This PDF file contains the front matter associated with SPIE Proceedings Volume 10737, including the Title Page, Copyright information, Table of Contents, Author listing, and Conference Committee listing.

Heating, cooling, and lighting for buildings represent 40 percent of our nation’s energy consumption. Smart windows can reduce energy needs by up to 40 percent by regulating the transmission of visible and near-infrared light. This talk will highlight a self-powered smart-window technology that uses UV-harvesting organic solar cells for onboard power. Highlighted in the Wall Street Journal, implementation of such smart windows can simultaneously provide energy savings and increase occupant comfort.

Squaraines continue to attract attention for their use in non-linear optics, fluorescence bioimaging and organic photovoltaics applications because of their strong, broad NIR absorbance and optoelectronic properties that depend on both excitonic and intermolecular charge transfer (ICT) couplings in the solid state. Our previous theoretical work demonstrates splitting of the H-aggregate with coupling to the ICT that goes beyond Kasha’s exciton model. This ICT splitting leads to the panchromatic absorption profile in the solid state, but the impact of the ICT on excited state diffusion and dynamics remains unclear. Here, we employ subpicosecond transient absorption spectroscopy to probe the excited state photophysics of an anilino-squaraine and its aggregates. Our samples are designed with a continuum of intermolecular separation from monomers in solution, through solid solution thin films, to the fully condensed phase, demonstrating the increasing contribution of short-range intermolecular charge transfer. We measure excited state kinetics that confirm species assignments and we show the effect of ICT states on exciton diffusion. The experimental results are in excellent agreement with our theoretical modeling. Finally, we correlate this combination of theory and excited state characterization with the measured efficiency in small molecule organic photovoltaic devices. Our remarkable results explain the importance of excitonic and ICT couplings for future applications driven by rational optoelectronic material design.

Multi-junction device architectures represent a promising strategy to further advance the efficiency of organic solar cells. For solution-processed organic solar cells, tandem and triple junction cells have been reported in the past. Yet, several challenges remain, both in developing new photoactive materials as well crating new recombination layers that serve to interconnect the subcells. We developed new versatile charge recombination layers for solution-processed multi-junction solar cells in n-i-p and p-i-n architectures. The new recombination layers provide an essentially lossless contact in each case, without the need of adjusting the formulations or deposition conditions for six different tandem cells and three different triple-junction solar cells, employing a range of different photoactive layers. The approach permitted realizing complex devices in good yields, providing a power conversion efficiency up to 10%. We will also present a first example of a quadruple-junction polymer solar cell, featuring four different and complementary band gap absorber layers that absorb light up to 1150 nm. The quadruple junction cell is reaches a power conversion efficiency of about 7.5% with an open-circuit voltage of 2.46 V. Measuring the external quantum efficiency (EQE) of the quadruple cells has been accomplished using a protocol using bias light of different wavelengths, involving optical modeling and correcting for the build-up electric field. At present, the efficiency of the quadruple-junction polymer cell is mainly limited by bimolecular recombination in the active layers.

Interface engineering is a critical strategy for improving the performance of polymer solar cells. A good interfacial material should fulfill several requirements including 1) good charge selectivity to improve the charge collection efficiency at the corresponding electrodes, 2) matched energy levels with the conduction band and valence band of the light harvesting film to maximize the photovoltage of the solar cells, 3) high conductivity to minimize the interfacial resistance loss and forming Ohmic contact with the electrodes.[1] In addition to interface engineering, optical management is another powerful method to enhance the performance of polymer solar cells by maximizing the light harvesting property of the devices. The capability to use optical model to precisely predict the light propagation property and charge generation rate within the devices allows us to design optimal device architectures with maximum performance. In this talk I will discuss how to combine these two key strategies to improve performance of polymer solar cells. The design of new conjugated polymer-based interfacial materials with desired electrical conductivity, energy levels and processibility allows us to improve the charge collection efficiency and compatibility for polymer solar cells based on fullerene [2,3] and non-fullerene acceptors.[4] Finally I will also discuss how to combine both interface engineering and optical modeling to design and fabricate very high performance tandem [5,6] and semitransparent polymer solar cells.[7] References [1] H.-L. Yip, A. K.-Y. Jen, Energy Environ. Sci., 5, 5994 (2012). [2] Z. Wu, H.-L. Yip, F. Huang, Y. Cao, et al, J. Am. Chem. Soc., 138, 2004 (2016). [3] K. Zhang, H.-L. Yip, F. Huang, Y. Cao, et al, Adv. Mater., 27, 3607 (2015). [4] C. Sun, H.-L. Yip, J. Hou, F. Huang, Y. Cao, et al, Energy Environ. Sci., 10, 1784 (2017). [5] K. Zhang, K. Gao, F. Huang, X. Peng, L. Ding, H.-L. Yip, Y. Cao, et al, Adv. Mater., 28, 4817 (2016). [6] M. Li, K. Gao, X. Wan, H.-L. Yip, X. Peng, Y. Cao, Y. Chen, et al, Nat. Photonics, 11, 85 (2017). [7] H. Shi, H.-L. Yip, Y. Cao, et al, Adv. Energy Mater. , 10.1002/aenm.201701121.

Hole transport layer (HTL) plays a critical role for achieving high performance solution-processed optoelectronics including organic electronics. For organic solar cells (OSCs), the inverted structure has been widely adopted to achieve prolonged stability. However, there are limited studies of p-type inorganic semiconductor-based effective HTL on top of organic active layer (hereafter named as top HTL) for inverted OSCs. Currently, the p-type top HTLs are mainly two-dimensional (2D) materials, which have vertical conduction limitation intrinsically and is too thin to function as practical HTL for large area optoelectronic applications. Here, we demonstrate a novel self-assembled quasi three-dimensional (3D) nanocomposite as a p-type top HTL [1]. Remarkably, the novel HTL achieves ~15 times enhanced conductivity and ~16 times extended thickness compared to the 2D counterpart. By applying this novel HTL in inverted OSCs covering fullerene and non-fullerene systems, device performance is significantly improved. The champion power conversion efficiency (PCE) reaches 12.13%, which is the highest reported performance of solution processed HTL based inverted OSCs. Furthermore, the stability of OSCs is dramatically enhanced compared with conventional devices. The work contributes to not only evolving the highly stable and large scale OSCs for practical applications but also diversifying the strategies to improve device performance. [1] J. Cheng, H. Zhang, Y. Zhao, J. Mao, C. Li, S. Zhang, K.S. Wong, J. Hou, W.C. H. Choy, "Self-assembled Quasi-3D Nanocomposite: A Novel p-Type Hole Transport Layer for High Performance Inverted Organic Solar Cells", Adv Funct. Mater., DOI:10.1002/adfm.201706403.

