Low-cost Hole-transporting Materials Research Articles

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.

Highly Efficient Phenoxazine Core Unit Based Hole Transport Materials for Hysteresis-Free Perovskite Solar Cells.

Two novel simple-constructed and low-cost hole transport materials (HTMs) POZ9 and POZ10, incorporating a phenoxazine (POZ) core unit, were designed and synthesized for application in perovskite solar cells (PSCs). The typically semblable molecular structure of POZ9 and POZ10 cause them to possess similar energy levels. However, their photovoltaic performances are quite different from each other because of the small variations of N-substitution on POZ ring. The PSCs based on POZ10, which contains three N, N-di-4-methoxyphenylamino units, achieved a power conversion efficiency (PCE) of 19.4%, while the PSC adopting POZ9 as HTM obtained a lower PCE of 17.1%. Moreover, the light intensity dependence research showed that POZ10 has a better hole transporting ability and can efficiently resist the charge recombination.

Triazatetrabenzcorrole (TBC) as efficient dopant-free hole transporting materials for organo metal halide perovskite solar cells

An easily prepared, cost-effective and solution processable tetraphenoxy substituted phosphorous triazatetrabenzo corrole (PVTBC) was synthesized and tested as the hole transporting material (HTM) for MAPbI3 perovskite solar cells (PSC). This phthalocyanine analogue HTM works efficiently in the absence of any dopant or additive for the device composed of FTO/c-TiO2/m-TiO2/MAPbI3/HTM/Au. The optimized undoped mesoscopic solar cell device based on the PVTBC compound exhibited a good power conversion efficiency (PCE) of 16.1% (measured at 100 mW cm2 illumination, AM 1.5G), together with a good long-term stability under ambient conditions, in contrast to the PCE of 11.2% for Spiro-OMeTAD under the same condition. This result paves the way for further developing TBC as a new type of low cost HTMs.

Nitrogen doped cuprous oxide as low cost hole-transporting material for perovskite solar cells

Substituting expensive traditional hole transporting material (HTM) with cheaper inorganics is a key factor for perovskite photovoltaics commercialization. Cu2O is a promising p-type semiconductor exhibiting good band-alignment with perovskite. However, due to solvent and temperature incompatibility, Cu2O is typically employed in inverted configuration, where an even more expensive, unstable Phenyl-C61-butyric acid methyl ester is necessary as an electron-transporting layer. Therefore, we explored the use of sputtered nitrogen-doped Cu2O directly on halide-perovskite as a HTM. With a thin interfacial layer, efficiency of 15.73% was achieved. This work indicates the possibility of low cost sputtered inorganics as HTM for efficient perovskite photovoltaics.

Optical and structural properties of ZnO thin films prepared by spray pyrolysis for enhanced efficiency perovskite solar cell application

In the present study, ZnO thin films are prepared successfully by simple and flexible spray pyrolysis technique on fluorine doped tin oxide glass substrate. The thermal spray deposition process is optimized by varying all the major parameters simultaneously: (1) precursor concentration; (2) substrate temperature; (3) carrier gas pressure; and (4) solution flow rate. Optical, structural and morphological properties are appropriately studied in detail. Most influencing parameters were observed to be the precursor concentration and substrate temperature, affecting film thickness, roughness, the band gap, and efficiency. It is confirmed that prepared films are uniform and crystalline in nature with a preferred orientation in (002) plane. The optical band gap is favourably in the range of 3.0–3.2 eV. At optimum conditions, the appropriate band gap (3.05 eV), thickness (89 nm) and roughness (2.38 nm) is achieved. The prepared films are consequently applied for perovskite solar cell preparation. Additionally, CuI (low-cost hole transport material) and carbon (counter electrode) are utilized. The efficiency of the device at optimum conditions is obtained to be maximum (3.21%). Maximum open circuit voltage (0.52 V), the maximum short circuit current (10.11 mA cm−2) and maximum fill factor (61%) is also achieved by the optimum device.

Improved performance and air stability of perovskite solar cells based on low-cost organic hole-transporting material X60 by incorporating its dicationic salt

The development of an efficient, stable, and low-cost hole-transporting material (HTM) is of great significance for perovskite solar cells (PSCs) from future commercialization point of view. Herein, we specifically synthesize a dicationic salt of X60 termed X60(TFSI)2, and adopt it as an effective and stable “doping” agent to replace the previously used lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) for the low-cost organic HTM X60 in PSCs. The incorporation of this dicationic salt significantly increases the hole conductivity of X60 by two orders of magnitude from 10−6 to 10−4 S cm−1. The dramatic enhancement of the conductivity leads to an impressive power conversion efficiency (PCE) of 19.0% measured at 1 sun illumination (100 mW cm−2, AM 1.5 G), which is comparable to that of the device doped with LiTFSI (19.3%) under an identical condition. More strikingly, by replacing LiTFSI, the PSC devices incorporating X60(TFSI)2 also show an excellent long-term durability under ambient atmosphere for 30 days, mainly due to the hydrophobic nature of the X60(TFSI)2 doped HTM layer, which can effectively prevent the moisture destroying the perovskite layer. The present work paves the way for the development of highly efficient, stable, and low-cost HTM for potential commercialization of PSCs.

