Graded Band Gap Research Articles

Method to Determine the Recombination Characteristics of Minority Carriers in Graded-Band-Gap Solar Cells

In solar-cell technology, band-gap grading has been introduced to enhance the efficiency of charge collection, by inducing drift motion of charge carriers. Characterizing such systems accurately is tricky, though, due to technical difficulty in clearly separating carrier diffusion from drift. This study uses wavelength-dependent lateral photocurrent in a generic device structure to simultaneously characterize the electrical qualities of both the light absorber and the back surface in a graded-band-gap solar cell. The technique is readily applicable to many types of photoactive devices, because it works well without any knowledge of the actual band-gap grading.

Wavelength-Dependent Charge Carrier Dynamics for Single Pixel Color Sensing Using Graded Perovskite Structures.

We report smart color-sensing devices of two-dimensional lead halide perovskites that exhibit a graded band gap across the film. We observe that the device short-circuit photocurrent is strongly dependent on excitation wavelength λ, and this arises through photoabsorption at different depths in the sample due to the graded bandgaps present. This λ signature in the response of the device is observed in case of steady-state excitation when incident from the high bandgap side of the film, where a complete reversal in the polarity of the photocurrent Iph(t) is obtained as the excitation wavelength is spanned across the visible spectrum. The transient photocurrent reveals λ-specific response arrived from a combination of positive and negative Iph(t) components. The uniqueness of Iph(t) as a function of incident λ can be utilized to examine spectral purity without dispersive optical elements. An equivalent circuit model description provides a possible glimpse into the physical sources involved in contributing to these features.

Is It Possible To Develop Complex S-Se Graded Band Gap Profiles in Kesterite-Based Solar Cells?

This work presents the development of a novel chalcogenization process for the fabrication of Cu2ZnSn(S,Se)4 (CZTSSe or kesterite)-based solar cells that enable the generation of sharp graded anionic compositional profiles with high S content at the top and low S content at the bottom. This is achieved through the optimization of the annealing parameters including the study of several sulfur sources with different predicted reactivities (elemental S, thiourea, SnS, and SeS2). As a result, depending on the sulfur source employed, devices with superficially localized maximum sulfur content between 50 and 20% within the charge depletion zone and between 10 and 30% toward the bulk material are obtained. This complex graded structure is confirmed and characterized by combining multiwavelength depth-resolved Raman spectroscopy measurements together with in-depth Auger electron spectroscopy and X-ray fluorescence. In addition, the devices fabricated with such graded band gap absorbers are shown to be fully functional with conversion efficiencies around 9% and with improved VOC deficit values that correlate with the presence of a gradient. These results represent one step forward toward anionic band gap grading in kesterite solar cells.

Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading.

The power conversion efficiency of an ultrathin CuIn1-ξGaξSe2 (CIGS) solar cell was maximized using a coupled optoelectronic model to determine the optimal bandgap grading of the nonhomogeneous CIGS layer in the thickness direction. The bandgap of the CIGS layer was either sinusoidally or linearly graded, and the solar cell was modeled to have a metallic backreflector corrugated periodically along a fixed direction in the plane. The model predicts that specially tailored bandgap grading can significantly improve the efficiency, with much smaller improvements due to the periodic corrugations. An efficiency of 27.7% with the conventional 2200-nm-thick CIGS layer is predicted with sinusoidal bandgap grading, in comparison to 22% efficiency obtained experimentally with homogeneous bandgap. Furthermore, the inclusion of sinusoidal grading increases the predicted efficiency to 22.89% with just a 600-nm-thick CIGS layer. These high efficiencies arise due to a large electron-hole pair generation rate in the narrow-bandgap regions and the elevation of the open-circuit voltage due to a wider bandgap in the region toward the front surface of the CIGS layer. Thus, bandgap nonhomogeneity, in conjunction with periodic corrugation of the backreflector, can be effective in realizing ultrathin CIGS solar cells that can help overcome the scarcity of indium.

