A new approach to high-efficiency solar cells by band gap grading in Cu(In,Ga)Se2 chalcopyrite semiconductors

High efficiencies in Cu(In,Ga)(S,Se)₂ solar cells result from alloying CuInSe₂ base material with the corresponding Ga- or S-containing compound. Compositional grading is one important issue in these devices. To obtain high efficiencies a reconstructed Cu-depleted absorber surface is essential. We consider this Cu/In grading non-intentional, process related and present a model which explains its importance. Another approach to improve performance is controlled intentional band gap grading via Ga/In and S/Se grading during the deposition. We show that appropriate grading can improve current and voltage of the device simultaneously. The key objective is to design a larger band gap for recombination and a lower band gap for absorption to energetically separate the mechanisms of carrier recombination and current generation.     

Chemical synthesis of Cd-free wide band gap materials for solar cells

Chemical methods are nowadays very attractive, since they are relatively simple, low cost and convenient for larger area deposition of thin films. In this paper, we outline our work related to the synthesis and characterization of some wide band gap semiconducting material thin films prepared by using solution methods, namely, chemical bath deposition and successive ionic layer adsorption and reaction (SILAR). The optimum preparative parameters are given and respective structural, surface morphological, compositional, optical, and electrical properties are described. Some materials we used in solar cells as buffer layers and achieved remarkable results, which are summarized.     

The effects of the band gap and defects in silicon nitride on the carrier lifetime and the transmittance in c-Si solar cells

In this paper, silicon nitride thin films with different silane and ammonia gas ratios were deposited and characterized for the antireflection and passivation layer of high efficiency single crystalline silicon solar cells. An increase in the transmittance and a recombination decrease using an effective antireflection and passivation layer can be enhanced by an optimized SiN_x film in order to attain higher solar cell efficiencies. As the flow rate of the ammonia gas increased, the refractive index decreased and the band gap increased. Consequently, the transmittance increased due to the higher band gap and the decrease of the defect states, which existed for the 1.68 and 1.80 eV in the SiN_x films. The interface trap density found in silicon can be reduced down to 1.0×10¹⁰ cm⁻² eV⁻¹ for the SiN_x layer deposited under the optimized silane to ammonia gas ratio. Reduction in the carrier lifetime of the SiN_x films deposited using a higher NH₃/SiH₄ flow ratio was caused by the increase of the interface traps and the defect states in/on the interface between the SiN_x and the silicon wafer. Silicon and nitrogen rich films are not suitable for generating both higher carrier lifetimes and transmittance. An improvement in the single c-Si solar cell parameters was observed for the cells with an optimal SiN_x layer, as compared to those with non-optimal SiN_x layers. These results indicate that the band gap and the defect states of the SiN_x films should be carefully controlled in order to obtain the maximum efficiency for c-Si solar cells.     

The limiting efficiency of band gap graded solar cells

Two fundamental mechanisms limit the maximum attainable efficiency of solar cells, namely the radiative recombination and Auger recombination. We show in this paper that proper band gap grading of the solar cell localizes the Auger recombination around the metallurgical junction. Two beneficial effects result from this Auger recombination localization; first the cell is less sensitive to the surface conditions, and second, the previous estimates for the limiting efficiency of solar cells by Shockley, Tiedje, and Green are revised upwardly. We calculate the optimum bandgap grading profile for several real material systems, including GaInAsP lattice matched to InP, and a-SiGe on a-Si substrate.     

A-Si:H buffer in a-SiGe:H solar cells

Profiled a-SiGe:H-buffer layers between the doped and the absorption layers of amorphous silicon germanium (a-SiGe:H) solar cells are routinely used to avoid bandgap discontinuities and high-defect densities at the p/i- and i/n interface. Here, we present a much simpler approach replacing the profiled a-SiGe:H-buffer layers at both interfaces by a-Si:H-buffer layers. It is demonstrated that for a-SiGe:H solar cells (thickness of the E_G=1.5 eV part is 54 nm) these structures yield similar open circuit voltage V_OC and fill factor (FF) compared to the bandgap profiled layer at the same short circuit current density j_SC. The influence of thickness, optical bandgap and position of the buffer layers on the solar cell performance is investigated.     

