UC Santa Cruz

This dissertation examines solar cell absorber materials that have the potential to replace silicon in solar cells, including several copper-based sulfides and perovskites. Earth-abundant absorbers such as these become even more cost-effective when used in a nanostructured solar cell. Atomic layer deposition (ALD) and chemical vapor deposition (CVD) deposit highly conformal films and hence are important tools for developing extremely thin absorber solar cells with scalability. Thus, the primary deposition techniques used for this work are the vacuum-based ALD and CVD. The primary characterization technique used is extended X-ray absorption fine structure (EXAFS), which has the ability to probe the local environment about different atoms, and can also give very precise ratios of elements using their fluorescence peaks.

The work on copper-based sulfides focused on examining the local structure of ZnS, ZnS/Cu_2S, and Cu_2SnS_3 composite films prepared with ALD and CVD. Individual thin films of Zinc Sulfide (ZnS) and Copper (I) Sulfide (Cu_2S) formed very successfully via ALD and resemble bulk structure. Yet multi-layer films of ZnS/Cu_2S, prepared using a wide range of parameters, produce films that are predominantly either ZnS or Cu_2S, with the other material being highly disordered. This can be attributed to the crystal structure mismatch of ZnS and Cu_xS, making ALD with these precursors unsuitable for a CuZnS alloy. Another copper-based material, Cu_2SnS_3, has a stable structure with good electrical and optical absorption properties. Composite films of Cu_2SnS_3 were made using CVD layers of Cu_2S and SnS_2 with an anneal step. The results highlight the importance of stoichiometry and phase control in copper-based ternary and quaternary materials.

Cu_2ZnSnS_4 (CZTS) has many desirable properties for a solar cell absorber, but the structures formed within the material remain difficult to characterize. This work investigates the local structure about the metal atoms in CZTS nanoparticles. The results suggest that the Sn may be substituting on to the Cu and Zn sites, excess SnS may be present, or clustering may occur for each element within the CZTS structure.

Finally, perovskites are highly promising solar cell materials, but degradation in structure from MAPbI_3 to PbI_2 remains a huge problem. XRD is a common method of characterizing the crystal structure, but it misses nanostructured regions, which are expected near mesoporous-TiO2. We formed thin films of lead iodide perovskites using solution-based deposition, a low-cost and low-energy alternative to vacuum-based techniques such as ALD and CVD. We use the EXAFS technique to explore the local structure of the perovskites in TiO_2. We find remnant PbI_2 even in fresh films, and the percentage increases as the MAPbI_3 degrades under light in dry conditions. The presence of TiO_2 accelerates the degradation.