Semiconductor-sensitized Solar Cells Research Articles

Bi2S3 Liquid-Junction Semiconductor-Sensitized SnO2 Solar Cells

Semiconductor-sensitized solar cells (SSCs) are usually fabricated by coating a layer of semiconductor film on a metal oxide TiO2 photoelectrode. This work demonstrates a new-structure solar cell−Bi2S3 SSCs fabricated on SnO2 photoelectrodes. Bi2S3 nanoparticles were grown using the successive ionic layer adsorption and reaction process. The Bi2S3-coated SnO2 electrodes were fabricated to form liquid-junction solar cells. The best cell yielded a short-circuit current density Jsc of 4.20 mA/cm2, an open-circuit voltage of 0.16 V, and a power conversion efficiency η of 0.20% under 1 sun. At a reduced light intensity of 10.9% sun, Jsc = 0.90 mA/cm2 (which is equivalent to 8.26 mA/cm2 under 1 sun) the η increased to 0.27%. The absorbed-photon-to-collected-electron efficiency (APCE) spectrum covered a spectral range of 350−900 nm with a maximal APCE of 64% at λ = 490 nm. These results and the large optical absorption coefficient (α∼105 cm−1) indicate that Bi2S3 can be used as an absorber for solar cells.

AgBiS2 Semiconductor-Sensitized Solar Cells

We present a new ternary semiconductor sensitizer-AgBiS2 for solar cells. AgBiS2 nanoparticles were grown using a two-stage successive ionic layer adsorption and reaction process. Post annealing transformed the double-layered structure into AgBiS2 nanoparticles of ∼16 nm in diameter. Liquid-junction semiconductor-sensitized solar cells were fabricated from the synthesized AgBiS2 semiconductor. The best cell exhibited a short-circuit current density Jsc of 7.61 mA/cm2, an open-circuit voltage of 0.18 V, a fill factor of 38.6%, and a power conversion efficiency η of 0.53% under 1 sun. The η increased to 0.76% at a reduced light intensity of 0.148 sun. The external quantum efficiency (EQE) spectrum covered the spectral range of 350–850 nm, with an average EQE of ∼54% over the main spectral region of 450–650 nm. The Jsc under 0.148 sun was equal to 1.69 mA/cm2, a respectable Jsc for a new sensitizer.

Trap and Transfer. Two-Step Hole Injection Across the Sb2S3/CuSCN Interface in Solid-State Solar Cells

In solid-state semiconductor-sensitized solar cells, commonly known as extremely thin absorber (ETA) or solid-state quantum-dot-sensitized solar cells (QDSCs), transfer of photogenerated holes from the absorber species to the p-type hole conductor plays a critical role in the charge separation process. Using Sb2S3 (absorber) and CuSCN (hole conductor), we have constructed ETA solar cells exhibiting a power conversion efficiency of 3.3%. The hole transfer from excited Sb2S3 into CuSCN, which limits the overall power conversion efficiency of these solar cells, is now independently studied using transient absorption spectroscopy. In the Sb2S3 absorber layer, photogenerated holes are rapidly localized on the sulfur atoms of the crystal lattice, forming a sulfide radical (S(-•)) species. This trapped hole is transferred from the Sb2S3 absorber to the CuSCN hole conductor with an exponential time constant of 1680 ps. This process was monitored through the spectroscopic signal seen for the S(-•) species in Sb2S3, providing direct evidence for the hole transfer dynamics in ETA solar cells. Elucidation of the hole transfer mechanism from Sb2S3 to CuSCN represents a significant step toward understanding charge separation in Sb2S3 solar cells and provides insight into the design of new architectures for higher efficiency devices.

Investigation on New CuInS2/Carbon Composite Counter Electrodes for CdS/CdSe Cosensitized Solar Cells

The search for semiconductor-sensitized solar cell (SSC) counter electrode alternatives has been a continuous effort and long ongoing work, while the studies in counter electrode kinetic performance and stability are important to improve the overall efficiency. Here, a ternary chalcopyrite compound CuInS2 is first employed as counter electrode (CE) material for CdS/CdSe cosensitized solar cells. Besides, in order to increase the electron transfer activity at the counter electrode/electrolyte interface and stability, an appropriate amount of active carbon/carbon black mixture is introduced to afford CuInS2/carbon composite electrodes. Electron transfer processes in CuInS2-based electrodes are investigated in detail with the aid of electrochemical impedance spectroscopy and I-E measurement. Up to 4.32% of the light-to-electricity conversion efficiency has been achieved for the CdS/CdSe SSCs with the CuInS2/carbon composite electrode. Besides, a preliminary long-term stability test reveals that the new CuInS2/carbon composite counter electrode exhibits good stability after being kept in the dark at room temperature and without current flow for 1000 h.

