Deep-level impurities in CdTe/CdS thin-film solar cells

Abstract

We have studied deep-level impurities in CdTe/CdS thin-film solar cells by capacitance–voltage (C–V), deep-level transient spectroscopy (DLTS), and optical DLTS (ODLTS). CdTe devices were grown by close-spaced sublimation. Using DLTS, a dominant electron trap and two hole traps were observed. These traps are designated as E1 at EC−0.28 eV, H1 at EV+0.34 eV, and H2 at EV+0.45 eV. The presence of the E1 and H1 trap levels was confirmed by ODLTS. The H1 trap level is due to Cu-induced substitutional defects. The E1 trap level is believed to be a deep donor and is attributed to the doubly ionized interstitial Cu or a Cu complex. The E1 trap is an effective recombination center and is a lifetime killer.

Studies of Basic Electronic Properties of CdTe-Based Solar Cells and Their Evolution During Processing and Stress: Final Technical Report, 16 October 2001 - 31 August 2005

“…The C Cd Cu The H3 signature with an E a of 0.47 eV was only detected in cells with no intentionally added Cu. This energy level has been detected by others [67,68] and in two instances has been attributed to [65,69] in strong discrepancy with the theoretical estimates for this defect [20]. Besides, we note that H1 which we attributed to the singly ionized remains detectable…”

Electron and hole drift mobility measurements on thin film CdTe solar cells

“…There have been many previous studies of trapping in CdTe solar cells with deep level transient spectroscopy (DLTS) and other methods. 27 These typically report the location of the trap level within the energy gap, but not mobility-lifetime products. One transient photoconductivity measurement has been reported showing a 6 ls lifetime for holes; 28 this is consistent with the deep-trapping lifetime in the present work, which is about 1 ls for holes.…”

Process Development and Basic Studies of Electrochemically Deposited CdTe-Based Solar Cells; Annual Technical Report, Phase II, 16 May 1999-13 May 2000

“…Five deep levels were also revealed in the NREL CSS devices [17,18] of the level energy : E1*=E c -0.28eV, H1*=E V +0.35eV, H2*=E V +0.45eV, H3*=E V +0.79eV, and H4*=E V +0.93eV. These energies are close to those found in the CSM cells except the H2* and H2 levels.…”

“…[11] observed in the ED-grown CdTe devices a minority carrier trap level with an activation energy of E c -0.35eV. In as-grown CSS CdTe/CdS cells the E1 level has an activation energy of E c -0.28 eV [17,18]. After stressing of a cell this level shifts into the band gap by 0.22eV [17].…”