**5. Conclusion**

This chapter gives the review of the CIGS solar cells regarding the heterostructures, materials, technology and research advances. The CIGS solar cells consist of a number of films which are deposited onto a rigid or flexible substrate. Mo deposited by magnetron sputtering serves as a nontransparent back-contact. The Mo film is covered by the actual Cu(In,Ga)Se2 film. The coevaporation method and the two-stage process are two of the most important techniques to fabricate Cu(In,Ga)Se2. The heterojunction is formed by depositing a very thin n-type buffer layer (typically CdS) and an n-type wide gap transparent window layer (usually heavily doped ZnO). The CdS buffer layer is usually fabricated by a chemical bath deposition method. The magnetron sputtering is extensively used to deposit the ZnO window layer. The chapter also states the key findings in our research and provides suggestions for future research. One of the plausible solutions for boosting solar cells' efficiency is to implant nanostructures such as ZnO nanorod arrays in the conventional thin film photovoltaic devices. ZnO nanostructures can be grown on top of the CIGS solar cells' window layers as an antireflective coating layer or implanted into the solar cells. On one hand, the implanted nanostructures will decrease the reflection and increase the light path due to light coupling effects. On the other hand, the ZnO nanostructures will put the electrode close to the photoinduced carrier generation area with larger carrier collection function. It will assist in boosting the solar cells' efficiency by carrier collection enhancement.
