Charge Injection, Carriers Recombination and HOMO Energy Level Relationship in Perovskite Solar Cells

Flexible, Transparent, and Bifacial Perovskite Solar Cells and Modules Using the Wide-Band Gap FAPbBr3 Perovskite Absorber

Perovskite solar cells (PSCs) offer impressive performance and flexibility, thanks to their simple, low-temperature deposition methods. Their band gap tunability allows for a wide range of applications, transitioning from opaque to transparent devices. This study introduces the first flexible, bifacial PSCs using the FAPbBr3 perovskite. We investigated the impact of optimizing electron and hole transport layers on the cells' bifaciality, transparency, and stability. PSCs achieved a maximum power conversion efficiency (PCE) of 6.8 and 18.7% under 1 sun and indoor light conditions (1200 lx), respectively, showing up to 98% bifaciality factor and an average visible transmittance (AVT) of 55%. Additionally, a P1-P2-P3 laser ablation scheme has been developed on the flexible poly(ethylene terephthalate) (PET) substrate for perovskite solar modules showing a PCE of 4.8% and high geometrical fill factor (97.8%). These findings highlight the potential of flexible, bifacial PSCs for diverse applications such as building-integrated photovoltaics (BIPV), agrivoltaics, automotive technology, wearable sensors, and Internet of things (IoT).

Poly(3-hexylthiophene)/perovskite Heterointerface by Spinodal Decomposition Enabling Efficient and Stable Perovskite Solar Cells

The best research-cell efficiency of perovskite solar cells (PSCs) is comparable with that of mature silicon solar cells (SSCs); However, the industrial development of PSCs lags far behind SSCs. PSC is a multiphase and multicomponent system, whose consequent interfacial energy loss and carrier loss seriously affect the performance and stability of devices. Here, by using spinodal decomposition, a spontaneous solid phase segregation process, in situ introduces a poly(3-hexylthiophene)/perovskite (P3HT/PVK) heterointerface with interpenetrating structure in PSCs. The P3HT/PVK heterointerface tunes the energy alignment, thereby reducing the energy loss at the interface; The P3HT/PVK interpenetrating structure bridges a transport channel, thus decreasing the carrier loss at the interface. The simultaneous mitigation of energy and carrier losses by P3HT/PVK heterointerface enables n-i-p geometry device a power conversion efficiency of 24.53% (certified 23.94%) and excellent stability. These findings demonstrate an ingenious strategy to optimize the performance of PSCs by heterointerface via Spinodal decomposition.

Hybrid Mesoporous TiO2/ZnO Electron Transport Layer for Efficient Perovskite Solar Cell

In recent years, perovskite solar cells (PSCs) have gained major attention as potentially useful photovoltaic technology due to their ever-increasing power-conversion efficiency (PCE). The efficiency of PSCs depends strongly on the type of materials selected as the electron transport layer (ETL). TiO2 is the most widely used electron transport material for the n-i-p structure of PSCs. Nevertheless, ZnO is a promising candidate owing to its high transparency, suitable energy band structure, and high electron mobility. In this investigation, hybrid mesoporous TiO2/ZnO ETL was fabricated for a perovskite solar cell composed of FTO-coated glass/compact TiO2/mesoporous ETL/FAPbI3/2D perovskite/Spiro-OMeTAD/Au. The influence of ZnO nanostructures with different percentage weight contents on the photovoltaic performance was investigated. It was found that the addition of ZnO had no significant effect on the surface topography, structure, and optical properties of the hybrid mesoporous electron-transport layer but strongly affected the electrical properties of PSCs. The best efficiency rate of 18.24% has been obtained for PSCs with 2 wt.% ZnO.

Promotion Strategies of Hole Transport Materials by Electronic and Steric Controls for n-i-p Perovskite Solar Cells

Hole transport materials (HTMs) play a requisite role in n-i-p perovskite solar cells (PSCs). The properties of HTMs, such as hole extraction efficiency, chemical compatibility, film morphology, ion migration barrier, and so on, significantly affect PSCs' power conversion efficiencies (PCEs) and stabilities. Up till now, researchers have devoted much attention to developing new types of HTMs as well as promoting pristine HTMs using numerous strategies. In this Review, we summarize the design strategies of various common HTMs for n-i-p PSCs are comprehensively discussed from two separate aspects (additive and non-additive engineering). Additive engineering generally tunes electronic properties of HTMs while non-additive engineering basically modifies their steric structures. Critical analysis and comparison between these design strategies are provided, considering the overall PCEs and stabilities of PSCs. Finally, a brief perspective on future promising design strategies for HTMs is given, in order to fabricate efficient and stable n-i-p devices for the commercialization of PSCs.