Correlation between carrier recombination and valence band offset effect of graded Cu(In,Ga)Se2 solar cells

Carrier recombination in Cu(In,Ga)Se2 (CIGS) solar cells is one of the critical factors determining cell performance loss. Additionally, heterointerface recombination is the most detrimental recombination pathway based on the Shockley–Read–Hall mechanism. Various methods have been applied to impede interface recombination, and the introduction of a valence band offset (ΔEV) produced by a homogeneous Cu-deficient layer (CDL) has been proved to be effective for improving cell efficiency. However, the relationship between ΔEV and recombination processes has not yet been fully clarified. In this work, through a Se interval irradiation process, ΔEV was incorporated into CIGS solar cells with bandgap grading so as to explore the correlation between the ΔEV effect and heterointerface recombination, which was demonstrated by both SCAPS-1D simulation and experiment. Moreover, illumination-dependent open-circuit voltage (Suns–Voc) and temperature-dependent open-circuit voltage (Voc–T) measurements were performed to quantitatively estimate the recombination rates at the interface, in the space-charge region (SCR), and in the quasi-neutral region (QNR), contributing to elaboration of the mechanism by which ΔEV serves as a hole barrier at the interface, thereby reducing heterointerface recombination and enhancing cell performance.

CuZnInSe3‐based solar cells: Impact of copper concentration on vibrational and structural properties and device performance

AbstractCuZnInSe3 (CZISe) is an interesting alternative for the acknowledged Cu(In,Ga)Se2 absorber layer in thin film solar cells. While the partial replacement of scarce and expensive indium and gallium by zinc decreases manufacturing costs, the solid solution between CuInSe2 and ZnSe opens interesting options for band gap tuning and grading. Its potential as an absorber layer in photovoltaic devices has been demonstrated by obtaining 7.4 and 7.6 % efficiency in CZISSe‐ and CZISe‐based devices, respectively. On the other hand, the inherent complexity of the quaternary CZISe together with a lack of fundamental insights puts a limit to its current development. We present insights on the influence of the copper content ([Cu]/([Zn] + [In]) ratio) on the structural and optoelectronic properties of CZISe as well as the formation of secondary phases. By means of XRD and Raman scattering analyses, in addition to the sphalerite CZISe structure, a chalcopyrite Cu‐In‐Zn‐Se phase was found for high copper concentrations. On the contrary, for low Cu concentrations, unambiguous indications of a new ordered vacancy compound (OVC)–like phase formation both in XRD patterns and in Raman spectra were found. Conditions of pre‐resonant Raman scattering were applied to emphasize the new found phase and to estimate its concentration. Finally, the influence of each phase on the optoelectronic parameters and performance of solar cells with efficiencies of up to 7.4 % was studied.

Numerical reproduction of a perovskite solar cell by device simulation considering band gap grading

Recent progress in halide perovskite solar cells exceeding 20% efficiency is remarkable. However, the understanding of device operation based on reasonable device model and physical parameters is not sufficient. In this study, experimental current density-voltage (J-V) characteristic and external quantum efficiency (EQE) of a 20%-efficient perovskite solar cell with mixed halide of I and Br in a literature were numerically reproduced by a device simulator called Solar Cell Capacitance Simulator (SCAPS), where device parameters extracted by fitting experimental J-V and EQE curves were used. It is demonstrated that the simulated EQE curve was not matched with the experimental one under the assumption of uniform band gap energy (Eg) resulting from a constant Br content throughout the perovskite absorber. On the other hand, the experimental J-V and EQE curves were reasonably reproduced by the simulation under the assumption of single-graded Eg of the absorber, resulting from the graded Br content. Moreover, the perovskite solar cell with Eg-grading absorber has theoretically showed higher open-circuit voltage and power conversion efficiency (PCE) than those with uniform Eg. The results are useful to not only improve the understanding of device operation but also indicate possibility of the further improvement of PCE using Eg-grading perovskite absorber.

