Efficient Charge Extraction Research Articles

Balancing Charge Extraction for Efficient Back‐Contact Perovskite Solar Cells by Using an Embedded Mesoscopic Architecture

AbstractAs the performance of organic–inorganic halide perovskite solar cells approaches their practical limits, the use of back‐contact architectures, which eliminate parasitic light absorption, provides an effective route toward higher device efficiencies. However, a poor understanding of the underlying device physics has limited further performance improvements. Here a mesoporous charge‐transporting layer is introduced into quasi‐interdigitated back‐contact perovskite devices and the charge extraction behavior with an increased interfacial contact area is studied. The results show that the incorporation of a thin mesoporous titanium dioxide layer significantly shortens the charge‐transfer lifetime and results in more efficient and balanced charge extraction dynamics. A high short‐circuit current density of 21.3 mA cm–2 is achieved using a polycrystalline perovskite layer on a mesoscopic quasi‐interdigitated back‐contact electrode, a record for this type of device architecture.

An antimonene modified hole extraction layer for high efficiency PEDOT:PSS-free nonfullerene organic solar cells

In this paper, a bilayer hole extraction layer (HEL) with solution-processed molybdenum trioxide (MoO 3 ) and two-dimensional (2D) material of antimonene was developed to achieve high performance nonfullerene organic solar cells (NF–OSCs). The application of antimonene facilitates effective charge extraction and lowered recombination loss, achieving improved photovoltaic performance. By inserting the antimonene layer, power conversion efficiency ( PCE ) of devices with MoO 3 HEL was increased from 8.92% to 11.30% in OSCs with non-fullerene systems of PBDB-T-2F:IT-4F, which was even much higher than that of the devices with PEDOT:PSS HEL (10.59%). Results make it clear that the solution-processed bilayer MoO 3 /antimonene HEL shows great potential for application in high performance PEDOT:PSS-free NF–OSCs. • A bilayer HEL with solution-processed MoO 3 and antimonene was developed to achieve high performance PEDOT:PSS-free NF-OSCs. • The MoO 3 /antimonene HEL favors the efficient operation of NF–OSCs, achieving suppressed trap-limited recombination and enhanced charge extraction efficiency.

Enhanced Charge Selectivity via Anodic-C60 Layer Reduces Nonradiative Losses in Organic Solar Cells.

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

Impact of Vertical Inhomogeneity on the Charge Extraction in Perovskite Solar Cells: A Study by Depth-Dependent Photoluminescence.

In perovskite solar cells (PSCs), the vertical inhomogeneities which include uneven grains, voids, and grain boundaries are closely linked to the underlying charge transport layer which controls the nucleation and grain growth in the perovskite film. Herein, the vertical inhomogeneity of perovskite films in the device structure is analyzed by depth-dependent photoluminescence (PL) achieved with different excitation wavelengths. An analytical representation between vertical inhomogeneity and depth-dependent PL, parametrized with a factor, b, is introduced to understand the relation between inhomogeneity and charge recombination. Lower values of b correlate to lower vertical inhomogeneity and hence reduced recombination. The analytical representation is validated in two sets of devices that show remarkable differences in perovskite film morphology, device based on mesoporous TiO2 and planar SnO2. By exploring the morphological properties and the PL emission from different depths across the device structures, we show that the lower vertical inhomogeneity leads to more efficient charge carrier extraction in planar SnO2-based devices. Moreover, the SnO2-based devices exhibit lower Urbach energy, which concurs with the slow transient photovoltage decay, suggesting less defects and recombination losses. This work provides a broader understanding of the impact of vertical inhomogeneity on the charge extraction efficiency and presents a methodology to study quantitatively the inhomogeneity of perovskite films in device structures.

Black/red phosphorus Z-scheme hybrid with novel photosynthesis-inspired electrolyte additives for enhanced photoelectrochemical activity

