If you look on the datasheet of any NPN transistor its base-emitter is turned on when its base is positive about 0.7V from its emitter.
When the base voltage is negative it does not conduct but causes it to break down (it conducts like a zener diode) and be damaged at 5V or more.
Therefore the diode is added so that the base does not get a negative voltage that would damage it.
The 60Hz from the mains electricity is a sinewave. A sinewave is not supposed to have harmonics because harmonics are distortion.
I have never heard of "clean" harmonics because all harmonics are "dirty". Light dimmers cause harmonics.
Something that conducts symmetrically at the peaks of the sinewave cause it to be a squarewave that has only odd harmonics: 1, 3, 5, 7, 9 etc.
Something that conducts on only one polarity of the sinewave produces even harmonics: 2, 4, 6, 8, 10 etc.