Avoid Left and Center Field
Home plate faces east at Target Field, meaning that the sun will set almost directly behind home plate. Seats directly behind home plate and closest to home plate (infield) -- especially down the first base line -- will see shade first. Left and center field seats will remain in the sun for the longest period of time.
Consider High-Number Rows in the Upper Deck
An impressive overhang covering nearly half of the Upper Deck keeps many of the higher numbered rows (5+) covered even during afternoon games.