Interfacial Charge Recombination Research Articles

Facile NaF Treatment Achieves 20% Efficient ETL-Free Perovskite Solar Cells.

Electron transporting layer (ETL)-free perovskite solar cells (PSCs) exhibit promising progress in photovoltaic devices due to the elimination of the complex and energy-/time-consuming preparation route of ETLs. However, the performance of ETL-free devices still lags behind conventional devices because of mismatched energy levels and undesired interfacial charge recombination. In this study, we introduce sodium fluoride (NaF) as an interface layer in ETL-free PSCs to align the energy level between the perovskite and the FTO electrode. KPFM measurements clearly show that the NaF layer covers the surface of rough underlying FTO very well. This interface modification reduces the work function of FTO by forming an interfacial dipole layer, leading to band bending at the FTO/perovskite interface, which facilitates an effective electron carrier collection. Besides, the part of Na+ ions is found to be able to migrate into the absorber layer, facilitating enlarged grains and spontaneous passivation of the perovskite layer. As a result, the efficiency of the NaF-treated cell reaches 20%, comparable to those of state-of-the-art ETL-based cells. Moreover, this strategy effectively enhances the operational stability of devices by preserving 94% of the initial efficiency after storage for 500 h under continuous light soaking at 55 °C. Overall, these improvements in photovoltaic properties are clear indicators of enhanced interface passivation by NaF-based interface engineering.

Managing Challenges in Organic Photovoltaics: Properties and Roles of Donor/Acceptor Interfaces

AbstractOrganic photovoltaics (OPVs) have demonstrated increasing potential for use in large‐area, flexible, and light‐weight applications. To date, the rapid development of nonfullerene acceptors (NFAs) and their conjugated polymeric donors have increased the efficiency of OPV by over 19%. Nevertheless, OPV is still suffering from high energy loss, which primarily derives from the donor (D)/acceptor (A) interfacial charge recombination. In particular, the voltage loss occurring at the D/A interface accounts for the current bottleneck, hampering further enhancement of the OPV efficiency. In this review, the recent discovery of D/A interfacial photophysics in NFA‐based OPVs, including the comparison with its fullerene‐based counterpart, is covered. Additionally, the factors governing interfacial energy loss, such as interfacial energetics and local morphologies, which causes the trade‐off relationship between photovoltage and photocurrent in OPV are highlighted. Accordingly, the control of D/A interfacial properties to create an “ideal” interface for charge generation in OPVs is reviewed; and emphasized that the D/A interfacial modifications can serve as a powerful tool to manage the challenges in OPVs path toward future practical applications.

Maximizing Merits of Undesirable δ‐FAPbI3 by Constructing yellow/black Heterophase Bilayer for Efficient and Stable Perovskite Photovoltaics

AbstractYellow‐phase formamidinium lead iodide (δ‐FAPbI3) as a degradation product of black perovskite phase is usually unwelcome in perovskite solar cells (PSCs). However, it also shows high potential in passivating defects and protecting perovskite films from moisture intrusion due to its water‐stable nature. Herein, the utilization of δ‐FAPbI3 to construct a yellow/black heterophase bilayer is reported, in which the perovskite layer is shielded by in situ formed δ‐FAPbI3 species on the surface, for PSCs with enhanced efficiency and stability. It is found that the δ‐FAPbI3 layer with a broader bandgap than the black‐phase perovskite can efficiently suppress the charge recombination at the interface, offering a high PSC efficiency of 23.10%. More importantly, the δ‐FAPbI3‐modified devices exhibit preferable stability against various external stresses to that of the control ones. Compared to those using the disliked δ‐FAPbI3 in the literature, the strategy, which maximizes its merits in reducing the interfacial charge recombination and stabilizing the perovskite structure, is more simple yet effective to realize highly efficient and stable PSCs.

Alkali Metal Cations Modulate the Energy Level of SnO2 via Micro-agglomerating and Anchoring for Perovskite Solar Cells.

