local charge carrier density (i.e. , electron and/or hole concentrations), and (for bipolar devices such as OLEDs) the local recombination rate. Other device characteristics that can be simulated are light ernission(voltage) and light emission(current density) for light emitting devices, and voltage (absorbed light) and current density (absorbed light) for light absorbing devices like solar cells and photodetectors. Here the notation A(B) is a shorthand for “ A as a function of B . The data generated by the calculations of the system can be placed in hies t o subsequently manipulate the data for displaying t o a user the optimal device geometry. 

Practice of the invention does not depend critically on how it is implemented. In all cases, simulation methods are computer implemented, but the details of that computer implementation don’tmatter. Some non-limiting examples follow. A soft ware product that provides semiconductor device modeling as described herein is one such implementation. The simulation model can be implemented, for example, as embedded software in a device that can run on e.g., a microcontroller with memory, or it can be more complex such as software running with electronic components, e.g. a personal computer or a computer cluster. Another implementation is as a non- transitory computer readable medium, the medium readable by a processing circuit or general purpose computer and storing instructions run by the processing circuit or general purpose computer for performing methods as described herein. Alternatively the software comprising code for applying a simulation model as described herein can be downloadable via a network and loaded into memory of a computer used t o perform the simulation. Another implementation is as a system or apparatus having at least one processor and at least one memory device connected t o the processor configured t o receive and perform methods as described herein. 

A non-limiting example of a structure that can be simulated according t o the present approach is an OLED structure having a metal cathode, an electron trans port layer (ETL), an emissive layer (EML), a hole transport layer (HTL), an indium tin oxide (ITO) layer and a glass substrate. I such a layer structure light emission occurs predominantly from the EML. Fig. 3A shows an example of such a structure. I these layers, only electrons flow in the electron transport layer, arid only holes flow in the hole transport layer, whereas the recombination and radiative decay oc curs almost entirely in the emissive layer. This is shown in Fig. 4. By selecting the appropriate thicknesses of the layers it is possible t o determine precisely where re- 

