Ab Initio Simulation of Charge Transfer at the Semiconductor Quantum Dot/TiO2 Interface in Quantum Dot‐Sensitized Solar Cells

Quantum dot‐sensitized solar cells (QDSSCs) have emerged as a promising solar architecture for next‐generation solar cells. The QDSSCs exhibit a remarkably fast electron transfer from the quantum dot (QD) donor to the TiO₂ acceptor with size quantization properties of QDs that allows for the modulation of band energies to control photoresponse and photoconversion efficiency of solar cells. To understand the mechanisms that underpin this rapid charge transfer, the electronic properties of CdSe and PbSe QDs with different sizes on the TiO₂ substrate are simulated using a rigorous ab initio density functional method. This method capitalizes on localized orbital basis set, which is computationally less intensive. Quite intriguingly, a remarkable set of electron bridging states between QDs and TiO₂ occurring via the strong bonding between the conduction bands of QDs and TiO₂ is revealed. Such bridging states account for the fast adiabatic charge transfer from the QD donor to the TiO₂ acceptor, and may be a general feature for strongly coupled donor/acceptor systems. All the QDs/TiO₂ systems exhibit type II band alignments, with conduction band offsets that increase with the decrease in QD size. This facilitates the charge transfer from QDs donors to TiO₂ acceptors and explains the dependence of the increased charge transfer rate with the decreased QD size.     

Investigation of Interfacial Charge Separation at PbS QDs/(001) TiO2 Nanosheets Heterojunction Solar Cell

In the recent years, the heterojunction solar cells based on quantum dots (QDs) have attracted attention due to strong light absorbing characteristics and the size effect on the bandgap tuning. This paper reports on the kinetics of interfacial charge separation of PbS QDs/(001) TiO₂ nanosheets heterojunction solar cells. PbS QDs are deposited using a bifunctional linker molecule on two different TiO₂ films, i.e., TiO₂ nanosheets (with 001 dominant exposed facet) and TiO₂ nanoparticles (with 101 dominant exposed facet). Upon bandgap excitation, electrons are transferred from the PbS QDs conduction band to the lower lying conduction band of TiO₂. Based on the ultrafast pump‐probe laser spectroscopy technique, the kinetics of charge separation is scrutinized at the PbS/TiO₂ interface. The interfacial charge separation at PbS/TiO₂ nanosheets films made of (001) dominant exposed facets is five times faster than that on (101) dominant exposed facets TiO₂ nanoparticles. The quantum yields for charge injection are higher for the (001) TiO₂ nanosheets than the (101) TiO₂ nanoparticles due to enhanced interfacial interaction with (001) surface compared to the (101) nanoparticles. The superior interfacial charge separation at PbS/(001) nanosheets respect to PbS/(101) nanoparticles is consistent with the higher photocurrent and enhanced power conversion efficiency in the PbS QDs/(001) TiO₂ heterojunction solar cell. The use of (001) TiO₂ nanosheets can be a better alternative to conventional mesoporous TiO₂ films in QD heterojunction solar cells and perovskites‐based heterojunction solar cells.     

Accurate analysis of electron transfer from quantum dots to metal oxides in quantum dot sensitized solar cells

Electron transfer rate from quantum dot (QD) to metal oxide (MO) in quantum dot sensitized solar cells (QDSSCs) has an important role in the efficiency. In this work, we analyse the electron transfer rate from CdSe, CdS and CdTe QDs to TiO₂, ZnO and SnO₂ MOs by extending the related equations with considering various effects, based on the Marcus theory. In this regard, the effects of QD diameter, QD–MO spacing, the crystalline defects, temperature, and the reorganizational energy, on the electron transfer rate are investigated. The results show that, the maximum electron transfer rate is achieved for CdTe QD with the mentioned three MOs. Moreover, in order to direct the designer to reach the appropriate QDs–MOs combinations for obtaining the maximum electron transfer rate, the average electron transfer rate for various combinations is calculated. For the verification of simulation method, a part of work has been compared with the previous experimental and theoretical results, which indicates the correctness of our simulation algorithm.     

