Investigation of Hole-Transporting Poly(triarylamine) on Aggregation and Charge Transport for Hysteresisless Scalable Planar Perovskite Solar Cells

A Pyrrole Modified 3,4-Propylenedioxythiophene Conjugated Polymer as Hole Transport Layer for Efficient and Stable Perovskite Solar Cells

Despite the outstanding electric properties and cost-effectiveness of poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives, their performance as hole transport layer (HTL) materials in conventional perovskite solar cells (PSCs) has lagged behind that of widely used spirobifluorene-based molecules or poly(triaryl amine). This gap is mainly from their poor solubility and energy alignment mismatch. In this work, the design and synthesis of a pyrrole-modified HTL (PPr) based on 3,4-propylenedioxythiophene (ProDOT) are presented for efficient and stable PSCs. As a result of the superior defects passivation ability, excellent contact with perovskite, enhanced hole extraction, and high hydrophobicity, the unencapsulated PPr-based PSCs showed the peak PCE of 21.49% and outstanding moisture stability (over 4000 h). This work highlights the potential application of ProDOT-based materials as HTL for PSCs and underscores the importance of the rational design of PEDOT and its derivatives.

Real-Time Ultraviolet Monitoring System with Low-Temperature Solution-Processed High-Transparent p-n Junction Photodiode with a Fast Responsive and High Rectification Ratio

We introduce an enhanced performance organic-inorganic hybrid p-n junction photodiode, utilizing poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) and ZnO, fabricated through a solution-based process at a low temperature under 100 °C. Improved interfacial electronic structure, characterized by shallower Gaussian standard deviation of the density-of-state distribution and a larger interface dipole, has resulted in a remarkable fold increase of ∼102 in signal-to-noise ratio for the device. This photodiode exhibits a high specific detectivity (2.32 × 1011 Jones, cm × Hz × W - 1 ) and exceptional rectification ratio (5.47 × 104 at ±1 V). The primary light response, concentrated in the optimal thickness of the PTAA layer, contributes to response over the entire UVA region and rapid response speed, with rise and fall times of 0.24 and 0.64 ms, respectively. Furthermore, this work demonstrates immense potential of our device for health monitoring applications by enabling real-time and continuous measurements of UV intensity.

Dissecting the Role of the Hole-Transport Layer in Cu2 AgBiI6 Solar Cells: An Integrated Experimental and Theoretical Study

Perovskite-inspired materials (PIMs) provide low-toxicity and air-stable photo-absorbers for several possible optoelectronic devices. In this context, the pnictogen-based halides Cu2AgBiI6 (CABI) are receiving increasing attention in photovoltaics. Despite extensive studies on power conversion efficiency and shelf-life stability, nearly no attention has been given to the physicochemical properties of the interface between CABI and the hole transport layer (HTL), which can strongly impact overall cell operations. Here, we address this specific interface with three polymeric HTLs: poly(N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine) (poly-TPD), thiophene-(poly(3-hexylthiophene)) (P3HT), and poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA). Our findings reveal that devices fabricated with poly-TPD and P3HT outperform the commonly used Spiro-OMeTAD in terms of device operational stability, while PTAA exhibits worse performances. Density functional theory calculations unveil the electronic and chemical interactions at the CABI-HTL interfaces, providing new insights into observed experimental behaviors. Our study highlights the importance of addressing the buried interfaces in PIM-based devices to enhance their overall performance and stability.

Advances in Hole Transport Materials for Layered Casting Solar Cells

Huge energy consumption and running out of fossil fuels has led to the advancement of renewable sources of power, including solar, wind, and tide. Among them, solar cells have been well developed with the significant achievement of silicon solar panels, which are popularly used as windows, rooftops, public lights, etc. In order to advance the application of solar cells, a flexible type is highly required, such as layered casting solar cells (LCSCs). Organic solar cells (OSCs), perovskite solar cells (PSCs), or dye-sensitive solar cells (DSSCs) are promising LCSCs for broadening the application of solar energy to many types of surfaces. LCSCs would be cost-effective, enable large-scale production, are highly efficient, and stable. Each layer of an LCSC is important for building the complete structure of a solar cell. Within the cell structure (active material, charge carrier transport layer, electrodes), hole transport layers (HTLs) play an important role in transporting holes to the anode. Recently, diverse HTLs from inorganic, organic, and organometallic materials have emerged to have a great impact on the stability, lifetime, and performance of OSC, PSC, or DSSC devices. This review summarizes the recent advances in the development of inorganic, organic, and organometallic HTLs for solar cells. Perspectives and challenges for HTL development and improvement are also highlighted.

