Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandem solar cells

Doping of Si quantum dots is important in the field of Si quantum dots-based solar cells. Structural, optical and electrical properties of Si QDs formed as multilayers in a SiO₂ matrix with various phosphorus (P) concentrations introduced during the sputtering process were investigated for its potential application in all-silicon quantum dot tandem solar cells. The formation of Si quantum dots was confirmed by transmission electron microscopy. The addition of phosphorus was observed to modify Si crystallization, though the phosphorus concentration was found to have little effect on quantum dot size. Secondary ion mass spectroscopy results indicate minimal phosphorus diffusion from Si QDs layers to adjacent SiO₂ layers during high-temperature annealing. Resistivity is significantly decreased by phosphorus doping. Resistivity of slightly phosphorus-doped (0.1 at% P) films is seven orders of magnitude lower than that of intrinsic films. Dark resistivity and activation energy measurements indicate the existence of an optimal phosphorus concentration. The photoluminescence intensity increases with the phosphorus concentration, indicating a tendency towards radiative recombination in the doped films. These results can provide optimal condition for future Si quantum dots-based solar cells.     

Synthesis and characterization of boron-doped Si quantum dots for all-Si quantum dot tandem solar cells

Multiple layers of Si quantum dots (QDs) in SiO₂ with a narrow size distribution were synthesized by a co-sputtering technique. Structural, electrical and optical properties of Si QD/SiO₂ multilayer films with various boron (B) concentrations introduced during the sputtering process were studied. X-ray photoelectron spectroscopy (XPS) revealed B-B/B-Si bonding, which suggests possible boron inclusion in the nanocrystals. The addition of boron was observed to suppress Si crystallization, though the boron concentration was found to have little effect on the QD size. Reductions in film resistivity were observed with the increase in boron concentration, which is believed to be a consequence of an increase in carrier concentration. This is supported by a large decrease in the activation energy accompanying the drop in resistivity, consistent with the Fermi energy moving towards the valence bands. The photoluminescence (PL) intensity was found to decrease with increase in boron concentration.     

Multi-stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs

We report GaAs-based quantum dot (QD) solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a highly stacked and well-aligned InGaAs QD structure of over 30 layers without using a strain compensation technique by the intermittent deposition of InGaAs layers. Moreover, there was no degradation in crystal quality. The external quantum efficiency of multi-stacked InGaAs QD solar cells extends the photo-absorption spectra toward a wavelength longer than the GaAs band gap, and the quantum efficiency increases as the number of stacking layers increases. The performance of the QD solar cells indicates that the novel InGaAs QDs facilitate the fabrication of highly stacked QD layers that are suitable for solar cell devices requiring thick QD layers for sufficient light absorption.     

Tunnel current through a miniband in InGaAs quantum dot superlattice solar cells

We report the tunnel current through a miniband in In_0.4Ga_0.6As quantum dot (QD) superlattice solar cells fabricated using molecular beam epitaxy. High-quality and well-aligned In_0.4Ga_0.6As QD superlattice structures with an interdot spacing of 3.5 nm were grown without using a strain balancing technique. 10-stack In_0.4Ga_0.6As QD superlattice solar cells had a high open circuit voltage and good cell characteristics even when the interdot spacing was reduced to 3.5 nm. Moreover, a short-circuit current density increases as the interdot spacing decreases. From the temperature dependence of the external quantum efficiency for QD solar cells with different interdot spacings, we observed the tunnel current through a miniband in QD superlattices with an interdot spacing of 3.5 nm.     

Fabrication of CdS quantum dot sensitized solar cells via a pressing route

CdS quantum dots have been self-assembled on the surface of dispersed nanocrystalline TiO₂ particles, and a light-harvesting electrode has been fabricated from the resulting sensitized P25 particles using the pressing route. The spectroscopic and photochemical properties of photosensitized nanocrystalline TiO₂ electrodes were studied. The results indicate that electrode preparation by the pressing route may lead to partial loss or damage of the CdS coating and creation of regions that are inaccessible to the redox electrolyte. Nevertheless, the pressing method using pre-coated powders shows promise as a low cost method for the preparation of photoelectrodes in sensitized-solar cells.     

Quantum dot integration in heterostructure solar cells

InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm² light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm², shows an open circuit voltage V_oc of 0.7 V, a short circuit current I_sc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).     

