FIG 3 - uploaded by Yong Zhang
Content may be subject to copyright.
Source publication
We apply an array of correlated spatially-resolved techniques, including l-Raman/photoluminescence/
reflectance/laser-beam-induced-current in conjunction with scanning electron microscopy and
atomic force microscopy, to study the impact of the microscopic-scale thickness inhomogeneity of
CdS layer in a Cu2ZnSnSe4 thin-film solar cell. Thicker CdS r...
Context in source publication
Context 1
... 1. As expected and indeed observed, in Fig. 2(f), the CdS rich regions (the large island and dark spots in the optical image) yield much smaller photocurrents, reduced by as much as a factor of 2 or more, than the general area. A few dark spots are circled in both Figs. 2(a) and 2(f) to show the correlation. Furthermore, there is a generally anti-correlation between the reflectance and LBIC data (Figs. 2(c) and 2(f)). These findings suggest that improving the thickness uniformity of CdS will improve the average short-circuit current of the device at least to some extent. More interesting findings come from the laser power dependence of the LBIC mapping data. In Fig. 3 (at 532 nm) and Fig. 4 (at 633 nm), we compare the reflectance mapping with the LBIC mapping of the same area under two represen- tative low and high laser powers. The corresponding histogram plots are also given with the LBIC data being converted into EQEs. By “low power,” we mean the power level that can yield an average EQE comparable to the macroscopic probe, whereas by “high power” the EQE of the general area shows significant degradation, but the power is not as high as to cause permanent damage to the material. Note that the macroscopic specular reflectance from the device surface is rather small, $ 0.85% at 532 nm and 1.2% at 633 nm, because the rough sample surface leads to significant diffuse reflectance or scattering. With the use of high NA objective lens, we were able to capture a major portion of the scattered light, yielding much higher “reflectance,” for instance, 5.1% in average (not including the large CdS island) at 532 nm. For the LBIC mapping data, at the low
...
Similar publications
We have studied the photocarrier localization and recombination dynamics in Cu2ZnSnS4 single crystals at room temperature. The band-gap energy and tail states below the band edge were evaluated by a combination of photoluminescence (PL), PL excitation, photocurrent, and femtosecond transient reflectivity spectroscopy. The photocarriers are rapidly...
We examined the 4 K photoluminescence spectra of over a dozen Cu2ZnSnS4 films and eight devices. We show that samples deficient in zinc show on average a higher quasi donor-acceptor pair (QDAP) density. However, the QDAP density in samples with the same metal composition also varies widely. Devices prepared with similar metal compositions show diff...
Citations
... The μ-HSI technique can also extend its application in other fields where light emitted or reflected from device can be collected. For instance, to efficiently detect defects or failure points so as to improve quality of μ-displays, or, to take place of laser-beam-induced-current mapping in thin-film solar cell study with higher efficiency [12]. In this case, the incident laser shall be filtered out before entering the μ-HSI tester. ...
Based on Microscopic hyperspectral imaging (
$\mu $
-HSI) technique, a measurement technique is proposed to precisely determine two-dimensional (2D) pixel-level photometric and colorimetric mass-distributions of micro-displays (
$\mu $
-displays). Light from halogen lamp coupled into a 1”-integrating cube is used as calibration light source for spectral radiant distribution (SRD). Therefore, optical and colorimetric parameters of each pixel on
$\mu $
-displays can be easily obtained. Active-matrix organic light-emitting diode (AMOLED) and Micro-LED samples are tested by the proposed method and conventional spectroradiometer with satisfactory consistency. Besides, detailed differences between pixels are accessible for further evaluation from the 2D pixel-level luminance and color distributions of
$\mu $
-displays.
... Then, the impact of individual defects is thoroughly analyzed by applying an array of correlative and spatially resolved techniques, including electroluminescence (EL), PL, micro-Raman, and microscale illuminated current-voltage (I-V) characteristics. [22] Finally, the atomic-scale defect structure for the same defects are determined using high-resolution TEM. It has been shown previously that defects may be modified or mutate during device operation [23] or under high density photoexcitation. ...
... The reason is that only a very small area is illuminated in the microscopic measurement, which is approximately equivalent to a circuit with one diode behaving as a solar cell and many parallel diodes under forward bias, or treating the whole nonilluminated area as the electrode area, thus, resulting in higher dark current. [22] This condition tends to reduce V oc and FF or distort the light I-V curve, as evident in Table 1, although to much lesser extent in the current device, because it has an overall lower dark current than other thin-film solar cells. [22] It is interesting that for the two higher laser powers, illuminating a defect-free site with a focused laser beam yields substantially higher efficiency than under uniform illumination of simulated sun light. ...
