You can convert your query string using php function strtolower();
http://php.net/manual/en/function.strtolower.php
Or you can set up a new field type with LowerCase filter in schema.xml
https://cwiki.apache.org/confluence/display/solr/Tokenizers#Tokenizers-LowerCaseTokenizer
<fieldType name="newFilter" class="solr.TextField" omitNorms="false" positionIncrementGap="100" multiValued="true">
<analyzer type="index">
<tokenizer class="solr.WhitespaceTokenizerFactory"/>
<filter class="solr.LowerCaseFilterFactory"/>
</analyzer>
<analyzer type="query">
<tokenizer class="solr.WhitespaceTokenizerFactory"/>
<filter class="solr.StopFilterFactory" words="stopwords.txt" ignoreCase="true"/>
<filter class="solr.LowerCaseFilterFactory"/>
</analyzer>
</fieldType>
Example:
In: "I just LOVE my iPhone!"
Out: "i", "just", "love", "my", "iphone"