The role of spin in the degradation of organic photovoltaics

Nature Communications Springer Nature 12 (2021) 471

I Ramirez, A Privitera, S Karuthedath, A Jungbluth, J Benduhn, A Sperlich, D Spoltore, K Vandewal, F Laquai, M Riede

Stability is now a critical factor in the commercialization of organic photovoltaic (OPV) devices. Both extrinsic stability to oxygen and water and intrinsic stability to light and heat in inert conditions must be achieved. Triplet states are known to be problematic in both cases, leading to singlet oxygen production or fullerene dimerization. The latter is thought to proceed from unquenched singlet excitons that have undergone intersystem crossing (ISC). Instead, we show that in bulk heterojunction (BHJ) solar cells the photo-degradation of C<sub>60</sub> via photo-oligomerization occurs primarily via back-hole transfer (BHT) from a charge-transfer state to a C<sub>60</sub> excited triplet state. We demonstrate this to be the principal pathway from a combination of steady-state optoelectronic measurements, time-resolved electron paramagnetic resonance, and temperature-dependent transient absorption spectroscopy on model systems. BHT is a much more serious concern than ISC because it cannot be mitigated by improved exciton quenching, obtained for example by a finer BHJ morphology. As BHT is not specific to fullerenes, our results suggest that the role of electron and hole back transfer in the degradation of BHJs should also be carefully considered when designing stable OPV devices.

Halide Segregation in Mixed-Halide Perovskites: Influence of A-Site Cations.

ACS Energy Lett 6 (2021) 799-808

AJ Knight, J Borchert, RDJ Oliver, JB Patel, PG Radaelli, HJ Snaith, MB Johnston, LM Herz

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br0.5I0.5)3 and FA0.83Cs0.17Pb(Br0.4I0.6)3 films. We report evidence for low-barrier ionic pathways in MAPb(Br0.5I0.5)3, which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA0.83Cs0.17Pb(Br0.4I0.6)3 lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.

Electron spin as fingerprint for charge generation and transport in doped organic semiconductors

Journal of Materials Chemistry C Royal Society of Chemistry (RSC) (2021)

A Privitera, R Warren, G Londi, P Kaienburg, J Liu, A Sperlich, AE Lauritzen, O Thimm, A Ardavan, D Beljonne, M Riede

<p>We use the electron spin as a probe to gain insight into the mechanism of molecular doping in a p-doped zinc phthalocyanine host across a broad range of temperatures (80–280 K) and doping concentrations (0–5 wt% of F6-TCNNQ).</p>

Nanotechnology for catalysis and solar energy conversion.

Nanotechnology 32 (2021) 042003-

U Banin, N Waiskopf, L Hammarström, G Boschloo, M Freitag, EMJ Johansson, J Sá, H Tian, MB Johnston, LM Herz, RL Milot, MG Kanatzidis, W Ke, I Spanopoulos, KL Kohlstedt, GC Schatz, N Lewis, T Meyer, AJ Nozik, MC Beard, F Armstrong, CF Megarity, CA Schmuttenmaer, VS Batista, GW Brudvig

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: 'high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al 'Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure-property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. The contribution by Megarity and Armstrong presents the 'electrochemical leaf' for improvements in electrochemistry and beyond. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society.

Chain Conformation Control of Fluorene-Benzothiadiazole Copolymer Light-Emitting Diode Efficiency and Lifetime.

ACS applied materials & interfaces 13 (2021) 2919-2931

B Wang, H Ye, M Riede, DDC Bradley

The β-phase, in which the intermonomer torsion angle of a fraction of chain segments approaches ∼180°, is an intriguing conformational microstructure of the widely studied light-emitting polymer poly(9,9-dioctylfluorene) (PFO). Its generation can in turn be used to significantly improve the performance of PFO emission-layer-based light-emitting diodes (LEDs). Here, we report the generation of β-phase chain segments in a copolymer, 90F8:10BT, containing 90% 9,9-dioctylfluorene (F8) and 10% 2,1,3-benzothiadiazole (BT) units and show that significant improvements in performance also ensue for LEDs with β-phase 90F8:10BT emission layers, generalizing the earlier PFO results. The β-phase was induced by both solvent vapor annealing and dipping copolymer thin films into a solvent/nonsolvent mixture. Subsequent absorption spectra show the characteristic fluorene β-phase peak at ∼435 nm, but luminescence spectra (∼530 nm peak) and quantum yields barely change, with the emission arising following efficient energy transfer to the lowest-lying excited states localized in the vicinity of the BT units. For ∼5% β-phase chain segment fraction relative to 0% β-phase, the LED luminance at 10 V increased by ∼25% to 5940 cd m<sup>-2</sup>, the maximum external quantum efficiency by ∼61 to 1.91%, and the operational stability from 64% luminance retention after 20 h of operation to 90%. Detailed studies addressing the underlying device physics identify a reduced hole injection barrier, higher hole mobility, correspondingly more balanced electron and hole charge transport, and decreased carrier trapping as the dominant factors. These results confirm the effectiveness of chain conformation control for fluorene-based homo- and copolymer device optimization.

