The SHERLOC Calibration Target on the Mars 2020 Perseverance Rover: Design, Operations, Outreach, and Future Human Exploration Functions

Marc D. Fries, Carina Lee, Rohit Bhartia, Joseph Razzell Hollis, LutherW. Beegle, Kyle Uckert, et al.

Space Science Reviews, 2022, 218:46

Abstract: The Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) is a robotic arm-mounted instrument onboard NASA’s Perseverance rover. SHERLOC combines imaging via two cameras with both Raman and fluorescence spectroscopy to investigate geological materials at the rover’s Jezero crater field site. SHERLOC requires in situ calibration to monitor the health and performance of the instrument. These calibration data are critically important to ensure the veracity of data interpretation, especially considering the extreme martian environmental conditions where the instrument operates. The SHERLOC Calibration Target (SCT) is located at the front of the rover and is exposed to the same atmospheric conditions as the instrument. The SCT includes 10 individual targets designed to meet all instrument calibration requirements. An additional calibration target is mounted inside the instrument’s dust cover. The targets include polymers, rock, synthetic material, and optical pattern targets. Their primary function is calibration of parameters within the SHERLOC instrument so that the data can be interpreted correctly. The SCT was also designed to take advantage of opportunities for supplemental science investigations and includes targets intended for public engagement. The exposure of materials to martian atmospheric conditions allows for opportunistic science on extravehicular suit (i.e. “spacesuit”) materials. These samples will be used in an extended study to produce direct measurements of the expected service lifetimes of these materials on the martian surface, thus helping NASA facilitate human exploration of the planet. Other targets include a martian meteorite and the first geocache target to reside on another planet, both of which increase the outreach and potential of the mission to foster interest in, and enthusiasm for, planetary exploration. During the first 200 sols (martian days) of operation on Mars, the SCT has been analyzed three times and has proven to be vital in the calibration of the instrument and in assisting the SHERLOC team with interpretation of in situ data.

In situ Identification of Paleoarchean Biosignatures Using Colocated Perseverance Rover Analyses: Perspectives for in situ Mars Science and Sample Return

Keyron Hickman-Lewis, Kelsey R. Moore, Joseph J. Razzell Hollis, Michael L. Tuite, Luther W. Beegle, Rohit Bhartia, John P. Grotzinger, Adrian J. Brown, Svetlana Shkolyar, Barbara Cavalazzi, and Caroline L. Smith

Astrobiology 2022, 22 (9) DOI:10.1089/ast.2022.0018

Abstract: The NASA Mars 2020 Perseverance rover is currently exploring Jezero crater, a Noachian–Hesperian locality that once hosted a delta–lake system with high habitability and biosignature preservation potential. Perseverance conducts detailed appraisals of rock targets using a synergistic payload capable of geological characterization from kilometer to micron scales. The highest-resolution textural and chemical information will be provided by correlated WATSON (imaging), SHERLOC (deep-UV Raman and fluorescence spectroscopy), and PIXL (X-ray lithochemistry) analyses, enabling the distributions of organic and mineral phases within rock targets to be comprehensively established. Herein, we analyze Paleoarchean microbial mats from the *3.42 Ga Buck Reef Chert (Barberton greenstone belt, South Africa) —considered astrobiological analogues for a putative ancient martian biosphere—following a WATSON–SHERLOC–PIXL protocol identical to that conducted by Perseverance on Mars during all sampling activities. Correlating deep-UV Raman and fluo- rescence spectroscopic mapping with X-ray elemental mapping, we show that the Perseverance payload has the capability to detect thermally and texturally mature organic materials of biogenic origin and can highlight organic–mineral interrelationships and elemental colocation at fine spatial scales. We also show that the Perseverance protocol obtains very similar results to high-performance laboratory imaging, Raman spectroscopy, and mXRF instruments. This is encouraging for the prospect of detecting microscale organic-bearing textural biosignatures on Mars using the correlative micro-analytical approach enabled by WATSON, SHERLOC, and PIXL; indeed, laminated, organic-bearing samples such as those studied herein are considered plausible analogues of biosignatures from a potential Noachian–Hesperian biosphere. Were similar materials discovered at Jezero crater, they would offer opportunities to reconstruct aspects of the early martian carbon cycle and search for potential fossilized traces of life in ancient paleoenvironments. Such samples should be prioritized for caching and eventual return to Earth.

