Depletion Region Width Research Articles

Achieving High Doping Density in pH-neutral Conjugated Polyelectrolyte Toward Effective Hole-Transporting Materials for Organic Solar Cells.

The lack of effective and non-corrosive hole-transporting layer (HTL) materials has remained a long-standing issue that severely restricts the performance of organic solar cells (OSCs). Most pH-neutral conjugated polyelectrolytes (CPEs) exhibit inferior performance to the acid-doped HTL materials due to their low doping density. In this study, a series of pH-neutral CPEs is designed and synthesized with high doping density as HTL materials. Through an elaborate synthetic route, two sulfonate-terminating alkoxyl side chains can be introduced into thiophene, by which the electron-rich, highly soluble, and chemically stable thiophene monomer is synthesized to enable the subsequent polymerization. The CPE PTT-F exhibit a remarkable self-doping property with an enhanced doping density from 2.01 × 1017 to 7.02 × 1018 cm-3. The high work function and the increased doping density of PTT-F-based HTL decrease the depletion region width from 38.4 to 8.1nm at the anode interface, which minimized the energy loss in hole transport. Consequently, a binary OSC modified by PTT-F-based HTL achieve a high PCE of 18.8%. To the best of the knowledge, this is the highest PCE for OSC employing CPE-based HTL. The results from this work demonstrate an encouraging achievement of realizing exceptional hole collection ability in pH-neutral CPEs.

In situ seed layer bandgap engineering leading to the conduction band offset reversion and efficient Sb2Se3 solar cells with high open-circuit voltage

Sb2Se3 solar cells have achieved a power conversion efficiency (PCE) of over 10%. However, the serious open-circuit voltage deficit (VOC-deficit), induced by the hard-to-control crystal orientation and heterojunction interface reaction, limits the PCE of vapor transport deposition (VTD) processed Sb2Se3 solar cells. To overcome the VOC-deficit problem of VTD processed Sb2Se3 solar cells, herein, an in-situ bandgap regulation strategy is innovatively proposed to prepare a wide band gap Sb2(S,Se)3 seed layer (WBSL) at CdS/Sb2Se3 heterojunction interface to improve the PCE of Sb2Se3 solar cells. The analysis results show that the introduced Sb2(S,Se)3 seed layer can enhance the [001] orientation of Sb2Se3 thin films, broaden the band gap of heterojunction interface, and realize a “Spike-like” conduction band alignment with ΔEc = 0.11 eV. In addition, thanks to the suppressed CdS/Sb2Se3 interface reaction after WBSL application, the depletion region width of Sb2Se3 solar cells is widened, and the quality of CdS/Sb2Se3 interface and the carrier transporting performance of Sb2Se3 solar cells are significantly improved as well. Moreover, the harmful Se vacancy defects near the front interface of Sb2Se3 solar cells can be greatly diminished by WBSL. Finally, the PCE of Sb2Se3 solar cells is improved from 7.0% to 7.6%; meanwhile the VOC is increased to 466 mV which is the highest value for the VTD derived Sb2Se3 solar cells. This work will provide a valuable reference for the interface and orientation regulation of antimony-based chalcogenide solar cells.

16.48% Efficient solution-processed CIGS solar cells with crystal growth and defects engineering enabled by Ag doping strategy

Solution-processed Cu(In,Ga)Se2 (CIGS) solar cells suffer from serious carrier recombination and power conversion efficiency (PCE) loss because of the poor film properties and easy formation of defects. Herein, we propose Ag&Se co-selenization strategy to enhance the crystallization and passivate harmful defects of the CIGS films. The formation of Ag-Se phase during the selenization process enables the formation of large grains and suppresses the deep level defects. It is found that Ag doping can enlarge the depletion region width, lower the Urbach energy and prolong the carrier lifetime. As a result, a champion solution-processed CIGS solar cell presents a high efficiency of 16.48% with the highly improved open-circuit voltage (VOC) of 662 mV and fill factor (FF) of 75.8%. This work provides an efficient strategy to prepare high quality solution-processed CIGS films for high-performance CIGS solar cells.

