Symposium ES18—Frontiers in Organic Photovoltaics

Excitons play a central role in photogeneration in organic detectors and solar cells. The efficiency of the exciton dissociation process is directly related to molecular structure of the donor and acceptor molecules and the film morphology. In our work, we have focused on understanding the microscopic nature of these interactions in both fullerene and nonfullerene based junctions. We have developed quantum mechanical models coupled with molecular dynamics simulations to understand the role that morphology plays on the energy of the charge transfer state – i.e. the intermediate between exciton and free polaron – and ultimately its ability to generate free charge at a low expense of energy. We find that quantum confinement of the exciton by crystalline domains in dilute donor-acceptor blends can result in substantial energy shifts in the state, thereby impacting its binding energy.[1, 2] Further, we have developed models that quantitatively relate the energy loss in nonfullerene acceptors to the molecular structure itself. Ultimately, this understanding of exciton and CT state energetics has led to very high efficiency (>15%) organic solar cells, with prospects of reaching 20% in the not distant future.[3, 4] The models, experiments and implications of these findings will be discussed in my presentation. Finally, if there remains time, I will discuss confinement effects in novel quantum well structures comprising a combination of organic and inorganic semiconductors, including 2D solids.[5]

[1] X. Liu, K. Ding, A. Panda, and S. R. Forrest, "Charge transfer states in dilute donor-acceptor blend organic heterojunctions," ACS Nano, vol. 10, p. 7619, 2016.
[2] K. Ding, X. Liu, and S. R. Forrest, "Charge Transfer and Collection in Dilute Organic Donor-Acceptor Heterojunction Blends," Nano Letters, vol. 18, p. 3180, 2018.
[3] X. Che, Y. Li, Y. Qu, and S. R. Forrest, "High Fabrication Yield Organic Tandem Photovoltaics Combining Vacuum- and Solution-Processed Subcells with 15% Efficiency," Nature Energy, vol. 3, p. 422, 2018.
[4] Y. Li, J. D. Lin, X. Liu, Y. Qu, F. P. Wu, F. Liu, Z. Q. Jiang, and S. R. Forrest, "Near Infrared Ternary Tandem Solar Cells," Advanced Materials, 1804416, 2018.
[5] A. Panda, C. K. Renshaw, A. Oskooi, K. Lee, and S. R. Forrest, "Excited State and Charge Dynamics of Hybrid Organic/Inorganic Heterojunctions. II. Experiment," Phys. Rev. B, vol. 90, p. 045303, 2014.

The power conversion efficiencies (PCEs) of solution-processed single-junction organic photovoltaic (OPV) devices continue increasing and are approaching the 15% milestone. The recently-emerging non-fullerene acceptors (NFAs) have significantly accelerated this development. Further reducing the recombination losses including radiative and non-radiative pathways is crucial for minimizing the energy losses and pushing the performance of organic solar cells (OSCs) to the theoretical limit.[1-2] In particular, the non-radiative open-circuit voltage (V_OC) losses in OSCs is generally much higher compared to other photovoltaic technologies. Identifying intrinsic and extrinsic factors affecting the non-radiative V_OC losses in OSCs is crucial for next generation molecular and device engineering.[3]

In this contribution, we report on analysis of V_OC losses of representative solution-processed bulk-heterojunction OSCs based on NFAs with some of them achieving a remarkably low non-radiative V_OC loss of 0.20 V, which is beginning to align with the state-of-the-art solar technologies based on silicon, CIGS and perovskite solar cells.[4-7] Analysis on the non-radiative recombination losses with its relation to intrinsic material's photo-physical properties, e.g., absorption coefficient, stokes shift, the energy-levels, photoluminescence efficiency, as well as extrinsic thin film properties, e.g., energetic and structure disorders, will be presented.

References
[1] D. Baran, R. S. Ashraf, D. A. Hanifi, M. Abdelsamie, N. Gasparini, J. A. Rohr, S. Holliday, A. Wadsworth, S. Lockett, M. Neophytou, C. J. Emmott, J. Nelson, C. J. Brabec, A. Amassian, A. Salleo, T. Kirchartz, J. R. Durrant, I. McCulloch, Energy Environ. Sci. 9, 3783-3793 (2016).
[2] N. Li, I. McCulloch, C. J. Brabec, Energy Environ. Sci. 11, 1355-1361, (2018).
[3] J. Yao, T. Kirchartz, M. S. Vezie, M. A. Faist, W. Gong, Z. He, H. Wu, J. Troughton, T. Watson, D. Bryant, J. Nelson, Phys. Rev. Applied 4, 014020 (2015).
[4] X. Du et al., to be submitted.
[5] X. Liu, X. Du, J. Wang, C. H. Duan, X. F. Tang, T. Heumueller, G. Liu, Y. Li, Z.i Wang, J. Wang, F. Liu, N. Li, C. J. Brabec, F. Huang, Y. Cao. Adv. Energy Mater. 1801699 (2018).
[6] B. B. Fan, X. Du, F. Liu, W.K. Zhong, L. Ying, R. X. Tang, K. An, J. Xin, N. Li, W. Ma, C. J. Brabec, F. Huang,Y. Cao. Nat. Energy, DOI:10.1038/s41560-018-0263-4 (2018).
[7] P. Zhu, B. B. Fan, X. Du , X. Tang, N. Li , F. Liu, L. Ying, Z. Li, W. Zhong, C. J. Brabec, F. Huang, Y. Cao. ACS Applied Materials & Interfaces 10 (26). 22495–22503, (2018).

Energetic disorder is a defining feature of organic semiconductors and devices made of them, exerting a significant impact on transport physics and device performance. I will describe several recent results that emphasise the importance of disorder for charge generation in organic solar cells.

Although disorder is often thought to diminish device performance, it actually enhances charge generation in organic solar cells. Combined with the effect of entropy, it suffices to explain how the interfacial charges are able to overcome their strong Coulomb interaction [1]. Indeed, when entropy and disorder are included in charge separation models, the barrier to separation is dramatically reduced or absent altogether [1]. This conclusion holds even if charge delocalisation is included in the model [2]. Furthermore, kinetic simulations show that, when it comes to predicting the efficiency of charge separation, disorder is a fundamental parameter, which cannot be encapsulated using an effective measure like the bulk mobility (as is done in several standard theories) [3].

The influence of disorder on charge generation can also be used to measure the disorder, which had proven difficult to determine unambiguously in the past. In particular, energetic disorder can be determined from the dependence of the internal quantum efficiency of organic solar cells on the frequency of the incident light [4]. Experimental results for a number of systems suggest that disorder in organic semiconductors could be larger than previously reported. We also outline how the disorder of interfacial charge-transfer states can be measured from their absorption and emission spectra [5].


References
1. S. Hood and I. Kassal, “Entropy and Disorder Enable Charge Separation in Organic Solar Cells”, J. Phys. Chem. Lett. 7, 4495 (2016).
2. A. Gluchowski, K. Gray, S. Hood, and I. Kassal, “Increases in the Charge Separation Barrier in Organic Solar Cells Due to Delocalization”, J. Phys. Chem. Lett. 9, 1359 (2018).
3. J. Zhou, L. Jiang, C. Birch, and I. Kassal, “Bulk Charge-Carrier Mobility is an Imperfect Predictor of Charge-Generation Efficiency in Organic Photovoltaics”, submitted (2018).
4. S. Hood, N. Zarrabi, P. Meredith, I. Kassal, and A. Armin, “Measuring energetic disorder in organic semiconductors using the photo-generated charge-separation efficiency”, submitted (2018).
5. D. Balzer and I. Kassal, manuscript in preparation (2018)

Ternary blend polymer solar cells can harvest the sun light over a wide wavelength range from visible to near-IR region because of the complementary absorption bands. As a result, photovoltaic performances have been improved. In this talk, I will talk about photovoltaic performance of ternary blend polymer solar cells with different blend combinations: 1) a wide-bandgap conjugated polymer, a fullerene acceptor, and a near-IR dye, 2) a wide-bandgap conjugated polymer, a low-bandgap polymer, and a fullerene acceptor, and 3) a wide-bandgap conjugated polymer, a low-bandgap conjugated polymer, and a non-fullerene acceptor. First, I will talk about ternary blend polymer solar cells based on a wide-bandgap conjugated polymer (P3HT), a fullerene acceptor (PCBM), and a near-IR dye (SiPc). In this case, the key to success is dye location: dye molecules should be located at a donor/acceptor interface in ternary blend solar cells. We have demonstrated that dye location can be controlled by careful tuning of surface energy of the three materials. We also found that heterostructured dye molecules with different ligands are spontaneously located at the interface and hence can improve photocurrent generation most effectively. Second, I will talk about ternary blend polymer solar cells based on a wide-bandgap conjugated polymer (PDCBT), a low-bandgap polymer (PTB7-Th), and a fullerene acceptor (PCBM). Because of complementary absorption bands of the two donor polymers, the photocurrent can be improved in comparison with that of binary blend counterpart solar cells. Interestingly, a fill factor is also improved for the ternary blend solar cells, suggesting that charge transport is more efficient in ternary blends rather than in binary blends. Third, I will talk about ternary blend polymer solar cells based on a wide-bandgap conjugated polymer (PDCBT), a low-bandgap conjugated polymer (PBDB-T), and a non-fullerene acceptor (IT-M). Similarly to the above case, the photocurrent and fill factor are also improved in this ternary blend solar cell. Remarkably, the external quantum efficiency is as high as ~80% even at the IT-M absorption band in a near-IR wavelength region. As a result, more than 11% efficiency was obtained. This finding suggests that non-fullerene acceptors are suitable as near-IR harvesting materials.

References
1. Xu H.; Ohkita, H.; Tamai, Y.; Benten, H.; Ito S. Adv. Mater., 2015, 27, 5868.
2. Kim, H. D.; Shimizu, R.; Ohkita, H. Chem. Lett., 2018, 47, 1059-1062.
3. Wang, Y.; Kim, H. D.; Wang, B.; Ohkita, H. J. Photopolym. Sci. Technol., 2018, 31, 177-181.

Near infrared (NIR) organic semiconductors have attracted interest due to their relevance for enabling efficient semitransparent organic solar cells (OSCs). A series of narrow bandgap electron acceptors is designed and synthesized for efficient NIR organic solar cells. Extending π conjugation of donor frameworks leads to an intense intramolecular charge transfer, resulting in broad absorption profiles with band edge reaching 1000 nm. When blended with a donor polymer PTB7Th, some of these acceptors exhibit efficient charge transfer even with a small energetic offset, so as to achieve a large photogenerated current over 22 mA cm⁻² with small energy losses (≈0.49 eV) in solar cell devices. With an intense NIR absorbance, PTB7Th:IOTIC2F based cells achieve a power conversion efficiency of 12.1% with good visible transparency (52% transmittance from 370 to 740 nm). Analysis of film morphology reveals that processing with solvent additives enhances crystalline features of acceptor components, while keeping an appropriate level of donor:acceptor intermixing in the binary blends. The incorporation of the third component, ITIC2F, into the PTB7Th:IOTIC2F blends increases the device efficiency up to 12.9%. This work provides an efficient molecular design strategy to optimize nonfullerene acceptor properties for efficient NIR organic photovoltaics.

The field of non-fullerene organic solar cells (OSC) has experienced rapid development during the past few years, mainly driven by the development of novel non-fullerene acceptors and matching donor semiconductors with device efficiencies approaching 15%. However, organic solar cell material development has progressed via a trial-and-error approach with limited understanding of the materials’ structure–property relationships and the underlying device physics of non-fullerene devices. In addition, the availability of hundreds of donor and acceptor semiconductors creates a large pool of possible donor–acceptor combinations, which poses a daunting challenge for rationally screening material combinations. Resonant soft X-ray scattering (RSoXS) has proven to be a powerful technique to measure domain purity in OSC active layer blends. In polymer-fullerene systems, RSoXS has shown that higher average domain purity is generally correlated to device performance. A simple interpretation of RSoXS purity measurements is complicated by the consideration that real morphologies may be composed of three or more phases, with mixed amorphous regions in addition to relatively pure, aggregated donor and acceptor domains. Whilst charges are understood to migrate to the aggregated domains due to a favorable electronic landscape, these charges might be effectively trapped in aggregates if there is insufficient number of percolating pathways in the amorphous mixed regions. Conversely, if the mixed regions have a composition much beyond the percolation threshold or if the volume faction of the mixed regions is large, then recombination is enhanced. Therefore, understanding the role of domain purity over different length scales and controlling the mixed amorphous phase for maximum charge creation and minimum recombination remains a principal challenge for non-fullerene OSCs.

We will summarize several of our RSoXS studies that compare non-fullerene acceptor based organic solar cells having different primary chemical structures of the constituents and with significant processing variations. Our analysis suggests that mixed regions are often detrimental to good performance in these systems, typically having a pronounced negative impact on FF. Optimizing performance continues to be a delicate balance of competing factors and the volume fraction of the ideal morphology remains to be determined. However, the miscibility of blend components, which might be probed using near-edge X-ray absorption spectroscopy as a screening tool, can guide expectations of achievable phase purity and ultimately the achievable performance of the active layer.

Gaining insight into the morphology of semi-crystalline polymer films has drawn significant interest during the last decades.^[1] One key application of these polymers are organic solar cells that can play a vital role in the biggest challenge humanity faces in the current century: Reducing mankind’s carbon footprint in a quest to combat climate change. We present a tabletop imaging technique that employs a femtosecond laser system and a photoemission electron microscope. Our set-up allows the determination of the degree of order, an index for the crystallinity, and the molecular orientation within the highly ordered regions, as well as their correlation with the excited state lifetime. All these informations are gathered with a spatial resolution of up to 50 nm.^[2] In addition we will present the orientation persistence length (OPL) as a way to represent the morphology, that has previously been imaged with our technique, in a single graph. We are convinced that this procedure will enable a more direct correlation of data, acquired with an imaging technique, and results from performance or conductivity measurements gathered by e.g. time-resolved microwave conductivity measurements (TRMC) or impedance spectroscopy. These alternating current techniques displace charge carriers over a certain distance, depending on the frequency of operation. Consequently, the resulting mobilities depend, among other things, on the size of the crystalline domains and their respective orientation mismatch.^[3] The OPL, derived from the data gathered with our imaging technique, yields the average orientation mismatch and domain size for a given spatial distance. We
believe that this direct correlation of the OPL with charge carrier mobilities determined by alternating current techniques will greatly improve the fundamental understanding of the relevant processes in organic semiconductor films.

References
[1] H. Hoppe, N. S. Sariciftci, Journal of Materials Research 2004, 19, 1924–1945.
[2] A. Neff, F. Niefind, B. Abel, S. C. B. Mannsfeld, K. R. Siefermann, Advanced Materials 2017, 1701012.
[3] F. C. Grozema, L. D. A. Siebbeles, The Journal of Physical Chemistry Letters 2011, 2, 2951–2958.

