Energy Level Mismatch Research Articles

An Efficient and Stable Inverted Structure Organic Solar Cell Using ZnO Modified by 2D ZrSe2 as a Composite Electron Transport Layer

AbstractAs an electron transport layer (ETL) widely used in organic solar cells (OSCs), ZnO has problems with energy level mismatch with the active layer and excessive defects on the ZnO surface, which can reduce the efficiency of OSCs. Here, ZnO/ZrSe2 composite is fabricated by modifying ZnO with 2D ZrSe2. The XPS and first‐principles calculation (FPC) show that ZnO obtains electrons from ZrSe2 and forms interfacial dipoles toward the active layer, which decreases the work function of ZnO, thus reducing the interface barrier and favoring the collection of electrons in OSCs. At the same time, after ZrSe2 modification, the oxygen vacancy density on the ZnO surface decreases, thus improving the conductivity of ZnO. More importantly, the femtosecond transient absorption (Fs‐TA) shows that ZrSe2 selectively traps holes from the active layer, which prevents the holes from entering the ZnO, thereby reducing the probability of electron recombination. Finally, ZnO/ZrSe2 composite is used as a novel ETL in OSCs with PBDB‐T: ITIC, PM6:Y6 and PM6: L8‐BO as active layers, and obtaining 12.09%, 16.34%, and 18.24% efficiency, respectively. This study provides a method for the interface modification of ZnO, and further investigates the role of 2D nanosheets in the interface modification.

Hexylammonium Acetate-Regulated Buried Interface for Efficient and Stable Perovskite Solar Cells.

Due to current issues of energy-level mismatch and low transport efficiency in commonly used electron transport layers (ETLs), such as TiO2 and SnO2, finding a more effective method to passivate the ETL and perovskite interface has become an urgent matter. In this work, we integrated a new material, the ionic liquid (IL) hexylammonium acetate (HAAc), into the SnO2/perovskite interface to improve performance via the improvement of perovskite quality formed by the two-step method. The IL anions fill oxygen vacancy defects in SnO2, while the IL cations interact chemically with Pb2+ within the perovskite structure, reducing defects and optimizing the morphology of the perovskite film such that the energy levels of the ETL and perovskite become better matched. Consequently, the decrease in non-radiative recombination promotes enhanced electron transport efficiency. Utilizing HAAc, we successfully regulated the morphology and defect states of the perovskite layer, resulting in devices surpassing 24% efficiency. This research breakthrough not only introduces a novel material but also propels the utilization of ILs in enhancing the performance of perovskite photovoltaic systems using two-step synthesis.

Buried interface modification toward efficient perovskite solar cell based on PVK hole transport layer

Poly (9-vinylcarbazole) (PVK) is an attractive hole transport material for organometallic-halide perovskite solar cell (PSC) because of its excellent hole transport ability and great film-forming property. Nowadays, the PVK hole-transport-layer (HTL) based PSC suffered from low power conversion efficiency (PCE), which mainly caused by bad interface contact between PVK and perovskite film, including the energy level mismatch. Doing PVK is an effective and facial method to modify the buried interface of perovskite film. In this work, 1,1-bis [(di-4-tolylamino)phenyl]cyclohexane (TAPC) was added into PVK HTL. Experimental results show that a highest occupied molecular orbital (HOMO) level elevation was achieved after doping TAPC. Meanwhile perovskite films with increased crystallinity, reduced defect density and enhanced light absorbance were obtained. Finally, the PCE of PSCs were enhanced from 10.81% of the control device to 15.65% of PVK + 40% TAPC device. The reason that responsible for the energy level lifting, morphology improvement and performance enhancement were thoroughly investigated and analyzed. This work provides an alternative option for the dopant in PVK to improve the buried interface of perovskite film, and therefore, the performance of PSCs.

