The company’s board of directors has started the search for a replacement.
Hall’s decision to leave Vattenfall comes after nearly six years with the company.
He told reporters in a conference call that it was a "personal decision", and added: "It has been a very interesting and good period (as CEO), and I believe that Vattenfall has come to a good strategic position with good growth prospects for the future."
Hall added that he believed that Vattenfall's board members would look for his successor both internally and externally, and that he expected to leave later this autumn.
He said that key challenges for his successor would be maintaining Vattenfall's competitiveness in offshore wind and picking strategic markets for the developer. Hall added that his successor would need to assess whether Vattenfall should invest heavily in green hydrogen.
"These are questions that we have to dwell on and we have to know exactly where we should place ourselves," Hall added.
In Vattenfall’s latest interim results, the Swedish utitility recorded a SEK 1.6 billion (€154 million) loss for the first half of the year — down from a SEK 7.6 billion profit one year earlier.
It also recorded a loss of SEK 8.5 billion in the second quarter — down from a SEK 1.2 billion profit in the same period last year.
Hall explained that these losses were largely due to huge impairment losses: Vattenfall wrote down a German coal-fired power plant by SEK 9.1 billion due to the impending phaseout of fossil fuels, and its Swedish and Danish wind assets by SEK 1.5 billion due to falling electricity certificates prices.
Chief financial officer Anna Borg explained that the affected wind farms were mostly older, onshore projects in Sweden and Denmark.
Vattenfall’s underlying operating profit for both the first half of the year and the second quarter fell year on year, as the company was hit by low electricity prices.
Underlying operating profit fell 2.3% to SEK 13 billion in the first half of 2020 and 22.9% to SEK 2.8 billion in the April-June quarter.
Meanwhile, underlying operating profit from wind rose 8.1% to SEK 2 billion for the first half of the year, despite an underlying operating loss from wind of SEk 144 million - down from a SEK 365 million profit one year earlier. Vattenfall explained that its second quarter earnings from wind were negatively impacted by lower wind speeds, lower electricity prices and lower availability.