Back-contact Silicon Solar Cells Research Articles

Qualification of laser-weld interconnection of aluminum foil to back-contact silicon solar cells

Laser welding can be used to interconnect high-efficiency back-contact silicon solar cells with low-cost Al foil. This interconnection approach is relatively new and, thus, requires detailed vetting of its reliability before being adopted commercially. In this study, we weld 50-μm-thick Al foil to Sunpower back-contact cells and observe that the laser-weld adhesion, module fill factor, and reliability through thermocycling are all highly correlated to each other. A JMP statistical model built from adhesion data reveals that the statistically significant parameters to improve laser-weld adhesion are the laser pulse energy, pulse density, and pattern. Increasing the laser pulse energy and density improves the foil adhesion to the cell metallization, which is likely because of the improved melting of the Sn capping layer on the Cu electrode of the cells, as identified by cross-sectional microscopy. 94.4% of the modules fabricated using laser welds with a mean adhesion above 0.8 mJ lost less than 5% of their initial maximum power after 200 thermocycles, which is the IEC 61215 criterion for any single accelerated stress test. Additionally, 90.0% of modules fabricated with an as-fabricated module series resistance below 1.9 Ω cm passed thermocycling. Thus, laser-weld adhesion and the as-fabricated module series resistance can be used in the further development of new laser-weld settings and as quality control parameters in the manufacturing of these modules.

Design of Large Poly‐Si on Oxide Interdigitated Back Contact (POLO IBC) Silicon Solar Cells with Local Al–p+ Contacts in the Constraints of Measurement and Module Integration

Interdigitated back contact (IBC) silicon solar cells with a passivating n‐type poly‐Si on oxide emitter and an aluminum‐doped p+ base contact on M2‐sized Ga‐doped p‐type Cz wafers are reported. The Al‐doped base contact forms during the firing of the printed contacts and allows for a lean process flow. The device optimization balances recombination at the base contacts against resistive losses and respects constraints set by the need of interconnecting cells in a module and contacting the cells temporally by a measurement chuck. A special sample holder is designed for measuring the Isc–Voc curve of the IBC cell with a busbar‐less metal grid. The pseudo‐efficiency is 24.7%. All fingers of each polarity are connected with wires and an efficiency of 22.3% is measured. The comparison of simulations and measurements reveals that the cell has 23.4% efficiency without the series resistance losses due to the wires. A huge part of the resistive losses in the cell are the transport losses of the majorities in the base dissipating a power that corresponds to 0.76%abs efficiency and the resistive losses at the Al‐doped base contact (0.29%abs).

Realization and simulation of interdigitated back contact silicon solar cells with dopant-free asymmetric hetero-contacts

Crystalline silicon (c-Si) solar cells using interdigitated back contact (IBC) configurations are one of the most promising candidates to reach the practical efficiency limits of c-Si solar cells. However, the complexity of the process flow hinders the mass production of the IBC cells with conventional doped regions. One of the simple fabrication methods is to introduce the dopant-free carrier-selective contacts, which utilizes the fabrication processes with low temperature, e.g., the thermal evaporation or the spin coating. In this paper, we investigated efficiency close to 20% silicon IBC solar cells with dopant-free asymmetric hetero-contacts. In this solar cell configuration, the high work function material MoOx was chosen as the hole transporting layer, while the low work function material LiF was chosen as the electron transporting layer, respectively. The simulation results indicate that the perspective efficiency exceeding 22% for this type of cells is achievable with the optimized pitch width and improved passivation quality of the contacts, which has a great potential for the industrialization of IBC solar cells with simple fabrication processes.

Simulation and characterization of planar high-efficiency back contact silicon solar cells

Short-circuit current, open-circuit voltage, and photoconversion efficiency of silicon high-efficiency solar cells with all back contact (BCSC) with planar surfaces have been calculated theoretically. In addition to the recombination channels usually considered in this kind of modeling, namely, radiative, Auger, Shockley–Read–Hall, and surface recombination, the model also takes into account the nonradiative trap-assisted exciton Auger recombination and recombination in the space charge region. It is ascertained that these two recombination mechanisms are essential in BCSCs in the maximum power operation regime. The model results are in good agreement with the experimental results from the literature.

