This PDF file contains the front matter associated with SPIE Proceedings Volume 11094, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

The performance of molecular/macromolecular and hybrid solar cells depends on understanding and controlling the interfaces between the component materials. Molecularly tailoring interfaces offers an effective and informative means to selectively modulate charge transport, molecular self-assembly, and exciton dynamics at hard matter-soft matter and soft-soft matter interfaces. Such interfaces can act as filters to facilitate extraction of “correct charges” while blocking extraction of “incorrect charges” at electrode-active layer and active layer-active layer interfaces in almost all types of solar cells. Such interface engineering can also suppress carrier-trapping defects at interfaces and stabilize such interfaces against de-cohesion and the ingress of oxidants. For soft matter-soft matter interfaces, interfacial tailoring also enhances charge separation and photocurrent generation. In this lecture, challenges and opportunities in controlling the structures of solar cell interfaces are illustrated in the following areas:1) modulating charge transport by active layer molecular/microstructural organization,[1],[2] 2) controlling exciton splitting and carrier generation at active layer donor-acceptor interfaces,[3],[4] 3) tuning donor-acceptor combinations for maximum performance,[5],[6] 4) modulating charge transport across electrode-soft matter interfaces in polymer and perovskite cells.[7] Rational interface engineering along with improved donor and acceptor structures, guided by theoretical/computational analysis, affords large fill factors, efficiencies greater than 14%, and enhanced cell durability. All this must of course be accomplished using environmentally benign synthetic processes.[8] REFERENCES [1] Manley, E.F.; Strzalka, J.; Fauvell, T.J.; Jackson, N.E.; Marks, T.J.; Chen, L.X. Advan. Mater. 2018, 30, 1703933. [2] Manley, E.F.; Harschneck, T.; Eastham, N.D.; Leonardi, M.J.; Zhou, N.; Chang, R.P.H.; Chen, L.X.; Marks, T.J. Advan. Energy Mater. 2019, 1800611. [3] Eastham, N.D.; Dudnik, A.S.; Aldrich, T.J.; Manley, E.F.; Fauvell, T.J.; Hartnett, P.E.; Wasielewski, M.R.; Chen, L.X.; Facchetti, A.F.; Chang, R.P.H.; Marks, T.J. Chem. Mater. 2017, 29, 4432–4444. [4] Wang, G.; Eastham, N.D.; Aldrich, T.J.; Ma, B.; Manley, E.F.; Chen, Z.; Chen, L.X.; Olvera de la Cruz, M.; Chang, R.P.H.; Facchetti, A.; Marks, T.J. Advan. Energy Mater. 2018, 8, 1702173. [5] Eastham, N.D.; Logsdon, J.L.; Manley, E.F.; Aldrich, T.J.; Leonardi, M.J.; Wang, G.; Powers-Riggs, N.E.; Young, R.M.; Chen, L.X.; Wasielewski, M.R.; Melkonyan, F.S.; Chang, R.P.H.; Marks, T.J. Advan. Mater. 2018, 30,1704263. . [6] Fallon, K.J.; Santala, A.; Wijeyasinghe, N.; Manley, E.F.; Goodeal, N.; Leventis, A.; Freeman, D.M.E.; Al-Hashimi, M.; Chen, L.X.; Marks, T.J.; Anthopoulos, T.D.; Bronstein, H. Advan. Funct. Mater. 2017, 27, 1704069. [7] Liao, H.-C.; Tam, T.L.D; Guo, P.; Wu, Y.; Manley, E.; Huang, W.; Seo, C. M.; Wasielewski, M.R.; Kanatzidis, M.G.; Chen, L.C.; Facchetti, A.; Chang, R.P.H.; Marks, T.J. Advan. Energy Mater. 2016, 1600502. [8] Aldrich, T.J.; Matta, M.; Zhu, W.; Swick, S.M.; Stern, C.; Schatz, G.C.; Facchetti, A.; Melkonyan, F.S.; Marks, T.J.;, J. Amer. Chem. Soc. 2019, in press. DOI: 10.1021/jacs.8b13653.

