Tema: Comprensión de la entonación

The advent of modern dye-sensitized solar cells began with the adaptation of high-surface area TiO2 nanoparticle films35. These mesoporous films allowed for sufficient light harvesting efficiencies to and thus significant increases in the photocurrent density possible in the devices. The first example of a NiO p-type DSSC was by He et. al.44, which was used in tandem with a TiO2 photoanode to produce a larger photopotential than the TiO2 or NiO DSSC alone.

The poor performance of NiO DSSCs relative to their TiO2 counterparts necessitated additional research but little progress was made. A comprehensive study of commercially- available chromophores was performed by Nattestad and coworkers39_{, which showed that}

Coumarin-343, C343, was the most optimal chromophore to date. This result however should not be taken as definitive. C343, see Figure 1.7, has been reported to have significant photocurrent generation from the iodide/triiodide electrolyte rather than light absorption from the

chromophore57, 63. These findings are shown to be corroborated by our own study in Section 2.14.

The most significant progress made with NiO-based DSSCs was made by Licheng Sun38, 58, 80 _{with the advent of the P1 chromophore, Figure 1.7. The electron donor-π bridge-electron} accepter, D-π-A, configuration allows for long-lived charge separated states by spatially separating the HOMO and LUMO. More simply, the photoexcited electron moves from the triphenylamine, electron donor, through the thiophene, π-bridge, to the malononitrile, electron acceptor, physically pushing the electron away from the surface of the nickel oxide.

While the P1 chromophore has been commonly reported with photocurrent densities ranging from 1-to-2 mA·cm-2_{, several reports from Gibson et. al.}38, 81-82 _{now show current} densities in excess of 5 mA·cm-2. The reported increase was due to a substantial increase in the internal quantum efficiency of the electrodes. This enhancement in the material quality however was not confirmed by Wu and coworkers.56 Internal verification of the enhancement is

forthcoming.

Peter Bäuerle and Udo Bach reported another D-π-A chromophore deemed PMI-6T-TPA, which refers to the structure perylene monoimide-six thiophenes-triphenylamine83. Upon first report, the chromophore was capable of producing ~5 mA·cm-2 of photocurrent density with an impressive 218 mV of photopotential. The chromophore represented a nearly 4-fold increase in overall photoconversion efficiency in p-type DSSCs83. The chromophore was then utilized on “microballs” of NiO84_{, which showed nearly equivalent photoconversion efficiency but showed} that the chromophore could generate photocurrent densities of 7 mA·cm-2.

The material quality utilized in NiO devices was then explored by testing “stoichiometric NiO” fabricated at 950 °C85_{. Due to poor electronic coupling of the film to the FTO substrate as} well as low surface area, poor photocurrent densities were reported but a Voc of 350 mV was obtained. With the inclusion of a dense adhesion layer of NiO, photocurrent densities of 1 mA·cm-2 were reported with a photovoltage of >320 mV. To our knowledge, this is the highest reported photovoltage for NiO reported with an iodide/triiodide electrolyte.

In order to maintain the higher photocurrent densities, the annealing conditions were then optimized showing an enhancement in nanoscale crystallinity by annealing first at 450 °C and then ramping to 550 °C for 15 minutes46_{. Zhang and coworkers reported minimal loss of surface} area but significant increase in the photopotential, from 200 to 294 mV. This allowed the record

photoconversion efficiency for p-DSSCs to again rise to 0.61 %. It is important to note however that the NiO material utilized in this study is commercially available, Advanced Materials Inframat Part Number 28N-0801.

With the nanoscale crystallinity tuned and a highly optimized chromophore, the

electrolyte was then tuned. Cobalt-based electrolytes had been previously shown to create open- circuit voltages of ~350 mV86-87, but with the optimized PMI-6T-TPA chromophore Voc’s of 709

mV allowed the record photoconversion efficiency to rise to 1.3 %88_{. This significant rise in} photopotential is due to the larger potential energy difference between the Nernstian potential of the electrolyte and the valence band edge of NiO. Tuning the electrolyte again with an iron-based allowed for another significant increase in the record photoconversion efficiency to 2.55 %73 though the authors note the requirement for a dense blocking layer of NiO to decrease the dark current with an iron-based electrolyte. The iron electrolyte allowed for an increase in the Jsc to 7.65 mA·cm-2 but did not further increase the Voc even though the potential difference between the electrolyte and the valence band had increased. Notably, the authors state that a significantly thicker film, ~4.2 µm, was utilized indicating that the iron-based electrolyte is likely providing additional enhancement in dye regeneration. Significantly, a fill factor of 60% was reported.

Other methods to improve the material quality have been met with on moderate success. Doping NiO with cobalt89 was found to lower the valence band edge and thus modestly increase the Voc to 160 mV. Lithium doping51 significantly reduced trap state densities, but offered only minor enhancement in DSSC performance. Doping with magnesium90 was found to also increase DSSC device performance by increasing Jsc from 3-to-5 mA·cm-2. However, work by Gibson and coworkers found a decrease in Jsc and only a slight increase in Voc for magnesium doping81_.

Various groups have also reported deposition of alumina onto NiO with moderate success. Uehara reported a solution-phase deposition, which retarded the rate detrimental electron injection but did not significantly improve device performance61. Natu and Wu also reported a vapor-phase deposition through atomic layer deposition. Interestingly, they optimized device performance to a single cycle of alumina corresponding to 0.2-0.3 nm of deposition, which provided slightly increased photocurrent densities as well as an increase in Voc to ~170 mv56_.