Enhanced Charge Extraction Research Articles

NaBr-Modified CsPbI2Br Improves the Comprehensive Performance of the Solar Cells

Perovskite solar cells based on the CsPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Br photoactive layer have received much attention recently because of the suitable bandgap and good thermal stability for CsPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Br, as compared with other perovskite photoactive materials. However, the poor quality for the normally prepared CsPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Br layers severely restricts realization of high performance for the related solar cells. Herein, we report a facile low-temperature solution process, in which NaBr is introduced into the CsPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Br precursor solution as an additive, to prepare high-quality CsPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Br layers with the improved crystallinity and reduced defect/trap density, as well as the reduced surface roughness and enhanced resistance to moisture and Ag diffusion. Thanks to the enhanced charge extraction, suppressed carrier recombination because of the improved CsPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Br quality, the power conversion efficiency (PCE) of the corresponding solar cells has a remarkably increase to 14.70% compared with 13.26% for the control device with the pristine CsPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Br layer. Moreover, the improved operation stability for the unencapsulated devices with the NaBr-modified CsPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Br layers is observed, i.e., ∼90.1% retention of the initial PCE after 200 h aging in air with the relative humidity of 30%–40%, much more stabler than the control device.

Above 23% Efficiency by Binary Surface Passivation of Perovskite Solar Cells Using Guanidinium and Octylammonium Spacer Cations

One of the important factors in the performance of perovskite solar cells (PSCs) is effective defect passivation. Dimensional engineering technique is a promising method to efficiently passivate non‐radiative recombination pathways in the bulk and surface of PSCs. Herein, a passivation approach for the perovskite/hole transport layer interface is presented, using a mixture of guanidinium and n‐octylammonium cations introduced via GuaBr and n‐OABr. The dual‐cation passivation layer can provide an open‐circuit voltage of 1.21 V with a power conversion efficiency of 23.13%, which is superior to their single cation counterparts. The mixed‐cation passivation layer forms a 1D/2D perovskite film on top of 3D perovskite, leading to a more hydrophobic and smoother surface than the uncoated film. A smooth surface can diminish non‐radiative recombination and enhance charge extraction at the interface making a better contact with the transport layer, resulting in improved short‐circuit current. In addition, space charge‐limited current measurements show a three times reduction in the trap‐filled limit voltage in the mixed‐cation passivated sample compared with unpassivated cells, indicating fewer trapped states. The shelf‐life stability test in ambient atmosphere with 60% relative humidity as well as light‐soaking stability reveal the highest stability for the dual‐cation surface passivation.

Multifunctional Two-Dimensional Polymers for Perovskite Solar Cells with Efficiency Exceeding 24%

The passivation of the intrinsic surface defects of perovskites by organic functional materials has a great potential to retard charge recombination and enhance charge extraction. However, unsatisfactory device performance and a lack of in-depth understanding of the defect passivation mechanism make rational molecule design for efficient solar cells a great challenge. Herein, two solution-processable two-dimensional (2D) conjugated polymers, namely, 2DP-F and 2DP-O, have been synthesized for perovskite solar cells (PSCs). It is found that these materials could passivate surface defects, transport and extract hole carriers, hamper moisture invasion, and impede diffusion of Li+ cations into the perovskite film. As a result, champion efficiencies of 23.31% and 24.08% were achieved for 2DP-F- and 2DP-O-based devices, respectively, coupled with dramatically improved stability. These results indicate that our proposed 2D polymers could be promising multifunctional materials for further boosting the efficiency and improving the stability of PSCs.

Realizing 19.05% Efficiency Polymer Solar Cells by Progressively Improving Charge Extraction and Suppressing Charge Recombination.

Improving charge extraction and suppressing charge recombination are critically important to minimize the loss of absorbed photons and improve the device performance of polymer solar cells (PSCs). In this work, highly efficient PSCs are demonstrated by progressively improving the charge extraction and suppressing the charge recombination through the combination of side-chain engineering of new nonfullerene acceptors (NFAs), adopting ternary blends, and introducing volatilizable solid additives. The 2D side chains on BTP-Th induce a certain steric hindrance for molecular packing and phase separation, which is mitigated by fluorination of side chains on BTP-FTh. Moreover, by introducing two highly crystalline molecules as the second acceptor and volatilizable solid additive, respectively, into the BTP-FTh-based host blend, the molecular crystallinity is significantly improved and the blend morphology is finely optimized. As expected, enhanced charge extraction and suppressed charge recombination are progressively realized, contributing to the largely improved fill factor (FF) of the resultant devices. Accompanied by the enhanced open-circuit voltage (Voc ) and short-circuit current density (Jsc ), a record high power conversion efficiency (PCE) of 19.05% is realized finally.

