Deep Donor Defect Research Articles

Mechanism of Defect Passivation in Sb2Se3 Solar Cells via Buried Selenium Seed Layer

AbstractQuasi‐1D antimony selenide (Sb2Se3) is known for its stable phase structure and excellent light absorption coefficient, making it a promising material for high‐efficiency light harvesting. However, the (Sb4Se6)n ribbons align horizontally, increasing defect interference and limiting vertical carrier transport. Herein, a novel strategy of burying selenium (Se) seed layers to reduce lattice mismatch at the heterojunction interface, promote crystal orientation, mitigate deep donor defects, increase P‐type carrier concentration, and purify the PN junction, is proposed. Admittance spectroscopy reveals that Sb2Se3 solar cells with Se seed layers have higher activation energies for defect states and significantly lower defect densities (1.2 × 1014, 2.7 × 1014, and 1.3 × 1015 cm−3 for D1, D2, and D3) compared to an order of magnitude higher densities in Sb2Se3 solar cells without a Se seed layer. First‐principles calculations support these findings, showing that Se seed layers create a Se‐rich environment, reducing selenium vacancies (VSe), antimony on selenium sites (SbSe), and interface defects. This dual passivation mechanism suppresses defect formation and activation, increasing carrier concentration and open‐circuit voltage (VOC). Ultimately, employing this novel method, a VOC of 498.3 mV and an efficiency of 8.42%, the highest performance reported for Sb2Se3 solar cells prepared via vapor transport deposition (VTD), are achieved.

Comprehensive study of photoluminescence and device properties in Cu2Zn(Sn1−xGex)S4 monograins and monograin layer solar cells

Ge-alloying of Cu2ZnSnS4 is a promising strategy for producing wide-bandgap absorber materials suitable for use in tandem structures or indoor solar cell applications. Incorporating Ge can suppress Sn-related defects while maintaining the kesterite structure and p-type conductivity throughout the entire range of composition. In this study, Cu2Zn(Sn1−xGex)S4 monograins were synthesized in molten salt by the synthesis-growth method, where value x was varied from 0 to 1, with a step of 0.2. The inclusion of Ge into the crystals was confirmed by energy-dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray diffraction analysis. The bandgaps of Cu2Zn(Sn1−xGex)S4 solid solutions were determined as Eg = 1.50–2.25 eV by external quantum efficiency measurement. A detailed study of temperature and laser power-dependent photoluminescence (PL) for powder crystals with x = 0, x = 0.2 and x = 0.4 revealed that the dominant recombination mechanisms originate from defect clusters involving a shallow acceptor and deep donor defect. Ge-alloying helped to suppress the harmful SnZn donor defects, but at the same time shifted the defects' energy levels deeper into the bandgap. The obtained activation energies indicate that the acceptor defect becomes shallower with the inclusion of Ge. At the mid-temperature range (T = 60–200 K), the presence of two different recombinations was revealed in the system, originating from very closely located PL emission bands.

Quasiparticle self-consistent GW band structures and phase transitions of LiAlO2 in tetrahedrally and octahedrally coordinated structures

A first-principles computational study is presented of various phases of LiAlO$_2$.The $\beta$ and $\gamma$ tetrahedral phases are found to be very close in energy with the $\gamma$ phase having the lowest energy. The octahedral $\alpha$ phase is a high-pressure phase and the transition pressure from the $\gamma$ and $\beta$ phases to $\alpha$ is determined to be about 1 GPa. The electronic band structures are determined using the quasiparticle self-consistent (QS) $GW$ method. The effective masses of the band edges and the nature of the band gaps are presented. The lowest energy $\gamma$ phase is found to have a pseudodirect gap of 7.69 eV. The gap is direct at $\Gamma$ but corresponds to a dipole forbidden transition. The imaginary part of the dielectric function and the absorption coefficient are calculated in the long-wavelength limit and the random phase approximation, without local field or electron-hole interaction effects for each phase and their anisotropies are discussed. Si doping on the Al site is investigated as a possible $n$-type dopant in $\gamma$-LiAlO$_2$ using a 128 atom supercell corresponding to 3.125 \% Si on the Al sublattice in the generalized gradient approximation and a smaller 16 atom cell with 25 \% Si in the QS$GW$ approximation. The Si is found to significantly perturb the conduction band and lower the gap but a clearly separated deep donor defect level is not found. However, the donor binding energy is still expected to be relatively deep, of order a few 0.1 eV in the hydrogenic effective mass approximation.

