Graded Bandgap Solar Cells Research Articles

Performance Improvement of Graded Bandgap Solar Cell via Optimization of Energy Levels Alignment in Si Quantum Dot, TiO2 Nanoparticles, and Porous Si

Charge carriers’ generation from zinc includes silicon quantum dots (ZnSiQDs) layer sandwiched in-between porous silicon (PSi) and titania nanoparticles (TiO2NPs) layer-based solar cell is an efficient way to improve the cell’s performance. In this view, ZnSiQDs layer with various QDs sizes have been inserted, separating the PSi and TiO2NPs layers to achieve some graded bandgap quantum dot solar cells (GBQDSCs). In this process, ZnSiQDs of mean diameter 1.22 nm is first prepared via the top-down method. Next, ZnSiQDs have been re-grown using the bottom-up approach to get various mean diameters of 2.1, 2.7 and 7.4 nm. TiO2NPs of mean diameter in the range of 3.2 to 33.94 nm have been achieved via thermal annealing. The influence of different ZnSiQDs sizes on the designed GBGQDSCs performance has been determined. The proposed cell attains a short circuit current of 40 mA/cm2 and an efficiency of 4.9%. It has been shown that the cell performance enhances by optimizing the energy levels alignment in the PSi, ZnSiQDs, TiO2NPs layers.

Analytical study of gradual photovoltaic CuInGaSe2 based solar cell performances

The most significant challenge for environment and renewable energy researchers is to achieve good performance, as well as cost efficiency in terms of solar generators especially in the PV sector. Regarding the thin film solar cells based on Cu(In,Ga)Se2, the present work dissertation is concerned with the performance of a graded band gap solar cell based on (CIGS). The aim is to determine the influence of physical and geometrical parameters on performance. Results indicate that the increase in electric field resulting from the gradient of the band gap dismantles the effects of surface recombination. The results obtained are an efficiency of 26 % for Eg0=1.67 eV, Eg1=1.02 eV and a thickness of layer p and n d1=1µm, d2=2µm respectively.

Investigations on Optical, Material and Electrical Properties of aSi:H and aSiGe:H in Making Proposed n+aSi:H/i-aSi:H/p+aSiGe:H Graded Bandgap Single-junction Solar Cell

Objective: This work identifies materials that satisfy refractive index, optical band gap, composition profile, conductivity, hall mobility, carrier type and carrier concentration to utilize them in making thin film photovoltaic cells. Methods: We fabricated phosphorous doped amorphous silicon (n+ aSi:H), boron doped amorphous silicon germanium(p+ aSiGe:H) and intrinsic amorphous silicon (i-aSi:H). A detailed and systematic characterization of the fabricated layers was done. The phosphorous doped amorphous silicon (n+ aSi:H) showed an optical band gap of 1.842 eV and an electron mobility of 295.45 cm2V-1s-1. The boron doped amorphous silicon germanium (p+ aSiGe:H) exhibited an optical band gap of 1.74 eV and a hole mobility of 158.353 cm2V-1s-1. The intrinsic amorphous silicon (i-aSi:H) has an optical band gap of 1.801 eV. The films of n+ aSi:H, i-aSi:H and p+ aSiGe:H can be utilized for fabricating graded band gap single junction thin film solar cells, as they are semiconducting materials with varying band gaps in the range of 1.74 eV to 1.84 eV. The tailoring of band gap achieved by the proposed material combination has been presented using its energy band diagram. Results: In this work, we are proposing a single junction graded band gap solar cell with aSi:H and aSi- Ge:H alloys of varying doping to achieve grading of band gap, which improves the efficiency while keeping the cell compact and light. Conclusion: As a first step in the validation, we have simulated a thin film solar cell using SCAPS1D simulation software with the measured parameters for each of the layers and found that it successfully performs as solar cell with an efficiency of 14.5%.

