Effects of CdS buffer layers on photoluminescence properties of Cu(In,Ga)Se2 solar cells

Photoluminescence (PL) have been studied on Cu(In,Ga)Se₂ (CIGS) thin films, CdS/CIGS and CIGS solar cells, to clarify the carrier recombination process. The chemical-bath deposition (CBD) of the CdS buffer layer on the CIGS thin film leads to (i) the enhancement of near-band-edge PL intensity by a factor of 2–3, (ii) change in energy of the defect-related PL and (iii) the slight change in the decay time. They are related not only to the minimization of the surface recombination but also to the modification of native defects at the Cu-poor surface of CIGS by the occupation of Cd atom at the Cu site. A donor–acceptor pair PL at low-temperature and temperature-dependent PL have been studied. They are discussed in terms of the impurity and defect levels created in the CIGS film during the CBD-CdS process.     

Theoretical maximal efficiency for a silicon solar cell with a two infrared-photon absorption in an inserted sub-structure

One of the possible optimized device designs far silicon solar cell photocurrent enhancement, consists of a cell having an inserted sub-structure with extrinsic gap levels. A middle-gap impurity and defect level band may actually allow a two infrared photon absorption. The junction near local defect layer design (Li et al., 1992) was assumed to enhance the sub-band-gap light absorption but it also enhances the recombination mechanisms strongly. Kuznicki (1993) has proposed another design with an L-H interface insertion at the edges of a continuous sub-structure to avoid extra recombination. The maximal photocurrent due to an additional infrared absorption calculated in this way is smaller than ΔI_ph = 16.8 mA cm⁻². In the case when the widths of the absorbing sub-structure are negligible compared to the width of the emitter, the simulated maximal efficiency can vary from 30.87 mW cm⁻² to 40.51 mW cm⁻².     

Numerical simulation of the impurity photovoltaic effect in silicon solar cells

Recently, the impurity photovoltaic effect (IPV) was proposed to improve the solar cell performance. Free electron–hole pairs can be generated in a two-step process involving an impurity level in the energy gap and two lower-energy photons: first electrons are optically excited from the valence band to the defect level and then from the defect level to the conduction band. The IPV effect will thus enhance the long-wavelength response of the cell.A significant amount of theoretical work has been carried out on IPV effect in the literature, particularly on silicon solar cells with indium impurities as defect. However, the lack of an easily available solar cell simulator including the IPV effect is a handicap.In this work, the numerical solar cell simulator SCAPS of the ELIS group was extended to include IPV in collaboration between the ELIS and the LPDS groups. Also, some special features are implemented, such as the calculation of electron and hole photoemission cross-sections of the impurity using the model of Lucovsky. The functionality of new SCAPS version was checked against existing results in the literature. Also, new results are presented such as the evolution of solar cell parameters with the indium density. We find that increasing indium concentration can improve silicon solar cell parameters, especially the short-circuit current and the efficiency, without drastically decreasing the open-circuit voltage. This is possible if a suitable structure for the cell is chosen. The optimum indium density should be equal around the base region density to obtain a positive benefit from the IPV effect.Light trapping, which is related to the internal reflectance at the front and the back of the cell, is very important in the IPV study. Reflectivity at the front and the back should exceed 99.9% to obtain a real efficiency increase. We calculate an improvement of about 6 mA/cm² in the photocurrent, and about 2% for the efficiency, which is due to the enhancement of long-wavelength absorption by the IPV effect.     

A thermal failure model of solar cells and its experimental studies

Within silicon, silver is an impurity with fast diffusivity and deep levels. It forms effective recombination centres in silicon acting as either acceptor or donor levels. That has been confirmed by a depth-profile analysis with the SIMS. The silver atoms do exist near the barrier region of a solar cell with Ti/Pd/Ag electrodes heated at 245°C for 308 h. The open-circuit voltage at low injection decreases as recombination actions increase in the barrier region. According to these phenomena, an estimation for the lifetime of solar cells is given by using acceleration stress tests.     

Modeling of light-induced degradation of amorphous silicon solar cells

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.     