TiO<Subscript>2</Subscript> Nanorod Arrays Sensitized with CdS Quantum Dots for Solar Cell Applications: Effects of Rod Geometry on Photoelectrochemical Performance

CdS quantum dot (QD) sensitized TiO₂ nanorod array (NRA) film electrodes with different rod geometries were fabricated via a solvothermal route followed by a sequentialchemical bath deposition (S-CBD) process. By controlling the solution growth conditions, the rod geometries, especially the tip structures, of the TiO₂ NRAs were tuned. The results indicated that the vertically aligned hierarchical NRAs possessed conically shaped tip geometry, which was favorable for film electrodes due to the reduced reflectance, enhanced light harvesting, fast charge-carrier separation and transfer, suppression of carrier recombination, sufficient electrolyte penetration and subsequent efficient QD assembly. CdS QD sensitized TiO₂ NRA film electrodes with tapered tips exhibited an enhanced photoelectrochemical (PEC) performance, a photocurrent intensity of 5.13 mA/cm² at a potential of 0 V vs. saturated calomel electrode, an open-circuit potential of −0.68 V vs. saturated calomel electrode and an incident photon to current conversion efficiency (IPCE) of 22% in the visible-light region from 400 to 500 nm. The effects of rod geometry on the optical absorption, reflectance, hydrophilic properties and PEC performance of bare TiO₂ and CdS QD sensitized TiO₂ NRA film electrodes were investigated. The mechanism of charge-carrier generation and transfer in these CdS QD sensitized solar cells based on vertically aligned TiO₂ nanorods is discussed.     

Electronic energy alignment at the PbSe quantum dots/ZnO(101̅0) interface

A number of solar energy conversion strategies depend on exciton dissociation across interfaces between semiconductor quantum dots (QDs) and other electron or hole conducting materials. A critical factor governing exciton dissociation and charge transfer in these systems is the alignment of electronic energy levels across the interface. We probe interfacial electronic energy alignment in a model system, sub-monolayer films of PbSe QDs adsorbed on single crystal ZnO(101?0) surfaces using ultraviolet photoemission spectroscopy. We establish electronic energy alignment as a function of quantum dot size and surface chemistry. We find that replacing insulating oleic-acid capping molecules on the QDs by the short hydrazine or ethanedithiol molecules results in pinning of the valence band maximum (VBM) of QDs to ZnO substrate states, independent of QD size. This is in contrast to similar measurements on TiO₂(110) where the alignment of the PbSe QD VBM to that of the TiO₂ substrate depends on QD size. We interpret these findings as indicative of strong electronic coupling of QDs with the ZnO surface but less with the TiO₂ surface. Based on the measured energy alignment, we predict that electron injection from the 1s_e level in photo-excited PbSe QDs to ZnO can occur with small QDs (diameter ? = 3.4 nm), but energetically unfavorably for larger dots (? = 6.7 nm). In the latter, hot electrons above the 1s_e level are necessary for interfacial electron injection.     

Development of gel polymer electrolytes for application in quantum dot-sensitized solar cells

A methylcellulose–polysulfide gel polymer electrolyte has been prepared for application in quantum dot-sensitized solar cells (QDSSCs) having the configuration FTO/TiO₂/CdS/ZnS/SiO₂/electrolyte/Pt(cathode). The electrolyte with the composition of 30.66 wt.% methylcellulose, 67.44 wt.% Na₂S, and 1.90 wt.% sulfur exhibits the highest conductivity of 0.183 S cm^?1 with the lowest activation energy of 6.14 kJ mol^?1. CdS quantum dot sensitizers have been deposited on TiO₂ film via the successive ionic layer absorption and reaction (SILAR) method. The QDSSC fabricated using the highest conducting electrolyte and CdS QD prepared with five SILAR cycles exhibits a power conversion efficiency (PCE) of 0.78%. After deposition of zinc sulfide (ZnS) and silicon dioxide SiO₂ passivation layers, the PCE of the QDSSC with photoanode arrangement of TiO₂/CdS(5)/ZnS(2)/SiO₂ increased to 1.42%, an improvement in performance by 82%.     

