Reduced Interface Recombination Research Articles

Revealing the reason for enhanced CZTSSe device performance after Ag heavily doped into absorber surface.

Ag-doping treatment is a popular method for enhancing the performance of kesterite-structured Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Among the various methods, incorporating a high concentration of Ag+ into an absorber surface has proven to be particularly effective. However, the exact mechanisms behind this improvement are still unclear. This study aims to investigate the key factors that improve device performance through simulation. Specifically, the influence of the change in the carrier density, CuZn antisite defects, interface defect density, and formation of an n-type AZTSSe surface after heavy surface Ag doping have been examined. The simulation results indicate that the formation of an n-type AZTSSe layer on an absorber surface can significantly improve the open circuit voltage (VOC) and overcome the efficiency saturation problem induced by severe interface recombination for CZTSSe devices with a negative conduction band offset (CBO), compared to other affecting factors. This is because the modified conduction band alignment and the realization of interface-type inversion reduce interface recombination and retard the Fermi level pinning. However, the formation of interface-type inversion does not significantly improve CZTSSe devices with a positive CBO, as these devices already have weaker interface recombination. This work implies that the formation of an n-type AZTSSe layer is crucial for further improving the performance of CZTSSe devices with a negative CBO and can pave the way for improving the performance of thin film solar cells with severe interface recombination.

Band offsets and point-defect charges of the aluminum and hafnium oxides in contact with the Cu(In,Ga)Se2 chalcopyrite

Surface passivation of CuInSe2 (CIS) and related Cu(In,Ga)Se2 (CIGS) chalcopyrite materials by depositing selected dielectric layers has been a major research activity aiming to reduce interface recombination and increase the electrical efficiency of chalcopyrite-based thin-film solar cells. The present study reports calculations based on density-functional theory and ab-initio thermodynamics that examine the origin of field-effect passivation from alumina and hafnia two wide-gap, predominantly ionic insulators that have exhibited promising passivation qualities in silicon-based microelectronics. The source of fixed charges within the bulk lattices of both oxides was studied by determining the thermodynamically most favorable charge states of their native defects within the admissible ranges of the metal and oxygen chemical potentials. An alignment of the electron bands based on the branch-point energies was performed in order to correctly place the defect charge-transition levels with respect to the band edges of the CIS and the CIGS materials. The trends and predictions of the sign of the fixed charges in either insulator were obtained as a function of temperature, oxygen partial pressure and Fermi-level position inside the band gaps of CIS and CIGS. The findings are discussed in connection with existing experimental studies that extracted the magnitude and polarity of the fixed charges of both alumina and hafnia by analyzing the electrical properties of the CIGS/insulator interfaces.

Improving Photocurrent in Sb2Se3 Thin‐Film Solar Cells Through Sb2S3 Electron Reflector Layer: A Simulation Study

AbstractIn this paper, antimony sulfide (Sb2S3) material is utilized as an electron reflector layer (ERL) in antimony selenide (Sb2Se3) solar cells. The primary objective of this simulation study is to mitigate recombination at the back contact and improve cell performance. It is demonstrated that incorporating Sb2S3 ERL into a traditional Sb2Se3 cell yields enhancements in both the short‐circuit current density (Jsc) and carrier collection rate. This enhancement is assigned to the reduced interface recombination rate at the back electrical contact. The strong electric field at the p+‐Sb2S3/p‐Sb2Se3 electrical contact creates a barrier with a height of 0.34 eV in the conduction band that hinders the electron carriers' access to the back contact. This barrier effectively improves cell efficiency. The use of this method has demonstrated a significant improvement in the external quantum efficiency (EQE) at longer wavelengths, specifically ranging from ≈800 to 1100 nm. Using this method, the Sb2Se3 active layer thickness is reduced to 1 µm, which reduces the fabrication cost. After optimizing the device, the cell efficiency enhances from 9.2% to 13.1%, resulting in a significant improvement compared to traditional Sb2Se3 solar cells without an ERL, thanks to the incorporation of a suitable Sb2S3 ERL.

