Symposium EN04—Next-Generation Organic Photovoltaics—Fundamentals and Applications for Flexible, Stretchable and Wearable Devices

Organic photovoltaics (OPVs) have advantages in that they are lightweight and flexible and prepared by cost effective roll-to-roll coating and printing techniques. OPV efficiency close to 19% was reported in a small cell, which showing the great potential of OPV technology as an alternative source of green energy. To further develop OPVs from lab-scale researches into industrial applications, OPV technology prepared at a small area needs to be transferred to a large scale such as modules. It is highly important to develop new organic materials including conjugated polymers and understand the fundamental working mechanism of the organic materials. In this presentation, we developed new conjugated polymers for large-area OPV and studied the working mechanism of organic materials and photovoltaic performance in large area OPVs. We successfully transferred the material and device technologies realized at a small area below 0.2 cm2 into a large area of 60 cm2 using non-spin-coating methods. This presentation will introduce the relationship of organic materials and photovoltaic properties of large-area OPV devices.

In bulk heterojunction organic solar cells (OSCs), charge generation and transport pathways are dictated by the phase behavior of the donor:acceptor active layer. Unlike in well-studied fullerene-based OSCs, non-fullerene (NF) acceptors are partially miscible in the polymer donor matrix, allowing for an amorphous mixed phase with a complex charge transfer (CT) network. This mixed phase plays an important role in OSC performance; however, little is known about the amount, or even presence, of mixed NFA-polymer regions in high performance NF OSCs.
In this study, we examine the influence of blend composition on the phase behavior and interfacial CT state in the state-of-the-art NF blend PBDB-T-2F:BTP-4F (colloquially known as PM6:Y6). We find that PM6-rich blends are dominated by an amorphous mixed phase, with a small fraction of donor crystallites. In contrast, Y6-rich blends can phase separate upon annealing, resulting in larger fractions of pure-donor and pure-acceptor crystallites. These results are supported by thermal analysis and grazing incidence wide-angle X-ray scattering (GIWAXS), where we have identified an in-plane backbone feature that has been previously misassigned to a second order lamellar peak. Finally, using highly sensitive Fourier transform photocurrent spectroscopy (FTPS) to measure quantum efficiency, we relate the composition-dependent phase behavior to favorable CT energy cascades and sub-bandgap disorder. Our results present a tunable phase space that will help develop design rules to optimize high-performing NF-based solar cells.

Blends of conjugated and commodity polymers allow for design and balance between the conjugated polymer’s electronic properties and the commodity polymer’s variable physical properties. This balance is vital in flexible wearable organic electronics such as electronic skin, biosensors, or haptics, which require increased flexibility and durability, while still maintaining good electronic performance. Understanding the fundamental interactions between conjugated polymers and the matrix polymers is essential for designing and optimizing organic electronic devices. It is often difficult to study such materials with microscopy techniques, due to reduced contrast between components, as well as the small length scales that are involved. Through the use of selectively deuterated components and careful system design, small angle x-ray and neutron scattering (SAXS, SANS) can be used to investigate the nano and micro scale structures. Using these scattering methods, we have probed the morphology of conjugated polymer blends in both solid and solution states, allowing us to characterize both the phase separated domains and self-assembled structures of the conjugated polymers.
Initial research on polystyrene and conjugated polymer blends has shown strong relationships between morphology and resulting electronic properties [1]. Recently, solution state SANS with contrast matching was used to investigate the effect of the matrix polymer on the phase separation and assembly of the conjugated polymer. Moreover, the work has also been expanded to study blends of conjugated polymers with thermoplastic elastomeric-triblock copolymers (PS-PI-PS and PS-PBD-PS). SAXS and USAXS were used to examine the phase separated structure of the tri-block matrix polymer as a function of the conjugated polymer concentration and identity. We observed a significant shift in the characteristic peak of the tri-block polymer, indicating that the addition of the conjugated polymer forced the tri-block’s phase-separated lamellar structure to swell. We have also observed alignment of the lattice structures with the addition of heat and shear forces, suggesting a process for manipulating the orientation of the lattice for material optimization. We aim to develop models to relate the morphology of elastomeric conjugated polymer blends to their composition and processing and to identify relationships between resulting structures and the electronic properties. Ultimately, we aim to inform the optimization of flexible conductive polymeric materials.
[1] Wolf, C. M. et al. Blend Morphology in Polythiophene−Polystyrene Composites from Neutron and X-ray Scattering. doi:10.1021/acs.macromol.0c02512

With the recent reports on power conversion efficiencies surpassing 18%, organic photovoltaic is reaching new heights and is considered as an encouraging contender to other co-existing technologies. The rapid progress is due to the design and emergence of variety of both novel donors and non-fullerene acceptors. Despite this, the necessity of the design combined with efficient systems and the photo-physical processes is still lacking, and also tracking the associated loss mechanisms in devices is still elusive. Herein, we focus on the impact of the ionization energy in selected non-fullerene acceptors IT-4F, NCBDT-4Cl, BTCIC, IEICO-4F, IT-DM and O-IDTBR paired with the PBDB-T-SF and PBDB-T-2Cl donors in bulk heterojunction solar cells. More specifically, the investigation of the impact of the energetic offset on photophysical parameters and losses will be carried out. With combination of time resolved spectroscopy and electro-optical measurements we infer that the PBDB-T-SF:IT-4F with the highest offset exhibit the highest photocurrent (22 mA/cm²) and fill factor (64%) due to significantly less geminate - and non-geminate recombination and field independent charge generation. In contrast, the systems with the lowest offset O-IDTBR and IEICO-4F paired with both donors yield lowest photocurrent and fill factor due to considerable geminate recombination, poor charge generation, carrier mobility, and substantial field dependent charge generation. In addition to this, the IT-DM based system, with an intermediate offset displays a photocurrent of 12 mA/cm² and a fill factor of 49% caused by substantial non-geminate recombination compared to the IT-4F based device. We specifically infer a good photoluminescence quenching but limited internal quantum efficiency in O-IDTBR and IT-DM based systems when paired with both donors. Our findings provide important performance-charge separation relations for the design of novel polymer:NFA systems.

Organic solar cells (OSCs) are currently experiencing a second golden age thanks to the development of novel non-fullerene acceptors (NFAs). Remarkably, high-performance NFA blends exhibit short-circuit currents (J_sc) above 25 mAcm^-2, pointing to excellent charge generation despite a low energy offset at the donor/acceptor (D/A) heterojunction [1]. Whilst OSCs match their inorganic competitors in terms of current production, they still have to optimise their voltage loss and fill factor (FF), rarely displaying fill factor of 80% and above. We have previously shown that most state-of-the-art organic solar cells exhibit a sufficiently large drift length to guarantee efficient charge extraction at short circuit, but that they still suffer from too small diffusion lengths, typically half of the active layer thickness, of photogenerated carriers limiting their FF [2]. In this regard reducing nonradiative recombination of charge transfer (CT) state - appearing at the donor/acceptor interfaces in organic solar cells - is essential to achieve high FF and open-circuit voltage (V_OC) [3]. Whereas the role of energetic disorder in OSC for charge generation and extraction is discussed extensively, the impact on nonradiative recombination is largely unexplored. Herein, we present a study of fullerene and non-fullerene systems to introduce a new understanding of OSCs with reduced nonradiative recombination losses. Our results clearly point out that a high FF is possible when nonradiative recombination of the CT states is suppressed by reducing the energetic disorder. This study provides a rationale to explain and further improve the V_OC and FF of organic solar cells with PCEs > 20%, which is an essential step towards their commercialisation. These findings open the route for the next prospect in the design of materials.

REFERENCES:

L. Perdigón-Toro, H. Zhang, A. Markina, J. Yuan, S. M. Hosseini, C. M. Wolff, G. Zuo, M. Stolterfoht, Y. Zou, F. Gao, D. Andrienko, S. Shoaee, D. Neher, Adv. Mater. 2020, 1906763.
N. Tokmoldin, S. M. Hosseini, L. P. Toro, Y. Zou, H. Y. Woo, D. Neher, S. Shoaee, Adv Energy Mater. 2021, 11, 2100804.
L. Q. Phuong, S. M. Hosseini, O. J. Sandberg, Y. Zou, H. Y. Woo, D. Neher, S. Shoaee, Sol. RRL. 2020, 123, 2000649.

Organic conjugated materials have many favourable properties that make them interesting for a variety of electronic applications. The aim of my group is to understand the fundamental processes underlying their functionality. We use ultrafast spectroscopic techniques, such as transient absorption (TA) and time-domain terahertz (TD-THz) spectroscopies, to investigate charge carriers in organic semiconductors. While femtosecond TA measurements bring insights to the nature and evolution of the photoexcited species, we use TD-THz spectroscopy to gain information about the charge transport properties on the nanoscale. After presenting an overview of our experimental techniques, I will show results about charge generation in highly efficient solar cell materials based on organic polymer:fullerene [1] polymer:non-fullerene blends [2] with negligible driving force for interfacial charge transfer. Efficiencies beyond 18% have recently been achieved by combining low bandgap conjugated polymers with small-molecule non-fullerene acceptors (NFAs). I will discuss how charge generation and recombination processes depend on parameters such as the charge transfer driving force, the short-range charge mobility and the morphology.
[1] R. Shivhare, G. J. Moore, A. Hofacker, S. Hutsch, Y. Zhong, M. Hambsch, T. Erdmann, A. Kiriy, S. C.B. Mannsfeld, F. Ortmann, N. Banerji. Advanced Materials (2021), 2101784 .
[2] Y. Zhong, M. Causa’, G. J. Moore, P. K., B. Xiao, F. Günther, J. Kublitski, R. Shivhare, J. Benduhn, E. BarOr, S. Mukherjee, K. M. Yallum, J. Réhault, S.C.B. Mannsfeld, D. Neher, L. J. Richter, D. M. DeLongchamp, F. Ortmann, K. Vandewal, E. Zhou, N. Banerji. Nature Communications (2020), 11, 833

Organic photovoltaic (OPV) materials are a ground-breaking technology that harvests light in contexts where typical inorganic solar cells would not be very effective. Thanks to their transparence and flexibility, they can be easily incorporated into windows or textiles. However, many challenges remain in the development of this technology, in particular related to the relatively low power conversion efficiencies (PCEs). The most efficient OPVs consist of a blend of electron donors (D), typically polymers, and electron acceptors (A), typically small molecules. These two materials form a bulk heterojunction (BHJ) consisting of regions of neat polymer and neat acceptor molecules. Charge generation generally relies on excitation of either the donor or acceptor, exciton diffusion to a D-A interface by energy transfer, and charge transfer and separation at the D-A interface.
Historically the accepter role has been fulfilled by fullerene derivatives, however in recent years, PCEs have nearly doubled thanks to the introduction of non-fullerene acceptors (NFAs). The newest generations of NFAs, such as ITIC-4F and BTP-4F, consist of an electron-deficient core and electron withdrawing end-groups that lead to an intramolecular electron pushing and pulling effect. These NFAs yield a photocurrent in the absence of a donor. Therefore, there is some charge generation mechanism in these neat materials and the neat domains of the BHJs featuring these NFAs.
Our study employs solvatochromism to observe the ground and excited state dipole moments in ITIC-4F and BTP-4F, and to quantify their intramolecular charge transfer character. Furthermore, transient absorption spectroscopy reveals an exciton decay pathway that is likely feeding into charge separation in films of the neat NFAs. This manifests as a fast decay process in the exciton dynamics and a concurrent rise of polaron features. The solvatochromism reveals that the change in dipole moment between the ground and excited states of these two acceptors are comparable, although the charge yield in BTP-4F film is much higher. This means that the charge separationis influenced by molecular packing effects, not solely to the changes in the dipole moment upon excitation. Electromodulated Differential Absorption (EDA) spectroscopic techniques revealed the increased electron mobility and charge extraction in BTP family NFAs.
Due to conformation-locking S---O interactions in the NFAs, the ITIC and BTP families are part of two different symmetry point groups: the BTP family has C_2v symmetry and the ITIC family has C_2h symmetry. Increased PCE caused by increased electron mobility has been documented as one effect of a change in symmetry point group^1,2. We posit that the difference in symmetry point group between the ITIC and BTP family NFAs is also driving the mechanism of exciton dissociation to charges in neat NFA films and devices. Furthermore, we show that this mechanism is contributing to the charge population in polymer:NFA blends featuring BTP-4F.

References:
1. Zhenghui Yao, Yaokai Li, Shuixing Li, Jiale Xiang, Xinxin Xia, Xinhui Lu, Minmin Shi, and Hongzheng Chen, ACS Applied Energy Materials 2021 4 (1), 819-827

2. Ran Hou, Miao Li, Xueqing Ma, Hao Huang, Hao Lu, Qingqing Jia, Yahui Liu, Xinjun Xu, Hai-Bei Li, and Zhishan Bo, ACS Applied Materials & Interfaces 2020 12 (41), 46220-46230

Organic solar cell performance continues to improve near 20% and beyond. To realize such a threshold and eventually limiting performance, detailed and quantitative models of how structure affects device dynamics is needed. In particular understanding how structure of the donor-acceptor interfaces encourage charge generation and limit recombination are still lacking. For example, debate continues around disorder and mixing at the interface providing beneficial driving forces for separation versus recombination centers. This is due to the difficulty in effectively measuring interfacial properties in a bulk heterojunction as well as sample variability from solution printing that increases uncertainties. I will discuss our work in combining advanced X-ray characterization of interfaces and measurements of interfacial charge generation on the exact same samples. Combining diffraction with resonant microscopy and scattering enables us to measure domain interfacial width, while time-delayed collection field technique enables direct measurements of interfacial charge generation under device operating fields like maximum power point. We apply this strategy to a variety of systems that exhibit diverse generation and recombination properties and find that for all systems investigated thus far, sharper interfaces that limit mixing and disorder near interfaces both generate more charge and hinder recombination at device relevant photonic fluxes and fields. Thus, targeting increasingly discrete interfacial morphology through molecular architecture and processing strategies will result in device performance that increasingly approaches thermodynamic limits and commercialization of this exciting technology.

