Study of organic-inorganic hybrid solar cells

Abstract

Conventional solar cells based on bulk silicon have been considered as a promising candidate to deal with the world energy crisis. However, the fabrication of such kind of solar cells needs high temperature, high vacuum and high purity materials. These conditions inevitably result in expensive manufacturing cost and limited production area. Therefore, it is desirable to develop novel photovoltaic devices with easy fabrication, low cost and availability for large area production to replace the commercial silicon-based solar cells. Following the invention of organic semiconductors and the fast development of thin film technology, organic solar cells have been considered to be the third-generation photovoltaic devices. Organic materials have several important advantages, including low cost, solution processability and flexibility, making organic solar cells a very attractive area to be investigated. In this project, organic solar cells composed of poly(3-hexylthiophene) (P3HT) and titanium oxide (TiO₂) have been studied. P3HT is a p-type semiconducting polymer that has high light absorption coefficient in the visible region and is soluble in common organic solvents. Therefore P3HT can be used for producing thin film devices by simple printing or coating techniques and a few hundred nanometer thick P3HT film is enough to absorb sufficient light to generate electricity. TiO₂ is a n-type semiconductor which exhibits higher electron mobility than that of organic semiconductors. More importantly, TiO₂ can form a staggered heterojunction with P3HT and show photovoltaic effect. In our experiments, a TiO₂ thin film has been coated on an indium tin oxide (ITO) electrode by spin-coating and subsequently annealed for crystallization. In order to enhance the performance of the solar cell, aligned TiO₂ nanofibers have been fabricated on the TiO₂ thin film by electrospinning to increase the interfacial area between TiO₂ and P3HT. Afterward, P3HT film of ~200nm thick has been spin-coated on the TiO₂ nanofibrous film and annealed in a nitrogen-filled glovebox. Gold electrode is then evaporated on top of the P3HT. Consequently, J-V curve of the device has been measured under a solar simulator with AM1.5 filter to characterize the photovoltaic parameters of such hybrid solar cell. Considering the absorption range of TiO₂ and P3HT, the cell only utilizes a narrow part of the solar spectrum. Hence, we aim to improve the absorption by adding an inorganic semiconductor interlayer. Surface modification of cadmium sulphide (CdS) on electrospun TiO₂ nanofibers using electrochemical deposition has been employed to improve the performance of the solar cell. Because the band gap energy of CdS is between those of TiO₂ and P3HT, it provides a different absorption peak in the solar spectrum. The effect of such modification has been analysed by measuring the external quantum efficiency and J-V characteristics of the device. In addition, different microscopy techniques, such as field emission scanning electron microscopy and transmission electron microscopy, have been employed to investigate the morphology of nanofibers before and after the deposition. In an ordinary organic solar cell, a reflective electrode is commonly adopted to ensure a sufficient absorption of light in the active layer. But it is also feasible to use a transparent electrode in the organic solar cell to collect light from both sides. Here, we use graphene as the top electrode of an organic solar cell since graphene is a fascinating material bearing flexible, metallic and transparent properties. After the transfer of graphene on the active layer, a hydrophilic thin layer of poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) is added to serve as the electron blocking layer. Moreover, due to the doping effect of PEDOT:PSS on graphene, the series resistance of the device is dramatically reduced. In the end, we obtain a semi-transparent device that can absorb light from both sides. The performance of the device under two single-sided and double-sided illuminations has been characterized. In summary, our results indicate that the efficiency of a hybrid solar cell can be optimized by various strategies, including interfacial modification, control of the nanostructure of heterojunction and broadening the light absorption region of the active layer. In addition, the fabrication of semi-transparent solar cells which can collect light from both sides is also a meaningful way to enhance the energy harvesting of the devices in real applications.