Heterojunction Photovoltaic Devices Research Articles

TiO2/Se heterojunction photovoltaic device made from Se/Te/Se stacked precursor

This paper presents an innovative approach for forming crystallized selenium (c-Se) using a Se/Te/Se stacked precursor. The aim is to enhance the adhesion between the TiO2 and Se layers to improve the power conversion efficiency of TiO2/Se heterojunction photovoltaic devices. Our previous study revealed that the accumulation of Te near the TiO2/Se interface resulted in insufficient shunt resistance, leading to increased dark leakage current. This issue arises because Te possesses a high carrier density (∼1018 cm−3) and narrow bandgap energy (0.26 eV). However, a Te additive layer remains necessary to passivate the unbonded TiO2/Se interface and to maximize the adhesion between the absorber and the substrate. Previous research on Se-based photovoltaic devices demonstrated that an extremely thin (∼0.5 nm) and homogeneous Te adhesion layer could be formed beneath the Se photo-absorbing layer and subsequently crystallized through air-annealing post-deposition. However, achieving precise and homogeneous sub-nanometer Te films using a compact resistance heating evaporation system in a laboratory setting proved challenging so far. To address these limitations, we propose a novel technique for forming the Se absorber layer by post-annealing a stacked 1st-Se/Te/2nd-Se precursor on a TiO2 window layer. Through experimentation, we achieved the highest power conversion efficiency of 4.49 % without antireflective coating, accompanied by Voc 0.795V, JSC 11.13 mA/cm2, and FF 0.507 when the 1st-Se thickness was 5 nm. Furthermore, the impact of decreasing Te/(Te + Se) near the TiO2 window layer on photovoltaic performance was theoretically predicted using modelling by SCAPS-1D. Overall, this work introduces a promising method for enhancing the formation of c-Se, improving adhesion between TiO2 and Se layers, and enhancing the power conversion efficiency of TiO2/Se heterojunction photovoltaic devices.

Enhancing bio-photovoltaic cell efficiency: A study in transport property

Recently, bio-solar cells have garnered significant attention owing to their relatively high efficiency and cost-effectiveness, positioning them as alternative devices for solar energy harvesting. This study delves into the optical and electrical transport characteristics of a novel bilayer bio-solar cell using the total system transfer matrix method, theoretically. These bilayer bio-solar cells consist of two distinct layers of Chlorophyll-a and Chlorophyll-b, forming a heterojunction in the bio-solar cell architecture. Within these solar cells, photon absorption by the active layers of chlorophyll generates excitons and quasi-particles (bound electron and hole pairs), which subsequently diffuse through the collecting layer, resulting in significant recombination losses. The influence of the thickness of chlorophyll adsorbent active layers on the short circuit current density and quantum efficiency is examined through numerical calculations. Furthermore, we develop models for the solar cell parameters of thin film heterojunction photovoltaic devices based on Chlorophyll-a/Chlorophyll-b. The results indicate efficient short circuit current and quantum efficiency values (or power conversion efficiency) of 72 A/m2 and 3.30 %, respectively. It is demonstrated that optimizing the thickness of bio-solar cells based on these findings may enhance electron transport, leading to higher photocurrents and improved overall efficiency. These simulation outcomes pave the way for optimizing the thickness of bio-solar cells and advancing towards higher efficiencies.

Utilizing machine learning to enhance performance of thin-film solar cells based on Sb2(S x Se1-x )3: investigating the influence of material properties.

Antimony chalcogenides (Sb2(S x Se1-x )3) have drawn attention as a potential semiconducting substance for heterojunction photovoltaic (PV) devices due to the remarkable optoelectronic properties and wide range of bandgaps spanning from 1.1 to 1.7 eV. In this investigation, SCAPS-1D simulation software is employed to design an earth abundant, non-toxic, and cost-effective antimony sulfide-selenide (Sb2(S,Se)3)-based thin-film solar cell (TFSC), where tungsten disulfide (WS2) and cuprous oxide (Cu2O) are used as an electron transport layer (ETL) and hole transport layer (HTL), respectively. The PV performance parameters such as power conversion efficiency, open-circuit voltage (V oc), short-circuit current (J sc), and fill factor (FF) are assessed through adjustments in material properties including thickness, acceptor concentration, bulk defect density of the absorber, defect state of absorber/ETL and HTL/absorber interfaces, operating temperature, work function of the rear electrode, and cell resistances. This analysis aims to validate their collective impact on the overall efficiency of the designed Ni/Cu2O/Sb2(S,Se)3/WS2/FTO/Al TFSC. The optimized physical parameters for the Sb2(S,Se)3 TFSC lead to impressive PV outputs with an efficiency of 30.18%, V oc of 1.02 V, J sc of 33.65 mA cm-2, and FF of 87.59%. Furthermore, an artificial neural network (ANN) machine learning (ML) algorithm predicts the optimal PCE by considering five semiconductor parameters: absorber layer thickness, bandgap, electron affinity, electron mobility, and hole mobility. This model, which has an approximate correlation coefficient (R 2) of 0.999, is able to predict the data with precision. This numerical analysis provides valuable data for the fabrication of an environmentally friendly, economical, and incredibly non-toxic efficient heterojunction TFSC.

