A way to keep the absorber cooler while extracting the same amount of energy from it is more airflow. On solar air heating collectors, it is relatively easy to get most of the suns energy into the collector absorber. The difficult part of air collector design is getting the heat transferred from the absorber into the air.
A simpler and I think perfectly acceptable solution is to install a thermal switch that goes into the collector and turns the collector fan on when the air in the collector hits the temperature you set (usually about 80F to 90F). Many people use this setup for solar air heating collectors -- its very simple and it works well.
DIY solar air heating collectors are one of the better solar projects. They are easy to build, cheap to build, and offer a very quick payback on the cost of the materials to build them. They also offer a huge saving over equivalent commercially made collectors. Two of the more popular designs are the pop can collector and screen absorber collector.
On just about all solar thermal collectors, the sun shines through the glazing, and hits the collector absorber heating it. The air flows through the inlet and over or inside or through the absorber picking up heat as it goes. This heated air then flows out the collector outlet and into the room being heated.
The controller will allow you turn set the collector fan to come on when the collector is the number of degrees you set warmer than the house air. Most of these are used on solar water heating systems, but don't see why they would not work for an air system.
Vertical collectors facing east or west do gain some heat, but not nearly as much as south facing collectors. You can get a pretty exact numbers by using the PVWatts calculator. PVWatts is intended for sizing PV systems, but it has one column in its output that gives solar radiation by month on the collector you define.