Direct observation and evolution of electronic coupling between organic semiconductors

Physical Review Materials American Physical Society (APS) 5 (0) 015601

SV Kesava, MK Riede

Understanding Dark Current-Voltage Characteristics in Metal-Halide Perovskite Single Crystals


EA Duijnstee, VM Le Corre, MB Johnston, LJA Koster, J Lim, HJ Snaith

Coarse and fine-tuning of lasing transverse electromagnetic modes in coupled all-inorganic perovskite quantum dots

Nano Research Springer Science and Business Media LLC 14 (2021) 108-113

Y Park, G Ying, A Jana, V Osokin, CC Kocher, T Farrow, RA Taylor, KS Kim

<jats:title>Abstract</jats:title> <jats:p>Inorganic perovskite lasers are of particular interest, with much recent work focusing on Fabry-Pérot cavity-forming nanowires. We demonstrate the direct observation of lasing from transverse electromagnetic (TEM) modes with a long coherence time ∼ 9.5 ps in coupled CsPbBr<jats:sub>3</jats:sub> quantum dots, which dispense with an external cavity resonator and show how the wavelength of the modes can be controlled via two independent tuning-mechanisms. Controlling the pump power allowed us to fine-tune the TEM mode structure to the emission wavelength, thus providing a degree of control over the properties of the lasing signal. The temperature-tuning provided an additional degree of control over the wavelength of the lasing peak, importantly, maintained a constant full width at half maximum (FWHM) over the entire tuning range without mode-hopping.</jats:p>

Terahertz Conductivity Analysis for Highly Doped Thin-Film Semiconductors


AM Ulatowski, LM Herz, MB Johnston

Highly efficient photoluminescence and lasing from hydroxide coated fully inorganic perovskite micro/nano-rods

Advanced Optical Materials Wiley (2020)

G Ying, A Jana, V Osokin, Y Park, R Taylor, T Farrow

The effect of surface passivation on the photoluminescence (PL) emitted by CsPbBr3 micro/nano‐rods coated with Pb(OH)2 is investigated, where a high quantum yield and excellent stability for the emission are found. The CsPbBr3/Pb(OH)2 rods generally present a peak that is blue shifted compared to that seen in rods without a hydroxide cladding at low temperatures. By increasing the temperature, it is further shown that the passivated surface states are very robust against thermal effects and that the PL peak intensity only drops by a factor of 1.5. Localized stimulated emission at defect states found within larger rods is also demonstrated, clarified by spatially resolved confocal PL mapping along the length of the rods. The diffusion parameter of the carrier density distribution is measured to be 5.70 µm for the sky‐blue emission, whereas for the defect lasing site it is found to be smaller than this excitation spot size.

Impact of tin fluoride additive on the properties of mixed tin-lead iodide perovskite semiconductors

Advanced Functional Materials Wiley 30 (2020) 2005594

KJ Savill, AM Ulatowski, MD Farrar, MB Johnston, HJ Snaith, LM Herz

Mixed tin‐lead halide perovskites are promising low‐bandgap absorbers for all‐perovskite tandem solar cells that offer higher efficiencies than single‐junction devices. A significant barrier to higher performance and stability is the ready oxidation of tin, commonly mitigated by various additives whose impact is still poorly understood for mixed tin‐lead perovskites. Here, the effects of the commonly used SnF2 additive are revealed for FA0.83Cs0.17SnxPb1−xI3 perovskites across the full compositional lead‐tin range and SnF2 percentages of 0.1–20% of precursor tin content. SnF2 addition causes a significant reduction in the background hole density associated with tin vacancies, yielding longer photoluminescence lifetimes, decreased energetic disorder, reduced Burstein–Moss shifts, and higher charge‐carrier mobilities. Such effects are optimized for SnF2 addition of 1%, while for 5% SnF2 and above, additional nonradiative recombination pathways begin to appear. It is further found that the addition of SnF2 reduces a tetragonal distortion in the perovskite structure deriving from the presence of tin vacancies that cause strain, particularly for high tin content. The optical phonon response associated with inorganic lattice vibrations is further explored, exhibiting a shift to higher frequency and significant broadening with increasing tin fraction, in accordance with lower effective atomic metal masses and shorter phonon lifetimes.

