Adeline Sugianto

Advanced Bulk Defect Passivation for Silicon Solar Cells

Through an advanced hydrogenation process that involves controlling and manipulating the hydrogen charge state, substantial increases in the bulk minority carrier lifetime are observed for standard commercial grade boron-doped Czochralski grown silicon wafers from 250-500 μs to 1.3-1.4 ms and from 8 to 550 μs on p-type Czochralski wafers grown from upgraded metallurgical grade silicon. However, the passivation is reversible, whereby the passivated defects can be reactivated during subsequent processes. With appropriate processing that involves controlling the charge state of hydrogen, the passivation can be retained on finished devices yielding independently confirmed voltages on cells fabricated using standard commercial grade boron-doped Czochralski grown silicon over 680 mV. Hence, it appears that the charge state of hydrogen plays an important role in determining the reactivity of the atomic hydrogen and, therefore, ability to passivate defects.

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.

Rear junction laser doped solar cells on CZ n-type silicon

N-type silicon (Si) has been shown to have generally higher bulk lifetimes and far better post illumination performance stability compared to boron doped p-type materials of similar crystallographic quality. In particular, the high minority carrier diffusion lengths in n-type wafers makes the rear emitter n+np+ structure an attractive option, especially when incorporated with screen printing as a simple and cost effective way to create an Al-alloyed junction on the back surface. However, when screen printing is used to apply the front contacts, its wide metal lines and its requirement for a heavy Phosphorus-doped front surface significantly reduces the performance of this simple device with the latter limiting the blue wavelength response and surface passivation quality. Recently, the laser doping process has been shown capable of overcoming these major drawbacks due to its ability to produce a selective emitter. In this present work, an innovative application of the laser doping process in the fabrication of such rear Al-alloyed emitter n+np+ device enables an excellent energy conversion efficiency of 18.2% to be achieved on commercial grade n-type CZ wafers (148.6cm2).

Investigation of Al-Doped Emitter on N-Type Rear Junction Solar Cells

This brief models the junction discontinuities of a rear Al-doped p⁺ emitter (np⁺) formed by screen printing and firing. Theoretical fitting of the suns-<i>V</i> _oc data to the circuit model shows that not only do the junction discontinuities deteriorate cell <i>V</i> _oc, for the case of p-type cells, but they also reduce cell fill factor on n-type cells through increased junction recombination and nonlinear shunts.

Hydrogen Passivation of Laser-Induced Defects for Laser-Doped Silicon Solar Cells

Hydrogen passivation of laser-induced defects (LasID) is shown to be essential for the fabrication of laser-doped solar cells. On first-generation laser-doped selective emitter solar cells where open-circuit voltages were predominately limited by the full-area back surface field, a 10-mV increase and 0.4% increase in the pseudo-fill factor were observed through hydrogen passivation of defects generated during the laser doping process, resulting in an efficiency gain of 0.35% absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices and can result in improvements in implied open-circuit voltage on test structures up to 50 mV. On n-type PERT solar cells, an efficiency gain of 0.7% absolute was demonstrated with increases in open-circuit voltage and pseudo-fill factor by applying a short low-temperature hydrogenation process using only hydrogen within the device. This process was also shown to improve the rear surface passivation, increasing the short-circuit current of approximately 0.2 mA/cm2 of wavelengths from 950 to 1200 nm compared with that achieved using an Alneal process. Subsequently, an average efficiency of 20.54% was achieved.

High efficiency at module level with almost no cell metallisation: Multiple wire interconnection of reduced metal solar cells

Perpendicular multiple busbar wires have proven an effective way of interconnecting standard screen printed solar cells with high reliability and low cell to module loss. The technology is also an effective way to interconnect plated solar cells, where conventional soldered interconnects may be problematic. However, the full benefits of this interconnection technology can be fully realized on plated cell structures with drastically reduced plated metallization. This paper presents a new selective emitter plated cell structure with metallization reduced to around 1 μm thickness, interconnected using perpendicular multiple busbar wires. Metal usage on the cell is reduced by more than 90% compared to conventional plated or screen print cells and the use of Ag almost eliminated, while high efficiency at the module level is achieved along with environmental durability.

Improved adhesion for plated Solar cell metallisation

Experts predict that copper plated metal contacts will eventually become the dominant metallisation for silicon wafer-based technologies once several key issues are solved. Of particular importance is the adhesion strength and hence durability of such plated contacts with many of the largest cell manufacturers currently nervous about considering such metallisation for large-scale manufacturing due to concerns in this area. A new approach for enhancing the adhesion strength for plated contacts involves establishing laser-machined anchor points in the silicon surface which when plated act to enhance both the ohmic contact and the mechanical adhesion strength for the metallisation. Cells manufactured on a production line using this innovation typically lose 0.1% in efficiency in absolute terms relative to identically manufactured cells that do not use the anchor points. BP Solars Saturn cells using a similar approach have demonstrated that such plated contacts are at least as durable if not more durable than screen-printed contacts installed at the same time 20 years ago.

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.

20.3% efficiency rear passivated silicon solar cells with local back contact using commercial P-Cz wafers

In this paper, progress results on the next generation Pluto technology were reported. In the next generation Pluto, we focused on the rear surface design, by improvement of the back internal reflection and passivation we got 20.3% cell efficiency with very high Jsc. Still there is further improvement of the cell design to get high FF and Eff.