Investigation of polymer/inorganic hybrid photovoltaic device using quantum dots as the interface modifier

CdS quantum dots (QDs) were introduced as an interface modifier in the poly(3-hexylthiophene) (P3HT)/TiO₂ nanorod arrays hybrid photovoltaic device. The presence of CdS QDs interlayer was found to provide enhanced light absorption, increased interfacial recombination resistance at the P3HT/TiO₂ interfaces, thus leading to a lower recombination rate of the electrons due to the stepwise structure of band edge in P3HT/CdS/TiO₂, which accounts for the observed enhanced photocurrent and photovoltage of the hybrid solar cells. The optimized performance was achieved in P3HT/CdS/TiO₂ hybrid solar cells after deposition of CdS QDs for 10 cycles, with a power conversion efficiency of 0.57 %, which is nearly ten times higher than that of P3HT/TiO₂. The findings indicate that inorganic semiconductor quantum dots provide effective means to improve the performance of polymer/TiO₂ hybrid solar cells.     

The Dynamic Resistance of CdS/CdSe/ZnS Co-Sensitized TiO2 Solar Cells

Quantum dots' sensitized solar cells (QDSSCs) can create the high-performance and low-cost photovoltaic in the future. In this study, we synthesized the film of TiO₂/CdS/CdSe/ZnS photoanodes by successive ionic layer adsorption reaction (SILAR) method. The absorption spectra, photoluminescent spectra and electrochemical impedance spectra (EIS) of the film TiO₂/CdS/CdSe/ZnS photoanodes show that the structure of energy levels in the conduction band (CB) of photoanode materials CdS, CdSe, and ZnS quantum dots (QDs) can absorb a great number of photons in each region and inject stimulated electrons quickly into the conduction band (CB) of TiO₂. Furthermore, we also studied the influence of the SILAR cycles on the dynamic resistance, the lifetime of electrons in QDSSCs through Nyquist and Bode.     

Effects of 1064 nm laser on the structural and optical properties of nanostructured TiO2 thin film

TiO₂ thin film has been widely used as photoelectrode in dye-sensitized solar cells. It can also be used in quantum dot synthesized solar cells. Study of its effects in different spectrum of light is important for its use in solar cells. We have reported effects of 1064 nm laser on the surface morphology, structural and optical properties of nanostructured TiO₂ thin film deposited on glass substrates using sol-gel spin coating technique. Q-Switched Nd:YAG pulsed laser at various power densities is used in this study. Surface morphology of the film is investigated using X-ray diffraction (XRD) and atomic force microscopy technique. The XRD pattern of as deposited TiO₂ thin film is amorphous and after laser exposure it became TiO₂ anatase structure. Atomic force microscopy of the crystalline TiO₂ thin film shows that the grain size increases by increasing laser power density. The calculations of the band gap are carried out from UV/Visible spectroscopy measurements with JASCO spectrometer. For laser power density of 25 MW/cm² there is an increase in the transmission and it decreases at the value of 38 MW/cm² and band gap decreases with increasing laser power density. Photoluminescence spectra of the crystalline TiO₂ thin film indicate two broad peaks in the range of 415 and 463 nm, one for band gap peak (415 nm) and other for oxygen defect during film deposition process.     

Natural dye-based photoelectrode for improvement of solar cell performance

In the present paper, photovoltaic studies of dye-sensitized solar cells (DSSCs) based on betacyanin/TiO₂ and betacyanin/WO₃–TiO₂ have been done. The cell performances were compared through I–V curves and wavelength dependant photocurrent measurements for the two new types of DSSCs. The TiO₂-coated DSSC showed the photovoltage and photocurrent of 300 mV and 4.96 mA/cm², whereas the cell employing WO₃–TiO₂ photoelectrode showed the values 435 mV and 9.86 mA/cm², respectively. The conversion efficiency of TiO₂ based dye-sensitized solar cell was found to be 0.69 %, while WO₃–TiO₂-based cell exhibited a higher conversion efficiency of 2.2 %. The better performance of the WO₃–TiO₂ dye-sensitized solar cell photoelectrode is thought to be due to an inherent energy barrier at the electrode/electrolyte interface leading to the reduced recombination of photoinduced electrons.     

