Organic and perovskite solar cells: Working principles, materials and interfaces

A Density Functional Theory Study of the Physico-Chemical Properties of Alkali Metal Titanate Perovskites for Solar Cell Applications

The urgent need to shift from non-renewable to renewable energy sources has caused widespread interest in photovoltaic technologies that allow us to harness readily available and sustainable solar energy. In the past decade, polymer solar cells (PSCs) and perovskite solar cells (Per-SCs) have gained attention owing to their low price and easy fabrication process. Charge transport layers (CTLs), transparent conductive electrodes (TCEs), and metallic top electrodes are important constituents of PSCs and Per-SCs, which affect the efficiency and stability of these cells. Owing to the disadvantages of current materials, including instability and high cost, the development of alternative materials has attracted significant attention. Owing to their more flexible physical and chemical characteristics, ternary oxides are considered to be appealing alternatives, where ATiO3 materials-a class of ternary perovskite oxides-have demonstrated considerable potential for applications in solar cells. Here, we have employed calculations based on the density functional theory to study the structural, optoelectronic, and magnetic properties of ATiO3 (A=Li, Na, K, Rb, and Cs) in different crystallographic phases to determine their potential as PSCs and Per-SCs materials. We have also determined thermal and elastic properties to evaluate their mechanical and thermal stability. Our calculations have revealed that KTiO3 and RbTiO3 possess similar electronic properties as half-metallic materials, while LiTiO3 and CsTiO3 are metallic. Semiconductor behavior with a direct band gap of 2.77 eV was observed for NaTiO3, and calculations of the optical and electronic properties predicted that NaTiO3 is the most appropriate candidate to be employed as a charge transfer layer (CTL) and bottom transparent conducting electrode (TCE) in PSCs and Per-SCs, owing to its transparency and large bandgap, whereas NaTiO3 also provided superior elastic and thermal properties. Among the metallic and half-metallic ATiO3 compounds, CsTiO3 and KTiO3 exhibited the most appropriate features for the top electrode and additional absorbent in the active layer, respectively, to enhance the performance and stability of these cells.

Unveiling the Potential of Two-Terminal Perovskite/Organic Tandem Solar Cells: Mechanisms, Status, and Challenges

Perovskite/organic tandem solar cells (PO-TSCs) demonstrate exceptional suitability for emerging applications such as building-integrated photovoltaics, wearable devices, and greenhouse farming. By leveraging the distinctive attributes of perovskite and organic materials, which encompass expanded solar spectrum utilization, chemically benign solubility, and soft nature, PO-TSCs position themselves as ideal candidates for high-performance semi-transparent photovoltaics (ST-PVs). Despite these advantages, their development significantly lags behind other perovskite-based counterparts, such as perovskite/perovskite, perovskite/silicon, and perovskite/Cu(In, Ga)Se2. To address existing challenges and unlock the full potential of PO-TSCs, an exploration of the fundamental mechanisms governing tandem photovoltaic devices is embarked. Delving into critical aspects such as charge generation/separation, energy level alignment, and material choices becomes pivotal for optimizing PO-TSC performance. The investigation of monolithic two-terminal PO-TSCs offers insights into achievements and barriers, recognizing the competitive landscape with other TSC counterparts. Further scrutiny of perovskite absorbers and organic absorbers in TSCs reveals strategies aimed at enhancing stability and efficiency. The discussion extends to interconnection layers, elucidating their role in optimizing light transmission and balancing carrier recombination. In conclusion, a compelling outlook on the dynamic landscape of PO-TSCs is presented, highlighting the remarkable efficiency progression and signaling their potential to revolutionize solar energy harvesting technologies.

An additive manufacturing approach based on electrohydrodynamic printing to fabricate P3HT:PCBM thin films

Additive manufacturing (AM) enables the production of high value and high performance components with applications from aerospace to biomedical fields. We report here on the fabrication of poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) thin films through the electrohydrodynamic atomization (EHDA) process and its integration as absorber layer for organic solar cells. Prior to the film fabrication, the optimization of the process was carried out by developing the operating envelope for the P3HT:PCBM ink to determine the optimal flow rate and the appropriate applied voltage to achieve a stable-cone deposition mode. The EHDA printed thin-film's topography, morphology and optical properties were systematically analyzed. The root-mean-square roughness was found to vary significantly with the annealing temperature and the flow rate and ranged from 1.938 to 3.345 nm. The estimated film mass and thickness were found between 3.235 and 23.471 mg and 597.5 nm to 1.60 µm, respectively. The films exhibited a broad visible absorption spectrum ranging from ~ 340 to ~ 600 nm, with a maximum peak λmax located at ~ 500 nm. As the annealing temperature and the flow rate were increased, discernible alterations in the PCBM clusters were consequently observed in the blends of the film and the size of the PCBM clusters has decreased by 3% while the distance between them was highly reduced by as much as 82%.

