Development of Highly Conducting, Crystalline and Amorphous Transparent Conductors for Flexible Photovoltaics

Description

Currently, transparent conducting oxides (TCOs) are widely used as transparentconductors for thin film solar cells. Because of the potential of roll-to-roll, large-areaprocessing on flexible low-cost substrates, flexible solar cells are a very attractivetechnology for the low-cost photovoltaic market. For flexible solar cells, an additionalrequirement for the transparent electrode is that the deposition must be carried out attemperatures lower than the deformation temperature of flexible polymer substrates.In this proposal we plan develop highly conducting and transparent CdO-basedtransparent conductors for flexible solar cell applications. This work built upon our priorefforts on the development of CdO-based TCOs for full spectrum solar cells. We havedemonstrated that because of its high carrier mobilities, CdO can achieve a highconductivity of >30000 S/cm with transparency widow of 400-1500 nm. This makesthem good candidates for full spectrum photovoltaic applications. However, our previousefforts focused on CdO-based TCOs deposited at elevated temperatures of 200-500C onglass substrates. Low temperature growth of most metal oxides resulted in amorphousor nano-crystalline materials, giving rise to degraded electrical and optical properties.Nevertheless, CdO is known to have a surface electron accumulation layer, suggestingthat grain boundaries in CdO do not form barriers for electron transport. Hence it isexpected that CdO thin films grown at low temperature can still achieve high electronconcentration and mobility.Recently, a new class of low temperature grown amorphous oxides, the amorphous oxidesemiconductors (AOSs) has been developed. They have the many unique propertiesincluding large electron mobilities ~10-20 cm^2/Vs and excellent surface flatness thatcan be exploited as TCOs for flexible solar cells. We note that since CdO and Ga2O3crystallize in distinctly different crystal structure and they have very different band gapenergies of 2.2 and 4.8 eV, respectively, co-depositing CdO and Ga2O3 at lowtemperature is expected to form amorphous alloys with a wide band gap span. Given themany desirable properties of CdO, Cd-Ga-O AOSs is also expected to be a good TCOmaterial for flexible PV applications.In this proposal, the low temperature synthesis and properties of CdO as well as alloys ofCdO-Ga2O3 over the entire composition range will be explored. In particular, we willoptimize both CdO and Cd-Ga-O AOS films as transparent conductors on flexiblesubstrates and demonstrate the improved performance of flexible solar cells fabricatedon these new transparent conductors.?