Gating of Two-Dimensional Electron Systems in (In,Ga)As/(In,Al)As Heterostructures: The Role of Intrinsic (In,Al)As Deep Donor Defects

We present an analysis of gated $(\mathrm{In},\mathrm{Ga})\mathrm{As}$/$(\mathrm{In},\mathrm{Al})\mathrm{As}$ heterostructures, a device platform to realize spin-orbitronic functionalities in semiconductors. The phenomenological model deduced from our magnetotransport experiments allows us to correlate the gate response of the two-dimensional electron systems with the design parameters of the heterostructure, in particular the indium concentration. We explain the occurrence of metastable electrostatic configurations showing reduced capacitive coupling and provide gate-operation strategies to reach classical field-effect control in such heterostructures. Our study highlights the role of the intrinsic $(\mathrm{In},\mathrm{Al})\mathrm{As}$ deep donor defects, as they govern the dynamics of the electrostatic response to gate-voltage variations through charge trapping and unintentional tunneling.

Vacancy Defects in Ga2O3: First-Principles Calculations of Electronic Structure.

First-principles density functional theory (DFT) is employed to study the electronic structure of oxygen and gallium vacancies in monoclinic bulk β-Ga2O3 crystals. Hybrid exchange–correlation functional B3LYP within the density functional theory and supercell approach were successfully used to simulate isolated point defects in β-Ga2O3. Based on the results of our calculations, we predict that an oxygen vacancy in β-Ga2O3 is a deep donor defect which cannot be an effective source of electrons and, thus, is not responsible for n-type conductivity in β-Ga2O3. On the other hand, all types of charge states of gallium vacancies are sufficiently deep acceptors with transition levels more than 1.5 eV above the valence band of the crystal. Due to high formation energy of above 10 eV, they cannot be considered as a source of p-type conductivity in β-Ga2O3.

Effect of oxygen vacancies on the electronic structure and dielectric properties of SrAl2O4: A first-principles study

In this study, both the generalized gradient approximation (GGA) and the GGA-1/2 methods were applied to examine the effect of oxygen vacancies on the electronic and optical properties of bulk monoclinic SrAl2O4. Since the monoclinic structure of SrAl2O4 has two non-equivalent sites for oxygen atoms, we can distinguish between two kinds of oxygen vacancies (VO) in this material. These two kinds of VO are labeled VOI and VOII. Our results show that although these two types of vacancies have comparable formation energies, their electronic and optical properties are significantly different. The formation energy of the VO defect under Sr-rich, Al-rich, and O-poor conditions was less than 1 eV, which means that it could exist at high concentrations. Moreover, the results of density of states (DOS) analysis show that the oxygen vacancy had a deep donor defect featuring a fully occupied electronic level located deep in the band gap of SrAl2O4. This electronic level was located at Ev+2.9eV in case of VOI and at Ev+3.3eV in case of VOII. This defect induced a noticeable change in the spectrum of optical absorption of SrAl2O4. Our results show that the formation of oxygen vacancies can significantly enhance the UV absorption-related properties of this material.