A comprehensive review of the current progresses and material advances in perovskite solar cells

Recently, perovskite solar cells (PSCs) have attracted ample consideration from the photovoltaic community owing to their continually-increasing power conversion efficiency (PCE), viable solution-processed methods, and inexpensive materials ingredients. Over the past few years, the performance of perovskite-based devices has exceeded 25% due to superior perovskite films achieved using low-temperature synthesis procedures along with evolving appropriate interface and electrode-materials. The current review provides comprehensive knowledge to enhance the performance and materials advances for perovskite solar cells. The latest progress in terms of perovskite crystal structure, device construction, fabrication procedures, and challenges are thoroughly discussed. Also discussed are the different layers such as ETLs and buffer-layers employed in perovskite solar-cells, seeing their transmittance, carrier mobility, and band gap potentials in commercialization. Generally, this review delivers a critical assessment of the improvements, prospects, and trials of PSCs.

Self-supported CsPbBr3/Ti3C2Tx MXene aerogels towards efficient photocatalytic CO2 reduction

Aerogels, especially MXene aerogels, are an ideal multifunctional platform for developing efficient photocatalysts for CO₂ reduction because they are featured by abundant catalytic sites, high electrical conductivity, high gas absorption ability and self-supported structure. However, the pristine MXene aerogel has almost no ability to utilize light, which requires additional photosensitizers to assist it in achieving efficient light harvesting. Herein, we immobilized colloidal CsPbBr₃ nanocrystals (NCs) onto the self-supported Ti₃C₂T_x (where T_x represents surface terminations such as fluorine, oxygen, and hydroxyl groups) MXene aerogels for photocatalytic CO₂ reduction. The resultant CsPbBr₃/Ti₃C₂T_x MXene aerogels exhibit a remarkable photocatalytic activity toward CO₂ reduction with total electron consumption rate of 112.6 μmol g^-1h^-1, which is 6.6-fold higher than that of the pristine CsPbBr₃ NC powders. The improvement of the photocatalytic performance is presumably attributed to the strong light absorption, effective charge separation and CO₂ adsorption in the CsPbBr₃/Ti₃C₂T_x MXene aerogels. This work presents an effective perovskite-based photocatalyst in aerogel form and opens a new avenue for their solar-to-fuel conversions.

Triazine: An Important Building Block of Organic Materials for Solar Cell Application

Since the beginning of the 21st century, triazine-based molecules have been employed to construct different organic materials due to their unique optoelectronic properties. Among their applications, photovoltaics stands out because of the current need to develop efficient, economic, and green alternatives to energy generation based mainly on fossil fuels. Here, we review all the development of triazine-based organic materials for solar cell applications, including organic solar cells, dye-sensitized solar cells, and perovskite solar cells. Firstly, we attempt to illustrate the main synthetic routes to prepare triazine derivatives. Then, we introduce the main aspects associated with solar cells and their performance. Afterward, we discuss different works focused on the preparation, characterization, and evaluation of triazine derivatives in solar cells, distinguishing the type of photovoltaics and the role of the triazine-based material in their performance (e.g., as a donor, acceptor, hole-transporting material, electron-transporting material, among others). Throughout this review, the progress, drawbacks, and main issues of the performance of the mentioned solar cells are exposed and discussed. Finally, some conclusions and perspectives about this research topic are mentioned.

Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors

Energy loss caused by exciton binding energy (E_b) has become a key factor that restricts further advancement of organic solar cells (OSCs). Herein, we used transient mid-IR spectroscopy to study direct photogeneration of free charge carriers in small-molecule acceptors (SMAs) Y6 and IDIC as well as polymerized SMAs (PSMAs) PYFT and PZ1. We found that free carrier concentration is higher in PSMAs than in their corresponding SMAs, indicating reduced exciton E_b, which is then confirmed by ultraviolet photoelectron spectroscopy, low-energy inverse photoemission spectroscopy, and film absorption spectra measurements. The measured E_b values of PYFT and PZ1 are 0.24 and 0.37 eV, respectively, smaller than those of Y6 (0.32 eV) and IDIC (0.47 eV). This work not only provides a method to directly monitor the photogenerated free carriers in OSC materials but also demonstrates that polymerization is an effective strategy to reduce the E_b, which is crucial to decrease the energy losses in high-performance OSCs.

Device Modeling of Organic Photovoltaic Cells with Traditional and Inverted Cells Using s-SWCNT:C60 as Active Layer

This research work presents a thorough analysis of Traditional Organic Solar Cell (TOSC) and novel designed Inverted OSC (IOSC) using Bulk Hetero-Junction (BHJ) structure. Herein, 2D photovoltaic device models were used to observe the results of the semiconducting Single Wall Carbon Nanotube (s-SWCNT):C₆₀-based organic photovoltaic. This work has improved the BHJ photodiodes by varying the active layer thickness. The analysis has been performed at various active layer thicknesses from 50 to 300 nm using the active material s-SWCNT:C₆₀. An analysis with various parameters to determine the most effective parameters for organic photovoltaic performance has been conducted. As a result, it has been established that IOSC has the maximum efficiency of 10.4%, which is higher than the efficiency of TOSC (9.5%). In addition, the active layer with the highest efficacy has been recorded using this material for both TOSC and IOSC Nano Photodiodes (NPDs). Furthermore, the diode structure and geometrical parameters have been optimized and compared to maximize the performance of photodiodes.

