A new method for in-situ Sb doping of CdTe that uses a modified vapor transport deposition system is described. This modification enables control of the Sb concentration with a pyrolysis stage to enhance the doping efficiency. CdTe:Sb films under different deposition conditions are characterized by SEM, XRD, and CV measurements for determining morphology, crystal structure, and hole concentration. Variations of the Sb dopant heater and pyrolyzer temperatures do not affect the CdTe morphology and crystal structure. However, CV measurements show that a higher dopant heater or pyrolyzer temperature leads to higher hole concentration. In this study, CdTe: Sb films achieve a hole concentration of 10 16 cm −3 and 10% doping efficiency when the dopant heater is 600C and the pyrolyzer temperature is 1100C. This demonstrates a path to produce high hole concentration polycrystalline CdTe film with a low concentration of dopant-induced defect.

Sulfur passivation of different Czochralski Si surfaces along with hydrogenated amorphous silicon nitride capping layer has been reported. The effect of capping layer thickness and deposition temperature on sulfur reacted surface has a strong effect on initial passivation quality and its stability. Sulfur passivation on phosphorus diffused n-type textured Czochralski Si wafers capped with a 100 nm stacked hydrogenated amorphous silicon nitride layer demonstrates improved stability under heat and light with saturation current density <80 fA/cm(2). A stack layer of a low temperature (<= 300(omicron)C) hydrogenated amorphous silicon nitride followed by a high temperature (approximate to 450(omicron)C) hydrogenated amorphous silicon nitride is adopted to achieve good thermally stable surface passivation. Sulfur-passivation with only a low temperature hydrogenated amorphous silicon nitride capping layer exhibits unstable behavior after thermal treatment due to blister/pinhole formation at >= 300OC and possible oxidation of the S-passivated surface. Photoluminescence imaging shows increased defect recombination loss due to disrupted surface passivation after thermal treatment. Fourier transform infrared spectroscopic studies demonstrate that these blister formations were caused by hydrogen effusion from Si-H and N-H bonds dissociation upon thermal processing. Time of flight secondary ion mass spectroscopy exhibits oxidation of S-reacted surface after thermal treatment that degrades the passivation quality.

This study explores the influence of residual lead iodide (PbI2) on the performance of FAPbI3 perovskite solar cells fabricated using a two-step close-space vapor transport process. The cells are fabricated in an n-i-p structure with the PbI2 at the interface between the SnO2 electron contact and the FAPbI3. The PbI2 content is quantified by a reaction extent parameter (y), determined from relative x-ray diffraction peak intensities. When y is within the optimal range of 0.01 to 0.1, the PbI2 enhances solar cell performance by passivating the contact. However, excessive PbI2 leads to low photogenerated carrier collection, while low PbI2 levels (y < 0.01) fail to adequately passivate material imperfections. These findings highlight the critical role of precise PbI2 control in achieving high-efficiency perovskite solar cells.

The fabrication of CdZnTe solar cells is complicated by the removal of lattice-bound Zn during CdCl2-based anneal treatments. Fundamental understanding of the chemistry of CdZnTe with chloride treatment is essential to advance the technology. We investigate alternate and manufacturable chloride chemistry using SbCl3 for CdZnTe-based alloy. New SbCl3 treatments of CdZnTe show promising performance while maintaining Zn composition compared to CdCl2. Effects of varying reaction conditions on chemical mechanisms are presented.

The chalcopyrite alloy (Ag,Cu)(In,Ga)Se-2 is a highly efficient thin film solar cell absorber, reaching record efficiencies above 23%. Recently, a peculiar behavior in the bond length dependence of (Ag,Cu)GaSe2 was experimentally proven. The common cation bond length, namely Ga-Se, decreases with increasing Ag/(Ag + Cu) ratio even though the crystal lattice expands. This is opposite to the behavior observed for Cu(In,Ga)Se-2, where all bond lengths increase with increasing lattice size. To better understand this peculiar bond length behavior, element-specific bond lengths of (Ag,Cu)InSe2 and Ag(In,Ga)Se-2 alloys are determined using extended x-ray absorption fine structure spectroscopy. They show that the peculiar bond length dependence occurs only for (Ag,Cu) alloys, independent of the species of common cation (In or Ga). The bond lengths are used to determine the anion displacements and to estimate their contribution to the bandgap bowing. Again, both behaviors differ significantly depending on the type of alloyed cation. A valence force field approach, relaxing bond lengths and bond angles, is used to describe the structural distortion energy for a comprehensive set of I-III-VI2 and II-IV-V-2 chalcopyrites. The model reveals bond angle distortions as main driving factor for the tetragonal distortion and reproduces the literature values with less than 10% deviation. In contrast, the peculiar bond length dependence is not reproduced, demonstrating that it originates from electronic effects beyond the scope of this structural model. Thus, a fundamental understanding of bond length behavior and tetragonal distortion is achieved for chalcopyrite materials, benefiting their technological applications such as high efficiency thin film photovoltaics.