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.     

Polycrystalline thin-film solar cells – A review

This paper reviews recent progress in polycrystalline thin-film solar cell research and development. Results from both small area cells and larger area modules/submodules are discussed. Emphasis is given to results from deposition techniques with potential for fabrication of large-area cells of the type CdS/CdTe and CdS/Cu(In,Ga)Se₂. Small area high-efficiency cell results are discussed in terms of manufacturability issues for large-area cells. A discussion of recent successes in electrodeposition of high-quality absorbing layers of Cu(In,Ga)Se₂ is given. Hybrid approaches involving final adjustment of local and overall stoichiometries using post-deposition vacuum evaporation of selected substituent elements and thermal anneals are also discussed.     

Influence of the layer thickness and hydrogen dilution on electrical properties of large area amorphous silicon p–i–n solar cell

The aim of this work is to present data concerning the optimization of performances of a large area amorphous silicon p–i–n solar cell (30×40 cm²) deposited by plasma enhanced chemical vapour deposition (PECVD) at 27.12 MHz. In this work the solar cell was split into small areas of 0.126 cm², aiming to study the device performance uniformity, where emphasis was put on the role of the n-layer thickness. The solar cells were studied through the spectral response behaviour in the 400–750 nm range as well as by the behaviour of the AC impedance. Solar cells with fill factor of 0.58, open circuit voltage of 0.83 V, short circuit current density of 17.14 mA/cm² and an efficiency of 8% were obtained at growth rates higher than 0.3 nm/s.     

Au–Cu/p–CdTe/n–CdO/glass-type solar cells

The heterostructure design proposed by us for the photovoltaic (PV) solar cell is: Au–Cu/p–CdTe:Sb/n–CdO:F/glass. The CdO:F films were grown by the sol–gel method, in conditions in order to get low resistivity 4.5×10⁻⁴ Ω-cm and an optical transmission higher than 85%. The CdTe:Sb films were prepared by means of the RF sputtering technique, in conditions to get resistivity value around 10⁶ Ω-cm, high crystalline quality and higher grain size. The Au–Cu contacts were thermally evaporated. For the study of PV-heterostructure a systematic variation of the preparation parameters were carried out. The parameters involved in the manufacture of the cell, in order to look for the highest efficiency were: (A) For the deposit of the p-CdTe:Sb films, a low argon pressure of 2.5 m Torr and high substrate temperature of 450 °C. The CdTe:Sb film thickness was varied in the interval 4.5–11 μm. (B) For the activation of the heterostructure: (i) The treatment temperature in vacuum, after the CdTe is deposited, was varied in the 350–550 °C range and (ii) the treatment temperature in Ar atmosphere, after the heterostructure is dipped in CdCl₂ solution, was studied in the 400–510 °C range. (C) Optimization of the Cu–Au contact with the adequate Cu-film thickness. The highest energy conversion efficiency (η) value was 5.48%. This work reports a systematic study of the parameters involved in the solar cell manufacture, for the search of a better value of η.     

Theoretical analysis of the optimum energy band gap of semiconductors for fabrication of solar cells for applications in higher latitudes locations

In this work some results of theoretical analysis on the selection of optimum band gap semiconductor absorbers for application in either single or multijunction (up to five junctions) solar cells are presented. For calculations days have been taken characterized by various insolation and ambient temperature conditions defined in the draft of the IEC 61836 standard (Performance testing and energy rating of terrestrial photovoltaic modules) as a proposal of representative set of typical outdoor conditions that may influence performance of photovoltaic devices. Besides various irradiance and ambient temperature ranges, these days additionally differ significantly regarding spectral distribution of solar radiation incident onto horizontal surface. Taking these spectra into account optimum energy band gaps and maximum achievable efficiencies of single and multijunction solar cells made have been estimated. More detailed results of analysis performed for double junction cell are presented to show the effect of deviations in band gap values on the cell efficiency.     