A large scientific figure made of 5 parts on a black background. Part A shows a colour image of a mottled brown-grey rock, part B shows a black and white version of the same image with coloured circles overlaid, depicting different types of minerals detected by SHERLOC. Parts C, D, and E are false-colour rectangular maps showing the distribution of different types of minerals detected by PIXL.

The Power of Paired Proximity Science Observations: Co-Located Data from SHERLOC and PIXL on Mars

Joseph Razzell Hollis, Kelsey R. Moore,, Sunanda Sharma, Luther Beegle, John P. Grotzinger, Abigail Allwood, William Abbey, Rohit Bhartia, Adrian J. Brown, Benton Clark, Edward Cloutis, Andrea Corpolongo, Jesper Henneke, Keyron Hickman-Lewis, Joel A. Hurowitz, Michael W. M. Jones, Yang Liu, Jesús Martinez-Frías, Ashley Murphy, David A. K. Pedersen, Svetlana Shkolyar, Sandra Siljeström, Andrew Steele, Mike Tice, Alan Treiman, Kyle Uckert, Scott VanBommel, Anastasia Yanchilina

Icarus 2022, 387, 115179

Abstract: We present a synthesis of PIXL elemental data and SHERLOC Raman spectra collected on two targets investigated by the Perseverance rover during the first year of its exploration of Jezero Crater, Mars. The Bellegarde target (in the Máaz formation) and Dourbes target (in the Séítah formation) exhibit distinctive mineralogies that are an ideal case study for in situ analysis by SHERLOC and PIXL. Each instrument alone produces valuable data about the chemistry and spatial distribution of mineral phases at the sub-millimeter scale. However, combining data from both instruments provides a more robust interpretation that overcomes the limitations of either instrument, for example: 1) Detection of correlated calcium and sulfur in Bellegarde by PIXL is corroborated by the co-located detection of calcium sulfate by SHERLOC. 2) Detection of sodium and chlorine in Dourbes is consistent with either chloride or oxychlorine salts, but SHERLOC does not detect perchlorate or chlorate. 3) A Raman peak at 1120 cm–1 in Dourbes could be sulfate or pyroxene, but elemental abundances from PIXL at that location are a better match to pyroxene. This study emphasizes the importance of analyzing co-located data from both instruments together, to obtain a more complete picture of sub-millimeter-scale mineralogy measured in situ in Jezero crater, Mars, by the Perseverance rover.

A scientific figure on a white background depicting several different Raman spectra, which appear as horizontal lines with upward spikes at different positions, measured in Raman shift. The lines are labelled Halite, Kaolinite, Olivine, Quartz, Fluorapatite, Gypsum, Calcite, and Colemanite.

A Deep-Ultraviolet Raman and Fluorescence Spectral Library of 62 Minerals for the SHERLOC Instrument onboard Mars 2020

Joseph Razzell Hollis, William Abbey, Luther W. Beegle, Rohit Bhartia, Bethany L. Ehlmann, Jasper Miura, Brian Monacelli, Kelsey Moore, Austin Nordman, Eva Scheller, Kyle Uckert, Yen-Hung Wu

Planetary and Space Science, 209 (2022) 105356

Abstract: We report deep-ultraviolet (DUV) Raman spectra as measured by a SHERLOC analog instrument (248.6 nm excitation) for 92 samples representing 62 distinct minerals, including borates, carbonates, sulfates, phosphates, halides, metal oxides & hydroxides, silicates & phyllosilicates. We found that DUV Raman is capable of detecting the majority of these minerals, with major mineral peaks occurring at ~500, ~850, 950–1200, and ~3600 cm–1, and that detection thresholds will be better for the SHERLOC flight instrument than the analog used in this study. Minerals can be classified (e.g., sulfate vs carbonate, or pyroxene vs olivine) based on the number of major peaks and their general positions. Identification of specific mineral phases is possible based on precise Raman peak positions, provided the difference between spectrally similar minerals is at least 10 cm–1 to overcome the esti- mated instrumental uncertainty of 5 cm–1 for all peak positions reported in this study. A number of silicate minerals did not produce measurable Raman signal, and iron-rich minerals tend to be more difficult to detect due to significant UV absorption. Many Mars-relevant minerals expected to occur in Jezero crater should be detectable to SHERLOC even during short-exposure survey scans. This library will help inform the detection and identification of mineral phases in Martian samples using the SHERLOC instrument onboard the Mars 2020 Perseverance rover.

