The vast majority of the hole transporting materials require the use of chemical doping as an essential step for preparation of efficient perovskite solar cells. An oxidized organic hole‐transporting ...material, obtained during a doping procedure, could potentially be one of the weak links in the device composition. It is not uncommon for the solar cell to heat up under summer sun; therefore, all device components must possess some degree of resistance to repetitive thermal stress. In the current publication, a series of oxidized hole‐transporting materials have been synthesized and their long‐term stability investigated. During thermal stability testing of the films, kept at 100 °C under an inert atmosphere, it was observed that oxidized HTMs start to degrade and partly revert to original unoxidized material. It is known that oxidized HTM, formed during doping, is responsible for the increased conductivity and ultimately for better efficiency of hole extraction process in the PSC device; therefore, observed instability of the oxidized HTMs in the thin films at elevated temperatures could be one of the causes of drop in conductivity reported for the doped spiro‐OMeTAD. It could also potentially be one of the reasons why perovskite solar cells lose their efficiency under prolonged thermal stress.
An oxidized hole transporting material, formed during doping, is responsible for increased conductivity and better efficiency of hole extraction process in perovskite solar cells. During long‐term thermal stability testing, degradation and reversion back to original unoxidized material was observed. Reported instability could be one of the causes of drop in hole transporting material's conductivity and ultimately in device's performance.
Organic halide salt passivation is considered to be an essential strategy to reduce defects in state-of-the-art perovskite solar cells (PSCs). This strategy, however, suffers from the inevitable ...formation of in-plane favored two-dimensional (2D) perovskite layers with impaired charge transport, especially under thermal conditions, impeding photovoltaic performance and device scale-up. To overcome this limitation, we studied the energy barrier of 2D perovskite formation from ortho-, meta- and para-isomers of (phenylene)di(ethylammonium) iodide (PDEAI
) that were designed for tailored defect passivation. Treatment with the most sterically hindered ortho-isomer not only prevents the formation of surficial 2D perovskite film, even at elevated temperatures, but also maximizes the passivation effect on both shallow- and deep-level defects. The ensuing PSCs achieve an efficiency of 23.9% with long-term operational stability (over 1000 h). Importantly, a record efficiency of 21.4% for the perovskite module with an active area of 26 cm
was achieved.
The unprecedented emergence of perovskite‐based solar cells (PSCs) has been accompanied by an intensive search of suitable materials for charge‐selective contacts. For the first time a ...hole‐transporting self‐assembled monolayer (SAM) as the dopant‐free hole‐selective contact in p–i–n PSCs is used and a power conversion efficiency of up to 17.8% with average fill factor close to 80% and undetectable parasitic absorption is demonstrated. SAM formation is achieved by simply immersing the substrate into a solution of a novel molecule V1036 that binds to the indium tin oxide surface due to its phosphonic anchoring group. The SAM and its modifications are further characterized by Fourier‐transform infrared and vibrational sum‐frequency generation spectroscopy. In addition, photoelectron spectroscopy in air is used for measuring the ionization potential of the studied SAMs. This novel approach is also suitable for achieving a conformal coverage of large‐area and/or textured substrates with minimal material consumption and can potentially be extended to serve as a model system for substrate‐based perovskite nucleation and passivation control. Further gains in efficiency can be expected upon SAM optimization by means of molecular and compositional engineering.
A novel concept for the formation of the hole selective layer in efficient perovskite solar cells is presented. Carbazole‐based material is synthesized and used for the formation of a self‐assembled monolayer on top of the indium tin oxide transparent conductive substrate. Power conversion efficiency as high as 17.8% is achieved.
The small‐molecule‐based hole‐transporting material methoxydiphenylamine‐substituted carbazole was synthesized and incorporated into a CH3NH3PbI3 perovskite solar cell, which displayed a power ...conversion efficiency of 16.91 %, the second highest conversion efficiency after that of Spiro‐OMeTAD. The investigated hole‐transporting material was synthesized in two steps from commercially available and relatively inexpensive starting reagents. Various electro‐optical measurements (UV/Vis, IV, thin‐film conductivity, hole mobility, DSC, TGA, ionization potential) have been carried out to characterize the new hole‐transporting material.
Quick and easy: The readily synthesized methoxydiphenylamine‐substituted carbazole (V886), a hole‐transporting material, has been used in perovskite solar cells with a power conversion efficiency of 17 %. This is the second highest conversion efficiency after that of solar cells with the hole transporter Spiro‐OMeTAD.