OPVs based on solution processable small molecule donors have rapidly improved in performance over the last 5 years playing catch-up with polymeric counterparts. Much of the success in this material subclass has been with liquid crystalline donor molecules, which have been able to achieve high charge carrier mobilities and slow bimolecular charge recombination rates.[1] Recent work has produced the liquid crystalline benzodithiophene-quarterthiophene-rhodanine (BQR) molecule as an alternative to the high performing, but less thermally stable benzodithiophene-terthiophene-rhodanine (BTR).[2] When blended with PC71BM, BQR-based devices can achieve a power conversion efficiency over 9%, a high fill factor (FF) of 0.7, and also maintain good performance with a thick active layer. To further understand the structure-property relationships that yield high performance in this blend, we compare detailed morphological characterization from X-ray scattering [3] and transmission electron microscopy with a rigorous electrical characterization using a new high-throughput impedance-photocurrent device analysis (IPDA) measurement setup. Recently, IPDA has been demonstrated to produce reliable measurements of the charge carrier mobility and bimolecular recombination characteristics under steady-state operating conditions,[4] and we expand on this work by demonstrating its utility for understanding the physical origins of fill factor changes upon thermal annealing. Altogether, we quantify how changes in the domain purity and domain size during thermal annealing each impact the charge carrier mobility and bimolecular recombination coefficient to produce a given FF value, and this work reveals a new mechanism by which domain purity impacts OPV device performance.
[1] S. D. Collins, N. A. Ran, M. C. Heiber, and T.-Q. Nguyen, “Small is Powerful: Recent Progress in Solution-Processed Small Molecule Solar Cells” Adv. Energy Mater. 7, 1602242 (2017).
[2] P. B. Geraghty, C. Lee, J. Subbiah, W. W. H. Wong, J. L. Banal, M. A. Jameel, T. A. Smith, and D. J. Jones, “High Performance p-Type Molecular Electron Donors for OPV Applications via Alkylthiophene Catenation Chromophore Extension” Beilstein J. Org. Chem. 12, 2298 (2016).
[3] A. J. Bourque, S. Engmann, A. Fuster, C. R. Snyder, L. J. Richter, P. B. Geraghty, and D. J. Jones, “Impact of Thermal Annealing on Device Performance and Morphology for a Liquid Crystalline Small Molecule Donor” (In Preparation)
[4] M. C. Heiber, T. Okubo, S.-J. Ko, B. R. Luginbuhl, N. A. Ran, M. Wang, H. Wang, M. A. Uddin, H. Y. Woo, G. C. Bazan, and T.-Q. Nguyen, “Measuring the Competition Between Bimolecular Charge Recombination and Charge Transport in Organic Solar Cells under Operating Conditions” Energy Environ. Sci. 11, 3019 (2018).

Organic solar cells (OSCs) have drawn much interest due to their apparent advantages including light weight, low-cost production, semitransparency, flexibility and large-area manufacturing. In addition to the synthesis of donor and acceptor materials, interface engineering and device structure, the morphology of the active layer plays a crucial role in determine the power conversion efficiency (PCE) of device. Considerable research has demonstrated that the donor and acceptor should form an interpenetrating network, comprising a pure donor and acceptor phase of 10-20 nm domain size with large interfacial mixed phase, to guarantee efficient exciton separation, carrier transport and carrier collection. However, non-fullerene molecules has two-dimensional structure, its strong electronic coupling is only expected for molecules oriented with their π systems parallel to each other, leading to a weak driving force for nucleation. Moreover, non-fullerene molecules have large molecular volume, inhibiting the process of molecular diffusion, which is not benefit for the crystal growth process. As a result, the crystallization of non-fullerene molecules was suppressed, inhibiting the formation of interpenetrating networks.
Herein, we regulated the crystallization dynamics to enhance the crystallinity of non-fullerene molecules, i.e., regulating the crystallization process of donor and acceptor to reduce the nucleation barrier and enhance molecular diffusion capability. Take P3HT:O-IDTBR blends for an example, during film-forming process the crystallization of donor and acceptor tend to occur simultaneously. It would reduce the π-π interaction between like molecules, thus lead to a high nucleation barrier, which is not benefit for obtaining a film with high crystallinity. Here, TCB is selected as cosolvent to modify the film-forming kinetics due to its selective solubility and high boiling point. As a result, the crystallization process of P3HT and O-IDTBR was separated, which increase the chance of π-π electronic coupling among donors and acceptors, resulting in high crystallinity for both P3HT and OIDTBR. Another factor to influence the formation of interpenetrating networks is the molecular diffusion capability. For example, for PBDB-T:ITIC blends, due to the competition between crystallization and phase separation, the polymers are prone to form a network first, which inhibits the molecular diffusion of ITIC, resulting in weak crystallinity of ITIC. We proposed to change the crystallization sequence of donor and acceptor, i.e., ITIC crystallized first, then the polymer crystallized. Thus, molecular diffusion of ITIC could be improved without the confinement of polymer, leading to an enhanced crystallinity. The enhanced crystallinity of non-fullerene molecules is beneficial to the formation of interpenetrating network, which facilitates the electron transport and suppresses the bimolecular recombination. This work demonstrates that manipulation of crystallization sequence of donor and acceptor may be a key to further optimize the morphology of NF-OSCs.

We present the novel characterization of the morphology and molecular ordering at the nanoscale structure of organic photovoltaics (OPVs) using 4-dimensional scanning transmission electron microscopy (4D-STEM). 4D-STEM utilizes the new-generation fast pixelated STEM detector that enables fast acquisition of electron nanodiffraction patterns from the entire area of the OPV with oversampling. The fast acquisition rate of the new detector is the key to minimize the radiation damage and carbon contamination on the sample when combined with low electron dose and in situ annealing. Real space reconstruction of the nanodiffraction data based on the intensity fluctuation analysis can reveal the detailed spatial and orientation maps of the structure with unprecedented resolution and precision. The new information we can gain from this new approach includes (i) the size, orientation, distribution and volume fraction of each type of ordered domains, (ii) how different domains (e.g. donor and acceptor domains) are correlated in terms of their structure and orientation at the interface, and (iii) how processing conditions or doping affects the molecular ordering and morphology in great detail. We will demonstrate the results acquired from polymer OPV systems, blended with fullerene or non-fullerene acceptors, and also from more recent high-performance ternary systems, and show how their nanoscale structure and morphology relate to the charge transport, photovoltaic properties, and mechanical stability of the films.

Polymer solar cells (PSCs) are lead-free and can now be processed from relatively benign solvents with high efficiency of >14%. For many years, progress has relied mostly on intuition and trial-and-error approaches. For example, it was conceptually well known that fundamental molecular interactions must control the achievable PSC morphology, quantifying these interactions and predicting important transient and final device morphology parameter such as the composition of the mixed domains and their relation to performance remained unattainable for a long time. This presentation will review the history of this winding road, the lost tracks, the parallel paths, and some of the blind alleys of the field to understand “miscibility” and the phase behavior of PSC blends. Recent progress indicates that we might converge on a coherent understanding of structure-function relationships and that computer simulations might help to screen materials combinations for their suitability of high performing system. In some cases, the amorphous-amorphous phase behavior of a range of fullerene- and NFA-based systems has been determined and the metastable miscibility gap (the phase boundary) could be parameterized and thus quantified by the temperature-dependent Flory-Huggins interaction parameter χ_e(T). This has allowed to clearly show that there are two regimes: i) low χ systems with shallow quenches, where the best performance is achieved when the mixed domains reach the miscibility limit, and ii) high χ systems with deep quenches/low miscibility were the mixed domains have to be quenched into a composition that is close to the percolation threshold for electron transport. The deep quench systems are intrinsically unstable and further purification of the mixed domains often leads to burn-in degradation, although some highly vitrified systems have been discovered. We will review these developments and arguments and present some preliminary data that also hints that there might be a relationship between χ and the propensity for a system to vitrify and thus be stabilized.

Fused-ring electron acceptors (FREAs), as a family of non-fullerene (NF) acceptors, have achieved tremendous success in pushing the power conversion efficiency of organic solar cells. Here, the detailed molecular packing motifs of two extensively studied FREAs--ITIC and ITIC-Th are reported. It is revealed for the first time the long-range structure ordering along the backbone direction originated from favored end group π-π stacking. The backbone ordering could be significantly enhanced in the ternary film by the mutual mixing of ITIC and ITIC-Th, which gives rise to an improved in-plane electron mobility and better ternary device performance. The backbone ordering might be a common morphological feature of FREAs, providing explanations to previously observed small open circuit voltage loss and superior performance of FREA-based devices and guiding the future molecular design of high-performance NF acceptors.

Fullerene derivatives are dominantly used electron acceptors in organic solar cells (OSCs) for decades owing to their high electron mobility and fine phase separation. However, the drawbacks of fullerene derivatives, such as high cost, poor absorption, limited energy levels tunability and morphological instability, provide an opportunity for the development of non-fullerene electron acceptors. So far, numerous non-fullerene electron acceptors, especially small molecule electron acceptors, are designed and synthesized with outstanding performances recently. Among them, fused-ring electron acceptors are the most famous ones with good structural planarity and rigidity, which successfully boost the PCEs of OSCs over 14%. In this presentation, I will introduce couple of small electron acceptors with unfused ring cores. PCEs over 12% can be obtained for polymer solar cells based on the electron acceptors with unfused ring cores. A high PCE over 14% is delivered for tandem polymer solar cells based on the newly designed NIR photoresponsive fullerene-free accepto

Development of non-fullerene electron acceptors for organic solar cells
The power conversion efficiency (PCE) of single junction organic solar cells has increased significantly during the last decade and now approaching the threshold considered necessary for commercialisation. During this period, the structural diversity of semiconducting donor polymers for solar cells has increased dramatically, enabling accelerated development of bulk heterojunction (BHJ) organic solar cells based on polymer donor materials and molecular fullerene derivatives. However, both the fullerenes, and the low bandgap polymers typically suffer from low absorption coefficients due to weak oscillator strength. Our approach is to use P3HT as a p-type hole acceptor, and design highly absorbing, low-bandgap n-type small molecules to replace fullerenes. These fullerene acceptors not only have weak absorption, but also poor tunability of absorption over the longer wavelengths of the solar spectrum; morphological instability in thin film blends over time; high synthetic costs and limited scope for synthetic control over electronic and structural properties. For these reasons, we have developed new, synthetically simple electron acceptor materials, based on rhodanine end groups, which have much larger absorption coefficients than fullerenes, coupled with high lying LUMO energy levels, to maximize cell voltages. In BHJ devices with P3HT donor polymer, the rhodanine molecules were demonstrated to outperform fullerenes. The highest performing P3HT devices have power conversion efficiencies approaching 8%, based on a ternary blend of two rhodanine acceptors. We also demonstrate performances of over 12% with one non-fullerene acceptors in combination with lower bandgap polymers, deposited from non-chlorinated solvents.

Tapping energy from sunlight can be a sustainable way to meet the increasing world energy demand. Organic photovoltaics (OPVs) are a viable technology in this direction. For the technology to be marketable, issues of low efficiency, high instability and the short lifetime of organic solar cells (OSCs) must be addressed. Recently, the advent of non-fullerene acceptors (NFA) made it possible for OSCs to break the 10% efficiency barrier hardly attained by fullerene acceptors (FA). This is because of the interesting possibilities offered by this new class of acceptors which the FAs are bereft of, namely, the broad/strong absorption, the facile synthesis, the ease of tuning their properties and the apparent enhanced stability.^[1] Thus, in the past five years more than hundreds of NFA with applications in the OPV field were synthesized, thus, allowing for a notable current record efficiency of above 14%^[2,3] in single junction solar cells and 17.3%^[4] in a multijunction solar cells. Hence, there is an impressive shift in interest towards the NFAs. However, there has been little work on the stability of this new type of materials and devices. More importantly, there is very little to no comparative work on the photo-stability of FA vs. NFA solar cells, to really ascertain the pros and cons of the two systems.
In this work, we show the photo-stability of solar cells based on two workhorse acceptors, in both conventional and inverted structure, namely ITIC (as NFA) and [70]PCBM (as FA) blended with either PBDB-T or PTB7-Th polymer. We find that irrespective of the polymer, the cell structure, or the initial efficiency, the [70]PCBM devices are more photo-stable than the ITIC ones. This observation, however, opposes the generally assumed claim that NFA solar cells are more stable. Finally, we propose ternary blend (D:A:A) made of the PBDB-T polymer and the two acceptors as a path to making the solar cell devices more stable. These findings suggest that there is still work left to be done for OSCs to achieve an acceptable level of stability.


[1] G. Zhang, J. Zhao, P. C. Y. Chow, K. Jiang, J. Zhang, Z. Zhu, J. Zhang, F. Huang, H. Yan, Chem. Rev. 2018, acs. chemrev.7b00535
[2] S. Zhang, Y. Qin, J. Zhu, J. Hou, Adv. Mater. 2018, 1800868, 1
[3] S. Li, L. Ye, W. Zhao, H. Yan, B. Yang, D. Liu, W. Li, H. Ade, J. Hou, J. Am. Chem. Soc. 2018, jacs. 8b02695
[4] L. Meng, Y. Zhang, X. Wan, C. Li, X. Zhang, Y. Wang, X. Ke, Z. Xiao, L. Ding, R. Xia, H. Yip, Y. Cao, Y. Chen, Science 2018, 2612, 1

Over the last few years, the development of fullerene-free organic photovoltaic (OPV) cells have achieved great attention, and the power conversion efficiencies (PCEs) of small molecular acceptor-based OPV devices have surpassed 14% recently. The non-fullerene small molecules have the advantage of easily tuned absorption spectra and molecular energy levels, contributing greatly to the rapid increasing PCEs of the resulting OPV devices. Among the varied kinds of non-fullerene acceptors, the ones with acceptor-donor-acceptor structures like ITIC show the outstanding performance. In our work, we tuned the intramolecular charge transfer (ICT) effect in this kind of molecules and designed a series of highly efficient materials.
First, we weakened the ICT effect of ITIC via replacing its benzene terminal groups with thiophene units to synthesize the ITCC, then we further added methyl groups on thiophene rings to decrease the ICT effect to prepare the ITCC-m.[1] As thiophene rings and methyl groups have relatively electron-donating properties than benzene rings, the ICT effect in ITIC is weakened gradually in ITCC and ITCC-m. When compared to ITIC, the ITCC and ITCC-m show upshifted the lowest unoccupied molecular orbital (LUMO) levels and blue-shifted absorption spectra. In the OPV cells fabricated with a polymer PBDB-T as the donor material, the ITCC- and ITCC-m yielded much higher open-circuit voltages. Then, we adopted the contrary molecular design strategy to enhance the ICT effect in the small molecular acceptors. Taking IEIC as an example, we introducing the electron-donating alkoxy groups and electron-withdrawing fluorine atoms on the donor and acceptor parts of IEIC, respectively, to synthesize the IEICO and IEICO-4F.[2-3] Both of the acceptors show much-redshifted absorption spectra and thus obtained considerable enhancement of short-circuit current densities in the resulting OPV cells. Recently, we used the chlorination to replace the fluorination to further enhance the ICT effect and synthesized the IEICO-4Cl, which has very weak absorption in the visible range.[4] When blending different donors, the colors of the blend films were changed accordingly. We selected three polymer donor with varied colors and fabricated the semi-transparent OPV devices, which showed very good photovoltaic performance. We also integrating the advantages of the wide bandgap and narrow bandgap acceptors to fabricate the double-junction OPV cells. For example, we used the ITCC-m and IEICO to fabricate the front and rear devices in the tandem cells, respectively, a high PCE of 13.8% was recorded. After that, we replaced the IEICO with IEICO-4F with broader absorption spectrum, the PCE approached to 15%.
In our recent work, in order to understand the efficient charge transfer mechanism in these non-fullerene-systems, we studied the distributions of the molecular electrostatic potential (ESP) on these molecules.[5] As there is much difference in electronegative on the different part of the non-fullerene molecules, the theoretical calculations suggest that most parts of this kind of acceptor have positive ESP values. By contrast, the polymer donors usually have negative ESP on the backbones. Therefore, we think the ESP difference will induce an intermolecular filed to assist the charge transfer between the donors and acceptors in the active layers.

