RESUMEN

Lead halide perovskite and organic semiconductors are promising classes of materials for photodetector (PD) applications. State-of-the-art perovskite PDs have performance metrics exceeding silicon PDs in the visible. While organic semiconductors offer bandgap tunability due to their chemical design with detection extended into the near-infrared (NIR), perovskites are limited to the visible band and the first fraction of the NIR spectrum. In this work, perovskite-organic heterojunction (POH) PDs with absorption up to 950 nm are designed by the dual contribution of perovskite and the donor:acceptor bulk-heterojunction (BHJ), without any intermediate layer. The effect of the energetics of the donor materials is systematically studied on the dark current (Jd) of the device by using the PBDB-T polymer family. Combining the experimental results with drift-diffusion simulations, it is shown that Jd in POH devices is limited by thermal generation via deep trap states in the BHJ. Thus, the best performance is obtained for the PM7-based POH, which delivers an ultra-low noise current of 2 × 10-14 A Hz-1/2 and high specific detectivity of 4.7 × 1012 Jones in the NIR. Last, the application of the PM7-based POH devices as NIR pulse oximeter with high-accuracy heartbeat monitoring at long-distance of 2 meters is demonstrated.

RESUMEN

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has been considered as the most promising absorber material for inorganic thin-film solar cells. Among the three main interfaces in CZTSSe-based solar cells, the CZTSSe/Mo back interface plays an essential role in hole extraction as well as device performance. During the selenization process, the reaction between CZTSSe and Mo is one of the main reasons that lead to a large open circuit voltage (VOC) deficit, low short circuit current (Jsc), and fill factor. In this study, 2D Ti3C2-MXene was introduced as an intermediate layer to optimize the interface between the CZTSSe absorber layer and Mo back contact. Benefiting from the 2D Ti3C2-MXene intermediate layer, the reaction between CZTSSe and Mo was effectually suppressed, thus, significantly reducing the thickness of the detrimental Mo(S,Se)2 layer as well as interface recombination at the CZTSSe/Mo back interface. As a result, the power conversion efficiency of the champion device fabricated with the 2D Ti3C2-MXene intermediate layer was improved from 10.89 to 13.14% (active-area efficiency). This study demonstrates the potential use of the 2D Ti3C2-MXene intermediate layer for efficient CZTSSe solar cells and promotes a deeper understanding of the back interface in CZTSSe solar cells.

RESUMEN

Vapor-transport deposition (VTD) method is the main technique for the preparation of Sb2Se3 films. However, oxygen is often present in the vacuum tube in such a vacuum deposition process, and Sb2O3 is formed on the surface of Sb2Se3 because the bonding of Sb-O is formed more easily than that of Sb-Se. In this work, the formation of Sb2O3 and thus the carrier transport in the corresponding solar cells were studied by tailoring the deposition microenvironment in the vacuum tube during Sb2Se3 film deposition. Combined by different characterization techniques, we found that tailoring the deposition microenvironment can not only effectively inhibit the formation of Sb2O3 at the CdS/Sb2Se3 interface but also enhance the crystalline quality of the Sb2Se3 thin film. In particular, such modification induces the formation of (hkl, l = 1)-oriented Sb2Se3 thin films, reducing the interface recombination of the subsequently fabricated devices. Finally, the Sb2Se3 solar cell with the configuration of ITO/CdS/Sb2Se3/Spiro-OMeTAD/Au achieves a champion efficiency of 7.27%, a high record for Sb2Se3 solar cells prepared by the VTD method. This work offers guidance for the preparation of high-efficiency Sb2Se3 thin-film solar cells under rough-vacuum conditions.

RESUMEN

The kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have shown a continuous rise in power conversion efficiencies in the past years. However, the encountered interfacial problems with respect to charge recombination and extraction losses at the CdS/CZTSSe heterojunction still hinder their further development. In this work, an additional plasmonic local electric field is imposed into the CdS/CZTSSe interface through the electrostatic assembly of a two-dimensional (2D) ordered Au@SiO2 NP array onto an aminosilane-modified surface absorber. The interfacial electric properties are tuned by controlling the coverage particle distance, and the finite-difference time domain (FDTD) simulation demonstrates that the strong near-field enhancement mainly occurs near the p-n junction interface. It is shown that the imposed local electric field leads to interfacial electrostatic potential (Velec) augmentation and improves the charge extraction and recombination processes. These electric benefits enable remarkable improvements in open-circuit voltage (Voc) and short-circuit current (Jsc), leading to the cell efficiency being increased from 10.19 to 11.50%. This work highlights the dramatic role of the plasmonic local electric field and the use of the 2D Au@SiO2 NP array to modify a surface absorber instead of the extensively used ion passivation, providing a new strategy for p-n junction engineering in kesterite photovoltaics.