Trace Doping: Fluorine-Containing Hydrophobic Lewis Acid Enables Stable Perovskite Solar Cells

With the rapid development in perovskite solar cell (PSC), high efficiency has been achieved, but the long-term operational stability is still the most important challenges for the commercialization of this emerging photovoltaic technology. So far, bi-dopants lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI)/4-tert-butylpyridine (t-BP)-doped hole-transporting materials (HTM) have led to state-of-the art efficiency in PSCs. However, such dopants have several drawbacks in terms of stability, including the complex oxidation process, undesirable ion migration and ultra-hygroscopic nature. Herein, a fluorine-containing organic Lewis acid dopant bis(pentafluorophenyl)zinc (Zn-FP) with hydrophobic property and high migration barrier has been employed as a potential alternative to widely employed bi-dopants Li-TFSI/t-BP for poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). The resulting Zn-FP-based PSCs achieve a maximum PCE of 20.34 % with hysteresis-free current density-voltage (J-V) scans. Specifically, the unencapsulated device exhibits a significantly advanced of operational stability under the International Summit on Organic Photovoltaic Stability protocols (ISOS-L-1), maintaining over 90 % of the original efficiency after operation for 1000 h under continuous 1-sun equivalent illumination in N2 atmosphere in both forward and reverse J-V scan.

Thermal Properties of Polymer Hole-Transport Layers Influence the Efficiency Roll-off and Stability of Perovskite Light-Emitting Diodes

While the performance of metal halide perovskite light-emitting diodes (PeLEDs) has rapidly improved in recent years, their stability remains a bottleneck to commercial realization. Here, we show that the thermal stability of polymer hole-transport layers (HTLs) used in PeLEDs represents an important factor influencing the external quantum efficiency (EQE) roll-off and device lifetime. We demonstrate a reduced EQE roll-off, a higher breakdown current density of approximately 6 A cm^-2, a maximum radiance of 760 W sr^-1 m^-2, and a longer device lifetime for PeLEDs using polymer HTLs with high glass-transition temperatures. Furthermore, for devices driven by nanosecond electrical pulses, a record high radiance of 1.23 MW sr^-1 m^-2 and an EQE of approximately 1.92% at 14.6 kA cm^-2 are achieved. Thermally stable polymer HTLs enable stable operation of PeLEDs that can sustain more than 11.7 million electrical pulses at 1 kA cm^-2 before device failure.

Enhanced performance of flexible quantum dot light-emitting diodes using a low-temperature processed PTAA hole transport layer

Low-temperature processing is important for improving the stability and performance of flexible quantum dot light-emitting diodes (QLEDs). In this study, QLEDs were fabricated using poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) as a suitable hole transport layer (HTL) material owing to its low-temperature processability and vanadium oxide as the low-temperature solution-processable hole injection layer material. The maximum luminance and highest current efficiency of the QLEDs on a glass substrate with an optimal PTAA HTL was 8.9 × 10⁴ Cd/m² and 15.9 Cd/A, respectively, which was comparable to that of conventional devices. The QLEDs on a flexible substrate showed a maximum luminance of 5.4 × 10⁴ Cd/m² and highest current efficiency of 5.1 Cd/A. X-ray and ultraviolet photoelectron spectroscopies were used to investigate the chemical state and interfacial electronic structure according to the materials and the state changes of the HTL, respectively. The interfacial electronic structure showed that PTAA exhibited a better hole transport ability owing to its low hole injection barrier ([Formula: see text]). Moreover, QLEDs with a PTAA HTL could operate as photosensors under reverse bias conditions. These results indicate that the low-temperature-processed PTAA HTL is suitable for improving the performance of flexible QLEDs.

