Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy

A potassium thiocyanate additive for hysteresis elimination in highly efficient perovskite solar cells

Potassium thiocyanate as a cheap additive effectively eliminates the hysteresis effect of perovskite solar cells.

High efficiency dye-sensitized solar cells with VOC–JSC trade off eradication by interfacial engineering of the photoanode|electrolyte interface

Interfacial modification of the photoanode|electrolyte interface using oleic acid (OA) is thoroughly investigated in this present study. The overall photoconversion efficiency of 11.8% was achieved under the illumination of 100 mW cm⁻² with an optical filter of AM 1.5 G. OA molecules were meant to be adsorbed on to the vacant areas of the TiO₂ and the OA moieties leached out the aggregated C106 dye molecules from the TiO₂ surface. There was a strong spectral overlap between the absorption spectrum of donor (OA) and the emission spectrum of acceptor (C106), leading to effective Förster Resonance Energy Transfer (FRET) between OA and C106 and suggested an excellent opportunity to improve the photovoltaic performances of DSSCs. UV-vis DRS and UPS analysis revealed that OA molecules created new surface (mid-gap energy) states (SS) in TiO₂ and these SS played a major role in the electron transport kinetics. Mott–Schottky analysis of DSSCs under dark conditions was carried out to find the shift in the flat band potential of TiO₂ upon OA modification. Surprisingly, no trade off between V_OC and J_SC was observed after interfacial modification with OA. The dynamics of charge recombination and electron transport at the photoanode|electrolyte interface were studied in detail using electrochemical impedance spectroscopy.

V_OC–J_SC trade off is eliminated. Newly created surface states by OA in TiO₂ facilitated the charge transfer kinetics.

Diverging surface reactions at TiO2- or ZnO-based photoanodes in dye-sensitized solar cells

Surface reactions of electrolyte additives and consequences for cell properties are studied and assigned to characteristics specific for both semiconductors.

Highly Reproducible Large-Area Perovskite Solar Cell Fabrication via Continuous Megasonic Spray Coating of CH3 NH3 PbI3

A simple, low-cost, large area, and continuous scalable coating method is proposed for the fabrication of hybrid organic-inorganic perovskite solar cells. A megasonic spray-coating method utilizing a 1.7 MHz megasonic nebulizer that could fabricate reproducible large-area planar efficient perovskite films is developed. The coating method fabricates uniform large-area perovskite film with large-sized grain since smaller and narrower sized mist droplets than those generated by existing ultrasonic spray methods could be generated by megasonic spraying. The volume flow rate of the CH₃ NH₃ PbI₃ precursor solution and the reaction temperature are controlled, to obtain a high quality perovskite active layer. The devices reach a maximum efficiency of 16.9%, with an average efficiency of 16.4% from 21 samples. The applicability of megasonic spray coating to the fabrication of large-area solar cells (1 cm² ), with a power conversion efficiency of 14.2%, is also demonstrated. This is a record high efficiency for large-area perovskite solar cells fabricated by continuous spray coating.

Interfacial Modification of Photoanode|Electrolyte Interface Using Oleic Acid Enhancing the Efficiency of Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are useful devices in converting renewable solar energy into electrical energy. In DSSCs, the triiodide reduction at the surface of TiO₂ is one of the detrimental processes that limit the realization of high efficiencies of the device. To alleviate the active sites available on the semiconductor surface for this detrimental process, the interfacial modification of the dye-adsorbed TiO₂|electrolyte interface has been attempted by coadsorption of oleic acid (OA) over the TiO₂ surface. Thus, the modified cell exhibited a higher efficiency (η) of 12.9% under one sun illumination when compared with that of the unmodified cell (η = 11.1%). To provide an insight into the OA anchoring and dynamics of electron transport at the photoanode|electrolyte interface, molecular spectroscopic and electrochemical impedance spectroscopic analyses were carried out. A red shift in the optical absorption spectrum was observed after the addition of OA to dye-adsorbed TiO₂. The binding of OA to TiO₂ surface was found to be through bridging bidentate type. Mott-Schottky analyses of the DSSCs under dark conditions were made to probe the shift in the Fermi level of TiO₂ upon OA modification. In addition, the Förster resonance energy transfer (FRET) has been found between OA and N719 dye. Thus, the red shift in the optical absorption, enhanced electron-transfer kinetics, and FRET contributes to the observed enhancement in the efficiency of the device containing OA-modified photoanode.

