Interdigitated Back Contact Solar Cells Research Articles

Optimization of Front Diffusion Profile in Bifacial Interdigitated Back Contact Solar Cell

The diffusion profiles of the front floating emitter (FFE) and front surface field (FSF) in a bifacial interdigitated back contact solar cell are optimized. The optimization results revealed that the FFE and FSF schemes are beneficial for enhancing the cell performance at the front and rear sides, respectively. Lighter doping is particularly better for the FSF scheme, and the FFE scheme requires a large diffusion depth for improving the performance. Increasing the area of the rear emitter boosts the performance of the cell, and an FFE scheme with 90% rear emitter area is found to be the best design. Quantum efficiency mapping demonstrated that the FFE scheme suppresses the loss at the back surface field region, thereby enhancing the performance of the total cell. The FSF scheme improves the quantum efficiency for the entire region by enhancing the carrier transport in the vertical direction. Furthermore, loss analysis revealed that the FFE scheme suppresses the recombination loss at the maximum power point, which is an important advantage over the FSF scheme.

Carrier Transmission Mechanism-Based Analysis of Front Surface Field Effects on Simplified Industrially Feasible Interdigitated Back Contact Solar Cells

Interdigitated back contact (IBC) n-type silicon solar cells with a different front surface layer doping concentration were fabricated and studied and the influence of the front surface doping level was analyzed via simulation (PC1D). The IBC cells were processed by industrially feasible technologies including laser ablation and screen printing; photolithography was not used. A maximum efficiency of up to 20.88% was achieved at an optimal front surface field (FSF) peak doping concentration of 4.8 × 1019 cm−3 with a sheet resistance of approximately 95 Ω/square, corresponding to Jsc = 40.05 mA/cm2, Voc = 671 mV and a fill factor of 77.70%. The effects of the front surface doping level were studied in detail by analyzing parameters related to carrier transmission mechanisms such as minority carrier concentration, minority carrier lifetime and the saturation current density of the FSF (J0e). The influence of the front surface recombination velocity (FSRV) on the performance of IBC solar cells with different FSF layer doping concentrations was also investigated and was verified by examining the variation in the minority carrier density as a function of the distance from the front surface. In particular, the impact of the FSF doping concentration on the Jsc of the IBC cells was clarified by considering carrier transmission mechanisms and the charge-collection probability. The trends revealed in the simulations agreed with the corresponding experimental data obtained from the fabricated IBC solar cells. This study not only verifies that the presented simulation is a reasonable and reliable guide for choosing the optimal front surface doping concentration in industrial IBC solar cells but also provides a deeper physical understanding of the impact that front surface layer doping has on the IBC solar cell performance considering carrier transmission mechanisms and the charge-collection probability.

Effect of novel encapsulants and backsheets on short circuit current in interdigitated Back contact solar cells based PV modules

Due to zero shading and higher packing density, interdigitated back contact (IBC) solar cells based PV modules offer low optical losses as compared to conventional PV modules. The optical losses in PV modules are mainly governed by reflection losses. In this work, loss in short circuit current density (JSC) due to reflection losses from two types of coupons, (1) back contact solar cell having glass-glass packaging with Aluminium foil as back sheet (2) back contact solar cell with conventional Tedlar as back sheet, are evaluated by incorporating two novel encapsulants, thermoplastic polyolefin (TPO) and polybutadiene ionomer (PBI), along with conventionally used ethylene vinyl acetate (EVA). The loss in JSC in case of “(1)” with TPO, PBI and EVA are found as 3.043 mA/cm2, 3.048 mA/cm2 and 3.11 mA/cm2 respectively. While, the loss in JSC in case of “(2)” with TPO, PBI and EVA are found as 2.88 mA/cm2, 2.95 mA/cm2 and 3.024 mA/cm2 respectively. Although lesser losses are observed in case of Tedlar based coupons, the results obtained from Aluminum foil based coupons are still very much comparable and its lower cost makes it a good choice to replace Tedlar in future. Moreover, the results clearly showed that TPO and PBI performs better as compared to conventional EVA with an added advantage of being less prone to encapsulant discoloration.

