Rear Solar Cells Research Articles

Investigation of polysilicon passivated contact's resilience to potential-induced degradation

We present clear evidence of excellent resilience to potential-induced degradation (PID) from polysilicon passivated contacts implemented on the rear of n-type solar cells. Under the stress conditions of −1000 V, 50 °C, 30% relative humidity with aluminum foil, no damage was caused to the passivated contact consisting of an ultrathin silicon oxide (SiOx) film and an n+-doped polysilicon (poly-Si) layer after 168 h. With +1000 V bias and under the same chamber conditions, the SiOx/poly-Si (n+) passivated contact showed a slight change that translated into about 1% module power loss after 168 h, which is significantly lower than the 5% threshold recommended by IEC 62804-1 PID test standard. Furthermore, the SiOx/poly-Si (n+) passivated contact, even when encapsulated with ethylene-vinyl acetate copolymer films having a low volume resistivity in the range of 5 × 1014 Ω⸱cm, exhibited good stability under high-voltage stress. The experimental results were also validated by a generic device simulation, where the SiOx/poly-Si (n+) stack was shown to be immune to the surface polarization effect. In addition, a promising cell-level solution (i.e., using a stack of aluminium oxide and silicon nitride) to the polarization-type PID for n-type passivated emitter rear totally diffused silicon solar cells was also demonstrated.

Measuring the light recovery factor of backsheets in photovoltaic modules

We measure the short circuit current improvement of various commercial backsheets for passivated emitter and rear solar cells (PERC) in photovoltaic (PV) modules. We quantify this improvement by a light recovery factor k defined as the share of the light reflected by the backsheet around the cell in a PV module laminate, which is converted into electrical current by the solar cells in the module relative to a reference rear surface. The presented measurement method allows measuring the light recovery factor as function of the incident light beam angle, which are important for the calculation of the yield analysis in the field. Simulations of the recovery factor with the optical ray tracing program DIADALOS are performed in order to support the interpretation of the results. For measuring the absolute light recovery probability of a backsheet one should use a reference backsheet with an absorbance of close to 100% to avoid systematic reflection errors especially for high incident light beam angles > 50°. The ranking of various recovery factors are compared to the ranking measured by reflectance measurements defined in standard IEC 62788-2 Ed.1 for backsheets. The standard shows a substantial deviation between the ranking of recovery factor and the reflectance which can be corrected by using an adapted definition for the reflectance measurement method.

In-situ characterization of electron-assisted regeneration of Cz-Si solar cells

We examine the regeneration kinetics of passivated emitter and rear solar cells (PERCs) fabricated on boron-doped p-type Czochralski-grown silicon wafers in darkness by electron injection via application of a forward bias voltage at elevated temperature (140 °C) in order to discriminate between electronic and photonic effects. Based on these dark regeneration experiments, we address the existing inconsistency regarding the measured linear dependence of the regeneration rate constant on the excess carrier density. Using the method of dark regeneration by current injection into the solar cell, we are able to measure the total recombination current of the solar cell at the actual regeneration temperature under applied voltage, i.e., at the physically relevant regeneration conditions. The direct comparison of the regeneration rate constant as a function of electronically injected carrier concentration in the dark and the regeneration rate constant during illumination clearly shows that the regeneration is a purely electronically stimulated effect and that photons are not directly involved.

Solder Interconnection of Aluminum Foil Rear Side Metallization for Passivated Emitter and Rear Solar Cells

Foil metallized (FolMet) solar cells combine the high-efficiency PERC technology and a cost-efficient rear side metallization based on a 9 µm thin aluminum foil. Laser fired contacts are used to attach the foil to the rear side and allow for the mechanical and electrical contact to the cell. Direct soldering on aluminum is hampered by a native oxide film formed immediately when exposed to air. To realize the cell interconnection by a standard solder process the Al foil is coated by sputter deposition or roll cladding of solderable layers. This work evaluates the solderability of 200 µm thick coated Al foils with the common solder interconnection process established in photovoltaics using standard copper ribbons for module integration. Our analysis reveals for both coating approaches strong initial mechanical adhesion of > 4 N/mm after soldering and very low contact resistivities of < 1.6 µ∙cm2. The contact resistivity shows no degradation after isothermal aging within 1000 hours at 85 °C. The mechanical adhesion of the sputter coated Al foils remains at 3 N/mm under thermal aging of more than 1000 h at 85 °C whereas the adhesion for the Al foils coated by roll cladding drops strongly to values of < 1 N/mm already after 50 hours. SEM images indicate that this effect is caused by re-oxidation of the aluminum surface supported by micro cracks in the solderable top layer.

