Gregory Andrew Manoukian

Group V doping is a key strategy to achieve the equilibrium hole concentrations in cadmium telluride (CdTe) photovoltaics required to exceed 25% power conversion efficiency. While arsenic‑doped devices have surpassed 23% efficiency, low dopant activation and dopant‑related defects continue to limit open‑circuit voltage (VOC). Here, we report on antimony (Sb) incorporation during vapor transport deposition of CdTe and graded CdSeTe absorbers and its impact on photoluminescence and carrier dynamics. Sb incorporation was controlled by the dopant source temperature during deposition, followed by CdCl₂ treatment; and graded CdSeTe with the highest dopant‑temperature was also subjected to varied CdCl₂ treatments. Sb concentrations up to ~3x10¹⁷ cm⁻³ were achieved. Steady‑state photoluminescence spectra and optical pump – terahertz probe decays were largely unchanged across a ten‑fold increase in Sb concentration in both CdTe and graded CdSeTe, indicating that moderate Sb incorporation does not intrinsically degrade radiative properties or carrier dynamics. In contrast, high Sb source temperature introduced new deep defect‑related emission in CdTe, broadened sub‑bandgap emission in graded CdSeTe, and showed substantially faster decays, consistent with increased carrier trapping or recombination associated with processing‑induced defects. An alternative CdCl₂ heat treatment increased hole concentrations from ~1x10¹⁵ to 3x10¹⁶ cm⁻³ and increased VOC but at the cost of increased trapping and reduced fill factor. These results highlight the critical interplay between dopant incorporation and post‑deposition processing in governing defect chemistry and device‑relevant optoelectronic properties in Sb‑doped CdTe and CdSeTe absorbers.

The impact of high temperature annealing (HTA) treatments on the performance of Sb-doped CdSeTe solar cells containing a CdSe0.25Te0.75/CdTe front stack and Sb-doped CdTe main absorber was studied. The HTA treatment of the thermally evaporated CdSe0.25Te0.75/CdTe front stack converts the mixed-phase film into a single zinc blende structure through recrystallization of the photo-inactive wurtzite phase. Subsequently, HTA treatment of the full device stack after vapor transport deposition of Sb-doped CdTe (CdTe:Sb) absorber promotes Se-Te intermixing and enhances CdTe:Sb grain growth. Comprehensive characterization using x-ray diffraction, scanning electron microscopy, cathodoluminescence spectrum imaging, and time-resolved terahertz spectroscopy revealed that the combined HTA treatments significantly improved film crystallinity, reduced defect densities, and enhanced carrier dynamics. With HTA treatments, the open-circuit voltage (VOC) of CdSeTe:Sb devices increased from ∼ 400 mV to ≥ 600 mV, and when combined with optimized CdCl2 treatment, the device’s VOC reached 849 mV. The findings demonstrate the potential of HTA treatments to improve the structural and electrical properties of CdSexTe1-x solar cells.

