Estrutura eletrônica de materiais orgânicos para aplicações em camadas ativas de células solares
Carregando...
Data
2016-11-28
Autores
Oliveira, Eliézer Fernando de [UNESP]
Título da Revista
ISSN da Revista
Título de Volume
Editor
Universidade Estadual Paulista (Unesp)
Resumo
Materiais orgânicos vêm sendo utilizados em diversos tipos de dispositivos eletrônicos e optoeletrônicos, sendo um dos destaques seu uso em células solares. Atualmente, células solares que utilizam materiais orgânicos na camada ativa já atingem eficiências em torno de 12% na conversão da energia solar em elétrica. Apesar destes materiais apresentarem vantagens em relação ao custo do produto final e processamento comparado aos inorgânicos, a comunidade científica ainda se depara com problemas relacionados às propriedades intrínsecas, como por exemplo, baixa estabilidade à oxidação e energias de bandgap elevadas; deste modo, ainda é viável encontrar novos materiais orgânicos que superem tais problemas. Este trabalho teve como finalidade a busca de novos materiais orgânicos, mais especificamente polímeros conjugados, para aplicações em camada ativa de células solares utilizando métodos teóricos de modelagem de materiais em níveis de teoria semi-empírico e DFT. A primeira parte deste trabalho focou no estudo de novos copolímeros de comonômeros similares e novos homopolímeros derivados de P3HT. Aqui foi possível verificar que, através de substituições químicas realizadas no P3HT, pode-se encontrar novos polímeros com valores mais apropriados para as energias dos orbitais moleculares de fronteira e maiores mobilidades de carga, propriedades que estão intimamente relacionadas à eficiência e à estabilidade que a célula solar pode atingir. Um dos novos polímeros que apresentou potencial para aplicação em camadas ativas de células solares foi o P3HT fluorado, que posteriormente foi confirmado através de trabalhos experimentais. Para os copolímeros de comonômeros similares, verificou-se que as propriedades eletrônicas e ópticas destes materiais apresentam uma dependência linear com as mesmas propriedades e a proporção de monômeros dos homopolímeros que foram utilizados como comonômeros na construção do copolímero; o destaque maior é que através desta relação linear pode-se projetar novos copolímeros de comonômeros similares antes de uma possível síntese. A segunda parte deste trabalho avaliou as metodologias de modelagem de materiais orgânicos baseadas na (TD)DFT que seriam viáveis para a correta descrição teórica das propriedades ópticas. Verificou-se que uma confiável previsão da energia de transição vertical pode ser obtida por meio do funcional M06HF, adicionando um fator de correção de -0,75 eV nas energias obtidas com ele. Em relação à absorção óptica do estado excitado, o funcional B3LYP não prevê corretamente as energias de transição entre os estados excitados, ao passo que resultados mais confiáveis podem ser obtidos com os funcionais BHLYP e CAM-B3LYP comparado aos dados experimentais disponíveis.
Organic materials have been used in various types of electronic and optoelectronic devices, with prominent application in solar cells. Solar cells in which the active layers are composed of organic materials already reach efficiencies of around 12% in the conversion of solar energy into electricity. Although these materials have advantages in relation to the final cost and processing compared to the inorganics, the scientific community still faces problems related to intrinsic properties, such as low oxidation stability and high bandgap energy; thus, it is desirable to find new organic materials that overcome these problems. This work aimed at the search for new organic materials – specifically conjugated polymers – for applications in active layers of solar cells employing theoretical methods of material modeling of semi-empirical and DFT theory levels. The first part of this work focuses on the study of new copolymers of similar comonomers and new P3HT derivatives. Here it was observed that, through chemical substitutions made in P3HT, one may find new polymers with appropriate values for the energies of the frontier molecular orbitals and increased charge mobility, properties that are closely related to the efficiency and stability that the solar cell can achieve. One of the novel polymers found which have potential for use in active layers was the fluorinated P3HT, later confirmed by experimental works. For copolymers of similar comonomers, it was found that the electronic and optical properties of these materials exhibit a linear dependence with the same properties and the monomers proportion of the homopolymers that were used as comonomers in the construction of the copolymer; most important is that through this linear relationship one may design new copolymers of similar comonomers before a possible synthesis. The second part of this study evaluated the methodologies for modeling of organic materials based on (TD)DFT that would be viable for a proper theoretical description of the optical properties. It was found that a reliable prediction of the vertical transition energy from the ground state of conjugated polymers can be obtained with the M06HF functional adding a correction factor of -0.75 eV in its obtained energies. Regarding the excited state optical absorption, the B3LYP functional do not correctly predicts the transition energy between the excited states, while more reliable results can be obtained with BHLYP and CAM-B3LYP functionals compared to the available experimental data.
Organic materials have been used in various types of electronic and optoelectronic devices, with prominent application in solar cells. Solar cells in which the active layers are composed of organic materials already reach efficiencies of around 12% in the conversion of solar energy into electricity. Although these materials have advantages in relation to the final cost and processing compared to the inorganics, the scientific community still faces problems related to intrinsic properties, such as low oxidation stability and high bandgap energy; thus, it is desirable to find new organic materials that overcome these problems. This work aimed at the search for new organic materials – specifically conjugated polymers – for applications in active layers of solar cells employing theoretical methods of material modeling of semi-empirical and DFT theory levels. The first part of this work focuses on the study of new copolymers of similar comonomers and new P3HT derivatives. Here it was observed that, through chemical substitutions made in P3HT, one may find new polymers with appropriate values for the energies of the frontier molecular orbitals and increased charge mobility, properties that are closely related to the efficiency and stability that the solar cell can achieve. One of the novel polymers found which have potential for use in active layers was the fluorinated P3HT, later confirmed by experimental works. For copolymers of similar comonomers, it was found that the electronic and optical properties of these materials exhibit a linear dependence with the same properties and the monomers proportion of the homopolymers that were used as comonomers in the construction of the copolymer; most important is that through this linear relationship one may design new copolymers of similar comonomers before a possible synthesis. The second part of this study evaluated the methodologies for modeling of organic materials based on (TD)DFT that would be viable for a proper theoretical description of the optical properties. It was found that a reliable prediction of the vertical transition energy from the ground state of conjugated polymers can be obtained with the M06HF functional adding a correction factor of -0.75 eV in its obtained energies. Regarding the excited state optical absorption, the B3LYP functional do not correctly predicts the transition energy between the excited states, while more reliable results can be obtained with BHLYP and CAM-B3LYP functionals compared to the available experimental data.
Descrição
Palavras-chave
Células solares orgânicas, Modificações químicas, Modelagem de materiais, Propriedades de transporte, Polímeros condutores, Copolímeros, Organic solar cells, Chemical modifications, Materials modeling, Transport properties, Conducting polymers, Copolymers