Nanocomposites of Nickel Oxide and Zirconia for the Preparation of Photocathodes with Improved Performance in p-Type Dye-Sensitized Solar Cells

Abstract

14 In p-type dye sensitized solar cells (p-DSCs) with nickel oxide (NiO) based photocathodes one of 15 the main causes of their relatively poor photoconversion performances is the fast recombination 16 between the photoinjected holes in the valence band of the p-type semiconductor and the reduced 17 form of the redox shuttle (typically I). As a matter of fact, recombination phenomena at the 18 NiO/electrolyte interface heavily limit both photovoltage and photocurrent. Different approaches 19 have been adopted to minimize such an unwanted process: these range from the pretreatment of the 20 electrode surface with NaOH to the employment of passivating organic molecules (e.g. CDCA) in 21 the sensitizing solution and/or in the electrolyte solution. The present contribution describes the 22 implementation of the addition of zirconia (ZrO2) nanoparticles in nanostructured NiO films as anti23 recombination agent in p-DSCs due to the electro-inactivity of ZrO2. ZrO2 nanoparticles with 24 diameter, Ø, of 20 nm, and NiO nanoparticles with Ø < 50 nm were dispersed together in the paste 25 precursor for screen-printing. Different compositions of the mixture of NiO and ZrO2 nanoparticles 26 were considered. From the combined analysis of the electrochemical and photoelectrochemical 27 properties of different nanocomposites it was concluded that the molar ratio ZrO2/NiO had the 28 optimal range of 2-5 % for realizing photocathodes more efficacious than sole nanostructured NiO. 29 Among the nanocomposite photoelectrodes the one obtained from the inclusion of 2% of ZrO2 30 nanoparticles produced the better photoelectrochemical performance being the short-circuit current 31 density JSC = 2.037 mA/cm 2 and the overall efficiency \uf068 = 0.088% when P1 is the sensitizer. These 32 results show an increase up to 40% compared to the un-modified NiO electrode. The unexpectedly 33 low efficiency of electrode with molar ratio of zirconia in nickel oxide of 5% was associated to an 34 insufficient dye-loading on NiO, in combination to the increase of the percentage of the 35 photoelectrochemically inert ZrO2 additive. The electrochemical impedance spectroscopy (EIS) 36 data of the complete device under illumination confirmed that the improvement is mainly due to an 37 increase of the recombination resistance, Rrec, ongoing from sole nanostructured NiO (Rrec = 56.3 \uf057) 38 to the electrode obtained from the nanocomposite with molar ratio ZrO2/NiO = 0.02 (Rrec = 70.3 \uf057). 39 40 41 42 * corresponding author, email: [email protected] [email protected] 43 44 45 46 INTRODUCTION 47 48 Among photovoltaic technologies the monocrystalline silicon-based devices and lead iodide based 49 perovskite solar cells are capable to reach conversion efficiencies up to 20% under solar 50 irradiation. When indoor illumination with diffuse features is considered as source of luminous 51 energy the dye-sensitized solar cell (DSC) appears as the most effective choice despite the fact 52 that the highest efficiency of a DSC is below 15%. More recently, Gräetzel and co-workers 53 reported overall efficiency up to 30% when the source intensity is as low as 100 lux. Conversions 54 up to 40% could be theoretically achieved by creating a p-n junction (i.e. by coupling a photoanode 55 to a photocathode) when the solar radiation is considered as excitation source. Such a tandem 56 configuration would sensibly reduce also the costs of production of the corresponding device. The 57 theoretical limit of 40% is still far to be reached because of the generally poor performance of 58 photocathodes. To our knowledge, the best performance reported so far for a p-type DSC (p-DSC) 59 is lower than 2% under 1 Sun of illumination. One of the main causes of this is the fast 60 recombination reaction that occurs between the photoinjected holes in the valence band (VB) of the 61 photocathode (usually made of NiO) and the reduced form of the redox shuttle (typically the 62 iodide anion). In fact, recombination phenomena at the NiO/electrolyte interface heavily limit 63 both photovoltage and photocurrent. The photoinjected holes are mainly localized onto NiO surface 64 in correspondence of the electron-deficient Ni sites. Different approaches have been adopted to 65 minimize such an unwanted reaction. The rational design of a sensitizer with bulky 66 substituents could help to keep iodide distant from the holes localized on the electrode surface. 