Suns-Voc Measurements Research Articles

Monolithic Selenium/Silicon Tandem Solar Cells

Selenium is experiencing renewed interest as a promising candidate for the wide bandgap photoabsorber in tandem solar cells. However, despite the potential of selenium-based tandems to surpass the theoretical efficiency limit of single-junction devices, such a device has never been demonstrated. In this study, we present the first monolithically integrated selenium/silicon tandem solar cell. Guided by device simulations, we investigate various carrier-selective contact materials and achieve encouraging results, including an open-circuit voltage of Voc=1.68V from suns-Voc measurements. The high open-circuit voltage positions selenium/silicon tandem solar cells as serious contenders to the industrially dominant single-junction technologies. Furthermore, we quantify a pseudo fill factor of more than 80% using injection-level-dependent open-circuit voltage measurements, indicating that a significant fraction of the photovoltaic losses can be attributed to parasitic series resistance. This work provides valuable insights into the key challenges that need to be addressed for realizing higher efficiency selenium/silicon tandem solar cells. Published by the American Physical Society 2024

Gamma-ray dose threshold for MAPbI3 solar cells.

In this work, we report on the effects observed in MAPbI3 polycrystalline films and solar cells under moderate gamma-ray doses of 3-21 kGy. We applied several instrumental techniques such as photoluminescence spectroscopy, time-resolved photoluminescence, Suns-VOC measurement, and impedance spectroscopy to characterize exposed samples. We observed a nonlinear dependency of such characteristics as PL intensity, career lifetime, ideality factor, and recombination resistance on the exposure dose. Small doses of 3-5 kGy annihilate some of the defect centers in the material, which results in improved carrier extraction and prolonged carrier lifetime, while with larger doses of 10 kGy and above, nonradiative recombination becomes predominant. In this way, we revealed a gamma-ray threshold for MAPbI3 films of around 10 kGy, above which it is not recommended to exploit this material. In space environment, yearly doses rarely exhibit 0.1 kGy (10 krad), and the MAPbI3 material has a sufficient margin of safety for space applications. Moreover, this unusual behaviour opens up the opportunity to use gamma-ray sources as an effective method to improve the quality of defective polycrystalline perovskite films before actual exploitation in an ionizing radiation-free environment.

Advanced carrier lifetime analysis method of silicon solar cells for industrial applications

Over the last few years, the silicon photovoltaic industry has been searching for efficient technologies to reduce the cost of cost/Wpeak by improving their conversion efficiency. However, with the silicon solar cell efficiency approaching the theoretical limit, it has become important to increase the efficiency distribution of cells by managing the loss factors existing in the production process. Although some companies are managing the efficiency distribution through in-line analysis equipment, a preventive action for reducing process defects has not yet been accomplished. To prevent process defects in advance, big data management is required to correlate the process conditions and solar cell parameters. Hence, an analysis method that can simply evaluate the detailed properties of completed solar cells is a key technique in future PV industries. In this paper, we introduce an advanced injection-dependent carrier lifetime analysis logic that can subdivide detailed recombination factors by fitting the carrier lifetime graph obtained through Suns-Voc measurement with a theoretical graph. The industrial applicability of this analysis method was tested with 160 commercially manufactured solar cells, demonstrating the expected quantitative gains of low-efficiency solar cells. The results show that the proposed method helps in determining the process priority for improving the efficiency distribution and provides a research direction for increasing the absolute efficiency.

Analysis of variation in recombination characteristics due to light and heat in industrial silicon solar cells