Quantum dot solar concentrators: Electrical conversion efficiencies and comparative concentrating factors of fabricated devices

A novel, non-tracking concentrator is described, which uses nano-scale quantum dot technology to render the concept of a fluorescent dye solar concentrator (FSC) a practical proposition. The quantum dot solar concentrator (QDSC) comprises quantum dots (QDs) seeded in materials such as plastics and glasses that are suitable for incorporation into building façades. Photovoltaic (PV) cells attached to the edges convert direct and diffuse solar energy collected into electricity for use in the building. Small scale QDSC devices were fabricated. Devices have been characterised to determine current, voltage and power readings. Electrical conversion efficiencies, fill factors and comparative concentrating factors are reported.     

Efficient polysulfide electrolyte for CdS quantum dot-sensitized solar cells

A polysulfide electrolyte considering simultaneously the penetration of the electrolyte in a mesoscopic TiO₂ film and the ion dissociation in the solution is developed for application in a CdS-sensitized solar cell (CdS-DSSC). A methanol/water (7:3 by volume) solution was found to be a good solvent for fitting the requirement mentioned above. The optimal composition of the electrolyte, based on the performance of the CdS-DSSCs, was found to contain 0.5 M Na₂S, 2 M S, and 0.2 M KCl. By using a photoelectrode prepared after 4 cycles of chemical bath deposition, FTO/TiO2/CdS-4, the efficiency of the CdS-DSSC obtained for this polysulfide electrolyte is 1.15% under the illumination of 100% sun (AM1.5, 100 mW cm⁻²). This efficiency is less than that obtained using I⁻/I₃⁻ redox couple (1.84%), mainly caused from the smaller values of fill factor and open circuit potential. However, the CdS sensitizer is stable and, furthermore, a much higher short circuit current and IPCE (80%) are obtained by using the polysulfide electrolyte.     

New Hamiltonian for a better understanding of the quantum dot intermediate band solar cells

The quantum dot intermediate band solar cell has the potential for very high conversion efficiency. However, the cells manufactured so far show efficiencies below the expectations mainly because the sub-bandgap photocurrent associated to the quantum dots is too low and because of a substantial reduction of the voltage. We present a new Hamiltonian for the use with the k·p method with low computing power demands. With it, we show here the fundamentals that explain the low light absorption coefficient and, consequently, the low photocurrent observed. We also prove that the bandgap of the host material, GaAs in our case, is reduced by the introduction of the quantum dots, which also explains the voltage reduction. The model is justified by the agreement with internal quantum efficiency measurements. It opens the path for improvement and suggests changes for increasing the photocurrent and for the compensation of the voltage reduction.     

CdSe quantum dot-single wall carbon nanotube complexes for polymeric solar cells

The development of lightweight, flexible polymeric solar cells which utilize nanostructured materials has been investigated. Incorporation of quantum dots (QDs) and single wall carbon nanotubes (SWNTs) into a poly(3-octylthiophene)-(P3OT) composite, has been shown to facilitate exciton dissociation and carrier transport in a properly structured device. Optimization towards an ideal electron acceptor for polymeric solar cells that exhibits high electron affinity and high electrical conductivity has been proposed in the form of QD-SWNT complexes. Specifically, the synthesis of CdSe-aminoethanethiol-SWNT complexes has been performed, with confirmation by microscopy (SEM, TEM, and AFM) and spectroscopy (FT-IR and optical absorption). Polymer composites containing these complexes in P3OT have been used to fabricate solar cells which show limited efficiency due to recombination and surface effects, but an open-circuit voltage (V_OC) of 0.75 V. However, evaluation of the optical absorption spectra for these nanomaterial-polymeric composites has shown a marked enhancement in the ability to capture the available irradiance of the air mass zero (AM0) spectrum.     

A nickel-complex sensitiser for dye-sensitised solar cells

We report the first example of a Ni(II) complex that demonstrates sensitiser function in a Dye-Sensitised Solar Cell (DSSC). Complexes [Ni(dcbpy)(qdt)] (1), [Ni(decbpy)(qdt)] (2) and [Ni(decbpy)Cl₂] (3) (where dcbpy = 4,4′-dicarboxy-2,2′-bipyridine; decbpy = 4,4′-di(CO₂Et)-2,2′-bipyridine; and qdt = quinoxaline-2,3-dithiolate) have been prepared. Characterisation was carried out using electrochemical, spectroscopic and computational techniques. Intensive visible transitions of 1 and 2 have been assigned predominantly to Ligand-to-Ligand Charge Transfer (LLCT) from the qdt to the diimine ligand, suggesting appropriate charge separation for application in a photoelectrochemical device. TiO₂ sensitised with 2, following charge injection, processes a recombination time significantly long for photovoltaic function. In a DSSC, using redox electrolyte, photocurrents and photovoltages of 0.293 mA and 521 mV were observed, with optimum values requiring TiCl₄ post-treatment of TiO₂ and co-adsorption of Chenodeoxycholic acid (Cheno). Although photovoltaic function was observed, the low photocurrent is attributed to a short-lived excited state lifetime resulting in poor charge injection from the Ni(II) sensitiser.     

