GCEP award #40654: High-Efficiency, Low-Cost Thin Film Solar Cells

Several key advances were accomplished within this project in the last year. These advances are subdivided in materials development, device development and modeling and simulation. Highly doped ZnO nanowires were synthesized and characterized. The goal is to use them as a solution-processable alternative to ITO for transparent electrodes. Appropriate characterization techniques for these nanomaterials were developed this year. We were able to independently determine carrier mobility and charge density in the nanowires. The most highly-doped material has a resistivity of the order of a few mΩ.cm. We found that the concentration of dopant atoms exceeds the carrier density by up to one order of magnitude suggesting that at high dopant density a compensating impurity sequesters the free charge. Simultaneously, Ag nanowire mats are being developed for the same purpose. A plating process was explored to improve the sheet resistance. Ag nanowire electrodes were demonstrated on flexible substrates owing to their low processing temperature. The last set of materials under development is a family of doped Cu-In-S nanowires, which will constitute the active layer of the cells. Initial devices were made with Ag nanowire electrodes. Single-junction organic cells performed as well as the ITO control cells. Simple tandem unconstrained cell stacks were fabricated as well. The two cells operated independently, as predicted, confirming the advantage of the unconstrained geometry. Finally, optical, electrical and device simulation tools were developed. An optical simulation program was written to rigorously solve for the trasmission through Ag nanowire meshes as a function of nanowire density. Furthermore, a separate program was developed to calculate the sheet resistance of Ag nanowire meshes with random geometry as a function of wire density. The latter program is being adapted to work with ZnO nanowires. Finally, a full device simulation tool has been developed to estimate what materials or cell architecture parameters limit performance. This program helps quantify the advantage of the unconstrained cell architecture compared to conventional current-matched architectures. A particularly attractive feature of the unconstrained architecture is its relative robustness with respect to spectrum (i.e. time of the day). Introduction and Background The goal of the project is to develop materials and processing techniques that will allow the fabrication of low-cost multi-junction solar cells entirely by solution-processing or other low-cost techniques amenable to roll-to-roll (R2R) fabrication such as lamination. Device modeling will help identify the best device architecture. A specific device design goal consists of developing an unconstrained multi-terminal multi-junction photovoltaic cell. We propose to take advantage of nanostructured materials to fabricate high-efficiency multi-junction cells using solution-based processing techniques. As a result, our cells will be amenable to large-area fabrication, thereby dramatically reducing their cost. Finally, our materials will allow to operate the individual cells independently, rather than in series, thereby bypassing current matching requirements. Results Materials Development ZnO nanowires Figure 1: High-resolution TEM micrographs of ZnO nanowires doped with Al (a,b) and Ga (c,d). The wires shown in b and d were grown in the presence of a surfactant (oleic acid). Degeneratively doped ZnO nanowires will be used as solution-processable transparent conductors. ZnO is known to form nanostructures of varied shapes. A synthesis in non-aqueous solvent was adapted. ZnO nanowires were obtained by decomposing Zn acetate in tri-octylamine at 365°C. These wires can be doped with with Al or Ga during synthesis by introducing Ga nitrate and Al acetate in the synthesis bath. In order to study dopant incorporation, the nanowires are characterized by scanning electron microscopy (SEM), scanning Auger spectroscopy, x-ray diffraction and inductively-coupled plasma atomic emission spectroscopy (ICP-AES). The typical range of dopant concentration used is 1 to 5 atomic % in solution. ICP-AES allow to determine the Al:Zn and Ga:Zn ratio in the wires. It is found that the dopant incorporation rate in the nanowires is approximately 50%. As a result, extremely high Al and Ga concentrations are obtained (up to 2x10 cm), which is remarkable given the low temperatures involved in the synthesis. Within the range of concentration we investigated, no phase separation is observed by x-ray diffraction. The formation of amorphous phases however cannot be excluded. In fact, in the absence of a surfactant during the synthesis process, an amorphous shell is observed by high-resolution TEM around Al-doped ZnO nanowires (Fig. 1a). The surfactant stabilizes the growth of Gadoped ZnO nanowires as well (Fig. 1d). Scanning Auger electron spectroscopy confirms that the dopants are distributed evenly within the nanowires. Figure 2: Electron mobility as a function of Al% in the nanowires (left) and carrier density in the nanowires as a function of Al% (right) obtained from optical absorption