Designing a Z-scheme heterojunction is of great significance to achieve high separation of photogenerated carriers and strong photo-redox properties of two semiconductor materials. In photosynthesis, the effects of photocatalytic electrolytes (adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH)) on chemical energy has attracted numerous interests. In this case, developing photocatalytic electrolytes with more active site and efficient charge extraction and separation would be beneficial for superior photocatalytic activity in Z-scheme system. Herein, we synthesized a Black/Red (BP/RP) phosphorus Z-scheme hybrid via a feasible two-steps sonochemical method, and the adding photocatalytic electrolytes ATP and NADPH in KOH solution endow the system with new vitality in photocatalysis. Consequently, photoelectrochemical (PEC) tests demonstrated the as-prepared BP/RP Z-scheme hybrid exhibits enhanced photocurrent of 9 μA/cm2 at 0.5 V compared to the sole RP, while adding NADPH and ATP, the photocurrent of BP/RP Z-scheme hybrid shows dramatic increment of 30 μA cm−2 at a bias potential of 0.6 V. The improved photocatalytic activity is ascribed to the abundant active sites, higher carrier mobility of the BP/RP Z-scheme system, as well as the efficient photogenerated carrier separation and the prolonged lifetime of photocatalytic electrolytes. This work demonstrated the photocatalytic electrolytes can meet the unity of coordination and efficiency of achieving efficient PEC catalytic in the Z-Scheme system, and provides positive ideas for the possibility and superiority for other 2D materials in the field of ultrafast Photonics, photodetectors, etc.

Fluorinating Dopant-Free Small-Molecule Hole-Transport Material to Enhance the Photovoltaic Property.

For the stability and commercial development of the perovskite solar cells (PVK-SCs), synthesizing high-efficiency dopant-free hole-transport materials (DF-HTMs) and exploring how the DF-HTM structure affects the photovoltaic performance is inevitable. Two small-molecule DF-HTMs based on 2,2'-bithiophene as a central part (denoted by BT-MTP and DFBT-MTP) were designed and synthesized. DFBT-MTP, with two more fluorine atoms substituted on the 2,2'-bithiophene group, exhibited enhanced photovoltaic property as DF-HTMs, including larger backbone planarity, declining highest occupied molecular orbit (HOMO) energy level, increasing hole transportation, more effective passivation, and efficient charge extraction. With fluorinated DFBT-MTP being applied as DF-HTMs in p-i-n PVK-SCs, an efficiency of 20.2% was achieved, showing ∼35% efficiency increase compared with the nonfluorinated BT-MTP-based devices. The leading power conversion efficiency (PCE) indicates that the fluorinated compounds should be a promising direction for exploring high-performance DF-HTMs in the p-i-n PVK-SCs.

Rational construction of dual cobalt active species encapsulated by ultrathin carbon matrix from MOF for boosting photocatalytic H2 generation

Dual cocatalyst which integrates the merits of metal and metal oxide has been endowed with competitive advantages over noble metals for achieving efficient H2 evolution. In this case, developing spatially separated dual cocatalyst in carbon substrate with more efficient charge extraction and separation would be beneficial for further enhancing the photocatalytic activity. Herein, we utilized ultrathin two-dimensional (2D) Co-based MOF (Co-MOF) nanosheets as specific precursors for preparing carbon matrix encapsulated dual cobalt active species (Co and CoOx) on the surface of CdS for high-efficient photocatalytic H2 evolution under visible-light irradiation. The fundamental roles of 2D-MOF derived carbon matrix towards rational construction of dual cobalt active species have been visualized by combining a series of in-situ and ex-situ temperature-dependent analytic strategies. The carbon matrix as well as co-existed dual cobalt active species bring great improvement on charge carrier transportation and photogenerated electron-hole separation for enhanced photocatalytic activities, and an approximate 12.5-fold enhancement of H2 generation performance from 0.161 mmol h−1 of CdS to 1.997 mmol h−1 of the newly formed CdS-Co-CoOx@C with the apparent quantum efficiency of 43.7 % at 420 nm has been achieved. This work might provide the basic understanding and new opportunities in manipulating MOF structure through pyrolysis to fabricate high efficient cocatalysts for versatile photocatalytic reactions.

Thiazole‐Modified C3N4 Interfacial Layer for Defect Passivation and Charge Transport Promotion in Perovskite Solar Cells

Despite the conspicuous achievements in perovskite solar cells (PSCs), further improvement of the power conversion efficiency (PCE) is hindered by substantially detrimental carrier recombination resulting from the high interfacial charge defect density and inferior charge transport kinetics. Herein, an interface engineering strategy is developed to introduce a Lewis base thiophene or thiazole–modified C3N4 layer at the electron transfer layer (ETL)/perovskite interface to constitute a stepwise energy band alignment and passivate defects at interfaces of the perovskite film. Attributed to its well‐matched energy level with TiO2 and perovskite, the charge extraction efficiency and charge transfer dynamics can be promoted remarkably, greatly inhibiting charge recombination at the interface. Furthermore, thiophene and thiazole can donate the lone pair electrons in S or N atoms to undercoordinated Pb2+, which effectively passivates the electronic trap states caused by halogen vacancies, thereby greatly minimizing trap‐assisted nonradiative recombination in the PSCs. Eventually, the thiazole–C3N4/perovskite‐based devices acquire an outstanding efficiency of 19.23%, supported by an enhanced open‐circuit voltage (VOC) of 1.11 V with improved moisture stability. This work provides an avenue for interfacial energy level modulation and defect passivation strategies for a rational interface microstructure design for meliorating the performance of PSCs.

