Colloidal quantum dot solar cells

In recent years colloidal quantum dots solar cells have been the subject of extensive research. A promising alternative to existing silicon solar cells, quantum dot solar cells are among the candidates for next generation photovoltaic devices. Colloidal quantum dots are attractive in photovoltaics research due to their solution processability which is useful for their integration into various solar cells. Here, we review the recent progresses in various quantum dot solar cells which are prepared from colloidal quantum dots. We discuss the preparation methods, working concepts, advantages and disadvantages of different device architectures. Major topics discussed in this review include integration of colloidal quantum dots in: Schottky solar cells, depleted heterojunction solar cells, extremely thin absorber solar cells, hybrid organic-inorganic solar cells, bulk heterojunction solar cells and quantum dot sensitized solar cells. The review is organized according to the working principle and the architecture of photovoltaic devices.     

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。     

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.     

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.     

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.     

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.     

A theoretical approach on the strain-induced dislocation effects in the quantum dot solar cells

The numbers of the quantum dot layers that can be embedded in the active region of the quantum dot intermediate band solar cells affects on the photocurrent and also can produce strain-induced dislocations in the cell. To enhance the absorption of the low energy photons in the system, the number of the quantum dot layers needs to be increased, but in this way, dislocations and defects of the cell non-radiative recombination will also increase. In this paper, the characteristics of intermediate band solar cells containing 10, 20, and 50 InAs quantum dot layers embedded in the active region of the cells have been considered and compared. There are an optimum number of quantum dot layers for significant absorption of low energy photons. Furthermore, for a cell with 10 QD layers, the current–voltage characteristics and internal quantum efficiency have been investigated for different values of minority carriers recombination lifetimes (or diffusion lengths) and electron filling factors. Electron filling factor, gives a design constraints for the size of the quantum dots and distance between the layers. The results showed that the perfect cells need to be considered from two aspects; first, from the optimum number of the quantum dot layers to control the strain-induced dislocations that produce non-radiative recombinations and reduce the photocurrent and second, the dots spacing and size that need to be justified for wavefunction penetration into barrier region that reduces the non-radiative recombinations.     

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.     

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).     

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.     

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。     

Internal quantum efficiency for solar cells

The total internal quantum efficiency (IQE) of a flat-band p–n homojunction silicon solar cell and contributions of the three regions to it are numerically evaluated. It is found that both the spatial widths of the cell and the surface recombination velocities have significant impacts on the IQEs. By a linear transformation and a proper approximation, the differential equation of the minority carrier density in a textured cell becomes the same form as for the flat cell. What makes differences is that texturization slightly enhances the IQEs for photons with longer wavelengths while notably increasing external quantum efficiency. Hence it plays a good role for getting a better performance of a solar cell. It is considered that the results in the present are of universal technical importance both in designing solar cells and their surface structures.     

On quantum efficiency of nonideal solar cells

A simple approach for the calculation of the width of the space charge region (and consequently the concentration of uncompensated acceptors), which takes into account the effect of the series resistance on the quantum efficiency of anisotype asymmetrical thin film heterojunction solar cells (on the example of CdS/CdTe solar cells), was proposed. The effect of the light dependent series resistance and current transport mechanism was also taken into consideration.     

Advances in Bragg stack quantum well solar cells

Strain-balanced quantum well solar cells (SB-QWSC) extend the photon absorption edge beyond that of bulk GaAs by incorporation of quantum wells in the i-region of a p–i–n device. The addition of a distributed Bragg reflector (DBR) can substantially increase the photocurrent with little or no detriment to the dark-current. Experimental results are presented that show improvements of DBR cell efficiencies over SB-QWSC's without DBR's. In addition, at high dark-current levels appropriate to high concentration, we observe that the dark-currents of the SB-QWSC's exhibit ideal diode behaviour. We present evidence that the ideality n=1 dark-current is reduced in the DBR cells and discuss the possible efficiency improvements if the dark-current is radiatively dominant.     