Multifunctional surface modification of carbon-based all-inorganic CsPbI2Br solar cells by 3-(trifluoromethyl) phenyltrimethyl ammonium iodide

The preparation process of carbon-electrode perovskite solar cells (C-PSCs) without hole transport layers (HTLs) is simple, cost-effective, and the as-prepared C-PSCs exhibit excellent thermal stability. However, there are numerous defects at the interfaces of C-PSCs and grain boundaries. Due to the presence of these defect states, non-radiative recombination of charge carriers occurs, ultimately resulting in significant VOC (voltage open circuit) loss and an energy level mismatch between device structures, which seriously affect the photoelectric conversion efficiency (PCE) and stability of C-PSCs. Therefore, this work uses 3-(trifluoromethyl)-phenyltrimethyl-ammonium iodide (TFPTAI) as a surface modifier to modify the surface of CsPbI2Br. Due to the strong electronegativity of fluorine atoms (F), acting as electron-withdrawing groups, they promote the separation of positive and negative charge centers in the molecule. This, in turn, can effectively induce strong binding of ammonium cations (NH+) with negatively charged defects, consequently suppressing the non-radiative recombination of charge carriers. In addition, thanks to the ordered arrangement of F atoms, a hydrophobic protective umbrella is constructed on the perovskite surface layer, effectively improving the stability of the device. The highest PCE of the as-prepared device is 14.02%, surpassing the optimal PCE of the original device at 12.46%. Simultaneously, the surface-modified device exhibits excellent environmental stability.

Completely annealing-free flexible Perovskite quantum dot solar cells employing UV-sintered Ga-doped SnO2 electron transport layers

The electron transport layer (ETL) is a critical component in perovskite quantum dot (PQD) solar cells, significantly impacting their photovoltaic performance and stability. Low-temperature ETL deposition methods are especially desirable for fabricating flexible solar cells on polymer substrates. Herein, we propose a room-temperature-processed tin oxide (SnO2) ETL preparation method for flexible PQD solar cells. The process involves synthesizing highly crystalline SnO2 nanocrystals stabilized with organic ligands, spin-coating their dispersion, followed by UV irradiation. The energy level of SnO2 is controlled by doping gallium ions to reduce the energy level mismatch with the PQD. The proposed ETL-based CsPbI3-PQD solar cell achieves a power conversion efficiency (PCE) of 12.70%, the highest PCE among reported flexible quantum dot solar cells, maintaining 94% of the initial PCE after 500 bending tests. Consequently, we demonstrate that a systemically designed ETL enhances the photovoltaic performance and mechanical stability of flexible optoelectronic devices.

Enhancing Stability and Efficiency of Inverted Inorganic Perovskite Solar Cells with In-Situ Interfacial Cross-Linked Modifier.

Inverted inorganic perovskite solar cells (PSCs) is potential as the top cells in tandem configurations, owing to the ideal bandgap, good thermal and light stability of inorganic perovskites. However, challenges such as mismatch of energy levels between charge transport layer and perovskite, significant non-radiative recombination caused by surface defects, and poor water stability have led to the urgent need for further improvement in the performance of inverted inorganic PSCs. Herein, the fabrication of efficient and stable CsPbI3-x Brx PSCs through surface treatment of (3-mercaptopropyl) trimethoxysilane (MPTS), is reported. The silane groups in MPTS can in situ crosslink in the presence of moisture to build a 3-dimensional (3D) network by Si-O-Si bonds, which forms a hydrophobic layer on perovskite surface to inhibit water invasion. Additionally, -SH can strongly interact with the undercoordinated Pb2+ at the perovskite surface, effectively minimizing interfacial charge recombination. Consequently, the efficiency of the inverted inorganic PSCs improves dramatically from 19.0% to 21.0% under 100mW cm-2 illumination with MPTS treatment. Remarkably, perovskite films with crosslinked MPTS exhibit superior stability when soaking in water. The optimized PSC maintains 91% of its initial efficiency after aging 1000 h in ambient atmosphere, and 86% in 800 h of operational stability testing.

