Analysis of anomalous current–voltage characteristics of silicon solar cells

The current–voltage characteristics of solar cells, under illumination and in the dark, represent a very important tool for characterizing the performance of the solar cell.The PC-1D computer program has been used to analyze the deviation of the dark current–voltage characteristics of p–n junction silicon solar cells from the ideal two-diode model behavior of the cell, namely the appearance of “humps” in the I–V characteristics. The effects of the surface recombination velocity, the minority-carrier lifetimes in the base — and emitter regions of the solar cell, as well as the temperature dependence of the I–V characteristics have been modeled using PC-1D.It is shown that the “humps” in the I–V characteristics arise as a result of recombination within the space-charge region of the solar cell, occurring when conditions for recombination are different from the simple assumptions of the Sah–Noyce–Shockley theory.     

Dye-sensitized solar cells: Scale up and current–voltage characterization

Large size, dye-sensitized solar cells (DSSC) have been prepared on silver grid embedded, transparent conductive oxide (TCO) glass substrate by screen printing method. Under one sun condition (AM 1.5, P_in 100 mW cm⁻²), achieved active area energy conversion efficiency of 5 cm×5 cm device approaches that of small-size DSSC prepared at similar condition. To improve the accuracy of efficiency measurement, current–voltage characterizations were carried out with shadow mask and different light reflective backgrounds. It was found that transparent and stripe-like non-active area, arising due to current collector and overcoat layer, of large size DSSC does not strongly influence the energy conversion efficiency.     

Determination of solar cell parameters from its current–voltage and spectral characteristics

An algorithm for the calculation of solar cell parameters (series and parallel resistance, diode coefficient, reverse current density) calculation from its current–voltage characteristics at fixed illumination intensity is proposed. The possibility of determining the p–n junction depth on the basis of spectral dependencies of diode photocurrent at different values of the applied bias voltage is shown.     

Constant voltage I–V curve flash tester for solar cells

Flash testers are commonly used for measuring solar cells and modules but in their usual implementation are complex, expensive, and susceptible to transient errors. This work presents a new tester design that is simple, low cost, and reduces transient errors by use of a constant-voltage cell-bias circuit. A novel feature of the system is that it extracts a family of I–V curves over a decade range of light intensity, which provides comprehensive information on cell performance. The new design has been tested and used extensively.     

In situ investigation of water transport in an operating PEM fuel cell using neutron radiography: Part 2 – Transient water accumulation in an interdigitated cathode flow field

An interdigitated cathode flow field has been tested in situ with neutron radiography to measure the water transport through the porous gas diffusion layer in a PEM fuel cell. Constant current density to open circuit cycles were tested and the resulting liquid water accumulation and dissipation rates with in-plane water distributions are correlated to measured pressure differential between inlet and outlet gas streams. The effect of varying the reactant gas relative humidity on liquid water accumulation is also demonstrated. These results provide evidence that the reactant gas establishes a consistent in-plane transport path through the diffusion layer, leaving stagnant regions where liquid water accumulates. A simplified permeability model is presented and used to correlate the relative permeability to varying gas diffusion layer liquid water saturation levels.     

Monitoring current–voltage characteristics and energy output of silicon photovoltaic modules

Photovoltaic (PV) system designers use performance data of PV modules to improve system design and make systems more cost effective. The collection of this valuable data is often not done due to the high costs associated with data acquisition systems. In this paper, we report on the design of a low-cost current–voltage (I–V) measuring system used to monitor the I–V characteristics of PV modules. Results obtained from monitoring seven crystalline silicon modules between October 2001 and November 2002 are presented and discussed. Results obtained also show the value of being able to continuously monitor the current–voltage characteristics of PV modules.     

Highly and quickly stabilized p–i–n/p–i–n-type protocrystalline silicon multilayer tandem solar cells

We have developed p–i–n/p–i–n-type protocrystalline silicon (pc-Si:H) multilayer tandem solar cells. The purpose of this work is to make a thin film silicon solar cell with low degradation by combining the virtues of a pc-Si:H multilayer and tandem structure. The usefulness of the pc-Si:H multilayer as a low degradation top and bottom cell was confirmed when we achieved a low degradation ratio of 10.0%. Notably, this tandem cell stabilized rapidly, within 1 h. Nanocrystalline silicon (nc-Si) grains embedded in a pc-Si:H multilayer were detected with the aid of a planer transmission electron microscope. The isolated nc-Si grains may suppress the photocreation of dangling bonds due to non-radiative recombination in an a-Si:H matrix. Because of these embedded nc-Si grains, the pc-Si:H multilayer has a fast and high light-induced metastability.     

Degradation of transparent metal–insulator–semiconductor solar cells due to heating effects

All the output parameters of the metal–insulator–semiconductor solar cells are degraded after heating. Also the dark current and the non-ideality factor are increased with heating. A reduction in the built-in potential has been detected. The capacitance–voltage–frequency measurements indicate the presence of interface states. These states are heavily occupied by electrons. Heating will increase the density of these states and consequently reduce the barrier height and the overall cell efficiency.     