An Ab Initio Perspective on the Key Vacancy Defects of KMgF3

Perovskite-type KMgF3, which represents a distinct class of multifunctional materials, has drawn immense interest in the advanced technological fields due to its various attractive properties. However, these properties are strongly influenced by the presence of various vacancy point defects in the host crystal structure. This drives us to gain a detailed knowledge of the defect chemistry in KMgF3 by investigating geometry, electronic structure, defect formation energies, and charge transition levels. To achieve reliable results, we have adopted hybrid density functional calculations. To find out the stable growth condition for all possible vacancy defects and vacancy cluster defects in KMgF3, calculations of defect formation energies under different chemical potential range have been carried out. Among all the vacancy defects considered in this study, the calculated formation energy for VMg in a −2 charge state is found to be a more negative value as the Fermi energy level approached toward the conduction band minimum irrespective of the chemical potential range. The present study explored the role of different vacancy defects in creating impurity states in the band gap region by detailed analysis of electronic structure in each case. It has been observed that the impact of anion vacancy and their aggregation on the electronic structure of KMgF3 is much more prominent than that of cation vacancy. The electrical and optical properties of KMgF3 in the presence of various defects have been described on the basis of calculated thermodynamic and optical charge transition levels, respectively. It has been revealed that the cation vacancy defects are not likely to form deep defect states, while anion vacancy and their aggregation act as deep donor defects. The mixed cluster vacancy of K and F pair behaves differently with respect to Mg and F pair in the optical property. The cation vacancies have hardly any contribution to the observed optical spectrum, which is consistent with experimental observation. Thus, an unambiguous and ultimate clarification of the fundamental absorption and emission processes in KMgF3 has been accomplished in the present study.

Ab-Initio Calculations of Oxygen Vacancy in Ga2O3 Crystals

Abstract Gallium oxide β-Ga2O3 is an important wide-band gap semiconductor. In this study, we have calculated the formation energy and transition levels of oxygen vacancies in β-Ga2O3 crystal using the B3LYP hybrid exchange-correlation functional within the LCAO-DFT approach. The obtained electronic charge redistribution in perfect Ga2O3 shows notable covalency of the Ga-O bonds. The formation of the neutral oxygen vacancy in β-Ga2O3 leads to the presence of deep donor defects with quite low concentration. This is a clear reason why oxygen vacancies can be hardly responsible for n-type conductivity in β-Ga2O3.

Defect Engineering in Earth‐Abundant Cu2ZnSn(S,Se)4 Photovoltaic Materials via Ga3+‐Doping for over 12% Efficient Solar Cells

AbstractThe efficiency of earth‐abundant Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is considerably lower than the Shockley–Queisser limit. One of the main reasons for this is the presence of deleterious cation disordering caused by SnZn antisite and 2CuZn+SnZn defect clusters, resulting in a short minority carrier lifetime and significant band tailing, leading to a large open‐circuit voltage deficit, and hence, low efficiency. In this study, Ga‐doping is used to increase the CZTSSe solar cell efficiency to as high as 12.3%, one of the highest for this type of cells. First‐principles calculations show that the preference of Ga3+ occupying Zn and Sn sites has a benign effect on suppressing the formation of the SnZn deep donor defects by upwardly shifting the Fermi level, which is further confirmed by deep‐level transient spectroscopy characterization. Besides, the Ga dopants can also form defect‐dopant clusters, such as GaZn+CuZn and GaZn+GaSn, which also have positive effects on suppressing the band‐tailing states. The defect engineering via Ga3+‐doping may suppress the band‐tailing defect with a decreased Urbach energy, elevate the minority carrier lifetime, and in the end, enhance the VOC from 473 to 515 mV. These results provide a new route to further increase CZTSSe‐based solar cell efficiency by defect engineering.

Study of point defects in wide-bandgap Cu2CdGeS4 microcrystals by temperature and laser power dependent photoluminescence spectroscopy

We present temperature and laser power dependent photoluminescence (PL) study of high quality wide-bandgap Cu2CdGeS4 microcrystals. At T = 10 K three PL bands were detected at about 1.919 eV (#1), 1.855 eV (#2) and 1.748 eV (#3). The temperature and laser power dependencies indicate that the properties of PL bands can be explained by donor- acceptor pair model, where the #1 and #2 bands result from a recombination between distant pairs involving the same shallow acceptor VCu with EA ≈ 30 meV and different deep donor defects. The #3 PL band originates from the deep donor-deep acceptor pair recombination where the depth of deep acceptor defect is more than 157 meV. Detailed analysis of the PL spectra show the absence of deep potential or band gap fluctuations in this material making it suitable for photovoltaic applications.

