Carrier Transport Layers Research Articles

Perovskite based low-powered hybrid UV photodetector with RF sputtered metal oxide carrier transport layers

The authors here have reported two low-powered UV photodetectors designed with and without NiO as a hole transport layer (HTL) having the hybrid heterostructure of ZnO/CH3NH3PbI3 (Device1) and ZnO/CH3NH3PbI3/NiO (Device2) respectively. Both the devices showed good photoelectric parameters in UV region under self-powered condition. However, in Device1 due to its high dark current (390 nA), the responsivity (Rλ) and specific detectivity (D*) were degraded which were improved in Device2 (low dark current of 1.7 nA) after incorporating the NiO as an HTL. The maximum D* values of Device1 and Device2 in UV region are found to be of 5.5 × 108 Jones and 7.8 × 109 Jones at 0 V respectively. Moreover, an ultra-fast device response was obtained for Device2 with rise and fall times of 50 ms and 40 ms respectively. Also, the accumulation capacitance and conductance were found to be higher for Device2 making it a good candidate for low power photodetection application.

Targeted passivation and optimized interfacial carrier dynamics improving the efficiency and stability of hole transport layer-free narrow-bandgap perovskite solar cells

Narrow-bandgap mixed Sn-Pb perovskite solar cells (PSCs) have showcased great potential to approach the Shockley-Queisser limit. Nevertheless, the practical application and long-term deployment of mixed Sn-Pb PSCs are still largely impeded by the rapid oxidation of Sn2+ ions and under-optimized carrier transport layer (CTL)/perovskite interfaces that would inevitably incur serious interfacial charge recombination and device performance degradation. Herein, we successfully removed the hole transport layer (HTL) by incorporating a small amount of organic phosphonic acid molecules into perovskites, which could preferably interact with Sn2+ ions (relative to Pb2+ analogues) at the grain boundaries (GBs) throughout the perovskite film thickness via coordination bonding, thus effectively retarding the oxidation of Sn2+, passivating the defects and suppressing the non-radiative recombination. Targeted modification effectively reinforced built-in potential by ∼100 mV, and favorably induced energy level cascade, thus accelerating spatial charge separation and facilitating the hole extraction from perovskite layer to underlying conductive electrodes even in the absence of HTL. Consequently, enhanced power conversion efficiencies up to 20.21% have been achieved, which is the record efficiency for the HTL-free mixed Sn-Pb PSCs, accompanied by a decent photovoltage of 0.87 V and improved long-term stability over 2400 h.

CsPbBr3 Quantum Dots‐Sensitized Mesoporous TiO2 Electron Transport Layers for High‐Efficiency Perovskite Solar Cells

The carrier transport layer plays a critical role in high efficiency and stable perovskite solar cells (PSCs). Herein, CsPbBr3 quantum dots‐sensitized mesoporous TiO2 (CPB–TiO2) electron transport layer for fabricating high‐quality perovskite films with enhanced phase stability is developed. The CPB–TiO2 layer enhances the perovskite crystallinity, phase stability, and reduces the nonradiative carrier recombination in PSCs. As a result, the CPB–TiO2 significantly improves the power conversion efficiency of FAPbI3 PSCs from 23.11% to 24.46% and reduces the device hysteresis. The CPB–TiO2–FAPbI3 device shows enhanced stability, retaining 90% of its initial efficiency after 500 h operation at maximum power point tracking under 1 sun illumination, whereas the control device degrades to 80% of initial performance in the first 200 h. In addition, this strategy is applicable to CsPbI3 perovskite, which provides a new and general strategy for preparing high‐efficiency PSCs.

Superior performance and ultrafast response from CH3NH3PbI3 based UV–visible broadband photodetector using 1D carrier transport layers

We have designed and fabricated two perovskite-based photodetectors (PPDs) with 2D and 1D metal oxide carrier transport layers having the structure of ZnO thinfilm (TF)/CH3NH3PbI3/NiO TF (Device1) and ZnO nanowire (NW)/CH3NH3PbI3/NiO NW (Device2). Vertical and porous 1D carrier transport layers were fabricated using the advanced glancing angle deposition (GLAD) technique. The structure with 1D carrier transport layers in Device2 showed an enhanced optical absorption and emission as compared to 2D layered structure in Device1. The Device2 would enhance the responsivity (Rλ) upto 134 times in UV (390 nm) and 1350 times in visible (450 nm) region as compared to Device1. Moreover, the maximum detectivity (D*) values of 3.43 × 1011 Jones and 3.53 × 1011 Jones were observed for Device2 and 0.343 × 1011 and 0.35 × 1010 Jones for Device1 in UV and visible regions respectively. In addition to that, the Device2 also showed an external quantum efficiency (EQE) of 3.2 × 103% and 2.9 × 103% in UV and visible regions respectively. Furthermore, the Device2 showed an ultrafast device response with rise time and fall times of 0.17 s and 0.13 s respectively. The 1D carrier transport layers in PPDs can be used as good candidate for future broadband photodetectors (PDs).

