CdTe Grain Boundaries Research Articles

Computational design of passivants for CdTe grain boundaries

CdTe is the second most-widely deployed photovoltaic (PV) material due to its high efficiency and low manufacturing costs. Currently, polycrystalline CdTe (poly-CdTe) has a record efficiency of ~22%, which is still well below the theoretical limit (~30%). Polycrystalline CdTe films that have incorporated Cl or Se show higher efficiency, possibly due to the segregation of these ions to the grain boundaries (GBs), where they may passivate the dangling bonds. However, the efficiency enhancement mechanisms of passivants in CdTe GBs, and the feasibility of employing alternative passivants, have not been well explored. Here, we present a systematic computational study of CdTe GBs with potential passivants, namely S, P, As, Se, and Sb on Te sites, and Na, Mg, Al, Sc, Cu, and Zn on Cd sites. Density functional theory (DFT) calculations were performed on GB dislocation core structures derived from scanning transmission electron microscopy (STEM) images of a model GB (bicrystal). We computed the segregation thermodynamics, electronic density of states, and charge variations near doped CdTe GBs. We find that segregation of impurities to GBs is thermodynamically favorable. For a Te-terminated core, Se, S, and P on Te sites effectively reduce midgap states. For both Cd- and Te- terminated dislocation cores, Sc and Al reduce midgap states when substituted for Cd atoms. The greatest improvement was achieved with co-doping, i.e. simultaneously substituting Te with Se and substituting Cd with Cu or Al. The elimination of midgap states is predicted to increase the photovoltaic efficiency of CdTe by reducing the recombination at grain boundaries. • CdTe solar cell efficiency depends highly on grain boundary properties. • Large scale first principles computational study was performed on realistic grain boundary models. • For a Te-terminated core, Se, S, P, and As on Te sites effectively reduce midgap states and improve efficiency. • For both Cd- and Te- terminated dislocation cores, Sc and Al on Cd sites reduce the midgap states and improve efficiency.

Chlorine passivation of grain boundaries in cadmium telluride solar cells

Cadmium telluride is the most commercially important second generation thin-film photovoltaic, with a record solar cell conversion efficiency of 22.1%. However as-deposited cells are 5% efficient and require a cell activation treatment with ${\mathrm{CdCl}}_{2}$ at about 400 ${}^{\ensuremath{\circ}}\mathrm{C}$ to reach commercially viable efficiencies. Such a treatment is a routine process during CdTe module manufacturing. However, the precise mechanisms at work for this remarkable efficiency enhancement are not well understood. In this paper, atomistic modeling techniques are used to improve the fundamental understanding of the structural and electronic properties of CdTe by modeling the effects of chlorine and other elements with their interaction with extended defects and grain boundaries (GBs). Studies at high spatial resolution with nanoscale secondary ion mass spectrometry, transmission electron microscopy (TEM), and energy dispersive x-ray analysis show that chlorine atoms are concentrated at grain boundaries in CdTe after the ${\mathrm{CdCl}}_{2}$ treatment. Density functional theory calculations show that both ${\mathrm{Cl}}_{\mathrm{Te}}$ and ${\mathrm{Cl}}_{i}$ are stabilized at the grain boundaries compared to bulk CdTe. Similar defect formation energies of these defects suggest both will be present at the grain boundaries. As expected, four single-particle levels are present in the $\mathrm{\ensuremath{\Sigma}}3$ (112) GB band gap, which explains the low efficiencies prior to treatment. ${\mathrm{Cl}}_{\mathrm{Te}}$ substitutions passivate one of these levels and partially passivate another two. Remarkably, further addition of ${\mathrm{Cl}}_{i}$ fully passivates the remaining single-particle levels. This passivation of single-particle levels is most likely to be the primary cause of the efficiency enhancement on chlorine treatment. Further to this, alternative halogens were then trialed as activation treatments. All halogens show similar electronic effects and their defect formation energies follow ionic radii trends.

Passivating Grain Boundaries in Polycrystalline CdTe.

