Influence of TCO dry etching on the properties of amorphous-silicon solar cells

The transparent conductive oxide (TCO) plays an important role in the performance of amorphous-silicon solar cells. Three types of TCO are widely used at laboratory scale (SnO2, ITO and ZnO:Al), but industrial amorphous-silicon modules are almost always made using SnO2, owing to their lower cost. The present paper describes some investigations on the influence of TCO on device shunt resistance and proposes a simple method to prevent potential short-circuits caused by unwanted peaks on the surface of SnO2. A number of cells have been made with the structure SnO2/p–i–n/Al. Only 15% of these devices have shown shunt resistance above 1 MΩ cm2. Atomic-force microscopy (AFM) of the SnO2 surface has revealed the presence of some spikes higher than 500 nm (the overall thickness of the amorphous-silicon layers), which have been considered potentially responsible for short-circuits. With this hypothesis, a procedure has been devised, aimed at eliminating such irregularities inside the reactor chamber before growing the amorphous-silicon layers. After cleaning and drying the substrates, they are put in the PECVD chamber and subject to a CF4 plasma. Immediately, the three amorphous-silicon layers are deposited and the cell is completed. The proportion of cells having a shunt resistance above 1 MΩ cm2 has gone up to 60% using this approach. Independent tests have confirmed that the optical properties of the SnO2:F are not negatively affected by this dry-etching process.