A comparative life-cycle assessment of photovoltaic electricity generation in Singapore by multicrystalline silicon technologies

In this work, we proposed the impact of the rear laser process design applied to screen-printed passivated emitter and rear cell (PERC) structure. We compared different laser pattern design 1) line (reference), 2) Dash (2∶1), and 3) Dash (5∶1) on a high-efficiency level using 156 mm × 156 mm p-type Cz mono silicon wafer. It can be found that employing the laser pattern of dash 2:1 outperforms all other laser pattern due to its excellent aluminum back surface field. The average open-circuit voltage of the laser pattern of Dash 2∶1 PERC solar cells is also larger than all other opening ratio solar cells. Besides, by using the laser pattern of Dash 2∶1 PERC solar cells, the conversion efficiencies of 20.6% for the best PERC solar cell and 20.4% for the average of PERC solar cells are measured. Besides, the PERC solar cells also passed the PID test which condition was indicated to be 85 degree Celsius and 85% RH in climatic chamber for 96 hours. Observed the power loss < 3% and no EL darkened area were found after PID test for the PERC solar cells.

Commercial passivated emitter and rear cell (PERC) devices are critically hindered by a thus-far unidentified degradation mechanism called light- and elevated temperature-induced degradation (LeTID). In contrast to PERC devices, aluminum back-surface field (Al-BSF) devices are markedly more resistant to the same degradation mechanism. In this work, we employ device simulations to elucidate the differences between Al-BSF and PERC degradation behavior and thus accelerate the search for the root cause of LeTID. We find that a difference in defect activation under degradation conditions is a more likely explanation than enhanced sensitivity to bulk lifetime for PERC compared to Al-BSF devices. By employing device simulation for the two architectures under high illumination intensity, we identify a narrow parameter space for the LeTID defect precursor. When combined with experimental observations, this may yield important new information about LeTID defect formation.

Light‐induced degradation (LID) is a well‐known problem faced by p‐type Czochralski (Cz) monocrystalline silicon (mono‐Si) wafer solar cells. In mono‐Si material, the physical mechanism has been traced to the formation of recombination active boron‐oxygen (B–O) complexes, which can be permanently deactivated through a regeneration process. In recent years, LID has also been identified to be a significant problem for multicrystalline silicon (multi‐Si) wafer solar cells, but the exact physical mechanism is still unknown. In this work, we study the effect of LID in two different solar cell structures, aluminium back‐surface‐field (Al‐BSF) and aluminium local back‐surface‐field (Al‐LBSF or PERC (passivated emitter and rear cell)) multi‐Si solar cells. The large‐area (156 mm × 156 mm) multi‐Si solar cells are light soaked under constant 1‐sun illumination at elevated temperatures of 90 °C. Our study shows that, in general, PERC multi‐Si solar cells degrade faster and to a greater extent than Al‐BSF multi‐Si solar cells. The total degradation and regeneration can occur within ∼320 hours for PERC cells and within ∼200 hours for Al‐BSF cells, which is much faster than the timescales previously reported for PERC cells. An important finding of this work is that Al‐BSF solar cells can also achieve almost complete regeneration, which has not been reported before. The maximum degradation in Al‐BSF cells is shown to reduce from 2% (relative) to an average of 1.5% (relative) with heavier phosphorus diffusion.

Upgraded metallurgical-grade silicon (UMG-Si) has the potential to reduce the cost of photovoltaic (PV) technology and improve its environmental profile. In this contribution, we summarize the extensive work made in the research and development of UMG technology for PV, which has led to the demonstration of UMG-Si as a competitive alternative to polysilicon for the production of high-efficiency multicrystalline solar cells and modules. The tailoring of the processing steps along the complete Ferrosolar’s UMG-Si manufacturing value chain is addressed, commencing with the purification stage that results in a moderately compensated material due to the presence of phosphorous and boron. Gallium is added as a dopant at the crystallization stage to obtain a uniform resistivity profile of ∼1 Ω cm along the ingot height. Defect engineering techniques based on phosphorus diffusion gettering are optimized to improve the bulk electronic quality of UMG-Si wafers. Black silicon texturing, compatible with subsequent gettering and surface passivation, is successfully implemented. Industrial-type aluminum back surface field (Al-BSF) and passivated emitter and rear cell (PERC) solar cells are fabricated, achieving cell efficiencies in the range of those obtained with conventional polysilicon substrates. TOPCon solar cell processing key steps are also tested to further evaluate the potential of the material in advanced device architectures beyond the PERC. Degradation mechanisms related to light exposure and operation temperature are shown to be insignificant in UMG PERC solar cells when a regeneration step is implemented, and PV modules with several years of outdoor operation demonstrated similar performance to reference ones based on poly-Si. Life cycle analysis (LCA) is carried out to evaluate the environmental impact of UMG-based PV technology when compared to poly-Si-based technology, considering different scenarios for both the manufacturing sites and the PV installations.

