Variable light biasing method to measure component I–V characteristics of multi-junction solar cells

We present a new technique to measure component current–voltage (I–V) curves of individual sub-cells integrated in a monolithic multi-junction solar cell. This new approach, compared to all previously reported ones, is well suited for thin-film silicon p–i–n structures where the so-called shifting approximation, which supposes that illumination only shifts the I–V curve without changing its shape, is not valid. Moreover, the proposed method is particularly resistant to problems related to electrical shunts. The principle of this method lies in coupling the level of a selective light bias with the level of measured electrical current in order to fix the voltage of a selected sub-cell while sweeping over the current axis. When one of the sub-cells has a fixed voltage, it is then possible to get the I–V characteristics of the second one, shifted by a fixed voltage value. This measurement procedure is simple and requires no modeling. The accuracy of the method is evaluated by numerical simulations of a thin-film silicon p–i–n photodiode. Our technique is then successfully experimentally tested on a specially prepared three-terminal amorphous/microcrystalline silicon tandem solar cell.