Efficient energy transfer mitigates parasitic light absorption in molecular charge-extraction layers for perovskite solar cells

Nature Communications Springer Science 11 (2020) 5525

HJ Eggimann, JB Patel, MB Johnston, LM Herz

Organic semiconductors are commonly used as charge-extraction layers in metal-halide perovskite solar cells. However, parasitic light absorption in the sun-facing front molecular layer, through which sun light must propagate before reaching the perovskite layer, may lower the power conversion efficiency of such devices. Here, we show that such losses may be eliminated through efficient excitation energy transfer from a photoexcited polymer layer to the underlying&#xA0;perovskite. Experimentally observed energy transfer between a range of different polymer films and a methylammonium lead iodide perovskite layer was used as basis for modelling the efficacy of the mechanism as a function of layer thickness, photoluminescence quantum efficiency and absorption coefficient of the organic polymer film. Our findings reveal that efficient energy transfer can be achieved for thin (&#x2264;10&#xA0;nm) organic charge-extraction layers exhibiting high photoluminescence quantum efficiency. We further explore how the morphology of such thin polymer layers may be affected by interface formation with the perovskite.

Molecular doped organic semiconductor crystals for optoelectronic device applications

Journal of Materials Chemistry C 8 (2020) 14996-15008

Z Qin, C Gao, WWH Wong, MK Riede, T Wang, H Dong, Y Zhen, W Hu

© The Royal Society of Chemistry. For semiconductors, doping is an efficient approach to tune their energy bandgaps, charge transport, and optical properties which could enable the improvement of the corresponding performances and open up the possibility of multifunction integration. Recently, significant advances have been achieved in molecular doped organic semiconductors, especially doped organic semiconductor single crystals (OSSCs) which have features of well-defined packing structures, long-range molecular orders, and low-density defects for fundamental studies and improved properties. In this review, we will give a summary of the exciting progress of molecular doped OSSCs from the aspects of selection criteria of molecular dopants, general growth methods, and resulting optoelectronic properties as well as their applications in optoelectronic devices. Finally, a brief conclusion is given with challenges and perspectives of molecular doped OSSCs and their related promising research directions in this field. This journal is

Organic solar cells—the path to commercial success

Advanced Energy Materials Wiley (2020)

M Riede, D Spoltore, K Leo

Organic solar cells have the potential to become the cheapest form of electricity, beating even silicon photovoltaics. This article summarizes the state of the art in the field, highlighting research challenges, mainly the need for an efficiency increase as well as an improvement in long‐term stability. It discusses possible current and future applications, such as building integrated photovoltaics or portable electronics. Finally, the environmental footprint of this renewable energy technology is evaluated, highlighting the potential to be the energy generation technology with the lowest carbon footprint of all.

Thermally stable passivation toward high efficiency inverted perovskite solar cells

ACS Energy Letters American Chemical Society (2020)

RDJ Oliver, Y-H Lin, AJ Horn, CQ Xia, JH Warby, MB Johnston, AJ Ramadan, HJ Snaith

Although metal halide perovskite photovoltaics have shown an unprecedented rise in power conversion efficiency (PCE), they remain far from their theoretical PCE limit. Among the highest efficiencies to date are delivered when polycrystalline films are enhanced via “molecular passivation”, but this can introduce new instabilities, in particular under severe accelerated aging conditions (e.g., at 85 °C in the dark or under full spectrum simulated sunlight). Here, we utilize a benzylammonium bromide passivation treatment to improve device performance, achieving the champion stabilized power output (SPO) of 19.5 % in a p-i-n device architecture. We correlate the improved device performance with a significant increase in charge carrier diffusion lengths, mobilities, and lifetimes. Furthermore, treated devices maintain an increased performance during 120 h combined stressing under simulated full spectrum sunlight at 85 °C, indicating that enhancement from this passivation treatment is sustained under harsh accelerated aging conditions. This is a crucial step toward real-world operation-relevant passivation treatments.