Textured p-type crystalline silicon surfaces obtained by multi-step plasma process for SHJ solar cells

P-type monocrystalline silicon wafers were textured by means of a multi-step plasma etching technique with the aim of greatly reducing their reflectance and effectively exploiting the highest possible amount of solar radiation in heterojunction photovoltaic devices. Suitable surface morphologies, characterized by high light scattering, have to be obtained without damaging the c-Si electrical properties. For this reason, some aspects of the plasma etching process have been studied and improved. Modifications have been made to obtain both “cleaner” processes and energetically lower ion bombardment. By operating at high pressure (0.4 mbar), a process controlled by diffusion of reactive species towards the substrate was favoured. Another innovative aspect consisted in two “reconditioning” plasma processes of the Si surface before the actual silicon removal step with SF6 /O2 gas mixtures. A strong reduction in the average optical reflection down to 5% was obtained coupled with a minority carriers lifetime (τeff) of about 950 μs, value comparable to that of the untreated silicon. An interesting surface morphology, consisting in “U" shaped cavities, was obtained. Texture irregularities were easily removed by acid etching. The moderate depth of the cavities is very promising for an effective passivation with a-Si:H thin films.

Field-Effect Passivation of the Cu2O/ZnO Transparent Heterojunction Photovoltaic Device Using Ga2O3 Thin Film

Photovoltaics (PV) would be more promising if light could generate electric power invisibly. This will endow more degrees of freedom to PV cells for wide-range deployment. Transparent photovoltaic (TPV) devices are the groundwork of the building-integrated photovoltaic (BIPV) systems that provide aesthetics to the buildings and solar energy convertor modules. TPV is adequate for the BIPV system of windows. Heterojunctions between p-type Cu2O and n-type ZnO have emerged as one of the most promising TPV devices. However, their inadequate transparency and a deficit of open-circuit voltage (VOC) and short-circuit current density (JSC) remained an open challenge. In this work, we have fabricated a thin Ga2O3 buffer layer embedded transparent ZnO/Cu2O heterojunction photovoltaic device, showing an average visible transmittance of more than 50%. The insertion of the Ga2O3 buffer layer remarkably increases the JSC and VOC to ∼860% and ∼41%, respectively. The TPV device also demonstrated a high JSC of 3.41 mA/cm2 under UV light. The Ga2O3 layer provides a graded conduction band alignment and passivates the interface by the field-effect passivation mechanism. The Ga2O3 buffer layer embedded device also demonstrated a broadband photodetection with remarkably high responsivity and detectivity of 250 mA/W and 5 × 1011 Jones at self-biased conditions.

Gate‐Tunable Photovoltaic Efficiency in Graphene‐Sandwiched PdSe2 Photodetectors with Restrained Carrier Recombination

AbstractThe exploration of various two‐dimensional (2D) materials provides a promising platform for constructing van der Waals heterojunction (vdWH) photovoltaic devices. The photovoltaic efficiency of such devices is still a challenge due to the recombination of photogenerated carriers resulting from the intrinsic doping property of materials. Here, PdSe2‐based graphene‐sandwiched vertical devices with high photovoltaic efficiency are constructed. As the graphene‐sandwiched structure limits the diffusion length to tens of nanometers, the carrier recombination during diffusion is spontaneously reduced. Moreover, due to the low‐resistance contact of graphene/PdSe2, the Fermi level and carrier density of PdSe2 can still be significantly tuned by gate voltage, which leads to a sharp enhancement of built‐in field and photovoltaic effect. As a result, the Gr/PdSe2/MoS2/Gr vdWH photovoltaic device shows a high power conversion efficiency of 3.9% and an open circuit voltage of 0.6 V under 650 nm laser illumination at room temperature. The present work provides an efficient scheme for the design of high‐efficiency 2D material photovoltaic devices.

