V. New Device Architecture Design and Optimization
for DSSC Commercialization
Dye-sensitized solar cells (DSSCs) have aroused intense interest and been regarded as one of the most prospective solar cells, due to low-cost, flexibility, simple device fabrication and high conversion efficiency under weak light, in comparison to the conventional photovoltaic devices. According to PV Installation Market Share’ projection, DSSCs will have been significantly increasing in demand in a near future. Very recently, G2E in Swiss and G24i in UK including Korean and Japan companies have demonstrated commercial and prototyped components based on DSSC technology with liquid electrolytes. However, the unit costs, long-term device stability and power conversion efficiency must be further improved for real-life applications. For these purposed, new strategy on materials paradigm for low-cost, long-term stable, highly efficient dye-sensitized solar cells will be developed to give right answers in overcoming the limitation of the existing DSSC technology for the practical use.
The optimization of tandem-structured dye-sensitized solar cells (T-DSSCs) has been investigated for improving DSSC performances by expanding the photo-response into the near-IR region. The irradiated near-IR light must penetrate into the bottom cell without losing photons at the top cell. And there are two kinds of T-DSSCs, such as serial-connected (ST-DSSC) and parallel-connected (PT-DSSC) dye-sensitized tandem solar cells. For ST-DSSCs, the short-circuit photo current density(Jsc) of the top and bottom cells should be same, and for PT-DSSCs, the open-circuit voltage(Voc) of the top and bottom cells should be identical. When these conditions are fulfilled in each case, the conversion efficiency of both ST-DSSC and PT-DSSC could be optimized to give the maximized efficiency.
For use as counter electrodes (CE) in dye-sensitized solar cells (DSSCs), nitrogen-doped graphene nanoplatelets (NGnP) were synthesized using a simple two-step reaction sequence. The resultant NGnP were deposited on FTO glass substrates by an electrostatic spray (e-spray), and their electrocatalytic activities were evaluated for Co(bpy)33+/2+ redox couple in electrolyte solution. Their charge transfer resistances were much lower than that of Pt CE. Moreover, the NGnP5 and NGnP6 CEs showed better electrochemical stability under prolonged cycling potential. DSSCs fabricated with the NGnP5 and NGnP6 CEs were superior to that with the Pt CE particularly in short circuit current and PCE at one sun illumination by a factor of 1.05.
Triblock copolymers containing PAN (poly(acrylonitrile) were synthesized by RAFT polymerization for highly efficiency quasi-solid state dye-sensitized solar cells (DSSCs), for the first time. The PAN has been widely used in the DSSC to create a polymeric gel-state electrolyte. But, its solubility is not good enough to use as a gel-state electrolyte because of strong interaction between the molecules inside. In order to improve the solubility of PAN, we introduced comonomers of ethylene oxide and amide-based monomer for the preparation triblock copolymers, as a result, the amide-based segments increased not only their solubility but also their molecular weights. Triblock copolymers-based quasi-solid state DSSCs exhibited the open-circuit photovoltage (Voc) of 0.770V, short circuit photocurrent density (Jsc) of 17.83 mA cm-2, and the fill factor (FF) of 75.1%, yielding an overall photovoltaic conversion efficiency(PCE) of 10.32%, which is higher than a liquid electrolyte-based DSSC with a value of 9.89%. In addition, the normalized efficiency and long-term device stability of gel-state DSSCs under light-soaking (100 mW cm−2) at 50 °C as a function of time is decribed as below.
