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.     

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.     

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.     

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.     

Investigated optical studies of Si quantum dot

Further study of the quantum dot potential for Si is presented. This potential has been calculated by means of our recent empirical model. The indirect energy gap (Γ-X) is calculated using the full potential-linearized augmented plane wave (FP-LAPW) method. The Engel-Vosko generalized gradient approximation (EV-GGA) formalism is used to optimize the corresponding potential for energetic transition and optical properties calculations of Si. The refractive index and transverse effective charge are predicted as a function of dot diameter that is in turn used to test the validity of our model. The obtained results show a reasonable agreement in comparison with experimental data and theoretical results.     

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.     

The use of the self-calibration method for accurate determination of silicon solar cell internal quantum efficiency

The application of the self-calibration method as a means of increasing the accuracy of spectral response, SR, and internal quantum efficiency, Q(λ), measurements is discussed. The principle of the method is the precise calculation of Q(λ_m) of a test cell for light at λ_m≈0.8 μm, where the response is weakly dependent on the emitter and base parameters. The ratio of the calculated and measured Q(λ_m) values gives the corresponding factor for shifting the experimental spectral response curve. The sequence of calculations is described, and an algorithm of the necessary operations for a computer is developed. Several examples of the use of the self-calibration method for correction of SR measurements of solar cells with low shunt resistance demonstrate its very high effectiveness. The corrected Q(λ) values follow the respective actual data with very high accuracy even when the measured SR is decreased by factor 2–3 due to low shunt resistance of the solar cell.     

Investigation of Cu metallization for Si solar cells

For application of copper metallization to silicon solar cells, electrical resistivity of the electroplated Cu was investigated for different annealing conditions: the rapid thermal annealing (RTA) and the vacuum annealing at various temperatures. The characteristics of Ti as the diffusion barrier were also observed. The specific contact resistance between Si and Ti/Cu was measured using Kelvin test pattern. For 8-min electroplated sample, the lowest resistivity of 2.1 μΩ cm was obtained at 300°C RTA condition. For Cu with Ti barrier, 400°C 2 min vacuum-annealed sample showed etch pits whereas 400°C RTA showed no etch pits. A vacuum annealing at 450°C for 30 min reduced the specific contact resistance to 7.2×10⁻⁶ Ω cm².     

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.     

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.     

Rapid thermal technologies for high-efficiency silicon solar cells

This paper shows that rapidly formed emitters in less than 6 min in the hot zone of a conveyor belt furnace or in 3 min in an rapid thermal processing (RTP) system, in conjunction with a screen-printed (SP) RTP Al-BSF and passivating oxide formed simultaneously in 2 min can produce very simple high-efficiency n⁺-p-p⁺ cells with no surface texturing, point contacts, or selective emitter. It is shown for the first time that an 80 Ω/□ emitter and SP Al-back surface field (BSF) formed in a high throughput belt furnace produced 19% FZ cells and greater than 17% CZ cells with photolithography (PL) contacts. Using PL contacts, we also achieved 19% efficient cells on FZ, >18% on MCZ, and 17% boron-doped CZ by emitter and SP Al-BSF formation in <10 min in a single wafer RTP system. Finally, manufacturable cells with 45 Ω/□ emitter and SP Al-BSF and Ag contacts formed in the conveyor belt furnace gave 17% efficient cells on FZ silicon. Compared to the PL cells, the SP cell gave 2% lower efficiency along with a decrease in J_sc and fill factor. This loss in performance is attributed to a combination of the poor blue response, higher series resistance and higher contact shading in the SP devices     

