Dye-sensitized solar cells using cobalt electrolytes: the influence of porosity and pore size to achieve high-efficiency

Diffusion impediments in redox shuttles are a major limitation to realizing high power conversion efficiencies (PCEs) in dye sensitized solar cells (DSCs) using cobalt complexes. Mass transport limitations are strongly dependent on the pore size and the porosity of mesoporous TiO2 films. Herein, we examine the major factors responsible for mass transport limitations and identify ways to minimize them, achieving highly efficient cobalt based DSCs. Depending on the electrolyte and dye used, the pore size and the porosity of the TiO2 films must be modulated to facilitate diffusion of cobalt complexes through pores. The influence of porosity, pore size and viscosity of the electrolyte used on the performance of DSCs with different max. obtainable JSCs are studied in detail. These effects were studied using two dye solns., Y123 (0.2 mM) and a cocktail soln. of SM342/Y123 (0.2 mM each, 7 : 1). The JSC is affected primarily by lowered pore sizes as a result of the TiCl4 post-treatment. Transient photocurrent decay measurements were performed on devices featuring photoanodes fabricated with differing porosities and pore sizes. This study indicates that in cells featuring only Y123, recombination rates decreased with decreasing porosity of TiO2 films (i.e. higher TiCl4 concns. used in their post-treatment). Conversely, in DSCs fabricated with the SM342/Y123 dye cocktail, an intermediate TiCl4 post-treatment concn. (20 mM) afforded the lowest recombination rate, resulting in the preservation of a high open-circuit potential (VOC). It was found that this intermediate concn. regime is optimal in order to achieve an efficiency of over 12.7% at full sun intensity with the SM342/Y123 dye cocktail. Finally, it was concluded that the porosity and pore size must be modulated for different dyes and electrolytes to minimize diffusion limitations of the redox species.

A new terpyridine cobalt complex redox shuttle for dye-sensitized solar cells

Filippo de Angelis, Gabriele Marotta, Mohammad Khaja Nazeeruddin

A new cobalt-terpyridine redox shuttle (Co(Cl-phenyl-terpyridine)(2)) has been synthesized, characterized, computationally investigated and then tested in photovoltaic devices, in conjunction with the prototypical N719 and Z907 ruthenium dyes. Lower efficiencies compared to the reference Co(bpy)(3) redox couple were obtained, probably due to increased recombination processes. Improvements in the devices performances were obtained by using chenodeoxycholic acid as surface co-adsorbent and as an electrolyte additive. Moreover, to better rationalize the photovoltaic results, the effect of two different counterions (TFSI and PF6) has been investigated for the reference Co(bpy)(3) electrolyte, finding a reduced efficiency for the TFSI-based electrolyte. (C) 2013 Elsevier B.V. All rights reserved.