Modified SnO2 with Alkali Carbonates as Robust Electron-Transport Layers for Inverted Organic Solar Cells

We report for the first time that alkali carbonates (Li₂CO₃, K₂CO₃, and Rb₂CO₃) based on a low-temperature solution process can be used as interfacial modifiers for SnO₂ as robust electron-transport layers (ETL) for inverted organic solar cells (iOSCs). The room-temperature photoluminescence, the electron-only devices, and the impedance studies altogether suggested the interfacial properties of the alkali carbonates-modified SnO₂ ETLs, which were much better than those based on the SnO₂ only, provided efficient charge transport, and reduced the charge recombination rates for iOSCs. The iOSCs using the polymer donor poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] and the fullerene acceptor phenyl-C₇₀-butyric acid methyl ester as the active layer showed the average power-conversion efficiencies (PCEs) based on ten devices of 6.70, 6.85, and 7.35% with Li₂CO₃-, K₂CO₃-, and Rb₂CO₃-modified SnO₂ as ETLs, respectively; these are more than 22, 24, and 33% higher than those based on the SnO₂ only (5.49%). Moreover, these iOSC devices exhibited long-term stabilities, with over 90% PCEs remaining after the devices were stored in ambient air for 6 weeks without encapsulations. We believe that alkali carbonates-modified SnO₂ approaches are an effective way to achieve stable and highly efficient iOSCs and might also be suitable for other optoelectronic devices where an ETL is needed, such as perovskite solar cells or organic light-emitting diodes.

Electrolyte tuning in dye-sensitized solar cells with N-heterocyclic carbene (NHC) iron(II) sensitizers

We demonstrate that the performances of dye-sensitized solar cells (DSCs) sensitized with a previously reported N-heterocyclic carbene iron(II) dye in the presence of chenodeoxycholic acid co-adsorbant, can be considerably improved by altering the composition of the electrolyte while retaining an I^-/I₃ ^- redox shuttle. Critical factors are the solvent, presence of ionic liquid, and the use of the additives 1-methylbenzimidazole (MBI) and 4-tert-butylpyridine (TBP). For the electrolyte solvent, 3-methoxypropionitrile (MPN) is preferable to acetonitrile, leading to a higher short-circuit current density (J _SC) with little change in the open-circuit voltage (V _OC). For electrolytes containing MPN, an ionic liquid and MBI (0.5 M), DSC performance depended on the ionic liquid with 1-ethyl-3-methylimidazolium hexafluoridophosphate (EMIMPF) > 1,2-dimethyl-3-propylimidazolium iodide (DMPII) > 1-butyl-3-methylimidazolium iodide (BMII) ≈ 1-butyl-3-methylimidazolium hexafluoridophosphate (BMIMPF). Omitting the MBI leads to a significant improvement in J _SC when the ionic liquid is DMPII, BMII or BMIMPF, but with EMIMPF the removal of the MBI additive results in a dramatic decrease in V _OC (542 to 42 mV). For electrolytes containing MPN and DMPII, the effects of altering the MBI concentration have also been investigated. Although the addition of TBP improves V _OC, it causes significant decreases in J _SC. The best performing DSCs with the NHC-iron(II) dye employ an I^-/I₃ ^--based electrolyte with MPN as solvent, DMPII ionic liquid (0.6 M) with no or 0.01 M MBI; values of J _SC = 2.31 to 2.78 mA cm^-2, V _OC = 292 to 374 mV have been achieved giving η in the range of 0.47 to 0.57% which represents 7.8 to 9.3% relative to an N719 reference DSC set at 100%. Electrochemical impedance spectroscopy has been used to understand the role of the MBI additive in the electrolytes.

A Phosphonic Acid Anchoring Analogue of the Sensitizer P1 for p-Type Dye-Sensitized Solar Cells

We report the synthesis and characterization of the first example of an organic dye, PP1, for p-type dye-sensitized solar cells (DSCs) bearing a phosphonic acid anchoring group. PP1 is structurally related to the benchmarking dye, P1, which possesses a carboxylic acid anchor. The solution absorption spectra of PP1 and P1 are similar (PP1 has λmax = 478 nm and εmax = 62,800 dm3 mol−1 cm−1), as are the solid-state absorption spectra of the dyes adsorbed on FTO/NiO electrodes. p-Type DSCs with NiO as semiconductor and sensitized with P1 or PP1 perform comparably. For PP1, short-circuit current densities (JSC) and open-circuit voltages (VOC) for five DSCs lie between 1.11 and 1.45 mA cm−2, and 119 and 143 mV, respectively, compared to ranges of 1.55–1.80 mA cm−2 and 117–130 mV for P1. Photoconversion efficiencies with PP1 are in the range 0.054–0.069%, compared to 0.065–0.079% for P1. Electrochemical impedance spectroscopy, open-circuit photovoltage decay and intensity-modulated photocurrent spectroscopy have been used to compare DSCs with P1 and PP1 in detail.

