His early, pre-war work was characterized by “despair” in Le Mur and La Nausée, but he took a different direction after that.
The change in Sartre may have actually begun shortly before his imprisonment. In February 1940, after Sartre had enlisted and served several months in the war, his friend Simone de Beauvoir noted a change: he was interested in running for political office after the war. This was shocking; the Sartre of despair would have found such a thought hopelessly optimistic.
Life as a soldier and then as a prisoner had changed Sartre. He re-evaluated his concepts of fraternity, involvement, dignitiy, freedom, and hope - and what it meant to live in “good faith” or “bad faith”.
While in the German POW camp, he began work on two works: L'Age de raison and L'Etre et le Néant. One of his lectures, presented to fellow prisoners, was about the concept of death as presented in the work of Heidegger, Rilke, and Malraux. He tutored a fellow prisoner by reading together with him through the works of Heidegger.
In the prison camp, Sartre developed an attitude of hope, as seen in the play Bariona. The new imperative is that man is to use his freedom to commit himself to a cause precisely because there is hope. Again, his friend Simone de Beauvoir noted the change: when he was released from the POW camp and allowed to return to Paris. In fact, Simone de Beauvoir commented that Sartre “had developed during his internment a moral fiber that, at first” she “found disconcerting, but it was not long before” she “fell in step with him.”
Sartre's later career was tinged again with bits of “despair”, though never as bleak as his early period. Ironically, he was never so hopeful as when imprison in the German POW camp.
Desautels, Alfred “The Sartre of Stalag 12D” The French Review vol. LV no. 2 December 1981