For thermoelectric and other device applications there has been great interest in the chemical doping of conjugated polymer films. Solution doping followed by film deposition generally produces poor-quality films, but this issue can be alleviated by sequential doping: a pure polymer film is deposited first, and the dopant is then added as a second processing step, preserving the quality and structure of the original polymer film. In this paper, we compare two methods for sequential doping of conjugated polymer films: evaporation doping, where a controlled thickness of dopant is added via thermal sublimation to a temperature-controlled polymer film, and sequential solution doping, where the dopant is spin cast from a solvent chosen to swell but not dissolve the underlying polymer film. To compare these two different types of sequential doping, we examine the optical, electrical, and structural properties of poly(3-hexylthiophene-2,5diyl) (P3HT) films doped by each method with the small-molecule dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) as a function of the polymer film thickness. Although each method intercalates dopant in fundamentally unique ways, we find that both vapor and solution doping methods produce films that share many of the same properties. Interestingly, both methods can produce doped P3HT films with conductivities of ∼5 S/cm and comparable thermoelectric properties, even for films as thick as 400 nm. For the evaporation method, an "overhead" dopant film thickness of ∼6 nm is required, either to promote reorganization of existing crystallites or to fill preexisting trap states in the polymer film. After the overhead amount has been deposited, the thickness of the dopant layer that must be evaporated to reach the optimal electrical conductivity is ∼1/3 that of the underlying polymer film. For a given P3HT film thickness, the amount of evaporated dopant needed to produce the highest conductivity corresponds to a thiophene monomer to ionized dopant ratio of ∼8.5:1. For solution processing, with the appropriate choice of solvent and dopant concentration, we show that P3HT films as thick as 2 μm can be doped to achieve conductivities of ∼5 S/cm and thermoelectric power factors approaching 2 μW/mK 2 . For either method, if excess dopant is applied, it remains in neutral form either in the amorphous regions or on top of the film, reducing the conductivity by increasing the film thickness. For both methods, UV−vis absorption can be used as a quick proxy to easily monitor whether saturation doping levels have been reached or exceeded. Fourier transform infrared spectroscopy (FTIR) and grazing-incidence wide-angle X-ray scattering (GIWAXS) both show that vapor-doped films and thicker solution-doped films have improved morphologies that result in more mobile carriers. Overall, we demonstrate that it is a straightforward process to select a sequential doping method for a desired application: evaporation doping is more amenable to large-area films, while solution doping is lower cost ...

Social distancing during the COVID-19 pandemic presents mental health and academic obstacles for students. As mental health can strongly influence academic performance, addressing the loss of community by transitioning to distance education, midsemester, is imperative. This work draws upon the community and wellness benefits associated with online gaming and applies it to ChemDraw. The goal is to improve the class’ wellness and social community while teaching valuable organic chemistry skills. In this paper, a molecule, speed-drawing tournament (Molecule Madness) is presented along with reflections describing the impacts on student wellness and organic chemistry skills. Gamification of ChemDraw is achieved by (1) treating at-home practice as video-game training and (2) using video-conferencing software as a medium for multiplayer gameplay. As the primary focus is placed on education rather than entertainment, ChemDraw is subclassified as a serious educational game. Community is developed by in-game, online chat: observers, announcers, and competitors all interact with each other in real time. The tournament provided students an event to look forward to, helped maintain/improve class community, and developed organic chemistry skills (molecular drawing, history of molecules, and conversational IUPAC) in addition to increased appreciation for IUPAC nomenclature. Although an online community cannot substitute for in-person classroom experiences, this activity supplements the loss of community associated with social distancing and could be extended to enhance community within a face-to-face environment. Additionally, ideas are presented that illustrate how other chemistry classes and topics can be gamified in computer environments (both single- and multiplayer) by drawing upon well-established gaming modes (story mode, maker mode, and speed runs). Overall, this work highlights the supplemental role that chemistry video games can play in learning chemistry: by engaging students in an alternative, yet familiar to many format, classroom community and student confidence are maintained/strengthened.