This thesis is a culmination of efforts to study and identify optoelectronic effects in cadmium telluride (CdTe)-based photovoltaic (PV) absorbers. Specifically, it elaborates on characterization of films alloyed with selenium (Se) in Chapter 3 or doped with antimony (Sb) in Chapter 4. The goal of this research is to investigate fundamental impacts of processing conditions on absorber properties from alloying and doping using non-contact optical probes. Polycrystalline thin film PV are desirable for their short time and low energy intensity to manufacture compared to single-crystal silicon PV. Thus far, CdTe-based absorbers are the only successful thin film technology on the global PV market, comprising ~4% of global module production. The power conversion efficiency from the detailed balance limit for CdTe, is ~31%, however CdTe-based technologies to date have only reached 23.1% efficiency. Despite successful commercialization, CdTe shows a large deficit in open circuit voltage (V_[OC]), which represents a viable direction for technological improvement. Research efforts to support CdTe manufacturing have improved efficiency to 23.1% at lab scale and ~19.5% at the commercial scale in the last 10 years. Improvements are largely the result of incorporation and optimization of: 1) selenium alloy gradients and 2) the transition from copper doping to arsenic doping. Both changes have been implemented commercially; however, the full extent of microstructural and electronic effects of the processing is not fully understood. Both Se-alloying and As-doping were critical to enhance performance. However, there are known problems and knowledge gaps with both Se and As in CdTe, and both can be correlated to harmful defect chemistry. Selenium alloying improved performance greatly, but it is induces defects in quantities that can limit V_[OC]. The identities of Se-related defects are not known precisely, and their behaviors have only recently been studied. Films containing selenium show also show native n-type conductivity, which reduces p-type doping ability and can result in compensation. Arsenic-doped CdSeTe are the current state-of-the-art devices; however, dopants show low activation, leading to inactive impurities. Inactive impurities can form new defects of varying impact, in the worst cases causing extreme electronic disorder, but in general reducing or limiting performance in devices. The exact contributions of each impurity or defect complex on performance limitations require detailed study. Optical spectroscopies are non-destructive and non-contact methods that can be applied to absorber films or full devices. Photoluminescence (PL) and Time Resolved Photoluminescence (TRPL), as well as Optical Pump Terahertz-Probe (OPTP) and terahertz Time Domain Scans (TDS) are valuable and complementary experimental that are used in this work to inform on band and defect structures, as well as carrier transport and recombination, with regard to processing. In general, developing a deeper understanding of how chemistry and processing affect thin film electronic behavior can help improve performance. Se-alloying appears to increase carrier lifetimes, yet estimates from TPRL do not match performance expectations from device models. In Chapter 3, we report the results extracted from fitting a photocarrier transport and recombination model to TRPL and OPTP transients. We quantify effects of Se concentration on carrier dynamics and recombination in CdSe_[x]Te_[1-x]. Bulk carrier and surface lifetimes are improved with Se alloying, however, carrier mobilities are reduced. We show that carrier trapping in shallow defects can complicate interpretations of carrier lifetime from TRPL decays. We show that a model incorporating trapping physics results in good description of dynamics of Se alloyed CdTe films. These results demonstrate the need for both careful parameter estimation for devices and controlled processing with Se. Additionally, using terahertz TDS analysis, we show that the incorporation of Se does not fundamentally change the carrier mobility, and there are instead other causes for mobility reduction. Incorporation of group V dopants in place of traditional copper led to a boost in device V_[OC] and efficiencies; however, low activation and defect chemistry of state-of-the-art dopants such as As or P can lead to unintended V_[OC] deficits. Sb was proposed for its size match with the Te-anion site and demonstrated high activation in CdTe. In Chapter 4, we show that Sb-doped films achieve high carrier concentrations (>2x10¹⁶ cm⁻³) in graded CdSeTe devices with aggressive CdCl₂, at the cost of inducing defect luminescence ~350 meV below the gap energy, which can limit V_[OC]. In samples with mild CdCl₂, Sb doping has no deleterious effects on carrier dynamics or emission spectrum in CdTe or CdSeTe up to 3x10¹⁷ cm⁻³. High dopant heater temperature during processing results in complex chemistry and transport physics in newly identified radiative defects. While it has yet to be demonstrated, this work suggests the combination of high doping and sufficient optoelectronic performance is possible. Ultimately, many material properties of relevance to device performance can be probed with optical spectroscopies, which can be used to enhance understanding of processing-structure-property-performance relationships. The methods used in this thesis are well suited for initial analyses and screening of experimental thin films as well as deeper correlation between processing variables and their potential impacts on device physics and performance.

In this study, we developed a new method for in situ Sb doping of CdTe thin films combining vapor transport deposition with a Group V pyrolyzer to address Sb doping concentration and doping efficiency. The Sb doped CdSeTe (CdSeTe:Sb) films were deposited in solar cell structures under variations of Sb dopant source heater, vapor pyrolyzer temperature, and Cd vapor excess. Results indicate that although these parameters do not affect the CdTe morphology or crystal structure, they critically influence doping efficiency and trap concentration. Capacitance–voltage measurements show that a higher dopant heater (T_D) or pyrolyzer (T_P) temperature leads to higher net carrier concentration, achieving a net carrier concentration of 10¹⁶ cm⁻³ and 20% doping efficiency with a T_D/T_P combination of 600 °C/1100 °C. By tuning the Cd/Sb flux ratio during CdSeTe:Sb deposition, the lowest defect concentration is achieved at Cd/Sb of 1.4:1, which produced the best V_OC CdSeTe:Sb cell. This demonstrates a path to produce high net carrier concentration polycrystalline CdTe thin film with a low concentration of dopant-induced defects.

Noble-transition metal alloys offer emergent optical and electronic properties for near-infrared (NIR) optoelectronic devices. We investigate the optical and electronic properties of CuxPd1−x alloy thin films and their ultrafast electron dynamics under NIR excitation. Ultraviolet photoelectron spectroscopy measurements supported by density functional theory calculations show strong d-band hybridization between the Cu 3d and Pd 4d bands. These hybridization effects result in emergent optical properties, most apparent in the dilute Pd case. Time-resolved terahertz spectroscopy with NIR (e.g., 1550 nm) excitation displays composition-tunable electron dynamics. We posit that the negative peak in the normalized increment of transmissivity (ΔT/T) below 2 ps from dilute Pd alloys is due to non-thermalized hot-carrier generation. On the other hand, Pd-rich alloys exhibit an increase in ΔT/T due to thermalization effects upon ultrafast NIR photoexcitation. CuxPd1−x alloys in the dilute Pd regime may be a promising material for future ultrafast NIR optoelectronic devices.