Inhomogeneous microscopic carrier transport is difficult to study, but important in many condensed-matter applications. For example, the role of grain boundaries (GBs) in polycrystalline ...semiconductors has been controversial for 20 years. In cadmium telluride (CdTe) solar cells, electron-beam-induced current (EBIC) measurements consistently demonstrate enhanced current collection along GBs, which is argued as evidence for interpenetrating CdTe p-n current-collection networks critical to high efficiency. Conversely, cathodoluminescence (CL) measurements consistently indicate that GBs are deleterious low-lifetime regions. Here, we apply transport imaging (TI) in conjunction with spatially correlated EBIC, CL, and scanning Kelvin probe force microscopy measurements to understand carrier drift, diffusion, and recombination in polycrystalline CdTe. We simultaneously observe GB potential wells, reduced carrier lifetime at GBs, and seemingly contradictory enhanced GB current collection, and then describe their coexistence with microscopic TI and physical arguments. The results provide visualization of inhomogeneous transport that is critical to understanding and engineering polycrystalline solar technology.
Display omitted
Inhomogeneous local carrier transport is critical in polycrystalline semiconductorsVisualize local charge transport inside single grains and across grain boundariesCoexistence of carrier drift, diffusion, and recombination in polycrystalline CdTe
Inhomogeneous local carrier transport is critical in polycrystalline semiconductor applications. Xiao et al. resolve the coexistence with microscopic transport imaging and other physical arguments. The results provide visualization of local charge transport inside individual grains and across grain boundaries, which is critical to understanding and engineering polycrystalline solar technology.
Abstract
After a focused effort over the last decade, order‐of‐magnitude improvements in doping and electro‐optical characteristics (radiative efficiency, carrier lifetime, and passivation) have been ...reported for polycrystalline CdSeTe solar cells. Surprisingly, this did not result in higher solar cell voltages regardless of device contacting layers, absorber grading profiles, and other changes in device architecture. From detailed evaluation of radiative emission and carrier dynamics in CdSeTe heterostructures and devices, it is shown that the complexity introduced to the absorber to achieve lifetime and passivation metrics resulted in charge carrier trapping, which now negatively affects CdSeTe absorbers. It is found that the defects with activation energy
E
a
≈ 0.14‐0.22 eV dominate radiative emission and carrier dynamics in undoped CdSeTe, and electronic potential fluctuations with the amplitude
γ
≈ 45–60 meV are present in As‐doped CdSeTe/CdTe. Because of potential fluctuations, radiative voltage is reduced by ≈ −100 mV, to = 1020‐1050 mV (for 1.4 eV bandgap). For record‐efficiency solar cells with
V
OC
≥900 mV, radiative and nonradiative recombination voltage losses are comparable, and future research needs to focus on reducing dopant compensation which causes potential fluctuations. This represents a paradigm shift for CdTe solar cells, with non‐radiative bulk recombination no longer representing a dominant voltage loss pathway.
Here, we report on nm-scale electrical potential imaging throughout As-doped and Cu-doped CdSeTe absorbers using Kelvin probe force microscopy (KPFM). The potential imaging was conducted both ...laterally and vertically on beveled films using ion milling at small glancing angle. KPFM images electrical potential on the beveled surface and assesses defect charging in the subsurface region within a screening length from the beveled surface. We found that the grain boundaries were positively charged and that there were significant potential fluctuations in both grain boundary versus grain interior and intragrain. We further found that these potential fluctuations decreased significantly toward the front interface. Time of flight secondary ion mass spectrometry imaging shows that Se content increased toward the front interface, consistent with Se passivation of defects. The potential fluctuation was induced by defect charging, and the results elucidate different details of the defect configurations and grain structures of the films with different CdCl2 treatment temperatures in the As-doped CdSeTe. The defect configurations in the region near the front interface can be a main factor contributing to the device performance difference. Our potential imaging provides insights about the defects throughout the absorber films, and shows that the potential fluctuation has a direct correlation to the Voc deficit.