N-type tin oxide (SnO2) films are commonly used as an electron transport layer (ETL) in perovskite solar cells (PSCs). However, SnO2 films are of poor quality due to facile agglomeration under a low-temperature preparation method. In addition, energy level mismatch between the SnO2 and perovskite (PVK) layer as well as interfacial charge recombination would cause open-circuit voltage loss. In this work, alkali metal oxalates (M-Oxalate, M = Li, Na, and K) are doped into the SnO2 precursor to solve these problems. First, it is found that the hydrolyzed alkali metal cations tend to change colloid size distribution of SnO2, in which Na-Oxalate with suitable basicity leads to most uniform colloid size distribution and high-quality SnO2-Na films. Second, the electron conductivity is enhanced by slightly agglomerated SnO2-Na, which facilitates the transmission of electrons. Third, alkali metal cations increase the conduction band level of SnO2 in the sequence of K+, Na+, and Li+ to promote band alignment between ETLs and perovskite. Based on the optimized film quality and energy states of SnO2-Na, the best PSC efficiency of 20.78% is achieved with a significantly enhanced open-circuit voltage of 1.10 V. This work highlights the function of alkali metal salts on the colloid particle distribution and energy level modulation of SnO2.

D-A-π-A organic dyes with fluorenyl-substituted bulky donor for efficient dye-sensitized solar cells

Improving light-harvesting efficiency and suppressing interfacial charge recombination are of paramount importance to the development of Cu (II/I) redox shuttle-based dye-sensitized solar cells (DSSCs). Here, we present a rational molecular engineering on D-A-π-A organic sensitizers featuring with fluorenyl-substituted bulky donor (HY57, HY60, and HY61) for Cu (II/I) mediated DSSCs. By gradually enhancing conjugation and rigidity of the π-spacer and auxiliary acceptor moieties, the light-harvesting and electron-injection efficiencies can be simultaneously improved. The large size of fluorenyl-substituted arylamine donor together with abundant alkyl chains decorated on the molecular structures assist dense assembly of dye monolayer at surface of nanocrystalline TiO2, which largely suppress interfacial charge recombination losses. When applied in Cu (II/I) mediated DSSCs, the combination of an auxiliary acceptor of phenanthrene-fused-quinoxaline (PFQ) and a π-spacer of cyclopentadithiophene (CPDT) in dye HY61 results in a simultaneous enhancement in short-circuit current (JSC) and open-circuit voltage (VOC), thus improving the power conversion efficiency (PCE) from 7.3% for reference dye HY59 to 10.3% for HY61. This study demonstrates that rigid and fused building block is promising towards constructing highly efficient organic sensitizers.

Synergetic Regulation of Oriented Crystallization and Interfacial Passivation Enables 19.1% Efficient Wide‐Bandgap Perovskite Solar Cells

AbstractWide‐bandgap (WBG) perovskite solar cells (PSCs) suffer from severe voltage loss, which significantly limits the enhancement of photovoltaic performance. Here, 4‐fluoro‐phenylethylammonium iodide (FPEAI) is used as a dual‐functional agent for oriented crystallization and comprehensive passivation of WBG PSCs. The additive of FPEAI promotes crystals to grow along with the (100) orientation with improved crystallinity and to spontaneously form Ruddlesden–Popper 2D perovskite on the grain boundary and surface of 3D crystals, which can passivate defects and protect the perovskite film from moisture erosion as well as suppressed ion migration. In addition, the 2D/3D heterostructure induces a matched energy‐level alignment, which mitigates the detrimental interfacial charge recombination at the interface of the 3D perovskite and hole transport layer. Consequently, the modified WBG PSCs exhibit an improved open‐circuit voltage to 1.3 V and a fill factor of 77.8%, leading to a remarkable power conversion efficiency of 19.1% with negligible hysteresis. Furthermore, the WBG PSCs maintain 85% of the original efficiency after 1000 h in air, demonstrating outstanding humidity stability. This work indicates that FPEAI can be used as a dual‐functional agent to significantly enhance the efficiency of WBG PSCs.