Junction Piezotronic Transistor Arrays Based on Patterned ZnO Nanowires for High-Resolution Tactile and Photo Mapping.

Recently, a great deal of interest has been focused on developing sensors that can measure both pressure and light. However, traditional sensors are difficult to integrate into silicon (Si)-based integrated circuits. Therefore, it is particularly important to design a sensor that operates on a new principle. In this paper, junction piezotronic transistor (JPT) arrays based on zinc oxide (ZnO) nanowire are demonstrated. And the JPT arrays show high spatial resolution pressure and light mapping with 195 dpi. Because ZnO nanowires are arranged vertically above the p-type Si channel's center of the transistor, the width of the heterojunction depletion region is constricted by the positive piezoelectric potential generated by strained ZnO. In addition, photogenerated charge carriers can be created in the Si channel when JPT is stimulated by light, which increases its electrical conductivity. Consequently, the external pressure and light distribution information can be obtained from the variation in the output current of the device. The prepared JPT arrays can be compatible with Si transistors, which make them highly competitive and make it possible to incorporate both pressure and light sensors into large integrated circuits. This work will contribute to many applications, such as intelligent clothing, human-computer interaction, and electronic skin.

Effect of AZO seed layers on the ultraviolet photoresponse of Ag/ZnO NRs Schottky junctions

Al-doped ZnO (AZO) thin films with different layers were prepared by sol-gel method. The SEM, XRD and AFM for three layers thin films demonstrates high crystal quality and low roughness. ZnO nanorods arrays (NRs) with different AZO seed layers were fabricated using hydrothermal process, and the effect of AZO seed layers on the structural, optical, and electrical characteristics was investigated. When there are three AZO seed layers present, ZnO NRs exhibit fewer surface states, smaller interface charge transfer resistance (Rce = 6.1 × 103Ωand Rct = 6.6 × 106Ω), longer carrier lifetimes (τe = 4.0 ms), higher carrier concentration (ND = 4.73 × 1019 cm−3), and narrower depletion region width W = 3.1 nm. Under weak UV light 381 nm (5.68 mW/cm2) at a bias −0.5 V, Ag/ZnO NRs Schottky devices with three AZO seed layers display a comparatively strong responsivity 34.64 mA/W. Appropriate AZO seed layers are favorable for highly efficient ZnO NRs-based photoelectric conversion devices.

Broadband mid-infrared photodetectors utilizing two-dimensional van der Waals heterostructures with parallel-stacked pn junctions

Optimizing the width of depletion region is a key consideration in designing high performance photovoltaic photodetectors, as the electron-hole pairs generated outside the depletion region cannot be effectively separated, leading to a negligible contribution to the overall photocurrent. However, currently reported photovoltaic mid-infrared photodetectors based on two-dimensional heterostructures usually adopt a single pn junction configuration, where the depletion region width is not maximally optimized. Here, we demonstrate the construction of a high performance broadband mid-infrared photodetector based on a MoS2/b-AsP/MoS2 npn van der Waals heterostructure. The npn heterojunction can be equivalently represented as two parallel-stacked pn junctions, effectively increasing the thickness of the depletion region. Consequently, the npn device shows a high detectivity of 1.3 × 1010 cmHz1/2W−1 at the mid-infrared wavelength, which is significantly improved compared with its single pn junction counterpart. Moreover, it exhibits a fast response speed of 12 μs, and a broadband detection capability ranging from visible to mid-infrared wavelengths.

Indoor organic photovoltaic module with 30.6 % efficiency for efficient wireless power transfer

Organic photovoltaic (OPV) cells possess substantial advantages for indoor applications. However, the underdeveloped cathode interlayer hinders the industrialization of indoor OPV cells. Here, we introduce a phenanthroline derivative with weak nucleophilicity, dicarbolong-phenanthroline (DCP), as the cathode interlayer and comprehensively examine its utilization in indoor OPV cells. DCP exhibits thin depletion region width, which is beneficial for the charge extraction at the interface, thereby contributing to improved power conversion efficiency (PCE). Moreover, DCP presents high adaptability for coating process due to its thickness insensitivity. A DCP-based OPV module is fabricated by blade-coating method, reaching a high maximum output voltage of 4.00 V and a remarkable PCE of 30.6 % at 1000 lux. The OPV module can be successfully applied for efficient wireless power transfer. Furthermore, the DCP-based cell demonstrates outstanding device stability with a lifetime of 32000 hours under indoor lighting. These results demonstrate the promising potential of DCP-based OPV cells for indoor applications.