An ITO-Free Kesterite Solar Cell.

Photovoltaic thin film solar cells based on kesterite Cu2 ZnSn(S, Se)4 (CZTSSe) have reached 13.8% sunlight-to-electricity conversion efficiency. However, this efficiency is still far from the Shockley-Queisser radiative limit and is hindered by the significant deficit in open circuit voltage (VOC ). The presence of high-density interface states between the absorber layer and buffer or window layer leads to the recombination of photogenerated carriers, thereby reducing effective carrier collection. To tackle this issue, a new window structure ZnO/AgNW/ZnO/AgNW (ZAZA) comprising layers of ZnO and silver nanowires (AgNWs) is proposed. This structure offers a simple and low-damage processing method, resulting in improved optoelectronic properties and junction quality. The ZAZA-based devices exhibit enhanced VOC due to the higher built-in voltage (Vbi ) and reduced interface recombination compared to the usual indium tin oxide (ITO) based structures. Additionally, improved carrier collection is demonstrated as a result of the shortened collection paths and the more uniform carrier lifetime distribution. These advances enable the fabrication of the first ITO-free CZTSSe solar cells with over 10% efficiency without an anti-reflectivecoating.

Efficiency Boost of (Ag0.5,Cu0.5)(In1‐x,Gax)Se2 Thin Film Solar Cells by Using a Sequential Process: Effects of Ag‐Front Grading and Surface Phase Engineering

AbstractPost‐selenization‐fabricated (using elemental Se vapor) Cu(In,Ga)Se2 solar cell efficiency is limited by a low open‐circuit voltage, which is attributable to the Ga‐deficient surface and insufficient grain growth. In this study, a band‐grading structure is demonstrated by combining Ag‐front and Ga‐back grading in selenized (Ag,Cu)(In,Ga)Se2 (ACIGSe) absorbers with a properly designed precursor structure (Mo/CuGa/In/AgGa) and high Ag content ([Ag]/([Ag]+[Cu]) = 0.5). The phase evolution during post‐selenization reveals that the precursor structure suppresses Ag2Se formation and promotes the ACIGSe phase formed at a low temperature with enhanced grain growth. A widened surface bandgap by Ag‐front grading substantially increases the open‐circuit voltage. Furthermore, Ag addition promotes ordered vacancy compound (OVC) formation on the front surface to enlarge the valence band offset, which in turn reduces interface recombination. Furthermore, the OVC phase also assists interface passivation. Promoting surface OVC phase by Ag addition is also validated by first‐principles calculations. Furthermore, the K‐doped CuGa precursor is used for a ACIGSe absorber to address the significantly reduced carrier density by the Ag addition. With a band‐grading structure and surface OVC phase, the superior device achieves an efficiency of > 19%, the highest efficiency by post‐selenization with an elemental Se source.

Energy Band Alignment by Solution-Processed Aluminum Doping Strategy toward Record Efficiency in Pulsed Laser-Deposited Kesterite Thin-Film Solar Cell.

Kesterite-based Cu2ZnSnS4 (CZTS) thin-film photovoltaics involve a serious interfacial dilemma, leading to severe recombination of carriers and insufficient band alignment at the CZTS/CdS heterojunction. Herein, an interface modification scheme by aluminum doping is introduced for CZTS/CdS via a spin coating method combined with heat treatment. The thermal annealing of the kesterite/CdS junction drives the migration of doped Al from CdS to the absorber, achieving an effective ion substitution and interface passivation. This condition greatly reduces interface recombination and improves device fill factor and current density. The JSC and FF of the champion device increased from 18.01 to 22.33 mA cm-2 and 60.24 to 64.06%, respectively, owing to the optimized band alignment and remarkably enhanced charge carrier generation, separation, and transport. Consequently, a photoelectric conversion efficiency (PCE) of 8.65% was achieved, representing the highest efficiency in CZTS thin-film solar cells fabricated by pulsed laser deposition (PLD) to date. This work proposed a facile strategy for interfacial engineering treatment, opening a valuable avenue to overcome the efficiency bottleneck of CZTS thin-film solar cells.

