펄스레이저 증착법으로 제작된 ZnTe:Cr 기반의 중간밴드 태양전지에 대한 연구

Abstract

Recently, low-cost, high efficiency solar cells have attracted tremendous interest for finding a renewable and clean energy source. The power conversion efficiency (PCE) of a single junction solar cell with an Intermediate Band (IB) in the band gap, which is called intermediate band soalr cell (IBSC), is theoretically predicted to be about 63 % which is much higher than that of a triple junction solar cell. There are several kinds classified as impurity doping, quantum dots, and highly mismatched alloys. And, several theoretical and experimental works which dope impurities into semiconductors have been recently attempted. In this thesis, the high performance ZnTe:Cr IBSC is fabricated by using pulsed laser deposition (PLD). And, for confirming IB in ZnTe:Cr absorber layer of ZnTe:Cr IBSC, the characterization method and analysis are performed. Firstly, ZnTe:Cr and undoped ZnTe thin films are grown on Al2O3 substrate by using PLD prior to realizing ZnTe:Cr IBSC. In X-ray diffraction (XRD) measurement, there is no significant change of crystal structure between the ZnTe:Cr and undoepd ZnTe, and the crystal structure of the thin films shows cubic zinc-blende structure. From UV-VIS spectroscopy, ZnTe:Cr thin film has more enhanced absorption coefficient than undoped ZnTe, and IB is located in band gap (2.14 eV) of ZnTe:Cr thin film. From Hall effect measurement, ZnTe:Cr thin film shows p-type conductivity with doping concentration of 1.2x1014cm-3. To find the optimum condition of emitter layer for ZnTe:Cr IBSC, ZnO:Al thin films are grown at different partial oxygen pressure, p(O2), and growth temperature. Among the ZnO:Al thin films grown at different p(O2), the ZnO:Al thin film grown at p(O2) of 1 mTorr exhibits the highest transmittance of 87.58%, the lowest resistivity, and the highest mobility. Next, ZnTe:Cr IBSC (ZnO:Al/ZnTe:Cr/Si) is fabricated after the electrical and optical properties of ZnTe:Cr and ZnO:Al layer are analyzed. Chromium (Cr) is uniformly distributed in the ZnTe:Cr absorber layer with an atomic concentration of about 3.5at.%. And, from spectroscopy ellipsometry measurement, the ZnTe:Cr absorber layer in ZnTe:Cr IBSC has higher absorption coefficients than those of undoped ZnTe absorber layer in undoped ZnTe SC in the photon energy range below band gap of ZnTe (2.14 eV). Under AM 1.5 Global illumination, ZnTe:Cr IBSC shows a large short circuit current of 21.18 mA/cm2, an open circuit voltage of 0.46 V, a fill factor of 0.58, and a PCE of 5.65 %, which is the highest reported PCE in an IBSC based on impurity-doped ZnTe. For further improving PCE of ZnTe:Cr IBSC, ZnO:Al emitter layer grown at p(O2) of 1 mTorr is used as emitter layer of ZnTe:Cr IBSC, and the value of PCE for the ZnTe; Cr IBSC with the ZnO:Al emitter layer is 8.05 %. Additionally, in order to grow p-ZnTe thin films with high hole concentration, N-doped ZnTe (ZnTe:N) thin films are grown at different partial pressure of N2, p(N2), from 0.1 to 70 mTorr by using PLD. From XRD measurement, the crystallinity of the ZnTe:N thin film is degraded with increasing p(N2) from 0.1 to 70 mTorr, while all ZnTe films have a cubic zinc-blende structure. In x-ray photoelectron spectroscopy, ZnTe:N thin film grown at p(O2) of 70 mTorr has the binding energy of the N-Zn bond positioned at 397.1 eV, which results in high hole concentrations of 6.65x1019 cm-3. From these results, it is concluded that the high p-type conductivity of ZnTe:N thin films originates from substantial nitrogen incorporation at substitutional Te sites.