Ah Reum Jeong

Enhanced exciton separation through negative energy band bending at grain boundaries of Cu2ZnSnSe4 thin-films

Local surface potential of Cu2ZnSnSe4 thin-films was investigated by Kelvin probe force microscopy. The surface potential profile across grain boundaries (GBs) shows a rise of 200–600 meV at GBs in a Cu-poor and Zn-poor film with 3.8% efficiency, which means positively charged GBs. In contrast, the GBs in a Cu-poor and Zn-rich film with 2% efficiency exhibit lowering of surface potential by 40 meV. The results indicate that GBs of Cu2ZnSnSe4 films play a role for exciton separation and governing defects for high efficiency could be not only CuZn but also VCu as explained theoretical predictions.

Synthesis and physical properties of Cu(In,Ga)Se 2 nanoparticles and CuGaSe 2 thin-films for tandem cell photovoltaic applications

For tandem cell photovoltaic applications, we have investigated Cu(In,Ga)Se2 (CIGS) nanoparticles and CuGaSe2 (CGS) thin-films. Alternative structures with metal Mo and transparent conductive oxide (TCO) back contacts were also studied.

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.

Carrier transport and low temperature device properties of Cu 2 ZnSnSe 4 thin-films with different Cu and Zn/Sn ratio

Earth abundant and non-toxic quaternary semiconductor Cu2ZnSnSe4 (CZTSe) is regarded as one of the alternatives for solar cell applications due to low cost and competitive photovoltaic performance. In this study, CZTSe thin-films with different Cu and Zn/Sn composition were investigated. On the other hand, for improving solar cell properties, it is important to understand from electron-hole transport to solar cell performance. The carriers in photovoltaic chalcopyrite thin-films behave differently in the vicinity of grain boundaries, which eventually effects on efficiency of the solar cell. Kelvin probe force microscopy measured local surface potential which indicates positive in the vicinity of grain boundaries in a Cu-poor CZTSe not a Cu-rich thin-film. Local current was measured by conductive atomic force microscopy under external bias reveals that carrier polarity changes due to bias direction. The electron-hole transport effects on a solar cell device which is influenced by the thin-film composition. The only stoichiometric (Cu/(Zn+Sn)=0.8, Zn/Sn=1.2) CZTSe thin-film reveals 1.5% of conversion efficiency and distinctive solar cell properties as a function of temperature. The conversion efficiency as well as Jsc and Voc are decreased under 200 K, which is particular on the contrary to those of Cu(In,Ga)Se2 thin-film. For improving CZTSe solar cell performance, it is essential to understand compositional effect and local electrical characteristics on CZTSe.

Kelvin probe microscopic studies on grain boundaries of Cu(In, Ga)Se 2 and Cu 2 ZnSnSe 4 thin-films grown by co-evaporation methods

Cu2ZnSnSe4 (CZTSe) thin-films is a promising candidate for absorber layer as an alternative to Cu(In, Ga)Se2 (CIGS) solar cells. However, they have been recorded lower efficiency than that of CIGS until now. In CIGS, local electrical property is an important issue for high efficiency such as Na interstitial in the grain boundaries (GBs). Therefore, difference between CIGS with CZTSe in local electrical property can be one of the clues for explaining lower efficiency of CZTSe thin-film solar cells than CIGS. We studied local surface potential using Kelvin probe force microscopy (KPFM) around GBs. The results reveal difference of electron-hole transport behavior on both thin-films at GBs. Eventually, we can understand relation between solar cell efficiency and local electrical property.

Nano-scale observation of charge transport and potential distribution of photovoltaic Cu(In,Ga)Se 2 thin-films

Understanding of grain boundary (GB) is critical for photovoltaic applications since electron-hole recombination at GBs determines the conversion efficiency. However, our local electrical and optical analysis shows positive potential at GBs in Cu(In,Ga)Se2 (CIGS), which suppresses the recombination at GBs. We report on a direct measurement of potential distribution and local electrical transport on the surface of photovoltaic CIGS using a nano-scale electrical characterization of Kelvin probe microscopy and conductive atomic force microscopy. This reveals that the positively charged surface potential at GB is expected to increase the minority carrier collection and the enhanced current at GB leads to large carrier mobility and electron-hole separation at the GBs. Micro-Raman scattering results helps to analyze electrical behavior from defect analysis.