Thermal stability of hybrid solar cells containing spiro-OMeTAD as hole-transporting layer is investigated. It is demonstrated that fully symmetrical spiro-OMeTAD is prone to crystallization, and ...growth of large crystalline domains in the hole-transporting layer is one of the causes of solar cell degradation at elevated temperatures, as crystallization of the material inside the pores or on the interface affects the contact between the absorber and the hole transport. Suppression of the crystal growth in the hole-transporting layer is demonstrated to be a viable tactic to achieve a significant increase in the solar cell resistance to thermal stress and improve the overall lifetime of the device. Findings described in this publication could be applicable to hybrid solar cell research as a number of well-performing architectures rely heavily upon doped spiro-OMeTAD as hole-transporting material.
Self‐assembled monolayers (SAMs) based on Br‐2PACz (2‐(3,6‐dibromo‐9H‐carbazol‐9‐yl)ethylphosphonic acid) 2PACz (2‐(9H‐Carbazol‐9‐yl)ethylphosphonic acid) and MeO‐2PACz ...(2‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl)ethylphosphonic acid) molecules were investigated as hole‐extracting interlayers in organic photovoltaics (OPVs). The highest occupied molecular orbital (HOMO) energies of these SAMs were measured at −6.01 and −5.30 eV for Br‐2PACz and MeO‐2PACz, respectively, and found to induce significant changes in the work function (WF) of indium‐tin‐oxide (ITO) electrodes upon chemical functionalization. OPV cells based on PM6 (poly(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo1,2‐b:4,5‐b’dithiophene))‐alt‐(5,5‐(1’,3’‐di‐2‐thienyl‐5’,7’‐bis(2‐ethylhexyl)benzo1’,2’‐c:4’,5’‐c’dithiophene‐4,8‐dione)) : BTP‐eC9 : PC71BM (6,6‐phenyl‐C71‐butyric acid methyl ester) using ITO/Br‐2PACz anodes exhibited a maximum power conversion efficiency (PCE) of 18.4 %, outperforming devices with ITO/MeO‐2PACz (14.5 %) and ITO/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT : PSS) (17.5 %). The higher PCE was found to originate from the much higher WF of ITO/Br‐2PACz (−5.81 eV) compared to ITO/MeO‐2PACz (4.58 eV) and ITO/PEDOT : PSS (4.9 eV), resulting in lower interface resistance, improved hole transport/extraction, lower trap‐assisted recombination, and longer carrier lifetimes. Importantly, the ITO/Br‐2PACz electrode was chemically stable, and after removal of the SAM it could be recycled and reused to construct fresh OPVs with equally impressive performance.
Super SAM: Two self‐assembled monolayers (SAMs; Br‐2PACz and MeO‐2PACz) are investigated as hole‐extracting interlayer in organic photovoltaics and compared against the widely used PEDOT : PSS. Cells based on the ternary bulk‐heterojunction blend PM6 : BTP‐eC9 : PC71BM and ITO/Br‐2PACz as the anode exhibit the highest power conversion efficiency of 18.4 %, outperforming devices with ITO/MeO‐2PACz (14.5 %) and even ITO/PEDOT : PSS (17.5 %).
Metal‐halide perovskites offer great potential to realize low‐cost and flexible next‐generation solar cells. Low‐temperature‐processed organic hole‐transporting layers play an important role in ...advancing device efficiencies and stabilities. Inexpensive and stable hole‐transporting materials (HTMs) are highly desirable toward the scaling up of perovskite solar cells (PSCs). Here, a new group of aniline‐based enamine HTMs obtained via a one‐step synthesis procedure is reported, without using a transition metal catalyst, from very common and inexpensive aniline precursors. This results in a material cost reduction to less than 1/5 of that for the archetypal spiro‐OMeTAD. PSCs using an enamine V1091 HTM exhibit a champion power conversion efficiency of over 20%. Importantly, the unsealed devices with V1091 retain 96% of their original efficiency after storage in ambient air, with a relative humidity of 45% for over 800 h, while the devices fabricated using spiro‐OMeTAD dropped down to 42% of their original efficiency after aging. Additionally, these materials can be processed via both solution and vacuum processes, which is believed to open up new possibilities for interlayers used in large‐area all perovskite tandem cells, as well as many other optoelectronic device applications.
A new group of aniline‐based enamine hole‐transporting materials is synthesized, characterized, and tested in perovskite solar cells, yielding a champion power conversion efficiency over 20%. The investigated materials are obtained via one‐step synthesis procedure, without the use of a transition metal catalyst, from a very common and inexpensive precursor—aniline.
