Reduced Carrier Recombination Research Articles

Efficient Narrow-Bandgap Mixed Tin-Lead Perovskite Solar Cells via Natural Tin Oxide Doping.

Narrow-bandgap (NBG) mixed tin/lead-based (Sn-Pb) perovskite solar cells (PSCs) have attracted extensive attention for use in tandem solar cells. However, they are still plagued by serious carrier recombination due to inferior film properties resulting from the alloying of Sn with Pb elements, which leads to p-type self-doping behaviors. This work reports an effective tin oxide (SnOx ) doping strategy to produce high-quality Sn-Pb perovskite films for utilization in efficient single-junction and tandem PSCs. SnOx can be naturally oxidized from tin diiodide raw powders and successfully incorporated into Sn-Pb perovskite films. Consequently, Sn-Pb perovskite films doped with SnOx exhibit dramatically improved morphology, crystallization, absorption, and more interestingly, upward-shifted Fermi levels. The resulting narrow-bandgap Sn-Pb PSCs with natural SnOx doping have considerably reduced carrier recombination, therefore delivering a maximum power conversion efficiency (PCE) of 22.16% for single-junction cells and a remarkable PCE of 26.01% (with a steady-state efficiency of 25.33%) for two-terminal all-perovskite tandem cells. This work introduces a facile doping strategy for the manufacture of efficient single-junction narrow-bandgap PSCs and their tandem solar cells.

Self-Powered UV Photodetector of TiO2 with BaTiO3 Surface Modification and Light-Controlled Logic Circuits Application.

One-dimensional (1D) metal oxides with excellent carrier transport and light absorption properties can be applied to photodetectors (PDs), facilitating device miniaturization, portability, and integration. Surface modification of 1D semiconductors can reduce carrier recombination in PDs as a way to increase photocurrent and decrease dark current of PDs. Herein, ultrathin BaTiO3 (BTO) shell layers are grown on the surface of TiO2 nanorod arrays (NRs) by in situ conversion using hydrothermal reaction, and the self-powered TiO2-BTO NRs PDs are constructed. The effect of the thickness of BTO shell layers on the photoresponse characteristics of self-powered TiO2-BTO NRs PDs is investigated by controlling the Ba2+ conversion concentration. The results show that the BTO shell layer reduces the dark current of the PDs because of the decreased interfacial transfer resistance and improved transfer of photogenerated carriers for building a "bridge" of carrier transport between BTO and TiO2 due to the formation of Ti-O-Ti bonds. Moreover, the presence of the spontaneous polarization electric field in BTO enhances the photocurrent and response speed of PDs. The self-powered TiO2-BTO NRs PDs are integrated in series and parallel to realize the functions of "and" and "or" gates of light-controlled logic gates. The ability to convert light signals into electrical signals for the self-powered PDs in real time demonstrates its great potential for optoelectronic interconnection circuits, which has important application prospects in the field of optical communication.

Design and simulation investigations on electron transport layer-free in tin-based p-n homojunction perovskite solar cells

Inverted tin-based perovskite solar cell (PSC) devices with the electron transport layer-free (ETL-free) have drawn tremendous attention. Nonetheless, the photovoltaic performance of the device still lags behind that of conventional structural devices. In this context, a novel ETL-free inverted PSC with p-type MASnI3/n-type MASnI3 all-perovskite homojunction as light-harvester has been modeled and investigated by using wxAMPS software. This innovative PSC configuration not only promotes carrier separation and oriented transmission but also reduces carrier recombination losses. The simulation results reveal that NiO stands out as the best alternative. To enhance the device's overall efficiency, the optimal hole transport layer and perovskite layer thickness should be 30 nm and 200 nm, respectively. Lower defect densities in the perovskite layer and interface layers can favor the reduction of carrier recombination losses and efficient carrier transfer. Additionally, superior-performance photovoltaic devices can be further realized with a moderate perovskite layer acceptor doping density of 1E14 cm−3. With these optimized values, the optimal power conversion efficiency (PCE) of 20.5609 % has been obtained for the PSC devices. This work indicates that the proposed inverted homojunction PSC with ETL-free will be a valuable reference for future research in optoelectronic and electro-optical devices.

