What is there to gain from further research of solar materials? You can just look in a book and make a decent model with existing information.

Look at this picture of a cup. When you draw a picture of a cup, does the picture give you all the information about the cup? The picture may be a very good representation of the cup, but it still would not tell you everything about the cup. You can't tell if it is a representation of an actual cup or a cardboard cut-out. You can't see the NSF logo on the opposite side of the cup. You don't know how the cup transfers heat and if it is heavy. Even if you knew that there was hot chocolate in the cup, you wouldn't necessarily know what type of hot chocolate it was or if there were any impurities. You wouldn't know if there was a layer of caramel at the bottom of the cup that did not dissolve or if all of the hot chocolate mix had dissolved. Similarly, books don't give the entire story; real materials are not "ideal" and it is important to learn how and why real materials are not ideal. This knowledge is necessary if you want to find new uses for a material or optimize a material for a given application. 

Actual materials have defects--and not just the defects that you added or want. In a compound, you have composition variations that can change the band gap locally. This is especially true in the compounds that we study. From these composition variations, you can even get other compounds to form along with the desired material. Even for a given compound, that compound is generally stable over a range of compositions. Think of it this way.

In this se_tanc3 1 left owt or repl@ced s3veral lett3rs, but tha sent3nce 1s $till understood.

Many more letters would need to change for it to be a different sentence or in a different language. The crystal structure (grammar and basic spelling rules in this case) are still obeyed, but it is full of mistakes. The sentence can't really mean anything else than what it would have said without the mistakes, but it slightly alters the way that meaning is interpreted (either I was being sloppy or text too much). In a semiconductor these defects can lead to electrons existing where they shouldn't or in different concentrations than they would for the basic crystal structure, changing the electronic properties.

Our research investigates and predicts these stability windows based on a discipline called thermodynamics. Changes in band gap, which can be caused by composition variations and secondary compounds/phases can cause charges to get stuck and not be able to be used to generate current in a device.

This is the crystal structure (the way the atoms are packed) in CZTSSe, the material that we study. This unit is repeated throughout the structure like tiles in the floor. In a given unit, however, an atom can be missing (called a vacancy) or can be stuck in the wrong position (for example, if grey (zinc) atom were found where a orange (copper) atom is in the picture). This can change how the material behaves. (Photo from Wikipedia Commons)

Atoms can pack incorrectly (in ways that are not necessarily the lowest energy and/or in ways that don't agree with the ideal crystal structure of the compound). You can get copper atoms where zinc should be or you can have nothing where a copper atom should be. These issues do not even require impurities to exist. As you can see, we have our work cut out for us, but it is oh so worth it!