Processes taking place at contacts are of particular importance in organic and perovskite solar cells where selective contacts that are able to efficiently collect majority carriers, simultaneously blocking minority carriers are desired. The surface recombination velocity S_R, describing the quality of the contact interface, is a key parameter in obtaining an increased understanding of the kinetics taking place at contacts in thin-film devices [1]. We have extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory taking the effect of built-in voltage, diffusion and band-bending into account [2] and show how we can experimentally quantify loss mechanisms in charge collection [3-4]. We have derived analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish various (loss) mechanisms for contact related effects in thin film solar cells [2-4]. References [1] O. Sandberg, M. Nyman, R. Österbacka, Physical Review Applied 1, 024003 (2014) [2] O. Sandberg, M. Nyman, R. Österbacka, Organic Electronics 15, 3413-3420 (2015) [3] A. Sundqvist, M. Nyman, O. Sandberg, S. Sandén, J.-H. Smått, and R. Österbacka, Advanced Energy Materials, 1502265 (2016) [4] O.J. Sandberg, et. al, Physical Review Letters, 118, 076601 (2017).

We show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation [1,2]. We explain how this order and interfacial roughness generate electrostatic forces that drive charge separation and prevent carrier trapping across a donor-acceptor interface [2]. Comparing several of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a trade-off between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices. By accounting for long-range mesoscale fields, we obtain the ionization energies in both crystalline [3] and mesoscopically amorphous systems with high accuracy [4]. [1] C. Poelking, M. Tietze, C. Elschner, S. Olthof, D. Hertel, B. Baumeier, F. Wuerthner, K. Meerholz, K. Leo, D. Andrienko, Nature Materials, 14, 434, 2015 [2] C. Poelking, D. Andrienko, J. Am. Chem. Soc., 137, 6320, 2015 [3] M. Schwarze, W. Tress, B. Beyer, F. Gao, R. Scholz, C. Poelking, K. Ortstein, A. A. Guenther, D. Kasemann, D. Andrienko, K. Leo, Science, 352, 1446, 2016 [4] C. Poelking, D. Andrienko J. Chem. Theory Comput., 12, 4516, 2016

It is established that the nanomorphology plays an important role in performance of bulk-heterojunction (BHJ) organic solar cells. From intense research in polymer-fullerene systems, some trends are becoming apparent. For example, small ~10 nm domains, high crystallinity, and low miscibility are typically measured in high-performance systems. However, the generality of these concepts for small-molecule (SM) BHJs is unclear. We present a comprehensive study of performance, charge generation and extraction dynamics, and nanomorphology in SM-fullerene BHJ devices to probe these critical structure-property relationships in this class of materials. In the systems investigated, small domains remain important for performance. However, devices composed of highly mixed domains with modest crystallinity outperform those consisting of pure/highly crystalline domains. Such a result points to an alternative ideal morphology for SM-based devices that involves a predominant mixed phase. This stems from SM aggregation in highly mixed domains that both maximize interface for charge generation and establish continuous pathways for efficient charge extraction. Such a morphological paradigm should be considered in future SM systems in pursuit of high-efficiency large-scale solar power production.

Solution processable polymer-based organic photovoltaics offer tremendous opportunities for applications requiring flexible, translucent or aesthetically pleasing designs, with potential for low-cost roll-to-roll mass production. However, progress in moving the organic photovoltaic technology from lab-scale to commercial applications has been slow, generating skepticism around the commercial viability of the technology. Organic photovoltaic research is often carried out on small-area research cells fabricated under inert conditions using techniques such as spin coating that do not translate to mass production. While high research cell performance can be achieved, the materials selected or conditions used for fabrication are typically not amenable to scale-up. This paper will focus on solving some of the technical challenges associated with scaling polymer-based bulk heterojunction organic photovoltaics to high-performance large-area modules. Efforts to develop materials that are stable to fabrication in air, have good solubility, and enable deposition of thick (>300 nm) photoactive layers are described, leading to organic photovoltaic modules reaching 8% power conversion efficiency (PCE).

Herein, we present our current efforts on fundamental research towards large area organic photovoltaic devices using nonfullerene acceptors (NFAs). First, we present a short review of the main highlights of the state-of-the art in large-area organic solar cells (OSCs) coating. We then present our guidelines to prepare OSCs in an environmentally friendly way from the synthesis of the organic compounds to the choice of the solvent for the coating solutions and the actual OSCs coating. As a starting point, we paired a perylene diimide (PDI) acceptor, PDI₂-EH, to the donor polymer PBDB-T to compare spin-coated and slot-die coated OSC devices. Considering the poor solubility of this bulk-heterojunction system in non-halogenated solvents, we focused our efforts to slot-die coat the soluble PDI₂-EH acceptor on glass and polyethylene terephthalate (PET) substrates from non-halogenated solvents such as toluene, o-xylenes and anisole. We also explored the influence of different UV/ozone treatments for cleaning the PET substrates. Overall, this study presents practical considerations for laying foundations to proceed with an environmentally responsible upscale of OSC coatings.

With over 2500 technical papers published in 2017 on the use of perovskite materials for photovoltaic applications, multiple organisations are endeavoring to commercialise this technology. We will review the status of a promising approach which combines a wide bandgap perovskite solar cell with a silicon solar cell in a tandem cell configuration. We will discuss long term reliability test data, scaling and pilot production challenges, the annualised energy yield of tandem and other multi-junction modules, as well as cost of ownership (CoO) models using an example factory process and LCOE calculations that validate the economic benefit of these solar cells.