Impact of oxygen incorporation on interface optimization and defect suppression for efficiency enhancement in Cu2ZnSn(S,Se)4 solar cells

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells suffer from severe carrier recombination, limiting the photovoltaic performance. Unfavorable energy band alignment at the p-n junction and defective front interface are two main causes. Herein, oxygen incorporation in CZTSSe via absorber air-annealing was developed as a strategy to optimize its surface photoelectric property and reduce the defects. With optimized oxygen incorporation conditions, the carrier separation and collection behavior at the front interface of the device is improved. In particular, it is found that oxygen incorporated absorber exhibits increased band bending, larger depletion region width, and suppressed absorber defects. These indicate the dynamic factors for carrier separation become stronger. Meanwhile, the increased potential difference between grain boundaries and intra grains combined with the decreased concentration of interface deep level defect in the absorber provide a better path for carrier transport. As a consequence, the champion efficiency of CZTSSe solar cells has been improved from 9.74% to 12.04% with significantly improved open-circuit voltage after optimized air-annealing condition. This work provides a new insight for interface engineering to improve the photoelectric conversion efficiency of CZTSSe devices.

Spatiotemporal evolution of ultrafast photocarrier dynamics across WS2-ReS2 lateral interface.

2D lateral heterostructures possess atomically sharp lateral interfaces, while understanding of their ultrafast photocarrier dynamics from a spatiotemporal viewpoint is rather elusive. In this study, we have investigated the spatiotemporal evolution of photocarrier dynamics across the 1D lateral interface of a WS2-ReS2 2D lateral heterostructure utilizing femtosecond laser pump-probe. The nontrivial band offset across the 1D lateral interface markedly mediates the spatiotemporal photocarrier transfer and transport processes. Subsequently, a hole accumulation region on the WS2 side and an electron accumulation region (1DEG) on the ReS2 side have been spatially identified by correlating ultrafast photocarrier signals. The measured width of the unilateral depletion region is 1360 ± 160nm. Our work has provided substantial insights into mediated photocarrier dynamics in the 2D lateral heterostructure, which will benefit explorations in 2D interfacial physics and 2D lateral optoelectronic devices.

Piezojunction effect in heterojunctions under external bias for ultrasensitive strain sensing

This paper investigates for the first time the piezojunction effect in heterojunctions under external bias for ultrasensitive strain sensing. As a proof of concept, we used sensing devices made of 3C-SiC/Si heterostructure with vertically aligned electrodes. Applying the beam bending method to characterize the sensing effect, the bending strain was introduced along the typical orientation [100] or [110] on (100) Si plane. Experimental results show a linear relationship between the relative change in the forward current and the applied strain from 0 to 500 ppm, decreasing under the tensile strain while increasing under the compressive strain. At the forward bias of 8 V, the obtained gauge factors (GFs) are 199.7 for [100] orientation and 173.1 for [110] orientation, which significantly enhance about 630 % and 540 % compared to the highest GF of n-type 3C-SiC in the literature. Interestingly, the GFs of the n+-3C-SiC/p-Si heterostructure are positive in contrast to the negative GFs of n-3C-SiC thin films. The results were explained by the strain modulation on the band split and electron mass shift along the out-of-plane direction as well as by the change in the barrier height, depletion region width, and carrier concentrations under the forward bias. The ultrasensitive piezojunction effect in the 3C-SiC/Si heterojunction demonstrated in this study can pave the way toward developing ultrasensitive mechanical sensors.