RESUMEN

Semiconductor/co-catalyst coupling is considered as a promising strategy to enhance the photoelectrochemical (PEC) conversion efficiency. Unfortunately, this model system is faced with a serious interface recombination problem, which limits the further improvement of PEC performances. Here, a FeNiOOH co-catalyst with abundant oxygen vacancies on BiVO4 is fabricated through simple and economical NaBH4 reduction to accelerate hole transfer and achieve efficient electron-hole pair separation. The photocurrent of the BV (BiVO4 )/Vo-FeNiOOH system is more than four times that of pure BV. Importantly, the charge transfer kinetics and charge carrier recombination process are studied by scanning photoelectrochemical microscopy and intensity modulated photocurrent spectroscopy in detail. In addition, the oxygen vacancy regulation proposed is also applied successfully to other semiconductors (Fe2 O3 ), demonstrating the applicability of this strategy.

Asunto(s)

RESUMEN

The number of publications on perovskite solar cells (PSCs) continues to grow exponentially. Although the efficiency of PSCs has exceeded 25.5%, not every research laboratory can reproduce this result or even pass the border of 20%. Unfortunately, it is not always clear which dominating mechanism is responsible for the performance drop. Here, a simple method of light intensity analysis of the JV parameters is developed, allowing an understanding of what the mechanisms are that appear in the solar cell and limit device performance. The developed method is supported by the drift-diffusion model and is aimed at helping in the explanation of parasitic losses from the interface or bulk recombination, series resistance, or shunt resistance in the perovskite solar cell. This method can help not only point toward the dominating of bulk or interface recombination in the devices but also determine which interface is more defective. A detailed and stepwise guidance for such a type of light intensity analysis of JV parameters is provided. The proposed method and the conclusions of this study are supported by a series of case studies, showing the effectiveness of the proposed method on real examples.

RESUMEN

Although the traditional Cu-poor architecture addresses many limitations for Cu2ZnSn(S,Se)4 solar cells, its further development still encounters a bottleneck in terms of efficiency, primarily arising from the inferior charge transport within the quasineutral region and enlarged recombination at back contact. On the contrary, the electrical benign kesterite compound with higher Cu content may compensate for these shortages, but it will degrade device performance more pronouncedly at front contact because of the Fermi level pinning and more electric shunts. Based on the electric disparities on their independent side, in this work, we propose a new status of Cu component by exploring a large grain/fine grain/large grain trilayer architecture with higher Cu content near back contact and lower Cu content near front contact. The benefits of this bottom Cu-higher strategy are that it imposes a concentration gradient to drive carrier diffusion toward front contact and decreases the valence band edge offset in the rear of the device to aid in hole extraction. Also, it maintains the Cu-poor architecture at the near surface to facilitate hole quasi-Fermi level splitting. In return, the local Cu component engineering-mediated electric advances contribute to the highest efficiency of 12.54% for kesterite solar cells using amine-thiol solution systems so far.

RESUMEN

Multication metal-halide perovskites exhibit desirable performance and stability, compared to their monocation counterparts. However, the study of the photophysical properties and the nature of defect states in these materials is still a challenging and ongoing task. Here, we study bulk and interfacial energy loss mechanisms in solution-processed MAPbI3 (MAPI) and (CsPbI3)0.05[(FAPbI3)0.83(MAPbBr3)0.17]0.95 (triple cation) perovskite solar cells using absolute photoluminescence (PL) measurements. In neat MAPI films, we find a significantly smaller quasi-Fermi level splitting than for the triple cation perovskite absorbers, which defines the open-circuit voltage of the MAPI cells. PL measurements at low temperatures (∼20 K) on MAPI films demonstrate that emissive subgap states can be effectively reduced using different passivating agents, which lowers the nonradiative recombination loss at room temperature. We conclude that while triple cation perovskite cells are limited by interfacial recombination, the passivation of surface trap states within the MAPI films is the primary consideration for device optimization.

RESUMEN

Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1-sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non-radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open-circuit voltage and the internal quasi-Fermi level splitting (QFLS), the transport resistance-free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity-dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non-radiative fill factor and open-circuit voltage loss. It is found that potassium-passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit.