This work presents a comprehensive analysis of the variation in performance parameters of monocrystalline and multicrystalline silicon solar cells when subjected to illumination at elevated temperature. Higher degradation and better regeneration in performance parameters namely open circuit voltage (VOC), short circuit current density (JSC) and fill factor (FF) are observed for monocrystalline solar cells. Suns-VOC measurement clearly indicate that the recombination current densities within the space charge region (J02) and rest of the solar cells (J01) together play an important role in light and elevated temperature induced degradation (LeTID) and subsequent regeneration characteristics. During the degradation phase, both J01 and J02 changed significantly for monocrystalline and multicrystalline solar cells. During regeneration, even though J01 improved for both the types of solar cells, notable reduction in J02 was observed only for monocrystalline solar cells. FF loss analysis shows that the recombination in the space charge region is majorly responsible for degradation and regeneration in FF. In contrast to most of the previous studies, we report similar reduction in external quantum efficiency (EQE) at 984 nm, 877 nm and 658 nm and propose that the formation of defects related to LeTID are not only in the deep bulk, but are distributed throughout the bulk. Absence of any improvement in EQE after prolonged exposure to light and heat suggests that defect transformation and gettering are not sufficient enough to regenerate JSC in multicrystalline solar cells. EQE maps at 407 nm suggest that LeTID behavior of emitter is different from that of bulk.

In-Situ LID and Regeneration of Al-BSF Solar Cells from Different Positions of a B-Doped Cz-Si Ingot

In this paper, five groups of industrial aluminium back-surface-field (Al-BSF) solar cells were made from silicon wafers from different locations of a B-doped Czochralski silicon ingot. Then, we performed the first LID (45 °C, 1 sun, 12 h), regeneration (100 °C, 1 sun, 24 h), and second LID (45 °C, 1 sun, 12 h) treatments on the cells, and measured the in-situ changes of their I-V characteristic parameters by using an I-V tester during the experiment. The cells were also characterized by Suns-Voc measurement, full-area light beam induced current scanning, and external quantum efficiency measurement at the four breakpoints of treatments (before and after the first LID, after regeneration and the second LID). It was found that the LID and regeneration of the Al-BSF solar cells can be explained by the LID and regeneration reaction of B-O defects and the LID caused by dissociation of Fe-B pairs. After regeneration, the relative decay rate of efficiency decreased from 2.75–3.8% during the first LID to 0.42–1.23% during the second LID, indicating that regeneration treatment (100 °C, 1 sun, 24 h) can improve the anti-LID ability of Al-BSF solar cells.

Bulk defect characterization in metalized solar cells using temperature-dependent Suns-Voc measurements

Extracting the parameters, energy level and electron-to-hole capture cross-section ratio, of efficiency-limiting bulk defects in silicon solar cells is a critical step in identifying those defects and potentially eliminating their impact. Typically, this is achieved on specially prepared test structures. However, in some cases, this is not possible, especially in mass production lines when only completed solar cells are available. In this study, a method that is based on temperature-dependent Suns-V oc measurements is introduced to extract the defect parameters in metalized solar cells. The method is validated by comparing the parameters of the boron-oxygen-related defect extracted from cells and those extracted from wafers using the commonly used temperature- and injection-dependent lifetime spectroscopy. It is shown that this method has the benefit of a more accurate lifetime at low injection levels compared with photoconductance-based lifetime measurement since it is not impacted by minority carrier traps. The proposed technique is then applied to determine the parameters of the defect causing light-induced degradation in gallium-doped silicon solar cells. We determined an energy level, with respect to the intrinsic level, of −0.26 ± 0.04 eV and a capture cross-section ratio of 34 ± 2 for this defect. Finally, a sensitivity analysis is performed by considering the system’s limited measurement temperature range. The findings demonstrate the potential of the temperature-dependent Suns-V oc method as a fast and easy-to-apply method for defect characterization in metalized cells. • Parameters of bulk defects in cells can be extracted using Sun-V oc (T) method. • The method is validated by extracting the parameters of BO-related defects. • The parameters of LID-related defects in Ga-doped cells are extracted with the method. • Sensitivity analysis is performed to assess the limitations of the method.

Spray deposited TiO2 thin films for large-area TiO2/p-Si heterojunction solar cells

Carrier selective contacts (CSC) have the potential to lower the cost of photovoltaic (PV) cells. In the present work p-Si/TiO2 heterojunction solar cells have been fabricated using titanium dioxide (TiO2) as an electron-selective layer. Thin uniform anatase TiO2 films have been deposited using a commercially viable spray deposition technique over large area Si substrates. With a simple architecture of Al (200 nm fingers)/Al (15 nm thin film)/TiO2/c-Si(p)/Ag (200 nm), a highest conversion efficiency of 1.56% has been achieved for the TiO2 carrier selective contact based solar cell with an active area of ∼3.84 cm2. The minority carrier lifetime value of the TiO2 coated Si wafer was found to be less than that of the uncoated wafer, indicating the inability of the anatase TiO2 layer to provide surface passivation. Suns-VOC measurements yielded comparable values of the implied open circuit voltage for both the uncoated and the TiO2 coated Si wafers. Spray deposition technique can be used for scalable fabrication of carrier selective contact based heterojunction solar cells.