Design, Fabrication, and Characterization of n-Si IBC Solar Cells Using PERC Technology

In this article, high-efficiency n-Si interdigitated back contact solar cells (IBC) were fabricated with the perc-like process. The cell was demonstrated with the efficiency of 22.16% (cell area = 10 × 10 cm), open-circuit voltage of 685 mV, short-circuit current density of 41.21 mA/cm2, and fill factor of 78.48%. The low fill factor can be attributed to the high base recombination taking place under high-level injection. Fill factor over 81% and, therefore, cell efficiency over 23% is expected when wafers of high bulk lifetime ( τ bulk ≥ 10 ms) or low resistivity ( ρ ≤ 1 Ω·cm) are used. The light I–V curve was fully characterized with the optical and electronic properties obtained from independent measurements.

High‐Efficiency Interdigitated Back Contact Silicon Solar Cells with Front Floating Emitter

Silicon interdigitated back contact (IBC) solar cells with front floating emitter (FFE‐IBC) put forward a new carrier transport concept of “pumping effect” for minority carriers compared with traditional IBC solar cells with front surface field (FSF‐IBC). Herein, high‐performance FFE‐IBC solar cells are achieved theoretically combining superior crystalline silicon quality, front surface passivation, and shallow groove structure using 2D device model. The improvement of minority carrier transport capacity is realized in the conductive FFE layer through optimizing the doping concentration and junction depth. It is shown that the shallow groove on the rear side of FFE‐IBC solar cells can effectively enhance the carrier collection ability by means of minimizing the negative impact of undiffused gap or surface p–n junction. The high efficiency exceeding 25% can be realized on silicon FFE‐IBC solar cells with the novel cell structure and optimized cell parameters, where the back surface field and emitter region width can be made for the same with only a slight sacrifice of photocurrent density and conversion efficiency. It is demonstrated theoretically that the realization of high‐efficiency and low‐cost silicon IBC solar cells is feasible due to the increase of the module fabrication tolerance.

Back-Contact Perovskite Solar Cells

Interdigitated back-contact (IBC) architectures are the best performing technology in crystalline Si (c-Si) photovoltaics (PV). Although single junction perovskite solar cells have now surpassed 23% efficiency, most of the research has mainly focussed on planar and mesostructured architectures. The number of studies involving IBC devices is still limited and the proposed architectures are unfeasible for large scale manufacturing. Here we discuss the importance of IBC solar cells as a powerful tool for investigating the fundamental working mechanisms of perovskite materials. We show a detailed fabrication protocol for IBC perovskite devices that does not involve photolithography and metal evaporation. The interview is available at https://youtu.be/nvuNC29TvOY.

Numerical Simulations of IBC Solar Cells Based on Poly-Si Carrier-Selective Passivating Contacts

This paper presents an analysis of physical mechanisms related to operation and optimization of interdigitated back contact (IBC) poly-silicon-based devices. Concepts of carrier selectivity and tunneling are used to identify the parameters that impact on the fill factor. Then, based on technology computer-aided design (TCAD) numerical simulations, we describe the device performance in terms of transport and passivation. A validation of the model is performed by matching measured and simulated R, T, and external quantum efficiency spectra and electrical parameters. As result of such process, the opto-electrical losses of the reference device are identified. Then, we execute a study of the impact of process parameters on the performance of the IBC device under analysis. Assuming a uniform SiO2 layer, simulation results reveal that both n-type and p-type poly-Si contacts can be theoretically perfect (i.e., approx. lossless), if assuming no interface recombination but considering tunneling of both carrier types. In other words, there exists an optimum oxide thickness (1 nm) for which majority carriers tunneling works already very well, and minority tunneling is still low enough to not result in significant recombination. Moreover, SiO2 thickness up to maximum 1.6 nm is crucial to achieve high efficiency. Regarding rear geometry analysis, the efficiency curve as a function of emitter width peaks at 70% of pitch coverage. Further, it is shown that diffused dopants inside crystalline silicon make the device resilient to passivation quality. Finally, the calibrated model is used to perform an optimization study aiming at calculating the performance limit. The estimated performance limit is 27.3% for a 100-μm-thick bulk, 20-nm-thick poly-silicon layers, silver as rear contact, and double ARC.

