Bono struggled to write U2's 'personal' new album.

The Irish rocker and his band mates, The Edge, Adam Clayton and Larry Mullen Jr - who released their new record 'Songs of Innocence' for free to all iTunes Store customers and U2's website subscribers yesterday(10.09.14) - found composing the tracks hard as it required them to revisit past memories.

Bono told Rolling Stone: ''We wanted to make a very personal album.

''Let's try to figure out why we wanted to be in a band, the relationships around the band, our friendships, our lovers, our family.

''The whole album is first journeys -- first journeys geographically, spiritually, sexually. And that's hard. But we went there.''

In one particularly painful track entitled 'Iris', Bono opens up about his late mother, Iris Hewson, who died when he was just 14-years-old.

He explained: ''Forty years ago, my mother fell at her own father's funeral, and I never spoke with her again.

''Rage always follows grief, and I had a lot of it, and I still have, but I channeled it into music and I still do. I have very few memories of my mother, and I put a few of them in a song called 'Iris.'''

The autobiographical album also features collaborations with Ryan Tedder, Lykke Li, Danger Mouse and Paul Epworth - who the 54-year-old singer credits for the success of their latest offering.

He said: ''I think having them around really helped.

''Some of the music out there now that people call pop, it's not pop - it's just truly great. And we wanted to have the discipline of the Beatles or the Stones in the Sixties, when you had real songs. There's nowhere to hide in them: clear thoughts, clear melodies.''