With demonstrated power conversion efficiencies close to 23%, perovskite-based photovoltaics is already able to compete with established technologies like silicon, CdTe and CIGS. However, next to high efficiencies, the potential low-cost fabrication of devices with sufficient stability under real-world conditions is of key importance for the future economic prospects of the perovskite technology. In this contribution, we report on a novel inexpensive architecture for efficient and highly reproducible, all-evaporated perovskite solar cells. Our evaporated CH3NH3PbI3 absorber is sandwiched between nickel oxide as hole transport material and C60 as electron transport material. By replacing the highly expensive hole transport layer Spiro-MeOTAD by electron-beam deposited nickel oxide and the gold back electrode by copper, we reduce the cost of materials on the lab-scale to one third of the price of the common stack based on Spiro-MeOTAD. At the same time, extraordinary stable devices even at operating temperatures of 80°C are achieved. Stabilized power conversion efficiencies under standard test conditions exceed values of 14%. Moreover, the vacuum deposition combines the ease of controlled deposition and a simple upscaling, making it a favorable process for industry. A homogenous and reproducible deposition on substrates with an area of up to 8x8 cm² is demonstrated by light beam induced current mapping, which is a fundamental requirement for the fabrication of larger prototype modules. Finally, as an inverted architecture with the anode deposited on top of the substrate the investigated layer stack is a promising candidate for two-terminal tandem devices on top of CIGS or p-type silicon.

Colorful semitransparent organic photovoltaic cells (OPVs) are increasing in demand due to their application in building-integrated photovoltaics. In general, the colors in OPVs have been determined by the absorption properties of the active material, requiring one to use different active materials to achieve distinct colors. However, such a strategy presents challenges in fabrication, costs and implementation as different processes are needed to produce differently colored OPVs. Moreover, the photovoltaic performance cannot stay uniform from different active materials in colored OPVs. In this work, we present a simple solution to such problems by incorporating transmissive Fabry-Perot-type color filters (CFs) as the electrode in an OPV employing one type of active material. From this modification, we achieve widely tunable colors covering the whole visible range with high spectral purity, peak transmission efficiencies surpassing 25% at the expense of charge generation that is only a fraction of that of an opaque OPV, and uniformity in device performance regardless of transmitted color. Because the CF is spatially removed from the charge generation and transport pathway, the optical characteristics are largely decoupled from the electronic characteristics. This provides one the freedom to explore creative designs without having to consider the electronic properties and uniformity in performance. Furthermore, integration of CFs into OPVs ensures the transmitted colors to be spectrally pure, bidirectional, and of high saturation represented by sub-100nm resonance widths, providing optical characteristics suitable for use in colorful power generating windows.

The voltage loss in organic photovoltaics (OPVs) is defined as the difference between the open circuit voltage and the voltage offset between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied MO (HOMO) of the donor forming the charge dissociating heterojunction. Widely employed fullerene acceptors show significant voltage losses, approaching 0.8 V, thereby reducing the potential efficiencies of OPVs. In this work, we show that ternary blends of two non-fullerene acceptors with a polymer donor are effective in significantly reducing the voltage losses in near-infrared (NIR) OPVs. A narrow energy gap non-fullerene acceptor, FIDC, sharing similar HOMO energies with the acceptor BT-CIC [1], (absorption up to 1000 nm) is blended with the polymer PCE-10. The power conversion efficiency (PCE) of the ternary cell is increased from 10.7% in a BT-CIC:PCE-10 binary cell to 12.6% in the BT-CIC:FIDC:PCE-10 ternary cell. Further, the short-circuit current density is increased from 22.3 mA cm-2 to 25.5 mA cm-2. Importantly, the ternary cell decreased the voltage loss from 0.61 to 0.54 V. The ternary devices showed larger open circuit voltage of 0.70V than either the analogous BT-CIC (0.69 V) or FIDC (0.66 V) binary devices [2,3]. This work points to a simple means for increasing the materials available for spanning the visible and NIR spectra in very high efficiency, low loss OPVs. 1. Li, Y.; Lin, J.-D.; Che, X.; Qu, Y.; Liu, F.; Liao, L.-S.; Forrest, S. R. J. Am. Chem. Soc. 2017, 139, 17114. 2. Ameri, T.; Khoram, P.; Min, J.; Brabec, C. J. Adv. Mater. 2013, 25, 4245. 3. Fu, H.; Wang, Z.; Sun, Y. Solar RRL 2018, 2, 1700158.

Fullerene-free organic solar cells (OSCs) have attracted significant interest in the research community over the past few years. Their efficiency has risen rapidly, with multiple reports of record power conversion efficiencies (PCEs) breaching 14%. While encouraging, these performance metrics are often achieved with the utilization of toxic halogenated solvents for the fabrication process, which is less attractive for large-scale manufacturing. Dimeric perylene diimide (PDI) electron transport materials are currently considered amongst the key candidates for the realization of low-cost, highefficiency “green-processed” OSCs. The low-cost synthetic versatility of the PDI skeleton allows for a range of chemical “fine-tuning” and the chromophore has excellent photochemical stability and strong light absorption in the visible region.

This report will detail our research into OSCs using PDI dimers as the electron acceptors and active layer fabrication using non-halogenated solvents. PTB7-Th was chosen as the donor material, owing to its good solubility in nonhalogenated solvents, complementary light absorption and suitable energy level alignment for pairing with our PDI acceptors. Two different PDI dimers having linear and branched alkyl chains are studied.

We have previously shown that PTB7-Th:PDI based solar cells with active layers processed from 2Me-THF, o-xylene, or 1,2,4-trimethylbenzene could reach PCEs from 5-6%. The processing solvent can be extended to toluene with solar cells exhibiting PCEs of 5%. Thus, this work highlights the many processing options for the PTB7-Th / N-annulated PDI dimer active layer combination.

Although fullerene derivatives (e.g. PC61BM/PC71BM) are being widely used as electron acceptors in organic solar cells (OSCs), their obvious drawbacks, such as the high cost, poor absorption, limited energy levels tunability and morphological instability, have become the bottlenecks to hinder the further advancement of OSCs. Therefore, the exploration of non-fullerene electron acceptors is motivated in recent years, and the efficiencies of fullerene-free OSCs have been boosted over 13%. In this presentation, I will focus on the molecular design for highly efficient and thermally stable small molecule electron acceptors based on fused diketopyrrolopyrrole (DPP) and perylene diimide (PDI) building blocks. A new strategy of unfused-ring core is put forward to synthesize thenovel electron acceptors for OSC applications. The highly efficient and thermally stable non-fullerene organic solar cells over 11% have been fabricated by carefully designing the non-fullerene acceptors.