In Situ Cesium Modification at Interface Enhances the Stability of Perovskite Solar Cells

A consensus has been reached that organic transport layer (e.g., Spiro-OMeTAD) in perovskite solar cell (PSC) is prone to be impact by mobile ions in perovskite film during long-term operation. Here, we incorporate cesium acetate as a buffer layer into perovskite solar cells to mitigate this detrimental behavior, in which cesium acetate is sandwiched between perovskite and organic transport layer. The mobile ions that migrate toward the organic transport layer (e.g., MA⁺) are gradually consumed by cesium acetate, resulting in cesium-rich perovskite at the interface. This in situ reaction and the subsequent Cs incorporation greatly enhance the operational stability of PSC without efficiency loss. The optimized PSC presents a power conversion efficiency of 20.9% with an open-circuit voltage of 1.18 V, maintaining ∼80% of its initial efficiency after 4500 min continuous operation at maximum power point. This new strategy opens up a new opportunity for fabricating stable perovskite solar cells.

Surface Modification of Methylamine Lead Halide Perovskite with Aliphatic Amine Hydroiodide

By spin-coating method, a thin layer of dodecylamine hydroiodide (DAHI) is introduced to the surface of perovskite CH₃NH₃PbI _xCl_{3- x}. This layer of DAHI successfully changes the surface of perovskite from hydrophilic to hydrophobic as revealed by the water contact angle measurement. Significantly enhanced fluorescence intensity and prolonged fluorescence lifetime are found for these modified films in comparison to those of unmodified perovskite films, suggesting that the number of structure defects is reduced dramatically. The compatibility between the perovskite and hole transfer layer (HTL) is also improved, which leads to more efficient hole collection from the perovskite layer by HTL as revealed by the fluorescence spectra, fluorescence decay dynamics, as well as the transient photocurrent measurements. Moreover, the perovskite solar cells (PSCs) fabricated from these modified perovskite films exhibit significantly improved humidity stability as well as promoted photoelectron conversion efficiency (PCE). The result of this research reveals for the first time that the layer of aliphatic amino hydroiodide is a multiple functions layer, which can not only improve the humidity stability but also promote the performance of PSCs by reducing the defect number and improve the compatibility between perovskite and HTL. Because the structure of aliphatic amines can be functionalized with myriad of other groups, this perovskite modification method should be very promising in promoting the performance of PSCs.

Improving the parameters of electron transport in quantum dot sensitized solar cells through seed layer deposition

Here we investigate the effect of seed layer deposition on electron-transport parameters of chemical-bath-deposited (CBD) CdSe quantum dot sensitized solar cells (QDSCs). Fill factors were systematically improved to more than 0.6 through reduced recombination after seed layer deposition. Considering the beneficial effects of seed layer deposition, noticeably higher efficiency values were systematically obtained in cells with the seed layer (2–3.19%) in comparison to cells without a seed layer (0.03–0.46%) depending on the TiO₂ photoanode particle size. Electron-transport parameters in cells, including chemical capacitance, recombination resistance, the diffusion coefficient, electron life time and small perturbation diffusion lengths of electrons were examined by modeling the experimental impedance spectroscopy data. We showed that a seed layer enhanced recombination resistance in cells, while the photoanode conduction band position was not affected. Higher diffusion lengths of electrons were obtained after seed layer deposition, correlated to the reduced electron recombination rate by redox electrolyte through seed layer deposition. As a general conclusion we report that while the seed layer generally is deposited to increase light absorption, at the same time this could be applied in order to systematically enhance charge-transport properties in cells and it has a clear application in the optimization of QDSC performance.

It is proved that the seed layer deposition could be systematically applied in order to enhance the charge transport in the cells.

Photo-induced surface modification to improve the performance of lead sulfide quantum dot solar cell

The solution-processed quantum dot (QD) solar cell technology has seen significant advancements in recent past to emerge as a potential contender for the next generation photovoltaic technology. In the development of high performance QD solar cell, the surface ligand chemistry has played the important role in controlling the doping type and doping density of QD solids. For instance, lead sulfide (PbS) QDs which is at the forefront of QD solar cell technology, can be made n-type or p-type respectively by using iodine or thiol as the surfactant. The advancements in surface ligand chemistry enable the formation of p-n homojunction of PbS QDs layers to attain high solar cell performances. It is shown here, however, that poor Fermi level alignment of thiol passivated p-type PbS QD hole transport layer with the n-type PbS QD light absorbing layer has rendered the photovoltaic devices from realizing their full potential. Here we develop a control surface oxidation technique using facile ultraviolet ozone treatment to increase the p-doping density in a controlled fashion for the thiol passivated PbS QD layer. This subtle surface modification tunes the Fermi energy level of the hole transport layer to deeper values to facilitate the carrier extraction and voltage generation in photovoltaic devices. In photovoltaic devices, the ultraviolet ozone treatment resulted in the average gain of 18% in the power conversion efficiency with the highest recorded efficiency of 8.98%.