Spectral response of a-Si:H p–i–n solar cells

The spectral response of a typical thin-film a-Si:H p–i–n solar cell has been investigated using the simulation RAUPV2. The peak in the external quantum efficiency has been observed to shift towards the violet part of the spectrum on decreasing the cell thickness. Moreover, the height of the peak increases as cell thickness is decreased. This is correlated with an enhancement in cell performance for thinner cells, due to a general increase in the drift field within the cell. The external quantum efficiency of a cell with an optimal concentration of phosphorous in the intrinsic layer has also been investigated. The external quantum efficiency for this cell is similar to that of the thinner cell, and is associated with the enhancement of the drift field near the p/i interface that is brought about by the phosphorous doping of the intrinsic layer. However, the integrated recombination for the thinner cell and the phosphorous-profiled cell differ significantly at long wavelengths, despite their similarity at shorter wavelengths. This effect is due to the weakening of the drift field near the n/i interface in the phosphorous-profiled cell.     

High-rate microcrystalline silicon deposition for p–i–n junction solar cells

We have developed a high-rate plasma process based on high-pressure and silane-depletion glow discharge for highly efficient microcrystalline silicon (μc-Si:H) p–i–n junction solar cells. Under high-rate conditions (2–3 nm/s), we find that the deposition pressure becomes the dominant parameter in determining solar-cell performance. With increasing deposition pressure from 4 to 7–9 Torr, short-circuit current increases by 50% due to a remarkable improvement in quantum efficiencies at the visible and near infrared. As a result, the maximum efficiency of 9.13% has been achieved at an i-layer deposition rate of 2.3 nm/s. We attribute the improved performance of high-pressure-grown μc-Si:H solar cells to the structural evolution toward denser grain arrangement that prevents post-oxidation of grain boundaries.     

Design strategy for a polymer electrolyte membrane fuel cell flow-field capable of switching between parallel and interdigitated configurations

Novel water management strategies are important to the development of next generation polymer electrolyte membrane fuel cell systems (PEMFCs). Parallel and interdigitated flow fields are two common types of PEMFC designs that have benefits and draw backs depending upon operating conditions. Parallel flow fields rely predominately on diffusion to deliver reactants and remove byproduct water. Interdigitated flow fields induce convective transport, known as cross flow, through the porous gas diffusion layer (GDL) and therefore are superior at water removal beneath land areas which can lead to higher cell performance. Unfortunately, forcing flow through the GDL results in higher pumping losses as the inlet pressure for interdigitated flow fields can be up to an order of magnitude greater than that for a parallel flow field. In this study a flow field capable of switching between parallel and interdigitated configurations was designed and tested. Results show, taking into account pumping losses, that using constant stoichiometry the parallel flow field results in a higher system power under low current density operation compared to the interdigitated configuration. The interdigitated flow-field configuration was observed to have lower overvoltage at elevated current densities resulting in a higher maximum power and a higher limiting current density. An optimal system power curve was produced by switching from parallel to interdigitated configuration based on which produces a higher system power at a given current density. This design method can be easily implemented with current PEMFC technology and requires minimal hardware. Some of the consequences this design has on system components are discussed.     

Optimisation of interdigitated back contacts solar cells by two-dimensional numerical simulation

In this paper we present the results of the simulation of interdigitated back contacts solar cell on thin-film (50 μm) silicon layer. The influence of several parameters (surface recombination rate, substrate thickness and type, diffusion length, device geometry, doping levels) on device characteristics are simulated using the accurate two-dimensional numerical simulator DESSIS that allows to optimise the cell design.     

Effect of design parameters on the efficiency of the solar cells fabricated using SOI structure

Thin cells demand new grid design concepts in that all the contacts (to the emitter and base) be located on the front surface. Hence, the aim of the investigation is to determine the potential and the basic limitation of the design. With this concept, an interdigitated front grid structure was realized and cells were fabricated through a set of photolithography processes. Confirmed efficiencies of up to 11.5% were achieved on bonded silicon-on-insulator wafers with a cell thickness of 50 μm in the case of finger spacing more than 1000 μm and a base width of 35 μm. It was also shown from the results that to adapt the design rules for optimizing the base fraction and the shadowing fraction was noted as an important technique to realize high-efficiency thin silicon solar cells.     

Photovoltaic cells based on cadmium sulphide–phthalocyanine heterojunction

Hybrid photovoltaic (PV) cells based on cadmium sulphide (CdS) single crystal and phthalocyanine (Pc) films have been developed and their PV performance was measured. Five different Pcs have been selected as candidates for the PV cell, PcCu, PcMn, PcZn, PcMg, and PcVO. It was found that all the chosen Pcs are capable of forming a hybrid heterojunction with the CdS surface, and that illumination results in charge separation at the interface. However, the performance of the In/CdS/Pc/Au device was dependent on the Pc used. PV cells with PcMg and PcZn showed the best results. An unoptimized cell with the PcZn film showed an open-circuit voltage V_oc=0.595 V, a short-circuit current density J_sc=1.88 μA/cm², a fill factor FF=0.265, and a power conversion efficiency PCE=3.0×10⁻⁴% under the AM1.5 conditions.     

