Excimer Laser Crystallised Polysilicon Solar Cells

Hydrogenated amorphous silicon (a-Si:H) is one of the most widely used materials for photovoltaics. The direct band gap coupled with the high absorption coefficient and reduced costs for deposition compared to crystalline silicon, are the  main advantages in adopting such a material system. The high absorption coefficient of a-Si:H also allows one to reduce the thickness of the active layer used. However, light induced degradation based on the Stabler-Wronski effect (SWE) is a major drawback. Due to the SWE the devices will degrade 15-30% from its initial performance. Investigations carried out to improve the performance of a-Si:H for applications pointed to crystallisation as a potential solution. Solid phase crystallisation and laser crystallisation are the two main methods for crystallisation. Excimer laser crystallisation may prove to be superior due to its ability to use cheap substrates, reduced processing times and induce higher mobilities. Furthermore, the high absorption coefficient of a-Si:H, especially in UV range, provides an advantage of using cheap substrates such as glass, plastic and so on. This is largely because the excimer laser will mostly be absorbed within a few nanometres from the surface as it operates in the ultra-violet (UV) wavebands with short pulse duration. The ability to produce bigger grains and improved broadband absorption are key reasons in applying laser crystallised a-Si:H for photovoltaics.

Pre-deposited a-Si:H (400nm) on ITO coated glass with a  p+ a-SiC:H (10 nm) was laser crystallised using Lambda Physik (LPX 210i) KrF pulsed excimer laser operating at 248 nm with 25 ns full width half maximum pulse duration. The samples were kept in the chamber, mounted on a translational stage, with a base pressure of 0.05 Pa. The laser was scanned 2.5 mms^-1 with a pulse repetition rate of 50 Hz. A 4x8 mm² pulse with an asymmetric peak profile along the length, and constant energy was utilized along the width. The samples were crystallised 40, 80, 120 and 160 mJcm^-2 and 5-8 mm² area of 60 nm Al metal contacts were evaporated to the devices after HF etching. The device structure is shown in the figure.

The cross sectional TEM image shows the crystallised silicon with grains; small grains in the middle and amorphous silicon  at the bottom at 160 mJcm^-2. The J-V measurements were taken under AM 1.5 simulated solar irradiation. The devices show higher series resistance compared to the amorphous silicon devices. After studying the TEM image, we can conclude that the efficiency degradation was mainly due to grain mismatch leading to higher defects densities. At present, methods are being employed to reduce the series resistance leading to higher efficiencies.