Alison Margaret Anne Bennett

Performance of thermoplastic ionomer encapsulant material with advanced emitter solar cells

The solar industry is currently driving technology towards higher module power outputs. This force has put pressure on cell manufacturers to maximize efficiency and module producers to minimize packaging losses. This study will consider the cell efficiency as a function of emitter type and module power as a function of encapsulant. Three types of emitter are investigated: standard homogeneous emitter (HE), lightly doped emitter (LDE) and selective emitter (SE). A portion of the device efficiency gain from advanced emitters is from improved short wavelength quantum efficiency (300 - 500 nm). In order to take advantage of this improved blue response, transparency in short wavelengths of module encapsulants should be maximized. Unfortunately the UV (ultra violet) portion of the solar spectrum causes encapsulant aging effects like yellowing, brittleness, and metal corrosion. Therefore more advanced module packaging materials are desired that achieve high transmission across a broad spectrum, yet withstand aging and environmental stress due to UV exposure. Two types of encapsulant will be compared, a thermoset EVA (ethylene vinyl acetate) to represent the industry benchmark versus an ionomer-based thermoplastic with improved blue light transmission. The three emitter types are compared at the device level for IV performance and quantum efficiency. Single-cell mini-modules are constructed using the two different encapsulants. These are measured for IV performance pre- and post-lamination. With cell IQE, glass and encapsulant complex index of refraction, one can calculate the expected changes with emitter and encapsulant selection. The mini-module Jsc results are compared to simulation using a ray tracing, thin film stack model. It is concluded that cell efficiency gains from advanced emitters are maintained at the module level, plus there is a 1.3 - 1.5%rel Jsc gain of ionomer over EVA. Comparing a standard cell encapsulated in EVA to SE encapsulated in ionomer, a 4.3%rel power gain is measured.