= 아 래 =
1. 일 시 : 2013. 3. 19 (화), 16:00 ~
2. 장 소 : 응용공학동 1층 영상강의실
3. 연 사 : 신병하 박사 (IBM T.J. Watson Research Center)
4. 제 목 : High Efficiency Thin Film Solar Cells with Earth-Abundant Cu2ZnSn(SXSe1-X)4Absorbers
The currently leading thin film photovoltaic technologies are based on a family of chalcogenides. Notably, Cu(In,Ga)Se2 (CIGS) and CdTe based thin film solar cells have achieved a cell efficiency over 20% and over 17%, respectively. However, these technologies suffer from scarcity and/or toxicity of the constituent elements. To overcome these issues, researchers have begun to look for a non-toxic and earth-abundant alternative such as Cu2ZnSn(S, Se)4 (CZTSSe). CZTSSe has suitable optical properties for photovoltaic applications: the compound has large absorption coefficients and its band gap can be readily tunable—by adjusting the relative amounts of the chalcogens in the compound, i.e., sulfur and selenium—from 1.0 eV to 1.5 eV, thereby covering the band gap the range predicted to yield the highest power conversion efficiencies. We have employed thermal co-evaporation and a brief post-deposition annealing under chalcogen (S or Se) vapor to prepare CZTSSe films using Knudsen-type elemental sources (Cu, Zn, and Sn) and valved crackers (S and Se). Completing the device structure with a Mo back contact, CdS buffer layer, ZnO transparent conducting oxide, and Ni/Al fingers, we have achieved a power conversion efficiency of 8.4% from pure sulfide CZTS solar cells (bandgap ~1.45 eV)—which is the record efficiency for pure sulfide CZTS fabricated by any method—and 8.9% from pure selenide CZTSe solar cells (bandgap ~1.0 eV).
In the first part of the talk, I will present our extensive structural, optical, and electrical characterization of the 8.4% CZTS record cell. I will highlight issues that may be responsible for limiting the efficiency—the formation of secondary phases, an interfacial reaction to form MoS2, a short recombination lifetime of photo-generated carriers, open-circuit voltage deficit, a relatively high series resistance that diverges at low temperatures, and CZTS/CdS band alignment.
In the second part of the talk, I will report on structural properties and device results of full selenide CZTSe solar cells. Compared to similarly-prepared pure sulfide CZTS devices, CZTSe devices exhibit a much thicker interfacial MoSe2 reaction layer between CZTSe and a Mo back contact. We find that the control of an interfacial MoSe2 layer thickness and the introduction of an adequate Se partial pressure during a post-deposition annealing are essential to achieve high efficiency CZTSe solar cells; a reverse correlation between device performance and the MoSe2 thickness is observed, and an insufficient Se partial pressure leads to the formation of defects within the bandgap as revealed by photoluminescence measurements. I will discuss the use of a TiN diffusion barrier which suppresses the formation of MoSe2 and leads to 8.9% efficiency CZTSe cells. If time permits, I will also discuss the formation kinetics of the MoSe2 interfacial layer.