This chapter introduces monofacial passivated emitter and rear cells (PERC) and bifacial PERC+ solar cells which are the mainstream solar cell technology in the photovoltaic (PV) industry today. The authors describe the PERC solar cell design as well as major technology development steps in the past decades such as the bifacial PERC+ design. The process technology to manufacture PERC solar cells is covered, whereas the specific aspects of the screen-printed Ag front and Al rear metal contacts are introduced in detail in later sections. The busbar design of PERC metal grids for module interconnection is also discussed. Finally, an outlook on the future improvement opportunities of PERC and PERC+ solar cells is given, in particular regarding the metal contact design and its impact on conversion efficiency.

This paper reviews the development of the passivated emitter and rear cell (PERC) silicon solar cell in the 1980s, which set several efficiency records, but was not taken up commercially at the time. Following extensive development of suitable fabrication processes, materials, and production tools, the PERC solar cell is now on track to become the dominant commercial solar cell. Since photovoltaics (PV) itself is on track to become the dominant energy generation technology, the PERC is having a global impact in both energy generation and greenhouse gas emission reduction. Assuming an average growth rate of annual PV installations of 25%, PV mitigation of greenhouse gases will reach about 5% in 2022, including 2% from PERCs, with much higher values expected later in the 2020s. This review focuses on the period of development of the PERC during the 1980s.

Passivated emitter and rear cell (PERC) solar cells with p-type silicon (Si) substrate have become the mass production factor in the global photovoltaic (PV) market. However, recombination loss at the Si/dielectric interface restricts the performance of the PERC device toward higher efficiency. A local front and back surface doping are utilized to prevent such recombination by creating n/n+ or p/p+ junction in conventional Si solar cells. However, it is hard to achieve a p/p+ based back surface field (BSF) layer using conventional doping methods in PERC solar cell owing to the device architecture. Therefore, in this manuscript, an alternative approach has been discussed to create the p/p+ based BSF in PERC solar cell without altering the device architecture. Here, silicide electrode-based electrostatic doping (ED) is used to induce p+ doping near the back surface in an upright regular pyramid textured-based PERC device. Initially, conventional fully aluminum screen-printed rear electrode-based PERC device is designed and simulated using a process and device simulator to which reflected 25.67% conversion efficiency. Afterward, the concept of ED has been introduced in the conventional PERC device to induce a p+ region at the backside. The silicide electrode with work function (WF) of 5.4 eV (Pd 2 Si) and 5.2 eV (WSi 2 ) plays an important role in the formation of p-type doping especially near the Si/dielectric interface at the backside. The performance of the proposed ED-PERC device has been investigated using PV parameters and the current density-voltage (J-V) curve. Results depict that higher conversion efficiency of 26.32% has been achieved with higher WF silicide Pd 2 Si (5.4 eV) owing to the induced heavily doped p+ region. Optimized PERC device with metal silicide Pd 2 Si (5.4 eV) delivers short circuit current density (J SC ) of 41.87 mA/cm2, open-circuit voltage (V OC ) of 0.745 V, and fill-factor (FF) of 84.38%. The reported study of silicide ED-based BSF in PERC devices may open a window for further enhancement in PERC device performance.

Passivated emitter and rear cell (PERC) silicon solar cells are currently being migrated to mainstream production in recent years. However, emitter recombination loss has become the main limiting factor in the PERC devices. The emitter region is formed using ion implantation followed by a diffusion process, and the process variables such as dose (cm-2), energy (keV), diffusion time (s), and temperature (oC) play a crucial role in the formation of the emitter region with desired properties such as peak doping concentration as well as the thickness of emitter region. The influence of these process variables on the performance of PERC solar cell is not investigated in the literature using process and device simulations. Therefore, to examine the emitter region performance in terms of ion implantation dose of phosphorus, an industrial standard stacked nitride-based front and back dielectric antireflective coated PERC solar cell has been designed and simulated by using Silvaco TCAD based process and device simulators. The objective is to analyze the influence of ion implantation dose of phosphorus on the performance of the PERC device while keeping the rest of the process-related parameters intact. A comprehensive analysis of the proposed research work has been studied using the current density-voltage (J-V), EQE curves and photovoltaic (PV) parameters. The PERC solar cell exhibits the highest conversion efficiency of 25.2% at the optimized phosphorus dose of 5×1015 cm-2. The PERC device under consideration reflects short circuit current density (J SC ) of 41.78 mA/cm2, open-circuit voltage (V OC ) of 0.72 V, and fill-factor (FF) of 83.57%. The reported study may open a window for the experimental work to understand the influence of phosphorus ion implantation dose on the quality of the emitter region and for further enhancement of conversion efficiency of the PERC solar cell.