Thermal Laser Separation of PERC and SHJ Solar Cells

This article investigates thermal laser separation (TLS) on p-type Czochralski-grown silicon (Cz-Si) passivated emitter and rear cells (PERC) and n-type Cz-Si heterojunction (SHJ) solar cells. The TLS comprises of two laser-based processes: the crack initiation by a scribe laser and the crack propagation by a cleave process leading to separated cells with smooth edge surfaces. The sole impact of the cleave process on the cells' passivation layers is examined by performing it without the initial scribe, not separating the samples. By means of photoluminescence imaging, different cleave laser powers PC on passivated PERC and SHJ cell precursors are investigated, and the optimum PC values are then chosen for the processing of metallized and fired host cells with half cell and shingle cell layouts. Both, the cleave process as well as the complete TLS process are then separately performed in order to investigate the effect of each individual process by SunsVOC measurements taken before and after processing. For monofacial PERC cells, a drop in pseudofill factor ΔpFF = - 0.3%abs is recorded after TLS of the host cell into five shingle cells with 31.35 mm cell width. The effect on bifacial SHJ cells is stronger with ΔpFF = - 2.1%abs. Thereby, the cleave process itself does not induce significant losses in the cell performance for both cell types. The main losses are attributed to the recombination at the newly created unpassivated edges after complete TLS.

Monitoring of Photovoltaic System Performance Using Outdoor Suns-VOC

Summary In-field characterization of photovoltaics is crucial to understand performance and degradation mechanisms, subsequently improving overall reliability and lifespans. Current outdoor characterization is limited by logistical difficulties, variable weather, and requirements to measure during peak production hours. We capitalize on Suns-VOC, which is widely used for laboratory measurements of single solar cells and discuss the barriers in extending the technique to outdoor systems. We demonstrate the normalization of measurements using both backsheet temperature sensors and on-site weather stations. Despite weather variation, VOC, ideality factor, and pseudo fill factor all fall within 1% of the laboratory measurements. It is also demonstrated that monitoring the system VOC at 0.05 to 0.1 suns, during minimal power production, provides a figure of merit that can indicate early degradation of the system. Extensive simulations show that shading portions of a system has minimal effect on measurements, allowing the technique to be used in all weather conditions.

Low-Cost High-Sensitive Suns–$V_{\text{oc}}$ Measurement Instrument to Characterize c-Si Solar Cells

Measuring open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> ) versus light intensity (Suns) in solar cells permits the access of cell performance without the series resistance effect. This article shows the implementation of a Suns-Voc measurement system that consists of an off-the-shelf photograph flash lamp, a digital oscilloscope, and a specifically designed photodetector board with three silicon photodiodes that covers a light intensity measurement range from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> suns. The whole system is controlled via universal serial bus (USB) ports from a personal computer allowing the measurement of cell characteristics (pseudo-dark and pseudo-light current density-voltage curves, as well as the pseudo fill factor and pseudo efficiency) automatically in a very short time.

Device design rules and operation principles of high-power perovskite solar cells for indoor applications