RESUMEN

2D Ruddlesden-Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite-based cells. Herein, 2D (CH3 (CH2 )3 NH3 )2 (CH3 NH3 )n -1 Pbn I3 n +1 perovskite cells with different numbers of [PbI6 ]4- sheets (n = 2-4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open-circuit voltage (VOC ) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C60 interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi-Fermi level splitting matches the device VOC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

RESUMEN

Copper zinc tin sulfo-selenide (CZTSSe) is a promising light-absorbing material of thin-film solar cells because of its low material cost especially when it is prepared by cost-effective processes like the electrochemical deposition process. The CZTSSe thin-film solar cells, however, suffer from the relatively low efficiency, mostly because of the significant charge recombination. Given that the surface recombination is one of the major recombination paths, controlling the surface roughness, and thus the interfacial area is one of the key factors for improving their device performances. In this study, we demonstrated a simple but effective strategy for reducing the surface roughness during the electrochemical deposition process of the CZTSSe thin films. By adopting an initial nucleation stage with higher deposition currents ahead of the steady-state galvanostatic deposition, the surface of the copper-zinc-tin (CZT) precursor and CZTSSe thin films became significantly smoother and uniform (ΔRrms: -43.8% for CZT, -28.9% for CZTSSe). The effects of the surface roughness on the photovoltaic properties of the CZTSSe thin-film solar cells have been investigated systematically with various characterization techniques like the diode analysis, lifetime measurement, and the temperature dependency of the open-circuit voltage. The device with the smoother surface exhibited higher open-circuit voltage and fill factor, mostly because of the significantly reduced charge recombination, leading to the high conversion efficiency of 8.64% (active).

RESUMEN

The interfacial properties of organolead halide perovskite solar cells (PSCs) affect the exciton and charge-transport dynamics significantly. Thus, proper modification of the interfaces between perovskite and charge-transport layers is an efficient method to increase the power conversion efficiency (PCE) of PSCs. In this work, we explore the effect of a nonionic surfactant, that is, Triton X-100 (TX) additive, in the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hole-transport layer. The electronic structure of TX-modified PEDOT:PSS is investigated with ultraviolet/X-ray photoelectron spectroscopy and X-ray absorption spectroscopy with various TX concentrations. The surface of the TX-modified PEDOT:PSS layer showed high TX content, and thus the semimetallic properties of PEDOT:PSS were suppressed conspicuously by its insulating nature. With the TX-modified PEDOT:PSS, the PCE of methylammonium lead iodide (MAPbI3) PSCs increased significantly. To elucidate the origin of the improved device performance, the electrical properties and photoluminescence were investigated comprehensively. Consequently, it was found that the TX additive inhibits interface recombination between PEDOT:PSS and MAPbI3, which is caused by the suppression of semimetallic properties of the PEDOT:PSS surface. Hence, we fabricated flexible PSCs successfully using a graphene electrode and TX-modified PEDOT:PSS.

RESUMEN

High-efficiency hole transport layer free perovskite solar cells (HTL-free PSCs) with economical and simplified device structure can greatly facilitate the commercialization of PSCs. However, eliminating the key HTL in PSCs results usually in a severe efficiency loss and poor carrier transfer due to the energy-level mismatching at the indium tin oxide (ITO)/perovskite interface. In this study, we solve this issue by introducing an organic monomolecular layer (ML) to raise the effective work function of ITO with the assistance of an interface dipole created by Sn-N bonds. The energy-level alignment at the ITO/perovskite interface is optimized with a barrier-free contact, which favors efficient charge transfer and suppressed nonradiative carrier recombination. The HTL-free PSCs based on the ML-modified ITO yield an efficiency of 19.4%, much higher than those of HTL-free PSCs on bare ITO (10.26%), comparable to state-of-the-art PSCs with a HTL. This study provides an in-depth understanding of the mechanism of interfacial energy-level alignment and facilitates the design of advanced interfacial materials for simplified and efficient PSC devices.

RESUMEN

Contactless time-resolved optical pump-probe and external quantum efficiency measurements were performed in epitaxially grown free-standing wurtzite indium arsenide/indium aluminum arsenide (InAs-InAlAs) core-shell nanowires on Si (111) substrate from 77 to 293 K. The first independent investigation of Shockley-Read-Hall, radiative, and Auger recombination in InAs-based NWs is presented. Although the Shockley-Read-Hall recombination coefficient was found to be at least 2 orders of magnitude larger than the average experimental values of other reported InAs materials, the Auger recombination coefficient was reported to be 10-fold smaller. The very low Auger and high radiative rates result in an estimated peak internal quantum efficiency of the core-shell nanowires as high as 22% at 77 K, making these nanowires of potential interest for high-efficiency mid-infrared emitters. A greater than 2-fold enhancement in minority carrier lifetime was observed from capping nanowires with a thin InAlAs shell due to the passivation of surface defects.

