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Since the 1970s, installed solar photovoltaic capacity has grown tremendously to 230 gigawatt worldwide in 2015, with a growth rate between 1975 and 2015 of 45%. This rapid growth has led to concerns regarding the energy consumption and greenhouse gas emissions of photovoltaics production. We present a review of 40 years of photovoltaics development, analysing the development of energy demand and greenhouse gas emissions associated with photovoltaics production. Here we show strong downward trends of environmental impact of photovoltaics production, following the experience curve law. For every doubling of installed photovoltaic capacity, energy use decreases by 13 and 12% and greenhouse gas footprints by 17 and 24%, for poly- and monocrystalline based photovoltaic systems, respectively. As a result, we show a break-even between the cumulative disadvantages and benefits of photovoltaics, for both energy use and greenhouse gas emissions, occurs between 1997 and 2018, depending on photovoltaic performance and model uncertainties.

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The specifications of photovoltaic modules show performance under standard testing conditions (STC), but only limited information relating to performance at non-STC conditions. While performance is affected by irradiance, temperature, spectral composition of irradiance, angle-of-incidence of the irradiance and other parameters, specifications only partly give detail to consumers or retailers about the effect of irradiance and temperature. In this study, we characterise and analyse the performance of eight different, commercially available photovoltaic modules. We establish the effect of four different parameters on module performance: irradiance, temperature, spectral composition of irradiance (via the parameter average photon energy) and angle-of-incidence, by performing linear and nonlinear optimisation of physical or empirical models. Furthermore, we characterise the operating conditions and analyse the seasonal and annual development and contribution of the four parameters to energy losses or gains relative to STC operating conditions. We show a comprehensive way of presenting the deviation of performance from STC, combining the variation in operating conditions and the resulting variation in performance. Our results show that some effects on performance are attributable to the semiconductor material used in the modules (spectral composition and temperature), while especially angle-of-incidence effects seem more related to the type of glass used on as the front cover of the module. Variation in irradiance and module temperature generally affect performance the strongest, resulting in a performance effect ranging from +2.8% to -3.2% and -0.5% to -2.2%, respectively. The combined effect of all parameters results in an annual yield deviation ranging from +1.2% to -5.9%.

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/m 2 , results for current designs show that life-cycle GHG emissions could be 32 gCO 2 -eq/kWh for complete SHJ photovoltaic (PV) systems (module efficiencies of 18.4%), compared with 38 gCO 2 -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 (Ä 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 gCO 2 -eq/kWh for SHJ systems and 25 gCO 2 -eq/kWh for monocrystalline system, while EPBT could drop to 0.9 and 1.2 years, respectively.