Synergetic modification on buried and upper surfaces of perovskites with nitrogen-doped carbon quantum dots for efficient and stable solar cells

Tin oxide (SnO2), which is used as an electron transport layer in perovskite solar cells (PSCs), however, has nonnegligible defects. The surface of SnO2 films contains hydroxyl groups (–OH), which increases film defects and affects the quality of the upper perovskite. The energy level mismatch is also found in SnO2, which leads to severe nonradiative recombination and unsatisfactory photovoltaic performance. Meanwhile, excessive PbI2 residues on the perovskite surface serve as catalytic decomposition sites, significantly undermining device stability. In this study, nitrogen-doped carbon quantum dots (N-CQDs) were synthesized by using human hair as a raw material and were employed as an additive for interface between SnO2 and perovskites to improve interfacial charge transport. In addition, these N-CQDs were also used as a surface modifier on top of the perovskite layer to reduce residual PbI2. These results significantly increase the power conversion efficiency (PCE) of the related devices from 19.22% to 21.49%. The CQDs-modified device also shows significantly enhanced operational stability that when exposed to 30% humidity, the PCE of the optimized device retains 86% of its initial value after 1200 h. This study emphasizes the great potential of N-CQDs as a multifunctional interfacial modifier for fabrication of efficient and stable PSCs.

Interface modification for efficient carbon-electrode CsPbI2Br perovskite solar cells using ionic liquid.

All-inorganic CsPbI2Br, as a promising photovoltaic (PV) material, have attracted extensive research attention in society for its outstanding thermal stability and appropriate trade-offs. Carbon-based perovskite solar cells (C-PSCs) without hole transporting layer (HTL) have shown great potential in terms of cost-effectiveness and stability. However, the inevitable defects on the surface of CsPbI2Br films severely hampers the development of high-efficiency CsPbI2Br C-PSCs. Surface engineering has emerged an effective approach to overcome this challenge. Herein, 1-decyl-3-methylimidazolium tetrafluoroborate (DMTT) ionic liquid was introduced between CsPbI2Br and carbon electrode to reduce non-recombination of charges, decrease defect states, minimize the energy-level mismatch, and greatly enhance the device stability. As a result, the HTL-free CsPbI2Br C-PSCs combined with DMTT as an interface modification achieved a higher power conversion efficiency (PCE) of 12.47% than that of the control devices with a PCE of 11.32%. Furthermore, without any encapsulation, the DMTT-optimized C-PSC remained approximately 84% of its initial PCE after over 700 h under room temperature and 25% relative humidity (RH) conditions. Additionally, when exposed to a temperature of 65 °C for over 400 h, the device still retained 74% of the initial PCE, demonstrating its thermal stability.

Optimizing Energy Levels and Improving Film Compactness in PbS Quantum Dot Solar Cells by Silver Doping.

PbS quantum dot (QD) solar cells harvest near-infrared solar radiation. Their conventional hole transport layer has limited hole collection efficiency due to energy level mismatch and poor film quality. Here, how to resolve these two issues by using Ag-doped PbS QDs are demonstrated. On the one hand, Ag doping relieves the compressive stress during layer deposition and thus improves film compactness and homogeneity to suppress leakage currents. On the other hand, Ag doping increases hole concentration, which aligns energy levels and increases hole mobility to boost hole collection. Increased hole concentration also broadens the depletion region of the active layer, decreasing interface charge accumulation and promoting carrier extraction efficiency. A champion power conversion efficiency of 12.42% is achieved by optimizing the hole transport layer in PbS QD solar cells, compared to 9.38% for control devices. Doping can be combined with compressive strain relief to optimize carrier concentration and energy levels in QDs, and even introduce other novel phenomena such as improved film quality.

Buried Interface‐The Key Issues for High Performance Inverted Perovskite Solar Cells

AbstractInterface engineering is known for effectively improving interfacial contact and passivating defects to enhance device performance of inverted perovskite solar cells (PSCs). Currently, most of works focus on surface passivation, while the buried interface is equally important. The film quality of perovskite layer greatly relies on the buried interface, leaving a pronounced impact on overall device performance. In addition, resolving defects and energy level mismatch at buried interface remains challenging. Optimizing the buried interface becomes a promising approach for high‐efficiency inverted PSCs. This review summarizes recent advances in buried interface engineering and emphasize the importance of corresponding characterization techniques. The various functions of buried interface engineering are carefully discussed, including crystallization modulation, defect passivation, energy level alignment, chemical reaction inhibition, chemical bridge, dipole cancellation and novel buried interfacial techniques. Finally, current challenges and prospects are put forward that should be addressed to further improve device performance of inverted PSCs.

