Multi-junction polymer solar cells: Status and challenges (Conference Presentation)

Multi-junction device architectures represent a promising strategy to further advance the efficiency of organic solar cells. For solution-processed organic solar cells, tandem and triple junction cells have been reported in the past. Yet, several challenges remain, both in developing new photoactive materials as well crating new recombination layers that serve to interconnect the subcells. We developed new versatile charge recombination layers for solution-processed multi-junction solar cells in n-i-p and p-i-n architectures. The new recombination layers provide an essentially lossless contact in each case, without the need of adjusting the formulations or deposition conditions for six different tandem cells and three different triple-junction solar cells, employing a range of different photoactive layers. The approach permitted realizing complex devices in good yields, providing a power conversion efficiency up to 10%. We will also present a first example of a quadruple-junction polymer solar cell, featuring four different and complementary band gap absorber layers that absorb light up to 1150 nm. The quadruple junction cell is reaches a power conversion efficiency of about 7.5% with an open-circuit voltage of 2.46 V. Measuring the external quantum efficiency (EQE) of the quadruple cells has been accomplished using a protocol using bias light of different wavelengths, involving optical modeling and correcting for the build-up electric field. At present, the efficiency of the quadruple-junction polymer cell is mainly limited by bimolecular recombination in the active layers.