Reducing Voltage Losses in Organic Photovoltaics Requires Interfacial Disorder Management

Thanks to the introduction of non‐fullerene acceptors, efficiencies of organic photovoltaics are now approaching 20%. Closing the gap with inorganic photovoltaics requires minimizing voltage losses without penalizing charge extraction, for which microstructure control is crucial. However, the complex interplay between microstructure and charge generation, recombination, and extraction has so far not been unraveled. Here, a systematic study linking device performance to distinct microstructural features via machine learning is presented. Building bi‐layer devices allows to separately study the influence of aggregation and disorder on the energies and lifetimes of bulk and interfacial states. Unambiguous assignments of specific structural motifs to the device photophysics are thus possible. It is found that the control of aggregation‐caused quenching is decisive for the exciton splitting efficiency and thus the carrier generation. Furthermore, the static disorder at the donor–acceptor interface controls the nonradiative recombination by shifting the excited state population from the bulk toward the interface. Finally, the amount of disorder in the bulk is found decisive for charge extraction. The finding that charge generation, recombination, and extraction are controlled by distinct structural features, is the key to optimizing these motifs independently, which will pave the way for organic photovoltaics toward the detailed balance limit.In this work, the aggregation‐related photophysical properties of bulk and interfacial states are disentangled through the construction of bilayer organic solar cells. Machine learning is used to enable the assignment of specific structural motifs to elementary device photophysics. The results highlight those different aspects of structural features control loss in voltage, current, and fill factor. imageDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Hellenic Foundation for Research and Innovation http://dx.doi.org/10.13039/501100013209China Scholarship Council http://dx.doi.org/10.13039/50110000454

Indacenodithienothiophene-Based Ternary Organic Solar Cells

One of the key aspects to achieve high efficiency in ternary bulk-hetorojunction solar cells is the physical and chemical compatibility between the donor materials. Here, we report the synthesis of a novel conjugated polymer (P1) containing alternating pyridyl[2,1,3]thiadiazole between two different donor fragments, dithienosilole and indacenodithienothiophene (IDTT), used as a sensitizer in a host system of indacenodithieno[3,2-b]thiophene,2,3-bis(3-(octyloxy)phenyl)quinoxaline (PIDTTQ) and [6,6]-phenyl C70 butyric acid methyl ester (PC71BM). We found that the use of the same IDTT unit in the host and guest materials does not lead to significant changes in the morphology of the ternary blend compared to the host binary. With the complementary use of optoelectronic characterizations, we found that the ternary cells suffer from a lower mobility-lifetime (μτ) product, adversely impacting the fill factor. However, the significant light harvesting in the near infrared region improvement, compensating the transport losses, results in an overall power conversion efficiency enhancement of ~7% for ternary blends as compared to the PIDTTQ:PC71BM devices

Thermally stable blue emitting terfluorene block copolymers

Spectroscopic and morphological studies on a series of rod-coil block copolymers containing terfluorene segments as the rigid blocks and polystyrene as the flexible parts demonstrate an increase in the photoluminescence intensity and the exciton lifetime as well as formation of isolated spheres as thin films upon thermal annealing in air (200 degrees C for 30 min). Moreover, no appearance of the low energy emission band centered at 520 nm was found after the same thermal treatment which permits these copolymers to emit pure blue light

Optimization of the power conversion efficiency in high bandgap pyridopyridinedithiophene-based conjugated polymers for organic photovoltaics by the random terpolymer approach

We report that the organic photovoltaic (OPV) performance of wide band gap pyridopyridinedithiophene-based conjugated polymers can be significantly improved by employing the random terpolymer approach for the development of new pyridopyridinedithiophene-based conjugated polymers. This is demonstrated by the synthesis of the alternating copolymer (P1) consisting of 3,3′-difluoro-2,2′-bithiophene and pyridopyridinedithiophene and the random terpolymer (P2) containing pyridopyridinedithiophene 3,3′-difluoro-2,2′-bithiophene and thiophene. OPV devices fabricated by P1 and P2 in combination with PC61BM and PC71BM in an inverted device configuration exhibited power conversion efficiencies (PCEs) of 1.5% and 4.0%, respectively. We identified that the main reason for the enhanced performance of the OPV devices based on the P2 random copolymer was the improved morphology (miscibility) between P2 and PCBM as compared to P1. More specifically, atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies revealed that the P1 based films showed rougher surface with clear crystallization/precipitation of the polymer chains even after the addition of chloronaphthalene (CN) to the chloroform processing solvent which significantly limited the short circuit current density (JSC), fill factor (FF) and overall performance of the prepared photovoltaic devices. On the other hand, P2 based films showed better miscibility with the acceptor particularly when processed using 5% CN containing chloroform solvent giving a respectable improvement in the PCE of the photovoltaic devices

Optimization of the power conversion efficiency in high bandgap pyridopyridinedithiophene-based conjugated polymers for organic photovoltaics by the random terpolymer approach

We report that the organic photovoltaic (OPV) performance of wide band gap pyridopyridinedithiophene-based conjugated polymers can be significantly improved by employing the random terpolymer approach for the development of new pyridopyridinedithiophene-based conjugated polymers. This is demonstrated by the synthesis of the alternating copolymer (P1) consisting of 3,3?-difluoro-2,2?-bithiophene and pyridopyridinedithiophene and the random terpolymer (P2) containing pyridopyridinedithiophene 3,3?-difluoro-2,2?-bithiophene and thiophene. OPV devices fabricated by P1 and P2 in combination with PC61BM and PC71BM in an inverted device configuration exhibited power conversion efficiencies (PCEs) of 1.5% and 4.0%, respectively. We identified that the main reason for the enhanced performance of the OPV devices based on the P2 random copolymer was the improved morphology (miscibility) between P2 and PCBM as compared to P1. More specifically, atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies revealed that the P1 based films showed rougher surface with clear crystallization/precipitation of the polymer chains even after the addition of chloronaphthalene (CN) to the chloroform processing solvent which significantly limited the short circuit current density (JSC), fill factor (FF) and overall performance of the prepared photovoltaic devices. On the other hand, P2 based films showed better miscibility with the acceptor particularly when processed using 5% CN containing chloroform solvent giving a respectable improvement in the PCE of the photovoltaic devices

Enhancement of the Power-Conversion Efficiency of Organic Solar Cells via Unveiling an Appropriate Rational Design Strategy in Indacenodithiophene-alt-quinoxaline pi-Conjugated Polymers

We report on the photovoltaic parameters, photophysical properties, optoelectronic properties, self-assembly, and morphology variations in a series of high-performance donor-acceptor (D-A) pi-conjugated polymers based on indacenodithiophene and quinoxaline moieties as a function of the number-average molecular weight ((M) over bar (n)), the nature of aryl substituents, and the enlargement of the polymer backbone. One of the most important outcome is that from the three optimization approaches followed to tune the chemical structure toward enhanced photovoltaic performance in bulk heterojunction solar cell devices with the fullerene derivative [6,6]-phenyl-C-71-butyric acid methyl ester as the electron acceptor, the choice of the aryl substituent is the most efficient rational design strategy. Incorporation of thienyl rings as substituents versus phenyl rings accelerates the electron-hole extraction process to the respective electrode, despite the slightly lower recombination lifetime and, thus, improves the electrical performance of the device. Single-junction solar cells based on ThIDT-TQxT feature a maximum power-conversion efficiency of 7.26%. This study provides significant insights toward understanding of the structure-properties-performance relationship for D-A pi-conjugated polymers in solid state, which provide helpful inputs for the design of next-generation polymeric semiconductors for organic solar cells with enhanced performance.11Nsciescopu