The characteristics of Cu(In, Ga)Se2 thin-film solar cells by bandgap grading

We investigated the characteristics of Cu(In, Ga)Se2 solar cells with bandgap (Eg) grading. Two precursor types were employed: Mo/Cu0.75Ga0.25/In/Ga2Se3 (CIGSe-1) and Mo/Cu/In/Ga2Se3 (CIGSe-2). In CIGSe-1, the range of depths with a high Ga content is wider than that in CIGSe-2; thus, the region in which the main electron-trapping clusters and high-population deep donor defects can form is larger, and the defect density is higher. In the defect energy level range, various other defects and defect clusters exist with a defect density of 2.83 × 1015 cm−3 within the CIGS-1 absorber layer and 2.37 × 1015 cm−3 within the CIGS-2 absorber layer. The average efficiency values are 5.71% for CIGSe-1 and 6.82% for CIGSe-2. Additionally, the average VOC deficit (Eg/q − VOC) values are 0.758 V for CIGSe-1 and 0.731 V for CIGSe-2. As a result, in the 7 CIGSe-2 samples, the open-circuit voltage and efficiencies are improved. Thus, it is demonstrated that appropriate Eg grading in a CIGSe layer with a wider Eg on the back surface can result in improved performance.

Formation of the front-gradient bandgap in the Ag doped CZTSe thin films and solar cells

Abstract The graded bandgap of kesterite based absorber layer is an important way to achieve high efficiency solar cells. Incorporation of Ag into CZTSSe thin films can adjust the bandgap and thus reduce the VOC-deficit and improve the quality of crystallization. However, the distribution of Ag is difficult to control due to the quick diffusion of Ag under the high temperature. In this study, we achieve the front Ag-gradient in kesterite structured compound films by prealloying followed by selenization process at 550 °C. AgZn3, Ag3Sn, and Sn–Ag–Cu alloy phases were formed during prealloying stage at 250 °C. After prealloying process, Ag tends to distribute at the front surface of the ACZTSe thin films. Combining the results of experiment and SCAPS simulation, the significantly VOC improvement of devices is ascribed to the formation of the front Ag-gradient bandgap structure in the absorber layer. This facile prealloying selenization process affords a feasible method to design the graded bandgap structure absorber layers, which will promote the fabrication of high efficient graded bandgap structure solar cells.

Selenization of CuInS2 by rapid thermal processing – an alternative approach to induce a band gap grading in chalcopyrite thin-film solar cell absorbers?

Characterization of selenium-treated CuInS2 reveals a band gap grading and a surface Cu enrichment, opening new chalcopyrite absorber tailoring opportunities.

RbF post deposition treatment for narrow bandgap Cu(In,Ga)Se2 solar cells

Multi-junction solar cells are known to have a considerably increased efficiency potential over their typical single junction counterparts. In order to produce low cost and lightweight multi-junction devices, the availability of suitable narrow (<1.1 eV) bandgap bottom cells is paramount. A possible absorber for such a bottom cell is the Cu(In,Ga)Se2 (CIGS) compound semiconductor, one of the most efficient thin film materials to date.In this contribution we report on the RbF post deposition treatment of narrow bandgap CIGS absorbers grown with a single bandgap grading approach. We discuss the necessary deposition conditions and the observed improvements on solar cells performance. A certified record efficiency of 18.0% for an absorber with 1.00 eV optoelectronic bandgap is presented and its suitability for perovskite/CIGS tandem devices is shown.

CIGS thin film solar cells with graded-band gap fabricated by CIS/CGS bilayer and CGS/CIS/CGS trilayer systems

High efficiency CuIn1-xGaxSe2 (CIGS) thin film solar cells are usually deposited on Mo-coated soda-lime glass (SLG) substrates by the three-stage co-evaporation process which is intended to create double-graded band gap. It requires short total deposition time and rapid changing of substrate temperature in order to obtain the double-graded band gap which is quite difficult for high source-temperatures. When the total deposition time cannot be made short enough, uniform band gap is rather achieved. In this work, the depositions of CIS/CGS bilayer are employed in order to create back grading while CGS/CIS/CGS trilayers are for the double-grading, owing to non-uniformity and different diffusivity of In and Ga constituents in the films. The bilayers with back grading show the increasing short-circuit current density (Jsc) due to the assisting back surface field, but the open-circuit voltage (Voc) is relatively low due to the reduction of Ga content at the front surface. For the CGS/CIS/CGS trilayers, double grading and the increasing of Voc are observed when compared with the bilayers due to the increasing Ga content near the junction. The efficiencies of the devices fabricated from the CIS/CGS bilayer and CGS/CIS/CGS trilayer absorbers show the maximum value of 12.5% and 15.5%, respectively. The external quantum efficiency (EQE) of the bilayer and trilayer absorbers exhibit the enhancement in the long wavelengths compared to that of the three-stage process absorbers.