Co9S8/CdS heterostructures as efficient photocatalysts for degradation of organic pollutant

Because of its superior coupling and composition matching, narrow band gap semiconductor heterojunction can increase redox activity, reduce carrier recombination, and enhance light absorption. When heterojunction photocatalysts are constructed properly, it is thought to be an efficient technique to boost photocatalytic activity. The Co9S8/CdS heterostructures in this study were created using a two-step solvothermal technique. The as-prepared Co9S8/CdS heterostructures exhibit outstanding photocatalytic degradation performance of methyl orange (MO) owing to their unique structure and compositional properties. Co9S8/CdS heterostructures can offer a remarkable MO degradation rate that can reach 97% in 25 min when illuminated by visible light. In addition to demonstrating better long-term stability after numerous cycling photocatalytic tests, experiments with radical trapping have shown that ·O2− is crucial for the photocatalytic degradation of MO. The pertinent photogenerated charge transfer and catalytic mechanisms of Co9S8/CdS heterostructures are suggested and examined.

Ultrafast Deposition of NiFe Metal-organic Framework Catalysts Boosts BiVO4 Photoanode for Efficient Solar Water Splitting.

The severe photocorrosion of BiVO4 limits its application in solar energy conversion in the long-term photoelectrochemical stability test. Herein, we synthesized a Fe@Ni-MOFs/BiVO4 photoanode by a simple ultrasonic method and ultrafast deposition, which avoids the problems of damaging the surface structure of photoelectrode. The Fe@Ni-MOFs/BiVO4 shows a photocurrent density of 4.89 mA cm-2 at 1.23 VRHE with an onset potential of 0.25 VRHE under one sun illumination. Importantly, a stability over 30 h at 0.7 VRHE can be obtained, which is so far the best stability character for MOFs-based cocatalysts decorated on BiVO4. During operation, Fe@Ni-MOFs transforms into a homogeneous and active hydroxide layer covering the surface, which exposes more active sites for OER reactions. This work demonstrates a simple strategy for MOFs co-catalysts to obtain fast hole transfer capability and reduce carrier recombination, thereby improving PEC performance.

A facile surface trap passivation of perovskite absorber enabled by tetrabenzocorrolazine dye for efficient and stable planar perovskite solar cells

The trap-site passivation of perovskite absorbers is very important for reducing charge recombination and resultant energy loss in high-performance perovskite solar cells (PSCs). To achieve the successful role of a passivating agent, an appropriate molecular design strategy with specific functional groups should be considered for effective passivation of the defective antisites on the surface of perovskite absorber layers. Here, we designed a tetrabenzocorrolazine phosphorus dye (RC1), that is capable of electron-donating properties with a hydroxyl functionalization, as a multi-functional passivation agent for high-performance PSCs. It is found that the surface properties of the perovskite absorber were stabilized as RC1 interacts with Pb2+ and Cs + cations of the perovskite lattice, providing much improved optoelectronic properties and their charge extraction/collection capability. Moreover, the RC1-passivation successfully reduced carrier recombination and internal energy loss, leading to hysteresis-free power conversion efficiency of 18.922% (14.466% for the control device). The RC1 also retains long-term stability for 2000 h with only 10% efficiency loss of the fresh device. Thus, this study demonstrates a successful passivating capability of functionalized tetrabenzocorrolazine phosphorus dye for high-performance PSCs.

Type-II van der Waals heterostructures of GeC, ZnO and Al2SO monolayers for promising optoelectronic and photocatalytic applications

Two-dimensional materials stacked via van der Waals (vdW) forces provide a revolutionary route toward high-performance optoelectronic and renewable energy devices. Here, we report vdW heterostructures (vdWHs) consisting of GeC, ZnO and Al2SO monolayers on first-principles computations. GeC (ZnO)–Al2SO vdWHs are both stable type-II semiconductors with indirect (direct) band gaps. This significantly suppresses the recombination of photogenerated charge carriers across the interface, making them promising for light detection and harvesting applications. Charge transfer from GeC (Al2SO) layer to Al2SO (ZnO) layer leads to p-doping in GeC (Al2SO) and n-doping in Al2SO (ZnO) of GeC (ZnO)–Al2SO vdWHs. In contrast to pristine monolayers, higher carrier mobility promotes charge transfer to the surface and reduces carrier recombination in GeC (ZnO)–Al2SO vdWHs. Further, the absorption spectra indicate redshift (blueshift) and reveal more solar light is absorbed by GeC (ZnO)–AlS2O vdWHs in the visible (ultraviolet) region. The band edge positions suggest that GeC–Al2SO vdWHs can reduce water into H2 but fails to perform an oxidation reaction at pH = 0. More interestingly, ZnO–Al2SO vdWHs can perform redox reactions, making them prominent for overall water-splitting reactions. Our computational findings provide a path for the design of vdWHs for future optoelectronic and photovoltaic devices.