Optimization of four terminal rear heterojunction GaAs on Si interdigitated back contact tandem solar cells

High-efficiency, four-terminal tandem solar cells composed of thin GaAs films mechanically stacked onto interdigitated back contact silicon solar cells with a glass interlayer are demonstrated. The optimal thickness of the absorber layer of a rear heterojunction GaAs subcell for use in four terminal tandem solar cells was studied. GaAs top cells with absorber layer thicknesses of 1.5, 1.9, 2.3, 2.8, and 3.5 μm were fabricated on glass and mechanically stacked onto interdigitated back-contact Si bottom cells. All tandem cells were found to have efficiencies above 30% under the AM1.5 G spectrum demonstrating a relatively weak sensitivity to thickness in the four-terminal configuration. We found the 2.8 μm absorber layer cell to have the highest top cell and tandem cell efficiency at 26.38% and 32.57%, respectively. Optical modeling with transfer matrix method for the planar top cell and Lambertian light trapping in the textured Si subcell, along with drift-diffusion Hovel equations, were used to show photon recycling enhancement to the effective diffusion length and VOC of the top cell as a result of the low-index glass interlayer.

Laser Patterned Electroplated Copper Contacts for Interdigitated Back Contact Silicon Solar Cells

Metallization of interdigitated back contacts for Si-based solar cells (IBC), where both the n- and p-contacts are processed on the rear side of the device, currently requires costly and labor-intensive masking, photolithography, and/or screen printing approaches, which are difficult for scale-up to high-throughput high-volume manufacturing. Electrodeposition (ED) offers a low cost, scalable, and selective approach for IBC metallization, but is potentially complicated by the close geometry of the contacts and the need for simultaneous plating to both contacts to achieve high processing throughput. Direct laser patterning of a thin seed metal film offers a fast single-step process for defining contact areas, reducing both device handling and chemical use, and allows simple variation of contact design to implement external electrical connections for ED, as well as rapid testing of alternative pattern dimensions and the subsequent effects on device performance. A simple and flexible direct laser scribing approach for processing of IBCs through patterning of thin Ni seed layers prior to Cu electroplating will be presented.IBC patterns were laser-scribed on blanket evaporated Ni seed layers on a-SiNx:H-coated textured-Si wafers (Ni/a-SiNx:H/Si) prior to plating. Patterning was carried out at optimum laser powers to achieve electrical isolation of the Ni contacts. Pattern designs, with 12 mm long n- and p-fingers of 400 and 1200 µm in width, respectively, were fabricated to include tabs for external contacting for ED (Fig. 1a). Contact patterns were prepared using either double-pass-focused or single-pass-defocused laser conditions during ablation. Samples processed with either single (Fig. 1b) or double (Fig. 1c) laser passes show well-defined ~100 µm cuts, however, those patterned with the latter condition showed a ~20 µm laser-induced trench along the center of all scribe lines, due to overlap of the beam paths, and which was not observed for patterns processed with a single pass. Plating conditions from acidified CuSO4-based baths were optimized for deposit reproducibility, uniformity, and adhesion on substrates of non-patterned thin evaporated Ni seed layers on 150 µm textured-Si wafers. Films of growth rate ~0.1 µm/min were achieved, with contact resistance (RC) ~5 mΩ.cm2, and excellent adhesion, enduring pull tests of >5 N where the Si wafer broke before any peeling of the Cu layer.With plating to laser-patterned IBC substrates, significant Cu features grow from the edges of the laser cuts (Fig. 1d), eventually shorting the fingers and leading to unwanted deposition outside of the contact areas. Shorting was not observed with plating to IBC Ni seed patterns that had been evaporated through a shadow mask, suggesting residual conductive phases in the scribes are responsible for the growth of the shorts. Brief chemical etches were carried out after scribing to remove residues and, following Cu plating, etched samples showed attenuated growths into the laser scribes with the contacts remaining electrically isolated. However, for single laser pass samples, ~40 µm wide parasitically-plated Cu stripes (Fig. 1e) were observed along the center of the laser cuts throughout the scribed pattern. Energy-dispersive x-ray spectroscopy (EDS) mapping confirmed the stripe consists of discrete Cu islands, isolated from the growing layer, that eventually coalesce during plating (Fig. 1f). This Cu-stripe feature was not observed for double laser pass samples, indicating differences in properties of the scribes with the different laser patterning conditions.The formation of the Cu-stripe is postulated to be the result of laser ablation and chemical etch damage to the insulating a-SiNx:H layer that opens pinholes to the more conductive c-Si wafer (Fig. 1g). These higher conductivity points produce localized conduits of enhanced current density, sufficient to nucleate Cu, and allow subsequent rapid growth of islands. In contrast, with plating to double pass processed samples, the more extensive laser damage to the a-SiNx:H layer exposes a significantly larger area of the Si wafer, which results in lower current densities insufficient to nucleate Cu. These observed effects of damage to both underlying layers and to growth habits of subsequent films, highlight the need for careful optimization and control of laser conditions during fabrication of photovoltaic devices.Front-junction Si-based solar cells with Al back contacts were etched and re-contacted with ED Cu to IBC-patterned Ni seed layers that had been evaporated through a shadow mask. Device performance showed little change, maintaining ~18% efficiency, with decreased device series resistance through improved RC. These results confirm ED Cu as a feasible and industrially-relevant approach for processing high quality metal contacts for Si-based solar cells. Figure 1

