Bifacial Crystalline Silicon Research Articles

Textured anti-reflection and down-conversion composite functional films for high-efficiency solar cells.

Advances in high-efficiency solar cells introduce photon management challenges, including the difficult texturization of flat surfaces and low photon utilization at short wavelengths. While bifacial crystalline silicon solar cells have a front pyramid structure and SiN x layers reduce reflections, managing photons on the flat backside remains a challenge. To enhance light utilization, a soft nanoimprint technique was utilized to create pyramid micro-structured polyurethane films doped with europium (Eu3+) complex. These films, which possess anti-reflection and down-conversion properties, can be applied externally to various high-efficiency solar cells without compromising electrical performance. Research on the backside of bifacial PERC solar cells revealed that the optimal composite functional film increases the integrated current by 5.70%, with a 1.27% gain from down-conversion effects. This specialized film presents a novel approach to interface matching for different types of solar cells.

Corrosion effects in bifacial crystalline silicon PV modules; interactions between metallization and encapsulation

This study addresses the influence of different encapsulation materials on performance losses in bifacial PV modules after extended damp heat testing. The widely used ethylene vinyl acetate (EVA) is compared to polyolefin elastomers (POE) and thermoplastic polyolefins (TPO). We show that modules based on n-type bifacial solar cells (tunnel oxide passivated contact, TOPCON) are more prone to degradation than p-type (Passivated emitter rear cell, PERC) based laminates. More specifically, the front side metallization of these n-type cells corrodes under extended damp heat testing, yielding a decreased fill factor in the I–V characteristics and the appearance of dark spots and dark areas in electroluminescence imaging. The newer generation of encapsulant materials (POE and TPO), slowly replacing EVA in the modules commercialized today, demonstrate an improved protection of the metallization. Apart from the nature of the encapsulant, the cell type and the presence or absence of glass cover plates are also shown to influence the metal grid corrosion. To better understand the mechanisms at play, post-mortem analysis was carried out on small sized degraded PV laminates, by drilling out circular samples (“coring”) that could be analyzed in depth. The analyses revealed a weakened bond between the metallization and the silicon emitter, ultimately causing metal grid delamination. The proposed mechanism behind this delamination starts with the degradation of lead glass, which is part of the cell metallization grid and contains lead oxide (PbO). This compound is susceptible to dissolution in acidic environment.

Electrical performance analysis and optimization of monofacial and bifacial crystalline silicon solar cells

This paper presents the investigations and performance analysis of monofacial and bifacial crystalline silicon solar cells with PC1D simulation software. The fundamental limitation in the monofacial solar cell’s performance is its inability to absorb all the incoming solar radiation since the albedo effect (ground-reflected light that can be captured by the rear of the solar cell) is often neglected. So, the efficiency of the monofacial cell will be lower due to poor and incomplete optical absorption. Bifaciality helps to enhance the capturing of light in the solar cell, which means that the rear of the cell is exposed to solar radiation to produce electrical power. The primary focus of our work is to determine which solar cell offers better device performance and conversion efficiency by analyzing various parameters of the solar cell like surface texturing, emitter doping, bulk doping, minority carrier lifetime, bulk and surface recombination rates, front and rear reflectance, among other parameters. The other parameters are maintained at an optimal range to achieve the highest conversion efficiency. Our work has shown that the bifacial solar cell can be as efficient as 28.15%, which is much better than the 22.65% efficiency of the monofacial solar cell.

Architecture of symmetrical bifacial perovskite/Si/perovskite PV modules and LCOE comparison in bifacial applications

Perovskite on silicon tandem technologies offer a promising way to overcome the efficiency limit of single-junction crystalline silicon modules and are therefore widely studied. Besides the cost reduction potential of using tandem modules in utility-scale PV plants, the application of bifacial PV modules offers additional opportunities to reduce costs by increasing the energy yield. However, the bifaciality factor of tandem PV modules using bifacial crystalline silicon solar cells is typically lower than that of single-junction bifacial modules, considering the asymmetric architecture of the module with higher front-side than rear-side efficiency. In this study, we assess a symmetrical bifacial triple-junction perovskite/silicon/perovskite PV module architecture which allows for a significant improvement of the PV module rear side efficiency. Cost of ownership, energy yield and levelized cost of energy modeling show the superior performance of such modules compared to crystalline silicon and perovskite/silicon tandem modules specifically in vertical mounting applications that are used e.g.in AgriPV or noise protection PV walls on highways.

