New diagnostic tool to assess thin-film solar-cell reliability

Abstract

Few subjects trigger as much passion as alternative or, more specifically, ‘green’ energy. Economical use of sunlight has been a key goal of scientists, engineers, and governments for over 50 years, given that the sun provides us hourly with enough energy to meet the annual electricity requirements of the entire world population.1 Solar cells (or photovoltaics: PVs) convert sunlight directly into electricity. Until recently, most PVs were based on single-crystal silicon. Yet, lack of grid parity (which refers to equal cost compared with existing, grid-based electricity sources) has discouraged their use. Recently, the PV industry has shifted towards adopting more exotic, thin-film materials, driven by the high sunlight-to-electricity conversion efficiencies attainable for cadmium telluride2 (CdTe) and copper indium gallium selenide—Cu(In,Ga)Se2—cells. Grid parity for thin-film modules (a packaged set of series-connected cells) is now almost certain.4 However, questions persist about their long-term reliability and, ultimately, the true cost of electricity: the longer a PV module functions, the cheaper it is. To achieve lower costs, materials like CdTe and Cu(In,Ga)Se2 are deposited quickly, often using low growth (or fabrication) temperatures. These conditions result in polycrystalline structures with sizes and thicknesses on the order of microns, which (although packaged using technologically advanced materials) are inherently more sensitive to long-term, outdoor exposure (10–20 years) than their monocrystalline counterparts. We employ a design-of-experiment approach combined with accelerated lifetime testing (ALT) to correlate the type of cellfabrication process with performance and reliability.5 ALTs enable us to estimate the long-term robustness of solar cells within a short period of time. Most current ALTs apply heat to accelerate degradation and measure changes in either the module’s power or the cell’s current-density/voltage (J–V) output characteristics to quantify reliability. Figure 1. Arrhenius plot for calculating degradation-activation energies in cadmium telluride (CdTe) solar cells. Time to fail was set arbitrarily as the time in which cells lose 10% of their initial performance, which is a function of temperature, T. Ea: Diffusion-activation energy.