Design, Fabrication And Application Of Novel Indium Free Transparent Conductive Films

Transparent conductor is an important opto-electronic multi-function film. It is widely used in flat panel displays, organic light emitting diodes, and most of the thin film solar cells, etc. Recently, energy crisis and global warming lead to widely investigatation on the solar cells in the whole world. Transparent conductive films (TCF) play a very important role in most thin film solar cells. Among them, In₂O₃: Sn (ITO), SnO2:F (FTO) and ZnO:Al (AZO) are the most widely used ones. However, ITO has high cost. Both FTO and AZO still have to improve their conductivity and transmittance in visible region. The purpose of this thesis is to develop novel indium free TCFs for solar cells that have the properties with room temperature fabricating process, low sheet resistance and high transmittance. The traditional technology for TCF is doping, it always requires high temperature deposition methods, and has difficulty in reach low sheet resistance and high transmittance in visible range. Dielectric/metal/dielectric (DMD) TCF has advantage in realizing these two performances. By optical anti-reflection design, high transparency in visible region can be easily accomplished; with metal layer sandwiched between two dielectrics, three layers forms a parallel connection, high conductivity can be achieved. This thesis is focus on studies of indium free DMD TCF. Indium free multifunctional TCFs have been developed through room temperature fabricating process. By optical design we got the optimal layer thicknesses of DMD. The novel results are as following:(1) Through theoretical calculation and experimental results we found that two dielectric layer thicknesses affect the transparent region, while Ag layer thichness decided the transmittance and electrical performances in DMD structure. (2) We adopted WO₃, ZnSe and Sb₂O₃ as dielectric materials, and Ag as metal layer, respectively. By inputting optical index n, k of each material, each layer's optimal thickness of WO₃/Ag/WO₃ (WAW), ZnSe/Ag/ZnSe (ZAZ) and Sb₂O₃/Ag/ Sb₂O₃ (SAS) for good transmittance were theoretically modeled. Then based on the designed structure, we fabricated WAW, ZAZ and SAS transparent conductive films by electron beam evaporation under room temperature.WAW with a work function of 6.33 eV, and a low sheet resistance of 12Ω/â–¡were achieved. This is the highest work function TCO to the best of our knowledge, which is desirable for applications in many opto-electronic devices. This result suggests that the work function of WAW depends little on the Ag work function. A green organic light emitting device (OLED) adopted WAW as anode, showed a very low turn on voltage of 1.9 V, which broke the thermodynamic limit of 2.35 V for the green device. It indicates that the extremely high work function of WAW can enhance the hole injection significantly. The polymer solar cells adopted WAW as anode showed 75.2% higher energy conversion efficiency and 0.2 V higher open voltage than those of ITO anode device.ZnSe is an IR material that is widely used in military field, however it is high absorptive below 0.65um that restricts its application in visible region. Through the optimal design, we achieved ZAZ TCF with an average transmittance of 64% in visible region (400-700nm), and a peak transmittance of 83%. It has an electron density of 1.208 1020 cm-3, an electron mobility of 17.22 cm2V-1s-1, and resistivity of 2.867 10-5Ω?cm, respectively. The work function is as high as 5.13 eV, which is comparable to high work function metal Aurum.We firstly adopted Sb₂O₃ in DMD structure, and fabricated Sb₂O₃/Ag/Sb₂O₃ (SAS) deep UV transparent conductive films with low resistivity. SAS with the thickness of 51nm/18nm/32nm has carrier concentration of 7.722E+21 cm-3, mobility of 21.92 cm2V-1s-1, resistivity of 3.688E-05Ω?cm, work function of 5.22 eV, and the sheet resistance of 8Ω/â–¡. We firstly reached such low sheet resistance in deep UV region with a high transmittance of 50% at 280nm, 60% at 286nm, 70% at 296nm, 80% at 306nm, and 92% at 335nm. Furthermore, by changing the layer thickness, adjustable high transparent region with transparency over 80% can be accomplished within 306⁸00nm. The room temperature fabricated SAS shows good thermal stability that can sustain 400 degrees Celsius annealing. SAS film is firm on glass substrate, and can be processed by lithography. The polymer solar cell adopted SAS as anode is better than ITO anode device both in open voltage and short current density. These outstanding performances allow SAS to have potential applications in future.