Comparison of Free Energy Loss Analysis and Synergistic Efficiency Gain Analysis for PERC Solar Cells

The optimization of a solar cell requires a detailed knowledge of the efficiency limiting power losses. The free energy loss analysis (FELA) and the synergistic efficiency gain analysis (SEGA) both allow for studying these power losses. We compare both approaches for industrial passivated emitter and rear solar cells. The resistive losses calculated with both approaches are in good agreement with each other. In contrast, we find deviations of more than 20% for losses because of charge carrier recombination. From an analytical description of both methods, we conclude that these deviations are caused by a shift of the maximum power point voltage in an improved cell which is not considered in the FELA. The deviation of the losses critically depends on the specific loss channel investigated. In contrast with implied front-side power gains which are usually underestimated, rear-side or absorber gains can be over or underestimated by the FELA depending on the magnitude of the power loss. These differences follow from the Fermi level splitting which is affected differently by recombination currents at different locations. For an accurate determination of the efficiency improvement potential of individual recombination loss channels, we therefore recommend to perform a SEGA.

The devisable reflection-enhanced lumpy silver particle and its application in thin film amorphous silicon solar cell

A tailored large-size plasmonic core-protuberances silver particle with unique optical properties with promising applications in thin-film solar cells was numerically and experimentally studied. The designed particle possesses an extremely enhanced electromagnetic near-field excited by localized surface plasmons resonance and its far field angular plots of light scattering power indicated an enhanced back-scattering ability. When embedded the lumpy particles into the rear of thin-film amorphous silicon solar cells, light trapping in broad wavelength range and ideal short circuit density of the cells increased. A simple one-step reaction was established to acquire the structured particles. The strengthening backward scattering capacity in a broadband light region was also verified experimentally.

The effects of bifacial deposition of ALD AlOx on the contact properties of screen-printed contacts for p-type PERC solar cells

Most c-Si solar cells adopt monofacial thin film structures and consequently much effort, particularly at the equipment design level, is devoted to ensure monofacial thin film processing. This often leads to more complex production equipment with lower throughput. However, in some cases, bifacial processing can be tolerated, and it is shown that a bifacial deposition of aluminium oxide (AlOx) using atomic layer deposition (ALD) can even result in improved performance of the solar cell. The merits of ALD AlOx at the rear of p-type PERC solar cells are well established and this work shows that a thin AlOx layer on the front of a PERC solar cell can also significantly reduce the contact resistance of screen printed Ag without affecting the optical properties of the solar cell. In this work, a equidistant linear transmission line method (TLM) pattern is used to characterise the contact resistance and specific contact resistivity. This technique has the advantage of being able to measure the fingers of completed cells, and this also explores the error introduced by measuring the resistance across interjacent fingers. A 3 or 5 nm AlOx film reduces the contact resistivity to 0.06 mΩ.cm2 and 0.11 mΩ.cm2, respectively, significantly lower than a value of 0.25 mΩcm2 achieved with an uncapped reference. Also, the peak temperatures required to achieve a good contact was lower, and the optimum firing temperature window was found to be wider, in comparison to the reference. A 10 nm ALD AlOx film is found to have a detrimental effect on the contact resistance which cannot be mitigated by a higher firing temperature.

Optical and electrical characterization of poly-Si/SiOx contacts and their implications on solar cell design

The scope of this paper lies on the phenomenon of free-carrier absorption (FCA) in heavily phosphorus-doped poly-Si layers, applied at solar cells featuring poly-Si/SiOx passivating contacts at the rear. Firstly, FCA is investigated on test structures featuring poly-Si contacts of different thickness and doping level. Secondly, these passivating contacts are integrated into the rear of solar cells featuring a boron-diffused emitter at the front. The infrared (IR) response of the solar cells is analyzed and FCA losses are quantified. In agreement with theory, it is shown that Jsc losses due to FCA increase with poly-Si doping level and thickness. For instance, a total Jsc loss of ~0.5 mA/cm² is obtained for a 145 nm thick poly-Si layer with a doping concentration of 1.9x1020 cm-3.