Strengthened Buried Interface via Metal Sulfide Passivation Toward High‐Performance CsPbBr3 Perovskite Solar Cells

Although SnO2 has been widely used as the electron transport material (ETM) of the perovskite solar cells (PSCs), the energy level mismatch at the SnO2/CsPbBr3 buried interface is as high as 1 eV, which is disastrous for the CsPbBr3‐based PSCs. Herein, a buffer layer of metal sulfide (CdS, ZnS) is introduced to solve this problem. The power conversion efficiency (PCE) of CsPbBr3 PSCs has been increased from 8.16% to 9.48% for ZnS‐treated SnO2 (ZnS‐SnO2), and a champion efficiency of 10.61% has been achieved in CdS‐treated SnO2 (CdS‐SnO2) devices. Aside from the reduced energy loss, the mobility of the SnO2 ETM has been greatly enhanced after the metal sulfide treatment. The CdS‐SnO2 devices also enjoy the benefits of reduced defect density and speeded carrier extraction, contributing to an almost 30% performance enhancement. This 10.61% PCE is among the highly efficient CsPbBr3‐based PSCs reported to date. Finally, CdS‐SnO2 devices survive a harsh damp heat test (120 °C with a relative humidity of 50%) for a month with less than 15% efficiency loss, demonstrating the superior stability of our CsPbBr3 PSCs.

Surface p-type band bending and energy level alignment minimizes voltage loss in perovskite solar cells

Due to the significant energy level mismatch and contact energy loss between the perovskite absorber layer and p-type hole-transporting layer (HTL), there exists a large hole transport barrier between the perovskite absorber layer and HTL, which hinders hole transport and leads to lower open circuit voltage (VOC) in perovskite devices. Therefore, the surface contact process between the perovskite film and the hole-transporting layer becomes particularly important. Here, we doped cesium silver bismuth bromide (Cs2AgBiBr6) quantum dots (QDs) into ethyl acetate (EA) as an anti-solvent to achieve the formation of a bulk heterojunction structure with quantum dot solution on the surface of the MAPbI3 perovskite film. The perovskite film exhibits appropriate band edge bending and forms a p-type semiconductor. This facilitates the directed transport of photo-induced charge carriers to the hole-transporting layer, reducing carrier recombination losses and enhancing the collection efficiency of holes by the HTL. Through characterization experiments, we have found that this method significantly improves the VOC and photovoltaic conversion efficiency (PCE) of perovskite solar cells. The perovskite solar cells fabricated using this method show a better PCE with a VOC of 1.06 V.

A dual passivation strategy based on F/N co-doped coal-based graphene quantum dots for high-efficiency carbon-based perovskite solar cells

F/N-GQDs were used to synergistically passivate defects of perovskite layers, suppressing nonradiative recombination, optimizing energy level alignment, and improving hole extraction ability, greatly enhancing the photoelectric performance of C-PSCs.

Red Fluorescent Carbon Dots with Alkyl Chain Achieving Stable Electroluminescence via an In Situ Electric Excitation

AbstractThe energy barrier between the highest occupied molecular orbital (HOMO) of the emission layer (EML) and the hole transport layer material (HTL) restricts the development of carbon dots (CDs) based light‐emitting diodes (LEDs). Here, the fabrication of red fluorescent CDs (RCDs) by a one‐step solvothermal method is reported. These RCDs have a photoluminescence quantum yield (PLQY) of 47.97% and a full width at half maximum (FWHM) of 26 nm. This study also shows the RCDs‐based LEDs fabrication with Poly(9,9‐dioctylfluorene‐co‐N‐(4‐butylphenyl) diphenylamine (TFB) as HTL. In normal case, these devices are not able to work due to the large interfacial energy barrier between RCDs and TFB. While, the RCDs‐LEDs can overcome through interfacial energy barriers and achieve stable carrier injecting by a simple in situ electric excitation at the current of 50 mA cm−2. This work provides a new strategy to overcome the obstacle of mismatch of interfacial energy levels in the LEDs by an in situ electric excitation.