Performance Enhancement by the Introduction of Additional Narrow Band Gap Bottom Layer of aSi0.64Ge0.36: H on Proposed p + aSi:H/i-aSi: H/n+ aSi0.73Ge0.27: H Thin Film Solar Cells

This work is the continuation of our previous work entitled "Investigations on optical, material and electrical properties of aSi:H and aSiGe:H in making proposed n+aSi:H/iaSi:H/p+aSiGe:H graded band gap solar cells." In this work, we present an additional bottom layer made of increased germanium content: aSi0.64Ge0.36:H to the previously recommended p+aSi:H/i-aSi:H/n+aSi0.73Ge0.27:H photovoltaic cell to strengthen the absorption spectrum and thereby boosting the attainment of the solar cell. Moreover, the overall active layer thickness is reduced from 430 nm of previous work to 395 nm of proposed work. This work includes the fabrication of samples of epitaxially grown aSiGe:H thin films of varying band gap made with Plasma Enhanced Chemical Vapour Deposition (PECVD) technique succeeded by their characterisation. The establishment of band gap tailoring by varying the germane (GeH4) gas flow rate is thoroughly investigated through optical characterisation. The growth chemistry of PECVD made aSi0.64Ge0.36:H layer has been analysed and the presence of respective radicals has been verified using Fourier Transform Infra Red (FTIR) spectroscopy. In accordance with the measured band gaps, p+ aSi:H/i-aSi:H/n+aSi0.73Ge0.27:H/naSi0.64Ge0.36:H solar cell has been proposed. A comprehensive inquiry on optimisation of the recommended structure has been made by varying the optical band gap and thickness of the bottom most aSi0.64Ge0.36:H layer of the structure. All the cell parameters including open circuit voltage (Voc), short circuit current density (Jsc), maximum power point voltage (Vm), maximum power point current density (Jm), Fill factor (FF) and conversion efficiency (η) has been calculated using SCAPS1D solar simulator. Furthermore, C–V characteristics and Mott-Schottky plot of the proposed structure has been evaluated. The introduction of narrow band gap amorphous silicon germanium (aSi0.64Ge0.36:H) at the bottom has remarkably enhanced Jsc and η to 15.54 mA/cm2 and 15.15% respectively, which is better compared to reported amorphous silicon photovoltaic cells having single junction.

Analysis, optimisation and experimental validation of [formula omitted]aSi:H/i-aSi:H/p+aSiGe:H graded band gap single junction solar cell

In this paper, we present a single junction graded band gap thin film solar cell using heavily phosphorous doped amorphous silicon, intrinsic amorphous silicon and heavily boron doped amorphous silicon germanium. The first part of the work presents a rigorous analysis of J-V characteristics of recommended photovoltaic structure under short circuit (SC), open circuit (OC), dark and AM 1.5G illumination standard. Further, optimisation of thickness of active layers of the suggested n+ aSi:H/i aSi:H/p+aSiGe:H solar cell structure is done using SCAPS1D solar simulator. The active layer thickness of the proposed solar cell is 430 nm only. Low active layer thickness and absence of multiple junctions reduces the material requirement, complexity and cost of the proposed solar cell. Furthermore, fabrication of individual layers and overall summary of their characterisation have been done. Finally, the proposed structure has been fabricated and validated its J-V characteristics. The fabricated solar cell has short circuit current density (Jsc) of 11.67 mA/cm2, open circuit voltage (Voc) of 1.18 V, fill factor (FF) of 0.857 and conversion efficiency (η) of 11.80% which is on par with other announced single junction amorphous silicon solar cells. Here, we are reporting a single junction graded band gap solar cell using combination of aSi:H and aSiGe:H alloys with varying doping levels for the first time, which is better in conversion efficiency while compact and light.

