Energy from renewable sources is considered today the only long-term viable solution to the current energy crisis. Appropriate storage solutions, such as batteries, are, however, always required in such systems since energy from renewable sources such as wind solar or tidal has an intermittent character. The efficiency of solar cells, irrespective of the type, has steadily and significantly been improved in the past decade. Today, they are championed as one of the most relevant renewable energy conversion devices in both academic and political media. In a complete solar energy system, solar cells convert solar radiation into electric energy, the excess being stored in lead-acid or Li-ion batteries. So far, solar cells and the batteries were implemented as separated devices. Here, we explored possibilities to develop one single hybrid device, i.e., by integrating the conversion function of the solar cell with the storage function of a battery. We used an organic solar cell and combined it with a lithium-ion battery.

Some critical aspects have to be considered beforehand. First, the materials used in the solar cell and the battery have to be chemically compatible since they will be eventually sharing the same case. Second, it is important to find two electrode materials with a potential difference that matches the solar cell voltage (approximately 1.5 V). In addition, the goal for the solar cell is to reach high efficiencies and reasonable open circuit potentials.

The organic homo-tandem¹ solar cell was built in inverted device architecture. PTB7-Th (Poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethyl­hexyl)­carbonyl]thieno[3,4-b]thiophenediyl})) was used as donor and O-IDTBR ((5Z, 5'Z)-5,5'-((7,7'-(4,4,9,9-Tetraoctyl-4,9-dihydro-s-indaceno[1,2-b: 5,6(Benzo[c][1,2,5]thia­dia­zol-7,4-diyl) bis (methanolyliden)) bis (3-ethyl-2-thioxothiazolidin-4 -ein)) as non-fulle­rene acceptor. The intermediate layer was MoO₃/Al/PFN-Br. The battery was assembled with carbon-coated lithium vanadium phosphate² (LVP-C, cathode), LP30 (1M lithium hexa­fluoro­phosphate (LiPF₆) in ethylene carbonate (EC) and dimethyl carbonate (DMC); EC:DMC=1:1 (v/v)) and vanadium disulphide³ (VS₂, anode) in a Swagelok cell.

Full cell measurements of the Cu|VS₂|LP30|LVP-C|Al battery in a Swagelok cell showed a constant capacity of 74 mAh/g over 50 cycles. The tandem solar cells reached an open circuit potential (V_OC) of 1.9 V and a conversion efficiency slightly above 6 %. The first combined measurements were carried out with the solar cell encapsulated in epoxy resin and connected to the battery. The battery cycled between 0.4 and 1.4 V and was charged with a current density between 25 – 35 mA/cm² from the solar cell during illumination. To assemble the hybrid device, the Al-supported LVP-C cathode was directly placed on the Ag contact layer of the solar cell (the positive electrode) followed by the separator and the negative electrode of the battery. The battery was activated by soaking it with electrolyte. Finally, the whole solar-cell battery stack was encapsulated with ultraviolet-curable epoxy resin. First measure­ments lead to a successful proof of principle − the integrated battery was successfully charged by the solar cell.

These very first results are encouraging and prove the validity of this relatively simple but potentially powerful concept. Further work is scheduled to refine the concept and improve the performances of the hybrid solar cell - battery device.

Acknowledgement

The work was financed within the project SOLABAT funded by the Klima- und Energiefonds of FFG (Österreichische Forschungsförderungsgesellschaft. Moreover, we thank the Austrian Federal Ministry for Science, Research and Economy as well as the Christian-Doppler Forschungsgesellschaft for the financial support.

References

1. Kaltenhauser, V., Rath, T., Edler, M., Reichmann, A. & Trimmel, G. Exploring polymer/nanoparticle hybrid solar cells in tandem architecture. RSC Adv. 3, 18643 (2013).
2. Huang, C., Chen, D., Huang, Y. & Guo, Y. Electrochimica Acta Sol – gel synthesis of Li₃V₂(PO₄)₃/C cathode materials with high electrical conductivity. Electrochim. Acta 100, 1–9 (2013).
3. Rout, C. S. et al. Synthesis and characterization of patronite form of vanadium sulfide on graphitic layer. J. Am. Chem. Soc. 135, 8720–8725 (2013).