Investigation of background electron emission in the LUX detector

Physical Review D 102 (2020)

DS Akerib, S Alsum, HM Araújo, X Bai, J Balajthy, A Baxter, EP Bernard, A Bernstein, TP Biesiadzinski, EM Boulton, B Boxer, P Brás, S Burdin, D Byram, MC Carmona-Benitez, C Chan, JE Cutter, L De Viveiros, E Druszkiewicz, A Fan, S Fiorucci, RJ Gaitskell, C Ghag, MGD Gilchriese, C Gwilliam, CR Hall, SJ Haselschwardt, SA Hertel, DP Hogan, M Horn, DQ Huang, CM Ignarra, RG Jacobsen, O Jahangir, W Ji, K Kamdin, K Kazkaz, D Khaitan, EV Korolkova, S Kravitz, VA Kudryavtsev, E Leason, BG Lenardo, KT Lesko, J Liao, J Lin, A Lindote, MI Lopes, A Manalaysay, RL Mannino, N Marangou, DN McKinsey, DM Mei, M Moongweluwan, JA Morad, ASJ Murphy, A Naylor, C Nehrkorn, HN Nelson, F Neves, A Nilima, KC Oliver-Mallory, KJ Palladino, EK Pease, Q Riffard, GRC Rischbieter, C Rhyne, P Rossiter, S Shaw, TA Shutt, C Silva, M Solmaz, VN Solovov, P Sorensen, TJ Sumner, M Szydagis, DJ Taylor, R Taylor, WC Taylor, BP Tennyson, PA Terman, DR Tiedt, WH To, L Tvrznikova, U Utku, S Uvarov, A Vacheret, V Velan, RC Webb, JT White, TJ Whitis, MS Witherell, FLH Wolfs, D Woodward, J Xu, C Zhang

© 2020 American Physical Society. Dual-phase xenon detectors, as currently used in direct detection dark matter experiments, have observed elevated rates of background electron events in the low energy region. While this background negatively impacts detector performance in various ways, its origins have only been partially studied. In this paper we report a systematic investigation of the electron pathologies observed in the LUX dark matter experiment. We characterize different electron populations based on their emission intensities and their correlations with preceding energy depositions in the detector. By studying the background under different experimental conditions, we identified the leading emission mechanisms, including photoionization and the photoelectric effect induced by the xenon luminescence, delayed emission of electrons trapped under the liquid surface, capture and release of drifting electrons by impurities, and grid electron emission. We discuss how these backgrounds can be mitigated in LUX and future xenon-based dark matter experiments.

Molecular Quadrupole Moments Promote Ground-State Charge Generation in Doped Organic Semiconductors


A Privitera, G Londi, M Riede, G D'Avino, D Beljonne

Atomic-scale microstructure of metalhalide perovskite

Science American Association for the Advancement of Science 370 (2020) eabb5940

J Kim, H Snaith, M Johnston, L Herz, AJ Borchert, M Rothmann

Hybrid organic-inorganic perovskites are exciting materials for solar-energy applications whose microscopic properties are still not well understood. Atomic-resolution (scanning) transmission electron microscopy, (S)TEM, has provided invaluable insights for many crystalline solar-cell materials, and is used here to successfully image CH(NH2)2PbI3 thin films with low electron-radiation dose. Such images reveal a highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI2 interfaces, with a striking absence of long-range disorder in the crystal. We demonstrate that beaminduced degradation of the perovskite leads to an initial loss of CH(NH2)2 + ions, leaving behind a partially unoccupied perovskite lattice, which explains the unusual regenerative properties of these materials. We further observe aligned point defects and climbdissociated dislocations. Our findings thus provide an atomic-level understanding of technologically important lead-halide perovskites.

The Elusive Nature of Carbon Nanodot Fluorescence: An Unconventional Perspective

The Journal of Physical Chemistry C American Chemical Society (ACS) 124 (2020) 22314-22320

M Righetto, F Carraro, A Privitera, G Marafon, A Moretto, C Ferrante

Coarse and fine-tuning of lasing transverse electromagnetic modes in coupled all-inorganic perovskite quantum dots

Nano Research Springer Science and Business Media LLC (2020)

Y Park, G Ying, A Jana, V Osokin, CC Kocher, T Farrow, RA Taylor, KS Kim

&lt;jats:title&gt;Abstract&lt;/jats:title&gt; &lt;jats:p&gt;Inorganic perovskite lasers are of particular interest, with much recent work focusing on Fabry-P&#xE9;rot cavity-forming nanowires. We demonstrate the direct observation of lasing from transverse electromagnetic (TEM) modes with a long coherence time &#x223C; 9.5 ps in coupled CsPbBr&lt;jats:sub&gt;3&lt;/jats:sub&gt; quantum dots, which dispense with an external cavity resonator and show how the wavelength of the modes can be controlled via two independent tuning-mechanisms. Controlling the pump power allowed us to fine-tune the TEM mode structure to the emission wavelength, thus providing a degree of control over the properties of the lasing signal. The temperature-tuning provided an additional degree of control over the wavelength of the lasing peak, importantly, maintained a constant full width at half maximum (FWHM) over the entire tuning range without mode-hopping.&lt;/jats:p&gt;