Also, there is maybe a better way of displaying the data to find a better equation that fits.
Anytime you are using a high order polynomial, you have to wonder if there is a better equation for the data. Which equation to use in regression/curve-fitting is not really an Excel question, but more of a data analysis question. First part of choosing an equation is understanding the physical process involved. In many cases, knowing how others have modeled the process is useful.

Just looking at the data, you might consider a more sigmoidal function http://en.wikipedia.org/wiki/Sigmoid_function like the logistics function or a cumulative distribution function. Many of these functions are non-linear, so they can't be regressed using the LINEST() or chart trendline algorithms. You would have to write your own spreadsheet using Solver to regress the parameters for these kinds of functions, but they would seem more likely to fit the kind of data you show.