Demonstration of an Integrated Inorganic–Organic IoT-Enabled System With PV and Electrochromic Devices for Autonomous Smart Windows

Smart windows may incorporate integrated stacks of photovoltaic and electrochromic devices, whose opacity/transparency can be synergistically modulated (and/or wirelessly controlled) by autonomously generated voltages. This article demonstrates a proof of concept of an Internet of Things (IoT)-enabled system comprised of an integrated bulk (organic) heterojunction photovoltaic device with an (inorganic) electrochromic device. First, fully organic (poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2 -thienyl-2’,1’,3’-benzothiadiazole)]:phenyl-C71-butyric-acid-methyl) photovoltaic devices were fabricated, and their performance were evaluated with respect to optical transparency and power conversion efficiency (PCE). Upon varying the hole transport layers and substrates, the strongest performing devices exhibited a PCE of 3.2%, an open-circuit voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_{\text{oc}}$</tex-math></inline-formula> ) of 0.9 V, and a short-circuit current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$J_{\text{sc}}$</tex-math></inline-formula> ) of 10–15 mA/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{^{2}}$</tex-math></inline-formula> . Second, tungsten trioxide WO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> electrochromic films were inkjet printed on conductive and transparent ITO-coated PET substrates of varying mechanical flexibility, including PDMS with a network of embedded Ag nanowires and indium- tin-oxide-coated polyethylene terephthalate. The printed electrochromic devices demonstrated clear switching behavior under external bias, with a coloration time of 8 s, a bleaching time of 12 s, and an optical modulation of 0.5874 at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda$</tex-math></inline-formula> = 525 nm. Finally, the photovoltaic and electrochromic devices were connected, with a network configuration of the former to provide requisite autonomous power for the demonstration of opacity modulation (or light transmission properties) of the latter, utilizing a Particle IoT controller that was switched wirelessly with a smartphone.

Phenothiazine-Based Donor-Acceptor Polymers as Multifunctional Materials for Charge Storage and Solar Energy Conversion.

The increasing energy demand for diverse applications requires new types of devices and materials. Multifunctional materials that can fulfill different roles are of high interest as they can allow fabricating devices that can both convert and store energy. Herein, organic donor-acceptor redox polymers that can function as charge storage materials in batteries and as donor materials in bulk heterojunction (BHJ) photovoltaic devices are investigated. Based on its reversible redox chemistry, phenothiazine is used as the main building block in the conjugated copolymer design and combined with diketopyrrolopyrrol and benzothiadiazole as electron-poor comonomers to shift the optical absorption into the visible region. The resulting polymers show excellent cycling stability as positive electrode materials in lithium-organic batteries at discharge potentials of 3.6-3.7V versus Li/Li+ as well as good performances in BHJ solar cells with up to 1.9% power conversion efficiency. This study shows that the design of such multifunctional materials is possible, however, that it also faces challenges, as essential properties for good device function can lead to diametrically opposite requirements in materials design.

Device Modelling and Optimization of Nanomaterial-Based Planar Heterojunction Solar Cell (by Varying the Device Dimensions and Material Parameters).

The objective of this work is to model a multi-disciplinary (multi-physics) organic photovoltaic (OPV) using mathematical modeling and analyzing the behavior of a standard planar heterojunction (PHJ) or bi-layer thin-film photovoltaic device, supporting the optimization of an efficient device for future production and assisting in evaluating and choosing the materials required for the efficient device. In order to increase photodiode performance, the device structure and geometrical properties have also been optimized and evaluated. In this work, the effects of varying the device size and transport parameters on the performance parameters of a PHJ OPV structure comprised of Indium Tin Oxide as the anode (ITO), semiconducting single-wall carbon nanotube (s-SWCNT) as the donor, fullerene C70 as the acceptor, and Aluminium (Al) as the cathode have been analyzed. The conclusion suggests that a highly effective ITO/s-SWCNT/C70/Al PHJ solar cell may be fabricated if the suggested device is appropriately built with a thin layer and a high exciton diffusion length, bi-molecular recombination coefficient, and improved mobility charge carriers, in particular hole mobility in the cell’s donor layer. In addition, the displayed current–voltage (I–V) characteristics of the proposed PHJ device are clearly indicated, with the ITO/s-SWCNT/C70/Al combination having the greatest short-circuit current density (Jsc) value of 5.61 mA/cm2, open-circuit voltage (Voc) of 0.7 V, fill factor (FF) of 79% and efficiency (ɳ) of 3.1%. Results show that the electrical performance of organic solar cells is sensitive to the thickness of the photoactive substance. These results open the path for developing inexpensive and highly efficient solar cells.

