Industrial Passivated Emitter And Rear Cell Research Articles

Performance optimization of PERC solar cells based on laser ablation forming local contact on the rear

Abstract To improve the photoelectric conversion efficiency (η) of the solar cell, a green wavelength (532 nm) laser source in a nanosecond range was used to ablate the passivated emitter and rear cell (PERC) to form the contact holes. If the laser ablation hole opening process was not set properly, the diameter or the external expansion of holes would be too large, causing the decline of the PERC performance. The Gaussian distribution of the laser is regulated by the output power (P o) and the repetition frequency (f rep) of the incident pulse laser, so that the optimized morphology of holes is obtained on the back of the PERC solar cells. After the contact holes are screen printed by the aluminum paste, the local back surface field is finally formed. The experimental results showed that the outward expansion decreases obviously with the increase of laser P o. Second, the spacing of the holes decreases with the increase of the laser f rep. It was found that under the laser P o of 33.0 W and f rep of 1,400 kHz, the η of the industrial PERC solar cells was the highest. The Quokka simulations indicated that small outward expansion, small diameter, and long spacing of holes would further decrease the recombination parameter in the rear surface. With the optimized morphology of contact holes and the low contact resistance, the PERC cell’s calculated V oc and η improvements were 6.5 mV and 0.48%, respectively, which was verified with experimental findings.

Process and device simulations aimed at improving the emitter region performance of silicon PERC solar cells

Achieved levels of silicon-based passivated emitter and rear cell (PERC) solar cells laboratory and module-level conversion efficiencies are still far from the theoretically achievable Auger limit of 29.4% for silicon solar cells, prominently due to emitter recombination and resistive losses. The emitter region in PERC devices is formed by using either ion implantation followed by a diffusion process or POCl3 diffusion. In ion-implanted emitter-based PERC, the process variables such as dose, energy, diffusion time, and temperature play a vital role in defining the characteristics of the emitter region. Detailed investigation of these parameters could provide a pathway to mitigate the recombination as well as resistive losses; however, it requires a considerable budget to optimize these parameters through a purely experimental approach. Therefore, advanced industrial standard process and device simulation are perceived in this work to carry out the comprehensive study of process variables. Investigation of ion implantation and diffusion process parameters on the PV performance of an upright pyramid textured, industrial standard stacked dielectric passivated PERC solar cell is carried out to deliver 22.8% conversion efficiency with improved PV parameters such as short circuit current density (J SC) of 40.8 mA cm−2, open-circuit voltage (V OC) of 686 mV, and fill-factor (FF) of 81.54% at optimized implantation and diffusion parameters, such as implantation dose of 5 × 1015 cm−2 with energy 30 keV followed 950 °C diffusion temperature and 30 min of diffusion time. The performance of the optimized PERC device is compared with already published large area screen printed contact-based device. This work may open up a window for the experimental work to understand the influence of process parameters on the emitter region to develop the highly efficient PERC solar cell in the future.

Ultra-thin Al2O3 capped with SiNx enabling implied open-circuit voltage reaching 720 mV on industrial p-type Cz c-Si wafers for passivated emitter and rear solar cells

In this study, we report on the passivation quality of atomic layer deposition grown ultra-thin Al2O3 and Al2O3 capped with plasma-enhanced chemical vapor deposition deposited SiNx on Cz p-type wafers for the rear side of a passivated emitter and rear cell (PERC). Different activation recipes using N2, forming gas (FG), and two-step annealing for different durations are investigated before SiNx deposition. The effect of different Al2O3 thicknesses and corresponding activation processes on the Al2O3/SiNx passivation performance, after a high temperature firing step, is studied to reach a new optimization toward higher efficiency and lower cost. A record high iVoc of 720 mV is obtained after firing step from Al2O3/SiNx stacks with Al2O3 thickness as thin as ∼2 nm with FG annealing. Our results demonstrate that, under well-optimized process conditions, ultra-thin Al2O3 thicknesses provide superior passivation quality as compared to the larger thicknesses which are commonly applied in the PERC industrial line and the potential for further improvement of industrial PERC solar cells in terms of cost reduction and efficiency.