Temperature Dependence of the Internal Quantum Efficiency of Cu(In,Ga)Se2-Based Solar Cells

We measured the temperature-dependent internal quantum efficiency (IQE) of Cu(In,Ga)Se2-based (CIGS) solar cells. The largest differences in IQE spectra measured between 100 and 300 K were observed in the wavelength range, corresponding to the light absorbed exclusively in the CIGS layer. Absorbers in the investigated cells were grown using a one-stage process. Since all elements are supplied at a constant rate, the obtained layers are free of the band gap grading, therefore collection in the bulk of the layer is not affected by a quasi-electrical field. This allows us to discuss temperature changes in collection solely in terms of recombination. We associate the change in IQE with recombination via defects present in the bulk of absorber. The two cases of donor and acceptor defects are discussed. Using SCAPS software and basic handbook formulas that describe the emission and capture rates of carriers, we estimate a range of basic parameters of the possible bulk defects in CIGS that are responsible for the temperature change of IQE spectra. Our results suggest that IQE may be controlled by shallow defects of ionization energy of 45 and 60 meV for the donor and acceptor cases, respectively. We calculate IQE spectra at different temperatures. The temperature change of simulated spectra reproduces the same tendency as experimental characteristics.

Simulation of optimum band structure of HTM-free perovskite solar cells based on ZnO electron transporting layer

Expensive gold electrode and hole transporting material (HTM) currently used in perovskite solar cells are the major economic constraints to the scaling-up of this promising technology. Designing high-efficiency perovskite solar cell with graded band-gap structure to replace HTM and with low-cost electrode is urgently needed for real cell production. In our work numerical simulation of perovskite solar cell with the configuration of FTO/ZnO/CH3NH3Pb(I1-xBrx)3/Carbon is performed using SCAPS-1D program. The band gap of CH3NH3Pb(I1-xBrx)3 absorber is tuned in the range of 1.5 eV to 2.3 eV by variation of the Br doping content. Both the uniform band gap and graded band gap absorber were examined. Based upon the simulated results, a promising efficiency of 17.89% can be realized with a back grading profile that consists of a graded thickness of 50 nm as well as 1.9 eV band gap at back surface. This work will provide guidelines for further efficiency enhancement of low cost perovskite solar cells.

Studies on the graded band-gap copper indium di-selenide thin film solar cells prepared by electrochemical route

A graded band-gap CuInSe2 (CIS) thin film solar cell (TFSC) having glass/FTO/CdS/CIS multilayer/Au structure has been fabricated. A simple and low-cost electrodeposition technique is used to deposit the multilayers of CIS onto fluorine doped tin oxide (FTO) coated glass substrate. A conventional three-electrode geometry consisting, FTO, graphite and Ag/AgCl as a working, counter and reference electrodes, respectively was used for electrodeposition. Structural characterization was carried out using X-ray diffraction (XRD) and Raman spectroscopy, which revealed the chalcopyrite tetragonal CIS structure with a quite Cu-rich surface which reduces upon selenization. The morphology of the as grown and selenized CIS multilayer thin films was studied by using atomic force microscopy (AFM) which shows the compact and uniform layer formation. The depth profile distribution of individual elements in both as-grown and selenized CIS multilayer thin films has been determined using secondary ion mass spectroscopy (SIMS). SIMS results revealed that the proposed graded band gap structure is retained even after selenization. The presence of Cu+, In3+ and Se2− oxidation states were confirmed using X-ray photoelectron spectroscopy (XPS). A single layer and multilayer CIS solar cell devices yielded ∼5.10% and ∼7.20% power conversion efficiency, respectively. In the present work, pH 3 buffer solution helps to improve the morphology of CIS layer which gives the better power conversion efficiency as compared to the previously reported value.