Organic Photovoltaics (OPVs) offer sustainable, integrated energy harvesting solutions. While the optoelectronic processes in opaque OPVs have been studied, the influence of selective transparency has not been investigated systematically. This work is the first to take advantage of a simulation-based approach to unravel the effect of (semi)transparency on exciton generation, multi-mechanism non-geminate recombination dynamics, and extraction processes in emerging semitransparent (ST-)OPVs and discusses the impact of series and shunt resistance with respect to the systematically changed average visible transmittance (AVT). [1]

Specifically, we demonstrate that the relative contribution of bulk-trap assisted recombination is increased in devices with higher AVT, compared to bimolecular and surface-trap assisted recombination mechanisms. As a result, bulk trap-assisted recombination reduces the V_oc more drastically in transparent OPVs, indicating the need for high-purity and morphologically optimized active layers with low bulk-trap concentration. At the same time, the effect of surface recombination at the active layer/electrode interfaces becomes less pronounced for systems with larger AVT. The investigation of the series resistance effect has yielded the conclusion that more transparent devices are less affected by high series resistance. Together with the reduced impact of the surface recombination, this suggests that a wider range of materials and deposition techniques can be considered for the transparent electrical contacts in terms of their compatibility with the active layer and the optical transmittance/sheet resistance ratio.

Our results, pointing out practical aspects that require special considerations in the fabrication of efficient ST-OPVs, will be of interest to a wide range of material scientists and device engineers working in the emerging field of ST-OPV. The findings are also anticipated to promote efforts in laboratories interested in better understanding of optoelectronic processes in state-of-the-art narrow bandgap non-fullerene OPVs.

1. N. Schopp, V.V. Brus, T.-Q. Nguyen, On Optoelectronic Processes in Organic Solar Cells: from Opaque to Transparent, Advanced Optical Materials 9 (2021) 2001484 DOI: 10.1002/adom.202001484

Efforts to understand the electronic processes and the contributions of different loss mechanisms have contributed to the immense progress that OPVs underwent in the past decade. While different advanced techniques exist to assess the losses caused by geminate or non-geminate recombination and extraction, specialized experimental setups are often required. This work is the first to employ only standard measurements (JV characteristics and EQE measurement) coupled with optical transfer-matrix method simulations to deconvolute the electronic processes under short-circuit conditions.
Specifically, we propose a simple method that allows us to quantify geminate recombination losses, to determine the mobility-lifetime product (μτ) as a single integrated parameter for charge transport and non-geminate recombination, and to quantify extraction losses under short-circuit conditions. We apply our approach to three device systems in an inverted or conventional structure and find excellent agreement of our results for μτ with those obtained by Impedance Spectroscopy. We discuss the implications of our detailed analysis of the losses on the performance of the solar cells and, lastly, demonstrate that our method can accurately predict the thickness-dependent short-circuit current.

Y6 is a recently developed non-fullerene electron acceptor (NFA) with dithienothiophen[3.2-b]-pyrrolobenzothiadiazole as the central unit and improves the performance of organic photovoltaics (OPVs) in combination with many electron-donor polymers. Although Y6 has desirable electronic properties for OPVs, the origin of its superiority as an acceptor is unclear. In this study, we investigate empirically why Y6 is an excellent acceptor by comparing Y6 with F8IC, which is an analog that has a similar electronic structure, in bulk heterojunction (BHJ) OPVs with various electron acceptor concentrations. The difference in performance between Y6 and F8IC appears only at high concentrations, suggesting that it is originates from the aggregation structures of the NFAs in the BHJs. Electric current loss analysis reveals that exciton loss and non-geminate recombination are suppressed more strongly in Y6 OPVs than in F8IC OPVs. Variable angle spectroscopic ellipsometry shows that Y6 molecules align in the in-plane direction at high concentrations, which contributes to the high charge generation rate via efficient exciton harvesting.

The control of microstructure during the active layer processing of bulk-heterojunction solar cells is critical to obtain elevated fill factors and overall performance. With this aim, additives can help to promote optimum morphologies while they can negatively affect the scalability and device stability. Post-processing can equally improve the morphology and even to enhance the crystallinity, while their effect on performance and lifetime of polymer/non-fullerene solar cells is still unclear. Here, we show a systematic comparison between different post-processing treatments, including thermal, vacuum and solvent vapor annealing on TPD-3F polymer and IT-4F non-fullerene acceptor, comparing their effects on performance and materials properties. While all treatments improved device performance compared to the control devices, the vapor annealed ones showed remarkable power conversion efficiencies close to 14% with a 76% fill factor and superior short-circuit currents. Moreover, this treatment demonstrated to improve device stability under illumination and under ambient conditions. The increased crystallinity of the acceptor molecules, as revealed by GIWAXS, led to increased photogenerated currents, with a more effective exciton dissociation and ultimately boosting the FF. Overall, our study points the attention on the role of post-processing in organic solar cell fabrication, and contributes towards a new generation of efficient and stable additive-free organic solar cells.

The currently best organic solar cells suffer from relatively large voltage losses due to non-radiative recombination as compared to inorganic or perovskite solar cells. Further enhancement of the power conversion efficiency to values over 20% will require a reduction of these losses, inevitably corresponding to an increase in the electroluminescence quantum efficiency of the devices.^[1] For a large number of donor-acceptor combinations, we have observed that non-radiative voltage losses decrease with increasing charge-transfer-state energies, consistent with non-radiative decay being facilitated by a common high frequency molecular vibrational mode.^[2] We further identify small molecule donor-acceptor blends with an optical gap in the visible spectral range, with strongly reduced non-radiative losses as compared to systems with a gap in the near infrared (NIR).^[3] This highlights the possibility of a simultaneous occurrence of a high photovoltaic quantum efficiency as well as a high electroluminescence quantum efficiency, occurring in a single organic donor-acceptor blend. For photovoltaic blends with strong absorption in the NIR, we show that the lowest non-radiative decay rates correspond to systems with the narrowest emission linewidths and steepest absorption tails.^[4]

[1] Vandewal et al, Nature Materials 8, 904 (2009)
[2] Benduhn et al, Nature Energy 2, 17053 (2017)
[3] Ullbrich et al, Nature Mater. 18, 459 (2019)
[4] Liu et al, joule 5, 2365 (2021)

Transparent solar cells can be integrated into surfaces of buildings and vehicles to provide point-of-use power without impacting aesthetics. Though most demonstrations of transparent photovoltaics (TPVs) have focused on absorbing near-infrared photons, UV-harvesting TPVs can be far more transparent and better suited for low-power electronics that prioritize aesthetics. Organic solar cells employing contorted hexabenzocoronene and its derivatives as chromophores have been shown good candidates for this application due to their strong UV light absorption, visible transparency, modular synthesis, and chemically tunable electronic structure, and large-area scalability. In this presentation, we report the operational stability of such devices. We find the solar cells that include peripherally halogenated chromophores degrade rapidly and exhibit S-shape J-V curves upon photo-exposure during aging experiments, while control cells employing non-halogenated derivatives as donors with archetype C₇₀ as an acceptor to show excellent stability. We find that this degradation is primarily caused by aggregation of the halogenated acceptor materials during operation, which generates voids at the active layer-electrode interface. These results suggest that halogenation of chromophores, a commonly utilized molecular modification method, can negatively impact the stability of materials and reduce device operational lifetimes, which should be assessed as part of the material design process.

The indoor photovoltaic market continues to grow at the same pace as the sensor market, fueled by the continued emergence of the Internet of Things (IoT). One promising strategy to power the IoT are flexible, printable organic photovoltaics (OPVs), which can efficiently harvest/recycle ambient lighting to power small electronic devices. Additional attributes can include recyclability of devices and substrates, preferential color selection for appearance/aesthetics, and low-cost. For wearables in particular, OPVs offer an order of magnitude (or more) in power-per-weight over conventional PV technologies (CIGS, Si, GaAs, etc).
Towards these applications, OPVs devices have a number of interfaces that synergistically lead to high power conversion efficiencies but can also contribute to degradation. Blends of donor and acceptor materials are responsible for both free carrier generation and charge transport through the photoactive layer but phase segregation with time can lead to decreased photoconductivity and reduced fill factors. Charge transfer reactions with oxygen and water can result in chemical changes to the semiconductors, yielding trap states and/or increased recombination pathways. Charge selective contacts are responsible for selectively harvesting one carrier but for metal oxide-based contacts in particular, unpassivated Lewis acid-base sites can result in mid-gap states that promote surface recombination and decreased open-circuit voltages and fill factors.
This talk will focus on a large characterization tool box that allows for investigations of interconnected properties and functionalities of various interfaces in organic photovoltaics from the atomic-to-molecular-to-nanometer-to-device length scales. Emphasis will be placed on understanding contributions to stability and power conversion efficiency retention, with considerations to aesthetics (necessary for wearables and building integrated PV), bond and redox chemistry, microstructure, and photoconductivity. New approaches to operando degradation (degradation of layers under relevant electric fields and charge fluxes) and subsequent characterization of buried interfaces will be discussed.

Organic bulk heterojunction photovoltaic solar cells (OPV), with record efficiency approaching 19% and proven high stability, are becoming a credible technology to participate in the global renewable energy bouquet. In this context, OPV manufacturers need materials in sufficient quantity and certified quality. Batch-to-batch variations must be eliminated and impurities content should be carefully controlled, could them be organic or metallic traces. This presentation will show a panorama of what is known, to date, concerning the effects of impurities in the raw semiconductor on solar cells. Our recent research will be presented describing our strategies to identify critical impurities and to estimate the tolerance threshold of solar cell. If initial post-fabrication efficiency of solar cells is obviously examined, our studies also explore the impurity effects on the stability of solar cells under illumination. Care is taken to understand the fundamental mechanisms associated with impurity effects.

Organic photovoltaic cells (OPVs) have the potential of becoming a productive renewable energy technology if the requirements of low cost, high efficiency and prolonged lifetime are simultaneously fulfilled. Since the efficiencies of OPVs have already exceeded 19% through the development of non-fullerene acceptors (NFAs)^1,2, and production costs have been estimated to be competitive with incumbent solar technologies³, the remaining challenge is to demonstrate OPVs with an operational lifetime adequate to meet the application requirements. Here, we demonstrate that instability of high efficiency NFA solar cells often observed in past work arises primarily from chemical changes at organic/inorganic interfaces bounding the bulk heterojunction active region. We find that these instabilities can almost be completely eliminated by adding protective buffer layers between the active region and charge transporting layers, as well as the integration of a simple solution processed ultraviolet filtering layers to the distal surface of the glass substrate. Using accelerated aging by exposure of light illumination intensities up to 27 suns, and operation temperatures as high as 65°C leads to an extrapolated intrinsic lifetime, that corresponds to a decrease in efficiency to 80% of its initial value, of > 5.6 ×10⁴ h⁴. This is equivalent to 30 years outdoor exposure. Thus, we find that solution-processed NFA-based OPVs can have long intrinsic operational lifetimes meeting many of the demands required in solar energy harvesting applications.
References:
1. Y. Cui, Y. Xu, H. Yao et al., "Single-Junction Organic Photovoltaic Cell with 19% Efficiency" Adv. Mater., 2021, 33, 2102420.
2. J. Wang, Z. Zheng, Y. Zu et al., "A Tandem Organic Photovoltaic Cell with 19.6% Efficiency Enabled by Light Distribution Control" Adv. Mater., 2021, 33, 2102787.
3. B. Lee, L. Lahann, Y. Li, et al., “Cost estimates of production scale semitransparent organic photovoltaic modules for building integrated photovoltaics” Sustain. Energy Fuels, 2020, 4, 5765.
4. Y. Li, X. Huang, K. Ding et al., “Non-fullerene Acceptor Organic Photovoltaics with Intrinsic Operational Lifetimes over 30 Years” Nat. Commun., 2021, 12, 5419.

The demand for low dimensional, micro powered and wireless indoor electronic devices has been increasing. To power-up those devices, organic photovoltaic (OPV) cells are being employed. The OPV cells exhibit good spectral matching and mechanical flexibility, and can harvest artificial indoor light energy efficiently [1]. Hole transport layer (HTL) is an important component of an OPV cell. Hole transport layer (HTL) is an important component of an OPV cell. Water stable, low temperature processable poly (3, 4-ethylene dioxythiophene): poly (4-styrenesulfonic acid) (PEDOT:PSS) based HTL is commonly used in the indoor OPV cells. However, strongly acidic, highly hydrophilic and expensive PEDOT:PSS resulted in the development of cheaper, mildly acidic and less humidity sensitive alternative hole transport material (HTM) for the indoor OPV cells [2]. Here, we utilized an economic and low acidic, water-stable PSS doped Polypyrrole (PPY) as HTM for aoly [4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b]Dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-ffluorothieno[3,4-b]thiophene-)-(2-carboxylate-2-6-diyl)]:phenyl-C70-butyric acid methyl ester (PTB7-th:PC₇₀BM) active material-based indoor OPV cell. The device was also optimized by adjusting the concentrations of PPY and PSS. As a result, under the 1-sun light source, the optimized PPY:PSS-based device showed 130% higher efficiency than the PEDOT:PSS-based device. Under the LED 1000 lx light source, the PPY:PSS-based device showed 115% higher efficiency than the PEDOT:PSS-based device.