A critical component of any donor-acceptor (D-A) bulk heterojunction organic solar cell is the appearance of inter-molecular charge-transfer (CT) electronic states at their D-A interfaces. These electronic states play a determining role in the photo-physical processes that transform the energy of the absorbed sunlight into electrical power. Here, through integrated multiscale theoretical simulations, we have illustrated how factors such as the details of the molecular packing at the D-A interfaces, the electronic polarization effects, and the extent of electron/hole delocalization around the interface impact the nature of the CT states. Moreover, we have also discussed how the hybridization between the CT and local-exciton (LE) states impacts the spectroscopy characteristics of D-A blends, the recombination rates and consequently the voltage losses, which need to be minimized.

Aqueous dispersions of silver nanowires state an environmentally friendly avenue for highly conductive, yet transparent top electrodes for semi-transparent perovskite solar cells. However, for the well-known chemical instability of halide perovskites upon exposure to water, there are no reports of successful aqueous processing on top of perovskite devices. Here, we show that electron extraction layers of AZO/SnOx [1,2], with the SnOx grown by low temperature atomic layer deposition, provide outstanding protection layers, which even afford the spray coating of AgNW electrodes (sheet resistance Rsh =15 Ohm/sq and a transmittance of 90%) from water-based dispersions without damage to the perovskite. The layer sequence of the inverted cells is ITO/PTAA/perovskite/PCBM/AZO/SnOx/top-electrode. In devices without the ALD SnOx, aqueous spray processing decomposes the perovskite layers. Interestingly, the direct interface of Ag-NW/SnOx comprises a Schottky barrier, with characteristics strongly dependent on the charge carrier density of the SnOx. For a carrier density below 10^18 cm^-3, S-shaped J-V characteristics are found, that successively vanish upon UV-light soaking. For our low-T SnOx with 10^16 cm^-3, the insertion of a thin interfacial layer with a high charge carrier density (10^20 cm^-3), e.g. 10nm of ITO, is found to afford high performance semitransparent PSCs with an efficiency of 15%. Most importantly, compared to ITO electrodes Ag-NW based electrodes provide a key to achieve a higher transmittance in the IR, which is desirable for tandem Si/PSCs. [1] K. Brinkmann et al., Nat. Commun. 8, 13938 (2017). [2] L. Hoffmann et al. ACS Applied Mater. & Interfaces 10, 6006 (2018).

The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells (PSCs) with both low cost and boosted high power conversion efficiency (PCE) over 22%. Despite the achievements, MAPbI3 suffers from inherent instability over ambient operation conditions due to the low formation energy of the material itself and the high hydrophilicity of the organic cations. Efforts such as developing novel device architectures as well as exploring novel materials have been tried to improve the device stabilities. Among them, the two-dimensional (2D) perovskites that are crafted using bulkier alkylammonium cations in place of methylammonium exhibit appealing environmental stability. However, the insulating alkylammonium spacer cations hinder charge transport and limit the efficiencies of the devices based on such perovskites. In this scenario, an exploration of alternative yet effective organic spacer cations that increase the charge transfer is imperative to enhance the efficiency. Herein, we design such an alternative bi-functional conjugated cation AB. We report the first time the incorporation of AB in 2D/3D perovskites and its implementation on solar cells. The use of bi-functional conjugated cations enhances significantly the performance of the cells, reaching a highest power conversion efficiency of 15.6% with improved stability. The efficiency remained around 90% of the initial value after 100 h continuous illumination, much more stable than MAPbI3 perovskite. By comparing this cation with a mono-functional cation and a bi-functional non conjugated cation with similar structure, we found that the bi-functional conjugated cation not only benefits the growth of perovskite crystals in the mesoporous network, but also facilitates the charge transport. Our approach helps explore new rational designs of cations in perovskites.