Temperature dependence of photocurrent components on enhanced performance GaAs/AlGaAs multiple quantum well solar cells

The performance of Al_0.36Ga_0.64As p/i/n solar cells with multiple quantum wells (MQW) of GaAs/Al_0.36Ga_0.64As in the i-region has been investigated at various temperatures, ranging from −10°C to 100°C, and compared with that of conventional solar cells composed of either the quantum well material (GaAs) or the barrier material (Al_0.36Ga_0.64As) alone. The dark currents of the MQW cells were found to lie between those of the conventional cells. The increase of dark current with temperature was accompanied by a slight decrease of the diode ideality factor. A linear dependence of open-circuit voltage (V_oc) on temperature was observed for all cells when illuminated with a 100W halogen lamp. V_oc for the MQW cells was found to be independent of the number of wells, lying between the V_oc's for the two conventional cells. The MQW cells exhibited performance improvement with temperature when compared to the conventional cells and there was a significant enhancement in the short-circuit current with temperature of those MQW cells that exhibited poorer performance at lower temperatures. Theoretical calculations have quantified the contribution of the tunneling current component to the total observed photocurrent at the various temperatures examined. It was found that tunneling currents are present at all temperatures and can be the dominant component in MQW cells of thinner wells at low temperatures. These results suggest that GaAs/Al_0.36Ga_0.64As MQW structures, of good-quality material, when processed as conventional solar cells with antireflective coatings should deliver more output power under intense illumination than conventional solar cells composed of the quantum well material alone.     

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.     

Characteristic features of the energy spectrum and multiple exciton generation in nanoparticles of PbSe and PbS

This paper considers the formation of molecular terms of lead selenide and lead sulphide and shows a correlation between the molecular term structure and maximum quantum yield of a solar energy converter in the case of the multiexciton generation effect.     

Linker-free quantum dot sensitized TiO2 photoelectrochemical cells

Aerosol based techniques were used to characterize and deposit quantum dots (QDs). Using an electrospray-assisted characterization technique, the mobility diameter of CdSe QDs was successfully measured in real-time. The electrospray technique was also used to deposit CdSe QDs onto nanostructured TiO₂ films. Compared to conventional methodologies such as dip coating with linker-containing molecules or chemical bath deposition, an electrospray system enabled uniform deposition of QDs over the nanostructured TiO₂ surface in a short processing time. As-deposited films were annealed to enhance binding between the QDs and TiO₂ surface. These QD-decorated TiO₂ films were used in photoelectrochemical cells, for which the photoenergy conversion efficiencies were tested. Optimization of the deposition time of the QDs resulted in increased efficiencies. Multiple layers of QDs caused a decrease in energy-conversion efficiency, likely due to inhibition of the transportation of photogenerated electrons into the TiO₂ structure. The energy-conversion efficiency trends were supported by time-resolved photoluminescence decay data.     

The application of quantum well solar cells to thermophotovoltaics

We discuss the advantages of quantum well solar cells (QWSCs) for thermophotovoltaic (TPV) applications and illustrate them with InP/InGaAs and GaInAsP/InGaAs QWSCs which were designed for other applications and have not been optimised for TPV. It is shown that an InP p-i-n solar cell with 15 lattice matched InGaAs quantum wells (QWs) in the i region has an increase in open circuit voltage (V_oc) of (1.7 ± 0.1) times that of a control cell of InP with InGaAs in the i-region under an illuminating spectrum close to that expected from an ideal ytterbia emitter. Also, using an InGaAsP quaternary cell of band gap wavelength of 1.1 Am with 60 InGaAs QWs under the same illuminating spectrum the current density is increased by a factor of (2.4 ± 0.1) over that of the InP QWSC. The quaternary cell also absorbs longer wavelengths without any significant loss in V_OC. Better temperature coefficients for the former quantum well solar cell than the control cell are observed in a spectrum approximating a black body at 3000 K. Further advantages of QWs for narrow band and broad band illuminating spectra are discussed.