Atomic Layer Deposition of Highly Transparent Platinum Counter Electrodes for Metal/Polymer Flexible Dye‐Sensitized Solar Cells

Atomic layer deposition (ALD) is used to deposit Pt nanoparticles at low temperature (25–150 °C) to fabricate highly transparent counter electrodes (CEs) for flexible dye‐sensitized solar cells (DSCs). The Pt nanoparticles (NPs) are deposited for different number of ALD cycles on indium tin oxide (ITO)/polyethylene naphthalate (PEN) substrates. Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM) are used to assess the Pt NP loading, density, and size. There is a trade‐off between transparency and catalytic activity of the CE, and the best cell performances of back‐side‐illuminated DSCs (≈3.7% efficiency) are achieved for Pt ALD at temperatures in the range of 100–150 °C, even though deposition at 25 °C is also viable. The best cell produced with ALD platinized CE (100 cycles at 100 °C) outperforms the reference cells fabricated with electrodeposited and sputtered Pt CEs, with relative improvements in efficiency of 19% and 29%, respectively. In addition, these parameters are used to fabricate a large area CE for a sub‐module (active area of 17.6 cm²), resulting in an efficiency of 3.1%, which demonstrates the scalability of the process.     

Titanium Dioxide Mesoporous Electrodes for Solid‐State Dye‐Sensitized Solar Cells: Cross‐Analysis of the Critical Parameters

We report a comparative study on the use of four different mesoporous titanium dioxide (TiO₂) photo‐electrodes for the fabrication of solid‐state dye‐sensitized solar cells (sDSSCs). The photovoltaic parameters of the device correlate with several intrinsic properties of the film, based not only on its morphological features, as commonly considered in standard characterizations, but also on the transport and the electronic properties of the photo‐electrode. These properties differ significantly for TiO₂ electrodes processed using different colloidal pastes, and are decisive for the photovoltaic efficiency, ranging from 3.7% up to 5.1%. In particular, the dielectric permittivity of each mesoporous layer (ε_eff) and the number of traps (N_t) determined by the space‐charge‐limited current (SCLC) theory are found to be a bottle‐neck for the charge transport, greatly influencing the fill factor (FF) and open circuit voltage (V_oc) of the cells. In addition, a direct correlation between TiO₂ surface potential with the V_oc was established. Cross‐analysis of key macroscopic parameters of the films prior to integration in the devices, in particular focusing on the determination of the capacitance and surface potential shift of the TiO₂ mesoporous anode, represents a straightforward yet powerful method to screen and select the most suitable TiO₂ for applications in sDSSCs.     

Parasitic Absorption and Internal Quantum Efficiency Measurements of Solid‐State Dye Sensitized Solar Cells

The internal quantum efficiency (IQE) of solid‐state dye sensitized solar cells (ssDSCs) is measured using a hybrid optical modeling plus absorptance measurement approach which takes into account the parasitic absorption of the hole transport material (HTM). Across device thicknesses of 1 to 4 microns, ssDSCs sensitized with Z907 and TT1 dyes display relatively constant IQEs of approximately 88% and 36%, respectively, suggesting excellent charge collection efficiencies for both dyes but poor carrier injection for TT1 devices. The addition of more coadsorbent is shown to increase the IQE of TT1 up to approximately 58%, but significantly lowers dye loading. Finally, optical losses due to absorption by the HTM are quantified and found to be a significant contribution to photocurrent losses for ssDSCs sensitized with poor absorbers such as Z907, as the weak absorption of the dye gives the HTM opportunity for significant parasitic absorption within the active layer.     

Towards Long‐Term Photostability of Solid‐State Dye Sensitized Solar Cells

The solid‐state dye‐sensitized solar cell (DSSC) was introduced to overcome inherent manufacturing and instability issues of the electrolyte‐based DSSC and progress has been made to deliver high photovoltaic efficiencies at low cost. However, despite 15 years research and development, there still remains no clear demonstration of long‐term stability. Here, solid‐state DSSCs are subjected to the severe aging conditions of continuous illumination at an elevated temperature. A fast deterioration in performance is observed for devices encapsulated in the absence of oxygen. The photovoltaic performance recovers when re‐exposed to air. This reversible behavior is attributed to three related processes: i) the creation of light and oxygen sensitive electronic shunting paths between TiO₂ and the top metal electrode, ii) increased recombination at the TiO₂/organic interface, and iii) the creation of deep electron traps that reduce the photocurrent. The device deterioration is remedied by the formation of an insulating alumino‐silicate shell around the TiO₂ nanocrystals, which reduces interfacial recombination, and the introduction of an insulating mesoporous SiO₂ buffer layer between the top electrode and TiO₂, which acts as a permanent insulating barrier between the TiO₂ and the metal electrode, preventing shunting.     