In this work, we report on the design principles of high-power perovskite solar cells (PSCs) for low-intensity indoor light applications, with a particular focus on the electron transport layers (ETLs). It was found that the mechanism of power generation of PSCs under low-intensity LED and halogen lights is surprisingly different compared to the 1 Sun standard test condition (STC). Although a higher power conversion efficiency (PCE) was obtained from the PSC based on mesoporous-TiO2 (m-TiO2) under STC, compared to the compact-TiO2 (c-TiO2) PSC, c-TiO2 PSCs generated higher power than m-TiO2 PSCs under low-intensity (200–1600 Lux) conditions. This result indicates that high PCE at STC cannot guarantee a reliable high-power output of PSCs under low-intensity conditions. Based on the systemic characterization of the ideality factor, charge recombination, trap density, and charge-separation, it was revealed that interfacial charge traps or defects at the electron transport layer/perovskite have a critical impact on the resulting power density of PSC under weak light conditions. Based on Suns-VOC measurements with local ideality factor analyses, it was proved that the trap states cause non-ideal behavior of PSCs under low-intensity light conditions. This is due to the additional trap states that are present at the m-TiO2/perovskite interface, as confirmed by trap-density measurements. Based on Kelvin probe force microscopy (KPFM) measurements, it was confirmed that these traps prohibit efficient charge separation at the perovskite grain boundaries when the light intensity is weak. According to these observations, it is suggested that for the fabrication of high-power PSCs under low-intensity indoor light, the interface trap density should be lower than the excess carrier density to fill the traps at the perovskite's grain boundaries. Finally, using the suggested principle, we succeeded in demonstrating high-performance PSCs by employing an organic ETL, yielding maximum power densities up to 12.36 (56.43), 28.03 (100.97), 63.79 (187.67), and 147.74 (376.85) μW/cm2 under 200, 400, 800, and 1600 Lux LED (halogen) illumination which are among the highest values for indoor low-intensity-light solar cells.

Ceramic Roller Inline Diffusion With Lowest Standard Deviation for Crystalline Si Solar Cells

Crystalline silicon solar cells and modules have dominated terrestrial photovoltaic market. Inline diffusion as an alternative emitter fabrication method has been focused on due to its lower budget cost and higher throughput. In this study, phosphorus emitter with less metallic contamination was realized by means of ceramic roller inline diffusion (CRID) and diluted H3PO4 solution (5%) as a dopant source. A wide range of homogeneous sheet resistance ( R sq) (40–130 Ω/sq) was obtained at temperature increases from 810 to 870 °C with duration fixed on 20 min. A standard deviation (SD) of 2.1 Ω/sq was gained by CRID at the target R sq of 75 Ω/sq compared with the SD of 4.0 Ω/sq by reference tube diffusion (TD). The R sq can be adjusted from 71.1 to 115.6 Ω/sq by a removal of predefined “dead layer” in 1% KOH solution, which allows SD within a range of 2.1 to 5.2 Ω/sq. For a target R sq of 85 Ω/sq, cells with efficiency SD of 0.02% were achieved by CRID, which were reduced by 76.8% relative to that by TD. The results in combination with Suns-Voc measurement indicate that CRID cells with uniform emitters have high potential for efficiency enhancement based on our investigation, which are catering for higher cell performance with multibusbar design in the near future.

Influence of the transition region between p- and n-type polycrystalline silicon passivating contacts on the performance of interdigitated back contact silicon solar cells

Passivating contacts based on thin tunneling oxides (SiOx) and n- and p-type semi-crystalline or polycrystalline silicon (poly-Si) enable high passivation quality and low contact resistivity, but the integration of these p+/n emitter and n+/n back surface field junctions into interdigitated back contact silicon solar cells poses a challenge due to high recombination at the transition region from p-type to n-type poly-Si. Here, the transition region was created in different configurations—(a) p+ and n+ poly-Si regions are in direct contact with each other (“pn-junction”), using a local overcompensation (counterdoping) as a self-aligning process, (b) undoped (intrinsic) poly-Si remains between the p+ and n+ poly-Si regions (“pin-junction”), and (c) etched trenches separate the p+ and n+ poly-Si regions (“trench”)—in order to investigate the recombination characteristics and the reverse breakdown behavior of these solar cells. Illumination- and injection-dependent quasi-steady state photoluminescence (suns-PL) and open-circuit voltage (suns-Voc) measurements revealed that non-ideal recombination in the space charge regions with high local ideality factors as well as recombination in shunted regions strongly limited the performance of solar cells without a trench. In contrast, solar cells with a trench allowed for open-circuit voltage (Voc) of 720 mV, fill factor of 79.6%, short-circuit current (Jsc) of 41.3 mA/cm2, and a conversion efficiencies (η) of 23.7%, showing that a lowly conducting and highly passivating intermediate layer between the p+ and n+ poly-Si regions is mandatory. Independent of the configuration, no hysteresis was observed upon multiple stresses in reverse direction, indicating a controlled and homogeneously distributed breakdown, but with different breakdown characteristics.

