Efficient Quantum Dot Solar Cells Research Articles

Simulation of Efficient Lead Sulfide Colloidal Quantum Dot Solar Cell using Spiro-OMeTAD as Hole Transport Layer

In recent years, the advancement of solar cell technology is increased by leaps and bounds and it is also used to achieve a solution for the worldwide huge need for generation of energy and electricity. The colloidal quantum dot (CQD) offers a size-tuned bandgap and materials processing compatibility with a range of substrates. QDSC (Quantum dot solar cell) have advantages such as low cost, high efficiency, and replaces bulky material (Cadmium Selenide, Lead Selenide etc over traditional solar cell. “Despite these advantages, it lags due to carrier recombination in the Quasi-Neutral Region (QNR). The performance of the solar cell greatly depends on the electron transport layer (ETL) and hole transport layer (HTL). To investigate the feasibility of a highperformance device, a comparative investigation of the PbS-EDT and Spiro-OMeTAD hole transport layers has been done. For this, we have varied the various parameters upon which performance of solar cells is dependent in order to maximise the performance. All simulations study has been performed using SCAPS-1D simulator. The overall maximum optimized performance of the photovoltaic solar cell of 16.29% is obtained using TiO2 and PbS-TBAI(tetrabutylammonium iodide) as a ETL and absorber layer respectively. Our research demonstrates that an efficient quantum dot solar cell could be fabricated experimentally using the optimal device structure.

Thiocyanate‐ and Thiol‐Functionalized p‐Doped Quantum Dot Colloids for the Development of Bulk Homojunction Solar Cells

Progress in device engineering and surface passivation strategies has led to steady progress in colloidal quantum dot (QD) solar cells. Bulk homojunction (BHJ) device architecture has several advantages over the conventional planar junction in developing QD solar cells. Herein, surface ligand chemistry is utilized to control the doping type and dispersibility of oppositely doped PbS QDs to develop BHJ solar cells. Thiocyanate and thiol ligand combination is introduced to develop p‐PbS QD ink, which is blended with halide‐passivated n‐PbS QDs to build BHJ solar cells. It is shown that BHJ solar cells are benefited from high energy offset and higher hole mobility. This leads to the superior carrier extraction from a thicker active layer without compromising fill factor and open circuit voltage. Power conversion efficiency has reached 10.7% in 530 nm‐thick BHJ solar cells, a 24% improvement over the best performing planar solar cells. With the help of the 1D solar cell capacitance simulator, it is shown that a 15% efficient QD solar cell can be realized by further improving the hole mobility above 0.1 cm2 V−1 s−1.

Eco-friendly and stable silver bismuth disulphide quantum dot solar cells via methyl acetate purification and modified ligand exchange

Abstract From the point of view of environmentally-friendly, non- or less toxic photovoltaic material and fabrication method are desired by both the academia and industrial community. However, most efficient quantum dot solar cells have been fabricated based on heavy metal and toxic solvent consumption, which could cause serious ecological problems. Quantum dot solar cells were studied and explored based on lead-free active material silver bismuth disulphide, less toxic solvent purification of methyl acetate and lab-developed ligand exchange process. Twice ligand exchanges with tetramethylammonium iodide were applied for every deposited quantum dot layer kept in static. Methanol rinse was applied to the quantum dot film after each ligand exchange process to promote the ligand exchange and clean the organic residuals. Decent power conversion efficiency and good stability were demonstrated for the fabricated silver bismuth disulphide quantum dot solar cells. The best power conversion efficiency of 4.57% was achieved, with open-circuit voltage of 0.445 V, short-circuit current of 18.87 mA/cm2 and fill factor of 54.4%. Over 95% of the original power conversion efficiency was maintained after 150 days both in N2-filled glove box and ambient atmosphere with relative humidity of 30%. This work demonstrated fabrication of quantum dot solar cells based on non- or less-toxic quantum dots and solvent consumption. Which would benefit the development and commercialization of quantum dot solar cell technology.

Transition metal – Graphene oxide nanohybrid materials as counter electrodes for high efficiency quantum dot solar cells

Abstract We report the synthesis of new nanocomposite hybrids, based on graphene oxide (GO) incorporating Cu and Co transition metals. GO was produced through graphite oxidation using powerful oxidizing reagents and consecutively was functionalized with the use of CTAB and PSS surfactants. The resulting materials were further decorated with Cu and/or Co using an electrostatic self-assembly process through solvothermal treatment with the corresponding metal chlorides in an autoclave. The influence of metal precursor concentration in the final material characteristics was examined, using mass ratios of 1/4.5 and 1/9 for GO/Cu and GO/Co, and 1/4.5, 1/9 and 1/18 for the bimetallic GO/(Cu-Co) nanohybrid materials, respectively. The produced nanohybrid materials (GO/Cu, GO/Co and GO/(Cu-Co)) were characterized morphologically, structurally and electrochemically and consequently, they were incorporated in quantum dot sensitized solar cells (QDSCs). The novel hybrid materials work effectively as counter electrodes (CE) in CdS-ZnS/CdSe-based QDSCs presenting high photocurrent density (Jsc) and open-circuit voltage (Voc) values leading to enhanced photovoltaic performance. Optimization of the electrocatalytic activity as a function of the GO/(Cu-Co) mass ratio results in cells with a power conversion efficiency (PCE) of 8.73%.