Achieving the low interfacial tension by balancing crystallization and film-forming ability of the cathode interlayer for organic solar cells

A series of molecules with imide units bridged by the core of thiophene-based groups, namely N-dimethylaminopropyl-4-thiophene-1,8- naphthalimide (NT), bis(N-dimethylaminopropyl)-4-thiophene-1,8-naphthalimide (NTN), and bis(N-dimethylaminopropyl)-4-bithiophene-1,8-naphthalimide (N2TN), have been reported as cathode interfacial materials (CIMs) to realize low interfacial tension with the blend in organic solar cells (OSCs). We evaluated the Ohmic contact between the active layer and these cathode interlayers basedon various characterizations, which is of great significance for further understanding these imide-based interlayers. It turned out that the homogeneous and continuous NTN interlayer as a CIM balanced the factors of crystallization and film-forming property, and broke through the limitation of poor conductivity and high aggregation in our previous work. Moreover, compared with NT and N2TN, the NTN interlayer achieve a combination of good solubility in methanol, efficient electron mobility, and aligned work function. These advantages of NTN are conducive to the realization of high-efficient interfacial electron collection and transfer, thus improving the short-circuit current density (J_SC) and filling factor (FF) of devices. Therefore, the binary OSCs (PM6:Y6) based on NTN engineered aluminium-cathode with excellent stability demonstrate a maximum power conversion efficiency (PCE) of 16.56 %, which is higher than NT (PCE = 1.34 %) and N2TN (PCE = 13.90 %). The enhanced performance is ascribed to the improvement of JSC and FF, which is originated from the outstanding conductivity and high-quality interface of NTN. Surprisingly, the PM6:Y6-based semitransparent device with NTN obtain a PCE of 13.43 % with an average visible transmittance of 17.79 %, which is better than traditional PDINO. This study highlights a potential strategy for enhancing the performance of OSCs by the interface engineering via decreasing the interfacial intension.

The evolution of triphenylamine hole transport materials for efficient perovskite solar cells

In recent years, the dramatic increase in power conversion efficiency (PCE) coupled with a decrease in the total cost of third-generation solar cells has led to a significant increase in the collaborative research efforts of academic and industrial researchers. Such interdisciplinary studies have afforded novel materials, which in many cases are now ready to be brought to the marketplace. Within this framework, the field of perovskite solar cells (PSCs) is currently an important area of research due to their extraordinary light-harvesting properties. In particular, PSCs prepared via facile synthetic procedures, containing hole transport materials (HTMs) with versatile triphenylamine (TPA) structural cores, amenable to functionalization, have become a focus of intense global research activity. To optimize the efficiency of the solar cells to achieve efficiencies closer to rival silicon-based technology, TPA building blocks must exhibit favourable electrochemical, photophysical, and photochemical properties that can be chemically tuned in a rational manner. Although PSCs based on TPA building blocks exhibit attractive properties such as high-power efficiencies, a reduction in their synthetic costs coupled with higher stabilities and environmental considerations still need to be addressed. Considering the above, a detailed summary of the most promising compounds and current methodologies employed to overcome the remaining challenges in this field is provided. The objective of this review is to provide guidance to readers on exploring new avenues for the discovery of efficient TPA derivatives, to aid in the future development and advancement of TPA-based PSCs for commercial applications.

Polymer Photoelectrodes for Solar Fuel Production: Progress and Challenges

Converting solar energy to fuels has attracted substantial interest over the past decades because it has the potential to sustainably meet the increasing global energy demand. However, achieving this potential requires significant technological advances. Polymer photoelectrodes are composed of earth-abundant elements, e.g. carbon, nitrogen, oxygen, hydrogen, which promise to be more economically sustainable than their inorganic counterparts. Furthermore, the electronic structure of polymer photoelectrodes can be more easily tuned to fit the solar spectrum than inorganic counterparts, promising a feasible practical application. As a fast-moving area, in particular, over the past ten years, we have witnessed an explosion of reports on polymer materials, including photoelectrodes, cocatalysts, device architectures, and fundamental understanding experimentally and theoretically, all of which have been detailed in this review. Furthermore, the prospects of this field are discussed to highlight the future development of polymer photoelectrodes.

Highly efficient hole injection from Au electrode to fullerene-doped triphenylamine derivative layer

Triphenylamine derivatives are superior hole-transport materials. For their application to high-functional organic semiconductor devices, efficient hole injection at the electrode/triphenylamine derivative interface is required. Herein, we report the design and evaluation of a Au/fullerene-doped α-phenyl-4'-[(4-methoxyphenyl)phenylamino]stilbene (TPA) buffer layer/TPA/Au layered device. It exhibits rectification conductivity, indicating that hole injection occurs more easily at the Au/fullerene-doped TPA interface than at the Au/TPA interface. The Richardson-Schottky analysis of the device reveals that the hole injection barrier (Φ_B) at the Au/fullerene-doped TPA interface decreases to 0.021 eV upon using C₇₀ as a dopant, and Φ_B of Au/TPA is as large as 0.37 eV. The reduced Φ_B of 0.021 eV satisfies the condition for ohmic contact at room temperature (Φ_B [Formula: see text] 0.025 eV). Notably, C₇₀ doping has a higher barrier-reduction effect than C₆₀ doping. Furthermore, a noteworthy hole-injection mechanism, in which the ion-dipole interaction between TPA and fullerenes plays an important role in reducing the barrier height, is considered based on cyclic voltammetry. These results should facilitate the design of an electrode/organic semiconductor interface for realizing low-voltage driven organic devices.

