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Nicole Moody, Samuel Sesena, Dane W. deQuilettes, Benjia Dak Dou, Richard Swartwout, Anna Johnson, Udochukwu Eze, Roberto Brenes, Matthew Johnston, Vladimir Bulovic, and Moungi G. Bawendi are members of Tata-MIT GridEdge Solar, an interdisciplinary research program at the Massachusetts Institute of Technology working toward scalable design and manufacturing of lightweight, flexible solar cells. Joseph T. Buchman and Christy L. Haynes are part of the Center for Sustainable Nanotechnology, a multi-institutional partnership aimed at understanding the fundamental chemical and physical processes that govern the transformations and interactions of nanoparticles in the environment.

Summary Today’s approach to deploying solar photovoltaics (PV) implicitly assumes that module technology is fixed. Solar panels are installed and expected to operate for the system life of 30 years or more. However, many PV technologies are improving rapidly along several dimensions, including cost, power conversion efficiency, and reliability. Periodic module replacement or planned repowering takes advantage of this technological improvement and counteracts predictable degradation. Here, we show that a module replacement strategy allows a competitive levelized cost of electricity to be achieved with an initial module lifetime of less than 15 years, assuming backward compatibility with the original system design. We also assess the life-cycle environmental impacts of module replacement and find that all commercial PV technologies offer benefits in the majority of impact categories, regardless of the replacement strategy, compared to today’s electric generation mix. Module replacement can thus accelerate the market introduction and decarbonization impact of emerging PV technologies that have achieved a competitive module efficiency (≥20%), cost (≤$0.30/W), and lifetime (≥10 years) and have the potential to improve further on all three metrics but lack decades-long field deployment experience.

Metal halide perovskites are generating enormous excitement for use in solar cells and light-emission applications, but devices still show substantial non-radiative losses. Here, we show that by combining light and atmospheric treatments, we can increase the internal luminescence quantum efficiencies of polycrystalline perovskite films from 1% to 89%, with carrier lifetimes of 32 μs and diffusion lengths of 77 μm, comparable with perovskite single crystals. Remarkably, the surface recombination velocity of holes in the treated films is 0.4 cm/s, approaching the values for fully passivated crystalline silicon, which has the lowest values for any semiconductor to date. The enhancements translate to solar cell power-conversion efficiencies of 19.2%, with a near-instant rise to stabilized power output, consistent with suppression of ion migration. We propose a mechanism in which light creates superoxide species from oxygen that remove shallow surface states. The work reveals an industrially scalable post-treatment capable of producing state-of-the-art semiconducting films. Metal halide perovskites are exciting materials for low-cost optoelectronic devices such as solar cells and LEDs. In order to reach the theoretical efficiency limits for both applications, any parasitic non-radiative charge-carrier recombination losses, such as those mediated by carrier trapping, must be eliminated. At present, perovskite materials still suffer from substantial non-radiative decay, particularly under solar illumination conditions, and are therefore yet to reach their full potential. Perovskite single crystals have very low trap concentrations but their controlled growth into devices does not lend themselves to the advantages offered by solution-processing thin films such as roll-to-roll depositions. Here, we demonstrate the use of light and atmospheric treatments on polycrystalline perovskite films, resulting in minimal non-radiative losses and properties approaching those of perovskite single crystals and even the best crystalline semiconductors reported to date. The authors demonstrate the use of light and atmospheric treatments on polycrystalline perovskite thin films, resulting in properties approaching those of the best crystalline semiconductors reported to date. The results translate to exceptional photovoltaic device performances with rapid rises to stabilized power output consistent with an inhibition of ionic migration.

