Publication date: 31st March 2013
We report on the optimization of thin film silicon tandem junction solar cells for applications in photoelectrochemical water splitting. Thin film silicon technology stands out as an attractive choice for water splitting applications, because of its low-cost production, its earth-abundance and versatility. The requirement to generate a photovoltage over 1.23 V, the thermodynamic 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 sustaining device efficiency.
Tandem junction solar cells consist of two sub-cells connected in series. In this work, we investigate two types of 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 from Photothermal Deflection Spectroscopy measurements and the crystallinityICRS of µc-Si:H from Raman spectroscopy. Solar cells were investigated by current-voltage and Quantum Efficiency measurements.
By varying the substrate temperature and SiH4 to total gas-flow concentration (SC), we show that in the case of a-Si:H/a-Si:H tandem cells, the optical and electrical properties of the sub-cells can be tuned to improve VOC of the tandem device. An optimum is found, when the substrate temperature for the intrinsic a-Si:H layers is decreased to 120°C, which results in an VOC of 1.87 V with 10.4% efficiency of the tandem solar cell. The increase in VOC is attributed to wider E04 of the individual a-Si:H sub-cells. For µc-Si:H-layers, the VOC is related to the intrinsic layer crystallinity, which depends on SC during deposition. µc-Si:H single junction devices with SC of 4.8% promote 535 mV and provide crystallinity 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 growth control of low-crystalline µc-Si:H-layers is needed. When connecting a 60%-crystalline µc-Si:H sub-cell with the optimized a-Si:H top-cell, mentioned above, we could achieve a VOC of 1.41 V and an efficiency of 10.8%.
Our results evidence that a direct application of silicon based photoelectrochemical cells fulfills the main thermodynamic requirements and may provide an efficient and low cost solution to generate H2 formation to a level which is sufficient for technological use.
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