Understanding the Interfacial Energy Structure and Electron Extraction Process in Inverted Organic Solar Cells with Phosphine‐Doped Cathode Interlayers

Comprehensive SummaryCathode interlayers (CILs) play an essential role in achieving efficient organic solar cells (OSCs). However, the electronic structure at the electrode/CIL/active layer interfaces and the underlying mechanisms for electron collection remain unclear, which becomes a major obstacle to develop high‐performance CILs. Herein, we investigate the relationship of the electron collection abilities of four cross‐linked and n‐doped CILs (c‐NDI:P0, c‐NDI:P1, c‐NDI:P2, c‐NDI:P3) with their electronic structure of space charge region at heterojunction interface. By accurately calculating the depletion region width according to the barrier height, doping density and permittivity, we put forward that the optimal thickness of CIL should be consistent with the depletion region width to realize the minimum energy loss. As a result, the depletion region width is largely reduced from 13 nm to 0.8 nm at the indium tin oxide (ITO)/c‐NDI:P0 interface, resulting in a decent PCE of 17.7% for the corresponding inverted OSCs.

Reducing the Depletion Region Width at the Anode Interface via a Highly Doped Conjugated Polyelectrolyte Composite for Efficient Organic Solar Cells.

In the realm of organic solar cells (OSCs), the width of the depletion region at the anode interface is a critical factor that adversely impacts the open-circuit voltage (Voc) and the power conversion efficiency (PCE). To address this challenge, a novel approach involving a conjugated polyelectrolyte (CPE)-based composite, PCP-2F-Li:POM, has been developed. This composite serves as a solution-processed hole transport layer (HTL), effectively minimizing the depletion region width in high-performance OSCs. The innovative aspect of PCP-2F-Li:POM lies in its "mutual doping" mechanism. Polyoxometalate (POM) is utilized as a dopant, facilitating the formation of p-doped CPE and n-doped POM within the composite. This results in a substantial increase in doping density, nearly 2 orders of magnitude higher than that observed in unmodified CPE. Consequently, the width of depletion region is markedly reduced, shrinking from 76.4 to 6.0 nm. This reduction plays a pivotal role in enhancing hole transport via the tunneling effect. The practical impact of this development is notable. It leads to an increase in Voc from 0.84 to 0.86 V, thereby contributing significantly to an impressive PCE of 18.04% in OSCs. Moreover, the compatibility of PCP-2F-Li:POM with large-area processing techniques underscores its potential as a viable HTL material for future practical applications. Additionally, its contribution to the enhanced long-term stability of OSCs further bolsters its suitability for practical applications.

Using thin-film transistor with thick oxygen-doped Si–Zn–Sn–O channel and patterned Pt/NiO capping layer to enhance ultraviolet light sensing performance

Improving the photodetection performance of thin-film transistor (TFT)-based UV photodetectors (UVPDs), using thick channel layers to promote photocurrent (I ph) or using thin channel layers to suppress dark current (I dark) is typically a trade-off. In this work, UVPDs based on oxygen-doped Si-Zn-Sn-O (SZTO) TFT with a stack of Pt/NiO capping layers (CLs) to release the trade-off between I dark and I ph are demonstrated. The Pt CL creates a wide depletion region in the channel layer to allow the use of thick channels, but still maintains low I dark, while the NiO CL forms a pn heterojunction to provide additional photogenerated carriers and enhance I ph under UV irradiation. Experimental results show that the proposed 95 nm-thick oxygen-doped SZTO TFT with a stack of Pt/NiO dual CLs exhibits an excellent photoresponsivity of 2026 A W−1 and photosensitivity of 9.3 × 107 A A−1, which are about 76× and 82.5× higher than a conventional 45 nm-thick SZTO TFT under 275 nm UV irradiation.

Building a depletion-region width modulation model and realizing memory characteristics in PN heterostructure devices.

