Role of Charge-Carrier Dynamics Toward the Fabrication of Efficient Air-Processed Organic Solar Cells.

Over the past couple of decades, immense research has been carried out to understand the photo-physics of an organic solar cell (OSC) that is important to enhance its efficiency and stability. Since OSCs undergoes complex photophysical phenomenon, studying these factors has led to designing new materials and implementing new strategies to improve efficiency in OSCs. In this regard, the invention of the non-fullerene acceptorshas greatly revolutionized the understanding of the fundamental processes occurring in OSCs. However, such vital fundamental research from device physics perspectives is carried out on glovebox (GB) processed OSCs and there is a scarcity of research on air-processed (AP) OSCs. This review will focus on charge carrier dynamics such as exciton diffusion, exciton dissociation, charge-transfer states, significance of highest occupied molecular orbital-offsets, and hole-transfer efficiencies of GB-OSCs and compare them with the available data from the AP-OSCs. Finally, key requirements for the fabrication of efficient AP-OSCs will be presented from a charge-carrier dynamics perspective. The key aspects from the charge-carrier dynamics view to fabricate efficient OSCs either from GB or air are provided.

Thickness Tolerance in Large-Area Organic Photovoltaics with Fluorine-Substituted Regioregular Conjugated Polymer.

Large areas and simple processing methods are necessary for the commercialization of organic photovoltaics (OPVs). However, the efficiency drop due to the variation in thickness of OPVs limits their large-scale applications. Regioregular polymers with good crystallinity and packing properties that exhibit high charge mobility and extraction ability can help overcome these limitations. In this study, a regioregular polymer named PDBD-2FBT was synthesized. The crystallinity and packing properties of PDBD-2FBT were enhanced by a simple thermal treatment. Using PDBD-2FBT material as a donor and Y6-HU as an acceptor, we fabricated binary blend OPV devices. The devices with optimized active layer thickness achieved a power conversion efficiency (PCE) of 14.14%. A PCE of 13.18% was maintained even in thick-film conditions (400 nm), and thickness tolerance was observed. Based on the thickness tolerance, a 5-line module measuring 36 cm2 was fabricated via the bar-coating method, and a PCE of approximately 10% was achieved.

Prospects of glove-box versus air-processed organic solar cells.

In the search for alternate green energy sources to offset dependence on fossil fuels, solar energy can certainly meet two needs with one deed: fulfil growing global energy demands due to its non-depletable nature and lower greenhouse gas emissions. As such, third generation thin film photovoltaic technology based organic solar cells (OSCs) can certainly play their role in providing electricity at a competing or lower cost than 1st and 2nd generation solar technologies. As OSCs are still at an early stage of research and development, much focus has been placed on improving power conversion efficiencies (PCEs) inside a controlled environment i.e. a glove-box (GB) filled with an inert gas such as N2. This was necessary until now, to control and study the local nanomorphology of the spin-coated blend films. For OSCs to compete with other solar energy technologies, OSCs should produce similar or even better morphologies in an open environment i.e. air, such that air-processed OSCs can result in similar PCEs in comparison to their GB-processed counterparts. In this review, we have compared GB- vs. air-processed OSCs from morphological and device physics aspects and underline the key features of efficient OSCs, processed in either GB or air.

Mechanically Stable Flexible Organic Photovoltaics with Silver Nanomesh for Indoor Applications.