Bulk heterojunction (BHJ) photovoltaics based on blends of conjugated polymers and fullerenes require an optimized nanoscale morphology. Casting BHJ films using solvent additives such as 1,8diiodooctane (DIO), 1,8-octanedithiol (ODT), chloronapthalene (CN), or diphenyl ether (DPE) often helps achieve this proper morphology: adding just a few volume percent of additive to the casting solution can improve polymer/fullerene mixing or phase separation, so that solvent additives have become staples in producing high-efficiency BHJ solar cells. The mechanism by which these additives improve BHJ morphology, however, is poorly understood. Here, we investigate how these additives control polymer/fullerene mixing by taking advantage of sequential processing (SqP), in which the polymer is deposited first and then the fullerene is intercalated into the polymer underlayer in a second processing step using a quasi-orthogonal solvent. In this way, SqP isolates the role of the additives' interactions with the polymer and the fullerene. We find using ellipsometry-based swelling measurements that when adding small amounts of lowvapor-pressure solvent additives such as DIO and ODT to solutions of poly(3-hexylthiophene-2,5-diyl) (P3HT), poly [(4,4′bis(3-(2-ethyl-hexyl)dithieno[3,2-b:″,3′-d]silole)-2,6-diyl-alt-(2,5-bis(3-(2-ethyl-hexyl)thiophen-2yl)thiazolo[5,4-d]thiazole)] (PSEHTT), or poly[4,8-bis(2-ethylhexyloxy)-benzol[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one)thieno-[3,4-b]thiophene-2,6-diyl] (PBDTTT -C), the additives remain in the polymer film, leading to significant swelling. Twodimensional grazing-incidence wide-angle X-ray scattering measurements show that the swelling is extensive, directly affecting the polymer crystallinity. When we then use SqP and cast phenyl-C 61 -butyric acid methyl ester (PCBM) onto DIO-swollen polymer films, X-ray photoelectron spectroscopy and neutron reflectometry measurements demonstrate that vertical mixing of the PCBM in additive-swollen polymer films is significantly improved compared with films cast without the additive. Thus, lowvapor-pressure solvent additives function as cosolvent swelling agents or secondary plasticizers, allowing fullerene to mix better into the swollen polymer and enhancing the performance of devices produced by SqP, even when the additive is present only in the polymer layer. DIO and ODT have significantly different fullerene solubilities but swell polymers to a similar extent, demonstrating that swelling, not fullerene solubility, is the key to how such additives improve BHJ morphology. In contrast, higher-vapor-pressure additives such as CN and DPE, which have generally high polymer solubilities, function by a different mechanism, improving polymer crystallinity.

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The polymer chain orientation and degree of crystallinity within a polymer:fullerene bulk heterojunction (BHJ) photovoltaic can greatly impact device performance. In general, a face-on chain orientation is preferred for charge conduction through sandwich-structure photovoltaic devices, but for many conjugated polymers, an edge-on conformation is energetically favored. In this work, we examine the effects of different processing techniques on photovoltaics based on the poly [4,8-bis(2ethylhexyloxy)]-phenyl-C 71 -butyric-acid-methylester (PC 71 BM) materials combination. We examine the extent of polymer crystallinity and crystalline domain orientation using both traditional blend-casting (BC), where the polymer and fullerene are cast from a single, codissolved solution, as well as sequential processing (SqP), where the polymer film is deposited first, and then the fullerene is infiltrated into the polymer film in a second solution processing step. We show using two-dimensional grazing-incidence wide-angle X-ray scattering (GIWAXS) that BC leads to a disordered, isotropic polymer network in the resulting BHJ film with a correspondingly poor device efficiency. By contrast, SqP preserves the preferred face-on chain orientation seen in pure polymer films, yielding higher short-circuit currents that are consistent with the increased hole mobility of face-on oriented polymer chains. We also study the effects of the widely used processing additive 1,8-diiodooctane (DIO) on polymer chain orientation and crystallinity in photovoltaic devices made by both processing techniques. We show that DIO results in increased polymer crystallinity, and in devices made by BC, DIO also causes a partial recovery of the face-on PBDTTT-C domain orientation, improving device performance. The face-on chain orientation in SqP devices produces efficiencies similar to those of optimized BC devices made with DIO but without the need for solvent additives or other postprocessing steps.