| 학회 | 한국재료학회 |
| 학술대회 | 2013년 봄 (05/23 ~ 05/24, 여수 엠블호텔(THE MVL)) |
| 권호 | 19권 1호 |
| 발표분야 | 제24회 신소재 심포지엄-The 2nd International Symposium on Green Energy 2013 |
| 제목 | Development of ZnO-based window and buffer layer for CIGS solar cells by MOCVD and ALD process |
| 초록 | The conversion efficiency of lab-scale thin film devices exceeds 20%, which consists of ZnO:Al/i-ZnO/CdS/CIGS/Mo/glass. The Al-doped ZnO (Zno:Al) is commonly deposited by RF magnetron sputtering that might cause the plasma damage to the underlying buffer and absorber layers. For commercial product, it is desirable to replace CdS with Cd-free buffer. To minimize the plasma damage we investigated B-doped ZnO films using LP-CVD to replace the Al-doped ZnO. Diethyl zinc (DEZ) and H2O vapor were used as a Zn and an O source, respectively, and B2H6 was used as a doping gas. The mobility of the films decreases continuously with an increase in the B doping concentration. This is attributed to the increased ionized impurity scattering caused by the high carrier concentration and grain boundary scattering caused by the reduced grain size. Moreover, it was found that the transparency of the heavily doped ZnO thin films in the NIR wavelength range was decreased with an increase in the carrier concentration due to the free carrier absorption. To solve the problem, we fabricated a high-mobility ZnO thin film with both high conductivity and high transparency in the NIR wavelength range by introducing a new technique, named pulse B doping. The pulse B-doped ZnO thin film also exhibited a well-textured surface morphology with large pyramid-shaped grains and a minimum resistivity value of about 1.0x10-3 Ω․cm. In addition, due to the lower carrier concentration and larger surface grains, the pulse-boron-doped ZnO films exhibited enhanced total transmittance in the near-infrared wavelength range and diffuse transmittance in the overall wavelength range compared to conventional boron-doped ZnO thin films. We utilized ZnO as a buffer layer for CIGS solar cells. In the case, Mg must be added in ZnO to increase the band gap and to minimize the conduction band offset. A ZnMgO buffer layer was deposited by ALD using DEZ and bis-ethyl-cyclopentadienyl-magnesium as metal precursors wit, H2O vapor as a reactant. The Mg content in the Zn1-xMgxO films were closely matched to the ALD cycle ratio of ZnO and MgO, implying that the composition of the films can be precisely controlled by adjusting the ALD cycle ratio of each monolayer. We confirmed that the band gap energy of the Zn1-xMgx O films increases from 3.25 to 3.76 eV as the Mg content increases. The optimum thickness of Zn1-xMgxO film for CIGS cell application was found to be 40 nm. The experimental results suggest that the optimum Mg content in Zn1-xMgxO film is 20%. Post annealin and light soaking were performed for CIGS solar cells to enhance the device performance after cell fabrication. The post annealing at 200 °C for 2 minutes increased the open circuit voltage due to defect recovery. The light soaking improved the fill factor and open circuit voltage. Using these two post treatments, we obtained conversion efficiency of 15%. However, this cell efficiency is still low compared to the world’s best efficiency of 18.1% from Uppsala University. Based on our result, we found that the main reason for this difference in the cell efficiency is the low fill factor. The fill factor was improved by reducing the series resistance of CIGS solar cells using a bilayer buffer with two different Mg contents, which has lower resistivity compared to Zn0.8Mg0.2O film. The CIGS solar cell fabricated using the bilayer buffer showed the efficiency of over 15%. Ref: 1) C. S. Lee et al., J. Electrochem. Soc. 2011, 158, H482-H486. 2) C. S. Lee et al., ECS Trans. 2011, 41, 213-219. 3) C.S. Lee et al, Phys. Chem. Chem. Phys. 2012, 14, 4789-4795 |
| 저자 | Byung Tae Ahn1, Chang Soo Lee2 |
| 소속 | 1Department of Materials Science and Engineering, 2KAIST |
| 키워드 | Cu(In; Ga)Se2 solar cell; ZnO window; ZnMgO buffer; MOCVD; ALD |