Theoretical Analysis of All‐Inorganic Wide Bandgap Perovskite/Sn‐Based Narrow Bandgap Perovskite Tandem Solar Cells

Compared with the single‐junction perovskite solar cells, the perovskite/perovskite tandem solar cells have the advantages of lower cost and higher power conversion efficiency (PCE). Herein, both two‐terminal (2‐T) and four‐terminal (4‐T) perovskite/perovskite tandem solar cells with all‐inorganic perovskite as the top cell absorption layer and narrow bandgap perovskite MASn0.5Pb0.5I3 material as the bottom cell absorption layer are studied. To effectively improve the photon absorption ratio and performance of the 4‐T tandem device, both reflection and parasitic absorption should be reduced. Afterward, by optimizing the doping concentration of the carrier transport layer, a 4‐T all‐perovskite tandem solar cell with a high PCE of 30.45% is obtained. For the 2‐T all‐perovskite tandem device, the all‐inorganic perovskites with different halogen components (CsPbI3−xBrx, 0 ≤ x ≤ 3) are used as the absorption layer of the top cell, respectively. Through the optimization of the current matching of the subcell, the photoelectric field distribution, the parasitic absorption of the device, etc., an optimal PCE of 27.86% is obtained based on 2‐T CsPbI2Br/MASn0.5Pb0.5I3 tandem device. This study provides a guide for achieving high performance perovskite/perovskite tandem solar cells.

Relevance of Long Diffusion Lengths for Efficient Halide Perovskite Solar Cells

Carrier diffusion length is not a reliable indicator of performance in some halide perovskite solar cells; instead, experiments and modeling show that properties of the carrier transport layers limit the performance.

Application of Natural Molecules in Efficient and Stable Perovskite Solar Cells.

Perovskite solar cells (PSCs), one of the most promising photovoltaic technologies, have been widely studied due to their high power conversion efficiency (PCE), low cost, and solution processability. The architecture of PSCs determines that high PCE and stability are highly dependent on each layer and the related interface, where nonradiative recombination occurs. Conventional synthetic chemical materials as modifiers have disadvantages of being toxic and costly. Natural molecules with advantages of low cost, biocompatibility, and being eco-friendly, and have improved PCE and stability by modifying both functional layers and interface. In this review, we discuss the roles of natural molecules on PSCs devices in terms of the perovskite active layer, interface, carrier transport layers (CTLs), and substrate. Finally, the summary and outlook for the future development of natural molecule-modified PSCs are also addressed.

ZnO‐Doped In2O3 Front Transparent Contact Enables >24.0% Silicon Heterojunction Solar Cells

Transparent conductive oxide (TCO) film acts as the window layer and carrier transport layer of silicon heterojunction (SHJ) solar cells. The optoelectrical and contact properties of TCO films, as well as the sputtering damage, limit the further efficiency improvement. Herein, the microstructure, morphology, and optoelectrical properties of sputtering‐prepared zinc‐doped indium oxide (IZO) are analyzed. The substrate temperature has a significant impact on the film growth and the cell performance. The IZO film is amorphous when substrate temperature is below 200 °C, while it seems the emergence of nanocrystals with the increased temperature, resulting in the highest conductivity (3070 S cm−1) and carrier mobility (47.9 cm2 V−1 s−1). In addition, the low carrier concentration and surface potential of IZO promote less optical parasitic absorption and higher work function. The IZO films are applied as the contact layer of SHJ solar cells. An impressive efficiency of 24.02% on an M2‐sized wafer is achieved, indicating great potential for the mass production application after careful optimization.