Accurate capacitance measurements in Cu(In,Ga)Se2 solar cells as well as any other material system are important for extracting accurate doping profiles, trap energies, and trap concentrations, but non-Ohmic back contacts can impact the measured capacitance. In this case it is demonstrated that a five-element small-signal model accurately fits the capacitance and conductance frequency dependence where two elements represent and are correlated to the back contact, two others represent the semiconductor junction, and the fifth is a series resistance. A temperature-dependent contact conductance (GC) is found to cause the inflection in the capacitance-frequency (C-f) measurements, which is shown to be responsible for the signature commonly observed in admittance spectroscopy. Good agreement is observed between measured data and simulations using the five-element model across a wide range of temperatures. This analysis shows the importance of performing a C-f measurement before subsequent capacitance measurements to choose an appropriate measurement frequency.

While the development of methylammonium lead iodide (MAPbI3) perovskite in photovoltaic research has matured for solution-based fabrication methods, the growth of high-quality material from vapor processes continues to be difficult due to challenges in vapor chemistry control. Postdeposition methylamine (MA) vapor treatment is an approach to improve the morphology and crystallinity of MAPbI(3) films, producing highly oriented, large-grain perovskite films. Herein, experiments to characterize the effects of substrate temperature, MA partial pressure, and MA exhaust flow rate on the liquefaction and recrystallization processes that occur during MA treatments are described. Improved morphology and crystallinity following MA treatment indicate that substrate temperature plays a significant role in both processes during treatment. The enhancement in electronic properties after treatment is confirmed by steady-state and time-resolved photoluminescence spectroscopy. The solar cells made from the films treated at the optimal treatment conditions have shown open circuit voltages over 1 V and fill factors up to 70%. The result of this study is a useful pathway for the fabrication of high-quality MAPbI(3) films via all-vapor process manufacturing.

Methylamine (MA) treatment of vapor-processed perovskite films is used to improve performance of perovskite solar cells (PSC) by a significant increase in open-circuit voltage. In sequential processing of PSCs, the presence of residual PbI2 in the film impacts device performance. This study aims to understand the role of residual PbI2 after MA treatment and its effect on PSCs. We characterize properties of MA-treated PbI2 films and MA-treated perovskite films with different amounts of PbI2 from two-step vapor processing. Our findings suggest that MA interacts with PbI2 and changes its optical properties. However, the role of excess PbI2 in MA-treated and as-deposited PSCs does not change, with Voc improvement after MA treatment depending on the amount of PbI2 in the as-deposited film. An optimized MA-treatment gives approximately a 10% increase in VOC compared to untreated PSC devices.

The effect of high-temperature annealing (HTA) treatments and Cd-excess during the in-situ doping of CdSeTe:Sb is investigated. Optimized treatments eliminate the wurtzite phase from the as-deposited CdSeTe layer, while facilitating Se intermixing and grain size enhancement before CdC12 treatment. The improved device stack quality results in a VOC improvement of 250 mV. VOC is further improved 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 VOC CdSeTe:Sb cell with VOC = 849mV, despite a decreased carrier concentration due to the harsh CdC12 treatment.

The conversion efficiency of CdTe solar cells may be improved by bandgap engineering, i.e., changing the bandgap value through the addition of Se in the absorber. The Se alloying enables a short-circuit current density improvement, as it leads to a bandgap energy value decrease. Furthermore, it has been associated with increased minority carrier lifetimes, assuring high open-circuit voltage values. An Se gradient profile control can further optimize the solar cell performance. Thus, an optical model baseline of the CdSexTe1-x (CST) compound was developed. Spectroscopic ellipsometry measurements were conducted to accurately extract the optical constants of ten CST layers deposited through coevaporation with x varying from 0 to 1. Using the measured dielectric function spectra from the discrete CST layers with varying x, and considering the composition-induced shift in the critical point energies, an energy-shift model was employed to develop the accurate optical library for the CST compound for any x value to provide data for future modeling and optimization. The library accuracy was validated through optical simulations of the quantum efficiency of a graded CST solar cell using the finite-difference time-domain method by replicating the Se profile in the absorber layer measured through secondary ion mass spectrometry.

