Dopant-free Hole-transporting Materials Research Articles

N,N-Di-para-methylthiophenylamine-Substituted (2-Ethylhexyl)-9H-Carbazole: A Simple, Dopant-Free Hole-Transporting Material for Planar Perovskite Solar Cells

A dopant-free hole-transporting material (HTM), with (2-ethylhexyl)-9H-carbazole as core and N,N-di-p-methylthiophenylamine as end groups, termed CMT, has been designed and synthesized by a simple method. For the first time, four methylthiol groups have been introduced, rather than methoxy groups, at the para position of the diphenylamine. Under AM 1.5 illumination at 100 mW cm–2, perovskite solar cells based on CH3NH3PbI3 with pristine CMT as the HTM achieved a power conversion efficiency of 13.05%, with a short-circuit current density of 21.82 mA cm–2, an open-circuit voltage (VOC) of 1.03 V, and a fill factor of 58.23%. The value of VOC is comparable to that of the device based on 2,2′,7,7′-tetrakis(N,N-di-para-methoxy-phenylamino)-9,9′-spirobifluorene, which was 1.02 V.

A Rising Star: Truxene as a Promising Hole Transport Material in Perovskite Solar Cells

Truxene and its derivatives have been extensively employed for various applications and are considered as promising dopant-free hole transport materials (HTMs) in perovskite solar cells (PSCs). Yet, a systematic exploration of their performance for this specific application remain lacking. Here, multiscale simulations are used to investigate the key structure–property relationships of truxene derivatives featuring distinct variations to the parent core. Specifically, the role of heteroatoms, alkyl chains, and substitution site on the most relevant electronic, transport, and stability properties to high-performing PSCs are assessed. Our findings demonstrate that each of the considered truxenes are potential alternatives to the current state-of-the-art HTM Spiro-OMeTAD.

Quinoidal 2,2′,6,6′‐Tetraphenyl‐Dipyranylidene as a Dopant‐Free Hole‐Transport Material for Stable and Cost‐Effective Perovskite Solar Cells

AbstractWe report on the use of 2,2′,6,6′‐tetraphenyldipyranylidene (DIPO‐Ph4), a large quinoidal planar π‐conjugated heterocycle, as a simple, easy‐to‐synthetize, and efficient dopant‐free hole‐transport material in perovskite solar cells (PSCs). PSCs using pristine DIPO‐Ph4 show photon conversion efficiencies up to 10.1 %, which is higher than a PSC utilizing dopant‐free spiro‐OMeTAD (5.1 %). DIPO‐Ph4‐based PSCs exhibit a short‐circuit current density of 19.52 mA cm−2 and an open‐circuit voltage of 0.933 V, values that are superior to dopant‐free spiro‐OMeTAD and comparable to doped spiro‐OMeTAD. These better performances find their origin in a higher HOMO level (−4.74 eV) and a much higher hole mobility (2×10−2 cm2 V−1 s−1). Finally, PSCs based on DIPO‐Ph4 possess a superior stability compared to doped‐spiro‐OMeTAD‐based devices when tested over 600 h.

Dopant-Free Hole-Transporting Materials for Stable and Efficient Perovskite Solar Cells.

Molecularly engineered novel dopant-free hole-transporting materials for perovskite solar cells (PSCs) combined with mixed-perovskite (FAPbI3 )0.85 (MAPbBr3 )0.15 (MA: CH3 NH3+ , FA: NH=CHNH3+ ) that exhibit an excellent power conversion efficiency of 18.9% under AM 1.5 conditions are investigated. The mobilities of FA-CN, and TPA-CN are determined to be 1.2 × 10-4 cm2 V-1 s-1 and 1.1 × 10-4 cm2 V-1 s-1 , respectively. Exceptional stability up to 500 h is measured with the PSC based on FA-CN. Additionally, it is found that the maximum power output collected after 1300 h remained 65% of its initial value. This opens up new avenue for efficient and stable PSCs exploring new materials as alternatives to Spiro-OMeTAD.

Non-Conjugated Polymer as an Efficient Dopant-Free Hole-Transporting Material for Perovskite Solar Cells.

