Light- and elevated temperature-induced degradation (LeTID) causes long-term instabilities, especially in passivated emitter and rear cells, leading to severe performance loss of commercial modules. ...Despite the hundreds of LeTID reports available in the literature, there is little consensus regarding the underlying defects and defect formation mechanism responsible for this degradation and its exact electronic properties. Recently, it has been shown that a form of carrier-induced degradation similar to that of LeTID is also observed in some high-purity silicon crystals grown by the float-zone method. In this work, using deep level transient spectroscopy and lifetime spectroscopy, we study the role of nitrogen on the degradation. Intentional contaminated samples with different and known level of nitrogen have been specially grown and characterized for this study. We detect the appearance of a set of majority carrier traps in the degraded state of the samples with activation energies 0.1 (H85), 0.43 (H270A), 0.39 (H270B), and 0.46 eV (H200) with respect to the valence band, from which H270A appears to correlate with the degradation. The results show that the extent of degradation in the nitrogen-rich samples is at least double that of the nitrogen-lean samples.
In this work, we study the electronic properties of the boron-oxygen precursor defect responsible for light-induced degradation in crystalline silicon via deep-level transient spectroscopy. Even ...though this degradation has been known for many decades, and has resulted in severe silicon solar cell degradation, its root cause is poorly understood. The detection of a deep defect correlated to BO degradation has only been reported recently using deep-level transient spectroscopy. The resulting trap has been labeled as H390. In this work, we confirm the existence of this trap and report on the observation of a second defect (H400), which appears to be similarly associated with the BO light-induced degradation. To the best of our knowledge, this level has not been previously reported and could result from a second latent form of the BO light-induced degradation defect, a topic that has been largely debated in the literature. The concentration of both levels increases following dark annealing and decreases following minority carrier injection. The two levels disappear during BO activation and appear during BO deactivation, confirming a link between the observed levels and the latent state of the BO defect. Furthermore, consistent with previous studies, the two traps are metastable and change configurations after hole capture. Finally, we determine the minority carrier capture cross-section of the active BO defect by comparing the latent defect concentration with the minority carrier lifetime.
Abstract
This paper investigates the use of consumer flatbed scanners for the use of monitoring solar cell precursors. Two types of scanners are investigated a contact image scanner and scanners with ...more conventional optical setups. The contact image sensor is found to be more suitable as it does not require additional flat field calibration. The scanners’ ability to monitor variation in sample texture was investigated by monitoring the reflection of multi-crystalline and mono-crystalline textured wafers. For a baseline, a comparison was made to a high-end tool used in industry. Both good qualitative agreement and statistical correlation were achieved between the scanner and industry tool for the isotropic multi-crystalline wafers.
In photoconductance (PC) based carrier lifetime measurement, artificially high values at low-to-medium injection levels are often observed because of the presence of minority carrier traps in the ...sample. In this paper, we demonstrate that the traps in n-type Czochralski silicon are in a transient regime under the typical quasi-steady-state measurement condition. The trap-associated apparent lifetime in such a transient regime is different from the one in the steady-state condition; thus, traps should be analyzed according to their actual measurement condition. We demonstrate and subsequently propose a method to extract the majority carrier capture the cross section of traps from transient PC decay measurements. Based on different assumptions, the proposed method is complementary to the method proposed by Hornbeck and Haynes. The proposed method is demonstrated with transient PC measurements of a trap in n-type Czochralski silicon wafers.
With the remarkable advances in semiconductor processing, devices such as solar cells have fewer and fewer defects that impact their performance. Determination of the defects that currently limit the ...device performance, predominantly by increasing the charge carrier recombination rate, has become more challenging with standard methods like deep level transient spectroscopy. To circumvent this limitation, the photovoltaic community is attempting to use the measurement of the charge carrier recombination rates to identify the remaining defects, as this approach is intrinsically sensitive to the defects that limit the cell's/sample's performance/lifetime. This article reviews this new approach, contrasting it with the developments that have occurred with deep-level transient spectroscopy, finding several critical limitations in the current assumptions, and providing suggestions for an improved strategy.
Epitaxial monolithic III–V/Si tandem solar cells are one of the most promising technologies to be adopted by the industry after the efficiency of the current market dominating single junction silicon ...solar cell saturates at its fundamental limit. One of the key limitations of this technology is the degradation of silicon wafers during in situ annealing in the molecular beam epitaxy chamber. Determining the nature of contaminants in this process is key to improve the efficiency of epitaxial tandem solar cells. However, to date, the nature of contaminants from molecular beam epitaxy chambers remains unknown. In this work, we use photoluminescence imaging, lifetime spectroscopy, and deep level transient spectroscopy to measure the electronic properties of extrinsic impurities incorporated during annealing in the molecular beam epitaxy chamber. Photoluminescence images reveal that at least two impurities diffuse into silicon wafers during molecular beam epitaxy annealing. One is highly localized, while the other one is distributed uniformly across the whole wafer. Phosphorus diffusion is found to confine the localized impurity within the diffused layer but is ineffective at preventing the indiffusion of other impurities. Lifetime spectroscopy shows that metastable impurities with characteristic similar to Cr and CrB in our molecular beam epitaxy annealed silicon wafers. No evidence of Fe or FeB was observed. The emission rates and concentrations of the electrically active defects were measured with deep-level transient spectroscopy: The emission rates of detected defects do not match that of known Cr-related defects.
The absorptance of a silicon solar cell determines the upper limit of its short-circuit current and, thus, its efficiency. Traditional methods used to determine the absorptance require contact to the ...sample or are affected by parasitic absorption. This paper demonstrates that through the combination of the spectral response of photoluminescence with photoluminescence imaging, the contactless determination of spatially resolved absorptance is possible. The demonstrated method is based on the comparison of two photoluminescence images, each acquired using different excitation wavelengths (808 and 1060 nm). Thus, the relative absorptance between these specific wavelengths is determined. Experimental verification of the method is performed with passivated monocrystalline silicon wafers of varying absorptances. The demonstration of spatially resolved extraction of absorptance is then performed with a multicrystalline silicon wafer with a strong lateral variation in absorptance at 1060 nm.
Outdoor Photoluminescence imaging of crystalline silicon photovoltaic modules in full daylight via contactless switching of the operating point was recently demonstrated. That previous method ...requires an optical modulator to be placed on at least two positions of each module under test and two or more separate sets of image pairs need to be acquired and combined into a merged PL image.Here we demonstrate a modified approach which enables outdoor PL image acquisition of an entire module from just one image pair. The method still enables image acquisition without any modification of the electrical wiring within the system but is overall simpler and thereby enables significantly higher sample throughput for outdoor PL image acquisition in PV power plants. It also enables imaging of several PV modules at once. The method relies on partial shading of up to five series connected modules and PL image detection of the remaining non-shaded series connected modules within the same module string. In this paper the underlying principles and technical details of this novel method are discussed. Experimental data demonstrating the feasibility of the method are presented, including PL images acquired on state-of-the art bifacial half-cell PV modules.