High-Performance Planar Perovskite Solar Cells with a Reduced Energy Barrier and Enhanced Charge Extraction via a Na2WO4-Modified SnO2 Electron Transport Layer.

Tin oxide (SnO2) has been commonly used as an electron transport layer (ETL) in planar perovskite solar cells (p-PSCs) because it can be prepared by a low-temperature solution-processed method. However, the device performance has been restricted due to the limited electrical performance of SnO2 and its mismatched energy level alignment with the perovskite absorber. Considering these problems, sodium tungstate (Na2WO4) has been employed to modify the SnO2 ETL. The conduction band minimum of SnO2 increases and the defects at the ETL/perovskite interface decrease by the modification of the SnO2 ETL with Na2WO4, thus reducing the energy barrier between the ETL and perovskite. In addition, the electron extraction ability has been enhanced and the interface recombination between the ETL and perovskite has also been inhibited. As a result, the photovoltaic performance of p-PSCs based on the modified ETL has been improved, and a champion power conversion efficiency of 21.16% has been achieved compared with the control device of 17.30% with an open circuit voltage increased from 1.075 to 1.162 V.

Self-boosting non-hydrolytic synthesis of Cl-passivated SnO2 nanocrystals for universal electron transport material of next-generation solar cells

Recently, tin oxide (SnO2) has attracted great attention as a promising electron transport layer (ETL) material for perovskite solar cells (PSCs) due to its high electron mobility, low-temperature fabrication and superior photocatalytic stability. However, there remains room for further utilization of the full potential of this material and improvement in power conversion efficiency (PCE) of devices, including establishment of controllable synthetic process and efficient surface modification. In this study, we first report on the novel tert-butyl alcohol (t-BuOH)-mediated non-hydrolytic synthesis of SnO2 nanocrystals (NCs) with effective pre-Cl-passivation without additional treatment. In this reaction, the by-products, tert-butyl chloride and water, are readily hydrolyzed to t-BuOH and chloride ions, which can be continuously reused and boost the reaction. Consequently, Cl effectively passivates the surface of SnO2 NCs as well as facilitates the dispersion stability, leading to reduced trap density and enhanced charge extraction characteristics of the SnO2 ETL. The device fabricated using the t-BuOH-mediated SnO2 NCs exhibits improved device efficiency, excellent reproducibility, reduced hysteresis, and longer device stability compared to the device using SnO2 NCs synthesized by conventional non-hydrolytic process. For the regular type (n-i-p) PSCs, the device achieves a PCE of 20.2% and maintains 97% of initial efficiency after 70 days under ambient conditions. In addition, unnecessary high temperature annealing and additional treatment of the non-hydrolytically synthesized SnO2 NCs enable their direct deposition on the perovskite layer, thereby leading to inverted type (p-i-n) PSCs with the best PCE of 17.0%.

Hysteresis-Free and Efficient Perovskite Solar Cells Using SnO2 with Self-assembly L-Cysteine Layer

Perovskite solar cells (PSCs) with SnO2 electron transport layer (ETL) still face the challenge of low efficiency with serious hysteresis due to the nonradiative recombination at the SnO2/perovskite interface. Herein, a self-assembly modification layer with multifunctional properties using L-Cysteine (Cys) is applied to solve the above issues. Through applying this novel self-monolayer between SnO2 and perovskite film, the trap state is greatly alleviated as well with an enhanced charge extraction. In addition, the naked -NH2 and sulfur atom of this monolayer will affect the crystallization dynamics of perovskite film, resulting in an improved film quality. Compared with the control device with a PCE of 18.55%, the monolayer contacts engineered device shows a significantly increase PCE over 21% along with negligible hysteresis. More interestingly, monolayer contacts engineered PSCs exhibit superior stability that over 90% of its initial PCE remains after stored at 50% relative humidity for 50 days. This work provides a promising method to fabricate planar n-i-p perovskite devices with high efficiency, stability, and repeatability.

