In search of hidden defects

The standard ab initio approach of modeling defects is prone to missing the correct structures. This can severely affect theoretical predictions material performance, ranging from photovoltaics catalysis, highlighting necessity an efficient method assess defect configurational landscape. Crystalline play a key role in today’s technology. They “make or break” device performance range important technological applications. Well-known cases include solar panels, batteries electric vehicles, thermoelectrics for waste heat recovery, and LEDs phone screens medical imaging, yet they are also crucial pacemakers, radiation detectors, quantum computers, catalysts, gas sensors, many others.1Huang Y.-T. Kavanagh S.R. Scanlon D.O. Walsh A. Hoye R.L.Z. Perovskite-inspired materials beyond – design devices.Nanotechnology. 2021; 32: 132004Crossref PubMed Scopus (39) Google Scholar These imperfections strikingly ambivalent. act as dopants, creating free carriers boosting electronic conduction, other “killer” centers that quench charge transport. Such detrimental extremely (PV), where traps both electrons holes. promotes non-radiative recombination sabotages photovoltaic performance—to point defect-mediated currently represents largest challenge improving PV efficiencies emerging technologies.1Huang Recently, performance-limiting effects such deleterious were defied by new type material. Astonishingly, immune killer traps! Despite promise these “defect-tolerant” materials, which allow high with low-cost synthesis, their development still its infancy. Researchers have disentangled chemical features underlying resistance,1Huang but we lacking universal principle. While further insights gained, scientists continue tackle present conventional materials.2Park J.S. Kim S. Xie Z. Point engineering thin-film cells.Nat. Rev. Mater. 2018; 3: 194-210Crossref (175) Their dilute concentration—beyond resolution most characterization techniques—hinders definitive identification. It only through synergy experiment theory, latter aiding unmask atomistic origins observed spectroscopic fingerprints, behavior be thoroughly understood. With this in-depth understanding, researchers aim mitigate and, perhaps more exciting, discover novel defect-tolerant materials. Recent years seen major progress ability techniques accurately model behavior, even during cell operation, now predict “defect-limited” efficiency material.3Kim Márquez J.A. Unold T. Upper limit imperfect crystals first principles.Energy Environ. Sci. 2020; 13: 1481-1491Crossref issue are, you might expect, sensitive atomic structure defect. Slightly different geometries change reducing maximum third (e.g., kesterites4Kim Park Identification kesterite cells.ACS Energy Lett. 496-500Crossref (93) Scholar) not affecting it at all perovskites1Huang Scholar). makes identification absolutely reliability prediction. Standard methods calculating defects, however, severe limitation traced back inherent dependence local optimization algorithms on initial configuration. If begins within minimum energy basin (such tetrahedral state Figure 1), gradient-based optimizers will often relax down metastable configuration, failing locate true global minimum. This, combined myriad locally stable configurations may exhibit, poses tremendous identifying ground-state. But, do handful missed structures really make difference? Well, overlooking actual ground-state render subsequent analysis obsolete. lead inaccuracies concentrations resulting erroneous prediction harmless fact traps, vice versa. unfortunately common occurrence therefore compromise derived properties, well any strategy designed improve passivating computed traps. Clearly, presents hindrance robustness predictions. importance properly was recently demonstrated et al.5Kavanagh Rapid Recombination Cadmium Vacancies CdTe.ACS 6: 1392-1398Crossref (7) work highlights structural complexity prolific cadmium vacancy (VCd) CdTe, impact performance. Indeed, exhaustive exploration potential surface (PES), unmasked three distinct low-energy neutral VCd; namely, tetrahedral, Te dimer, bipolaron shown 1. Of these, identify tellurium dimer elusive overlooked works. So why tricky so hard find? As guess, ideal undistorted resided basin, thereby trapping region. In order escape distorted prior geometry relaxation basis intuition electron counting, unravelling