Research On Solid-state High Temperature Solar Cell Based On Photon Enhanced Thermionic Emission

Photovoltaic(PV) solar cells and solar thermal converters are two conventional categories of technologies that have been used in solar energy conversion. Hybrid systems combining PV solar cells and solar thermal converters use the full spectrum of solar radiation and are predicted to have high efficiency.Photon enhanced thermionic emission(PETE) is a newly proposed novel concept in solar energy conversion which can be effective at elevated temperatures. Combining thermal converter with PETE device, the total conversion efficiency can be boosted. The existing design of PETE device is based on vacuum structure, which makes it extremely difficult to fabricate practical devices. Therefore, the current research mostly stays at the theoretical level, and the reports of experimental progress are rare.In this paper, based on PETE effect, a novel Solid-state high temperature solar cell(SHTSC) is proposed, which can harvest solar energy as electricity at elevated temperatures. By theoretical researching its working process, preparing and testing the samples, the mechanism of SHTSC is clarified. The effects of structure and material parameters on the characteristic are discussed and the feasibility of SHTSC is verified. The specific contents are as follows:1. Based on PETE effect, a Solid-state high temperature solar cell(SHTSC) is proposed to harvest solar energy at elevated temperatures. Carrier separation and extraction are achieved by selective contacts that preferentially extract electrons or holes. Using the balance between the carrier generation and loss, a ―rate equation‖ model is presented to estimate the theoretic limit of efficiency of SHTSC. At 600 K and an incident solar radiation concentration of 1000, theoretical conversion efficiency higher than 30% can be achieved. The waste heat of SHTSC could be used by a secondary thermal converter, boosting the total efficiency of the hybrid system above 50%. The analyses show that GaAs and AlxGa1-xAs can be used as absorber and barrier materials, respectively. The effect of barrier condition on the charge extraction property is discussed.2. Based on a 1D steady-state continuity equation, a ―diffusion-emission‖ model is presented to evaluate the effects of device structure and material parameters on performance and efficiency of SHTSC. Calculation results show that the device with PN-structure is more efficient compared with the one with PIN-structure. The optimal absorber thickness is predicted in the range of 1~2 μm. The electron diffusion length that longer than 5 μm is favourable for enhancing efficiency. Our analyses show that the front interface strongly affects conversion efficiency, which emphasises the need to reduce interface recombination losses. Our analyses also show that SHTSC is based on diffusion/ballistics process so that the temperature coefficient of open circuit voltage is much smaller than that of conventional PV cells.3. A new device structure with a grated bandgap window-layer is proposed to improve the performance of SHTSC. The bandgap gradation is achieved by varying the proportion of Al in the AlxGa1-xAs window layer. The built-in electric field induced by the compositional grade will not only reduce back surface recombination losses but also efficiently collect photogenerated electrons in the window layer, enhancing the conversion efficiency. A ―drift/diffusion-emission‖ model is developed to analyze the characteristics of the proposed device. Calculation shows that when S1= 107 cm/s the efficiency of SHTSC with grated bandgap window-layer is 5 times higher than that of SHTSC without window layer. The window layer parameters are designed and optimized.4. Based on the theoretical research, the GaAs/AlGaAs epitaxial wafers are designed, fabricated, and characterized. After preparation process, we obtain SHTSC sample with uniform bandgap window layer P138 and SHTSC sample with graded bandgap window layer P139. We test and analyze the output performance of these SHTSC samples. The measured temperature coefficients of open circuit voltage at 1 sun for sample P138 and sample P139 are 1.81 mV/K and 1.77 mV/K, respectively. At 64 sun, the measured temperature coefficients of open circuit voltage for sample P138 and sample P139 are 1.43 mV/K and 1.38 mV/K, respectively. These values are consistent with the theoretical value and are much lower than that of conventional PV cells, which verify the effectiveness of SHTSC. The measured short-circuit current and open circuit voltage of sample P139 are larger than that of sample P138, which shows the effectiveness of graded bandgap window layer to improve the performance of SHTSC. Combining a Fresnel condenser and a thermal converter, the concentrated solar hybrid system is established. Under flux concentration of 64, the conversion efficiency of this demonstration system is 6.41%.In this paper, we propose the SHTSC which is based on the PETE effect and selective contact to separate and extract photogenerated carriers. We prepare the SHTSC samples for the first time and verify the effectiveness of SHTSC experimentally. We propose using graded bandgap window layer to improve the performance of SHTSC, and prove its validity. Through this research, the new method for harvesting solar energy at higher temperatures is explored, which will promote the development of solar hybrid systems and enhance the core competitiveness of China’s solar energy industry.