- type ▁sem icon duct or ▁it ▁forms ▁the ▁core ▁component ▁of ▁a ▁phot ov olta ic ▁( sol ar ) ▁cell ▁and ▁a ▁C d S / C u 2 S ▁solar ▁cell ▁was ▁one ▁of ▁the ▁first ▁efficient ▁cells ▁to ▁be ▁reported ▁( 1 9 5 4 ) ▁when ▁do ped ▁with ▁for ▁example ▁Cu + ▁(" activ ator ") ▁and ▁Al 3 + ▁(" co activ ator ") ▁C d S ▁lum ines ces ▁under ▁electron ▁beam ▁exc itation ▁( c ath od ol um ines c ence ) ▁and ▁is ▁used ▁as ▁ph osph or ▁both ▁polym orph s ▁are ▁pie zo elect ric ▁and ▁the ▁hex ag onal ▁is ▁also ▁py ro elect ric ▁elect rol um ines c ence ▁C d S ▁cry stal ▁can ▁act ▁as ▁a ▁solid ▁state ▁las er ▁In ▁thin - film ▁form , ▁C d S ▁can ▁be ▁combined ▁with ▁other ▁layers ▁for ▁use ▁in ▁certain ▁types ▁of ▁solar ▁cells . ▁C d S ▁was ▁also ▁one ▁of ▁the ▁first ▁sem icon duct or ▁materials ▁to ▁be ▁used ▁for ▁thin - film ▁trans ist ors ▁( T FT s ). ▁However ▁interest ▁in ▁comp ound ▁sem icon duct ors ▁for ▁T FT s ▁largely ▁w an ed ▁after ▁the ▁emer gence ▁of ▁am orph ous ▁sil icon ▁technology ▁in ▁the ▁late ▁ 1 9 7 0 s . ▁Th in ▁films ▁of ▁C d S ▁can ▁be ▁pie zo elect ric ▁and ▁have ▁been ▁used ▁as ▁trans du cers ▁which ▁can ▁operate