Se Inter-Diffusion Limits Absorber Layer Grain Growth in CdSe - CdTe</mml:…

Diffusion of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mi>Se</a:mi></a:math> from the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:mrow><d:mi>Cd</d:mi><d:mi>Se</d:mi></d:mrow></d:math> window layer into the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><g:mrow><g:mi>Cd</g:mi><g:mi>Te</g:mi></g:mrow></g:math> absorber improves the short circuit current density by narrowing the band gap and increasing the carrier lifetime. Thicker <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><j:mrow><j:mi>Cd</j:mi><j:mi>Se</j:mi></j:mrow></j:math> layers, however, show a dramatic loss in photocurrent collection due to <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><m:mi>Se</m:mi></m:math> over-alloying. Electron microscopy investigations show that this decrease in performance is due to the formation of small grains (∼783 nm average diameter), which exhibit grain boundary porosity in the <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><p:mi>Se</p:mi></p:math> inter-diffusion region. The larger grain boundary area and void free surfaces give rise to higher levels of nonradiative recombination, and therefore, a lower photocurrent. It is proposed that the small grain size is due to a drag force exerted by segregated <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><s:mi>Se</s:mi></s:math> solute atoms on a moving grain boundary, while faster <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><v:mi>Se</v:mi></v:math> diffusion along the grain boundaries results in vacancy build up and porosity due to the Kirkendall effect. The results indicate that the device processing conditions must be carefully controlled such that the negative effects of <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><y:mi>Se</y:mi></y:math> alloying (i.e., smaller grains, Kirkendall voids) do not undermine its benefits.