Chee Mun Chong

18.5% laser-doped solar cell on CZ p-type silicon

For many years, the selective emitter approach has been well-known to yield cell efficiencies well above those achieved by conventional screen-printed cells. A simple and effective way of forming a selective emitter can be achieved by laser doping to simultaneously pattern the dielectric with openings as narrow as 8 µm, and create heavy doping beneath the metal contacts. In conjunction with laser doping, light-induced plating (LIP) is seen as an attractive approach for forming metal contacts on the laser-doped regions, without the need for aligning masks or other expensive, long laboratory processes. As laser-doping is gaining increasing interests in the PV industry, selection of the most appropriate laser and processing conditions is important to ensure high yields in a production environment. In this work, we have identified a suitable laser that enables good ohmic contacts for a wide range of laser scan speeds. Sheet resistances of laser-doped lines as low as 2 ohms/sq was achieved at a scan speeds of <1 m/s, while a sufficiently high doping (∼20 ohms/sq) is still achievable at scan speeds up to 6 m/s. Optimization of the laser parameters in this work lead to a cell efficiency of 18.5% being achieved with the laser-doped selective emitter (LDSE) structure. The cell also has an excellent pseudo fill factor (pFF) of 82.3% and a local ideality factor n nearing unity. This indicates there is minimal laser-induced damage and junction recombination as a result of the laser doping process.

18% efficient polycrystalline silicon solar cells

Over the past 7 yr, there has been marked improvements in crystalline silicon solar cell performance, with the highest independently confirmed cell efficiency increasing from 17.1% to 24.2%. Work directed at transferring some of these improvements to polycrystalline silicon cells is described. Applying a high-efficiency crystalline cell sequence has given efficiencies as high as 17.8% with the addition of a phosphorous pretreatment step and modifications of a rear Al alloying step and cell processing temperatures. Surface texturing is identified as an important area requiring attention to obtain the highest possible efficiency. Laser texturing has given the best results to date for polycrystalline substrates. Results are described for a laser-textured, laser-grooved cell processing sequence with the potential to produce polycrystalline cells having efficiencies well above 18%.<<ETX>>

A techno-economic analysis method for guiding research and investment directions for c-Si photovoltaics and its application to Al-BSF, PERC, LDSE and advanced hydrogenation

A techno-economic analysis method is used to analyse industry standard mono crystalline silicon photovoltaic technologies – Aluminium Back Surface Field (Al-BSF) and Passivated Emitter and Rear Cell (PERC), together with promising process variations – Laser Doped Selective Emitter (LDSE) and three implementations of advanced hydrogenation. Building on a previously reported manufacturing cost and uncertainty analysis method, the impact of uncertainty in module performance and market price is added to estimate the manufacturers gross margin. Two additional interpretation methods are described – (i) simultaneous Monte Carlo and (ii) contribution to variation – that help distinguish the impact of small differences between sequences, and identify the most important factors (cost, performance or market) affecting commercial viability. Combining these methods allows a rapid commercial viability assessment without requiring exact data on all the inputs. The analysis indicates that PERC is more commercially attractive than Al-BSF, with a median improvement in manufacturers margin >5%. Al-BSF + LDSE is found to reduce manufacturers margin. The PERC + LDSE and PERC + advanced hydrogenation sequences are estimated to provide a median margin improvement >2% compared to PERC alone. The advantage of PERC + LDSE depends strongly on delivering the expected 0.9%abs cell efficiency gain with high production yields and receiving a selling price premium for higher power modules; less important is the production cost of the LDSE process steps. The advantage of advanced hydrogenation depends strongly on achieving the expected 0.2%abs as-produced efficiency gain with high production yield, as well as realising a 0.5% selling price premium for being “CID-Free”; these factors outweigh the production costs of the alternative hydrogenation processes.

LEDs for the Implementation of Advanced Hydrogenation Using Hydrogen Charge-State Control

Light-induced degradation (LID) of p-type Cz solar cells has plagued the industry for many decades. However, in recent years, new techniques for solving this LID have been developed, with hydrogen passivation of the boron-oxygen defects appearing to be an important contributor to the solution. Advanced hydrogenation approaches involving the control of the charge state for the hydrogen atoms in silicon to enhance their diffusivity and reactivity are developed and evaluated in this work for commercial application using a prototype industrial tool in conjunction with solar cells manufactured on commercial production lines. This prototype tool, unlike the previous successful laser-based laboratory approaches, is based on the use of LEDs for controlling the charge state of the hydrogen atoms. The illumination from the LEDs is also used in this work to passivate process-induced defects and contamination from the respective production lines with significant improvements in both efficiency and stability. The results indicate that the low-cost LED-based industrial tool performs as well as the laser-based laboratory tool for implementing these advanced hydrogen passivation approaches.

Copper plated contacts for large-scale manufacturing

Copper plated contacts have the potential to become the dominant front metallization technology over silver contacts if several key issues can be solved. These are addressed in this work. Through large batches of cells produced at Suntech on Plutos 0.5 GW production line, it is shown that: adhesion concerns can be over come by optimized nickel seed layers or laser formed anchor points; long-term reliability can also be solved by an optimized nickel seed layer in conjunction with deep laser doping; and an absence of large-scale high-throughput inline plating equipment is no longer an issue. Average device efficiencies are 19-20% for Al-BSF and over 20% for PERC structures with record open circuit voltages but with a small efficiency loss when anchor points are applied.

Stability of hydrogen passivated UMG silicon with implied open circuit voltages over 700mV

Interstitial hydrogen is used to passivate defects in upgraded metallurgical grade (UMG) Czockralski silicon. It is observed that the quality of the UMG material can be improved progressively, with the passivated defects appearing to be stable to subsequent light soaking. New defects however continue to form for a prolonged period of light soaking, requiring subsequent further hydrogen passivation to restore the open circuit voltages to over 700 mV for the UMG wafers. By repeatedly applying the advanced hydrogenation process, the quality and stability of UMG wafers are improved to the point of being comparable to that for Czockralski wafers produced from semiconductor grade silicon purified by the Siemens Process.

19.1 % laser-doped selective emitter P-type multi-crystalline UMG silicon solar cell

In this paper standard screen printed solar cells and laser-doped selective emitter solar cells are fabricated on p-type multi-crystalline silicon wafers from three upgraded-metallurgical grade feedstock suppliers. A cell efficiency of UMG material is demonstrated above 17 % using standard screen printing technology and a full area aluminum back surface field. By employing a laser-doped selective emitter and self-aligned light-induced plated contacts, a 1-2 % absolute increase in cell efficiency is obtained with a peak efficiency of 19.1 %, when still employing a full area aluminum-back surface field.

Study of p-type laser doping using ALD AlO x as a dopant source

The formation of heavily doped p⁺ region on p-type silicon substrates using laser doping through a layer of ALD AlO_x is studied. A 532 nm continuous wave (CW) laser is used to incorporate Al atoms from the AlO_x layers to form p⁺ silicon. The p⁺ regions formed through laser doping are found to be affected by various parameters such as laser speed and power. Sheet resistances as low as 10 Ω/□ were achieved using a power of 15 W and laser scanning speed of 0.5 m/s. The impact of the laser doping process on effective minority carrier lifetime is investigated using only the AlO_x layer as a dopant source, as well as with the addition of a Boron dopant source. Laser doping boron have better protection by introducing a lifetime drop from 67 μs to 57.7 μs after laser doping comparing to the lifetime drop of laser doping AlO_x from 62.5 μs to 46.1 μs. However, the addition of a boron spin on dopant source may induce voids in the laser doped region.