![\"\"](\"https:\/\/wtheiss.com\/wordpress\/wp-content\/uploads\/2021\/03\/layer-stack.png\")
<\/a><\/p>\nThe reflectance of the stack is easily calculated \u2013 this is standard in CODE:<\/p>\n
If you are interested how much light is absorbed in one of the active layers you have to generate a spectrum of type \u2018Layer absorption\u2019 in the list of spectra. Please note that this spectrum type works for layer stacks which consist of either \u2018thick layers\u2019 or \u2018thin films\u2019 \u2013 you should not have \u2018rough interface\u2019 or \u2018thickness averaging\u2019 layers in the stack.<\/p>\n \u2018Layer absorption\u2019 objects require the selection of the active layer. A control element for this is available in the upper left corner:<\/p>\n In addition, you have to set a parameter called \u2018Number of sources\u2019. CODE divides the layer of interest into several sublayers and computes the local electric field inside each sublayer. Then the absorbed power is computed and added up for all sublayers. \u2018Number of sources\u2019 is the number of sublayers \u2013 it should be selected high enough so that the final results do not change significantly if you change the number of sources somewhat.<\/p>\n In our example the fraction of light absorbed in the a-Si layer is this:<\/p>\n What really counts for the performance of the solar cell is the number of electron-hole pairs generate by the absorbed light. Not every photon may generate an electron-hole pair that contributes to the current of the cell. The spectrum type \u2018Charge carrier generation\u2019 is used here: It works like the type \u2018Layer absorption\u2019 discussed above, but multiplies the absorbed fraction by an internal efficiency function.<\/p>\n Each spectrum of type \u2018Charge carrier generation\u2019 has subobjects called \u2018Local absorption\u2019 and \u2018Generation efficiency\u2019. The local absorption subobject shows the local absorption of light within the stack:<\/p>\n Starting at z=0 on the left we see the absorption within the Al layer, followed by the a-Si and mc-Si absorption. Finally, on the right side, the TCO absorbs in the UV and the NIR.<\/p>\n The subobject \u2018Generation efficiency\u2019 is used to define the spectral function that returns the probability of the generation of an electron-hole pair by the absorption of a photon. Per default setting the internal efficiency is one:<\/p>\n However, you can click on \u2018Definition\u2019 to open a list that allows more flexibility:<\/p>\n Or a superposition of many oscillators or any user-defined function. The parameters of each term show up as potential fit parameters \u2013 this allows to tune the model and get some information about the generation process.<\/p>\n For simplicity in this example the internal efficiency is 1.0.<\/p>\n Having defined \u2018Charge carrier generation\u2019 objects for both active layers (mc-Si and a-Si) CODE knows the relation of absorbed photons and generated electron-hole pairs for each wavelength. In order to compute a macroscopic property we have to integrate over the whole spectrum. This is done by objects of type \u2018photocurrent\u2019 in the list of integral quantities. For each active layer you have to define one photocurrent object and assign the proper spectrum:<\/p>\n Editing a photocurrent object you can define the spectrum of the incident light and its integrated, total power. Please note that CODE uses the unit mA\/cm^2 for the current density:<\/p>\n Next you have to define the spectrum of the incident radiation. Should the graph be missing press \u2018Update\u2019 to see it \u2013 or press \u2018a\u2019 on your keyboard for automatic scaling. Our example uses an AM1.5 spectrum. Do not worry about normalization \u2013 CODE will do that for you. You just have to provide the wanted spectral shape:<\/p>\n Finally, the total power of the illumination is entered in W\/m^2:<\/p>\n Now CODE knows all parameters and computes the current density generated in the active layer.<\/p>\n In our example we have 2 active layers which may have different layer absorption and also different current densities:<\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a>In this list you can define a spectral dependence of the generation efficiency, using terms that you usually use for dielectric functions. You can use oscillators like this one:<\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n