Atomic layer deposition of ZnO films and their application to solar cells

ZnO films have been successfully grown by atomic layer deposition (ALD) using diethylzinc (DEZn) and H2O as reactant gases. The processing window of self-limiting growth is for the first time observed for the substrate temperature from 105 to 165 degrees C in this study. The films had a good uniformity in thickness. Furthermore, a resistivity of 6.4 x 10(-4) Omega cm was obtained for the boron-doped films with a thickness of only 200 nm. A remarkable feature of ALD-ZnO films is the stability of electrical properties as compared with the MOCVD or photo-MOCVD-ZnO films. For the films grown by MOCVD and photo-MOCVD, a strong variation of the electrical properties was observed in the undoped films after air exposure. On the other hand, the ZnO films grown by ALD exhibited excellent stability regardless of whether they were doped or not. For the application of ALD-ZnO films to the TCO of a-Si, we newly proposed a two-step process to grow textured and low resistive ZnO films. In this process, ZnO films with a bilayer structure were deposited, the first layer with textured morphology was grown by the MOCVD method, then the second layer with lower resistivity was deposited using the ALD technique. It was found that electrical properties of ZnO films can be improved with this technique, and an improvement in the stability of the electrical properties was also observed. The a-Si solar cell with a ZnO layer grown by the two-step method whose area was about 1cm(2) showed the stabilized efficiency of 8.2% with an i-layer thickness of 300 nm.