Exploring the properties of carbazole-based derivatives as hole transport materials from first principle and MD simulation

Abstract Although the perovskite solar cell (PSC) without hole transport materials (HTM) has been developed, the power conversion efficiency (PCE) is still less than 15%. Exploring efficient and low cost HTMs is still the urgent requirement for the further development of efficient PSCs. Three new HTMs, H5, H6, and H7, are designed on the basis of X25, in which N-p-methoxyphenyl-1-naphthylamine, 9,9-dimethyl-acridine, and 9,9-dimethyl-10-phenyl-9,10-dihydroacridine are utilized as side groups, respectively, along with the 9-(4-methoxyphenyl)-9H-carbazole core. The performance of new designed molecules is evaluated from various aspects including frontier molecular orbital, absorption spectrum, and hole mobility. More importantly, the interfacial properties between HTM and CH3NH3PbI3 (110) surface are explored. After adsorption, the band gap of CH3NH3PbI3 (110) surface is almost kept but the energy levels of HTMs are varied indicating that it is necessary to study the adsorption properties rather than only the isolated HTM molecule. Actually, the side groups in H5, H6, and H7 have been employed as side groups to constitute molecules applied in some relevant regions, which would provide an alternative pathway to develop new HTM rather than synthesis of new groups.

Over 20% PCE perovskite solar cells with superior stability achieved by novel and low-cost hole-transporting materials

The exploration of alternative low-cost molecular hole-transporting materials (HTMs) for both highly efficient and stable perovskite solar cells (PSCs) is a relatively new research area. Two novel HTMs using the thiophene core were designed and synthesized (Z25 and Z26). The perovskite solar cells based on Z26 exhibited a remarkable overall power conversion efficiency (PCE) of 20.1%, which is comparable to 20.6% obtained with spiroOMeTAD. Importantly, the devices based-on Z26 show better stability compared to devices based on Z25 and spiroOMeTAD when aged under ambient air of 30% or 85% relative humidity in the dark and under continuous full sun illumination at maximum power point tracking respectively. The presented results demonstrate a simple strategy by introducing double bonds to design hole-transporting materials for highly efficient and stable perovskite solar cells with low cost, which is important for commercial application.

Molecular engineering to enhance perovskite solar cell performance: Incorporation of benzothiadiazole as core unit for low cost hole transport materials

Through molecular engineering, a benzothiadiazole unit is incorporated as the core structure to generate a new hole transport material JY5. Cyclic voltammetry measurement suggests matched energy level of JY5 with the perovskite materials. TGA and DSC show excellent thermal stability of JY5. Hole mobility and PL decay experiments indicate good hole transport ability in JY5. As a result, perovskite solar cells utilizing JY5 as HTM outperforms the biphenyl centered analogue, and a champion efficiency of 16.87% was achieved among the HTMs studied in this work, with short-circuit current (Jsc) of 21.06 mA cm−2, open-circuit voltage (Voc) of 1.06 V, and fill factor (FF) of 0.76. The high efficiency as well as low costs make it potential alternative to the highly cost Spiro-OMeTAD.

3D hole-transporting materials based on coplanar quinolizino acridine for highly efficient perovskite solar cells.

Over the past five years, perovskite solar cells (PSCs) have gained intense worldwide attention in the photovoltaic community due to their low cost and high power conversion efficiencies (PCEs). One of the most significant issues in achieving high PCEs of PSCs is the development of suitable low-cost hole-transporting materials (HTMs). Here, we put forward a new concept of HTMs for PSCs: a 3D structure with a core of coplanar quinolizino acridine, derived from the conventional concept of 2D triphenylamine HTMs. A cheaper Ag nanolayer was utilized to replace Au as the counter electrodes, and the title HTM TDT-OMeTAD was synthesized via an easy four-step synthesis (total yield: 61%) to achieve the low cost and convenient manufacture of PSCs. Compared with the conventional 2D triphenylamine HTM, TTPA-OMeTPA, PSC devices based on the 3D HTM TDT-OMeTPA showed a significant improvement in PCE from 10.8% to 16.4%, even outperforming Spiro-OMeTAD (14.8%). TDT-OMeTAD's highest PCE mainly results from it having the highest open-circuit voltage (Voc) of 1.01 V in this work, which is proven to be due to the higher hole mobility, matching energy levels, higher hydrophobicity and the smaller dark current. Moreover, an incident photon-current conversion efficiency (IPCE) test and time-resolved photoluminescence (TRPL) have been carried out to observe the better hole injecting efficiency and photoelectric conversion capability of TDT-OMeTPA based PSCs than Spiro-OMeTAD. The TDT-OMeTPA based PSCs exhibited >75% reproducibility (PCE > 15%) and retained 93.2% of the initial PCE after >500 hours.