Hydrogen bonding drives the self-assembling of carbazole-based hole-transport material for enhanced efficiency and stability of perovskite solar cells

Designing a hole-transport material (HTM) that guarantees effective hole transport while self-assembling at the perovskite|HTM interface with the formation of an ordered interlayer, has recently emerged as a promising strategy for high-performance and stable perovskite solar cells (PSCs). Hydrogen bonding (HB) is a versatile multi-functional tool for the design of small molecular HTMs. However, to date, its employment is mostly limited to p-i-n inverted PSCs. This study demonstrates the advantages of a novel HTM design that can self-assemble into a long-range ordered interlayer on the perovskite surface via HB association. A hydro-functional HTM (O1) is compared to a reference HTM (O2) that cannot form HB due to the replacement of the amide group of O1 with a plain butyl alkyl chain in O2. As a result, O1-based n-i-p PSCs display enhanced hole extraction reaction, suppressed interfacial charge recombination, reduced hysteresis effect, and an increase in Voc (by 60 mV), FF (>11% increase), and overall power conversion efficiency, PCE (32% increase) compared to the case of HB-free O2-based devices. Remarkable stability is observed for unencapsulated O1 cells, with a T80 lifetime of 35.5 h under continuous maximum power point tracking in air. This work emphasizes the role of HB-directed self-assembling in simultaneously enhancing both the PCE and stability of popular n-i-p PSCs. This study paves the way for the development of new hydro-functional charge-transport material designs for efficient and stable PSCs.

Location effect of triptycene on the photovoltaic performance of carbazole-based dyes

Due to the unique three-dimensional spatial structure and special electronic properties, triptycene has been applied in dye-sensitized solar cells. Herein, two dyes JY76/JY77 featuring triptycene at the different substitution position of carbazole donor have been synthesized, and location effect of triptycene on the photovoltaic performance of devices has been investigated. Lateral derived triptycene endows JY77 much more enhanced capability of impeding the interface charge recombination than JY76 which triptycene was decorated at terminal. Finally, JY77-sensitized solar cells exhibit around 20% higher power conversion efficiency (PCE) than that of JY76. The photovoltaic properties are further tuned by co-sensitization strategy, and better PCEs are obtained. Under AM 1.5G irradiation, cells fabricated by the synergic adsorption of JY77 and JY75, achieve the highest PCE of 8.01%.

Interface Effects in Triazine‐Based g‐C3N4/MAPbI3 Van der Waals Heterojunctions: A First‐Principles Study

2D materials (TDMs) have demonstrated their great potential as functional materials in perovskite solar cells (PSCs) to boost conversion efficiency. As a versatile TDM, g‐C3N4 has far less applications in PSCs than in medical science, energy storage, supercapacitors, and catalysis. Is 2D g‐C3N4 naturally incompatible with perovskites limiting their cooperation? Herein, the capacity of two kinds of triazine‐based g‐C3N4 as interfacial modifiers for CH3NH3PbI3 (MAPbI3) perovskite based on density functional theory is discussed. Due to the existence of two feasible atomically exposed surfaces in MAPbI3 perovskite, denoted as PbI and MAI interfaces, four heterojunction structures are constructed. The interfacial carrier transport kinetics and charge recombination region distribution of the heterojunctions are systematically investigated. The results show that the NPCN/MAI heterojunction facilitates the separation and transport of charges due to its type‐II energy band alignment and smaller charge recombination region. Herein, a promising interface‐modified 2D material for perovskite photoactive layer is introduced and an avenue to reduce the charge recombination loss in PSCs is provided.

Combining Polymer Zwitterions and Zinc Oxide for High-Performance Inverted Organic Solar Cells.

Zinc oxide (ZnO) is a widely used cathode interlayer material in inverted organic solar cells (OSCs). However, there are lots of surfaces or bulk film defects in ZnO layers, which degrade solar cell performance. Here, the typical phosphorylcholine- and sulfobetaine-based polymer zwitterions (PMPC and PDMAPS) are synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization to modify ZnO interlayers for inverted OSCs. The polymer zwitterions can efficiently passivate the defects in ZnO films and thus increase the conductivity of the ZnO interlayers. Both PMPC and PDMAPS modified ZnO interlayers show some general advantages in improving the performance of fullerene-based and non-fullerene-based OSCs. The highest efficiency of 16.69% is achieved by using PMPC modified ZnO interlayers in PM6:Y6 based solar cell devices, which is among the best performance in inverted OSCs. Such an improvement in device performance is attributed to the work function reduction of the polymer zwitterions modified ZnO films, which provides an efficient cathode platform to extract and transport electrons from the active layers, to the benefit of suppressing interfacial charge recombination. As a result, the organic-inorganic hybrid composites (ZnO: polymer zwitterions) show efficient interfacial modification to align energy levels at the device interface, which have promising application prospects in organic electronics.

