Heat treatment induced structural modification of boron-doped ZnO layers

Complete text of publication follows. In the case of solar cells, to optimize the properties of photovoltaic (PV) modules one of the important issues is the diffusion of sodium from the glass substrate into the layered structure. There are many discussions about the role of the Na during preparation of PV modules: i) selenisation of Mo interlayer between the ZnO and active layers to obtain a lower series electrical resistance without changing the transmittance of the ZnO layer; ii) the diffusion of Na from glass substrate to a-Si:H/c- Si or Cu(In,Ga)Se2 (CIGS) layer. In certain cases the ZnO layers must be annealed in air or in hydrogen atmosphere to improve electrical conductivity. At the same time, the annealing temperature induces diffusion of Na. Thus, detailed investigation of appropriate layered materials is required. Present work was initiated in the framework of developing the solar cell technology. The goal was to investigate suitable substrates appropriate for desirable Na diffusion. Borondoped ZnO layers were prepared by Low Pressure Chemical Vapour Deposition (LPCVD) method on three different types of substrate: soda-lime windows glass (s1) and Schott AF-45 alkali-free thin glass (s2) were selected for our experiments. Polished Si substrate (s3) was used as a reference sample. After sample preparation procedure, the samples were annealed in air at 300 deg C. As it can seen in Fig.1, significant structural changes occurred at the surface of boron-doped ZnO layers in the form of bubble formation. We experienced that Na diffusion directly connected with hydrogen precipitation. Formation of bubbles observed was most probably due to enhanced hydrogen accumulation from the ZnO:B structure or decomposition of physisorbed precursor. After the annealing, the bubbles on the sample surface were blown up and cracked, i.e. were destroyed on polished Si and Soda-lime glass substrates. However, the samples prepared on AF 45 Schott alkali-free thin glass substrate had visually much less bubbles originated from the ZnO film. Acknowledgements. This work was supported by the National Office for Research and Technology (Grant nos. OTKA 73424 and TFSOLAR2).