Enhanced charge separation by interchain hole delocalization in nonfullerene acceptor‐based bulk heterojunction materials

AbstractBulk heterojunction (BHJ) composites show improved power conversion efficiencies when optimized in terms of morphology using various film processing methods. A reduced carrier recombination loss in an optimized BHJ was characterized previously. However, the driving force that leads to this reduction was not clearly understood. In this study, we focus on the decreased carrier recombination loss and its driving force in optimized nonfullerene acceptor‐based PTB7‐Th:IEICO‐4F BHJ composites. We demonstrate that the optimized BHJ shows deactivation in the sub‐nanosecond nongeminate carrier recombination process. The driving force for this deactivation was determined to be the improved interchain hole delocalization between the polymers. An enhanced interchain hole delocalization was observed using steady‐state photoinduced absorption (PIA) spectroscopy. In particular, increased splitting between the polaron PIA bands was noted. Moreover, improved interchain hole delocalization was observed for other state‐of‐the‐art BHJ materials, including D18:Y6 with optimized morphologies.

Porphyrins-Assisted Cocatalyst Engineering with CoOV Bond in BiVO4 Photoanode for Efficient Oxygen Evolution Reaction.

The application of photoelectrochemical (PEC) water splitting is limited by the sluggish surface oxygen evolution reaction (OER) kinetics. OER kinetics can be effectively improved through cocatalyst engineering. However, the tardy transfer process and serious recombination of carriers are the key factors restricting the cocatalyst development. Taking BiVO4 as an example, a Co-modified heme film rich in large conjugated ring structures is introduced onto the photoanode surface using a solvothermal method. This film functions as an efficient cocatalyst. It considerably reduces the surface overpotential, promotes the transfer of photogenerated holes, and boosts the kinetics of OER by specifically affecting the formation of OOH*. Simultaneously, the formed CoOV bonds induce strong interaction at the photoanode/cocatalyst interfaces, reducing the recombination of photogenerated carriers. Consequently, the onset potential of the optimized photoanode decreases from 0.45 to 0.07V and the photocurrent density at 1.23V versus reversible hydrogen electrode boosts to 5.3mA cm-2 . This work demonstrates a facile strategy for designing cocatalysts to obtain rapid hole transfer capability and reduced carrier recombination for improved PEC performance.

CuSCN Modified Back Contacts for High Performance CZTSSe Solar Cells

AbstractOptimization of the back contact interface is crucial for improving the performance of Cu2ZnSnS4 (CZTS) thin film solar cells. In this paper, self‐depleted CuSCN is deployed as an intermediate layer at the Mo/CZTS interface to improve the quality of the back contact. This CuSCN layer, obtained via aqueous solution processing, reduces the thickness of Mo(S,Se)2 and eliminates multi‐layer crystallization of the absorber by suppressing the undesirable reaction between Mo and Se during the selenization process. By regulating the selenium infiltration into the CZTS precursor films during the selenization process, highly crystalline, single‐layer Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers are realized. The single‐layer CZTSSe absorber exhibits reduced carrier recombination, enhanced carrier density and increased work function. The improved back contact and absorber layer enables 11.1% power‐conversion‐efficiency to be achieved.

Green-solvent-processed formamidinium-based perovskite solar cells with uniform grain growth and strengthened interfacial contact via a nanostructured tin oxide layer.

