Spectrometric characterization of multi-junction organic solar cells

Many absorber materials used for the fabrication of organic solar cells cover a rather narrow wavelength region in their Spectral Response (SR). Therefore, the use of several absorbing layers with different SR in multi-junction solar cells is of high benefit in order to cover a wider part of the solar spectrum and, in addition, can be realized with a straightforward extension of the manufacturing process of single junction devices. Thus, recently organic multi-junction devices have gained great interest and attracted a broad research activity. In consequence, precise and reliable determination of the performance parameters of such devices constitutes an increasing demand at calibration laboratories such as Fraunhofer ISE CalLab PV Cells. The presented procedures allow for determining solar cell parameters of such technologies in accordance to standard test conditions (STC) traceable to international units. The existing experience with measuring III-V multi-junction solar cells at Fraunhofer ISE with a multi-source solar simulator was translated to measurements on encapsulated thin film organic multi-junction solar cells manufactured within a cooperation of Heliatek GmbH and IAPP. The paper reports the methodology of the calibration of a multi-junction organic solar cell. The SR of the component cells were determined applying suitable LED bias light and a reverse bias voltage chosen to prevent measurement artefacts. With the results we were able to perform a spectrometric characterization for this organic tandem solar cell and verify that a close current matching under the AM1.5g spectrum was achieved. The knowledge of the current at the current matching point enables to calculate the absolute SR of the sub cells. Also specific considerations which have to be taken into account when measuring encapsulated thin film (e.g. organic) solar cells will be discussed.