Intrinsic point defects in lead-free organic inorganic hybrid double perovskite (OIHDP) (MA)2KBiCl6

Organic Inorganic Hybrid Double Perovskite (OIHDP) (MA)2KBiCl6 is an alternative material greatly potential for CH3NH3PbCl3. We have made a report of the narrow and long pentahedral region of (MA)2KBiCl6, which is stably according to chemically potential calculation by the first principles method. It is also found that a deep acceptor defect VMA could be easily formed in such kind of (MA)2KBiCl6, leading to the tendency of intrinsic p-type conductivity. A deep donor defect BiMA is obviously suppressed under a Cl-rich growth environment, and a deep acceptor defect ClMA is more likely to be produced with Cl-poor conditions. These results indicate that an increased abundance of Cl elements is going firmly with a reduced generation of BiMA, while the subsequent situation is that defects in ClMA are easier to be made than those in VMA. A great richness of Cl is necessary for an achieved effect of the inhibited deep defect BiMA, only through which the degraded photoelectric performance caused by deep defects could be avoided. Moreover, combination with the former experimental results, we found that the richness of Cl (halide ion) is necessary for an achieved good performance of solar cell devices.

Origin of Band-Tail and Deep-Donor States in Cu2ZnSnS4 Solar Cells and Their Suppression through Sn-Poor Composition.

By comparing optical spectral results of both Sn-rich and Sn-poor Cu2ZnSnS4 (CZTS) with the previously calculated defect levels, we confirm that the band-tail states in CZTS originate from the high concentration of 2CuZn + SnZn defect clusters, whereas the deep-donor states originate from the high concentration of SnZn. In Sn-rich CZTS, the absorption, reflectance, and photocurrent (PC) spectra show band-tail states that shrink the bandgap to only ∼1.34 eV, while photoluminescence (PL) and PC spectra consistently show that abundant CuZn + SnZn donor states produce a PL peak at ∼1.17 eV and abundant SnZn deep-donor states produce a PL peak near 0.85 eV. In contrast, Sn-poor CZTS shows neither bandgap shrinking nor any deep-donor-defect induced PL and PC signals. These results highlight that a Sn-poor composition is critical for the reduction of band-tailing effects and deep-donor defects and thus the overcoming of the severe open-circuit voltage (Voc) deficiency problem in CZTS solar cells.

The Effect of Indium Doping on Deep Level Defects and Electrical Properties of CdZnTe

CdZnTe (CZT) ingots doped with different concentrations of indium (2 ppm, 5 ppm, 8 ppm, and 11 ppm) were grown by the Vertical Bridgman Method. The charge transport behaviors of CZT wafers were characterized by Thermally Stimulated Current (TSC), Time of Flight technique (TOF) and Current–Voltage measurements (I–V). TSC results indicate that the concentration of deep donor defects $$ {\hbox{Te}}_{\rm{Cd}}^{{ 2 { + }}} $$ is reduced significantly by increasing indium dopant content from 2 ppm to 8 ppm, while that of indium related traps, $$ {\hbox{In}}_{\rm{Cd}}^{ + } $$ and A-centers, is sharply increased. Hecht fitting and TOF results indicate that the electron mobility keeps nearly unchanged for different dopant concentrations in the region between 2 ppm and 5 ppm, but the lifetime increased greatly with increasing indium dopant concentration. Therefore, (μτ)e value was increased with higher indium dopant. The up-shift of Fermi level is also observed in the temperature-dependent I–V result with the increasing of indium dopant content. Large Schottky barriers are found in detectors with higher indium concentration. High voltage x-ray response results show that the channel number shifts to the low energy side for 2 ppm dopant samples compared with best performance 5 ppm dopant samples, while the full-energy peaks are broadened for 8 ppm and 11 ppm dopant samples.