Suppressing Halide Segregation in Wide-Band-Gap Mixed-Halide Perovskite Layers through Post-Hot Pressing

Mixed-halide perovskites (MHPs) have attracted attention as suitable wide-band-gap candidate materials for tandem applications owing to their facile band-gap tuning. However, when smaller bromide ions are incorporated into iodides to tune the band gap, photoinduced halide segregation occurs, which leads to voltage deficit and photoinstability. Here, we propose an original post-hot pressing (PHP) treatment that suppresses halide segregation in MHPs with a band gap of 2.0 eV. The PHP treatment reconstructs open-structured grain boundaries (GBs) as compact GBs through constrained grain growth in the in-plane direction, resulting in the inhibition of defect-mediated ion migration in GBs. The PHP-treated wide-band-gap (2.0 eV) MHP solar cells showed a high efficiency of over 11%, achieving an open-circuit voltage (V_oc) of 1.35 V and improving the maintenance of the initial efficiency under the working condition at AM 1.5G. The results reveal that the management of GBs is necessary to secure the stability of wide-band-gap MHP devices in terms of halide segregation.

Influence of the Electron Selective Contact on the Interfacial Recombination in Fresh and Aged Perovskite Solar Cells

In this work, we have used TiO2 and SnO2 layers as electron selective contact (ESC) in n-i-p perovskite solar cells configuration. To study and compare the ion migration kinetics of these ESC, CsFAMAPbIBr and MAPbI3-based devices were fabricated and characterised in fresh (1 day) and aged (28 days) conditions. Depending on the ESC and perovskite composition, devices reveal a different progression over time in terms of hysteresis and performance. Using transient photovoltage (TPV) and transient photocurrent (TPC) techniques, we studied the kinetics of carrier extraction and recombination, which showed that aged devices present slower recombination kinetics compared to their fresh counterparts, revealing a positive effect of the aging process. Finally, transient of the transient, derived from the TPV technique, discloses that TiO2 accumulates more charges in the ESC/perovskite interface compared to SnO2 and that the ion migration kinetics are directly related to the perovskite composition.

A Comparison of Charge Carrier Dynamics in Organic and Perovskite Solar Cells

The charge carrier dynamics in organic solar cells and organic-inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are compared. The similarities and differences in charge generation, charge separation, charge transport, charge collection, and charge recombination in these two technologies are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated. In particular, the impact of exciton binding energy, charge transfer states, bimolecular recombination, charge carrier transport, sub-bandgap tail states, and surface recombination is evaluated, and the lessons learned from transient optical and optoelectronic measurements are discussed. This perspective thus highlights the key factors limiting device performance and rationalizes similarities and differences in design requirements between organic and perovskite solar cells.

Perovskite Quantum Dots as Multifunctional Interlayers in Perovskite Solar Cells with Dopant-Free Organic Hole Transporting Layers

Perovskite solar cells (PSCs) with organic hole transporting layers (o-HTLs) have been widely studied due to their convenient solution processing, but it remains a big challenge to improve the hole mobilities of commercially available organic hole transporting materials without ion doping while maintaining the stability of PSCs. In this work, we demonstrated that the introduction of perovskite quantum dots (QDs) as interlayers between perovskite layers and dopant-free o-HTLs (P3HT, PTAA, Spiro-OMeTAD) resulted in a significantly enhanced performance of PSCs. The universal role of QDs in improving the efficiency and stability of PSCs was validated, exceeding that of lithium doping. After a deep examination of the mechanism, QD interlayers provided the multifunctional roles as follows: (1) passivating the perovskite surface to reduce the overall amount of trap states; (2) promoting hole extraction from perovskite to dopant-free o-HTLs by forming cascade energy levels; (3) improving hole mobilities of dopant-free o-HTLs by regulating their polymer/molecule orientation. What is more, the thermal/moisture/light stabilities of dopant-free o-HTLs-based PSCs were greatly improved with QD interlayers. Finally, we demonstrated the reliability of the QD interlayers by fabricating large-area solar modules with dopant-free o-HTLs, showing great potential in commercial usage.

