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All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Self-regulating surface chemistry for more robust highly durable solid oxide fuel cell cathodes

Solid oxide fuel cells (SOFCs) offer direct conversion of hydrogen or hydrocarbon fuels to electricity with high energy conversion efficiency (>50%). Impurities that block active reaction sites on the electrode surface can markedly degrade the oxygen exchange rate, and lead to a reduction in long term durability of such devices. In this work, we propose to develop electrodes that are tolerant to Si which is one of the most prevalent cathode impurities. This work draws on our recent method to fully recover oxygen exchange kinetics following Si induced aging of ceria containing cathodes. We take advantage of elements, which over time during SOFC operation, are released by exsolution to actively trap (i.e. getter) Si impurities in a self-regulating chemical fashion. The objective is to evaluate impurity scavenging by added reactive elements that are intentionally exsolved from the SOFC cathode electrode during operation. In this study, we chose to focus our effort on one cathode material: Pr doped ceria (PCO). This material has the advantage of not containing strontium, an element known to segregate to the surface of the well-known perovkite based cathodes materials, thereby degrading their performance. As for the Si getter, we choose to focus on lanthanum, which is added in PCO in solid solution.

03 NATURAL GAS↗
DOE OSTI2022

Enhancing methane production of anaerobic digested sewage sludge by advanced wet oxidation & steam explosion pretreatment

Anaerobic digestion (AD) of sewage sludge is associated with lowered carbon conversion efficiency due to the recalcitrance of microbial cells in the Waste Activated Sludge (WAS) from the aeration basins with 50 % of the dry weight still being present after AD. The effect of Advanced Wet Oxidation & Steam Explosion (AWOEx) pretreatment on the methane yield at variable temperatures (165–200 °C), residence time (15–45 min) and oxygen dosage (1%–10% based on VS concentration) was tested in this study. Central composite design (JMP® Pro) was used to design 19 runs of AWOEx pretreatment of dewatered digested sewage sludge (DADSS) at different operating conditions. After pretreatment the DADSS was fed to 19 bioreactors operated in semi-continues mode at 37 °C. The highest average methane yields of 183 mL/g VS and 170 mL/g VS were found for the bioreactors receiving DADSS pretreated at 165 °C with a retention time of 15 min and 10 % O2 (Condition 1), and, 182.5 °C with a retention time of 15 min and 5.5 % O2 (Condition 2). This corresponds to an increase in the methane production of 156.2 % and 140.5 % compared to the methane production from the DADSS without pretreatment.

09 BIOMASS FUELS↗
DOE OSTI2018

3D/2D multidimensional perovskites: Balance of high performance and stability for perovskite solar cells

Here, the power conversion efficiency achieved in three-dimensional (3D) perovskite solar cells (PSCs) is already competitive with that of commercial silicon solar cells. Achieving long-term stability under operation conditions is critical for reaching market success for PSC applications. Despite impressive progress over the past few years on fundamental understandings and technical innovations for improving PSC stability, the stability of PSCs against moisture/heat/light is still a key focus of research efforts for PSC development. Recent studies suggest that the long hydrophobic organic spacer in two-dimensional (2D) perovskites is helpful to improve perovskite stability, but 2D perovskites seem limited on obtaining high-performance solar cells due to its wide optical bandgaps and limited charge transport. To overcome this challenge, 3D/2D multidimensional perovskites - with an intermediate dimensionality between 3D and 2D - has recently emerged as a potential candidate to simultaneously maintain long-term stability and high performance. In this review, we first present a detailed discussion of the structure of 3D/2D multidimensional perovskites and their unique properties. Second, we discuss the stability of 3D/2D multidimensional perovskite and 2D perovskite as interface engineering layer-based solar cells. Finally, we summarize and outline the perspectives toward high-performance 3D/2D multidimensional perovskite-based solar cells.

