[Show abstract][Hide abstract] ABSTRACT: A quaternary oxide, CuSnW2O8 (CTTO), has been predicted by density functional theory(DFT) to be a suitable material for sustainable photovoltaic applications. CTTO possesses band gaps of 1.25 eV (indirect) and 1.37 eV (direct), which were evaluated using the hybrid functional (HSE06) as a post-DFT method. The hole mobility of CTTO was higher than that of silicon. Further, optical absorption calculations demonstrate that CTTO is a better absorber of sunlight than Cu2ZnSnS4 and CuInxGa1−xSe2 (x = 0.5). In addition, CTTO exhibits rigorous thermodynamic stability comparable to WO3, as investigated by different thermodynamic approaches such as bonding cohesion, fragmentation tendency, and chemical potential analysis. Chemical potential analysis further revealed that CTTO can be synthesized at flexible experimental growth conditions, although the co-existence of at least one secondary phase is likely. Finally, like other Cu-based compounds, the formation of Cu vacancies is highly probable, even at Cu-rich growth condition, which could introduce p-type activity in CTTO.
No preview · Article · Dec 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Electron-beam-induced damages in methylammonium lead triiodide (MAPbI(3)) 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 (FAPbI(3)) and CsPbI3 thin films. FAPbI(3) shows similar behavior as MAPbI(3), 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 preview · Article · Nov 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: We present a 2-D numerical model simulating cathodoluminescence (CL) measurements on CdTe. The model is used to analyze the impact of material parameters on the measured CL intensity to establish when grain-boundary (GB) recombination velocity SGB can be determined accurately from CL contrast. In addition to GB recombination, grain size and its ratio to the carrier diffusion length can impact CL measurements. Holding the grain interior and GB recombination rates constant, we find that as the grain size increases and exceeds the diffusion length, the observed CL contrast increases. For small-grain-size material, surface recombination lowers the overall intensity of the CL signal but does not significantly impact CL contrast. For large grains, high-surface recombination velocity can decrease the CL contrast. The model is combined with experimental results to quantify the SGB in polycrystalline CdTe before and after the CdCl2 treatment and to predict the impact of GB recombination on device performance.
Full-text · Article · Nov 2015 · IEEE Journal of Photovoltaics
[Show abstract][Hide abstract] ABSTRACT: Organometal-halide perovskite solar cells have greatly improved in just a few years to a power conversion efficiency exceeding 20%. This technology shows unprecedented promise for terawatt-scale deployment of solar energy because of its low-cost, solution-based processing and earth-abundant materials. We have studied charge separation and transport in perovskite solar cells - which are the fundamental mechanisms of device operation and critical factors for power output - by determining the junction structure across the device using the nanoelectrical characterization technique of Kelvin probe force microscopy. The distribution of electrical potential across both planar and porous devices demonstrates p-n junction structure at the TiO 2 /perovskite interfaces and minority-carrier diffusion/drift operation of the devices, rather than the operation mechanism of either an excitonic cell or a p-i-n structure. Combining the potential profiling results with solar cell performance parameters measured on optimized and thickened devices, we find that carrier mobility is a main factor that needs to be improved for further gains in efficiency of the perovskite solar cells.
[Show abstract][Hide abstract] ABSTRACT: ZnSnN 2 is an Earth-abundant semiconductor analogous to the III-Nitrides with potential as a solar absorber due to its direct bandgap, steep absorption onset, and disorder-driven bandgap tunability. Despite these desirable properties, discrepancies in the fundamental bandgap and degenerate n-type carrier density have been prevalent issues in the limited amount of literature available on this material. Using a combinatorial RF co-sputtering approach, we have explored a growth-temperature-composition space for Zn 1+x Sn 1-x N 2 over the ranges 35–340 • C and 0.30–0.75 Zn/(Zn+Sn). In this way, we identified an optimal set of deposition parameters for obtaining as-deposited films with wurtzite crystal structure and carrier density as low as 1.8 x 10 18 cm-3. Films grown at 230 • C with Zn/(Zn+Sn) = 0.60 were found to have the largest grain size overall (70 nm diameter on average) while also exhibiting low carrier density (3 x 10 18 cm-3) and high mobility (8.3 cm 2 V-1 s-1). Using this approach, we establish the direct bandgap of cation-disordered ZnSnN 2 at 1.0 eV. Furthermore, we report tunable carrier density as a function of cation composition, in which lower carrier density is observed for higher Zn content. This relationship manifests as a Burstein-Moss shift widening the apparent bandgap as cation composition moves away from Zn-rich. Collectively, these findings provide important insight into the fundamental properties of the Zn-Sn-N material system and highlight the potential to utilize ZnSnN 2 for photovoltaics.
Full-text · Article · Sep 2015 · Journal of Materials Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Both randomly oriented and highly (220) textured thin films of zinc phosphide (Zn3P2) were grown by the close-space sublimation method. The effect of deposition parameters, such as pressure and substrate temperature, on the texture evolution has been established. It was found that the deposition temperature plays a dominant role in determining the preferred orientation whereas the ambient pressure (below 10 Torr) does not greatly affect the film texture. We further found that the microstrain changes from tensile at lower deposition temperatures to compressive at higher deposition temperatures. The preferred orientation also had a strong impact on the electrical resistivity of the films. The results provide guidance on the selection of substrate and deposition parameters to grow Zn3P2 thin films with desirable properties.