Focus-Induced Photoresponse: a novel way to measure distances with photodetectors

We present the Focus-Induced Photoresponse (FIP) technique, a novel approach to optical distance measurement. It takes advantage of a universally-observed phenomenon in photodetector devices, an irradiance-dependent responsivity. This means that the output from a sensor is not only dependent on the total flux of incident photons, but also on the size of the area in which they fall. If probe light from an object is cast on the detector through a lens, the sensor response depends on how far in or out of focus the object is. We call this the FIP effect. Here we demonstrate how to use the FIP effect to measure the distance to that object. We show that the FIP technique works with different sensor types and materials, as well as visible and near infrared light. The FIP technique operates on a working principle, which is fundamentally different from all established distance measurement methods and hence offers a way to overcome some of their limitations. FIP enables fast optical distance measurements with a simple single-pixel detector layout and minimal computational power. It allows for measurements that are robust to ambient light even outside the wavelength range accessible with silicon.

Ultrasonic-Assisted Spin-Coating: Improved Junction by Enhanced Permeation of a Coating Material within Nanostructures

Over the last decades, the spin-coating (SC) technique has been widely used to prepare thin films of various materials in the liquid phase on arbitrary substrates. The technique simply relies on the centrifugal force to spread a coating solution radially outward over the substrate. This mechanism works fairly well for solutions with low surface tension to form thin films of reasonable junctions on smooth substrates. Here, we present a modified SC technique, namely, ultrasonic-assisted spin-coating (UASC), to form thin films of coating solution having high surface tension on rough substrates with excellent junctions. The UASC technique couples SC with an external ultrasonic wave generator to provide external perturbation to locally break down big drops of the coating material into smaller droplets via Rayleigh instability. Because of their lower mass, these tiny droplets gain low momenta and move slowly both in radial and azimuthal directions, giving them an enough time to effectively permeate within pores, thereby yielding excellent junctions. Furthermore, we also investigated the effect of junction improvement on conventional and inverted bulk heterojunction organic solar cells. Intriguingly, the organic solar cells fabricated by the UASC method showed an improved efficiency compared to typical SC owing to efficient charge transfer across the junction. These results clearly imply that UASC is a simple and powerful technique which can significantly enhance the device performance by improving the junction. Moreover, we believe that UASC can be more effective for the preparation of devices composed of multilayers of different materials having complicated nanostructures.

Comparison of charge transfer dynamics in polypyridyl ruthenium sensitizers for solar cells and water splitting systems

Standard ruthenium components of dye-sensitized solar cells (sensitizer N719) and dye-sensitized photoelectrochemical cells (sensitizer RuP and water oxidation catalyst RuOEC) are investigated in the same solar cell configuration to compare their photodynamics and charge separation efficiency. The samples are studied on time scales from femtoseconds to seconds by means of transient absorption, time-resolved emission and electrochemical impedance measurements. RuP shows significantly slower electron injection into a mesoporous titania electrode and enhanced fast (sub-ns) electron recombination with respect to those of N719. Moreover, RuOEC is found to be responsible for partial light absorption and electron injection with low efficiency. The obtained results reveal new insights into the reasons for the lower charge separation efficiency in water splitting systems with respect to that in solar cells. The important role of the initial processes occurring at the dye-titania interface within the first nanoseconds in this efficiency is emphasized.

Cosensitized Quantum Dot Solar Cells with Conversion Efficiency over 12

The improvement of sunlight utilization is a fundamental approach for the construction of high-efficiency quantum-dot-based solar cells (QDSCs). To boost light harvesting, cosensitized photoanodes are fabricated in this work by a sequential deposition of presynthesized Zn-Cu-In-Se (ZCISe) and CdSe quantum dots (QDs) on mesoporous TiO₂ films via the control of the interactions between QDs and TiO₂ films using 3-mercaptopropionic acid bifunctional linkers. By the synergistic effect of ZCISe-alloyed QDs with a wide light absorption range and CdSe QDs with a high extinction coefficient, the incident photon-to-electron conversion efficiency is significantly improved over single QD-based QDSCs. It is found that the performance of cosensitized photoanodes can be optimized by adjusting the size of CdSe QDs introduced. In combination with titanium mesh supported mesoporous carbon as a counterelectrode and a modified polysulfide solution as an electrolyte, a champion power conversion efficiency up to 12.75% (V_oc = 0.752 V, J_sc = 27.39 mA cm^-2 , FF = 0.619) is achieved, which is, as far as it is known, the highest efficiency for liquid-junction QD-based solar cells reported.