14 SOLAR ENERGY↗
DOE OSTI2014

Origins of electrostatic potential wells at dislocations in polycrystalline Cu(In,Ga)Se{sub 2} thin films

Thin-film solar cells based on Cu(In,Ga)Se{sub 2} (CIGSe) reach high power-conversion efficiencies in spite of large dislocation densities of up to 10{sup 10}–10{sup 11} cm{sup −2}. The present work gives insight into the structural and compositional properties of dislocations in CIGSe thin films, which are embedded in a complete solar cell stack. These properties are related to the average electrical potential distributions obtained by means of inline electron holography. At a part of the dislocations studied, the average electrostatic potential shows local minima, all with depths of about −1.4 V. The measured average electrostatic potential distributions were modeled in order to reveal possible influences from strain fields, excess charge, and also compositional changes at the dislocation core. Cu depletion around the dislocation core, as evidenced by atom-probe tomography, explains best the measured potential wells. Their influences of the strain field around the dislocation core and of excess charge at the dislocation core are small. A structural model of dislocations in CIGSe thin films is provided which includes a Cu-depleted region around the dislocation core and gives a possible explanation for why decent photovoltaic performances are possible in the presence of rather large dislocation densities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Cu Based Dilute Alloys for Tuning the C 2+ Selectivity of Electrochemical CO 2 Reduction

Electrochemical CO 2 reduction is a promising technology for replacing fossil fuel feedstocks in the chemical industry but further improvements in catalyst selectivity need to be made. So far, only copper-based catalysts have shown efficient conversion of CO 2 into the desired multi-carbon (C 2+ ) products. This work explores Cu-based dilute alloys to systematically tune the energy landscape of CO 2 electrolysis toward C 2+ products. Selection of the dilute alloy components is guided by grand canonical density functional theory simulations using the calculated binding energies of the reaction intermediates CO*, CHO*, and OCCO* dimer as descriptors for the selectivity toward C 2+ products. Here, a physical vapor deposition catalyst testing platform is employed to isolate the effect of alloy composition on the C 2+ /C 1 product branching ratio without interference from catalyst morphology or catalyst integration. Six dilute alloy catalysts are prepared and tested with respect to their C 2+ /C 1 product ratio using different electrolyzer environments including selected tests in a 100-cm 2 electrolyzer. Consistent with theory, CuAl, CuB, CuGa and especially CuSc show increased selectivity toward C 2+ products by making CO dimerization energetically more favorable on the dominant Cu facets, demonstrating the power of using the dilute alloy approach to tune the selectivity of CO 2 electrolysis.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Closely packed, low reorganization energy $π$-extended postfullerene acceptors for efficient polymer solar cells

New organic semiconductors are essential for developing inexpensive, high-efficiency, solution-processable polymer solar cells (PSCs). PSC photoactive layers are typically fabricated by film-casting a donor polymer and a fullerene acceptor blend, with ensuing solvent evaporation and phase separation creating discrete conduits for photogenerated holes and electrons. Until recently, n-type fullerene acceptors dominated the PSC literature; however, indacenodithienothiophene (IDTT)-based acceptors have recently enabled remarkable PSC performance metrics, for reasons that are not entirely obvious. Herein we report two isomeric IDTT-based acceptors 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-benz-(5, 6)indanone))-5,5,11,11-tetrakis(4-nonylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]di-thiophene (ITN-C9) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-benz(6,7)indanone))-5,5,11,11-tetrakis(4-nonylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITzN-C9) that shed light on the exceptional IDTT properties vis-à-vis fullerenes. The neat acceptors and blends with fluoropolymer donor poly{[4,8-bis[5-(2- ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5- b′ ]dithiophene2,6-diyl]- alt -[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo4H,8H-benzo[1,2- c :4,5- c′ ]dithiophene-1,3-diyl]]} (PBDB-TF) are investigated by optical spectroscopy, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetry, single-crystal X-ray diffraction, photovoltaic response, space-charge-limited current transport, atomic force microscopy, grazing incidence wide-angle X-ray scattering, and density functional theory-level quantum chemical analysis. The data reveal that ITN-C9 and ITzN-C9 organize such that the lowest unoccupied molecular orbital-rich end groups have intermolecular π−π distances as close as 3.31(1) Å, with electronic coupling integrals as large as 38 meV, and internal reorganization energies as small as 0.133 eV, comparable to or superior to those in fullerenes. ITN-C9 and ITzN-C9 have broad solar-relevant optical absorption, and, when blended with PBDB-TF, afford devices with power conversion efficiencies near 10%. Performance differences between ITN-C9 and ITzN-C9 are understandable in terms of molecular and electronic structure distinctions via the influences on molecular packing and orientation with respect to the electrode.