Effects of Gd<Superscript>3+</Superscript> modifications on the photoelectrochemical properties of TiO<Subscript>2</Subscript>-based dye-sensitized solar cells

In this paper, TiO₂ particles (~30 nm) modified with Gd₂O₃-coating layer (~2 nm) for dye-sensitized solar cells (DSSCs) were fabricated via the hydrothermal method. Among the solar cells based on the Gd³⁺-doped TiO₂ photoanodes, the optimal conversion efficiency was obtained from the 0.025Gd³⁺-modified TiO₂-based cell, with a 17.7% improvement in the efficiency as compared to the unmodified one (7.18%). This enhancement was probably due to the improved UV radiation harvesting via a down-conversion luminescence process by Gd³⁺ ions, enhancement of visible light absorption and improved dye loading capacity. In addition, after Gd modification, a thin coating could be formed on the TiO₂ nanoparticles, which worked as an energy barrier and resulted in a lower charge recombination.     

ZnS overlayer on in situ chemical bath deposited CdS quantum dot-assembled TiO2 films for quantum dot-sensitized solar cells

ZnS overlayers were deposited on the CdS quantum dot (QD)-assembled TiO₂ films, where the CdS QDs were grown on the TiO₂ by repeated cycles of the in situ chemical bath deposition (CBD). With increasing the CdS CBD cycles, the CdS QD-assembled TiO₂ films were transformed from the TiO₂ film partially covered by small CdS QDs (Type I) to that fully covered by large CdS QDs (Type II). The ZnS overlayers significantly improved the overall energy conversion efficiency of both Types I and II. The ZnS overlayers can act as the intermediate layer and energy barrier at the interfaces. However, the dominant effects of the ZnS overlayers were different for the Types I and II. For Type I, ZnS overlayer dominantly acted as the intermediate layer between the exposed TiO₂ surface and the electrolyte, leading to the suppressed recombination rate for the TiO₂/electrolyte and the significantly enhanced charge-collection efficiency. On the contrary, for Type II, it dominantly acted as the efficient energy barrier at the interface between the CdS QDs and the electrolyte, leading to the hindered recombination rate from the large CdS QDs to the electrolyte and thus enhanced electron injection efficiency.     

Electrostatic assembly of CdTe quantum dots with different charged ligands into TiO2 porous film for solar cells

We have demonstrated an approach for the electrostatic assembly of CdTe quantum dots (QDs) with different charged ligands as sensitizers, achieving high coverage and good dispersion in TiO₂ porous films. The CdTe QD-sensitized TiO₂ porous films were subjected to thermal annealing in a high vacuum chamber to remove the ligand linker, resulting in the formation of direct heterojunctions between the bare CdTe QDs and TiO₂ for a favorable charge transfer. The as-received CdTe QD-sensitized TiO₂ porous films were employed as photoanodes for quantum dot-sensitized solar cells (QSSCs), and the photocurrent density reached as high as 4.69 mA/cm² under a standard illumination condition of simulated AM 1.5G (100 mW/cm²).     

The electric field effect on binding energy of hydrogenic impurity in zinc-blende GaN/AlxGa1−xN spherical quantum dot

Within the framework of effective mass approximation, the binding energy of a hydrogenic donor impurity in zinc-blende GaN/Al_xGa_1−xN spherical quantum dot (QD) is investigated using the plane wave basis. The results show that the binding energy is highly dependent on impurity position, QD size, Al content and external field. The binding energy is largest when the donor impurity is located at the centre of the QD and the binding energy of impurity is degenerate for symmetrical positions with respect to the centre of QD without the external electric field. The maximum of the donor binding energy is shifted from the centre of QD and the degenerating energy levels for symmetrical positions with respect to the centre of QD are split in the presence of the external electric field. The binding energy is more sensitive to the external electric field for the larger QD and lower Al content. In addition, the Stark shift of the binding energy is also calculated.     

