Highly doped layers as efficient electron-hole recombination contacts for tandem organic solar cells

A key feature of stacked organic solar cells is an efficient recombination contact at the interface between the solar cells in the stack. Here, an electron current has to be converted into a hole current without loss of energy. Furthermore, the recombination contact has to be highly transparent. We present a new approach for small molecule organic solar cells using highly doped organic layers. Our approach adapts the use of tunnel diodes known from inorganic tandem solar cells. We compare a metal cluster based recombination contact reported in literature to the new approach using different organic tandem solar cell structures. For this purpose, current-voltage characteristics of adequate solar cells are measured. The experiments show that highly doped layers as recombination contacts in tandem organic solar cells are superior to the metal cluster based approach. The proposed concept allows an addition of the open circuit voltages of the subcells of a tandem solar cell, without absorption or reflection at the recombination contact. The results further show that our concept does not depend on the specific choice of materials as it is seen for metal cluster based recombination contacts. It therefore represents a general approach which is compatible to mass manufacturing. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3467786]