Cost effective process for high-efficiency solar cells

A new method for patterning the rear passivation layers of high-efficiency solar cells with a mechanical scriber has been developed and successfully adapted to fabricate high-efficiency passivated emitter and rear cell (PERC). Three types of the rear contact patterns: dot patterns with a photolithography process, line and dashed line patterns with a mechanical scriber process have been processed in order to optimize the rear contact structure. An efficiency of 19.42% has been achieved on the mechanical-scribed (MS)-PERC solar cell on 0.5 Ω cm p-type FZ-Si wafer and is comparable to that of conventional PERC solar cells fabricated by using photolithography process. The mechanical scriber process shows great potential for commercial applications by achieving high efficiency above 20% and by significantly reducing the fabrication costs without an expensive photolithography process. Low-cost Ni/Cu metal contact has been formed by using a low-cost electroless and electroplating. Nickel silicide formation at the interface enhances stability and reduces the contact resistance resulting in an energy conversion efficiency of 20.2% on 0.5 Ω cm FZ wafer.     

Ribbon Si solar cells with efficiencies over 18% by hydrogenation of defects

We have fabricated 4 cm² solar cells on String Ribbon Si wafers and edge-defined film-fed grown (EFG) Si wafers with using a combination of laboratory and industrial processes. The highest efficiency on String Ribbon Si wafer is 17.8% with an open circuit voltage (V_oc) of 620 mV, a short circuit current density (J_sc) of 36.8 mA/cm² and a fill factor (FF) of 0.78. The maximum efficiency on EFG Si is 18.2% with a V_oc of 620 mV, a J_sc of 37.5 mA/cm² and a FF of 0.78. These are the most efficient ribbon Si devices made to date, demonstrating the high quality of the processed Si ribbon and its potential for industrial cells. Co-firing of SiN_x and Al by rapid thermal processing was used to boost the minority carrier lifetime of bulk Si from 3–5 μs to 70–100 μs. Photolithography-defined front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The effects of firing temperature and time were studied to understand the trade-off between hydrogen retention and Al-doped back surface field (Al-BSF) formation. Excellent bulk defect hydrogenation and high-quality thick Al-BSF formation was achieved in a very short time (1 s) at firing temperatures of 740–750 °C. It was found that the bulk lifetime decreases at annealing temperatures above 750 °C or annealing time above 1 s due to dissociation of hydrogenated defects.     

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.     

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.     

Didactic software for solar cells and materials parameters analysis

The authors present a package of computer applications gathered in a form of virtual laboratory as a package of the virtual exercises to solar cells parameters analysis at a student university laboratory. The applications may also be used to analyze some aspects of silicon materials or solar cell structures as a research tool. The package consists of three blocks of exercises. Each of the blocks solves different tasks and is based on different software platforms (LabWindows, Visual Basic, Java and Flash). The range of the tasks is relatively wide: from analysis of the role of parameters of the material (silicon) on the I–V and spectral curves of a solar cell up to simulation of the method of solar quality silicon parameter testing—evaluation of a value of the minority carriers diffusion length from surface photovoltage spectra.     

Classification of shunting mechanisms in crystalline silicon solar cells

The efficiency of a solar cell is given by its average electrical parameters. On inhomogeneous materials and especially on large-area solar cells the inhomogeneity of the short circuit current, the open circuit voltage and the fill factor are important factors to reach high and stable efficiencies and may limit the overall performance of the device.A locally increased dark forward current (shunt) reduces the fill factor and the open circuit voltage of the whole cell. The inhomogeneity of the forward current in a solar cell can be measured using lock-in thermography. The quantitative and voltage-dependent evaluation of these thermographic investigations of various solar cell types on mono- or multi-crystalline silicon enables the classification of the different shunting mechanisms found. By further microscopic investigations the physical reasons for the increased dark forward currents can be determined.It turns out that a high density of crystallographic defects like dislocation tangles or microdefects can be responsible for an increased dark forward current. Unexpectedly, grain boundaries in solar cells on multicrystalline silicon do not show any measurable influence on the local dark forward current. In most cases shunts caused by process-induced defects are dominating the current–voltage characteristic at the maximum power point of the solar cell. In commercial solar cells shunts at the edges are most important, followed by shunts beyond the grid lines.