Thermophotovoltaic Cells With Bi₂Te₃/SnO Heterojunctions

Thermophotovoltaic（TPV）cells are a technology that can directly convert thermal energy into electrical energy,which has potential applications in many fields.In order to avoid the influence of the intrinsic excitation effect,the TPV cells currently studied usually require the band gap of the semiconductor to be greater than 0.3 e V,so that it can absorb the radiation of a high temperature heat source with a temperature above 1300 K.For more popular low-temperature（400-600 K）heat sources,the infrared photons radiated by them cannot be absorbed by conventional TPV cells due to the low energy of the photons.Thus,the band gap of the semiconductor used to make TPV cells must be less than 0.3 e V to match the infrared radiation emitted by low-temperature heat sources.Bismuth telluride（Bi₂Te₃）,as a semiconductor with a band gap of only 0.14-0.17 e V,can absorb long-wave thermal radiation.It is suitable for the preparation of TPV cells that convert low-temperature thermal radiation into electricity.Previously prepared Bi₂Te₃/Si TPV cells confirm the feasibility of converting low-temperature radiation into electricity.However,due to the mismatch of valence band levels between Bi₂Te₃ and Si,the transport of carriers is hindered and results in low conversion efficiency of the cells.Stannous oxide（SnO）,as a p-type semiconductor,has a valence band energy level of-4.4e V.It is close to that of Bi₂Te₃,which is expected to solve the energy level matching problem of Bi₂Te₃/Si heterojunctions.In order to prepare a heterojunction with better performance,Bi₂Te₃/SnO heterojunctions are studied in this thesis.The details are as follows:1.Bi₂Te₃ thin films and SnO thin films are prepared by magnetron sputtering.The results show that the prepared Bi₂Te₃ thin films are n-type polycrystalline semiconductors with a carrier concentration of about 10²⁰ cm^-3.And SnO thin films are p-type amorphous semiconductor thin films with a carrier concentration of about 10¹⁹ cm^-3.2.The role of different metals/Bi₂Te₃ contacts on TPV cells is explored.By depositing two different metal electrodes on the same TPV cell,the effects of metal/Bi₂Te₃ Schottky junctions on TPV cells are investigated,including energy band structure,IV curves,open circuit voltages and short circuit currents.It is concluded that the built-in electrical fields of the Schottky junctions are the main factors influencing the properties of TPV cells.3.Bi₂Te₃/SnO heterojunctions have typical rectification characteristics of pn junctions.It is also found that the built-in electrical field direction of the heterojunction is from p-type SnO to n-type Bi₂Te₃,which is opposite to that of the conventional pn junctions.The reason is that the Fermi level of SnO is higher than that of Bi₂Te₃,which makes electrons diffuse from SnO to Bi₂Te₃.Bi₂Te₃/SnO TPV cells can convert infrared radiation（<8.3μm）emitted by a low-temperature heat source（400-600 K）into electricity,whose conversion efficiency is better than that of Bi₂Te₃/Si TPV cells.However,the built-in electrical field of Schottky junctions,the potential barriers at the interface of the heterojunctions,low absorption coefficient and carrier recombination caused by defects lead to the low conversion efficiency of Bi₂Te₃/SnO TPV cells.