Electrodeposition of CdS thin-films from cadmium acetate and ammonium thiosulphate precursors

Cadmium sulphide (CdS) thin-films have been electrodeposited using two electrode system to be used as the hole back diffusion barrier (hbdb) layer for graded bandgap solar cells with p-type windows. Cadmium acetate dihydrate [Cd(CH3COO)2·2H2O] and ammonium thiosulphate [(NH4)2S2O3] have been used as the cadmium (Cd) and sulphur (S) precursors respectively. In this work, CdS layers have been grown on glass/FTO (fluorine doped tin oxide) substrates at cathodic potentials ranging from 1300 to 1460 mV in order to find the best growth voltage. N-type conductivity is observed for all the layers and band-gap ranged between ~ 2.36 and ~ 2.40 eV for as-deposited layers and ~ 2.31 and ~ 2.36 eV for air-annealed layers. X-ray diffraction (XRD) analysis revealed cubic/hexagonal mixed crystallinity for the as-grown layers which indicates a tendency of transiting towards hexagonal structure upon annealing. Compositional and morphological characteristics of the layers have been investigated with energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) respectively.

Perovskite solar cells: a deep analysis using current\u2013voltage and capacitance\u2013voltage techniques

Perovskite solar cells exhibiting ~ 14–15% efficiency were experimentally measured using current–voltage (I–V) and capacitance–voltage (C–V) techniques in order to extract material and device properties, and understand the action of photovoltaic (PV) operation. Deep analyses were carried out on dark- and illuminated I–V curves, and dark C–V curves. Results were compared with those of graded bandgap solar cells fabricated on inorganic n-type window layers. These analyses according to a physicist’s point of view lead to understand the perovskite solar cell as a graded bandgap solar cell built on a p-type window layer. I–V and C–V results show very similar behaviour and the principle of PV action is identical. Once the stability issues with perovskites are solved, these devices have very high potential of producing next generation solar cells reaching at least mid-20% efficiency values.

Rationally Controlled Synthesis of CdSexTe1-x Alloy Nanocrystals and Their Application in Efficient Graded Bandgap Solar Cells.

CdSexTe1−x semiconductor nanocrystals (NCs), being rod-shaped/irregular dot-shaped in morphology, have been fabricated via a simple hot-injection method. The NCs composition is well controlled through varying molar ratios of Se to Te precursors. Through changing the composition of the CdSexTe1−x NCs, the spectral absorption of the NC thin film between 570–800 nm is proved to be tunable. It is shown that the bandgap of homogeneously alloyed CdSexTe1−x active thin film is nonlinearly correlated with the different compositions, which is perceived as optical bowing. The solar cell devices based on CdSexTe1−x NCs with the structure of ITO/ZnO/CdSe/CdSexTe1−x/MoOx/Au and the graded bandgap ITO/ZnO/CdSe(w/o)/CdSexTe1−x/CdTe/MoOx/Au are systematically evaluated. It was found that the performance of solar cells degrades almost linearly with the increase of alloy NC film thickness with respect to ITO/ZnO/CdSe/CdSe0.2Te0.8/MoOx/Au. From another perspective, in terms of the graded bandgap structure of ITO/ZnO/CdSe/CdSexTe1−x/CdTe/MoOx/Au, the performance is improved in contrast with its single-junction analogues. The graded bandgap structure is proved to be efficient when absorbing spectrum and the solar cells fabricated under the structure of ITO/ZnO/CdSe0.8Te0.2/CdSe0.2Te0.8/CdTe/MoOx/Au indicate power conversion efficiency (PCE) of 6.37%, a value among the highest for solution-processed inversely-structured CdSexTe1−x NC solar cells. As the NC solar cells are solution-processed under environmental conditions, they are promising for fabricating solar cells at low cost, roll by roll and in large area.