Characterisation of direct epitaxial silicon thin film solar cells on a low-cost substrate

Direct epitaxial crystalline silicon thin film (CSiTF) solar cells on low-cost silicon sheets from powder (SSP) ribbons have been prepared using rapid thermal chemical vapour deposition (CVD) growth. The characterisation of CSiTF solar cells was investigated by electron and spectrally resolved light beam induced current (EBIC and SR-LBIC, respectively). All EBIC measurements were performed on both the front-side surface as well as on the cross-section of CSiTF solar cells. The electrical recombination was detected by EBIC and compared with their morphologies. The results of EBIC scan show that recombination centres are situated at grain boundaries (GBs); higher the density of grain, higher the recombination activities (higher contrast). Recombination of different intensity (strong and weak) takes place at vertical GBs. Compared with the high recombination at GBs, the contrast in intragrain is low. The dark contrast of the GBs and intragrain defects is clearly reduced near the surface due to the passivation by hydrogen, which indicates that the minority carrier diffusion length decreases gradually with the depth perpendicular to the surface. The diffusion length was determined by SR-LBIC. The results show that the diffusion length distribution is quite inhomogeneous over the whole cell area. A maximum L_eff of about 25 μm and mean values around 15 μm are calculated for the best solar cell.     

Low cost processing of CIGS thin film solar cells

A set of low cost techniques with realistic potential for direct manufacturing costs reduction were developed in the last five years while the industrial Cu(In,Ga)Se₂ (CIGS) solar cell production is based on vacuum processes, which require high initial investment into production machines. The common properties of these low cost techniques are the use of simple and fast non-vacuum deposition methods and the prefixing of the film-composition on a molecular level in a precursor layer, which is chemically and thermally treated to form a high quality CIGS film. The paste coating approaches use premixed inks which are applied by doctor-blade coating to yield solar cell efficiencies of 13.6%, with the potential to reach 15% and more in the next years. The choice of the precursor material has to be made with respect to the used selenization conditions to avoid detrimental impurity phases. A new precursor material is discussed, which allows fast conversion in selenium atmosphere and was used to produce solar cells with 6.7% efficiency. The CIGS film thickness has to be increased for complete absorption of the incident light.     

How lifetime fluctuations, grain-boundary recombination, and junctions affect lifetime measurements and their correlation to silicon solar cell performance

Two-dimensional simulations of quasi-steady-state photoconductance (QSSPC), carrier density imaging (CDI), photoconductive decay (PCD), and solar cell performance are performed on silicon models that incorporate grain-boundary recombination or lifetime fluctuations on the distance scale of 5 μm to 5 mm. The relationships between the lifetime measurement, actual recombination rates, and solar cell performance vary widely based on beam size, measurement technique, and recombination profile. The strengths and weaknesses of different measurement methods and the ability of analytical models to predict aggregate solar cell performance are examined and compared with earlier studies. Lifetime measurements in the presence of a junction are shown to be distorted by charge separation.     

Temperature-dependent quantum efficiency analysis of graded-gap Cu(In,Ga)Se2 solar cells

Despite the high solar cell efficiencies achieved with Cu(In,Ga)Se₂ (CIGS) absorbers, key parameters such as the carrier diffusion length and recombination lifetime are still under investigation. Here, we extract lifetime and diffusion length from temperature-dependent internal quantum efficiency (IQET) spectra of state of the art high efficiency CIGS solar cells. Two-parameter fits to the measured IQE curves using a model for double-graded gap solar cells show very good agreement in the studied temperature range T=146–293 K, allowing the extraction of the electron recombination lifetime in the absorber and the collection probability in the front region of the cell. The obtained results agree with current literature values obtained by other characterization techniques. Furthermore, the temperature dependence of the recombination lifetime is explained by Shockley–Read–Hall recombination through a single bulk defect level with an activation energy of 200 meV.     

Hall effect analysis of liquid phase epitaxy silicon for thin film solar cells

We use variable temperature Hall effect measurements to determine the doping concentration, impurity compensation, and mobility of n- and p-type liquid phase epitaxy (LPE) silicon layers that are grown from indium solutions onto silicon substrates. Our theoretical analysis of carrier concentration versus temperature data considers temperature-dependent effective masses, Fermi-Dirac statistics, multiple majority impurity levels, excited impurity states, and the temperature dependence of the Hall scattering factor. The measured Hall mobilities and computed compensation ratios in these LPE silicon thin films are within the range of values that have been measured in bulk silicon crystals. Such LPE layers are therefore suitable for the fabrication of high efficiency silicon thin film solar cells.     