Directional Transformation of Heterometallic Oxo Clusters: A New Approach to Prepare Wide‐Bandgap Cathode Interlayers for Perovskite Solar Cells

AbstractTypical wide‐band gap cathode interlayer materials are difficulty in reducing interface recombination without limiting charge transport in perovskite solar cells (PSCs). Here, a lead‐doped titanium‐oxo cluster protected by S‐containing ligands is introduced at the interface of perovskite and SnO2. By in situ heating, the cluster is transformed into PbSO4‐PbTi3O7heterostructure. The oxygen atoms from sulfate ion in heterostructure connect with iodine from perovskite to boost interfacial electron extraction and reduce charge recombination. While the yielded metallic interface between PbSO4and PbTi3O7promotes the electron transport across the interface. Finally, an efficiency as high as 24.2 % for the modified PSC is obtained. The heterostructure well‐stabilize the interface of perovskite and SnO2, to greatly improve the device stability. This work provides a novel strategy to prepare wide‐band gap cathode interlayer by directional transformation of heterometallic oxo clusters.

Directional Transformation of Heterometallic Oxo Clusters: A New Approach to Prepare Wide-Bandgap Cathode Interlayers for Perovskite Solar Cells.

Typical wide-band gap cathode interlayer materials are difficulty in reducing interface recombination without limiting charge transport in perovskite solar cells (PSCs). Here, a lead-doped titanium-oxo cluster protected by S-containing ligands is introduced at the interface of perovskite and SnO2 . By in situ heating, the cluster is transformed into PbSO4 -PbTi3 O7 heterostructure. The oxygen atoms from sulfate ion in heterostructure connect with iodine from perovskite to boost interfacial electron extraction and reduce charge recombination. While the yielded metallic interface between PbSO4 and PbTi3 O7 promotes the electron transport across the interface. Finally, an efficiency as high as 24.2 % for the modified PSC is obtained. The heterostructure well-stabilize the interface of perovskite and SnO2 , to greatly improve the device stability. This work provides a novel strategy to prepare wide-band gap cathode interlayer by directional transformation of heterometallic oxo clusters.

A Strategy to Optimize Nickel Oxide/Crystalline Silicon Heterocontact of HJT Solar Cells

NiOx is a p-type semiconductor material with wide band-gap (3.6 ∼ 4.0 eV), good thermal and chemical stability. In terms of energy band structure, NiOx/n-Si possesses low valence band offset to allow hole and high conduction band offset to block electron, NiOx is thus a promising hole-selective layer for n-type c-Si based heterojunction (HJT) solar cells. However, intrinsic NiOx suffers from low carrier concentration and poor conductivity, which severely limits its development in photovoltaics. This study aimed to obtain Ag-doped NiOx film with high carrier concentration and low resistivity by co-sputtering using high-purity Ag and NiOx target for high efficiency solar cells. The results show that appropriate Ag doping can increase the acceptor concentration of the film, promote the tunnelling effect, reduce interface recombination, and thus improve the device efficiency. When Ag content is 2.4%, the fill factor of Al/ITO/NiOx:Ag/SiOx/c-Si/SiOx/Al solar cell is increased from 52.97% to 68.42%, and the power conversion efficiency reaches 9.11%. Combined with the analysis of AFORS-HET simulation, the mechanism how Ag doping works in the hetrojunction is revealed.