A new non-conjugated polymer (PVCz-OMeDAD) with good solution processability was developed to serve as an efficient dopant-free hole-transporting material (HTM) for perovskite solar cells (PSCs). PVCz-OMeDAD was simply prepared by the free-radical polymerization of vinyl monomers, which were synthesized from low-cost raw materials through three high-yield synthesis steps. The combination of the flexible non-conjugated polyvinyl main chain and hole-transporting methoxydiphenylamine-substituted carbazole side chains endowed PVCz-OMeDAD with excellent film-forming ability, a suitable energy level, and high hole mobility. As a result, by using an ultra-thin (≈30 nm) PVCz-OMeDAD film as cost-effective dopant-free polymer HTM, the conventional n-i-p-type PSCs demonstrated a power conversion efficiency (PCE) up to 16.09 %, suggesting the great potential of the polymer film for future low-cost, large-scale, flexible PSCs applications.

Simple dopant-free hole-transporting materials with p-π conjugated structure for stable perovskite solar cells

Two simple hole-transporting materials, Me-QTPA and Me-BPZTPA, which consist of p-π conjugated structure, have been synthesized and studied in solid-state perovskite solar cells. Me-QTPA and Me-BPZTPA show outstanding thermal stabilities and appropriate HOMO levels; in addition, these two materials show wide band gaps, thus they can block the electron transport and hence suppress the carrier recombination. The solution-processed CH3NH3PbI3-based device using dopant-free Me-QTPA and Me-BPZTPA can achieve a power conversion efficiency of 9.07% and 8.16%, respectively. The perovskite solar cells with dopant-free Me-QTPA show better performance than the cells with dopant-free spiro-OMeTAD, especially in long-term stability. The power conversion efficiency for the perovskite solar cells with dopant-free Me-QTPA remains almost constant after 600h. The dopant-free Me-QTPA layer shows strong hydrophobicity with a contact angle of 101.6° to water, which indicates that Me-QTPA has a promising long-term stability at room temperature.

Readily synthesized dopant-free hole transport materials with phenol core for stabilized mixed perovskite solar cells

Abstract With the dramatic development of the power conversion efficiency (PCE) of perovskite solar cells (PVSCs), device lifetime has become one of the extensive research interests and concerns. To enhance the device durability, developing high performance dopant-free hole transport materials (HTMs) is a promising strategy. Herein, two new C3 -symmetric HTMs with phenol core, TCP-OH and TCP-OC 8 are readily prepared and show ultra-wide energy band-gap and excellent film-formation property. PCEs of 16.97% and 15.28% are achieved with pristine TCP-OH and TCP-OC 8 film as HTMs, respectively, even though their hole mobilities are as low as 10 −6 cm 2 V −1 s −1 . Phenol acts as hole trap in traditional concept, however, TCP-OH shows higher hole mobility than that of TCP-OC 8 . Moreover, TCP-OH shows higher glass transition temperature and better matching band alignment than those of TCP-OC 8 . Phenol shows great potential as building block for HTMs as it is beneficial to enhance hole mobility of HTMs. Moreover, our study demonstrates an interesting viewpoint to design HTMs with the balance of hole mobility and electron blocking effect.

Recent progress of dopant-free organic hole-transporting materials in perovskite solar cells**Project supported by the Scientific Research Starting Foundation for Overseas Introduced Talents of College of Chemistry, Nankai University.

Organic–inorganic hybrid perovskite solar cells have undergone especially intense research and transformation over the past seven years due to their enormous progress in conversion efficiencies. In this perspective, we review the latest developments of conventional perovskite solar cells with a main focus on dopant-free organic hole transporting materials (HTMs). Regarding the rapid progress of perovskite solar cells, stability of devices using dopant-free HTMs are also discussed to help readers understand the challenges and opportunities in high performance and stable perovskite solar cells.

A novel perovskite solar cell design using aligned TiO2 nano-bundles grown on a sputtered Ti layer and a benzothiadiazole-based, dopant-free hole-transporting material.