Tailored Polymeric Hole-Transporting Materials Inducing High-Quality Crystallization of Perovskite for Efficient Inverted Photovoltaic Devices.

For achieving high-performance p-i-n perovskite solar cells (PSCs), hole transporting materials (HTMs) are critical to device functionality and represent a major bottleneck to further enhancing device stability and efficiency in the inverted devices. Three dopant-free polymeric HTMs are developed based on different linkage sites of triphenylamine and phenylenevinylene repeating units in their main backbone structures. The backbone curvatures of the polymeric HTMs affect the morphology and hole mobility of the polymers and further change the crystallinity of perovskite films. By using PTA-mPV with moderate molecular curvature, p-i-n PSCs with high efficiency of 19.5% and long-term stability can be achieved. The better performance is attributed to the more effective hole extraction ability, higher charge-carrier mobility, and lower interfacial charge recombination. Furthermore, these three polymeric HTMs are synthesized without any noble metal catalyst, and show great advantages in future application owing to the low-cost.

Designed p-type graphene quantum dots to heal interface charge transfer in Sn-Pb perovskite solar cells

In Sn-Pb based low bandgap perovskite solar cells (LB-PSCs), the large energy loss of ~0.5 eV lags its development, originating from unfavorable band alignment at the interface and serious charge recombination in the device. Here, judiciously designed graphene quantum dots by functional N/Cl doping are applied at PEDOT:PSS/perovskite interface to modulate the energy level and charge transfer simultaneously. Among three designed graphene quantum dots, the N, Cl codoped one (N,Cl-GQDs) induces the best band alignment, because of the enhanced p-doping. Besides, N,Cl-GQDs best regulate the charge distribution and passivate the defect states at the interface by π-conjugated effect and electronegative Cl, paving the way for efficient interface charge transport. Upon N,Cl-GQDs optimization, the FA0.6MA0.4Sn0.6Pb0.4I3 based PSCs achieves impressive efficiency of 21.5% (1 sun, AM1.5) with a high photovoltage of ~0.89 V (energy loss of ~0.36 eV) and fill factor over 80%, which are among the best parameters in reported LB-PSCs. The N,Cl-GQDs modified device exhibits the best shelf stability after 1000 h, due to the least trap states and reformative interface. This work highlights designed graphene quantum dots for photovoltaic applications.

Room Temperature Fabrication of SnO2 Electrodes Enabling Barrier‐Free Electron Extraction for Efficient Flexible Perovskite Photovoltaics

AbstractRoom temperature‐processed electron transport layers (RT‐ETLs) demonstrate vast potential to be used in fabricating high‐performance flexible perovskite solar cells (PSCs) in an energy‐saving manner. However, the RT‐ETL normally suffers from inferior crystallinity, mismatched energy level, and high surface trap‐state density, which would result in under‐optimized interfacial electron extraction and undesirable interfacial charge recombination at ETL/perovskite interface, thus limiting the device performance. Herein, a novel strategy is demonstrated to prepare annealing‐free RT‐ETL based on precrystalline metal ion‐modified SnO2 nanocrystals, which perfectly optimizes the interfacial energy level alignment between ETL and perovskite layer, achieving nearly zero‐barrier charge transfer at the interface. As a result, the charge extraction has been remarkably accelerated and the interfacial charge recombination has been largely suppressed, leading to a ≈26% enhancement in device efficiency. The best‐performing flexible PSCs achieve efficiencies up to 19.3%, accompanied by outstanding mechanical strength under repeated bending cycle tests, which, to the best of the knowledge, is one of the highest reported values for the flexible perovskite photovoltaics fabricated with RT‐ETLs.