Organic solar cell (OSC) technology has recently achieved over 13% efficiency through the synthesis of novel non-fullerene small molecule acceptors (NFAs), which can be processed from benign solvents as low-cost third generation photovoltaics[1,2]. Of critical importance to OSCs is understanding the morphological and thermal stability of the active layers governed by thermodynamics and kinetics as an intrinsic stability process which cannot be controlled by encapsulation[3,4]. Here we highlight the importance of ductility of donor polymers on nucleation and growth of micro-size small molecule crystals which leads to the catastrophic failure of the solar cells in the long-term operating condition We consider three high performance polymers P3HT, PBnDBT-FTAZ, and PffBT4T-C9C13 blended with EH-IDTBR as the model systems to investigate the thermal stability of state of the art non-fullerene OSCs, where elevated temperatures were used to accelerate the crystal formation and imitate the long-term operation conditions of OCSs. We also propose an easy accessible method using differential scanning calorimetry (DSC) to investigate the thermal behavior of NFA in the blends. Although non-fullerene solar cells have shown to have better overall morphological stability compared to their fullerene counterparts, our results suggest that catastrophic failure due to micro-size crystal formation in non-fullerene systems can happen at a rate similar to fullerene systems unless the right donor polymer is chosen to suppress the crystallization of small molecule. It is also shown and argued that the growth rate of small molecule crystals can be reduced upon mixing of NFAs with semi-crystalline polymers, such as P3HT with a higher overall density compared to amorphous donor polymers, i.e. PBnDT-FTAZ. Our findings may pave a way to understand and predict the morphological stability of non-fullerene OSCs. References [1] L. Ye, Y. Xiong, Q. Zhang, S. Li, C. Wang, Z. Jiang, J. Hou, W. You, H. Ade, Adv. Mater. 2017, DOI: 10.1002/adma.201705485. [2] S. Holliday, R. S. Ashraf, A. Wadsworth, D. Baran, S. A. Yousaf, C. B. Nielsen, C.-H. Tan, S. D. Dimitrov, Z. Shang, N. Gasparini, M. Alamoudi, F. Laquai, C. J. Brabec, A. Salleo, J. R. Durrant, I. McCulloch, Nat. Commun. 2016, 7, 11585. [3] M. Ghasemi, L. Ye, Q. Zhang, L. Yan, J. H. Kim, O. Awartani, W. You, A. Gadisa, H. Ade, Adv. Mater. 2017, 29, 1604603. [4] L. Ye, H. Hu, M. Ghasemi, T. Wang, B. A. Collins, J.-H. Kim, K. Jiang, J. Carpenter, H. Li, Z. Li, T. McAfee, J. Zhao, X. K. Chen, J. Y. L. Lai, T. Ma, J.-L. Bredas, H. Yan, H. Ade, Nat. Mater. 2018, DOI: 10.1038/s41563-017-0005-1.

Non-fullerene acceptors (NFAs) provide an exciting alternative to traditional fullerene containing organic photovoltaics and are already out-competing fullerenes in terms of stability and efficiency. One class of NFA that has proved most promising is an A-D-A motif not dissimilar to a push/pull conjugated polymer. O-IDTBR, a planar A-D-A NFA has achieved the highest efficiency for a P3HT containing device, with an impressive 7.7% being obtained when in a ternary blend with a secondary twisted acceptor, O-IDFBR. Surprisingly this ternary blend not only shows an improvement in JSC, FF and VOC but also exhibits a greatly improved operational stability compared to the binary IDTBR:P3HT device. Our overarching goal is to understand this improved stability. Here we undertake the first steps to understanding the stability of these materials by studying the molecular origin of degradation for neat and blends containing these novel NFAs. We find by using in-situ resonant Raman spectroscopy, and supported by molecular DFT simulations, that both molecules undergo a two phase degradation. That being an oxygen mediated, photo-induced conformational change, most likely a torsion of the Core-BT dihedral, which then induces further irreversible degradation. It is also found that annealing O-IDTBR greatly enhances stability. We then investigate the miscibility of the two NFAs with P3HT and how this in-turn affects blend stability. The impact of such molecular conformational changes of these non-fullerene acceptors on device stability will be discussed.

Solution-processed thin-film perovskite solar cells (PSCs), where the record efficiency has rocketed from 3.8% to 22.7% - comparable to commercial silicon-based solar cells - in just eight years, offer unprecedented promise of low-cost, high-efficiency renewable electricity generation. Organic-inorganic halide perovskites (OIHPs) at the heart of PSCs have unique structures, which entail rotating organic cations inside inorganic cages, imparting them with desirable optical and electronic properties. To exploit these properties for PSCs application, the reliable deposition of high-quality OIHP thin films over large areas is critically important. The microstructures and grain-boundary networks in the resulting polycrystalline OIHP thin films are equally important as they control the PSC performance and stability. Fundamental phenomena pertaining to synthesis, crystallization, coarsening, and microstructural evolution involved in the processing of OIHP thin films for PSCs will be discussed with specific examples. Additionally, the discovery of Pb-free, Ti-based all-inorganic halide perovskites will be presented, together with the demonstration of viable PSCs based on these new halide perovskites. The overall goal of our research is to have deterministic control over scalable processing of tailored halide perovskite thin films with desired compositions, microstructures, and grain-boundary networks for large-area, high-efficiency, and stable PSCs.

