Effect of bottom cell properties on micromorph tandem device performance

Micromorph tandem solar cells represent an elegant way of overcoming the efficiency limits of single-junction solar cells and reducing the light-induced degradation of amorphous silicon films. Micromorph devices have been realised on Asahi U-type TCO-covered glass substrates by very high frequency plasma-enhanced chemical vapour deposition (VHF-PECVD) at 100 MHz at low substrate temperature (150 degrees C). For the bottom cell, different growth regimes were explored by changing both chamber pressure and plasma power, with the aim of finding interesting regimes for industrial application. The effect of the structural composition of the microcrystalline absorber layer on the electrical parameters of the device was investigated. The highest efficiency (11.1%) is reached at 67 Pa and power to pressure ratio of 0.3 W/Pa. On the other hand, using a larger power to pressure ratio (0.5 W/Pa), high short circuit current density and constant efficiency over a wide silane concentration range were obtained. Homogeneity problems and low response at high wavelengths were found at large pressure (200 Pa). An evaluation of micromorph device stability was also carried out.