An improved circuit model for polymer solar cells

Authenticity of conventional circuit model, to interpret the characteristics of polymer solar cells (PSCs) is examined. Conventional circuit model is found to be quite limited, and various assumptions used there are not valid for PSCs. By understanding the nature of photovoltaic characteristics, through detailed investigations, we developed an improved circuit model, which explains correctly the behavior of PSCs under different environmental conditions. Investigations are carried out on the solar cells, made of the blend of regioregular poly(3‐hexylethiophene) (P3HT) and phenyl [6,6] C₆₁ butyric acid methyl ester (PCBM). The model is developed by treating both the dark and illuminated characteristics separately, even the characteristics were dealt with separately in reverse and forward biases. The formulated equivalent circuit model helps us in explaining many other important features, observed in the characteristics of PSCs. Copyright © 2013 John Wiley & Sons, Ltd.     

Use of the derivative of the EBIC signal for determining the grain boundary parameters in a polycrystalline solar cell

The grain boundary parameters such as diffusion length and effective recombination velocity are determined from the derivative i of the short-circuit current I_ac. This is equal to the first harmonic of the I_ac and is measured by a selective nanovoltmeter. From i it is also possible to estimate the localization of the edge of the boundary layer.     

An evaluation by solar simulation of radiation damage in silicon solar cells

From the viewpoint of the space power-systems designer, the most useful data for radiation-damaged solar cells is that of output power as a function of cell voltage, temperature, and radiation. This paper reviews the available results from laboratory radiation experiments where solar simulators were used. The solar cells studied were 1 and 10 ohm-cm n-on-p boron-doped cells, 5 and 10 ohm--cm aluminum-doped cells, and dendritic drift-field cells. Most of the experiments use 1 MeV electrons with some data for 0.5 to 2 MeV electrons and 0.5 to 2.7 MeV protons. Comparisons are made between types of cells on the basis of maximum power output and power at a fixed voltage. A fixed voltage is determined for each cell type using the value of cell voltage at maximum power after a 1 MeV electron fluence of 10¹⁶e/cm². There is an apparent lack of agreement among experimental results in the order of 3 or 4 percent, due to spectral variations between simulators. Another reason for the spread in data is attributed to differences that may occur from one group of cells to another, even from the same manufacturer. However, taking this into account, the average power at fixed voltage for the 1 ohm-cm cells is greater than the average for 10 ohm-cm up to a fluence of 5 × 10¹⁵e/cm², where a crossover occurs, and the 10 ohm-cm cells became superior.     

An experimental study of drift-field silicon solar cells

Silicon solar cells were constructed with drift fields of various widths and magnitudes. Both initial performance and performance after irradiation with up to 10¹⁶1 MeV electrons/cm²are compared with theory. Behavior is much as expected if the radiation damage is assumed to vary with doping level. This latter assumption leads to the conclusion that little change in cell performance occurs because of the field. Such a result was not anticipated at the start of the investigation, since the neglect of the capture cross-section variation gave the prediction of appreciably improved radiation tolerance.     

An approach toward 20-percent-efficient silicon solar cells

This paper shows that oxide surface passivation coupled with optimum multilayer antireflective coating can provide ∼3 percent (absolute) improvement in solar cell efficiency. Use of single-layer AR coating, without passivation, gives cell efficiencies in the range of 15- 15.5 percent on high-quality 4-Ω . cm as well as 0.1-0.2-Ω . cm float-zone silicon. Oxide surface passivation alone raises the cell efficiency to ≥ 17 percent. An optimum double-layer AR coating on oxide-passivated cells provides an additional ∼5-10 percent improvement over a single-layer AR-coated cell, resulting in cell efficiencies in excess of 18 percent. Experimentally observed improvements are supported by model calculations, and an approach to ≥20 percent efficient cells is discussed.     

基于InGaAs渐变缓冲层的太阳能电池的研究

This paper uses an InGaAs graded buffer layer to solve the problem of lattice mismatch and device performance degradation. In the graded buffer layer, we choose the ＂transition layer＂ and the ＂cover layer＂ to accommodate the 3.9% mismatch. No threading dislocations were observed in the uppermost part of the epitaxial layer stack when using a transmission electron microscope （TEM）. We analyze the factors which influence the saturation current. Simulation data shows that the cells grown by metal organic vapor phase epitaxy （MOVPE） have considerable open circuit voltage, short circuit current, and photoelectric conversion efficiency. Finally we propose that InP may have great development potential as a substrate material.     

An effective bilayer cathode buffer for highly efficient small molecule organic solar cells

Novel organic/ultrathin low work function metal bilayer cathode buffers for small molecule organic solar cells are proposed. Ultrathin low work function metal layers possess a high built-in electric field for effective carrier extraction and a high cathode reflectivity for maximum absorption in the photoactive layers. This leads to a significant increase of short circuit current density and fill factor of cells. By integrating this bilayer cathode buffer with DTDCTB:C₆₀ small molecular heterojunction, the device exhibits a high power conversion efficiency of up to 5.28%, which is an improvement of 22% compared to a device with a traditional single organic layer buffer.     