We explore the dynamic interaction of charge carriers between band-edge states and sub-band trap states in PbS quantum dot (QD) solids using time-resolved spectroscopy. In monodisperse arrays of 4- to 5-nm diameter PbS QDs, we observe an optically active trap state ∼100–200 meV below the band edge that occurs at a frequency of 1 in ∼2,500 QDs. Uncoupled QD solids with oleic acid ligands show trap-to-ground-state recombination that resembles Auger recombination. In electronically coupled QD solids, we observe entropically driven uphill thermalization of trapped charge carriers from the trap state to the band edge via two distinct mechanisms: Auger-assisted charge transfer (∼35 ps) and thermally activated hopping (∼500 ps). Photophysical characterization combined with atomistic simulations and high-resolution electron microscopy suggest that these states arise from epitaxially fused pairs of QDs rather than electron or hole traps at the QD surface, offering new strategies for improving the optoelectronic performance of QD materials., Department of Energy (DOE), Office of Basic Energy Sciences (Awards DE-SC0010538,DE-SC0019345), National Science Foundation (Awards 1452857 and 1122374)

We present an all solution, two-step method for the production of methylammonium lead iondide (MAPbI3) perovskite thin films. Initially, PbS thin films were deposited on GaAs substrates. Subsequently, PbS films were transformed into PbI2 following treatment with polyiodide solutions. In a second conversion step, MAPbI3 films were obtained by reacting the films with methylammonium iodide solutions. The effect of the solvent on lead iodide morphology and consequently on MAPbI3 morphology was demonstrated. Conversion rate was slower with increasing solvent polarity. Optimization of conversion parameters is crucial for reproducibly obtaining continuous and adhesive perovskite films. Photoluminescence measurements showed that chemical treatments can substantially enhance light emission from the films. This route for all-solution-processed perovskites enables control over film morphology while maintaining a simple and low-cost process.

We demonstrate all thermal vapor-deposited CsSnBr 3 perovskite solar cells. The cell architecture consists of a planar CsSnBr 3 heterojunction where the perovskite layer is grown by thin sequential deposition of SnBr 2 and CsBr allowing for immediate reaction. While other halides of CsSnX 3 (e.g. X=Cl, I) are extremely air sensitive and change color within seconds, illumination tests performed on unencapsulated CsSnBr 3 devices in ambient air show no change over the span of hours. Further, we evaluate the impact of low-temperature annealing and incorporation of various fluoride passivating dopant profiles via vapor deposition. Our results demonstrate an exciting alternative pathway to develop all-inorganic lead-free perovskite photovoltaics.

We report the use of isobenzofulvene–C60 adducts in bulk heterojunction organic solar cells, synthesized via the [4 + 2] cycloaddition of C60 with an in situ generated isobenzofulvene intermediate. The LUMO energy levels of these adducts are 20–180 meV higher than that of PCBM ([6,6]-phenyl-C61-butyric acid methyl ester). This large increase of the LUMO level is attributed to cofacial π-orbital interactions between the fullerene surface and the isobenzofulvene π–system (aromatic ring and double bond). Raised LUMO levels of fullerenes, together with their desirably slow recombination dynamics, led to higher open-circuit voltages (VOC) in bulk heterojunction polymer solar cells (up to 0.75 V for bisadducts) relative to cells tested in parallel using the well-known PCBM as the fullerene acceptor. In addition to enhanced VOC, the short-circuit current densities (JSC) were improved in the devices containing the epoxide analogs of the isobenzofulvene–C60. Notably the epoxide derivative of the monoadduct (IBF–Ep) exhibited ∼20% enhancement of power conversion efficiency (PCE) compared to reference P3HT:PCBM solar cells. A combination of optical and electronic methods was used to investigate the origin of the PCE enhancement observed with these new fullerene acceptors with particular attention to the increased VOCs.

We fabricate the thinnest (1.3 μm) and lightest (3.6 g/m2) solar cells yet demonstrated, with weight-specific power exceeding 6 W/g, in order to illustrate the lower limits of substrate thickness and materials use achievable with a new processing paradigm. Our fabrication process uniquely starts with growth of an ultra-thin flexible polymer substrate in vacuum, followed by deposition of electrodes and photoactive layers in situ. With this process sequence, the entire cell—from transparent substrate to active layers to encapsulation—can be fabricated at room temperature without solvents and without breaking vacuum, avoiding exposure to dust and other contaminants, and minimizing damage risk associated with handling of thin substrates. We use in situ vapor-phase growth of smooth, transparent, and flexible parylene-C films to produce ultra-thin, lightweight molecular organic solar cells as thin as 2.3 μm including encapsulation with a second parylene-C film. These parylene-based devices exhibit power conversion efficiencies and fabrication yields comparable to glass-based cells. Flexible solar cells on parylene membranes can be seamlessly adhered to a variety of solid surfaces to provide additive solar power.

