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.     

Statistical theory of multiple exciton generation in quantum dot solar cells

The high efficiency of quantum dot solar cells is determined by the so-called multiple exciton generation (MEG) effect in quantum dots when a single photon is absorbed. The present work proposes a statistical approach to the process of simultaneous generation of many excitons in quantum dots when a high-energy photon is absorbed. This approach is based on the statistical Fermi approach to simultaneous generation of elementary particles (nucleons and π-mesons) in the nucleon-nucleon collision. Analysis of the results shows that the exciton quantum yields calculated using the statistical approach agree rather well with the experimental data.     

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.     

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.     

A new approach to modelling quantum dot concentrators

Luminescent collectors have advantages over geometric concentrators in that tracking is unnecessary and both direct and diffuse radiation can be collected. However, development has been limited by the performance of luminescent dyes. We have recently proposed a novel concentrator in which the dyes are replaced by quantum dots (QDs). Advantages over dyes include that the absorption threshold can be tuned by choice of dot diameter, and that the red shift between absorption and luminescence is related to the spread of dot sizes. In this paper we discuss how we have developed a self-consistent thermodynamic model for planar concentrators which allows for re-absorption by the QDs.     

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.     

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).     

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。     

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.     

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.     

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.     

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.     

A theoretical simulation of light scattering of nanocrystalline films in photoelectrochemical solar cells

A Monte Carlo simulation method of light scattering in nanocrystalline films based on solutions of Maxwell's equations is proposed. A nanocrystalline film is assumed to be superposition of randomly distributed nanoparticles and deviation of the nanocrystalline film from the randomness. Since a scattering field of the randomly and densely distributed nanoparticles can be neglected, a scattering field of the nanocrystalline film results in a sum of scattering fields of the deviant parts in the nanocrystalline film. In the method, configurations of the deviant parts are simulated with a random function of a computer language. A simulation converges in small number of the configuration patterns. The simulation theoretically demonstrates that almost all of the incident light to the nanocrystalline films in the photoelectrochemical solar cells penetrate without scattering.     

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.     

Modelling the quantum efficiency of cadmium telluride solar cells

A simple analytical model is presented describing the quantum efficiency of cadmium telluride (CdTe) solar cells. This model is based on a consistent set of parameters that were extracted from electrical and optical measurements. These measurements also reveal the CdTe solar cells to mainly rely on carrier generation as well as carrier collection within the space-charge region. Recombination in this part of the cell is hence taken into account. As a result, quantum efficiency spectra can be closely fitted by an expression that includes the lifetime of the minority carriers and the width of the space-charge region as free variables. The comparison of the calculated quantum efficiency curves with the experimental ones gives fundamental insight into the specific operation of CdTe solar cells.