A Newton-Raphson method approach to adjusting multi-source solar simulators

NASA Glenn Research Center has been using an inhouse designed X25 based multi-source solar simulator since 2003. The simulator is set up for triple junction solar cells prior to measurements by adjusting the three sources to produce the correct short circuit current, Isc, in each of three AM0 calibrated sub-cells. The past practice has been to adjust one source on one sub-cell at a time, iterating until all the sub-cells have the calibrated Isc. The new approach is to create a matrix of measured Isc for small source changes on each sub-cell. A matrix, A, is produced. This is normalized to unit changes in the sources so that A×Δs= Δi_sc. This matrix can now be inverted and used with the known Isc differences from the AM0 calibrated values to indicate changes in the source settings, Δs = A^-1×Δisc This approach is still an iterative one, but all sources are changed during each iteration step. It typically takes four to six steps to converge on the calibrated Isc values. Even though the source lamps may degrade over time, the initial matrix evaluation is not performed each time, since measurement matrix needs to be only approximate. Because an iterative approach is used the method will still continue to be valid. This method may become more important as state-of-the-art solar cell junction responses overlap the sources of the simulator. Also, as the number of cell junctions and sources increase, this method should remain applicable.