A Quasi-Quantum Well Sensitized Solar Cell with Accelerated Charge Separation and Collection

Semiconductor-sensitized solar cell (SSSC) represents a new generation of device aiming to achieve easy fabrication and cost-effective performance. However, the power of the semiconductor sensitizers has not been fully demonstrated in SSSC, making it actually overshadowed by dye-sensitized solar cell (DSSC). At least part of the problem is related to the inefficient charge separation and severe recombination with the current technologies, which calls on rethinking about how to better engineer the semiconductor sensitizer structure in order to enhance the power conversion efficiency (PCE). Herein we report on using for the first time a quasi-quantum well (QW) structure (ZnSe/CdSe/ZnSe) as the sensitizer, which is quasi-epitaxially deposited on ZnO tetrapods. Such a novel photoanode architecture has attained 6.20% PCE, among the highest reported to date for this type of SSSCs. Impedance spectra have revealed that the ZnSe/CdSe/ZnSe QW structure has a transport resistance only a quarter that of, but a recombination resistance twice that of the ZnSe/CdSe heterojunction (HJ) structure, yielding much longer electron diffusion length, consistent with the resulting higher photovoltage, photocurrent, and fill factor. Time-resolved photoluminescence spectroscopy indicates dramatically reduced electron transfer from ZnO to the QW sensitizer, a feature which is conducive to charge separation and collection. This study together with the impedance spectra and intensity modulated photocurrent spectroscopies supports a core/shell two-channel transport mechanism in this type of solar cells and further suggests that the electron transport along sensitizer can be considerably accelerated by the QW structure employed.

MnTe semiconductor-sensitized boron-doped TiO2 and ZnO photoelectrodes for solar cell applications

We report a new tailoring MnTe semiconductor-sensitized solar cells (MnTe SSCs) using successive ionic layer adsorption and reaction (SILAR) technique. X-ray diffraction and SAED patterns reveal the orthorhombic MnTe and cubic MnTe2 phases were grown on boron-doped TiO2 and ZnO nanoparticles. The diameter of MnTe NPs ranged from 15 to 30nm on both B-doped metal oxide structures. The energy gaps of metal oxide become narrower after boron doping, which have an advantage for enhancing the light absorption from UV to visible region. Also, the energy gap of MnTe NPs on B-doped metal oxide was determined ∼1.27–1.30eV. The best power conversion efficiency (η) of 0.033% and 0.030% yielded from B-doped TiO2/MnTe(7) and B-doped ZnO/MnTe(9), respectively. The reduction in power conversion efficiency by 103% and 91% was due to the absence of boron doping into TiO2 and ZnO nanostructures, respectively.

Photoelectrochemical performance of a sub-micron structured film with poly(3-methylthiophene) (P3MT)-modified CdTe/ZnO shell-core sub-micron tube arrays

A sub-micron structured film with a poly(3-methylthiophene) (P3MT)-modified CdTe/ZnO shell-core sub-micron tube array has been prepared by a series of electrodeposition processes, and a semiconductor-sensitized solar cell based on this structure was also fabricated. Vertically oriented ZnO sub-micron tubes were obtained on an indium tin oxide (ITO) substrate, and then CdTe nanocrystals and a thin P3MT layer were electrodeposited sequentially onto the walls of the ZnO sub-micron tubes. A suitable thickness of CdTe and P3MT could improve the photovoltaic properties of the solar cell, which was attributed to the enhancement in the light absorption and the decrease in the recombination of photogenerated carriers. In addition, a p-n heterojunction formed between the interface of CdTe and P3MT played an important part in the efficient separation and fast transport of photogenerated carriers in the sub-micron structure. A power conversion efficiency of 1.20% was obtained with this type of solar cell.