Optimization of micron size passivated contact and doping level for high efficiency interdigitated back contact solar cells

The Interdigitated back contact (IBC) solar cell is one of the most efficient commercial solar cells in the PV market. In order to improve the IBC solar cell efficiency, the entire solar cell structure must be well designed. The first issue is the solar cell’s doping level, including the p+ and n+ surfaces. The second is that the contact and passivation must be optimized. In this paper, we analyzed the recombination parameter (Jo) and specific contact resistivity (ρc) which are influenced by doping level, contact size and passivation. We found that the Jo decreases regularly as the ratio of contact size decreases in the diffusion wafer. An industrial, cost effective IBC solar cell fabrication with laser process was developed. The IBC solar cell with micron size contact and special doping level gave Voc of 690.0 mV, Jsc of 42.0 mA/cm2, FF of 81.0%, and η of 23.5%. The software 3D Quokka@ was used to calculate the IBC solar cell efficiency through simulation using a device model. Through the energy loss analysis, the simulation indicated that IBC solar cell with 25.0% efficiency could achieve with optimized doping level, micron size passivated contact, low contact resistance in the high bulk lifetime wafer.

Record-high solar-to-hydrogen conversion efficiency based on a monolithic all-silicon triple-junction IBC solar cell

We present a record-high solar-to-hydrogen conversion efficiency (STH) for monolithic all-silicon multi-junction solar devices. The device is based on an interdigitated back-contact silicon solar cell, in which the p- and n-regions are connected in a combination of series and parallel contacts, in order to triple the photovoltage compared to a single-junction cell. Our best device provides an open-circuit voltage of 1.99 V, larger than the water redox potential of 1.23 V plus overpotentials at the electrodes, as well as a short-circuit current density of 12.6 mA/cm2. Coupled to a sulphuric acid electrolysis system, with platinum and ruthenium dioxide electrodes, our device shows an STH of 14.5% at 1.63 V.

High-efficiency black IBC c-Si solar cells with poly-Si as carrier-selective passivating contacts

In this work, we present the application of poly-Si carrier-selective passivating contacts (CSPCs) as both polarities in interdigitated back-contacted (IBC) solar cell architectures. We compared two approaches to form a gap between the back-surface field (BSF) and emitter fingers. It is proved that the gaps prepared by both approaches are efficient in preventing carriers’ recombination. To minimize the reflection losses, we developed a novel modulated surface texturing (MST) structure as anti-reflection coating (ARC). It is obtained by superposing a nano-textured SiO2 layer on the conventional micro-textured pyramids, which are passivated with a-Si:H / SiNx:H layers. This approach decouples the light harvesting from the Si surface passivation, which potentially results in the highest possible optical and electrical performances of the solar cells. The reflectance (R) of the MST-ARC is very close to that of the high-aspect ratio nano-structured silicon (black-Silicon), achieving R < 1% between 450 and 1000 nm. The J0 of MST-ARC passivated Si surface (6.3 fA/cm2) is the same as that of standard a-Si:H/SiNx:H layers passivated pyramidally-textured Si surface. By applying this novel MST-ARC in our IBC solar cell, the highest JSC observed in a device is 42.2 mA/cm2 with a VOC as high as 701 mV. A spectral response enhancement in case of the MST-ARC cell is observed over the whole wavelength range with respect to the cell with standard SiNx:H ARC. The highest efficiency achieved in this work is 23.0%, with the potential to reach 24.0% in short term by using more conductive metal fingers.

Realization of interdigitated back contact silicon solar cells by using dopant-free heterocontacts for both polarities

For crystalline-silicon (c-Si) solar cells, the interdigitated back contact (IBC) structure has been long known as an efficient way to approach the theoretical limit of efficiency. However, the complexity of fabricating this kind of devices as well as the high dependence on expensive vacuum systems pose concerns about their commercial potential. Here, we demonstrate a novel c-Si IBC solar cell featuring dopant-free heterocontacts for both polarities, i.e. a solution-proceeded PEDOT:PSS film as hole-transporting layer (HTL) and an evaporated magnesium-oxide film as electron-transporting layer (ETL). Our innovatively buried ETL method provides substantial simplification on the architecture and fabrication of the IBC cells and makes it possible to adapt solution-proceeded HTLs while keeping good passivation in gap regions. The IBC solar cell shows an efficiency of 16.3%, with a promising short-circuit current density (Jsc) up to 38.4 mA/cm2. A thorough simulation concerning the influence of pitch size, surface recombination rate (at ETL and gap regions) was conducted, revealing a readily achievable Jsc of 41 mA/cm2 and a PCE beyond 22%. Our findings demonstrated a feasibility of using solution method to fabricate high efficiency dopant-free IBC solar cells.

