Trying to break up string of BigIntegers into digits - not working

I have the following code:

    // Project Euler 16: Sums the digits of 2^1000.
using System;
using System.Numerics;
namespace Project16
{
    class Hello 
    {
        static void Main() 
        {
            BigInteger summand = 0;
            BigInteger bigInteger = BigInteger.Pow(2, 1000);
            string stringbigInteger = bigInteger.ToString();
            BigInteger[] stringbigIntegerArray = new BigInteger[stringbigInteger.Length+1];
            for (int i = 0; i <= stringbigInteger.Length;i++)
            {
                //summand += stringbigIntegerArray[i];
                stringbigIntegerArray[i] = BigInteger.Parse(stringbigInteger[i]);
                Console.WriteLine(stringbigIntegerArray[i]); //outputs 2^1000 at i = 302
            }
            Console.WriteLine(summand);
            Console.ReadKey();
        }
    }
}

And inside the for loop, where I have stringbigIntegerArray[i], theoretically it should give the first digit to stringbigIntegerArray[0], the second to stringbigIntegerArray[1], and so on. However it gives the entire string to stringbigIntegerArray[301]. How can I fix this?