Electron-beam-induced damages in methylammonium lead triiodide (MAPbI3) perovskite thin films were studied by cathodo­luminescence (CL) spectroscopy. We find that high-energy electron beams can significantly alter perovskite properties through two distinct mechanisms: (1) defect formation caused by irradiation damage and (2) phase transformation induced by electron-beam heating. The former mechanism causes quenching and broadening of the excitonic peaks in CL spectra, whereas the latter results in new peaks with higher emission photon energy. The electron-beam damage strongly depends on the electron-beam irradiation conditions. Although CL is a powerful technique for investigating the electronic properties of perovskite materials, irradiation conditions should be carefully controlled to avoid any significant beam damage. In general, reducing acceleration voltage and probing current, coupled with low-temperature cooling, is more favorable for CL characterization and potentially for other scanning electron-beam-based techniques as well. We have also shown that the stability of perovskite materials under electron-beam irradiation can be improved by reducing defects in the original thin films. In addition, we investigated effects of electron-beam irradiation on formamidinium lead triiodide (FAPbI3) and CsPbI3 thin films. FAPbI3 shows similar behavior as MAPbI3, whereas CsPbI3 displays higher resistance to electron-beam damage than its organic–inorganic hybrid counterparts. Using CsPbI3 as a model material, we observed nonuniform luminescence in different grains of perovskite thin films. We also discovered that black-to-yellow phase transformation of CsPbI3 tends to start from the junctions at grain boundaries.

No abstract

CdTe-based solar cells exhibiting 19% power conversion efficiency were produced using widely available thermal evaporation deposition of the absorber layers on SnO 2 -coated glass with or without a transparent MgZnO buffer layer. Evaporating CdSe and CdTe sequentially by thermal evaporation and subsequent CdCl 2 annealing establishes efffective CdSeTe band grading as well as dense, large-grain films. These results show that high-performance II−VI photovoltaics can be made by inexpensive, commercially available evaporation systems without the need to build customized equipment, enabling CdTe photovoltaics research and manufacturing to be more accessible to the broader photovoltaics community.

Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells.

We conducted cathodoluminescence (CL) spectrum imaging and electron backscatter diffraction on the same microscopic areas of CdTe thin films to correlate grain-boundary (GB) recombination by GB “type.” We examined misorientation-based GB types, including coincident site lattice (CSL) Σ = 3, other-CSL (Σ = 5–49), and general GBs (Σ > 49), which make up ∼47%–48%, ∼6%–8%, and ∼44%–47%, respectively, of the GB length at the film back surfaces. Statistically averaged CL total intensities were calculated for each GB type from sample sizes of ≥97 GBs per type and were compared to the average grain-interior CL intensity. We find that only ∼16%–18% of Σ = 3 GBs are active non-radiative recombination centers. In contrast, all other-CSL and general GBs are observed to be strong non-radiative centers and, interestingly, these GB types have about the same CL intensity. Both as-deposited and CdCl2-treated films were studied. The CdCl2 treatment reduces non-radiative recombination at both other-CSL and general GBs, but GBs are still recombination centers after the CdCl2 treatment.

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.

Halide perovskite solar cells have achieved a certified efficiency of 25.2%, surpassing CdTe and CIGS, which have long been regarded as the most-efficient thin-film photovoltaic materials. As this exciting class of materials continues to mature, researchers will require characterization techniques capable of exposing the interplay between structure, chemistry, and optoelectronic properties to inform processing strategies and increase both device efficiencies and long-term stability. Cathodoluminescence microscopy is an ideal technique to provide such information due to the high spatial resolution and robust optical information acquired. Here, the current body of work related to cathodoluminescence analysis of halide perovskite materials for optoelectronic applications is surveyed. This review demonstrates how cathodoluminescence can monitor degradation due to environmental stressors, phase segregation resulting from material processing, and other halide perovskite-centric material issues. A persistent concern associated with e-beam-based analysis of halide perovskites is what effect the electron beam has on the material properties being probed.Addressing this, a detailed discussion is provided on the origin of the cathodoluminescence signal and a review of studies focused on revealing changes in the properties of halide perovskites resulting from This article is protected by copyright. All rights reserved. e-beam excitation. Finally, a perspective on future opportunities to expand the role of cathodoluminescence analysis for halide perovskites is provided.

Reducing recombination in polycrystalline solar cells by orders of magnitude is currently one of the greatest challenges for increasing thin-film solar cell efficiency to theoretical limits. The question of how to do this has been a challenge for the thin-film community for decades. This work indicates that effective interface passivation is critical. Here, polycrystalline Al2O3/CdSeTe/Al2O3/glass heterostructures are grown, and a combination of spectroscopic, microscopic, and time-resolved electro-optical measurements demonstrates that the interface recombination velocity at alumina/thin-film interfaces can be less than 100 cm/s. This is three orders of magnitude less than typical CdTe interfaces without passivation, commensurate with single-crystal epitaxial CdMgSeTe/CdSeTe/CdMgSeTe double heterostructures, and enables minority-carrier lifetimes in polycrystalline CdSeTe well above 100 ns. Microscopic interfacial electric-field measurements identify the field effect as a potential mechanism for polycrystalline Al2O3/CdSeTe interface passivation. The results provide guidance for modeling and interface passivation in devices and indicate future paths to realize highly efficient thin-film solar cells.