The emergence of non-fullerene acceptors together with the continuous optimization of transport interlayers has recently led the field of organic photovoltaics to power conversion efficiencies over 18%. While the use of industrially adaptable fabrication techniques such as doctor blade-coating has been already explored for the active layer deposition, it is rarely reported for the fabrication of interlayers. The thickness constraint due to the low conductivities of the materials used as interlayers, limits their selection based on the suitability for printing technologies. The wide bandgap and high electron mobility tin oxide (SnO₂) nanocrystals are an attractive option for scalable transport layer fabrication, thanks to their water-based processability and thickness tolerance. Nevertheless, SnO₂/organic interface are often defective, determining an ineffective charge extraction due to carrier trapping and/or to the increase of the energy band offset with consequent lowering of the V_OC. To overcome this problem, several passivation agents deposited on top of the spin-coated films have been reported to improve device performance. Here, inspired by the need to accelerate the lab-to-fab device processing transition, blade-coating is employed to fabricate SnO₂ electron transporting layers for inverted organic solar cells. With the aim of studying the effect of trap passivation on scalable devices, a simple and environmentally friendly passivation strategy based on the interaction with several alkanolamines is explored, showing a remarkable boost in short-circuit current over 20 mA/cm² and fill factors of 70 %. We focus our research in understanding the differences in trap density on SnO₂ layers before and after the passivation and we put a strong focus on device stability. Using x-ray photoelectron spectroscopy and infrared spectroscopy we are able to describe the corresponding chemical interactions with the passivating agents and successfully explain the observed changes in device performance and stability.

As organic solar cell (OSC) efficiencies continue to climb there remains a need to address device stability, both intrinsic operational stability as well as mechanical stability in flexible applications. In this talk, we discuss how the thermomechanical behavior of the polymer donor and small molecule acceptor (SMA) informs the expected morphological and mechanical stability of OSCs. We show that organic semiconductors that have a high elastic modulus and glass transition temperature have the greatest morphological stability. Interestingly these systems also show poor miscibility highlighting the relationship between processing, morphology, and stability. In addition, we show how the thermal relaxation behavior of polymer: SMA and polymer: polymer blends dictate film ductility and fracture toughness. Specifically, we find that suppression of secondary relaxations in polymer semiconductors with the addition of the SMA plays a key role in mechanical stability. Through this analysis, clear correlations emerge between film toughness and molecular diffusion highlighting trade-offs between morphological and mechanical stability. The talk will end with an outlook of approaches to overcome limitations associated with this morphological and mechanical stability trade-off to achieve high-performance mechanically robust flexible organic solar cells.

Current state-of-the-art organic solar cells (OSC) has an active layer comprised of a conjugated polymer-small molecule acceptor (SMA) blend. The material selection and local morphology is not only critical for power conversion efficiency, these factors also dictate the thermomechanical behavior of the film. Understanding the thermomechanical behavior of the active layer is important to understand the expected mechanical stability as well as inform processing strategies and device stability. Given the films are typically quenched into non-equilibrium morphology, it is important that the mechanical behavior is characterized under these same conditions. Here, we introduce a kirigami-inspired thin-film (KIT) test method to test the dynamic mechanical behavior of bulk heterojunction films in device relevant morphologies. In this approach a kirigami inspired substrate is used to support that active layer. This substrate has a cut pattern that effectively transfers the applied load to the film of interest. Using this method we show that the thermomechanical behavior of the active layer is dependent on processing conditions which has important implications for mechanical stability. We then use this method to highlight a number of thermal relaxation events of neat polymers, SMAs and polymer:SMA blends. In particular, we show how the SMA structure impacts the polymer relaxation behavior and how it ultimately relates to mechanical stability.

Flexible transparent electrodes are an irresponsible, crucial component of future generation of flexible, portable and wearable optoelectronic devices, including but not limited to smart windows, antiglare mirrors and touchscreens. Although indium tin oxide is currently a dominating material in the market of transparent conductive electrodes (TCEs), its future applications are limited by the high cost of the sputtering process, the scarcity of the indium resource and the low mechanical flexibility of ITO. In efforts towards replacing ITO, metal nanowires emerge as the most promising materials, thanks to their excellent electrical and thermal conductivities and cost-effective solution processability. In this talk, I will present some of our recent results on the preparation of cost-effective, stable, high-performance nanowire TCEs achieved by carefully designing the structure of nanowires (such as a core/shell structure) and their film deposition process, and on their application in smart windows, electrochromic devices and solar cells [1-3]. For instance, in one case, we wrapped cost-effective copper (Cu) nanowires with reduced graphene oxide to enhance the stability of Cu nanowires, without compromising their properties. With this core/shell strategy, we were able to achieve cost-effective TCEs with high optical transparency and electrical conductivity, as well as significantly improved stability under various testing conditions. We further engaged these TCEs in suspended particle devices, which demonstrated the large variation in transmittance (42%) between the “on” and “off” states of an electric bias and impressively fast switching time and superior stability. These devices hold a great promise for practical smart window applications. References: [1] Advanced Energy Materials 2018, 1703658; [2] Advanced Functional Materials 2021, 2010022; [3] Journal of Materials Chemistry A 2020, 8, 8620-8628.

The continuous development of multifunctional wearable electronics leads to a need of portable, light weight, and real-time power supply. Dyes-sensitized solar cells are one of the main types of photovoltaic devices adequate for integration into textiles. Among them, fiber dye-sensitized solar cells (FDSSCs), owing to the thread-like structure and device flexibility, represent a promising platform for fully fiber-based devices. However, on the textiles, the optimal solar conversion efficiency of FDSSCs cannot be achieved due to the face-down and side surface areas not being fully covered by the top-incident sunlight. To achieve the full potential of FDSSCs and, simultaneously, maintain their wearability, wearable luminescent solar concentrators (LSCs) could be applied to FDSSCs. LSCs have attracted considerable attention in recent years for their advantages in absorbing diffusive light and increasing the cost-effectiveness of solar cells; however, the compatibility with flexible photovoltaics and the energy transfer (ET) efficiency still require improvement.
In our recent work [1], amphiphilic polymer conetworks (APCNs) are employed as polymer matrices for wearable LSCs owing to their flexibility and wearability. Furthermore, with the assistance of APCNs’ nanophase separated hydrophobic and hydrophilic domains, hydrophobic (Lumogen Red, acceptor) and hydrophilic (fluorescein, donor) luminescent materials are loaded in adjacent nanometer-separated domains. This results in high ET rates and broaden the acceptor’s absorption range, rendering a more efficient down conversion emission. With this straightforward synthesis procedure, we could achieve high ET rates between dye pairs via FRET and photon recycling. This two energy transfer mechanisms were confirmed by steady-state and dynamic photoluminescence methods, showing a ~100% total ET between donors and acceptors. The developed nanostructure-assisted ET system is not limited to the dyes investigated here, but can be directly extended to a wide variety of dyes (Rhodamine B, HPTS, DCM, and Lumogen Yellow) and quantum dots (CsPbBr3 and CdSe/ZnS).
Herein [2], we present wearable LSCs with breathable and flexible polymer matrix hosting luminescent dyes as companions to assist FDSSCs fully utilize the advantage of harvesting light from all directions, enlarge the photon harvesting area, and recycle the lost photons. The prototype demonstrated in this work possessed a remarkable FDSSCs power conversion enhancement of 84% and a sustained efficiency even after 1000 bending cycles. This is the first showcase of the integration of FDSSCs and wearable LSCs as energy harvesting textiles for use in future wearable electronics systems.
References
[1] Huang, C. -S., et al. (2020). Nano-domains assisted energy transfer in amphiphilic polymer conetworks for wearable luminescent solar concentrators. Nano Energy, 76, 105039.
[2] Huang, C. -S., et al. (2021). Energy harvesting textiles: using wearable luminescent solar concentrators to improve the efficiency of fiber solar cells. Journal of Materials Chemistry A. accepted.

Understanding the interplay between film morphology, photophysics, and its relation to device performance in bulk heterojunction organic photovoltaics remains challenging.
Using the well-defined morphology of both vapordeposited neat C₆₀, as well as a6T:C₆₀ and TAPC:C₆₀ blends, where the dilute-donor blends (<10% donor) show a higher efficiency that the completely intermixed (50% donor) blends, we see the morphological effects on charge generation, separation and recombination on an ultra-fast time scale. With the use of ultrafast transient spectroscopy we are able to follow each step of the charge generation process and give an explanation for the unusual efficiency trends in these organic solar cells.
First, we explore the nature of Frenkel and Charge Transfer excitons in neat C₆₀ and then how photocurrent is generated over the entire fullerene absorption range in the blends, giving advantage to the dilute-donor blend with more C₆₀. [1,2]
Second, we put a magnifying glass in the charge separation process by using a novel technique to show an energy cascade between interfacial and bulk C₆₀ electrons that assists free charge generation in the TAPC:C₆₀ blends, and then being able to separate and model the dynamics between interfacial CT state and separated charges in the a6T:C₆₀ blends. [2]
Third, we identify a fast (<1 ns) recombination channel, where free electrons recombine with trapped holes on isolated donor molecules which should harm the performance of dilute solar cells. We, however, give evidence that this recombination is mitigated when electrons are rapidly extracted in efficient devices, reminding us that more than just the active layer needs to be considered when designing efficient organic solar cells. [2]
[1] J. Phys. Chem. Lett. 2018, 9, 8, 1885–1892
[2] J. Phys. Chem. Lett. 2020, 11, 14, 5610–5617

Organic photovoltaics (OPVs) have been in a boom in terms of their efficiencies with the discovery of non-fullerene acceptors (NFAs) and ternary blend systems then have gotten new opportunities for commercialization. However, manufacturing technology is still lagging behind. Here we show a new research approach to develop OPVs via industrial roll-to-roll (R2R) slot die coating in conjunction with in-situ formulation technique and machine learning (ML) technology. The formulated PM6:Y6:IT-4F ternary blends deposited on continuously moving substrates resulted in high-throughput fabrication of OPVs with various compositions. The system was used to produce training data for ML prediction. Composition/deposition parameters, referred to as deposition densities, and efficiencies of 2218 devices were used to screen ML algorithms and to train an ML model based on Random Forest regression algorithm. Generated model was used to predict high-performance formulations and the prediction was experimentally validated by confirming the thickness tolerance tendency and fabricating 10.2% of efficiency devices, the highest efficiency for R2R-processed OPV so far.

Nanofibers' fabrication has recently attracted researchers' interest, especially the fabrication process using the electrospinning method. That attraction came from the capability of electrospinning to create oriented and controlled nanofibers with specific dimensions and structures. The photocatalytic properties of Cu-doped zirconia nanofibers fabricated by the electrospinning method have not been studied extensively due to the difficulties during the nanocomposite preparation. Since the most influential parameters on the fabrication process led by the electrospinning method are the concentration of the spinning solution; which is the Cu-doped zirconia composite in our case, the potential applied on the needle during the electrospinning process and the dimensions of the spun layer. In this proposal, we suggest using these characters combined with the co-axial electrospinning method with a sheath solution of Copper oxide and ZrO₂ as a core solution to improve the photocatalytic activity of its fabricated product. Besides, to make the best use of the chemicals and time, I propose using Molecular Dynamics and DFT computational calculations to predict the efficiency of preparation, optimization methods, and the resulting electronic properties before starting with the experimental work stage.

The development of environmentally friendly and sustainable energy source became important in future generation and also a resolution for the climate change issue. Recently, organic photovoltaics (OPVs) have gained much attention due to the possible low-cost fabrication, good flexibility, lightweight, and transparency that can provide various applications such as windows PV, building integrated PV (BIPV), indoor power source, or façade PV, different from silicon-based PVs. However, typically used processing media for the fabrication of OPVs is yet highly toxic halogenated solvents and small amount of harmful processing additives.

In this study, efficient OPV based on using green solvent media was developed which showed a power conversion efficiency (PCE) of 7-8% without using traditional chlorinated solvents such as chlorobenzene. The use of alternative non-halogenated solvent combined with a new non-toxic, non-halogenated processing additive was also investigated and demonstrated comparable or even higher PCEs compared to typical 1,8-diiodooctane (DIO) solvent processing additive. The result shows potential possibility for fully green fabrication of highly efficient next generation OPVs.

In an organic solar cell (OSC), the interface between the electron donor and acceptor materials facilitates the key charge generation and recombination processes that directly govern the power conversion efficiency. Therefore, it is expected that OSC performance will be extremely sensitive to any disturbances in this critical region. Here, we challenge the accepted requirement that donor and acceptor molecules must have close intermolecular contacts for efficient OSC operation. This is achieved by ‘encapsulating’ the donor polymer by linking two parts of the backbone together using alkoxy bridging groups. Thus, encapsulation introduces bulk to the molecule and prevents the close approach of neighbouring species[1]
We demonstrate this approach in a model system based on the benchmark PM6:Y6 blend[2]. We have synthesised an encapsulated PM6 derivative, ‘EP’, and fabricated OSC devices with the acceptor material Y6. Surprisingly, we find that despite significantly disrupting the interfacial morphology, the power conversion efficiency of our EP:Y6 device remains comparable to the reference PM6:Y6 blends. Transient absorption measurements indicate that while charge transfer is slightly slower in the encapsulated system, this process remains efficient despite the more distant donor-acceptor interactions. These results indicate that the donor-acceptor interface may not be as critical as previously suggested for obtaining high performance OSC blends. In addition, we find evidence that increasing the interfacial donor-acceptor separation through encapsulation also suppresses undesirable non-geminate recombination into the Y6 triplet exciton, providing a novel strategy for overcoming this critical loss pathway in OSCs[3].