Organic-inorganic hybrid hole transport layers with SnS doping boost the performance of perovskite solar cells

Perovskite solar cells (PSCs) have demonstrated excellent photovoltaic performance which currently rival the long-standing silicon solar cells’ efficiency. However, the relatively poor device operational stability of PSCs still limits their future commercialization. Binary sulfide is a category of materials with promising optoelectrical properties, which shows the potential to improve both the efficiency and stability of PSCs. Here we demonstrate that the inorganic tin monosulfide (SnS) can be an efficient dopant in 2,2′,7,7′-tetrakis(N,N-di-p-methoxy-phenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) to form a composite hole transport layer (HTL) for PSCs. SnS nanoparticles (NPs) synthesized through a simple chemical precipitation method exhibit good crystallization and suitable band matching with the perovskites. The introduction of SnS NPs in Spiro-OMTAD HTLs enhanced charge extraction, reduced trap state density, and shallowed trap state energy level of the devices based on the composite HTLs. Therefore, the resulting solar cells employing SnS-doped spiro-OMeTAD HTLs delivered an improved stabilized power output efficiency of 21.75% as well as enhanced long-term stability and operational stability. Our results provide a simple method to modify the conventional spiro-OMeTAD and obtain PSCs with both high efficiency and good stability.

Sn‐Pb Mixed Perovskites with Fullerene‐Derivative Interlayers for Efficient Four‐Terminal All‐Perovskite Tandem Solar Cells

AbstractInterfacial engineering is the key to high‐performance perovskite solar cells (PSCs). While a wide range of fullerene interlayers are investigated for Pb‐based counterparts with a bandgap of &gt;1.5 eV, the role of fullerene interlayers is barely investigated in Sn‐Pb mixed narrow‐bandgap (NBG) PSCs. In this work, two novel solution‐processed fullerene derivatives are investigated, namely indene‐C60‐propionic acid butyl ester and indene‐C60‐propionic acid hexyl ester (IPH), as the interlayers in NBG PSCs. It is found that the devices with IPH‐interlayer show the highest performance with a remarkable short‐circuit current density of 30.7 mA cm−2 and a low deficit in open‐circuit voltage. The reduction in voltage deficit down to 0.43 V is attributed to reduced non‐radiative recombination that the authors attribute to two aspects: 1) a higher conduction band offset of ≈0.2 eV (&gt;0 eV) that hampers charge‐carrier‐back‐transfer recombination; 2) a decrease in trap density at the perovskite/interlayer/C60 interfaces that results in reduced trap‐assisted recombination. In addition, incorporating the IPH interlayer enhances charge extraction within the devices that results in considerable enhancement in short‐circuit current density. Using a NBG device with an IPH interlayer, a respectable power conversion efficiency of 24.8% is demonstrated in a four‐terminal all‐perovskite tandem solar cell.

Alkali Metal Chloride-Doped Water-Based TiO2 for Efficient and Stable Planar Perovskite Photovoltaics Exceeding 23% Efficiency.

TiO2 is one of the most broadly employed electron transport materials in n-i-p structure perovskite solar cells (PSCs). Low-temperature non-hydrolyzed sol-gel method is developed to prepare TiO2 in order to simplify the fabrication process and match with the planar structure PSCs. Conventional low-temperature TiO2 film using organic solvents as dispersants makes direct doping challenging due to limited solubility. Here, a newly developed water-based TiO2 solution is directly doped with different alkali chlorides, resulting in better conductivity, compatible energy level matching, and enhanced charge extraction in terms of electron transport layer (ETL) for PSCs. As a result, a power conversion efficiency of 23.15% is achieved based on NaCl-doped TiO2 with competitive storage stability and light stability. The water-based TiO2 ETL for more general doping of various solutes opens up a new avenue for environmental-friendly manufacturing superior ETL toward high-efficiency and stable perovskite photovoltaic devices.

Perovskite solar cells with atomically coherent interlayers on SnO2 electrodes.

In perovskite solar cells, the interfaces between the perovskite and charge-transporting layers contain high concentrations of defects (about 100 times that within the perovskite layer), specifically, deep-level defects, which substantially reduce the power conversion efficiency of the devices1-3. Recent efforts to reduce these interfacial defects have focused mainly on surface passivation4-6. However, passivating the perovskite surface that interfaces with the electron-transporting layer is difficult, because the surface-treatment agents on the electron-transporting layer may dissolve while coating the perovskite thin film. Alternatively, interfacial defects may not be a concern if a coherent interface could be formed between the electron-transporting and perovskite layers. Here we report the formation of an interlayer between a SnO2 electron-transporting layer and a halide perovskite light-absorbing layer, achieved by coupling Cl-bonded SnO2 with a Cl-containing perovskite precursor. This interlayer has atomically coherent features, which enhance charge extraction and transport from the perovskite layer, and fewer interfacial defects. The existence of such a coherent interlayer allowed us to fabricate perovskite solar cells with a power conversion efficiency of 25.8 per cent (certified 25.5 percent)under standard illumination. Furthermore, unencapsulated devices maintained about 90 per cent of their initial efficiency even after continuous light exposure for 500 hours.Our findings provide guidelines for designing defect-minimizinginterfaces between metal halide perovskitesand electron-transporting layers.