Perovskite Solar Cells (PSC) have attracted great attention due to the high efficiencies achieved in the past few years (up to 24.2 %). Perovskite semiconductors show excellent light absorption and large charge-carrier mobilities. In addition, device fabrication is low cost and easily up-scalable. However, the current density-voltage curve (J-V) shows hysteresis and devices suffer from stability issues which are still poorly understood. Among all perovskite materials, mixed-cation lead mixed-halide PSC have become very popular due to their high efficiencies and reasonably good stabilities^1,2. On the other hand, Impedance Spectroscopy (IS) is a very valuable non-destructive technique to obtain information about dynamical mechanisms occurring both in the bulk and at the interfaces³ . In this work, J-V curves and the impedance response have been measured for CsFAPbIBr-based PSC from 1 Hz up to 1 MHz, under different illumination levels (from 0.06 mW/cm² to 100 mW/cm² ) both at 0 V (short circuit) and at V_oc (open circuit). Impedance spectra show two significant arcs, associated to different recombination and charge accumulation mechanisms. IS data have been fitted to a circuital model that consists of a low-frequency RCPE subcircuit in series with a high frequency resistance, all shunted with a high-frequency capacitance. Dependence of the circuital parameters with V_oc and I_sc will be discussed.

In recent years, highly flexible solar cells have been gathering great interest as a power source for operating wearable and/or on-skin electronic devices which are necessary technologies for the Internet of things (IoT) society. By adapting such flexible solar cells, wearable and on-skin devices can be free from the troubles of replacement of batteries and contact problems of external wirings. In this talk, our approaches for ultra-thin organic solar cells will be introduced with three important keywords, namely 1. high power conversion efficiency (PCE) , 2. strethchability/flexibility, and 3. robustness against the environmental conditions. We succeeded to fabricate organic solar cells with total thickness of 3 μm and achieved excellent PCE up to 10.5% for free-standing ultra-thin solar cells. There are trade-off relations between ultimate thinness and environmental stabilities such as air, water, and heating. We tackled to solve these problem and developed technologies to improve thermal stability and water stability of ultra-thin organic solar cells. Especially, high thermal stability of more than 100 degrees Celsius allows hot-melt adhesion process onto textiles, which enabled wearable power source systems. Additionally, we developed self-powered on-skin sensor systems by integrating our solar cells with ultra-thin organic electrochemical transistors and monitored heart-beat rate with such integrated devices.

Advancement of organic photovoltaic (OPV) technology towards commercial products requires the development of materials that are amenable to large area print manufacturing. This manuscript will highlight efforts by the OPV program at Phillips 66 to solve issues related to material scale-up, through material design combining good processability and high photovoltaic performance. An internally developed polymer/phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) blend that combines these critical parameters achieved a certified power conversion (PCE) of 12.14% in 2017.

Semi-transparent Organic Solar Cells for Greenhouse Application Yuan Xiong1*, Eshwar Ravishankar2, Jennifer Swift3, Harald Ade1*, Ronald Booth2, Melodi Charles4, Reece Henry1, Brendan O’Connor2, Jeromy James Rech5, Carole Saravitz3, Heike Sederoff4, Long Ye1, Wei You5 1. Department of Physics, Organic and Carbon Electronics Lab (ORaCEL), North Carolina State University, Raleigh, NC 27695, USA 2. Department of Mechanical and Aerospace Engineering and ORaCEL, North Carolina State University, Raleigh, NC 27695, USA 3. Department of Plant Biology, North Carolina State University, Raleigh, NC 27695, USA 4. Department of Plant and Microbial Biology, North Carolina State University, Raleigh, NC 27695, USA 5. Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA E-mail: yxiong8@ncsu.edu; hwade@ncsu.edu Semitransparent organic solar cells (ST-OSCs) show great potential in building-integrated photovoltaics due to the advantages in solution processability, flexibility, and transparency. Herein, we present a systematic study on the application of high-performance ST-OSC filters in a greenhouse by utilizing three representative systems with different spectral responses, namely, FTAZ:PC71BM[1], FTAZ:IT-M[2, 3], and PTB7-Th:IEICO-4F[4]. Specifically, the cultivation of red leaf lettuce is conducted in a controlled environment growth chamber, which is possible to duplicate any climate, and under different ST-OSC filters. In principle, the ST-OSCs absorb a portion of the solar spectrum for power generation and lettuce utilizes the penetrated light for photosynthesis. Furthermore, we quantitatively investigate the leaf area and number profiles, plant biomass, and photosynthetic rate under the as-prepared ST-OSC filters treatments. On the base of statistical analysis after the growth cycle, we can identify the best ST-OSC for plant growth. These results thus pave the way to integrate ST-OSCs with greenhouses. [1] S. C. Price, A. C. Stuart, L. Yang, H. Zhou, W. You, Journal of the American Chemical Society 2011, 133, 4625. [2] L. Ye, Y. Xiong, Q. Zhang, S. Li, C. Wang, Z. Jiang, J. Hou, W. You, H. Ade, Advanced Materials 2018, 30, 1705485. [3] Y. Xiong, L. Ye, A. Gadisa, Q. Zhang, J. J. Rech, W. You, H. Ade, Advanced Functional Materials 2019, 29, 1806262. [4] X. Song, N. Gasparini, L. Ye, H. Yao, J. Hou, H. Ade, D. Baran, ACS Energy Letters 2018, 3, 669.