Organometal halide perovskites have captured wide interest as a promising material for low-cost and high-efficiency solar cells. The power conversion efficiency (PCE) of the organometal halide perovskite solar cells (PSCs) has reached 22% within a few years of its advent. Through recent studies of PSCs, the composition of organometal halide perovskites is recognised as one of the key factors in the improvement of the PCE. As for the organic cation methylammonium (MA), the mixing of formamidinium (FA) extended the absorption edge from 800 nm to 850 nm, and then the photocurrent of PSC was enhanced. However, the pure formamidinium lead iodide (FAPbI3) contained the non-perovskite phase (δ-phase), which resulted in the low PCE of the perovskite solar cells. To suppress the creation of the δ-phase, the methylammonium chloride or bromide (MACl or MABr, respectively) was incorporated into the perovskite layer, and the PCE was improved to more than 20%. It was also verified that the incorporation of caesium cations (Cs+) into the A-site of perovskite inhibited the creation of the δ-phase, which considerably promoted the stability of the perovskite phase and the reproducibility of PSCs. Recently, it was reported that the incorporation of rubidium cations (Rb+) into the A-site of the perovskite promoted the PCE to more than 21%, although the ionic radius of Rb+ (148 pm) was smaller than that of Cs+ (169 pm) and, according to the Goldschmidt's tolerance factor, the formation of a stable perovskite structure was considered to be difficult. The above findings motivated us to investigate potassium cations (K+) with an ionic radius (133 pm) for improving the photovoltaic performance of PSCs. Potassium is abundant in the earth’s crust (21, 000 ppm), as compared to caesium (90 ppm) or rubidium (3 ppm), and it is available cheaply. On the other hand, there is a big issue with I-V hysteresis of PSCs that causes uncertainty as to its real PCE. Several hypotheses, such as ion migration, polarisation, defect traps, and capacitance, have been discussed to explain I-V hysteresis. In the conventional structure of PSCs, the difference of the conduction band edge between the TiO2 and the perovskite forms a small barrier at the interface, which retards the transportation of electrons from the perovskite to the TiO2.As for the electron transport layer (ETL), when TiO2 with a relatively higher conduction band minimum (CBM) was replaced by SnO2 or Cl-caped TiO2 with relatively lower CBM, the I-V hysteresis was diminished, indicating the disappearance of the electron transfer barrier. These results suggested that the band engineering of the organometal halide perovskite by composition tuning is important for diminishing I-V hysteresis. In this study, we explored the feasibility of incorporating K+ into the perovskite absorber. The results revealed that incorporating a small amount of K+ into the double organic cation perovskite absorber (FA0.85MA0.15Pb(I0.85Br0.15)3) improved the photovoltaic performance of PSCs significantly, and K+ incorporation diminished I-V hysteresis. To understand the mechanism of the phenomena, we investigated the influence under various K+ ratios in the perovskite absorber. References 1)"Hysteresis-free Perovskite Solar Cells Made of Potassium-doped Organometal Halide Perovskite", Zeguo TANG, Takeru BESSHO, Fumiyasu AWAI, Takumi KINOSHITA, Masato M. MAITANI, Ryota JONO, Takurou N. MURAKAMI, Haibin WANG, Takaya KUBO, Satoshi UCHIDA, Hiroshi SEGAWA Scientific Reports 2017, 7, 12183. 2)"Modulations of Various Alkali Metal Cations on Organometal Halide Perovskites and Their Influence on Photovoltaic Performance", Zeguo TANG, Satoshi UCHIDA, Takeru BESSHO, Takumi KINOSHITA, Haibin WANG, Fumiyasu AWAI, Ryota JONO, Masato M. MAITANI, Jotaro NAKAZAKI, Takaya KUBO, Hiroshi SEGAWA Nano Energy 2018, 45, 184-192.

The realization of very high efficiency, stable perovskite solar cells fabricated on a large scale at low cost, has the potential to further lower the cost of photovoltaics. This necessitates an understanding of the properties required of the perovskite material, including the carrier mobility. Perovskite cells also feature mobile ionic species, and the impact of these ions on cell performance – and in particular, to what extent and under what circumstances they may limit device performance – is not well understood. Here, we employ an advanced numerical model that allows for the presence of mobile ionic species to probe the relationship between carrier mobility, the presence of ionic species as well as different possible recombination mechanisms within the cell. We show that a high electron and hole conductivity throughout the device is key to avoiding transport losses. For devices operating significantly below their radiative limit, achieving a sufficiently high conductivity requires high carrier mobilities of at least 10cm²/V-s. It is shown that the presence of a single mobile ionic species can lead to effective doping of the perovskite bulk, which is detrimental to cell performance by lowering the conductivity of one type of carrier. The results also indicate that increasing cell VOC closer to its radiative limit is also beneficial for reducing transport losses and pushing cell performance closer to its theoretical limit.

The tandem configuration consisting of two or more solar cells is practically the only approach to overcome the Shockley-Queisser limit, as evidenced by the III-V multijunction solar cells. From theoretical calculation, it has been found that the combination of a top cell with a large bandgap energy (e.g. 1.5~1.7 eV) and a bottom cell with a low bandgap energy (e.g. 1.0~1.1 eV) can lead to a conversion efficiency higher than 30%. Given that the bandgap energy of most commercial single junction solar cells is around 1.1 eV, the perovskite solar cells with a bandgap energy around 1.6 eV must be a very promising candidate for the top cell of tandem solar cells. In this presentation, I will discuss the essential requirements for preparing highly performing perovskite top cells of perovskite-based tandem solar cells. Firstly, the strategies for improving the performance of the p-i-n type planar perovskite solar cell, mostly focusing on the interfacial charge transfer, will be introduced. After a series of interfacial engineering procedures to the charge extraction layers, a conversion efficiency as high as 19% could be achieved. Secondly, strategies for fabricating transparent perovskite solar cells with a TCO top electrode layer will be discussed. Finally, some of the recent results on the highly efficient (> 23%) tandem solar cells incorporating the transparent perovskite top cell will be introduced.

Perovskite solar cells (PSCs) have been attracted much attention due to their high-energy conversion effi-ciency and low production cost. However, the issues of stability and toxicity are still remained. Our group focuses on the fundamental studies of PSCs devices, including development of a series of materials for compact layer, active layer, hole transfer layer, as well as back electrode of PSCs. In this lecture, we will introduce current progress of several types of Pb-free perovskites solar cells and remaining issues, as well as present the recent results of our group in development of Pb-free perovskites and application for solar cells. Several Pb-free perovskites have been designed and synthesized in our gourp. The electronic and optical properties of the lead-free halide double perovskites Cs2NaBX6 (B = Sb and Bi; X = Cl, Br and I) were investigated by first-principles calculation to ascertain the potential application of solar energy conversion. The designed double perovskites also were synthesized and carried out the characterization. Another work is preparation of composite active layer of bismuth triiodide (BiI3) and a layered perovskite (CH3NH3)3Bi2I9 (MBI) by a simple solution method. The nano/macrostructures of composite active layer were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). We found that the nanostructures and morphology of Pb-free perovskites affect significantly the performance of the solar cells fabricated. References: 1. H.Zhou, Y.Shi, K.Wang, Q. Dong, and T. Ma., J.Physical Chemistry C. 2015, 31, 285 2. Q. Dong, Y. Shi, and T. Ma. J. Phys. Chem. C, 2015, 119 (19), 1021 3. K.Wang, Q. Dong, and T. Ma.,,Adv. Mater. 2016, DOI: 10.1002/adma.201505241 4. T. Xu, T. Ma, Phys. Chem. Chem. Phys., 2016,18, 27026-27050 5. S.Zhao, L. Gao, S. Hayase, T. Ma, RSC Advances, 2016, 6, 31968-31975

