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Final Report Summary - NLO FOR PV (NonLinear Optics for Photovoltaics)

Increasing the efficiency of renewable energy sources is one of the most important challenges to mankind. In solar energy, the Shockley Queisser limits the efficiency of single junction photovoltaic (PV) cells and sets the maximum efficiency for Si PV at about 30%. This is because of two constraints: i. The energy PV generates at each conversion event is set by its bandgap, irrespective of the photon’s energy. Thus, energetic photons lose most of their energy to heat. ii. PV cannot harness photons at lower energy than its bandgap. Therefore, splitting energetic photons, and fusing two photons each below the Si bandgap to generate one higher-energy photon that match the PV, push the potential efficiency above the Shockley Queisser limit. Nonlinear optics (NLO) offers efficient frequency conversion, yet it is inefficient at the intensity and the coherence level of solar and thermal radiation.
In this research we aim to develop new thermodynamic concepts and use nano-technology for converting inefficient parts of the solar spectrum to wavelengths most suitable for energy harvesting. The project objective includes the demonstration of solar powered laser, up-conversion of sub-bandgap solar radiation and splitting of energetic solar photons.
Along the research progress we discovered what we believe is a breakthrough in fundamental science as well as applied. First we experimentally demonstrated a new up-conversion method that is suitable for solar radiation. Second we experimentally demonstrated a new thermodynamic concept for up-conversion and harvesting of thermal losses in photovoltaic cells. These new directions are within the framework of this grant and open the way to present disruptive technology in photovoltaics. As a results of this achievements the group published five papers and won an ERC starter award on “New thermodynamic ideas for Frequency Conversion and Photovoltaics (ThforPV)”
Specifically, as part of the CIG project, our main achievements thus far include:
1. Solar-powered-lasers: Such lasers are the missing link between NLO and photovoltaics. Yet currently they are limited to operate above 3000 suns, inapplicable for PVs. We aim to demonstrate solar-powered-lasers that operates at low solar concentration (below 10 suns), based on our earlier experiments demonstrating, for the first time, incoherent pumping for high-Q micro-lasers. We have validated this new concept, and experimentally demonstrated record efficiency solar powered lasers operating at 200 suns. These findings were presented in invited talks in Europe and at a conference in the US. We also publication one paper in Scientific reports a paper that summaries this work. Our next step was to implement cascaded energy transfer, which was proven to reduce losses and power threshold by two orders of magnitude. This is expected to reach the aimed power threshold of one sun. A 2nd paper on the subject is currently under review.
2. “Entropy driven up-conversion”: This is a new thermodynamic concepts for up-converting partially-incoherent low-energy photons to emission that matches solar cells, thereby allowing such wasted energy to be harvested. This concept is based on coupling "hot phonons" to Near-IR emitters, while the bulk remains at low temperature. We recently demonstrate experimentally entropy-driven ten-fold up-conversion of 10.6μm excitation to 1μm at internal efficiency of 27% and total efficiency of 4%. This is more efficient by orders of magnitude from any prior art, and opens the way for efficient up-conversion of thermal radiation. These findings were presented at few invited talks in Europe and at a conference in the US and in recent publication in ASC photonics. Also a patent was filed on the subject. Our current effort is to generate new up-conversion frequencies. A paper on this issue is currently under preparations.
3. "Thermally assist photoluminescence for ultra-efficient PV". This is a new thermodynamic concepts for harvesting the otherwise lost thermalization in single junction PVs with theoretical efficiencies as high as 70%, and low operating temperatures. We support the theory by experimental validation. We show orders of magnitude enhancement in the amount of accessible photons for high-bandgap PV, compared to conventional thermal photovoltaics concepts. Such high efficiencies at practical temperatures show a promising route for realizing photovoltaic systems at much higher efficiency than ever before. As a result of this demonstration we are asked to join publication with major groups in the field , on 3rd generation photovoltaics. We are currently in pursuing to demonstrate such fully operational device, with another publication is currently under review. These findings were presented in few invited talks and a patent was filed on the subject.
As part of my academic work, I was able to guaranty extra funding for the research from external sources. Many of the grants share similar goals as the CIG project. These include collaboration with Weizmann institute on the solar powered laser (Helmsley and FTA grants) and individual grant (ISAF) Also, I have recently established Israeli-German collaboration between, Assist. Prof. Dirk Dorfs from Leibniz University Hannover, Prof Efrat Lifshitz from the Technion and myself (as the leading PI in this joint grant). The topic of the grant is multi exciton generation in quantum dots for excitonic fission. These collaborations on related topics to this CIG project are part of our effort to share our knowhow within the European research community. Finally, I received an ERC starting award on “new thermodynamic ideas for Frequency Conversion and Photovoltaics (ThforPV)” which is based on the CIG funded research.
In Parallel to my research, I made important contribution to the Technion and to my department; As part of my position negotiations I insisted to re-open the "Optical engineering" course of study for under-graduates at our department. This effort, I believe, will contribute greatly to the electro-optics industry and academy in Israel. Last year I was nominated as the head of the program. The program currently includes 15 students, and three professors. I also teach several courses including: Optical system design, Radiation heat transfer, and transport at the nano-scale.

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