Using first-principles density functional calculations, we investigate the structure and properties of three different grain boundaries (GBs) in the solar absorber material CdTe. Among the low ∑ value symmetric tilt GBs ∑3 (111), ∑3 (112), and ∑5 (310), we confirm that the ∑3 (111) is the most stable one but is relatively benign for carrier transport as it does not introduce any new states into the gap. The ∑3 (112) and ∑5 (310) GBs, however, are detrimental due to gap states induced by Te–Te and Cd–Cd dangling bonds. We systematically investigate the segregation of O, Se, Cl, Na, and Cu to the GBs and associated electronic properties. Our results show that co-doping with Cl and Na is predicted to be a viable approach passivating all gap states induced by dangling bonds in CdTe.

Molecular Dynamics Calculations of Grain Boundary Mobility in CdTe

Molecular dynamics (MD) simulations have been applied to study mobilities of Σ3, Σ7 and Σ11 grain boundaries in CdTe. First, an existing MD approach to drive the motion of grain boundaries in face-centered-cubic and body-centered-cubic crystals was generalized for arbitrary crystals. MD simulations were next performed to calculate grain boundary velocities in CdTe crystals at different temperatures, driving forces, and grain boundary terminations. Here a grain boundary is said to be Te-terminated if its migration encounters sequentially … planes, where “·” and “−” represent short and long spacing respectively. Likewise, a grain boundary is said to be Cd-terminated if its migration encounters sequentially … planes. Grain boundary mobility laws, suitable for engineering time and length scales, were then obtained by fitting the MD results to Arrhenius equation. These studies indicated that the Σ3 grain boundary has significantly lower mobility than the Σ7 and Σ11 grain boundaries. The Σ7 Te-terminated grain boundary has lower mobility than the Σ7 Cd-terminated grain boundary, and that the Σ11 Cd-terminated grain boundary has lower mobility than the Σ11 Te-terminated grain boundary.

Molecular statics simulation of CdTe grain boundary structures and energetics using a bond-order potential

The structure and energetics of coincidence site lattice grain boundaries (GB) in CdTe are investigated by mean of molecular statics simulations, using the Cd–Zn–Te bond-order potential (second iteration) developed by Ward et al (2012 Phys. Rev. B 86 245203; 2013 J. Mol. Modelling 19 5469–77). The effects of misorientation (Σ value) and interface plane are treated separately, complying with the critical need for full five-parameter characterization of GB. In addition, stoichiometric shifts, occurring between the inner interfaces and their adjacent atomic layers, are also predicted, revealing the energetic preference of Te-rich boundaries, opening opportunities for crystallography-based intrinsic interface doping. Our results also suggest that the intuitive assumption that Σ3 boundaries with low-indexed planes are more energetically favorable is often unfounded, except for coherent twins developing on {111} boundary planes. Therefore, Σ5, 7 or 9 boundaries, with lower interface energy than that of twin boundaries lying on different facets, are frequently encountered.

Cathodoluminescence spectrum imaging analysis of CdTe thin-film bevels

We conducted T= 6 K cathodoluminescence (CL) spectrum imaging with a nanoscale electron beam on beveled surfaces of CdTe thin films at the critical stages of standard CdTe solar cell fabrication. We find that the through-thickness CL total intensity profiles are consistent with a reduction in grain-boundary recombination due to the CdCl2 treatment. The color-coded CL maps of the near-band-edge transitions indicate significant variations in the defect recombination activity at the micron and sub-micron scales within grains, from grain to grain, throughout the film depth, and between films with different processing histories. We estimated the grain-interior sulfur-alloying fraction in the interdiffused CdTe/CdS region of the CdCl2-treated films from a sample of 35 grains and found that it is not strongly correlated with CL intensity. A kinetic rate-equation model was used to simulate grain-boundary (GB) and grain-interior CL spectra. Simulations indicate that the large reduction in the exciton band intensity and relatively small decrease in the lower-energy band intensity at CdTe GBs or dislocations can be explained by an enhanced electron-hole non-radiative recombination rate at the deep GB or dislocation defects. Simulations also show that higher GB concentrations of donors and/or acceptors can increase the lower-energy band intensity, while slightly decreasing the exciton band intensity.