Green-solvent-processed perovskite solar cells (PSCs) have reached an efficiency of 20%, showing great promise in safe industrial production. However, the nucleation process in green-solvent-based deposition is rarely optimized, resulting in randomized crystallization and much lowered reported efficiencies. Herein, a nanostructured tin oxide nanorods (SnO2-NRs) substrate is utilized to prepare a high-quality formamidinium (FA)-based perovskite film processed from a green solvent of triethyl phosphate (TEP) with a low toxic antisolvent of dibutyl ether (DEE). Compared with SnO2 nanoparticles, the oriented SnO2-NRs can accelerate the formation of heterogeneous nucleation sites and retard the crystal growth process of the perovskite film, resulting in a high-quality perovskite film with uniform grain growth. Furthermore, a chlorine-terminated bifunctional supramolecule (Cl-BSM) is introduced to passivate the increasing interfacial defects due to the vast contact area in SnO2-NRs. Correspondingly, the substrate design of SnO2-NRs with Cl-BSM increases the power conversion efficiency (PCE) of green-solvent-processed PSCs to 22.42% with an open-circuit voltage improvement from 1.02 to 1.12 V, which can be attributed to the uniform grain growth and reduced carrier recombination at the SnO2-NRs/perovskite interface. More importantly, the photo and humidity stabilities of the unencapsulated device for up to 500 and 1000 hours are also achieved with negligible interfacial delamination after aging. This work provides a new perspective on the future industrial scale production of PSCs using environment-friendly solvents with compatible substrate design.

An ultrafast and self-powered MoSxSe2-x/Si photodetector with high light-trapping structures and a SiOx interface layer.

MoSxSe2-x nanofilms, as a typical metal dichalcogenide, have attracted great interest, due to their adjustable bandgap and distinctive electronic and optical properties. However, the inherent bandgap of MoSxSe2-x and the strong interface recombination impede the actualization of a high-sensitivity photodetector (PD). Few-layer MoSxSe2-x nanofilms were prepared with vertically orientation at 450 °C, which would be a less restrictive choice of substrates. Herein, a self-powered MoSxSe2-x/SiOx/Si photodetector was fabricated which exhibits unprecedented performance with excellent reproducibility and stability from 405 nm to 980 nm, a high responsivity (0.450 A W-1), normalized detectivity (4.968 × 1012 Jones) and ultrafast photoresponse (τr = 1.20 μs, τf = 4.92 μs) at zero bias under 980 nm incident laser illumination with a density of 200 μW cm-2. Significantly, the self-powered PD is capable of detecting ultraweak IR signals below 200 μW cm-2 with high on-off ratios. More importantly, an oxidized atomic layer is generated through the wet oxidation in the Piranha solution. The PD can work well at high frequencies even at 100 kHz, which shows its potential application in high-frequency photoelectric devices and health monitors. Summing up, this work not only suggests that an ultrathin SiOx interface layer can reduce carrier recombination via simple interface engineering, but also proposes a novel strategy for the preparation of high-performance and low-cost optoelectronic devices.

Modifying a D-A-π-A-D HTM system for higher hole mobility by the meta-substitution strategy to weaken the electron-donating ability of the donor unit: a DFT study.

Tuning the electron-donating ability (EDA) of the donor units of hole transporting materials (HTMs) is an efficient strategy to modulate the optoelectronic properties of HTMs. Based on this strategy, we first theoretically investigated the effects of the EDA of donor units on D-A-π-A-D architectural HTMs. The results show that the enhanced EDA of the donor unit leads to larger hole reorganization energy and poorer molecular stability of HTMs. In contrast, meta-substitution of side groups is an effective strategy to reduce the EDA of the donor unit. We found that the application of the meta-substitution strategy in the D-A-π-A-D system not only successfully improves the molecular stability, but also achieves higher hole mobility by promoting the electronic coupling between the molecular dimers and decreasing the hole reorganization energies simultaneously. Studies on interfacial properties indicate that intermolecular coupling also synergistically enhances the interfacial charge extraction performance and reduces carrier recombination. In conclusion, by utilizing the meta-substitution strategy to reduce the EDA of donor units on D-A-π-A-D architectural HTMs, we successfully designed four superior performance HTMs mD1, mD2, mD3, and mD4.

Towards low-temperature processing of efficient γ-CsPbI3 perovskite solar cells.