This work highlights the utilization of a novel hole-transporting material (HTM) derived from benzothiadiazole: 4-(3,5-bis(trifluoromethyl)phenyl)-7-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole (CF-BTz-ThR) and aligned TiO2 nano-bundles (TiO2 NBs) as the electron transporting layer (ETL) for perovskite solar cells (PSCs). The aligned TiO2 NBs were grown on titanium (Ti)-coated FTO substrates using a facile hydrothermal method. The newly designed CF-BTz-ThR molecule with suitable highest occupied molecular orbital (HOMO) favored the effective hole injection from perovskite deposited aligned TiO2 NBs thin film. The PSCs demonstrated a power conversion efficiency (PCE) of ∼15.4% with a short circuit current density (Jsc) of ∼22.42 mA cm-2 and an open circuit voltage (Voc) of ∼1.02 V. The efficiency data show the importance of proper molecular engineering whilst highlighting the advantages of dopant-free HTMs in PSCs.

A solution-processable copper(ii) phthalocyanine derivative as a dopant-free hole-transporting material for efficient and stable carbon counter electrode-based perovskite solar cells

A solution-processable copper(ii) phthalocyanine derivative CuPc-TIPS has been explored as a dopant-free hole-transporting material in carbon counter electrode-based perovskite solar cells.

A simple and dopant-free hole-transporting material based on (2-ethylhexyl)-9H-carbazole for efficient planar perovskite solar cells

Planar perovskite solar cells based on CMO as the HTM showed a high power conversion efficiency of 15.92%.

Efficiency Enhancement of Hybrid Perovskite Solar Cells with MEH-PPV Hole-Transporting Layers.

In this study, hybrid perovskite solar cells are fabricated using poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly(3-hexylthiophene-2,5-diyl) (P3HT) as dopant-free hole-transporting materials (HTMs), and two solution processes (one- and two-step methods, respectively) for preparing methylammonium lead iodide perovskite. By optimizing the concentrations and solvents of MEH-PPV solutions, a power conversion efficiency of 9.65% with hysteresis-less performance is achieved, while the device with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′spirobifluorene (Spiro-OMeTAD) doped with lithium salts and tert-butylpyridine (TBP) exhibits an efficiency of 13.38%. This result shows that non-doped MEH-PPV is a suitable, low-cost HTM for efficient polymer-based perovskite solar cells. The effect of different morphologies of methylammonium lead iodide perovskite on conversion efficiency is also investigated by incident photon-to-electron conversion efficiency (IPCE) curves and electrochemical impedance spectroscopy (EIS).

Rational Design of Dipolar Chromophore as an Efficient Dopant-Free Hole-Transporting Material for Perovskite Solar Cells.

In this paper, an electron donor-acceptor (D-A) substituted dipolar chromophore (BTPA-TCNE) is developed to serve as an efficient dopant-free hole-transporting material (HTM) for perovskite solar cells (PVSCs). BTPA-TCNE is synthesized via a simple reaction between a triphenylamine-based Michler's base and tetracyanoethylene. This chromophore possesses a zwitterionic resonance structure in the ground state, as evidenced by X-ray crystallography and transient absorption spectroscopies. Moreover, BTPA-TCNE shows an antiparallel molecular packing (i.e., centrosymmetric dimers) in its crystalline state, which cancels out its overall molecular dipole moment to facilitate charge transport. As a result, BTPA-TCNE can be employed as an effective dopant-free HTM to realize an efficient (PCE ≈ 17.0%) PVSC in the conventional n-i-p configuration, outperforming the control device with doped spiro-OMeTAD HTM.