Self-passivation rule and structure of CdTe Σ3 (112) grain boundaries

The theoretical study of grain boundaries (GBs) in polycrystalline semiconductors is currently stalemated by their complicated nature, which is difficult to extract from any direct experimental characterization. Usually, coincidence-site-lattice models are constructed simply by aligning two symmetric planes ignoring various possible reconstructions. Here, we propose a general self-passivation rule to determine the low-energy GB reconstruction and find new configurations for the CdTe \ensuremath{\Sigma}3 (112) GBs. First-principles calculations show that it has lower formation energies than the prototype GBs adopted widely in previous studies. Surprisingly, the reconstructed GBs show self-passivated electronic properties without deep-level states in the band gap. Based on the reconstructed configurations, we revisited the influence of $\mathrm{CdC}{\mathrm{l}}_{2}$ post-treatment on the CdTe GBs and found that the addition of both Cd and Cl atoms in the GB improves the photovoltaic properties by promoting self-passivation and inducing $n$-type levels, respectively. The present study provides a new route for further studies of GBs in covalent polycrystalline semiconductors and highlights that previous studies on the GBs of multinary semiconductors, which are based on the unreconstructed prototype GB models, should be revisited.

Direct Observation of CdCl2 Treatment Induced Grain Boundary Carrier Depletion in CdTe Solar Cells Using Scanning Probe Microwave Reflectivity Based Capacitance Measurements

Device grade polycrystalline CdTe films, grown by rf-sputtering (yielding up to 13% efficient devices) and close-spaced sublimation (CSS) (yielding up to 16% efficient devices), have been analyzed using scanning microwave impedance microscopy (sMIM). In order to study the effect of Cl on the electronic properties of grain boundaries in CdTe, sMIM based capacitance measurements were performed on the surfaces of as-deposited and CdCl2 annealed CdTe films. The measurements were performed with an sMIM instrument operating at ∼3 GHz probe signal frequency in conjunction with a combination of ac and dc voltage biases applied to the tip. It was found that CdCl2 annealed CdTe samples deposited using both CSS and sputtering exhibit carrier depletion along the grain boundaries as compared to the grain bulk. The grain boundary carrier depletion was not observed in samples that have not been CdCl2 annealed and hence have no incorporated Cl.

Direct observation of electrical properties of grain boundaries in sputter-deposited CdTe using scan-probe microwave reflectivity based capacitance measurements

Polycrystalline CdTe in 12% efficient solar cells has been studied using scanning microwave impedance microscopy (sMIM). The CdS/CdTe junctions were grown on transparent-conducting-oxide-coated soda lime glass using rf sputter deposition. sMIM based capacitance measurements were performed on the exposed surface of CdCl2 treated CdTe adjacent to thermal-evaporation-deposited Cu/Au back contacts. The sMIM instrument was operated at ∼3 GHz, and capacitance measurements were performed as a function of ac and dc voltage biases applied to the tip, with and without sample illumination. Although dc capacitance measurements are affected by sample topography, the differential capacitance measurement was shown to be topography independent. It was found that the grain boundaries exhibit a depleted carrier concentration as compared to the grain bulk. This depletion effect is enhanced under photo-generated carrier separation or under sufficiently large probe tip biases opposite to the majority carrier charge.

Stability and electronic structure of the low-Σ grain boundaries in CdTe: a density functional study

Using first-principles density functional calculations, we investigate the relative stability and electronic structure of the grain boundaries (GBs) in zinc-blende CdTe. Among the low-Σ-value symmetric tilt Σ3 (111), Σ3 (112), Σ5 (120), and Σ5 (130) GBs, we show that the Σ3 (111) GB is always the most stable due to the absence of dangling bonds and wrong bonds. The Σ5 (120) GBs, however, are shown to be more stable than the Σ3 (112) GBs, even though the former has a higher Σ value, and the latter is often used as a model system to study GB effects in zinc-blende semiconductors. Moreover, we find that although containing wrong bonds, the Σ5 (120) GBs are electrically benign due to the short wrong bond lengths, and thus are not as harmful as the Σ3 (112) GBs also having wrong bonds but with longer bond lengths.