Inorganic cesium lead iodide (CsPbI3) perovskite solar cells (PSCs) have attracted enormous attention due to their excellent thermal stability and optical bandgap (∼1.73 eV), well-suited for tandem device applications. However, achieving high-performance photovoltaic devices processed at low temperatures is still challenging. Here we reported a new method for the fabrication of high-efficiency and stable γ-CsPbI3 PSCs at lower temperatures than was previously possible by introducing the long-chain organic cation salt ethane-1,2-diammonium iodide (EDAI2) and regulating the content of lead acetate (Pb(OAc)2) in the perovskite precursor solution. We find that EDAI2 acts as an intermediate that can promote the formation of γ-CsPbI3, while excess Pb(OAc)2 can further stabilize the γ-phase of CsPbI3 perovskite. Consequently, improved crystallinity and morphology and reduced carrier recombination are observed in the CsPbI3 films fabricated by the new method. By optimizing the hole transport layer of CsPbI3 inverted architecture solar cells, we demonstrate efficiencies of up to 16.6%, surpassing previous reports examining γ-CsPbI3 in inverted PSCs. Notably, the encapsulated solar cells maintain 97% of their initial efficiency at room temperature and under dim light for 25 days, demonstrating the synergistic effect of EDAI2 and Pb(OAc)2 in stabilizing γ-CsPbI3 PSCs.

Two-dimensional Bi2S3/MoS2 van der Waals heterostructure for self-powered photodetectors

Two-dimensional (2D) semiconductor materials have been widely applied for optoelectronic devices, but fast charge recombination severely affects device performance. The construction of a 2D heterostructure is a proven strategy for effectively reducing carrier recombination. 2D Bi2S3/MoS2 heterostructure is prepared by a sequential vapor deposition method. This heterostructure displays a fast response/recovery time of 4/15 ms and a high responsivity of 218.3 μA W−1. The improved photodetection performance is mainly ascribed to the internal electric field-induced efficient charge transfer in the type-II Bi2S3/MoS2 heterostructure. This study provides a guideline to extend other heterostructure photodetectors and improve the photodetection performance.

Interfacial electronic and vacancy defect engineering coupling of the Z-scheme CsSnBr3/SnS2 heterostructure for photovoltaic performance: a hybrid DFT study

Interfacial coupling of Z-scheme CsSnBr3/SnS2 heterostructure induces a narrowing band gap and reduces carrier recombination rate. Coupling interface and Br vacancy defects of CsSnBr3 has excellent photovoltaic performance.

Enhancement of Perovskite Solar Cells by TiO2-Carbon Dot Electron Transport Film Layers

The high performance of perovskite solar cells was produced with the help of an electron transport layer (ETL) and hole transport layer. The film ETL (mesoporous (meso)-TiO2/carbon dot) boosted the efficiency of the perovskite solar cells. A perovskite cell was fabricated by a coating of carbon dot on a meso-TiO2 ETL. The fabricated meso-TiO2/carbon dot-based device has decreased the pin-holes of the perovskite film layer compared to the meso-TiO2-based device, which boosted 3% of the averaged PCE value of the devices. The UV-visible spectroscopy confirmed that the meso-TiO2/carbon dot ETL showed better absorbance, that is, absorbed more incident light than meso-TiO2 ETL to generate higher power conversion efficiency. Coating of carbon dot on meso-TiO2 reduced carrier recombination, and fadeaway of the perovskite film cracks. The X-ray diffraction spectra displayed the removal of the perovskite component after spin-coating of carbon dot to the meso-TiO2 ETL, indicating that the suppression of non-radiative recombination improves the device performance compared to meso-TiO2 ETL. The stability after four weeks on the performance of the device was improved to be 92% by depositing carbon dot on meso-TiO2 ETL compared to the meso-TiO2 ETL-based device (82%). Thus, the high-quality perovskite cell was fabricated by coating carbon dot on a meso-TiO2 ETL, because the electron transport between ETL and perovskite film layer was improved by the injection of electrons from carbon dot.