14 SOLAR ENERGY↗
DOE OSTI2014

Distinguishing the importance of fullerene phase separation from polymer ordering in the performance of low band gap polymer: Bis-fullerene heterojunctions

It is known, one way to improve power conversion efficiency (PCE) of polymer based bulk-heterojunction (BHJ) photovoltaic cells is to increase the open circuit voltage (V oc ). Replacing PCBM with bis-adduct fullerenes significantly improves V oc and the PCE in devices based on the conjugated polymer poly(3-hexyl thiophene) (P3HT). However, for the most promising low band-gap polymer (LBP) system, replacing PCBM with ICBA results in poor short-circuit current (J sc ) and PCE although V oc is significantly improved. The optimization of the morphology of as-cast LBP/bis-fullerene BHJ photovoltaics is attempted by adding a co-solvent to the polymer/fullerene solution prior to film deposition. Varying the solubility of polymer and fullerene in the co-solvent, bulk heterojunctions are fabricated with no change of polymer ordering, but with changes in fullerene phase separation. The morphologies of the as-cast samples are characterized by small angle neutron scattering and neutron reflectometry. A homogenous dispersion of ICBA in LBP is found in the samples where the co-solvent is selective to the polymer, giving poor device performance. Aggregates of ICBA are formed in samples where the co-solvent is selective to ICBA. Furthermore, the resultant morphology improves PCE by up to 246%. Finally, a quantitative analysis of the neutron data shows that the interfacial area between ICBA aggregates and its surrounding matrix is improved, facilitating charge transport and improving the PCE.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2019

(CH 3 NH 3 ) AuX 4 ·H 2 O (X=Cl, Br) and (CH 3 NH 3 ) AuCl 4 : Low-Band Gap Lead-Free Layered Gold Halide Perovskite Materials

Perovskite solar cells have recently enabled power conversion efficiency comparable to established technologies such as silicon and cadmium telluride. Ongoing efforts to improve the stability of halide perovskites in ambient air has yielded promising results. However, the presence of toxic heavy element lead (Pb) remains a major concern requiring further attention. Herein, we report three new Pb-free hybrid organic–inorganic perovskite-type halides based on gold (Au), (CH 3 NH 3 )AuBr 4 ·H 2 O ( 1 ), (CH 3 NH 3 )AuCl 4 ·H 2 O ( 2 ), and (CH 3 NH 3 )AuCl 4 ( 3 ). Hydrated compounds 1 and 2 crystallize in a brand-new structure type featuring perovskite-derived 2D layers and 1D chains based on pseudo-octahedral AuX 6 building blocks. In contrast, the novel crystal structure of the solvent-free compound 3 shows an exotic non-perovskite quasi-2D layered structure containing edge- and corner-shared AuCl 6 octahedra. The use of Au metal instead of Pb results in unprecedented low band gaps below 2.5 eV for single-layered metal chlorides and bromides. Moreover, at room temperature the three compounds show a weak blue emission due to the electronic transition between Au-6s and Au-5d, in agreement with the density function theory (DFT) calculation results. These findings are discussed in the context of viability of Au-based halides as alternatives for Pb-based halides for optoelectronic applications.

13 HYDRO ENERGY↗
DOE OSTI2023

Modeling the Environment-Dependent Kinetics of Oxygen Reduction Reaction – a Continuum Model for Electric Double Layer