Scalable Solution-Processed Hybrid Electron Transport Layers for Efficient All-Perovskite Tandem Solar Modules.

All-perovskite tandem solar cells offer the potential to surpass the Shockley-Queisser (SQ) limit efficiency of single-junction solar cells while maintaining the advantages of low-cost and high-productivity solution processing. However, scalable solution processing of electron transport layer (ETL) in p-i-n structured perovskite solar subcells remains challenging due to the rough perovskite film surface and energy level mismatch between ETL and perovskites. Here, scalable solution processing of hybrid fullerenes (HF) with blade-coating on both wide-bandgap (≈1.80 eV) and narrow-bandgap (≈1.25 eV) perovskite films in all-perovskite tandem solar modules is developed. The HF, comprising a mixture of fullerene (C60 ), phenyl C61 butyric acid methyl ester, and indene-C60 bisadduct, exhibits improved conductivity, superior energy level alignment with both wide- and narrow-bandgap perovskites, and reduced interfacial nonradiative recombination when compared to the conventional thermal-evaporated C60 . With scalable solution-processed HF as the ETLs, the all-perovskite tandem solar modules achieve a champion power conversion efficiency of 23.3% (aperture area = 20.25 cm2 ). This study paves the way to all-solution processing of low-cost and high-efficiency all-perovskite tandem solar modules in the future.

Engineering a novel role for Yb3+ to overcome thermal quenching and improve photothermal conversion efficiency as well as temperature sensing performance

In this paper, two types of phosphors, Cs3GdGe3O9:Er3+ and Cs3GdGe3O9:Yb3+/Er3+, were synthesized using the high-temperature solid-state method, and a new role for Yb3+ was successfully achieved. The presence of low phonon energy in the matrix and the low energy level mismatch between the 2F5/2 and the 4I11/2 enables the construction of a phonon-assisted channel between Yb3+ and Er3+ through doping with Yb3+ ions. This channel overcomes thermal quenching and achieves thermal enhancement at high temperatures. Additionally, Yb3+ increases the chance of non-radiative transition within the matrix, effectively converting the 980 nm laser into heat energy. This leads to a significant change in the surface temperature of the sample, greatly improving the photothermal conversion efficiency. Moreover, Yb3+ greatly enhances the emission intensity of green light, resulting in a maximum relative sensitivity of 1.11 % K−1 for temperature measurements in the Er3+ single-doped system and 1.28 % K−1 in the Yb3+/Er3+ co-doped system, based on the Er3+ thermally coupled energy level. These results demonstrate that this paper provides a new idea for materials to overcome thermal quenching and improve the efficiency of photothermal conversion and temperature sensing performance.

Mutually Tuned Dual Additive Engineering Synergistically Enhances the Photovoltaic Performance of Tin-Based Perovskite Solar Cells.

Tin-based perovskite solar cells (T-PSCs) have become the star photovoltaic products in recent years due to their low environmental toxicity and superior photovoltaic performance. However, the easy oxidation of Sn2+ and the energy level mismatch between the perovskite film and charge transport layer limit its efficiency. In order to regulate the microstructure and photoelectric properties of tin-based perovskite films to enhance the efficiency and stability of T-PSCs, guanidinium bromide (GABr) and organic Lewis-based additive methylamine cyanate (MAOCN) are introduced into the FA0.9PEA0.1SnI3-based perovskite precursor. A series of characterizations show that the interactions between additive molecules and perovskite mutually reconcile to improve the photovoltaic performance of T-PSCs. The introduction of GABr can adjust the band gap of the perovskite film and energy level alignment of T-PSCs. They significantly increase the open-circuit voltage (Voc). The MAOCN material can form hydrogen bonds with SnI2 in the precursor, which can inhibit the oxidation of Sn2+ and significantly improve the short-circuit current density (Jsc). The synergistic modulation of the dual additives reduces the trap-state density and improves photovoltaic performance, resulting in an increased champion efficiency of 9.34 for 5.22% of the control PSCs. The unencapsulated T-PSCs with GABr and MAOCN dual additives prepared in the optimized process can retain more than 110% of their initial efficiency after aging for 1750 h in a nitrogen glovebox, but the control PSCs maintain only 50% of their initial efficiency kept in the same conditions. This work provides a new perspective to further improve the efficiency and stability of T-PSCs.