[1] Yao, H., Ye, L., Hou, J., Jang, B., Han, G., Cui, Y., Su, G. M., Wang, C., Gao, B., Yu, R., Zhang, H., Yi, Y., Woo, H. Y., Ade, H. and Hou, J., Adv Mater 2017, 29, 1700254.
[2] Yao, H., Chen, Y., Qin, Y., Yu, R., Cui, Y., Yang, B., Li, S., Zhang, K. and Hou, J., Adv Mater 2016, 28, 8283-8287.
[3] Yao, H., Cui, Y., Yu, R., Gao, B., Zhang, H. and Hou, J., Angew. Chem. 2017, 56, 3045-3049.
[4] Cui, Y., Yang, C., Yao, H., Zhu, J., Wang, Y., Jia, G., Gao, F. and Hou, J., Adv Mater 2017, 29.
[5] Yao, H., Qian, D., Zhang, H., Qin, Y., Xu, B., Cui, Y., Yu, R., Gao, F. and Hou, J., Chin J Chem 2018, 36, 491-494.

Organic materials and devices offer the possibility for a novel electronics realized by low-cost processing on flexible substrates. Despite their limited mobilities, organic materials excellent optoelectronic properties which allow devices with excellent performance in photovoltaic and photodetector applications. Also, devices based on organic materials can display surprisingly high stability if well encapsulated from water vapor and oxygen.

In this talk, we will first present studies about the influence of defect states. We investigate defects in a system composed of the small-molecule oligothiophene derivative DCV5T-Me blended with C-60, having power conversion efficiencies above 8% when used in a solar cell. Impedance spectroscopy shows a Gaussian distribution of trap states below the electron transport level. Comparison with drift-diffusion simulations show that the trap states mainly lead to a decrease of fill factor. Further, we discuss some new materials for solar cells and photodetectors based on bodipy and merocyanine dyes. In particular the former class allows excellent devices absorbing in the infrared spectral range with high stability.

Intrinsic photodegradation of organic solar cells, particularly bulk heterojunction (BHJ), remains a key commercialization barrier. Two types of photogenerated defects in BHJ films and related systems have recently been explored via electron paramagnetic resonance (EPR): (a) deeply trapped holes and electrons in polyelectrolyte-fullerene assemblies [1] and (b) carbon dangling bonds (C DBs) [2]; the latter were invoked in support of simulations [3,4]. In both cases, the generated EPR defect signature observed in photodegraded films weakens over several days. This talk will present new results of broadly examined various donor/acceptor structures, including of BHJ blend films with a non-fullerene acceptor, to obtain a comprehensive understanding of photodegradation. Evidence for C DBs vs deeply trapped holes and electrons will be discussed, given that the defects are generated largely by blue/UV irradiation rather than longer wavelengths. This observation clearly supports C DB formation over deeply trapped charges. The role of “hot” polarons in donor:acceptor interface C DB formation will also be discussed.
[1] R. C. Huber et al., Science 348, 1340 (2015).
[2] F. Fungura et al., Adv. Ener. Mater. 7, 1601420 (2017).
[3] J. E. Northrup, Appl. Phys. Express 6, 121601 (2013).
[4] S. Shah and R. Biswas, J. Phys. Chem. C 119, 20265 (2015).

Solution-processed polymer solar cells and perovskite solar cells are a promising class of photovoltaic devices because of noticeable progress in the device performance with their many advantages such as flexibility and easy fabrication with low cost. The power conversion efficiency (PCE) has gradually increased over the last few decades and recently exceeded 14% for polymer solar cells. On the other hand, it has been steeply improved up to 23% for perovskite solar cells. In other words, the PCE of polymer solar cells still lag far behind that of perovskite counterparts. This is mainly ascribed to the difference in voltage loss (photon energy loss, E_loss = E_g − qV_OC) between the two devices. The E_loss is typically in the range of about 0.7−1.1 eV for polymer solar cells while it is reported to be about 0.4−0.5 eV for perovskite solar cells. This indicates that such a voltage loss is dependent upon different mechanisms. In this research, we therefore studied the origin of E_loss in both polymer solar cells and perovskite solar cells. By measuring the temperature dependence of V_OC, we discussed the difference in E_loss between the two devices. As a result, we found that there are temperature-independent and -dependent voltage losses in polymer solar cells whereas there is only a temperature-dependent voltage loss in perovskite solar cells. This discrepancy results from different charge generation and recombination mechanisms between them. On the basis of these experimental data, we further discuss the potential strategies for further improvement in V_OC for both devices.

References
1. H. D. Kim et al., ACS Appl. Mater. Interfaces, 2017, 9, 19988.
2. H. D. Kim et al., Chem. Lett., 2017, 46, 253.

The presence of tie chains that connect between crystallites can critically impact the electrical properties of conjugated polymers. Yet, the community has not yet been able to directly visualize them, let alone quantify their content in conjugated polymers. We applied the Huang-Brown model, a framework commonly used to elucidate the structural origins of mechanical properties in polyolefins, to quantify the tie-chain contribution to charge transport in a series of model poly(3-hexylthiophene), P3HT, and its blends. Plotting field-effect mobility as a function of tie-chain content, as extracted from the Huang-Brown model, collapses the data on a single curve not previously seen when the charge transport property is plotted against molar mass. We find a threshold tie-chain fraction of 10^-3, below which intercrystallite connectivity limits macroscopic charge transport. X-ray paracrystallinity analysis of these P3HT films suggests intracrystallite disorder to be the bottleneck that limits charge transport when crystallites are connected. Our study affirms the importance of connectivity between crystalline domains, with the Huang-Brown model implicating long polymer chains with rigid backbone to facilitate macroscopic charge transport.

Minimizing energy loss is a key aspect to transcend the current limitations on the performance of organic photovoltaics (OPV). However, an inherent limit has set for an organic bulk-heterojunction (BHJ) blends from prominent non-geminate recombination through non-radiative charge transfer states. Our recent study on charge recombination in BHJ blends comprising a donor with a non-fullerene acceptor has shown both high photovoltaic internal quantum efficiency and high external electroluminescence quantum efficiency. Crystallographic and spectroscopic studies reveal that the fully-fused rigid molecular acceptors are not only intrinsically highly luminescent but also meets the criteria in achieving intrinsically radiative recombination within the blend, by promoting narrow distribution of charge transfer states with small structural reorganization. These results provide the important demonstration of radiative non-geminate charge recombination in an efficient OPV blend, a key step in moving towards new OPV materials and selection criteria if they are to approach the theoretical efficiency limits. In this talk, further advances in design, synthesis, processing, and tuning of the properties of fused-ring non-fullerene acceptors and corresponding polymer and small molecule donors that enable significantly enhanced performance of OPV will be discussed. At the end, several new device architectures to make tandem cells and semitransparent solar cells will be discussed to explore the full promise of these solar cells.

Conjugated polymers and molecules are very important for their applications as semiconducting materials in organic electronics. In comparison to many p-type semiconductors, n-type semiconductors are very rare. Most of molecular designs of n-type semiconductors are based on the amide units. It is important but challenging to develop new design concept of n-type semiconductors. Based on organoboron chemistry,^[1,2] we now use boron-nitrogen coordination bond (B←N) as a powerful strategy to develop n-type semiconductors.^[3]
We have proposed two approaches to design n-type conjugated polymers using B←N unit. The one is to replace a C−C unit by a B←N unit in conjugated polymers to transform a p-type polymer to a n-type polymer.^[4] The other approach is to construct novel electron-deficient building block based on B←N unit for n-type polymer.^[5] Following these two approaches, polymer electron acceptors containing B←N unit have been developed for all-polymer solar cells with power conversion efficiency of exceeding 8%.^[6]
Moreover, we use B←N unit to design n-type polyarenes.^[7] A series of polyarenes containing B←N unit have been developed. They exhibit low-lying LUMO energy levels and high electron affinity. They have been used as electron-transporting semiconductors for solution-processed organic thin-film transistors. The electron mobility has reached 0.21 cm² V^–1 s^–1. These results indicate that organoboron chemistry is a new toolbox to develop organic and polymer semiconductors.

Reference
[1] Dou, C.; Saito, S.; Mstsuo, K.; Hisaki, I.; Yamaguchi, S. Angew. Chem. Int. Ed. 2012, 51, 12206.
[2] Dou, C.; Saito, S.; Yamaguchi, S. J. Am. Chem. Soc. 2013, 135, 9346.
[3] Dou, C.; Ding, Z.; Zhang, Z.; Xie, Z.; Liu, J.; Wang, L. Angew. Chem. Int. Ed. 2015, 54, 3648.
[4] Zhao, R.; Dou, C.; Xie, Z.; Liu, J.; Wang, L. Angew. Chem. Int. Ed. 2016, 55, 5313.
[5] Dou, C.; Long, X.; Ding, Z.; Xie, Z.; Liu, J.; Wang, L. Angew. Chem. Int. Ed. 2016, 55, 1436.
[6] Long, X.; Ding, Z.; Dou, C.; Zhang, J.; Liu, J.; Wang, L. Adv. Mater. 2016, 28, 6504.
[7] Min, Y.; Dou, C.; Tian, H.; Geng, Y.; Liu, J.; Wang, L. Angew. Chem. Int. Ed. 2018, 57, 2000.

The ability to control the translation of molecular properties to bulk properties remains crucial in our development of high performance materials for organic electronics. We investigate the effect of differing core substituents on the benzodithiophene-based high-performing organic photovoltaic (OPV) material BTR. Through sidechain engineering four novel p-type small-molecule semiconducting materials based on the BTR molecular structure are designed and synthesized to carry out this study, with one of our analogues possessing an excellent PCE of over 9% in simple device architectures. Through grazing incidence wide-angle X-ray scattering (GIWAXS) depth-dependent studies we find that this efficiency is realized through an optimal active-layer morphology comprised of a spectrum of crystallite orientations. We go on to establish that high solubility in the spin-coating solution during OPV active layer thin film formation significantly improves the formation of this beneficial crystallite orientation. These results further our understanding of these high-performing class of materials and demonstrate the vital importance of solubility considerations to OPV morphology.

The charge-transfer electronic states appearing at the donor-acceptor interfaces in organic solar cells mediate exciton dissociation, charge generation, and charge recombination. Assessing their nature is thus crucial to a thorough understanding of device operation and efficiency. To date, this characterization has been carried out, both theoretically and experimentally, on the basis of models that only involve the charge-transfer state and the ground state. Here, we show that it is essential to go beyond such a two-state model and to consider explicitly as well the electronic and vibrational couplings with the local absorbing state on the donor and/or acceptor. We describe a three-state vibronic model that we have recently developed and allows us: (i) to provide a reliable description of the optical absorption features related to the charge-transfer states; (ii) to underline the erroneous interpretations stemming from the application of the semi-classical two-state model; and (iii) to rationalize how the hybridization between the local-excitation state and charge-transfer state can lead to lower non-radiative voltage losses and higher power conversion efficiencies. Our three-state approach is applied to a number of organic photovoltaic blends.

Charge separation in organic photovoltaics (OPV) is generally less efficient than in inorganic PV technologies due to the large binding energy of the photogenerated excitons and the spatially localized charge carriers in molecular semiconductors.This has motivated intense research into the basic processes that govern charge separation in OPV. An increasing number of experimental and theoretical reports show a correlation between molecular vibrational modes, charge delocalization, and the separation of photogenerated excitons. As a result, many researchers have recognised that the electrostatic band diagram of the donor-acceptor system is not sufficient to explain charge separation in OPV. In this talk, evidence demonstrating dynamic charge separation in OPV is discussed in light of recent results from the theory of open quantum systems. These theoretical results point to the need for a self-oscillating internal degree of freedom, acting as a microscopic piston in order to explain how a solar cell behaves as a heat engine, producing an inexhaustible photovoltage and photocurrent under illumination. The dynamic picture of energy conversion in OPV opens up new possibilities for the design and development of efficient new OPV systems.

In the domain of organic solar cells (OSC), the long-lasting dominance of fullerene-based acceptors has been recently overcome. Different mechanisms that could explain this enormous boost in solar cell efficiency have been suggested. Recent theoretical studies highlight the dominant role that the energy disorder and the entropy play on the charge separation efficiency at the donor-acceptor (D-A) interface. [1-2] However, implicit contributions from driving forces other than the energy disorder make unclear the individual impacts of different factors. Comparative studies between the impacts of each mechanism are thus still missing but highly desirable for the design of new materials.

Aiming to obtain deeper insights on the mechanisms impacting the charge separation efficiency at the D-A interface, here we focus on the relative impact of two driving forces: the energy bending (EB) and the energy disorder. We firstly discuss results obtained by means of density functional theory (DFT) methods, allowing to (i) demonstrate the presence of a substantial driving force (EB) between the donor-acceptor interface and the pure donor- and acceptor domains; (ii) identify the reasons for this driving force (geometric deformations and direct-contact intermolecular polarization). [3] In a second time, we compare the impact of the EB and of the energy disorder on the charge separation efficiency by means of Kinetic Monte-Carlo (KMC) simulations. Our results suggest large impact of the EB on the charge separation efficiency, which is much stronger than the effect of the energy disorder. [4]
References:
[1] Hood, S. et al. J. Phys. Chem. Lett. 2016, 7, 4495
[2] Vithanage, D. A., et al. Nature Communications, 2013, 4, 2334.
[3] Sini, G., et al., Adv. Energy Mater. 2018, 1702232
[4] L. Sousa, V. Coropceanu, D. A. da Silva Filho, G. Sini. Manuscript in preparation.