Leakage Current Mitigation of Photovoltaic System Using Optimized Predictive Control for Improved Efficiency

This paper proposes an optimized predictive control strategy to mitigate the potential leakage current of grid-tied photovoltaic (PV) systems to improve the lifespans of PV modules. In this work, the PV system is controlled with an optimized predictive control algorithm that selects the switching voltage vectors intelligently to reduce the number of computational burdens. Thus, it improves the dynamic performance of the overall system. This is achieved through a specific cost function that minimizes the change in common-mode voltage generated by the parasitic capacitance of PV modules. The proposed controller does not require any additional modulation schemes. Normalization techniques and weighting factors are incorporated to obtain improved results. The steady state and dynamic performance of the proposed control scheme is validated in this work through simulations and a 600 W experimental laboratory prototype.

A Comparison of Charge Carrier Dynamics in Organic and Perovskite Solar Cells

The charge carrier dynamics in organic solar cells and organic-inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are compared. The similarities and differences in charge generation, charge separation, charge transport, charge collection, and charge recombination in these two technologies are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated. In particular, the impact of exciton binding energy, charge transfer states, bimolecular recombination, charge carrier transport, sub-bandgap tail states, and surface recombination is evaluated, and the lessons learned from transient optical and optoelectronic measurements are discussed. This perspective thus highlights the key factors limiting device performance and rationalizes similarities and differences in design requirements between organic and perovskite solar cells.

Bioapplication of cyclodextrin-containing montmorillonite

Recent progresses in the integration of CDs and montmorillonite, as well as applications of CD-containing montmorillonite hybrid host systems are summarized in this review. Several efficient synthesis strategies, such as ion exchange, metal coordination, supramolecular strategies, polymerizations and organic synthesis methods, have been discussed during the preparation of CDs/montmorillonite hybrid composites. In particular, diverse instrumental techniques were highly recommended for characterizing the as-obtained hybrid systems, including their chemical composition and structures, crystallinity, surface/self-assembled morphologies, as well as other particular physiochemical properties, providing a direct guide for promoting the desired structures and exploring various applications. It should be noted that the introduction of functional groups, as well as the integration of CDs and montmorillonite granted the thus obtained CD-containing montmorillonite hybrid host systems a lot of unique features, providing great opportunities for expanding the practical applications to a series of biological and environmental areas, such as biosensors, sorption and decontamination of bio/environmental hazardous materials, biostudies about aqueous dispersity, stability and biocompatibility, drug loading and target delivery, controlled and sustained drug release, as well as antibacterial.

Interface modification of an electron transport layer using europium acetate for enhancing the performance of P3HT-based inorganic perovskite solar cells

In recent years, although the power conversion efficiency (PCE) of thermally stable all-inorganic CsPbI₃ perovskite solar cells (PSCs) had shown a great progress, the most reported CsPbI₃ PSCs suffered from the large open-circuit voltage (V_oc) loss, which is related to severe nonradiative recombination and a mismatch in energy level at the transport layer/perovskite interface. In this work, europium acetate (EuAc₃) as a multifunction interface material is chosen to modify the TiO₂/perovskite interface, the crystal quality of CsPbI₃ perovskite films is improved, and both bulk and interfacial defects are reduced effectively. Meanwhile, the energy levels arrangement between TiO₂ and CsPbI₃ perovskites is also optimized, corresponding the raised built-in electric field afford a strength force to accelerate the transport and extraction of charge carriers from CsPbI₃ perovskites to TiO₂. As a result, the performance of CsPbI₃ PSCs is largely enhanced with the PCE of 16.76%. When an Ag electrode was replaced by Au, the PCE further improves to 17.92%, which is the highest for CsPbI₃ PSCs with P3HT as the HTL ever reported. Besides, the CsPbI₃ PSC with the EuAc₃ modification layer maintains 84% of the initial PCE under continuous UV irradiation for 250 h in a nitrogen filled glovebox, being obviously higher than the control devices with only 40% of the initial PCE after UV irradiation for 100 h in the same environment.

A Review of Integrated Systems Based on Perovskite Solar Cells and Energy Storage Units: Fundamental, Progresses, Challenges, and Perspectives

With the remarkable progress of photovoltaic technology, next-generation perovskite solar cells (PSCs) have drawn significant attention from both industry and academic community due to sustainable energy production. The single-junction-cell power conversion efficiency (PCE) of PSCs to date has reached up to 25.2%, which is competitive to that of commercial silicon-based solar cells. Currently, solar cells are considered as the individual devices for energy conversion, while a series connection with an energy storage device would largely undermine the energy utilization efficiency and peak power output of the entire system. For substantially addressing such critical issue, advanced technology based on photovoltaic energy conversion-storage integration appears as a promising strategy to achieve the goal. However, there are still great challenges in integrating and engineering between energy harvesting and storage devices. In this review, the state-of-the-art of representative integrated energy conversion-storage systems is initially summarized. The key parameters including configuration design and integration strategies are subsequently analyzed. According to recent progress, the efforts toward addressing the current challenges and critical issues are highlighted, with expectation of achieving practical integrated energy conversion-storage systems in the future.