A facile approach to improve the performance and stability of perovskite solar cells via FA/MA precursor temperature controlling in sequential deposition fabrication

Hybrid perovskite solar cells (PSCs) have introduced as a good revolution photovoltaic to meet the green world energy demands. The crystallinity improvement of PSCs is a practical approach to improve the efficacy of PSCs. Double cation formamidinium (FA)/methylammonium (MA) PSCs were fabricated by using a simple sequential deposition process. The mesoporous titanium dioxide (TiO 2 ) layers with 200 nm thickness were used as the electron transport layer (ETL). It was observed that with changing the FA/MA precursor temperature could achieve more desirable perovskite films for photovoltaic applications. Our measurements show that with pre-heating of FA/MA solution at 55 °C the defects on the perovskite layer are well passivated. Besides, the perovskite with better crystallinity and smooth morphology, and the defect-free surface was achieved. It is delightful that perovskite with 55 °C reveals stronger photon absorption and better charge collection. Using the pre-heating approach, a redshift in edge absorption of the perovskite films was observed, indicating a better light-harvesting phenomenon in the corresponding perovskite films. Through optimization of the pre-heating temperature of FA/MA solution, a champion PCE of 13.77% was yielded for PSCs higher than of 10.05% one for pre-heating. Also, modified mixed cation devices revealed higher stability behavior in atmospheric conditions than the reference devices. • The optoelectrical properties of mixed cation perovskite are modified using different FA/MA solution temperature. • This modification results in enhancing of the average device power conversion efficiency reaching to 13.7%. • The enhancement in performance originates from improved perovskite film formation and more efficient charge extraction.

Ligand-bridged charge extraction and enhanced quantum efficiency enable efficient n–i–p perovskite/silicon tandem solar cells

27%-efficient perovskite/silicon tandem solar cells are achieved in n–i–p configuration by developing novel electron and hole selective contacts, which combine high broadband transparency with efficient charge extraction.

Gate tunable vertical geometry phototransistor based on infrared HgTe nanocrystals

Infrared nanocrystals are promising building blocks for the design of low-cost infrared sensors. Vertical geometry diode is, among possible geometries, the one that has led to the best performance so far. However, this geometry suffers from a lack of tunability after its fabrication, slowing down possible improvements. Here, we demonstrate gate control on a vertical diode in which the active layer is made of HgTe NCs absorbing in the extended short-wave infrared (2.5 μm). To reach this goal, we take advantage of the electrostatic transparency of graphene, combined with the high capacitance LaF3 ionic glass to design a gate tunable photodiode. The latter behaves as a work function-tunable electrode which lets the gate-induced electric field tune the carrier density within the nanocrystal film. In particular, we show that the gate allows to tune the band profile leading to more efficient charge extraction and thus an enhanced photoresponse (×4 compared to the device with a floating gate). This work also demonstrates that photoelectron extraction can still be improved in the existing diode, by better controlling the doping profile of the stack.

Highly Efficient and Air-Stable Heterostructured Perovskite Quantum Dot Solar Cells Using a Solid-State Cation-Exchange Reaction.

Perovskite quantum dots (PQDs) have expanded the scalability of perovskite materials by their high crystallinity, band-gap tunability, and surface ligand-driven functionalities in the colloidal state across optoelectronics as well as photovoltaics. To improve PQD performance in applications, however, defect control has emerged as a major challenge given the increased PQD surface area. Herein, we have developed a heterostructured PQD solar cell by combining CsPbI3 and FAPbI3 (FA, formamidinium) PQD layers to introduce a multinary PQD layer based on a solid-state A-site cation-exchange strategy. A heterostructure, including the solid-state diffusion-driven multinary PQD layer, creates an internally graded heterojunction for more efficient charge extraction. The best PQD cell achieves a power conversion efficiency (PCE) of 16.07% with negligible hysteresis. Furthermore, this architecture offers significantly enhanced stability with reduction of trap-assisted recombination as compared to cells of a monocompositional PQD layer. The unencapsulated device retains a 96% PCE after 1000 h in ambient storage.

Planar Heterojunction Organic Photodetectors Based on Fullerene and Non-fullerene Acceptor Bilayers for a Tunable Spectral Response.