Photoelectrocatalytic Synthesis of Hydrogen Peroxide by Molecular Copper-Porphyrin Supported on Titanium Dioxide Nanotubes

We report on a self-assembled system comprising a molecular copper-porphyrin photoelectrocatalyst, 5-(4-carboxy-phenyl)-10,15,20-triphenylporphyrinatocopper(II) (CuTPP-COOH), covalently bound to self-organized, anodic titania nanotube arrays (TiO₂ NTs) for photoelectrochemical reduction of oxygen. Visible light irradiation of the porphyrin-covered TiO₂ NTs under cathodic polarization up to -0.3 V vs. Normal hydrogen electrode (NHE) photocatalytically produces H₂O₂ in pH neutral electrolyte, at room temperature and without need of sacrificial electron donors. The formation of H₂O₂ upon irradiation is proven and quantified by direct colorimetric detection using 4-nitrophenyl boronic acid (p-NPBA) as a reactant. This simple approach for the attachment of a small molecular catalyst to TiO₂ NTs may ultimately allow for the preparation of a low-cost H₂O₂ evolving cathode for efficient photoelectrochemical energy storage under ambient conditions.

Impedance investigation of the highly efficient polymer solar cells with composite CuBr2/MoO3 hole transport layer

Developing an air-stable, low-cost, non-toxic, and high-transparency charge buffer layer is a critical strategy to achieve the high photoelectric conversion efficiency of polymer photovoltaic cells. This paper reports the remarkable improvement of device performance by employing a combination of copper bromide (CuBr₂) and molybdenum trioxide (MoO₃) (CuBr₂/MoO₃) as the hole transport layer (HTL) of inverted-type polymer solar cells (PSCs). The bulk transport processes and resistive capacitance elements in the operating PTB7:PC₇₁BM bulk heterojunction PSCs were characterized using impedance spectroscopy. The impedance response was modeled using two equivalent circuital models, which are the general transmission line circuit (GTLC) model and the electrochemical polarization model. The effective carrier lifetime, conductivity, and mobility for both devices were extracted from the models. The improved hole transport at the anode and the efficient electron transport blocking decreased interface recombination and contact resistance, resulting in improved power conversion efficiency (PCE) values ranging from 7.30% to 9.56%. These results suggest that quantitative interpretation and modeling of the impedance spectroscopy results provide an effective way to unravel the operating mechanism of photovoltaic devices.

Scale-Up of the Electrodeposition of ZnO/Eosin Y Hybrid Thin Films for the Fabrication of Flexible Dye-Sensitized Solar Cell Modules

The low-temperature fabrication of flexible ZnO photo-anodes for dye-sensitized solar cells (DSSCs) by templated electrochemical deposition of films was performed in an enlarged and technical simplified deposition setup to demonstrate the feasibility of the scale-up of the deposition process. After extraction of eosin Y (EY) from the initially deposited ZnO/EY hybrid films, mesoporous ZnO films with an area of about 40 cm² were reproducibly obtained on fluorine doped tin oxide (FTO)-glass as well as flexible indium tin oxide (ITO)-polyethylenterephthalate (PET) substrates. With a film thickness of up to 9 µm and a high specific surface area of up to about 77 m²·cm^-3 the ZnO films on the flexible substrates show suitable properties for DSSCs. Operative flexible DSSC modules proved the suitability of the ZnO films for use as DSSC photo-anodes. Under a low light intensity of about 0.007 sun these modules achieved decent performance parameters with conversion efficiencies of up to 2.58%. With rising light intensity the performance parameters deteriorated, leading to conversion efficiencies below 1% at light intensities above 0.5 sun. The poor performance of the modules under high light intensities can be attributed to their high series resistances.

Consequences of changes in the ZnO trap distribution on the performance of dye-sensitized solar cells

The well-established indoline dye D149 and three indoline dyes with improved binding stability (DN91, DN216 and DN285) were used as sensitizers for electrodeposited porous ZnO and studied in dye-sensitized solar cells and showed very similar cell characteristics. After storage of the cells in the dark for four weeks, an increase in short-circuit current density was observed. Electrochemical impedance spectroscopy served to detect a shift of the conduction band edge energy to lower energies and, hence, an increased injection efficiency of electrons from the excited state of the sensitizer to the ZnO conduction band is detected as the main cause of this change. Additional photoelectrochemical experiments at different illumination intensities, intensity modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS), transient photocurrent measurements, as well as measurements of the open-circuit photovoltage decay (OCVD) were performed to confirm this conclusion and, further, to prove a reduced rate of recombination for these injected electrons which prevented a corresponding decrease in the open-circuit voltage for many cells, frequently observed earlier. It is therefore shown how the long-term stability of ZnO-based DSSCs can be improved.

Understanding the role of silica nanospheres with their light scattering and energy barrier properties in enhancing the photovoltaic performance of ZnO based solar cells

The present study discusses the design and development of a dye sensitized solar cell (DSSC) using a hybrid composite of ZnO nanoparticles (ZnO NP) and silica nanospheres (SiO₂ NS). A ≈22% enhancement in the overall power conversion efficiency (PCE, η) was observed for the device fabricated with a binary hybrid composite of 1 wt% SiO₂ NS and ZnO NP compared to the pristine ZnO NP device. A systematic investigation revealed the dual function of the silica nanospheres in enhancing the device efficacy compared to the bare ZnO NP based device. Sub-micron sized SiO₂ NS can boost the light harvesting efficiency of the photoanode by optical confinement, resulting in increased propagation length of the incident light by multiple internal reflections, which was confirmed by UV-Vis diffused reflectance spectroscopy. Electrochemical impedance spectroscopic (EIS) analysis showed a reduced recombination of photo-generated electrons to the I^-/I₃^- redox shuttle in the case of the composite photoanode. The higher recombination resistance (R_ct) in the case of a 1 wt% composite indicates that the SiO₂ NS serves as a partial energy barrier layer to retard the interfacial recombination (back transfer) of photo-generated electrons at the working electrode/electrolyte interface, increasing the device efficiency.

