Gee Yeong Kim

Efficient Carrier Separation and Intriguing Switching of Bound Charges in Inorganic–Organic Lead Halide Solar Cells

We fabricated a mesoporous perovskite solar cell with a ∼14% conversion efficiency, and we investigated its beneficial grain boundary properties of the perovskite solar cells through the use of scanning probe microscopy. The CH3NH3Pb(I0.88,Br0.12)3 showed a significant potential barrier bending at the grain boundary and induced passivation. The potential difference value in the x = 0.00 sample is ∼50 mV, and the distribution of the positive potential is lower than that of the x = 0.12 sample. We also investigated the polarization and hysteretic properties of the perovskite thin films by measuring the local piezoresponse. Specifically, the charged grain boundaries play a beneficial role in electron-hole depairing and in suppressing recombination in order to realize high-efficiency perovskite solar cells.

Trapping charges at grain boundaries and degradation of CH3NH3Pb(I1−x Br x )3 perovskite solar cells

The electrical properties of CH3NH3Pb(I1-x Br x )3 (x = 0.13) perovskite materials were investigated under ambient conditions. The local work function and the local current were measured using Kelvin probe force microscopy and conductive atomic force microscopy, respectively. The degradation of the perovskite layers depends on their grain size. As the material degrades, an additional peak in the surface potential appears simultaneously with a sudden increase and subsequent relaxation of the local current. The potential bending at the grain boundaries and the intragrains is the most likely reason for the change of the local current surface of the perovskite layers. The improved understanding of the degradation mechanism garnered from this study helps pave the way toward an improved photo-conversion efficiency in perovskite solar cells.

Effects of Mo back-contact annealing on surface potential and carrier transport in Cu2ZnSnS4 thin film solar cells

The effects of Na on Cu2ZnSnS4 (CZTS)-based solar cells have been examined with respect to surface potential and carrier transport. The Mo back-contact was annealed in a furnace for 10 minutes under a nitrogen atmosphere at different temperatures and CZTS thin films were subsequently grown by sputtering and sulfurization. The thickness of MoS2, formed during the sulfurization process, decreased as the Mo annealing temperature increased. Interestingly, the Na contents diffused from soda lime glass has increased as well. The current and surface potential near CZTS grain boundaries were investigated by Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM) were used. Surface current increased with increasing annealing temperature and surface potential increased up to approximately 50 mV near GBs, which led to inhibition of electron–hole recombination and an increase in minority carrier collection near GBs. This observation explains the improvement of solar cell open circuit voltage (VOC) and current density (JSC).

Identification of marcasite in pyrite FeS2 thin films and the films' carrier transport characteristics

In this study, we characterized FeS2 thin films grown by the non-vacuum spin-coating method. We annealed the samples under a sulfur atmosphere at different sulfurization temperatures. The phase transformation from marcasite-containing pyrite to pure pyrite occurs between 350 and 400 °C. The structural phase formation on the films depends on the sulfurization temperature. It is known that pure pyrite thin films are more suitable for solar cells. FeS2 thin films with and without the marcasite phase were investigated in terms of their local electrical properties and carrier transport by conductive atomic force microscopy and Kelvin probe force microscopy. Interestingly, the pure pyrite thin film shows less conducting behavior than the mixed phase sample because the mixed phase thin film has other residues on the surface. Pure pyrite shows two major work functions at 4.64 and 4.70 eV, and the mixed phase sample has multiple surface potential peaks below 4.63 eV, which is the main pyrite FeS2 work function. Pure-phase pyrite thin films are promising for earth-abundant solar cell applications.

Influence of the ZnS precursor thickness on high efficiency Cu2ZnSn(S,Se)4 thin-film solar cells grown by stacked-sputtering and selenization process

CZTSSe thin-films were deposited by stacked sputtering methods (ZnS/SnS/Cu) and annealed with selenization. We adjusted the thickness of the ZnS precursor layer in CZT precursors. A 337 nm thickness of ZnS precursor was shown an efficiency of up to 9.1%. We investigated the secondary phases by Raman spectroscopy and Kelvin probe force microscopy with depth profiles. The Cu2SnSe3, ZnSe, and MoSe2 secondary phases appeared near the back contact region. The phase distributions of the CZTSSe thin-films are different depending on ZnS precursor thickness with different depths. This phase characterization can describe the influences to the device performance of the CZTSSe thin-film solar cells.

Effect of selenization on local current and surface potential of sputtered Cu 2 ZnSn(S, Se) 4 thin-films with 8% conversion efficiency

Cu2ZnSn(S, Se)4 (CZTSSe) is emerged as a promising material because of non-toxic, inexpensive and earth abundant more than Cu(In, Ga)Se2. The highest conversion efficiency of CZTSSe is 11.1 % at IBM based on hydrazine process [1]. We have achieved CZTSSe thin-film solar cell 8% conversion efficiency by sputtering process. CZTSSe thin-films show different efficiency depending on selenization process. Depending on the different selenization process, conversion efficiencies in each sample are definitely distinguished from 8.06 to 3.17%. We investigated local electrical properties on these samples. The local surface potential is also critically different, which is ~ 200 mV on 8.06% film and ~ 70 mV on 3.17% film. From these results, we can suggest that selenization process can affect to local electrical characteristic as well as improving solar cell performances.