Narrow band gap high conducting nc-Si1-xGex:H absorber layers for tandem structure nc-Si solar cells

Abstract

Advantages of spectral sensitivity in the infrared range and good stability against light exposure together have made nanocrystalline silicon–germanium alloy (nc-Si1-xGex) as an ideal absorber material at the bottom-cell in multi-junction Si thin film solar cells. Evolution of good quality SiGe thin films with sustained nanocrystallinity, particularly at low growth temperature (TS), is a challenging task in view of very different chemical reaction behaviors controlled via different energy requirements of two frequently used source gases, e.g., SiH4 and GeH4. Growth of nc-SiGe:H thin films have been pursued at TS ∼220 °C, using H2-diluted (by ∼96%) (SiH4 + GeH4) plasma in 13.56 MHz RF-PECVD, by varying the RF power applied to the parallel plate capacitively coupled electrodes of the reactor. At lower RF power Ge-dominated low band gap SiGe network has been produced with Si component in mostly amorphous configuration that leads to a low electrical conductivity. At high RF power Si–Si network becomes highly crystalline; however, Ge content reduces drastically. Breaking of weaker Ge–Ge bonds creates defects via dangling bonds and increased grain boundary defects around Si nanocrystals together produce relatively wider band gap SiGe network with degraded electrical conductivity. The novelty and significance of the present investigation remains in realizing the narrowing of the optical band gap (Eg ∼1.499 eV) even at lower Ge content, through moderate crystallization in the SiGe network and a simultaneous high electrical conductivity of the nc-SiGe:H films (σD ∼2.38 × 10−3 S cm−1) obtained at a high growth rate (∼9.5 nm/min), via optimization of the applied RF power at a moderate level (∼100 W). In such circumstances nanocrystallization of the network plays leading role in narrowing the optical band gap, overriding the effect arising out of the restricted presence of Ge.