Thermoelectric Mechanism And Photovoltaic-Thermoelectric Hybrid Properties In Organic And Perovskite Thin-Film Materials And Devices

Thermoelectric is known as direct conversion between heat and electricity,and has been regarded as potential candidates to develop renewable-energy sources.Compared with conventional bulk thermoelectric materials,low dimensional materials including organic or organic hybrid thin-film materials have becoming promising thermoelectric materials due to their advantages such as light weight,flexibility,low cost of solution processing and easy preparation in large area.However,the disadvantages of low conversion efficiency and poor stability limit the development of organic composite films.In order to improve the thermoelectric performance of thin film materials and devices,it is necessary to extensively study and understand the specific thermoelectric transport mechanism of organic composites and thin films,and to regulate the electronic and phonon transport process,so as to realize the efficient energy conversion.In this thesis,we put forward newly developed methods such as photoexcitataion,interfacial effects and polarization effects to control thermoelectric properties and solve the coupling among Seebeck coefficient,electrical conductivity and thermal conductivity based on the organic and organic-inorganic hybrid perovskite thin films,providing experimental methods and theoretical bases to enhance the thermoelectric properties.Considering the low power conversion efficiency of organic thermoelectric thin films,we demonstrated an efficient photovoltaic-thermoelectric hybrid device and confirm the feasibility to achieve high power conversion efficiency.Our study present an interesting example to explore and design novel thermoelectric materials and devices.More details of the research are summarized as four parts listed below:(1)The mechanism on enhancement of thermoelectrics for organic thin films under photoexcitation.The p type and n type organic thin films based on MEH:PPV and PCBM have been fabricated.We found that the Seebeck coefficient and electrical conductivity were both enhanced under photoexcitation states.Consequently,the internal mechanism of the decoupling of the thermoelectric parameters was excluded through the quantitative analysis for the carrier mobility and concentration of organic thin films under components blending and photoexcitation states.The benefits for the decoupling of Seebeck coefficient and electrical conductivity contribute to the enhancement of carrier mobility and interfacial polarization under photoexcitation.(2)Enhancing the thermoelectric performance of organic-inorganic hybrid perovskites polycrystalline thin films by using interfacial effects.The thermoelectric properties of MAPbI₃thin films were studied.By means of the addictive of Bi I₃ component,the highest Seebeck coefficient in vertical direction was improved by 3 times,and the electrical conductivity was enhanced by two orders of magnitude,while the thermal conductivity remained nearly unchanged,which resolved the problem of the coupling of these three parameters.Due to the Bi I₃ addictive,the grain boundaries and interface of perovskite polycrystalline thin films were modified,with passivated defects and supressed ion migration effect,leading to the enhanced charge transport and thermal stability of perovskite thin films.In addition,the Bi I₃ addictive induced the improved surface polarization,which successfully resolved the coupling problem among thermoelectric parameters with simultaneously enhanced Seebeck coefficient and electrical conductivity and limited improvement of thermal conductivity.(3)Enhancing Seebeck effects by using ion migration induced polarization gradient.The impact of ion migration on thermoelectric transport for MAPbI₃polycrystalline thin films was studied for the first time.The thermoelectric performance of MAPbI₃polycrystalline thin films in low temperature were characterized by the repeated process of freezing-heating-freezing.The electron Seebeck coefficient and ion Seebeck coefficient in perovskites were calculated quantitatively,and enhanced electron Seebeck coefficient modulated by ion migration was extensively studied.The extremely high electron Seebeck coefficient was 5 m V K^-1,which had been improved by almost three order of magnitude.The sharply enhanced Seebeck coefficient contributed to the ion migration induced polarization difference with acts as another driving force to drift the carriers under temperature difference aside from the conventional entropy difference.(4)Efficient perovskite photovoltaic-thermoelectric hybrid device.An efficient perovskite photovoltaic-thermoelectric hybrid device was demonstrated by integrating the hole-conductor-free perovskite solar cell with a Bi₂Te₃based thermoelectric generator.The integrated thermoelectric generator effectively reduced the operating temperature of the perovskite solar cell thus improved the device performance and stability,while at the same time it converted the thermal energy into electricity via Seebeck effect.Consequently,an optimized hybrid device was obtained with the open-circuit voltage of 1.29 V and a maximum increased efficiency of 5%compared with the sole photovoltaic device.This study confirmed that the photovoltaic-thermoelectric hybrid device is an effective approach to enhance the power conversion efficiency.