Benzothiadiazole Aryl-amine Based Materials as Efficient Hole Carriers in Perovskite Solar Cells

Four hole transport materials (HTMs) based on a benzothiadiazole (BT) central core have been synthesized and successfully employed in triple-cation mixed-halide perovskite solar cells (PSCs), reaching 18.05% solar to energy conversion efficiency. The synthesis of these HTMs follows the push-and-pull approach to modulate the HOMO energy level by combining the BT group as an electron acceptor and diphenyl- and triphenyl-amines as electron donors. Here we show that despite adjusting the HOMO energy level to that of the perovskite is a believed requisite to achieve efficient interfacial hole transfer, additional factors must be taken into account to design novel and efficient HTMs, such as a high hole mobility, solubility in organic solvents, and thermal stability.

Interfacial and structural modifications in perovskite solar cells

The rapid and continuous progress made in perovskite solar cell (PSC) technology has drawn considerable attention from the photovoltaic research community, and the application of perovskites in other electronic devices (such as photodetectors, light-emitting diodes, and batteries) has become imminent. Because of the diversity in device configurations, optimization of film deposition, and exploration of material systems, the power conversion efficiency (PCE) of PSCs has been certified to be as high as 25.2%, making this type of solar cells the fastest advancing technology until now. As demonstrated by researchers worldwide, controlling the morphology and defects in perovskite films is essential for attaining high-performance PSCs. In this regard, interface engineering has proven to be a very efficient way to address these issues, obtaining better charge collection efficiency, and reducing recombination losses. In this review, the interfacial modification between perovskite films and charge-transport layers (CTLs) as well as CTLs and electrodes of PSCs has been widely summarized. Grain boundary (GB) engineering and stress engineering are also included since they are closely related to the improvement in device performance and stability.

Improved Carrier Collection and Hot Electron Extraction Across Perovskite, C60, and TiO2 Interfaces

The use of C₆₀ as an interfacial layer between TiO₂ and methylammonium lead iodide perovskite is probed to reduce the current-voltage hysteresis in perovskite solar cells (PSCs) and, in turn, to impact the interfacial carrier injection and recombination processes that limit solar cell efficiencies. Detailed kinetic analyses across different time scales, that is, from the femtoseconds to the seconds, reveal that the charge carrier lifetimes as well as the charge injection and charge recombination dynamics depend largely on the presence or absence of C₆₀. In addition, we corroborate that C₆₀ is applicable in hot carrier PSCs as it is capable of extracting hot carriers generated throughout the early time scales following photoexcitation.

Hot electron injection into semiconducting polymers in polymer based-perovskite solar cells and their fate

Metal halide perovskites are known to possess upon photoexcitation long-lived hot carriers. By using femtosecond laser transient absorption spectroscopy, we probed in the current work interfacial charge transfer, that is, hot electrons and holes in methylammonium lead iodide perovskite. The focus was, on the one hand, on titanium dioxide as an electron transporting material and, on the other hand, on several organic semiconducting materials as hole transporting materials in perovskite solar cells. An unexpected carrier loss pathway for hot electrons was found in the form of injection into the low lying LUMOs of several organic semiconducting materials. Of great importance is the fact that the final photocurrents of perovskite solar cells scale with the suppression of this newly discovered loss pathway.

Interfacial recombination kinetics in aged perovskite solar cells measured using transient photovoltage techniques

The reduction of interfacial charge recombination kinetics in perovskite solar cells is key to increase device photovoltaic efficiencies. Thus, it is necessary to fully understand which are the major carrier losses and, thereafter, how they can be minimized. Transient Photovoltage (TPV) has been widely used to study carrier recombination in solar cells under operando conditions. Interestingly, a novel negative transient deflection appears in perovskite solar cells when carrying out TPV measurements and it has been related to the ionic accumulation at the perovskite interfaces, which is a process that requires great attention to fully understand how perovskite solar cells work. Herein, we moved one step further and continuously monitored the evolution of the negative trace with the aging of the perovskite solar cell. Importantly, we demonstrated that the negative signal changes with aging of the solar cells and such a change can be directly related to the enhancement of the open circuit voltage and fill factor of the devices and thus the solar cell efficiency. We postulate that this increase in efficiency is due to better/faster ion redistribution within the perovskite material.

