The role of secondary phase precipitation on grain boundary electrical activity in Cu2ZnSnS4 (CZTS) photovoltaic absorber layer material

Abstract

Cathodoluminescence is used to measure the recombination velocity of the heterointerfaces between Cu2ZnSnS4 (CZTS) and CuxSnySz, SnS secondary phases precipitated along the grain boundaries as well as ZnS precipitated within the CZTS grain interiors. The CZTS/CuxSnySz and CZTS/ZnS heterointerfaces had recombination velocities smaller than the bulk carrier diffusion velocity while the opposite is true for the CZTS/SnS heterointerface. Secondary phases having crystal structures compatible with CZTS (e.g., ZnS, Cu2SnS3) are likely to form heterointerfaces with small misfit strain and hence low interfacial recombination velocity. The precipitation of such secondary phases along grain boundaries in CZTS provides a novel mechanism for grain boundary passivation. However, it is not known if grain boundary passivating secondary phases would necessarily increase the overall photovoltaic device efficiency since other factors, such as the band gap of the secondary phase compared to the Shockley-Queisser ideal value and the nature of the heterointerface between CZTS (i.e., type-I vs type-II), also affect device operation and must therefore be taken into consideration.

Nanometre-scale optical property fluctuations in Cu2ZnSnS4 revealed by low temperature cathodoluminescence

“…Further evidence is obtained via room temperature panchromatic CL imaging where the precipitates are found to be highly luminescent (Fig. 3f), consistent with previous observations for ZnS in CZTS [23]. Note that the coverage of ZnS precipitates appears to be higher in the CL image compared to the backscattered image and EDX chemical maps.…”

Luminescence of Cu2ZnSnS4 polycrystals described by the fluctuating potential model

“…Analysis of secondary phases and their potential impact on PV devices is reported elsewhere. 25 It should be noted that a full analysis of larger bandgap secondary phases using PL will require a shorter laser wavelength than the 458 nm Ar þ laser line used in these measurements. The PL intensity of the main CZTS feature is obtained through fitting a series of Gaussian peaks to the full PL spectra.…”

“…In general, it has been considered that the presence of secondary phases, particularly when their fundamental energy gap is larger than CZTS, will be detrimental to device performance. 14 Recent work 25 has shown that this may not be the case. The presence of secondary phases at grain boundaries in polycrystalline CZTS alters the grain boundary recombination velocity potentially increasing PV conversion efficiency.…”

Opposing effects of stacking faults and antisite domain boundaries on the conduction band edge in kesterite quaternary semiconductors

“…Since current thin-film technologies mostly rely on polycrystalline materials, physical properties of extended defects, especially grain boundaries (GBs) have been investigated to understand their effects on the device efficiency [10][11][12][13][14][15][16]. Other extended defects like stacking faults (SFs) and antisite domain boundaries (ADBs) have been less documented as compared to the GBs, but since SFs in CdTe act as electron barriers and reduce the efficiency [17][18][19][20], SFs in CZTS should be investigated.…”