Session M67: Defect Design for Energy Materials and Devices

Thermal and electronic properties of layered EuM1-xPn compounds with tunable vacancy concentrations from x = 0-0.5

AMX compounds belonging to the ZrBeSi structure type are able to tolerate a large vacancy concentration - up to 50% - on the M site in the planar hexagonal MX layers.  Their tolerance for disorder makes AMX compounds exciting for thermoelectric applications by providing a route to amorphous-like lattice thermal conductivity.   Here, we investigate the impact of vacancies on the thermal and electronic properties of EuM_1-xPn compounds (M = Cu, Zn, and Pn = Sb, Bi).  The fully-ordered compound, EuCuSb, is a stiff material with high electronic mobility and high thermal conductivity.  Replacing Cu1+ by Zn2+ leads to a composition of EuZn_0.5Sb, in which 50% of the Zn site remains vacant to maintain overall charge-balance.  The transition from a fully-occupied hexagonal net to a one with a quarter of the atoms missing has wide-ranging consequences;  with increasing vacancy concentration, we observe a drastic, non-linear expansion of the average bond lengths in the hexagonal net.  Despite the absence of long-range ordering, pair distribution function analysis from synchrotron X-ray diffraction reveals significant local structural distortions caused by Zn vacancies, which leads to strong phonon scattering.   The high vacancy concentrations also cause the elastic stiffness to decrease significantly, leading to a corresponding decrease in sound velocity.  This, combined with increased scattering, leads to a precipitous drop in lattice thermal conductivity in the vacancy-rich EuZn_0.5Sb samples.    Naturally, the electronic mobility is suppressed by the high defect concentration.  However, we find that isovalent alloying with Bi on the Sb site can increase the electronic mobility from approximately 50 to 100 cm²/Vs by reducing the effective mass.   Simultaneously, substituting Bi further reduces the lattice thermal conductivity by reducing the sound velocity.     

Effects of Te doping on the optical and electronic properties of a-Se photodetectors

Amorphous selenium (a-Se) is a high resistivity semiconductor used in UV and X-ray detectors due to its high gain, low dark current, and ability to achieve avalanche multiplication at relatively low fields (60-70 V/um). The 2.2 eV band gap of a-Se limits the low energy detection range to UV-blue/green wavelengths; incorporation of Te has been shown to lower the gap and decrease resistivity. However, it has also been determined that the inclusion of Te generates deep and shallow trap states within the a-Se material, lowering both hole and electron mobilities. Still, studies have demonstrated that Te inclusion at 15 wt % can lower the threshold for impact ionization, providing a renewed interest in investigations of Te as a dopant for a-Se detectors targeting lower energy wavelength detection.

In this work, we will review the optoelectronic properties of a-Se_1-xTe_x (x < 0.3) devices and neat films fabricated by thermal evaporation. Characterization will include a study of the absorption coefficient, including the optical bandgap and defect states; hole and electron mobilities as a function of applied field; and conversion efficiency and gain, including a detailed analysis of the onset field for avalanche multiplication.   

Structure, Electronic Properties and Defect Chemistry of Delafossite CuRhO2 Bulk and Surfaces

CuRhO₂ in the delafossite structure is a promising, highly stable photocathode material for solar water splitting, yet the fundamental bulk and surface properties of CuRhO₂ that are relevant to such application have rarely been studied. Here, we present a comprehensive computational study of the bulk and majority (001) surface of CuRhO₂ using density-functional theory at the meta-GGA and hybrid functional levels. First, we examine the structure, thermodynamic and electronic properties of pristine and defective bulk CuRhO₂, addressing the potential role of bulk defects in the behavior of charge carriers. Our calculations show that Cu vacancies and Cu antisite defects introduce shallow acceptor levels in line with the results for bulk CuFeO₂. The computed surface stability diagram for CuRhO₂(001) under vacuum conditions shows that Cu antisite defects can be favorable also at the surface. Analysis of the electronic structure of pristine and defective surfaces allows us to identify the surface (trapping) states and their dependence on the local structure. Finally, we discuss the influence of bulk and surface defects on photoelectrochemical performance.   