Pathways for efficiency improvements of industrial PERC silicon solar cells

The global manufacturing capacity of Passivated Emitter and Rear Cell (PERC) devices on p-type Czochralski-grown silicon (Cz-Si) wafers is increasing rapidly. This paper analyses various industrial process improvements carried out in our lab to improve the efficiency of large-area Cz-Si PERC solar cells from 20.7% to 21.9%. The key improvements presented in this paper include the transition from line to dot-shaped laser openings in the rear dielectric stack, a dual printing approach for the front fingers and implementation of a laser-doped selective-emitter structure. A nanosecond (ns) pulsed laser source with 532 nm wavelength was used for both the rear dielectric ablation and the front laser doping. By adopting a systematic power loss analysis approach and targeting the largest power loss mechanisms at various stages of development, the best efficiency of the solar cells in this experimental study was improved from 20.7% to 21.9%. Pathways for further improvements in efficiency are analysed, including the application of passivated-contact structures in a PERC-like cell.

Efficiency improvement of passivated emitter and rear cells using annealing process before surface passivation

Thermal annealing and effective surface passivation are significant to achieve high conversion efficiency (η) of passivated emitter and rear cells (PERC). Thus, we added an annealing process before front and rear surface passivation in the existing production line. We found that the front recombination current (J0,f) and the rear recombination current (J0,r) of annealed PERCs are both lower than baseline PERCs without annealing process, which means better defect repair and passivation effect were achieved. And the average η of annealed PERCs reaches 21.79%, which is 0.23% absolutely higher than that of the baseline PERCs. After that, we further explored the influence of gas composition (0%, 5% and 10% oxygen concentration in nitrogen atmosphere) during annealing process. We found that as the oxygen concentration increases, J0,fs and J0,rs of annealed PERCs (0%, 5%, 10%) are all lower than those of baseline PERCs. Meanwhile, the annealed PERCs (0%, 5%, 10%) get the average η of 21.77%, 21.82% and 21.84% respectively, possessing absolute efficiency increase over the baseline PERCs with the average η of 21.67%. The present work could help enhance the understanding of passivation property and improve the performance of industrial PERC cells.

Optimization of rear surface roughness and metal grid design in industrial bifacial PERC solar cells

The bifacial p-type silicon (p-Si) passivated emitter and rear cells (PERCs) are predicted to dominate the industrial bifacial solar cells. In this work, we have investigated the impact of different rear surface morphologies on the electrical performance of bifacial PERCs with both five-busbar (5BB) and nine-busbar (9BB) grid design. The passivation and optical properties with differing rear surfaces are evaluated on semi-device structures. The depth of local aluminum back surface field is hardly affected by the rear surface morphology. The calculated efficiency loss analysis indicates that the negative electrical impact with enlarged rear surface area is more serious for rear side than that of front side. The batch conversion efficiency of 9BB bifacial PERCs increases by 0.2%–0.3% comparing to 5BB ones depending on the rear surface roughness. Consequently, a highest front-side average efficiency of 22.57%, with a champion efficiency of 22.75%, has been achieved for 9BB bifacial p-Si PERCs with a nearly planar rear surface. A highest bifaciality of 78.7% is realized for both 5BB and 9BB bifacial PERCs with the roughest rear surface. We have further simulated the relative enhancement of electric generation to compare the performance of bifacial PERCs in practical application.

Potential-Induced Degradation of Bifacial PERC Solar Cells Under Illumination

Bifacial passivated emitter and rear cell (PERC) solar cells are promising candidates to reduce levelized cost of electricity since a rear-side power gain can be achieved with minor changes in cell production. However, in addition to the potential-induced degradation of the shunting type (PID-s), some bifacial solar cells may exhibit another degradation related to their rear side. Using three types of purchased industrial bifacial PERC cells encapsulated in our laboratory, we show that a rear-side PID test under simultaneous illumination can cause up to 14% power degradation, which is related to increased carrier recombination at the rear side of the cell. Thus, we find that the degradation under realistic field conditions is strong enough to significantly reduce the bifaciality gains. This is of particular importance as only some of the investigated cells recover under dark conditions. Microstructural analysis of the PID-stressed rear side reveals localized permanent structural damages of the passivation layer leading to an increased rear-side recombination.