Chromatic Titanium Photoanode for Dye-Sensitized Solar Cells under Rear Illumination

Titanium (Ti) has high potential in many practical applications such as biomedicine, architecture, aviation, and energy. In this study, we demonstrate an innovative application of dye-sensitized solar cells (DSSCs) based on Ti photoanodes that can be integrated into the roof engineering of large-scale architectures. A chromatic Ti foil produced by anodizing oxidation (coloring) technology is an attractive roof material for large-scale architecture, showing a colorful appearance due to the formation of a reflective TiO₂ thin layer on both surfaces of Ti. The DSSC is fabricated on the backside of the chromatic Ti foil using the Ti foil as the working electrode, and this roof-DSSC hybrid configuration can be designed as an energy harvesting device for indoor artificial lighting. Our results show that the facet-textured TiO₂ layer on the chromatic Ti foil not only improves the optical reflectance for better light utilization but also effectively suppresses the charge recombination for better electron collection. The power conversion efficiency of the roof-DSSC hybrid system is improved by 30-40% with a main contribution from an improvement of short-circuit current density under standard 1 sun and dim-light (600-1000 lx) illumination.

Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface.

Effect on absorption and electron transfer by using Cd(ii) or Cu(ii) complexes with phenanthroline as auxiliary electron acceptors (A) in D–A–π–A motif sensitizers for dye-sensitized solar cells

Four new polymeric metal complex dyes (PBDTT-PhenCd, PBDTT-PhenCu, PPV-PhenCd and PPV-PhenCu) with donor–acceptor–π-bridge-acceptor (D–A–π–A) structure were designed and synthesized.

Quantum dot-sensitized solar cells

A comprehensive overview of the development of quantum dot-sensitized solar cells (QDSCs) is presented.

A comparative study of the density of surface states in solid and hollow TiO2 microspheres

The density of surface states and charge transport of TiO₂ microspheres with different structures were investigated.

Performance enhancement of a dye-sensitized solar cell by peripheral aromatic and heteroaromatic functionalization in di-branched organic sensitizers

Suppression of back reaction and enhanced photoinduced intramolecular electron transfer through peripheral functionalization of triphenylamino based dibranched donor–acceptor dyes.

Enhanced crystallinity of CH3NH3PbI3 by the pre-coordination of PbI2–DMSO powders for highly reproducible and efficient planar heterojunction perovskite solar cells

The pre-coordinated PbI₂–DMSO powders improved efficiency and reproducibility of perovskite solar cells with enhanced crystallinity of the CH₃NH₃PbI₃ film.

Origin of the effects of PEG additives in electrolytes on the performance of quantum dot sensitized solar cells

It has been well established that polymer additives in electrolyte can impede the charge recombination processes at the photoanode/electrolyte interface, and improve performance, especially V_oc, of the resulting sensitized solar cells. However, there are few reports about the effect of electrolyte additives on counter electrode (CE) performance. Herein, we systematically investigated the effect of polyethylene glycol (PEG) additives with various molecular weights (M_w from 300 to 20 000) in polysulfide electrolyte on the performance of two representative CdSe and Zn–Cu–In–Se (ZCISe) quantum dot sensitized solar cells (QDSCs), and explored the mechanism of the observed effects. Electrochemical impedance spectroscopy measurements indicate that all PEG additives can improve the charge recombination resistance at the photoanode/electrolyte interface, therefore suppressing the unwanted charge recombination process, and enhancing the V_oc of the resulting cell devices accordingly. On the CE side, with the increase of M_w of PEG additives, the initial effect of reducing the charge transfer resistance at the CE/electrolyte interface evolves into an increasing resistance; accordingly the initial positive effect on FF turns into negative one. Accordingly, low M_w PEG can improve efficiency for both CdSe (increasing from 6.81% to 7.60%) and ZCISe QDSCs (increasing from 9.26% to 10.20%). High M_w PEG is still effective for CdSe QDSCs with an efficiency of 7.38%, but falls flat on ZCISe QDSCs (with an efficiency of 9.11%).

The origin for the effect of PEG additives in polysulfide electrolyte on the performance of both photoanode and counter electrode was explored, and a facile and general route for remarkably improving photovoltaic performance of QDSCs was offered.