Stabilizing Polar Phases in Binary Metal Oxides by Doping

       Ferroelectric materials have a spontaneous electric polarization that can be switched between two or more orientations with an applied electric field. They enable devices such as nonvolatile random-access memories, ferroelectric field-effect transistors, and ferroelectric tunnel junctions. The discovery of ferroelectricity in simple binary metal oxides, such as Ga₂O₃, HfO₂ and ZrO₂, is attractive as they can be easily integrated with Si devices. However, the ferroelectric phase in these binary metal oxides is usually metastable and can only be stabilized on proper substrates under specific growth conditions. Here, we show that doping in these binary metal oxides can efficiently modulate the relative stability of their polar and non-polar phases. By using a polarized charge-spring model and DFT calculations, we show that doping facilitates charge redistribution on anion lattice because of the difference of their on-site Coulomb energy. We further show that experimentally the doping enhanced stabilization of the polar phase can be achieved using cation- or anion-substitution, interlayer charge transfer or even by the formation of point defects.     

Influence of rapid thermal annealing on nitrogen incorporation in GaAsN alloys

GaAsNBi alloys are promising for infrared optoelectronic devices due to their bandgap tunability without significant lattice parameter variation. N-related defects, which often limit device performance, can be manipulated by rapid-thermal annealing (RTA).  For example, RTA has been shown to suppress persistent photoconductivity, increase photoluminescence efficiency, and improve electron mobility in GaAsN.  Although RTA-induced decreases in [(N-N)As] have been suggested, direct evidence has not yet been presented.  Here, we report on the influence of RTA on the dominant N interstitial incorporation and dissociation mechanisms for GaAsN alloys with N compositions ranging from 0.006 to 0.025.  X-ray rocking curves suggest that GaAsN films are coherently strained and compositionally stoichiometric, with minimal RTA-induced out-diffusion.  Raman spectroscopy reveals a RTA-induced reduction in vibrational modes associated with (N-N)As.  Finally, channeling nuclear reaction analysis reveals an RTA-induced decrease (increase) in the total [100] ([111]) yields, suggesting an increase in the fractions of Nsub and (N-As)As interstitial complexes. We will discuss these results along with a comparison of combined Monte-Carlo-molecular-dynamics simulations of channeling yields.   

Design of Defect-Tolerant Materials for Photovoltaic Applications

What makes a good photovoltaic (PV) material? The emergence of hybrid halide perovskites as a class of PV absorbers has spurred researchers to reconsider conventional wisdom about this question. Prior to 2015, every research cell exceeding 20% efficiency had been composed of exclusively inorganic materials synthesized at extreme purity either from the melt or via vapor deposition. Hybrid organic-inorganic perovskites are fabricated via solution synthesis, leading to a microstructure riddled with defects, yet their device efficiency now routinely rivals that of conventional materials. How can such a “messy” material achieve such high performance?

The answer that has emerged to this riddle is the notion of defect tolerance: the idea that in certain materials, naturally abundant (i.e. low formation-energy) defects simply aren’t as detrimental to device performance as defects in materials like silicon or gallium arsenide. A detailed understanding of this phenomenon is vital to the future of PV materials design, especially given the continuing challenges of toxicity and instability of hybrid perovskite materials.

In this talk, I will discuss figures of merit for PV absorbers broadly and how defect tolerance factors in. I will outline contributions from myself and others in the evolving theory of the origins of defect tolerance as well as what questions remain unanswered. Finally, I will discuss approaches for accelerating the design and screening of novel PV materials, and the importance of tight connections between theoretical/computational and experimental work in this effort.   

X-ray Microscopy Studies on Dopant Activation and its Correlation to Current Collection Efficiencies in CdTe Solar Absorbers

Copper and arsenic have been used as primary dopants in CdTe photovoltaic absorbers. However, both atomic species suffer from very low dopant activation.  Furthermore, Cu and As concentrations in these devices are on the order of 10¹⁴ cm⁻³ and 10¹⁶ cm⁻³, respectively, which makes it notoriously difficult to correlate nanoscale distributions to the local charge transport properties. To measure and correlate these properties, measurement techniques require high sensitivity to elemental concentration, large penetration depth, and operando compatibility.  In this work, we use nanoscale X-ray microscopy to correlate chemical distribution – structure – electrical properties through X-ray Fluorescence, X-ray Absorption Near edge spectroscopy, and X-ray Beam Induced Current at a pixel-to-pixel level (50-120nm).