[1] A. Leventis et al., J. Am. Chem. Soc., vol. 140, pp. 1622–1626, 2018.
[2] J. Yuan et al., Joule, vol. 3, pp. 1140–1151, 2019.
[3] A. J. Gillett et al., Nature, vol. 597, pp. 666–671, 2021.

Organic photovoltaics based on π-conjugated non-fullerene acceptors (NFA) have gained enormous interest over the past few years. More precisely, the recent development in fused-ring systems led to several promising photovoltaic devices, with power conversion efficiencies exceeding 15%. Since the complexity of the molecular designs has grown exponentially, the development of new materials with specific properties has become a complex process. Therefore, many studies employ computational modelling, in particular density functional theory (DFT), to anticipate material electronic properties. Such approaches provide useful information about proposed organic semiconductors, such as optical absorption, frontier orbital energy levels and molecular geometries. However, the accuracy of the common methods for recent organic semiconductors has not been explored. Thus, we evaluated a series of DFT functionals and Hartree-Fock theory (HF) for a collection of 14 common NFAs. Computational results were compared with physical properties from experimental data. By applying empirical corrections from linear fits, mean absolute errors between theoretical and experimental results below 0.05 eV could be achieved for HOMO and LUMO energies as well as maximum absorption energies. These optimized computational methods were then used to design new NFAs for organic photovoltaics.

Over the past decades, color sensors have been of great interest among researchers for conveying information. The color sensors are required to detect any deformation or structural change occurring in our surrounding infrastructures as well as the internal activities in the operating devices. To fabricate an efficient sensor, researchers are in search of a material that can exhibit large structural change in response to a small applied voltage. Many investigations have been carried out to explore the applications of organic light-emitting diodes (OLEDs) for testing the devices subjected to stress. Among them, exciplex based OLED has demonstrated various advantages over simple structured OLED. Enormous efforts have been devoted to discover its applications in the field of science and engineering. However, a very few investigations have been carried out to explore the applications of exciplex.
Here, we suggest another way to applicate the exciplex device for the voltage sensor. The color of exciplex device changes as the recombination zone changes according to the driving voltage. Using color variability of exciplex device, a visible voltage sensor was fabricated. 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl1-H-benzimidazole) (TPBi), and tris(4-carbazoyl-9-ylphenyl)amine (TCTA) were used as hole transport layer (HTL), electron transport layer (ETL) and excitons blocking layer (EBL). In the construction of emissive layer (EML) composed of TAPC and TPBi, the color coordinates changes for each driving voltage according to the composition ratio of two materials. With 2:1 (TAPC:TPBi) composition ratio, the color changes from reddish yellow to white by increasing the voltage, and it changes from blue to white with 1:1 composition ratio. Furthermore, in the case of 1:2, the variation of color is similar to 1:1, however, the degree of change with applied voltage is larger than 1:1. Hence, we implemented a sensor device that changes from two colors (reddish yellow, blue) to white according to the applied voltage. This application can be used as a safety device in a limited space due to noise and can also be employed as a sensor for electronic device equipment.

Polymer solar cells (PSCs) using a bulk heterojunction (BHJ) have attracted attention because of their unique advantages such as lightweight, flexibility, and low cost. In recent years, significant efforts have been devoted to the development of novel materials, device optimization, and morphology control to achieve enhanced power conversion efficiencies (PCEs) over 18% in single junction PSCs. Single-junction PSCs based on the PM6 copolymer show good photovoltaic performance in single junction PSCs only when treated with a halogenated solvent such as chlorobenzene (CB) or chloroform (CF). However, PM6 copolymer-based PSCs processed with nonhalogenated solvents exhibit relatively low PCE due to undesirable morphological properties, including high aggregation, rough surface morphology, and unfavorable edge-on orientation.
Recently, terpolymers consisting of three different monomers in the polymer backbone have emerged as promising p-type polymers. The incorporation of a third monomer into a D–A copolymer backbone can create numerous combinations that may have a synergetic effect on physicochemical properties such as miscibility, energy levels, charge-carrier mobility, and crystallinity.
Herein, we synthesized and characterized a novel series of 1D/2A terpolymers (PBTPBD) composed of 4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-F), 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD), and 1,3-bis-(4-hexylthiophen-2-yl)-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (HT-TPD) for the development of high-efficiency and long-lived PSCs processed with ecofriendly nonhalogenated solvents. The terpolymers (i.e., PBTPBD-25, PBTPBD-50, and PBTPBD-75) were prepared with different ratios of HT-TPD to BDD corresponding to an HT-TPD content of 25%, 50%, and 75%, respectively. The optical, molecular packing, charge-transport, and morphological properties of the PBTPBD terpolymers were controlled by adjusting the composition ratio of the HT-TPD and BDD segments. The PBTPBD-50:IT-4F blended film showed superior hole and electron mobility, which resulted in excellent charge-carrier balance. Therefore, the corresponding PBTPBD-50:IT-4F PSC, processed with o-xylene, exhibited a high PCE of 13.64%, which is 13% higher than that of PM6:IT-4F PSCs. Furthermore, the PBTPBD-50:IT-4F PSC maintained 82% of the initial PCE even after 204 days at 85 ^oC, which is the highest thermal stability ever achieved with a PSC processed with an ecofriendly nonhalogenated solvent.

As a variety of non-fullerene small molecule acceptors (SMAs) have been developed to improve power conversion efficiency (PCE) of organic solar cells (OSCs), the pairing of the SMAs with optimal polymer donors (P_Ds) is an important issue. Herein, a systematic investigation is conducted with the development of the SMA series, named C6OB-H, C6OB-Me, and C6OB-F, which contain distinctive terminal substituents –H, –CH₃, and –F, respectively. These SMAs are paired with two P_Ds, PBDT-H and PBDT-F. Interestingly, the P_D/SMA pairs with similar terminal groups yield enhanced molecular compatibility and energetic interactions, which suppress voltage loss while improving blend morphology to enhance simultaneously the open–circuit voltage, short–circuit current, and fill factor of the OSCs. In particular, the OSC based on the PBDT-F:C6OB-F blend sharing fluorine terminal groups achieves the highest PCE of 15.2%, which outperforms those of PBDT-H:C6OB-F (10.1%) and PBDB-F:C6OB-H OSCs (11.2%). Furthermore, the PBDT-F:C6OB-F OSC maintains high PCEs with active layer thicknesses between 85 and 310 nm. In contrast, the PCE of PBDT-H:C6OB-F-based OSC already drops by 80% from 10.1% to 2.1% when the active layer thickness increases from 100 to 200 nm. This study establishes an important P_D/SMA pairing rule in terms of terminal functional groups for achieving high-performance OSC.

PEDOT:PSS (poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate)) is a well-known commercial polyelectrolyte that is employed in a variety of electronic applications. It is commonly utilized as a charge transport layer or transparent conductive electrodes in organic photovoltaics due to qualities such as high transparency in the visible range, good film forming properties, high work function, and high electrical conductivity. Different concentrations of different amino acids were added to the PEDOT:PSS solution in this study to see how they affected morphology, work function, and conductivity. To monitor the changes after doping, the produced electrodes with doped PEDOT:PSS were evaluated using several characterisation techniques. The influence of modified electrodes on the performance of organic solar cells was also investigated.

The development of new materials and production systems and energy-saving pursue the objective of ensuring a suitable transition to a sustainable and reliable future based on the use of clean, locally accessible, and safe energy. Recently there have been intensive efforts in exploring innovative solar cell structures with high performance and profitable manufacturing costs. To this end, this work aims to use functionalized multiwalled carbon nanotubes (mwcnt) with titanium dioxide nanoparticles (TiO₂) by a mechanical-thermal process, and then with a spray method to obtain a hybrid material of graphene and mwcnt functionalized with TiO₂, characterizing the structural and morphological properties of the resulting composites. Raman shows bilayer graphene was obtained, and in TEM was observed a bilayer graphene/mwcnt/TiO₂ interactions obtained by a simple coating method.

In light harvesting and sensing devices such as organic solar cells (OSCs) and organic photodetectors (OPDs), accurate tuning of molecular orbital energy levels and absorption wavelengths are necessary to optimize device efficiency. Much of the interest arises due to the tunability of their energy levels and absorption properties through designing the molecular structure of organic/polymeric semiconductors. However, a critical challenge arises in narrowing the bandgap with respect to the counterbalance between driving force and charge separation. However, the benefits of near-infrared (NIR) organic semiconductors remain comparatively unexplored in the context of OSCs and OPDs. In this study, we controlled conformational asymmetry in nonfullerene small molecules for simultaneously achieving bandgap narrowing and optimal energy level alignment. This design approach leads to efficient charge generation without scarifying voltage losses in OSCs and high photoresponse with low dark currents in OPDs.

Near-infrared organic photodetectors (NIR-OPD), in contrast to inorganic photodetectors, have attracted considerable attention for their differentiated advantages such as flexibility substrate compatibility, tailorable light absorption property, high absorption coefficient, low-cost manufacturing, and room temperature operation. Especially in the NIR region, NIR detection between 900 and 950 nm is very important for low-noise communication in the surrounding environment. It is also necessary to achieve a low dark current for high NIR detection. In this study, we used a NIR absorbing conjugated polymer (PNIR) and a non-fullerene acceptor (ITIC) to significantly inhibit the dark current of the bulk heterojunction (BHJ), thereby demonstrating a high-performance NIR-OPD at 900-950 nm. Systematic characterization of fullerenes and non-fullerene acceptors through electrical, structural, and morphological analysis probed that ITIC effectively reduced charge recombination of OPDs, resulting in high detectivity of over 3.2 × 10¹¹ Jones at 900–950 nm through reduction of dark current. The results presented in this study show that the use of non-fullerene receptors for NIR-OPD of BHJ type is a strategic approach to achieve low dark current and high detectivity.

In recent, organic solar cells (OSCs) have been advanced dramatically and had big attention as one of the most promising solar cells. However, it is still a challenge to manufacture OSCs with high efficiency and long-term stability in all layers. For solving these problems gradually, many researchers have studied insightfully about each layer including active layers and charge transporting layers in the device structure of OSCs. In particular, the introduction of the charge transporting layers can minimize charge recombination at the interface between the active layer and metal electrode. As the most commonly used hole transporting layer (HTL), PEDOT:PSS has been utilized as an anode interfacial layer to improve the contact between HTL and electrode and to increase hole collection in OSCs. Nevertheless, PEDOT:PSS layer has several drawbacks including moisture absorbing property, high acidity, and some electrical interruption properties, leading to direct damages in device performance. Therefore, it is required to develop new HTLs for improving the device performance and stability. In this study, we demonstrate oxidized recycling carbon nanoparticles (NPs) as an efficient HTL for OSCs for the first time. For carbon neutrality, the development of the recycling oxidized carbon NPs can suggest new applications and technologies. We prepared inverted OSCs including the interlayers with oxidized carbon soot (OCS) and functionalized OCS (F-OCS) NPs, enhancing the hole transporting properties and minimizing the contact resistance. As a result, the high device performance of 14.22% (PM6:Y6/OCS) and 14.17% (PM6:Y6/F-OCS) compared with PM6:Y6-based devices (13.52%) were achieved. The effects of OCS-based HTLs were thoroughly analyzed by using UV-Vis absorption, AFM, TEM, and conductivity measurements.

Flexible and transparent electrodes are essential to fabricate flexible optoelectronics such as LCD, OLED, OPV. Silver nanowire (AgNW)-based electrodes are considered as the most suitable alternative transparent electrode to replace ITO electrode and achieve flexible devices due to good electrical, optical, mechanical properties. Here, we investigated major mechanism to improve electrical property of AgNW-based electrode by introducing sol-gel ZnO layer. The electrical properties are improved by two major effects, which are electrical bridge effect and the decrease in interconnection resistance due to capillary force. We introduced ZnO layer to AgNW-based electrode with different architecture to decouple each effect. All architectures with ZnO layer had lower resistance than reference electrode without ZnO layer. In particular, the architecture in which the electrical bridge effect is dominant had lower resistance than the architecture in which the decrease of interconnection resistance due to capillary force is dominant. To theoretically calculate each effect on the resistance of electrode, we constructed the equivalent circuit based on electrical path in each architecture. Similar to experimental results, the electrical bridge effect rather than the decrease of interconnection resistance due to capillary force affected the improvement of the electrical properties of the AgNW-based electrode. To apply such AgNW-based electrode to optoelectronics, patterning process is essential. However, general patterning process such as photolithography is too complex and expensive. So, to fabricate AgNW-based optoelectronic devices economically and ecofriendly, we developed the novel patterning process for AgNW-based electrode by controlling the adhesion between AgNW and substrate. Particularly, when using flexible substrate such as PET as a bottom substrate, we achieved to simultaneously pattern AgNW exposed and embedded electrodes through selective surface modification. In addition, we figured out the critical adhesion to success patterning process by exploring various substrates and AgNWs with different properties. Then, AgNW-exposed and embedded electrodes were applied to various optoelectronics such as OPV, LC cell, and each device showed well-operating performance. But, when fabricating an organic solar cell, it showed a low power conversion efficiency. So, to improve its power conversion efficiency, we fabricated the microlens arrays of various diameter (0.5µm~10µm) and confirmed that the optical properties and PCE can be dramatically increased with microlens array. When we examined optical properties by UV-VIS spectroscopy, the total transmittance was almost the same regardless of the diameter of microlens. But, the specular transmittance was sharply decreased with increasing diameter of microlens. It indicated that the dispersive light increased with increasing diameter of microlens. Because dispersive light has longer optical path, the increase of PCE was attributed to the increase of the probability of light absorption and excitons generation in photoactive layer. In addition, experimental performance of OPVs with microlens array was well matched with theoretically expectation which was calculated from optical properties of the microlens arrays.