Ambient stability of wet chemically passivated germanium wafer for crystalline solar cells

Surface passivation has been recognized as a crucial step in the evaluation of minority carrier lifetime of photovoltaic materials as well as in the fabrication of high efficient solar cells. Dilute acids of HF and HCl are employed for germanium (Ge) surface passivation. An effective lifetime of passivated Ge wafers has been evaluated by a microwave photoconductive decay (μ-PCD) measurement. Surface recombination velocities, S, of H- and Cl-terminated Ge surfaces are 23 and 37 cm/s, respectively. The stability of passivated Ge surfaces against exposure to air has also been examined. The HCl-passivated Ge surfaces are found to be more robust than HF-passivated surfaces.     

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.     

Application of real-time spectroscopic ellipsometry for characterizing the structure and optical properties of microcrystalline component layers of amorphous semiconductor solar cells

Over the past few years, we have applied real-time spectroscopic ellipsometry (RTSE) to probe hydrogenated amorphous silicon (a-Si:H)-based solar cell fabrication on the research scale. From RTSE measurements, the microstructural development of the component layers of the cell can be characterized with sub-monolayer sensitivity, including the time evolution of (i) the bulk layer thickness which provide the deposition rates, and (ii) the surface roughness layer thickness which provide insights into precursor surface diffusion. In the same analysis, RTSE also yields the optical properties of the growing films, including the dielectric functions and optical gaps. Results reported earlier have been confined to p-i-n and n-i-p cells consisting solely of amorphous layers, because such layers are found to grow homogeneously, making data analysis relatively straightforward. In this study, we report the first results of an analysis of RTSE data collected during the deposition of an n-type microcrystalline silicon (μc-Si:H) component layer in an a-Si:H p-i-n solar cell. Such an analysis is more difficult owing to (i) the modification of the underlying i-layer by the H₂-rich plasma used in doped μc-Si:H growth and (ii) the more complex morphological development of μc-Si:H, including surface roughening during growth.     

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.     

An initial investigation of amorphous silicon-selenium solar cells

Amorphous silicon-selenium solar cells, where a selenium alloy thin film was used as an intrinsic layer, were successfully fabricated by plasma-enhanced chemical vapor deposition. The cells have a simple p-i-n structure. The maximum conversion efficiency of the cells was 5.1 %, measured under 100 mW/cm [2] ELH white light illumination. The fabricated selenium alloy solar cells exhibited stable characteristics under white light soaking, with respect to those of amorphous silicon comparison solar cells, fabricated under similar conditions. Measurement of the quantum efficiency showed that this cell can be used as a top cell in a tandem cell structure.     

Voltage and current loss in semiconductor solar cells from MCV and simultaneous IV measurements

We present a method for determining the voltage and current loss in solar cells using the modulation capacitance voltage measuring technique. Both losses can be investigated in the forward biased mode and the generator mode. The method is especially suitable for characterising particular loss components due to band offsets and interface recombination in heteroemitter solar cells. Nonlinear loss-versus-current characteristics were generally found for a-Si:H(p⁺), SIPOS(n⁺) and ZnO(n⁺) emitters on c-Si. The losses are strongly modified by the preparation which affects the heterojunction interface. These results can give insight into effects of recombination and band offsets in both bands.     

Incorporation and critical concentration of oxygen in a-Si:H solar cells

For different process conditions, series of hydrogenated amorphous silicon p-i-n solar cells with various oxygen concentrations in the intrinsic absorber layer were fabricated by plasma-enhanced chemical vapor deposition at 13.56 MHz using process gas mixtures of SiH₄ and H₂. Oxygen was introduced into the gas phase during the deposition process by a controllable leak in the chamber wall and the amount of oxygen supply is characterized by the oxygen base pressure p_b. It is found that for a certain deposition regime defined by silane and H₂ flows, deposition pressure and substrate temperature the oxygen incorporation follows an expected dependence on the ratio p_b/r_d with r_d the deposition rate. This relation is not valid for the comparison of different deposition regimes. A high hydrogen flow is found to reduce the oxygen incorporation strongly. The photovoltaic parameters of the solar cells were measured in the initial state as well as after 1000 h of light-soaking. The critical oxygen concentration (i.e. the upper limit of incorporated oxygen not leading to a decay of the solar cell performance) was determined for each regime in the initial and light-soaked state. For all deposition regimes, the results show no difference in these critical oxygen concentrations for the initial and light-soaked state. The critical oxygen concentration, is found to differ for the different process regimes and turns out to be the highest (approximately 1×10²⁰ cm⁻³) for the deposition regime with the highest hydrogen flow rate, which interestingly is the regime with the lowest oxygen incorporation at a given p_b/r_d ratio. This combination makes the regime of high hydrogen gas flow suitable for depositing high-efficiency solar cells at high base pressure.     