Dendritic-Like Molecules Built on a Pillar[5]arene Core as Hole Transporting Materials for Perovskite Solar Cells

Multi-branched molecules have recently demonstrated interesting behaviour as charge-transporting materials within the fields of perovskite solar cells (PSCs). For this reason, extended triarylamine dendrons have been grafted onto a pillar[5]arene core to generate dendrimer-like compounds, which have been used as hole-transporting materials (HTMs) for PSCs. The performances of the solar cells containing these novel compounds have been extensively investigated. Interestingly, a positive dendritic effect has been evidenced as the hole transporting properties are improved when going from the first to the second-generation compound. The stability of the devices based on the best performing pillar[5]arene material has been also evaluated in a high-throughput ageing setup for 500 h at high temperature. When compared to reference devices prepared from spiro-OMeTAD, the behaviour is similar. An analysis of the economic advantages arising from the use of the pillar[5]arene-based material revealed however that our pillar[5]arene-based material is cheaper than the reference.

Morphology and surface analyses for CH3NH3PbI3 perovskite thin films treated with versatile solvent-antisolvent vapors

Organometal halide perovskite (CH3NH3PbI3) semiconductors have been promising candidates as a photoactive layer for photovoltaics. Especially for high performance devices, the crystal structure and morphology of this perovskite layer should be optimized. In this experiment, by employing solvent-antisolvent vapor techniques during a modified sequential deposition of PbI2-CH3NH3I layers, the morphology engineering was carried out as a function of antisolvent species such as: chloroform, chlorobenzene, dichlorobenzene, toluene, and diethyl ether. Then, the optical, morphological, structural, and surface properties were characterized. When dimethyl sulfoxide (DMSO, solvent) and diethyl ether (antisolvent) vapors were employed, the CH3NH3PbI3 layer exhibited relatively desirable crystal structures and morphologies, resulting in an optical bandgap (E g) of 1.61 eV, crystallite size (t) of 89.5 nm, and high photoluminescence (PL) intensity. Finally, the stability of perovskite films toward water was found to be dependent on the morphologies with defects such as grain boundaries, which was evaluated through contact angle measurement.

Polariton-assisted excitation energy channeling in organic heterojunctions

Exciton-polaritons are hybrid light-matter states resulting from strong exciton-photon coupling. The wave function of the polariton is a mixture of light and matter, enabling long-range energy transfer between spatially separated chromophores. Moreover, their delocalized nature, inherited from the photon component, has been predicted to enhance exciton transport. Here, we strongly couple an organic heterojunction consisting of energy/electron donor and acceptor materials to the same cavity mode. Using time-resolved spectroscopy and optoelectrical characterization, we show that the rate of exciton harvesting is enhanced with one order of magnitude and the rate of energy transfer in the system is increased two- to threefold in the strong coupling regime. Our results exemplify two means of efficiently channeling excitation energy to a heterojunction interface, where charge separation can occur. This study opens a new door to increase the overall efficiency of light harvesting systems using the tool of strong light-matter interactions.

A review of experimental and computational attempts to remedy stability issues of perovskite solar cells

Photovoltaic technology using perovskite solar cells has emerged as a potential solution in the photovoltaic makings for cost-effective manufacturing solutions deposition/coating solar cells. The hybrid perovskite-based materials possess a unique blend from low bulk snare concentrations, ambipolar, broad optical absorption properties, extended charge carrier diffusion, and charge transport/collection properties, making them favourable for solar cell applications. However, perovskite solar cells devices suffer from the effects of natural instability, leading to their rapid degradation while bared to water, oxygen, as well as ultraviolet rays, are irradiated and in case of high temperatures. It is essential to shield the perovskite film from damage, extend lifetime, and make it suitable for device fabrications. This paper focuses on various device strategies and computational attempts to address perovskite-based solar cells' environmental stability issues.

Influence of Fluorinated Components on Perovskite Solar Cells Performance and Stability

Several valuable scientific investigations have been conducted these last few years in materials design and device engineering for perovskite solar cells (PSCs) to make them competitive compared to traditional silicon-based photovoltaic technologies. Consequently, high power conversion efficiency beyond 25% is nowadays reported. However, their long-term stability remains a significant challenge to overcome. Herein, the influence of fluorinated compounds on each layer of PSCs devices and their impact on the resulted device performances and stability is spotlighted. The fluorinated compounds exhibit attractive properties due to their very high electronegativity attributed to the fluorine atom, and their strong hydrophobicity. Thus, the introduction of these compounds is found to be a successful strategy to positively suppress the surface trap states, enhancing charge collection and reducing interfacial charge recombination. Besides, a better film quality and better energy level alignment is obtained, resulting in the improvement of device photovoltaic parameters such as the open-circuit voltage (V_oc ), short-circuit current (J_sc ), and fill factor (FF), and then, the device's overall power conversion efficiency (PCE). Their long-term stability is also found to further be improved.

Tuning the work function of nickel oxide using triethoxysilane functionalized monolayers

The work function of nickel oxide (NiOx) electrodes was tuned by the covalent attachment of commercially available as well as specially synthesized triethoxysilane functionalized molecules with a range of dipole moments. The presence of the silane molecular layers on the NiOx surface was verified using Fourier Transform Infrared (FTIR) spectroscopy and contact angle measurements. While these tests indicated the surface coverage was incomplete, Kelvin probe measurements showed that the coverage was sufficient to change the work function of the NiOx across a range of ∼900 meV. Density functional theory (DFT) calculations of the dipole moments of the isolated molecules correlated well with the measured work function changes.