Organic solar cells (OSCs) have attracted much interest in recent years because of their own merits, such as lightweight, flexibility, and color tunability as well as low-cost fabrication using solution process. Non-fullerene acceptors (NFAs) have contributed significantly to the progress of OSCs. Core engineering of small molecule acceptors (SMAs) is crucially important for enhancing device efficiency for non-fullerene organic solar cells (NF-OSCs). The most commonly used SMAs (e.g., Y6) utilize thienotiophene as the side core, which has restricted their absorption ranges due to the weak electrondonating ability and short conjugation length. In this study, A New Y6 derivative was designed and synthesized to compensate for these shortcomings. UV–vis absorptions for optical property of new SMA was recorded in diluted solution state and thin-film state, the maximum absorption peaks (λmax) were found to be 804 nm and 916 nm. CV for electrochemical property measurement results show that HOMO and LUMO energy levels are −5.50 eV and −4.08 eV. As a result, red-shifted absorption and up-shifted HOMO levels compared with commonly used SMAs. So, this work helps shed light on engineering the molecular conformation of NFAs to achieve high efficiency OSCs.

In the past several decades, development of electronic devices has usually been focused on higher speed, larger capacity and smaller size. Meanwhile, recent popular demand has rather shifted towards comfortable electronic devices that are flexible, foldable, stretchable and eventually wearable, and it is imperative that flexible power generation systems are developed in order to operate such electronic devices. That being said, the presence of transparent conducting oxides (TCOs) such as indium tin oxide (ITO) is disadvantageous to the development of flexible organic-inorganic hybrid perovskite solar cells (OIHP-SCs) due to the brittle nature of ITO which is easily breakable during bending. Herein, a flexible TCO-free OIHP-SC is demonstrated by using lithium bis(trifluoromethane)sulfonimide (Li-TFSI) as a co-dopant for a single-layer graphene transparent conducting electrode and a poly(triarylamine) hole-transporting material (HTM) on a flexible polydimethylsiloxane substrate. The optical and electrical properties of the Li-TFSI-doped graphene substrate are measured by controlling the doping amount, where the best conditions for charge extraction are established at a doping concentration of 20 mM Li-TFSI, thus optimizing the device photovoltaic performance. As a result, a highest power conversion efficiency of 19.01 % is demonstrated by the flexible TCO-free OIHP-SC device with an active area of 1 cm². In addition, the flexible TCO-free OIHP-SCs exhibit good bending stability after 5000 bending cycles at radii of 6, 4, and 2 mm and excellent light soaking stability under 1 Sun light intensity over 1000 h.

Many research results have been reported on the topic of failure and deterioration of CIGS photovoltaic modules. However, the mechanism of the deterioration process due to the burning hot spots generated by partial shading has not been fully elucidated. To improve the performance and reliability of CIGS modules, it is important to study the hot-spot generation mechanism. In this paper, we present an analysis method for CIGS modules using electrical and thermal equivalent circuits and propose a hot-spot model for CIGS modules to describe numerically the degradation process under partial shade stress.
To elaborate on the above, we conducted a partial shade stress test on three series-connected large-area CIGS modules. Each module measures 156 cm × 62.6 cm and consists of 102 cells connected in series. Then, we observed the generation of hot spots on the shaded cells via infrared imaging. That is, we confirmed that the partial shading stress gave unpredictable and irreversible damage to the modules. We constructed an electrical and thermal equivalent circuit of the CIGS modules to numerically emulate the internal electrical and thermal changes induced in the shaded module by partial shade stress. In addition, we modeled a hot-spot generation model in which the shunt resistance of a CIGS cell is irreversibly degraded by dissipated power exceeding a critical dissipated power (P_th).
In this study, the simulation results showed that the simulated and measured current-voltage characteristics of three CIGS modules are consistent under partial shade conditions when using the proposed hot-spot generation model. The results also showed that partial shading induces a high reverse voltage in the shaded cell, resulting in high power consumption. For reference, the module induced high power consumption of up to ~34 mW/cm² at a 5% shade ratio, which is ~1500 times higher than that of unshaded cells. According to the hot-spot generation model for the best-fit simulation to the measurements, the minimum deteriorated shunt resistance was estimated to be ~18.5 Ω/mm² (only ~0.1% of the initial normal value). And P_th values were estimated at ~2.4 mW/cm². Each of these values is an estimate from the center of the hot-spot region.
Based on this study, we expect that further studies will be conducted to reproduce the internal physical property changes in response to the external environment. The results of this study indicate that the P_th value can be used as a reliability parameter to estimate the durability of thin-film photovoltaic modules with respect to deterioration caused by partial shade stress. The presented modeling methods can be used to improve the reliability of CIGS PV modules as well as the reliability of various thin-film PV modules.

The charge transport ability of polymer acceptors (P_As) is an essential property for achieving high power conversion efficiencies (PCEs) of corresponding all-polymer solar cells (all-PSCs). However, the electron mobilities (µes) of most PAs are inferior to those of their small molecule acceptor (SMA) counterparts. In this study, we design a new series of the polymerized SMA-based PA materials (Y5-A-B, A= selenophene (Se)/ biselenophene (BiSe) and B= In/Mix/Out) for enhancing the µe and, thus, PCEs of all-PSCs. In detail, i) donating moieties (Se/BiSe) and ii) backbone regioregularities (In/Mix/Out) are two-dimensionally controlled to yield six different PAs. We successfully demonstrate the importance of this two-dimensional control on the structural, crystalline, and electrical properties of the PAs. In particular, the µe values in both organic field-effect transistors and all-PSCs are significantly varied. The most interesting finding here is that the In/Mix/Out regioisomer effects on the PA¬ properties and device performances are completely different depending on the Se/BiSe donating unit employed in the PA. In the case of the Y5-Se PA series, the PCEs of all-PSCs increase, in terms of the SMA regioisomer, in the order of Out (7.52 %) < Mix (9.33%) < In (13.38%). In contrast, for the Y5-BiSe PA series, this trend in the PCEs of associated all-PSCs is reversed, that is, the PCEs decrease in the order Out (10.67%) > Mix (9.58%) > In (8.52%). These contrasting effects of the regioisomers depending on the PA backbone structure is attributed to their different effects on the planarity and intermolecular assembly of the PAs. Additionally, Y5-Se-In blend exhibits the improved blend morphology with smaller domain size and appropriate domain purity than other PA blends, resulting in the highest PCE of 13.38% among the all PA systems. Overall, we observe a strong, direct relationship between the µe values of the PAs and PCEs of all-PSCs, as evidenced by highest µ_e value from Y5-Se-In PA that affords the highest PCE of all-PSCs.

In this work, we develop a series of BDT-8ttTPD-based polymer donors to produce optimal crystallinity and hole mobility in nonfullerene organic solar cells. To overcome the amorphous characteristics of BDT unit, we employ planar 8ttTPD unit as an accepting counterpart, and moreover, replace the hydrogen atoms attached to the third positions of side thienyl groups with halogen atoms to yield PBDT-X (X= H, F, and Cl) polymer donors. Synergistic effects of incorporated 8ttTPD unit and the halogenated 2D side chain generate significantly enhanced charge transport and recombination properties of the OSCs, which is mainly attributed to optimized crystallinity and hole mobility of the polymer donors. As a result, the PBDT-Cl:Y6 OSCs achieve the highest power conversion efficiency (PCE) of 15.63% with simultaneous improvements of open-circuit voltage, short-circuit current density, and fill factor, outperforming the PCEs of PBDT-H:Y6 (11.84%) and PBDT-F:Y6 (14.86%).

Due to their unique combination of mechanical, esthetical and intrinsic electro-optical properties, the class of organic photovoltaics can open novel routes towards creative niche applications not accessible for other photovoltaic technologies. While commercial Si-based solar cells are conventionally used to power electrical applications at mild temperature conditions, here we investigate the potential of organic photovoltaics for more ‘extreme’ environments, ranging from polar regions to desert environments, up to (aero)space conditions.
While Si solar cells paradoxically suffer decreasing power output at high solar radiation intensity due to coinciding increasing temperatures, organic photovoltaics can improve their performance at these conditions.
The goal of this contribution is to explore the ‘temperature’ frontiers - or thermal window of operation - in which organic solar cells can operate. The cross-disciplinary approach followed is based on a combination of experiments and simulations going boldly beyond the conventional temperature ranges reported in literature. For the simulations the in-house developed simulation program SimiConductor has been introduced, providing a good description of the behaviour of the various photovoltaic performance parameters in a broad temperature range. This combined experimental/theoretical approach has resulted in defining a temperature window of operation, which can be a guideline for using and further developing organic photovoltaics for solar energy generation in extreme worlds.

Organic photovoltaics (OPVs) are a promising technology for low-cost renewable energy in applications such as wearable solar cells and semitransparent photovoltaics because of their tunable bandgap and solution processability. The core of an OPV device is the bulk heterojunction (BHJ), which is composed of donor and acceptor molecules or polymers that form an interpenetrating network of domains. Photons absorbed in the BHJ form bound electron-hole pairs that diffuse to the donor-acceptor interface and split into free charges, where the charges then travel to their respective electrodes. In the ideal BHJ morphology, domains are 10 to 50 nm in size and exhibit high crystallinity with continuous pathways to the transport layers. The BHJ is typically prepared by depositing a solution of donor and acceptor material and relying on thermodynamic and kinetic processes for film formation. Due to a wide range of processing conditions available, and difficulty in maintaining exact parameters, reproducibility of OPV has remained a challenge. Furthermore, specialized characterization methods must be used to probe the BHJ morphology. Grazing incidence X-ray scattering (GIWAXS) is one such technique and is used to look at length scales ranging from single to tens of nanometers; ideal for investigating the molecular packing and crystallinity of donor and acceptor materials. One interesting donor material to investigate reproducibility and stability in OPVs is X2, a small molecule donor. X2 has demonstrated robust device efficiencies over wide ranges of donor-acceptor ratios and processing conditions. Determining the molecular packing of X2 may explain the reasons for its robustness, which could then be exploited for improved reproducibility and stability in other OPV systems. GIWAXS data on neat films of X2 has been collected over a range of temperatures and is currently being analyzed to determine its molecular packing structure for the first time. Preliminary results show that X2 forms a highly crystalline structure and that it remains stable over wide temperature ranges. The results from this data analysis are expected to help guide further rational design of reproducible and efficient OPV devices.

In recent decade mixed cation approach has been widely used to enhance the performance of perovskite solar cell. The prevailing method of producing mixed cation perovskite formamidinium methylammonium lead tri-iodide (FAxMA1-xPbI3) though existing conventional method has inherited some problems of low phase stability and trap density. Herein we are producing mixed cation perovskite through an unorthodox method which will enhance phase stability and trap density to a new high level. Here mixed cation FAxMA1-xPbI3 will be engineered by treating intermediate with varying concentration of chloride cation. The grain size, crystallinity and phase stability of the prepared film perovskite will be tested to reckon the exact composition of the precursor leading to smooth morphology and better crystalline stability. The conversion efficiency and retention time of the perovskite solar cell would be tested to get higher quality Perovskite solar cell.

Hybrid perovskite solar cells (with chemical formula ABX₃) are of great interest due to the recently measured power conversion efficiency of greater than 25% (but theoretically, 33.7%). Perovskite structures are easily customisable, with a range of options for A, B and X. This enables us to both tune the electronic band gap and the stability by varying the composition. Two promising perovskites are the CH₃NH₃PbI₃ (MAPI) and CH(NH₂)₂PbBr₃ (FAPB) structures. With these, we have two choices, we can use solid state solution (mixing) approaches, or we can structure them in a desired form, such as a superlattice. Both approaches have advantages and disadvantages. In order to compromise for a more desirable bandgap for stand-alone PV and better structural stability, the hybrid perovskite constituents are show great promise in the superlattice form, with one configuration showing the low band gap of 1.29 eV.

We present a theoretical investigation of the structure and electronic properties of superlattices and solid state solutions performed using first–principles density functional theory. We include the role of vibrational entropy and assess the room temperature stability of the cubic phases for these systems. For solid state solution we show that by varying the ratio of FA to MA and the ratio of I to Br, we can potentially tune the band gap and effective masses (and hence electronic transport). For MAPI(n)/FAPB(m) superlattices, our results show that the electronic direct gap is evaluated and we show that these systems in general possess a lower band gap than either of the two bulks. We also evaluate the stability of these structures, showing that the cubic phase to be favourable at room temperature. Our results show that these cubic superlattice structures present an increased stability of this phase compared to the hexagonal phase and the bulk of the constituents.