Near-ultraviolet organic light emitting diodes using melem

This study fabricates near-ultraviolet (NUV)-organic light-emitting diodes (NUV-OELDs) using 2,5,8-triamino-tri-s-triazine (melem) as the emitting layer. Melem exhibits NUV emission owing to thermally activated delayed fluorescence. However, the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap of melem is large, thus rendering difficulty in carrier injection from the electrodes and causing the carrier injection layer or carrier transport layer to emit light. Therefore, in this study, the device structure of the NUV-OLED is optimized to obtain melem-derived luminescence using carrier transport materials with a large HOMO–LUMO gap. Consequently, NUV-OLED with emission in the NUV region is successfully realized.

Solution-Processed Organic–Inorganic Semiconductor Heterostructures for Advanced Hybrid Phototransistors

Organic/inorganic hybrid phototransistors are an emerging class of advanced optoelectronic devices. The separated organic high absorption layer and inorganic carrier transport layer endow hybrid phototransistors with remarkable sensing performance in a broad spectrum. The flexibility, ambient stability, and process compatibility of organic and metal oxide semiconductors also render them promising for large-area matrix integration and wearable applications. Moreover, extraordinary photoelectric behaviors, such as bidirectional photoresponses and the versatility of in-sensor computing functions, including feature extraction and image enhancement, can be obtained by rational customization of the hybrid heterojunctions. In this Spotlight on Applications, we review the recent progress of organic/inorganic hybrid phototransistors and their applications in photodetection, sensor arrays, flexible electronics, and in-sensor computing.

Enhanced performance of a photodetector based on a graphene/CVD-grown dendritic ReS2/Ta2O5 vertical heterojunction

Here, a dendritic ReS2/Ta2O5/graphene structure with tunneling contacts and work-function optimized carrier transport layer achieved one of the highest photoresponsivity among devices fabricated with synthesized transition metal dichalcogenide films.

Slowing the hot‐carrier cooling by an organic small molecule in perovskite solar cells

AbstractHarvesting hot carriers (HCs) before thermalizing to the perovskite crystal lattice is very promising for advancing the efficiency of perovskite solar cells (PSCs) towards the Shockley–Queisser limit. Thus, it is very crucial to slow down the HCs cooling process in lead halide perovskites. Herein, we investigated the HCs cooling dynamics of perovskite films in the presence of an organic small molecule, 2,5‐thiophenedicarboxylic acid (TDCA). We found that adding TDCA into perovskite films can efficiently retard HCs cooling process, which favors extracting the excess energies of HCs through carrier transport layers and reducing electron–hole recombination, resulting in enhanced performance in TDCA modified device. Moreover, TDCA can effectively passivate unsaturated Pb2+ defects of perovskite films while promoting a preferential crystal orientation. Consequently, the PSCs efficiency is enhanced to 22.85% (TDCA) from 19.96% (control). These findings provide important insight for understanding HCs transfer dynamics and developing next‐generation perovskite‐based HCs optoelectronic devices.image

Implementation of Tunneling Junction Passivated Contact Concept in Flexible CIGS Solar Cells

AbstractIn today's state‐of‐the‐art high‐efficiency silicon solar cells need to be inserted a thin insulating layer in order to reduce the recombination losses between the carrier transport layer and Si surface, which can form a tunneling junction (TJ), thus increasing the performance of the TJ solar cells comparable with the pn junction structure. However, the copper indium gallium selenium (CIGS) solar cells inevitably lead to the losses of the carrier transport due to the interface which is widely assumed as the pn‐heterojunction. Herein, the TJ solar cells, aiming to enhance the performance of the solar cells, are fabricated by inserting the TiO2 between CIGS/CdS interface deposited by atomic layer deposition (ALD). By inserting the TiO2 insulating layer, the CIGS/TiO2/CdS structure can be effectively reduced the interface recombination, which leads to a reduced band bending in the p‐CIGS surface and compromises its field‐effect passivation. As a result, the CIGS solar cell with the tunneling junction achieves the 15.57% based on the stainless steel (SS) substrate, by introducing a barrier layer and doping NaF. These results provide an important preliminary foundation for the development of the CIGS solar cells with the tunneling junction structure.

