Singlet exciton fission in nanostructured organic solar cells

Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60). Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60). We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.     

量子点太阳电池的探索

阐述了探索量子点太阳电池的重要意义与物理构想,简要介绍了两种不同结构组态的量子点太阳电池的光伏性能,如p-i-n量子点太阳电池和量子点敏化太阳电池.对发生在各种量子点(PbSe、PbS、PbTe、CdSe和Si)中的因碰撞电离而导致的多激子产生效应及其研究进展进行了重点评述,并提出了设计与制作量子点太阳电池的若干技术对策.可以预期,具有超高能量转换效率、低制作成本与高可靠性的量子点太阳电池的实现,有可能对未来的光伏技术与产业产生革命性的影响.     

Silicon quantum dot/crystalline silicon solar cells

Silicon (Si) quantum dot (QD) materials have been proposed for 'all-silicon' tandem solar cells. In this study, solar cells consisting of phosphorus-doped Si QDs in a SiO(2) matrix deposited on p-type crystalline Si substrates (c-Si) were fabricated. The Si QDs were formed by alternate deposition of SiO(2) and silicon-rich SiO(x) with magnetron co-sputtering, followed by high-temperature annealing. Current tunnelling through the QD layer was observed from the solar cells with a dot spacing of 2?nm or less. To get the required current densities through the devices, the dot spacing in the SiO(2) matrix had to be 2?nm or less. The open-circuit voltage was found to increase proportionally with reductions in QD size, which may relate to a bandgap widening effect in Si QDs or an improved heterojunction field allowing a greater split of the Fermi levels in the Si substrate. Successful fabrication of (n-type) Si QD/(p-type) c-Si photovoltaic devices is an encouraging step towards the realization of all-silicon tandem solar cells based on Si QD materials.     

A lossless negative dielectric constant from quantum dot exciton polaritons

Prospects for a lossless negative dielectric constant material for optical devices are studied. Simulations show that with sufficient gain, a mixture of two semiconductor quantum dots (QDs) can produce an effective dielectric constant that is lossless and negative. This permits, in concept, arbitrarily small scaling of the optical mode volume, a major goal in the field of nanophotonics. The proposed implementation of a lossless negative dielectric constant material based on colloidal QDs opens a tractable path.     

Quantum dot size dependent J-V characteristics in heterojunction ZnO/PbS quantum dot solar cells

The current-voltage (J-V) characteristics of ZnO/PbS quantum dot (QD) solar cells show a QD size-dependent behavior resulting from a Schottky junction that forms at the back metal electrode opposing the desirable diode formed between the ZnO and PbS QD layers. We study a QD size-dependent roll-over effect that refers to the saturation of photocurrent in forward bias and crossover effect which occurs when the light and dark J-V curves intersect. We model the J-V characteristics with a main diode formed between the n-type ZnO nanocrystal (NC) layer and p-type PbS QD layer in series with a leaky Schottky-diode formed between PbS QD layer and metal contact. We show how the characteristics of the two diodes depend on QD size, metal work function, and PbS QD layer thickness, and we discuss how the presence of the back diode complicates finding an optimal layer thickness. Finally, we present Kelvin probe measurements to determine the Fermi level of the QD layers and discuss band alignment, Fermi-level pinning, and the V(oc) within these devices.     

Solution-processed PbS quantum dot infrared photodetectors and photovoltaics

In contrast to traditional semiconductors, conjugated polymers provide ease of processing, low cost, physical flexibility and large area coverage. These active optoelectronic materials produce and harvest light efficiently in the visible spectrum. The same functions are required in the infrared for telecommunications (1,300-1,600 nm), thermal imaging (1,500 nm and beyond), biological imaging (transparent tissue windows at 800 nm and 1,100 nm), thermal photovoltaics (>1,900 nm), and solar cells (800-2,000 nm). Photoconductive polymer devices have yet to demonstrate sensitivity beyond approximately 800 nm (refs 2,3). Sensitizing conjugated polymers with infrared-active nanocrystal quantum dots provides a spectrally tunable means of accessing the infrared while maintaining the advantageous properties of polymers. Here we use such a nanocomposite approach in which PbS nanocrystals tuned by the quantum size effect sensitize the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) into the infrared. We achieve, in a solution-processed device and with sensitivity far beyond 800 nm, harvesting of infrared-photogenerated carriers and the demonstration of an infrared photovoltaic effect. We also make use of the wavelength tunability afforded by the nanocrystals to show photocurrent spectra tailored to three different regions of the infrared spectrum.     