Direct evidence of enhanced chlorine segregation at grain boundaries in polycrystalline CdTe thin films via three‐dimensional TOF‐SIMS imaging

AbstractFor more than 25 years, the CdTe photovoltaic research and manufacturing communities have been subjecting CdTe materials to a CdCl2 treatment or activation step to improve performance. However, little work has been carried out using imaging to elucidate the spatial distribution of chlorine in the CdTe devices after this treatment. This work addresses fundamental questions about the spatial distribution of chlorine in the CdTe absorber material after a CdCl2 treatment comparable to industrial practices. We used a state‐of‐the‐art, time‐of‐flight secondary ion mass spectrometer (ION‐TOF GmbH) (Muenster, Germany) with a lateral resolution of about 80 nm to complete three‐dimensional depth‐profiling and imaging of two CdTe devices. The results clearly demonstrate enhanced chlorine concentration along grain boundaries, supporting the hypothesis that chlorine plays an important role in passivating grain boundaries in CdTe solar cells. The results are discussed in terms of possible passivation mechanisms, and the effect of chlorine on grain interiors and grain boundaries. The data are also used to estimate the free energy of segregation of chlorine to grain boundaries in CdTe. Copyright © 2014 John Wiley & Sons, Ltd.

Study of Selected Grain Boundaries in CdTe by Aberration-corrected STEM

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Possible effects of oxygen in Te-rich Σ3 (112) grain boundaries in CdTe

Using density functional theory calculation, we show that oxygen (O) exhibits an interesting effect in CdTe. The Te atoms with dangling bonds in a Te-rich rich Σ3 (112) grain boundary (GB) create deep gap states due to strong interaction between Te atoms. However, when such a Te atom is substituted by an O atom, the deep gap states can be shifted toward the valence band, making the site no longer a harmful non-radiative recombination center. We find that O atoms prefer energetically substituting these Te atoms and induce significant lattice relaxation due to their smaller atomic size and stronger electronegativity, which effectively reduces the anion–anion interaction. Consequently, the deep gap states are shifted to lower energy regions close to or even below the top of the valence band.

Effect of Copassivation of Cl and Cu on CdTe Grain Boundaries

Using a first-principles method, we investigate the structural and electronic properties of grain boundaries (GBs) in polycrystalline CdTe and the effects of copassivation of elements with far distinct electronegativities. Of the two types of GBs studied in this Letter, we find that the Cd core is less harmful to the carrier transport, but is difficult to passivate with impurities such as Cl and Cu, whereas the Te core creates a high defect density below the conduction band minimum, but all these levels can be removed by copassivation of Cl and Cu. Our analysis indicates that for most polycrystalline systems copassivation or multipassivation is required to passivate the GBs.

Understanding the defect physics in polycrystalline photovoltaic materials

We use the combination of high-resolution electron microscopy and density-functional theory to study the atomic structure and electronic effects of structural defects, such as lamellar twins, stacking faults, and double-positioning twin boundaries in polycrystalline photovoltaic materials such as Si, CdTe, and CuInSe 2. We find that individual lamellar twins and stacking faults do not create deep levels in all these materials. However, areas with high density of these defects can form buried wurtzite layers that introduce a barrier to the majority carriers. Double-positioning twin boundaries, which contain dislocation cores, create deep levels in Si and CdTe. Surprisingly, however, they do not create deep levels in CuInSe 2. These results may explain the fact that Si and CdTe solar cells usually require special passivation, whereas CuInSe 2 solar cells do not. Our further study on the passivation effects indicates that grain boundaries in Si cannot be passivated completely by H alone. On the other hand, grain boundaries in CdTe can be passivated well by Cl and I.