Two Channels of Charge Generation in Perylene Monoimide Solid‐State Dye‐Sensitized Solar Cells

The mechanism of charge generation in solid‐state dye‐sensitized solar cells using triarylamine‐substituted perylene monoimide dyes is studied by vis‐NIR broadband pump‐probe transient absorption spectroscopy. The experiments demonstrate that photoinduced electron injection into the TiO₂ can only occur in regions where Li⁺, from the commonly used Li‐TFSI additive salt, is present on the TiO₂ surface. Incomplete surface coverage by Li⁺ means that some dye excitons cannot inject their electron into the TiO₂. However it is observed in the solar cell structure that some of the dye excitons that cannot directly inject an electron still contribute to free charge generation by the previously hypothesized reductive quenching mechanism (hole transfer to the solid‐state hole transporter followed by electron injection from the dye anion into the TiO₂). The contribution of reductive quenching to the quantum efficiency of charge generation is significant, raising it from 68% to over 80%. Optimization of this reductive quenching pathway could be exploited to maintain high quantum efficiency in dyes with greater NIR absorption to achieve overall enhancements in device performance. It is demonstrated that broadband NIR transient spectroscopy is necessary to obtain population kinetics in these systems, as strong Stark effects distort the population kinetics in the visible region.     

Solid‐State Dye‐Sensitized Solar Cells Using a Novel Class of Ullazine Dyes as Sensitizers

Here we present the photovoltaic performance of solid‐state dye‐sensitized solar cells (DSCs) using a series of ullazine‐based metal‐free organic sensitizers and spiro‐MeOTAD as a hole‐transport material. A maximum of 4.95% power conversion efficiency measured under standard AM 1.5G illumination (100 mW cm^?2) was achieved with the best performing ullazine dye, and was further improved to 5.40% through co‐sensitization with the triphenylamine‐based organic sensitizer, D35. This study investigates the effect of the molecular structure of the ullazine sensitizer on the performance in solid‐state DSCs.     

Quantum‐Dot Sensitized Solar Cells Employing Hierarchical Cu2S Microspheres Wrapped by Reduced Graphene Oxide Nanosheets as Effective Counter Electrodes

Hierarchical Cu₂S microspheres wrapped by reduced graphene oxide (RGO) nanosheets are prepared via a one‐step solvothermal process. The amount of graphene oxide used in the synthesis process has a remarkable effect on the features of Cu₂S microspheres. Compared to Pt and Cu₂S electrodes, RGO‐Cu₂S electrodes show better electrocatalytic activity, greater stability, lower charge‐transfer resistance, and higher exchange current density. As expected, RGO‐Cu₂S electrodes exhibit superior performance when functioning as counter electrodes in CdS/CdSe quantum dot‐sensitized solar cells (QDSSCs) using a polysulfide electrolyte. A power conversion efficiency up to 3.85% is achieved for the QDSSC employing an optimized RGO‐Cu₂S counter electrode, which is higher than those of the QDSSCs featuring Pt (2.14%) and Cu₂S (3.39%) counter electrodes.     

Beneficial Effects of Liquid Crystalline Phases in Solid‐State Dye‐Sensitized Solar Cells

Two novel double‐alkyl functionalized imidazolium ionic liquid crystals have successfully been utilized to demonstrate the benefits of the liquid crystalline phase on the ssDSSC performance. In particular, a good balance between dye regeneration and hole transport is only realized in the liquid crystalline phase. Devices that employ a single component ionic liquid based electrolyte show a remarkably stable efficiency during 1000 h under outdoor operation temperature conditions and 1 sun illumination.     

Integrated Design of Organic Hole Transport Materials for Efficient Solid‐State Dye‐Sensitized Solar Cells

A series of triphenylamine‐based small molecule organic hole transport materials (HTMs) with low crystallinity and high hole mobility are systematically investigated in solid‐state dye‐sensitized solar cells (ssDSCs). By using the organic dye LEG4 as a photosensitizer, devices with X3 and X35 as the HTMs exhibit desirable power conversion efficiencies (PCEs) of 5.8% and 5.5%, respectively. These values are slightly higher than the PCE of 5.4% obtained by using the state‐of‐the‐art HTM Spiro‐OMeTAD. Meanwhile, transient photovoltage decay measurement is used to gain insight into the complex influences of the HTMs on the performance of devices. The results demonstrate that smaller HTMs induce faster electron recombination in the devices and suggest that the size of a HTM plays a crucial role in device performance, which is reported for the first time.     

Design Principles of FRET‐Based Dye‐Sensitized Solar Cells with Buried Quantum Dot Donors

In this article, the physics of FRET is demonstrated for an architecture of dye‐sensitized solar cells, in which the quantum dot “antennas” that serve as donors are incorporated into the solid titania electrode, providing isolation from electrolyte quenching, and potentially increased photostability. The energy transferred to the dye acceptor from the quantum dot donor, in addition to the direct light absorption by the dye, finally induce dye excitation and electron injection to the metal oxide semiconductor electrode. We use time‐resolved photoluminescence measurements to directly show achievement of FRET efficiencies of up to 70%, corresponding to over 80% internal quantum efficiency when considering radiative energy transfer as well. The various parameters governing the FRET efficiency and the requirements for high efficiency FRET‐based cells are discussed. Since both buried donors inside the electrode and donors solubilized in the electrolyte have both been shown to achieve high energy transfer efficiencies, and as the two methods take advantage of different available volumes of the electrode to introduce donors providing the excess absorption, synergy of the two methods is highly promising for achieving panchromatic absorption within a thin electrode.     

