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.     

Strained and strain-balanced quantum well devices for high-efficiency tandem solar cells

The state of GaAs/InGaAs quantum well solar cell research is reviewed. The effect of strain upon the GaAs/InGaAs cells is discussed and the limits to a strained GaAs/InGaAs cell established. The strain-balance approach is suggested as a means of overcoming the limits inherent to the strained approach and the principle is demonstrated in two differing device configurations. The strain-balance devices show enhanced efficiencies over their strained counterparts and in one case, comparable efficiency to a good GaAs control cell. The application of these cells to tandem structures is discussed, indicating the potential for a substantial efficiency enhancement.     

Modelling the light absorption in organic photovoltaic devices

Electromagnetic reflection, transmission and absorption properties are basically important for the optical characterization of multilayers used in optoelectronic and photovoltaic devices. They describe the interaction of incident light with the layers of the system. Depending on the thicknesses and optical constants of the individual layers, the interaction of a light source with a multilayer causes distinct distributions of the electric field and energy absorption density. Consequently the optical modelling of an organic bilayer photovoltaic device, in which the incident sunlight must be absorbed in a very narrow region near the active interface, has to take into account the influence of the optical parameters and the thicknesses of the device layers in order to gain optimal energy conversion. We focussed on the electrodynamic behavior of organic photovoltaic bilayer devices with varied layer thicknesses. We found a sensitive response of the maxima of the absorption density inside the solar cell to even fine changes in the thicknesses of the active layers. We also investigated the electrodynamic behavior of a photovoltaic device in dependence on the incident light wavelength. As a new and interesting result, our investigations showed a good correlation between measured photo current and calculated absorption density and no good correlation between measured photo current and calculated square of the electric field.     

Thermographic sampling technique applied to microelectronics and photovoltaic devices

We investigated the energy turnover due to current flow across microelectronic devices and the energy loss during the light into current conversion by solar cells by use of a sampling technique of a space resolved infrared thermographic video imaging system. The change of temperature is correlated with current induced heating or cooling. The sampling technique increases the signal to noise ratio, the sensitivity of the system and suppresses misleading temperature drifts and effects due to infrared reflection of the surrounding or inhomogeneous composition of the sample. This video technique provides space resolved information about the maximum power point and of the efficiencies of solar cells, and may also be applied to macroscopic power systems such as the entire fields of photovoltaic panels.     

Study on GaAs/GaAlAs MQW structure for photovoltaic applications

GaAs/GaAlAs MQW structures with varying well widths having narrowest wells on the top layers and gradually wider wells for the inner layers were studied and confirmed by Auger spectroscopy and photoluminescence measurements. Multiplication of quantum wells gives stronger photoluminescence due to higher density of quantized states in the structure. This MQW structure was experimented in the photoconductivity and photocurrent measurements at room temperature. It is found that at low multiplication of quantum wells in MQW structure, the photoconductivity effect was mainly controlled by bulk material as shown in the spectral response. Photocurrent at low bias voltage shows relatively better spectral sensitivity at shorter wavelength. The photoconductivity and photocurrent measurements indicate that appropriate MQW structures acting as broader absorbers due to graded characters of quantized energy states are needed for photovoltaic application. Integration of MQW structure to solar cell structure was also investigated.     

Properties of GaAs/InGaAs quantum well solar cells under low concentration operation

Multi-quantum well GaAs/In_0.19Ga_0.81As solar cells have been measured under low concentration levels (1–10 suns) of AM1.5 illumination. An efficiency of 22% has been obtained at a ratio of 4 suns as opposed to 18% under 1 sun AM1.5 conditions. We explain the improvements in conversion efficiency in terms of an enhancement in minority-carrier lifetime under concentration. Even when the concentration ratio is low, the high-injection regime can be achieved since the carrier concentration in the intrinsic layer is very low. The existence of a high concentration of defects at 0.36 eV below the conduction band in the base layer has been observed by the DLTS analysis. Enhancement of the minority-carrier lifetime under concentration is thought to be due to filling of recombination centers by the injection minority carriers.     

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.     

Self-calibration as a tool for high-accuracy determination of silicon solar cell internal quantum efficiency

Light nonuniformity, uncertainty in the illuminated photoactive area, and relative, but not absolute radiometric data for the reference detector, can be the reasons for the inaccuracy or impossibility of solar cell spectral response and quantum efficiency determination. The use of a self-calibration principle permits minimization of the errors caused by the above factors. This principle consists of quite precise calculation of the internal quantum efficiency Q(λ_m) of the test cell at λ_m≈0.8 μm, where the cell response is weakly dependent on emitter and base parameters. Experimentally determined short- and long-wavelength internal quantum efficiencies, Q(0.4) and Q(0.95), respectively, based on relative radiometric data for a reference detector, are used as starting data for the Q(λ_m) calculation. The ratio of the calculated to measured Q(λ_m) values gives the correction factor for shifting the experimental quantum efficiency curve. Computer modeling supports the assumption that uniform deviation of measured Q(λ) can be precisely corrected by calculation. Analysis of the accuracy of the self-calibration method demonstrates very small uncertainties in the corrections of quantum efficiency measurements, attainable for many practical situations. Confirmation of correctness of the proposed method is shown by analysis of the results of spectral response measurements of several solar cells.     

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.     

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.     