Polymerizable Ammonium Salt Interlayer for Efficient and Stable NiOx-Based Perovskite Solar Cells

Although the research of formal perovskite solar cells (PSCs) has achieved great breakthroughs, the use of doped small-molecule organic hole transport layers limits the improvement of stability. PSCs with an inverted form show great potential in a more stable device structure. In this paper, a NiOx/3D perovskite interface in inverted PSCs was modified with a polymerizable amine salt (2-methylallylamine hydroiodide (CTBAI)) interlayer. The double bonds contained in CTBAI thermally induced polymerization of a thin film after heat treatment, resulting in the formation of amine salts with longer molecular chains. The modified layer can improve crystallinity and reduce defects in the film, as well as optimize the interface energy level and reduce interface recombination. The efficiency of NiOx-based inverted PSCs based on the optimized interface increased from 17.0% to 18.2%. After 280 h of MPP testing, the encapsulated device tested in ambient air (RH = 50–60%) demonstrated a robust light stabilization and maintained over 93% efficiency.

Investigating the role of copper in arsenic doped Cd(Se,Te) photovoltaics

The open circuit voltage (VOC) deficit in Cd(Se,Te)-based photovoltaics remains a critical obstacle for pushing the technology closer to theoretical performance limits. Arsenic doping has become a dominant and promising route to achieve the higher p-type carrier concentrations necessary for higher VOC, but challenges associated with this alternate defect chemistry and higher doping density have hindered progress. Here we show that while arsenic doping enables high carrier concentrations (>1016 cm−3), co-doping with copper can provide a boost to VOC without a significant change to carrier concentration. A large data set is initially used to explore current-voltage and capacitance-voltage trends associated with arsenic doped devices with and without copper. A smaller subset is then used to probe these trends using a wide variety of characterization techniques. Copper is found to facilitate reduced interface recombination and potentially improved bulk absorber characteristics, though the mechanisms for these improvements are not yet clear. Despite the improved performance of co-doped devices, VOC is still far below its potential especially for highly doped devices. Low emitter doping in conjunction with high absorber doping seems to be a plausible cause for this significant deficit, though other device properties may exacerbate this problem.

9.3% Efficient Flexible Cu2ZnSn(S,Se)4 Solar Cells with High‐Quality Interfaces via Ultrathin CdS and Zn0.8Sn0.2O Buffer Layers

The buffer layer plays a critical role in high‐performance flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cell. The conventional CdS buffer layer causes the photoelectric performance loss and the pollution of toxic Cd. Herein, the ultrathin CdS and Zn0.8Sn0.2O (ZTO) buffer layer engineering is proposed to reduce interface recombination and Cd content. An ultrathin CdS layer acts as the interface passivation layer to protect the CZTSSe layer and passivate its surface defects. To solve the problems of decreased carrier collection capacity caused by thinning the CdS layer, the ZTO layer with low resistivity and high carrier concentration is obtained by doping Sn into the ZnO layer. The systematic study indicates that the ultrathin CdS and ZTO buffer layers‐based solar cells realize high‐quality interface and band matching. Ultimately, 9.3% efficiency of the ultrathin CdS and ZTO‐based solar cell with 448 mV open‐circuit voltage is obtained, which is higher than that of the standard CdS buffer layer‐based device (8.5% with 419 mV). The study provides a new idea for achieving efficient flexible CZTSSe solar cells through heterojunction interface management.

Gradient doped nickel oxide hole selective heterocontact and ultrathin passivation for silicon photovoltaics with efficiencies beyond 20%

A prominent feature of photovoltaic devices is their ability to drive photo-generated charges to pathways of asymmetrical conductivity, guiding them to the corresponding electrodes. Under this guideline, the crystalline silicon (c-Si) solar cells dominate the photovoltaic (PV) market for decades and several optoelectronic losses and technological limitations are gradually emerging by now. To circumvent these issues, here, we replaced the doped-silicon emitter layer with a gradient copper-doped nickel oxide (Gd-NiOx), and incorporated a silicon oxide passivation (SOP) layer at the Gd-NiOx/c-Si interface, to construct the novel c-Si PV devices. Interestingly, it was observed that the Gd-NiOx hole selective layer can strengthen the built-in field by minimizing the front electrode/hole-selective layer barrier width and the SOP can reduce interface recombination simultaneously, thereby yielding a remarkable 20.3% efficiency for the proof-of-concept device. We believe the design proposed in this may be of interest for applications in PVs and other optoelectronic devices.