Poly-crystalline silicon-oxide films as carrier-selective passivating contacts for c-Si solar cells

The poly-Si carrier-selective passivating contacts (CSPCs) parasitically absorb a substantial amount of light, especially in the form of free carrier absorption. To minimize these losses, we developed CSPCs based on oxygen-alloyed poly-Si (poly-SiOx) and deployed them in c-Si solar cells. Transmission electron microscopy analysis indicates the presence of nanometer-scale silicon crystals within such poly-SiOx layers. By varying the O content during material deposition, we can manipulate the crystallinity of the poly-SiOx material and its absorption coefficient. Also, depending on the O content, the bandgap of the poly-SiOx material can be widened, making it transparent for longer wavelength light. Thus, we optimized the O alloying, doping, annealing, and hydrogenation conditions. As a result, an extremely high passivation quality for both n-type poly-SiOx (J0 = 3.0 fA/cm2 and iVoc = 740 mV) and p-type poly-SiOx (J0 = 17.0 fA/cm2 and iVoc = 700 mV) is obtained. A fill factor of 83.5% is measured in front/back-contacted solar cells with both polarities made up of poly-SiOx. This indicates that the carrier transport through the junction between poly-SiOx and c-Si is sufficiently efficient. To demonstrate the merit of poly-SiOx layers' high transparency at long wavelengths, they are deployed at the back side of interdigitated back-contacted (IBC) solar cells. A preliminary cell efficiency of 19.7% is obtained with much room for further improvement. Compared to an IBC solar cell with poly-Si CSPCs, a higher internal quantum efficiency at long wavelengths is observed for the IBC solar cell with poly-SiOx CSPCs, thus demonstrating the potential of poly-SiOx in enabling higher JSC.

Electrical Characterization of n-ZnO/c-Si 2D Heterojunction Solar Cell by Using TCAD Tools

This paper investigates recent developments in Interdigitated Back Contact (IBC) solar cell physics and technology by using ATLAS Silvaco device simulator. The work has been focused on improvement of structural and electrical properties by using zinc oxide layer to passivate the front surface of a crystalline silicon (c-Si) substrate. In a silicon cells, the device performance crucially depends on the quality of the n-ZnO/c-Si heterojunction. Simulation results for various front dielectric material, emitter doping concentration, and semiconductor material for the front surface field (FSF) layer on the considered structure are reported and analyzed. These variations have a direct impact on the electrical device characteristics. We could determine the critical parameters of the cell and optimize its main parameters to obtain the highest performance for IBC solar cell. Therefore, we present our best results obtained recently and some guidelines to improve still more the efficiency of the devices. The optimization at 300 K led to the following results Jsc = 41.89 mA/cm2, Voc = 0.727 V, FF = 85.23 %, P $_{\max }=$ 259.95 W/m-2, and η =25.99 % which are close with those found in different works. The structure simulation will simplify the manufacturing processes of solar cells; will thus reduce the costs while producing high outputs of photovoltaic conversion. Finally, we finish by a summary of the main advantages of this technology taking into account all the parameters described above.

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] .

Interdigitated back-contacted crystalline silicon solar cells with low-temperature dopant-free selective contacts

A new Al2O3/TiO2/Mg electron-contact scheme together with a V2Ox-based hole-contact is applied to cold-IBC solar cells achieving efficiencies beyond 19%.

Quantification of pn-Junction Recombination in Interdigitated Back-Contact Crystalline Silicon Solar Cells

Interdigitated back-contact (IBC) solar cells based on diffused crystalline silicon comprise a series of pn -junctions which border at the rear surface of the wafer. In this work, it is established that the presence of these pn -junctions can induce significant additional charge-carrier recombination, which affect the conversion efficiency of IBC cells through a reduction in fill factor and open-circuit voltage. Using specialized test structures with varying length of pn -junctions per area of solar cell (i.e., with varying junction density), the magnitude of the recombination at the pn -junction was determined. For nonpassivated rear surfaces, a second-diode recombination current density per unit of junction density J 02 of ∼61 nA·junction–1·cm–1 was measured, whereas for surfaces which were passivated by either SiN $_{x}$ or Al 2O3/SiN $_{x}$ , J 02 was reduced to ∼0.4 nA·junction–1·cm–1. Therefore, passivation of defects at the rear surface was proven to be vital in reducing this characteristic recombination current. Finally, by optimizing the p - and n -type dopant diffusion process recipes, J 02 recombination could be suppressed. The improved doping recipes led to an increase in conversion efficiency of industrial “mercury” IBC solar cells by ∼1% absolute.