The ability to tune the optoelectronic properties of polymers through co-polymerisation, while obviously attractive in itself for applications such as photovoltaics, also presents a dimensionality headache that arises from the combination of a large number of possible monomers and the way they can be combined. To illustrate this numerically, consider a pool of 500 different monomers. Combining these monomers in all possible ways results in 125,250 ordered binary co-polymer compositions, increasing to over 250,000 when we consider that each repeat unit, if asymmetric, has two isomers. With more complex repeat units, i.e. three- and four-component co-polymers, we arrive at billions of possible combinations. These astronomically large numbers make it impossible to explore this compositional space experimentally, even with high-throughput robotic synthesis and characterisation techniques, or even computationally using standard computational chemistry approaches based around Density Functional Theory.
To address this issue from a computational perspective we have developed a computational workflow [1], in which a fast but accurate tight-binding method [2-4] is used to calculate the properties of hundreds to thousands of co-polymers and as such generates sufficient data to train a deep neural network (DNN). This DNN can subsequently predict the properties of hundred thousands to millions of co-polymers in a near instant. In our contribution we will briefly highlight the underlying machinery and discuss its predictions, both in terms of the general features of property space and co-polymers with particularly desirable properties for photovoltaics, for ordered binary co-polymers.
[1] L.Wilbraham, E. Berardo, L. Turcani, K.E. Jelfs, M.A. Zwijnenburg, J. Chem. Inf. Model. 2018 DOI: 10.1021/acs.jcim.8b00256.
[2] S. Grimme, C. Bannwarth, P. Shushkov, J. Chem. Theory Comput. 2017, 13, 1989– 2009.
[3] V. Ásgeirsson, C.A. Bauer, S. Grimme, Chem. Sci. 2017, 8, 4879– 4895.
[4] S. Grimme, C. Bannwarth, J. Chem. Phys. 2016, 145, 054103.

The non-adiabatic molecular dynamics (NA-MD) of the charge transfer (CT) process in the boron subphtalocyanine chloride (SubPc)/fullerene (C₆₀) interface is studied using our developed Libra-X software package, which is based on an interface of the Libra NA-MD library and the GAMESS electronic structure software. Particularly, the following aspects of the simulation protocol are addressed: (1) the selection of the potential employed to treat interatomic interactions and its effect on the structures of the complex and CT rates; (2) the selection of the electronic structure methodology employed; and (3) the selection of the trajectory surface hopping (TSH) methodology employed. Our analysis of the electronic structure indicates that orbital localization is affected by the distortion of the SubPc conical structure and that the CT process in SubPc/C₆₀ is facillitated by the ‘‘breathing’’ motion of SubPc. This study explains that the selection of the TSH methodology and electronic decoherence are essential for the CT simulation. Our analysis is extended to CT in SubPc/(C₆₀)_n models by increasing the number of C₆₀ molecules up to n = 4. It is revealed that the details of the interfacial SubPc/(C₆₀)_n geometry are crucial for the CT timescale. Finally, we find that the calculated CT timescale is within 2.2–5.0 ps, which is comparative to the experimentally observed timescale in the order of magnitude of ~10 ps. The implemented open-source Libra-X package is freely avaiable on the Internet at https://github.com/Quantum-Dynamics-Hub/Libra-X.

We fabricated a semi-transparent organic photovoltaic (OPV) using a carbon nanotube (CNT) sheet as the top transparent electrode and an indium tin oxide (ITO) as the bottom electrode. A bulk heterojunction layer of poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-Phenyl C₆₁ butyric acid methyl ester (PCBM) was used as the photovoltaic layer. We investigated the illumination direction dependence on the photovoltaic properties of OPVs. It was found that the power conversion efficiency under the CNT side illumination showed the lower value than that under the ITO side illumination. Especially, the short-circuit current density (J_SC) was decreased significantly.
A simple reason for the asymmetric photovoltaic properties was considered as the difference between the transparency of the two electrodes. The transparency of the ITO electrode (including the glass substrate) was higher than that of the CNT-sheet electrode. However, the decrease ratio of the J_SC was much larger than the difference of the transparencies. In addition, the external quantum efficiency (EQE) spectrum of the CNT side illumination showed a decrease of EQE in the absorption region of P3HT. These results inferred that there were other reasons for the asymmetricity.
To investigate the origin of the asymmetricity, we fabricated semi-transparent OPVs with a wide range of thickness of the photovoltaic layer (50 ~ 5000 nm) and investigated the illumination direction dependence of the photovoltaic properties. It was found that the discrepancy of the J_SC between the ITO side and the CNT side was increased with the increase of thickness. In the present OPV, electrons and holes were extracted from the ITO electrode and the CNT electrode, respectively. Hence, the difference of the illumination direction results in the difference of the transfer distance of electrons and holes. We considered that a difference between electron and hole for an annihilation probability during the transfer through the bulk heterojunction layer was the main reason for the asymmetric J_SC. We estimated an effective extraction length (EEL) of each carrier from the thickness dependence of the asymmetric ratio of J_SC. Our results indicated that the EEL of electron was shorter than that of hole. It should be noted that, in many reports, the electron mobility in the P3HT:PCBM blend film is higher than the hole mobility: the trend of mobility does not explain our results. We consider that the EEL is another useful index for the understanding of the photovoltaic mechanism in the bulk heterojunction layer.

Recently, it has been proposed that electron delocalization can promote free carrier generation from bound charge transfer (CT) excitons at organic donor/acceptor interfaces. However, how the charge separation pathway depends on the orientation of the delocalized electronic wavefunction is unclear. Here, we compare the charge separation dynamics between two model interfaces: ZnPc/C₆₀ and ZnPc/F₈ZnPc, which have a fullerene acceptor and a planar non-fullerene acceptor respectively. The temporal evolution of coherent sizes and binding energies of the CT excitons are determined by time resolved two photon photoemission spectroscopy (TR-TPPE). We observed two distinctly different charge separation pathways at the two interfaces. For the ZnPc/C₆₀ interface, hot and delocalized CT excitons relax into CT excitons with larger binding energies in a few ps. On the other hand, the binding energy of the CT exciton decreases as a function of time for the ZnPc/F₈ZnPc interface, which indicates that the CT exciton gradually transforms into free carriers. We propose that the distinctly different dynamics is related to the orientation of the electronic wavefunction. For the ZnPc/C₆₀ interface, the ZnPc has an edge-on orientation, which results in the hole confined near the interface. For the ZnPc/F₈ZnPc interface, both molecules have a face-on orientation. Hence, both electron and hole delocalize along the normal direction of the interface, which favors charge separation.

This work is supported by NSF DMR-1351716.

Substituting fullerenes in organic bulk heterojunction (BHJ) solar cells with organic small molecules or polymers as electron acceptor can improve performances in solar cells. A lot of research is therefore being dedicated to design of novel organic acceptors. Copolymers of 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene or BODIPY have been reported for photovoltaic applications. However, to the best of our knowledge in most of these reports the BODIPY polymers have been used as the donor material in BHJ and the application of this class of materials as electron acceptor in organic BHJ solar cells have not been explored.
In this work we have synthesized four (two pairs) donor-acceptor copolymers of BODIPY with Benzodithiophene (P1and P2) and Fluorene (P3 and P4). Each pair of polymers exhibit structural isomerism of the polymer backbone due to the difference in the point of attachment on the BODIPY (α or β) to the donor moiety (Benzodithiophene or fluorene). The polymers are synthesized using Stille and Suzuki coupling reactions. The α connected polymers (P1 and P3) have a coiled backbone as compared to the β connected polymers (P2 and P4). The backbone isomerism is seen to alter the HOMO and LUMO energy levels of the polymer. While all the polymers have low lying HOMO (< -5.2 eV) and hence good ambient stability, the energy of the HOMO in the aconnected polymers is than that in their bconnected counterparts. The polymers show narrow band gap (~ 1.5 eV) and absorption ranging from 700 nm to 900 nm which is influenced by the structural isomerism. The thermal stability of the α connected polymers (P1and P3) is better than the β connected ones (P2and P4). Space charge limited current (SCLC) measurements were performed to obtain the charge carrier mobility values in the polymers. Interestingly, the isomerism of the polymer backbone has no significant effect on the charge transport property of the polymers. All of the polymers exhibit ambipolar charge transport property with mobility values of the order of 10^-5cm²/V.s for the BODIPY-Benzodithiophene polymers and 10^-6cm²/V.s for the BODIPY-Fluorene polymers. Photoluminescence quenching experiments on blend of these polymers with P3HT provide evidence of photoinduced charge transport form P3HT to these polymers. However, it was observed from Time resolved photoluminescence (TRPL) studies that charge transport is more efficient in the case of the α connected polymers than their β connected counterparts. This observation is supported by results from Density functional theory calculations which show greater degree of delocalization of the LUMO orbitals in the α connected polymers, whereas in case of the β connected polymers are the LUMO orbitals are mostly localized on the BODIPY units. The better electron acceptor property of the aconnected polymers translates into better photovoltaic properties of the all polymer solar cells fabricated using P3HT:P1/P3as compared to P3HT:P2/P4. The best efficiency of these polymer solar cells is ~ 0.3 %. The low photoconversion efficiencies can be attributed to the low charge carrier mobility and hence low I_SC. Modifying the alkyl side chain on the backbone of the polymer might be instrumental in improving the charge carrier mobility. Nevertheless, this study provides a crucial insight into the effect of LUMO delocalization on the ability of a material to perform as an electron acceptor. These results could be used as roadmap to the design of BODIPY based novel electron acceptor for fullerene free solar cells.

Squaraine based organic photovoltaic (OPV) devices have shown promise through growing power conversion efficiencies (PCE) but more significantly through the improved mechanistic understanding of the role of aggregation on such efficiencies. When new squaraines are synthesized to test our mechanistic understanding, the devices first need to be optimized before conclusions are drawn. The optimization process will benefit from streamlining and we present here a Design Of Experiments (DOE) approach.The goal therefore is to determine what individual manufacturing parameters and what interactions between multiple parameters significantly affect PCE, so that future organic photovoltaics devices can be optimized more quickly. A 6 factor, 2 level DOE was set up with the six factors chosen being the blend ratio, the solvent additive, the concentration of the active layer solution, the spin speed, the annealing time and annealing temperature. We stress the value of the DOE approach, in that it will efficiently allow us to quantify the significance of the interactions between parameters from simple PCE measurements, which cannot be formally recognized through an isolated single variable study. However, we have maintained a single binary blend of squaraine (DBSQ(OH)₂) and fullerene PC₆₁BM to ensure that the PCE is only affected by changes to the parameters. We expect to see one or two significant parameters and one significant interaction. For example, Dimethylacetamide (DMA) was chosen as one of the solvent additives because we expected the highly polar solvent to impact the molecular aggregation. Initial results have quickly shown strong significance in this parameter, underlining the value of our approach. In this way, we not only standardize the parameters that will be manipulated during device optimization, but we also learn about how these parameters are entwined so as to develop even more far-reaching mechanistic perspectives.

Despite the rapidly growing demand for wearble and stretchable sensors, the understanding on the mechanical properties of n-type conjugated polymers has lagged significantly behind that of their p-type conuterpart. The understanding on the mechanical properties of semicrystalline n-type conjugated polymers is very important for developing the stretchable electronics. In this work, we investigated the intrinsic mechanical properties of naphthalene diimide (NDI) based n-type conjugated polymer, P(NDI2OD-T2) via controlling the number-average molcular weight (M_n), varing from low to very high M_n = 15, 20, 48, 103 and 163 kg mol^-1. While we observed the general increasing trend of tensile properties as a function of M_n, a sharp transition in the strain at fracture and toughness values was observed between 48 and 103 kg mol^-1 with an increase by a factor of 26 and 160, indicating the presence of the critical molecular weight. This distinct transition from brittle to ductile is mainly attributed to the presence of large fraction of amorphous regions including tie molecules and interchain entanglements, which can effectively dissipate a substantial strain energy. The molecular weight dependence of mechanical behavior coincide well with thermal, viscoelastic and microstructural and thin film morphological properties. Therefore, our work suggests design rule for n-type conjugated polymer having good compromise between mechanical reliability and electrical performance for producing highly flexible and stretchable electronics.

In this study, we designed and synthesized a new A-D-A type SMs with thiophene-phenylene based central donor unit (P3T4) and different acceptor terminal groups of dicyanovinylene (VCN) and dicyanoindenedione (INCN). The P3T4 central donor unit is designed to allow intramolecular non-covalent Coulomb interaction, which has the feature that the torsional angle is minimized to improve planarity. We investigated the influence of terminal end-groups on intermolecular packing and the resulting electrical and photovoltaic characteristics. A small change in the end-group structure of the SMs induces a significant variation in the torsional structures, molecular packing, and pristine/blend film morphology. P3T4-VCN displayed a highly planar conformation and strongly crystalline morphology even in as-cast film, exhibiting morphological tolerance toward all types of processing treatments. However, the P3T4-INCN with a marginally tilted chain conformation exhibited much weaker intermolecular interactions in the film. After morphology optimization, the fullerene-based bulk-heterojunction solar cell of tilted P3T4-INCN exhibits a power conversion efficiency (PCE) of 5.68%, which is significantly superior to that of P3T4-VCN:PC71BM (PCE = 1.29%). The clear morphological changes in P3T4-INCN:PC71BM (with processing treatments) were observed through the measurements of grazing incidence X-ray scattering (GIXS), resonant soft X-ray scattering (RSoXS), and atomic force microscopy (AFM). However, highly crystalline P3T4-VCN was insensitive to post treatments and did not exhibit noticeable morphological changes. Our results demonstrate the importance of the terminal end-group for the design of A–D–A type SMs and their sensitivity toward the post-processing treatments in optimizing their performance.

Although high temperature operation (i.e., beyond 150 Celsius) is of great interest for many electronics applications, it is fundamentally challenging to achieve stable carrier mobilities for organic semiconductors at elevated temperatures. We report a general strategy to make thermally-stable high-temperature semiconducting polymer blends, composed of interpenetrating semicrystalline conjugated polymers and high glass-transition temperature insulating matrices. When properly engineered, such polymer blends display a temperature-insensitive charge transport behavior with hole mobility exceeding 2.0 cm²/Vs across a wide temperature range from room temperature up to 220 Celsius in thin-film transistors.

Solution-processed organic semiconductors are a promising class of materials, as they have the potential to be more economically viable than conventional inorganic photovoltaic devices, where fast return-on-investment and scalable, inexpensive manufacturing are a priority. The self-assembled molecular morphology strongly affects device performance and unlimited candidate molecules and processing protocols make it challenging to identify which combinations lead to nanostructures that result in the most efficient devices. Efficiently elucidating the links between morphology and final device performance is therefore a critical prerequisite for the large-scale adoption of these materials.