Impurity photovoltaic effect in GaAs solar cell with two deep impurity levels

The impurity photovoltaic (IPV) effect has been extensively investigated for silicon solar cells with indium impurities. A small positive effect on the efficiency was theoretically predicted, provided that efficient light trapping schemes are applied. Since IPV effects are predicted to be more pronounced in wider band gap materials and possibly also by introducing more than one impurity level, we carried out a numerical study on GaAs solar cells with copper impurities. Indeed, copper introduces multiple acceptor type mid-gap levels in GaAs, two of which are investigated here: one at 0.14 eV and another at 0.40 eV above the valence band edge. We used a solar cell device simulator (SCAPS) specially adapted to include the IPV effect. We varied the impurity concentration and the light trapping parameters of the device, and calculated the occupation probability of the impurity levels, the generation and recombination through these levels (thermal and optical) and the solar cell characteristics: quantum efficiency, QE(λ), short-circuit current J_sc, open-circuit voltage V_oc and efficiency η.A significant IPV effect with extended response in the infrared is calculated when efficient light trapping is present. When an internal reflection coefficient R=0.99 can be reached at both the front and the back surface of the solar cell, an increase of the short-circuit current densities 2 mA/cm² can be obtained. The consequences of two levels instead of one are calculated and discussed, leading to a trade-off between J_sc improvement and a V_oc decrease.     

Electrical/electronic effects of titanium and iron impurities in EFG and FZ solar cell silicon: SPV/EBIC analysis

In this paper results on surface photovoltage (SPV) and electron beam induced conductivity (EBIC) studies of edge-defined film-fed growth (EFG) and floating zone (FZ) silicon solar cell materials (both p-type) are presented. A systematic comparison based on minority carrier diffusion length and carrier recombination is made between: (i) samples contaminated with Ti and/or Fe, (ii) samples gettered by phosphorous diffusion, and (iii) as-received samples. Deep level transient spectroscopy (DLTS) measurements, together with the iron-boron (FeB) pairing kinetics [1] have successfully been used to detect the presence of Fe in the samples. Even though this process is effective in revealing Fe impurities in p-type FZ silicon it is evidently not suitable for Fe identification in p-type EFG silicon. Ti, like Fe, is found to be a prominent lifetime-limiting metallic impurity in both EFG and FZ silicon. Phosphorous diffusion is proven to be an effective external gettering technique for fast-diffusing impurities such as Fe, but not for Ti.     

Thin film poly-Si formation for solar cells by Flux method and Cat-CVD method

Two methods were examined for the formation of poly-Si films. One is flux method and the other is Cat-CVD method. Flux method was used for forming poly-Si seed films on glass substrates covered with rear electrode. Poly-Si films of a few μm grain size and of mainly (1 1 1) crystalline orientation were obtained at less than 600°C. To make the seed films function as BSF layer for solar cell, boron doping was applied and carrier concentration of 2×10¹⁹/cm³ was obtained which is suitable for highly efficient solar cells. Cat (catalytic)-CVD method was examined for forming poly-Si photo-active layers on the seed films. The films showed deposition gas pressure-dependent crystalline orientations and there was no amorphous incubation layer in (1 1 1) oriented films by Cat-CVD method when deposited on (1 1 1) oriented seed films prepared by Flux method. The electrical properties of the film are insufficient at present, may be due to high defect density and the film structure which allows impurity contaminations of oxygen and carbon after film deposition. Although the film quality needs to be improved, poly-Si films whose crystal fraction is more than 85% were obtained at deposition rate of up to around 40 Å/s. This result indicates high potential of Cat-CVD method for high throughput photo-active formation process necessary for low production cost thin film silicon solar cells.     

Minority carrier lifetime of silicon solar cells from quasi-steady-state photoluminescence

Minority carrier lifetime is the most crucial material parameter for the performance of a silicon solar cell. While numerous methods exist to determine carrier lifetime on solar cell precursors prior to metallization, only very few techniques are capable of implicitly extracting effective minority carrier lifetimes from metallized silicon samples. In this paper, a measurement technique for effective minority carrier lifetime on silicon solar cells and metallized cell precursors via quasi-steady-state photoluminescence is presented. The setup requirements for this measurement technique are elaborated, experimental evidence of the reliability of such measurements down to carrier lifetimes in the range of a microsecond is provided, and a lifetime calibration of spatially resolved photoluminescence images of solar cells via the presented measurement technique is sketched. Finally, the very good agreement between the obtained effective carrier lifetime and the corresponding open circuit voltage of a solar cell is demonstrated.     

Photogeneration and recombination of carriers in hydrogenated amorphous silicon germanium alloys

The spectral dependence of carrier generation and recombination loss in a-SiGe:H samples having band gaps between 1.74 eV and 1.35 eV was studied in the Schottky barrier solar cell device structure. The active a-SiGe:H layers were of identical thickness (1 μm). The effect of hydrogen dilution of the source gases (silane and germane) on the generation and recombination characteristics was also studied. It has been observed that the net optical generation of carriers increases almost linearly with the lowering of the band gap but is independent of the localised states. Net recombination loss, on the other hand, increases with defect states but cannot be fully explained by taking into account the neutral defect centers alone. Within the above mentioned band gap range, it has been observed that the competing processes of generation and recombination resulted in a maximum short circuit current for a-SiGe:H samples of 1.44 eV band gap.     