Impact of contact integrity during thermal stress testing on degradation analysis of copper-plated silicon solar cells

In this study, silicon solar cells with copper-plated front side metallisation were exposed to long-term reliability thermal stress conditions and the material integrity of the plated contacts after stress testing was investigated using imaging and electrical measurements. Significant voltage ‘bend-back’ was observed in Suns-VOC measurements at high illumination intensities (> 1Sun) following thermal stress testing at 200°C for 500h of laser-ablated cells with a nickel/copper/silver plated front metal grid. Using a combination of focussed ion beam milling, high resolution imaging and energy dispersive X-ray spectroscopy, it was shown that large voids can form between the silver capping layer and the main copper stack during thermal annealing. However, even more revealing was the detection of a new metal layer comprising largely of diffused copper overlying the silver capping. The cause of the Schottky ‘bend-back’ behaviour was theorised to be due to increased contact resistance arising from the voids which are presumed to form as a result of grain boundary diffusion of copper through the silver capping layer. Errors of 5–10% in the determination of pFF from Suns-VOC occur as a result, with the scale of the error dependent on the capping method and sintering conditions. Collapsing the voids was subsequently shown to remove the Schottky behaviour and improve reliability of the fitted diode parameters extracted from Suns-VOC measurements.

Totally Vacuum-Free Processed Crystalline Silicon Solar Cells over 17.5% Conversion Efficiency

In this work, we introduce a totally vacuum-free cost-efficient crystalline silicon solar cells. Solar cells were fabricated based on low-cost techniques including spin coating, spray pyrolysis, and screen-printing. A best efficiency of 17.51% was achieved by non-vacuum process with a basic structure of &lt;AI/p+/p−Si/n+/SiO2/TiO2/Ag&gt; CZ-Si p-type solar cells. Short circuit current density (JSC) and open circuit voltage (VOC) of the best cell were measured as 38.1 mA·cm−2 and 596.2 mV, respectively with fill factor (FF) of 77.1%. Suns-Voc measurements were carried out and the detrimental effect of the series resistance on the performance was revealed. It is concluded that higher efficiencies are achievable by the improvements of the contacts and by utilizing good quality starting wafers.

Magnesium thin film as a doping-free back surface field layer for hybrid solar cells

In this work, a magnesium (Mg) thin film with a low work function is introduced to obtain a downward energy band at the rear surface of a hybrid solar cell to achieve the function of a back surface field (BSF) similar to the conventional n-n+ high-low junction, i.e., favouring the majority carrier transport and suppressing minority carrier recombination. The open circuit voltages (Voc) of the hybrid solar cells with the Mg BSF layer achieve a clear improvement over those containing only the conventional metal electrode (e.g., Ag), resulting in an increase in the power conversion efficiency (PCE) of the hybrid solar cells from 9.2% to 12.3%. The Suns-Voc measurement determined using a Sinton tool gives Voc as a function of light intensity and shows that Voc increases linearly with the light intensity for the solar cell with the Mg BSF but increases first and then stabilises for the cells without the Mg BSF. This result is attributed to a difference in back surface recombination, further confirming the efficacy of an Mg thin film as a BSF. This work indicates that the BSF of hybrid solar cells could be achieved by a doping-free, simple, and room temperature process.

Correlation of the crystal orientation and electrical properties of silicon thin films on glass crystallized by line focus diode laser

In this work, crystallographic orientation of polycrystalline silicon films on glass formed by continuous wave diode laser crystallization was studied. Most of the grain boundaries were coincidence lattice Σ3 twin boundaries and other types of boundaries such as, Σ6, Σ9, and Σ21 were also frequently observed. The highest photoluminescence signal and mobility were observed for a grain with (100) orientation in the normal direction. X-ray diffraction results showed the highest occupancies between 41 and 70% along the (110) orientation. However, the highest occupancies changed to (100) orientation when a 100nm thick SiOx capping layer was applied. Suns-Voc measurement and photoluminescence showed that higher solar cell performance is obtained from the cell crystallized with the capping layer, which is suspected from increased occupancies of (100) orientation.