We demonstrate an open-source, multi-scale computational pipeline, MorphCT, that seamlessly combines molecular dynamics, quantum chemical calculations, machine learning, and kinetic Monte Carlo simulations. MorphCT encompasses length-scales from angstroms to microns and time-scales from femtoseconds to microseconds, linking molecular morphologies to device properties with excellent experimental agreement. We evaluate both donor and acceptor molecules across several classes of semiconductor (perylenes, fullerenes, ladder molecules, polymers, and block-copolymers) for electronic device applications, while identifying optimal processing conditions for device performance. This information is expected to be useful to device manufacturers to help focus further research efforts and identify a set of design rules for producing the most efficient devices.

Despite many alluring advantages of polymer solar cells (PSCs) including low-cost fabrication process and device flexibility, comparatively low efficiency of PSCs has been a main hurdle for their full commercialization. There have been huge impressive efforts to overcome this drawback by developing active materials with high internal quantum efficiency, designing layer arrangement for high open-circuit voltage, and applying optical nanostructures that can enhance the photon absorption. Recently, novel approach of employing the spectral upconversion medium underneath cells has been arising to enable additional absorption for the solar photons with energy below the bandgap of active materials. While successful demonstrations of photovoltaics with the upconversion systems have been made for silicon or III-V solar cells, attaining meaningful enhancement of the performance of polymer-based thin film solar cells by such upconversion system is still challenging, because a reflective metallic electrode that can substantially increase the optical path length inhibits the penetration of the below-bandgap photons for excitation of the upconversion medium behind the cells, as well as of consequently upconverted above-bandgap photons. Herein we report an alternative design of PSCs to achieve the maximum benefits from the upconversion process without compromising pre-existing absorption of the incident solar photons. An opaque thick (~100 nm) silver (Ag) cathode of the typical PSCs (ITO/PFN/PTB7-Th:PC₇₁BM/MoO₃/Ag) is replaced with a multilayer electrode consisting of Ag (20 nm)/NPB (220 nm)/Ag (20 nm)/MoO₃ (100 nm) that can selectively transmit the photons with wavelengths corresponding to the excitation (~980 nm) or emission (~540 nm) of NaYF₄:Yb³⁺,Er³⁺ upconversion luminophors, thereby provoking the upconversion process and retrieving the resulting upconverted photons. To incorporate the highly effective upconversion system, a separately prepared upconversion medium deposited on a plasmonically nanostructured plate is attached to the manipulated PSCs by using a polymeric adhesive without physical or chemical damages. The optimally designed PSCs with the plasmonically enhanced upconversion medium present a boosted level of the short-circuit current, even compared to the typical PSCs.

As organic photovoltaic (OPV) devices reach efficiencies that make them attractive for commercial implementation, it is becoming increasingly important to understand the thermo-mechanical behavior of the active layers to ensure reliable operation. This is particularly important to exploit intrinsic mechanical advantages of polymers to make physically robust flexible solar cells. Currently, high efficiency OPV devices are typically processed by blending a donor polymer with a small-molecule acceptor. These polymer-small molecule films, however, often demonstrate poor mechanical behavior and are prone to fail due to the detrimental effect of the small molecules. All-polymer solar cells (all-PSCs) provide a promising alternative that can be thermally stable, and mechanically robust. However, the mechanical behavior of all-polymer films is dependent upon a number of factors of the individual polymers and interactions between the polymers. Here, we present the mechanical behavior of two high performing all-PSCs consisting of PBDT-TS1:PPDIODT and PBNDT-FTAZ:PNDI2OD-T2. We focus on the fracture energy, crack onset strain, and storage and loss moduli of the films, and explore how the film morphology, polymer ratios, and polymer molecular weights impact both mechanical behavior and device performance. We also compare these results to their polymer:fullerene counterparts. We show that the all-PSC active layers can have a number of mechanical advantages over polymer:fullerene films including greater ductility and toughness and discuss the key molecular and morphological features that govern thermo-mechanical behavior. For example, we show that for the PBNDT-FTAZ:PNDI2OD-T2 system, increasing the molecular weight of each polymer leads to both improved power conversion efficiency and cohesive fracture energy resulting in high performance mechanically robust flexible solar cells.

Direct arylation polymerization (DArP) has emerged as a viable alternative to conventional methods, such as Stille and Suzuki-Miyaura polymerizations, for conjugated polymer synthesis. DArP utilizes C-H activation, circumventing the need for the functionalization of monomers with a transmetalating reagent, reducing the overall number of synthetic steps and associated chemical waste. Through extensive study, conditions have been realized that allow for the polymer products with undetectable levels of undesired couplings (donor-donor, acceptor-acceptor, or branching defects). The incorporation of polymers prepared using DArP into electronic devices, such as organic photovoltaic applications has remained limited, however. We report the preparation of a series of perfectly-alternating conjugated polymers incorporating the acceptor thieno[3,4-c]pyrollodione and various donors and their incorporation into polymer bulk-heterojunction (BHJ) solar cells. The focus was to discover conjugated polymers that can be synthesized from simple building-blocks and their structure-property relationships unraveled. The aryl groups incorporated into the TPD copolymers include bithiophene (BT), thienothiophene (TT), thienylene-vinylene-thienylene (TVT), and ethylenedioxythiophene (EDOT). The BT based copolymer was shown to have the highest performance with a short-circuit current (J_sc) of 10.54 mA cm^-2, an open-circuit voltage (V_oc) of 0.74 V, and a fill-factor (FF) of 0.61 affording a power conversion efficiency of 4.76% with PC₆₁BM as the acceptor and fabricated under ambient conditions. This methodology is then applied to the polymer, poly[(4,4′-bis(2-butyloctoxycarbonyl-[2,2′-bithiophene]-5,5-diyl)-alt-(2,2′-bithiophene-5,5′-diyl) (PDCBT), which is made of simple constituents and the structure can be easily modified through the incorporation of various monomers to generate a library of polymer donors for BHJ solar cell applications using DArP.

Fluorine (F) substitution on conjugated polymers in polymer solar cells (PSCs) has a diverse influence of molecular properties and device performance. We synthesize a series of three D-A type conjugated polymers (PBT, PFBT and PDFBT) based on dithienothiophene and benzothiadiazole units with different numbers of F atoms to find the effect on F substitution by comparing the molecular interactions of the polymer in PSCs. In the UV-Vis absorption spectra, the pre-aggregation behavior of fluorinated polymers in o-DCB proves that polymers have strong intermolecular interactions even in solution state. Besides, more closely packed structures of F substitution polymers are observed in polymer:PC₇₁BM blends by GIXD which is beneficial for charge transport and ultimately, for current density (4.3, 13.0 and 14.5 mA cm^-2 for PBT, PFBT and PDFBT, respectively) and for the fill factor (0.41, 0.68 and 0.69 for PBT, PFBT and PDFBT, respectively) in PSCs. Consequently, the PCE of PSCs reached 7.3% without any additional treatment (annealing, solvent additive, etc) in the polymer containing di-fluorinated BT (PDFBT) which is much higher than non-fluorinated BT (PBT ~ 1%) and mono-fluorinated BT (PFBT ~ 6%).

Ternary solar cells have become an important architecture for organic bulk-heterojunction solar cells in recent years. By adjusting the composition of the active layer, the open circuit voltage (V_oc) of the solar cell can be tuned across the composition range of the active layer. It was theoretically proposed that the maximum offset between HOMO/LUMO energies of synergistic donors/acceptors couldn't exceed 0.3 eV without pinning the V_oc. Recently it was demonstrated that, the ternary blend active layer containing poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] as donor and two fullerene based acceptors having an offset of upto 0.5 eV between their LUMO energies can still result in solar cells exhibiting composition dependent V_oc. Although this shows that 0.3 eV is not the limit, the maximum achievable energy offset is still unknown for tuning of V_oc. We tried to address the issue by using a pair of two random donors consisiting of poly(3-hexylthiophene-co-3-hexylesterthiophene) (P3HT-co-3HET) and poly(3-hexylthiophene-co-3-hexoxythiophene) (P3HT-co-3HOT). This specific system was used because previous studies showed that we can easily tune the HOMO levels of the respective polymers by varying the composition of the individual monomers and can achieve a maximum offset of 1.2 eV between HOMO levels of P3HT-co-3HET and P3HT-co-3HOT. We have studied the influence of polymer surface energy on polymer-polymer miscibility, and judicially functionalized the polymers to ensure adequate mixing for alloy formation. The characterization of the ternary blends and solar cell performance will be discussed.

We designed and synthesized a series of narrow bandgap conjugated copolymers by combining two conjugated polymers with various molar ratios of naphthobis[1,2,5]thiadiazole (NT) unit. All-polymer solar cells (all-PSCs) based on the resulting ternary copolymers NT40 and N2200 exhibited an impressively high power conversion efficiency (PCE) over 8.0%, which obviously outperformed those of obtained from devices based on the NT00:NT100:N2200 blended films. Detailed characterization of the morphology of the films revealed that the incorporation of the NT unit can improve π-π stacking and a degree of phase separation conducive to exciton diffusion and charge transport. It is interesting to note that the resulting copolymer NT95, which has much higher ratio of NT unit, can be integrated with a wide-bandgap electron-donating copolymer PBTA-BO to construct all-PSCs with N2200, which presents an impressively high PCE over 10%. The improved photovoltaic performance is attributable to the combined effects of the extended absorption profile, a favorable film morphology, and more efficient charge transfer. Of particular interest is that these ternary blend films were processed using a non-halogenated solvent, 2-methyltetrahydrofuran, which is promising for practical applications. These findings lend credence to the ternary approach as a facile and promising strategy for achieving high-performance all-PSCs.

All polymer solar cells (PSCs) have attracted a considerable attention owing to their advantages such as their film-forming properties and the mechanical stability of their active layers. However, the major disadvantages of polymer/polymer blend films include poor miscibility, difficulty in controlling crystallinity, nonuniform internal phase composition, and large-scale phase segregation, all of which adversely affect the performance of the final PSC device. To overcome these shortcomings, much effort is being expended to develop a single-component active layer in PSCs which exhibits several key advantages such as simplified devices, stable internal morphology, and efficient exciton diffusion, dissociation, charge transport rather than a binary blend active layer. To date, few studies on single active material polymer solar cell (SAMPSCs) have been reported and the development of SAM is need for the realization of high-performing organic solar cell.
In this presentation, we introduced a very intriguing conjugated block copolymers (CBCs) containing a p-type oligomeric unit and an n-type oligomeric unit. CBC showed very broad complementary absorption owing to the wide band gap of the p-type block and the narrow band gap of the n-type block, and also showed complete PL quenching because of the forming of photoinduced inter- and intramolecular charge transfer states. A CBC-based SAMPSC exhibited an unusually high PCE of ~4.00 % and excellent shelf-life under ambient conditions compared with the PSC fabricated with the blend film composed of p/n-type blocks. This is the first study to report excellent shelf-life for a CBC-based SAMPSC under ambient conditions.

Terpolymer is designed by introducing a third monomer into the binary D–A copolymer structure and has a various property such as the absorption wavelength range, molecular energy level, charge carrier mobility, and solubility can be precisely tailored, often leading to improvements in the carrier mobility and power conversion efficiency (PCE) via synergistic effects between the three monomers. In other words, the chemical structures and distinct properties of the conjugated polymers can be finely controlled by this strategy.
In this presentation, terpolymer, Ter-3MTTPD was synthesized using 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b0]dithiophene as a donor and methyl thiophene-3-carboxylate (3MT) and 5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) as acceptor units to investigate its fundamental properties. The absorption spectrum of Ter-3MTTPD was intriguingly observed to be similar to the spectrum obtained by simply adding the absorption spectra of copolymers, Co-3MT and Co-TPD. This resulted in efficient complementary absorption properties after mixing NDI-Se as n-type polymer with Ter-3MTTPD. In terms of morphology, a blend film of Ter-3MTTPD:NDI-Se exhibited a relatively smooth surface and fine internal morphology, and the BHJ-type all-polymer solar cell (all-PSC) based on it provided a maximum efficiency of 7.7 %. This was further supported by the results of time-resolved photoluminescence (TRPL) in Ter-3MTTPD:NDI-Se blend film states and also showed the excellent shelf-life (~1000 h) of the all-PSC at ambient conditions compared to the other two copolymers.

In this study, we incorporated molybdenum disulfide (MoS₂) nanosheets into sol–gel-processed zinc oxide (ZnO) to form ZnO:MoS₂ composites for use as electron transport layers (ETLs) in inverted polymer solar cells featuring a binary blend active layer. We could effectively tune the energy band of the ZnO:MoS₂ composite film from 4.45 to 4.22 eV by varying the content of MoS₂ up to 0.5 wt%, such that the composite was suitable for use in bulk heterojunction photovoltaic devices based on poly[bis(5-(2-ethylhexyl)thien-2-yl)benzodithiophene–alt–(4-(2-ethylhexyl)-3-fluorothienothiophene)-2-carboxylate-2,6-diyl)] (PTB7-TH):phenyl-C₇₁-butryric acid methyl ester (PC₇₁BM). In particular, the power conversion efficiency (PCE) of the PTB7-TH:PC₇₁BM (1:1.5, w/w) device incorporating the ZnO:MoS₂ (0.5 wt%) composite layer as the ETL was 10.1%, up from 8.8% for the corresponding device featuring ZnO alone as the ETL—a relative increase of 15%. Incorporating a small amount of MoS₂ nanosheets into the ETL flattened the morphology of the ETL and resulted in enhanced fill factors and PCEs. We used ultraviolet photoelectron spectroscopy, synchrotron grazing-incidence wide-/small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy to characterize the energy band structures, internal structures, surface roughnesses, and morphologies, respectively, of the ZnO:MoS₂ composite films.

The device performance of bulk heterojunction (BHJ) polymer solar cells is very sensitive to the blend film nanomorphology including phase separation, molecular orientation, and crystalline nature, which depends significantly on the manufacturing process conditions such as temperature, humidity, and types of solvents and processing additives used. Especially in large-areas required for commercialization, these difficulties in controlling the nanoscale morphologies of both phases may debase the reproducibility of the devices. Such sensitivity to processing conditions can be alleviated easily in multilayer heterojunction structures because individual layer properties including thickness and crystalline nature can be independently controlled, thereby simplifying morphology optimization and improving device performance. However, the multilayer heterojunction structures could be limited to certain materials and solvents combinations because the top layer should be sequentially deposited from a semi-orthogonal solvent with respect to the bottom. Recently, we reported a novel technique that can form large-scale organic thin films while keeping high-quality nanomorphology in a very short time based on a Marangoni flow¹. The film formed from the process can be easily stamped and transferred on various substrates, suggesting the multilayer heterojunction structure can be effectively formed with high performing nanomorphology though the process. To obtain insight in the parameters controlling the formation of multilayer heterojunction structure, we study a bilayer structured device stamped poly[(2,5-bis(2- hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT) donor film on top of poly([N,N’-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)) (P(NDI2OD-T2)) acceptor film using the aforementioned technique. Morphology studies reveal that more ordered molecular packing with strong face-on stacking and partially penetrated PPDT2FBT molecules into the pre-deposited P(NDI2OD-T2) layer upon the solvent vapor annealing (SVA) process. Lastly, we discuss factors that control orientations of polymer backbones, which have great impact on the performance of optoelectronic devices.