Synthesis and characterization of CdSexTe1−x sensitized TiO2 solar cell with enhanced photovoltaic properties

Water-soluble CdSexTe1−x alloyed quantum dots (AQDs) sensitized TiO2 electrodes and their performances in semiconductor-sensitized solar cell were investigated. We found that the photovoltaic performance was affected by the alloying degree of AQDs. Compared to pure CdSe and CdTe QDs, CdSexTe1−x AQDs showed better photovoltaic property. Maximum short circuit photocurrent density (1.448mA/cm2) and open circuit voltage (0.46V) were obtained when Se/Te molar ratio of AQDs was 0.4/0.6. Maximum power conversion efficiency (0.588%) was obtained by TiO2 nano-tubes (NTs) electrode. We attribute this to enhanced photo-absorbing efficiency and photo-induced charges separation of the AQDs, which were proved by the UV–vis absorption and fluorescence lifetime data.

Influence of Nanoporous Oxide Substrate on the Performance of Photoelectrode in Semiconductor-Sensitized Solar Cells

. In the development of high-performance SSSCs,it is of significant importance to understand the effect of oxides on the electron injection and chargerecombination as these two are major factors in dictating solar cell performance. In addition, elucidating therelationship between these two critical processes and solar cell performance in each oxide is critical in buildingup the basic foundation of SSSC s. In this study, ultraf ast pump-probe laser spectroscopy and open-circuit decayanalysis were conducted to examine the characteristics of three representative oxides (TiO

Effective light harvesting in CdS nanoparticle-sensitized rutile TiO2 microspheres

In this article, we focused on cadmium sulphide (CdS) nanoparticles (NPs) sensitized rutile TiO2 microspheres (TMS) as a photoanode, which has been studied systematically. Hydrothermally grown TMS are sensitized with CdS NPs by simple and cost effective successive ionic layer adsorption and reaction (SILAR) technique. The synthesized CdS and CdS-TMS samples were characterized by Fourier Transform Raman Spectroscopy (FT-Raman), X-ray photoelectron spectroscopy (XPS) and FESEM characterization techniques. The CdS and CdS-TMS samples were further used for their photoelectrochemical (PEC) performance and were tested in Na2SNaOHS and Na2S electrolytes respectively. When CdS nanoparticles are coated on TMS, it exhibits improved photoelectrochemical (PEC) performance with maximum short circuit current of (JSC) 2.75mA/cm2. The Judiciously optimized CdS-decorated TiO2 morphology will be helpful for effective light-harvesting and fast electron-transfer from CdS to conduction band (CB) of TiO2. The highest power conversion efficiency of semiconductor-sensitized solar cells (SSSCs) i.e. CdS NP-sensitized TMS for the first time is 2.34% which is much higher than the bare CdS sample.

One-Dimensional Core/Shell Structured TiO2/ZnO Heterojunction for Improved Photoelectrochemical Performance

One-dimensional array grown on optically transparent electrode holds a promise as a photoelectrode for photoelectrochemical water splitting; however, its crystal structure is rutile, imposing constraints on the potent use of this nanostructure. To address this issue, a heterojunction with type-II band alignment was fabricated using atomic layer deposition (ALD) technique. One-dimensional core/shell structured /ZnO heterojunction was superior to in the photoelectrochemical water splitting because of better charge separation and more favorable Fermi level. The heterojunction also possesses better light scattering property, which turned out to be beneficial even for improving the photoelectrochemical performance of semiconductor-sensitized solar cell.

Electrodeposited AgInSe2 onto TiO2 films for semiconductor-sensitized solar cell application: The influence of electrodeposited time

The influence of electrodeposited time (EDT) on Ag–In–Se species growth onto TiO2 films for possible semiconductor-sensitized solar cells (SSSCs) application was investigated. XRD analysis illustrated that the Ag–In–Se film was predominantly comprised by AgInSe2 species with tetragonal body structure and crystal size of 6.05–7.50nm when EDT was in the region of 15–60min at a bias of −1.25V (verse Hg/Hg2SO4 (MSE)). Scanning electron microscope (SEM) indicated a high porosity of AgInSe2/ITO morphology, permitting electrolytes freely percolated through these films. The prepared AgInSe2 films exhibited n-type semiconductor behavior with two band gap energies at 1.27 and 1.80eV. Photoelectrochemical measurement reflected that open circuit potential varied little with EDT, however, significant change was associated with short circuit current and fill factor (FF), causing the AgInSe2/TiO2 films with EDT of 45min exhibited the best solar to electricity conversion efficiency of 0.26%. The AgInSe2/TiO2 films with EDT of 45min demonstrated the longest electron lifetime according to the open circuit voltage decay analysis.

