| AVS 59th Annual International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Thursday Sessions |
| Session EN+AS-ThA |
| Session: | Characterization of Energy Materials and Systems |
| Presenter: | D.S. Albin, National Renewable Energy Laboratory |
| Authors: | D.S. Albin, National Renewable Energy Laboratory T.A. Gessert, National Renewable Energy Laboratory R. Dhere, National Renewable Energy Laboratory S.-H. Wei, National Renewable Energy Laboratory J. Ma, National Renewable Energy Laboratory D. Kuciauskas, National Renewable Energy Laboratory A. Kanevce, National Renewable Energy Laboratory R. Noufi, National Renewable Energy Laboratory |
| Correspondent: | Click to Email |
Capacitance-voltage (CV) measurements are commonly used to characterize semiconductor junctions. A common observation when performing such measurements on polycrystalline CdTe solar cells is that the measured capacitance is a strong function of the voltage scan direction. These results in a noticeable hysteresis in the C-V profile when capacitance data is collected using both forward (fwd) and reverse (rev) voltage scan directions. Similarly, hysteresis curves for the usual derived quantities such as net carrier density, Na, and depletion width, W, naturally follows. We have recently observed that in particular, the hysteresis in calculated carrier acceptor density, Na,hys, arbitrarily defined as Na,fwd – Na,rev decreases as the CdTe growth temperature is reduced. At higher CdTe growth temperatures, this value is positive and shifts to negative values at lower growth temperatures. This behavior is believed to reflect a transition of the CdTe stoichiometry in CdS/CdTe solar cells from Cd-poor at higher temperatures to Cd-rich at lower temperatures based upon recently published CdTe binary phase diagrams.
The impact of Cd/Te stoichiometry on cell performance is suspected. Cd-poor stoichiometry favors the formation of Cd-vacancies (VCd), a beneficial acceptor, but it also simultaneously increases the formation of the Te antisite (TeCd), an important recombination center in CdTe. Recent theoretical calculations using the hybrid Heyd-Scuseria-Ernzerhof (HSE) functional also suggests that Te interstitials (Tei) may be an important recombination center under Cd-poor conditions. Increased Cd chemical potential will reduce the formation of these recombination centers, but also reduce hole carrier density. Thus, an optimal growth condition, which could include extrinsic p-type doping, may be needed for leading to higher performance CdTe solar cells.
Not discussed to date however is whether stoichiometry has an inherent impact on the stability of CdTe solar cells. In this talk, we contrast accelerated lifetime study results for CdTe cells grown at different growth temperatures. Open-circuit voltage (Voc) stability in cells grown at lower temperatures was noticeably improved. Na,hys in the latter cells was considerably smaller. Both Voc and FF were well correlated with this capacitance-derived parameter. Na,hys was observed to be relatively unchanged in devices where Voc did not degrade. Finally, time-resolved photoluminescence lifetimes of nearly 10 ns were measured in these cells made at lower temperatures relative to values of around 2 ns measured in higher temperature devices.