[1] J. Lee et al., Nature Communications, 7, 12374 (2016).

High efficiency organic photovoltaic devices have relied on the development of new donor and acceptor materials to optimize opto-electronic properties and control charge-transfer rates. To date, the highest single-junction organic photovoltaic devices have demonstrated ~14% power conversion efficiencies using non-fullerene acceptor molecules and tandem devices with efficiencies ~17%. For both donors and acceptors, push-pull of the electron density between different parts of the molecule allows for facile tuning of the band gap for maximizing the absorption of the solar spectrum, while simultaneously altering the redox properties to control the open circuit voltage and maximizing the photo-generated exciton to free carrier dynamics for optimal short-circuit current. Of these, small molecule-based materials hold exceptional promise over polymeric systems, with reduced batch-to-batch variations, high open circuit voltages and higher hole mobilities. Yet small molecules have more confined electron density, which could make the materials more susceptible to degradation effects.

This work focuses on the degradation chemistries of a class of small molecule donors inspired by benzodithiophene terthiophene cores (BDT-3T) with rhodanine end caps, which have demonstrated 9% efficiency in single junction devices and >11% in ternary cells. Herein we show that the same opto-electronic properties can be achieved for three additional molecules by simply changing the end cap to benzothiazoleacetonitrile, pyrazole, or barbituric acid functional groups. This allows us to evaluate the stability of the 3T-BDT core as well as the role of the stability of the different end groups, each of which has unique spectroscopic signatures. Time-dependent degradation is monitored using both UV-vis and FT-IR spectroscopies, with changes in transmission related to the CIE color coordinate system. Changes in frontier orbitals and electronic structure are monitored using a combination of laboratory and synchrotron-based photoelectron spectroscopies and correlated to density functional theory predictions of degradation products.

We develop a new series of aqueous solvent-soluble fullerene derivatives and successfully fabricate polymer solar cells (PSCs) using only aqueous solvents, essential to adapt for industrial criteria. In order to optimize the processability of the fullerenes in aqueous solvents and the device performances, different fullerene mono-adducts were designed by introducing oligoethylene glycol (OEG) side-chains having different lengths and the number of branches. As the results, we achieve the power conversion efficiency (PCE) of 1.4 % for the PSCs processed by aqueous solvent in air. Significantly, the new fullerene derivative showed the superior electron mobility (1.30 × 10^-4 cm² V^-1 s^-1), which is three different order higher than that of the previously reported alcohol soluble fullerenes owing to our design of fullerene-adducts that produce efficient packing between the fullerenes while ensuring sufficient solubility in aqueous solvents. Femtosecond transient absorption spectroscopy revealed the acceptor side chain to markedly impact geminate and/or nongeminate charge recombination in the PSCs. In addition, optimizing side-chains of these fullerenes produced the well-intermixed blend morphology with aqueous solvent-soluble p-type polymers to provide hole and electron pathway. Our results provide important guidelines for the design of the electroactive materials for real human-benign fabrication of the PSCs and other organic-electronics.

While the performance of laboratory-scale polymer solar cells (PSCs) continues to grow, there are still challenges on the way to realizing efficient module devices, such as the low compatibility of the thickness-sensitive interlayer and active layer with large area coating techniques, the tremendous power loss on enlarged electrode, the frequent need for toxic solvents and tedious optimization processes used during device fabrication.
To avoid the shortcomings of these problems and achieve high-performance large area PSC modules, we carried out a series of studies.^[1] Firstly, we developed a simple and efficient approach for achieving a large area cathode interlayer with controlled thickness at the nanometer-scale using an electrostatic layer-by-layer self-assembly (eLbL) process. The eLbL films exhibited excellent cathode modification ability and can be integrated into the current large area device processing techniques.^{[2, 3]} Then, we employed a newly developed amine-containing conjugated polymer PF3N-2TNDI as the ETL and PEDOT:PSS as the HTL to construct the interconnection layer (ICL) for conventional tandem PSCs, in which the device short current density (J_sc) was largely suppressed to minimize the power loss on the electrode.^[4-6] And then, We developed a highly efficient thickness-insensitive PSCs based on PTB7-Th:PC₇₁BM that processed with single-component green solvent 2-methylanisole, in which both junction thickness limitation and solvent toxicity issues are simultaneously addressed. 16 cm² and 93 cm² PSC with the doctor-blade coating with a state-of-the-art power conversion efficiency of 7.5% and 4.5% were obtained.^[7-9] Lately, we have developed highly efficient (12.9%) non-fullerene PSCs by using sequential bilayer deposition method from non-halogenated solvents. More importantly, bilayer structure devices show less dependence on the processing condition (e.g., D/A ratio, solvent, annealing and so on). Using this bilayer processing method, PSCs can be scaled up to a larger area (1 cm²) while maintaining a high performance of 11.4% using the doctor-blade coating technique. This method offers a truly compatible processing technique for printing large area PSC modules.

Reference
[1] K. Zhang, Z. Hu, C. Sun, Z. Wu, F. Huang*, Y. Cao, Chem. Mater., 2017, 29, 141.
[2] K. Zhang, Z. Hu, R. Xu, X.-F. Jiang, H.-L. Yip*, F. Huang*, Y. Cao, Adv. Mater., 2015, 27, 3607.
[3] K. Zhang, R. Xu, W. Ge, M. Qi, G. Zhang, Q.-H. Xu, F. Huang*, Y. Cao, X. Wang*, Nano Energy, 2017, 34, 164.
[4] K. Zhang, K. Gao, R. Xia, Z. Wu, C. Sun, J. Cao, L. Qian, W. Li, S. Liu, F. Huang*, X. Peng*, L. Ding*, H.-L. Yip*, Y. Cao, Adv. Mater., 2016, 28, 4817.
[5] K. Zhang, B. Fan, R. Xia, X. Liu, Z. Hu, H. Gu, S. Liu, H.-L. Yip, L. Ying*, F. Huang*, Y. Cao Adv. Energy Mater., 2018, 8, 1703180.
[6] K. Zhang, R. Xia, B. Fan, X. Liu, Z. Wang, S. Dong, H.-L. Yip, L. Ying*, F. Huang*, Y. Cao, Adv. Mater., 2018, 30, 1803166.
[7] K. Zhang, Z. Chen, A. Armin, S. Dong, R. Xia, H.-L. Yip, S. Shoaee*, F. Huang*, Y. Cao, Solar RRL, 2017, 1700169.
[8] S. Dong, K. Zhang*, F. Huang*, Sci. China Chem., 2018, DOI:10.1007/s11426-018-9350-4.
[9] S. Dong, K. Zhang*, B. Xie, J. Xiao, H.-L. Yip, H. Yan, F. Huang*, Y. Cao, Adv. Energy Mater., 2018, Accepted.

This report covers the morphological development of an ambient processed polymer:fullerene (DT-PDPP2T-TT:PC₇₁BM) blend system deposited via blade coating from chloroform based co-solvent solutions. Due to its high molecular weight and strong aggregation behavior, this polymer exhibits remarkably low solubility and is thus challenging to solution process. Upon addition of 7.5 %_v of 1,2-dichlorobenzene (DCB), 1,8-diiodooctane (DIO), or diphenyl ether (DPE) to the chloroform host solvent, a more ideal phase separation is introduced leading to a drastic enhancement of the power conversion efficiencies from 1 % to 6 %. With all cosolvents, the emergence of an isotropic nanofibrillar texture was observed in dry films by AFM. To understand the origin of these morphological changes as a result of the addition of the co-solvent, in-situ real-time optical reflection and X-ray scattering methods were used to characterize the film drying kinetics. We find that all three co-solvents act as nucleating agents. The increased nucleation density leads to a reduced domain size and enhanced polymer crystallinity relative to films cast from chloroform alone. The resulting percolating bulk-heterojunction network is able to perform well even in relatively thick (~250 nm) films, making this system amenable to continuous roll-to-roll processing methods. Hyperspectral microscopy, a novel technique to acquire spatially resolved absorption and fluorescence data on thin films, is used to confirm that chloroform-only cast films are coarsely phase separated relative to the co-solvent cast films. For films cast from chloroform-only, strong polymer fluorescence is observed, indicating that large polymer domains exist in the blend films.

Polymer solar cells (PSC) boasting a range of advantages including low cost, light weight and superior mechanical properties represent an important family in the third-generation solar cells. In the past few years, a variety of non-fullerene small-molecule acceptors have been reported, which afforded outstanding solar cell performances and are proven competent for replacing fullerene derivatives as the acceptor materials. Compared to the small molecule-polymer blends, solar cells comprising all-polymer active layers presumably present more stable donor-acceptor phase separation morphology and thus potentially superior long-term stability. The bottleneck of further enhancing the power conversion efficiency (PCE) of all-PSCs currently lies with the performance of polymer acceptors. Thus far, the most extensively studied polymer acceptors are naphthalenediimide (NDI) polymers, and pertinent all-PSCs have achieved impressive PCEs up to over 10%. Another important type of polymer acceptors is perylenediimide (PDI) polymers. Compared to NDI polymers, more versatile synthetic methods and diverse chemical structures have been developed with PDI polymers, which allow for more convenient tuning of the electronic and morphology properties. Our group have endeavored to improve the performance of all-PSCs by designing new high-performance PDI polymer acceptors. Particularly, we reported the application of a PDI-vinylene polymer acceptor (PDI-V) in all-PSCs. With PTB7-Th as the donor, PCE of 7.57% was attained. More recently, we designed and synthesized a new polymer acceptor NDP-V, which was chemically modified from PDI-V and contained an enlarged polycyclic aromatic dicarboximide, namely naphthodiperylene diimide (NDP), as the main building block, and the PCE of all-PSC using this polymer acceptor was boosted to over 8.5%. More recently, as an effort to fine tuning the phase separation morphology, the donor polymer structure was further modified by adding chlorine substituents in the backbone. Upon adding the new polymer as a co-donor to form a ternary blend active layer, the best PCE of NDP-V based all-PSC was further improved to over 9%. Finally, since perylene and naphthalene dicarboximide polymers both bring about outstanding performance, we are currently trying to develop new acceptor polymers by further diversifying the composing aromatic dicarboximide units. A series of new polycyclic aromatic dicarboximide monomers have been designed and synthesized, which will be applied to preparing new polymer acceptors.

So called ternary device structure is being deployed to overcome the limitation of the binary solar cells for energy conversion. One of the most important tasks during design and fabrication of organic devices is to determine the energy levels of the organic materials and understand the energy level alignment at the interfaces involving semiconducting molecules and polymers. Introducing the third component in the ternary device structure means that there will be more interfaces in the devices. Therefore, it is urgent to find a general strategy to design efficient ternary solar cells from intrfacial energetic view to improve the performance.
A correlation between CV-derived oxidation potentials with UPS-derived IP is obtained for organic semiconductor films typically used in organic solar cells, enabling complementarily use of these two technologies [Sol. RRL 2018, 2, 1800122]. In organic devices, energy level alignment at interface can be described by integer charge transfer (ICT) model. Based on energy-level diagrams obtained with the ICT model, we design ternary solar cells by adding non-fullerene acceptor to binary blends, demonstrating an unprecedented increase in Voc beyond the binary reference values and a significant improvement of PCE, Jsc and thermal stability simultaneously. [Nano Energy, 2017, 37, 24]. Ternary solar cells with minimum voltage loss were also developed by combining two donor materials featuring complementary optical absorption and the same ionization potential and positive ICT energy, matching the fullerene acceptor’s negative ICT energy. This design concept is shown to enhance dissociation of all polymer donor and fullerene acceptor excitons and to suppress bimolecular and trap assisted recombination [Adv. Energy Mater. 2017, 6, 1700390].

Ternary organic solar cells (TOSCs), consisting of multiple donor or acceptor materials in one photoactive layer, recently have emerged as one of the most promising strategy for broadening the light absorption spectrum and enhancing the power conversion efficiency (PCE) of photovoltaic devices compared with traditional binary solar cells. To enable efficient separation, transport and collection of charges, the photoactive layer morphology plays a dominating role for high-performance OSCs because there are some critical challenging issues such as controlling the phase separation and domain size of the device components. We propose that differentiating all-three components from each other in the TOSCs by using advanced characterization techniques can lead to a better understanding on the phase behavior and improvement of the device performance. Here, we demonstrate high-efficiency OSCs by combining the organic semiconductor components complementing with each other, such as donors(D) (PffBT4T-2OD (C₆₂H₈₈F₂N₂S₅)_n, PTB7-Th (C₄₉H₅₇FO₂S₆)_n, PCDTBT ((C₄₃H₄₇N₃S₃)_nC₁₂H₁₀)) and acceptors(A) (PC70BM (C₈₂H₁₄O₂), IDIC (C₆₆H₆₆N₄O₂S₂), O-IDTBR (C₇₂H₈₈N₆O₂S₈)), and their photovoltaic properties and morphology modulation were studied through optimizing processing parameters such as polymer concentration, D:A1:A2 ratio, and thermal annealing. The effect of these parameters on the structural, morphological, electrical and photovoltaic properties were systematically investigated by performing UV-Vis absorption, Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) and Energy-Filtered Transmission Electron Microscopy (EFTEM). This work shows ternary systems composed of morphologically compatible acceptors at their comparable loadings can achieve fibril-like nanostructure features resulting in improving phase separation, a remarkable high PCE (>10%), significant enhancement of short-circuit current due to broadening of the absorption spectrum, and the fill factor. We attribute that the high morphology compatibility of the ternary system can benefit to optimize electron/hole mobility and diminish recombination. This study reveals the correlation of the morphology characteristic with the device performance and offer new insight from the perspective of morphology modulation for constructing efficient TOSCs.

Bulk heterojunction polymer solar cells (BHJ PSCs) and all-polymer solar cells (BHJ all-PSCs) have received great amount of interest in organic electronic applications due to their unique features such as flexibility, fine tuning of optoelectronic properties and large area application. Even though, the power conversion efficiency (PCE) of PSCs and all-PSCs reached high PCE of 13 and 10%, respectively, the study towards stability and green solvent processable π-conjugated polymers are limited. In this scenario, we have developed two new series of fluorine containing π-conjugated polymers (P1-P3) and (NAP-1 and NAP-2). Especially, the fluorinated alkoxyphenylthiophene based 2D π-conjugated polymers (P1-P3) owns good compatibility with n-type polymers in all-PSCs and delivered a maximum PCE of 7.3% for P2 based devices in inverted all-PSCs. The second series of alkoxynapthylthiophene substituted benzodithiophene based 2D π-conjugated polymers NAP-1, NAP-2 which effectively foresee the usage of green solvents and delivered a maximum PCE of 8.3% for NAP-2 in conventional BHJ PSCs without any additional treatments. In addition, we designed and synthesized a new efficient solution processable electron injection layer (EIL) PO-TAZ based on 1,3,5-triazine moiety. The application of PO-TAZ as an EIL in inverted PSCs delivered a high PCE of 10% with enhanced device stability.