This study identifies potential future markets and deployment challenges for vertically mounted photovoltaic (PV) panels in the United States (U.S.). Target photovoltaic (PV) module metrics are determined for economically competitive installations comparable to the grid-supplied commercial electricity price in the contiguous U.S. The study is motivated by the emergence of third-generation PV materials that promise low cost modules with weight per unit area that are an order of magnitude lower than silicon panels, thereby enabling vertical installation of PV modules on existing structures with minimal mechanical reinforcement. Vertically-mounted PVs on building facades are the largest potential market, between 50 GWp and 550 GWp, with other smaller markets adding an additional 15 GWp of capacity. Conservatively, assuming that the third-generation lightweight and monofacial PV modules have 15% efficiency, cost $0.68/Wp and a 10-year lifetime, when mounted vertically facing south, their levelized cost of generated electricity is between 18.3 and 23.1 ¢/kWh, dependent on the region of the U.S. East or west facing monofacial modules result in a higher levelized cost between 24.1 and 28.4 ¢/kWh. Bifacial PV modules facing east–west achieve a grid-comparable cost between 11.8 and 14.2 ¢/kWh. With further improvement of the monofacial PV module performance to 19% efficiency, 10-year lifetime, and $0.20/Wp when vertically mounted facing south, the levelized cost of their generated electricity would be between 10.1 and 12.7 ¢/kWh, comparable to the average 2018 grid price in the U. S.

Morphology control is demonstrated in chemical solution deposited lead selenide thin films on fluorine-doped tin oxide substrates. Trisodium citrate co-complexing agent was utilized as an additional degree of control over deposition parameters, in addition to the commonly used hydroxide complexing agent. Films were polycrystalline in all cases, and control over grain size and thickness was obtained upon varying trisodium citrate-to-hydroxide ratio. The morphology evolves from initially 7 nm grains with random orientation, continuing to columnar growth, and finally to grains of about 50 nm in diameter. For each trisodium citrate concentration a specific pH was determined in which deposition rate was maximal, resulting in columnar films up to 1,100 nm in thickness obtained after only 30 min of deposition.

In organic photovoltaics (OPVs), active layer thicknesses are limited by exciton diffusion length. Thus, non-planar surfaces and device architectures are desired for enhancing the light absorption in OPVs. The oxidative chemical vapor deposition (oCVD) process enables the formation of conformal films of conducting polymers on complex surface structures. oCVD poly(3,4 ethylenedioxythiophene) (PEDOT) is demonstrated to be compatible with a wide range of nano- to macro-scale textured and non-planar architectures that have been demonstrated to enhance light absorption in photovoltaics by various mechanisms of light trapping, such as lengthening optical pathways and taking advantage of reflective light bouncing. Here, conformal oCVD (PEDOT) layers are demonstrated over submicron features including submicron nanowedges and nanocones (with 70–100 nm groove depth and 100 nm pitch) and square gratings (50–350 nm groove depth and 139–833.3 nm pitch). In contrast, solution-applied PEDOT:PSS exhibits blanketing, thinning at the top of the features, and welling up of material in the bottom of the features, thus failing to conformally coat the complex surfaces. The application of oCVD PEDOT to macro-scale 3D OPV architectures was also explored. Enhanced active layer absorbance is shown using tetraphenyldibenzoperiflanthene (DBP) as the absorbing layer over the PEDOT.

An array of optoelectronic applications requires micro- and nanoscale patterning of molecular organic films. A subtractive patterning technique is developed to define micron-sized features of nanometer range thickness on a variety of hole transporting and hole blocking materials. Lateral resolution of patterned features is controlled by micron-scale stamp shapes, while the removal of the patterned nanoscale film thicknesses (sub-10 nm to over 40 nm) is correlated to strength of interfacial and intermolecular forces. Using this process, a multi-color organic light-emitting device is fabricated, demonstrating the ability of contact patterning to manipulate organic molecular films on the micro- and nanoscales.