Interface layer modulation of an all-inorganic perovskite solar cell to study the carrier transport mechanism

Over the past decade, organic halide perovskite has been an excellent absorber in solar cells. However, their stability issues have forced the research community to search for purely inorganic perovskites. In this work, we designed a perovskite solar cell based on a purely inorganic Cs0.8Rb0.2SnI3 absorber layer with inorganic carrier transport layers using SCAPS-1D simulation software. Using this exemplary architecture, we have discussed different carrier movement mechanisms in the conduction and valence bands through the interfaces of the absorber and carrier transport layers by instigating two interface layers. Our results illustrate the variation in device performance with the electron affinities and diffusion lengths. In addition, the study demonstrates a positive impact of Rb doping in CsSnI3 based perovskite solar cells. The absorber thickness and defect density optimization are also executed to maximize the solar cell performance. The optimized electron affinities of the interface layers are found to be 4.1 and 4.3 eV, respectively. The final model of the device has achieved a photo-conversion efficiency of 23% with a short circuit current of about 30 mA cm−2. The optimized model covers the entire visible solar spectrum with a quantum efficiency of >90%.

Cathode Buffer Layer for Energy‐Level Matching and Interface Passivation

Perovskite solar cells (PSCs) have demonstrated high‐power conversion efficiency (PCE) and exhibit great application potential in photovoltaic systems. Generally, a typical PSC is accompanied with multiinterfaces; therefore, charge extraction and transport in PSCs are strongly affected by these interfaces. In fact, interfacial energy‐level mismatch of carrier transport layer and anode/cathode is also deeply limiting the electrical performance of PSCs. Herein, a low‐temperature‐processed indium tin oxide (ITO‐LT) cathode buffer layer is developed and used for optimizing the energy‐level alignment and suppressing defects at the electron transfer layer and cathode interface. It is revealed that a 3 nm ITO‐LT cathode buffer layer with an appropriate energy alignment and effective defect suppression could enhance interfacial charge transfer, resulting in a PCE of 21.13% in a planar PSC.

In-depth understanding the effect of electron-withdrawing/-donating groups on the interfacial carrier dynamics in naphthalimide-treated perovskite solar cells

Surface defect passivation of perovskite films through chemical interaction between specific functional groups and defects has been proven to be an effective technique for enhancing the performance and stability of perovskite solar cells (PSCs). However, an in-depth understanding of how these passivation materials affect the intrinsic nature of charge-carrier transfer kinetics in PSCs remains shielded so far. Herein, we have designed two naphthalimide-based perovskite surface passivators having electron-withdrawing (-CF3, NSF) or electron-donating (-CH3, NSC) substituents for use in PSCs. Transient absorption spectroscopy (TA) measurements confirmed how the electron-withdrawing and electron-donating groups can efficiently turn the hot carriers (HCs) cooling and injection, and interface recombination in the device. We found that NSC-passivated perovskite samples exhibit faster hot-carriers (HCs) injection from the perovskite layer into carrier transport layers before cooling to the crystal lattice compared with the NSF-based and control ones with the order: NSC > NSF > control. Fast HCs injection is advantageous to minimize the charge-carriers recombination and improve PSCs performance. The carrier lifetime in NSC-treated device measured by nanosecond TA exhibits nearly ∼2 times longer than that of NSF-based device, which demonstrates the decreased charge-carrier recombination in NSC-treated device. As expected, the power conversion efficiency (PCE) of the NSC-treated PSCs is improved to 23.04% compared with that of the device treated with NSF (21.81%). Our findings provide invaluable guide for developing highly efficient passivators to further boost PSCs photovoltaic performance.

Cat-CVD SiN x as a gas barrier for application to perovskite solar cells

We investigated a silicon nitride (SiN x ) film prepared by catalytic chemical vapor deposition as a gas barrier for further improving stability of mixed-cation-halide perovskite (PVK) Cs0.05FA0.85MA0.1PbI2Br under dark air conditions. The SiN x film can be formed on the PVK capped with carrier transport layers such as 2,2′,7,7′-tetrakis[N,N-di(4-methoxylphenyl)amino]−9,9′-spirobifluorene (Spiro-OMeTAD) and phenyl-C61-butyric-acid-methyl-ester (PCBM)/aluminum-doped-zinc-oxide (AZO) with less degradation in its optical transmittance property and crystal structure. The PVK/Spiro-OMeTAD encapsulated by SiN x exhibits a slower reduction in average carrier lifetime after storage for 330 h at room temperature with ∼65%RH. The PVK/PCBM/AZO covered with SiN x shows a small blue-shift (8–10 nm) in the absorption band-edge of PVK and with less decrease in the transmittance in the long wavelength range for ∼500 h at 85 °C with ∼2%RH. These results demonstrate the effect of SiN x in preventing the degradation of PVK due to the interactions with moisture and oxygen in the air.