Here, for proton-exchange-membrane fuel cells (PEMFCs) to achieve broad commercialization, improved energy-conversion efficiency with minimal Pt-based electrocatalyst is required. Because the sluggish rate of oxygen reduction reaction (ORR) limits the efficiency of PEMFCs, the efficiency improvement requires a better understanding of ORR kinetics and mechanism to design better catalyst. To understand the ORR mechanism, theoretical and experimental analyses have been conducted. While previous studies reasonably explained the catalyst-dependent activity on single crystal catalysts in 0.1 M perchloric acid solution, the explicit effect of electrolyte and related microenvironments is not thoroughly understood. The change in the electrolyte alters the electric-double-layer (EDL) structure and thus the local microenvironment at the electrode/electrolyte interface. Thus, the structure of the EDL should be carefully analyzed to uncover the electrolyte-dependent reaction kinetics. In this talk, we propose a multiscale continuum model to predict the EDL structure and examine the effect of perchloric acid concentration on ORR activity on Pt (111). The model includes Density Potential Functional Theory (DPFT) for electron density and Modified Poisson Boltzmann equation for species’ density and electric potential. Also, the interaction between adsorbents and electric field is taken into account by minimizing the grand potential. After model validation with experimentally measured double-layer capacity data as a function of applied potential and concentration, the effect of the perchloric acid concentration (0.02 M – 0.2 M) on ORR activity is analyzed and discussed. It is shown that the model reproduces the specific activity obtained in the experiments when assuming the oxygen adsorption is limiting the rate, which can be attributed to the large energetic barrier for solvent reorganization. Then, extension of the model to PEMFC ionomer electrolytes will be introduced. Overall, the model framework and findings provide insights into the ORR mechanism and guidance on how to tailor catalyst materials for increased PEMFC performance.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2016

Properties of Cu 1–x K x InSe 2 alloys

Adding potassium to Cu(In,Ga)Se 2 absorbers has been shown to enhance photovoltaic power conversion efficiency. To illuminate possible mechanisms for this enhancement and limits to beneficial K incorporation, the properties of Cu 1-x K x InSe 2 (CKIS) thin-film alloys have been studied. Films with K/(K + Cu), or x, from 0 to 1 were grown by co-evaporation, and probed by XRF, EPMA, SEM, XRD, UV-Visible spectroscopy, current-voltage, and TRPL measurements. Composition from in situ quartz crystal and EIES monitoring was well correlated with final film composition. Crystal lattice parameters showed linear dependence on x, indicating complete K incorporation and coherent structural character at all compositions in the <100> and <010> lattice directions, despite the different symmetries of CuInSe 2 and KInSe 2 . The band gap energy showed pronounced bowing with x composition, in excellent agreement with experimental reports and semiconductor theory. Films of Mo/CKIS/Ni were non-ohmic, and increasing x from 0 to 0.58 decreased the apparent CKIS resistivity. Further evidence of decreased CKIS resistivity was observed with photoluminescence response, which increased by about half a decade for x > 0, and indicates increased majority carrier concentration. Minority carrier lifetimes increased by about an order of magnitude for films grown at x = 0.07 and 0.14, relative to CuInSe 2 and x ≥ 0.30. As a result, this is the first report of a Cu-K-In-Se film with >1 at.% K, and the observed property changes at increased x (wider band gap; lower resistivity; increased lifetime) comprise valuable photovoltaic performance-enhancement strategies, suggesting that CKIS alloys have a role to play in future engineering advances.

(Cu,K)InSe2↗
DOE OSTI2025

Innovative Polyhydroxyalkanoates (PHA) Production with Microbial Electrochemical Technology (MET)

The project “Innovative Polyhydroxyalkanoates (PHA) Production with Microbial Electrochemical Technology (MET)” addressed food waste disposal challenges by successfully converting food waste to bioplastics (known as PHAs). The novel process created by our team of researchers from universities, national labs, and industry substantially enhanced overall carbon conversion efficiency of food waste processing (> 50%), while reducing disposal costs (> 25%). The project showed economic viability potential at community scale through pilot-scale demonstration at a relevant scale (50 L reactor volume) with more than 100 hours of PHA production using realistic conditions. The project goal was to valorize food waste by shunting traditional anaerobic digestion processing and creating a value-added PHA processing route that improves the economics and sustainability of local, community-scale, wet organic waste treatment. First, the food waste undergoes microbial-based, dark fermentation to break down the food to small carbon chains known as volatile fatty acids (VFAs). Instead of microorganisms converting the VFAs into methane using normal anaerobic digestion processing, our innovative process inhibits methane production. This preserves the produced VFAs for extraction and use by a novel Haloferax mediterranei (HM) archaea, which effectively converts the VFAs to bioplastics. The project added microbial electrochemical cells (MEC) to the dark fermentation process to enhance the VFAs produced and optimize the type of bioplastics formed.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Grid-Integrated Production of Fischer-Tropsch Synfuels from Nuclear Power