67 The implementation of a NiO compact layer has been proved to reduce recombination phenomena 68 at the electrolyte/FTO interface. Metal-doping or UV irradiation of NiO electrode are feasible 69 approaches to tune the opto-electronic properties of photocathode but are as not effective in 70 reducing the interfacial recombination. An alternative route is the direct modification of the 71 photocathode. In a previous paper we showed that the treatment of NiO surface with soda has a 72 twofold effect: it reduces the surface concentration of superficial Ni sites and passivates the NiO 73 surface prior sensitization. The success of this method has been confirmed by the achievement of 74 a less dark film. Unfortunately, the reduction of the number of Ni sites lowered also the amount of 75 loaded sensitizer leading to a less performing device (lower photocurrent). We also tested CDCA 76 (chenodeoxycholic acid) in squaraines-based p-DSC. In that work CDCA (acting as both 77 disaggregating and passivating agent) was added in the sensitizer solution with a concentration of 78 20 mmol. The overall efficiency was enhanced by 25% due to the depression of dye aggregation. 79 Nevertheless, the amount of chemisorbed dye was lowered because of the competition between 80 sensitizer and CDCA in binding Ni sites. To avoid the latter phenomena, Odobel et al. dissolved 81 CDCA (50 mM) in the electrolyte. They reported an enhancement of the 20 % of the conversion 82 efficiency due mainly to a higher VOC whereas the JSC was substantially unchanged. The 83 employment of an insulating layer of Al2O3 was proposed by Uehara and coworkers 25 but it 84 diminished the electron injection of surface chemisorbed sensitizer more than the desired 85 recombination phenomena Natu and co-authors reported the implementation of a more efficient 86 Al2O3 insulating layer directly deposited onto the NiO electrode by Atomic Layer Deposition . 87 Yet, the enhancement of photoelectrochemical properties is modest. As far as we are aware, no 88 research group previously attempted the nanometric approach in the framework of DSCs with the 89 preparation of the nanocomposites here reported. In particular, throughout this work we described, 90 for the first time, the employment of ZrO2 nanoparticles, NPs, with diameter Ø < 20 nm) as not 91 electroactive additive in NiO electrodes for p-DSC application. ZrO2 is an insulating oxide with a 92 bandgap higher than 5 eV. We expect that the presence of zirconia nanoparticles, i.e. a 93 nanostructured version of ZrO2 with strong tendency of being finely dispersed on the electrode 94 surface, diminishes the portion of NiO exposed to the electrolyte thus diminishing the probability 95 with which Ni sites on the surface recombine with the redox shuttle. The effect of NiO dilution 96 imparted by of zirconia nanoparticles on the electrode surface brings necessarily about the 97 consequent minimization of recombination phenomena at the electrode/electrolyte interface as well 98 as flux of photoinjected charges in the photocathode. The purpose of this study is to evaluate to 99 which extent the presence of ZrO2 NPs favors the suppression of recombination without being 100 excessively detrimental against dye-loading and photoinjection on the NiO portions of the 101 nanocomposite. ZrO2 has been chosen because of its chemical inertness and long-term stability. The 102 formation of a mixed oxide of nickel and zirconium with a structure distinct from the ones of NiO 103 and ZrO2 has not been evidenced (vide infra) . Therefore, the attainment of a solid solution from 104 the mixing and the sintering of NiO and ZrO2 NPs is reasonably excluded. On these bases we 105 expect that the nanocomposites are actually constituted by two segregated oxides. 106 107 EXPERIMENTAL PART 108 109 The chemicals ethylcellulose, \uf061-terpineol, NiO nanopowders, ethanol and acetonitrile (ACN) were 110 purchased from Fluka or Sigma-Aldrich whereas ZrO2 nanoparticles were purchased from US 111 Research Nanomaterials. All chemicals were used without any further treatment of purification. 112 The experimental procedure to produce NiO/ZrO2 slurry consists on a modified version of the one 113 reported in our previous paper: an ethanol solution of NiO nanopowders (6 g), ZrO2 114 nanospheres (variable amount), \uf061-terpineol as solvent (20 g) and ethylcellulose as crosslinker were 115 mixed together under continuous stirring. Then this solution was slowly heated at 50 °C to let 116 completely evaporate the solvent. The resulting slurries were screen-printed over 2.2 mm thick 117 FTO/glass substrates (TEC7 from NSG), which were previously cleaned in an ultrasonic bath with 118 acetone for 10 min and successively with isopropyl alcohol for 10 min. The electrodes with 119 geometrical area of 0.36 cm, were annealed at 450 °C in oven for 30 minutes. The thickness of the 120 annealed samples ranged between 2 and 3 μm (evaluated with a Dektak 150 profilometer from 121 Veeco). The morphology has been investigated with a FESEM Auriga Zeiss Field Emission. EDX 122 (EDX Quantax Bruker, Resolution 123 eV (Mn K\uf061) was employed for the elemental analyses. The 123 amount of added ZrO2 varied from 0 (pure NiO) to 856 mg (corresponding to the molar ratio 124 ZrO2/NiO = 0.11). Six slurries have been prepared with different values of ZrO2/NiO molar ratio: 125 • Pure NiO as reference 126 • ZrO2/ NiO = 0.001, with 8 mg of ZrO2 127 • ZrO2/ NiO = 0.010, with 85.6 mg of ZrO2 128 • ZrO2/ NiO = 0.020, with 171.2 mg of ZrO