SEOUL, South Korea: Samsung Electronics, the world’s biggest smartphone maker, has pledged to convert its operations in the US, Europe and China to using only solar and other renewable energy by 2020. With its announcement Thursday, the tech giant joins Apple and other Silicon Valley companies in making such a commitment, but it faces bigger challenges due to its vast factory network, while other companies use outside contractors for manufacturing. Samsung, also a major producer of computer chips, said its plans include installing 42,000 square meters of solar panels this year at its headquarters in Suwon, South Korea. South Korea gets 6 percent of its energy from renewable sources, relying on coal and nuclear power for the rest.