Selenium compositional grading in CdTe-based thin-film solar cells substantively improves carrier lifetime and performance. However, where and how recombination lifetime improves has not been studied ...significantly. Here, we deposit a CdSexTe1−x/CdTe bilayer on MgZnO/SnO2/glass, which achieves a short-circuit current density greater than 28 mA/cm2 and carrier lifetimes as long as 10–20 ns. We analyze the grain structure, composition, and recombination through the thickness of the absorber using electron backscatter diffraction, Auger-electron spectroscopy, cathodoluminescence spectrum imaging, and time-resolved photoluminescence microscopy. Despite small CdSeTe grains near the pn-junction and significantly larger CdTe grains in the rest of the film, both time-resolved photoluminescence and cathodoluminescence reveal that the carrier lifetime in CdSeTe alloy regions is longer than in CdTe regions. The results indicate that Se both passivates grain boundaries and improves grain-interior carrier lifetime. However, these effects occur only where there is significant alloying, which is important for bandgap engineering.
CdTe solar cells have reached efficiencies comparable to multicrystalline silicon and produce electricity at costs competitive with traditional energy sources. Recent efficiency gains have come ...partly from shifting from the traditional CdS window layer to new materials such as CdSe and MgZnO, yet substantial headroom still exists to improve performance. Thin film technologies including Cu(In,Ga)Se2, perovskites, Cu2ZnSn(S,Se)4, and CdTe inherently have many grain boundaries that can form recombination centers and impede carrier transport; however, grain boundary engineering has been difficult and not practical. In this work, it is demonstrated that wide columnar grains reaching through the entire CdTe layer can be achieved by aggressive postdeposition CdTe recrystallization. This reduces the grain structure constraints imposed by nucleation on nanocrystalline window layers and enables diverse window layers to be selected for other properties critical for electro‐optical applications. Computational simulations indicate that increasing grain size from 1 to 7 µm can be equivalent to decreasing grain‐boundary recombination velocity by three orders of magnitude. Here, large high‐quality grains enable CdTe lifetimes exceeding 50 ns.
Controlling thin film grain structure is challenging and can help realize solar electricity costs below conventional sources. Here, large columnar CdTe grains are achieved by aggressive recrystallization that circumvents nucleation constraints imposed by depositing on different nanocrystalline layers. The resulting films have reduced grain boundary and bulk recombination, enabling excellent transport, and carrier lifetimes.
Polycrystalline photovoltaic materials offer low costs and good scalability; however, grain boundaries in these materials are extended defects, which can drastically increase carrier recombination. ...Interestingly, three leading polycrystalline thin-film technologies—cadmium telluride (CdTe), CuIn1-xGaxSe2 (CIGS), and perovskite solar cells (PSCs)—passivate absorber surfaces in the same way: via formation of low-dimensional, typically two-dimensional (2D), van der Waals materials. This has primarily occurred serendipitously through process optimization, but in some cases, 2D capping layers are intentionally incorporated to improve device performance. Here, evidence compiled from the literature, supplemented with new data where necessary, is presented to illustrate the existence of 3D/2D interfaces in CdTe, CIGS, and PSCs, and their correlation with improved passivation and device performance. This suggests that 3D/2D passivation might be a heretofore unappreciated key to successful polycrystalline thin-film photovoltaics. Finally, the desired attributes of successful low-dimensional layers are presented with rational design strategies for next-generation polycrystalline solar cells.
