In this question, Geoffc's answer stated that the ISS solar arrays are about 14% efficient. Higher efficiency panels were available when these arrays were manufactured. Given the high cost per pound of launching stuff to orbit, why were more efficient panels not used? This question estimates the mass of the arrays at 30 tons. Double the efficiency (as seems to be possible with triple-junction panels), and you'd be able to halve that.
As 2012rcampion noted you are using the wrong metric of cost in your efficiency figures.
Triple junction solar panels are extremely inefficient wrt weight when compared to single junction.
To understand why this is you need to understand what a triple junction PV is.
To do that I need to explain the major inefficiency of standard single junction PV is due to quantum effiency.
Each single junction PV has a semi conductor junction with an associated voltage or colour.
That PV junction will only give you energy for photons of that colour/voltage or bluer. So if you have a PV junction that is tuned to green, that panel would give no power under a red light.
Conversely higher energy photons (bluer) would only give off an electron with that same voltage (the rest of the energy lost as heat).
Triple junction solar panels literally stack a red, green and blue solar panel on top of each other to maximise both the number of photons turned into electricity and the electrical energy gained per photon.
But if you are stacking 3 panels you could just build 3 panels instead and place them side by side so they don't block each other.
So for a naive approach triple junction is 3 times heavier, but not 3 times as powerful.