Gallium arsenide (GaAs) is an important semiconductor material for high-cost, high-efficiency solar cells and is used for single-crystalline thin film solar cells and for multi-junction solar cells. The first known operational use of GaAs solar cells in space was for the Soviet Venera 3 mission, launched in 1965. The GaAs solar cells, manufactured by Kvant, were chosen because of their higher performance in high temperature environments. GaAs cells were then used for the Lunokhod rovers for the same reason. In 1970, the GaAs heterostructure solar cells were developed by the team led by Zhores Alferov in the USSR, achieving much higher efficiencies. In the early 1980s, the efficiency of the best GaAs solar cells surpassed that of conventional, crystalline silicon-based solar cells. In the 1990s, GaAs solar cells took over from silicon as the cell type most commonly used for photovoltaic arrays for satellite applications. Later, dual- and triple-junction solar cells based on GaAs with germanium and indium gallium phosphide layers were developed as the basis of a triple-junction solar cell, which held a record efficiency of over 32% and can operate also with light as concentrated as 2,000 suns. This kind of solar cell powered the Mars Exploration Rovers Spirit and Opportunity, which explored Mars’ surface. Also many solar cars utilize GaAs in solar arrays. GaAs-based devices hold the world record for the highest-efficiency single-junction solar cell at 29.1 % (as of 2019). This high efficiency is attributed to the extreme high quality GaAs epitaxial growth, surface passivation by the AlGaAs, and the promotion of photon recycling by the thin film design. Complex designs of AlxGa1−xAs-GaAs devices using quantum wells can be sensitive to infrared radiation (QWIP). Beyond solar energy GaAs diodes can be used for the detection of X-rays.