A Study on Nanostructures for Enhanced Light Absorption in Solar Cells

DoctorThin film solar cells have attracted much attention as renewable energy source due to their advantages of low-cost, large-area, and mechanically flexible as well as the possibility of using roll-to-roll fabrication processes. The high photo-conversion efficiency (PCE) was achieved up to 12% for hydrogenated amorphous silicon (a-Si:H) solar cells and 10% for organic photovoltaics (OPVs). However, further improvement in efficiency is required for commercial applications. To acquire highly efficient thin-film solar cells, both the light trapping methods and the charge transport is very important. A active layers in thin-film solar cells absorb only 40% of the incident photons because of small absorption coefficient (e.g. PTB7, α ~ 6 x 103 cm-1 at λ = 500 nm). Therefore, several methodology have been demonstrated nanostructures to improve the absorption of photons in top-illuminated PSCs. Nanostructures can act as scattering center for randomizing the incident light. The randomly scattered light shows enhanced light absorption in active layer. Plasmonic metal nanostructures leads to an increased optical path length inside the active layer and thereby enhanced the light absorption. But, there were some drawbacks in implementing such nano-patterns.[25-27] It is necessary to employ the complicated processing steps such as high-temperature annealing, inductively coupled plasma etching, and lithography. These high cost fabrication methods cannot be used in large-area thin-film solar cells. Here, we studied nanophotonics and demonstrated nanostructures to enhance the efficiency of a-Si:H solar cells and OPVs. In the first section, optical microcavity resonance were used to optimize the inverted top-illuminated polymer solar cells. In the second section, 3-dimensional nanostructure of Ag nanoparticles and ITO nano-branches were conducted for enhanced light trapping in OPVs. The effect of plasmonic light scattering and localized surface plasmon is discussed. In the last section of nanophotonics, hexagonal reflector is fabricated by nano-imprint lithography. This flexible nanostructure is successfully demonstrated in flexible a-Si:H solar cells, thereby resulting in enhanced photocurrent. In OPVs, an critical factor in determining the device performance is the charge transport from the photoactive layer to the electrodes. Excitons are generated in the photoactive layers under illumination and some excitons would dissociate into electrons and holes at the donor/acceptor interfaces. These separated charge carriers are collected at the anode and cathode electrodes, contributing to the photocurrent. At the same time, some excitons may directly diffuse to the cathode/anode and quench non-radiative, which significantly lowers the quantum efficiency and photocurrent. To obtain good interfacial properties between the active layer and the anode, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is commonly used as a hole extraction layer (HEL) to enhance the hole extraction from active layer. However, PEDOT:PSS deposited on indium tin oxide (ITO) anodes is highly acidic (pH 1.3-2.1), thereby resulting in the dissociation of ITO into In and Sn atoms. The hygroscopic nature of PEDOT:PSS also degrades lifetime. In this work, we developed hole transport layer (HTL) with the aim of understanding the factors influencing the device performance of OPVs. In the first series of experiment, the effects of surface energy and work function on device performance are explored by using UV-ozone treatment to get rid of PEDOT:PSS. In the second section, the solution-processed MoO3 were discussed based on surface energy, electrical conductivity and work function. For the last section, ultrafast method of laser-assisted synthesis of HyMoO3-x were demonstrated to get electrically conductive HTLs in OPVs