A novel semiconductor-sensitized solar cell based on P3HT@CdS@TiO2 core-shell nanotube array

In this paper, a novel semiconductor-sensitized solar cell with the photoanode based on poly (3-hexylthiophene) (P3HT)@CdS@TiO2 nanotube array was fabricated. Vertically oriented TiO2 nanotube arrays were prepared by a sol–gel based method using ZnO nanorod arrays as sacrificing template. CdS nanocrystals and a thin P3HT layer were electrodeposited onto the wall of TiO2 nanotube to prepare the photoanode of the semiconductor-sensitized solar cell. It has been demonstrated that the coating of P3HT on the surface of the CdS@TiO2 core-shell nanotube arrays could largely improve the photovoltaic performance of the solar cell. A power conversion efficiency of 1.16% was obtained with this type of solar cell.

Three-dimensional networks of ITO/CdS coaxial nanofibers for photovoltaic applications

When serving as the electrode of semiconductor sensitized solar cells (SSSCs), conductive transparent nanofibers can enlarge the surface area and accelerate the electron transport, and can thus be expected to enhance the final power conversion efficiency of the device. Here we report the fabrication of three dimensional networks of indium tin oxide (ITO) nanofibers on a conductive glass substrate with a low sheet resistance of 15 Ω sq−1 by using an electrospinning technique. A post-treatment involving polyethylene glycol impregnation and calcination is developed to realize the excellent electrical contact of ITO nanofibers with the substrate, and maintain the fibrous morphology of the nanofibers also. Light-harvesting CdS nanocrystals are electrodeposited onto ITO nanofibers, and then the obtained ITO/CdS coaxial nanofibers are constructed into SSSCs. We find that the performance of this novel SSSC is remarkably improved compared with CdS nanocrystals directly deposited on a flat conductive substrate, and an unprecedented photocurrent density of 9.27 mA cm−2 is achieved under the illumination of one sun (AM1.5, 100 mW cm−2). Our work indicates that the special designation of three-dimensional coaxial nanofibers can pave a new avenue to efficient photovoltaic devices.

Band engineered ternary solid solution CdSxSe1−x-sensitized mesoscopic TiO2 solar cells

The optical band gap of the light absorber and the alignment of its bands with the underlying wide band gap metal oxide are critical for efficient light harvesting and charge separation in semiconductor-sensitized solar cells (SSCs). In practice, these two requirements are however not always fulfilled concomitantly in SSCs. Favourable band alignment in CdSe-sensitized TiO2 requires utilization of quantum sized CdSe, which causes great losses in the harvesting of long wavelength photons due to quantum confinement effects. In the present study, ternary cadmium sulfoselenide (CdSxSe1-x), which has a tunable band gap between those of CdSe and CdS without reducing the dimension, was proposed as a sensitizer for TiO2. CdSxSe1-x was successfully synthesized by alternately depositing CdS and CdSe layers under ambient conditions. SSCs utilizing CdSxSe1-x-sensitized TiO2 yielded a power conversion efficiency of 4.05% under simulated AM1.5 100 mW cm(-2) illumination, rivalling the well-studied cascaded CdS/CdSe electrodes when an aqueous polysulfide solution was used as the electrolyte and Cu2S as the counter electrode. The findings of the present study provide an alternative and viable approach for optimizing the energetics of semiconductor sensitizers for efficient charge separation, while also maintaining good light harvesting.

Improvement of external quantum efficiency depressed by visible light-absorbing hole transport material in solid-state semiconductor-sensitized heterojunction solar cells

A mesoporous (mp)-TiO(2)/Sb(2)S(3)/P3HT [poly(3-hexylthiophene)] heterojunction solar cell displays reduced external quantum efficiency (EQE) at a wavelength of approximately 650 nm. This loss in EQE is due to incomplete charge carrier transport because the transportation of charge carriers generated in P3HT by the absorption of light into Sb(2)S(3) was inefficient, and consequently, the carriers recombined. The depression of the EQE was greatly relieved by introducing the porous structure formed by thermal decomposition of 2,2'-azobisisobutyronitrile (AIBN) into the P3HT layer.