ANALYTICAL AND GRAPHICAL METHODS OR DETERMINATION OF SOLAR CELL PARAMETERS AND INVESTIGATIONS OF SHADOWING EFFECT

In actual solar cells, the main power loss is due to the effect of the internal series resistance and the shunt resistance of the solar cell. Two methods; mathematical and graphical, were used to determine these two resistances for an Iraqi monocrystalline solar cell (type AI-Mansour). The results show that both of the series resistance (0·09 Ω) and the shunt resistance (210 Ω) can usually be neglected in an array performance evaluation for systems which don't use concentration arrangements

In addition to the series and shunt resistances computations, the analysis of the mismatching among solar cells as well as the power dissipation by shadowed or faulty cells for different module configurations are discussed in detail in this paper. As a result it was found that the maximum number of cells that can be safely series, parallel connected are 50 and 6 cells respectively.     

Luminescence and current-voltage characteristics of solar cells and optoelectronic devices

The transduction and conversion of light into work via a quantum process is dependent on the luminescent properties of the materials involved. Materials that can exhibit emission of light upon illumination are likely candidates for solar cells, detectors and optoelectronic devices. This radiative recombination in a material is directly related to the output device parameters, such as the current voltage characteristics. The chemical potential of the incoming light is a function of the photon energy and incident radiance. The maximum amount of work per particle, or voltage, that can be extracted by a solar converter is shown to be equal to chemical potential of the excitation, which can be inferred from the photoluminescence efficiency at ambient temperature. A discussion is made as to the use and optical properties of materials such as Si, GaAs, FeS₂, and organic dyes as efficient solar cell materials. In particular, the silicon I–V curve and luminescence are evaluated using the model, and shown to correspond to measured devices. A discussion is also made as to the extension of the luminescence model to the understanding of the light emitting diode, or LED. By allowing the absorber to remain as thin as possible, lower recombination fluxes and higher voltages are possible in solar cells and detectors.     

Photoluminescence properties of polycrystalline AgGaTe2

Due to its high absorption coefficient and close to optimal bandgap energy, AgGaTe₂ is a promising material for solar energy conversion. In order to avoid recombination losses, the study of the defect structure of solar cell materials is very important. This paper reports the results of photoluminescence experiments on polycrystalline AgGaTe₂. Two emission regions centred at 1.32 and 0.8 eV were found. The first region appears near the bandgap energy and comprises three bands that are identified by the theory of heavily doped semiconductors as the band-to-band (1.337 eV), the band-to-tail (1.317 eV) and the band-to-impurity (1.287 eV) recombination. The second deep PL region consists of two bands with the peak energies of 0.835 and 0.75 eV. Both these deep bands have rather low thermal activation energy; 18.5 and 20.8 meV, respectively. The possible origins of these bands are discussed.     

Effect of oxygen precipitation on the performance of Czochralski silicon solar cells

We have demonstrated the effect of oxygen precipitation on the performance of Czochralski (CZ) silicon solar cells. The oxygen precipitates in silicon substrates were formed by a low–high two-step annealing. With the increase of oxygen precipitation, the minority carrier diffusion length of CZ silicon solar cells decreases and, meanwhile, the leakage currents due to the carrier recombination at the defect states get increased. The external quantum efficiency (EQE) measurement shows that the decrease of the solar cell efficiency due to oxygen precipitates mainly takes place in the long wavelength range of light. The short-circuit current and open-circuit voltage of solar cells both become smaller, while the fill factor does not significantly change. The efficiency of solar cells is reduced to 12.7% from an original value of 17.5%. All these results suggest that the oxygen precipitation is a limitation factor for the improvement of solar cell efficiency, which should be strictly controlled during the crystal growth and cell fabrication.     

SOLAR CONVERSION PARAMETERS OF GRADED BASE SILICON SOLAR CELLS

A simple model for the photovoltaic processes in a diffused base silicon solar cell is presented. The practical profiles of impurity concentration are taken into account by using the method of integration by piecewise exponential approximations. The realistic variation of mobility and built-in field are considered. Such parameters of solar conversion as short circuit current, open circuit voltage and conversion efficiency are computed. Results show that these parameters are are sensitive functions of the impurity distribution in the graded base and the recombination velocity at the surface of the diffused layer. The model holds promise of its application for parametric study and optimization of the solar cell configuration. © 1997 John Wiley & Sons, Ltd.