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of Jsc, Voc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the Voc degradation while maximizing the short circuit current density (Jsc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in Jsc and Voc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells.

Pre-synthesized quantum dot deposition approach to obtain high efficient quantum dot solar cells

Quantum dot sensitized solar cells (QDSCs) appear to be one of the promising photovoltaic candidates, due to the lower cost of obtaining materials and assembling processes, as well as the advantages of their QD sensitizers which exhibit properties of tailoring the absorbance spectrum to near-infrared (NIR) regions, the multiple exciton generation (MEG), hot electron extraction, etc. However, the difficulty of QDs penetrating into TiO2 mesoporous film remains to be an obstacle for the development of QDSCs, which comes from (1) their larger size (1-10 nm) compared with dye molecules, (2) steric hindrance from the long chain organic ligands on the surface, and (3) the lack of terminal functional group of the ligand with affinity to TiO2. These issues imply the importance of implementing an efficient QD deposition method in the fabrication process. Based on summarizing the advantages and shortcomings, this review demonstrates the development of the QD deposition approaches in direct growth deposition methods: the chemical bath deposition (CBD) method, the successive ionic layer adsorption and reaction (SILAR) method, and the pre-synthesized QD deposition methods: linker-assisted deposition (LA), direct absorption (DA) and electrophoretic deposition (EPD). As an overall comparison to be taken for all these deposition approaches, the pre-synthesized QD deposition method has outperformed the direct growth deposition method due to the use of pre-synthesized high quality QD sensitizers for better performance in surface chemistry. Especially, the LA approach in this method exhibits its excellence of fast and uniform QD deposition with high coverage, as well as in building high efficiency QDSC devices. Specifically, the improved structure of the sensitizers such as the inverted type-I, type-II core/shell structures and alloyed configuration through surface ion-exchange, has been employed to boost the charge injection and depress the charge recombination, benefited from LA pre-synthesized QDs deposition method. The advantages of the LA method are fully illustrated by the examples of the most recent work in the achievement of reaching the record efficiency of QDSCs. Finally, outlooks have been given on possible approaches to realize further improvement of fabricating the QDSCs with excellent performance at higher levels.

Super sensitization: grand charge (hole/electron) separation in ATC dye sensitized CdSe, CdSe/ZnS type-I, and CdSe/CdTe type-II core-shell quantum dots.

Ultrafast charge-transfer dynamics has been demonstrated in CdSe quantum dots (QD), CdSe/ZnS type-I core-shell, and CdSe/CdTe type-II core-shell nanocrystals after sensitizing the QD materials by aurin tricarboxylic acid (ATC), in which CdSe QD and ATC form a charge-transfer complex. Energy level diagrams suggest that the conduction and valence band of CdSe lies below the LUMO and the HOMO level of ATC, respectively, thus signifying that the photoexcited hole in CdSe can be transferred to ATC and that photoexcited ATC can inject electrons into CdSe QD, which has been confirmed by steady state and time-resolved luminescence studies and also by femtosecond time-resolved absorption measurements. The effect of shell materials (for both type-I and type-II) on charge-transfer processes has been demonstrated. Electron injection in all the systems were measured to be <150 fs. However, the hole transfer time varied from 900 fs to 6 ps depending on the type of materials. The hole-transfer process was found to be most efficient in CdSe QD. On the other hand, it has been found to be facilitated in CdSe/CdTe type-II and retarded in CdSe/ZnS type-I core-shell materials. Interestingly, electron injection from photoexcited ATC to both CdSe/CdTe type-II and CdSe/ZnS type-I core-shell has been found to be more efficient as compared to pure CdSe QD. Our observation suggests the potential of quantum dot core-shell super sensitizers for developing more efficient quantum dot solar cells.

Electron Trap to Electron Storage Center in Specially Aligned Mn-Doped CdSe d-Dot: A Step Forward in the Design of Higher Efficient Quantum-Dot Solar Cell.

Specially aligned surface-accumulated Mn-doped CdSe (MnCdSe) quantum dots (QDs) have been synthesized to study the effect of dopant atom on charge-carrier dynamics in QD materials. EPR studies suggest that the (4)T1 state of Mn(2+) lies above the conduction band of CdSe, and as a result no Mn-luminescence was observed from MnCdSe. Femtosecond transient absorption studies suggest that Mn atom introduces structural defects in surface-doped CdSe, which acts as electron trap center in doped QD for the photoexcited electron. Bromo-pyrogallol red (Br-PGR) were found to form strong charge-trasfer complex with both CdSe and MnCdSe QDs. Charge separation in both the CdSe/Br-PGR and MnCdSe/Br-PGR composites was found to take place in three different pathways by transferring the photoexcited hole of CdSe/MnCdSe QDs to Br-PGR, electron injection from photoexcited Br-PGR to the QDs, and direct electron transfer from the HOMO of Br-PGR to the conduction band of both the QDs. Hole-transfer dynamics are found to be quite similar (∼1.1 to 1.3 ps) for both of the systems and found to be independent of Mn doping. However, charge recombination dynamics was found to be much slower in the MnCdSe/Br-PGR system as compared with that in the CdSe/Br-PGR system, which confirms that the Mn dopant act as the electron storage center. As a consequence, the MnCdSe/Br-PGR system can be used as a better super sensitizer in quantum-dot-sensitized solar cell to increase efficiency further.

