Recent progress on footsteps of organic light emitting diodes by engineering hole transport materials

New three-coordinate organoboron compounds functionalized by a (1-naphthyl)phenylamino group, B(mes)2(dbp-NPB) (1), B(db-NPB)3 (2), and B(dbp-NPB)3 (3), have been synthesized. A variable temperature 1H NMR study showed that the aryl groups around the boron center in these compounds have a rotation barrier ∼70 kJ mol−1. The new boron compounds are amorphous solids with Tg being 110 °C, 171 °C and 173 °C, respectively. The electronic properties of the new boron compounds were investigated by cyclic voltammetry and UV–visible spectroscopy. All three boron compounds are blue emitters in the solid state. In solution the emission spectra of the boron compounds shift toward a longer wavelength with increasing solvent polarity. In CH2Cl2, the emission quantum efficiency of the three compounds was determined to be 0.22, 0.27 and 0.23, respectively. Several series of electroluminescent (EL) devices where compounds 1–3 are used as either an emitter/electron transport material, a hole transport material, or a hole injection material have been fabricated and their performance has been compared to the corresponding devices of BNPB, a previously investigated molecule, NPB, a commonly used hole transport material, and CuPc, a commonly used hole injection material. The EL results indicate that the new boron compounds are not suitable as emitters/electron transport materials, but they are promising as hole transport and hole injection materials in EL devices.

As lead-based perovskite solar cells (PSCs) have a detrimental effect on the environment, lead-free Titaniumbased PSCs are drawing more attention to the researchers. Only experimental literature using lead-free Cesium Titanium Bromide (Cs2TiBr6) as the absorber layer has been published with an efficiency of 3.3%. However, analyzing the performance of this PSC is necessary through device simulation. In our work, we have studied the appropriate charge transport materials for Cs2TiBr6 based n-i-p structured PSC using SCAPS-1D simulation software. We find that MoO3 and SnO2 are suitable to be the hole transport material (HTM) and electron transport material (ETM) respectively. We also studied the optimum thickness of absorber layer, hole transport layer (HTL), and electron transport layer (ETL). The result provides an efficiency of - 5.82% which may have a positive impact on further research of Cs2TiBr6 based perovskite solar cell.

Tailoring electronic structure and charge transport in thiophene-based hole transport materials: An end-capping acceptor strategy for efficient perovskite solar cells

New organic molecules containing five different compounds, commonly called p-linkers, located between the triphenylamine units, were theoretically designed and analyzed in order to be proposed as new hole transport materials (HTMs) in perovskite solar cells, in total ten new molecules were analyzed. The electronic, optical and hole transport properties were determined, similarly, the relationship of these properties with their molecular structure was also investigated by Density Functional Theory (DFT) and Density Functional Tight Binding (DFTB) calculations. Eight of the ten analyzed compounds exhibited the main absorption band out of the visible region; therefore these compounds did not present an overlap with the absorption spectra of the typical methylammonium lead iodide (MAPI) hybrid-perovskite. The results showed that the Highest occupied molecular orbital (HOMO) levels of the compounds are higher than the perovskite HOMO level, and in some cases these are even higher than the Spiro-OMeTAD HOMO. The calculated electronic couplings and the reorganization energy values provided useful information in order to determine if the systems were hole or electron transport materials.

Theoretical investigations into optical and charge transfer properties of donor-acceptor 1,8-naphthalimide derivatives as possible organic light-emitting materials

Hole and electron transport materials: A review on recent progress in organic charge transport materials for efficient, stable, and scalable perovskite solar cells