Planar heterojunction (PHJ) organic photodetectors are potentially more stable than traditional bulk heterojunction counterparts because of the absence of uncontrolled phase separation in the donor and acceptor binary blend system. This work reports a new class of PHJ organic photodetectors based on the medium-band gap fullerene C60 and low-band gap fused-ring non-fullerene acceptor ID-MeIC bilayer structure, which allows a wide range of spectral response tuning across the UV-visible-near-infrared (UV-vis-NIR) region by tailoring individual layer thickness. The C60 layer not only increases the external quantum efficiency at 745 nm by 57% but also reduces dark currents by two orders of magnitude. More importantly, the p-type poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzi] is found to be the key compound to conduct the layer-by-layer fabrication as combined with n-type ID-MeIC for higher charge extraction efficiency. In light of the above information, PHJ organic photodetectors exhibited a specific detectivity of 6.5 × 1010 Jones to detect NIR light at 745 nm under -0.1 V. The linear dynamic range was estimated to be 80 dB. This work has demonstrated a feasible approach to develop a PHJ architecture with tunable spectral response in the UV-vis-NIR range toward long-term stable organic photodetectors for potential applications in flexible and wearable biomedical sensors.

Unravelling camphor mediated synthesis of TiO2 nanorods over shape memory alloy for efficient energy harvesting

We demonstrated a cost-effective and scalablechemical vapor deposition(CVD) process for the production of high quality and dense rutile titanium dioxide nanorods (R-TiO2 NR) based on camphor (Cinnamomum camphora) decomposition overshape memory alloy (nitinol). The topography of the camphor-based R-TiO2 NR was studied underfield emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM) which revealed high coverage of R-TiO2 NR over the substrate. The phase purity of R-TiO2 NR was established by X-ray diffraction measurements performed at different camphor concentrations and substrate temperatures. Raman spectroscopy and X-ray photoelectron spectroscopyconfirmed the chemical and the elemental composition of the nanorods grown over nitinol, which suggest the formation of high-quality R-TiO2 NR. Further, R-TiO2 NR was used as photoanode in 1 M KOH solution for efficient energy harvesting application by indicating a high photocurrent density of 550 µA/cm2 at 1.23 V vs RHE (reversible hydrogen electrode), indicating excellent light trapping mechanism with efficient charge separation and extraction. The proposed CVD process is a suitable method for large scale production of R-TiO2 NR over nitinol, which can be potentially employed in sensors and various other biomedical applications.

P-Phenylenediaminium iodide capping agent enabled self-healing perovskite solar cell

In this study, p-Phenylenediaminium iodide (PDAI) is used to in-situ growth of 2D (PDA)2PbI4 perovskite layer between (FAPbI3)0.85(MAPbBr3)0.15 3D perovskite and CuSCN as a cheap hole transport layer. The results indicate that the incorporation of 5 mg mL−1 PDAI leads to enlarged grain sizes, compact grain boundaries, reduced trap density, efficient charge extraction, and enhanced stability of perovskite film. Passivation of perovskite film with the appropriate amount of PDAI helps in achieving efficient perovskite solar cell with a PCE as high as 16.10%, a JSC of 21.45 mA cm−2, a VOC of 1.09 V, and FF of 70.21%, with negligible hysteresis and excellent moisture stability which remains 99.01% of its initial PCE value after 5 h in high relative humidity of 90 ± 5% and shows unchanged PCE after 1440 h in low relative humidity of 15 ± 5%. Most strikingly, this ultra-thin 2D passivation layer by the use of PDA cations as a bulky spacer not only passivates the defects on the surface of perovskite film but also induces self-healing properties in PSCs which can be rapidly recovered after keeping away from water vapor exposure. This study introduces the cheap and extra stable perovskite solar cells with outstanding self-healing ability towards commercialization.

High-Efficiency Organic Photovoltaic Cells With an Antimony Quantum Sheet Modified Hole Extraction Layer

2D materials with unique opto-electronic properties have shown immense potential for application in organic photovoltaics (OPVs). In this article, we reported a facile and efficient strategy to enhance the device performance of OPVs by doping 2D material of antimonene quantum sheets (AMQSs) into PEDOT:PSS emulsion. The interfacial properties between the hole extraction layer (HEL) and PBDB-T:ITIC-based active layer were properly meliorated, achieving boosted power conversion efficiency ( PCE ). Results indicate that the introduced AMQSs could help suppress the recombination loss and improve charge extraction efficiency. Over 8.09% enhancement of PCE is observed compared to that of the control cell with pure PEDOT:PSS. This work provides a simple and valid strategy of HEL modification to realize high efficiency OPVs.