No preview · Article · Aug 2015 · Journal of Electronic Materials
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Jul 2015 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: We present the latest developments in the characterization of thin-film solar cells based on the combination of elemental mapping from fluorescence measurements using synchrotron x-rays, with beam induced current from electron and x-ray beams. This is a powerful method to directly correlate compositional variations with charge collection efficiency. We compare different approaches for mapping solar cells both in cross-section and in plan view on CIGS and CdTe solar cells. Based on examples from our latest research, we discuss the experimental approaches and highlight the advantages and limitations of each technique. Finally, we present an outlook to experiments that will allow x-ray based characterization to enter new fields of research that were not accessible before.
[Show abstract][Hide abstract] ABSTRACT: Understanding the spatial composition inhomogeneity in CIGS solar cells can provide insight into a potentially large cause of decreased cell efficiency. We used synchrotron based nano x-ray fluorescence and nano x-ray beam induduced current to investigate the correlation between grain-to-grain compositional variations in Cu(In,Ga)Se2 absorber layers and variations in carrier collection efficiency. We found compositional variations as large as 5 at. % and current variations as large as 10%. Grains with higher average copper concentration and lower indium concentration showed increased carrier collection than neighboring grains with lower copper and higher indium concentration. We also investigated the overall composition dependence of carrier collection in CIGS films and show that 30% gallium content devices show a correlation between charge carrier collection, while 60% content devices do not. Index Terms — current measurement, microstructure, photovoltaic cells, synchrotron radiation, thin films
[Show abstract][Hide abstract] ABSTRACT: Three different characterization techniques show a unique region located at the CdS/CIGS interface. a) A line scan of STEM EDS (red arrow in (c)) shows variation of chemistry at the interface: a Cu-poor/In-rich region ~0-50 nm from the junction b) Atom probe tomography chemical data also shows a region ~0-50 nm from junction. Interestingly the relatively large amount of K segregation (~0.5 %) is inversely correlated with Cu/Ga+In. Perhaps, the K concentration may be able to tune carrier concentration at the interface. c) STEM High-Angle-Annular-Dark-Field image shows average contrast of average atomic number (z). There is a clear change in contrast for a ~50nm layer below the junction. This is indicative of a lower-z volume, perhaps due to an ordered vacancy compound where Cu-vacancies would be abundant. Recent progress has been made to Cu(In,Ga)Se 2 thin film solar cells' efficiency, pushing the record to above 21%. The latest advancements have been due in large part to the addition of alkali metals and band gap grading. Cells that currently achieve greater than 20.5%, are using potassium as well as sodium to enhance the effect. Where do these impurities reside? How do matrix element concentrations relate to the addition of alkali metals at the heterojunction and GB's? Another key factor for high efficiency cells is controlling the Ga/(In+Ga) (GGI) and Cu/(Ga+In) (CGI) levels at the CdS/CIGS interface and GB's and thereby the band gap and carrier densities, respectively. How do GGI and CGI relate to the addition of alkali metals?
[Show abstract][Hide abstract] ABSTRACT: ZnSnN2 is an Earth-abundant analog to the III-Nitrides with potential as a
solar absorber due to its direct bandgap, steep absorption onset, and the
possibility to tune its bandgap through introducing disorder into the cation
sublattice. Here we present a combinatorial study of ZnSnN2 that has achieved
dense crystalline films with photovoltaic-relevant carrier concentration. We
report evidence of a Burstein-Moss shift widening the apparent bandgap, and
tunable carrier concentration with cation composition. These results reaffirm
the potential of ZnSnN2 for energy-conversion applications.
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Mar 2015 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: We report on a local potential and resistance mapping of Cu2ZnSnSe4 (CZTSe) films using nm-resolution electrical scanning probe microscopies of scanning Kelvin probe force microscopy and scanning spreading resistance microscopy. We have conducted a comparative study with high-performance Cu2(In,Ga)Se2 (CIGSe) film. Both CZTSe and CIGSe were deposited by co-evaporation of elements in vacuum. The results show that the microelectrical properties of the two polycrystalline materials are similar-higher potential and lower resistance on the grain boundaries (GBs) than on grain surfaces-suggesting inverted GB carrier polarity of these films. The consistent GB properties in contrast to the large difference in photovoltaic output of the two materials suggest that factors other than the GBs are responsible for the low photovoltaic output of CZTSe device.
No preview · Article · Jan 2015 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: We report microscopic characterization studies of wide-bandgap Cu(In,Ga)Se2 photovoltaic thin films using the nano-electrical probes of scanning Kelvin probe force microscopy and scanning spreading resistance
microscopy. With increasing bandgap, the potential imaging shows significant increases in both the large potential features due to extended defects or defect aggregations and the potential fluctuation due to unresolvable point defects with single or a few charges. The resistance imaging shows increases in both overall resistance and resistance nonuniformity due to defects in the subsurface region. These defects are expected to affect open-circuit voltage after the surfaces are turned to junction upon device completion.
No preview · Article · Jan 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: The single-phase stability of Cu
2ZnSnS4 (CZTS), after an intrinsic defect was incorporated in it, has been examined here for the first time based on ab initio calculations. The stability analysis of such a non-stoichiometric-defect incorporated CZTS shows that the single-phase formation is unlikely at thermodynamic equilibrium conditions. In addition, the effective growth condition of CZTS is determined and quantified for all the elements (Cu-poor, Zn-rich, Sn-poor, and S-rich) to extract maximum photovoltaic efficiency from CZTS. These conditions promote (i) spontaneous formation of Cu
), which might benefit p-type conduction, and (ii) the co-existence of ZnS while suppressing other harmful defects and secondary phases. Further, the results presented here explain the unavailability of single-phase CZTS to date.
No preview · Article · Jan 2015 · Journal of Applied Physics