High‐Performance Inverted Perovskite Solar Cells with Sol–Gel‐Processed Sliver‐Doped NiOX Hole Transporting Layer

Nickel oxide (NiOX) has been established as a highly efficient and stable hole‐transporting layer (HTL) in perovskite solar cells (PSCs). However, existing deposition methods for NiOX have been restricted by high‐vacuum processes and fail to address the energy level mismatch at the NiOX/perovskite interface, which has impeded the development of PSCs. Accordingly, we explored the application of NiOX as a hybrid HTL through a sol–gel process, where a NiOX film was pre‐doped with Ag ions, forming a p/p+ homojunction in the NiOX‐based inverted PSCs. This innovative approach offers two synergistic advantages, including the enlargement of the built‐in electric field for facilitating charge separation, optimizing energy level alignment, and charge transfer efficiency at the interface between the perovskite and HTL. Incorporating this hybrid HTL featuring the p/p+ homojunction in the inverted PSCs resulted in a high‐power conversion efficiency (PCE) of up to 19.25%, significantly narrowing the efficiency gap compared to traditional n‐i‐p devices. Furthermore, this innovative strategy for the HTL enhanced the environmental stability to 30 days, maintaining 90% of the initial efficiency.

Insulating Electrets Converted from Organic Semiconductor for High‐Performance Transistors, Memories, and Artificial Synapses

AbstractElectrets are commonly used charged insulators that generate a quasi‐permanent electric field. However, when conventional electrets come into direct contact with semiconductors, the energy level mismatch at the interface results in low memory speed and high energy consumption of electret devices due to both charge injection and storage being non‐conducive. To address this, the n‐type semiconductor N,N′‐dioctyl‐3,4,9,10‐perylene tetracarboxylic diimide (C8‐PTCDI) is converted to C8‐PTCDI (D) via oxygen degradation. The resulting C8‐PTCDI (D) electrets, when charged using an electric field and/or light, retain the energy level of the n‐type semiconductors to facilitate charge trapping. They also exhibit deeper trap energy levels and increased trap density, thereby enhancing the sheet charge density of C8‐PTCDI (D) electrets (7.47 × 1012 cm−2). As a result, devices based on n‐type electrets demonstrate lower operation voltage (2 V) of transistors, lower operation voltage (20 V) of memories, and lower energy consumption (3.5 fJ per spike) of artificial synapses compared to those without n‐type electrets.

Interfacial Engineering of a PCBM/AZO Electron Transport Bilayer for Efficient and Stable Inverted Perovskite Solar Cells

AbstractThe reduced hysteresis index and low‐temperature processed fabrication of inverted p‐i‐n perovskite solar cells (PSC) have attracted substantial attraction for achieving higher photovoltaic performance through interfacial engineering. Despite having certain limitations, fullerene‐based electron transport layers have been used frequently utilized in mixed halide inverted perovskite solar cells. The energy level mismatch between PCBM and metal electrode creates a hindrance in efficient electron extraction and thus a limiting factor in attaining higher device performances. In this work, we report an efficient interlayer of aluminum‐doped zinc oxide (AZO) nanoparticles in‐between PCBM and the metal electrode to suppress interfacial recombination. The PCBM/AZO electron transport bilayer can effectively mitigate the imperfections of the PCBM layer alone. The PCBM/AZO electron transport bilayer with an optimal concentration of 2% Al dopant exhibited greatly improved power conversion efficiency (PCE) of 18.63%, VOC of 1.13 V, and FF of 73% with negligible hysteresis index of 0.04. Further, the optimal device exhibited remarkable stability by retaining 91% of the initial PCE after 200 hours confirming the suitable insertion of the AZO bilayer. The improved photovoltaic performance using PCBM/AZO bilayer can be attributed to higher electron transfer efficiency, suppressed interfacial recombination, and smooth surface morphology of AZO nanoparticles atop the PCBM layer.