Phenothiazine functional materials for organic optoelectronic applications

Phenothiazine (PTZ) is one of the most extensively investigated S, N heterocyclic aromatic hydrocarbons due to its unique optical, electronic properties, flexibility of functionalization, low cost, and commercial availability. Hence, PTZ and its derivative materials have been attractive in various optoelectronic applications in the last few years. In this prospective, we have focused on the most significant characteristics of PTZ and highlighted how the structural modifications such as different electron donors or acceptors, length of the π-conjugated system or spacers, polar or non-polar chains, and other functional groups influence the optoelectronic properties. This prospective provides a recent account of the advances in phenothiazine derivative materials as an active layer(s) for optoelectronic (viz. dye sensitized solar cells (DSSCs), perovskite solar cells (PSCs), organic solar cells (OSCs), organic light-emitting diodes (OLEDs), organic field-effect transistor (OFETs), chemosensing, nonlinear optical materials (NLOs), and supramolecular self-assembly applications. Finally, future prospects are discussed based on the structure-property relationship in PTZ-derivative materials. This overview will pave the way for researchers to design and develop new PTZ-functionalized structures and use them for various organic optoelectronic applications.

Electrically Enhanced Transition Metal Dichalcogenides as Charge Transport Layers in Metallophthalocyanine-Based Solar Cells

Transitional metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2) have found application in photovoltaic cells as a charge transporting layer due to their high carrier mobility, chemical stability, and flexibility. In this research, a photovoltaic device was fabricated consisting of copper phthalocyanine (CuPc) as the active layer, exfoliated and Au-doped MoS2, which are n-type and p-type as electron and hole transport layers, respectively. XRD studies showed prominent peaks at (002) and other weak reflections at (100), (103), (006), and (105) planes corresponding to those of bulky MoS2. The only maintained reflection at (002) was weakened for the exfoliated MoS2 compared to the bulk, which confirmed that the material was highly exfoliated. Additional peaks at (111) and (200) planes were observed for the Au doped MoS2. The interlayer spacing (d002) was calculated to be 0.62 nm for the trigonal prismatic MoS2 with the space group P6m2. Raman spectroscopy showed that theE 2 1 g (393 cm-1) and A1g (409 cm-1) peaks for exfoliated MoS2 are closer to each other compared to their bulk counterparts (378 and 408 cm-1, respectively) hence confirming exfoliation. Raman spectroscopy also confirmed doping of MoS2 by Au as the Au-S peak was observed at 320 cm-1. Exfoliation was further confirmed by SEM as when moving from bulky to exfoliated MoS2, a single nanosheet was observed. Doping was further proven by EDS, which detected Au in the sample suggesting the yield of a p-type Au-MoS2. The fabricated device had the architecture: Glass/FTO/Au-MoS2/CuPc/MoS2/Au. A quadratic relationship between I-V was observed suggesting little rectification from the device. Illuminated I-V characterization verified that the device was sensitive and absorbed visible light. Upon illumination, the device was able to absorb photons to create electron-hole pairs and it was evident that semipermeable junctions were formed between Au-MoS2/CuPc and CuPc/MoS2 as holes and electrons were extracted and separated at respective junctions generating current from light. This study indicates that the exfoliated and Au-MoS2 could be employed as an electron transporting layer (ETL) and hole transporting layer (HTL), respectively in fabricating photovoltaic devices.

Application of Metal-Organic Frameworks and Covalent Organic Frameworks as (Photo)Active Material in Hybrid Photovoltaic Technologies

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are two innovative classes of porous coordination polymers. MOFs are three-dimensional materials made up of secondary building blocks comprised of metal ions/clusters and organic ligands whereas COFs are 2D or 3D highly porous organic solids made up by light elements (i.e., H, B, C, N, O). Both MOFs and COFs, being highly conjugated scaffolds, are very promising as photoactive materials for applications in photocatalysis and artificial photosynthesis because of their tunable electronic properties, high surface area, remarkable light and thermal stability, easy and relative low-cost synthesis, and structural versatility. These properties make them perfectly suitable for photovoltaic application: throughout this review, we summarize recent advances in the employment of both MOFs and COFs in emerging photovoltaics, namely dye-sensitized solar cells (DSSCs) organic photovoltaic (OPV) and perovskite solar cells (PSCs). MOFs are successfully implemented in DSSCs as photoanodic material or solid-state sensitizers and in PSCs mainly as hole or electron transporting materials. An innovative paradigm, in which the porous conductive polymer acts as standing-alone sensitized photoanode, is exploited too. Conversely, COFs are mostly implemented as photoactive material or as hole transporting material in PSCs.