40% Efficiency enhancement in solar cells using ZnO nanorods as shell prepared via novel hydrothermal synthesis

Herein, rod-like ZnO nanostructures were synthesized via a novel hydrothermal route using Zn(OAc)₂, ethylenediamine and hydrazine as a new set of starting reagents. The as-synthesized products were characterized by techniques including XRD, EDS, SEM, XPS, Pl and FTIR. The prepared ZnO nanostructures were utilized as shell on TiO₂ film in DSSCs. Effect of precursor type, morphology and thickness of ZnO shell (number of electrophoresis cycle) on solar cells efficiency were well studied. Our results showed that ethylenediamine has crucial effect on morphology of synthesized ZnO nanostructures and using ZnO nanostructures leads to an increase in DSSCs efficiency compared to bare TiO₂ from 4.66 to 7.13% (~40% improvement). Moreover, highest amount of solar cell efficiency (7.13%) was obtained by using ZnO nanorods with two cycle of electrophoresis for deposition.     

Photovoltaic effect of TiO2 thick films with an ultrathin BiFeO3 as buffer layer

The photovoltaic (PV) effect of a bilayer anatase TiO₂/BiFeO₃ (BFO) film has been studied. The 20-nm ultrathin BFO layers were deposited on the fluorine-doped tin oxide (FTO) glass substrates by the chemical solution deposition method. An anatase TiO₂ layer is deposited subsequently on the BFO surface via a screen-printing technique. It is found that the FTO/TiO₂/Au cell exhibits negligible PV effect under solar exposure, while the one after introducing an ultrathin BFO film between TiO₂ and FTO leads to a considerable PV effect with an open-circuit voltage of ?0.58 V and a photocurrent density of 18.27 µA/cm². The FTO/BiVO₄ (BVO)/TiO₂/Au cell was constructed to investigate the underlying mechanism for the observed effect. A negligible PV effect of the FTO/BVO/TiO₂/Au cell indicates that the PV effect of the FTO/BFO/TiO₂/Au cell arises mainly from a built-in electric field in the BFO film induced by the self-polarization. Our work opens up a new path to utilize TiO₂ and may influence the future design of solar cells.     

Hydrothermal synthesis of TiO2 nanocrystals in different basic pHs and their applications in dye sensitized solar cells

In this research TiO₂ nanocrystals with sizes about 11–70 nm were grown by hydrothermal method. The process was performed in basic autoclaving pH in the range of 8.0–12.0. The synthesized anatase phase TiO₂ nanocrystals were then applied in the phtoanode of the dye sensitized solar cells. It was shown that the final average size of the nanocrystals was larger when the growth was carried out in higher autoclaving pHs. The photoanodes made of TiO₂ nanocrystals prepared in the pHs of 8.0 and 9.0 represented low amounts of dye adsorption and light scattering. The performance of the corresponding dye sensitized solar cells was also not acceptable. Nevertheless, the energy conversion efficiency was better for the state of pH of 9.0. For the photoanodes made of TiO₂ nanocrystals prepared at autoclaving pH of 10.0, the dye adsorption and light scattering were quite higher. The photovoltaic characteristics of the best cell in this state were 15.25 mA/cm², 740 mV, 0.6 and 6.8% for the short-circuit current density, open-circuit voltage, fill factor and efficiency, respectively. The photoanodes composed of TiO₂ nanocrystals prepared in autoclaving pHs of 11.0 and 12.0 demonstrated lower amount of dye adsorption and higher light scattering. This was quite considerable for the state of pH of 12.0. The energy conversion efficiencies were consequently decreased compared to that of the pH of 10.0. The optimum situation was finally discussed based on the nanocrystals size and its influence on the sensitization and light harvesting efficiency.     