White light emitting diode based on purely organic fluorescent to modern thermally activated delayed fluorescence (TADF) and perovskite materials

White organic/polymer light emitting diode (WOLED/WPLED) processed from solution has attracted significant research interest in recent years due to their low device production cost, device flexibility, easy fabrication over large area including roll to roll and ability to print in various designs and shapes providing enormous design possibilities. Although WOLEDs fabricated using solution process lack their thermally evaporated counterparts in terms of device efficiency, remarkable progress has been made in this regard in recent years by utilizing new materials and device structures. In the present review, we have summarized and extrapolated an excellent association of old and modern concept of cost-effective materials and device structure for realization of white light. In particular, this article demonstrated and focused on design, and development of novel synthesis strategy, mechanistic insights and device engineering for solution process low cost WOLEDs device. Herein, an overview of the prevailing routes towards white light emitting devices (WLEDs) and corresponding materials used, including polymer based WLED, small molecules emitters based thermally activated delayed fluorescence (TADF), perovskite light-emitting diodes (PeLEDs) and hybrid materials based LEDs, color down-converting coatings with corresponding best efficiencies ever realized. We presume that this exhaustive review on WLEDs will offer a broad overview of the latest developments on white SSL and stonework the approach en route for innovations in the immediate future.

High efficiency perovskite quantum dot solar cells with charge separating heterostructure

Metal halide perovskite semiconductors possess outstanding characteristics for optoelectronic applications including but not limited to photovoltaics. Low-dimensional and nanostructured motifs impart added functionality which can be exploited further. Moreover, wider cation composition tunability and tunable surface ligand properties of colloidal quantum dot (QD) perovskites now enable unprecedented device architectures which differ from thin-film perovskites fabricated from solvated molecular precursors. Here, using layer-by-layer deposition of perovskite QDs, we demonstrate solar cells with abrupt compositional changes throughout the perovskite film. We utilize this ability to abruptly control composition to create an internal heterojunction that facilitates charge separation at the internal interface leading to improved photocarrier harvesting. We show how the photovoltaic performance depends upon the heterojunction position, as well as the composition of each component, and we describe an architecture that greatly improves the performance of perovskite QD photovoltaics.

Metal halide perovskites have wide tunability in both material and device structure. Here Zhao et al. fabricate heterojunctions of colloidal perovskite quantum dots with different composition using layer-by-layer deposition and demonstrate improved photovoltaic performance with enhanced photocarrier harvesting.

Copper Iodide Interlayer for Improved Charge Extraction and Stability of Inverted Perovskite Solar Cells

Nickel oxide (NiO) is one of the most promising and high-performing Hole Transporting Layer (HTL) in inverted perovskite solar cells due to ideal band alignment with perovskite absorber, wide band gap, and high mobility of charges. At the same time, however, NiO does not provide good contact and trap-free junction for hole collection. In this paper, we examine this problem by developing a double hole transport configuration with a copper iodide (CuI) interlayer for efficient surface passivation. Transient photo-current (TPC) measurements showed that Perovskite/HTL interface with CuI interlayer has an improved hole injection; CuI passivation reduces the concentration of traps and the parasitic charge accumulation that limits the flow of charges. Moreover, we found that CuI protect the HTL/perovskite interface from degradation and consequently improve the stability of the cell. The presence of CuI interlayer induces an improvement of open-circuit voltage VOC (from 1.02 V to 1.07 V), an increase of the shunt resistance RSH (100%), a reduction of the series resistance RS (-30%), and finally a +10% improvement of the solar cell efficiency.

