Microscopic Analysis of Interdiffusion and Void Formation in CdTe_(1–x)Se_xand CdTe Layers

Abstract

The use of CdSe layers has recently emerged as a route to improving CdTe photovoltaics through the formation of a CdTe (1– x ) Se x (CST) phase. However, the extent of the Se diffusion and the influence it has on the CdTe grain structure has not been widely investigated. In this study, we used transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD) to investigate the impact of growing CdTe layers on three different window layer structures CdS, CdSe, and CdS/CdSe. We demonstrate that extensive intermixing occurs between CdS, CdSe, and CdTe layers resulting in large voids forming at the front interface, which will degrade device performance. The use of CdS/CdSe bilayer structures leads to the formation of a parasitic CdS (1– x ) Se x phase. Following removal of CdS from the cell structure, effective CdTe and CdSe intermixing was achieved. However, the use of sputtered CdSe had limited success in producing Se grading in CST.

Interrelation of the CdTe Grain Size, Postgrowth Processing, and Window Layer Selection on Solar Cell Performance

“…The variation of morphology with growth pressure of CdTe films grown on CdSe is broadly similar to those grown on CdS (Figure ), as might be expected given the similar crystal structure of both substrates. The grain size of films grown on CdSe substrates (Figure h) is smaller than for CdS substrates (Figure h), which is consistent with comparisons from EBSD measurements, and shows less variation with growth pressure. The average grain size for CdTe films grown on SnO 2 /CdSe substrates increases with growth pressure up to 200 Torr, which is not observed as clearly for growth directly onto SnO 2 .…”

20%-efficient polycrystalline Cd(Se,Te) thin-film solar cells with compositional gradient near the front junction

“…However, previous works have shown that this strategy does not work, because simply depositing these layers only leads to the formation of an individual wurtzite Cd(S,Se) layer (Supplementary Fig. 1b) with a hetero interface between with the Cd(Se,Te), as demonstrated by several groups 6,34,[38][39][40] . Firstprinciples calculations have shown that the mixing enthalpies of CdS 0.5 Se 0.5 , CdSe 0.5 Te 0.5 , CdS 0.5 Te 0.5 are 3, 8, 25 meV, respectively, suggesting the high propensity to form Cd(S,Se) 4 .…”

Understanding the Role of CdTe in Polycrystalline CdSexTe1–x/CdTe‐Graded Bilayer Photovoltaic Devices

“…Baines et al have made similar observations, where they attributed the formation of voids in the CdSe x Te 1-x / CdTe structure due to the intermixing of Se and Te atoms. [25] The presence of such voids and relatively small CdSeTe grains is expected to limit the open-circuit voltage and has been reported previously for several different thin-film solar cells. [24][25][26][27][28][29][30] The occurrence of voids in the front CdSeTe film could contribute in part to a lower V OC of 830 mV observed in the bilayer CdSe x Te 1-x /CdTe device in comparison with 845 mV observed for CdTe device.…”

“…[ 25 ] The presence of such voids and relatively small CdSeTe grains is expected to limit the open‐circuit voltage and has been reported previously for several different thin‐film solar cells. [ 24–30 ] The occurrence of voids in the front CdSeTe film could contribute in part to a lower V OC of 830 mV observed in the bilayer CdSe x Te 1– x /CdTe device in comparison with 845 mV observed for CdTe device.…”