Heavily phosphorus-doped silicon nanoparticles as intermediate layer in solar cell based on IFO/p-Si heterojunction

The goal of this work is to investigate the effect of heavily phosphorus-doped silicon nanoparticles (NP-n+)Si introduced between the p-type silicon substrate and indium fluorine oxide (IFO) thin film on the performance of the IFO/(NP-n+/pp+)Cz-Si (Czochralski silicon)/indium tin oxide (ITO) heterojunction solar cell. Nanoparticles were deposited on p-Si surface from ethanol-based colloidal suspension by ultrasonic spray coating. The IFO and ITO films were grown by ultrasonic spray pyrolysis. A reference solar cell without nanoparticles with IFO/(pp+)Cz-Si/ITO structure was also fabricated and studied. SEM, EDX, XPS, and FTIR spectroscopy, reflection and external quantum efficiency spectra, Suns–Voc measurements were used for the analysis. It was shown that the phosphorus doping level of (NP-n+) Si is of about ∼3×1020cm−3. After deposition, NPs are covered with SiOx layer which is removed by HF dip. The mean NPs size is of about ∼5.7nm. An increase in duration of NPs deposition from 7 to 19min leads to an increase in density of NPs from 5.6×1011 to 1.06×1012cm−2 and, consequently, to an increase in NPs surface coverage from 10.6% to 27.5%. IFO/(NP-n+/pp+)Cz-Si solar cell with NPs surface coverage of 27.5% have shown noticeably higher power conversion efficiency of 13.2% in comparison with 11.9% efficiency obtained for the reference cell. This result is achieved due to increased open-circuit voltage (571mV in the NPs-based solar cell against 484mV for the reference cell).

Determination of metal contact recombination parameters for silicon wafer solar cells by photoluminescence imaging

In order to reliably extract metal–silicon interface recombination parameters to aid optimization of silicon wafer solar cell designs, a test metallization pattern with regions of varying front metal contact fractions is screen printed to create special test cells, alongside solar cells with a standard H-pattern print. Both the test pattern and H-pattern cells are analyzed using intensity-dependent photoluminescence imaging (Suns-PL), and the H-pattern cells are additionally probed at each busbar to monitor their open-circuit voltage (via Suns-Voc measurements). The resultant data are analyzed in two ways, the first being a simple four parameter graphical fitting to the Suns-PL plots of the regions of interest, and the second being a detailed finite element method (FEM) based simulation and numerical fitting. By accounting for the lateral balancing currents in the FEM based simulation, test patterns with imaginary isolation across the edges of the mini cells could be used, which were the easiest to fabricate. The FEM method is more rigorous and is able to replicate simultaneously the Suns-PL characteristics of both test pattern and H pattern cells, using a common set of recombination parameters.

Comparing n- and p-type polycrystalline silicon absorbers in thin-film solar cells

We have investigated fine grained polycrystalline silicon thin films grown by direct chemical vapor deposition on oxidized silicon substrates. More specifically, we analyze the influence of the doping type on the properties of this model polycrystalline silicon material. This includes an investigation of defect passivation and benchmarking of minority carrier properties. In our investigation, we use a variety of characterization techniques to probe the properties of the investigated polycrystalline silicon thin films, including Fourier Transform Photoelectron Spectroscopy, Electron Spin Resonance, Conductivity Activation, and Suns-Voc measurements. Amphoteric silicon dangling bond defects are identified as the most prominent defect type present in these layers. They are the primary recombination center in the relatively lowly doped polysilicon thin films at the heart of the current investigation. In contrast with the case of solar cells based on Czochralski silicon or multicrystalline silicon wafers, we conclude that no benefit is found to be associated with the use of n-type dopants over p-type dopants in the active absorber of the investigated polycrystalline silicon thin-film solar cells.