Hierarchical self-assembled Cu2S nanostructures: Fast and reproducible spray deposition of effective counter electrodes for high efficiency quantum dot solar cells

The availability of a well-established procedure for fabricating reliable and reproducible counter electrodes for quantum dot sensitized solar cells is currently an issue, limiting both the functional performances of these devices and the possibility to compare results obtained in different laboratories. We present here a simple, cheap and fast method for Cu2S counter electrodes fabrication based on spray pyrolysis deposition. Application of prepared counter electrodes to SILAR-sensitized quantum dot solar cells results in high performance devices (photoconversion efficiencies as high as 3.75% and impressive incident photon-to-current-efficient higher than 90%) as well in excellent reproducibility.

Direct Work Function Measurement by Gas Phase Photoelectron Spectroscopy and Its Application on PbS Nanoparticles

Work function is a fundamental property of a material's surface. It is playing an ever more important role in engineering new energy materials and efficient energy devices, especially in the field of photovoltaic devices, catalysis, semiconductor heterojunctions, nanotechnology, and electrochemistry. Using ambient pressure X-ray photoelectron spectroscopy (APXPS), we have measured the binding energies of core level photoelectrons of Ar gas in the vicinity of several reference materials with known work functions (Au(111), Pt(111), graphite) and PbS nanoparticles. We demonstrate an unambiguously negative correlation between the work functions of reference samples and the binding energies of Ar 2p core level photoelectrons detected from the Ar gas near the sample surface region. Using this experimentally determined linear relationship between the surface work function and Ar gas core level photoelectron binding energy, we can measure the surface work function of different materials under different gas environments. To demonstrate the potential applications of this ambient pressure XPS technique in nanotechnology and solar energy research, we investigate the work functions of PbS nanoparticles with various capping ligands: methoxide, mercaptopropionic acid, and ethanedithiol. Significant Fermi level position changes are observed for PbS nanoparticles when the nanoparticle size and capping ligands are varied. The corresponding changes in the valence band maximum illustrate that an efficient quantum dot solar cell design has to take into account the electrochemical effect of the capping ligand as well.

Effect of Solar Concentration on the Thermodynamic Power Conversion Efficiency of Quantum-Dot Solar Cells Exhibiting Multiple Exciton Generation

Thermodynamic calculations show that all solar cells can convert solar photons into electricity or fuel with higher theoretical power conversion efficiencies under concentrated sunlight. For conventional (viz, present day) single-junction solar cells that produce at most one electron–hole pair per absorbed photon, the theoretical increase in efficiency is relatively small (absolute values of 38% at 500× vs 33% at 1×). However, when solar concentration is combined with multiple exciton generation (MEG) in semiconductor quantum dots, the increase in theoretical power conversion efficiency is greatly enhanced. For the ideal MEG case, where the threshold for exciton multiplication is twice the bandgap, Eg, the maximum thermodynamic efficiency increases to 75% at 500×, but the optimum Eg shifts to smaller values. If Eg is fixed at the 1-sun optimal level, then the maximum theoretical efficiency still increases markedly, becoming 62% at 500× for the staircase MEG characteristic (defined as producing N electron–...

Advances in highly efficient quantum dot solar cells

Advances in highly efficient quantum dot solar cells

Identification of candidate material systems for quantum dot solar cells including the effect of strain

AbstractHeterostructures that include self‐assembled quantum dots (SAQDs) have been suggested as model systems for the realization of novel high efficiency solar cells such as those based on intermediate bands (IBs). The lattice mismatch in the epitaxial growth of these structures, necessary for the formation of SAQDs, introduces strain throughout the structure, making the selection of materials systems with appropriate physical parameters problematic. The model solid theory is used to calculate the energy band edge alignment at Γ point of such quantum dot (QD) heterostructures including the effects of strain. With the modified band gaps due to strain, a materials search was performed for high efficiency QD solar cells among III‐V binaries and ternaries with negligible valence band offsets. This requirement of the valence band offset along with the limited band gap ranges for optimum efficiency results in only a few feasible materials systems being identified. The optimum barrier/dot material system found was Al0.57In0.43As/InP0.87Sb0.13 grown on lattice matched metamorphic buffer layer, but due to miscibility gap concerns it is suggested that the Al0.50In0.50As/InAs0.41P0.59 fully strained system may be preferred. Copyright © 2010 John Wiley &amp; Sons, Ltd.