Industrial Silicon Solar Cells Applying the Passivated Emitter and Rear Cell (PERC) Concept—A Review

Even though the passivated emitter and rear cell (PERC) concept was introduced as a laboratory-type solar cell in 1989, it took 25 years to transfer this concept into industrial mass production. Today, PERC-type solar cells account for 10% of the worldwide produced solar cells, and their share is expected to rapidly increase up to 35% within the next few years. Record efficiencies up to 22.1% of industrial PERC cells approach an efficiency of 22.8% of the lab-type PERC cell in 1989. This paper reviews the most important research results and technological developments of the past 25 years, which enabled the successful transfer of the lab-type PERC concept into industrial mass production. Particular attention is paid to the development of AlO x /SiN y layer stacks with excellent rear surface passivation properties and low production costs. In addition, we summarize the most important research results and technological improvements of industrially processed local aluminum rear contacts. Furthermore, we describe the most relevant process flows to manufacture industrial PERC cells and address silicon wafer material requirements regarding high and stable charge carrier lifetimes. An outlook is provided on future development opportunities, which may further increase the conversion efficiency and the energy yield of industrial PERC solar cells.

Simulation-based Efficiency Gain Analysis of 21.2%-efficient Screen-printed PERC Solar Cells

Passivated Emitter and Rear Cells (PERC) with efficiencies well above 20% are likely to become the next mass production technology. A quantification of all power loss mechanisms of such industrial PERC cells is helpful in prioritizing future efficiency improvement measures. We report on a numerical simulation of the power losses of a 21.2%-efficient industrial PERC cell using extensive experimental input data. Our synergetic efficiency gain analysis relies on deactivating single power loss mechanisms in the simulation at a time to access the full potential power gain related to that mechanism. The complete analysis therefore explains the efficiency gap between the industrial PERC solar cell and the theoretical maximum efficiency of a crystalline Si solar cell. Based on the simulations, the largest single loss mechanism is front grid shadowing followed by recombination in the emitter and its surface. All individual resistive losses, all individual optical losses and all (avoidable) individual recombination losses sum up to efficiency gains of 0.8%, 1.6%, and 1.3%, respectively, which is 3.7% in total. The efficiency gap between real and ideal solar cell is, however, much larger with 7.3%. The discrepancy is mainly due to the non-linear behaviour of recombination-based power losses which adds synergetic efficiency enhancements.

Model Based Continuous Improvement of Industrial p-type PERC Technology Beyond 21% Efficiency

In this work, we present our progress in the industrial p-type PERC technology [1, 2]. Based on device simulations we continuously develop an efficiency roadmap for a steady improvement of our PERC (passivated emitter and rear cell) process. Following this simulation based approach, we effectively improve the front side metallization and the emitter characteristics. Currently, our best prototype process has reached a conversion efficiency well over 21% which enables the manufacturing of a 60-cell based module with a power of 310W. Our best cell so far has a conversion efficiency of 21.5% which has been confirmed by the calibration laboratory of Fraunhofer ISE. This is to our knowledge the highest efficiency reported for industrial-size silicon solar cells with screen-printed metal front and rear contacts.

Sensitivity Analysis of Industrial Multicrystalline PERC Silicon Solar Cells by Means of 3-D Device Simulation and Metamodeling

How successfully an improved solar cell concept is transferred to mass production depends not only on the realized cell efficiency, but crucially on the stability of the fabrication process, i.e., on the distribution of the current-voltage (I-V) parameters. To model such distributions, we use three-dimensional (3-D) full-size device simulations of passivated emitter and rear cells (PERCs). The number of these time-consuming simulations is drastically reduced by changing all input parameters concurrently in a design of experiment approach. A simple polynomial response surface methodology (RSM) model is obtained from these simulations by regression analysis. The RSM contains all the mutual nonlinear interactions between the device parameters, and is therefore called a metamodel. The metamodel is applied: 1) to find maximum efficiency; 2) to compute how sensitively each device parameter influences the I-V parameters; 3) to explain, predict, and manipulate the distribution of the I-V parameters in mass production; and 4) to find an optimum starting point for experiments. For example, we demonstrate how the choice of the distance between the rear local point contacts leads to either maximal median efficiency but with a broad distribution, or to a slightly reduced median cell efficiency but with a narrow distribution and a reduced number of bad cells.