To further optimize the solar cell devices based on hybrid perovskites materials, understanding the major contributions of organic cations to ion migrauion and surface degradation is urgently needed. In this presentation, I show the tremendous impact of the structure of organic cations on halogen migration, vacancies, and interstitials, carrier lifetimes as well as surface degradation of perovskites using a combination of experimental and theoretical investigations. We found that Br- vacancies and interstitials have much lower formation energies and much higher density in MAPbBr3 compared to FAPbBr3 counterpart. The results also demonstrated clearly that the transition energy barrier for Br migration through vacancies within the bulk phase is much lower in MAPbBr3 than in FAPbBr3. Finally, we found that the rotation barrier of the organic cation is much higher in the case of FAPbBr3 than for MAPbBr3, which points to a much stronger H-bonding with Br- in the former case. Our results imply that incorporating organic cations with stronger H-bonding capacity, appropriate structure and more restricted motion inside the inorganic framework, is beneficial for suppressing ion migration and thus improving the performance of hybrid perovskite-based optoelectronic devices.

Recently, lead based organic inorganic hybrid perovskites (OIHP) have emerged in the photovoltaic field, achieving performance comparable to silicon-based technologies in few years of investigation.1,2 Despite the prodigious performance, there is a lack of information about many aspects related to the structure, homogeneity and stability of OIHP films. Synchrotron infrared nanospectroscopy (nano-FTIR) is a technique that uses an atomic force microscope (AFM) equipped with external optics which allow the focus of the synchrotron light onto the metallic AFM tip. The tip acts as an antenna and then we can obtain infrared spectra with resolution of the tip size (~20nm).3 Here, for the first time, the nano-FTIR technique was used to map the surface of the FAMA and CsFAMA perovskite films deposited onto Si/Au substrates before and after degradation in a hot plate in ambient atmosphere. The nano-FTIR images revealed a spatial heterogeneity in nanoscale in both samples. The infrared spectra collected in regions with small scattering nano-FTIR signal presented the main absorption of the formamidinium (FA) molecule at 1712 cm-1 that corresponds to symmetric C-N stretching. The regions with high scattering signal did not show any absorption related to FA, which indicated that these regions are rich in the inorganic elements/compounds. After degradation, we attributed the high scattering regions to lead iodide (PbI2), and this data were corroborated with X-ray diffraction analysis. Our results show that the perovskite films present high compositional heterogeneity even before degradation and that the nano-FTIR can be a powerful tool to monitor the perovskite composition at nanoscale. Acknowledgments: The authors would like to thank FAPESP, INEO, LNLS and CNPq for financial supports. References 1. W. S. Yang et al., “High-performance photovoltaic perovskite layers fabricated through intramolecular exchange,” Science 348(6240), 1234–1237 (2015) [doi:10.1126/science.aaa9272]. 2. W. S. Yang et al., “Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells,” Science 356(6345), 1376–1379 (2017) [doi:10.1126/science.aan2301]. 3. B. Pollard et al., “Infrared Vibrational Nanospectroscopy by Self-Referenced Interferometry,” Nano Lett. 16(1), 55–61 (2016) [doi:10.1021/acs.nanolett.5b02730].

Photocurrent generation of methylammonium lead iodide (CH₃NH₃PbI₃) hybrid perovskite solar cells is observed at the nanoscale using near-field scanning photocurrent microscopy (NSPM). We examine how the spatial map of photocurrent at individual grains or grain boundaries is affected either by sample post-annealing temperature or by extended light illumination. For NSPM measurements, we use a tapered fiber with an output opening of 200 nm in the Cr/Au cladded metal coating attached to a tuning fork-based atomic force microscopy (AFM) probe. Increased photocurrent is observed at grain boundaries of perovskite solar cells annealed at moderate temperature (100 °C); however, the opposite spatial pattern (i.e., increased photocurrent generation at grain interiors) is observed in samples annealed at higher temperature (130 °C). Combining NSPM results with other macro-/microscale characterization techniques including electron microscopy, x-ray diffraction, and other electrical property measurements, we suggest that such spatial patterns are caused by material inhomogeneity, dynamics of lead iodide segregation, and defect passivation. Finally, we discuss the degradation mechanism of perovskite solar cells under extended light illumination, which is related to further segregation of lead iodide.

In hybrid halide perovskite, the effectiveness of charge transport in relation to film microstructure and processing has remained elusive. In this study we succeeded in tuning grain size and grain boundary chemistry through solvent vapor annealing, which resulted in an increase in charge-carrier mobility by one order of magnitude. To understand the mechanism responsible for the enhanced charge transport, we performed a series of complementary measurements. Atomic force microscopy revealed an increase in grain size and uniformity, and optical microscopy showed a macroscopic reorganization of the film structure. X-ray diffraction measurements of the MAPbI3-xClx films confirmed the removal of preferential orientation after 20 min of solvent annealing at room temperature, in N,N-dimethylformamide. The presence of additional peaks was assigned to the formation of the solvent complex MAI:DMF:PbI2 and the PbI2:DMF ligand, and the content of these phases was monitored as a function of annealing time. Charge-carrier mobility was evaluated from field-effect transistor measurements in devices with gold top contacts and SiO2 bottom-gate dielectric. We obtained ambipolar transport, with both hole and electron mobility exceeding 10cm2/Vs at room temperature. We propose that this remarkable enhancement in electrical properties resulted from an increase in the grain size and passivation of grain boundaries via formation of intermediate solvent complexes formed from unreacted material. This work has allowed us to gain unprecedented insight into the impact of film morphology on charge transport in perovskite materials, an important milestone towards achieving high-performance optoelectronic devices such as transistors, photovoltaics, light emitting diodes, and photodetectors.

