Investigation of Energy Harvesting Materials for Organic and Hybrid Solar Cells

Abstract

Polymer solar cells (PSCs) have attracted great attentions because of their many advantages including flexibility, light weight, and low cost for fabrication. Moreover, hybrid solar cells (HySCs), based on organic and inorganic semiconductors, are also a promising way to enhance the efficiency of solar cells because they make a better use of the solar spectrum and are straightforward to fabricate. Among various strategies, design and choosing more efficient active layers are promising and efficient ways to maximize performance of solar cells. In this study, the active layers of PSCs and HySCs are presented with various strategy based on the active layer. Carefully designed p-type conjugated polymers and additives are studied in each polymer solar cells, and amorphous and crystalline Si are applied in HySCs. On the basis of theoretical considerations of the intramolecular charge transfer effect, a series of donor -acceptor conjugated polymers based on bis-benzothiadiazole (BBT) are designed. An electron-rich 2,7-carbazole (CZ) and electron-deficient BBT units poly[N-(2-decyltetradecyl)-2,7-carbazole-co-7,7’-{4,4’-bis-(2,1,3- benzothiadiazole)}] (PCZ-BBT), a PPV-type copolymer poly[N-(2-decyltetradecyl)- 2,7-carbazolevinylene-co-7,7’- {4,4’-bis-(2,1,3-benzothiadiazolevinylene)}] (PCZV-BBTV), and a tercopolymer based on carbazole, thiophene, and BBT poly[N-(2-decyltetradecyl)- 2,7-(di-2-thienyl)carbazole-co-7,7’-{4,4’- bis-(2,1,3-benzothiadiazole)}] (PDTCZBBT) have been designed to understand the influence of BBT acceptor structure on the PSC characteristics of the resulting materials. All the polymers provide a photovoltaic response when blended with a fullerene derivative as an electron acceptor. Moreover, the introduction of fluorine (F) atoms onto BBT-conjugated polymer backbone is an effective way to enhance the performance of BHJ solar cells. As an attempt to further understand the impact of F, two fluorinated analogues of PCDTBT, namely, PCDTFBT and PCDT2FBT, through inclusion of either one or two F atoms into the benzothiadiazole (BT) unit of the polymer backbone and the characterization their performance in solar cells. Considerable improvements in efficiency and thermal stability have been showed in the BHJ system by adding a diblockcopolymer P3HT-b-P (St89BAz11)-C60 as a compatibilizer additive. Small amounts of the additive alter the interfacial morphology between the P3HT and PCBM components, resulting in a noticeable difference in phase segregation of the BHJ films. The best performance is observed in the cell with10% additive, which exhibits substantially improved power conversion efficiency and thermal stability of the device. Another additive on the enhanced performance of BHJ solar cells composed of an iridium complex with pendant sodium cations (pqIrpicNa) as an energy donor, P3HT as an energy acceptor, polyethylene oxide (PEO) as an ion channel, and PCBM as an electron acceptor. With the iridium complex and PEO as additives, 20% increase in the efficiency of the PSC is observed. The enhancement of PSC stems from the morphology with the iridium complex and the enhanced mobility of the sodium cations toward the metal electrodes through the ion channel of PEO under sunlight, which results in an increased charge collection at the electrodes. The remarkable electronic properties of fullerenes have attracted great attention with different backgrounds to develop a wide variety of chemically modified fullerene derivatives. Fullerene derivatives are used as n-type organic materials for the solar cells. A fullerene-rich dendron and linear polymer and utilized polymer solar cells with P3HT donor conjugated polymer. Bi-functionalized materials which can act as p-type and n-type function are investigated as an ambipolar low band gap conjugated copolymer in BHJ solar cells. Ambipolar copolymer, poly[3,6-dithien-2-yl-2,5-di(2-decyltetradecanyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-50,50 ’-diyl-alt-benzo-2,1,3-thiadiazol-4,7-diyl] (PDTDPP-alt-BTZ), successfully works as the bifunctional material acting as both donor or acceptor. However, solar cell performances were limited because of fast recombination nature and broken charge balance of hole and electron, A hybrid solar cell comprised of hydrogenated amorphous silicon (a-Si:H), PC71BM, and poly-3,4-ethylenedioxythiophene poly styrenesulfonate (PEDOT:PSS) is studied for the properties. The devices were studied as a function of the thickness of the a-Si:H layer. It is observed that the open circuit voltage and the short circuit current density of the device depended on the thickness of the a-Si:H layer. 274 nm-thick layer of a-Si:H for the HySCs shows the best performance. High-efficiency hybrid solar cell of its type comprising p-type crystalline Si with an organic n-type C60 layer are studied. Fabrication parameters were thoroughly investigated and critical factors for the efficient operation of this type of device were found to include C60 thickness, doping of the C60 layer using tetrabutyl ammonium iodide, aging passivation and the incorporation of anti-reflection coatings. Using these versatile materials and method may offer the possibility to commercialize these photovoltaic cells.