Isolating p- and n-Doped Fingers With Intrinsic Poly-Si in Passivated Interdigitated Back Contact Silicon Solar Cells

Polycrystalline silicon on silicon oxide (poly - Si/SiO x ) passivating contacts enable ultrahigh-efficiency interdigitated back contact silicon solar cells. To prevent shunt between n- and p-type-doped fingers, an insulating region is required between them. We evaluate the use of intrinsic poly - Si for this isolation region. Interdigitated fingers were formed by plasma deposition of doped hydrogenated amorphous silicon through mechanically aligned shadow masks on top of a full-area intrinsic hydrogenated amorphous silicon ( a -Si:H) layer. High-temperature annealing then crystallized the a- Si:H to poly - Si and drove in the dopants. Two mechanisms were identified which cause contamination of the intrinsic poly - Si gap during processing. During deposition of doped fingers, we show using secondary ion mass spectrometry and conductivity measurements that the intrinsic gap becomes contaminated by doped a- Si:H tails several nanometers thick to concentrations of ∼1020 cm−3. Another source of contamination occurs during high-temperature annealing, where dopants desorb from doped regions and readsorb onto intrinsic a- Si:H. Both pathways reduce the resistivity of the intrinsic gap from ∼105 to ∼10−1 Ω·cm. We show that plasma etching of the a- Si:H surface before crystallizing with a capping layer can eliminate the contamination of the intrinsic poly-Si, maintaining a resistivity of ∼105 Ω·cm. This demonstrates masked plasma deposition as a dopant patterning method for Si solar cells.

Combined Optical-Electrical Optimization of Cd1-xZnxTe/Silicon Tandem Solar Cells.

Although the fundamental limits have been established for the single junction solar cells, tandem configurations are one of the promising approaches to surpass these limits. One of the candidates for the top cell absorber is CdTe, as the CdTe photovoltaic technology has significant advantages: stability, high performance, and relatively inexpensive. In addition, it is possible to tune the CdTe bandgap by introducing, for example, Zn into the composition, forming Cd1−xZnxTe alloys, which can fulfill the Shockley–Queisser limit design criteria for tandem devices. The interdigitated back contact (IBC) silicon solar cells presented record high efficiencies recently, making them an attractive candidate for the rear cell. In this work, we present a combined optical and electrical optimization of Cd1−xZnxTe/IBC Si tandem configurations. Optical and electrical loss mechanisms are addressed, and individual layers are optimized. Alternative electron transport layers and transparent conductive electrodes are discussed for maximizing the top cell and tandem efficiency.

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.

Hard mask processing of 20% efficiency back-contacted silicon solar cells with dopant-free heterojunctions

Single junction crystalline silicon (c-Si) solar cells featuring a conventionally doped interdigitated back contact heterojunction (IBC-SHJ) structure has approached a record efficiency of 26.6%, which is very close to the practical limit. However, integrating the interdigital p- and n-type amorphous silicon (a-Si:H) layers on the rear surface of Si substrate is of such complexity, posing problem of heavy dependences on expensive manufacturing techniques including plasma-enhanced chemical vapor deposition and photolithography. Its commercial potential is thus always being questioned, and to seek an alternative fabrication procedure, which adapts to cost-effective deposition parallel with simple patterning characteristics, has been a primary research target of related subjects. Here, we demonstrated 20.1% efficiency dopant-free IBC-SHJ solar cells by combining evaporated carrier-selective materials (MoOx and LiFx) and two-steps hard masks alignments, delivering substantial simplifications in the architecture and fabrication procedures. We investigated the effect of intrinsic a-Si:H films with different thicknesses on the passivation and contact resistance for both a-Si:H/MoOx and a-Si:H/LiFx contacts, showing 4 nm a-Si:H is better for high efficiency IBC-SHJ solar cells. We also found that the position of the metal target (electrode definition step) and isolation in between the busbar and the Si substrate are highly relevant to leakage and recombination and have great impact on the device performance. The dopant-free IBC-SHJ solar cells demonstrated here manifest enough confidence in our hard mask based fabrication procedure, with great potential for high performance-to-cost ratio in future.