In the recent years, organic phototransistors (OPTs) have attracted considerable interest due to their unique capability of delivering flexible and wearable photosensor modules for various applications in the rapidly growing field of flexible electronics [1–6]. Compared to conventional photodetectors and inorganic phototransistors, OPTs have an advantage of low-cost fabrication when it comes to their low processing temperature, as well as their mechanical flexibility and versatile chemical functionality of the organic materials [7,8]. OPT structure is identical to that of an organic field-effect transistor (OFET), where the channel layer simultaneously plays the sensing role. The three-electrode geometry of an OPT controls and amplifies signal, while reducing noise and hence improves photosensitivity by simply adjusting the gate voltage (V_G) [9-11].
Recent advances on OPTs have been particularly focused on detection in the ultraviolet (UV) and near infrared (NIR) regions. UV sensors are gaining increasing attention for utilisation in the health NIR light-sensing OPTs has become one of the most important foundations for advanced control and sensing systems such as night vision for cars and airplanes, light detection and ranging (LiDAR) sensors for autonomous cars and drones, probe beams for biomedical devices and diagnostics, and optical communications [12-15] and medical fields to prevent sunburn and skin cancer caused by sunlight [16,17]. Numerous researches have been dedicated to NIR sensors for medical diagnostics, for instance, pulse oximetry or monitoring blood pressure [18,19]. Home-assisted (bio)medical sensors have gained more demand than ever before, due to increased hospitalisation and hospitals overcrowding during the Covid-19 pandemic [20-23]. Photoplethysmography (PPG) sensors can be integrated into home-assisted kits and offer tremendous advantage to monitor cardiovascular and atherosclerosis disease markers, such as heart rate (HR), blood pressure (BP), heart rate variability (HRV), arterial oxygen saturation (SpO₂), arterial ageing. Most basic human characteristics can be evaluated by the flow of blood in the subcutaneous tissue. PPG sensors can detect these indicators by monitoring pulsatile changes in blood flow [24-27]. However, the NIR-absorbing organic materials are very rare because of the difficulty in synthesis to meet the narrow energy band (level) gap of ca. 0.89~1.65 eV, which corresponds to the wavelength (λ) range of ca. 750~1400 nm. In this regard, conjugated donor-acceptor (DA) polymers have been considered viable NIR-absorbing materials since their energy band gaps can be altered by combinations of electron-donating and electron-accepting monomers. Moreover, when used as gas and bio-sensors, sensitivity, selectivity and drift control are critical parameters of consideration.
In this work, we proposed novel, low-power, highly sensitive OFETs and OPTs based on natural and low-waste materials. We applied and modified aloe vera (aloin) and cellulose as natural dielectric materials to operate devices at low voltages. For the active material, we synthesised a selection of DA polymers based on thieno-thiophene (TT) and naphthalene diimide (NDI) as the donor and acceptor respectively, with different substituents to verify their electrical and sensing performances. The dielectric layers are separately characterised in parallel-plate capacitors and electrical and sensing (both as light-receiving device and chemical sensors) of the OFETs are fully tested. We believe that these natural, low-waste OFET devices can be used as gas, chemical and photoactivated light-receiving, sensing devices with low-operating voltage and good environmental and bias stress stability.

Luminescent solar concentrators (LSCs) were originally proposed for the efficient collection of solar energy and have since found a prominent place in building integrated photovoltaics. In its simplest form, an LSC device is composed of a transparent host matrix doped with fluorescent materials. The fluorophores absorb the incident light and typically re-emit it isotropically at a longer wavelength, due to the Stokes shift effect. The majority of the re-emitted light is trapped within the host matrix by means of total internal reflection, where it is waveguided towards the edges of the device. A thin solar cell module integrated on the sides of the LSC is then used for the photon-to-electron conversion. The ability of LSCs to concentrate light efficiently makes them a powerful photonic platform compatible with a diverse range of applications. Apart from photovoltaics, LSCs have been used in dark-field imaging, as chemical microreactors, in greenhouse coatings, and even in dynamic systems (where the time of photon arrival is of essence), such as in image recording and movement detection technologies and in free-space optical communications. In the latter case, LSCs were introduced as an efficient means to collect the diffuse light generated by fastly modulated light-emitting diodes (LEDs) in visible light communication (VLC) systems.

The first demonstrations of LSCs for VLC predominantly focused their attention on how to increase their optical gain (optical power out divided by optical power in) and field-of-view (range of solid angles for which the detector accepts light). However, little attention has been devoted to the performance of such devices in the time and frequency domains. Typically, fluorophore lifetimes are between 1 and 100 ns, which is orders of magnitude slower than the response time of currently employed high-quality communications semiconductor components, indicating that LSCs likely form the weakest link in optical communication channels. Worse still, the overlap between the absorption and emission spectra of fluorophores causes reabsorption, inducing further temporal delays and adding to the bandwidth (BW) limitations. In spite of LSCs potentially acting as barriers to high data-rate transmission, there is no existing study into the profound question of bandwidth limits in optical systems employing them.

This study makes a threefold contribution. i) We present a general analytical model for the prediction of the impulse response in arbitrary LSC geometries and show that for a large class of LSCs, for which reabsorption can be ignored, LSCs behave as simple low-pass, RC circuits. Crucially, we subsequently show that in the case of optically dense LSCs, for which multiple reabsorption events occur, the above equation becomes a strict upper bound and hence sets the fundamental bandwidth limit for any LSC integrated optical communication channel. ii) The next contribution of this work is the development of a powerful time-domain, Monte Carlo (MC) modelling framework for LSCs. With the MC model the received power and the system’s field-of-view can accurately be predicted, inferring fundamental quantities, such as the signal-to-noise ratio (SNR) and the bit-error ratio of the link. iii) Finally, we highlight a subtle trade-off between maximising the collection efficiency of LSCs and increasing their bandwidth, accentuating the importance of evaluating LSC dynamic performance when employed for use in optical communication systems.

References:
1. Portnoi, M., Haigh, P. A., Macdonald, T. J., Ambroz, F., Parkin, I. P., Darwazeh, I., & Papakonstantinou, I., "Bandwidth Limits of Luminescent Solar Concentrators as Detectors in Free-Space Optical Communication Systems". Light: Science & Applications, 10 (3), 2021

Efficient underwater solar cells could be used to power autonomous underwater vehicles and would allow for improved environmental monitoring and remote sensing without relying solely on batteries. However, this development has i) been lacking adequate guidelines for which active materials to employ, and ii) had challenges evaluating prototypes, as this requires either field deployment or access to large water tanks. To tackle these challenges, we here show that underwater solar cells can, in theory, exhibit efficiencies up to 65% in deep waters and that the optimum band gap shifts from 1.8 to 2.4 eV between shallow and deep waters and plateaus at > 2.1 eV at intermediate depths, highlighting the need to shift away from solar cell materials typically employed for terrestrial use. This wide range in large optimum band-gap energies opens the potential for a vast library of organic wide-band gap semiconductors to be explored for high-efficiency cells. Secondly, we present a simple bench-top characterization technique that does not require direct access to water, allowing for the exploration of materials without relying on field deployment. We use this technique to show that GaInP solar cells can operate at higher than 51% efficiency and that organic solar cells can outcompete commercial inorganic cells, such as Si and CdTe, when employed for underwater applications. The results presented here refocus the field of underwater solar cells in order to best screen emerging materials for their potential in underwater solar energy conversion.
1. Jenkins, P. & Walters, R. Photovoltaic Technology for Navy and Marine Corps Applications. 2017 IEEE 60th Int. Midwest Symp. Circuits Syst. 958–961 (2017). doi:10.1109/MWSCAS.2017.8053084
2. Röhr, J. A., Lipton, J., Kong, J., Stephen, A. & Taylor, A. D. Efficiency Limits of Underwater Solar Cells. Joule 4, 1–10 (2020).
3. Kong, J. et al. Underwater Organic Solar Cells via Selective Removal of Electron Acceptors near the Top Electrode. ACS Energy Lett. 4, 1034–1041 (2019).

Soft material and processing techniques have enabled the development of a variety of wearable electronics which requires the devices to be stretchable to form a conformal contact on human skin. However, the development of the stretchable display is greatly stagnant due to the lack of a protocol for the deposition of stretchable cathode materials. Alternatively, a direct solution process on the light-emitting layer will deteriorate the device's stability. In addition, conventional physical vapor deposition of rigid metal electrodes is not applicable for stretchable applications. Hence, the electrode materials that can be used as the cathode is greatly limited. In this research, we have developed a universal cathode lamination protocol with the aid of solvent annealing and hot pressing. The quality of the lamination is evaluated using the image-assisted analysis method and photoluminescence. Based on this lamination protocol, we have developed intrinsically stretchable light-emitting diodes (ISLED) with a turn-on voltage of lower than 4 V and maximum luminance over 1700 cd/m², indicating a well-contacted cathode-emitting layer interface. Furthermore, a three-inch five-by-five matrix has been successfully demonstrated with the application of tensile strain using the convex stretching method. This work could provide a guideline for developing an ISLED and stimulate considerable research on fundamental aspects of ISLEDs and furthermore into practical applications in both academia and industries.

Semitransparent organic solar cells (STOSCs) are a technology that combines the benefits of visible light transparency and light-to-electrical energy conversion. One of the greatest opportunities for STOSCs is their integration into windows and skylights in energy-sustainable buildings. For this application, the aesthetic aspects of solar cells may be as important as their electrical performance. Here, our strategy enables to achieve high-quality and colorful STOSCs using Fabry-Pérot etalon-type electrodes. These electrodes are composed of an antimony oxide (Sb₂O₃) cavity layer and two thin Ag mirrors. These dichroic tri-layer structures perform two functions as top conducting electrodes and color filters. These dual-function electrodes were applied to photovoltaic devices and displayed vivid colors, natural transparency, and good performance as compared to devices that use conventional metal electrodes. Furthermore, to achieve saturated colors and low photocurrent losses, active layer materials were selected such that their transmission peaks matched the transmission maxima of the electrodes. These strategies for colorful STOSCs result in power conversion efficiencies (PCEs) of up to 13.3% and maximum transmittances (T_MAX) of 24.6% in blue devices, PCEs of up to 9.71% and T_MAX of 35.4% in green devices, and PCEs of up to 7.63% and T_MAX of 34.7% in red devices.

Printed electronics represent a paradigm shift in the way we manufacture electronics, offering possibilities for more sustainable large-scale manufacturing in addition to new functionalities. Organic Semiconductors, which can be formulated as inks and then printed into various devices are poised to unlock even more potential applications in printed electronics. For large volume production runs of printed organic electronics technologies, it is expected that organic semiconductors will be manufactured on the kilogram to ton- scale. Thus, it is important that the large-scale synthesis of these materials is not only repeatable but also done in an environmentally responsible manner. This presentation will outline how Brilliant Matters is contributing to the growing field of printed solar cells by offering a scaleup platform for organic semiconductor materials.

Organic Photovoltaics (OPV) are yet to reach market readiness due to their relatively low power conversion efficiencies (PCE). However, the emergence of high-performance non-fullerene acceptors (NFA) has enabled a rapid increase in the PCE of state-of-the-art devices, with the record PCE of over 18 %, which is comparable to that of inorganic counterparts, bringing about a renaissance in OPV technologies. The key advantage of NFA over fullerene-based counterparts is their tunable opto-electronic properties. Energy levels can be tailored for minimum voltage loss and the absorption spectrum can be tuned to be complementary to that of donor materials so the integrated absorption covers a broad range of the solar spectrum. The latter approach can be further expanded by introducing multiple acceptors in so-called ternary or quaternary blend systems. Such an approach provides an opportunity to raise the PCE beyond 20 %, however, it also creates a new challenge in exploring numerous possible material combinations, together with thickness variations, thermal annealing conditions and processing additives. Once a market competitive PCE has been achieved on a laboratory scale, the next step would be translating the technology for commercial manufacturing.
One of the key advantages of OPV is the potential for low-cost manufacturing by industrial roll-to-roll (R2R) printing. However, rapid progress has been driven by material innovations and high PCEs have been demonstrated mostly by laboratory processes on glass substrates using R2R incompatible techniques. The PCEs of R2R OPV devices have been lagging far behind those of laboratory ones. An ideal solution to this problem is to explore the enormous parameter space using a new research method that can quickly screen a large number of parameters using industrial R2R printing methods.
Therefore, an automated R2R research platform has been developed to accelerate OPV research toward commercialization. A programmable in situ formulation system with slot die coating, a lossless deposition method, enabled the fabrication of over 10,000 unique OPV cells in a day with about 150 mg of materials (i.e. 15 µg of donor + acceptor per cell). A R2R testing system, with a matched capacity of over 10,000 tests per day, has also been developed and used to characterize R2R processed OPV devices. This research platform has enabled rapid progress of R2R-printed OPV and the demonstration of 10 % PCE fully-R2R-printed OPV. Despite the progress of OPV with vacuum-deposited metal electrodes, the efficiency of R2R OPV with printed back electrodes have been only up to 7 % and the result represents significant progress from previously reported results. The fabrication parameters and testing results have been digitalized to utilize digital technologies, i.e. artificial intelligence (AI) and machine learning (ML), to analyse such a huge experimental dataset. The research platform is being used to further optimize the formulation and processing parameters. The recent progress of the R2R printed OPV cells, as demonstrated by the fabrication of large-area OPV modules using the digitally optimised manufacturing parameters, will also be presented in the presentation.

Organic solar cells (OSCs) using non-fullerene acceptors (NFAs) have garnered a lot of attention during the last years and showed dramatic increases in the power conversion efficiency (PCE). PCEs higher than 18% for single-junction systems were achieved, but these high-performance organic photovoltaic cells are often processed with halogenated solvents. To accelerate the mass fabrication of OSCs, green solvent processing is crucial to reduce the harmful effect of halogenated solvents to human health and our environment. In this talk, I will discuss the design, synthesis, and performance of organic semiconductors processed from green solvents such as xylene and 2-methyl THF. A combination of characterization methods were employed to gain insight into the film morphology and solar cell performance.