Enhanced device performance of flexible organic solar cells (FOSCs) was achieved according to the development of organic solar cells (OSCs). OSCs are promising candidates as energy sources for low-power supply systems such as the Internet of Things (IoT) under indoor lighting environments. To apply FOSCs to flexible or wearable applications, they must be mechanically stable. In this study, we fabricated FOSCs with silver nanomesh (AgNM) as the bottom transparent conductive electrode (TCE). Instead of indium tin oxide (ITO), AgNMs were prepared using three pitches of 25, 50, and 100 µm with a square pattern, using a poly(ethylene terephthalate) (PET) substrate. Notably, the device using AgNMs with a pitch of 25 µm exhibited a power conversion efficiency (PCE) of 14.93% under 1 sun illumination and 17.91% under 1000 lux of light-emitting diode (LED) light conditions. Flexible devices using AgNMs maintained over 92% of their initial PCE under 1 sun illumination (PCE decreased to 12.98 from 14.04%) and over 92% when tested under 1000 lux of LED light illumination (PCE decreased to 16.57 from 17.91%) after 1000 instances of bending. These results demonstrate the advantages of using AgNMs as an alternative TCE under both 1 sun and indoor lightning environments and are promising candidates for flexible applications.

Simplified Y6-Based Nonfullerene Acceptors: In-Depth Study on Molecular Structure-Property Relation, Molecular Dynamics Simulation, and Charge Dynamics.

Two new Y6 derivatives of symmetrical YBO-2O and asymmetrical YBO-FO nonfullerene acceptors (NFAs) are prepared with a simplified synthetic procedure by incorporating octyl and fluorine substituents onto the terminal 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) moiety. By moving the alkyl substituents on the Y6 core to the terminal INCN moiety, the lowest unoccupied molecular orbital of the YBO NFAs increases without decreasing solubility, resulting in high open-circuit voltages of the devices. Molecular dynamics simulation shows that YBO-2O/-FO preferentially form core-core and terminal-terminal dimeric interactions, demonstrating their tighter packing structure and higher electron mobility than Y6, which is consistent with 2D grazing incidence X-ray scattering and space charge limited current measurements. In blend films, the hole transfer (HT) from YBO-2O/-FO to the polymer donor PM6 is studied in detail by transient absorption spectroscopy, demonstrating efficient HT from YBO-FO to PM6 with their suitable energy level alignment. Despite the simplified synthesis, YBO-FO demonstrates photovoltaic performance similar to that of Y6, exhibiting a power conversion efficiency of 15.01%. Overall, this design strategy not only simplifies the synthetic procedures but also adjusts the electrical properties by modifying the intermolecular packing and energy level alignment, suggesting a novel simplified molecular design of Y6 derivatives.

Alkylated Indacenodithiophene-Based Non-fullerene Acceptors with Extended π-Conjugation for High-Performance Large-Area Organic Solar Cells.

In this work, a series of A-D-A'-D-A-type electron acceptors based on alkylated indacenodithiophene (C8IDT), dicyanated thiophene-flanked 2,1,3-benzothiadiazole (CNDTBT), and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (FINCN) are synthesized in order to investigate the effect of substituents on their photovoltaic properties. The corresponding CNDTBT-C8IDT-INCN and CNDTBT-C8IDT-FINCN acceptors vary in their optical, electrochemical, morphological, and charge transport properties. The fluorinated-INCN-based acceptor (CNDTBT-C8IDT-FINCN) exhibits lower energy levels, improved absorptivity, narrower π-π spacing, and prominent fibrillar structures when it is blended with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo [1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PBDB-T). CNDTBT-C8IDT-FINCN exhibits a high power conversion efficiency (PCE) of 12.33% due to its high and well-balanced charge carrier mobility and distinct face-on orientation. Furthermore, large-area organic solar cells (OSCs) (active area: 55.45 cm2) with CNDTBT-C8IDT-FINCN exhibit a high PCE of 9.21%. This result demonstrates that CNDTBT-C8IDT-FINCN is a suitable and promising electron acceptor for large-area OSCs.

Synthesis of ITIC Derivatives with Extended π-Conjugation as Non-Fullerene Acceptors for Organic Solar Cells.