Stabilization of Perovskite Solar Cells: Recent Developments and Future Perspectives

Exceptional power conversion efficiency (PCE) of 25.7% in perovskite solar cells (PSCs) has been achieved, which is comparable with their traditional rivals (Si-based solar cells). However, commercialization-worthy efficiency and long-term stability remain a challenge. In this regard, there are increasing studies focusing on the interface engineering in PSC devices to overcome their poor technical readiness. Herein, the roles of electrode materials and interfaces in PSCs are discussed in terms of their PCEs and perovskite stability. All the current knowledge on the factors responsible for the rapid intrinsic and external degradation of PSCs is presented. Then, the roles of carbonaceous materials as substitutes for noble metals are focused on, along with the recent research progress in carbon-based PSCs. Furthermore, a sub-category of PSCs, that is, flexible PSCs, is considered as a type of exceptional power source due to their high power-to-weight ratios and figures of merit for next-generation wearable electronics. Last, the future perspectives and directions for research in PSCs are discussed, with an emphasis on their commercialization.

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.

Triarylamine/Bithiophene Copolymer with Enhanced Quinoidal Character as Hole-Transporting Material for Perovskite Solar Cells

Polytriarylamine is a popular hole-transporting materials (HTMs) despite its suboptimal conductivity and significant recombination at the interface in a solar cell setup. Having noted insufficient conjugation among the triarylamine units along the polymer backbone, we inserted a bithiophene unit between two triarylamine units through iron-catalyzed C-H/C-H coupling of a triarylamine/thiophene monomer so that two units conjugate effectively via four quinoidal rings when the molecule functions as HTM. The obtained triarylamine/bithiophene copolymer (TABT) used as HTM showed a high-performance in methylammonium lead iodide perovskite (MAPbI₃ ) solar cells. Mesityl substituted TABT forms a uniform film, shows high hole-carrier mobility, and has an ionization potential (IP=5.40 eV) matching that of MAPbI₃ . We fabricated a solar cell device with a power conversion efficiency of 21.3 % and an open-circuit voltage of 1.15 V, which exceeds the performance of devices using reference standard such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) and Spiro-OMeTAD.

Deciphering the Reduced Loss in High Fill Factor Inverted Perovskite Solar Cells with Methoxy-Substituted Poly(Triarylamine) as the Hole Selective Contact

A dopant-free polymeric hole selective contact (HSC) layer is ubiquitous for stable perovskite solar cells (PSCs). However, the intrinsic nonwetting nature of the polymeric HSC impedes the uniform spreading of the perovskite precursor solution, generating a terrible buried interface. Here, we dexterously tackle this dilemma from the perspective of dispersive and polar component surface energies of the HSC layer. A novel triarylamine-based HSC material, poly[bis(4-phenyl)(2,4-dimethoxyphenyl)amine] (2MeO-PTAA), was designed by introducing the polar methoxy groups to the para and ortho positions of the dangling benzene. These nonsymmetrically substituted electron-donating methoxy groups enhanced the polar components of surface energy, allowing more tight interfacial contact between the HSC layer and perovskite and facilitating hole extraction. When utilized as the dopant-free HSC layer in inverted PSCs, the 2MeO-PTAA-based device with CH₃NH₃PbI₃ as the absorber exhibited an encouraging power conversion efficiency of 20.23% and a high fill factor of 84.31% with negligible hysteresis. Finally, a revised detailed balance model was used to verify the drastically lessened surface defect-induced recombination loss and shunt resistance loss in 2MeO-PTAA-based devices. This work demonstrates a facile and efficient way to modulate the buried interface and shed light on the direction to further improve the photovoltaic performance of inverted PSCs with other types of perovskites.

Chemical and Electronic Investigation of Buried NiO1-δ, PCBM, and PTAA/MAPbI3-xClx Interfaces Using Hard X-ray Photoelectron Spectroscopy and Transmission Electron Microscopy

Identification and profiling of molecular fragments generated over the lifespan of halide perovskite solar cells are needed to overcome the stability issues associated with these devices. Herein, we report the characterization of buried CH₃NH₃PbI_3-xCl_x (HaP)-transport layer (TL) interfaces. By using hard X-ray photoelectron spectroscopy in conjunction with transmission electron microscopy, we reveal that the chemical decomposition of HaP is TL-dependent. With NiO_1-δ, phenyl-C₆₁-butyric acid methyl ester (PCBM), or poly(bis(4-phenyl) (2,4,6-trimethylphenyl)amine) (PTAA) as TLs, probing depth analysis shows that the degradation takes place at the interface (HaP/TL) rather than the HaP bulk area. From core-level data analysis, we identified iodine migration toward the PCBM- and PTAA-TLs. Unexpected diffusion of nitrogen inside NiO_1-δ-TL was also found for the HaP/NiO_1-δ sample. With a HaP/PCBM junction, HaP is dissociated to PbI₂, whereas HaP/PTAA contact favored the formation of CH₃I. The low stability of HaP solar cells in the PTAA-TL system is attributed to the formation of CH₃I and iodide ion vacancies. Improved stability observed with NiO_1-δ-TL is related to weak dissociation of stoichiometric HaP. Here, we provide a new insight to further distinguish different mechanisms of degradation to improve the long-term stability and performance of HaP solar cells.