... [22] This condition tends to reduce V oc and FF or distort the light I-V curve, as evident in Table 1, although to much lesser extent in the current device, because it has an overall lower dark current than other thin-film solar cells. [22] It is interesting that for the two higher laser powers, illuminating a defect-free site with a focused laser beam yields substantially higher efficiency than under uniform illumination of simulated sun light. One reason is that monochromatic irradiance on the long wavelength side of the solar spectrum can yield higher efficiency than the full solar spectrum under the same power. ...
Defects usually degrade device performance. Thus, many techniques and effort are devoted to studying semiconductor defects. However, it is rarely known: i) how individual defects affect device performance; ii) how the impact depends on the device operating conditions; iii) how the impact varies from one defect to another; and iv) how these variations are correlated to the microscopic‐scale defect structure. To address these crucial questions, an array of correlative and spatially resolved techniques, including electroluminescence, photoluminescence, Raman, I–V characteristics, and high‐resolution electron microscopy, are used to characterize dislocation defects in GaAs solar cells. Significantly, the study is carried out in a series mode and in operando. This approach provides quantitative and definitive correlation between the atomistic structure of defects and their explicit effects on device performance, thus giving unprecedented insight into defect physics.
... To obtain the actual laser power absorbed by the sample, surface reflection loss has been corrected by using a white light source and a UV enhanced aluminum mirror of known reflectance. The calibration was performed with a 100Â (NA = 0.9) lens to collect the light of both specular and most diffuse reflection [60]. Reflectivity (R) of MAPbI 3 , CdTe and GaAs were determined to be 0.22, 0.247 and 0.37, respectively at the excitation wavelength 532 nm, which match well with the calculated values from their refractive indexes in literatures [61][62][63]. ...
We compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination, over multiple orders of photo-excitation density. An analytic model is used to describe the excitation density dependence of PL intensity and extract the internal PL efficiency and multiple pertinent recombination parameters. A PL imaging technique is used to obtain carrier diffusion length without using a PL quencher, thus, free of unintended influence beyond pure diffusion. Our results show that perovskite samples tend to exhibit lower Shockley-Read-Hall (SRH) recombination rate in both bulk and surface, thus higher PL efficiency than the inorganic counterparts, particularly under low excitation density, even with no or preliminary surface passivation. PL lineshape and diffusion analysis indicate that there is considerable structural disordering in the perovskite materials, and thus photo-generated carriers are not in global thermal equilibrium, which in turn suppresses the nonradiative recombination. This study suggests that relatively low point-defect density, less detrimental surface recombination, and moderate structural disordering contribute to the high PV efficiency in the perovskite. This comparative photovoltaics study provides more insights into the fundamental material science and the search for optimal device designs by learning from different technologies.
... In this work, we have addressed the above questions through investigating and characterizing the impact of individual dislocations in GaAs and CdTe solar cells by applying an array of correlative and spatially-resolved techniques, including electroluminescence (EL), photoluminescence (PL), Raman, and current-voltage (I-V) characteristics [2]. Furthermore, we have determined the atomic-scale defect structures using highresolution TEM for the same defects that were first characterized with these techniques. ...
... a-Se may also have a weak feature at approximately 235 cm −1 due to the presence of crystalline-phase trigonal Se (t-Se) [21]. The Raman line near 303.6 cm −1 from the finished device S3 is known to be from CdS [22], which is the window layer of the CZTSe solar device. The origin of the broad band ranging from 380 to 390 cm −1 is unclear but common to the three samples. ...
While producing comparable efficiencies and showing similar properties when probed by conventional techniques, such as Raman spectroscopy, photoluminescence, and x-ray diffraction, two thin-film solar-cell materials with complex structures, such as quaternary compound Cu2ZnSnSe4 (CZTSe), may, in fact, differ significantly in their microscopic structures. In this work, laser-induced-modification Raman spectroscopy coupled with high spatial resolution and high-temperature capability is demonstrated as an effective tool to reveal the existence of microscopic scale variations between nominally similar alloys and, thus, to obtain additional structure information beyond what the conventional characterization techniques can offer. Specifically, CZTSe films prepared by sputtering and coevaporation methods that exhibit similar Raman and XRD features are found to behave very differently under high laser power and high-temperature Raman probe because the difference in their microscopic structures leads to different structure modifications in response to the external stimuli, such as light illumination and temperature. They are also shown to undergo different degrees of plastic changes and have different thermal conductivities as revealed by spatially resolved Raman spectroscopy. This technique provides a convenient way to assess and compare materials and provides complementary information for other structural characterization techniques, such as TEM and XRD.
... a-Se tended to have a weak feature at ~235 cm -1 , due to the presence of crystalline phase trigonal Se (t-Se) [18]. Raman line near 303.6 cm -1 from the finished device S3 is known to be from CdS [19], which is the window layer of the CZTSe solar device. The origin of the broad band ranging from 380 to 390 cm -1 is unclear, but common to the three samples. ...