Understanding the efficiency enhancement of perovskite solar cells with NaGdF4: Er3+/Yb3+ nanorods

Perovskite solar cells have drawn significant attention due to their excellent photovoltaic characteristics. Upconversion luminescence material modification is a good strategy to extend the device response to the near-infrared region, which can further increase device efficiency. By adding NaGdF 4 :Er 3+ /Yb 3+ nanorods into chlorobenzene as an additive in the antisolvent to modify the perovskite layer, we found that upconversion luminescence nanoparticles can effectively improve the perovskite solar cell performance. However, unexpectedly, the photocurrent contributed by the infrared part is lower. The efficiency increase of perovskite solar cells is contributed by defect passivation. Further research shows that nanoparticles can effectively fill the holes and gaps at the grain boundaries, eliminate surface defects of the perovskite, and modify the perovskite film, which reduces carrier recombination and effectively improves the device's performance. Finally, the power conversion efficiency of the champion device can reach 21.1%. This research demonstrates the function of upconversion luminescence nanoparticles in PSCs and provides a simple way to improve the device's efficiency. • The up-conversion luminescence NaGdF 4 :Er 3+ /Yb 3+ is introduced into the perovskite layer via anti-solvent chlorobenzene. • The champion device achieved an efficiency of 21.1% by passivating perovskite grain defects through NaGdF 4 nanocrystals. • The reason for the efficiency enhancement is the increase of carrier lifetime rather than spectrum broadening to infrared field.

Analysis of a novel method of alkalis treatment: Effect on energy band optimization and carrier recombination at the grain boundary

Alkali metal post-deposition treatment (PDT) is a key factor to approach high efficiency in Cu(In, Ga)Se 2 (CIGS) solar cells. In addition to single alkali element treatment, co-doping of light and heavy alkali elements is also applied to the preparation of CIGS absorber for higher efficiency. In this work, the details of different methods of alkali-treatments are revealed by combining several microstructural and compositional analysis methods, especially for the NaF-PDT/CsF-PDT process which introduces extra Na before Cs into the absorber. The ion exchange of Na and Cs in the grain boundaries (GB) of the absorber is analyzed in detail, which reveals alkali-treatment restrains the formation of large bandgap compounds at GB. Extensive material analysis shows that NaF/CsF-PDT can not only tailor the surface nanoscale morphology of CIGS films but also optimize the band structure at GB to reduce carrier recombination. Such modification gains the improvement of the highest power conversion efficiency (PCE) from 15.2% to 17.4% and average PCE from 14.3% to 16.5% (without antireflective coating) after NaF/CsF-PDT. According to the analysis of data, we propose a model of energy band at GB, which explains the reason why NaF/CsF-PDT shows a better effect on CIGS devices than CsF-PDT. These results could serve as a basis for the further understanding of the effects of alkali-PDT on CIGS film and help to achieve even higher efficiency. • Effects of alkali elements Na and Cs on CIGS solar cells are investigated by a series of characterization tools. • NaF/CsF-PDT can tailor the surface nanoscale morphology and optimize the band structure at CIGS films. • An energy band model of GBs is proposed to explain the effect of alkali metal co-doping. • The PCE improved from 15.2% to 17.4% after NaF/CsF-PDT.

Performance evaluation of WS2 as buffer and Sb2S3 as hole transport layer in CZTS solar cell by numerical simulation

AbstractThis study reports on performance enhancement of a Cu2ZnSnS4 solar cell introducing Sb2S3 as hole transport layer (HTL) along WS2 as buffer layer. We have investigated photovoltaic (PV) characteristics by utilizing SCAPS‐1D. A comparative analysis on PV performances between conventional CZTS/CdS and proposed Ni/Sb2S3/CZTS/WS2/FTO/Al solar cells is presented. It is revealed that “spike like” band structure at the CZTS/WS2 interface having smaller conduction band offset makes it potential alternative to commonly used CdS buffer. This report also evaluates that the Sb2S3 as an HTL inserted at the rear of CZTS enhances performances by reducing carrier recombination at back interface with appropriate band alignment. The impacts of thickness, carrier concentration of different layers, and bulk defect density in CZTS as well as the interface defects on cell outputs are analyzed. The influences of temperature, work function, and cell resistances are also examined. Optimum absorber thickness of 1.0 μm along doping density of 1017 cm−3 is selected. A maximum efficiency of 30.63% is achieved for the optimized CZTS cell. Therefore, these results suggest that Sb2S3 as HTL and WS2 as buffer layer can be employed effectively to develop highly efficient and low‐cost CZTS solar cells.