Influence of nitride and nitridation on the doping properties of PECVD-deposited BSG layers

High efficiency p-type passivated emitter and rear (PERC) silicon (Si) solar cells are becoming industrial standard with demonstrated efficiencies above 22.6 %. Alternatively to the PERC design there is the possibility to apply a full-area boron back-surface-field (B-BSF) to the solar cell rear side for implementation of passivated emitter and rear totally diffused solar cells (PERT). In this work we investigate plasma enhanced chemical vapor deposited (PECVD) boron silicate glass (BSG) layers employing the precursor gases nitrous oxide (N2O), silane (SiH4) and diborane (B2H6). We experimentally demonstrate the impact of nitride (N) content in the BSG on the boron (B) doping efficiency within a thermal drive-in step, i.e. a subsequent high temperature diffusion step post BSG-deposition. It is found that the N content of the BSG which is mainly controlled by the N2O:SiH4 precursor gas flow ratio has to be kept as low as possible to achieve optimal B doping results of the Si bulk material. In addition an in-situ nitridation process has been developed to prevent build-up of a boron rich layer (BRL) on the highly doped Si surface. PERT and PERC solar cells were processed from material with different base resistivities and equal solar cell efficiencies were achieved.

In-situ diagnostics of PECVD AlOx deposition by optical emission spectroscopy

Plasma enhanced chemical vapor deposition (PECVD) is ubiquitously used in the crystalline silicon photovoltaics industry to deposit surface passivation and anti-reflection coatings. Aluminum oxide deposited by PECVD is becoming an increasingly common rear surface passivation layer, due to the growing market share of solar cells with partial rear contacts, such as the passivated emitter and rear solar cell. In this study, we use in-situ monitoring to investigate the correlation between the PECVD plasma properties and the resulting aluminum oxide film properties. Although no linear correlation between the density of the constituent radicals (aluminum, oxygen, and hydrogen) in the plasma and the surface passivation quality was observed, we did identify optimum processing conditions for a high quality surface passivation layer using in-situ plasma monitoring. This study also highlights the limited knowledge that currently exists within the photovoltaic community regarding plasma characterization and its impact on device performance.

High-Efficiency Modules With Passivated Emitter and Rear Solar Cells—An Analysis of Electrical and Optical Losses

We process a photovoltaic (PV) module with 120 half passivated emitter and rear cells that exhibits an independently confirmed power of 303.2 W and a module efficiency of 20.2% (aperture area). The cells are optimized for operation within the module. We enhance light harvesting from the inactive spacing between the cells and the cell interconnect ribbons. Additionally, we reduce the inactive area to below 3% of the aperture module area. The impact of these measures is analyzed by ray-tracing simulations of the module. Using a numerical model, we analyze and predict the module performance based on the individual cell measurements and the optical simulations. We determine the power loss due to series interconnection of the solar cells to be 1.5%. This is compensated by a gain in current of 1.8% caused by the change of the optical environment of the cells in the module. We achieve a good agreement between simulations and experiments, both showing no cell-to-module power loss.

Integrated Front–Rear-Grid Optimization of Free-Form Solar Cells

Free-form solar cells expand solar power beyond traditional rectangular geometries. With the flexibility of being installed on objects of daily use, they allow making better use of available space and are expected to bring in new possibilities of generating solar power in the coming future. In addition, their customizable shape can add to the aesthetics of the surroundings. Evidently, free-form solar cells need to be efficient as well. One way to improve their performance is to optimize the metallization patterns for these cells. This work introduces an optimization strategy to optimize the metallization designs of a solar cell such that its performance can be maximized. For the purpose of optimization, we model an existing transparent free-form solar cell design, including front and rear electrode patterns, to validate it against previously published experimental results. The front and rear metallizations of this transparent free-form solar cell are subsequently redesigned using topology optimization. More than 50% improvement in output power is achieved by using topology optimization.

