We hear solar energy mentioned when climate change is discussed. How do we know how how solar fits into this?

Solar energy, itself, does not release greenhouse gases (although the energy required to make solar panels, themselves, often does--the precise "cleanness" depends on the specific technology, but modern technologies are cleaner than the status quo). At the same time, they absorb energy and pump it other places in the form of electricity (and, ideally, don't reflect much back to space). Consequently, solar panels change the energy distribution in their immediate vicinity. Computer models are used to predict the effects of solar cells on their local environment. Aixue He et al. recently published a paper discussing these effects. This paper was then picked up by the Washington Post. Solar panels, themselves, lower temperatures in their immediate vicinity. They absorb energy from their surrounding and convert it to electricity, which is sent into cities and towns. Those areas, then, use the electricity, often in ways that release more heat, resulting in warming of urban areas. It is important to note that these temperature increases are much less than those predicted without the use of solar or other clean energy sources.

It is important to note that precipitation is also affected by air temperature. Cold air can't hold as much moisture. If deserts are colder, they are often drier. Solar panels, where they are set up, decrease the local temperature, which A. He et al.'s study suggests would slightly reduce cloudiness and precipitation. It only takes a 2 C temperature decrease to reduce precipitation by 20 % in desert regions. For these reasons, it was found that placing solar panels in a mixture of urban and desert regions minimizes many of the local environmental impacts of solar cells.

The moral of the story is that no matter what you do, the environment is effected. It is just a question of degrees and in what way things are affected. 

What is graduate school like? How is it different from other schooling?

Graduate school is like no other beast. In the sciences, you both do research and take classes. When you start out, you are mostly taking classes but, as you progress, you do more and more research with fewer to no classes. By the time you graduate, there is little difference between being a graduate student and being a postdoc or research scientist. It is a transition and learning period. By the end, you are the world expert on the topic of your research. Answers are not in the back of a textbook or online. You make your own questions.

What does it mean that solar energy is growing? What are all these costs being thrown around?

According to the Motley Fool, 40 % of all new electricity generating capacity during the first half of 2015 came from solar energy. Yes, it still accounts for only 0.4 % of total electricity generation in the US, but this just demonstrates how much built up generation capacity that we have. While solar modules are being installed at a fast rate, the energy needed is also growing and building a power plant is a long-term commitment. It takes time to gain market share.

There are a lot of different metrics involved in calculating the cost of solar energy. One must account for the basic cost of a module, the installation costs, and the cost of electronics used to interface the module with the grid. For some applications, energy storage batteries must also be taken into account. Many reports of cost do not take all of these costs into account. Several years ago, people were really excited about module costs lowering below $1/Watt. As you can see, module costs are now well-below that price. Also, there is a price difference between industrial-scale and roof top (residential and non-residential) solar energy. Industrial scale production is less expensive than roof top energy. These costs also vary by location. It will cost more to install a solar panel in areas where installers lack experience.

Source: Deutsche Bank, Vishal Shah, for rooftop market

Therefore, when you hear people talking about solar and citing costs, always ask what they are referring to.

Why are we still researching new solar materials?

Credit: L.M. Peter, Phil. Trans. R. Soc. A 369 (2011) 1840-1856.

There are a variety of reasons. Most commercial solar technologies are currently based off silicon or cadmium telluride (CdTe). CdTe is relatively new to the market and has been instrumental to lowering the cost of cells since it can be processed using inexpensive technologies (it's more defect-tolerant) and only requires a very thin layer of material (~2 millionths of a meter, thinner than a human hair). Silicon, itself is very abundant and is very well understood thanks to the microelectronics industry. Unfortunately, though, silicon requires more material to make a good solar cell and that material needs to be of higher purity and quality. Silicon technologies, however, can capitalize on the existing knowledge band and manufacturing advances by the microelectronics industry to lower costs. My adviser always says: "Never bet against silicon".

That being said, these technologies have their limitations. Silicon, while raw material is cheap, getting it to the required purities for good devices is an expensive and energy-intensive process. Also, it can be expensive to get rid of defects that harm performance. For every solar cell, there is a trade-off between cost and performance. Record silicon cells are close to the theoretical maximum efficiency possible, although these record cells are costly to manufacture. CdTe requires special encapsulation because Cd (cadmium) and Te (tellurium) are toxic if released from the CdTe compound (cells based on these materials are great safe ways to dispose of Cd waste when refining zinc ores and Te waste when refining copper), raising the cost. CdTe developments have been instrumental in lowering the cost of solar cells and modules, although it is unclear how long that will continue. The Department of Energy expects a Te shortage ~2025, which will raise costs. As you can see from the chart above, Cd and Te are fairly scarce and current reserves are not enough to supply our current energy needs completely.

A material  known as CIGS (Cu(In,Ga)Se2) that has been researched for decades is just now being commercialized. These cells look to fulfill entirely new niches in the solar market. The cells are lightweight and flexible. The constituent elements have fewer scarcity concerns compared to CdTe, although competition with other industries (touch screen devices use In, as well), will likely limit production capacity (and raise costs). This is another potentially really inexpensive technology with good performance comparable to CdTe. People in these research groups actively research this material, as well as CdTe to further improve performance.

The material that we research, Cu2ZnSn(S,Se)4, is composed of earth-abundant and inexpensive elements, making it unlikely that the scarcity concerns that hinder the other technologies would limit it. Research focuses on improving the efficiency of cells using CZTS as an absorber layer. It also seems amazingly amenable to cheap fabrication methods. However, CZTS device performance isn't yet good enough to compete commercially. The record cell efficiency is 12.6 %, lower than module efficiencies of CIGS and CdTe commercial modules. We research these materials in order to improve efficiencies and make them competitive. Even if they don't pan out, we also learn about the broader materials system. For example, in my quest to learn about the CZTS material, I'm also learning about Cu2SnS3 and Cu4Sn7S16, which may also be of engineering importance, to name a few. Much of the understanding of CIGS helped in understanding CZTS.

Perovskite solar cells came out of nowhere and are achieving amazingly high efficiencies (although their lifetimes and tendency to degrade leave more to be desired). We don't know what the next technology will be, but it is always good to gain a better fundamental understanding of the world around us so that we have a knowledge base that can actually be applied intelligently.