We report on the optimization of thin film silicon tandem junction solar cells for applications in photoelectrochemical water splitting devices. Thin film silicon technology stands out as an attractive choice for water splitting applications, because it combines low-cost production, earth-abundance and versatility. The requirement to generate a photovoltage above 1.23 V, the electrochemical potential needed to electrolyze water, gives great importance to the latter characteristic, as thin film silicon solar cells can be adjusted to satisfy the specific thermodynamic requirement of different photoelectrochemical systems, i.e. provide an extended range of achievable voltages, without impairing device efficiency. Tandem junction solar cells consist of two sub-cells connected in series.
In this work, we investigate two types of p-i-n tandem solar cells: two amorphous (a-Si:H/a-Si:H) sub-cells and amorphous connected to microcrystalline (a-Si:H/µc-Si:H) sub-cells.a-Si:H and µc-Si:H layers were deposited by plasma enhanced chemical vapor deposition, using a mixture of SiH4, H2, CH4, B(CH3)3 and PH3 gases. The optical band gap E04 was evaluated using photothermal deflection spectroscopy measurements and the crystallinity ICRS of µc-Si:H was determined by means of Raman spectroscopy. Solar cells were investigated by current-voltage under AM 1.5 illumination and quantum efficiency measurements. The photoelectrochemical performance of the solar cells was evaluated in an aqueous 0.1 M H2SO4 solution, using a three-electrode configuration under halogen lamp irradiation.
By varying the substrate temperature and the SiH4 to total gas-flow concentration (SC) during deposition, we show that in the case of a-Si:H/a-Si:H tandem cells, the VOC can be significantly improved. An optimum is found, when the substrate temperature for the intrinsic a-Si:H layers with 4% SC is maintained at 120°C, which results in a VOC of 1.87 V with 10.0% efficiency. The increase in VOC is attributed to wider E04 of the individual a-Si:H sub-cells. For µc-Si:H solar cells, the VOC is strongly related to the intrinsic layer crystallinity, which can be controlled by varying the SC during deposition. µc-Si:H single junction devices with SC of 4.8% promote 540 mV and provide a crystallinity of up to 60%. By increasing SC up to 6%, photovoltages of 655 mV were achieved to the detriment of η and ICRS.
Here, further improvement in the control of the bulk and interface properties is needed. When connecting a 60%-crystalline µc-Si:H bottom-cell with the optimized a-Si:H top-cell we could achieve a VOC of 1.42 V and an efficiency of 10.8%. Based on photoelectrochemical experiments, we show the performance of the developed tandem solar cells, used as photocathodes, in contact to an electrolyte. a-Si:H/µc-Si:H photocathodes with a Pt back contact exhibit a photocurrent onset potential of 1.3 V vs. the reversible hydrogen electrode (RHE) and a high photocurrent of 9.0 mA/cm2 at 0 V vs. RHE.