How to export data in Oracle database into CSV file with UTF-8 format

I have an CSV file with WE8ISO8859P1 encoding generated from Oracle Stored Procedure. Is there any script line to define the output of CSV file in UTF-8 encoded format?

 CREATE OR REPLACE 
PROCEDURE export_main(dir VARCHAR2, file_name VARCHAR2)
  IS
 select_stmt VARCHAR2(100) := 'SELECT MTYPE || '','' || MNO FROM MAIN';
    cur INTEGER;
    file UTL_FILE.FILE_TYPE;
    row_value VARCHAR2(4000);
    ret INTEGER;
  BEGIN
    -- Open a cursor for the specified SELECT statement
    cur := DBMS_SQL.OPEN_CURSOR;
    DBMS_SQL.PARSE(cur, select_stmt, DBMS_SQL.NATIVE);
    ret := DBMS_SQL.EXECUTE(cur);

    -- All columns were concatenated into single value in SELECT
    DBMS_SQL.DEFINE_COLUMN(cur, 1, row_value, 4000);

    -- Open the file for writing
    --file := UTL_FILE.FOPEN(UPPER(dir), file_name, 'w', 32767);
file := UTL_FILE.FOPEN_NCHAR(UPPER(dir), file_name, 'w', 32767);
    -- Export rows one by one
    LOOP
       ret := DBMS_SQL.FETCH_ROWS(cur);
       EXIT WHEN ret = 0;

       -- Get the value
       row_value := NULL;
       DBMS_SQL.COLUMN_VALUE(cur, 1, row_value);

       -- Write the row to the file
      -- UTL_FILE.PUT_LINE(file, row_value);
UTL_FILE.PUT_LINE_NCHAR(file, TO_NCHAR(row_value));
    END LOOP;

     UTL_FILE.FCLOSE(file);
     DBMS_SQL.CLOSE_CURSOR(cur);

    EXCEPTION WHEN NOT_LOGGED_ON THEN
    DBMS_OUTPUT.PUT_LINE ('A program issues a database call without being connected to Oracle.');
  END;

Oracle database is encoded with WE8ISO8859P1 and postgres DB is encoded with UTF-8

I need to import CSV file into Postgres DB table using stored procedure where in postgres by default encoding is UTF-8