While producing comparable efficiencies and showing similar properties when probed by conventional techniques, such as Raman, photoluminescence and X-ray diffraction, two thin film solar cell materials with complex structures, such as quaternary compound CZTSe, may in fact differ significantly in their microscopic structures. In this work, laser induced modification Raman spectroscopy, coupled with high spatial resolution and high temperature capability, is demonstrated as an effective tool to obtain important structure information beyond that the conventional characterization techniques can offer, and thus to reveal the microscopic scale variations between nominally similar alloys. Specifically, CZTSe films prepared by sputtering and co-evaporation methods that exhibited similar Raman and XRD features were found to behave very differently under high laser power and high temperature Raman probe, because the differences in their microscopic structures lead to different structure modifications in response to the external stimuli, such as light illumination and temperature. They were also shown to undergo different degree of plastic changes and have different thermal conductivities as revealed by spatially-resolved Raman spectroscopy.
... With a 100x objective lens, whose numerical aperture is 0.9, the diffraction limit laser spot size is ~0.72 µm. µ-LBIC measurements were performed using the same system [6]Two gold plated probes are used to carry the LBIC from the solar cell to a current pre-amplifier (SR570) followed by a lock-in amplifier (SR830). With the help of a Si reference cell (PV Measurements, Inc.), the LBIC data were converted into external quantum efficiencies (EQEs). ...
... Histogram of the LBIC mapping in Fig. 1(c) shows that the majority part of the mapped region exhibits high EQEs from 45% to 55%. However, at higher power level, the general area of M3690_13 experienced drastic, about 85%, EQE droop, similar with the results reported in Ref. [6]. With only ~ 3% cell efficiency, the average EQE of M3652_13 is 58.4% at low power level. ...
... Unfortunately, some optical characterization work has been done in a rush without paying sufficient attention to the extreme structural instability, thus yielding results extrinsic to the material. Roughly speaking, if the photodegradation threshold of an ionic compound like Cu 2 ZnSnSe 4 is 2 orders of magnitude lower than covalent or mostly covalent semiconductors like Si and GaAs [10,11], MAPbI 3 is further lower by 2 orders more, as we demonstrate in this work. Therefore, extra caution is required in optical characterization. ...
... [9] Unfortunately, some optical characterization work has been done in a rush without paying sufficient attention to the extreme structural instability, thus, yielded results of extrinsic to the material. Roughly speaking, if the photo-degradation threshold of an ionic compound like Cu2ZnSnSe4 is two-order in magnitude lower than covalent or mostly covalent semiconductors like Si and GaAs, [10,11] MAPbI3 is further lower by two orders more, as to be demonstrated in this work. Therefore, extra-caution is required in optical characterization. ...
By performing spatially resolved Raman and photoluminescence spectroscopy with varying excitation wavelength, density, and data acquisition parameters, we have achieved a unified understanding towards the spectroscopy signatures of the organic-inorganic hybrid perovskite, transforming from the pristine state (CH3NH3PbI3) to fully degraded state (i.e., PbI2) for samples with varying crystalline domain size from mesoscopic scale (approximately 100 nm) to macroscopic size (cm), synthesized by three different techniques. We show that the hybrid perovskite exhibits multiple stages of structure transformation occurring either spontaneously or under light illumination, with exceptionally high sensitivity to the illumination conditions (e.g., power, illumination time and interruption pattern). We highlight four transformation stages (Stage 1 - 4, with Stage 1 being the pristine state) along a primary structure degradation path exhibiting distinctly different Raman spectroscopy features at each stage, and point out that previously reported Raman spectra in the literature reflect degraded structures of either Stage 3 or 4. Additional characteristic optical features of partially degraded materials under the joint action of spontaneous and photo degradation are given. This study offers reliable benchmark results for understanding the intrinsic material properties and structure transformation of this unique category of hybrid materials, and a straightforward method to monitor the structure degradation after the material is used in a device or characterized by other techniques. The findings are pertinently important to a wide range of potential applications where the hybrid material is expected to function in greatly different environment and light-matter interaction conditions.
Despite the long history of research that has focused on the role of defects on device performance, the studies have not always been fruitful. A major reason is because these defect studies have typically been conducted in a parallel mode wherein the semiconductor wafer was divided into multiple pieces for separate optical and structural characterization, as well as device fabrication and evaluation. The major limitation of this approach was that either the defect being investigated by structural characterization techniques was not the same defect that was affecting the device performance or else the defect was not characterized under normal device operating conditions. In this review, we describe a more comprehensive approach to defect study, namely a series mode, using an array of spatially-resolved optical, electrical, and structural characterization techniques, all at the individual defect level but applied sequentially on a fabricated device. This novel sequential approach enables definitive answers to key questions, such as: (i) how do individual defects affect device performance? (ii) how does the impact depend on the device operation conditions? (iii) how does the impact vary from one defect to another? Implementation of this different approach is illustrated by the study of individual threading dislocation defects in GaAs solar cells. Additionally, we briefly describe a 3-D Raman thermometry method that can also be used for investigating the roles of defects in high power devices and device failure mechanisms.