Idaho National Laboratory (INL) investigates the relative economic profitability of an integrated energy system (IES) coupling an NPP with a synfuel production process at selected case study locations across the United States. In the synfuel IES, a high-temperature steam electrolysis (HTSE) plant is thermally and electrically coupled with an NPP to produce zero-carbon hydrogen. The synthetic fuel is produced from this H 2 combined with a CO 2 supply using the reverse water gas shift process followed by the Fischer-Tropsch (FT) reaction. This analysis considers a system in which the CO 2 is sourced from regional CO 2 emitters via the construction and operation of pipeline-based CO 2 supply networks. Locating the FT plant at the same site as the NPP and HTSE plants enables the NPP to provide zero-carbon heat and power to the HTSE plant and zero-carbon power to the FT plant as well as avoid the requirement for long-distance H 2 product transport from the HTSE plant to the FT plant. Hydrogen storage is used to enable the NPP to dispatch power to the electrical grid (instead of the HTSE plant) when grid demand increases, thus enabling the FT plant to continue to operate in a steady-state production mode. The ability to cease hydrogen production for several hours within each day enables the NPP to provide power to the grid to balance the electricity market during peak periods and maximize revenues for the nuclear synfuel IES. The FT process design considered has a 99% carbon conversion efficiency. The use of nuclear energy and nuclear energy-derived hydrogen enables synfuel production to achieve this high level of carbon utilization. Additionally, the life-cycle carbon emissions of the nuclear-based synfuel production process are very low, with WTW emissions of approximately 25 gCO 2 e/MJ, including steam credits (generated from FT process excess heat), and approximately 7 gCO 2 e/MJ, if steam credits are excluded. This compares favorably with the WTW emissions of 90.5 gCO 2 e/MJ for a compression-ignition, direct injection (CIDI) vehicle with a fuel economy of 31.6 miles per gallon gasoline equivalent (MPGGE), using low-sulfur diesel produced using conventional petroleum production and refining processes. Several NPPs in various regions of the U.S. are considered as case study analyses. Supply locations and transportation via pipeline of the CO 2 feedstock to the NPP site are analyzed through the National Energy Technology Laboratory (NETL) CO 2 Transport Cost model. The team finds that the amount of CO 2 generated by different sectors is sufficient for the synfuel production process at all locations considered. The CO 2 transportation costs are functions of the distance of the source to the NPP location, the CO 2 capture cost at the source, and the quantity of CO 2 transported. Historical electricity prices for the NPP case study locations are collected and analyzed. Monthly average prices, price range, and duration of negative-price periods vary among these locations. For each location, an auto-regressive moving average (ARMA) model is trained on historical electricity price data. ARMA validation is done to ensure the synthetic price distributions represent one of historical prices with high fidelity. Synthetic time series from these ARMA models are used in a coupled dispatch and system optimization in the Holistic Energy Resource Optimization Network (HERON) to compute the differential net present value (NPV) of the IES. The team finds that this econometric is positive, ranging from $14M–1.3bn (2020) depending on the location. The optimal synfuel IES configuration to obtain this increase in NPV often maximizes the size of the synfuel production process with regards to the size of the NPP. However, the team shows that the NPP still plays a stabilizing role for the grid: In periods of high prices and high loads, more electricity from the NPP is sent to the grid. A high variability of electricity prices and extreme maximum prices tend to drive up electricity production. While it requires significant investment, the synfuel IES could increase the economic profitability for the existing fleet of LWRs across the country while still maintaining the grid stabilizer role of NPPs. During its lifetime, the main costs for the nuclear synfuel IES are the carbon feedstock transportation costs, followed by the capital expenses (CAPEX) and operation and maintenance (O&M) costs while the revenue comes first from the IRA H 2 production tax credit (PTC) and then from the sales of synfuel products. The profitability of the synfuel IES is most sensitive to the value of the hydrogen PTC and the synfuel products as well as the cost of the carbon feedstock, highlighting the importance of governmental incentives regarding hydrogen, carbon emissions, and synfuel in driving the deployment of future nuclear synfuel IESs.