Recently, organic-inorganic perovskite-based solar cells have become promising devices due to their unique properties in the photovoltaic field. However, the factor of toxicity, stability, high production cost and complicated fabrication processes of these devices is a challenge to their progress in commercial production. Here a numerical modelling of Caesium Tin–Germanium Tri-Iodide (CsSnGeI₃) as an efficient perovskite light absorber material is carried out. In this paper, different inorganic Hole Transport Materials (HTMs) such as Cu₂O, CuI, CuSbS₂, CuSCN and NiO have been analyzed with C₆₀ as the Electron Transport Material (ETM). We intend to replace the conventional hole and electron transport materials such as TiO₂ and Spiro-OMeTAD which have been known to be susceptible to light induced degradation. Moreover, the influence of the Electron Transport Layer (ETL) and the perovskite layer properties, bandgap, doping concentration and working temperature for various Hole Transport Layers (HTL) on the overall cell performance have been rigorously investigated. The design of the proposed PSC is performed utilizing SCAPS- 1D simulator and for optimum device an efficiency greater than 30% was obtained. The results indicate that CsSnGeI₃ and C₆₀ are viable candidates for use as an absorber layer and electron transport layer in high-efficiency perovskite solar cells, with none of the drawbacks that other PSCs have.

As the key properties of perovskite solar cells (PSCs), the hole extraction and transport capabilities of the hole transport material (HTM) affect the photovoltaic performance of PSCs to a considerable extent, while both capabilities can be adjusted by molecular planarity. Therefore, in this work, the molecular planarity of the HTM is systematically optimized to regulate the hole extraction and transport capabilities. Along with the improvement in planarity, the HTM's HOMO level is increased, leading to the enhancement of hole extraction capability. Meanwhile, the hole transport capability can also be improved due to the intensification of molecular stacking during the film formation. As a result, the planar HTM achieves a relatively high efficiency of 18.48 %, which is higher than that of spiro-OMeTAD. Accordingly, the molecular planarity presents an important impact on the photovoltaic performance of PSCs, providing us with a promising strategy for further optimization of efficient HTMs.

As a third-generation solar cell technology, perovskite solar cells are facing booming growth. However, typical perovskite solar cells have two critical challenges on the road to actual commercialization: toxicity and stability. Finding high-performance lead-free inorganic perovskites to replace conventional lead-based perovskites is an effective way to overcome both challenges. A lead-free inorganic perovskite Cs2TiBr6 has been demonstrated that possesses the impressive photoelectric property of direct bandgap and excellent environmental stability from recent experimental results. In addition, Cs2TiBr6 has been used as the absorber layer in a perovskite solar cell with an efficiency of 3.28%, which is lower than the efficiencies of typical lead-based perovskite cells. Therefore, it is significant to further explore the structure of Cs2TiBr6 solar cell for improving its power conversion efficiency. This work proposes a solar cell with the FTO/ETL/Cs2TiBr6/HTL/Au structure, then selects the most suitable hole and electron transport materials among some typical transport materials. Moreover, we optimize the absorber layer thickness. The optimized solar cell with the FTO/SnO2/Cs2TiBr6/PEDOT:PSS/Au structure possesses an efficiency of 7.92%. Our results show that lead-free inorganic perovskite Cs2TiBr6 used as an absorber in the solar cell has great potential. In addition, appropriate hole transport material, electron transport material, and absorber layer thickness can improve its performance.

A new series of carbazolo[4,3-c]carbazoles (1–3) have been examined as hole-transporting and emitting host materials in the fabrication of red phosphorescent OLEDs (PhOLEDs). The presence of different N-substituents including hydrogen, octyl and 4-butylphenyl attached to the carbazolo[4,3-c]carbazole skeleton was aimed to condition the charge transporting properties. Due to their resemblance in the electronic structure, these carbazolocarbazoles have been compared to a commonly used hole-transporting material such as N,N′-bis(naphthalen-1-yl)-N,N′-bisphenylbenzidine (NPB). Interestingly, the introduced structural differences endow compounds 1–3 with a wide range of hole-mobilities, which provide room for adjusting the carrier balance of OLEDs. Different approaches, including bi-layer and tri-layer architectures, have been employed for the successful fabrication of Ir(piq)2(acac) doped OLEDs using NPB and these novel carbazolocarbazoles as hole transport and host materials. The simplified bi-layer device demonstrated high performance with maximum efficiencies of 8.7%, 5.6 cd A−1 and 3.4 lm W−1 when using the N-alkylated derivative 2. Furthermore, the red PhOLEDs with tri-layer architecture using 3 as the HTL showed peak efficiencies of 12.2%, 8.7 cd A−1, and 9.3 lm W−1. In addition, both compounds 2 and 3 used in OLEDs exhibited superior performance to those of devices using NPB, demonstrating their high potential for employment in phosphorescent OLEDs.