An oligothiophene dye with triphenylamine as side chains for efficient dye-sensitized solar cells

A novel oligothiophene-cyanoacrylic acid photosensitizer with two triphenylamine side chains (7T-2TPA) is designed and synthesized for dye-sensitized solar cells. 7T-2TPA exhibits broad (250-600 nm) and strong absorption (ε = 5.0 × 10⁴ L mol⁻¹ cm⁻¹ at 496 nm). The optical band gap (E_g) is estimated from the onset absorption edge to be 2.07 eV. The oxidation potential E_ox and reduction potential E_red vs NHE of the dye is 0.93 and −1.14 V, respectively. Dye-sensitized solar cell (DSSC) based on 7T-2TPA exhibits an open-circuit voltage (V_oc) of 724 mV, a short-circuit current density (J_sc) of 16.28 mA cm⁻², a fill factor (FF) of 0.684 and a power conversion efficiency of 8.06%. The efficiency of 8.06% is similar to that for widely used N719-based cell fabricated and measured under the same conditions.     

An A-D-D-A-type small-molecule electron acceptor with chlorine substitution for high-efficiency polymer solar cells

Chlorination is a very effective technique to adjust molecular energy levels, absorption spectra and intermolecular π–π stacking of small-molecule acceptors (SMAs). On this basis, a new acceptor-donor-donor-acceptor (A-D-D-A)-type SMA IDT2-IC-4Cl with chlorinated end units was developed. Compared with the unchlorinated counterpart (IDT2-IC), the IDT2-IC-4Cl exhibited an efficient absorption ability in the range of 550–900 nm region. Moreover, the blend films of PBDB-T:IDT2-IC-4Cl exhibited better charge generation properties and more balanced charge mobilities as compared to those of PBDB-T:IDT2-IC blend films. Polymer solar cells (PSCs) based on PBDB-T:IDT2-IC-4Cl exhibited a power conversion efficiency (PCE) of 12.53% with a short-circuit current (J_sc) of 22.22 mA cm⁻² and a fill factor (FF) of 69.81, while the PBDB-T:IDT2-IC device yielded a PCE of 8.18% with a J_sc of 13.23 mA cm⁻² and a FF of 65.71. The results show that chlorination is an effective way to obtain high-performance SMAs.     

An analytical model for high-low-emitter (HLE) solar cells in concentrated sunlight

A current-voltage characteristic is derived for the high-low emitter (HLE) solar cell in concentrated sunlight. For high-level injection, the ambipolar approach is used to yield the complete information of the low emitter concentration region, including the ohmic drop, the Dember voltage, the minority carrier current density, the minority-carrier distribution and the electric field distribution. High doping effects including Auger recombination and bandgap narrowing are considered. The dependences of short-circuit current, open-circuit voltage, fill factor and conversion efficiency on the variations of the geometrical dimensions and material parameters are discussed in detail for silicon single crystal materials. It is shown that the maximum conversion efficiency of 22% at 100 suns AMO can be obtained for silicon high-low emitter solar cell.     

EBIC analysis of SiC mesa diodes

Mesa etched p⁺n 6H-SiC diodes were investigated using EBIC. The EBIC signal was analysed using TMA Medici and Monte Carlo simulations. Space charge characteristics obtained from EBIC and static device simulations were compared. Surface charge density and minority carrier lifetime of the low doped n-type region were determined. The charge generation volume is described by an analytical approximation based on calculations using single scattering Monte Carlo simulation code. Results show large positive surface charge density of the order of 1×10¹³ q/cm² present at the n-type SiC/SiO₂ interface. A common origin of the surface charge on n- and p-type SiC is suggested. The EBIC method, combined with simulation tools, was confirmed to be a valuable diagnostic tool for study of the junction termination and passivation in SiC devices.     

Design considerations for a-Si solar cells

Amorphous Si (a-Si) is potentially an attractive material for thin-film solar cells. However, its potential for high-efficiency devices to date has not been realized. In this paper, we examine the design considerations which can be used to develop high-efficiency devices from a-Si. The design analysis takes full advantage of the flexibility in bandgap and other material properties offered by a-Si. Several structures, such as light-trapped cells, graded layer cells, and tandem junction a-Si/a-Si cells are analyzed in detail for collection efficiency, Voc, and fill factors. It is shown that using these innovative structures, realistic conversion efficiencies exceeding 10 percent can be realized in a-Si in spite of the very low diffusion lengths (∼0.1 µm). Technological considerations for achieving such structures are also discussed.     

Polymer acceptors for all-polymer solar cells

Bulk-heterojunction polymer solar cells(PSCs)as a clean and renewable energy resource have attracted great atten-tion from both academia and industry[1-20].Recently non-fullerene PSCs based on polymer donors(PDs)and small mo-lecule acceptors(SMAs)have achieved remarkable success with the power conversion efficiencies(PCEs)over 18％[21-26].Among various PSCs,all-polymer solar cells(all-PSCs)consist of PDs and polymer acceptors(PAs),showing unique merits including superior stability and mechanical robustness.How-ever,the development of all-PSCs lag behind SMAs-based PSCs due to the scarcity of high-performance PAs[6].     