Electrical characterization of insulator-semiconductor systems based on graded band gap MBE HgCdTe with atomic layer deposited Al2O3 films for infrared detector passivation

The electrical properties of MIS structures based on graded band gap Hg0.77Cd0.23Te grown by the molecular beam epitaxy method with the Al2O3 dielectric formed by the plasma enhanced atomic layer deposition method were investigated. It is established that for such structures the recharge of slow states takes place and significantly distorts the form of the capacitance-voltage (C-V) characteristics. A technique is proposed for constructing the C-V curves of HgCdTe MIS structures, which makes it possible to eliminate the effect of slow surface states. It is shown that Al2O3 films are suitable for passivation of infrared detectors due to small flat band voltages leading to the realization of the depletion or weak inversion modes. The hysteresis of electrical characteristics is eliminated when an intermediate CdTe layer with a thickness of about 0.2 μm is grown in situ during the epitaxial growth.

Full-inorganic Sb2(S,Se)3 solar cells using carbon as both hole selection material and electrode

Sb2(S,Se)3 are considered as a promising absorber material for solar cells owing to its earth-abundant, nontoxic constituent, simple phase and long-term durability. In this work, a facile chemical bath deposition followed by post-selenization was introduced to prepare Sb2(S,Se)3 thin film, which leads to a graded band gap structure. The sulfoselenide Sb2(S,Se)3 thin films with smooth morphology were obtained through optimizing the selenization process. Meanwhile, a low cost and substitutable carbon electrode was used as hole selection material. Sequentially, the full-inorganic Sb2(S,Se)3 device with efficiency of 2.64% was also fabricated according to structure of FTO/CdS/Sb2(S,Se)3/Carbon, which is the highest value in CBD processed FTO/CdS/Sb2(S, Se)3/Carbon solar cells.

From sputtered metal precursors towards Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells with shallow back grading

Bandgap grading is often employed in thin film solar cell absorbers for creating the back surface field that can reduce interface recombination at the back contact. Here, we investigate different pathways to obtain back graded Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells based on a co-sputtered metal precursor and rapid thermal annealing route. The absorber bandgap can be precisely tuned for the whole compositional range of x = 0…1. While Ge does not accumulate towards the back in absorbers fabricated from uniform precursor, Ge-back graded absorbers can be obtained from stacked metal precursors. A linear back grading with a bandgap energy difference of up to 40 meV has been achieved. However, no significant improvement in open-circuit voltage and near-infrared response could be observed for the kesterite devices. This indicates that even steeper gradients are required to obtain an effective back surface field.

Low temperature incorporation of selenium in Cu2ZnSnS4: Diffusion and nucleation

Band gap grading of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells can be achieved by varying the Sr = [S]/([S] + [Se]) ratio in the absorber layer with depth. One approach is a two-step annealing process where the absorber is first sulfurized to Cu2ZnSnS4 (CZTS) followed by selenization to form CZTSSe. However, once nucleation of CZTSSe initiates, the rapid interchange of S and Se limits the control over the Sr ratio with depth. Here, we have studied incorporation of Se into CZTS and observed the behavior of Se below and up to the nucleation temperature of CZTSSe. Se diffusion at 337 and 360 °C is dominated by grain boundary diffusion while some increase of Se is also seen in the region from 100 to 800 nm from the surface. After selenization at 409 °C, recrystallization is observed and CZTSSe grains are formed. The recrystallization is more rapid for a smaller average grain size and is facilitated by diffusion of Na from the back contact. The grain boundary diffusion is identified with secondary ion mass spectrometry measurements by measuring the accumulation in the CZTS/Mo interface for three samples with different average grain size.

Simulation of graded bandgap on the performance of back-wall superstrate CIGS solar cells

In this work, a model of back-wall superstrate CuInxGa1-xSe2 (CIGS) thin film solar cell with Glass/SnO2:F/CIGS/CdS/ZnO/Ag was established by a simulation tool SCAPS. The influence of bandgap grading, accepter density and the thickness of CIGS layer on the performance of superstrate device has been systematically investigated. The simulation was carried by lighting through SnO2:F transparent contact. The simulation results showed that a efficiency of 15.8% could be reached when the graded-bandgap structure near the SnO2:F contact was established. However, only slight improvement on the efficiency could be achieved by the graded-bandgap in space charge region (SCR).The result indicates that the key reason which limits the efficiency of the back-wall device is the recombination from the SnO2:F/CIGS contact. In addition, in order to achieve high efficiency on the superstrate device, the accepter density in the CIGS layer should be kept below 1.0 × 1016 cm−3, and the thickness should be set at 600 nm.