Preparation of TiO2/ZrO2 Composite as Photoanode of Ruthenium-Based Dye-Sensitized Solar Cells

Photoanode of dye-sensitized solar cell (DSSC) is an inorganic semiconductor layer with a nanoporous structure that has a function as a charge carrier transport layer and dye storage. Oxide semiconductor material such as titanium dioxide (TiO2) is widely used as a photoanode because of its wide band gap (3.2 eV) and high active surface area (80 m2/g), but TiO2 has low electron diffusion with high recombination (electron-hole pair pairing). To improve the electron diffusion and reduce recombination, TiO2 was modified with zirconia (ZrO2). The synthesis process was carried out using sol-gel method with variations of ZrO2 concentrations at 5 mol%, 10 mol%, and 15 mol%. Composite of TiO2/ZrO2 was then applied as a photoanode of DSSC. XRF results show that in all variations of ZrO2, the composites of TiO2/ZrO2 has been formed. Based on the results of UV-Vis spectroscopy, it was shown that the energy gap of TiO2/ZrO2 decreased as the increasing of ZrO2 concentration. The current density-voltage measurement was done to investigate the effect of ZrO2 addition to the TiO2 photoanode using light irradiation with power intensity of 100 mW/cm2. There is an increasing of photocurrent from 1.63 mA/cm2 of DSSC with TiO2 photoanode to 2.79 mA/cm2 of DSSC with TiO2/ZrO2 10 mol% photoanode. The highest efficiency obtained from DSSC with TiO2/ZrO2 photoanode is 1.15%. It was found that the presence of ZrO2 in the composite of TiO2/ZrO2 increased electron transport and reduced charge recombination to improve the photocurrent of DSSC.

Electron-Transport Layers Employing Strongly Bound Ligands Enhance Stability in Colloidal Quantum Dot Infrared Photodetectors.

Solution-processed photodetectors based on colloidal quantum dots (CQDs) are promising candidates for short-wavelength infrared light sensing applications. Present-day CQD photodetectors employ a CQD active layer sandwiched between carrier-transport layers in which the electron-transport layer (ETL) is composed of metal oxides. Herein, a new class of ETLs is developed using n-type CQDs, finding that these benefit from quantum-size effect tuning of the band energies, as well as from surface ligand engineering. Photodetectors operating at 1450 nm are demonstrated using CQDs with tailored functionalities for each of the transport layers and the active layer. By optimizing the band alignment between the ETL and the active layer, CQD photodetectors that combine a low dark current of ≈1 × 10-3 mA cm-2 with a high external quantum efficiency of ≈66% at 1V are reported, outperforming prior reports of CQD photodetectors operating at >1400 nm that rely on metal oxides as ETLs. It is shown that stable CQD photodetectors rely on well-passivated CQDs: for ETL CQDs, a strongly bound organic ligand trans-4-(trifluoromethyl)cinnamic acid (TFCA) provides improved passivation compared to the weakly bound inorganic ligand tetrabutylammonium iodide (TBAI). TFCA suppresses bias-induced ion migration inside the ETL and improves the operating stability of photodetectors by 50× compared to TBAI.

Performance improvement for organic light emitting diodes by changing the position of mixed-interlayer

Organic Light-Emitting Diode (OLED) is presently the most sought-after display technology. It provides low-cost, flexible, rollable displays in addition to wide viewing angles and excellent colour qualities. Still, the organic displays have not reached at their best performance and there is a lot of scope for improvement in their performance. In addition to the injection layer, emission layer, transport layer, etc, researchers are looking forward to the charge carrier transport layer, spacer layer, mixed interlayer, etc. to further enhance the device performance. In this article, a depth analysis related to the impact of the position of the mixed interlayer is performed to analyze the impact on device performance. It is observed that on shifting mixed interlayer (MI) towards the cathode; luminescence and current density depict depreciation. However, on shifting MI towards anode there is a significant performance improvement. The complete analysis includes seven device structures, wherein the position of MI is varied. The best performing device depicts luminescence of 17139 cd/m2 and a current density of 84.6 mA/cm2, which is 40.05% higher for luminescence and 111.5% for current density than that of reference device. Additionally, the internal analysis of device structure is thoroughly evaluated using the cut line method to better understand the internal device physics in terms of the electric field, electron concentration, total current density, Langevin’s recombination rate, and Singlet exciton density.