Recent advances in organic solar cell material development based around non-fullerene electron acceptors in bulk heterojunctions have propelled power conversion efficiencies to >18%, with 20% on the horizon and 25% predicted. [1] These efficiencies are close to traditional inorganic semiconductor photovoltaics and thus focus is now turning to manufacturability and creating a viable solar cell technology. In these regards, achieving high power conversion efficiencies in so-called ‘thick junctions’ of order 300 nm and above is an acknowledged prerequisite for the use of high throughout printing techniques. This has proven extremely challenging to achieve with organic semiconductors due to their relatively poor transport properties and in particular photo-generated carrier bimolecular recombination. In this presentation we report the highest efficiency to date (16% with a Fill Factor >70%) in a thick junction binary organic solar cell based upon the PM6:BTP-eC9 system. Using a very accurate approach based upon temperature dependent ultra-sensitive EQE measurements we find that this system (and a similar one based upon PM6:Y6) have near unity charge generation yields (CGY > 99%). In this regime, we observe that a relatively small increase in CGY of only 0.5% leads to a 2.5 times reduction in bimolecular recombination relative to the Langevin limit – clearly demonstrating the subtle link between photo-generation and recombination in excitonic organic semiconductors. We observed a 1000-times-reduced recombination rate constant in the PM6:BTP-eC9 which delivers the superior thick junction performance than PM6:Y6. [2]

References:
[1] Armin et al., “A History and Perspective of NonFullerene Electron Acceptors for Organic Solar Cells”. Advanced Energy Materials, 2003570, 2021.
[2] Li et al., “Organic Solar Cells with Near-unity Charge Generation Yield” Energy & Environmental Science, 10.1039/d1ee01367j, 2021.

The properties of interfacial charge-transfer excitons affect almost every performance metric of interest in organic photovoltaics, from their open-circuit voltage to their external quantum efficiency and fill factor. However, charge-transfer excitons can be difficult to observe directly as they typically have low radiative efficiencies, weak oscillator strengths, and absorption spectra that overlap with those of monomolecular singlet excitons. Here, we employ a newly-synthesized set of UV-absorbing donor molecules that, when paired with acceptor B4PymPm, form strongly-emitting charge-transfer excitons with photoluminescence quantum yields of up to 11%. In addition, these heterojunctions can be integrated into efficient transparent photovoltaic cells that can sustain remarkably high reverse biases of more than 25V. Leveraging these unique properties, we conducted a detailed study of the voltage dependence of charge-transfer exciton photoluminescence and photocurrent in these devices, and performed voltage-dependent in operando pump-probe measurements that directly reveal the dynamics of charge transfer, polaron formation, and recombination in these systems. In aggregate, we were able to quantify the fate of all excitations in the devices with these measurements, evaluate the applicability of photovoltaic reciprocity theory, and correlate the properties of charge-transfer excitons to the molecular design of the donor molecules.

Interlayers in organic solar cells (OSCs) play a crucial role in determining their charge extraction properties, device performance, and stability. In this work, two naphthalene diimide (NDI)-based n-type polymers (i.e., P(NDIDEG-T) and P(NDITEG-T)), incorporating oligo(ethylene glycol) (OEG) side chains of different structures, are designed and utilized as new electron transport layers (ETLs) in OSCs. The hydrophilic OEG side chains enable the processing of these n-type polymers using eco-friendly water/ethanol mixtures. In addition, the polar OEG groups effectively modify the work function of the Ag cathode, enabling the polymers to function as efficient ETLs. Consequently, when these OEG-based ETLs are applied to PM6:Y6-based OSCs, a maximum power conversion efficiency (PCE) of 15.43% is achieved, which is substantially higher than that of a reference OSC without an ETL (9.93%) and comparable to that of an OSC with a representative high-performance PFN-Br ETL (15.34%). We demonstrate that the P(NDIDEG-T)-based OSCs show both enhanced PCEs and better reproducibility than their P(NDITEGT)-based counterparts, which are attributed to the lower surface tension and improved film uniformity of the P(NDIDEG-T) ETL. Also, the P(NDIDEG-T) ETL realizes OSCs with higher storage stability and more thickness-tolerant performance compared to the P(NDITEG-T) and PFN-Br ETLs. Our study provides useful guidelines for the design of ETLs suitable for the fabrication of high-performance, reproducible, and stable OSCs.

In 2000 Heinz Langhals and Susanne Kirner reported the N-annulation of the classic perylene diimide (PDI) dye.¹ Utility of this so called “N-annulated PDI” dye in the field of organic electronics was sparse until 2016. With reports of both sulfur and selenium annulated PDIs, our team in collaboration with Professor Henry Yan, re-introduced the N-annulated PDI as a useful building block for which to construct non-fullerene acceptors for organic photovoltaics.² N-annulation of the PDI chromophore destabilize the frontier molecular orbital energy levels leading to high voltage solar cell devices while the extra site for side-chain installation dramatically increases materials solubility, directs self-assembly, and allows for water soluble conjugated ionic salts to be readily formed. This presentation will highlight our teams work developing N-annulated perylene diimide derivatives to deliver non-halogenated solvent, roll-to-roll compatible processed organic photovoltaics.^3,4,5,6
European Journal of Organic Chemistry, 2000, 2000, 2, 365-380
Chemistry of Material, 2016, 28, 19, 7098-7109
Materials Horizons, 2020, 7, 2959-2969
ACS Applied Materials and Interfaces, 2020, 11, 42, 39010-39017
ACS Applied Materials and Interfaces, 2020, 11, 49, 46017-46025
ACS Applied Materials and Interfaces, 2021, 13, 41, 49096-49103

Organic solar cells offer the promise of renewable energy capture with an inexpensive, easy to produce, and mechanically flexible technology. Through two decades of research, power conversion efficiencies have risen dramatically. However, the materials used are far from cheap, and high-efficiency cells are difficult to produce at scale. While these challenges appear tractable, nearly 20 years of progress in understanding their underlying photophysics suggests that a radical rethinking of organic solar cell design may produce useful new directions. We present a design for a fully-conjugated block polymer solar cell that we term a double heterojunction. Detailed balance calculations suggest that device efficiencies approaching 30% are possible with exciton binding energies of 0.3 eV or less. The key to the design is a conjugated linker between the electron accepting and electron donating portions of the diblock system, called a bridge. Our calculations show that optimizing the energy level of this component is critical in achieving high efficiency devices. We will provide insight into the disappointing performance of more conventional diblock conjugated polymers and donor-bridge-acceptor dyads. In order to better describe general synthetic guidelines, we have performed self-consistent field calculations which suggests that a system composed in the following way: [D1-A1]-[D1-A2]-[D1-A2] results in optimal energy alignment of the central bridge segment. Although the calculations are relatively crude, the results provide a clear example of why fully conjugated block polymer solar cell materials are worthy of more intense study. Moreover, the synthetic complexity of this system points toward a dire need to leverage more sophisticated computational resources to identify tractable candidate systems. Time permitting, cells with leverage both the double heterojunction design and singlet fission processes will be discussed.

In most high-efficiency polymer solar cells (PSCs) studied to date, a bulk heterojunction (BHJ) film is used as an active layer by mixing various types of electron-accepting small molecules and donor polymers. The high-efficiency PSC mentioned above has already recorded a power conversion efficiency (PCE) in the range of 17-18%, thereby heralding its possibility for practical applications, However, in a recent study, it was found that the mechanical properties of a thin film, wherein a donor polymer and a small molecule acceptor were physically mixed, were unsuitable for flexible PSC device applications owing to poor strength, elongation at break and fracture toughness.
All-PSCs manufactured with donor and acceptor polymer blend films have recently been researched and found to have efficiencies greater than 10%. It is known that donor and acceptor polymer blend films used as the active layer of all-PSCs exhibit high tolerance against severe deformation, owing to their excellent mechanical properties. Therefore, for most flexible PSC devices, research has been commonly conducted using a donor-acceptor polymer blend film as the active layer. However, active layers made of binary polymer blends in a non-equilibrium state have the potential to exhibit large-scale phase separation. The gradual phase separation and mechanical fragility of polymer blend films, which are made of hard and soft polymers in BHJ matrices, ultimately deteriorate the performance and stability of flexible PSCs significantly under repeated bending, folding, or crumpling conditions over long periods of operation. To overcome these possible drawbacks, an attempt has been made to replace the donor-acceptor blend film with a new donor conjugated polymer, having n-type side-chain moieties, and main-chain conjugated copolymer (MCCC), containing donor and acceptor-based macromolecular species with high molecular weight in the polymer chain. Owing to the self-assembly nature of the donor and acceptor macromolecular species, certain copolymers preferentially generate nanoscale internal morphologies due to composition-dependent interactions between donor or acceptor individual domains.
In this presentation, the mechanical properties of an MCCC (e.g. PBDT2T-co-PNDI2T) containing donor- and acceptor-based macromolecular species in the polymer backbone were measured and compared with that of donor and acceptor polymer blend film (P(BDT2T):P(NDI2T)). It was observed that the MCCC active layer exhibited a higher crack-onset-strain and toughness compared to P(BDT2T):P(NDI2T) active layer. Correspondingly, the YTRON/polyethylene naphthalate (PEN) based flexible PSC with an MCCC active layer was used, which exhibited superior mechanical stability under multiple bending conditions. The results of this study indicate that flexible PSCs with excellent durability can be developed by replacing the polymer blend used for all-PSCs with MCCC as the active layer material for highly efficient future applications.

Approaches to fabricate flexible and stretchable devices have embraced extensive exploitation of materials for active coatings, functional printing and revolutionary modifications of deformable substrates. Based on these strategies, we have realized formation of metallic nanowire bundled network on nanocellulose substrate and textiles for flexible transparent conductors. To deliver stretchable conductors, liquid metal based multiphase urethane elastomer composites have been prepared. An all 3D-printed triboelectric nanogenerator with liquid metal based stretchable current collector and the elastomer matrix as the triboelectric active layer has been achieved. Energy generation can be harvested even under extreme strain states. The power output performance can be recovered despite severe mechanical damages due to the self-healing properties endowed by the supramolecular hydrogen bonding of the elastomer. Meanwhile, 3D printed kirigami stretchable piezoelectric sensor can be prepared using the elastic conductor, which allows it to operate in pressing mode with output voltage unperturbed up to 300% strain. The stretchable conductors can be incorporated into energy storage devices to realize stretchable batteries based on ionic liquid. Furthermore, it was found that perspiration or sweat can be used to sinter the Ag based stretchable conductor without the need of elevated temperature, making it suitable for many organic substrates.

Many types of conjugated polymers have recently been designed and synthesized to obtain a high power conversion efficiency (PCE) in all-polymer solar cells (All-PSCs) owing to good film formation properties and high mechanical stability for flexible devices. However, a polymer-polymer blend system traditionally has difficulty controlling the miscibility, which has a significant influence on their intrinsic properties, such as the different functional groups, molecular weight, and solubility. To overcome these disadvantages, a few researchers have proposed dual-functional materials for the single component active layer of BHJ PSCs. It can be expected that the use of polymer chains or small molecules containing well-separated donor and acceptor moieties together as the single component active material in a PSC may overcome the disadvantages of conventional BHJ All-PSCs bearing donor and acceptor polymers.

In this presentation, we introduce a new highly crystalline conjugated D-A block copolymer (PBDT2T-b-N2200) containing a wide-bandgap benzodithiophene-bithiophene carboxylate donor block (PBDT2T) and a narrow-band gap N2200-based acceptor block. PBDT2T-b-N2200 can exhibit wide complementary absorption within the 300–800 nm range. The maximum PCE of a non-halogenated solvent processed conjugated block copolymer (CBCP)-based PSC device was measured as 6.43 % after thermal annealing the active layer at 160 °C, which is remarkably higher than that of the blend film. Compared to those of a binary blend film using PBDT2T and N2200, a PBDT2T-b-N2200 film shows a dramatically improved surface and an internal morphology. This is well-matched with the time-resolved photoluminescence (TRPL) lifetime values and the distributed lifetime images. Interestingly, the annealed PBDT2T-b-N2200 films showed that DB and AB are arranged in a face-on manner and easily form fine domains, achieving percolation pathways for holes and electrons. More intriguingly, the corresponding CBCP-based PSCs exhibit a superior shelf-life time of their photovoltaic performance of over 1,020 h when stored under ambient conditions, without a significant change in morphology.

Organic semiconducting materials, which have been extensively researched in recent years, have the advantage that they can be applied to flexible devices such as organic photovoltaic cells (OPVs), organic light emitting diodes (OLEDs), and organic field effect transistors (OFETs). In order to realize a fully flexible optoelectronic device, a flexible transparent electrode (FTE) material that can replace the ITO electrode together with active materials is required. FTE materials for flexible organic devices must achieve high ductility, good mechanical durability, sufficient electrical conductivity and high optical transparency. Among various electrode materials, silver nanowires (AgNWs) have attracted considerable attention as alternative electrode materials for ITO because they can achieve high conductivity, transmittance, and flexibility at a low manufacturing cost.
In this study, simple patterning of sandwich-type AgNW-based FTEs with high transparency and conductivity was successfully demonstrated using a specific underlayer and an overcoat layer capable of photo-crosslinking. Crosslinked polyvinyl cinnamate (x-PVCn) film was used as the underlayer to achieve a uniform distribution of AgNWs and improve the adhesion simultaneously. A solution (PHT solution) containing poly(sodium-4-styrene sulfonate) (PSS-Na+), photo-crosslinkable 2,4-hexadiyne-1,6-diol (HDD), and the non-ionic surfactant Triton X–100 were used as an overcoat layer material on the AgNWs layer, and millimeter/centimeter scale patterns were easily formed by irradiating with 254 nm UV light under a shadow mask. The AgNW-based flexible electrode exhibited excellent electrical and optical properties while achieving a uniform film with a smooth surface and mechanical flexibility. In order to evaluate its potential as an FTE, the aforementioned AgNWs-based patterned electrode was applied to flexible PSCs. The FTE-based PSC device showed as high efficiency as ITO-based devices. In addition, during the mechanical bending test, the FTE-based device exhibited superior durability compared to the ITO-based flexible PSC.