Lithium Polystyrene Sulfonate as a Hole Transport Material in Inverted Perovskite Solar Cells

Despite the exceptional efficiency of perovskite solar cells (PSCs), further improvements can be made to bring their power conversion efficiencies (PCE) closer to the Shockley-Queisser limit, while the development of cost-effective strategies to produce high-performance devices are needed for them to reach their potential as a widespread energy source. In this context, there is a need to improve existing charge transport layers (CTLs) or introduce new CTLs. In this contribution, we introduced a new polyelectrolyte (lithium poly(styrene sulfonate (PSS))) (Li:PSS) polyelectrolyte as an HTL in inverted PSCs, where Li⁺ can act as a counter ion for the PSS backbone. The negative charge on the PSS backbone can stabilize the presence of p-type carriers and p-doping at the anode. Simple Li:PSS performed poorly due to poor surface coverage and voids existence in perovskite film as well as low conductivity. PEDOT:PSS was added to increase the conductivity to the simple Li:PSS solution before its use which also resulted in lower performance. Furthermore, a bilayer of PEDOT:PSS and Li:PSS was employed, which outperformed simple PEDOT:PSS due to high quality of perovskite film with large grain size also the large electron injection barrier (ϕ_e ) impeded back diffusion of electrons towards anode. As a consequence, devices employing PEDOT:PSS / Li:PSS bilayers gave the highest PCE of 18.64%.

Newfangled progressions in the charge transport layers impacting the stability and efficiency of perovskite solar cells

Organic-inorganic lead halide perovskite solar cells have rapidly emerged as a newfangled material for solar energy harnessing. Perovskite solar cells have succeeded in gaining a power conversion efficiency of 25% in the last year, further enhancement in the efficiency is anticipated due to advanced engineering of the different components making up the complete cell architecture with enhanced performance, stability and efficiency. Significant components of perovskite solar cell configurational architecture are the electron transport layer, active perovskite absorber layer, hole transport layer and counter electrode. Considering the profound role of transport layers in charge mobility, current review has particularly elucidated the advancements in the charge transport layers. The time duration of the review is from 2010 to 2021. However, the special focus has been laid on the recent articles. The influence of different organic and inorganic materials used for development of transport layers influencing the cell performance have been summarized. Materials used for transport layers have been modified by utilization of myriad of engineered substances through doping and surface functionalization strategies but every method have been marked by posing serious challenges towards the stability and efficiency of the cell and thus, hindering its commercialization. The review also provides an elucidation of the mechanical challenges and abatement strategies. These strategies are associated with the charge transport layers for enhancement of cell functionality.

Enhanced Thermoelectric Properties of Poly(3-hexylthiophene) through the Incorporation of Aligned Carbon Nanotube Forest and Chemical Treatments

Carbon nanotube/polymer composites have recently received considerable attention for thermoelectric (TE) applications. The TE power factor can be significantly improved by forming composites with carbon nanotubes. However, the formation of a uniform and well-ordered nanocomposite film is still challenging because of the creation of agglomerates and the uneven distribution of nanotubes. Here, we developed a facile, efficient, and easy-processable route to produce uniform and aligned nanocomposite films of P3HT and carbon nanotube forest (CNTF). The electrical conductivity of a pristine P3HT film was improved from ∼10-7 to 160 S/cm thanks to the presence of CNTF. Also, a further boost in TE performance was achieved using two additives, lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and tert-butylpyridine. By adding the additives to P3HT, the degree of interchain order increased, which facilitated the charge transport through the composite. Under the optimal conditions, the incorporation of CNTF and additives led to values of the Seebeck coefficient, electrical conductivity, and power factor up to rising 92 μV/K, 130 S/cm, and 110 μW/m K2, respectively, at a temperature of 344.15 K. The excellent TE performance of the hybrid films originates from the dramatically increased electrical conductivity and the improved Seebeck coefficient by CNTF and additives, respectively.

Poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine-Based Interfacial Passivation Strategy Promoting Efficiency and Operational Stability of Perovskite Solar Cells in Regular Architecture

The failure of perovskite solar cells (PSCs) to maintain their maximum efficiency over a prolonged time is due to the deterioration of the light harvesting material under environmental factors such as humidity, heat, and light. Systematically elucidating and eliminating such degradation pathways are critical to imminent commercial use of this technology. Here, a straightforward approach is introduced to reduce the level of defect-states present at the perovskite and hole transporting layer interface by treating the various perovskite surfaces with poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine (polyTPD) molecules. This strategy significantly suppresses the defect-mediated non-radiative recombination in the ensuing devices and prevents the penetration of degrading agents into the inner layers by passivating the perovskite surface and grain boundaries. Suppressed non-radiative recombination and improved interfacial hole extraction result in PSCs with stabilized efficiency exceeding 21% with negligible hysteresis (≈19.1% for control device). Moreover, ultra-hydrophobic polyTPD passivant considerably alleviates moisture penetration, showing ≈91% retention of initial efficiencies after 300 h storage at high relative humidity of 80%. Similarly, passivated device retains 94% of its initial efficiency after 800 h under operational conditions (maximum power point tracking under continuous illumination at 60 °C). In addition to interfacial passivation function, hole-selective role of dopant-free polyTPD is also evaluated and discussed in this study.

Stannite Quaternary Cu2M(M = Ni, Co)SnS4 as Low Cost Inorganic Hole Transport Materials in Perovskite Solar Cells

In this study, inorganic stannite quaternary Cu2M(M = Ni, Co)SnS4 (CMTS) is explored as a low-cost, earth abundant, environmentally friendly and chemically stable hole transport material (HTM). CMTS nanoparticles were synthesized via a facile and mild solvothermal method and processed into aggregated nanoparticle inks, which were applied in n-i-p perovskite solar cells (PSCs). The results show that Cu2NiSnS4 (CNiTS) is more promising as an HTM than Cu2CoSnS4 (CCoTS), showing efficient charge injection as evidenced by considerable photoluminescence quenching and lower series resistance from Nyquist plots, as well as higher power conversion efficiency (PCE). Moreover, the perovskite layer coated by the CMTS HTM showed superior environmental stability after 200 h light soaking in 50% relative humidity, while organic HTMs suffer from a severe drop in perovskite absorption. Although the obtained PCEs are modest, this study shows that the cost effective and stable inorganic CMTSs are promising HTMs, which can contribute towards PSC commercialization, if the field can further optimize CMTS energy levels through compositional engineering.

Unraveling the Impact of Hole Transport Materials on Photostability of Perovskite Films and p-i-n Solar Cells

We investigated the impact of a series of hole transport layer (HTL) materials such as Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), NiO_x, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA), and polytriarylamine (PTA) on photostability of thin films and solar cells based on MAPbI₃, Cs_0.15FA_0.85PbI₃, Cs_0.1MA_0.15FA_0.75PbI₃, Cs_0.1MA_0.15FA_0.75Pb(Br_0.15I_0.85)₃, and Cs_0.15FA_0.85Pb(Br_0.15I_0.85)₃ complex lead halides. Mixed halide perovskites showed reduced photostability in comparison with similar iodide-only compositions. In particular, we observed light-induced recrystallization of all perovskite films except MAPbI₃ with the strongest effects revealed for Br-containing systems. Moreover, halide and β FAPbI₃ phase segregations were also observed mostly in mixed-halide systems. Interestingly, coating perovskite films with the PCBM layer spectacularly suppressed light-induced growth of crystalline domains as well as segregation of Br-rich and I-rich phases or β FAPbI₃. We strongly believe that all three effects are promoted by the light-induced formation of surface defects, which are healed by adjacent PCBM coating. While comparing different hole-transport materials, we found that NiO_x and PEDOT:PSS are the least suitable HTLs because of their interfacial (photo)chemical interactions with perovskite absorbers. On the contrary, polyarylamine-type HTLs PTA and PTAA form rather stable interfaces, which makes them the best candidates for durable p-i-n perovskite solar cells. Indeed, multilayered ITO/PTA(A)/MAPbI₃/PCBM stacks revealed no aging effects within 1000 h of continuous light soaking and delivered stable and high power conversion efficiencies in solar cells. The obtained results suggest that using polyarylamine-type HTLs and simple single-phase perovskite compositions pave a way for designing stable and efficient perovskite solar cells.