Analysis of the electronic properties of all-electroplated ZnS, CdS and CdTe graded bandgap photovoltaic device configuration

All-electrodeposited ZnS, CdS and CdTe thin layers have been incorporated in a graded bandgap solar cell structure of glass/FTO/n-ZnS/n-CdS/n-CdTe/Au have been fabricated and an average conversion efficiency of 14.18% was achieved under AM1.5 illuminated condition. Based on former work in which 10% conversion efficiency was reported, optimisation has been made to the semiconductor layers, precursors, thicknesses and the post-growth treatment. These results demonstrate the advantages of multi-layer graded bandgap device configuration and the inclusion of gallium based post-growth treatment (CdCl2+Ga2(SO4)3) on the CdS/CdTe-based device structure. The fabricated devices were characterised using both current-voltage (I-V) and capacitance-voltage (C-V) techniques. Under dark I-V condition, a rectification factor (R.F.) of 104.8, ideality factor (n) of 1.60 and a barrier height (ϕb) >0.82 eV were observed. Under AM1.5 illuminated I-V condition, short-circuit current density (Jsc) of 34.08 mA cm−2, open-circuit voltage (Voc) of 730 mV, fill-factor (FF) of 0.57 and conversion efficiency of 14.18% were observed. Under dark C-V condition, doping density (ND) of 7.79×1014 cm−3 and a depletion width (W) of 1092 nm were achieved. In addition, the work demonstrates the capability of two-electrode system as a simplification to the conventional three-electrode system in the electrodeposition of semiconductors.

Investigating the electronic properties of multi-junction ZnS/CdS/CdTe graded bandgap solar cells

The fabrication of multi-junction graded bandgap solar cells have been successfully implemented by electroplating three binary compound semiconductors from II-VI family. The three semiconductor materials grown by electroplating techniques are ZnS, CdS and CdTe thin films. The electrical conductivity type and energy bandgap of each of the three semiconductors were determined using photoelectrochemical (PEC) cell measurement and UV–Vis spectrophotometry techniques respectively. The PEC cell results show that all the three semiconductor materials have n-type electrical conductivity. These two material characterisation techniques were considered in this paper in order to establish the relevant energy band diagram for device results, analysis and interpretation. Solar cells with the device structure glass/FTO/n-ZnS/n-CdS/n-CdTe/Au were then fabricated and characterised using current-voltage (I-V) and capacitance-voltage (C-V) techniques. From the I-V characteristics measurement, the fabricated device structures yielded an open circuit voltage (Voc) of 670 mV, short circuit current density (Jsc) of 41.5 mA cm−2 and fill-factor (FF) of 0.46 resulting in ∼12.8% efficiency when measured at room temperature under AM1.5 illumination conditions. The device structure showed an excellent rectification factor (RF) of 104.3 and ideality factor (n) of 1.88. The results obtained from the C-V measurement also showed that the device structures have a moderate doping level of 5.2 × 1015 cm−3.

Investigating the effect of GaCl3 incorporation into the usual CdCl2 treatment on CdTe-based solar cell device structures

The incorporation of GaCl3 into the usual CdCl2 post-deposition treatment solution of CdTe-based solar cells has been investigated. Both CdS and CdTe layers used in this work were prepared by electroplating technique and they are n-type in electrical conduction as observed from photoelectrochemical cell measurement technique. Before applying the chemical treatments to the device structures, the effect of GaCl3 incorporation into the usual CdCl2 treatment was first studied on the structural and optical properties of CdTe thin films. The results of the optical properties show that the bandgap of CdTe thin films treated with a mixture of GaCl3 + CdCl2 is closer to that of bulk CdTe layers than the ones treated only with CdCl2 solution. The structural properties also showed that CdTe thin films treated with GaCl3 + CdCl2 is more crystalline than CdTe thin films treated only with CdCl2 solution. The addition of GaCl3 into the CdCl2 solution have been seen to drastically enhance the solar-to-electric conversion efficiency of CdS/CdTe based solar cells. One of the effects of Ga incorporation into the usual CdCl2 treatment was seen in the series resistance reduction which ultimately leads to enhancement in the observed short-circuit current density, fill factor and overall solar cell efficiency. For the glass/FTO/n-CdS/n-CdTe device structures, the cell efficiencies were observed in the range 1.9–2.1% after being treated with CdCl2 solution only. When treated with CdCl2 + GaCl3, the efficiency increased to 6.1–6.4%. Subsequent study on multi-junction graded bandgap solar cells using the GaCl3 + CdCl2 chemical solution for the surface treatment of glass/FTO/n-ZnS/n-CdS/n-CdTe device structures results in solar cell efficiency >10%.