The realization of printed organic solar cells (OSCs) as a commercial technology is dependent on the development of high-performance photovoltaic materials suitable for large-scale device manufacture. In this study, the design, synthesis, and characterization of a series of A-D-A'-D-A-type molecular acceptors based on indacenodithienothiophene (IDTT) and thiophene-flanked 2,1,3-benzothiadiazole (DTBT) are reported. The synthesized molecular acceptors showed broader absorption ranges and narrower band gap energies than those of well-known 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno [2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC)-based molecular acceptors. Furthermore, the synthesized acceptors could tune the frontier molecular orbital energy levels, dipole moments, and their crystallinities by introducing fluorine (F) atoms and cyano (CN) groups on DTBT as a core A' unit. The cyano-substituted DTBT-based molecular acceptor (CNDTBT-IDTT-FINCN) showed a strong molar absorptivity and dipole moment, high hole/electron charge mobilities, and a favorable face-on orientation using films blended with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PBDB-T). An inverted organic photovoltaic (OPV) device using CNDTBT-IDTT-FINCN exhibits a power conversion efficiency (PCE) of 9.13% when using PBDB-T as a donor material in small cells (0.12 cm2). Sub-module devices with an active area of 55.45 cm2 are fabricated using bar-coating and exhibit PCEs of up to 7.50%. This demonstration of a high-efficiency large-area device makes CNDTBT-IDTT-FINCN a suitable and promising candidate for printed OPV devices.

Amine-Based Interfacial Engineering in Solution-Processed Organic and Perovskite Solar Cells.

Solution-processed organic solar cells (OSCs) and hybrid perovskite solar cells (PvSCs) generally require appropriate transparent electrode with a low work function, which improves the electron extraction, increases the built-in potential, and suppresses charge recombinations. Hence, interfacial modifiers between the cathode and the photoactive layer play a significant role in OSCs and PvSCs, as they provide suitable energy-level alignment, leading to desirable charge carrier selectivity and suppressing charge carrier recombinations at the interfaces. Here, we present a comprehensive study of the energy-level mapping between a transparent electrode and photoactive layers to enhance the electron-transport ability by introducing amine-based interfacial modifiers (ABIMs). Among the ABIMs, polyethylenimine ethoxylated (PEIE) incorporating inverted OSCs shows enhanced power conversion efficiencies (PCEs) from 0.32 to 9.83% due to large interfacial dipole moments, leading to a well-aligned energy level between the cathode and the photoactive layer. Furthermore, we explore the versatility of the PEIE ABIM by employing different photoactive layers with fullerene derivatives, a nonfullerene acceptor, and a perovskite layer. Promisingly, inverted nonfullerene OSCs and planar n-i-p PvSCs with PEIE ABIM show outstanding PCEs of 11.88 and 17.15%, respectively.

Performance and Uniformity Improvement in Ultrathin Cu(In,Ga)Se2 Solar Cells with a WO x Nanointerlayer at the Absorber/Transparent Back-Contact Interface.

Thinning CIGSe absorber layer to less than 500 nm is desirable for reducing the cost per unit watt of photovoltaic-generated electricity, and also, the semitransparent solar cell based on such a thin absorber can be used in bifacial and superstrate configurations if the back electrode is transparent. In this study, a WO x layer is inserted between Cu(In,Ga)Se2 (CIGSe) absorber and tin-doped indium oxide back-contact to enhance the hole collection at the back electrode. A WO x interlayer with a thickness of 6 nm is found to be optimum because it causes a ∼38% relative increase in the fill factor of a ∼450 nm thick CIGSe-based device compared to the reference device without a WO x interlayer. While fixing the thickness of CIGSe, increasing the WO x interlayer thickness to ≥6 nm results in decreases of solar cell parameters primarily because of the emergence of a GaO x interfacial layer at the CIGSe/WO x junction.

High-Efficiency Nonfullerene Polymer Solar Cells with Band gap and Absorption Tunable Donor/Acceptor Random Copolymers.