The passivated emitter and rear cell (PERC) concept is currently rapidly being introduced into industrial mass production and is expected to be the new silicon wafer based solar cell technology standard in the photovoltaics industry. In 2018, PERC-type solar cells accounted for approximately 40% of the worldwide produced solar cells and their share is expected to rapidly increase up to 70% within the next few years. Compared to the previous industrial silicon solar cell technology which applied a full-area aluminum rear contact, PERC cells passivate the rear silicon wafer surface with a dielectric layer and only locally contact the silicon wafer with an aluminum metal contact, which reduces charge carrier recombination and hence increases the conversion efficiency. Present record conversion efficiencies up to 22.8% of industrial PERC cells hence exceed the efficiency of conventional Al-BSF silicon solar cells by more than 2%abs. In addition, PERC solar cells can be made bifacial by substituting the full-area rear aluminum layer with an aluminum finger grid design. This so-called PERC+ solar cell design enables large volume industrial manufacturing of bifacial silicon solar cells which absorb stray light from the rear side and hence increase the energy yield by 5–25% depending on the detailed module installation conditions. This chapter describes the most important research results and technology developments of the past decades as well as the present status of industrial PERC and PERC+ solar cells.

Enhanced performance of silicon solar cells by application of low-cost sol–gel-derived Al-rich ZnO film

In order to maximize performance in all conditions of use, and to model exactly the performance of solar cells, it is very important to study the recombination parameters under different injection levels. In this paper, the recombination parameters and their effect on the output performance of solar cells are investigated under different injection levels for the full-area aluminum back surface field (Al-BSF) solar cell and passivated emitter and rear cell (PERC) solar cell for the first time. It is found that the recombination parameter J01 of the diffusion region and the recombination parameter J02 of the depletion region for the PERC solar cell are smaller than those of the Al-BSF solar cell under the same injection level. A new finding is that the recombination parameter J01 of Al-BSF solar cells increases quickly with the decreasing injection level compared with PERC solar cells. Finally, the J01/J02 of Al-BSF and PERC solar cells is investigated, and the effects of J01/J02 on the electrical parameters are also analyzed for Al-BSF and PERC solar cells under different injection levels. The obtained conclusions not only clarify the relationship between the recombination parameters and injection levels, but also help to improve cell processes and accurately model daily energy production.

Passivated emitter and rear cells (PERC) are considered to be the next generation of industrial-type screen-printed silicon solar cells. However, today there exist only few deposition methods for rear passivation layers which meet both, the high-throughput and low-cost requirements of the PV industry in combination with high-quality surface passivation properties. In this paper, we evaluate and optimize a novel deposition technique for AlO x passivation layers applying an inductively coupled plasma (ICP) plasma-enhanced chemical vapour deposition (PECVD) process. The ICP AlOx deposition process enables high deposition rates of up to 5 nm/s as well as excellent surface recombination velocities below 10 cm/s after firing. When applied to PERC solar cells the ICP AlOx layer is capped with a PECVD SiNy layer. We achieve independently confirmed conversion efficiencies of up to 20.1% for large-area (15.6x15.6 cm 2 ) PERC solar cells processed at ISFH with screen-printed metal contacts and ICP AlOx/SiNy rear side passivation on standard boron-doped Czochralski-grown silicon wafers. The internal quantum efficiency reveals an effective rear surface recombination velocity Srear of (90 ± 30) cm/s and an internal rear reflectance Rb of (91 ± 1)% which demonstrates the excellent rear surface passivation of the ICP AlOx/SiNy layer stack. PERC solar cells processed in the Q-Cells Research Line achieve efficiencies up to 19.6% with ICP AlOx/SiNy rear passivation which is comparable to the reference process at Q-Cells.

Design and simulations of 24.7% efficient silicide on oxide-based electrostatically doped (SILO-ED) carrier selective contact PERC solar cell

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.

Passivating contacts have recently considered as a superior carrier-selective contact approach for high-efficiency silicon-based photovoltaic devices. However, the conversion efficiencies of the silicon-based passivated emitter and rear cell (PERC) are limited by contact recombination losses that reduce their performance. Therefore, we investigated a new manufacturable silicide on oxide-based electrostatically doped (SILO-ED) carrier-selective contact to suppress the contact recombination losses and reduce the saturation current density (j 0). For the first time, double side electrostatic doping is introduced to the PERC devices to form the carrier selective passivating contacts. First, a conventional PERC device was designed and the effects of surface recombination velocity (SRV) at both contacts were studied. After that, single and double SILO-ED based contacts are introduced into the device and a systematic analysis is performed to understand the tunneling phenomena and improve the conversion efficiency compared to existing PERC cells. The front SILO-ED based device with back contact SRV of 10 cm s−1 showed a power conversion efficiency of 25.4% with j 0 (14.3 fA·cm−2). In contrast, the double SILO-ED device delivered 25.7% conversion efficiency by further suppressing the j 0 to 11.8 fA·cm−2 by implementing SILO-ED approach with two different metal silicides such as erbium silicide (ErSi2) and palladium silicide (Pd2Si) on front and rear contact surface. The champion double SILO-ED PERC solar cell delivered a conversion efficiency of 25.7% with an open circuit voltage (V OC) of 742 mV. The results reported in this study would help to develop superior passivating contact-based PERC solar cells for higher efficiencies.