Yet, location enough understand mechanism behind carrier capture. paramount consider By introducing intermediate process, accelerates overall electron-hole VCd 7 orders magnitude. Far trivial, findings dramatic implications turn benign trap reduce over 5%. Overall, results demonstrate assessing landscape, search equilibrium low ones. Properly catalytic activity doped ceria. Experimentally, had been doping CeO2 noble metals increased oxygen storage capacity (OSC; store release source power); reason enhancement To address issue, Kehoe al.6Kehoe A.B. Watson G.W. Role lattice distortions divalently CeO2.Chem. 2011; 23: 4464-4468Crossref (160) applied study effect divalent metal dopants quantify variation capacity. Interestingly, increase correlated magnitude distortion introduced metal. leads weakly bound atoms, easier remove, increasing activity. Crucially, found exploring several dopant followed optimization. Without critical, additional step, deep undergone would identified, burying relationship between enhancement. A familiar demonstration comes intrinsic silicon7Mercer J.L. Nelson Wright A.F. Stechel E.B. Ab calculations energetics Si defect.Model. Simul. Eng. 1998; 1-8Crossref (36) DX-centers gallium arsenide8Kundu Otte F. Carrete J. Erhart P. Li W. Mingo N. Madsen G.K.H. Effect chemistry thermal transport GaAs.Phys. 2019; 094602Crossref (5) aluminum alloys, studied detail due optoelectronic subject long-standing controversy regarding (Si) (GaAs, AlGaAs). case, theorists cautiously different, local, low-symmetry Td geometry, alternative explain puzzling experimental observations persistent photoconductivity. C3v DX-center relaxation, alongside nearby various extrinsic could finally reconcile properties macroscopic behavior. highlighted works, structures—and obtaining accurate thermodynamic properties—is widely acknowledged. Often, begin initial, defect, simply hoping does correspond (by considering single state) neglecting metastability. energy-lowering suspected, rely intuition, heuristics, indications distort accordingly, before optimization, arrangements across Yet requires significant manual researcher effort possible certain cases. burdensome limitations urge general, automated, methodology explore energetic landscape reside. prevailing has attacked Arrigoni Madsen.9Arrigoni M. Evolutionary computing machine learning discovering configurations.Npj Comput. 7: 71Crossref (2) developed combining evolutionary (ML) negotiate ground states. Essentially, uses genetic algorithm PES selecting, recombining, mutating set candidate generate lower ones, repeating process until converged solution reached. enhanced ML devised therein number calculations. Besides intelligently exploits localized nature two ways. First, constraining all-atom just surroundings second, larger displacements affected atoms (the nearest neighbors). well-grounded simplifications robust powerful method, tested systems, previously reported successfully identified. valuable progression field, vast computational requirements intricate implementation unsuitable majority investigations time. studies, full (and relevant extrinsic) typically considered, thus requiring efficient, minimal-effort solution. An solve closely related problem undertaken Deskins Pham.10Pham T.D. N.A. Efficient Method Modeling Polarons Using Electronic Structure Methods.J. Chem. Theory 16: 5264-5278Crossref (17) Though strictly focus polarons (localized states), core exceedingly similar present: drastic influence final obtained relaxations, compromising state. simple roughly selectively distorting optimization; essentially, elongating bonds polaronic site neighbors 0.1 Å–0.2 Å. simplicity, efficiently located oxides. Its success modest approaches exploit focusing regions similarity hand begs obvious question: defects? seems likely. Nevertheless, should encompass principles underly Jahn-Teller formation. synergistic leverage knowledge gathered half century research efforts crystalline imperfections, practicality coarse structure-searching techniques. Hence, general easily applicable procedure remains unsolved, waiting ingenious space reliable computationally feasible way. Until then, throughput compromised, limiting our understanding control Hope lost, however. above, worldwide actively tackling problem, accelerating path defect-engineered tailored whether application photovoltaics, cutting-edge technologies.