Investigation of deep levels in CdTe/CdS solar cells

Abstract

Polycrystalline CdTe/CdS solar cells were studied using double boxcar and correlation deep level transient spectroscopy (DLTS). The limitations of each technique as dictated by the polycrystalline nature of the CdTelCdS heterostructure were taken into consideration. Dark and illuminated J-V and dark C-V measurements were performed in order to monitor any changes in solar cell performance during the deep level studies. In order to avoid issues associated with the metastability of certain defects, the CdTe cells were kept in the dark during the DLTS measurements. Several traps were observed in the temperature range from 90K to 350K. Hole traps with activation energies of 0.32, 0.45, and 0.73 eV were found in most of the samples studied to date. An electron trap with E, m E, -0.14 eV was observed only in samples with low open-circuit voltages and fill factors and is believed to be a performance limiting defect. The results have been compared with published data in order to determine the chemical origin of the observed defects.

Machine-learned impurity level prediction for semiconductors: the example of Cd-based chalcogenides

“…SI-13 shows that there is a very high correlation between the two; the HSE values lie between the y=x and the y=x+1 lines. We also collected some experimentally measured defect levels in CdTe from the literature [50][51][52][53][54][55] and plotted a comparison between experiments, PBE ε(q 1 /q 2 ), and HSE ε(q 1 /q 2 ), for various defects in Fig. 5(d).…”

“…Charge transition levels (from +2/+1 to -1/-2) calculated at the HSE06 level of theory for selected impurity atoms in different sites in (a) CdTe, (b) CdSe and (c) CdS. In (d), we present a comparison between experimentally measured defect levels[50][51][52][53][54][55] and the corresponding PBE and HSE computed values in this work. V Cd refers to a Cd vacancy whereas V* Cd is the Vanadium at Cd site impurity.…”

Universal machine learning framework for defect predictions in zinc blende semiconductors

“… 3 , 10 It has been reported in past work that defect levels computed from semilocal GGA-PBE for well-known ZB semiconductors such as Si and GaAs can span the physical band gap of the material 25 , 50 ; that is, the defect transition levels calculated from PBE correspond well with experimental values up to the experimental band gap, even for transitions that are further from the VBM than the PBE-calculated band gap. To ascertain the accuracy of PBE defect and impurity levels in all II–VI, III–V, and IV–IV compounds, we scoured the published literature 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 (in a brute force manner, something we hope to replace with more efficient and comprehensive natural language processing-based searches in the future 67 , 68 ) and collected measured energy levels for 84 defects across the 34 compounds, adding to the set of 15 points collected for CdTe in ref 12 As presented in Figure 2 and Table S2 , the PBE predicted defect levels are highly correlated with experimental measurements, with a correlation coefficient (R 2 ) of 0.85. We find that the RMSE of PBE predictions compared with experiments is less than 0.2 eV for III–V and IV–IV compounds, and 0.24 eV for II–VI compounds, resulting in a total RMSE of approximately 0.21 eV.…”

Universal Machine Learning Framework for Defect Predictions in Zinc Blende Semiconductors

“…FIG. 2: Comparison of DFT computed defect levels with experimentally measured levels (obtained from publications [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] . Measured vs DFT RMSE values are also shown for different semiconductor types and for the combined set of points.…”

“…This contrast can be attributed to total energy differences in DFT being more accurate than using Kohn-Sham energy levels to estimate band edges 12,25,64 or band gaps 39 , or in the case of halide perovskites, a fortuitous cancellation of errors that leads to PBE being as accurate as HSE06+SOC 6,9 . To ascertain the accuracy of PBE defect and impurity levels in all II-VI, III-V, and IV-IV compounds, we scoured the published literature [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] (in a brute-force manner, something we hope to replace with more efficient and comprehensive natural language processing-based searches in the future 65,66 ) and collected measured energy levels for 84 defects across the 34 compounds, adding to the set of 15 points collected for CdTe in reference 12 . As presented in Fig.…”