Impact of the homogeneous junction breakdown in IBC solar cells on the passivation quality of Al2O3 and SiO2: degradation and regeneration behavior

Within the last years, many different approaches for the simplified fabrication of interdigitated back-contact (IBC) solar cells have been developed. Most of those concepts result in emitter and back-surface field (BSF) regions that are in direct contact to each other which leads to a controlled breakdown under reverse bias at the p+n+ junction. In this work, the influence of the reverse breakdown on the passivation quality of Al2O3 and SiO2 at the p+n+ junction is investigated, not only shedding light on the degradation but also on the regeneration behavior of the cells. It was found that cells with Al2O3 passivation on the back side degrade during reverse breakdown whereas sister cells with SiO2 passivation were rather unaffected. Consequently, the degradation seems to be related to the passivation layer. However, it is shown that the passivation can be regenerated even under normal operation condition. A possible explanation is the discharging of interface traps, which are getting recharged already at room temperature.

Silicon solar cells with heterojunction emitters and laser processed base contacts

In this work, we report on a novel structure of Interdigitated Back-Contacted (IBC) solar cells on c-Si p-type substrates that combines laser processed homojunction base contacts and silicon heterojunction (SHJ) emitters. These hybrid devices which can lead to potential benefits in device processing and/or conversion efficiency. In the proposed fabrication process special attention has been paid to the compatibility of both involved technologies: silicon heterojunction and laser doping from dielectric films. In particular, we focus on the surface passivation obtained by the heterojunction emitter after removing the aluminum oxide/silicon carbide (Al2O3/SiCx) layer stack needed for the laser doping process and previously deposited on the c-Si surface. A severe passivation degradation after plasma etching process to remove the top SiCx film is observed, despite leaving the Al2O3 film on the c-Si surface. Based on high-frequency capacitance-voltage characterization, an increase in the interface state density and a strong impact on the fixed charge density is deduced. Next, in order to choose an optimized metallization technology that could simultaneously contact both the ITO film and the p+ laser processed regions, we evaluate the contact quality of Titanium and Aluminum on ITO. Results show that Titanium is a better option with a specific contact resistance of 1.1 mΩ cm2. Finally, finished hybrid IBC solar cells with conversion efficiencies in the 18-19% range are reported.

Fabrication of a 22.8% Efficient Back Contact Solar Cell With Localized Laser‐Doping

Interdigitated back contact (IBC) solar cells offer one of the most promising routes towards highest device efficiencies but require finding of industrially feasible low cost production sequences. In this paper, the fabrication of a 22.8% efficient all‐laser‐doped IBC solar cell with localized boron and phosphorus doped contacts is reported. The laser‐based approach with localized emitter and back‐surface field regions reported here avoids costly patterning steps of the localized contacts. By means of numerical device simulations it is demonstrated that this cell is limited by recombination and contact resistivity of the localized contacts and an efficiency potential of 23.8% with improved laser parameters is demonstrated.

Improvement in IBC-silicon solar cell performance by insertion of highly doped crystalline layer at heterojunction interfaces

By inserting a thin highly doped crystalline silicon layer between the base region and amorphous silicon layer in an interdigitated back-contact (IBC) silicon solar cell, a new passivation layer is investigated. The passivation layer performance is characterized by numerical simulations. Moreover, the dependence of the output parameters of the solar cell on the additional layer parameters (doping concentration and thickness) is studied. By optimizing the additional passivation layer in terms of doping concentration and thickness, the power conversion efficiency could be improved by a factor of 2.5%, open circuit voltage is increased by 30 mV and the fill factor of the solar cell by 7.4%. The performance enhancement is achieved due to the decrease of recombination rate, a decrease in solar cell resistivity and improvement of field effect passivation at heterojunction interface. The above-mentioned results are compared with reported results of the same conventional interdigitated back-contact silicon solar cell structure. Furthermore, the effect of a-Si:H/c-Si interface defect density on IBC silicon solar cell parameters with a new passivation layer is studied. The additional passivation layer also reduces the sensitivity of output parameter of solar cell to interface defect density.