CdS量子点敏化TiO2纳米线束阵列太阳能电池的研究

采用水热合成技术,以盐酸、去离子水和酞酸丁酯为反应前驱物,在透明导电玻璃衬底(FTO)上生长TiO2纳米线束阵列,以化学浴沉积技术制备CdS量子点敏化TiO2纳米线束阵列光阳极。研究了CdS量子点敏化的循环周期对太阳能电池的光伏性能、单色光光子-电子转换效率、静态和动态光电流的特性的影响规律。结果表明:CdS量子点的大小和密度随着敏化循环周期的增加而增加,当敏化循环的周期为15次时,单色光光子-电子转换效率最高,电池的短路电流密度为0.61 mA/cm2,开路电压为0.65 V,填充因子为0.50,光电转换效率为0.20%。通过强度调制的光电流谱分析,得到光生电子在光阳极中的扩散系数为3.2×10-6cm2/s,传输时间为2.1×10-2s。     

Solution-processed bulk heterojunction organic solar cells with high polarity small molecule sensitizer

A new sensitizer molecule, HMBI (9,18-(di-2-hexyldecyl)-2,11-dimethoxy-9,18-dihydrobenzo[5,6]-s-indaceno[1,2-b]indeno[2,1-h]carbazol-6,15-dione), containing electron-donating carbazole and electron-accepting diketone units, has been synthesized for solution-processed bulk heterojunction organic solar cells. The HMBI material has good solubility in common organic solvents. Its HOMO and LUMO energy levels were found to be at 5.6 and 3.0 eV, respectively. It has absorption bands ranging from 300 to 500 nm. Dispersion of HMBI molecules in the P3HT/PCBM blend broadens the absorption bands over the spectral range of 350-500 nm. Uniform thin film devices doped with varying concentration of HMBI, incorporated within the P3HT/PCBM blend, were fabricated. The 3 wt% of HMBI doping produces an improvement in power-conversion efficiency (PCE) up to 11.5% compared with the reference P3HT/PCBM device. Efficient light harvesting caused by HMBI sensitizer molecules primarily yields increased carrier generation and short-circuit current. In addition, some morphological improvements in the P3HT/PCBM system may contribute to the generation of enhanced photocurrent.     

利用半导体量子点的高效太阳电池

1前言 第三代太阳电池的开发 太阳电池，主要是由做在半导体基片上的pn结组成。如图1所示若在pn结处射入太阳光，则在半导体内部激发出电子-空穴对，在内部电场作用下，电子向n侧。空穴向p侧迅速流动，产生光生电功率。现在广泛使用的是厚度200～350μm的单晶硅，及用铸造法制作的多晶硅太阳电池。最近为了降低硅太阳电池的制造成本，开发了薄膜太阳电池。在薄膜太阳电池的情形下由于光吸收层的厚度仅为0．2～3μm左右，使用的材料大大减少。现在还在开发非晶硅、微结晶硅、CdTe、Cu（InGa）、Se2（CIGS）等太阳电池。     

Ag2Se quantum-dot sensitized solar cells for full solar spectrum light harvesting

We report on the photovoltaic performance of Ag₂Se quantum-dot (QD) sensitized solar cells. The QDs are grown by the successive ionic layer adsorption and reaction process. The external quantum efficiency (EQE) spectrum of the assembled cells covers the entire solar power spectrum of 350-2500 nm with an average EQE of ∼80% in the short-wavelength region (350-800 nm) and 56% over the entire solar spectrum. The effective photovoltaic range is ∼7-14 times broader than that of the cadmium calcogenide system—CdS and CdSe. The photocurrent that Ag₂Se generates is four times higher than that of N3 dye. The best solar cell yields power conversion efficiencies of 1.76% and 3.12% under 99.4% and 9.7% sun, respectively. The results show that Ag₂Se QDs can be used as a highly efficient broadband sensitizer for solar cells.     

