2, National Renewable Energy Laboratory, Golden, Colorado, United States
CdTe solar cell technology holds immense potential for performance improvement through enhanced voltage output. When a carrier concentration exceeding 5x1016 cm-3 can be effected with adequate minority carrier lifetime (1-10 ns), open circuit voltages (VOC) more than 1 V are possible, higher than competing thin film and multi-crystalline silicon solar cell technologies . Consequently, in-situ substitutional p-type doping of CdTe solar cells processed using vapor transport deposition film growth technique was explored for P, As and Sb, and dopant densities exceeding 1016 cm-3 were reported . This process requires a post-deposition treatment (PDT) to electronically activate the dopant. We explore the process space by varying the time, temperature, and ambient atmosphere of the PDT and report the effect on VOC, NCV, device behavior.
In this work, a combination of conventional PDT techniques common to CdTe material system are explored for devices made with Sb-doped CdTe films. Devices prepared from doped films processed in identical deposition sequences are subject to a matrix of different annealing conditions with varying temperature, ambient and duration. The effect of the different treatment variables on doped devices will be evaluated by studying the doping efficiency, computed using capacitance-voltage (C-V) and secondary-ion mass spectroscopy (SIMS) measurements, and density of states from admittance spectroscopy measurements. Density of responding charge from C-V (NCV) vs W at 0V DC bias follows the trend expected for an abrupt p-n junction with uniform doping, giving us good confidence that NCV represents free carrier concentration.
NCV in devices annealed in an air ambient exceeded the Ncv for devices annealed in CdCl2 by more than an order of magnitude. These cells had the highest NCV among all the devices in the anneal matrix, exceeding 1x1016 cm-3. In general, we find that the presence of air during the PDT allows for higher doping and higher short circuit current, while CdCl2 is required for high open circuit voltages. Nearly equivalent doping was achievable with or without the presence of CdCl2. Open circuit voltages greater than 685 mV were measured on the devices treated with CdCl2 and short circuit currents as high as 23 mA/cm2 were obtained for those treated in an air ambient with CdCl2 coating. It is important to note that no Cu passivation was used to passivate the carbon-ink back contact. Typically Cu passivation increases VOC by 150-200mV. Further details of the anneal sequences performed, and the performance of the resulting devices will be discussed in the report.
 Brian McCandless, “CdTe Solar Cells: Processing Limits and Defect Chemistry Effects on Open Circuit Voltage,” Conf. Rec. 2013 MRS, MRS-13-1538-C13-04.R1, 2013.
 Brian McCandless et. al. “Enhancing p-type Doping in Polycrystalline CdTe Films”, IEEE Photovoltaics Specialists Conference, 2017.