Life cycle greenhouse gas emissions and energy payback time of current and prospective silicon heterojunction solar cell designs

Silicon heterojunction (SHJ) cells offer high efficiencies and several advantages in the production process compared to conventional crystalline silicon solar cells. We performed a life-cycle assessment to identify the greenhouse gas (GHG) footprint, energy payback time (EPBT) and cumulative energy demand of four different SHJ solar cell designs. We analyse these environmental impacts for cell processing and complete systems for both current and prospective designs. On the basis of in-plane irradiation of 1700¿kWh/m2, results for current designs show that life-cycle GHG emissions could be 32 gCO2-eq/kWh for complete SHJ photovoltaic (PV) systems (module efficiencies of 18.4%), compared with 38 gCO2-eq/kWh for conventional monocrystalline silicon systems (module efficiency of 16.1%). The EPBT of all SHJ designs was found to be 1.5¿years, compared with 1.8¿years for the monocrystalline PV system. Cell processing contributes little (=6%) to the overall environmental footprint of SHJ PV systems. Among cell processing steps, vacuum based deposition contributes substantially to the overall results, with 55–80%. Atomic layer deposition of thin films was found to have a significantly lower environmental footprint compared to plasma enhanced chemical vapour deposition and sputtering. Copper-based compared with silver-based metallization was shown to reduce the impact of this processing step by 74–84%. Increases in cell efficiency, use of thin silicon wafers and replacement of silver-based with copper-based metallization could result in life-cycle GHG emissions for systems to be reduced to 20 gCO2-eq/kWh for SHJ systems and 25 gCO2-eq/kWh for monocrystalline system, while EPBT could drop to 0.9 and 1.2¿years, respectively