Elsevier2013

A Material Approach to Dye-Sensitized Solar Cells

Philippe Pierre Labouchère

With growing concerns on future energy supplies, solar energy appears as an energy source whose potential remains to be tapped at a large scale. In the last two decades, dye-sensitized solar cells (DSCs) have been considered as a competitive means to convert solar energy into electricity thanks to their low manufacturing and environmental costs, simple fabrication and high efficiency. Indeed, record efficiencies as high as 13% have been obtained for liquid DSCs and recently, solid-state DSCs using solid hole transport materials as well as perovskite materials as sensitizers have showed efficiencies above 17%. From a physical point of view, charge carrier generation in a DSC takes place in a photoactive light harvester chemisorbed on a mesoscopic semiconductor oxide. Due to the excellent electronic overlap, the exciton separation occurs by the injection of the excited electrons and holes from the dye into the conduction band of the semiconductor and redox electrolyte, respectively. A high surface area mesoscopic photoanode is necessary to ensure a high-density bulk heterojunction and efficient light harvesting in the visible part of the solar spectrum due to effective dye loading. This implies that the DSC has a large, heterogeneous interface through which photogenerated electrons may be intercepted as a result of the slow electron transport. The latter is governed by an ambipolar diffusion mechanism controlled by trap-limited hopping through a long path to the transparent conductive electrode. It is however hindered by the low electron mobility in anatase TiO2 nanoparticles and the multiple grain boundaries in the mesoporous film. The work conducted during this thesis aimed at developing new architectures for the electrodes of DSCs using notably atomic layer deposition (ALD) and inverse opal host-guest structures with different semiconducting metal oxides. First, using a self-assembled polymeric template and ALD, we fabricated a porous ZnO inverse opal backbone. Advantages are numerous: a high electronic mobility of ZnO, a precise control of the metal oxide deposition using ALD and in situ growth of ZnO nanowires at low temperatures. Besides increasing the surface area available for photoactive dye molecules, single-crystal nanowires offer a defect-free path for photoinjected electrons. However, because of the relative instability of ZnO in acidic environments, a TiO2 overlayer was deposited by ALD. This proved to increase the recombination resistance of photogenerated electrons and increased their lifetime in the porous film. This host-guest approach was further transposed to three related fields in photo-electrochemistry using different ALD precursors. For water splitting, a niobium-doped tin oxide inverse opal backbone was constructed, which served as a host for hematite deposited by atmospheric pressure chemical vapour deposition or ALD. In the field of plasmonics, a TiO2 inverse opal host was made to act as a scaffold for silver nanoparticles. For solid-state DSCs, we strived to achieve an ultra-thin ZnO inverse opal, which acted as a host for perovskite crystals. Finally, we studied the effect of very thin TiO2 and ZnO overlayers deposited by ALD on top of mesoporous ZnO and TiO2, respectively. We were able to accurately monitor the thickness of overlayers and study the effects on photovoltaic parameters such as transport and recombination rates of photogenerated electrons, photocurrent, photovoltage, fill factor and efficiency.

EPFL2014

Novel Blue Organic Dye for Dye-Sensitized Solar Cells Achieving High Efficiency in Cobalt-Based Electrolytes and by Co-Sensitization

Anna Eriksson, Ulf Anders Hagfeldt, Kazuteru Nonomura, Enrico Polanski, Yasemin Saygili, Shaik Mohammed Zakeeruddin, Jiahuan Zhang

Blue and green dyes as well as NIR-absorbing dyes have attracted great interest because of their excellent ability of absorbing the incident photons in the red and near-infrared range region. A novel blue D-pi-A dye (Dyenamo Blue), based on the diketopyrrolopyrrole (DPP)-core, has been designed and synthesized. Assembled with the cobalt bipyridine-based electrolytes, the device with Dyenamo Blue achieved a satisfying efficiency of 7.3% under one sun (AM1.5 G). The co-sensitization strategy was further applied on this blue organic dye together with a red D-pi-A dye (D35). The successful co-sensitization outperformed a panchromatic light absorption and improved the photocurrent density; this in addition to the open-circuit potential result in an efficiency of 8.7%. The extended absorption of the sensitization and the slower recombination reaction between the blue dye and TiO2 surface inhibited by the additional red sensitizer could be the two main reasons for the higher performance. In conclusion, from the results, the highly efficient cobalt-based DSSCs could be achieved with the co-sensitization between red and blue D-pi-A organic dyes with a proper design, which showed us the possibility of applying this strategy for future high-performance solar cells.

2016

A Material Approach to Dye-Sensitized Solar Cells

Philippe Pierre Labouchère

EPFL2014

Dye-​sensitized solar cells using cobalt electrolytes: the influence of porosity and pore size to achieve high-​efficiency

A new terpyridine cobalt complex redox shuttle for dye-sensitized solar cells

A Material Approach to Dye-Sensitized Solar Cells

Novel Blue Organic Dye for Dye-Sensitized Solar Cells Achieving High Efficiency in Cobalt-Based Electrolytes and by Co-Sensitization

A Material Approach to Dye-Sensitized Solar Cells

Novel Blue Organic Dye for Dye-Sensitized Solar Cells Achieving High Efficiency in Cobalt-Based Electrolytes and by Co-Sensitization

A new terpyridine cobalt complex redox shuttle for dye-sensitized solar cells

Dye-sensitized solar cells using cobalt electrolytes: the influence of porosity and pore size to achieve high-efficiency