuit conditions) 2. We will now look more closely at the diffusion of excitons in organic materials. We know from empirical evidence that the diffusion length is typically 5~7nm, which is a serious obstacle in making efficient organic solar cells. For now let’s assume that diffusion is a random walk process. The standard equation for the diffusion length is τ D L D = , where D is the diffusivity and tau is the lifetime.

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MSE+302+Problem+Set+4 - MSE 302 Problem Set 4 Due 1 You...

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