Implementation of the IEC TS 60904-1-2 Measurement Methods for Bifacial Silicon PV Devices

Bifacial crystalline Silicon (Si) photovoltaic (PV) devices are attracting considerable interest from manufacturers and the market since they can enhance the performance in comparison with traditional monofacial PV devices. Technical specification IEC TS 60904-1-2 was published in 2019 and proposes several characterization methods for bifacial PV device testing based on single-side, double-sided and natural sunlight illumination. This article analyses the advantages, disadvantages, the suitability and the feasibility of the different methods and compares the electrical performance obtained by the proposed approaches at the European Solar Test Installation for the testing of bifacial Si devices. Deviations in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> below 0.9% for the different testing methods under test was obtained except for the double-source approach with a rear reflector (2.59%) where the measurement of the rear irradaince influence the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <u>P</u> </i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> .

Corrosion Effects in Bifacial Crystalline Silicon Pv Modules; Interactions between Metallization and Encapsulation

Corrosion Effects in Bifacial Crystalline Silicon Pv Modules; Interactions between Metallization and Encapsulation

High-efficiency n-type silicon PERT bifacial solar cells with selective emitters and poly-Si based passivating contacts

Bifacial crystalline silicon (c-Si) solar cells have currently attracted much attention due to the front high-efficiency and additional gain of power generation from the back side. Here, we have presented n-type passivated emitter and rear totally-diffused (n-PERT) bifacial c-Si solar cells featuring front selective emitter (SE) and polysilicon (poly-Si) based passivating contacts. The SE formation was scanned with laser doping based on front boron-diffusion p+ emitter. The poly-Si based passivating contacts consisting of nano-layer SiOx of ~1.5 nm thickness grown with cost-effective nitric acid oxidation and phosphorus-doped polysilicon exhibited excellent passivation for high open-circuit voltage. We have successfully achieved the large-area (156 × 156 mm2) n-PERT bifacial solar cells yielding top efficiency of 21.15%, together with a promising short-circuit current density of 40.40 mA/cm2. Theoretical calculation has further demonstrated that the optimal thickness of SiOx nano-layer will increase from 1.5 nm to 1.8 nm if the density of interface defect state decreases by one magnitude from 1 × 1010 cm−2/eV, and the cell efficiency can be improved up to 24.64% with open-circuit voltage over 0.720 V by optimizing the parameters of functional materials and interface layers. The present work has indicated that the commercialization of low-cost and high-efficiency n-PERT bifacial c-Si cells is possible due to the processes compatible with existing production lines.

Mass production of industrial tunnel oxide passivated contacts (i‐TOPCon) silicon solar cells with average efficiency over 23% and modules over 345 W

AbstractWe present an industrial tunnel oxide passivated contacts (i‐TOPCon) bifacial crystalline silicon (c‐Si) solar cell based on large‐area n‐type substrate. The interfacial thin SiO2 is thermally growth and in situ capped by an intrinsic poly‐Si layer deposited by low‐pressure chemical vapor deposition (LPCVD). The intrinsic poly‐Si layer is doped in an industrial POCl3 diffusion furnace to form the n+ poly‐Si at the rear, which shows an excellent surface passivation characteristics with J0 = 2.6 fA/cm2 when passivated by a SiNx:H layer deposited by plasma‐enhanced chemical vapor deposition (PECVD). With an industrial fabrication process, the cells are manufactured with screen‐printed front and rear metallization, using large‐area 6‐in. n‐type Czochralski (Cz) Si wafers. We demonstrate an average front‐side efficiency greater than 23% and an open‐circuit voltage Voc greater than 700 mV. These results are based on more than 20 000 pieces of cells from mass production on a single day, in an old conventional multicrystalline silicon (mc‐Si) Al‐back surface field (BSF) cell workshop, which has been upgraded to i‐TOPCon process. The best cell efficiency reaches 23.57%, as independently confirmed by Fraunhofer CalLab. A median module power greater than 345 W and a best module power greater than 355 W are demonstrated with double‐glass bifacial i‐TOPCon modules consisting of 120 pieces of half‐cut 161.7 mm pseudosquare i‐TOPCon cells with nine busbars.

Double-Sided Symmetrical and Crossed Emitter Crystalline Silicon Solar Cells With Heterojunctions for Bifacial Applications

In this paper, we propose symmetrical and crossed bifacial crystalline silicon solar cells with heterojunctions employing a double-sided emitter, for use in environments with ambient light. We perform real measurement-based simulation analysis by using Silvaco TCAD Atlas. At an albedo level of 30%, our symmetrically structured solar cell achieves a $\text{47.42}\,{\text{ mA/cm}}^{\text{2}}$ short-circuit current ( J sc) and a 29.72% conversion efficiency ( η ), due to the double-sided emitter's large area. These J sc and η values are, respectively, 14.2% and 16.1% better than for a conventional Interdigitated Back Contact (IBC) [1] . The crossed structure obtains an 84.34% fill factor (FF) and a 29.81% conversion efficiency, due to its surrounding electric field. Conversion efficiency is increased by 16.4% compared with a conventional IBC solar cell [1] . Also, conversion efficiencies for the symmetrical and crossed bifacial SHJs are increased by 11.60% and 11.94%, respectively, compared with the state-of-the-art IBC solar cells [2] .