We demonstrate the use of a vacuum-based, vapor phase technique for the deposition of a donor polymer for use in polymer solar cells. Unsubstituted polythiophene (PT), which is insoluble and infusible and thus typically difficult to process, is easily prepared by oxidative chemical vapor deposition (oCVD). The oCVD process results in a conductive PT film that is heavily doped with FeCl 3 , which is used as the oxidizing agent. A post-deposition methanol rinse sufficiently dedopes the film and removes spent oxidant, leaving semiconducting PT with an optical bandgap close to 2 eV. Drastic changes in the film color, absorption spectra, and film composition confirm the dedoping process. The resulting semiconducting PT is then applied as an electron donor in bilayer heterojunction solar cells with a thermally evaporated C 60 electron acceptor layer, resulting in power conversion efficiencies up to 0.8%. The absorption edge of the PT at ∼620 nm closely matches the edge present in the external quantum efficiency spectra, indicating that the oCVD PT contributes to the photocurrent of the devices. This demonstrates that the oCVD technique can be used in the processing and design of polymer active layers for polymer solar cells and hybrid small molecular organic solar cells without solubility, temperature, or substrate considerations.

We review our research efforts to develop solid state integrated devices that operate in the strong coupling limit of cavity quantum electrodynamics (QED) for eventual application in high speed optical switching, optical computing, and quantum computing. Our devices contain J-aggregates of (organic) cyanine dyes which, by virtue of their molecular arrangement and strong dipolar coupling, exhibit a collective narrow linewidth high oscillator strength optical transition. Using J-aggregates, the strong coupling limit can be reached at room temperature with large coupling strengths (Rabi-splitting >250 meV) in exciton–polariton microcavity structures. We demonstrate that high quality nanoscale thick J-aggregate films can be uniformly deposited over macroscopic substrates, engineered at the molecular level, and patterned into single or multi-dimensional photonic bandgap structures. Our unique methods for depositing J-aggregates enabled us to structure light emitting devices that demonstrated the first ever electrically pumped polariton emission, uniquely accomplished in room temperature operation. Additionally, we demonstrated critically coupled resonators that concentrate nearly all of the incident light into 5 nm thick J-aggregate films, yielding a record high effective absorption constant of 6.8 · 10 6 cm � 1 for films with thickness that is less than 1% of the incident light wavelength. Such remarkable optical properties, enabled by scalable deposition techniques, suggest that J-aggregates are a unique materials platform on which to demonstrate integrated exciton–polariton devices with the far reaching properties of polaritons in the optical domain. � 2007 Elsevier B.V. All rights reserved. PACS: 71.36.+c; 71.35.Aa; 85.60.Jb; 78.40.Me

We demonstrate exciton energy transfer from a thin film of phosphorescent dye fac tris(2-phenylpyridine) iridium (Ir(ppy)3 )t o a monolayer of colloidal CdSe/ZnS core/shell quantum dots (QDs). The energy transfer is manifested in time-resolved photoluminescence (PL) measurements as elongation of the QD PL time constant from 40 to 400 ns, and a concomitant 55% increase of time-integrated QD PL intensity. The observed PL dynamics are shown to be dominated by exciton diffusion within the Ir(ppy)3 film to the QD layer. � 2006 Elsevier B.V. All rights reserved.

The challenge of generating crystals of organic materials has been pursued by many research groups who aim to develop materials sets for active electronic and optoelectronic devices, including field-effect transistors [C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99; H.E. Katz, Z. Bao, J. Phys. Chem. B 104 (2000) 671; Y.Y. Lin, D.J. Gundlach, S.F. Nelson, T.N. Jackson, IEEE Trans. Electron Dev. 44 (1997) 1325], photodetectors [P. Peumans, V. Bulovic, S.R. Forrest, Appl. Phys. Lett. 76 (2000) 2650] and optical modulators [S.R. Marder, J.W. Perry, C.P. Yakymyshyn, Chem. Mater. 6 (1994) 1137]. Here we describe a method for in-plane growth of millimeter-scale crystalline organic needles from initially amorphous thin films of the molecular organic semiconductor tris(8-hydroxyquinoline)aluminum (Alq 3 ). The needles form when the vacuum-deposited amorphous films are exposed to chloroform vapor at room temperature and pressure, and can be as large as several microns thick, several microns wide, and one centimeter long (limited in length by the substrate dimensions). As such, the Alq 3 needles are more than 1000 times longer than any previously reported organic crystals formed via a similar solvent-vapor annealing process [F. Iwatsu, T. Kobayashi, N. Uyeda, J. Phys. Chem. 84 (1980) 3223; F. Iwatsu, J. Cryst. Growth 71 (1985) 629; A.M. Hor, R.O. Loutfy, Thin Solid Films 106 (1983) 291; B.A. Gregg, J. Phys. Chem. 100 (1996) 852; J.C. Conboy, E.J.C. Olson, D.M. Adams, J. Kerimo, A. Zaban, B.A. Gregg, P.F. Barbara, J. Phys. Chem. B 102 (1998) 4516; M. Brinkmann, J.C. Wittmann, C. Chaumont, J.J. Andre, Thin Solid Films 292 (1997) 192; F. Toffolo, M. Brinkmann, O. Greco, F. Biscarini, C. Taliani, H.L. Gomes, I. Aiello, M. Ghedini, Synth. Met. 101 (1999) 140]. Moreover, the Alq 3 needles are spatially separated from one another and oriented with their long axes parallel to lithographically pre-defined periodic submicron grooves in the substrate surface.