CISCuT––solar cells and modules on the basis of CuInS2 on Cu-tape

In this paper the work and current findings about thin film solar cell technology––CISCuT (CuInS₂ on Cu-tape) is reviewed. Taking current market requirements into account it is shown that CISCuT based solar cells and modules could satisfy the demands of the market. The results of this study show that especially flexible and lightweight cells and modules should be made available on the basis of CISCuT. The innovative reel-to-reel technology to make quasi-endless tapes of solar cells and the inseparable connected unique absorber growth is explained in more detail. It is shown that the growth process can be monitored and even be controlled by means of an on-line measurement of electrical properties, which are strongly correlated to the properties of the final solar cell. Investigations and modelling of cell physics result in a p-i-n like cell structure of the CISCuT solar cells. The efficiency potential is explained for this device connected with an outlook for further improvement of the cell performance. The current batch of cells with an efficiency of about 9% is demonstrated connected with an appropriate stability of cells. The efficiency losses during the module assembling process are discussed. Efficiencies of test modules up to 7% are reported.     

On the sensitivity of open-circuit voltage and fill factor on dangling bond density and Fermi level position in amorphous silicon p–i–n solar cell

A simple method for calculation of current–voltage characteristics of an amorphous silicon solar cell is described in terms of excitation current, J_G, and excitation voltage, V_G, the latter being defined in terms of separation of quasi-Fermi levels. Contrary to the usual method of calculating the short-circuit current and dark current separately and assuming a linear superposition, in the present method the calculations are done first in the open circuit where the neutrality of space charge can be assumed and then the current has been calculated in terms of a gradient in the quasi-Fermi levels. We find that depending on other parameters, the open-circuit voltage is a weak function of dangling bond density except in cases of very large degradation. The sensitivity of open-circuit voltage, V_oc, to light-induced degradation can further be reduced by moving the thermal equilibrium Fermi level above the upper dangling bond level. Fill factor deterioration is found to be mainly due to conductivity modulation and is higher for the lower values of thermal equilibrium Fermi level.     

Solar cells using iodine-doped polythiophene–porphyrin polymer films

Wet-type organic solar cells containing 5,10,15,20-3-tetrathienylporphyrin (TThP) and polythiophene (PTh) films were fabricated. The TThP/PTh film was prepared on indium-tin-oxide (ITO) glass using an electrochemical polymerization method in an n-Bu₄NPF₆/CH₂Cl₂ solution. It was found that a small amount of iodine doping of the film improved the incident photon-to-electron conversion efficiency (IPCE) of a solar cell consisting of a TThP/PTh film and an aqueous electrolyte. A HOMO level measurement suggested that a modified HOMO level of the low iodine-doped TThP/PTh film allowed a fast electron transfer from PTh to a porphyrin moiety. To obtain further improvement, a sandwich-type solar cell using a 5% (w/w) aqueous solution of acetonitrile containing 0.05 M iodine and 0.5 M lithium iodide as an electrolyte was then fabricated. The solar cell absorbed light in the 300–800 nm wavelength range, converting this to a cathodic photocurrent with a maximum IPCE of 32% at 430 nm under irradiation of 5.0×10¹⁴ photon cm⁻² s⁻¹. This value is about 10 times higher than that of the solar cells using an aqueous electrolyte. The total energy conversion efficiency (η) of the solar cell under simulated sunlight reached 0.12% for 2.59 mW cm⁻² at AM1.5 and 0.05% for 100 mW cm⁻² at air mass 1.5.     

Self-discharge of lithium–sulfur cells using stainless-steel current-collectors

The self-discharge behaviour of Li–S cell, is investigated through changes in the open-circuit voltage (OCV) and discharge capacity with storage time. A fresh Li–S cell experiences 72% sulfur utilization during the first discharge, as based on the theoretical capacity for the formation of Li₂S. After 30 days of storage, the OCV has fallen from 2.48 to 2.16 V and the discharge capacity has decreased from 1206 to 924 mAh g⁻¹ (based on sulfur). Analysis of the self-discharged sample by a variety of techniques shows the formation of lithium polysulfides, such as Li₂S_n (n ≥ 1) from the reaction of lithium and sulfur, which is related to the corrosion of the stainless current-collector. Stainless steel is not the most appropriate current-collector material for Li–S cells. The extent of self-discharge can be decreased by using a gold-coated current-collector that offers protection against corrosion.     

Multi-junction III–V solar cells: current status and future potential

Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm²) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.     

Pt–NiO nanophase electrodes for dye-sensitized solar cells

A new type of counter electrode comprising of Pt and NiO biphase was prepared an RF magnetron cosputtering system for a dye-sensitized solar cell (DSSC). Transmission electron microscope images, transmission electron diffraction patterns, and X-ray diffraction patterns of the Pt–NiO electrodes confirmed the formation of a nanosized Pt polycrystalline phase of 7 nm mixed with porous amorphous NiO phase. The short-circuit current density and cell efficiency were increased from 0.22 to 0.30 mA/cm² and from 2.1% to 2.8%, respectively, and almost constant open-circuit voltage and fill factor, 0.53 V and 63%, respectively, were observed.