Casting of a donor:acceptor bulk-heterojunction structure from a single ink has been the predominant fabrication method of solution-processed organic photovoltaics (OPVs). Despite the success of such bulk-heterojunction, the task of controlling its microstructure in a single casting process has been challenging and arduous and alternative approaches are desired. To achieve and even improve OPVs with a desirable microstructure, a facile and eco-compatible sequential deposition approach is demonstrated for nonfullerene polymer/small molecule pairs. Using a known weakly crystalline polymer FTAZ as the model material, we show the profound influence of casting solvent on the molecular ordering of the film, and thus the device performance and mesoscale morphology of sequentially deposited OPVs can be tuned[1]. Static and in-situ X-ray scattering indicate that applying the green solvent limonene is able to greatly promote the molecular order of FTAZ and form the largest domain spacing exclusively, which correlate well with the best efficiency in sequentially deposited devices. The sequentially cast device generally outperforms its control device[2] based on traditional single-ink bulk-heterojunction structure. Investigations of distinct material systems suggest that our approach be applicable to many conjugated polymers and nonfullerene acceptors, which yield consistently higher fill factors than traditional bulk-heterojunction devices. Moreover, the relationships between polymer:solvent interactions, thin-film microstructure, and device performance are discussed for these sequentially deposited devices. It is noted that polymer:solvent interaction parameter χ positively correlates with domain spacing in the devices. Our findings shed light on innovative approaches to rationally create ink-stable, environmentally friendly, and highly efficient nonfullerene solar cells. Reference [1] Ye, L.; Xiong, Y.; Chen, Z.; Zhang, Q.; Fei, Z.; Henry, R.; Heeney, M.; O’Connor, B.; You, W.; Ade, H. Adv. Mater. 2019, under review. [2] Ye, L.; Xiong, Y.; Zhang, Q.; Li, S.; Wang, C.; Jiang, Z.; Hou, J.; You, W.; Ade, H. Adv. Mater. 2018, DOI: 10.1002/adma.201705485.

Modest exciton diffusion lengths dictate the need for nanostructured bulk heterojunctions in organic photovoltaic (OPV) cells, however, this morphology compromises charge collection. Here, we reveal rapid exciton diffusion in films of a fused-ring electron acceptor that, when blended with a donor, already outperforms fullerene-based OPV cells. Temperature-dependent ultrafast exciton annihilation measurements are used to resolve a quasi-activationless exciton diffusion coefficient of at least 2 ×10-2 cm2 / s – substantially exceeding typical organic semiconductors, and consistent with the 20-50 nm domain sizes in optimized blends. Enhanced 3-dimensional diffusion is shown to arise from molecular and packing factors; the rigid planar molecular structure is associated with low reorganization energy, good transition dipole moment alignment, high chromophore density, and low disorder – all enhancing long-range resonant energy transfer. Relieving exciton diffusion constraints has important implications for OPVs; large, ordered, and pure domains enhance charge separation and transport, and suppress recombination, thereby boosting fill factors. Further enhancements to diffusion lengths may even obviate the need for the bulk heterojunction morphology.

