In order to break the barrier of almost 30 percent for the efficiency of silicon solar cells, the team at the Helmholtz-Zentrum Berlin together with international colleagues built organic layers into the silicon of the solar cell. These convert the energy of the high-energy photons (green and blue light) in such a way that the current yield in this energy range doubles.
Normally, a photon always generates a pair of charge carriers (exitons) consisting of a weakly bonded negatively charged electron and a positive hole. The pair is separated at the charge-selective contacts of the solar cell. The team led by HZB researcher Klaus Lips has succeeded in building the solar cell in such a way that certain photons from the light spectrum can each generate two pairs of charge carriers at once.
The effect they use for this occurs in certain organic molecule crystals and is called "singlet exciton fission" (SF). It becomes effective when the charge carrier pairs fulfil certain quantum physical conditions: all their spins must be aligned in parallel; they are then in a so-called triplet state. These triplet excitons are quite long-lived and very strongly bound to each other. One difficulty is however to tear apart the triplet pairs from the organic material at the interface to silicon so that the released positive and negative charge carriers can contribute to the current of the solar cell.
In an experiment, the HZB researchers have now shown that it is possible to separate the triplet pairs and that the quantum yield per photon can be doubled to 200 percent. This should increase the theoretical maximum efficiency of a silicon solar cell to around 40 percent.