Energy level alignment between a donor and an acceptor has a critical role in determining the open-circuit voltage ( VOC) in polymer solar cells (PSCs). Also, broad absorption of the photoactive layer is required to generate a high photocurrent. Herein, non-fullerene PSCs with D/A random copolymers and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b']dithiophene (ITIC) has been demonstrated. The D/A random copolymers are composed of a 2-ethylhexylthienyl-substituted benzo[1,2- b:4,5- b']dithiophene (BDT) donor unit (D) and a fluorinated thieno[3,4- b]thiophene (TT-F) acceptor unit (A). By controlling the D/A unit ratio in the polymer backbone, it is possible to modulate both the energy levels and absorption spectra of random copolymers. As the ratio of the donor unit in the polymer back bone increases, the highest occupied molecular orbital energy level is located deeper, leading to higher VOC. Also, the absorption spectra of random copolymers become blue-shifted with an increase of the donor unit ratio; it compensates the weak absorption region of ITIC. This complementary absorption enhances the photocurrent, leading to higher power conversion efficiency (PCE). Because of the optimization of the D/A ratio of random copolymers, a notable PCE of 10.27% can be achieved in PSCs with D5A and ITIC.

Effective Molecular Engineering Approach for Employing a Halogen-Free Solvent for the Fabrication of Solution-Processed Small-Molecule Solar Cells.

To utilize the potential of small-molecule-based organic solar cells, proper designs of the photoactive materials which result in reasonable performance in a halogen-free solvent system and thickness tolerance over a range are required. One of the best approaches to achieve these requirements is via the molecular engineering of small-molecule electron donors. Here, we have modified a previously reported dithienobenzodithiophene (DTBDT)-based small molecule (SM1) via the dimerization approach, that is, the insertion of an additional DTBDT into the main backbone of the small molecule (SM2). An SM1-based photoactive film showed severe pinhole formation throughout the film when processed with a halogen-free o-xylene solvent. On the other hand, the modified small-molecule SM2 formed an excellent pinhole-free film when processed with the o-xylene solvent. Because of the dimerization of the DTBDT in the SM2 core, highly crystalline films with compact lamellae and an enhanced donor/acceptor interdigitation were formed, and all of these factors led to a high efficiency of 8.64% with chloroform and 8.37% with the o-xylene solvent systems. To the best of our knowledge, this study represents one of the best results with the SM donor and fullerene derivative acceptor materials that have shown the device performance with halogen-free solvents.

Effects on Photovoltaic Performance of Dialkyloxy-benzothiadiazole Copolymers by Varying the Thienoacene Donor.

A series of four donor-acceptor alternating copolymers based on dialkyloxy-benzothiadiazole (ROBT) as an acceptor and thienoacenes as donor units were synthesized and tested for polymer solar cells (PSCs). These new polymers had different donor units with varied electron-donating ability (thieno[3,2-b]thiophene (TT), dithieno[3,2-b:2',3'-d]thiophene (DTT), benzo[1,2-b:4,5-b']dithiophene (BDT), and naphtha[1,2-b:5,6-b']dithiophene (NDT)) in the polymer backbone. To understand the effect of these thienoacenes on the optoelectronic and photovoltaic properties of the copolymers, we systematically analyzed and compared the energy levels, crystallinity, morphology, charge recombination, and charge carrier mobility in the resulting polymers. In this series, optimized photovoltaic cells yielded power conversion efficiency (PCE) values of 6.25% (TT), 9.02% (DTT), 6.34% (BDT), and 2.29% (NDT) with different thienoacene donors. The introduction of DTT into the thienoacene-ROBT polymer enabled the generation of well-ordered molecular packings with a π-π stacking distance of 3.72 Å, high charge mobilities, and an interconnected nanofibrillar morphology in blend films. As a result, the PSC employing the polymer with DTT exhibited the highest PCE of 9.02%. Thus, our structure-property relationship studies of thienoacene-ROBT-based polymers emphasize that the molecular design of the polymers must be carefully optimized to develop high efficient PSCs. These findings will help us to understand the impact of the donor thienoacene on the optoelectronic and photovoltaic performance of polymers.