Highly improved efficiency and stability of planar perovskite solar cells via bifunctional phytic acid dipotassium anchored SnO2 electron transport layer

The defects at the interface of SnO2 electron transport layers (ETLs) /perovskite layer hinder extraction and transfer of charge, which restrict the further improvement of the photoelectric conversion efficiency (PCE) of SnO2-based perovskite solar cells (PSCs). Herein, an effective bifunctional anchoring strategy is developed to passivate SnO2/perovskite interface defects by adequate chelate sites in polydentate phytic acid dipotassium (PAD) to anchor SnO2 colloids. It is confirmed that polydentate PAD anchoring can form new Sn-O-P bonds with uncoordinated Sn2+, which can efficiently passivate the SnO2 surface defects and alleviate the interfacial charge recombination between SnO2 and perovskite layer. The reduction of SnO2 surface defects enhances the electron transport efficiency and increases the conductivity of the PAD-SnO2 ETLs to 2.1 times that of SnO2 ETLs. Meanwhile, the anchoring group potassium ions (K+) in PAD can promote the in-plane growth of perovskite grains and increase the average grain size of perovskite from 560 nm to 850 nm. As a consequence, the PCE of the PAD-SnO2-based PSCs increased sharply to 21.61%, which is 10.7% higher than that of the PSCs made with SnO2 ETLs (19.52%). The unencapsulated PSCs deposited on PAD-SnO2 ETLs can remain 99.5% of their initial efficiency after 60 days under 25–30% humidity.

Zn-doped CdS/CdSe as efficient strategy to enhance the photovoltaic performance of quantum dot sensitized solar cells

Metal ion doping is a suitable approach for optimizing quantum dots (QDs), which can change the recombination dynamics and charge separation of QDs via creating electronic states within their band gap, and improve their optical and electrical properties. In this work, the transition metal Zn-doped CdS and CdSe QDs are performed by SILAR (successive ionic layer adsorption and reaction) method. And the sensitized solar cells (QDSSCs) based on the Zn-doped CdS and CdSe QDs are assembled. The effect of Zn-doping concentration on the optical properties and charge carrier transport characteristics of photoanodes is investigated. Based on these QDs sensitized TiO2 photoanodes, the assembled QDSSCs with Zn-dopant of a molar concentration of 1.6 mol % exhibited a maximum power conversion efficiency (PCE) of 5.59 %, which is increased by 26.76 % in comparison with that of the QDSSC (4.41%) without Zn-doping. The significantly enhanced PCE of QDSSCs was attributed to improved current density and open-circuit voltage, particularly arising from enhanced light absorbance by narrowing the band gap with the midgap states and upshift of Fermi level promoted by the high electrons concentration due to the ionization of Zn impurities. In addition, the Zn-doping contributes to attenuation of the interfacial charge recombination rate and prolongs the electron lifetime, resulting in more efficient charge collection in QDSSCs.

Ferroelectric Ba0.75Sr0.25TiO3 tunable charge transfer in perovskite devices

Interfacial charge recombination is a main issue causing the efficiency loss of the perovskite solar cells (PSCs). Here, ferroelectric Ba0.75Sr0.25TiO3 (BST) is introduced as a polarization tunable layer to promote the interfacial charge transfer of the PSCs. The coexistence of ferroelectric polarization and charge carriers in BST is confirmed by density functional theory (DFT) calculations. Experimental characterization demonstrates the polarization reversal and the existence of domain in BST film. The BST film conductivity is tested as 2.98 × 10−4 S/cm, which is comparable to the TiO2 being used as the electron transporting layer (ETL) in PSCs. The calculations results prove that BST can be introduced into the PSCs and the interfacial charge transfer can be tuned by ferroelectric polarization. Thus, we fabricated the BST-based PSCs with a champion power conversion efficiency (PCE) of 19.05% after poling.

Electron Transport Assisted by Transparent Conductive Oxide Elements in Perovskite Solar Cells.