Structural, electrical and photovoltaic characterization of Si nanocrystals embedded SiC matrix and Si nanocrystals/c-Si heterojunction devices

Thin films of Si nanocrystals (Si NCs) embedded in a silicon carbide (SiC) matrix (Si-NC:SiC) were prepared by alternating deposition of Si-rich silicon carbide (Si_1−xC_x) and near-stoichiometric SiC mutilayers (Si_1−xC_x/SiC) using magnetron cosputtering followed by a post-deposition anneal. Transmission electron microscopy and Raman spectroscopy revealed that the Si NCs were clearly established, with sizes in the range of 3–5 nm. Optical studies showed an increase in the optical band gap after annealing from 1.4 eV (as-deposited) to 2.0 eV (annealed at 1100 °C). P-type Si-NC:SiC/n-type crystalline silicon (c-Si) heterojunction (HJ) devices were fabricated and their electrical and photovoltaic properties were characterized. The diode showed a good rectification ratio of 1.0×10⁴ at the bias voltage of ±1.0 V at 298 K. The diode ideality factor and junction built-in potential deduced from current–voltage and capacitance–voltage plots are 1.24 and 0.72 V, respectively. Illuminated I–V properties showed that the 1-sun open-circuit voltage, short-circuit current density and fill factor of a typical HJ solar cell were 463 mV, 19 mA/cm² and 53%, respectively. The external quantum efficiency and internal quantum efficiency showed a higher blue response than that of a conventional c-Si homojunction solar cell. Factors limiting the cell's performance are discussed.     

Analytical model of mismatched photovoltaic fields by means of Lambert W-function

A new model of photovoltaic (PV) fields is introduced in this paper. It allows the simulation of a PV generator whose subsections, e.g. cells, groups of cells, panels or group of panels, work under different solar irradiation values and/or at different temperatures. Moreover, different nominal characteristics, rated power, production technology, shape and area can be settled for different subsections. Consequently, the proposed model is able to describe the behaviour of matched as well as mismatched PV fields. It results into a non linear system of equations, which includes bypass and blocking diodes models and is characterized by a sparse Jacobian matrix. The numerical model is reliable and requires a moderate computational burdensome, both in terms of memory use and processor speed. Numeric simulations confirm the accuracy and cheapness of the approach. The proposed model is used to simulate the drawbacks associated to mismatching during maximum power point tracking (MPPT) of the PV generator.     

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.     

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.     

两级聚光光伏系统中砷化镓电池特性的实验研究

针对一般聚光系统中光斑不均匀而导致电池性能下降的问题,设计并搭建了具有二级聚光器的碟式聚光光伏发电系统,介绍了系统的结构及工作原理,进行了户外实验。在相同聚光比条件下(150X),与单级聚光系统相比,三结砷化镓光伏电池的平均峰值功率为1.515 W/cm2,平均效率为29.29%,平均峰值功率和平均效率分别提高了23.32%和9.12%。     

Organic photovoltaic devices: influence of the cell configuration on its performences

On the basis of our thin film technology we have proceeded to the study of different cell configurations: two-layers D/A organic solar cells deposited by vacuum evaporation and bulk D/A heterojunction material based on a discontinuous D/A network thin film obtained by spin coating. We have also tested different transparent conductive oxides (TCO: ITO, ZnO). These TCO films have been used as under or upper electrode. The organic materials were zinc-phthalocyanine (Zn-Pc) or poly vinyl(N-carbazole) (PVK) as electron donor and 1,4-diaminoanthraquinone (1,4-DAAQ) as electron acceptor. A PEDOT/PSS thin film was often intercalated between the organic and the TCO in order to improve the current characteristics.Results can be summarized as follows:

• The cells efficiency does not depend strongly on the nature of the TCO used in the present work.

• The performance of the p–n bilayer ZnPc/1,4-DAAQ depends strongly on the surface roughness of the structure. The fill factor (FF) of the current–voltage characteristics varies from 0.2, when the structure is glass/TCO/(PEDOT/PSS)/ZnPc/1,4-DAAQ/Al, to 0.6, when the structure is glass/Al/DAAQ/ZnPc/ PEDOT/PSS/”mechanical contact”/TCO. It is shown that this behaviour is related to the columnar growth properties of the 1,4-DAAQ films.

• Best efficiency has been achieved with bulk D/A heterojunction material based on PVK/1,4-DAAQ blend (efficiency=0.5%). The electrochemical measure of its HOMO (5.7 eV) and LUMO (3.8 eV), by comparison with PVK (HOMO=5.7 eV, LUMO=2.2 eV), shows that these values should be optimized. Effectively, the electron affinity of the donor should be significantly smaller than that of the acceptor, while the ionization potential of the acceptor should be significantly greater than that of the donor, which is not the case presently.

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.     

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.     

Organic photovoltaic devices with a crosslinkable polymer interlayer

Organic photovoltaic devices with a photo-crosslinkable interlayer were fabricated. This photo-crosslinkable interlayer acted as a leakage current reducing buffer layer. The performance of the small area OPV cell (0.04 cm²) was enhanced by the increase in the short circuit current and the fill factor. When a larger area cell (1 cm²) was used, the performance of OPV cell increased when the appropriate interlayer thickness was used. In the case of a 10 cm×10 cm module, the power conversion efficiency was about double than that without the interlayer. The insertion of the interlayer increased the current extraction by lowering the barrier height and attenuated the fill factor reduction by enhancing the rectification with a better leakage current sealing. From this study, it is clearly proved that the insertion of the appropriate photo-crosslinkable layer improves the performance of OPV devices, the effect was especially evident for large area cells.     

Semiconductor septum electrochemical photovoltaic cell with electrodeposited CdSe thin films

The performance of a semiconductor septum electrochemical photovoltaic (SC-SEP) cell based on electrodeposited CdSe thin films is reported. Photovoltages and photocurrent densities of 1.06 V and 15 mA/cm², respectively, were obtained. The proper selection of the electrolyte/electrode combination in the back compartment is a key factor better performance. A low fill factor causes a relatively low conversion efficiency of about 3.5%.