Investigation of Potential-Induced Degradation in n-PERT Bifacial Silicon Photovoltaic Modules with a Glass/Glass Structure

Potential-induced degradation (PID) in n-type passivated emitter, rear totally diffused (n-PERT) bifacial crystalline silicon photovoltaic modules with a glass/glass structure is investigated. From front-side measurements, a significant loss in the short-circuit current ( I sc) and a relatively smaller loss in the open-circuit voltage ( V oc) and fill factor (FF) are observed due to PID. A similar degradation behavior is observed from the rear side, except that there is negligible change in I sc. External quantum efficiency and photoluminescence measurements reveal that the losses in I sc and V oc are most likely due to an increase in the front surface recombination. FF loss analysis and two-diode model fitting demonstrate that the FF loss is mainly attributed to an increased recombination in the space charge regions. Moreover, n-PERT bifacial silicon modules also suffer from PID when they are stressed from the rear side. Furthermore, some ethylene-vinyl acetate and polyolefin, which show high PID-resistance to conventional p-type technologies, are found to be not as effective in preventing PID in n-PERT technologies. However, a PID-free n-PERT bifacial module design is possible with the application of the sodium-free glass. Finally, the progression of PID is heavily dependent on the bias voltage and stress temperature.

Maximization of conversion efficiency based on global normal irradiance using hybrid concentrator photovoltaic architecture

AbstractMaximization of module conversion efficiency based on global normal irradiance (GNI) rather than direct normal irradiance (DNI) was experimentally demonstrated using a hybrid concentrator photovoltaic (CPV) architecture in which a low‐cost solar cell (a bifacial crystalline silicon cell) was integrated with a high‐efficiency concentrator solar cell (III–V triple‐junction cell) to harvest diffuse sunlight. The results of outdoor experiments showed that the low‐cost cell enhanced the generated power by factors of 1.39 and 1.63 for high‐DNI and midrange‐DNI conditions, respectively, and that the resultant GNI‐based module efficiencies were 32.7% and 25.6%, respectively. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley &amp; Sons Ltd.

Effective passivation of silicon surface by AZO films: Application in bifacial solar cells

Aluminum rich zinc oxide (AZO) film was made on silicon substrates by spin coating using a precursor containing Al and Zn in the 20% atomic ratio. The films were annealed in air atmosphere and in hydrogen ambient. The minimum reflectance (Rλ) in air annealed (AZO:Air) samples was ∼2.5% in the vicinity (±50nm) of 750nm, however, the value was marginally lower (∼2%) in hydrogen annealed (AZO:H2) samples. The average Rλ over 300–1200nm range was 18.5% and 14.1% respectively. An additional XRD peak of H2Al10O16 at 2θ=51.5° corresponding to (121) reflection plane was observed in the AZO:H2 film. The AZO films were applied on the front and back surfaces of bifacial crystalline silicon (c-Si) solar cells where an improvement both in short circuit current density (Jsc) and open circuit voltage (Voc) was observed. The values of Jsc and Voc after AZO:Air were increased by 23% and ∼10mV respectively w.r.t without AZO layer cells which on subsequent annealing in H2 raised to 32% and ∼21mV. Jsc (AZO:Air) was used to calculate the expected enhancement in Voc which matched with the measured increase in Voc. The improvement in Jsc is due to reduced reflectance and, hence, the observed increase in Voc (AZO:Air) is attributed to it. On the other hand, increase in Voc after AZO:H2 step was much more and cannot be accounted only by Rλ, and therefore, the additional enhancement in Voc is attributed to back surface passivation that has occurred after annealing of the solar cells in hydrogen ambient. This attribution is substantiated by the internal and external quantum efficiencies (IQE and EQE) after back and front illumination of the solar cells and minority carrier lifetime measurements. The IQE data was used to calculate the diffusion length (Ln) of the base region where the value of Ln did not change after AZO:Air layer (=130μm for Cell #1) but its value substantially increased (=244μm) after the AZO:H2 treatment. This observation further reiterates our claim that the AZO film has surface passivation property.

Luminescent layers for enhanced silicon solar cell performance: Up-conversion

An up-converting material can generate one high-energy photon for every two or more incident low-energy photons. This material when placed on the rear side of a traditional bi-facial photovoltaic solar cell has the potential to increase the overall efficiency of the PV device by utilising sub-bandgap photons that would otherwise be transmitted through the device. This paper reviews and examines the application of rare-earth doped inorganic phosphors for achieving external quantum efficiencies for sub-bandgap photons for a bi-facial crystalline silicon photovoltaic device. The mechanisms for up-conversion are discussed and recent experimental results on a bi-facial silicon photovoltaic-upconversion device are analysed.