A scientific figure in two parts on a white background. On the left the measured spectrum of adenosine shows multiple peaks that change in position with energy dosage, while above it two simulated spectra show how the adenosine spectrum is expected to change upon addition of a hydroxyl group to the molecule. On the right a diagram depicts how adenosine gains a hydroxyl group under UV light, and then, in the absence of clay, the central rings break

Detection and Degradation of Adenosine Monophosphate in Perchlorate-Spiked Martian Regolith Analogue, by Deep-Ultraviolet Raman Spectroscopy

Joseph Razzell Hollis, Teresa Fornaro, William Rapin, Jessica Wade, Álvaro Vicente-Retortillo, Andrew Steele, Rohit Bhartia, Luther W. Beegle

Astrobiology, 21 5 (2021) 511-525

Abstract: The search for organic biosignatures on Mars will depend on finding material protected from the destructive ambient radiation. Solar ultraviolet can induce photochemical degradation of organic compounds, but certain clays have been shown to preserve organic material. We examine how the SHERLOC instrument on the upcoming Mars 2020 mission will use deep-ultraviolet (UV) (248.6 nm) Raman and fluorescence spectroscopy to detect a plausible biosignature of adenosine 5′-monophosphate (AMP) adsorbed onto Ca-montmorillonite clay. We found that the spectral signature of AMP is not altered by adsorption in the clay matrix but does change with prolonged exposure to the UV laser over dosages equivalent to 0.2–6 sols of ambient martian UV. For pure AMP, UV exposure leads to breaking of the aromatic adenine unit, but in the presence of clay the degradation is limited to minor alteration with new Raman peaks and increased fluorescence consistent with formation of 2-hydroxyadenosine, while 1 wt % Mg perchlorate increases the rate of degradation. Our results confirm that clays are effective preservers of organic material and should be considered high-value targets, but that pristine biosignatures may be altered within 1 sol of martian UV exposure, with implications for Mars 2020 science operations and sample caching

Deep-Ultraviolet Raman Spectra of Mars-Relevant Evaporite Minerals under 248.6 nm Excitation

Joseph Razzell Hollis, Schelin Ireland, William Abbey, Rohit Bhartia, Luther W. Beegle

Icarus, 357 (2021) 114067

Abstract: We have measured the deep-ultraviolet (DUV) Raman spectra of a number of evaporite minerals that are relevant to lacustrine and fluvial environments found on Earth and Mars, and show that DUV Raman can provide detailed information on elemental composition and crystal structure. The minerals included three borates, eight carbonates, and seven sulfates, with each class of mineral exhibited very distinct spectra under 248.6 nm excitation, dominated by the various internal vibrations of the borate, carbonate or sulfate oxyanion. Major peak positions were similar to values reported under visible excitation: 1079 cm–1 for calcite, 1006 cm–1 for gypsum and 1078 cm–1 for colemanite. Within each class, minerals consisting different metallic cations could be distinguished by the position of the dominant vibrational mode and the relative intensities of the minor modes, ascribed to the electrostatic impact of the cation on the vibrational behavior of the oxyanion. There was also evidence that DUV Raman can reveal minor metallic components even if they are not apparent in XRD, as two of three calcite (CaCO3) samples exhibited a shoulder on the dominant peak consistent with perturbation by Mg. UV absorption by Fe2+/3+ was a major factor in determining measurable signal, with Fe-rich minerals exhibiting weak/undetectable spectra. Understanding the spectra of these evaporite minerals will be essential to interpreting and identifying complex mineral samples using this technique, and represents an important addition to the spectral standards library of DUV Raman spectroscopy