RESUMEN

The performance of ZnO/PbS colloidal quantum dot (CQD)-based heterojunction solar cells is hindered by charge carrier recombination at the heterojunction interface. Reducing interfacial recombination can improve charge collection and the photocurrent of the device. Here we report the use of a mixed nanocrystal (MNC) buffer layer comprising zinc oxide nanocrystals and lead sulfide quantum dots at the respective heterojunction interface. Remote trap passivation of the PbS CQDs taking place within this MNC layer reduces interfacial recombination and electron back transfer which in turn improves charge collection efficiency. Upon the addition of the MNC layer, the overall power conversion efficiency increases from 9.11 to 10.16% and Short-circuit current density (JSC) increases from 23.54 to 25.23 mA/cm2. Optoelectronic characterization of the solar cells confirms that the effects underlying device improvement are reduced trap density and improved charge collection efficiency due to the presence of the MNC buffer layer.

RESUMEN

Here, Zn2SnO4 nanorods/Cu4Bi4S9 (ZTO/CBS) and ZTO nanorods/CBS-graphene nanosheets (ZTO/CBS-GNs), as well as two types of bulk heterojunction (BHJ) solar cells with high flexibility were fabricated on stainless steel meshes (SSMs). The excellent photovoltaic responses of CBS-GNs and ZTO/CBS-GNs with incorporation of GNs were determined using surface photovoltage spectroscopy (SPS). The signals of time-resolved fluorescence response (TFR) and transient surface photovoltage (TPV) can provide more detailed information for transition, separation, and transport of photoinduced carriers. Besides, the ZTO nanorods/CBS-GNs cell exhibits the superior performance and the highest efficiency is 11.2%. The multichannel separation process from the TPVs indicates that the macro-photoelectric signals can be attributed to the photogenerated charges separated at the interface of CBS/GNs, rather than CBS/ZTO. The multi-interfacial recombination is the major carrier loss with electrical impedance spectroscopy (EIS) and the hole selective NiO can efficiently accelerate the charge extraction to the external circuit. The comprehensive signals of SPS, EIS, TFR, and TPV provide insights into transition, separation, recombination and shifting of carriers. Importantly, the BHJ flexible solar cells with high efficiency and facile, scalable production present a potential for application.

RESUMEN

To develop novel hole-transporting materials (HTMs) is an important issue of perovskite solar cells (PSCs), especially favoring the stability improvement and the cost reduction. Herein, we use ternary quantum dots (QDs) as HTM in mesoporous TiO2/CH3NH3PbI3/HTM/Au solar cell, and modify the surface of CuInS2 QDs by cation exchange to improve the carrier transport. The device efficiency using CuInS2 QDs with a ZnS shell layer as HTM is 8.38% under AM 1.5, 100 mW cm(-2). The electrochemical impedance spectroscopy suggested that the significantly enhanced performance is mainly attributed to the reduced charge recombination between TiO2 and HTM. It paves a new pathway for the future development of cheap inorganic HTMs for the high efficiency PSCs.

RESUMEN

A facile sol-gel and selenization process has been demonstrated to fabricate high-quality single-phase earth abundant kesterite Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic absorbers. The structure and band gap of the fabricated CZTSSe can be readily tuned by varying the [S]/([S] + [Se]) ratios via selenization condition control. The effects of [S]/([S] + [Se]) ratio on device performance have been presented. The best device shows 8.25% total area efficiency without antireflection coating. Low fill factor is the main limitation for the current device efficiency compared to record efficiency device due to high series resistance and interface recombination. By improving film uniformity, eliminating voids, and reducing the Mo(S,Se)2 interfacial layer, a further boost of the device efficiency is expected, enabling the proposed process for fabricating one of the most promising candidates for kesterite solar cells.

RESUMEN

Wide-gap semiconductors are often unipolar and can form type II bipolar heterostructures with large band discontinuities. We present such diodes with very high rectification larger than 1 × 10(10). The current is assumed to be entirely due to interface recombination. We derive the ideality factor for both symmetric and asymmetric diodes and find it close to 2 in agreement with experimental data from NiO/ZnO and CuI/ZnO type II diodes. The comparison with experimental results shows that the actual interface recombination rate is orders of magnitude smaller than its possible maximum value.