Analysis of the losses of industrial‐type PERC solar cells

The loss analysis of state‐of‐the‐art p‐type Czochralski‐grown silicon passivated emitter and rear solar cells (PERC) fabricated in a manner close to industrial production is presented in this paper. The 6‐inch solar cells are featuring a homogeneous emitter on the front side, an Al2O3 passivation layer and local contacts on the rear side. The peak energy conversion efficiencies obtained are 21.1% for a standard antireflection coating (ARC) and 21.4% for a double‐layer ARC. The loss analysis is based on an extended characterization of the solar cells and of special samples, which allow the separation of the contributions of each region of the solar cell including metallization. Their impact is determined experimentally on the open‐circuit voltage, the short‐circuit current density, and the fill factor. Based on the measurements, the devices are numerically simulated. The free energy losses are analyzed using the simulation model. Based on the results from the loss analysis, it is found that the integration of a selective emitter structure and the further improvement of the rear surface would allow for efficiencies above 22% in the short term.

Simultaneous Contacting and Interconnection of Passivated Emitter and Rear Solar Cells

The back end process of passivated emitter and rear cells (PERC) consists of at least one laser process and three screen-printing steps followed by the stringing and tabbing of the cells. To reduce the number of steps we have developed a process that metallizes the rear side including contact formation and simultaneously interconnects the cells. We attach an Al foil to an encapsulant layer. By laser processing we form ‘laser-fired and bonding contacts’ (LFBC) on the passivated rear side of the solar cells. The Al foil contacting the rear is laser welded to the Ag screen-printed front side metallization of the next cell and thus forms the cell interconnection. The laser contacts on the rear show a surface recombination velocity Scont for the contact regions of ▪ cm/s and a contact resistivity of 3.52 mΩcm2. We present a first proof-of concept module combining the in-laminate Ag-Al laser welding and the LFBC reaching an efficiency of 18.4%. In accelerated aging test modules show no degradation (< 1% in efficiency) after 100 humidity-free cycles.

Quantification of Void Defects on PERC Solar Cell Rear Contacts

Passivated emitter and rear (PERC) solar cells can show void formation within their metallized local rear contact. It is known that voids can cause enhanced rear side recombination. In this work, we present a study on void related current losses and the correlation to the local Al-doped silicon layer which forms the back-surface field (BSF) at the rear contact. At first, void related current losses have been quantitatively evaluated. By further microstructural investigation at lengthwise prepared rear contacts, a method for the analysis of BSF thickness distribution was developed. It turned out that BSFs within voids can vanish on both, small and large length scales. Regions with voids can be well passivated and result therefore in negligible current losses. The presented approach allows the quantitatively assessment of PERC solar cell rear contacts in terms of total current losses and void ratio.

Influence of Silver-Aluminium Alloy at Solar Cell Rear Side on Series Resistance and Open Circuit Voltage

The rear side metallization of industrial PERC solar cells was investigated regarding the implication on the silver-aluminium-alloy formation, in particular the effect of the high resistivity alloy formation and the etching of surface passivation layer. By changing the dimensions of the printed silver-pads the size of the silver-aluminium-overlap region at the pad-aluminium interface were varied. The influence on fill factor and open circuit voltage are evaluated. For a three busbar cell design with six pads per busbar, the fill factor decreases up to 2.2%abs, if the overlap length between aluminium and silver pad is reduced from 12.5 mm to 1 mm, whereas the open circuit voltage increases by 2.5 mV. These results are additionally confirmed by a 3D simulation which validates the measurement results and shows perspectives for reducing the series resistance effect of the silver-aluminium overlap region up to 40% by increasing the busbar count from 3 to 5.