Comparative advantages of Zn–Cu–In–S alloy QDs in the construction of quantum dot-sensitized solar cells

Alloyed structures of quantum dot light-harvesting materials favor the suppression of unwanted charge recombination as well as acceleration of the charge extraction and therefore the improvement of photovoltaic performance of the resulting solar cell devices. Herein, the advantages of Zn–Cu–In–S (ZCIS) alloy QD serving as light-harvesting sensitizer materials in the construction of quantum dot-sensitized solar cells (QDSCs) were compared with core/shell structured CIS/ZnS, as well as pristine CIS QDs. The built QDSCs with alloyed Zn–Cu–In–S QDs as photosensitizer achieved an average power conversion efficiency (PCE) of 8.47% (V_oc = 0.613 V, J_sc = 22.62 mA cm⁻², FF = 0.610) under AM 1.5G one sun irradiation, which was enhanced by 21%, and 82% in comparison to those of CIS/ZnS, and CIS based solar cells, respectively. In comparison to cell device assembled by the plain CIS and core/shell structured CIS/ZnS, the enhanced photovoltaic performance in ZCIS QDSCs is mainly ascribed to the faster photon generated electron injection rate from QD into TiO₂ substrate, and the effective restraint of charge recombination, as confirmed by incident photon-to-current conversion efficiency (IPCE), open-circuit voltage decay (OCVD), as well as electrochemical impedance spectroscopy (EIS) measurements.

Benefiting from the accelerative electron injection and retarded charge recombination, Zn–Cu–In–S alloy QD based QDSC achieved a PCE of 8.55%, which is 21%, and 82% higher than those of CIS/ZnS, and pristine CIS QDs based solar cells, respectively.

Improved performance induced by in situ ligand exchange reactions of copper bipyridyl redox couples in dye-sensitized solar cells

A reversible ligand exchange of the Cu(ii) redox shuttle is shown to reduce recombination, leading to excellent performance in DSSCs.

Photo-degradation of high efficiency fullerene-free polymer solar cells

Polymer solar cells are a promising technology for the commercialization of low cost, large scale organic solar cells. With the evolution of high efficiency (>13%) non-fullerene polymer solar cells, the stability of the cells has become a crucial parameter to be considered. Among the several degradation mechanisms of polymer solar cells, burn-in photo-degradation is relatively less studied. Herein, we present the first systematic study of photo-degradation of novel PBDB-T:ITIC fullerene-free polymer solar cells. The thermally treated and as-prepared PBDB-T:ITIC solar cells were exposed to continuous 1 sun illumination for 5 hours. The aged devices exhibited rapid losses in the short-circuit current density and fill factor. The severe short-circuit current and fill factor burn in losses were attributed to trap mediated charge recombination, as evidenced by an increase in Urbach energy for aged devices.

M(Al,Ni)-TiO2-Based Photoanode for Photoelectrochemical Solar Cells

This study presents the incorporation of Al and Ni cations onto the surface of TiO2 nanoparticles used as photoelectrode in dye sensitized solar cells (DSSCs). The incorporation of these cations was performed using the chemical bath deposition (CBD) technique. This process was applied up to three times to evaluate the semiconductors’ properties with respect to the amount of Al and Ni. The M(Al,Ni)-TiO2-based semiconductors were widely characterized using techniques such as X-ray fluorescence, X-ray diffraction, Raman spectroscopy, UV-Vis spectroscopy and X-ray photoelectron spectroscopy. The presence of (hydr)oxide species of Al(III) and Ni(II) was confirmed and anatase was the predominant crystalline phase obtained. Moreover, for both elements, a decrease in the band gap energy was observed, this being more pronounced after the incorporation of Ni. Furthermore, the use of the M(Al,Ni)-TiO2-based semiconductors as photoelectrodes in DSSCs led to an increase in the open-circuit voltage of up to 22% and 10% for the incorporation of Al and Ni, respectively. This increase can be reasonably explained by the negative shift of the flat band potential of the photoelectrodes. EIS measurements were performed to study the electron transport kinetics in the photoelectrode and the internal resistance in the DSSCs to understand the photocurrent density values obtained.

