Insights on the limiting factors of Cu2ZnGeSe4 based solar cells

Ge-alloyed kesterite thin-film solar cells: previous investigations and current status – a comprehensive review

“…53 They demonstrated that (1) Cu/(Zn + Ge) = 0.65–0.7 and (2) Zn/Ge = 1.05–1.15 should guarantee CZGSe to be mostly free of secondary phases including those related to Ge, as reported elsewhere. 47–52 This seems to be an argument in favour of Ge alloying since for similar Cu-poor and Zn-rich stoichiometry in standard CZTSe, traces of volatile Sn–Se secondary phases can be observed, leading to Sn-loss and defect formation. 2,12…”

“…In the continuation of these studies, equivalent experiments were carried out on the same precursor stack, though sputtered and not evaporated, in collaboration with multiple teams of researchers. [51][52][53] In order to improve the absorber crystallinity and mitigate secondary phase formation, the implementation of an optimized selenization recipe with an appropriate surface treatment was investigated. 51 The best recipe consisting in a 330 °C/480 °C two-step annealing in Se-GeSe 2 led to both high crystalline quality and low Urbach energy, 51,52 with only slight traces of ZnSe secondary phases in one study.…”

“…[51][52][53] In order to improve the absorber crystallinity and mitigate secondary phase formation, the implementation of an optimized selenization recipe with an appropriate surface treatment was investigated. 51 The best recipe consisting in a 330 °C/480 °C two-step annealing in Se-GeSe 2 led to both high crystalline quality and low Urbach energy, 51,52 with only slight traces of ZnSe secondary phases in one study. 51 In these two references, KCN etching seems efficient to remove detrimental phases from the top surface with possible non-toxic alternatives based on (NH 4 ) 2 S combined with KMnO 4 , but not from the bottom surface where a thin layer with smaller grains is observed.…”

“…53 They demonstrated that (1) Cu/(Zn + Ge) = 0.65-0.7 and (2) Zn/ Ge = 1.05-1.15 should guarantee CZGSe to be mostly free of secondary phases including those related to Ge, as reported elsewhere. [47][48][49][50][51][52] This seems to be an argument in favour of Ge alloying since for similar Cu-poor and Zn-rich stoichiometry in standard CZTSe, traces of volatile Sn-Se secondary phases can be observed, leading to Sn-loss and defect formation. 2,12 Investigations from independent groups attempted to adjust the precursor stack in this two-step process, namely with solidphase Se integration within the precursor stack, 54 or directly evaporating the CZGSe material with the targeted composition.…”

Analysis of Loss Mechanisms in CZGSe Thin-Film Kesterite Solar Cells: A Statistical Distribution for Defects and Traps

“…To reduce deep level defects in the bulk absorber layer, Ge +4 is implemented in place of Sn +4 as it reduces high recombination rate [16]. Normally, the bandgap of the kesterite absorber layer can be adjusted by changing the S/Se concentration or by placing the tin (Sn + ) molecules with silicon (Si + ) or germanium (Ge + ) [17][18][19][20]. It is investigated that Cu 2 ZnGeSe 4 (CZGSe) seems to be quaternary semi-conductor material groupings I 2 -II-IV-VI 4 which has been experimentally and theoretically demonstrated to be p-type semiconductors with a direct bandgap of 1.4eV and extremely high rate of photon energy absorption coefficient similar to CIGS and CZTS materials [16,17,21].…”