Owing to the low dielectric constant of organic materials, organic photovoltaic (OPV) is regarded as an excitonic solar cell that excitons are generated upon photo-excitation. Such intrinsic small dielectric constant (ε) in organic materials results in large exciton binding energy (Eb). That becomes a key detrimental factor limiting the further improvement in organic photovoltaic cells. Increasing the material dielectric constant seems to be a straight-forward strategy to reduce the strong coulombic attraction of the photo-generated electron-hole pairs. Despite the matter of importance, there are limited reports in measuring the Eb and ε in organic photovoltaic materials and the correlation between the dielectric constant and the exciton binding energy is unclear. Here, we extend our demonstration by using quantum efficiency measurement [1] and electro-absorption to access the transporting gap and exciton binding energy in pristine organic photovoltaic materials for polymeric donor, fullerene and non-fullerene small molecular acceptors. It is found that Eb varies from 0.3 eV to 1.2 eV in those prototypical materials and it apparently follows a second power law with the inverse of the dielectric constant of the materials, i.e. Eb ∝ 1 / ε2. Instead of widely assumed first-order dependence, this second order dependent relationship is firstly reported. Interestingly, we have also found that the binding energy is more dependent on the molecular-molecular interaction rather than the intrinsic properties of single molecule. In this presentation, we will also demonstrate how the higher dielectric material benefits the exciton dissociation at donor/acceptor interface. [1]: Ho-Wa Li, Zhiqiang Guan, Yuanhang Cheng, Taili Liu, Qingdan Yang, Chun-Sing Lee, Song Chen, Sai-Wing Tsang, On the Study of Exciton Binding Energy with Direct Charge Generation in Photovoltaic Polymers, Adv. Electron. Mater., 2016, 2 (11), 1600200.

In modern electronics, it is essential to create almost arbitrary band structures by adjusting the energy bands and the band gap. Until now, band structure engineering in organic semiconductors has not been possible, since they usually exhibit localized electronic states instead of energy bands. In a recent publication [1], we showed that it is possible to continuously shift the ionization energy (IE) of organic semiconductors over a wide range by mixing them with halogenated derivatives. This tuning mechanism is based on interactions of excess charges with the mean quadrupole field in the thin film. In this work, we raise the question whether the band structure engineering concept can be generalized to other organic semiconductor materials and even be used to tune the size of the band gap. As a model system we study oligothiophenes and in particular we address questions not only about the energy landscape, but also about the micro-structure and the molecular mixing in the film. For this purpose, we analyze optical measurements as well as photoelectron spectroscopy measurements of single and blended layers. Reference: [1] M. Schwarze et al., Science 352, 1446 (2016)

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

The recombination of electron and hole limits the amount of photocurrent that can be generated in a photovoltaic medium. For this reason the physical mechanisms behind free charge generation and recombination in organic photovoltaic diodes form an integral part of scientific research. The most recent studies on OPVs have established the importance of energy absorbance in individual components that form a D-A junction. Free charges can simultaneously be generated across the D-A interface. The fast transport of generated charges away from the D-A interface is required to minimize recombination loss. Free charges can be lost through recombination mainly through two routes- 1) recombination at the interface via CT states, and 2) In situations where excitons are generated far from the D-A interface recombination in the bulk of the organic layer far away from the interface (> LD). Recombination via CT states where an elecotron (hole) in acceptor recombines with a hole (electron) in donor phase is most probable and can be easily resolved via measuring the emission related to corresponding CT energy. However, isolating recombination within the bulk from that occurring at the D-A interface is not trivial, and for this reason it has been largely ignored. In this work, we will present field-dependent photocurrent measurements on the model photovoltaic diodes of tetracene/C60 and rubrene/C60 that unveil recombination hot spots through detailed spectral mapping. We will show that recombination zone starts first in the tetracene layer and shifts to the C60 layer as device thickness increases. Results show that shift in recombination zone has strong implications on effective open-circuit voltage generation in photovoltaic diodes. The spin character of charge recombination behind energy up-converted green electroluminescence observed will be discussed.

