The origin of an efficiency improving "light soaking" effect in SnO 2 based solid-state dye-sensitized solar cells
Energy and Environmental Science 5:11 (2012) 9566-9573
Abstract:
We observe a strong "light-soaking" effect in SnO 2 based solid-state dye-sensitized solar cells (SDSCs). Both with and without the presence of UV light, the device's short-circuit photocurrent and efficiency increase significantly over 20-30 minutes, until steady-state is achieved. We demonstrate that this is not due to improved charge collection and investigate the charge generation dynamics employing optical-pump terahertz-probe spectroscopy. We observe a monotonic speeding-up of the generation of free-electrons in the SnO 2 conduction band as a function of the light-soaking time. This improved charge generation can be explained by a positive shift in the conduction band edge or, alternatively, an increase in the density of states (DoS) at the energy at which photoinduced electron transfer occurs. To verify this hypothesis, we perform capacitance and charge extraction measurements which indicate a shift in the surface potential of SnO 2 of up to 70 mV with light soaking. The increased availability of states into which electrons can be transferred justifies the increase in both the charge injection rate and ensuing photocurrent. The cause for the shift in surface potential is not clear, but we postulate that it is due to the photoinduced charging of the SnO 2 inducing a rearrangement of charged species or loss of surface oxygen at the dye-sensitized heterojunction. Understanding temporally evolving processes in DSCs is of critical importance for enabling this technology to operate optimally over a prolonged period of time. This work specifically highlights important changes that can occur at the dye-sensitized heterojunction, even without direct light absorption in the metal oxide. © 2012 The Royal Society of Chemistry.Unraveling the function of an MgO interlayer in both electrolyte and solid-state SnO 2 based dye-sensitized solar cells
Journal of Physical Chemistry C 116:43 (2012) 22840-22846
Abstract:
The coating of n-type mesoporous metal oxides with nanometer thick dielectric shells is a route that has proven to be successful at enhancing the efficiency of some families of dye-sensitized solar cells. The primary intention is to introduce a "surface passivation layer" to inhibit recombination between photoinduced electrons and holes across the dye-sensitized interface. However, the precise function of these dielectric interlayers is often ambiguous. Here, the role of a thin MgO interlayer conformally deposited over mesoporous SnO 2 in liquid electrolyte and solid-state dye-sensitized solar cells is investigated. For both families of devices the open-circuit voltage is increased by over 200 mV; however, the short-circuit photocurrent is increased for the solid-state cells, but reduced for the electrolyte based devices. Through electronic and spectroscopic characterization we deduce that there are four distinct influences of the MgO interlayer: It increases dye-loading, slows down recombination, slows down photoinduced electron transfer, and results in a greater than 200 mV shift in the conduction band edge, with respect to the electrolyte redox potential. The compilation of these four factors have differing effects and magnitudes in the solid-state and electrolyte DSCs but quantitatively account for the difference in device performances observed for both systems with and without the MgO shells. To the best of our knowledge, this is the most comprehensive account of the role of dielectric shells in dye-sensitized solar cells and will enable much better interfacial design of photoelectrodes for DSCs. © 2012 American Chemical Society.A polyfluoroalkyl imidazolium ionic liquid as iodide ion source in dye sensitized solar cells
Organic Electronics Elsevier 13:11 (2012) 2474-2478
Solution-processed dye-sensitized ZnO phototransistors with extremely high photoresponsivity
Journal of Applied Physics 112:7 (2012)
Abstract:
We report the fabrication of light-sensing thin-film transistors based on solution processed films of ZnO, as the channel material, functionalized with an organic dye as the light sensitizer. Due to the presence of the dye, the hybrid devices show exceptionally high photosensitivity to green light of 10 6 and a maximum photoresponsivity on the order of 10 4 A/W. The high performance is argued to be the result of the grain barrier limited nature of electron transport across the polycrystalline ZnO film and its dependence on charge carrier density upon illumination with green light. In addition to the excellent photoresponsivity and signal gain, the hybrid ZnO-dye photoactive layer exhibits high optical transparency. The unique combination of simple device fabrication and distinctive physical characteristics, such as optical transparency, renders the technology attractive for application in large-area transparent photodetectors. © 2012 American Institute of Physics.The perils of solar cell efficiency measurements
Nature Photonics 6:6 (2012) 337-340