Fluorine and indium elements in F-doped SnO2 (FTO) and Sn-doped In2 O3 (ITO), respectively, significantly contribute toward enhancing the electrical conductivity of these transparent conductive oxides. In this study, fluorine was combined with indium to modify the SnO2 electron transport layer (ETL) through InF3 . Consequently, the modified perovskite solar cells (PSCs) showe the favorable alignment of energy levels, improved absorption and utilization of light, enhanced interfacial charge extraction, and suppressed interfacial charge recombination. After InF3 modification, the open circuit voltage (Voc ) and fill factor (FF) of the PSC were significantly improved, and the photoelectric conversion efficiency (PCE) reached 21.39 %, far exceeding that of the control PSC (19.62 %).

Highly Efficient Photoelectrochemical Hydrogen Production Using Nontoxic CuIn1.5Se3 Quantum Dots with ZnS/SiO2 Double Overlayers.

Quantum dots (QDs) are a promising material for photoelectrochemical (PEC) hydrogen (H2) production because of their attractive optical properties including high optical absorption coefficient, band-gap tunability, and potential multiple exciton generation. To date, QDs containing toxic elements such as Cd or Pb have been mainly investigated for PEC H2 production, which cannot be utilized in practice because of the environmental issue. Here, we demonstrate a highly efficient type II heterojunction photoanode of nontoxic CuIn1.5Se3 (CISe) QDs and a mesoporous TiO2 film. In addition, ZnS/SiO2 double overlayers are deposited on the photoanodes to passivate surface defect sites on the CISe QDs, leading to the enhancement of both photocurrent density and photostability. Due to a combination of a wide light absorption range of the CISe QDs and the reduced interfacial charge recombination by the overlayers, a remarkable photocurrent density of 8.5 mA cm-2 (at 0.5 VRHE) is obtained under 1 sun illumination, which is a record for the PEC sulfite oxidation based on nontoxic QD photoanodes.

Synergistic stabilization of CsPbI3 inorganic perovskite via 1D capping and secondary growth

Cesium lead iodide (CsPbI3) perovskite has gained great attention in the photovoltaic (PV) community because of its unique optoelectronic properties, good chemical stability and appropriate bandgap for sunlight harvesting applications. However, compared to solar cells fabricated from organic-inorganic hybrid perovskites, the commercialization of devices based on all-inorganic CsPbI3 perovskites still faces many challenges regarding PV performance and long-term stability. In this work, we discovered that tetrabutylammonium bromide (TBABr) post-treatment to CsPbI3 perovskite films could achieve synergistic stabilization with both TBA+ cation intercalation and Br-doping. Such TBA+ cation intercalation leads to one-dimensional capping with TBAPbI3 perovskite formed in situ, while the Br-induced crystal secondary growth helps effectively passivate the defects of CsPbI3 perovskite, thus enhancing the stability. In addition, the incorporation of TBABr can improve energy-level alignment and reduce interfacial charge recombination loss for better device performance. Finally, the highly stable TBABr-treated CsPbI3-based perovskite solar cells show reproducible photovoltaic performance with a champion efficiency up to 19.04%, while retaining 90% of the initial efficiency after 500 h storage without encapsulation.

Development of SnO2 Composites as Electron Transport Layer in Unencapsulated CH3NH3PbI3 Solar Cells

Improving morphological and electronic properties of the electron transport layer (ETL) is a critical issue to fabricate highly efficient perovskite solar cells. Tin dioxide is used as an ETL for its peculiarities such as low-temperature solution-process and high electron mobility and several handlings have been tested to increase its performances. Herein, SnO2:ZnO and SnO2:In2O3 composites are studied as ETL in planar n-i-p CH3NH3PbI3 solar cells fabricated in ambient air, starting from glass/ITO substrates. Morphological, electrical and optical properties of zinc- and indium-oxide nanoparticles (NPs) are investigated. First-principle calculations are also reported and help to further explain the experimental evidences. Photovoltaic performances of full devices show an improvement in efficiency for SnO2:In2O3–based solar cells with respect to pristine SnO2, probably due to a suppression of interfacial charge recombination between ITO/ETL and ETL/perovskite. Moreover, a better homogeneity of SnO2:In2O3 deposition with respect to SnO2:ZnO composites, conducts an increase in perovskite grain size and, consequently, the device performances.