Controlled Structure and Growth Mechanism behind Hydrothermal Growth of TiO2 Nanorods

Fabrication of uniform vertically-aligned titanium dioxide nanorods (TiO₂ NRs) was achieved by hydrothermal growth on a fluorine-doped tin oxide (FTO) glass substrate. The substrate was coated by a TiO₂ seed layer composed of titanium (IV) butoxide (TBO) as a precursor in an HCl solution. To reduce the amount of toxic substances used in this work, a minimal amount of HCl was used. On a larger scale, this method would require less precursor and therefore be a cost-savings. The aim of the present work is to achieve high crystalline orientations of TiO₂ NRs for low quantities of both TBO precursor and HCl solutions. Results showed that the 0.7% TBO TiO₂ NRs after 1.5 h of hydrothermal treatment exhibited the optimal crystalline orientation along [001] while the (002) plane is the dominant facet. The results demonstrate high transmittance of visible light and well-formed crystalline structures that offer a fast electron pathway along the length of the TiO₂ NRs with less grain boundaries. Lastly, TiO₂ NRs and their growth mechanism are discussed. This work offers a promising hydrothermal method for growing well-aligned TiO₂ single-crystal NRs that can be employed in solar cell applications.

Scalable, Alternating Narrow Stripes of Polyvinyl Alcohol Support and Unmodified PEDOT:PSS with Maintained Conductivity Using a Single-Step Slot Die Coating Approach

Slot die coating has been established as an economical approach for deposition of parallel narrow stripes, a constituent pattern feature in many printed device applications. However, the minimum feature size that can be achieved using this approach is constrained by wetting and liquid bridge phenomena at the deposition region. We hypothesize that pattern resolution and process control can be improved by co-depositing a support fluid to stabilize the pattern. Electrically conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is slot die-coated in parallel stripes on flexible poly(ethylene terephthalate) substrate, without wettability-enhancing dopants or substrate pretreatment. A miscible liquid phase, polyvinyl alcohol, is used as the support material. Feature size performance and conductivity of PEDOT:PSS stripe regions are evaluated across a range of process conditions. Narrow PEDOT:PSS stripes produced using our technique range from 400 to 850 μm and exhibit conductivity approaching 1.5 S cm^-1. This electrical performance falls within the upper range expected prior to standard conductivity-enhancing post-treatments. Significantly, dewetting effects normally present with undoped PEDOT:PSS on the plastic substrate are fully mitigated with our deposition technique. These results indicate high ease of processing and good feature size performance, with few inherent drawbacks to the functional properties of the patterned films.

Origin of Open-Circuit Voltage Losses in Perovskite Solar Cells Investigated by Surface Photovoltage Measurement

Increasing the open-circuit voltage (V_oc) is one of the key strategies for further improvement of the efficiency of perovskite solar cells. It requires fundamental understanding of the complex optoelectronic processes related to charge carrier generation, transport, extraction, and their loss mechanisms inside a device upon illumination. Herein, we report the important origin of V_oc losses in methylammonium lead iodide perovskite (MAPI)-based solar cells, which results from undesirable positive charge (hole) accumulation at the interface between the perovskite photoactive layer and the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-transport layer. We show strong correlation between the thickness-dependent surface photovoltage and device performance, unraveling that the interfacial charge accumulation leads to charge carrier recombination and results in a large decrease in V_oc for the PEDOT:PSS/MAPI inverted devices (180 mV reduction in 50 nm thick device compared to 230 nm thick one). In contrast, accumulated positive charges at the TiO₂/MAPI interface modify interfacial energy band bending, which leads to an increase in V_oc for the TiO₂/MAPI conventional devices (70 mV increase in 50 nm thick device compared to 230 nm thick one). Our results provide an important guideline for better control of interfaces in perovskite solar cells to improve device performance further.

Influence of Interfacial Traps on the Operating Temperature of Perovskite Solar Cells

In this paper, by developing a mathematical model, the temperature of PSCs under different operating conditions has been calculated. It is found that by reducing the density of tail states at the interfaces through some passivation mechanisms, the operating temperature can be decreased significantly at higher applied voltages. The results show that if the density of tail states at the interfaces is reduced by three orders of magnitude through some passivation mechanisms, then the active layer may not undergo any phase change up to an ambient temperature 300 K and it may not degrade up to 320 K. The calculated heat generation at the interfaces at different applied voltages with and without passivation shows reduced heat generation after reducing the density of tail states at the interfaces. It is expected that this study provides a deeper understanding of the influence of interface passivation on the operating temperature of PSCs.

Building an Organic Solar Cell: Fundamental Procedures for Device Fabrication

This laboratory experiment is designed to train undergraduate students in the fundamental steps followed in engineering solution-processed organic solar cells and to offer insight on the operating principles of said device. Bulk heterojunction (BHJ) organic solar cells represent a photovoltaic architecture which has attracted a lot of attention due to its promising properties; moreover, this architecture, due to its low cost and potential, is continuously being investigated and improved. This paper is intended as a useful step-by-step guide for students and researchers to learn how to construct such a device. Another primary objective of this article is to highlight the importance of optimizing device performance through enhancing the optical, electrical, and morphological properties of the materials selected as building blocks. Once a completed organic solar cell is made, students will also learn how to investigate and assess its performance through a series of spectroscopic, electrical, and morphological characterization measurements.

DFT Characteristics of Charge Transport in DBTP-Based Hole Transport Materials

To improve the hole-transport ability and photoelectric properties of perovskite solar cells, the ground-state geometry, frontier molecular orbital, and mobility of two organic molecules were investigated using density functional theory (DFT) with the Marcus hopping model. The absorption spectra were calculated using time-dependent DFT. The result indicated that the increase in the conjugated chain and change in the substituted group location from meta to para cause low mobility, which has a negative effect on the hole-transporting ability.