An Optical Model for Quantitative Raman Microspectroscopy

Joseph Razzell Hollis, David Rheingold, Rohit Bhartia, Luther W. Beegle

Applied Spectroscopy, 357 (2021) 114067

Abstract: Raman spectroscopy is an invaluable technique for identifying compounds by the unique pattern of their molecular vibrations, and is capable of quantifying the individual concentrations of those compounds provided that certain parameters about the sample and instrument are known. We demonstrate the development of an optical model to describe the intensity distribution of incident laser photons as they pass through the sample volume, determine the limitations of that volume that may be detected by the spectrometer optics, and account for light absorption by molecules within the sample, in order to predict the total Raman intensity that would be obtained from a given, uniform sample such as an aqueous solution. We show that the interplay between the shape and divergence of the laser beam, the position of the focal plane, and the dimensions of the spectrometer slit are essential to explaining experimentally observed trends in deep-UV (DUV) Raman intensities obtained from both planar and volumetric samples, including highly oriented pyrolytic graphite (HOPG) and binary mixtures of organic nucleotides. This model offers the capability to predict detection limits for organic compounds in different matrices based on the parameters of the spectrometer, and to define the upper/lower limits within which concentration can be reliably determined from Raman intensity for such samples. We discuss the potential to quantify more complex samples, including as solid phase mixtures of organics and minerals, that are investigated by the unique instrument parameters of the scanning habitable environments with Raman and luminescence for organics and chemicals (SHERLOC) investigation on the upcoming Mars 2020 mission.

A large scientific figure showing the DUV Raman spectrum of E. coli cells, and how the different peaks observed can be assigned to different molecules in DNA and protein. On the right, the cellular spectrum is deconvolved using the different standards, and below it are the chemical structures of the various compounds in question

The Cell and the Sum of Its Parts: Patterns of Complexity in Biosignatures as Revealed by Deep UV Raman Spectroscopy

Haley M. Sapers, Joseph Razzell Hollis, Rohit Bhartia, Luther W. Beegle, Victoria J. Orphan, Jan P. Amend

Frontiers in Microbiology, 10 (2019) 679

Abstract: The next NASA-led Mars mission (Mars 2020) will carry a suite of instrumentation dedicated to investigating Martian history and the in situ detection of potential biosignatures. SHERLOC, a deep UV Raman/Fluorescence spectrometer has the ability to detect and map the distribution of many organic compounds, including the aromatic molecules that are fundamental building blocks of life on Earth, at concentrations down to 1 ppm. The mere presence of organic compounds is not a biosignature: there is widespread distribution of reduced organic molecules in the Solar System. Life utilizes a select few of these molecules creating conspicuous enrichments of specific molecules that deviate from the distribution expected from purely abiotic processes. The detection of far from equilibrium concentrations of a specific subset of organic molecules, such as those uniquely enriched by biological processes, would comprise a universal biosignature independent of specific terrestrial biochemistry. The detectability and suitability of a small subset of organic molecules to adequately describe a living system is explored using the bacterium Escherichia coli as a model organism. The DUV Raman spectra of E. coli cells are dominated by the vibrational modes of the nucleobases adenine, guanine, cytosine, and thymine, and the aromatic amino acids tyrosine, tryptophan, and phenylalanine. We demonstrate that not only does the deep ultraviolet (DUV) Raman spectrum of E. coli reflect a distinct concentration of specific organic molecules, but that a sufficient molecular complexity is required to deconvolute the cellular spectrum. Furthermore, a linear combination of the DUV resonant compounds is insufficient to fully describe the cellular spectrum. The residual in the cellular spectrum indicates that DUV Raman spectroscopy enables differentiating between the presence of biomolecules and the complex uniquely biological organization and arrangements of these molecules in living systems. This study demonstrates the ability of DUV Raman spectroscopy to interrogate a complex biological system represented in a living cell, and differentiate between organic detection and a series of Raman features that derive from the molecular complexity inherent to life constituting a biosignature

A scientific figure illustrating the chemical structure of polytellurophene, a molecule made of repeating 5-membered atomic rings with a single non-C atom in each ring and an R group attached at one side. Underneath it is an illustration of three different molecules, P3HTe, P3DDTe, and P3EHTe, showing how the polymer chains (depicted as wiggly red lines) are more organised and parallel for P3HTe, less so for P3DDTe, and even less for P3EHTe.

Effects of Side-Chain Length and Shape on Polytellurophene Molecular Order and Blend Morphology

Joseph Razzell-Hollis, Franziska Fleischli, Ashlee A. Jahnke, Natalie Stingelin, Dwight S. Seferos, Ji-Seon Kim

