Improvement in performance of tandem solar cell by applying buffer layer, back reflector and higher crystallinity of the microcrystalline Si active layer of bottom subcell

Tandem-structured amorphous Si solar cells can absorb a broader range of solar spectra than a single-junction device. In a tandem cell, usually a thicker bottom subcell becomes necessary to absorb the required number of photons from long-wavelength light to generate the desired current density. Alternatively, an optoelectronically improved active layer and light-trapping scheme can also be used with a thinner bottom subcell. In this respect, we investigated bottom subcell of tandem solar cells with microcrystalline Si active layer. Three of the primary parameters that were progressively varied for the bottom subcells were the crystalline volume fraction (X_c) of the active layer, buffer layer at the i/n interface, and back reflector (BR). Here, the buffer layer and higher X_c are expected to improve respectively the electronic properties of the i/n interface and the active layer of the bottom subcell. Furthermore, a suitable BR can reflect the unabsorbed long-wavelength light back to the cell for further absorption; thereby raising its current density. We have observed that low-power deposition of the microcrystalline Si active layer (150 W), an 8 nm thick buffer layer at the i/n interface, and a zinc oxide BR layer gave best results in a tandem solar cell. An optimized tandem cell exhibited an open-circuit voltage, short-circuit current density, fill factor, and efficiency of 1.445 V, 12.32 mA/cm², 71.56%, and 12.74%, respectively.