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

Abstract

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 m, 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 laserdoping 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 laserdoped 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 laserdoped 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.

Impact of localized regions with very high series resistances on cell performance

“…As an identical amount of laser energy density was applied to scribe the cell pattern , the junctions of the two types of regions are approximately of the same depth. However, the surface concentration of the phosphorus dopant atoms of the former is typically above 10 11 /cm 3 , which is at least four orders of magnitude higher than that of the latter . As there are a significantly higher number of electrons available to participate in the plating process in the former, the Ni and Cu metal ions preferentially plate onto the more conductive laser‐doped regions where the phosphorus coating was applied.…”

Laser Enhanced Hydrogen Passivation of Silicon Wafers

“…The peak temperature of the sample in this case is illustrated in Figure 5. It was found that the peak temperature of the sample can reach 2200 K, which was much more than the melting temperature of silicon at 1687 K. The damage may therefore be due to the phase transformation of silicon from solid to liquid such that the expanded volume of silicon can fracture the silicon nitride layer [59,60]. It may therefore be concluded that high laser power densities such as 4.08 × 10 3 W/cm 2 could lead to the formation of laser damage.…”

Analysis of Losses in Open Circuit Voltage for an 18-μm Silicon Solar Cell

“…The Voc loss depends on the power, speed and wavelength of the laser, the structure of the solar cell and the subsequent annealing temperature. A minimum Voc loss of 6.1 mV has been reported [16]. Figure 6 illustrates this laser doping process on a test sample made of epi on FZ wafer, whose front surface was shallowly textured and passivated by SiOxNy.…”