A comparative study of different ETMs in perovskite solar cell with inorganic copper iodide as HTM

In the photovoltaic (PV) research community, the dramatic improvement in the power conversion efficiency of organic-inorganic metal halide perovskite-based devices has made them very appealing. The serious challenge, though, has to do with durability under different circumstances and difficulties with toxicity. In recent years, a significant number of papers have been published in PSCs based on these issues through the use of different electron transport materials(ETMs), hole transport materials (HTMs), and Perovskite materials. The hole transport material is responsible for a significant part of the expense of the components of a Perovskite solar cell (PSC), as the most effective systems so far have costly HTMs, such as spiro-OMeTAD and poly(triaryl amine). The inverted PSCs (p-i-n) give a wide variety of alternate HTLs, as the HTL is deposited underneath the perovskite layer. Any material with moderate hole mobility is, therefore, a possible substitute for replacing the expensive HTMs that have been used so far. In this review manuscript, we have compiled the various Cu-based materials added as HTMs in planar inverted p-i-n) structure. This manuscript appears to focus on the role and classification of various copper-based materials used as a hole transport layer affecting long-term stability, improving solar cell parameters, and thus improving overall device performance. Furthermore, a description of the overall device structure, preparation methods, and the effect of the thickness of the HTM layers on the overall solar cell parameters of the perovskite devices is also presented. We hope that this analysis will explicitly and extensively demonstrate their significance and strengths in the concerned domain by introducing and exploring the developments of Cu-based materials as HTMs in planar PSCs and provide inspiration for their further development.

Developing suitable hole transport materials is of utmost importance in the quest to enhance the performance of CsPbI2Br perovskite solar cells (PSCs). Among the various undoped hole transport materials (HTMs), D‐π‐A type polymers incorporating benzodithiophene (BDT) as the D unit and benzotriazole (BTA) as the A unit have shown promising potential. To further optimize the energy level and enhance the hole transport ability of these HTMs, we employed a fluorine substitution strategy to synthesize P‐BTA‐2F and P‐BTA‐4F based on the polymer P‐BTA‐0F. Subsequently, we investigated the impact of varying degrees of fluorine substitution on the properties of the polymer materials and the performance of the devices. As the number of F substitutions increases, the polymer energy level of the HTM gradually shifts downward, the face‐on stacking of the HTM strengthens, the hole mobility of the HTM increases, and the rate of hole extraction and transport becomes faster. Ultimately, the CsPbI2Br PSCs based on the P‐BTA‐4F HTM achieve the highest power conversion efficiency (PCE) of 17.68%. Those findings demonstrate that selecting an appropriate amount of fluorine substitution is crucial for regulating the performance of polymer HTMs and improving device efficiency.

Effect of arylamino-carbazole containing hole transport materials on the device performance and lifetime of OLED

Soln.-​processable perovskite solar cells have recently achieved certified power conversion efficiencies of over 20​%, challenging the long-​standing perception that high efficiencies must come at high costs. One major bottleneck for increasing the efficiency even further is the lack of suitable hole-​transporting materials, which ext. pos. charges from the active light absorber and transmit them to the electrode. In this work, we present a molecularly engineered hole-​transport material with a simple dissym. fluorene-​dithiophene (FDT) core substituted by N,​N-​di-​p-​methoxyphenylamine donor groups, which can be easily modified, providing the blueprint for a family of potentially low-​cost hole-​transport materials. We use FDT on state-​of-​the-​art devices and achieve power conversion efficiencies of 20.2​% which compare favorably with control devices with 2,​2',​7,​7'-​tetrakis(N,​N-​di-​p-​methoxyphenylamine)​-​9,​9'-​spirobifluorene (spiro-​OMeTAD)​. Thus, this new hole transporter has the potential to replace spiro-​OMeTAD.