Here is presented the impact of thermal (TA) and solvent vapor annealing (SVA) post-treatments in bulk heterojunction (BHJ)-film formation of OSCs based on DRCN5T small molecule (based on oligothiophenes [1]) as electron donor and [70]PCBM as electron acceptor, under the direct architecture ITO/PEDOT:PSS/DRCN5T:[70]PCBM/PFN/FM, where FM is an eutectic alloy composed of 32.5% Bi, 51% In and 16.5% Sn, that melt at 65 °C and is easily deposited as top electrode at low temperature (~ 90 °C) [2-4]. The evolution of thin active layer formation treated by TA (120 ºC/10 min), SVA (in chloroform/60 s) and the combination TA+SVA was studied by atomic force microscopy (AFM) in phase contrast mode in order to determine the size domains for each post-treatment (TA, SVA or TA+SVA). The results show domains up to 500 nm wide for active layers without further post-treatment and the PCE achieved for devices fabricated with these active layers was 2.15%. On the other hand, both TA and SVA post-treatments show domains in the range of 100 to 150 nm wide as maximum, which is favorable for an efficient transport of electronic species. The PCEs for the devices were 4.96 and 4.91% for active layers treated with TA and SVA, respectively. Finally, devices with active layers treated by the combination of TA+SVA reached PCEs of 7.63 %. References [1] B. Kan et al, J. Am. Chem. Soc. 2015, 137, 3886-3893. [2] E. Pérez-Gutiérrez, et al., ACS Appl. Mater. Interfaces 2016, 8, 28763. [3] D. Romero-Borja, et al., Synth. Met. 2015, 200, 91-98. [4] D. Barreiro-Argüelles et al., Solar Energy, accepted (2018). Acknowledgements: Ce-MIE-Sol 207450/27, CONACyT-SENER 24575 and CONACyT 281164 (LNMG) Mexico.

The fluorinated triazole (FTAZ) based conjugated polymers (e.g., PBnDT-FTAZ) have been used widely in organic solar cells, reaching efficiencies over 13% with non-fullerene acceptors. However, fluorination is just one structural variation that the triazole unit can be modified to. In recent year, we have explored a number of other structural variations. In particular, new synthetic strategies allow us to create a diverse set of triazole based conjugated molecular units bearing various electron accepting abilities. The structural differences of as-synthesized three new triazole acceptors have a significant impact on the optoelectronic properties of conjugated polymers incorporating these triazoles. Bulk heterojunction solar cells based on these new polymers, show a wide range of efficiencies (up to 8.6% with PCBM) with active layer thickness around 300 nm.

Organic semiconductors have been studied extensively with regards to their charge transport ability and have recently been demonstrated to be a viable option for large-area, commercial electronic devices and photonics. Thus far, various synthetic strategies for organic semiconductors have been reported to enhance their performance. Firstly, we developed small molecule organic semiconductors for solution processed solar cell because of high reproducibility, easy purification, and more over high crystallinity with oustanding stacking properties. ( JMC, 2, 4937,2014, ChemSusChem,8,9, 1548, 2015, ACS Applied Materials & Interfaces, 2016, 8, 34353, Solr Energy, 150, 9095, 2017, ACS Applied Materials & Interfaces, accepted, 2018, etc) Secondly, we developed polymer organic semiconductors for solution processed solar cell. ( Chem, Commum, 51, 8, 1524, 2015, ACS Applied Materials & Interfaces, 7, 16, 8859, 2015, Macomolecules, 48, 12, 2015, Advnaced Functional Materials, 25, 3833, 2015, Macromolecules, 49, 7844, 2016, Chemistry of Materials 5, 2135, 2017, etc) Finally, we developed p-type and n-type materials for perovskite solar cells (EES, 7, 1454, 2014, Advnced Energy Materials, Accepted, 2018)

Despite the high-efficiency of these lead-based perovskite solar cells, the problem associated from the toxic nature of lead has open a new research direction which focuses on lead-free perovskite materials. As an alternative, tin has been proposed to replace lead. The highest efficiency obtained with Sn only perovskite was 9 % which was based on 2D and 3D mixture of FASnI3. However, Sn-based perovskites are known to have low stability in air. The use of germanium-based perovskite in solar cell was first realized by Krishnamoorthy et. al. The measured solar cell performance was notably low, 0.11 % for CsGeI3 and 0.20 % for MAGeI3. A theoretical study exploring hybrid tin and germanium perovskite showed that it is possible to prepare a stable Sn-Ge perovskite material that absorbs the sunlight spectrum. In this study, a new type of SnGe mixed metal perovskite solar cells are reported with enhanced efficiency and stability. In this report, FA0.75MA0.25Sn1-xGexI3 (abbreviated as SnGe(x)-PVK) were used for the mixed metal SnGe perovskite. XRD spectra showed that the structure is perovskite. The structure of Ge-doped Sn perovskite was also discussed from the view point of band gap, conduction and valence band level, XPS analysis, and the urbach energy. It can be concluded that most of the Ge atoms passivate the surface of the Sn perovskite (graded structure).For SnGe(0)-PVK device, the averageJsc was 17.61 mA/cm2, VOC was 0.46 V, FF was 0.41 and PCE of 3.31 %. Upon doping with 5 wt% of Ge, the JSC increased up to 19.80 mA/cm2, FF improved up to 0.55 with an overall efficiency of 4.48 %. Upon increasing the Ge content more than 10wt%, all the photovoltaic parameters decreased significantly which resulted in an efficiency as low as 0.80 % for SnGe(0.2)-PVK device. After optimization, 7.75% of SnGe(5)-PVK device is reported. Significant effect on Ge doping was seen in the enhancement of the stability. The stability in air has been improved significantly with the Ge doping, retaining 80 % of its original performance, remarkable stability enhancement, compared with 10 % retention for non-doped sample. This work provides a platform for further research on lead-free Sn-Ge based perovskite solar cells.

Metal oxide transporting layer in organic-inorganic perovskite solar cells (PSCs) have a tremendous improvement in both aspects, first stability and second high power conversion efficiency (PCE) which provides a new platform for commercialization in near future. Herein we report for the first time a novel home-made ball milling technique for the synthesis of tin oxide (SnO2) nanoparticles (10~20 nm sizes) fabricated at composite temperature, employed as an electron transporting layer (ETL) in planar PSCs. A smartly designed ground SnO2 (G-SnO2) NPs which annealed at high temperature (≤ 300°C) and an additional layer of a SnO2 layer (C-SnO2) which converted from the precursor (SnCl2.2H2O), annealed at low temperature (≤ 200°C). This synergistic effect gives a pinhole-free layer of G-SnO2 NPs, which helps to improve the bonding and interlayer recombination between ETL and absorber layer. We fabricated C-SnO2, G-SnO2, and G-SnO2/C-SnO2 based PSCs, with champion PCE of 16.4%, 17.9% and 19.11% respectively, with an active area of 0.04 cm2. The G-SnO2 and G-SnO2/C-SnO2 based devices have long-term stability and less hysteresis compare to C-SnO2 based device.