Different hierarchical nanostructured carbons as counter electrodes for CdS quantum dot solar cells

CdS quantum dot sensitized solar cells based on TiO(2) photoanode and nanostructured carbon as well as Pt as counter electrodes using iodide/triiodide and polysulfide electrolytes were fabricated to improve the efficiency and reduce the cost of solar cells. Compared with conventional Pt (η = 1.05%) and CMK-3 (η = 0.67%) counter electrodes, hollow core-mesoporous shell carbon (HCMSC) counter electrode using polysulfide electrolyte exhibits much larger incident photon to current conversion efficiency (IPCE = 27%), photocurrent density (J(sc) = 4.31 mA.cm(-2)) and power conversion efficiency (η = 1.08%), which is basically due to superb structural characters of HCMSC such as large specific surface area, high mesoporous volume, and 3D interconnected well-developed hierarchical porosity network, which facilitate fast mass transfer with less resistance and enable HCMSC to have highly enhanced catalytic activity toward the reduction of electrolyte shuttle.     

The transitional heterojunction behavior of PbS/ZnO colloidal quantum dot solar cells

The nature of charge separation at the heterojunction interface of solution processed lead sulphide-zinc oxide colloidal quantum dot solar cells is investigated using impedance spectroscopy and external quantum efficiency measurements to examine the effect of varying the zinc oxide doping density. Without doping, the device behaves excitonically with no depletion region in the PbS layer such that only charge carriers generated within a diffusion length of the PbS/ZnO interface have a good probability of being harvested. After the ZnO is photodoped such that the doping density is near or greater than that of the PbS, a significant portion of the depletion region is found to lie within the PbS layer increasing charge extraction (p-n operation).     

TiO2 inverse opal based CdS/CdSe quantum dot co-sensitized solar cells

CdS and CdSe quantum dots were introduced as co-sensitizers into TiO₂ inverse opal quantum dot sensitized solar cells. Herein, the three-dimensionally ordered porous TiO₂ inverse opal film leads to a better infiltration of both sensitizers and hole transporting material, and the smaller surface area of TiO₂ inverse opal film is effectively offset by the incorporating of co-sensitization. It was found that the presence of CdS/CdSe co-sensitizers provides enhanced light absorption, and leads to a lower recombination rate of the electrons due to the stepwise structure of band edge in TiO₂/CdS/CdSe, which resulted in the observed enhanced photocurrent and energy conversion efficiency of the solar cells. A cell efficiency of 1.01 % has been attained.     

Efficient Mn-doped CdS quantum dot sensitized solar cells based on SnO2 microsphere photoelectrodes

Mn-doped CdS quantum dot sensitized solar cells based on SnO₂ microsphere photoelectrodes are prepared with successive ionic layer adsorption and reaction method. It is found that with Mn-doped CdS quantum dot sensitizers, the photovoltaic performance of the cells based on SnO₂ microsphere photoelectrodes can obviously be enhanced. The reasons are owing to the improved light absorption and the expanded light absorption edge by doping Mn in CdS quantum dots. The electrochemical impedance spectroscopy analysis found that the cells with Mn-doped CdS quantum dot sensitized SnO₂ microsphere photoelectrodes can efficiently suppress dark reaction, owing to the increased related resistance. Moreover, it is also found that the Mn-doped CdS quantum dot sensitized SnO₂ microsphere photoelectrode can increase the electron diffusion lifetime in the cell. The power conversion efficiency of the cell with 4 wt% Mn-doped CdS quantum dot sensitizers can attain to 2.80 %, with 53 % enhancement compared with that of the CdS quantum dot sensitized cell (1.83 %).     

CdS quantum dot sensitized nanocrystalline Gd-doped TiO2 thin films for photoelectrochemical solar cells

CdS quantum dot sensitized Gd-doped TiO₂ nanocrystalline thin films have been prepared by chemical method. X-ray diffraction analysis reveals that TiO₂ and Gd-doped TiO₂ nanocrystalline thin films are of anatase phase. The absorption spectra revealed that the absorption edge of CdS quantum dot sensitized Gd-doped TiO₂ thin films shifted towards longer wavelength side (red shift) when compared to that of CdS quantum dot sensitized TiO₂ films. CdS quantum dots with a size of 5 nm have been deposited onto Gd-doped TiO₂ film surface by successive ionic layer adsorption and reaction method and the assembly of CdS quantum dot with Gd-doped TiO₂ has been used as photo-electrode in quantum dot sensitized solar cells. CdS quantum dot sensitized Gd-doped TiO₂ based solar cell exhibited a power conversion efficiency of 1.18 %, which is higher than that of CdS quantum dot sensitized TiO₂ (0.91 %).     

