Inkjet Printing NiO-Based p-Type Dye-Sensitized Solar Cells
Résumé
Fabrication at low cost of transparent p-type semiconductors with suitable electronic properties is essential toward the scalability of many electronic devices, especially for photovoltaic and photocatalytic applications. In this context, the synthesis of mesoporous NiO films through inkjet printing of a sol−gel ink was investigated for the first time. Nickel chloride and Pluronic F-127, used as nickel oxide precursor and pore-forming agent, respectively, were formulated in a water/ ethanol mixture to prepare a jettable ink for Dimatix printer. Multilayer NiO films were formed, and different morphologies could be obtained by playing on the interlayer thermal treatment. At low temperature (30 °C), a porous nano-particulate−nanofiber dual-pore structure was observed. On the other hand, with a high temperature treatment (450 °C), nanoparticulate denser films without any dual structure were obtained. The mechanism for NiO formation during the final sintering step, investigated by means of X-ray photolectron spectroscopy, shows that a Ni(OH) 2 species is an intermediate between NiCl 2 and NiO. The different morphologies and thicknesses of the NiO films were correlated to their performance in a p-DSSC configuration, using a new push−pull dye (so-called " RBG-174 ") and an iodine-based electrolyte. Moreover, the positive impact of a nanometric NiO x layer deposited by spin-coating and introduced between FTO and the NiO mesoporous network is highlighted in the present work. The best results were obtained with NiO x /four layer-NiO mesoporous photocathodes of 860 nm, with a current density at the short circuit of 3.42 mA cm −2 (irradiance of 100 mW cm −2 spectroscopically distributed following AM 1.5). ■ INTRODUCTION Solar energy is the ideal source of energy because of its cleanliness, high power, and ready availability. However, for a massive use, the development of low-cost and efficient conversion devices is still needed. Among the different emerging cost-effective solar-cell technologies, nanocrystalline dye-sensitized solar cells (DSSCs) are promised to an industrial success. Indeed, DSSCs that consist of a photoactive anode and a passive cathode sandwiching an electrolyte (n-DSSCs) have shown high photon to energy conversion efficiencies (PCE), up to 13%. 1 Moreover, the propensity of DSSCs to operate under diffuse light conditions is an important asset regarding market-dominating silica-based technology, which require direct light for optimized performance. The idea of tandem-DSSC (t-DSSC), in which both the anode and cathode are photoactive, has been raised over the last twenty years. 2−4 These devices show theoretical PCEs over 40%, 5,6 which is superior to the one photoelectrode configuration. However, with record PCE at 2.42%, 3 t-DSSCs have not beaten yet classical n-DSSCs. Indeed p-DSSCs (which consist of one photocathode and a passive anode) have presented moderate yields compared to n-DSSCs, with record PCE at 2.51%, with an iron-based electrolyte. 7 These low yields stem from (1) the low short-circuit (J sc) current densities that can be delivered, where J sc values of 7.65 mA cm −2 were reported with the record p-DSSC device, and (2) the poor fill factors (FF) obtained, as FF of 0.51 in the same device were hardly reached. It is commonly assumed that the p-type oxide which is mostly used to build the photocathode, nickel oxide (NiO), is mainly responsible for the poor activity of the devices. However, a replacing material to NiO is yet to be found as p-type oxides are scarce. 8−10 Thus, research on NiO itself remains an important axis of work in the field of DSSCs. 11−16 This wide band gap semiconducting oxide should indeed also be a choice material for low-cost and carbon-free solar fuel production devices, 17 and then research on NiO, especially on its synthesis, is also crucial for this domain. A wide range of methods exists for the preparation of NiO, 16 among which is polymer-templated sol−gel synthesis, which has provided one of the current best NiO-based DSSCs. 4 However, for this preparation method, Scotch tape doctor-blading is used to deposit the ink onto the TCO glass substrate, and several authors reported a reproducibility issue. An alternative to doctor-blading is inkjet printing (IJP), and it