Memristive systems have potential applications in nonvolatile memories and even unexplored functionalities in electronics. However, progress has been delayed by difficulties in the controllability of memory behaviors and the dependence on material conductivity. Considering this, a new depletion-region width modulation model is proposed to realize and explain memory characteristics. The coexistence of memristive and memcapacitive behaviors is demonstrated in p-CuAlO2/n-ZnO, p+-Si/n-ZnO and p-NiO/n-ZnO heterostructure devices. A high external electric field induces the migration of oxygen ions and electrons/holes between the p-type and n-type semiconductor layers. It can regulate the oxygen vacancy concentration of the n-type side and cation vacancy concentration of the p-type side, changing the depletion-region width and modulating device conductivity and capacitance. Several essential synaptic functions were accurately imitated, including spike-timing-dependent plasticity (STDP) and "learning-experience" behaviors. This work provides new opportunities in fabricating a memristor and memcapacitor based on a PN heterostructure for synaptic simulation.

Mg content regulating built-in electric field for self-powered ZnO/MgxZn1-xO Schottky UV photodetector

In recent years, self-powered Schottky ultraviolet (UV) photodetectors (PDs) have attracted great attention due to their low manufacturing cost and zero-bias response. However, the development of high-performance self-powered Schottky devices is limited by the weak light response due to the internal weak electric field. Here, we introduce a MgxZn1-xO capping layer on ZnO and fabricate a ZnO/MgxZn1-xO heterojunction Schottky UV PD by using the sol-gel method. The improved device exhibits excellent self-powered characteristics and stability, specifically, an increase of 909% in photocurrent, an improvement of more than one order of magnitude in responsivity, and an improved linear dynamic region (LDR: 33.7 dB). The introduction of the MgxZn1-xO capping layer is responsible for the improved device performance, resulting in enhanced built-in electric field and depletion region width due to the increase of the Schottky barrier, thereby facilitating the separation and transport of photogenerated carriers. This work provides an effective way to fabricate high-performance self-powered ZnO Schottky UV PDs.

Ferroelectric Resistance Switching in Epitaxial BiFeO3/La0.7Sr0.3MnO3 Heterostructures.

BiFeO3/La0.7Sr0.3MnO3 (BFO/LSMO) epitaxial heterostructures were successfully synthesized by pulsed laser deposition on (001)-oriented SrTiO3 single-crystal substrates with Au top electrodes. Stable bipolar resistive switching characteristics regulated by ferroelectric polarization reversal was observed in the Au/BFO/LSMO heterostructures. The conduction mechanism was revealed to follow the Schottky emission model, and the Schottky barriers in high-resistance and low-resistance states were estimated based on temperature-dependent current-voltage curves. Further, the observed memristive behavior was interpreted via the modulation effect on the depletion region width and the Schottky barrier height caused by ferroelectric polarization reversal, combining with the oxygen vacancies migration near the BFO/LSMO interface.

Reducing Depletion Region Width at Electrode Interface via a Hole‐transport Layer for Over 18% Efficiency in Organic Solar Cells

AbstractThe large depletion region width at the electrode interface may cause serious energy loss in charge collection of organic solar cells (OSCs), depressing the open‐circuit voltage and power conversion efficiency (PCE). Herein, a pH neutral solution‐processed conjugated polyelectrolyte PIDT‐F:IMC as hole transport layer (HTL) to reduce the depletion region width in efficient OSCs is developed. By utilizing “mutual doping” strategy, the doping density of PIDT‐F:IMC is increased by more than two orders of magnitude, which significantly reduces the depletion region width at the anode interface from 55 to 7.4 nm, playing an effective role in decreasing the energy loss in hole collection. It is also revealed that the optimal thickness of HTL should be consistent with the depletion region width for achieving the minimum energy loss. The OSC modified by PIDT‐F:IMC shows a high PCE of 18.2%, along with an amazing fill factor of 0.79. Moreover, a PCE of 16.5% is achieved in the 1 cm2 OSC by using a blade‐coated PIDT‐F:IMC HTL, indicating the good compatibility of PIDT‐F:IMC with large‐area processing technology. The PIDT‐F:IMC‐modified OCS exhibits a lifetime of 400 h under operational conditions, which is ten times longer than that of the PEDOT:PSS device.