Development of a new SbCl3-based solution treatment for chloride activation of CdTe-based solar cells is described. Activation was confirmed with CdSeTe:Sb devices exhibiting V-OC > 550 mV and J(SC) > 25 mAcm(-2). Treatment optimization showed strong effects of annealing time and temperature on cell properties. Material characterization indicated surface conversion of CdSeTe to CdCl2 and Sb2Te3 with SbCl3 treatment. However, no reaction products were formed with treatment under inert conditions, which was coupled with poor device performance. Drying the SbCl3 films at controlled humidity conditions confirmed that exposure to high relative humidity is critical for CdSeTe activation, through hydrolysis of SbCl3. Mechanistic details of SbCl3 speciation during hydrolysis and annealing to activate CdSeTe are discussed.

Arsenic (As)-doped graded Cd(Se,Te) photovoltaics have achieved record efficiency of 22.4%, but further increases in efficiency have been limited by doping activation of only approximate to 1% and radiative voltage losses of approximate to 100 meV. Antimony (Sb) can also act as a p-type dopant of Cd(Se,Te) but has received relatively little attention. We measured carrier dynamics and photoluminescence of Sb-doped CdTe and CdSeTe films grown by vapor-transport deposition, with dopant activation up to approximate to 20%. Time resolved terahertz (TRTS) and photoluminescence (TRPL) data were fit by simulating the semiconductor equations, revealing bulk lifetimes of >20 ns in CdTe:Sb and >65 ns in CdSeTe:Sb and surface recombination velocities <1000 cm/s at alumina interfaces. We also qualitatively compare transients and PL spectra of doped and undoped films, with no apparent variation associated with hole concentrations up to 10(15) cm(-3). Preliminary variable temperature steady state photoluminescence (VT-SSPL) indicates dominant band-gap emission in CdSeTe:Sb and no evidence of potential fluctuations with Sb-doping up to this hole concentration.

Vapor transport (VT) deposition offers an industry-proven scale up process for perovskite solar cells (PSC) with many other potential advantages in process and waste handling. However, it requires a detail investigation of processing compatibility and necessary improvements of the PSC stack. This work demonstrates the necessity for appropriate transport layers onto which the VT deposited perovskite film is grown. This affects the critical interface quality for efficient charge carrier collection and fill factor. Furthermore, post-growth vapor treatment is shown to remarkably improve the bulk quality of VT grown methylammonium lead iodide perovskite films and open circuit voltage in PSC.

Our team has achieved ~20% doping activation of CdSeTe with antimony (Sb), whose shallow acceptor level makes it a viable alternative to As.1,2 Arsenic (As)-doped graded CdSeTe photovoltaics have achieved record efficiencies of 22.3%. Remaining challenges include doping activation of only ~2% in polycrystalline films and radiative voltage losses of ~100 mV. Here we use (transient) terahertz and photoluminescence spectroscopy of CdTe:Sb and CdSeTe:Sb to show sufficiently long bulk lifetimes to enable high-efficiency devices. Early results also indicate dominant bandgap emission in CdSeTe:Sb and a lack of potential fluctuations with Sb-doping that have proven detrimental for As-doped films. While more work is needed, these results show significant promise for Sb doping.

The effect of Sb inclusion in CdSeTe thin films was examined. CdSeTe alloys were grown by co-evaporation, either on 5 nm thick electron-beam evaporated Sb layers (Sb/CdSeTe) or coated with Sb post-deposition (CdSeTe/Sb). Sb inclusion improved CdSeTe grain structure, a critical factor for device performance, with average grain size increasing from 0.6 to 1.5 mu m. All samples crystallized on annealing to the cubic structure, with a change in film orientation from (111) to random or (220). The presence of Sb in CdSeTe/Sb or Sb/CdSeTe samples, however, had a limited effect on the carrier concentration compared to Sb-free samples. Time resolved photoluminescence showed Sb inclusion increased decay lifetimes, with tau(1) = 0.8 ns for and tau(2) = 6.9 ns for the Sb/CdSeTe sample.

The effect of air exposure on all-vapor processed perovskite solar cells using C-60 fullerene electron transport layers (ETLs) was investigated. C-60 is used in lead halide perovskite solar cells as an ETL to decrease hysteresis and improve stabilized power output. However, air exposure to n-i-p solar cells using C-60 ETLs without encapsulation or doping can result in performance degradation due to FF loss and the onset of s-shaped J-V curves. This is correlated to orders of magnitude increase in C-60 resistivity upon air exposure. Drift-diffusion simulations provide evidence that a change in the C-60 carrier concentration or mobility can lead to the FF loss and s-shaped J-V curve. The degradation does not occur when using inorganic ETLs but does occur in p-i-n architecture using C-60 ETLs, further confirming that the C-60 layer is the source of the degradation. This is an additional pathway for perovskite solar cell degradation upon air exposure beyond the instability of the perovskite itself. The loss of efficiency can be reduced in p-i-n solar cells using a LiF interlayer, and a better combination of hysteresis and air stability can be achieved in n-i-p solar cells using a C-60/SnO2 bilayer ETL.