Mechanical durability of the organic solar cell (OSC) active layer has been a major point of interest to achieve high-performance flexible OSCs. One major challenge is that in polymer: small molecular acceptor (SMA) bulk heterojunction (BHJ) films, the SMA embrittles the active layer. This results in poor fracture toughness in many high-performance OSCs that employ non-fullerene SMAs making them prone to failure. There remains a need to develop strategies to maintain high power conversion efficiency (PCE), maintain morphological stability, and improve mechanical stability. In this work, we have introduced a thermoplastic elastomer, styrene-ethylene-butylene-styrene (SEBS) into a PM6:Y6 BHJ to improve the film’s mechanical properties. This approach was inspired by the rubber toughening of commodity polymers. SEBS is a solution-processable elastomer that can be effectively integrated into the BHJ. We perform an optimization of the SEBS loading and processing conditions to maximize the PCE and toughness of the active layer. We find that SEBS loading as high as 10% by weight retains the PCE values as neat PM6:Y6 while improving film toughness. Additional SEBS improves toughness further but comes with a small decrease in PCE. For example, the fracture strain increases from 3% for the neat PM6:Y6 film to over 11% for the film with 20 wt.% SEBS. At this SEBS loading, the PCE of the solar cell was found to decrease from approximately 14% to 11%. To understand this behavior and optimize this ternary system, a detailed thermomechanical and morphological analysis is completed. We find that processing has a clear impact on the segregation behavior of the SEBS that impacts both device performance and mechanical behavior. Through this work, we show that active layers with elastomer additives is an effective strategy to improve the mechanical stability of organic solar cells.

Organic solar cells (OSCs) achieved rapid development in the last several years, particularly OSCs consisting of polymer semiconductor as the electron donor material and non-fullerene small molecule as the electron acceptor material. However, this type of OSCs show inferior device stability and poorer mechanical properties compared to all-polymer solar cells (all-PSCs), which contain polymer semiconductors as both the electron donor material and electron acceptor material. In spite of these distinctive advantages, all-PSCs suffer from low power conversion efficiencies (PCEs) due to the limited availability of n-type polymers with narrow bandgap and high absorption coefficient in long wavelength region.

To address this dilemma, we have been dedicating to the development of highly electron-deficient building blocks for constructing high-performance n-type polymers. We report here the design and synthesis of a series of imide-functionalized fused bithiophene imide derivatives (BTIn, n = 1-5) with up to 5 imide groups and 15 rings in a row. As highly electron-deficient building blocks with compact geometry and excellent solubilizing capability, these novel imide-functionalized (hetero)arenes are very promising building blocks for constructing n-type polymer semiconductors, such as alternating donor-acceptor polymers and structurally novel acceptor-acceptor (or all-acceptor) polymers. When applied in all-polymer solar cells, these BTIn-based acceptor polymers showed PCEs great than 16%. In addition, we have utilized strong electron-withdrawing cyano group to develop a very electron-deficient building block 5,6-dicyano-2,1,3-benzothiadiazole (DCNBT). The incorporation of this structurally novel DCNBT leads to a series of n-type polymers with ultra-narrow bandgap (down to 1.2 eV) and substantial absorption coefficient in long wavelength, which breaks the long-term bottleneck limiting the device performance of all-PSCs. When blended with wide bandgap polymer donor, the resulting all-PSCs show PCEs higher than 13%. The results demonstrating cyano is also a highly promising electron-withdrawing group to develop n-type polymers, and the emergence of such ultra-narrow bandgap polymer acceptors will guide further materials invention in terms of developing high-performance polymer acceptors for all-PSCs.

Recent years science fields and its applied technology advancement rapidly improving. In thin film technology such as organic light emitting diodes (OLED) and solar cells needed more flexible, transparent materials for its conductive and semiconductor layers. For this criteria graphene thin fims performing greatly as transparency and sheet resistance.
Indium tin oxide (ITO), the conventional conductive metal oxide thin film, has been widely used for transparent electrodes. However, a few drawbacks of ITO such as its high cost due to indium scarcity, complicated processing requirements, sensitivity to acid and basic environments, and cracking and delaminating when subjected to bending deem it unsuitable for applications that require flexibility and indium atoms diffuses into active layers of OLED and causes degradation. In this study we will prepare small molecule-based OLED solution-processed graphene anode with surface-active monolayers. Therefore, we will demonstrate current-voltage-luminescence measurement and conduct potential differences for work function. In addition, we will characterisize molecular electronic spectroscopic (UV-Vis and PL) properties and morphological changes by atomic force microscopy.

Electrochemical oxygen reduction has achieved significant prominence as a promising substitute for the conventional anthraquinone production process of hydrogen peroxide. Electrochemical synthesis offers an economical and environmentally friendly route through either H₂O or O₂. The electrosynthesis of H₂O₂ is a relatively unusual process because it involves reversible redox reactions of starting materials, intermediates, and products. Herein linker modulated Ni-based metal-organic nanosheets (Ni-MONs) catalysts for a economical electrosynthesis of H₂O₂ through an emerging electrochemical approach by oxygen reduction reaction is discussed as a modular system. The electronic structure of the Ni active sites was tuned by constructing analogous Ni-MONs with electron-withdrawing groups - fluorine (Ni-F-MON) and electron-donating groups (hydroxyl, amine) (Ni-OH-MON, Ni-Amine-MON) grafter onto their linkers. The electronic structures were examined through a series of cyclic voltammetric experiments and X-ray photoelectron spectroscopy. Our interpretation is that modifying the Ni electronic structure modulates the binding energy to the *OOH intermediate. Further, the electron-donating group of hydroxyl and amine linkers has increased the electronic density of the Ni²⁺. Amongst the Ni-MONs studied, the Ni-MONs with an electron-donating group –hydroxyl and amine have demonstrated higher activity and selectivity (> 90 ±3 %) towards the H₂O₂. Our understanding is that electron-donating groups ( hydroxyl and amine) linkers regulated the Ni catalytic activity. This study puts forward design principles towards attaining higher selectivity of electrosynthesis of H₂O₂ through rational modulation of electronic sites.

We study the thermal stability of inverted Cl-rich non-fullerene acceptor (NFA)-based organic photovoltaics (OPVs) with MoO_x as the anode buffer layer at temperatures up to 80^oC. Over time, the power conversion efficiency shows a thermally accelerated “check-mark” shaped degradation pattern, featured by an early decrease, followed by a long-term recovery. A high Cl concentration at the BHJ/MoO_x interface in the thermally aged device is found using energy dispersive X-ray spectroscopy (EDS). X-Ray photoelectron spectroscopy (XPS) shows that the MoO_x is chlorinated after thermal aging. With the help of bulk quantum efficiency (BQE) analysis, we propose an explanation to the mechanisms behind the check-mark shaped thermal degradation pattern. Furthermore, inserting a thin C₇₀ layer between the BHJ and MoO_x suppresses the thermal degradation mechanisms, resulting in 3 orders of magnitude increase in device lifetime at 80^oC.

The utilization of conjugated polymers as active materials in organic electronic devices is severely restricted by the synthetic procedures used to prepare such polymers. For the most part, conjugated polymers are prepared via Migita-Stille and Miyaura-Suzuki cross-coupling techniques, which require expensive and toxic organometallic intermediates. Direct (hetero)arylation polymerization (DHAP) makes accessible the synthesis of such materials through palladium-catalysed C-H activation, thereby avoiding many of the setbacks of other polymerization techniques. This reaction has been improved greatly over the last few years and can now be used to prepare some of the most highly-performing organic electronic materials reported in the literature. We will review in this lecture recent advances for the preparation of nearly defect-free conjugated polymers and their utilization in plastic solar cells.

Wearable devices for energy generation are required to be highly stretchable and mechanically stable to repeated deformations. During various body motions, both the stretching and compressive forces act on wearables. Piezoelectric nanogenerators (PENG) are desirable candidates for wearable electronics due to their simple structure and ease of generating energy. For such applications, polymer based devices are preferred, however, devices based on PVDF, a popular ferroelectric polymer, though flexible, suffer from poor stretchability. Herein, we demonstrate a first all polymer based ferroelectric device with high flexibility and stretchability.
This was achieved by electrospinning solutions of PVDF blended with an elastic polymer, in different compositions. Specific combinations of the two polymers were found to significantly improve both the piezoelectric properties and mechanical stretchability. The nanofibers from these compositions, having significantly high stretchability, piezoelectric coefficient, and electroactive phases were used to fabricate S-PENG devices by encapsulating nanofiber web between the two stretchable electrodes of graphene:PEDOT:PSS. The devices showed high efficiency, outstanding mechanical and electrical stability for at least 2000 cycles of stretching deformations.
The S-PENG devices were integrated with the wearables and evaluated for their performance during different body motions such as walking, knee and elbow bending, and finger movements. The generated energy could be stored in a capacitor, which got charged to ~ 650 mV in just 100 s. The approach was found to have a great potential for developing stretchable wearable devices for energy harvesting.

Organic solar cells (OSCs) have attracted abroad attention and extensive research interest due to their distinct advantages of light weight, flexibility, semitransparency and low cost. Recently, the performance of OSCs has been improved rapidly owing to the great efforts on innovative materials and device structures.
In this talk, we will present research works in Los Angles and Hong Kong, covering several aspects of OPV progress recently, including the following:
A dual-function p-type compatible interlayer to modify the interface of the hole-transporting layer and the ultrathin electrode of the semitransparent devices. [1] The modified semitransparent devices reach a highest PCE of 13.7% with an AVT of 22.2%.
Realizing and understanding of novel effective graded G-BHJ strategy via nonhalogenated solvent sequential deposition using nonfullerene acceptor (NFA) OSC systems. G-BHJ OSC via open-air blade coating achieved a record 16.77% PCE, and 17.54% from CF. [2]
Novel oligomer third component enabled 17.5% PCE ternary OPV, which representing the state-of-the-art OSCs processed by green solvent of o-xylene. [3] In particular, ultra-high EQE_EL of 0.05% and improve V_OC was achieved. Furthermore, a new morphology manipulation strategy leads to over 19% single junction OPV device (certified 18.5% PCE). [4]
A low cost, methanol processed copper phosphotungstate (Cu003) as anode interface layer (AIL) is developed. OPV device based on Cu003/PM6:BTP-eC9:PC71BM affords a remarkable efficiency of 18.2%. [5]
[1] ACS Nano 16, 1, 1231–1238 (2022)
[2] Nature Communications 12 (1), 4815 (2021)
[3] Advanced Materials, 2107659 (2022)
[4] Under review
[5] Nano Energy, 106923 (2022)

Developing polymer solar cells (PSCs) with high mechanical robustness is one of the most urgent tasks to ensure their operational reliability and applicability in wearable devices. Here, we present material design strategies to enhance the mechanical robustness of the PSCs as well as their power conversion efficiencies (PCEs); i) controlling molecular weights (MWs) of the polymer acceptors (P_A), ii) incorporating a high-MW P_A as an electro-active additive into binary blends, and iii) enhancing molecular miscibility between donor-acceptor in their blends. The superior mechanical robustness is demonstrated with P_A having MW above the critical molecular weight (M_c), which produces tie molecules and polymer entanglements that dissipate mechanical strain energy. And, long chains of the high-MW P_A act as electrical-bridges connecting adjacent crystalline domains of the blends, producing simultaneous enhancements of PCE and stretchability of the PSCs. In addition, the improved blend morphologies from enhanced donor-acceptor molecular miscibility induce substantial increases of PCE, mechanical toughness, and cohesion energy of the PSCs. With these contributions, the mechanically-robust, high-performance PSCs with over 17% PCE is developed. The material design guidelines and outlooks for the efficient and mechanically-robust PSCs will be presented. Finally, we employ these active materials in constructing intrinsically stretchable PSCs.

Top-performance organic solar cells (OSCs) consisting of conjugated polymer donors and nonfullerene small molecule acceptors (NF-SMAs) deliver rapid increases in efficiencies. Nevertheless, many of the polymer donors exhibit high stiffness and small molecule acceptors are very brittle, which limit their applications in wearable devices. Here we report a simple and effective strategy to improve the stretchability and reduce the stiffness of high-efficiency polymer:NF-SMA blends and simultaneously maintain the high efficiency by incorporating a low-cost commercial thermoplastic elastomer, SEBS. The microstructure, mechanical properties, and photovoltaic performance of PM6:N3 with varied SEBS contents, and the molecular weight dependence of SEBS on microstructure and mechanical properties were thoroughly characterized. This strategy for mechanical performance improvement exhibits excellent applicability in some other OSC blend systems, e.g., PBQx-TF:eC9-2Cl, PBDB-T:ITIC. More crucially, the elastic modulus of such ternary blends can be nicely predicted by a mechanical model. Therefore, incorporating thermoplastic elastomers is a widely applicable and cost-effective strategy to improve mechanical properties of nonfullerene OSCs and beyond.