First-Principles Nonequilibrium Green's Function Approach to Energy Conversion in Nanoscale Optoelectronics.

Understanding photon-electron conversion on the nanoscale is essential for future innovations in nano-optoelectronics. In this article, based on nonequilibrium Green's function (NEGF) formalism, we develop a quantum-mechanical method for modeling energy conversion in nanoscale optoelectronic devices. The method allows us to study photoinduced charge transport and electroluminescence processes in realistic devices. First, we investigate the electroluminescence properties of a two-level model with two different treatments of inelastic scatterings. We show the regime where self-consistency between electron and photon is important for correct description of the inelastic scatterings. The method is then applied to model single-molecule junctions based on the density-functional tight-binding approach. The predicted emission spectra are found to be in very good agreement with experimental measurements. For nanostructured materials, the method is further applied to study the photoresponse of a two-dimensional graphene/graphite-C3N4 heterojunction photovoltaic device. The simulations demonstrate clearly the impact of atomistic details on the optoelectronic properties of nanodevices. This work provides a practical theoretical framework that can be applied to model and design realistic nanodevices.

High‐Efficiency Graphene‐Oxide/Silicon Solar Cells with an Organic‐Passivated Interface

AbstractA breakthrough in graphene‐oxide/silicon heterojunction solar cells is presented in which edge‐oxidized graphene and an in‐plane charge transfer dopant (Nafion) are combined to form a high‐quality passivating contact scheme. A graphene oxide (GO):Nafion ink is developed and an advanced back‐junction GO:Nafion/n‐Si solar cell with a high‐power conversion efficiency (18.8%) and large area (5.5 cm2) is reported. This scalable solution‐based processing technique has the potential to enable low‐cost carbon/silicon heterojunction photovoltaic devices.

Impact of piezo-phototronic effect on ZnMgO/Se heterojunction photovoltaic devices

This paper investigates the impact of piezo-phototronic effects on the power conversion efficiency of ZnMgO/Se heterojunction photovoltaic devices. Sputter-deposited ZnMgO thin film and photo-absorbing Se were employed as the n-type window layer and p-type layer, respectively, to form a photovoltaic device on a flexible polyethylene terephthalate film. The applied strain was varied, and the changes due to the applied strain were recorded. The open-circuit voltage (Voc) increased from 0.59 V to 0.75 V when the applied strain was increased from − 0.41–0.40% by varying the bending angle. It has been reported that piezoelectric polarization of ZnMgO can decrease conduction band offset (CBO) between the ZnMgO and Se layers.

Effect of chlorine and bromine on the perovskite crystal growth in mesoscopic heterojunction photovoltaic device

Organic-inorganic hybrid perovskite solar cells are within the emerging photovoltaic technologies. The combination of different halogen ions, in certain fill fractions, is one of the methods to improve the perovskite film properties. Herein, fabrication and characterization of perovskite cells in standard mesoscopic architecture using one-step deposition method has been done. The role of the halogen ions (Chlorine or Bromine) on crystal structure growth and photoelectric performance has been investigated. X-ray diffraction, scanning electron microscopy, atomic force microscopy and optical microscopy analysis were performed. The microstructure, composition and morphology of CH3NH3PbI1.8Br1.2 and CH3NH3PbI1.8Cl1.2 films are dissimilar, although identical fabrication method was used. Same holds for optical properties, band gap energies of 1.84 eV and 1.63 eV, respectively, being obtained. Integrated in solar cells, the maximum power conversion efficiency of the Br based devices is beyond 10%, while for those based on Cl, the efficiency drops around 5%.