Accomplishment of Multifunctional π-Conjugated Polymers by Regulating the Degree of Side-Chain Fluorination for Efficient Dopant-Free Ambient-Stable Perovskite Solar Cells and Organic Solar Cells

We present an efficient approach to develop a series of multifunctional π-conjugated polymers (P1-P3) by controlling the degree of fluorination (0F, 2F, and 4F) on the side chain linked to the benzodithiophene unit of the π-conjugated polymer. The most promising changes were noticed in optical, electrochemical, and morphological properties upon varying the degree of fluorine atoms on the side chain. The properly aligned energy levels with respect to the perovskite and PCBM prompted us to use them in perovskite solar cells (PSCs) as hole-transporting materials (HTMs) and in bulk heterojunction organic solar cells (BHJ OSCs) as photoactive donors. Interestingly, P2 (2F) and P3 (4F) showed an enhanced power conversion efficiency (PCE) of 14.94%, 10.35% compared to P1 (0F) (9.80%) in dopant-free PSCs. Similarly, P2 (2F) and P3 (4F) also showed improved PCE of 7.93% and 7.43%, respectively, compared to P1 (0F) (PCE of 4.35%) in BHJ OSCs. The high photvoltaic performance of the P2 and P3 based photovotaic devices over P1 are well correlated with their energy level alignment, charge transporting, morphological and packing properties, and hole transfer yields. In addition, the P1-P3 based dopant-free PSCs and BHJ OSCs showed an excellent ambient stability up to 30 days without a significant drop in their initial performance.

Plasmonic Effect of Gold Nanostars in Highly Efficient Organic and Perovskite Solar Cells

Herein, a novel strategy is presented for enhancing light absorption by incorporating gold nanostars (Au NSs) into both the active layer of organic solar cells (OSCs) and the rear-contact hole transport layer of perovskite solar cells (PSCs). We demonstrate that the power conversion efficiencies of OSCs and PSCs with embedded Au NSs are improved by 6 and 14%, respectively. We find that pegylated Au NSs are greatly dispersable in a chlorobenzene solvent, which enabled complete blending of Au NSs with the active layer. The plasmonic contributions and accelerated charge transfer are believed to improve the short-circuit current density and the fill factor. This study demonstrates the roles of plasmonic nanoparticles in the improved optical absorption, where the improvement in OSCs was attributed to surface plasmon resonance (SPR) and in PSCs was attributed to both SPR and the backscattering effect. Additionally, devices including Au NSs exhibited a better charge separation/transfer, reduced charge recombination rate, and efficient charge transport. This work provides a comprehensive understanding of the roles of plasmonic Au NS particles in OSCs and PSCs, including an insightful approach for the further development of high-performance optoelectronic devices.

Molecular Engineering for Enhanced Charge Transfer in Thin-Film Photoanode

We developed three types of dithieno[3,2-b;2',3'-d]thiophene (DTT)-based organic sensitizers for high-performance thin photoactive TiO₂ films and investigated the simple but powerful molecular engineering of different types of bonding between the triarylamine electron donor and the conjugated DTT π-bridge by the introduction of single, double, and triple bonds. As a result, with only 1.3 μm transparent and 2.5-μm TiO₂ scattering layers, the triple-bond sensitizer (T-DAHTDTT) shows the highest power conversion efficiency (η = 8.4%; V_OC = 0.73 V, J_SC = 15.4 mA·cm^-2, and FF = 0.75) in an iodine electrolyte system under one solar illumination (AM 1.5, 1000 W·m^-2), followed by the single-bond sensitizer (S-DAHTDTT) (η = 7.6%) and the double-bond sensitizer (D-DAHTDTT) (η = 6.4%). We suggest that the superior performance of T-DAHTDTT comes from enhanced intramolecular charge transfer (ICT) induced by the triple bond. Consequently, T-DAHTDTT exhibits the most active photoelectron injection and charge transport on a TiO₂ film during operation, which leads to the highest photocurrent density among the systems studied. We analyzed these correlations mainly in terms of charge injection efficiency, level of photocharge storage, and charge-transport kinetics. This study suggests that the molecular engineering of a triple bond between the electron donor and the π-bridge of a sensitizer increases the performance of dye-sensitized solar cell (DSC) with a thin photoactive film by enhancing not only J_SC through improved ICT but also V_OC through the evenly distributed sensitizer surface coverage.

Revealing the correlation between charge carrier recombination and extraction in an organic solar cell under varying illumination intensity

The design and fabrication of better excitonic solar cells are the need of the hour for futuristic energy solutions. This designing needs a better understanding of the charge transport properties of excitonic solar cells. One of the popular methods of understanding the charge transport properties is the analysis of the J-V characteristics of a device through theoretical simulation at varied illumination intensity. Herein, a J-V characteristic of a polymer:fullerene based bulk heterojunction (BHJ) organic solar cells (OSCs) of structure ITO/PEDOT:PSS (∼40 nm)/PTB7:PC₇₁BM (∼100 nm)/Al (∼120 nm) is analyzed using one- and two-diode models at varied illumination intensity in the range of 0.1-2.33 Sun. It was found that the double diode model is better with respect to the single diode model and can explain the J-V characteristics of the OSCs correctly. Further, the recombination mechanism is investigated thoroughly and it was observed that fill factor (FF) is in the range of 62.5%-41.4% for the corresponding values of the recombination-to-extraction ratio (θ) varying from 0.001 to 0.023. These findings are attributed to the change in charge transport mechanism from trap-assisted to bimolecular recombination with the variation of illumination intensity.