Modeling and simulation of solar cells quantum well based on SiGe/Si

In recent years, the development of quantum well solar cells QWSCs (Quantum Well Solar Cells) has generated a great deal of interest. These configurations have shown good promise to optimize the low conversion efficiency of conventional solar cells because of the high rate of absorption losses present in them. In this work, we are interested in modeling and simulation of two different structures of solar cells, a simple solar cell based on silicon Si and a quantum well solar cell SiGe/Si. When a solar cell is compared to 80 quantum well layers of Si_0.8Ge_0.2with a pin solar cell based on Si. The short circuit current J_sc increases from 23.55 to 37.48 mA/cm² with a relative increase of 59.15% found. In addition, the limit of the absorption band of the lower energy photons extends from 1100 nm to 2000 nm.     

Improving the optical efficiency and concentration of a single-plate quantum dot solar concentrator using near infra-red emitting quantum dots

Low luminescent quantum yields and large overlap between quantum dot (QD) emission and absorption spectra of present commercially-available visible-emitting QDs have led to low optical efficiencies for single-plate quantum dot solar concentrators (QDSCs). It is shown that using near infra-red (NIR) emitting QDs, re-absorption of QD emitted photons can be reduced greatly, thereby diminishing escape cone losses thus improving optical efficiencies and concentration ratios. Using Monte-Carlo ray-trace modelling, escape cone losses are quantified for different types of QD. A minimum 25% escape cone loss would be expected for a plate with refractive index of 1.5 containing QDs with no spectral overlap. It is shown that escape cone losses account for ∼57% of incident photons absorbed in QDSCs containing commercially-available visible-emitting QDs.     

Fabrication and full characterization of state-of-the-art quantum dot luminescent solar concentrators

The fabrication and full characterization of luminescent solar concentrators (LSCs) comprising CdSe core/multishell quantum dots (QDs) is reported. TEM analysis shows that the QDs are well dispersed in the acrylic medium while maintaining a high quantum yield of 45%, resulting in highly transparent and luminescent polymer plates. A detailed optical analysis of the QD-LSCs including absorption, emission, and time-resolved fluorescence measurements is presented. Both silicon and GaAs solar cells attached to the side of the QD-LSCs are used to measure the external quantum efficiency and power conversion efficiency (2.8%) of the devices. Stability tests show only a minor decrease of 4% in photocurrent upon an equivalent of three months outdoor illumination. The optical data are used as input for a ray-trace model that is shown to describe the properties of the QD-LSCs well. The model was then used to extrapolate the properties of the small test devices to predict the power conversion efficiency of a 50×50 cm² module with a variety of different solar cells. The work described here gives a detailed insight into the promise of QD-based LSCs.     

Fullerene photoelectrochemical solar cells

The photoelectrochemistry of single crystal C₆₀ and fullerene photoelectrochemical solar cells is studied. Illuminated and immersed, C₆₀ is shown to drive oxidation of several solution-phase redox couples. Utilization of a photoelectrochemical solid/liquid junction, rather than solid-state photovoltaic junction, improves the observed photocurrent. Utilization of a single crystal, rather than a polycrystalline film, of C₆₀ decreases dark current to the extent that light-driven charge transfer dominates. The spectral response and current-voltage behaviour in several electrolytes is studied. A low-power fullerene photoelectrochemical solar cell, utilizing a regenerative polyiodide and ferri/ferrocyanide redox couple, is demonstrated.     

Graphene quantum dot reinforced hyperbranched polyamide proton exchange membrane for direct methanol fuel cell

The hyperbranched macromolecule (HBM) polyamide proton exchange membrane with uniform 3D matrix topology is beneficial to the enhancement of proton conductivity. In order to extend the application of HBM in direct methanol fuel cell (DMFC), graphene oxide (GO) and graphene quantum dot (GQD) are incorporated into HBM to enhance the proton/methanol selectivity of the membrane. The functional groups on GO and GQD surface would interact with –SO₃H groups in HBM by the hydrogen-bond interaction and participate in the proton conductive channel construction. And the GO and GQD in the composite can effectively prevent the permeation of methanol molecules. Most important, the HBM membrane filled by GQD (GQD-HBM) can effectively avoid large scale phase separation which occurs in HBM membrane filled by GO (GO-HBM) due to the greater size of GO. Proton conductivity of GQD-HBM (0.30 S/cm) is 17% improved compared with the pristine HBM while methanol permeability is significantly reduced by ca. 50% due to the physical barrier of GQD. The proton/methanol selectivity is therefore enhanced by more than 80% and the DMFC performance is also significantly enhanced.