Recently, non-fullerene n-type organic semiconductors (n-OS) have attracted significant attention as acceptors in organic photovoltaics (OPVs) due to their great potential to realize high power conversion efficiencies (PCEs). In this regard, a rational design of central fused ring unit of the n-OS molecules is crucial to maximize the state-of-the-art PCEs. Here, we report a new class of n-OS acceptor, Y series, that employ a ladder-type electron-deficient-core-based central fused ring to fine tune its absorption and energy levels. Among these new acceptors, the Y6-based OPVs exhibit a high efficiency of 15.7 % (both in conventional or inverted structures), and a certified efficiency of 14.9 % by an inverted structure. The electron-deficient-core-based fused ring reported in this work opens a new way in the molecular design of high performance n-OS acceptors for OPVs. References: [1] Nat. Commun., 2019, 10: 570 [2] Adv.Mater., 2019, 31: 1807577 [3] Joule, 2019, 4:1140-1151

Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous FREA small molecules have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. For example, of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close π-π stacking between the neighboring planar INCN groups, forming an effective charge transport pathway between molecules. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the π-π stacking distance by ~ 1 Å, but this change doesn’t affect the optical or electrochemical properties of the individual acceptor molecule. Overall, our results show that intermolecular interactions (especially π-π stacking between end groups) play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance.

A significant volume of literature exists that describe minor changes in composition and/or microstructure in perovskite solar cells (PSCs) as the driving force for incremental improvements in device performance. Many authors cite crystallinity as a fundamental driver of performance improvements, yet often do so without quantitatively defining crystallinity or, importantly, addressing the important questions behind their observations. Here I will discuss two recent case studies where processing modifications have been investigated as a means of controllably varying active layer crystallinity and describe the impact on device performance. i) We demonstrate the impact of active layer crystallinity on the accumulated charge and open-circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3,MAPI). We show that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where a small excess of methylammonium iodide (MAI) improves crystallinity increasing device Voc by ~200 mV and, in parallel, that the photoluminescence (PL) yield increases 15x, indicative of a suppression of non-radiative recombination pathways. This is coupled with the development of crystallographic texture (110) in the MAPI. In-situ transient optoelectronic measurements of the charge concentration in PSCs under operation suggest that the concentration of trapped charges (either at interfaces or in the bulk) is some 5x lower at matched Voc. We believe these trap states originate in/near the disordered or amorphous areas between MAPI crystallites, resulting at least in part from orientation mismatch between crystalline domains. ii) Secondly, we identify previously unobserved nanoscale defects residing within individual grains of solution processed MAPI thin films. Using scanning transmission electron microscopy (STEM) we identify the defects to be inherently associated with the established solution-processing methodology and introduce a facile processing modification to eliminate such defect formation. Specifically, defect elimination is achieved by co-annealing the as deposited MAPI layer with the electron transport layer PCBM resulting in devices that significantly outperform devices prepared using the established methodology, achieving PCE increases from 13.6 % to 17.7 %. The use of TEM allows us to correlate the performance enhancements to improved intra-grain crystallinity and show that highly coherent crystallographic orientation results within individual grains when processing is modified. Detailed optoelectronic characterization reveals that the improved intra-grain crystallinity drives an improvement of charge collection, a reduction of surface recombination at the MAPI/PCBM interface and a change in the density of local sub-gap states.

Rational Strategies to Stabilize the Morphology of Non-Fullerene Organic Solar Cells Huawei Hu, Long Ye, Masoud Ghasemi, Nrup Balar, Jeromy James Rech, Samuel J. Stuard , Wei You, Brendon O'Connor, Harald Ade* E-mail: hwade@ncsu.edu Organic photovoltaics (OPVs) are considered one of the most promising cost-effective options for utilizing solar energy. Recently, the OPV field has been revolutionized by the development of novel non-fullerene small molecular acceptors. With efficiencies now reaching 14% in many systems, the device stability and mechanical durability of non-fullerene OPVs have received less attention. Developing devices with both high performance and long-term stability remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable mechanical durability. Furthermore, many reports of OPV blends focus primarily on the device performance aspect of the solar cell and ignore the mechanical durability, which is an important consideration for OPV commercialization. Here we report a rational strategy to design nonfullerene OPVs that exhibit excellent thermal stability and storage stability while retaining high ductility with a two-donor polymer, non-halogenated ink. As a result, a highly efficient, stable, and ductile ternary nonfullerene OPV is achieved. The results indicate that synergistic enhancements can be achieved in more than one parameter. Our study indicates that improved stability and performance can be achieved in a synergistic way without significant embrittlement, which will accelerate the future development and application of non-fullerene OPVs. Reference H. Hu, L. Ye, M. Ghasemi, N. Balar, J. Rech, S. J. Stuard, W. You, B. O'Connor, H. Ade, Adv. Mater. 2019, Under review.