26%-efficient and 2 cm narrow interdigitated back contact silicon solar cells with passivated slits on two edges

Perimeter recombination is a relevant loss mechanism, in particular for cells with a large perimeter-to-area ratio and with poorly passivated edges, e.g., cut or cleaved solar cells for shingled modules. We experimentally demonstrate that cut edges can be well passivated during front-end processing. The resulting cells have an efficiency of 26%.The designated cell area of our lab-type highly efficient cells is smaller than the total area of the wafer. This causes recombination losses in the masked perimeter region. We separate the active cell area from the wafer on two sides of the cell by slits to reduce the transport of carriers into the perimeter region. We apply a diffusion model to describe impact of the slits on the perimeter recombination. The slits have an effective surface recombination velocity of down to 9 cm/s, depending on the resistivity of the base. For a base resistivity of 80 Ωcm, the average cell efficiency increases by 0.7 %abs as compared to embedded cells and by 2.3 %abs as compared to laser-cut cells due to the passivated slits.

Enhancing the efficiency of silicon solar cells using ZnO nanostructures prepared by microwave-assisted hydrothermal method

In this study, zinc oxide (ZnO) nanostructures were grown on interdigitated back contact silicon solar cells (IBC-SSCs) by using the microwave-assisted hydrothermal method. The effect of these ZnO nanostructures grown by different precursor concentrations on the conversion efficiency of solar cells was investigated. The as-prepared ZnO products were analyzed by XRD, SEM, EDS, UV–vis, and PL, then were grown onto IBC-SSCs. The IBC-SSCs conversion efficiency without any ZnO nanostructure was 8.88%, and with a ZnO nanostructure, it reached 12.15%. which effectively enhances the conversion efficiency of IBC-SSCs.

Effect of encapsulants on the thermomechanical residual stress in the back-contact silicon solar cells of photovoltaic modules – A constrained local curvature model

The fracture of the silicon cells and associated performance degradation is a major hindrance to the long-term viability of solar photovoltaics as a main-stream power source. The stress induced in the cells during the photovoltaic module integration (soldering and encapsulation) processes is significant as they may be high enough to cause cell fracture or propagate the pre-existing micro cracks. In a well-controlled cell soldering and encapsulation process in the industry, this stress may not cause significant cell cracking, but the stressed regions can still act as crack localization sites under the external operational loads. With the advent of thin silicon wafers to reduce material costs and increase productivity, the cells may become even more fragile and susceptible to fractures during the module fabrication processes. In this scenario, the PV industry is looking for methods and materials to reduce cell stresses. In this work we simulate the effect of the encapsulation polymers on cell stress and show that the encapsulant elastic modulus and thickness significantly affect cell stress, during the module fabrication and operation as well. The results suggest that the choice of the encapsulant can help to reduce the cell stress and improve the module reliability. The results further show that the effect of the encapsulant is much more significant for thinner cells, and that the effect of the front encapsulant is more significant compared to that of the back encapsulant. A physical model of constrained local curvature of the cell in the PV module is also proposed to explain the effects of the encapsulant modulus on the cell stress.

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.

Metallization Method for Interdigitated Back-Contact Silicon Solar Cells Employing an Insulating Resin Layer and a Ti/Ag/Cu Metal Stack

Interdigitated back-contact (IBC) silicon (Si) solar cells are attracting increased attention due to the potential of higher efficiency that comes from eliminating front-surface shading. However, as sources of recombination loss continue to be reduced due to improved processing and use of high-quality n-type wafers, parasitic absorption in the rear metal can limit cell performance. This paper reports experimental and simulation results that identify the optical advantages of an IBC metallization method, which employs a novolac resin layer to enhance the insulation of the cell from a metal electrode comprising a sputtered Ti/Ag metal stack and plated Cu. The polymer layer can electrically decouple the metal geometry from the doping geometry and also helps to reduce the parasitic absorption at the cell rear surface by increasing the distance between the metal and Si absorber. The optical properties of the Ti/Ag stacks formed over the novolac resin were comparable to those with an Ag reflector; however, the Ti/Ag reflector offered the additional benefit of contact resistivity of < 1 mΩ·cm2 to both the n+ and p+ Si regions of the diffused IBC cells.