15.3% efficient graded bandgap solar cells fabricated using electroplated CdS and CdTe thin films

Making use of previously designed and experimentally tested results of graded bandgap devices, and the comprehensive electrodeposition of semiconducting materials knowledge, a three layer n-n-p device structure was fabricated and tested for their electronic properties and solar cell performance. Glass/FTO/n-CdS/n-CdTe/p-CdTe/Au devices were fabricated and studied as a first step towards development of graded bandgap devices using electroplated materials. Efficiencies up to 15.3% were observed for lab-scale small devices.

Next Generation Solar Cells Based on Graded Bandgap Device Structures Utilising Rod-Type Nano-Materials

Current solar cells under research and development utilise mainly one absorber layer limiting the photon harvesting capabilities. In order to develop next generation solar cells, research should move towards effective photon harvesting methods utilising low-cost solar energy materials. This will lead to reduce the $W−1 figure for direct solar energy conversion to electrical energy. In this work, a graded bandgap solar cell has been designed to absorb all photons from the UV, visible and IR regions. In addition, impurity PV effect and impact ionisation have been incorporated to enhance charge carrier creation within the same device. This new design has been experimentally tested using the most researched MOCVD grown GaAs/AlGaAs system, in order to confirm its validity. Devices with high Voc ~ 1175 mV and the highest possible FF ~ (0.85–0.87) have been produced, increasing the conversion efficiency to ~20% within only two growth runs. These devices were also experimentally tested for the existence of impurity PV effect and impact ionisation. The devices are PV active in complete darkness producing over 800 mV, Voc indicating the harvesting of IR radiation from the surroundings through impurity PV effect. The quantum efficiency measurements show over 140% signal confirming the contribution to PV action from impact ionisation. Since the concept is successfully proven, the low-cost and scalable electrodeposited semiconducting layers are used to produce graded bandgap solar cell structures. The utilisation of nano- and micro-rod type materials in graded bandgap devices are also presented and discussed in this paper. Preliminary work on glass/FTO/n-ZnS/n-CdS/n-CdTe/Au graded bandgap devices show 10%–12% efficient devices indicating extremely high Jsc values ~48 mA·cm−2, showing the high potential of these devices in achieving higher efficiencies. The detailed results on these low-cost and novel graded bandgap devices are presented in a separate publication.

A simple model for the photocurrent density of a graded band gap CIGS thin film solar cell

Abstract A simple model for the photocurrent density of a linearly graded band gap Cu(In,Ga)Se2 solar cell is presented. Both generation and recombination mechanisms in the space charge region and absorber region of the cell are considered. The carrier collection function and effective absorption coefficient are introduced in the calculations to obtain a more realistic model. The results show that photocurrent density of the graded band-gap solar cell is higher than that with a constant averaged band gap. There is an optimum for grading strength or band gap widening of the absorber region. Recombination current reduces the photocurrent density with a lower reduction in the absorber material than in the depletion region. For longer diffusion lengths (or greater values of carrier collection factor), a higher photocurrent density is obtained except where collection probability is already unity everywhere in the absorber.

