First thing: I personally would avoid using "i" as a counter, since it could also signify the imaginary number. Use "ii" instead to avoid confusion.
Secondly, you are assigning the variables (DIR, hs, time, ...) in every loop. The first time through the loop (ii=1), only files(1) is defined. Because files(2) has not been created yet (ii has not reached 2), the code is throwing an error at hs=files(2).
Move the assignments of DIR, hs, etc to outside the loop.
Also, it appears you have mixed data types (strings and numbers). I would suggest that you use the "raw" data from Excel (both numbers and strings in a cell array), so use the 3rd output.
[~, ~, files{ii}] = xlsread(filename);