Journal of Physical Chemistry C, 121 (2017) 2088-2098

Abstract: We investigate the molecular order and thin film morphology of the conjugated polymer polytellurophene, in order to understand how the tellurium atom and the choice of side-chain influence the conjugated polymer’s backbone planarity and performance in organic transistors. We find that poly(3hexyltellurophene) (P3HTe) continues the trend from polythiophene (P3HT) to polyselenophene (P3HS): substitution with Tellurium leads to a more planar backbone, evident from the shifts of the C=C vibrational peak to lower wavenumbers (∼1389 cm–1) and a smaller optical band gap (∼1.4 eV). Resonant Raman spectroscopy revealed that molecular order was highly dependent on the structure of the P3ATe alkyl side-chain: a longer chains introduces kinetic hindrance, reducing the fraction of ordered phase obtained at room temperature, while a branched side-chain introduces steric hindrance, with intrinsic disorder present even when deposited at higher temperatures. When blended with the insulator HDPE, all three polymers exhibit little additional disorder and instead form phase-separated networks of high molecular order that are beneficial to percolated charge transport in transistors. We find that molecular order, as measured by Raman, correlates well with reported transistor mobilities and provides a greater understanding of the structure−property relationships that determine the performance of these novel organometallic polymers in electronic devices.

A scientific figure comprised of two diagrams showing different layered structures, flanking a plot showing how enhancement factor decreases dramatically as the distance X increases. X refers to the thickness of  the PEDOT:PSS layer in the left figure, which sits between a layer of silver and an organic film. The stack on the right shows a P3HT:F8TBT organic blend film on a quartz substrate, with a P3HT rich interface at the top and an F8TBT rich interface at the bottom interface

Interfacial Chemical Composition and Molecular Order in Organic Photovoltaic Blend Thin Films Probed by Surface-Enhanced Raman Spectroscopy

Joseph Razzell-Hollis, Quentin Thiburce, Wing C. Tsoi, Ji-Seon Kim

ACS Applied Materials & Interfaces, 8 (2016) 31469-31481

Abstract: Organic electronic devices invariably involve transfer of charge carriers between the organic layer and at least one metal electrode, and they are sensitive to the local properties of the organic film at those interfaces. Here, we demonstrate a new approach for using an advanced technique called surface-enhanced Raman spectroscopy (SERS) to quantitatively probe interfacial properties relevant to charge injection/extraction. Exploiting the evanescent electric field generated by a ∼7 nm thick layer of evaporated silver, Raman scattering from nearby molecules is enhanced by factors of 10−1000× and limited by a distance dependence with a measured decay length of only 7.6 nm. When applied to the study of an all-polymer 1:1 blend of P3HT and F8TBT used in organic solar cells, we find that the as-cast film is morphologically suited to charge extraction in inverted devices, with a top (anode) interface very rich in hole-transporting P3HT (74.5%) and a bottom (cathode) interface slightly rich in electron-transporting F8TBT (55%). While conventional, uninverted P3HT:F8TBT devices are reported to perform poorly compared to inverted devices, their efficiency can be improved by thermal annealing but only after evaporation of a metallic top electrode. This is explained by changes in composition at the top interface: annealing prior to silver evaporation leads to a greater P3HT concentration at the top interface to 83.3%, exaggerating the original distribution that favored inverted devices, while postevaporation annealing increases the concentration of F8TBT at the top interface to 34.8%, aiding the extraction of electrons in a conventional device. By nondestructively probing buried interfaces, SERS is a powerful tool for understanding the performance of organic electronic devices.


A scientific diagram comprised of 4 squares, each square is filled with a mixture of wiggly red lines and blue dots. The squares are labelled 0%, 20%, 40%, and 70%, the 0% square has no blue dots and lots of parallel red lines, the 20% and 40% show a mix of blue dots, wiggly red lines and parallel red lines, while the 70% square shows lots of blue dots and no parallel red lines, only wiggly ones.

Spectroscopic Investigations of Three-Phase Morphology Evolution in Polymer: Fullerene Solar Cell Blends

Joseph Razzell-Hollis, Saurav Limbu, Ji-Seon Kim

Journal of Physical Chemistry C, 120 20 (2016) 10806-10814

Abstract: Nanoscale morphology is critical to determining the device efficiency of bulk heterojunction organic solar cells, and the ideal structure is often described as a three-phase network with one well-mixed phase for efficient charge separation and two purer phases for efficient charge transport. In order to understand such nanoscale morphology, we have performed detailed spectroscopic investigations and identified the three-phase morphology evolution in one of the classic blend systems, P3HT:PCBM. The impact of different phases on polymer molecular (chain conformational) order and blend thermal and optical properties were monitored in situ using resonant Raman, absorption, and photoluminescence spectroscopy techniques. Semicrystalline P3HT was found to accommodate up to ∼25% PCBM (by weight) in its amorphous phase, with very little impact on either polymer molecular order or aggregation. Higher concentrations of PCBM resulted in a greater proportion of amorphous mixed phase and reduced polymer molecular order and aggregation. On the other hand, the formation of crystalline purer phases via phase separation was evident during in situ thermal annealing, revealing a consistent glass transition temperature (Tg) of ∼50 °C in blends with up to 50% wt PCBM. This indicates similar local chemical compositions in the amorphous mixed phase present in blends despite different overall blend ratios. A much higher Tg (80−100 °C) was observed for blends with >50% wt PCBM, indicating a stronger impact of PCBM on P3HT molecular order and thermal properties, requiring a higher annealing temperature to ensure formation of the preferred three-phase morphology.