Fully stretchable optoelectronics such as plastic solar cells and displays (e.g., light emitting diodes) have become a core issue in materials and engineering academia and industries. To realize such advanced technology, development of novel concepts in active materials including electrodes, transistors, photo-related organics and substrates should be accompanied with other skills Among them, organic-based transistors employing conducting polymers could be a good model to develop new logics. Various studies have attempted to enhance charge carrier transport by increasing crystallinity. However, these approaches are inevitably limited by the semicrystalline nature of conjugated polymers. Moreover, high-crystallinity conjugated polymers have proven inadequate for soft electronics applications because of their poor mechanical resilience. Herein, we demonstrate that π–π stacking can aid energy dissipation when the polymer film is subjected to strain; furthermore, the π–π stacking prevents the chains from irreversible sliding out of place due to the applied strain.

Optical management is a powerful strategy to enhance the performance of polymer solar cells by maximizing the light harvesting property of the devices. The capability to use optical model to precisely predict the light propagation property and charge generation rate within the devices allows us to design optimal device architectures with improved performance and stability.¹ Here I will discuss how to apply high throughput optical model to rapidly screen more than 10M device structures in order to identify the very best device design for extremely high performance tandem^2-4 and semitransparent polymer solar cells.^5-6 In addition, a multiple-function semitransparent polymer solar cell with both heat insulation and power generation properties will also be highlighted.⁷

References
[1] G. Zhang, R. Xia, Z. Chen, J. Xiao, H.-L. Yip, Y. Cao, et al, Advanced Energy Materials, 2018, 8, 1801609.
[2] K. Zhang, F. Huang, X. Peng, L. Ding, H.-L. Yip, Y. Cao, et al, Advanced Materials, 2016, 28, 4817.
[3] M. Li, X. Wan, H.-L. Yip, X. Peng, Y. Cao, Y. Chen et al, Nature Photonics, 2017, 11, 85.
[4] M. Li, X. Wan, L. Ding, H.-L. Yip, Y. Cao, Y. Chen, et al, Science, 2018, eaat2612.
[5] H. Shi, R. Xia, C. Sun, H.-L. Yip, et al, Advanced Energy Materials, 2017, 7, 1701121.
[6] Q. Xue, R. Xia, C. J. Brabec, H.-L. Yip, et al, Energy & Environmental Science, 2018, 11, 1688.
[7] C. Sun, R. Xia, H. Shi, F. Huang, H.-L. Yip, et al, Joule, 2018, 10.1016/j.joule.2018.06.006.

Carbon nanotubes (CNTs) have excellent charge carrier transport property and facilitates high flexibility of organic solar cells devices. In order to realize the use of CNTs in solar cells, we need to consider quality and electronic property of CNTs. We employ CNT transparent films made from aerosol CNTs which can be produced with floating iron catalysts prepared by thermal deposition of sublimated ferrocene. To obtain high electronic property of CNTs, we investigated Brønsted acid, Lewis acid, and electrophilic fullerenes for electronics application. We report CNTs-electrode-based flexible organic and perovskite solar cells. With replacing metal or metal oxide electrodes by CNT transparent electrodes, rare or expensive metal-free, highly flexible, potentially low cost, low hysteresis solar cells are realized. CNT electrodes can usually become hole-collecting electrode with acid doping, while we also developed electron-collecting CNT electrodes with penetration of fullerene derivatives into CNTs spatial networks. Both CNT cathode and anode electrodes can also be possible to be fabricated by fully solution process without vacuum deposition.
Also, we will discuss the use of fullerenes for improvement of performance in organic and perovskite solar cells. Topics will be 1) Use of C₆₀used as electron transport layers in normal-structure PSCs, 2) high-performance electron transport fullerene derivatives for inverted PSCs, 3) fullerene derivatives interpenetration into carbon nanotubes network to make transparent cathode, and 4) doping of Li⁺@C₆₀ into hole transport layers to realize stable PSCs with less than 10% efficiency loss under more than 1000 h continuous illumination.

References
J. Am. Chem. Soc. 2015, 137, 7982; Nano Lett. 2015, 15, 6665; Adv. Electron. Mater. 2016, 2, 1500341; Sci. Rep. 2016, 6, 31348; Adv. Energy Mater. 2017, 7, 1700449; Adv. Mater. 2017, 1702141; J. Phys. Chem. Lett. 2017, 8, 5395; J. Phys. Chem. C 2017, 121, 25743; J. Mater. Chem. A 2018, 6, 1382; J. Mater. Chem. A 2018, 6, 5746; Angew. Chem. Int. Ed. 2018, 57, 4607.

Light sensing and imaging represent an important important technological field, demanding the use of outstanding photodetectors (PDs). This means a low dark current and high responsivity, resulting in a low noise current and high detectivity. Currently used PDs in the visible and near infra-red spectral region are based on hard solid state materials, namely silicon and InGaAs alloys. While their performance is outstanding, they cannot provide flexible, low cost and lightweight devices and imagers. Organic semiconductors have been shown to be suitable for accomplishing these needs and have already successfully been applied in solar cells and light emitting diodes. However, organic PDs still lack a high detectivity, especially in the near-infrared which is mostly a consequence of their high dark currents (J_D) and noise currents at reverse bias voltages, where PDs usually operate.

Due to impurities and their disordered nature, trap and sub-gap states are often observed in organic materials. In this work, we investigate the generation of charge carriers through trap and charge-transfer (CT) states and their influence on J_D. When suppressing extrinsic shunt paths, we observe that J_D follows a trend with the energies (E_CT) of the subgap CT states formed at the charge generating donor-acceptor interface. Low energy CT states results in a reduced effective gap, over which charge carriers are thermally excited, resulting in increased dark currents. Furthermore, in specific donor:C₆₀ blends, we find trap concentrations of around 10¹⁶ cm^-3 with an energy of around 0.45 eV below the transport level of the acceptor material. We were able to vary the trap concentration in these blends by varying the donor concentration while keeping E_CT constant and observe that J_D increases with increasing trap concentration. Dark current-voltage characteristics were simulated by a drift-diffusion model. By including trap assisted generation (Shockley-Read-Hall like generation-recombination process), we show that the expected value of J_D increases four orders of magnitude and rules the dark JV characteristics, limiting the detectivity

By using electron/hole blocking layers and optimizing the energetic stack in our devices, we were able to achieve dark currents as low as 500 pA/cm² at -1V, leading to on/off ration of 10⁷. To the best of our knowledge, this value of dark current is among the best values reported in literature for organic devices. Moreover, our experimental findings point out to a physical process that might be present in several donor:acceptor structures, explaining the high J_D commonly observed in OPDs.

Electronic coupling and intermolecular charge transfer interactions strongly influence the excited state properties for organic solids, which dictate the effectiveness of these materials in optoelectronic devices. Excitonic coupling via long-range coulombic interaction is both theoretically and experimentally well demonstrated for many small molecule crystals. However, the spectroscopic properties of a variety of closely-packed push-pull dyes cannot be explained by established Frenkel exciton models. We will describe our theoretical work with squaraines, demonstrating how the quadrupole-quadrupole interaction must be accounted for when the intermolecular pi-stacking distance is small (~3-5 Å).
We present the results of subpicosecond transient absorption spectroscopy, which probes the excited state photophysics of an anilino-squaraine, known for its successful use in the active layer of organic photovoltaic devices. Our samples are designed with a continuum of intermolecular separations from monomers in solution, through solid solution thin films, to the fully condensed phase, demonstrating the increasing contribution of short-range quadrupolar interactions. We measure excited state kinetics that confirm species assignments. The experimental results are in excellent agreement with our theoretical modeling.
We further correlate this combination of theory and excited state characterization with our measured efficiencies in relevant small molecule organic photovoltaic devices.
The comprehensive explanation of device efficiencies and time-resolved excited state absorption spectroscopy with our excited state modeling leads us to recognize a far-reaching mechanistic bottleneck for molecular and polymeric bulk heterojunction devices. With the identification of such a bottleneck, we discuss significant predictions for improved materials research for optoelectronic devices in general.

Organic photovoltaics have several key advantages over other solar cell technologies that include the ability to tune absorption characteristics through material synthesis and device design, and the ability to make semitransparent devices with a broad range of transmittance across the solar spectrum. Exploiting these features may be the key to successful commercialization of this technology. One promising application is to integrate semitransparent organic solar cells with greenhouses. Greenhouses provide a controlled environment for plant growth that results in significant increases in yield while reducing water consumption compared to conventional farming. However, greenhouses typically require significantly more energy per kg of food produced. Reducing the need for external energy input has the opportunity to lower the cost of greenhouse-produced food, and enable production in a greater number of geographical locations. Given this desire, there has been interest in solar powered integrated greenhouses for over a decade. However, in many demonstrations the incorporation of traditional Silicon-based solar panels has shown poor performance due to the complete shading by these opaque panels. Here, we discuss the opportunity provided by semitransparent organic solar cells. The semitransparent solar cell absorption spectrum can be tuned to complement the absorption of plants rather than compete for the same light. This results in an opportunity to generate power without hindering plant growth. As part of this presentation, we show how the incorporation of semitransparent organic solar cells into greenhouses impacts the plant growth, power produced, and overall energy balance of the greenhouse. We use detailed models determined optimized solar cell designs and deployment strategies for various greenhouse geographical locations across the US. Furthermore, we experimentally demonstrate the modified growth behavior of red leaf lettuce under various semitransparent solar cells. Among the results, we show that the right combination of bulk heterojunction materials results in similar fresh weight of red leaf lettuce over its complete growth cycle compared to plant growth without solar cell coverage, highlighting the potential of this application.

Polymer solar cells (PSCs) have achieved the significant advancements in the past decade. The power conversion efficiencies (PCEs) have underwent a rapid increment to over 13% for binary PSCs and exceeded over 14% for ternary PSCs with the rapid developments of new materials and continuous innovations of the device engineering. Especially, the design of new materials plays the decisive role in pursuing higher PCEs for PSCs. Among the new emerged materials, the success of non-fullerene acceptors in PSCs in recent years have provided the great opportunity to pursue the promising photovoltaic performances for the practical applications. Currently, the PCEs of the fullerene-free PSCs have reached over 13%, which is comparable or even higher than its PC71BM-based counterparts.
Benzodifuran (BDF), a furan-based derivative, has recently received considerable attention in the developments of conjugated polymers for PSCs. Compared with BDT unit, the furan-based BDF possesses the advantages of i) more planar structure as the smaller oxygen size of furan unit; ii) the lower highest occupied molecular orbital (HOMO) energy level as the strong electron negativity of oxygen atom; iii) the denser π-π stacking behavior. In addition, the furan unit is also available from the biorenewable sources with the low cost. However, the current BDF-based polymers for PSCs in either the availabilities of the numbers of polymers or the device performances still largely lag behind the BDT-based counterparts.
In this meeting, we will report our works on the novel BDF-based conjugated polymers as donor polymer for polymer solar cell application. With the appropriate acceptor, a series of wide bandgap conjugated polymers were developed. We found that these BDF polymers possess the lower HOMO energy level and stronger π–π stacking compared to that of benzodithiophene-based counterparts. The photovoltaic performances of these BDF-based polymers were investigated in the fullerene-free PSCs with ITIC or m-ITIC as the electron acceptors. With the advantageous properties of BDF unit, the PCEs of fullerene-free PSCs were improved up to 10.28% with a higher Jsc and Voc, which is very competitive with the BDT-based counterparts. The optimized devices are among the best devices with a Voc of over 0.9 V and the PCE of more than 10%. The results demonstrate that the BDF-based polymers are the very promising candidates for the highly efficient polymer solar cells.

Organic photovoltaics (OPVs) offer several advantages over other solar energy technologies. The ability to solution-process OPVs presents an opportunity for highly scalable and cost-effective manufacturing. Despite this, significant research efforts focus on small area cells that are fabricated using techniques, such as spin coating, that are not amenable to scale-up. This talk will illustrate that the performance of photoactive layer materials in OPVs fabricated using spin coating is not a reliable measure of scale-up potential. Rather, the evaluation of a material’s performance using a scalable coating technique, such as slot-die coating, is crucial. Limitations in materials’ air stability, solids content, and solvent toxicity all present significant challenges to successful scale-up. This talk will highlight advances, such as the use of more air-stable non-fullerene electron acceptors and improvements in materials coating properties that have enabled the fabrication of >300 nm thick photoactive layer films for 12 cm² OPV modules reaching 8% power conversion processed in air using scalable techniques.

Organic photovoltaic (OPV) devices have attracted significant attention due to the low cost of fabrication, lightweight, and mechanical flexibility. Photogenerated charge transfer from active layer to carrier transport layer is critical for realizing efficient OPV devices. Defect states in the active layer or at the interface between active layer and charge transport layer can impede the charge transfer process by introducing traps, recombination centers, and limiting exciton diffusion. Here, we perform surface photovoltage spectroscopy (SPS) to study defect states in organic photovoltaic structures with different active layer systems. SPS measures surface photovoltage (SPV) as a function of incident photon energy, and has been used for obtaining electronic structure and electrically active defects of semiconductors. In SPS, sub-band illumination probes the transition between the band states and defect states within the bandgap. Here, we performed both steady-state (DC) and frequency-modulated (AC) SPS on bilayer structures containing different active layers (P3HT, P3HT:PC₆₁BM, PFBT₂Se₂Th:PC₇₁BM) and zinc oxide (ZnO) electron transport layer (ETL) to probe photogenerated transfer across this interface. AC SPS measures SPV signals of chopped monochromatic light of specific frequency. DC SPS measures stabilized SPV signals using continuous illumination from longer to shorter wavelengths. DC spectra are broader compared to AC spectra for all active layers. AC spectra overlaps with the absorption spectra of active layers, evident that AC SPS corresponds to band-to-band transitions. The sub-bandgap signals measured by DC SPS also include transitions with the bandgap states. When hole transport layer (HTL) was deposited on top of the active layer, the DC spectra are narrower than without the HTL, suggesting that the surface states are removed. In addition, we study DC SPS on dilute donor active layer (0,1, and 5wt% P3HT in PC₇₁BM) with conventional (on PEDOT) and inverted (on ZnO) architectures. We observed the positive SPV signal in the conventional structure, indicating that holes transferring to PEDOT and DC SPS magnitude increases with donor concentration. In the inverted structure, negative SPV signal indicates that electrons transferred to ZnO and DC SPS does not change with donor concentration due to the higher electron mobility of PC₇₁BM. These results give better insight on location and nature of sub-bandgap defects based on their SPS spectral signatures and frequency response.