Laser-induced graphene electrode based flexible heterojunction photovoltaic cells

Graphene based nanomaterials have attracted significant research interest due to their unique optoelectronic properties which can be tuned to tailor the functionalization in various photovoltaic (PV) applications. These PVs are capable of powering various wearable and flexible electronic devices such as touch-panels, light-emitting diodes (LEDs), sensors, high speed field effect transistors (FETs), etc., which can be a low-cost alternative to traditionally employed silicon solar cells. However, in terms of processing performance and electrical properties, current methods of manufacturing graphene-based PVs have numerous drawbacks. Therefore, there is a need for more effective fabrication methods with commercial feasibility and roll-to-roll processing of graphene-based PVs. This article is the first to present a laser-based patterning technique for fabricating graphene electrodes from polyimide that are compatible with flexible and thermally sensitive substrates for solar cell applications. A heterojunction PV, with ferroelectric Cr-doped BiFeO3 (BFCrO), was deposited on the flexible graphene as the energy harvesting layer sandwiched between p-NiO and n-WS2 window layers. Subsequently, its PV performance was compared with the similar multi-junction PV built on the Indium‑tin Oxide (ITO) one. The LIG electrode outperformed the ITO with its excellent stability, flexibility, and conductivity. The maximum power conversion efficiency (PCE) culminated was 5.20% which is around 5 folds enhancement when compared to the ITO based PV. Furthermore, after bending graphene electrode to 130°, the sheet resistance returned to its original value, while the resistance of ITO increased due to the cracking effect. With these unique properties, LIG electrodes can be promising for the development of flexible ferroelectric BFO based heterojunction PV devices.

Unsymmetrical zinc phthalocyanines containing thiophene and amine groups as donor for bulk heterojunction solar cells.

Photovoltaic technology is an alternative resource for renewable and sustainable energy and low costs organic photovoltaic devices such as bulk-heterojunction (BHJ) solar cells, which are selective candidates for the effective conversion of solar energy into electricity. Asymmetric phthalocyanines containing electron acceptor and donor groups create high photovoltaic conversion efficiency in dye sensitized solar cells. In this study, a new unsymmetrical zinc phthalocyanine was designed and synthesized including thiophene and amine groups at peripherally positions for BHJ solar cell. The structure of the targeted compound (4) was characterized comprehensively by FT-IR, UV–Vis, 1H-NMR, and MALDI-TOF MS spectroscopies. The potential of this compound in bulk heterojunction (BHJ) photovoltaic devices as donor was also researched as function of blend ratio (blend ratio was varied from 0.5 to 4). For this purpose, a series of BHJ devices with the structure of fluorine doped indium tin oxide (FTO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/ ZnPc:[6,6]- phenyl-C61- butyric acid methyl ester (PCBM) blend/Al with identical thickness of ZnPc:PCBM layer were fabricated and characterized. Photo current measurements in 4 revealed that the observed photo current maximum is consistent with UV-vis spectra of the compound of 4. Preliminary studies showed that the blend ratio has a critical effect on the BHJ device performance parameters. Photovoltaic conversion efficiency of 6.14% was achieved with 4 based BHJ device.

The investigation of current-voltage characteristics based on poly(2-methoxy,5-octoxy)-1,4-phenylenevinylene-PbSe composite solar cells

A bulk heterojunction photovoltaic device was prepared based on the blend of conjugated poly[(2-methoxy,5-octoxy)-1,4-phenylenevinylene] (MOPPV)-inorganic PbSe quantum dots. We studied the device performance of the active layer with different mass ratio, and the device performance was best when PbSe wt% was 50% in the MOPPV-PbSe composites, as well as, the short-current density(J sc), open-circuit voltage(V oc ), fill factor(FF) and the power conversion efficiency(η) of the device were 3.661mA/cm2, 0.326V, 23% and 0.277%, respectively. Results indicated that PbSe quantum dots in composite was helped to exciton dissociation, charge transfer, and mobility.

Direct growth of tungsten disulfide on gallium nitride and the photovoltaic characteristics of the heterojunctions

Photovoltaic device construct with a 2D nanomaterial atop a 3D wide bandgap semiconductor enables van Hove singularity induced enhanced light absorption on the front surface in close proximity (few atoms above) to the depletion region. Here, thin layers of tungsten disulfide (WS2) varying from mono to few layers were grown directly on both n-gallium nitride (GaN) and p-GaN via low pressure chemical vapor deposition. The conditions for WS2 growth were optimized for a larger coverage of the GaN substrate. The WS2 film was characterized using confocal Raman spectroscopy and x-ray photoelectron spectroscopy. Ultraviolet photoelectron spectroscopy measurement was conducted to elucidate the electronic band structures of WS2 on top of GaN. Afterwards, WS2/GaN heterojunction photovoltaic devices were fabricated. The current density–voltage (J–V) tests were conducted to illustrate the electrical performance. The device showed an open circuit voltage of 0.53 mV and a short circuit current density of 60 µA cm−2, 85% higher than that of a WS2/Si solar cell. An external quantum efficiency was measured to be near 60% for red and infrared, and above 50% in the violet region.