The characteristics of Cu(In, Ga)Se2 thin-film solar cells by bandgap grading

We investigated the characteristics of Cu(In, Ga)Se2 solar cells with bandgap (Eg) grading. Two precursor types were employed: Mo/Cu0.75Ga0.25/In/Ga2Se3 (CIGSe-1) and Mo/Cu/In/Ga2Se3 (CIGSe-2). In CIGSe-1, the range of depths with a high Ga content is wider than that in CIGSe-2; thus, the region in which the main electron-trapping clusters and high-population deep donor defects can form is larger, and the defect density is higher. In the defect energy level range, various other defects and defect clusters exist with a defect density of 2.83 × 1015 cm−3 within the CIGS-1 absorber layer and 2.37 × 1015 cm−3 within the CIGS-2 absorber layer. The average efficiency values are 5.71% for CIGSe-1 and 6.82% for CIGSe-2. Additionally, the average VOC deficit (Eg/q − VOC) values are 0.758 V for CIGSe-1 and 0.731 V for CIGSe-2. As a result, in the 7 CIGSe-2 samples, the open-circuit voltage and efficiencies are improved. Thus, it is demonstrated that appropriate Eg grading in a CIGSe layer with a wider Eg on the back surface can result in improved performance.

Effect of inversion layer at iron pyrite surface on photovoltaic device

Iron pyrite has great potential as a thin-film solar cell material because it has high optical absorption, low cost, and is earth-abundant. However, previously reported iron pyrite solar cells showed poor photovoltaic characteristics. Here, we have numerically simulated its photovoltaic characteristics and band structures by utilizing a two-dimensional (2D) device simulator, ATLAS, to evaluate the effects of an inversion layer at the surface and a high density of deep donor defect states in the bulk. We found that previous device structures did not consider the inversion layer at the surface region of iron pyrite, which made it difficult to obtain the conversion efficiency. Therefore, we remodeled the device structure and suggested that removing the inversion layer and reducing the density of deep donor defect states would lead to a high conversion efficiency of iron pyrite solar cells.

Compensation processes in high-resistivity CdZnTe crystals doped with In/Al

The high resistivity performances for aluminum/indium doped Cd0.9Zn0.1Te crystals (CZT:Al/CZT:In), grown via the modified vertical Bridgman method and under excess Cd/Te conditions, were investigated by the relationship between Al/In dopant behaviors and defects induced by crystal growth. Subsequently, their defects compensation processes responsible for high resistivity were proposed by thermally stimulated current spectroscopy. It was revealed that the donor levels from impurity Al/In are too shallow to account for the Fermi level pinned near the mid-gap of CZT materials. Considering the growth of CZT:Al crystal under Cd-rich condition, the doubly ionized Cd interstitial (Cdi2+) as a deep donor was formed with an activation energy of 0.554eV. Correspondingly, a deep donor level of doubly ionized Te antisite (TeCd2+) located at 0.704eV was observed in CZT:In crystal grown under Te-rich condition. In addition, the Fermi level dominated by deep level defects were evaluated at 0.716eV for CZT:Al and 0.740eV for CZT:In by the temperature-dependent resistivity using current-voltage measurements, which almost approach the mid-gap. We therefore suggest that these deep donor defects (TeCd2+/Cdi2+) can stabilize the Fermi level deep near the mid-gap and thus result in high resistivity.

Achieving high efficiency Cu2ZnSn(S,Se)4 solar cells by non-toxic aqueous ink: Defect analysis and electrical modeling

An extremely environment-friendly aqueous-ink process for Cu2ZnSn(S,Se)4 (CZTSSe) fabrication is reported. The aqueous ink is composed of SnSX nanoparticles, zinc ions from zinc nitrate (Zn(NO3)2), and metal complex made of copper nitrate (Cu(NO3)2) and thioacetamide (C2H5NS). The combinations of negatively surface-charged SnSX nanoparticles and cations form electric double layers and lead to a well-mixed and dispersed aqueous CZTSSe precursor ink, which eliminates the usage of hazard solvent. The results of X-ray diffraction, Raman spectroscopy, scanning electron microscope, and auger electron spectroscopy show that the CZTSSe film possesses a single phase, good crystallinity, and uniform composition after an annealing process. A high cell efficiency of 10.05% is achieved by using this novel aqueous-ink approach. To further investigate the possible root cause of efficiency limitation, the electrical properties of CZTSSe cells are studied. The existence of deep donor defects, which results in low carrier concentration as well as collapsed short-circuit current and fill factor at low temperature, is proposed. Our proposed model suggests that the relatively deep p-type defects (CuZn) as the major carrier source, the existence of deep n-type defects and short diffusion length are the key limitations for achieving high efficiency of CZTSSe solar cells.