Model calculations for metal-insulator-semiconductor solar cells

An analytical approach to calculating MIS solar cell properties has been formulated and utilized to study solar cell efficiency as a function of interfacial layer thickness, various interfacial film parameters and band gap. Three models are considered regarding interface state recombination kinetics. Calculations are presented for the case of interface states being in equilibrium with the metal (Model (I), in equilibrium with the majority carriers of the semiconductor (Model II) and in equilibrium with the minority carriers (Model III). It is found that in all three cases, the efficiency of low barrier height, Schottky barrier cells can be increased very significantly. For example, it is shown that for a band gap of 1.5 eV and a barrier height of 0.5 eV, it appears possible to increase the cell efficiency from essentially zero to 12%. If the barrier height is 1.0 eV, an efficiency of over 20% is possible. It is determined, however, that MIS solar cell performance is limited by leakage currents due to minority carrier diffusion back into the bulk. As a result, the upper limit of performance is defined by that for a homojunction. These calculations identify ranges of surface state density and interfacial barrier heights necessary for a good MIS solar cell.     

An approach toward 25-percent efficient GaAs heteroface solar cells

To approach one-sun 25% efficiency in GaAs solar cells, it is necessary to improve the basic understanding of internal loss mechanisms by a combination of characterization techniques and computer models. A methodology is developed to measure and evaluate minority-carrier transport properties such as lifetime and recombination velocity throughout the device structure in a 21.2% GaAs cell. It is found that this cell has a recombination velocity of 1.25×10⁵ cm/s at the AlGaAs/GaAs interface and a base minority-carrier lifetime of 8 ns. Guidelines are provided to increase the efficiency of this cell to 24% with slightly increased surface passivation and base lifetime using effective recombination velocity and device modeling computer programs. Further device modeling is performed to show that efficiencies of 25% can be obtained using a modified heteroface structure with a moderate surface recombination and their relation to device design are fully understood     

An improved equation for ionospheric absorption

The absorption equation of Laitinen and Haydon is limited to frequencies above about 5 MHz. Analysis of the data has led to a new equation which extends the lower limiting frequency to 2 MHz.     

Thin-film solar cells

The R&D status of cells and modules based on hydrogenated amorphous silicon (a-Si:H) and those based on CdTe and CuInSe₂ is reviewed. The stability of a-Si:H solar cells is still a major concern. Improvements have been achieved on an empirical basis by application of multijunction structures, optimization of interfaces, etc. Stabilized efficiencies of close to 10% have been reported. In parallel, the introduction of the ‘defect-pool model’ led to remarkable progress in understanding; it follows that a-SiGe:H instead of a-Si:H should be used for the i-layer (absorber). Improved cell engineering concepts, however, such as enhancement of the built-in electric field via reduction of the i-layer thickness and/or folded structures, are believed to be more promising. Polycrystalline thin-film cells based on CdTe and CuInSe₂ are not affected by inherent degradation mechanisms. the specific properties of these materials demand heterojunctions, and particular problems arise due to the polycrystallinity of the films and to the lattice mismatch and mismatch of the electronic band structures of the materials involved. These are discussed in conjunction with measures currently applied for optimizing solar cell performance. Both cell types exhibit eficiencies in the range 16-17%. Estimations of production costs and energy payback times of thin-film photovoltaic modules are reviewed (even below 1 US$ W_p⁻¹ and as low as 4 months, respectively) and environmental concerns, especially for Cd-containing cells, are summarized.     

Amorphous-silicon solar cells

The status of a-Si solar cell technology is reviewed. This review includes a discussion of the types of solar cell structure that are being used in commercial products. An overview of the development efforts under way involving new materials, such as alloys and microcrystalline films, and their impact on device performance is given. The status of stability in a-Si solar cells and projections for costs for large-scale manufacturing facilities are reviewed. The development of markets for a-Si photovoltaics is also discussed     

All-polymer solar cells

Organic solar cells(OSCs)show a promising commercializa-tion prospect with their power conversion efficiencies(PCEs)exceeding 18％[1-6].Among various types of OSCs,all-poly-mer solar cells(all-PSCs)with a physical blend of p-and n-type polymer as the active layer to harvest solar irradiation at-tract growing attention due to their unique advantages like ex-cellent morphological stability,and mechanical durability[7].Re-cently,great progresses have been achieved in this field includ-ing the development of high-performance polymer accept-ors and the advances in morphology regulation[8-13].Particu-larly,a PCE of 17.20％has been realized very recently by all-PSCs via properly aligned energy levels and optimal active-lay-er morphology[8].This achievement has significantly reduced the PCE gap between all-PSCs and small molecular acceptor-based OSCs,indicating the bright future of all-PSCs.There-fore,a highlight on these important progresses is timely and will effectively drive the development of all-PSCs.