Application of benzodithiophene based A–D–A structured materials in efficient perovskite solar cells and organic solar cells

In this work, we have designed and synthesized a novel molecular material, BDT-C1, in which the core unit, benzodithiophene (BDT), was functionalized by thiophene (TP) and benzo-[c][1,2,5]-thiadiazole (BTZ) derivatives to generate extended π–conjugation. BDT-C1 shows high hole mobility and high conductivity in its pristine form, in combination with appropriate energy level alignment with respect to [CH3NH3]PbI3 and PC70BM, qualifying the material as a good candidate for application both in perovskite solar cells (PSCs) as dopant-free hole transport material (HTM) and in OSCs as donor material. The champion PSCs based on BDT-C1 show an average conversion efficiency (PCE) of 13.4% (scan forward: 13.9%; scan backward: PCE=12.9%, scan rate: 10mV/s). Although the average efficiency obtained is slightly lower than that of reference devices based on the well-known doped HTM Spiro-OMeTAD (13.7%), the BDT-C1 based devices exhibit better stability. Moreover, BDT-C1 as a donor material in OSCs also shows good performance in combination with PC70BM as acceptor material, and an efficiency of 6.1% was obtained. The present results demonstrate that BDT-C1 works well as both donor material in OSCs as well as dopant-free HTMs for efficient PSCs.

Investigation on a dopant-free hole transport material for perovskite solar cells

In this work, we demonstrate a dopant free hole transport material for planar perovskite solar cells using a tetraphenylethene derivative, delivering an overall power conversion efficiency of 9.12% in the absence of additives.

A novel one-step synthesized and dopant-free hole transport material for efficient and stable perovskite solar cells

A simple one-step synthesized hole transport material was developed for dopant-free perovskite solar cells with a PCE of 15.4%.

High conductivity Ag-based metal organic complexes as dopant-free hole-transport materials for perovskite solar cells with high fill factors

Hole-transport materials (HTMs) play an important role as hole scavenger materials in the most efficient perovskite solar cells (PSCs). Here, for the first time, two Ag-based metal organic complexes (HA1 and HA2) are employed as a new class of dopant-free hole-transport material for application in PSCs. These HTMs show excellent conductivity and hole-transport mobility. Consequently, the devices based on these two HTMs exhibit unusually high fill factors of 0.76 for HA1 and 0.78 for HA2, which are significantly higher than that obtained using spiro-OMeTAD (0.69). The cell based on HA1-HTM in its pristine form achieved a high power conversion efficiency of 11.98% under air conditions, which is comparable to the PCE of the cell employing the well-known doped spiro-MeOTAD (12.27%) under the same conditions. More importantly, their facile synthesis and purification without using column chromatography makes these new silver-based HTMs highly promising for future commercial applications of PSCs. These results provide a new way to develop more low-cost and high conductivity metal-complex based HTMs for efficient PSCs.

Perovskite Solar Cells Employing Dopant-Free Organic Hole Transport Materials with Tunable Energy Levels.

Conjugated small-molecule hole-transport materials (HTMs) with tunable energy levels are designed and synthesized for efficient perovskite solar cells. A champion device with efficiency of 16.2% is demonstrated using a dopant-free DERDTS-TBDT HTM, while the DORDTS-DFBT-HTM-based device shows an inferior performance of 6.2% due to its low hole mobility and unmatched HOMO level with the valence band of perovskite film.

A novel phenoxazine-based hole transport material for efficient perovskite solar cell

Based on the previous research work in our laboratory, we have designed and synthesized a small-molecule, hole transport material (HTM) POZ6-2 using phenoxazine (POZ) as central unit and dicyanovinyl units as electron-withdrawing terminal groups. Through the introduction of a 2-ethyl-hexyl bulky chain into the POZ core unit, POZ6-2 exhibits good solubility in organic solvents. In addition, POZ6-2 possesses appropriate energy levels in combination with a high hole mobility and conductivity in its pristine form. Therefore, it can readily be used as a dopant-free HTM in perovskite solar cells (PSCs) and a conversion efficiency of 10.3% was obtained. The conductivity of the POZ6-2 layer can be markedly enhanced via doping in combination with typical additives, such as 4-tert-butylpyridine (TBP) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). Correspondingly, the efficiency of the PSCs was further improved to 12.3% using doping strategies. Under the same conditions, reference devices based on the well-known HTM Spiro-OMeTAD show an efficiency of 12.8%.

A dopant-free hole-transporting material for efficient and stable perovskite solar cells

A tetrathiafulvalene derivative (TTF-1) was introduced into perovskite solar cells as a dopant-free hole-transporting material, yielding an efficiency over 11%.