Improvement of Performance of Cu2ZnSn(S, Se)4 Solar Cells by Low‐Temperature Annealing of B‐Doped CdS

It is known that high carrier recombination at the p–n junction interface and low carrier separation ability (CSA) are two main problems resulting in low power conversion efficiency (PCE) of the Cu2ZnSn(S,Se)4 (CZTSSe) solar cell. To resolve these problems, one CZTSSe solar cell is prepared through substituting B‐doped CdS for CdS in a solar cell with the conventional structure of Ag/ITO/ZnO/CdS/CZTSSe/Mo/SLG and annealing B‐doped CdS/CZTSSe/Mo/SLG prior to deposition of ZnO, ITO, and Ag electrode. By optimizing the B doping content in the CdS and annealing temperature and time of the B‐doped CdS/CZTSSe, lattice mismatch between CdS and CZTSSe is decreased and width of depletion region is increased, leading to reduction in interfacial recombination, enhancement in CSA and intensity of incident light passing through the B‐doped CdS, and so increase in PCE from 7.89% of CZTSSe solar cell using CdS as buffer layer to 10.62%. A mechanism of the increment of the PCE induced by B doping and annealing is suggested. This work proposes a method of increasing PCE of CZTSSe solar cell and advances a deeper understanding of the mechanisms behind various parameters in CZTSSe solar cells through theoretical analysis and calculations.

Two-dimensional vertical-lateral hybrid heterostructure for ultrasensitive photodetection and image sensing

Constructing heterostructures of two-dimensional (2D) materials is critical for boosting their photoelectric performances, enabling a promising route to meet the harsh requirement of next-generation optoelectronic devices. However, the practical application of currently-studied lateral and vertical heterostructures is usually limited by poor photocarrier generation and transport abilities, primarily due to their inherent imbalance of depletion region and interface area. To solve this obstacle, we herein report a novel 2D hybrid heterostructure of bismuth selenide/oxyselenide with the coexistence of lateral and vertical interfaces. The unique hybrid structure provides the heterostructure with a huge interface area and a wide depletion region for sufficient photocarrier generation and separation, significantly improving the overall optoelectronic performances of the device. Specifically, the resultant photodetectors exhibit a remarkably decreased dark current and ultrafast photoresponse rate (τrise = 50 μs and τdecay = 20 μs). Moreover, our photodetector demonstrates outstanding image sensing capability for recording various complex patterns with high resolution over a wide spectral range from ultraviolet, visible to near-infrared regions. Our research provides a highly promising route for designing and fabricating novel geometries of 2D heterostructures to realize diverse photoelectric applications.

Molecular Ferroelectric with Directional Polarization Field for Efficient Tin‐Based Perovskite Solar Cells

AbstractDue to the relatively inferior dielectric constant, Sn‐based perovskites exhibit lower defect tolerance and insufficient dielectric shielding effect compared with Pb‐perovskites. Upgrading built‐in electric field (BEF) in Sn‐based perovskite solar cells (PSCs) can be effective to reduce large voltage deficit and improve poor performance caused by the low defect tolerance resulting from the intrinsic inferior dielectric of Sn‐based perovskites. Herein, 2‐methylbenzimidazole (MBI) molecular ferroelectric with low coercive field and high Curie‐temperature is introduced to construct an additional ferroelectric field in FASnI3‐based PSCs. The ferroelectric effect of MBI can promote exciton dissociation, enhance carrier population, and suppress the adverse effect of the residual defects on carriers, and the directional polarization of MBI in FASnI3 film can be driven by the BEF in PSCs to broaden the width of depletion region. Additionally, the MBI molecules with amine functional groups effectively regulate perovskite crystallization, passivate Sn‐related defects, and enhance the oxidation barrier. Profiting from the above advantages, the MBI‐modified device achieves a champion power conversion efficiency (PCE) of 12.91%, keeping over 94% average PCE after 1056 h in N2 glovebox for the unencapsulated device. This study highlights the significant role of molecular ferroelectrics in perovskite photovoltaics.