P-Type Indium-Doped Passivated Emitter Rear Solar Cells (PERC) on Czochralski Silicon Without Light-Induced Degradation

Solar cells fabricated on boron (B)-doped Czochralski (Cz) Si wafers in the photovoltaic industry are known to suffer from light-induced degradation (LID) in efficiency. This paper reports on promising LID-free large-area indium (In)-doped Cz Si solar cells. Two different commercial-grade B-doped Cz materials were included for comparison. To study the impact of LID on the cell structure, ion-implanted large-area (239 and 242.22 cm2) screen-printed full aluminum (Al) back-surface field (BSF) baseline cells, as well as higher performance passivated emitter rear cells (PERC) with oxide passivation and local Al BSF, were fabricated. In-doped PERC cells achieved 20.3% efficiency, while the B-doped cells gave efficiencies of 20.7% and 20.5% from low- (2 Ω⋅cm) and high-resistivity (6.2 Ω⋅cm) substrates, respectively. It was found that initial efficiency of In-doped PERC cells was ∼0.2% lower due to lower bulk lifetime and higher back-surface recombination velocity. However, In-doped PERC cells showed no LID and surpassed the B-doped PERC cell efficiency by 0.3–0.5% after 0.8-sun 48-h illumination at 37 °C.

Unassisted photoelectrochemical water splitting exceeding 7% solar-to-hydrogen conversion efficiency using photon recycling.

Various tandem cell configurations have been reported for highly efficient and spontaneous hydrogen production from photoelectrochemical solar water splitting. However, there is a contradiction between two main requirements of a front photoelectrode in a tandem cell configuration, namely, high transparency and high photocurrent density. Here we demonstrate a simple yet highly effective method to overcome this contradiction by incorporating a hybrid conductive distributed Bragg reflector on the back side of the transparent conducting substrate for the front photoelectrochemical electrode, which functions as both an optical filter and a conductive counter-electrode of the rear dye-sensitized solar cell. The hybrid conductive distributed Bragg reflectors were designed to be transparent to the long-wavelength part of the incident solar spectrum (λ>500 nm) for the rear solar cell, while reflecting the short-wavelength photons (λ<500 nm) which can then be absorbed by the front photoelectrochemical electrode for enhanced photocurrent generation.

Low-cost foil metallization using arc discharge for passivated emitter and rear solar cells

For the cost reduction of passivated emitter and rear cells (PERC), we propose a new rear contact formation method, in which an aluminum foil and an arc discharge system are used. The arc discharge system consists of inexpensive parts and does not contain any sophisticated part such as a laser ablation apparatus. Therefore, this system can save the cost of the rear contact forming process. We applied this technique to a test production of PERC. It is found that the arc discharge system can provide a similar performance to that attained by a conventional PERC production method.

Two-dimensional current flow in stringed PV cells and its influence on the cell-to-module resistive losses

In a c-Si wafer based photovoltaic (PV) module, the current generated in each solar cell must flow to the ribbons soldered onto its front and rear surfaces. Conventionally, a simplified current flow pattern is assumed in calculating the resistive power losses in the cell plane and the ribbons. However, it is found in this work that this approach leads to more than 30% overestimation for the resistive loss on the rear-side ribbons for a typical monofacial solar cell. This work uses a detailed two-dimensional network simulation, which accurately solves the current flow patterns in both cell surface planes. It is found from this work that the conventional approach is sufficiently accurate in the case of an H-pattern finger-busbar metallization scheme, but not with a full-area metallization pattern, as commonly used on the rear of silicon wafer solar cells. For a c-Si PV module using 3-busbar monofacial solar cells, the actual resistive loss on the rear metallization plane is only 67% of the value calculated conventionally. Also, 17% of the cell interconnection resistive loss comes from the rear metal sheet, which is often ignored. To correct the results of the conventional approach, we introduce correction factors for c-Si PV modules using monofacial cells. Using the correction factors we achieve a significantly better prediction of the module’s fill factor. We apply this approach to explain why the fill factor gain from replacing full-size solar cells by halved cells in a c-Si PV module is higher for bifacial than for monofacial solar cells. Experimentally, we find a 1.7% relative fill factor increase for monofacial halved-cell PV module, and a 2.0% relative increase for bifacial halved-cell PV module. The simulation results agree quite well with the experimental results. Since the influence from the cell metallization pattern on the cell-to-module resistive loss has not been investigated before, this work can be very useful correction and compensation for established methods analysing the cell-to-module losses. It also offers the guideline for PV module optimization in terms of further reducing the resistive loss.