Boosting the Efficiency of Non-fullerene Organic Solar Cells via a Simple Cathode Modification Method.

This work demonstrates a simple yet effective method to significantly improve the power conversion efficiency (PCE) of highly efficient non-fullerene organic solar cells by mixing two electron transport materials. The new electron transport layer shows an energy level better aligned with the active layer and an improved morphology that could reduce the active layer-electrode contact. These improvements lead to enhanced charge extraction, better charge selectivity, suppressed exciton recombination, and finally a boosted PCE in the PM6:Y6-based solar cells. When applied in conjunction with the non-halogenated solvent-processed PM6:PY-IT-based active layer, the mixed ETL also gives rise to a leading result for binary all-polymer solar cells (PCE of >16%) with a concurrent increase in VOC, JSC, and FF.

Cellulose‐Based Oxygen‐Rich Activated Carbon for Printable Mesoscopic Perovskite Solar Cells

Carbon electrodes (CEs) are demonstrated as the most stable and cost‐effective back electrodes for perovskite solar cells (PSCs), which influence the performance of related PSCs significantly. Herein, oxygen‐rich activated carbon (AC) is synthesized from the most abundant biomass resource, cellulose, via a feasible carbonization and oxidization process and applied in CEs for hole‐conductor‐free printable mesoscopic PSCs (p‐MPSCs). The obtained cellulose‐based activated carbon (CAC) exhibits a high specific area of 477.14 m2 g−1 and possesses a high oxygen content of 11.9%, promoting the wettability and contact between CEs and perovskites. Moreover, the high oxygen content also leads to an elevated work function of CEs. As a result, p‐MPSCs filled with (5‐AVA)0.03(MA)0.97PbI3 based on CEs containing CAC give an efficiency of 15.5%, whereas those devices based on CEs with no CAC give an efficiency of 13.8%. The improved efficiency benefits from the promoted fill factor and open circuit voltage due to the optimized energy level alignment and enhanced charge extraction by CAC. This work presents a good example for the value‐added utilization of cellulose in the energy conversion systems and offers a feasible strategy for preparing oxygen‐rich CE for PSCs with enhanced performance.

Dopant‐Free Hole‐Transporting Material with Enhanced Intermolecular Interaction for Efficient and Stable n‐i‐p Perovskite Solar Cells

AbstractDeveloping low‐cost, efficient, and stable dopant‐free hole‐transporting materials (HTMs) in perovskite solar cells (PVSCs) is essential to their commercial deployment. Herein, the synthesis of a novel spirofluorene‐dithiolane based small molecular HTM, SFDT‐TDM, through facile and low‐cost synthetic routes is reported. The CH…π interactions in adjacent SFDT‐TDM are beneficial for high hole mobility and the methylthio groups in SFDT‐TDM can serve as Lewis bases to passivate the defects on the surface of perovskite films, leading to suppressed non‐radiative recombination and enhanced charge extraction at the perovskite/HTM interface. As a result, CsxFA1−xPbI3 based PVSCs with SFDT‐TDM as the HTM realize champion power conversion efficiencies (PCEs) of 21.7% and 20.3% for small‐area (0.04 cm2) and large‐area (1.0 cm2) devices with negligible photocurrent hysteresis, respectively. Additionally, all‐inorganic CsPbI3−xBrx based PVSCs with SFDT‐TDM demonstrate an impressive PCE of 17.1% along with excellent stability. This work highlights the great potential of the spirofluorene core for exploring low‐cost and dopant‐free HTMs for PVSCs with high efficiency and stability.

Improving the Photovoltaic Performance of Flexible Solar Cells with Semitransparent Inorganic Perovskite Active Layers by Interface Engineering.

Inorganic perovskite CsPbBr3 has broad application prospects in photovoltaic windows, tandem cells, and other fields due to its intrinsic semitransparency, excellent photoelectric properties, and stability. In this work, a high-quality semitransparent CsPbBr3 film was prepared by a sequential vacuum evaporation deposition method without high-temperature annealing and successfully used as the active layer of flexible perovskite solar cells (F-PSCs) for the first time, achieving a power conversion efficiency (PCE) of 5.00%. By introducing an energy-level buffer layer of Cu2O between CsPbBr3 and Spiro-OMeTAD, the champion PCE has been further improved to 5.67% owing to the reduction of electron-hole recombination and enhanced charge extraction. The optimized devices present higher stability, which can maintain more than 95% of the initial efficiency even after continuous heating at 85 °C for 240 h. Moreover, the F-PSCs also exhibit excellent mechanical durability, and 90% of the original PCE can be retained after 1000 bending cycles at a curvature radius of 3 mm.