Chemical synthesis of CdS onto TiO2 nanorods for quantum dot sensitized solar cells

A quantum dot sensitized solar cell (QDSSC) is fabricated using hydrothermally grown TiO₂ nanorods and successive ionic layer adsorption and reaction (SILAR) deposited CdS. Surface morphology of the TiO₂ films coated with different SILAR cycles of CdS is examined by Scanning Electron Microscopy which revealed aggregated CdS QDs coverage grow on increasing onto the TiO₂ nanorods with respect to cycle number. Under AM 1.5G illumination, we found the TiO₂/CdS QDSSC photoelectrode shows a power conversion efficiency of 1.75%, in an aqueous polysulfide electrolyte with short-circuit photocurrent density of 4.04 mA/cm² which is higher than that of a bare TiO₂ nanorods array.     

Anomalous independence of multiple exciton generation on different group IV-VI quantum dot architectures

Multiple exciton generation (MEG) in PbSe quantum dots (QDs), PbSe(x)S(1-x) alloy QDs, PbSe/PbS core/shell QDs, and PbSe/PbSe(y)S(1-y) core/alloy-shell QDs was studied with time-resolved optical pump and probe spectroscopy. The optical absorption exhibits a red-shift upon the introduction of a shell around a PbSe core, which increases with the thickness of the shell. According to electronic structure calculations this can be attributed to charge delocalization into the shell. Remarkably, the measured quantum yield of MEG, the hot exciton cooling rate, and the Auger recombination rate of biexcitons are similar for pure PbSe QDs and core/shell QDs with the same core size and varying shell thickness. The higher density of states in the alloy and core/shell QDs provide a faster exciton cooling channel that likely competes with the fast MEG process due to a higher biexciton density of states. Calculations reveal only a minor asymmetric delocalization of holes and electrons over the entire core/shell volume, which may partially explain why the Auger recombination rate does not depend on the presence of a shell.     

Dark current and noise analyses of quantum dot infrared photodetectors

Because the dark current and the noise of quantum dot infrared photodetectors (QDIPs) can bring about a degradation in their performance, they have attracted more and more attention in recent years. In this paper, an algorithm used to evaluate the dark current of the QDIP is proposed, which is based on the algorithm including the common contribution of the microscale and the nanoscale electron transport. Namely, by accounting for the dependence of the drift velocity on the applied electric field, we greatly enhance the accuracy of the dark current calculation compared with that in the previous algorithm. This proposed algorithm is further used to estimate the noise current of QDIP, and the calculated results show a good agreement with the published data.     

Influences of silicon doping in quantum dot layers on optical characteristics of InAs/GaAs quantum dot infrared photodetector

We have investigated the effects of silicon doping concentration within thirty-period self-assembled quantum dot (QD) layers on quantum dot infrared photodetectors (QDIPs). The lens-shaped quantum dots with the dot density of 1 × 10¹¹ cm^− 2 were observed by atomic force microscope (AFM). From the high ratio of photoluminescence (PL) peak intensities from dot layer to that from wetting layer, we have concluded that high dot density caused the short diffusion length for carriers to be easily captured by QDs. Moreover, the Si-doped samples exhibited the multi-state transitions within the quantum dots, which were different to the single level transition of undoped sample. Besides, the dominant PL peaks of Si-doped samples were red-shifted by about 25 meV compared to that of the undoped sample. It should result from the dopant-induced lowest transition state and therefore, the energy difference should be equal to the binding energy of Si in InAs QDs.     

n-Type transition metal oxide as a hole extraction layer in PbS quantum dot solar cells

The n-type transition metal oxides (TMO) consisting of molybdenum oxide (MoO(x)) and vanadium oxide (V(2)O(x)) are used as an efficient hole extraction layer (HEL) in heterojunction ZnO/PbS quantum dot solar cells (QDSC). A 4.4% NREL-certified device based on the MoO(x) HEL is reported with Al as the back contact material, representing a more than 65% efficiency improvement compared with the case of Au contacting the PbS quantum dot (QD) layer directly. We find the acting mechanism of the hole extraction layer to be a dipole formed at the MoO(x) and PbS interface enhancing band bending to allow efficient hole extraction from the valence band of the PbS layer by MoO(x). The carrier transport to the metal anode is likely enhanced through shallow gap states in the MoO(x) layer.     

Enhanced photovoltaic performance of quantum dot sensitized solar cells with Ag-doped TiO2 nanocrystalline thin films

Ag-doped titanium dioxide (TiO₂) nanocrystalline thin films have been prepared by the sol–gel dip coating method and used as photoanode to fabricate quantum dot sensitized solar cells. The X-ray diffraction studies reveal the formation of anatase phase without any impurity phase. The surface morphology studied using scanning electron microscope shows uniform distribution of particles. The optical band gap was found to be 3.5 and 3.4 eV for CdS quantum dot sensitized TiO₂ and CdS quantum dot sensitized Ag-doped TiO₂ thin film respectively. The Ag-doped TiO₂ based solar cell exhibited a power conversion efficiency of 1.48 % which is higher than that of TiO₂ (0.9 %).