Synergistic effect of atomic layer deposition-assisted cocatalyst and crystal facet engineering in SnS2 nanosheet for solar water oxidation

The severe bulk recombination and sluggish oxygen evolution reaction (OER) dynamics of photoanodes severely restrict the application of photoelectrochemical (PEC) devices. To solve these two problems, crystallographic facet orientation and cocatalyst emergence with a high-quality photoanode/cocatalyst interface were realized through an air annealing-assisted strategy to treat atomic layer deposition (ALD)-modified SnS2 nanosheet arrays. Based on experimental observations and theoretical calculations, the reduced (001) crystal facet of SnS2 decreases the recombination of photogenerated carriers in the bulk and improves the carrier separation of the photoanode. Moreover, the unexpectedly formed ZnTiOxSy film decreases the overpotential of the surface OER, reduces interface recombination, and extends the carrier lifetime. These synergistic effects lead to significantly enhanced PEC performance, with a high photocurrent density of 1.97 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE) and a low onset potential of 0.21 V vs. RHE, which are superior to reported mostly SnS2-based photoanodes.

Additive MXene and dominant recombination channel in perovskite solar cells

Two-dimensional (2D) MXenes have emerged as attractive materials for application in perovskite solar cells (PSCs). Their unique properties arising from terminating functional groups to tune the work function (WF) of the layers in a PSC lead to a notable improvement in the photovoltaic efficiency. The ideality factor (nid) is used to infer the dominant recombination type in the PSCs. We study the effect of the added MXene in the bulk absorber and the electron transport layer (ETL), and at the interfaces on the ideality factor (nid), and then the obtained nid is used to infer the dominant recombination type in the PSCs. By using the concept of intensity-dependent quasi-Fermi level splitting (QFLS), conduction and valence band offsets, and drift–diffusion simulations, we show that the bulk recombination in the MXene-free cells dominates. Considering the obtained nid values, we explain that adding MXene just into the absorber layer leads to a suppressed bulk recombination compared with the case of MXene-free device. Meanwhile, devices with added MXene into the ETL and at the ETL/absorber interface show a notable reduced interface recombination. The study of the interfacial energy offsets and their correlation with the interface and bulk recombination currents reveals that in the cells with MXene, higher nid values are desirable and correlate with better cell performance.

Interface Engineering of CZTS/TiO2 Heterojunction Using Wide‐Bandgap Ga2O3 Passivation Interlayer for Efficient Charge Extraction

Field-Effect Passivation The implementation of Ga2O3 thin film effectively passivates the CZTS/TiO2 heterojunction by blocking holes away from the buffer layer and acts as an electron-selective contact. This hole-blocking layer reduces interface recombination evident from significant photoluminescence quenching. More information can be found in article number 2200001 by Mukesh Kumar and co-workers.

Analytical modeling and design optimization of a graphene/n-GaAs Schottky junction solar cell

A physics-based analytical model is important to understand the working mechanism through process parameters of any innovative material heterostructure. We present an analytical model to calculate the power conversion efficiency of solar cells based on graphene and III-V direct bandgap semiconductors. The model is comprehensively developed by incorporating several current densities obtained from both the generation and recombination processes. Moreover, to obtain a highly efficient Schottky junction solar cell, we propose an optimized structure of graphene/GaAs with lattice-matched passivation and carrier selective layers. The structure has the advantage of surface passivation and photon recycling that reduces interface recombination and ensures more electron–hole pair generation, respectively. It exhibits a theoretical efficiency of >18 % from the analytical model simulation which is later verified by numerical simulation using SCAPS 1D software. The analytical model will provide not only a better understanding of the solar cells’ operation but also a comparative study among them to achieve better efficiency in the future. In addition, the enhanced efficiency of the proposed structure will encourage further research in this field of study.