08 HYDROGEN↗
DOE OSTI2016

On the propagation and mode conversion of auroral medium frequency bursts

Abstract Auroral medium frequency (MF) bursts are broadband, impulsive radio emissions associated with local substorm onsets. MF bursts consist of a characteristic fine structure whereby the higher frequencies arrive 10–100 ms before the lower frequencies. LaBelle (2011a) proposed that MF bursts originate as Langmuir/Z mode waves on the topside of the ionosphere that mode‐convert to LO mode waves and propagate to ground level, with the fine structure resulting by propagation delays due to the topside ionospheric density profile. We investigate three aspects of this mechanism. First, full‐wave calculations are used to simulate the MF burst fine structure using a realistic ionospheric density profile. The delay between the highest and lowest frequencies is 21 ms. This value is smaller than the experimentally determined delays of ∼100 ms presented in Bunch and LaBelle (2009), but differences between the topside electron number density profile used in the simulations and the number density profile during disturbed conditions make comparisons only approximate. Second, the Landau damping of Langmuir/Z mode waves on the topside ionosphere is calculated, assuming the electron distribution function consists of a cold background population ( n e 0 ) and a warm secondary population ( n se ). The Landau damping is small when n se / n e 0 = 0.04% (consistent with Maggs and Lotko (1981)) but is significant when n se / n e 0 > 0.4%. Finally, full‐wave calculations are used to determine the mode conversion efficiency from Langmuir/Z mode waves to LO mode waves. These imply that waves would suffer an attenuation of wave energy density of approximately 5–10% if they are generated with their wave vectors in a narrow cone centered around the local magnetic field. Taken together, these calculations suggest that for small values of n se / n e 0 <0.4%, the mechanism proposed by LaBelle (2011a) is a plausible explanation for the origin of MF bursts.

Broughton, M. C.↗
DOE OSTI2015

Optical, Electrical, and Magnetic Studies of Organic Solar Cells Based on Low Bandgap Copolymer with Spin ½ Radical Additives

The charge photogeneration and recombination processes in organic photovoltaic solar cells based on blend of the low bandgap copolymer, PTB7 (fluorinated poly‐thienothiophene‐benzodithiophene) with C60‐PCBM using optical, electrical, and magnetic measurements in thin films and devices is studied. A variety of steady state optical and magneto‐optical techniques were employed, such as photoinduced absorption (PA), magneto‐PA, doping‐induced absorption, and PA‐detected magnetic resonance (PADMR); as well as picosecond time‐resolved PA. The charge polarons and triplet exciton dynamics in films of pristine PTB7, PTB7/fullerene donor–acceptor (D–A) blend is followed. It is found that a major loss mechanism that limits the power conversion efficiency (PCE) of PTB7‐based solar cell devices is the “back reaction” that leads to triplet exciton formation in the polymer donor from the photogenerated charge‐transfer excitons at the D–A interfaces. A method of suppressing this “back reaction” by adding spin½ radicals Galvinoxyl to the D–A blend is presented; this enhances the cell PCE by ≈30%. The same method is not effective for cells based on PTB7/C70‐PCBM blend, where high PCE is reached even without Galvinoxyl radical additives.

Basel, Tek↗
DOE OSTI2017

Precise Manipulation of Multilength Scale Morphology and Its Influence on Eco‐Friendly Printed All‐Polymer Solar Cells

Significant efforts have lead to demonstrations of nonfullerene solar cells (NFSCs) with record power conversion efficiency up to ≈13% for polymer:small molecule blends and ≈9% for all‐polymer blends. However, the control of morphology in NFSCs based on polymer blends is very challenging and a key obstacle to pushing this technology to eventual commercialization. The relations between phases at various length scales and photovoltaic parameters of all‐polymer bulk‐heterojunctions remain poorly understood and seldom explored. Here, precise control over a multilength scale morphology and photovoltaic performance are demonstrated by simply altering the concentration of a green solvent additive used in blade‐coated films. Resonant soft X‐ray scattering is used to elucidate the multiphasic morphology of these printed all‐polymeric films and complements with the use of grazing incidence wide‐angle X‐ray scattering and in situ spectroscopic ellipsometry characterizations to correlate the morphology parameters at different length scales to the device performance metrics. Benefiting from the highest relative volume fraction of small domains, additive‐free solar cells show the best device performance, strengthening the advantage of single benign solvent approach. This study also highlights the importance of high volume fraction of smallest domains in printed NFSCs and organic solar cells in general.