Understanding the Beneficial Role of Grain Boundaries in Polycrystalline Solar Cells from Single-Grain-Boundary Scanning Probe Microscopy

The superior performance of certain polycrystalline (PX) solar cells compared to that of corresponding single-crystal ones has been an enigma until recently. Conventional knowledge predicted that grain boundaries serve as traps and recombination centers for the photogenerated carriers, which should decrease cell performance. To understand if cell performance is limited by grain bulk, grain surface, and/or grain boundaries (GBs), we performed high-resolution mapping of electronic properties of single GBs and grain surfaces in PX p-CdTe/n-CdS solar cells. Combining results from scanning electron and scanning probe microscopies, viz., capacitance, Kelvin probe, and conductive probe atomic force microscopies, and comparing images taken under varying conditions, allowed elimination of topography-related artifacts and verification of the measured properties. Our experimental results led to several interesting conclusions: 1) current is depleted near GBs, while photocurrents are enhanced along the GB cores; 2) GB cores are inverted, which explains GB core conduction. Conclusions (1) and (2) imply that the regions around the GBs function as an extension of the carrier-collection volume, i.e., they participate actively in the photovoltaic conversion process, while conclusion (2) implies minimal recombination at the GB cores; 3) the surface potential is diminished near the GBs; and 4) the photovoltaic and metallurgical junction in the n-CdS/p-CdTe devices coincide. These conclusions, taken together with gettering of defects and impurities from the bulk into the GBs, explain the good photovoltaic performance of these PX cells (at the expense of some voltage loss, as is indeed observed). We show that these CdTe GB features are induced by the CdCl 2 heat treatment used to optimize these cells in the production process.

How Polycrystalline Devices Can Outperform Single‐Crystal Ones: Thin Film CdTe/CdS Solar Cells

Grain boundaries (GBs) participate in the photovoltaic energy conversion process in polycrystalline solar cells as efficient photocurrent collectors and transporters, as shown by high‐ resolution characterization of CdTe GBs in CdTe/CdS cells (see Figure). This suggests that structural defects can be advantageous for device performance, if properly designed, even in devices whose operation is based on physics of ideal, perfect solids.

CdSxTe1-x: bulk vapour growth, twin formation and the electrical activity of twin boundaries

CdSxTe1-x has been used as a model substance to investigate crystal growth from the vapour, twinning phenomena and the electrical properties of grain boundaries. A self-selecting vapour growth method produced solid solutions of CdSxTe1-x (x≈0.067) with compositional uniformity of <2{%} of x (i.e. <0.0013) as determined by x-ray diffraction and photoluminescence spectroscopy. The CdSxTe1-x crystals were highly twinned. Statistical tests showed that the twin boundaries are spatially correlated. A model for twin formation based on nucleation and grain size development is proposed. Band bending at grain and twin boundaries in CdTe and in CdSxTe1-x was measured using the scanning electron microscope remote electron beam induced current technique. Boundaries in CdTe have downward band bending, but those in CdSxTe1-x have upward band bending. Decoration of grain boundaries with Te inclusions reduced the band bending effect in CdTe. It is postulated that the grain boundary passivation in CdTe/CdS solar cells is partly caused by enrichment of the grain boundaries in CdTe with CdSxTe1-x, and partly by the Te enrichment that occurs during processing.

Effect of interfacial reactions on the photoelectric performance of In/CdTe/ITO and CrOxCdTe/ITO heterostructures

By evaporating undoped CdTe layers onto indium tin oxide (ITO), followed by a layer of In or CrOx, back-effect photovoltaic cells In/CdTe/ITO and CrOx/CdTe/ITO were prepared. It was shown that reducing the area of grain boundaries in CdTe, bringing the space-charge region farther away from the defect-rich region at the CdTe/ITO interface, and employing CrOx instead of In back contacts raise the quantum efficiency of the devices to 6%.