With rapid advances in the development of new conjugated polymers, non-fullerene acceptors, the power conversion efficiency (PCE) of OPVs has been increased over 14%. However, a major drawback for the commercialization of OPVs is their long-term stability under continuous operation. Especially, OPVs suffer from a rapid decrease in PCE during initial device operation, which is known as the “burn-in loss”. It is considered that the origin of the burn-in loss is mainly related with the instability of the BHJ morphology and/or interface rather than the photooxidation of the photoactive layer. We find that the photoactive layer prepared by a sequential solution deposition is more stable than that prepared by blend solution deposition. We also find that the burn-in loss is closely related with stability of photoactive layer / electron transporting layer interface.

Mixed halide, mixed cation lead perovskite films have been demonstrated to benefit tremendously from the addition of Cs and Rb into the perovskite formulation, resulting in high performance, enhanced reproducibility and stability. However, the root cause of these effects in these complicated systems is not well understood. We address the above challenge by tracking in situ the solidification of perovskite precursors during solution-casting using time-resolved grazing incidence wide-angle X-ray scattering (GIWAXS). In doing so, we can directly link the formation or suppression of different crystalline phases to the presence of Cs and/or Rb. In the absence of these elements, the multi-component perovskite film is inherently unstable, phase segregating into a solvated MAI-rich phase and a FABr-rich phase. Adding even one of the two (Cs or Rb) is shown to alter the solidification quite dramatically, promoting different solidification pathways. Importantly, the addition of both components in the optimal ratio can drastically suppress phase segregation and promotes the spontaneous formation of the desired perovskite phase. This result is also confirmed by elemental mapping of organic cations (FA+, MA+) and halide anions (I-, Br-) via time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Perovskite precursors with an optimal combination of additives (7% Cs, 3% Rb) result in solar cells with 20.1% power conversion efficiency (PCE), outperforming formulation excluding Cs and Rb (PCE=14.6%). We propose that the synergistic effect is due to the collective benefits of Cs and Rb on the formation kinetics of the perovskite phase, and on the halides redistribution throughout the film. Importantly, our study points to new design rules for tuning the crystallization pathway of multi-component hybrid perovskites.

All solid-state solar cells based on organometal trihalide perovskite absorbers have already achieved distinguished power conversion efficiency (PCE) to over 23% and further improvements are expected up to 25%. These novel organometal halide perovskite absorbers which possess exceptionally strong and broad light absorption enable to approach the performances of the best thin film technologies. To commercialize these great solar cells, there are many bottlenecks such as long-term stability, large scale fabrication process, and environmental issues. In this presentation, we introduce our recent efforts to improve long term stability and solve environmental issues, which will facilitate commercialization of Perovskite photovoltaic system. For examples, we introduce a recycling technology of perovskite solar cells, which will facilitate the commercialization as well as solve the environmental issues of perovskite solar cells. Also, we are going to show new interfacial layers and highly crystalline SnO2 nanoparticle layers for electron transport layer. Also, we will show a large scale coating methodology for enabling large size module fabrication by using a new solvent extractor, anisole. Also, stability issue of perovskite materials regarding charge generation and extraction will be discussed.

Organic photovoltaic cell performance is limited in part by a short exciton diffusion length (L_D). While state-of-the-art devices address this challenge using a morphology-optimized bulk heterojunction (BHJ), longer L_D would relax domainsize constraints and enable higher efficiency in simple bilayer architectures. One approach to increase L_D is to exploit long-lived triplet excitons in fluorescent materials. Though these states do not absorb light, they can be populated using a host-guest triplet-sensitized architecture. Photogenerated host singlets undergo energy transfer to a guest, which rapidly forms triplets that are transferred back to the long-lived host triplet state. Previous efforts have been focused on Pt- and Irbased guests. Here, a host-guest pairing of metal-free phthalocyanine (H₂Pc) and copper phthalocyanine (CuPc) is explored, advantageous as the guest also has strong and complementary optical absorption. In optimized devices (20 vol.% CuPc), the short-circuit current is enhanced by 20%. To probe the origin of the enhancement, the exciton L_D is measured using a device-based methodology that relies on fitting ratios of donor-to-acceptor internal quantum efficiency as a function of layer thickness. Compared with the neat H₂Pc, the L_D of the 20 vol.% CuPc doped layer increases from (8.5 ± 0.4) nm to (13.4 ± 1.6 nm), confirming the increased device current comes from enhanced exciton harvesting.

