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.
In the experiment, the researchers integrated a 100-nanometer thin organic layer of tetracene crystals into the surface of a silicon solar cell. Using spectroscopic investigations, they were able to detect the triplet charge carrier pairs in the tetracene layer. “The challenge was to integrate the tetracene layer in such a way that the current flow of the silicon solar cell is not significantly disturbed,” explains Klaus Lips, “since a poorly conducting organic layer borders on a well conducting silicon layer.” A constellation that quickly brings the current flow to a standstill.
The separation is achieved with an additional organic conductor called PEDOT:PSS. A further organic layer is therefore necessary. The boundary layers of the materials play a special role in this structure, which is why the X-ray light of the synchrotron BESSY II@HZB played an important role in the project in analytics.
The measurement results of the first silicon multiplier solar cell show: Tetracene absorbs the blue-green part of the light, the low-energy photons are absorbed by the silicon. Using a simulation, the researchers were able to estimate that currently around five to ten percent of the triplet pairs generated could be added to solar power.
Lips regards this as a huge success and announces follow-up experiments: “The additional current flow generated by the piggyback layer is not yet very large in the experiment currently presented, but with this solar cell structure we have shown that the approach works in principle. And we already know what we need to do to increase the yield of separated triplet excitons to up to 200 percent.”
The researchers published the results of their research in the journal Materials Horizons.