A scientific figure that demonstrates the change in Raman spectrum between pristine PTB7 degraded PTB7, namely a shift in the position of the strongest peak, increasing intensity of two minor peaks, and the appearance of a new peak.

Photochemical stability of high efficiency PTB7:PC70BM solar cell blends

Joseph Razzell-Hollis, Jessica Wade, Wing C. Tsoi, Ying Soon, James Durrant, Ji-Seon Kim

Journal of Materials Chemistry A, 2 (2014) 20189

Abstract: Thieno[3,4 b]thiophene-alt-benzodithiophene (PTB7) is a promising donor–acceptor copolymer that has achieved high efficiencies (7–9%) in organic solar cells but suffers from poor stability and degrades when exposed to light and oxygen. Using resonant Raman spectroscopy to examine the nature of this photo-oxidation, three main changes to the vibrations of the conjugated backbone are observed: (1) shift of the
benzodithiophene (BDT) C=C stretch peak at ~1489 cm–1 up to ~1499 cm–1; (2) increase in the relative intensity of coupled fused thiophene and benzene C=C stretch peaks at ~1535 and ~1575 cm–1; (3) appearance of a new peak at ~1650 cm–1; which suggest oxidation takes place on the BDT unit without loss of conjugation. In situ accelerated photo-degradation reveals that the observed oxidation is the initial step of degradation, which is followed by reductions in absorption and Raman scattering intensities that indicate the loss of chromophores by a second, more extensive oxidation step. Blending PTB7 with PC70BM is found to accelerate the polymer’s degradation, and further shift the BDT peak to ~1509 cm–1. Using density functional theory to simulate Raman spectra for several possible oxidised products, the initial oxidation is best described by hydroxylation of 3rd and 7th positions on the BDT donor unit.


A scientific figure comprised of a plot of the left and a diagram on the right. The plot shows a single broad Raman peak measured for 4 different films (thick vs thin, as-cast vs annealed), which gets narrower and shifts to the left for annealed films. The diagram depicts a thick layer made of a mixture of wiggly brown lines and blue dots, most of which are mixed well together, whereas the thin film shows more vertical separation of the two.

Directly probing the molecular order of conjugated polymer in OPV blends induced by different film thicknesses, substrates and additives

Joseph Razzell-Hollis, Wing C. Tsoi, Ji-Seon Kim

Journal of Materials Chemistry C, 1 (2013) 6235-6243

Abstract: In organic bulk heterojunction photovoltaic (OPV) devices, formation of a phase-separated morphology of the blend thin film with a high degree of molecular order is required for efficient device performance. Using resonant Raman spectroscopy we monitor in situ the P3HT molecular order in P3HT:PCBM blend films influenced by the substrate, film thickness and additives. We report that molecular order depends on substrate for as-cast films, consistent with vertical phase separation driven by a surface energy gradient, but is standardised to a highly ordered state by thermal annealing. In situ Raman spectroscopy reveals this phase transition to a more ordered state begins at 40–60 ºC for ~120 nm thick blend films, which corresponds to the glass transition temperature (Tg). Ultra-thin (<10 nm thick) blend films had greater P3HT order than the bulk and reorganised at lower temperatures, which we propose is due to a P3HT-rich interfacial layer at the film/air interface, and that extra disordered component retained despite annealing is due to P3HT trapped in a disordered state within the corresponding PCBM-rich substrate interface. Finally we probe how the 1,8-octanedithiol (ODT) additive improves P3HT molecular order in blends by increasing phase separation during deposition, finding that 3% ODT by volume presents a saturation point for improving molecular order, and the improvement is comparable to that by thermal annealing. Through in situ experiments and varied fabrication conditions, we have built an understanding of how processing conditions determine conjugated polymer molecular order in blends, with the aim of controlling morphology for higher OPV efficiencies.

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