This may be too late for the original poster, but for completeness, the way to achieve case insensitive behaviour from PostgreSQL is to set a non-deterministic collation.
Details are described in docs here. Reproducing relevant portion for completeness:
A collation is either deterministic or nondeterministic. A
deterministic collation uses deterministic comparisons, which means
that it considers strings to be equal only if they consist of the same
byte sequence. Nondeterministic comparison may determine strings to be
equal even if they consist of different bytes. Typical situations
include case-insensitive comparison, accent-insensitive comparison, as
well as comparison of strings in different Unicode normal forms. It is
up to the collation provider to actually implement such insensitive
comparisons; the deterministic flag only determines whether ties are
to be broken using bytewise comparison. See also Unicode Technical
Standard 10 for more information on the terminology.
To create a nondeterministic collation, specify the property
deterministic = false to CREATE COLLATION, for example:
CREATE COLLATION ndcoll (provider = icu, locale = 'und', deterministic = false);
This example would use the standard Unicode collation in a
nondeterministic way. In particular, this would allow strings in
different normal forms to be compared correctly. More interesting
examples make use of the ICU customization facilities explained above.
CREATE COLLATION case_insensitive (provider = icu, locale = 'und-u-ks-level2', deterministic = false);
CREATE COLLATION ignore_accents (provider = icu, locale = 'und-u-ks-level1-kc-true', deterministic = false);
All standard and predefined collations are deterministic, all
user-defined collations are deterministic by default. While
nondeterministic collations give a more “correct” behavior, especially
when considering the full power of Unicode and its many special cases,
they also have some drawbacks. Foremost, their use leads to a
performance penalty. Also, certain operations are not possible with
nondeterministic collations, such as pattern matching operations.
Therefore, they should be used only in cases where they are