Ionization of high-density deep donor defect states explains the low photovoltage of iron pyrite single crystals.

Iron pyrite (FeS2) is considered a promising earth-abundant semiconductor for solar energy conversion with the potential to achieve terawatt-scale deployment. However, despite extensive efforts and progress, the solar conversion efficiency of iron pyrite remains below 3%, primarily due to a low open circuit voltage (VOC). Here we report a comprehensive investigation on {100}-faceted n-type iron pyrite single crystals to understand its puzzling low VOC. We utilized electrical transport, optical spectroscopy, surface photovoltage, photoelectrochemical measurements in aqueous and acetonitrile electrolytes, UV and X-ray photoelectron spectroscopy, and Kelvin force microscopy to characterize the bulk and surface defect states and their influence on the semiconducting properties and solar conversion efficiency of iron pyrite single crystals. These insights were used to develop a circuit model analysis for the electrochemical impedance spectroscopy that allowed a complete characterization of the bulk and surface defect states and the construction of a detailed energy band diagram for iron pyrite crystals. A holistic evaluation revealed that the high-density of intrinsic surface states cannot satisfactorily explain the low photovoltage; instead, the ionization of high-density bulk deep donor states, likely resulting from bulk sulfur vacancies, creates a nonconstant charge distribution and a very narrow surface space charge region that limits the total barrier height, thus satisfactorily explaining the limited photovoltage and poor photoconversion efficiency of iron pyrite single crystals. These findings lead to suggestions to improve single crystal pyrite and nanocrystalline or polycrystalline pyrite films for successful solar applications.

Electronic structures and optical properties of rutile TiO2 with different point defects from DFT+U calculations

The electronic states and formation energies of four types of lattice point defects in rutile TiO 2 are studied using the first-principles calculations. The existence of oxygen vacancy leads to a deep donor defect level in the forbidden band, while the Ti interstitial forms two local states. It is predicted that oxygen vacancy prefers to combine with Ti-interstitial to form V O –Ti i dimer by a partial 3 d electron transfer from the Ti i to its neighboring V O . The charge distribution between a Ti interstitial and its neighboring Ti ions partially shields the Coulomb interactions. Lastly, optical properties of these defective lattices are discussed. • The four types defects including V O , V Ti , Ti i and V O –Ti i pair are studied. • Oxygen vacancy prefers to combine with Ti-interstitial to form V O –Ti i dimer. • The optical absorption of V O –Ti i defect shows two infrared absorption peaks.

Characterization of Zn-doped GaN grown by metal–organic vapor phase epitaxy

The point defects and photoluminescence (PL) spectra of gallium nitride (GaN) epilayers with Mg, Zn, and unintentional doping were investigated in this study. The concentration of point defects (Ga vacancy and its related complexes) in the Zn-doped GaN is consistent with that in the Mg-doped GaN, but lower than that in undoped GaN. It is suggested that Zn (Mg) atoms occupy Ga sites and suppress the formation of Ga vacancies. Comparing the blue luminescence (BL) band intensity of GaN:Zn with that of GaN:Mg, a factor of 10 strong PL intensity demonstrates that a moderate incorporation of Zn to GaN is likely to improve the structural quality of GaN. Detailed studies on 2.93 eV BL band for GaN:Zn reveal that the Zn related BL band behaves as a donor–acceptor pairs character. For the acceptor level, isolated ZnGa with the activation energy of 0.386 eV above the valence band is obtained from temperature-dependent PL measurements, whereas the deep donor defect responsible for the 2.93 eV band is deduced to be 164 meV below the conduction band. An ON–H complex model is suggested to explain the deep donor origin.