Field-Dependent Charge Collection Model for Thin Film Organic Photovoltaics

In this study, we developed a unified charge collection model using optical cavity dependent charge carrier generation and non-uniform built-in electric field distribution within a bulk heterojunction photovoltaic device. The charge collection model relies on the experimental inputs related to the charge carrier dynamics such as mobilities of charge carriers, recombination lifetime, and junction width of charge carrier species. Optical cavity modes and field strength were calculated using the experimental variable angle ellipsometry analysis of individual components of the devices. In order to evaluate the model, ambient processed PCDTBT:PC71BM based conventional and inverted derive architectures were utilized to underline the effect of unintentional doping and distinct optical cavity modes. The simulated external quantum efficiency and short-circuit current density profiles from the model were compared to the experimental results with differing active layers thicknesses and device architectures. The proposed charge collection model presented a high degree of correlation with the experimental results and underlined its validity for further application on other types of organic photovoltaic devices.

The hole transport mechanism of MoOx/a-Si: H(i)/n-Si heterojunction photovoltaic devices: the source of the ‘S-shaped’ behavior

For the purpose of introducing a high work-function layer to improve the implied voltage in asymmetric silicon-based heterojunction photovoltaic devices, molybdenum oxide (MoOx, 0 < x < 3) is applied to the device. However, besides the role of extracting holes on one side, another singularity behavior presented itself in the nonequilibrium state, i.e. a worsened photovoltaic peformance (an ‘S-shape’) of current density versus voltage (J–V) was revealed with an inappropriate chemical state of the MoOx film. The source of the ‘S-shaped’ behavior of an MoOx/a-Si: H(i)/n-Si heterojunction device was co-analyzed by x-ray photoelectron spectroscopy with depth profiling, ultraviolet photoelectron spectroscopy, current density-voltage representation, a minority carrier lifetime survey and automat for simulation of heterostructures software simulation. It was found that an amorphous SiOx interlayer was spontaneously formed during the deposition of MoOx film onto a-Si: H(i)/n-Si substrate, blocking the transport of holes. The decrease in the work-function of the MoOx layer is attributed to an oxidation reaction at the MoOx/a-Si: H(i) boundary zone, which results in the decline of hole selectivity. A rising O/Si ratio in the SiOx interlayer induces an augmentation of valence band offsets, which could be the best interpretation of the ‘S-shaped’ response, because of a barrier that hinders the thermionic emission of holes. Meanwhile, the thicker (>4 nm) SiOx layer leads to a lower tunneling probability for holes. The characteristic analysis of the MoOx/a-Si: H(i)/n-Si heterojunction device deepens the understanding of the hole transport mechanism of the device.

Understanding the efficacy of electron and hole transport layers in realizing efficient chromium doped BiFeO3 ferroelectric photovoltaic devices

In this work, we report the fabrication of Pt/i-n/ITO, Pt/p-i/ITO, and Pt/p–i–n/ITO heterojunction photovoltaic (PV) devices, where i is the lead-free 2% Cr doped BiFeO3 (BFCrO) ferroelectric, n is the WS2 electron transport layer (ETL), p is the NiO hole transport layer (HTL), and Pt and ITO are the top and bottom electrodes, respectively. In the tandem structure, the materials bandgaps were increased from anode to cathode side; hence the device could absorb the maximum solar light. The electrical performance parameters were compared for all the above-mentioned devices. The p-i-n device yielded an open-circuit voltage (VOC) and a short-circuit current density (JSC) of 0.60 V and 0.76 mA·cm−2, respectively, which leads to the improvement in an efficiency of about 70% as compared to the other two heterojunctions. The two depletion regions formed in the p-i-n heterostructure provide the necessary force to drive the carrier separation, which was confirmed through the electrochemical impedance spectroscopy. A band-diagram has been projected to understand the device operation mechanism. To reduce the structural defects in WS2 and further improve the PV performance parameters in p-i-n devices, sulfur passivation has been performed on the WS2 films. This process offered significant carrier extraction properties than the unpassivated one. The electrical performances for both the passivated and unpassivated devices were compared to elucidate the impact of sulfur passivation. Surprisingly, both the VOC and JSC were improved in the passivated devices, and the efficiency was remarkably increased to an additional 70% as compared to the unpassivated one.