Tursun Ablekim

Relationship of Open-Circuit Voltage to CdTe Hole Concentration and Lifetime

We investigate the correlation of bulk CdTe and CdZnTe material properties with experimental open-circuit voltage (Voc) through fabrication and characterization of diverse single-crystal solar cells with different dopants. Several distinct crystal types reach Voc > 900 mV. Correlations are in general agreement with Voc limits modeled from bulk minority-carrier lifetime and hole concentration.

Fabrication of single-crystal solar cells from phosphorous-doped CdTe wafer

A phosphorous doped CdTe boule was grown from melt using the modified vertical Bridgman technique. Single crystals prepared from the boule showed hole density of (5-10) × 1015 cm-3, resistivity ~(10-50) Ω.cm and mobility of ~50 cm2/v.s determined by Hall measurement. The two-photon excitation time-resolved photoluminescence (2PE-TRPL) measurement revealed a bulk minority carrier lifetime of ~47 ns. A 500μm thick single-crystal wafer was prepared and CdS/CdTe heterojunction solar cells were fabricated with ITO/CdS/CdTe/Cu/Au structure where all relevant thin film layers were deposited by DC/RF magnetron sputtering. An efficiency of ~11% and short circuit current density of as high as ~25 mA/cm2 has been obtained on a cell upon annealing at 320°C. The high short circuit current density could be attributed to the superior quality of the CdTe material with a relatively longer minority carrier lifetime.

Interface Characterization of Single-Crystal CdTe Solar Cells With VOC > 950 mV

Advancing CdTe solar cell efficiency requires improving the open-circuit voltage (VOC) above 900 mV. This requires long carrier lifetime, high hole density, and high-quality interfaces, where the interface recombination velocity is less than about 104 cm/s. Using CdTe single crystals as a model system, we report on CdTe/CdS electrical and structural interface properties in devices that produce open-circuit voltage exceeding 950 mV.

Defects in Undoped p-type CdTe Single Crystals

Recently, the device performance of Cadmium Telluride (CdTe) solar cells have improved significantly. However, little progress has been made in understanding the defected structures responsible for carrier trapping, which have limited further cell efficiency improvements. In order to better understand the underlying electrical compensation mechanism, an undoped CdTe bulk crystal boule was grown from melt with Te-rich stoichiometry using the Vertical Bridgman technique, where the growth process was controlled to maximize net acceptor density. The samples prepared from the boule indicate low resistivity of 100-500 Ω·cm, and Hall measurements provided net acceptor density of (1-2) × 1015 cm-3, which are comparable to relevant values reported from the CdTe films after CdCl2 treatment. Infrared microscopy suggests there are significant amounts of secondary phases (SPs) present in the samples, where the SPs are believed to be Te inclusions and precipitates. The point defects are characterized by thermoelectric effect spectroscopy (TEES), where we observed six TEES current peaks associated with the defect levels ranging from 110 to 700 mV within the bandgap.