Ye, Long↗
DOE OSTI2017

Correlated In Situ Low-Frequency Noise and Impedance Spectroscopy Reveal Recombination Dynamics in Organic Solar Cells Using Fullerene and Non-Fullerene Acceptors

Non-fullerene acceptors based on perylenediimides (PDIs) have garnered significant interest as an alternative to fullerene acceptors in organic photovoltaics (OPVs), but their charge transport phenomena are not well understood, especially in bulk heterojunctions (BHJs). Here, charge transport and current fluctuations are investigated by performing correlated low-frequency noise and impedance spectroscopy measurements on two BHJ OPV systems, one employing a fullerene acceptor and the other employing a dimeric PDI acceptor. In the dark, these measurements reveal that PDI-based OPVs have a greater degree of recombination in comparison to fullerene-based OPVs. Moreover, for the first time in organic solar cells, 1/f noise data are fit to the Kleinpenning model to reveal underlying current fluctuations in different transport regimes. Under illumination, 1/f noise increases by approximately four orders of magnitude for the fullerene-based OPVs and three orders of magnitude for the PDI-based OPVs. An inverse correlation is also observed between noise spectral density and power conversion efficiency. In general, these findings reflect that low-frequency noise spectroscopy is an effective in situ diagnostic tool to assess charge transport in emerging photovoltaic materials, thereby providing quantitative guidance for the design of next-generation solar cell materials and technologies.

1/f noise↗
DOE OSTI2018

High‐Performance Semitransparent Ternary Organic Solar Cells

Abstract Semitransparent organic solar cells (STOSCs) are ideal candidates for building‐integrated photovoltaics. Power conversion efficiency (PCE) is one of the most important parameters of STOSCs, which is typically limited by the intrinsic narrow absorption of thin active layer. Ternary structure is demonstrated as an efficient approach to broaden the light absorption and manipulate the morphology, thus leading to improved PCEs by introducing a third component into the binary system. However, STOSCs based on ternary systems are rarely studied. In this contribution, semitransparent ternary solar cells are fabricated using a wide‐bandgap ( E g = 2.10 eV) polymer donor, PBT1‐S, as the third component. It is found that the addition of a small amount of PBT1‐S to PTB7‐Th:PC 71 BM blend has a negligible influence on its average visible transmittance (AVT) and color perception. A PCE of 9.2% is achieved for the champion ternary device with 20% AVT, which is one of the highest values for STOSCs. The results indicate that ternary structure is a promising approach for the fabrication of high‐performance STOSCs.

Xie, Yuanpeng↗
DOE OSTI2018

A Highly Crystalline Fused‐Ring n‐Type Small Molecule for Non‐Fullerene Acceptor Based Organic Solar Cells and Field‐Effect Transistors

Abstract N‐type organic small molecules (SMs) are attracting attention in the organic electronics field, due to their easy purification procedures with high yield. However, only a few reports show SMs that perform well in both organic field‐effect transistors (OFETs) and organic solar cells (OSCs). Here, the synthesis and characterization of an n‐type small molecule with an indacenodithieno[3,2‐b]thiophene (IDTT) core unit and linear alkylated side chain (C 16 ) (IDTTIC) are reported. Compared to the state‐of‐the‐art n‐type molecule IDTIC, IDTTIC exhibits smaller optical bandgap and higher absorption coefficient, which is due to the enhanced intramolecular effect. After mixing with the polymer donor PBDB‐T, IDTIC‐based solar cells deliver a power conversion efficiency of only 5.67%. In stark contrast, the OSC performance of IDTTIC improves significantly to 11.2%. It is found that the superior photovoltaic properties of PBDB‐T:IDTTIC blends are mainly due to reduced trap‐assisted recombination and enhanced molecular packing coherence length and higher domain purity when compared to IDTIC. Moreover, a significantly higher electron mobility of 0.50 cm 2 V −1 s −1 for IDTTIC in OFET devices than for IDTIC (0.15 cm 2 V −1 s −1 ) is obtained. These superior performances in OSCs and OFETs demonstrate that SMs with extended π‐conjugation of the backbone possess a great potential for application in organic electronic devices.

Song, Xin↗