The study of grain boundaries in polycrystalline thin-film solar cells [electronic resource]
Keywords: Electrical Engineering.

Silicon is one of the most studied semiconductor materials with techniques to control and manipulate its properties well established by the integrated circuit (IC) industry. While silicon wafer-based technology constitutes about 85% of the photovoltaics (PV) market, there are compelling reasons to develop thin-film solar cells. These include potential material cost and energy savings achievable with thin absorber layers of direct band-gap thin film materials, and the potential for incorporation of thin film PV onto inexpensive, flexible, or building material substrates, which opens up the possibility of new applications. Given the success of silicon wafer-based solar cells in the PV market, crystalline silicon on glass is a natural choice for thin-film PV technology. However, the efficiency of this technology is still too low to compete in the PV market. In fact, its record efficiency is only 10.5% which was achieved by CSG Solar. In order to boost the efficiency of this technology, we explore the use of ion-beam assisted deposition (IBAD) to create biaxially-textured silicon films which can potentially have less defects at the grain boundaries. Experimentally, we have fabricated an operational biaxially-textured silicon solar cell whose Voc is likely to have been affected by intra-grain defects as shown by our transmission electron microscopy (TEM) image. In order to investigate the interplay between grain boundary and intra-grain defects on solar cell performance and determine the potential benefits of developing biaxially-textured film solar cells, I used Synopsys' two-dimensional technology computer-aided design (TCAD) Sentaurus simulation tool. In general, the simulations found that biaxially-textured silicon solar cells improve solar cell efficiencies but there is small improvement for devices that have both large grains and low intra-grain carrier lifetime. Among the various thin-film technologies, CdTe and Cu(In, Ga)(S, Se)2 (CIGSSe), dominate the thin-film PV market. However in recent years, the photovoltaic community has seen growing interest in CZTS-based thin-film solar cells which include Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(S, Se)4 (CZTSSe) solar cells. This is driven by their potential to replace Cu(In, Ga)Se2 (CIGSe) and CdTe solar cells which face material scarcity, toxicity and market acceptance issues. In polycrystalline CIGSe-based (CIGSe, CISe, CuGaSe2 (CGSe)) and CdTe solar cells, grain boundaries do not seem to affect solar cell efficiency as much. In fact, some studies have identified grain boundaries as the source of high efficiency in polycrystalline CIGSe-based and CdTe solar cells. CIGSe-based and CZTS-based films are similar in terms of growth methods, optoelectronic and crystallographic properties. Because of these similarities and the benign nature of grain boundaries in CIGSe-based and CdTe films, it would be useful to examine the properties of grain boundaries in these materials. Using scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (C-AFM) techniques, I investigated the electronic properties of grain boundaries in CIGSe, CZTS and CZTSSe solar cells. SKPM measurements carried out in this work reveal a higher positive surface potential at the grain boundaries as compared to the grain while C-AFM measurements show higher current flow in the vicinity of the grain boundaries. These two measurement results are similar to those obtained for high quality CIGSe and CdTe and together they demonstrate the enhanced minority carrier collection taking place at the grain boundaries of CZTS and CZTSSe. Although SKPM measurement are susceptible to topographical and geometric effects, we believe that this effect is not dominating in our measurements as topography and surface potential profile lines are not exact mirror images of one another and regions of similar height have different potentials and vice versa. Nonetheless, I used a technique that involves photoreduction of AgNO3 to provide convincing evidence that our SKPM result is not a result of experimental artifacts but is truly indicative of the higher positive potential at the grain boundary. Having benign or beneficial grain boundaries have been found to be essential for achieving high efficiencies in polycrystalline CIGSe and CdTe solar cells. In my investigation, I found that high efficiency CZTS and CZTSSe solar cells have similar grain boundary electronic properties as high efficiency CIGSe and CdTe solar cells. As such, it might be possible for CZTS and CZTSSe solar cells to achieve similar high efficiencies as CIGSe and CdTe solar cells if other defects (intra-grain, surface and interfacial) are not limiting efficiencies.