ALD-grown oxide protective layers on Ta3N5–Cu2O n–p nanoarray heterojunction for improved photoelectrochemical water splitting

It is highly challenging for coating protective layers on nanoarray photoelectrodes to achieve efficient charge extraction and suppressing corrosion of the electrolyte. Herein, atomic layer deposition was used to deposit a composite overlayer of Al-doped ZnO and TiO2 onto a Ta3N5–Cu2O heterojunction nanoarray photoanode, exhibiting a low onset potential of 0.40 V vs reversible hydrogen electrode (RHE), a high photocurrent density of 4.61 mA·cm−2 at 1.23 V vs RHE, and improved photoelectrochemical (PEC) stability, with the help of CoOOH as a cocatalyst. The improved PEC performances would result from that both the oxide overlayer and the cocatalyst layer contribute to the efficient charge extraction and stopping the electrolyte permeation from/and into the semiconductor, passivating the surface states, and improving the energetics at electrode–electrolyte interface.

Gold Nanoparticles Functionalized with Fullerene Derivative as an Effective Interface Layer for Improving the Efficiency and Stability of Planar Perovskite Solar Cells

AbstractTitanium dioxide (TiO2) is an extensively used electron transporting layer (ETL) in n–i–p perovskite solar cells (PSCs). Although, TiO2 ETL experiences the high surface defect together with low electron extraction ability, which causes severe energy loss and poor stability in the PSC. In this study, a new intermediate layer consisting of gold nanoparticles functionalized with fully conjugated fullerene C60 derivative (C60‐BCT@Au NPs) that enhances the interfacial contact at ETL/perovskite interface leading to a perovskite film with improved crystallinity and morphology is reported. Moreover, the studies demonstrate that the interface modification of the TiO2 ETL with C60‐BCT@Au NPs substantially improves the charge extraction efficiency from the perovskite layer and suppresses charge recombination processes. Consequently, the resulting device yields a champion efficiency of 19.08% as well as devaluation in hysteresis. In addition, the unencapsulated PSCs with c‐TiO2/C60‐BCT@Au NPs ETL retain 83% and 90% of their original PCEs after 500 h storage in air and exposure to continuous UV illumination for 200 h, respectively. This study provides an effective method to address the electron transporting issues between perovskite and c‐TiO2 ETL for developing stable and efficient PSCs.

Graphene‐Based Materials in Planar Perovskite Solar Cells

Metal halide perovskite solar cells (PSCs) have recently become the most promising new‐generation solar cells, with a breathtaking growth of efficiency from 3.8% to 25.2% in just one decade. Scientists have abandoned the traditional high‐temperature‐processed mesoscopic layer of the initial mesoscopic PSCs in designing and manufacturing planar PSCs. This new feature endows planar PSCs with possibilities of low‐temperature processibility and large‐scale production. Nevertheless, the advancement of planar PSCs remains limited by two bottlenecks: charge loss and device degradation. To address these two issues, researchers have adopted graphene‐based materials, which demonstrate tremendous potentials due to their superb optical transparency, outstanding carrier mobility, remarkable electrical conductivity, and superior physicochemical stability. Defects inside films and at interfaces are regulated by graphene, thereby contributing to more efficient charge extraction and suppressed charge recombination. The graphene protective layer enhances the moisture and heat stability of planar PSCs, thereby extending the lifetime of devices. Herein, the typical synthesis methods of graphene and the recent applications of graphene in planar PSCs are summarized and discussed. Furthermore, concluding perspectives on current challenges and the future development of graphene in planar PSCs are proposed.

Insights into the hole transport properties of LiTFSI-doped spiro-OMeTAD films through impedance spectroscopy

Hole transport materials are crucial for efficient charge extraction in perovskite solar cells to achieve high power conversion efficiency and stability. Herein, the hole transport properties of the 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) thin films with a dopant lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) are investigated through impedance spectroscopy. Upon doping, we observe a dispersive hole transport with nearly a 100-fold increase in the hole mobility compared with the pristine spiro-OMeTAD films. The hole mobilities slightly decrease with increasing electric fields for both films, exhibiting a negative electric field dependence of mobility due to the positional disorder. Subsequently, the charge carrier density of the LiTFSI-doped spiro-OMeTAD film is three orders of magnitude higher than that of the pristine film. The LiTFSI dopant induces two different electrical regions in the doped thin film, which can be reflected through impedance spectroscopy. The presented investigation through impedance spectroscopy is of high practical interest for the development of hole transport materials and the optimization of the transport layer doping in perovskite solar cells.