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.

Efficient post-treatment of CsPbBr3 film with enhanced photovoltaic performance

All-bromide inorganic perovskite CsPbBr3 has attracted enormous attention due to its excellent stability against thermal and high humidity environmental conditions. The wide bandgap of CsPbBr3 (2.3 eV) makes it a promising material for tandem and semitransparent solar cells. However, cesium based inorganic perovskite solar cells (PSCs) usually suffer from low device performance because of their imperfect film quality and mismatched energy levels with respect to charge transport materials. Herein, we describe a post-treatment strategy for CsPbBr3 films using formamidinium bromide (FABr). With this protocol, FA+ can partially substitute Cs+ in the bulk phase of CsPbBr3, improving energy level alignment and enhancing charge extraction. Meanwhile, grain boundaries could be effectively passivated by a second phase (FAyCs4−yPbBr6) generated in the ion exchange reaction. As a result, FABr-treated PSC exhibits a champion power conversion efficiency (PCE) of 8.25%, much higher than the pristine device with PCE of 6.74%. This work provides a feasible method to improve device performance of CsPbBr3 PSCs while maintaining their outstanding stability.

Toward efficient and stable operation of perovskite solar cells: Impact of sputtered metal oxide interlayers

AbstractPerovskite solar cells (PSCs) are an emerging photovoltaic technology. PSCs with a power conversion efficiency of 25.2% have been demonstrated. Apart from the encouraging progress made in the efficiency, continuous improvement in operational stability is essential if the PSC technology is to become one of the commercially viable photovoltaic options for applications in renewable energy. In parallel to the development of high‐performance PSCs using environmental‐friendly solution fabrication processes, incorporation of process‐compatible high‐quality metal oxide interlayers for enhancing the stability of PSCs, through passivating the interfacial defects at the perovskite/contact interfaces, enhancing charge extraction, and retarding of the moisture encroachment, is needed. This review discusses the recent research progresses made in the development of different metal oxide interlayers for applications in PSCs, highlighting the functions, process compatibility and advances in high performing stable PSCs.

Solution-processed PbCdS nanocrystals as a novel hole transport material for inverted CH3NH3PbI3 perovskite solar cells

Herein, we report a novel and effective strategy to prepare ternary PbCdS nanocrystals, where Cd2+ is successfully doped into PbS to form PbCdS via one step reaction. Moreover, the as-prepared PbCdS NCs are firstly applied in the hole transport layer of the inverted CH3NH3PbI3 solar cells. It is found that the well matched highest occupied molecular orbital (HOMO) energy levels of PbCdS NCs and CH3NH3PbI3 contribute to the enhanced charge extraction rate and boosted Voc. The inverted CH3NH3PbI3 solar cells based on PbCdS NCs as hole transport layer yield a peak power conversion efficiency up to 17.1% with average efficiency of 16.5%. Meanwhile, the as-prepared PbCdS NCs applied as anode material in the Li-ion batteries exhibit high capacity and excellent cyclability, indicating that PbCdS NCs can be a promising electrode material in Li-ion batteries as well.

Manipulation of Zinc Oxide with Zirconium Doping for Efficient Inverted Organic Solar Cells.

Solution-processed zinc oxide (ZnO) is one of the widely used electron transporting layers (ETLs) for organic solar cells (OSCs). However, low optical transparency along with thickness-sensitivity of ZnO ETL constrains the improvement of photovoltaic performance and large-scale fabrication compatibility. To resolve these issues, zirconium (Zr) doping is applied to tailor the optoelectronic and morphological properties of ZnO layer. This approach not only improves light transmittance with the suppressed parasitic absorption, but also provides an optimized surface morphology for enhancing charge extraction property and reducing potential of charge trap-assisted recombination. By using ZnO:Zr as ETL in inverted device configuration, the maximum power conversion efficiency (PCE) of PM6:Y6:PC71 BM solar cell devices is up to 17.2%, which makes an enhancement of 9.55% compared to ZnO-based devices (15.7%). As the thickness of ZnO:Zr ETL increases to ≈60nm, the presence of the lower parasitic absorption together with uniform surface morphology can help photovoltaic performance maintain above 15%, which is beyond the performance of the pristine ZnO-based device achieving only 11.9%. Such superiority of ZnO:Zr ETL is also validated by a series of well-known BHJ systems, where in comparison with the devices based on pristine ZnO ETL, a better photovoltaic performance from ZnO:Zr device can be achieved.