Photoconduction action spectra of regio-regular poly(3-hexylthiopene):TiO2 diodes

The development of polymer-based photovoltaic devices brings the promise of low-cost and lightweight solar energy conversion systems. This technology requires new materials and device architectures with enhanced efficiency and lifetime, which depends on the understanding of charge-transport mechanisms. Organic films combined with electronegative nanoparticles may form systems with efficient dissociation of the photogenerated excitons, thus increasing the number of carriers to be collected by the electrodes. In this paper we investigate the steady-state photoconductive action spectra of devices formed by a bilayer of regio-regular poly(3-hexylthiophene) (RRP3HT) and TiO₂ sandwiched between ITO and aluminum electrodes (ITO/TiO₂:RRP3HT/Al). Photocurrents were measured for distinct bias voltages with illumination from either side of the device. Heterojunction structures were prepared by spin coating a RRP3HT film on an already deposited TiO₂ layer on ITO. Symbatic and antibatic curves were obtained and a model for photocurrent action spectra was able to fit the symbatic responses. The quantum yield increased with the electric field, indicating that exciton dissociation is a field-assisted process as in an Onsager mechanism. Furthermore, the quantum yield was significantly higher when illumination was carried out through the ITO electrode onto which the TiO₂ layer was deposited, as the highly electronegative TiO₂ nanoparticles were efficient in exciton dissociation.     

Two-step annealed CdS/CdSe co-sensitizers for quantum dot-sensitized solar cells

CdS/CdSe co-sensitizers on TiO₂ films were annealed using a two-step procedure; high temperature (300 °C) annealing of TiO₂/CdS quantum dots (QDs), followed by low temperature (150 °C) annealing after the deposition of CdSe QDs on the TiO₂/CdS. For comparison, two types of films were prepared; CdS/CdSe-assembled TiO₂ films conventionally annealed at a single temperature (150 or 300 °C) and non-annealed films. The 300 °C-annealed TiO₂/CdS/CdSe showed severe coalescence of CdSe QDs, leading to the blocked pores and hindered ion transport. The QD-sensitized solar cell (QD-SSC) with the 150 °C-annealed TiO₂/CdS/CdSe exhibited better overall energy conversion efficiency than that with the non-annealed TiO₂/CdS/CdSe because the CdSe QDs annealed at a suitable temperature (150 °C) provided better light absorption over long wavelengths without the hindered ion transport. The QD-SSC using the two-step annealed TiO₂/CdS/CdSe increased the cell efficiency further, compared to the QD-SSC with the 150 °C-annealed TiO₂/CdS/CdSe. This is because the 300 °C-annealed, highly crystalline CdS in the two-step annealed TiO₂/CdS/CdSe improved electron transport through CdS, leading to a significantly hindered recombination rate.     

Applications of vertically oriented TiO2 micro-pillars array on the electrode of dye-sensitized solar cell

In this research, dye-sensitized solar cells based on TiO₂ micro-pillars fabricated by inductive couple plasma etcher were investigated by analyses of X-ray diffraction (XRD), scanning electron microscopy (SEM), contact angle, ultraviolet-visible absorption spectra (UV-vis), and current-voltage characteristics. X-ray diffraction patterns show that the TiO₂ anatase phase forms while sintering at 450 °C for 30 min. The SEM images reveal that the diameter and height of TiO₂ micro-pillars are about 3 and 0.8 μm, respectively. The measurements of contact angle between TiO₂ micro-pillars and deionized water (DI water) reveal that the TiO₂ micro-pillars is super-hydrophilic while annealed at 450 °C for 30 min.The absorption spectrum of TiO₂ micro-pillars is better than TiO₂ thin film and can be widely improved in visible region with N3 dye adsorbed. The results of current-voltage (I-V) characteristics analysis reveal that dye-sensitized solar cell with TiO₂ micro-pillars electrode has better I-V characteristics and efficiency than TiO₂ film electrodes. This result may be due to the annealed TiO₂ micro-pillars applied on the electrode of dye-sensitized solar cell can increase the contact area between TiO₂ and dye, resulting in the enhancement of I-V characteristics and efficiency for dye-sensitized solar cell.