The Molecular Weight Dependence of Thermoelectric Properties of Poly (3-Hexylthiophene)

Organic materials have been found to be promising candidates for low-temperature thermoelectric applications. In particular, poly (3-hexylthiophene) (P3HT) has been attracting great interest due to its desirable intrinsic properties, such as excellent solution processability, chemical and thermal stability, and high field-effect mobility. However, its poor electrical conductivity has limited its application as a thermoelectric material. It is therefore important to improve the electrical conductivity of P3HT layers. In this work, we studied how molecular weight (MW) influences the thermoelectric properties of P3HT films. The films were doped with lithium bis(trifluoromethane sulfonyl) imide salt (LiTFSI) and 4-tert butylpyridine (TBP). Various P3HT layers with different MWs ranging from 21 to 94 kDa were investigated. UV-Vis spectroscopy and atomic force microscopy (AFM) analysis were performed to investigate the morphology and structure features of thin films with different MWs. The electrical conductivity initially increased when the MW increased and then decreased at the highest MW, whereas the Seebeck coefficient had a trend of reducing as the MW grew. The maximum thermoelectric power factor (1.87 μW/mK2) was obtained for MW of 77 kDa at 333 K. At this temperature, the electrical conductivity and Seebeck coefficient of this MW were 65.5 S/m and 169 μV/K, respectively.

Synthesis of a side-chain hole transporting polymer through Mitsunobu post-functionalization for efficient inverted perovskite solar cells

Mitsunobu post-functionalization was utilized to construct a new efficient dopant-free side-chain hole transporting polymer for inverted perovskite solar cells, exhibiting a power conversion efficiency of 17.75% and a high fill factor over 81%.

Interfacial Modification and Defect Passivation by the Cross-Linking Interlayer for Efficient and Stable CuSCN-Based Perovskite Solar Cells

The study of the inorganic hole-transport layer (HTL) in perovskite solar cells (PSCs) is gathering attention because of the drawback of the conventional PSC design, where the organic HTL with salt dopants majorly participates in the degradation mechanisms. On the other hand, inorganic HTL secures better stability, while it offers difficulties in the deposition and interfacial control to realize high-performing devices. In this study, we demonstrate polydimethylsiloxane (PDMS) as an ideal polymeric interlayer which prevents interfacial degradation and improves both photovoltaic performance and stability of CuSCN-based PSC by its cross-linking behavior. Surprisingly, the PDMS polymers are identified to form chemical bonds with perovskite and CuSCN, as shown by Raman spectroscopy. This novel cross-linking interlayer of PDMS enhances the hole-transporting property at the interface and passivates the interfacial defects, realizing the PSC with high power-conversion efficiency over 19%. Furthermore, the utilization of the PDMS interlayer greatly improves the stability of solar cells against both humidity and heat by mitigating the interfacial defects and interdiffusion. The PDMS-interlayered PSCs retained over 90% of the initial efficiencies, both after 1000 h under ambient conditions (unencapsulated) and after 500 h under 85 °C/85% relative humidity (encapsulated).

Optimizing the Interface between Hole Transporting Material and Nanocomposite for Highly Efficient Perovskite Solar Cells

The performances of organometallic halide perovskite-based solar cells severely depend on the device architecture and the interface between each layer included in the device stack. In particular, the interface between the charge transporting layer and the perovskite film is crucial, since it represents both the substrate where the perovskite polycrystalline film grows, thus directly influencing the active layer morphology, and an important site for electrical charge extraction and/or recombination. Here, we focus on engineering the interface between a perovskite-polymer nanocomposite, recently developed by our group, and different commonly employed polymeric hole transporters, namely PEDOT: PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)], PEDOT, PTAA [poly(bis 4-phenyl}{2,4,6-trimethylphenyl}amine)], Poly-TPD [Poly(N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] Poly-TPD, in inverted planar perovskite solar cell architecture. The results show that when Poly-TPD is used as the hole transfer material, perovskite film morphology improved, suggesting an improvement in the interface between Poly-TPD and perovskite active layer. We additionally investigate the effect of the Molecular Weight (MW) of Poly-TPD on the performance of perovskite solar cells. By increasing the MW, the photovoltaic performances of the cells are enhanced, reaching power conversion efficiency as high as 16.3%.