Propeller-Shaped, Triarylamine-Rich, and Dopant-Free Hole-Transporting Materials for Efficient n-i-p Perovskite Solar Cells

By introducing six triarylamine groups to a hexaphenylbenzene (HPB) or a hexakis(2-thienyl)benzene (HTB) core, two propeller-shaped, triarylamine-rich, and low-cost hole-transporting materials (HTMs), which are termed as HPB-OMe and HTB-OMe, respectively, with considerable hole mobility, were obtained by easy synthetic routes. Solid-state planar perovskite CH₃NH₃PbI₃ solar cells (PSCs) with two new HTMs showed high power conversion efficiencies (12.9% for HPB-OMe and 17.3% for HTB-OMe in forward scans) under standard 100 mW cm^-2 AM 1.5G illumination without doping. A comparison of matched-degree of energy levels, hole-transporting ability, photovoltaic conversion efficiencies, and recombination of the two HTMs indicated that developing multi-triarylamine- and thiophene-rich molecules provides candidate and efficient dopant-free HTMs for PSCs.

MoS2 Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH3NH3PbI3 Perovskite Solar Cell with an Efficiency of over 20

Interface engineering of organic-inorganic halide perovskite solar cells (PSCs) plays a pivotal role in achieving high power conversion efficiency (PCE). In fact, the perovskite photoactive layer needs to work synergistically with the other functional components of the cell, such as charge transporting/active buffer layers and electrodes. In this context, graphene and related two-dimensional materials (GRMs) are promising candidates to tune "on demand" the interface properties of PSCs. In this work, we fully exploit the potential of GRMs by controlling the optoelectronic properties of molybdenum disulfide (MoS₂) and reduced graphene oxide (RGO) hybrids both as hole transport layer (HTL) and active buffer layer (ABL) in mesoscopic methylammonium lead iodide (CH₃NH₃PbI₃) perovskite (MAPbI₃)-based PSCs. We show that zero-dimensional MoS₂ quantum dots (MoS₂ QDs), derived by liquid phase exfoliated MoS₂ flakes, provide both hole-extraction and electron-blocking properties. In fact, on one hand, intrinsic n-type doping-induced intraband gap states effectively extract the holes through an electron injection mechanism. On the other hand, quantum confinement effects increase the optical band gap of MoS₂ (from 1.4 eV for the flakes to >3.2 eV for QDs), raising the minimum energy of its conduction band (from -4.3 eV for the flakes to -2.2 eV for QDs) above the one of the conduction band of MAPbI₃ (between -3.7 and -4 eV) and hindering electron collection. The van der Waals hybridization of MoS₂ QDs with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization-induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition of HTLs or ABLs onto the perovskite film, since the two-dimensional nature of RGO effectively plugs the pinholes of the MoS₂ QD films. Our "graphene interface engineering" (GIE) strategy based on van der Waals MoS₂ QD/graphene hybrids enables MAPbI₃-based PSCs to achieve a PCE up to 20.12% (average PCE of 18.8%). The possibility to combine quantum and chemical effects into GIE, coupled with the recent success of graphene and GRMs as interfacial layer, represents a promising approach for the development of next-generation PSCs.

Wide channel broadband CH3NH3PbI3/SnS hybrid photodetector: breaking the limit of bandgap energy operation

Perovskite-based hybrid organic-inorganic devices have recently demonstrated high potential in optoelectronics. Yet, the preparation of perovskite-based photodetectors over a desired scale without any complex architecture is still challenging. Herein, we proposed a new CH3NH3PbI3/SnS hybrid planar broadband (365 to 850 nm) photodetector, having a wide channel length of 6 mm. The growth of the device was studied by utilizing scanning electron microscopy, energy-dispersive X-ray mapping, X-ray diffraction, and optical spectroscopies. Furthermore, the efficient charge transfer from CH3NH3PbI3 to SnS was confirmed by employing time-correlated single photon counting. The pure SnS device generates 0.05 μA photocurrent at 365 nm, 4 mW cm-2, which is notably enhanced 140 times after embedding with CH3NH3PbI3. Further, the hybrid device shows a significant photoresponse even below the band gaps of individual CH3NH3PbI3 or SnS, which matches well with the density functional theory prediction. The observed results will create new opportunities to develop and design a low-cost, broadband, and efficient photodetector over a chosen horizontal area.

Intramolecular and interfacial dynamics of triarylamine-based hole transport materials

Hole injection across interfaces is fast but critically depends on good contact between the hole transfer and light-harvesting materials.

White perovskite based lighting devices

Hybrid organic–inorganic and all-inorganic metal halide perovskites have been one of the most intensively studied materials during the last few years.