Identification of electronic processes at buried interfaces of charge-selective contacts is crucial for photovoltaic and photocatalysis research. Here, transient surface photovoltage (SPV) is used to ...study the passivation of different hole-selective carbazole-based SAMs. It is shown that transient SPV and transient photoluminescence provide complementary information on charge transfer kinetics and trapping/de-trapping mechanisms, and that trap-assisted non-radiative recombination losses originate from electron trapping at the SAM-modified ITO/perovskite interface. The hole transfer rates and the density of interface electron traps, obtained by fitting SPV transients with a minimalistic kinetic model, depended strongly on the SAM’s chemical structure, and densities of interface traps as low as 109 cm−2, on par with highly passivated c-Si surfaces, were reached for Me-4PACz, previously used in record perovskite/silicon tandem solar cells. The extracted hole transfer rate constants and interface trap densities correlated well with the corresponding fill factors and open-circuit voltages of high-efficiency solar cells.
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•Loss mechanisms at buried interfaces identified by time-resolved surface photovoltage•Hole transfer rates obtained for ITO/carbazole-based SAMs/perovskite heterojunctions•Lowest density of interface traps (2 × 109 cm−2) found for the SAM “Me-4PACz”•Correlation with the performance of related record perovskite solar cells shown
The implementation of monolayers of small molecules (self-assembled monolayers, SAMs) at buried interfaces together with a deeper understanding of loss mechanisms bears tremendous opportunities for a further increase in the solar energy conversion efficiency of photovoltaic and photocatalytic devices. Carbazole-based SAMs (recently used in world record perovskite/silicon tandem solar cells) are fine-tuned by varying the size of molecules and by selecting specific functional groups. Separation of photogenerated charge carriers is a key for photovoltaic and photocatalytic solar energy conversion and can be directly investigated by surface photovoltage (SPV) techniques. The application of the time-resolved SPV together with kinetic modeling allows for the identification of dominating loss mechanisms as a function of the nature of SAMs at buried interfaces to the metal-halide perovskite absorber implemented in high-efficiency solar cells.
Dominating loss mechanisms were identified at hole-selective buried interfaces engineered with carbazole-based self-assembled monolayers between a metal halide perovskite absorber and a conductive metal oxide. The analysis of surface photovoltage transients with a minimalistic kinetic model allowed for the extraction of interfacial electron trap densities and hole transfer rates and their correlation with open-circuit voltages and fill factors of the corresponding high-efficiency solar cells is demonstrated.
Small-molecule hole transporting materials based on methoxydiphenylamine-substituted fluorene fragments were synthesized and incorporated into a perovskite solar cell, which displayed a power ...conversion efficiency of up to 19.96%, one of the highest conversion efficiencies reported. The investigated hole transporting materials were synthesized in two steps from commercially available and relatively inexpensive starting reagents, resulting in up to fivefold cost reduction of the final product compared with spiro-OMeTAD. Electro-optical and thermoanalytical measurements such as UV/Vis, thin-film conductivity, hole mobility, DSC, TGA, ionization potential and current voltage scans of the full perovskite solar cells have been carried out to characterize the new materials.
A series of new branched hole transporting materials (HTMs) containing two diphenylamine‐substituted carbazole fragments linked by a nonconjugated methylenebenzene unit is synthesized and tested in ...perovskite solar cells. Synthesis of the investigated materials is performed by a simple two‐step synthetic procedure providing a target product in high yield. The isolated materials demonstrate good thermal stability and majority of the investigated compounds exist in an amorphous state, which is advantageous as there is no risk of crystallization directly in the film. The highest charge drift mobility of µ0 = 4 × 10−4 cm2 V−1 s−1, measured at weak electric fields, is by ca. one order of magnitude higher than that of Spiro‐OMeTAD under identical conditions. From the perovskite solar cell testing results, it can be seen that performance of two new HTMs (V885 and V911) is on a par with Spiro‐OMeTAD. Due to the ease of synthesis, good thermal, optical and photophysical properties, this type of molecules hold great promise for practical application in commercial perovskite solar cells.
Branched hole transporting materials (HTMs), containing two diphenylamine substituted carbazole moieties linked by a nonconjugated fragment, are synthesized and tested in perovskite solar cells. They are synthesized by a simple two‐step synthetic procedure providing target products in high yield. Testing in perovskite solar cells indicates that performance of new HTMs is on a par with Spiro‐OMeTAD.
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