Page - 147 - in Photovoltaic Materials and Electronic Devices

1. Introduction Solar photovoltaic (PV) based electricity production is one of the significant ecofriendly methods to generate sustainable energy needed to mitigate the looming globalenergycrisis [1]. Despite technical improvements [2]andscaling[3],which have resulted in a significant reduction in crystalline silicon (c-Si) PV module costs, for continued PV industry growth [4,5], PV costs must continue to decline to reach a levelized cost of electricity [6] low enough to dominate the electricity market. One approach to reduced PV costs further is to transition to thin film PV technology [7]. Hydrogenated amorphous silicon (a-Si:H) based PV [8] have shown great potential for large scale [9] sustainable commercial production due to lower material costs and use of well-established fabrication techniques [10,11]. However, there is need to improve the efficiency of a-Si:H PV devices if they are to become the next dominant technology for solar cells commercialization. One method to improve a-Si:H PV performance is with optical enhancement [12]. Recent developments in plasmonic theory promise new light management methods for thin-film a-Si:H based solar cells [13–23]. However, previous work has shown these plasmonic approaches require the development of ultra-thin, low-loss and low-resistivity transparent conducting oxides (TCOs) [24]. Tin doped indium oxide (ITO), zinc oxide (ZnO) and tin oxide (SnO2) are the three most important TCOs and are already widely used in the commercial thin film solar cells [25]. In addition,aluminum-dopezinc oxide (AZO)andfluorine-dopedtinoxide (FTO) are among the other most dominant TCOs in various technological fields particularly the optoelectronic devices industry where TCOs have proved indispensable for applications such as photo electrochemical devices, light emitting diodes, liquid crystal displays and gas sensors [26,27]. ITOs can be prepared by direct current (DC) and radio frequency (RF) magnetron sputtering, electron beam evaporation, thermalvaporevaporation, spraypyrolysis, chemical solutiondeposition,andsol gelmethods[28–34]. RFmagnetronsputteringcanbeusedtocontrol theelectrical andopticalpropertiesof the ITOthinfilmsandisheavilyusedin industry [35]. Recent work by Vora et al. has emphasized the need for ultra-thin ITO top electrodes with low resistivity and high transmittances in the visible range of the electromagnetic spectrum as a prerequisite for the commercial realization of plasmonic-enhanced a-Si:H solar cells [36]. However, research by Gwamuri et al.has demonstratedthat fabricatingultra-thinITOfilms(sub-50nm)usingconversional methodspresentedanumberofchallengessincethereisatrade-offbetweenelectrical and optical properties of the films [37]. It was evidenced from their results that electricalpropertiesofRFsputterdepositedsub-50nmITOfilmsdegradeddrastically as their thickness is reduced, while theoptical properties of the samefilms wereseen to improvegreatly [37]. Tosolve thisproblem,anovelmethodinvolvingchemical shavingof thicker (greater than80nm)RFsputterdepositedfilmswasproposedand 147