Improved Open‐ Circuit Voltage in ZnO–PbSe Quantum Dot Solar Cells by Understanding and Reducing Losses Arising from the ZnO Conduction Band Tail

Colloidal quantum dot solar cells (CQDSCs) are attracting growing attention owing to significant improvements in efficiency. However, even the best depleted‐heterojunction CQDSCs currently display open‐circuit voltages (V_OCs) at least 0.5 V below the voltage corresponding to the bandgap. We find that the tail of states in the conduction band of the metal oxide layer can limit the achievable device efficiency. By continuously tuning the zinc oxide conduction band position via magnesium doping, we probe this critical loss pathway in ZnO–PbSe CQDSCs and optimize the energetic position of the tail of states, thereby increasing both the V_OC (from 408 mV to 608 mV) and the device efficiency.     

A Hierarchically Organized Photoelectrode Architecture for Highly Efficient CdS/CdSe‐Sensitized Solar Cells

A form of photoelectrode architecture suitable for inorganic semiconductor solar cells is reported. The developed architecture consists of hierarchically organized TiO₂ nanostructures with several tens of nanometer‐sized particles that have a large surface area and open channels with several hundred‐nanometer‐gaps perpendicular to the substrate. These are tailored by controlling the kinetic energy of the ablated species during pulsed laser deposition (PLD). To fabricate the solar cells, CdS and CdSe inorganic sensitizers are assembled onto the architecture by successive ionic layer adsorption and reaction and polysulfide solution is used as an electrolyte with lead sulfide counter‐electrodes. The inorganic semiconductor solar cells using the developed architecture (PLD‐TiO₂) show high energy conversion efficiencies of 5.57% compared to a conventional mesoporous TiO₂ film(NP‐TiO₂) (3.84%) with an optical mask at 1 sun of illumination. The improved cell performance of PLD‐TiO₂ is attributed to greater light‐harvesting ability, which results in the enhancement of the J_sc value. PLD‐TiO₂ absorbs more CdS/CdSe because of its larger surface area and excellent adhesion properties with fluorine‐doped tin oxide (FTO) substrates. Additionally, due to its unique channel‐shaped architecture, PLD‐TiO₂ has a longer electron lifetime compared to NP‐TiO₂.     

The Effect of Hole Transport Material Pore Filling on Photovoltaic Performance in Solid‐State Dye‐Sensitized Solar Cells

A detailed investigation of the effect of hole transport material (HTM) pore filling on the photovoltaic performance of solid‐state dye‐sensitized solar cells (ss‐DSCs) and the specific mechanisms involved is reported. It is demonstrated that the efficiency and photovoltaic characteristics of ss‐DSCs improve with the pore filling fraction (PFF) of the HTM, 2,2’,7,7’‐tetrakis‐(N, N ‐di‐ p ‐methoxyphenylamine)9,9’‐spirobifluorene(spiro‐OMeTAD). The mechanisms through which the improvement of photovoltaic characteristics takes place were studied with transient absorption spectroscopy and transient photovoltage/photocurrent measurements. It is shown that as the spiro‐OMeTAD PFF is increased from 26% to 65%, there is a higher hole injection efficiency from dye cations to spiro‐OMeTAD because more dye molecules are covered with spiro‐OMeTAD, an order‐of‐magnitude slower recombination rate because holes can diffuse further away from the dye/HTM interface, and a 50% higher ambipolar diffusion coefficient due to an improved percolation network. Device simulations predict that if 100% PFF could be achieved for thicker devices, the efficiency of ss‐DSCs using a conventional ruthenium‐dye would increase by 25% beyond its current value.     

Quantum Dot–Sensitized Solar Cells Featuring CuS/CoS Electrodes Provide 4.1% Efficiency

CuS, CoS, and CuS/CoS onto fluorine‐doped tin oxide glass substrates were deposited to function as counter electrodes for polysulfide redox reactions in CdS/CdSe quantum dot–sensitized solar cells (QDSSCs). Relative to a Pt electrode, the CuS, CoS, and CuS/CoS electrodes provide greater electrocatalytic activity, higher reflectivity, and lower charge‐transfer resistance. Measurements of fill factor and short‐current density reveal that the electrocatalytic activities, reflectivity, and internal resistance of counter electrodes play strong roles in determining the energy‐conversion efficiency (η) of the QDSSCs. Because the CuS/CoS electrode has a smaller internal resistance and higher reflectivity relative to those of the CuS and CoS electrodes, it exhibits a higher fill factor and short‐circuit current density. As a result, the QDSSC featuring a CuS/CoS electrode provides a higher value of η. Under illumination of one sun (100 mW cm^?2), the QDSSCs incorporating Pt, CuS, CoS, and CuS/CoS counter electrodes provide values of η of 3.0 ± 0.1, 3.3 ± 0.3, 3.8 ± 0.2, and 4.1 ± 0.2%, respectively.