Java, 169 Bytes

isProbablePrime is only valid for 32 bit integers, but it is correct for every 32 bit integer, so this is a valid solution.

(int n)->{int x=-1;for(int i=0;i<n;){x++;if(BigInteger.valueOf(x).isProbablePrime(15))i++;}String r="";while(x>0){x--;int m=x%26;r=(char)(m+97)+r;x=(x-m)/26;}return r;};