Barca still improving: Flick warns rivals after thrashing Dortmund
The Catalans put one foot into the semi-finals with the first leg thrashing of last year's runners-up at the Olympic Stadium.
Barca are the competition's top scorers with 36 goals and their dynamic attacking trio of Robert Lewandowski, Raphinha and Lamine Yamal strike fear into the hearts of opponents.
"I want to keep improving, today we saw some points we can improve on," Flick told reporters.
"We will analyse this game and I will tell the players they did very well, but there are still things to improve.
"I'm very happy with the result today but it's just the first step."
Raphinha netted his 12th Champions League goal of the season -- he is the top scorer in the competition -- and added two assists.
The Brazilian has 19 direct goal contributions this season in Europe, equalling former Barca great Lionel Messi's record for the club.
"I always thought he's an extraordinary player... the team is set on his shoulders, his dynamism," said the coach.
His opposite number Niko Kovac was disappointed with Dortmund's performance.
"It was a deserved defeat (but) we could have got a goal, as we all know the return leg will be very difficult," said coach Kovac.
"I am optimistic but I have to be a realist... it was a very big result, our chances of going through are minimal.
"We know it will be difficult but we have to have a spark of hope, because without that spark it will be impossible."