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).

3,4-Phenylenedioxythiophene (PheDOT) Based Hole-Transporting Materials for Perovskite Solar Cells.

Two new electron-rich molecules based on 3,4-phenylenedioxythiophene (PheDOT) were synthesized and successfully adopted as hole-transporting materials (HTMs) in perovskite solar cells (PSCs). X-ray diffraction, absorption spectra, photoluminescence spectra, electrochemical properties, thermal stabilities, hole mobilities, conductivities, and photovoltaic parameters of PSCs based on these two HTMs were compared with each other. By introducing methoxy substituents into the main skeleton, the energy levels of PheDOT-core HTM were tuned to match with the perovskite, and its hole mobility was also improved (1.33×10(-4) cm(2) V(-1) s(-1) , being higher than that of spiro-OMeTAD, 2.34×10(-5) cm(2) V(-1) s(-1)). The PSC based on MeO-PheDOT as HTM exhibits a short-circuit current density (Jsc) of 18.31 mA cm(-2) , an open-circuit potential (Voc ) of 0.914 V, and a fill factor (FF) of 0.636, yielding an encouraging power conversion efficiency (PCE) of 10.64 % under AM 1.5G illumination. These results give some insight into how the molecular structures of HTMs affect their performances and pave the way for developing high-efficiency and low-cost HTMs for PSCs.

Kesterite Cu2ZnSnS4 as a Low-Cost Inorganic Hole-Transporting Material for High-Efficiency Perovskite Solar Cells.

Kesterite-structured quaternary semiconductor Cu2ZnSnS4 (CZTS) has been commonly used as light absorber in thin film solar cells on the basis of its optimal bandgap of 1.5 eV, high absorption coefficient, and earth-abundant elemental constituents. Herein we applied CZTS nanoparticles as a novel inorganic hole transporting material (HTM) for organo-lead halide perovskite solar cells (PSCs) for the first time, achieving a power conversion efficiency (PCE) of 12.75%, which is the highest PCE for PSCs with Cu-based inorganic HTMs reported up to now, and quite comparable to that obtained for PSCs based on commonly used organic HTM such as 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD). The size of CZTS nanoparticles and its incorporation condition as HTM were optimized, and the effects of CZTS HTM on the optical absorption, crystallinity, morphology of the perovskite film and the interface between the perovskite layer and the Au electrode were investigated and compared with the case of spiro-MeOTAD HTM, revealing the role of CZTS in efficient hole transporting from the perovskite layer to the top Au electrode as confirmed by the prohibited charge recombination at the perovskite/Au electrode interface. On the basis of the effectiveness of CZTS as a low-cost HTM competitive to spiro-MeOTAD in PSCs, we demonstrate the new role of CZTS in photovoltaics as a hole conductor beyond the traditional light absorber.

Unipolar self-doping behavior in perovskite CH3NH3PbBr3

Recent theoretical and experimental reports have shown that the perovskite CH3NH3PbI3 exhibits unique ambipolar self-doping properties. Here, we show by density-functional theory calculation that its sister perovskite, CH3NH3PbBr3, exhibits a unipolar self-doping behavior—CH3NH3PbBr3 presents only good p-type conductivity under thermal equilibrium growth conditions. We further show that despite a large bandgap of 2.2 eV, all dominant defects in CH3NH3PbBr3 create shallow levels, which partially explains the ultra-high open-circuit voltages achieved by CH3NH3PbBr3-based thin-film solar cells. Our results suggest that the perovskite CH3NH3PbBr3 can be both an excellent solar cell absorber and a promising low-cost hole-transport material for lead halide perovskite solar cells.

High efficiency CH3NH3PbI(3−x)Clx perovskite solar cells with poly(3-hexylthiophene) hole transport layer

We fabricate perovskite based solar cells using CH3NH3PbI3−xClx with different hole-transporting materials. The most used 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (Spiro-OMeTAD) has been compared to the poly(3-hexylthiophene-2,5-diyl) (P3HT). By tuning the energy level of P3HT and optimizing the device's fabrication, we reached 9.3% of power conversion efficiency, which is the highest reported efficiency for a solar cell using P3HT. This result shows that P3HT can be a suitable low cost hole transport material for efficient perovskite based solar cells.