Reduced recombination through CZTS/CdS interface engineering in monograin layer solar cells

The power conversion efficiency of Cu2ZnSnS4 (CZTS) solar cells is still limited by deep defects, low minority carrier lifetime and high recombination rates at the CZTS/CdS interface. The objective of this study was to find an effective method to reduce interface recombination of CZTS monograin layer solar cells. A two-step heterojunction formation process was applied by controlling the intermixing of Cd and Cu in the CZTS/CdS interface, which resulted in improved device efficiency of up to 11.7%. Surface analysis by x-ray photoelectron spectroscopy confirmed Cd diffusion into the surface of CZTS after CdS air-annealing by forming an ultra-thin Cu2Zn x Cd1−x SnS4 layer. Moreover, external quantum efficiency measurements showed that the absorption edge shifts to longer wavelengths with the addition of Cd into the CZTS surface layer. This surface modification and replacement of a CdS:Cu buffer layer by fresh CdS greatly reduced the interface recombination and improved the junction quality, contributing to an enhancement of J SC ∼3 mA cm−2 (from 20.5 to 23.6 mA cm−2) and fill factor ∼14% (from 59.4% to 67.7%). The serial resistance of the CZTS monograin layer solar cells was significantly reduced from 2.4 Ω cm2 to 0.67 Ω cm2. To understand the electrical behavior of the highest-efficiency CZTS monograin layer solar cell in more detail, the temperature-dependent current–voltage characteristics were analyzed.

Facilitate hole transport with thin 2D perovskite capping layer to passivate interface defects of 3D perovskite solar cells using PEABr

Hybrid 2D/3D-perovskites are attracting more interest in the area of organometal halide perovskite solar cells (PSCs) because of their high efficiency and extended environmental stability. The 3D portion in 2D/3D perovskites, serves as an active material with exceptional optoelectronic characteristics (for example excellent light absorption and an extended carrier lifetime), while its 2D counterpart protects against environmental conditions (water and oxygen) and increase the stability of the whole structure. In this study, a hetero-structure 2D/3D is constructed via spin-coating a phenyl-ethyl ammonium bromide (PEABr) solution upon over of (FA0.6MA0.4)PbI3 3D perovskite films. As compared to their 3D analogs, the PSC devices based on optimized 2D/3D hetero-structures display substantial improvements in photovoltaic efficiency due to the passivation of cationic and halide vacancies on the surface of 3D perovskite for better energy level alignments, longer carrier lifetimes, and fewer defects, which facilitate hole transfer and transport to the hole transport layer and reduce interface recombination centers. The extremely-thin film of 2D perovskite on the top of 3D perovskite boots the stability and reduces the crystal imperfection of 3D perovskite. Furthermore, the PEABr-treated perovskite device has a PCE of 20.57% as compared with the control device, which has a PCE of 17.01%.

Improving Hole Transport and Extraction by Interface Engineering in Perovskite Solar Cells

Efficient carrier transport and extraction is essentially important for perovskite solar cells (PSCs). Here, an outstanding organic material tetraphenyldibenzoperiflanthene (DBP) is used to modify the perovskite/spiro‐OMeTAD interface as the intermediate layer. Using time‐resolved photoluminescence technique and dynamic analysis of photogenerated charge carriers, a significantly faster hole extraction and faster hole transport occurring in perovskite/DBP/spiro interface is demonstrated, this is ascribed to an intermixing layer of perovskite and thin DBP layer. The improved mobility has been further confirmed by the Hall measurement. As a consequence, the improved hole extraction and reduced interface recombination result in an improved performance of PSC, including improved conversion efficiency, mitigated J–V hysteresis, and amelioration of stability under humid conditions. This work provides a detailed insight into the transport and extraction of charge carriers improved by the intermediate layer of organic small molecules as an effective strategy for interface engineering by solution spin coating.