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Scientists in the United States have designed a microwire solar cell that could reportedly enable singlet fission coupling with silicon. The key to their achievement was an interface that transfers electrons and holes sequentially to silicon rather than both at the same time. Working with an effect known as singlet exciton fission (SF), scientists at the Massachusetts Institute of Technology (MIT) have demonstrated a novel silicon solar cell concept that could potentially overcome the quantum efficiency limit for conventional photovoltaic devices. Singlet exciton fission is an effect observed in certain materials whereby a single photon can generate two electron-hole pairs when absorbed in a solar cell, instead of just one. The effect has been observed by scientists since the 1970s, and while it has become a major area of ??research for some of the worlds leading institutes in the last decade, translating the effect into a viable solar cell has proven complex. Singlet fission solar cells can produce two electrons from one photon, making the cell more efficient. This occurs through a quantum mechanical process where a singlet exciton (an electron-hole pair) splits into two triplet excitons. “Until now, we only had indirect evidence that it’s possible to couple singlet exciton fission to silicon,” Marc A. Baldo, corresponding author of the research, told pv magazine . “The breakthrough for us was designing an interface that transfers electrons and holes sequentially to silicon instead of both at the same time.” In the study “ Exciton fission enhanced silicon solar cell,” recently published in Joule , the researchers explained that they designed and built a microwire (MW) cell with an interface based on a hafnium oxynitride (HfOxNy) film to enhance the coupling between tetracene (Tc) and silicon. Tc and its derivatives are prime candidates for SF because they can form charge-transfer and multi-excitonic states. The interface also included a thin layer of aluminum oxide (AlOx) for passivation, which prevents transferred charge carriers from immediately recombining on the silicon surface, as well as a layer of zinc phthalocyanine (ZnPc) as an electron donor material. “To minimize recombination on the backside, a backside field layer (BSF) with a junction depth of 1 µm and a localized back contact is added,” the scientists said. “A micrograting electrode is applied as the front electrode to efficiently collect the carriers.” The researchers performed a series of measurements on the cells performance and found that depositing ZnPc and Tc onto the device improved the short-circuit current density, with a negligible decrease in open-circuit voltage and fill factors, resulting in an overall improvement in power conversion efficiency. The analysis also showed that the maximum charge generation efficiency per absorbed photon in tetracene is around 138%, which the scientists said “comfortably exceeds” the quantum efficiency limit for conventional silicon solar cells. This technology will compete with double-junction concepts like perovskites on silicon, Baldo explained. Combining exciton fission with silicon avoids current-matching restrictions, and the approach promises the robustness under variable illumination and simplicity typical of single junctions. Theres still a lot of work to be done. Most importantly, we need to increase efficiency and demonstrate that the technology can be stable under sunlight. Observing photocurrent from exciton fission in a silicon solar cell is proof of concept that coupling to singlet exciton fission is a viable avenue for increasing the efficiency of silicon solar cells, Baldo concluded. I believe we can now affirm that exciton fission is a true technological contender in the competition for new solar cell technologies. In 2023, researchers at MIT and the University of Virginia announced plans to use acenes, which are benzene molecules with unique optoelectronic properties, in singlet fission solar cells. Their approach involved adding carbodicarbene ligands to acenes already doped with boron and nitrogen. In 2019, another MIT research group demonstrated how singlet exciton fission could be applied to silicon solar cells and lead to efficiencies of up to 35%. They claimed to be the first group to transfer the effect from one of the excitonic materials known to exhibit it, in this case also tetracene. They accomplished the feat by placing an additional layer of hafnium oxynitride just a few atoms thick between the silicon solar cell and the tetracene excitonic layer. The MIT researchers described their work as supercharging silicon solar cells and said it differs from more common approaches to increasing solar cell efficiency, which today focus more on tandem cell concepts. Were adding more current to the silicon instead of making two cells, they stated at the time. |