Our cross-section studies unveil the segregation of Cu and As and the transition of the maximum charge-collection efficiency from the ZnTe-CdTe interface to the CdS-CdTe interface as a function of processing conditions, which is correlated to the balance of acceptors and donors, and thus the activation of the dopants. We will show linear combination fitting of XANES and the presence of active species (e.g. Cu_Cd) as well as inactive species through the absorber.   

Revisiting the role of group-V acceptor impurities and their AX centers in CdTe

CdTe thin-film solar cells are commercially available technology, with active layers that are just a few microns thick and record power conversion efficiency of over 22%. Further improvements will rely on increasing hole concentration and carrier lifetimes, and reducing recombination rates.  At the  heart of this problem is the behavior of acceptor impurities such as Sb, As, and P.  While early theoretical results indicated a rather high ionization energy of 230 meV for Sb substituting on the Te site, and likely formation of AX compensation centers, leading to low hole concentrations, recent experimental observations indicate a much lower ionization energy of ~100 meV, and put into question the hole compensation by the formation of AX centers.  Understanding the behavior of these group-V impurities is crucial for designing the growth and processing of devices. Using electronic structure calculations based on hybrid density functional theory, we investigate formation energies and acceptor transition levels of P, As, and Sb in CdTe, and explore the formation of related AX centers. We pay special attention to the inclusion of spin-orbit coupling in the calculations and the size of the supercells to describe these acceptor impurities and related AX centers.  Our results indicate lower ionization energies than previously reported, and that AX centers are unlikely to play important role in the compensation of p-type conductivity.  Compensation by acceptor native defects are proposed to explain the experimental data.

This work was supported by the US DOE EERE Solar Energy Technologies Office, grant no. DE-EE0009344, and made use of NERSC supercomputer facility.   

Unconventional electronic transport near the insulator-metal transition in Co-doped pyrite FeS2 single crystals

Pyrite FeS₂ is a low-cost, sustainable, non-toxic 1-eV-gap semiconductor with unrealized potential in applications such as photovoltaics. From the fundamental perspective, issues such as surface conduction and the deep-donor nature of sulfur vacancies have strongly hindered the study of low T transport and phenomena such as the insulator-metal transition (IMT). In this work, a novel CoS₂-based contact scheme is used to access bulk transport, enabling wide-T-range transport studies of thoroughly characterized Co-doped FeS₂ single crystals. The IMT is found to occur near 3 × 10¹⁷ cm^-3 Hall density, with Efros-Shklovskii variable-range hopping below this, and electron-electron-interaction-corrected conductivity above this. In the vicinity of the IMT a host of intriguing phenomena emerge, including non-linear Hall effect with a non-monotonic T dependence, and non-saturating non-parabolic positive low-T magnetoresistance. Quantitative analysis of these effects points to strong effects of disorder in the vicinity of the IMT.   

First-principles study on enhancement of the oxygen diffusivity in La2−xSrxCuO4 by cation substitution and strain

Sr-doped lanthanum cuprate (La2−xSrxCuO4), one of the most studied high-TcTc superconductors, has drawn much attention as a candidate for cathode material in solid oxide fuel cells (SOFCs) due to its fast oxygen evolution capability. Although its large ionic conductivity has been phenomenologically explained by diffusion channels formed by interstitial oxygen and oxygen vacancy, its atomistic and systematic mechanism has not been understood clearly. Using density functional theory calculation, we identify vacancy diffusion paths with low activation energies leading to anisotropic oxygen diffusivity. It is found that not only the activation energy but also the formation energy of oxygen vacancy, especially at an apical site, can be drastically reduced by applying biaxial strain. We further reveal that Sr substitutional doing plays two competitive roles in the oxygen diffusivity: (1) increasing the equilibrium concentration of oxygen vacancy enhancing diffusion channel and (2) trapping oxygen vacancy around a Sr atom raising the activation energy. Our study would be applied to improve the efficiency of SOFCs.