Leading polycrystalline thin-film technologies—cadmium telluride (CdTe), CuIn1-xGaxSe2 (CIGS), and perovskite solar cells (PSCs)—are based on very different materials, defect chemistries, and device processing conditions, yet all achieve efficiencies rivaling that of crystalline silicon. We argue that a previously unappreciated key to their shared success is passivated (i.e., benign) surfaces via formation of two-dimensional (2D) van der Waals materials. In CdTe, the 2D material is CdCl2; in CIGS, it is XInSe2 (X = K, Rb, Cs) with X depending on the heavy-alkali post-deposition treatment; and in lead halide PSCs, PbI2 forms naturally, but many new, more stable, 2D materials have recently been incorporated. Importantly, this work shows that the large body of knowledge developed in PSCs on 3D/2D interface formation can, and should, be leveraged by other polycrystalline technologies. In many cases, the formation of 2D materials has been a consequence of empirical process optimization rather than by design. Here, we advocate for the active exploitation of 2D materials and their highly tunable properties both to further enhance the performance of leading technologies and as a strategy to elevate other polycrystalline technologies that have underperformed to date (e.g., GaAs). An important example of the tremendous benefit that can be gained by exploiting 2D material properties is the greatly enhanced device stability of 3D/2D PSCs. By outlining key attributes of successful 3D/2D systems along with rational design strategies for implementation, we invite researchers of diverse backgrounds—not just photovoltaics but functionalized materials more generally—to consider how the advanced field of 2D materials can be leveraged in their unique system. Moreover, this work presents an opportunity for 2D-material researchers to overcome some of their major challenges, such as commercialization, by leveraging the expertise of the photovoltaic industry.
Polycrystalline thin-film solar cells provide the lowest-cost pathway for scalable photovoltaic technologies. However, their many interfaces (i.e., grain boundaries) can drastically increase electron-hole recombination if not passivated (made benign). Here, we show that three of the highest-performing thin-film technologies—cadmium telluride (CdTe), CuIn1-xGaxSe2 (CIGS), and perovskites—all have naturally passivated surfaces via formation of two-dimensional capping layers. Key design strategies are proposed such that this passivation scheme can be employed in polycrystalline functionalized materials more generally.
Mg x Zn 1- x O (MZO) shows great promise to replace CdS as a buffer layer in CdTe-based solar cells. It is more transparent, and the MZO bandgap and electron density can be tuned, thus providing ...flexibility in controlling the conduction band offsets and recombination rates between transparent conductive oxide/MZO and MZO/CdSeTe interfaces. Integrating this material into solar cell devices has been frustrated by the common observation of abnormal current-voltage curves. Simulations indicate that this anomalous behavior can be attributed to front interface barrier effects. Experiments demonstrate that this common MZO interface problem can be resolved experimentally by surface preparation, preheat steps, and removing oxygen during absorber deposition and CdCl 2 treatment. Oxygen during the cell fabrication process is likely to alter MZO properties and MZO/CdSeTe band alignment. After addressing these interface issues and modest optimization, devices with high short-circuit density of 29 mA/cm 2 and efficiency above 16% are demonstrated.
We report x-ray absorption near edge structure (XANES) is a powerful tool to probe the fingerprint of local structures, and when coupled with X-ray microscopy, the small spot size enables one to ...probe very specific regions of interest in a device or material, e.g. interfaces/bulk, different grains, good/bad electrical areas. In this work, we investigate the use of linear combination fitting (LCF) of XANES spectra for the particular case of Cu doping in CdTe. We show that the experimental data seem to be accurately represented by standards of Cu2Te and its substoichiometric counterpart, Cu1.43Te. We use Cu in CdTe as a case study to evaluate the accuracy of linear combination fitting using simulated standards, given that experimental standards for certain phases (e.g. Cu1.43Te) or defect structures (e.g. vacancies) cannot be readily obtained. We discuss how spectral features of the FEFF9-simulated standards, fitting ranges, and noise levels all dictate the accuracy of this type of analysis. We show that the greater the spectral difference between the two standards, the better the LCF is able to differentiate between the two structures and to tolerate experimental noise. Finally, we estimate the error of the fitted weights for different spectral features and noise levels and propose a framework to study local structures semi-quantitatively by using binary mixtures of FEFF9-simulated standards.
PDF