Interfacial engineering of quantum dot-sensitized TiO2 fibrous electrodes for futuristic photoanodes in photovoltaic applications

Herein we report generic surface treatment approaches to improve the electronic interfaces of quantum dot-sensitized TiO2 fiber electrodes, thereby promoting their photoanode performance. Highly dense, continuous and nanostructured TiO2 fibrous membranes, without the inclusion of a scattering layer, unlike conventional TiO2 particulate electrodes, showed feasible photoconversion performance under the proposed interfacial engineering modification. The proposed interfacial treatment concerns fibrous membranes both before and after calcination. The chemical vapor pre-treatment on an as-deposited fibrous membrane using tetrahydrofuran (THF) reinforces the physical contact between the fibrous membrane and the transparent conducting substrate and reduces significantly the recombination rate. In the case of post-treatment by F-ion on a fibrous surface, together with the interfacial engineering approach, the ZnS surface passivation layer markedly improves the photoanode performance of the TiO2 fibrous membrane nearly to a factor of 3.2% with a remarkable open-circuit voltage Voc = 0.69 V and Jsc = 13 mA cm−2 under 1 sun illumination (100 mW cm−2). This report provides an excellent platform for studying and understanding the interfacial contacts and mechanisms related to the charge transfer at CdS/CdSe QD-sensitized TiO2 fibrous assemblies. Such implications of this interfacial treatment strategy can be successfully extended to a wide range of photoanode candidates in energy conversion systems and confirm the effectiveness of some alternative nanostructured electrodes for the development of semiconductor-sensitized solar cells.

Solar Cells: Structural and Electronic Properties of Semiconductor‐Sensitized Solar‐Cell Interfaces (Adv. Funct. Mater. 24/2011)

On page 4663, Christopher E. Patrick and Feliciano Giustino investigate the interface between TiO2 and Sb2S3 found in semiconductor-sensitized solar cells by means of first principle calculations. An atomistic model of the TiO2 and Sb2S3 interface is proposed and the interfacial energy-level alignment and ideal open-circuit voltage are determined. The analysis of other isostructural sensitizers indicates that anti-monselite (Sb2Se3) holds potential for delivering higher energy conversion efficiencies than the TiO2/Sb2S3 system.

Hole Transport and Recombination in All-Solid Sb2S3-Sensitized TiO2 Solar Cells Using CuSCN As Hole Transporter

All-solid semiconductor-sensitized solar cells lack models allowing their characterization in terms of the fundamental processes of charge transport and recombination. Nanostructured TiO2/Sb2S3/CuSCN solar cells were characterized by impedance spectroscopy, and a model was proposed for this type of cells. One important feature resulting from this analysis was the hole transport diffusion, which could be assimilated to a series resistance affecting the cell fill factor. The other important feature was the recombination rate, which could be described in a similar manner as other cells using nanostructured TiO2 electrodes and which had an important impact on the open circuit. A simulation of the current–voltage curves using such model allowed us to get an approximate quantification of the losses caused by each process and to evaluate the possible improvements on the performance of this kind of cell.

Fabrication of Poly(3-hexylthiophene)/CdS/ZnO Core–Shell Nanotube Array for Semiconductor-Sensitized Solar Cell

In this article, we designed and fabricated a novel semiconductor-sensitized solar cell using a poly(3-hexylthiophene) (P3HT)/CdS/ZnO core–shell nanotube array as the photoanode. A facile sequent electrodeposition process was employed to prepare the photoanode. The one-dimensional three-component core–shell ordered nanoarray structure was formed by electrodepositing CdS nanoparticles and a thin P3HT layer onto the walls of ZnO nanotubes, which enhanced the optical absorption in the visible region and presented an interface between n-type CdS and p-type P3HT to improve the charge separation. An energy conversion efficiency up to 1.28% was obtained with the designed nanostructured solar cell based on the P3HT/CdS/ZnO nanotube array.