Highly stabilized protocrystalline silicon multilayer solar cell using a silicon–carbide double p-layer structure

We have investigated a pin-type protocrystalline silicon (pc-Si:H) multilayer solar cell fabricated by employing a silicon–carbide double p-layer structure and a layered structure of multilayer processing through alternate H₂ dilution. The initial conversion efficiency is drastically improved by incorporating a hydrogen-diluted boron-doped amorphous silicon–carbide (p–a-SiC:H) buffer layer at the p/i interface. Remarkably, the pc-Si:H multilayer absorber exhibits superior light-induced metastability to a conventional amorphous silicon (a-Si:H) absorber. Therefore, we have successfully achieved a highly stabilized efficiency of 9.0% without using any back reflector.     

Metastable volume changes of hydrogenated amorphous silicon and silicon–germanium alloys produced by exposure to light

Exposure of hydrogenated amorphous silicon, a-Si:H, to light produces large-scale structural changes and increases the density of dangling Si bond defects acting as efficient carrier recombination centers. The latter is the well-studied Staebler–Wronski effect (SWE). All light-induced changes are metastable and disappear after annealing to approximately 200°C. This review focuses on one of the large-scale changes, namely that of the macroscopic density of the material. In all device quality materials, the initial stress is compressive with values typically in the range of 10⁸–10⁹ Pa. Exposure to light produces additional compressive stress, which can exceed 2×10⁷ Pa. The observed change of stress is due to a change of the volume of the unsupported material and not of its elastic modulus. The relative volume change, ΔV/V, at 300 K becomes detectable at values in excess of about 10⁻⁶ after only a few photons per Si atom have been absorbed. ΔV/V saturates above 10⁻³, under high-intensity light after an average of more than 10⁶ photons per Si atom have been absorbed. ΔV/V initially grows with t^0.50±0.04 under CW illumination producing carrier generation rate G in the range of 10²¹ to a few 10²³ cm⁻³ s⁻¹. The approach to saturation is well fitted by a stretched exponential function with stretch exponent close to 0.5. ΔV/V is approximately proportional to G. The fastest and largest photo-expansion has been observed in the so-called “edge material” between the amorphous and microcrystalline state, produced by plasma enhanced CVD from increasingly diluted silane/hydrogen gas mixtures. The quantum efficiency of volume expansion has been observed to increase with the photon energy of the light in contrast to the SWE. No volume increase is observed in Ge rich a-Si_1−xGe_x:H alloys and in hydrogenated microcrystalline material. Photo-expansion and the SWE show marked difference in spatial extend in the network, different evolution in time and different wavelength dependence. Hence, the two effects appear to be independent even though both involve hydrogen.     

Temperature dependence of the optical band gap and refractive index of poly(ethylene terepthalate) oligomer–DDQ complex thin film

The temperature dependence of the optical band gap and refractive index dispersion of thin film of poly(ethylene terepthalate) oligomer–DDQ charge transfer complex has been investigated. The absorption edge shifts to the lower energy as consequence of the thermal annealing on film and the fundamental absorption edge corresponds to a direct energy gap. The temperature coefficient of the optical band gap for the film was found as dE_g/dT = − 3.15 × 10⁻³ eV/K. The temperature dependence of the refractive index has also been investigated and it is observed that the refractive index changes by annealing temperatures.