Theoretical Analysis of AlxGa1-xAs/GaAs Graded Band Gap Solar Cell

The graded band gap AlxGa1-xAs/GaAs solar cell model has been proposed. This model consists of graded compositions material in the surface layer extending from y=0 to y=D, and uniform band gap from y=D to y=L. The model simulates the absorption, generation, current density, spectral response and the solar efficiency generated from the solar spectrum. These photovoltaic parameters have been calculated by a GRAD computer program. Electric field, mobility and diffusion length of holes gradient are assumed constant in the model to get an analytical solutions. The photovoltaic parameters, obtained from the model, are Jsc =33 mA/cm 2 , Voc= .91 V, FF= .87 and η= 31 %. The

Electrodeposition and characterisation of ZnTe layers for application in CdTe based multi-layer graded bandgap solar cells

Zinc Telluride (ZnTe) thin films have been deposited on glass/conducting glass substrates using low-cost aqueous electrodeposition (ED) method. The structural, optical and morphological properties of the resulting films have been characterized using X-ray diffraction (XRD), spectrophotometry and Scanning Electron Microscopy (SEM). It has been confirmed by XRD technique that the deposited layers are nano- and polycrystalline mixture for as deposited layers. Photoelectrochemical (PEC) studies revealed that the layers are p-type in electrical conduction. Optical absorption measurement has been used for the bandgap determination of the deposited layers. The bandgap of the polycrystalline ZnTe layers are in the range (2.65 – 2.75) eV for the ZnSO4 precursor, and (2.70 – 2.87) eV for the ZnCl2 precursor instead of (2.21 – 2.26) eV reported for crystalline ZnTe. This increase may be due to the quantum effect which confirmed by the absence of strong XRD peaks from these layers and/or inclusion of ZnO in the deposited layers.

Electrodeposition of CuInSe2 layers using a two-electrode system for applications in multi-layer graded bandgap solar cells

In order to develop low-cost, large area multi-layer graded bandgap solar cell structures, CuInSe2 layers were grown using a simplified two-electrode system. Atomic force microscopy (AFM) shows the layers grown consist of nano- and micro-size particles. Photoelectrochernical (PEC) cell, X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) measurements confirm that it is possible to grow CuInSe2 layers with p-, i- and n-type electrical conduction, as pre-determined for applications in multi-layer device structures. XRF, XPS and PEC measurements show that Cu-richness provides p-type conduction and In-richness provides n-type conduction in electrodeposited CuInSe2 layers. It is also possible to grow materials with different bandgaps in the range 1.00-1.90eV. The combination of these two properties allows growth of multi-layer structures and preliminary work on these devices show good rectifying properties and exhibit photovoltaic activity. These new developments will be presented in this paper. (c) 2006 Elsevier B.V. All rights reserved.

Numerical simulation of the limiting efficiency of the graded bandgap solar cell

The limiting efficiency of the compositionally graded bandgap solar cell is calculated using a 1-D numerical simulator. Our simulator calculates the limiting efficiency by solving the coupled semiconductor equations, namely, Poisson's equation, the current continuity equations, together with the Boltzmann photon equation, rather than the detailed balance equations that are usually used in such calculations. The non-avoidable radiative and Auger bulk losses are the only losses considered in the calculations. The effect of photon recycling on the cell's parameters is included in the calculations of radiative recombination rates. We verified numerically that bandgap grading, under optimum profile, increases the limiting efficiency of the solar cell, over the previously published values. The effect of the bandgap grading on desensitizing the surface of the cell is discussed.

Photon recycling in the graded bandgap solar cell

We derived a general integral expression for the carrier radiative recombination rate in solar cells. The photon Boltzmann equation is solved taking into account the photon recycling effect inside the cell and assuming arbitrary spatial variation of the absorption coefficient. This expression can thus be used for graded bandgap solar cells. Copyright © 2006 John Wiley & Sons, Ltd.

Third generation multi-layer graded band gap solar cells for achieving high conversion efficiencies—II: Experimental results

New designs of multi-layer graded band gap solar cell structures were experimentally tested using well-understood AlxGa(1-x)As materials grown by the MOVPE technique. Laboratory scale devices (0.5 mm diameter) were processed and measured for their performance as solar cells. Both V-oc (1110mV) and fill factors (similar to 83%) for the best devices have shown drastic improvements over existing cells and the short-circuit current densities measured are in the range (10-20) mA cm(-2). (c) 2005 Elsevier B.V. All rights reserved.