Nanostructured Perovskite Solar Cells

Over the past decade, lead halide perovskites have emerged as one of the leading photovoltaic materials due to their long carrier lifetimes, high absorption coefficients, high tolerance to defects, and facile processing methods. With a bandgap of ~1.6 eV, lead halide perovskite solar cells have achieved power conversion efficiencies in excess of 25%. Despite this, poor material stability along with lead contamination remains a significant barrier to commercialization. Recently, low-dimensional perovskites, where at least one of the structural dimensions is measured on the nanoscale, have demonstrated significantly higher stabilities, and although their power conversion efficiencies are slightly lower, these materials also open up the possibility of quantum-confinement effects such as carrier multiplication. Furthermore, both bulk perovskites and low-dimensional perovskites have been demonstrated to form hybrids with silicon nanocrystals, where numerous device architectures can be exploited to improve efficiency. In this review, we provide an overview of perovskite solar cells, and report the current progress in nanoscale perovskites, such as low-dimensional perovskites, perovskite quantum dots, and perovskite-nanocrystal hybrid solar cells.

Limitations of a polymer-based hole transporting layer for application in planar inverted perovskite solar cells

Planar inverted lead halide photovoltaics demonstrate remarkable photoconversion properties when employing poly(triarylamine) (PTAA) as a hole transporting layer. Herein, we elucidate the effect of ambient ultraviolet (UV) degradation on the structural and operational stability of the PTAA hole transporter through a series of rigorous optoelectrical characterization protocols. Due attention was given to the interplay between the polymer and perovskite absorber, both within the framework of a bilayer structure and fully assembled solar cells. The obtained results imply that UV degradation exerts a major influence on the structural integrity of PTAA, rather than on the interface with the perovskite light harvester. Moreover, UV exposure induced more adverse effects on tested samples than environmental humidity and oxygen, contributing more to the overall reduction of charge extraction properties of PTAA, as well as increased defect population upon prolonged UV exposure.

Doping strategies for small molecule organic hole-transport materials: impacts on perovskite solar cell performance and stability

Hybrid organic/inorganic perovskite solar cells (PSCs) have dramatically changed the landscape of the solar research community over the past decade, but >25 year stability is likely required if they are to make the same impact in commercial photovoltaics and power generation more broadly. While every layer of a PSC has been shown to impact its durability in power output, the hole-transport layer (HTL) is critical for several reasons: (1) it is in direct contact with the perovskite layer, (2) it often contains mobile ions, like Li+ - which in this case are hygroscopic, and (3) it usually has the lowest thermal stability of all layers in the stack. Therefore, HTL engineering is one method with a high return on investment for PSC stability and lifetime. Research has progressed in understanding design rules for small organic molecule hole-transport materials, yet, when implemented into devices, the same dopants, bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(iii) tri[bis(trifluoromethane)sulfonimide] (FK209), are nearly always required for improved charge-transport properties (e.g., increased hole mobility and conductivity). The dopants are notable because they too have been shown to negatively impact PSC stability and lifetime. In response, new research has targeted alternative dopants to bypass these negative effects and provide greater functionality. In this review, we focus on dopant fundamentals, alternative doping strategies for organic small molecule HTL in PSC, and imminent research needs with regard to dopant development for the realization of reliable, long-lasting electricity generation via PSCs.

The Impact of Hybrid Compositional Film/Structure on Organic⁻Inorganic Perovskite Solar Cells

Perovskite solar cells (PSCs) have been intensively investigated over the last several years. Unprecedented progress has been made in improving their power conversion efficiency; however, the stability of perovskite materials and devices remains a major obstacle for the future commercialization of PSCs. In this review, recent progress in PSCs is summarized in terms of the hybridization of compositions and device architectures for PSCs, with special attention paid to device stability. A brief history of the development of PSCs is given, and their chemical structures, optoelectronic properties, and the different types of device architectures are discussed. Then, perovskite composition engineering is reviewed in detail, with particular emphasis on the cationic components and their impact on film morphology, the optoelectronic properties, device performance, and stability. In addition, the impact of two-dimensional and/or one-dimensional and nanostructured perovskites on structural and device stability is surveyed. Finally, a future outlook is proposed for potential resolutions to overcome the current issues.