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.

A new interdigitated back contact silicon solar cell with higher efficiency and lower sensitivity to the heterojunction defect states

A new interdigitated back contact silicon heterojunction (IBC-SHJ) solar cell with two additional buffer layers is investigated. The buffer layers are two thin p-type crystalline silicon (c-Si) layers placed at front and back surface of c-Si/a-Si:H hetero-junction interfaces. Improvement of the overall efficiency and decrease in sensitivity of the output parameters to c-Si/a-Si:H interface defect density through the controlling of carrier concentration by introducing the new structure is achieved. The impact of new buffer layers on the cell performance is studied with numerical modeling. The main output parameters of the IBC-SHJ solar cell including buffer layers are compared with the conventional IBC-SHJ solar cell. It is found that the new IBC-SHJ solar cell has a higher fill factor due to reduced series resistance. With a deeper study on the impact of the buffer layers, it is realized that the field effect passivation has a key role in the improvement of the new cell performance. By modifying the energy band structure through the buffer layers, recombination at the c-Si/a-Si:H heterojunction interfaces is reduced due to the field effect passivation of the c-Si surface, and the efficiency of the solar cell is increased. Optimizing doping concentration and thickness of the buffer layers leads to an increase of 5% in fill factor and an increase of 1.2% in efficiency compared with the conventional similar solar cell.

Polyimide for silicon solar cells with double-sided textured pyramids

Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping over cells with front-side only texture. However, increased surface area, roughness and exposed <111> crystal planes of textured surfaces not only causes increased recombination, but also makes cells susceptible to shunting through pinholes in the dielectric at the sharp peaks and valleys of the textured pyramids. A polyimide film as an insulating interlayer film is investigated to circumvent the tradeoff between improved light trapping, increased recombination and increased shunt paths. When applied at the rear of the interdigitated back contact silicon solar cell structure, the polyimide film provides an excellent electrical insulation (> 1000 MΩ of insulation resistance) and increases photocurrent (~ 1.1 mA/cm2) owing to an increased rear internal reflectance. The polyimide is also compatible with metal annealing of passivating dielectrics such as silicon nitride. Optical simulation and experimental results are combined in a 3D semiconductor simulation (Quokka) to quantify the possible gain of implementing the double-sided texture in high efficiency silicon solar cells.

Characterization of Epitaxial Heavily Doped Silicon Regions Formed by Hot-Wire Chemical Vapor Deposition Using Micro-Raman and Microphotoluminescence Spectroscopy

We report on the characterization of heavily boron doped epitaxial silicon regions grown in a hot-wire chemical vapor deposition tool, using micro-Raman and photoluminescence spectroscopy. In particular, the use of this approach for emitter fabrication in an interdigitated back contact silicon solar cell is studied, by analyzing its suitability concerning selective growth, uniformity, anneal time, and luminescent defects. We show that by reducing the silane flow rate, both the required postanneal time and intensity of defect luminescence are reduced. Furthermore, we show that selective area growth does not affect either the quality of the films or the sharpness of the resulting lateral doping profile. The uniformity of the doping is shown to be better than that achieved using laser doping.

Novel hole selective CrOx contact for dopant-free back contact silicon solar cells

The dopant-free back contact solar cells are demonstrated based on hole selective contact material, chromium trioxide (CrOx, x < 3) with a low melting point for high stability and high performance. In this contribution, CrOx is first applied in silicon based solar cell as the emitter. Integrating 5 nm CrOx and 2 nm LiFx into solar cell as the emitter and the back surface filed, resulted in a device efficiency of 13.6%. For further improvement, the back contact solar cell reaching an efficiency of 15.8% was fabricated, by implementing the multilayer films of CrOx (5 nm)/Au (4 nm)/ CrOx (5 nm) as the emitter. The formation of Cr(OH)3 in ambient condition, resulted in a lower work function (4.8 eV) of the CrOx film. Furthermore, the multilayer back contact solar cell demonstrated a high stability due to CrOx covered with 500 nm Ag, when stored in ambient air longer than 170 days.