Tandem solar cells (TSCs) based on solution-processable semiconductors, including metal-halide perovskites and organic materials, show great promise for overcoming the Shockley-Queisser efficiency limit at low cost. However, difficulty in obtaining low-bandgap (<1.1 eV) perovskite and organic absorbers restricts the spectral range of solarenergy conversion, limiting the possibility of reaching ultrahigh efficiencies. Here we carry out detailed balance limit computations for a wide range of solution-processable materials in combination with a standard perovskite top-cell. Theoretical efficiency of 43% has been calculated for a tandem cell with a bandgap combination of 1.55 eV (perovskite) and 1.0 eV (bottom-cell material) under 1-sun illumination. We find that radiative coupling between the subcells contributes substantially (>11% absolute gain) to the ultimate efficiency via photon recycling. We emphasize the significance of using materials with high luminescence quantum efficiencies to benefit from this important effect. Initial laboratory demonstration of monolithic TSCs operating in the radiative-coupling regime is currently underway.

Intensive research on lead halide perovskites clarified that these materials are indeed suitable candidates for photovoltaic applications due to their excellent photoelectronic properties. Yet, lead is considered a major issue for commercialization. Concurrently, in the last five years, increasing research efforts have been made to replace lead with tin. Although partially successful, the present conversion efficiencies of tin halide perovskite solar cells are limited. Further performance improvements should be possible, if the underlying energy loss mechanisms in these devices can be clarified. Here, we investigated the energy loss mechanisms in lead-free CH₃NH₃SnI₃ (MASnI₃) solar cells as well as intrinsic photoelectronic properties of MASnI₃ to assess its potential for photovoltaics. Time-resolved photoluminescence (PL) measurements reveal that the short-circuit current (Jsc) in the MASnI₃ solar cell deviates from an ideal value as a result of fast recombination of photogenerated carriers in the perovskite layer. Consequently, a larger Jsc should be possible with longer carrier lifetimes. Furthermore, resonantly excited PL and temperature-dependent PL data clearly reveal that the intrinsic electron–longitudinal optical phonon coupling governs the broadening of optical transitions at around 300 K. By performing a detailed comparison of the data of MASnI₃ and MAPbI₃, it is shown that the intrinsic optical properties of tin and lead perovskites are similar to each other. Our results suggest that solar cells based on tin halide perovskites can compete with lead halide perovskite solar cells, if the carrier lifetimes can be improved.

Conjugated polymers doped with metal ions offer superior material properties in the development of next generation flexible PV technology [1]. Charge transport mechanism in metallated conjugated polymer with different solvent processing was described by A.F. Mitul in [1]. In this work, the modification of heavy metals e.g, Pt, Ru etc in organometllic solar devices are investigated. The variation in device performance i.e, open circuit voltage (Voc), external quantum efficiency (EQE) is explained in the light of nano scale morphology. Change of heavy metals in organicchemical structure provides differences in nanoscale morphology and hence, it describes the favorable condition for optimum device performance.

In this work, OPVs with an active layer composed by P3HT (poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C61- butyric acid methyl ester) was fabricated by spin coating technique and studied after post-thermal annealing. The devices were annealed at temperatures ranging from 150 °C to 175 °C, showing an increase in efficiency at 160 °C, decreasing after. In order to achieve a physical model for this behavior, dc and ac measurements, together morphology analysis was made and correlated. Under dc conditions, the overall figures of merit were measured and fitted to the physical models using genetic algorithms; by ac measurements, the capacitance and loss dependence on frequency were studied and equivalent circuit models were obtained. Capacitance – voltage behavior was also analyzed. The morphology of OPV active area film was investigated by Atomic Force Microscopy (AFM) in both tapping and current sensing methods. The OPVs exhibit efficiencies ranging from 1.2 to 3%, with fill-factors (FF) ranging from near 50% to near 70%. The relaxation frequencies can be correlated with the efficiency behavior, and with the micro electrical map obtained (and correlated with morphology) by AFM-current sensing. It was shown that how post-thermal annealing changes the micro electrical patterning and therefore, a suitable relationship with macroscopic behavior can be established.

In this work is reported an analysis of the external and internal quantum efficiency (EQE and IQE) and, its correlation with the power conversion efficiency (PCE) of organic photovoltaic (OPV) cells, as a function of the active layer thickness. It was used the bulk heterojunction architecture under the configuration ITO/PEDOT:PSS/PTB7- Th:PC71BM/PFN/Field’s Metal (FM) and the active film thickness range was 40-165 nm. FM is a eutectic alloy, composed by 32.5% Bi, 51% In and 16.5% Sn, that melt at 65°C and is easily deposited on top of the electron transport layer (ETL) at low temperature (~ 90 °C). EQE set up was a home-made and the IQE spectra for the active film thickness range were determined from EQE and net internal absorption spectra; net absorption was estimated through the transfer matrix method (TMM). It was observed a significant reduction of IQE with the increasing of the active layer thickness up to 120 nm. IQE decreases and consequently EQE and PCE too because of the reduction in charge carriers collection. It was calculated the short-circuit current density (Jsc) from these measurements and compared these values with those achieved by the J-V plots to verify EQE results. It is noteworthy that EQE takes into account the effect of optical losses by reflection and transmission, while IQE is related with the efficiency of the photons that are not reflected or transmitted out of the cell.

Post-treatments of the organic solar cells (OSCs) active layers, such as thermal, solvent vapor and applied external electric field, can lead to very different morphologies that directly impact the power conversion efficiency (PCE) of the OSCs. These post-treatments could improve bulk morphology, which reduce charge carrier recombination and, therefore, improving the photovoltaic performance of bulk heterojunction (BHJ). Here, thin solid films of the small molecule DRCN5T and DRCN5T:[70]PCBM blend, were analyzed through scanning tunneling microscopy (STM) and spectroscopy (STS) after different temperatures treatments. DRCN5T is an electro-donor compound used in the OSCs active layer, which reach ~10% of PCE. These very thin solid films (monolayers) exhibit a worm-like pattern without thermal annealing (TA) (amorphous behavior), however after applying TA at 120°C, the small molecule crystallizes: its structural geometry becomes a well-defined organized one.