Materials for Photovoltaics: State of Art and Recent Developments

In recent years, photovoltaic cell technology has grown extraordinarily as a sustainable source of energy, as a consequence of the increasing concern over the impact of fossil fuel-based energy on global warming and climate change. The different photovoltaic cells developed up to date can be classified into four main categories called generations (GEN), and the current market is mainly covered by the first two GEN. The 1GEN (mono or polycrystalline silicon cells and gallium arsenide) comprises well-known medium/low cost technologies that lead to moderate yields. The 2GEN (thin-film technologies) includes devices that have lower efficiency albeit are cheaper to manufacture. The 3GEN presents the use of novel materials, as well as a great variability of designs, and comprises expensive but very efficient cells. The 4GEN, also known as "inorganics-in-organics", combines the low cost/flexibility of polymer thin films with the stability of novel inorganic nanostructures (i.e., metal nanoparticles and metal oxides) with organic-based nanomaterials (i.e., carbon nanotubes, graphene and its derivatives), and are currently under investigation. The main goal of this review is to show the current state of art on photovoltaic cell technology in terms of the materials used for the manufacture, efficiency and production costs. A comprehensive comparative analysis of the four generations is performed, including the device architectures, their advantages and limitations. Special emphasis is placed on the 4GEN, where the diverse roles of the organic and nano-components are discussed. Finally, conclusions and future perspectives are summarized.

The Role of Graphene and Other 2D Materials in Solar Photovoltaics

2D materials have attracted considerable attention due to their exciting optical and electronic properties, and demonstrate immense potential for next-generation solar cells and other optoelectronic devices. With the scaling trends in photovoltaics moving toward thinner active materials, the atomically thin bodies and high flexibility of 2D materials make them the obvious choice for integration with future-generation photovoltaic technology. Not only can graphene, with its high transparency and conductivity, be used as the electrodes in solar cells, but also its ambipolar electrical transport enables it to serve as both the anode and the cathode. 2D materials beyond graphene, such as transition-metal dichalcogenides, are direct-bandgap semiconductors at the monolayer level, and they can be used as the active layer in ultrathin flexible solar cells. However, since no 2D material has been featured in the roadmap of standard photovoltaic technologies, a proper synergy is still lacking between the recently growing 2D community and the conventional solar community. A comprehensive review on the current state-of-the-art of 2D-materials-based solar photovoltaics is presented here so that the recent advances of 2D materials for solar cells can be employed for formulating the future roadmap of various photovoltaic technologies.

Efficient and stable perovskite solar cells based on perfluorinated polymers

Novel perfluorinated semiconductor compounds were introduced into the perovskite layer as additives and stable and efficient perovskite-based devices were achieved.

Additive-assisted one-step formed perovskite/hole conducting materials graded heterojunction for efficient perovskite solar cells

Solar cells based on organometallic perovskite materials have been intensively investigated as the most promising next-generation photovoltaic technology. The quality of perovskite film and the heterojunction between perovskite and charge transporting materials dominate the performance of resulting devices. Herein, we report a facile additive-assisted method to form perovskite/2, 2', 7, 7'-tetrakis (N, N-di-p-methoxyphenylamine)-9, 90-spirobifluorene (spiro-OMeTAD) graded heterojunction by one step instead of spin-coating two layers separately. The additives concentration in anti-solution is optimized to form a mixed layer where spiro-OMeTAD is dispersive in upper perovskite films with a vertical gradient, and a capping layer with appropriate thickness. The incorporation of spiro-OMeTAD in anti-solution tremendously improve the crystallinity of perovskite films while the graded heterojunction and the derived capping layer contribute to reduced interfacial losses. Moreover, poly(methyl methacrylate) as the second additive in anti-solution further passivates defects in perovskite films. As a result, we realize perovskite solar cells with a power conversion efficiency of 15.72% based on perovskite-graded heterojunction, which is far beyond the control devices. This study demonstrates an effective extension of heterojunction engineering to fabricate efficient perovskite solar cells using simplified procedures.

Does 1,8-diiodooctane affect the aggregation state of PC71BM in solution?

1,8-Diiodooctane (DIO) is an additive used in the processing of organic photovoltaics and has previously been reported, on the basis of small-angle X-ray scattering (SAXS) measurements, to deflocculate nano-aggregates of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in chlorobenzene. We have critically re-examined this finding in a series of scattering measurements using both X-rays and neutrons. With SAXS, we find that the form of the background solvent scattering is influenced by the presence of DIO, that there is substantial attenuation of the X-rays by the background solvent and that there appears to be beam-induced aggregation. All three factors call into question the suitability of SAXS for measurements on these samples. By contrast, small-angle neutron scattering (SANS) measurements, performed at concentrations of 15 mg ml-1 up to and including 40 mg ml-1, show no difference in the aggregation state for PC71BM in chlorobenzene with and without 3% DIO; we find PC71BM to be molecularly dissolved in all solvent cases. In situ film thinning measurements of spin-coated PC71BM solution with the DIO additive dry much slower. Optical imaging shows that the fullerene films possess enhanced molecular mobility in the presence of DIO and it is this which, we conclude, improves the nanomorphology and consequently solar cell performance. We propose that any compatible high boiling solvent would be expected to show the same behaviour.