A p‐Type NiO‐Based Dye‐Sensitized Solar Cell with an Open‐Circuit Voltage of 0.35 V

Abstract

In 2000, He et al. reported that the combination of a NiObased photoactive cathode with a TiO 2 -based photoanode produced a tandem device with an open-circuit voltage (V OC ) of 0.73 V, [1] which is the sum of the open-circuit potentials of the separate devices, where the TiO 2 -based electrode contributed 0.65 V and the NiO-based electrode 0.08 V. Tandem dye-sensitized solar cells (TDSCs), such as Hes, have suffered from the lack of research directed at the optimization of the photocathode, despite the potential they hold in developing higher voltage devices. Only recently have researchers begun to address issues associated with NiObased DSCs, such as low short-circuit photocurrents (J SC ) and poor fill factors (FF). [2][3][4][5][6] We present herein a three-fold increase in V OC (0.35 V), while maintaining a J SC of 1.7 mA cm À2 , for a p-type NiO dye-sensitized solar cell (p-DSC) employing a carefully designed "dyad" sensitizer and a cobalt-based redox mediator. We have almost quadrupled the p-DSC conversion efficiency from the previous record, 0.055 %, [4] to 0.20 %. Interestingly, the improvements within the p-DSCs translate to improvements in TDSC performance, namely achieving a V OC of 0.91 V, a FF of 0.62, and an efficiency of 0.55 %. The results reported herein present the most significant increase in photovoltaic effi-ciency of p-DSCs to date, emphasizing the importance of molecular engineering in sensitizer and redox-mediator development to achieve substantial gains in DSC performance.A schematic representation of a TDSC is shown in Figure 1. Absorption of light by the sensitizers, in separate photo-processes, causes an electron to be injected into the conduction band (CB) of the TiO 2 at the anode and a hole to be injected into the NiO at the cathode. These charges diffuse through the respective semiconductors to the SnO 2 :F charge collector. To complete the circuit, the oxidized and reduced forms of the redox mediator regenerate the dye at the cathode and anode, respectively.Often DSCs have a counter electrode of platinized conducting glass. In p-type devices, the V OC is the potential energy difference between the quasi-Fermi level close to the valence band (VB) and the Nernstian potential of the redox mediator. [7] Efforts to improve the V OC of TiO 2 -based n-type DSCs (n-DSCs) have involved modifications to the electrolyte [8,9] and doping of the semiconductor itself. [10,11] The various redox couples suffered from slow regeneration kinetics [12,13] or fast recombination with the semiconductor, [14] as a result the I À /I 3 À mediator, though problematic, has prevailed as the electrolyte of choice for n-DSCs. Previously, the p-side in a TDSC contributed little to the overall tandem performance, owing to the low V OC , in the presence of I À /I 3 À , and the low photocurrent. We report herein our rationaldesign effort directed at improving p-DSC performance.We present three p-DSCs composed of a cobalt mediator and three different sensitizing dyes: coumarin 343 (C343), a typical sensitizer for p...

Red-Absorbing Cationic Acceptor Dyes for Photocathodes in Tandem Solar Cells

“…Charge recombination between the photoreduced dye and the holes in the NiO is known to limit the APCE in p-type DSCs. 5,19,20,25 We have carried out visible and near-IR TA and TRIR experiments to determine the excited-state lifetimes of the dyes in solution and charge-separated state lifetimes of the dyes adsorbed on NiO. These experiments can help to determine Figure 7 shows the transient absorption spectra for the CAD 1 and CAD 2 excited states in dichloromethane solution 5 ps after excitation at 532 nm.…”

Visible Light-Driven Electron Transfer from a Dye-Sensitizedp-Type NiO Photocathode to a Molecular Catalyst in Solution: Toward NiO-Based Photoelectrochemical Devices for Solar Hydrogen Production

“…This is consistent with the already reported valence band bending of NiO in the presence of a high concentration of a small cationic species such as lithium cation which was added in the electrolyte of p-type DSC in order to enhance their open circuit voltage. 80,81 …”

Ultrafast and slow charge recombination dynamics of diketopyrrolopyrrole–NiO dye sensitized solar cells

“…A redox mediator such as triiodide or a cobalt 25,26 or iron 27 complex regenerates the dye and delivers electrons to the counter electrode in order to complete the photovoltaic circuit. 22,28,29 Recently, we found that simple diketopyrrolopyrrole based dyes (DPPBr and DPPCN2) and a "D-A" dyad (DPP-NDI are promising sensitizers for p-DSSCs and they can be used with both iodide and cobalt electrolytes (Chart 1). 13,14 Therefore, in order to achieve highly efficient systems, the charge recombination dynamic processes are the most critical factors to improve, in particular the competition between dye regeneration and recombination of the reduced dye with the hole in NiO.…”