Ladder type fused aromatic monomers have been at the forefront of conjugated semiconductor development, finding use in both donor polymers and non-fullerene acceptors for high efficiency solar cell blends. Here we discuss out recent efforts to develop flexible synthetic routes to a range of such monomers which allow the ready manipulation of the solubilizing sidechains, as well as the aromatic heterocycle in the fused unit. We show that the nature of the sidechain is important for both donor polymers, and non-fullerene acceptors. Changing from commonly used arylalkyl to simple alkyl sidechains is shown to have a positive impact on the performance of materials in single junction solar cells. The reasons behind these differences will be explored.

Over the last three decades, organic photovoltaic (OPV) cells achieved much significant progress, and power conversion efficiencies (PCEs) of over 14% were recorded.1 The development of organic active materials including electron donors and electron acceptors, device engineering, and device physics contribute greatly to the rapid increase of PCEs. In our work, we prepared many conjugated polymers and used them to fabricate the OPV devices for better performance. In 2008, we first incorporated the benzodithiophene units into the designing of polymer donors and achieved very good results.2 Then we systematically studied this type of polymers and summarized some useful strategies to improve their photovoltaic performance. For example, we used the conjugated side chains to modify the BDT units to improve the charge transport properties. Based on this method, we reported some highly efficient polymer donors for OPV cells. In 2014, we fabricated the OPV devices with over 10% efficiency using the BDT-based polymer PBDT-TS1 as an electron donor and PC71BM as an electron acceptor, which was among the top values in the OPV field.3 Over the last few years, the non-fullerene electron acceptors achieved much rapid progress. Many non-fullerene acceptors such as ITIC were reported, which showed very promising features in easily tuned optical absorption and molecular energy levels. We blended a BDT-polymer named PBDB-T with ITIC as active layers and first achieved over 10% efficiency for non-fullerene-based OPV cells.4 Then we further optimized the polymer donor and non-fullerene acceptor via introducing fluorine atoms on PBDB-T and ITIC, respectively.5 The resulting donor PBDB-T-SF and non-fullerene acceptor IT-4F showed synergistically down-shifted molecular energy levels. Interestingly, the newly prepared active layers had broad absorption spectra and higher absorption coefficient. Due to the considerable enhancement of short-circuit current densities, PCEs over 13% was obtained.
In our recent work, we designed some small molecules and used them as solid additives to optimize the morphology of the active layers. The small molecular additives were designed to have similar chemical structures with the terminal groups of highly efficient non-fullerene acceptors, resulting in very good mixability between them. Via the treatment of thermal annealing, the additive could volatilize from the active layer, and meanwhile, the intermolecular pi-pi stacking was enhanced greatly, which benefited the charge transport and led to better photovoltaic performance. Importantly, when compared with the OPV cells processed with some solvent additives with high boiling-point like DIO, the devices fabricated using volatilizable solid additives showed better device stability and reproducibility, which will benefit the largescale production of OPV cells.

References
(1) Hou, J.; Inganäs, O.; Friend, R. H.; Gao, F. Organic solar cells based on non-fullerene acceptors. Nat Mater 2018, 17, 119.
(2) Hou, J. H.; Park, M. H.; Zhang, S. Q.; Yao, Y.; Chen, L. M.; Li, J. H.; Yang, Y. Bandgap and molecular energy level control. of conjugated polymer photovoltaic materials based on benzo[1,2-b : 4,5-b']dithiophene. Macromolecules 2008, 41 (16), 6012.
(3) Zhang, S. Q.; Ye, L.; Zhao, W. C.; Yang, B.; Wang, Q.; Hou, J. H. Realizing over 10% efficiency in polymer solar cell by device optimization. Sci China Chem 2015, 58 (2), 248.
(4) Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganas, O.; Gao, F.; Hou, J. Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv Mater 2016, 28 (23), 4734.
(5) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J Am Chem Soc 2017, 139 (21), 7148.

Based on a moderate electron-deficient 6-alkyl-2-alkyl-5H-pyrrolo[3,4-f]benzotriazole-5,7(6H)-dione (TzBI) as the building block, we developed a series of donor-acceptor type of wide bandgap conjugated polymers based on benzothiophene derivatives as the electron-donating moiety. The resulting copolymers exhibited remarkable solubility in non-chlorinated solvent, such as 2-methyltetrahydrofuran, which are benefited from the incorporated solubilizing alkyl side chains in both imide and triazole moieties. By finely tuning the molecular structures of PTzBI-derivatives and non-fullerene acceptors of ITIC derivatives, we achieved an impressively high certified power conversion efficiency (PCE) over 12% at a relatively large device area of 1.0 cm². By combining FTPS and electroluminescence, we demonstrate that the Voc loss might be partially originated from a low but non-negligible charge transfer state. These findings highlight that the reduction of the bandgap to improve the efficiency requires a careful materials design to minimize non-radiative Voc losses. By incorporating siloxane substituents into the side chain of the PTzBI, we achieved PTzBI-Si that presents excellent solubility in non-chlorinated green solvents. The all-polymer solar cells constructed based on PTzBI-Si:N2200 gives a remarkable PCE of 10.1% and 11.2% for single junction and tandem all-polymer solar cells, respectively. Our results demonstrate the great potential of using PTzBI derivatives for constructing highly efficient polymer solar cells.

Forced planarization by covalently fastening adjacent aromatic units in the conjugated backbone strengthens the parallel p-orbital interactions to elongate effective conjugation length and facilitate electron delocalization, providing an effective way to reduce the band gap and enhances the intrinsic charge mobility. It is envisaged that the well-defined ladder-type small molecules can function as donor monomers to polymerize with acceptor units, leading to a new class of donor-acceptor semi-ladder copolymers that can be suitably used for solution-processable polymer solar cells. Therefore, elegant designs and synthesis have been devoted towards hybridizing electron-rich aromatic subunits into mutually fused structures in the anticipation of extracting their individual intrinsic advantages. Various accessible electron-rich aromatic and heterocycles building blocks can be chemically assembled to afford a range of fascinating ladder-type conjugated skeletons with tunable properties and functions. A series of donor-acceptor conjugated copolymers have been designed and synthesized.¹ On the other hand, the diformlyated ladder-type building blocks can be further end-capped with two acceptors to form a new class of A-D-A type small molecules. These molecules can serve as n-type materials to replace traditional fullerene materials. By combining with the suitable p-type polymers to form complementary absorption and matchable energy levels, the devices using ladder-type-based nonfullerene acceptors have achieved high power conversion efficiencies.^2-5 The applications of these materials in solar cells and OFET transistors will be discussed.
Reference
Wu, J.-S.; Cheng, S.-W.; Cheng, Y.-J.; Hsu, C.-S. Chemical Society Reviewers 2015, 44, 1113.
Lee, C.-H.; Lai, Y.-Y.; Hsu, J.-Y.; Huang, P.-K.; Cheng, Y.-J. Chemical Science 2017, 8, 2942-2951.
Cao, F.-Yi.; Tseng, C.-C.; Lin, F.-Y.; Chen, Y; Yan, H.; Cheng, Y.-J. Chemistry of Materials 2017, 29, 10045-10052
Cao, F.-Y.; Huang, W.-C.; Chang, S.-L.; Cheng, Y.-J. Chemistry of Materials 2018, 30, 4968-4977.
Chang, S.-L.; Cao, F.-Y.; Huang, W.-C.; Huang, P.-K.; Huang, K.-H.; Hsu, C.-S. Cheng, Y.-J. ACS Energy Letters 2018, 3, 1722–1729.

Over the past few decades, organic solar cells (OSCs) have made a significant progress, showing their great potential for low-cost, flexible, lightweight, portable and large-area energy-harvesting devices. Although PC₆₁BM and/or PC₇₁BM structures have been exploited successfully in OSC devices, efforts to modify the fullerene structures for further improving the device performance have been tried recently because fullerene derivatives have the inflexibility in molecular design, difficult purification, poor morphological stability, and limited light absorption in the visible region, etc. In recent years, nonfullerene acceptors have emerged as an alternative candidate of n-type materials to overcome the difficulties of fullerene derivatives in tuning optical and electronic properties. The strong and easily adjustable absorption characteristics of nonfullerene acceptors have been considered as a strong point compared to fullerene-type structures, showing a photovoltaic efficiency over ~14%. To further optimize the OSCs for next generation green energy sources, several important points need to be considered carefully. Here we discuss the fundamental correlations between molecular structure, blend morphology and device performance in fullerene- and nonfullerene based PSCs. I will also discuss the indoor light harvesting, single component OSCs and aqueous-based eco- and human-friendly fabrication of OSCs in detail.

In the current research, organic photovoltaics (OPVs) with various annealing temperature of TIPS-pentacene between ZnO (Zinc Oxide) interface layer and active layer were investigated for better current density. The TIPS-pentacene was dissolved in anisole overnight at room temperature, to give the blend solution for device fabrication and we prepared inverted bilayer electron transport layers (ETLs) Devices Structure based on poly[bis(5-(2-ethylhexyl)thien-2-yl)benzodithiophene–alt–(4-(2-ethylhexyl)-3-fluorothienothiophene)-2-carboxylate-2,6-diyl)] (PTB7-TH):phenyl-C₇₁-butryric acid methyl ester (PC₇₁BM). The ITO/ZnO/TIPS-Pentacene/PTB7-TH:PC₇₁BM/MoO₃/Ag-fabricated solar cell via spin coating provides superior photovoltaic, electrical and optical characteristics when compared with the ITO/ZnO/PTB7-TH:PC₇₁BM/MoO₃/Ag solar cell. As a result, annealing temperature of TIPS-pentacene is 100°C that significantly improves the power conversion efficiency from 8.81 to 9.59% in PTB7-TH/PC₇₁BM-based solar cells—a relative increase of 8%, which demonstrates a feasible approach to increase the efficiency of OPVs. We used synchrotron grazing-incidence wide-angle X-ray scattering, ultraviolet–visible spectroscopy, and atomic force microscopy to characterize the internal structures, transmittance, and surface roughness, respectively, of the ZnO/TIPS-Pentacene bilayer ETL films. Consequently, spin coating a slight thin film of TIPS-Pentacene small molecule onto the ZnO layer altered the morphology of the ZnO layer and resulted in enhanced current densities, fill factors, and PCEs for the devices.

One key advantage of solution-processable organic semiconductors is the opportunity of blending different materials in order to attain novel material properties and applications. The concept of ternary blend organic solar cells makes use of exactly that idea: three (or more) organic chromphores are combined to better match the solar irradiance spectrum and thus increase the amount of light absorbed, which in turn will increase the power output of the solar cell. However, charge transport limitations of many current generation polymer blends typically require rather low active layer thicknesses (around 100 nm) for optimum performance.¹ We have previously shown the design rules for non-fullerene acceptor (NFA)² and fullerene-based solar^3,4 cells with reduced charge recombination processes leading to a high short circuit current density (Jsc) and fill factor (FF) in ternary blends. Here, we report ternary organic solar cells based on a PTB7-Th (commonly known as PCE10) donor and various non-fullerene and fullerene acceptors, by investigating the improvements in photovoltaic parameters in terms of nanomorphology and photo-physics. We utilize a dual acting third component, the small molecule donor BIT-4F-T, which enables concomitant improvement in all binary systems we have tested. By adding this third component into PTB7-Th:IEICO-4F photovoltaic devices, we are able to achieve power conversion efficiencies in excess of 14%. This ternary system clearly outperforms binary reference devices, and its conversion efficiency is among the highest values reported in the literature for ternary organic solar cells. In this study, we further clarify the underlying mechanism contributing to the efficiency of this new blend by combining charge transport measurements and microstructural investigations. We find that adding 10% BIT-4F-T increases the photon harvesting window of the photoactive layers due to its complementary absorption. Moreover, BIT-4F-T also functions as a processing-aid in the ternary systems, increasing the overall volume fraction of the highly ordered region, and thus, improves charge generation and transport. These results motivated us to explore the dual effect of the third component on different non-fullerene and fullerene-based binary blends, resulting in highly efficient solar cells. Our findings reveal design rules for a third component moiety that can be used for improving all photovoltaic parameters simultaneously in ternary devices.


Gasparini et. al., Advanced Energy Materials, 2015, 5, 24.
Baran et. al., Nature Materials, 2017, 16, 363.
Gasparini et. al., Nature Energy, 2016, 1, 16118.
Gasparini et. al., Energy & Environmental Science, 2017, 10, 885.

When considering application in flexible and stretchable sensors as a power source, the bulk-heterojunction (BHJ) polymer solar cell (PSC) field have attracted tremendous attention owing to their advantages such as lightweight, flexibility, semi-transparency and roll-to-roll processability. In particular, the PSC devices are inevitably exposed to repetitive bending or stretching stress, mechanical stability of devices should be guaranteed. In this regard, the all-polymer solar cells (all-PSCs) have been considered to be a successful candidates achieving high stability and efficiency. Recently, there has been successful demonstrations that the all-PSCs exhibited superior mechanical endurance compared to fullerene based PSCs (PCBM-PSCs) due to ductile characteristic of polymer acceptor and interchain entanglements, in contrast to the brittle nature of PCBM molecules. Despite these important progresses, there is a lack of an in-depth understanding on mechanical properties of all-PSCs. In this work, we investigated the molecular weight effect of n-type polymer acceptor on the mechanical and electrical properties of all-PSCs via controlling the number-average molcular weight (M_n), varing from low to very high M_n = 15, 20, 48, 103 and 163 kg mol^-1. We observed the general increasing trend of power conversion efficiency (PCE) values as a function of M_n. In particular, the electron mobility of all-polymer blend film was found to increase by a factor of five from lowest M_n blend to highest M_n blend, while the hole mobilities are similar regardless of M_n. These enhancement of electron mobility induced the well balanced charge transport, resulting in large increment of Jsc values. Furthermore, while the low-M_n (15, 20 and 48 kg mol^-1) blend films exhibited relatively phase-separated BHJ morphology, the finely intermixed phase separation was seen in the high-M_n (103 and 163 kg mol^-1) blends, indicating large interfacial area between polymer donor and polymer acceptor, which are benefical for charge dissociation. The tensile and fracture properties followed similar trend in device peformance with the higher M_n blend films showing better mechanical robustness, but larger improvements were observed between 48 and 103 kg mol^-1 blend films. This better mechanical ductility of 103 and 163 kg mol^-1 blend films is mainly attributed to a larger fraction of amorphous regions where the interchain entaglements exist, therby allowing to dissipate a substantial strain energy. Furthermore, we also demonstrated the contrast in mechanical compliances depeding on different acceptor types (small molecule vs. polymer). Desptie the very high PCE devices based on ITIC have been reported recently, the active layer including ITIC showed much inferior mechanical stability compared to that of all-PSCs due to the brittle nature of ITIC.