Cu2Te 배면 전극을 이용한 p-type CdTe 태양전지의 ohmic contact 형성 및 CdTe 태양전지의 광전압 특성

Abstract

CdTe solar cells are leading candidate for cost-effective photovoltaics due to near-optimum band gap(1.4eV), high absorption coefficient, and manufacturability However CdTe films are highly resistive with a dark resistivity of larger than $10^{15} \Omega cm$. So one of the major problems to obtaining higher efficiency CdTe solar cells is the high resistivity and unstable contact to p-type CdTe. To achieve a stable efficiency for the CdS/CdTe solar cells, the formation of ohmic contact to CdTe films is important. In this work, CdS films were prepared by a chemical bath deposition(CBD) method on ITO/glass substrate. CdTe films were deposited on CdS/ITO/glass substrate by close spaced sublimation(CSS) with screen printed CdTe layer as a source, and these films were annealed in $CdCl_2$. Finally, carbon contact or evaporated $Cu_2$Te/Au contact were formed on CdTe/CdS/ITO/glass and CdTe films were deposited at the source temperature of $660^\circ C$ and at the substrate temperature of $590^\circ C$. Efficiency and fill factor of CdTe solar cells fabricated with carbon contact were 9.07 % and 0.667. But the carbon contact has some problems, such as thermal degradation and kink effect, due to small work function. So it is necessary to form any other contact to CdTe films. In this work. Au contact were formed at first. the Voc of CdTe solar cells fabricated with Au contact was lower than that of the carbon contact. Especially, it is lowered when the heat treatment temperature is $320^\circ C$ or higher and the thickness of CdTe films is $5 \mu m$ or below. It is due to Au diffusion. Excessive Au diffusion may reduce carrier life time so Voc is lowered. But other contact properties, contact resistivity and kink effect, are better. To obtain high Voc we must deposited diffusion barrier. The $Cu_2$Te with high work function metal is looked for controlling Au diffusion and for doping Cu into CdTe from $Cu_2$Te. So $Cu_2$Te/Au contact was studied in addition to Au contact. CdTe solar cells fabricated with as-deposited $Cu_2$Te/Au contact showed only high Voc and kink effect. When $Cu_2$Te films were annealed photovoltaic properties were improved. The efficiency of CdTe solar cells fabricated with annealed $Cu_2$Te/Au contact was 9.3%, the Voc and the fill factor were 0.79 and 0.612. And serese resistances were reduced due to reduction of contact resistance. $J_o$ is respectively small $9.56 \times 10^{-11}$ i.e. leakage current is very small in junction. It seems that $Cu_2$Te controll the Au diffusion. And kink effect is not appeared relative to carbon contact. It is expected that the $Cu_2$Te/Au can be applied to CdTe back contact materia with low and stable contact resistances.