Use TO_CHAR(1e-13, '0.9999999999999');
. You need to check for the proper format model to convert the scientific notation into string. I used '.', you could also use 'D' as decimal separator, but it depends on your NLS_NUMERIC_CHARACTER.
See the different number formats here, http://www.java2s.com/Tutorial/Oracle/0300__Conversion-Functions/FormatParameters.htm
Update I initially answered the opposite of the requirement, i.e. convert from scientific notation to regular number format. The update is about converting number into scientific notation.
In SQL Developer
, you would see something like this :

In SQL*Plus
, the same SQL
would convert the number into its scientific notation`.
SQL> WITH DATA AS(
2 SELECT to_number('0.0000000000001','0.99999999999999') num FROM dual
3 )
4 SELECT to_char(num, '9.9EEEE') num
5 FROM DATA;
NUM
---------
1.0E-13
SQL>
Without a format,
SQL> WITH DATA AS(
2 SELECT to_number('0.0000000000001','0.99999999999999') num FROM dual
3 )
4 select num from data;
NUM
----------
1.0000E-13
SQL>