The rapid progress in the field of perovskite solar cells has led to efficiencies approaching that of crystalline silicon in single junction devices, and increasing emphasis is now being placed on scalable fabrication strategies that have the potential to truly impact the photovoltaics industry. The standard fabrication route of sequential, layer-bylayer depositions to form the device stack is limited by the small range of selective materials that can withstand successive solvent exposures and thermal annealings. In order to overcome this limitation, we have developed a method to fabricate the device stacks through mechanical lamination under moderate pressures and temperatures. The procedure involves fabricating two transparent conductive oxide/transport material/perovskite half stacks and laminating them together at the perovskite/perovskite interface. This procedure previously achieved 10% efficiency in initial attempts based on methyl ammonium lead iodide (MAPI) perovskite compositions. Herein, we reported the fabrication conditions to achieve a reproducible lamination with a complex, multi-cation perovskite, [Cs_0.05(MA_0.17F A_0.83)_0.95P b(I_0.83Br_0.17)₃] (FAMACs), laminated at 18°C and with a pressure of 350 psi for 30 minutes. The resulting material showed a sharpening of the optical absorption edge and a slight reduction in the band gap from 1.622 eV to 1.616 eV, indicative of improved ordering and reduced defects. Concurrently, time resolved photoluminescence measurements revealed an increased luminescence lifetime from 85 ns to 123 ns.

Block copolymers containing electron rich conjugated donor block and electron poor conjugated acceptor block connected covalently via non-conjugated bridge units are promising for potential light harvesting applications due to fast photo induced charge separation and slow/controllable charge recombinations. Potential bicontineous and ordered nano-domain donor/acceptor phase separated morphologies of the block copolymer systems are also favorable for Frenkel type exciton dissociations. In this work, a DBfA type block copolymer is being evaluated for light harvesting applications where D is a conjugated donor block, fA is a conjugated and fluorinated acceptor block, and B is non conjugated bridge chain containing different number of methylene units. This study reveals that the photoluminescence PL quenching as well as photoelectric power conversion efficiencies of DBfA type block copolymers decreases from the bridge unit containing one methylene unit up to six methylene units, i.e., the shorter the chain the more efficient photoelectric conversions. However, in a DfA block copolymer where there is no bridge unit, the PL quenching or the photoelectric conversions are not better than the shortest bridge containing one methylene unit, i.e., the photo induced charge separation in DfA still fall in weak electronic coupling regime. This may be accounted for by a twist angle existence between the D and fA conjugated plains. Morphological and optoelectronic correlation studies reveals that the surface roughness of such block copolymer varies nonlinearly with thermal annealing temperature, and that a medium roughness exhibit best or optimal optoelectronic property.

Halide perovskite solar cells (PSCs) have increased their power energy conversion efficiency (PCEs) drastically in the last few years, becoming one of the most competitive photovoltaic technologies. However, instability problems and materials toxicity are two of the most important drawbacks that delays its commercialization. Different strategies have been proposed in order to increase the operational stability of the devices. One of these approaches is the replacement of the unstable hole transport layer (Spiro-OMeTAD) or the improvement of the stability to moisture through the application of semiconductor oxides as transport layers or scaffolds. In this respect, the carbon-based perovskite solar cells (C-PSCs) applies bilayer electrode made of the mesoporous TiO₂ (m-TiO₂), as the electron transport layer, and the mesoporous ZrO₂ (m-ZrO₂) used as scaffold. In this PSC configuration the application of a hole transport layer as well as the application of a back-metal electrode (e.g. Au) is avoided and replaced by a highly conductive porous carbon electrode. The final triple-layer TiO₂/ZrO₂/carbon is applied to fabricate printable mesoscopic solar cells, where the metal halide perovskite solution is infiltrated within the porous TiO₂/ZrO₂ bilayer and through the printed carbon layer. In this work, we present our most recent results on the application and optimization of C-based PSCs with champion PSCs efficiencies of ~14 %.