The luminescence of inorganic core-shell semiconductor nanocrystal quantum dots (QDs) can be tuned across much of the visible spectrum by changing the size of the QDs while preserving a spectral full width at half maximum (FWHM) as narrow as 30 nm and photoluminescence efficiency of 50% [Journal of Physical Chemistry B 101 (46) (1997) 9463] [1] . Organic capping groups, surrounding the QD lumophores, facilitate processing in organic solvents and their incorporation into organic thin film light-emitting device (LED) structures [Nature 370 (6488) (1994) 354] [2] . A recent study has shown that hybrid organic/inorganic QD-LEDs can indeed be fabricated with high brightness and small spectral FWHM, utilizing a phase segregation process which self-assembles CdSe(ZnS) core(shell) QDs onto an organic thin film surface [Nature 420 (6917) (2002) 800] [3] . We now demonstrate that the phase segregation process can be generally applied to the fabrication of QD-LEDs containing a wide range of CdSe particle sizes and ZnS overcoating thicknesses. By varying the QD core diameter from 32 A to 58 A, we show that peak electroluminescence is tuned from 540 nm to 635 nm. Increase in the QD shell thickness to 2.5 monolayers (∼0.5 nm) improves the LED external quantum efficiency, consistent with a Forster energy transfer mechanism of generating QD excited states. In this work we also identify the challenges in designing devices with very thin (∼5 nm thick) emissive layers [Chemical Physics Letters 178 (5–6) (1991) 488] [4] , emphasizing the increased need for precise exciton confinement. In both QD-LEDs and archetypical all-organic LEDs with thin emissive layers, we show that there is an increase in the exciton recombination region width as the drive current density is increased. Overall, our study demonstrates that integration of QDs into organic LEDs has the potential to enhance the performance of thin film light emitters, and promises to be a rich field of scientific endeavor.

We demonstrate a general method for tuning the luminescent emission spectrum of dipolar molecules by adjusting the strength of intermolecular dipole–dipole interactions using a doped guest–host molecular organic thin film system. Spectral shifting is achieved by introducing dopants that change the local electric field in the film. This `solid state solvation effect' is used to continuously tune, by up to 75 nm, the peak emission wavelength of organic light emitting devices consisting of triarylamine host materials doped with polar luminescent dyes DCM2, aluminum tris (8-hydroxyquinoline), or both. Red, orange, yellow, green, blue, and white emission is demonstrated with the same set of organic molecules.

We demonstrate red, orange, and yellow organic light-emitting devices (OLED) with the electroluminescent layer consisting of aluminum tris(8-hydroxyquinoline) (Alq3) doped with laser dye DCM2, with the emission color dependent on the concentration of DCM2. A peak emission wavelength shift of up to 50 nm is found to be due to strong polarization effects. For red and yellow–orange OLEDs, a maximum luminance of 1400 cd/m2 and 15200 cd/m2 is measured, with a luminance of 100 cd/m2 attained at 100 mA/cm2 (14 V) and 10 mA/cm2 (12 V), respectively. The current versus voltage dependencies of these devices are consistent with trap-limited conduction, unaffected by the presence of DCM2.

We describe a novel organic light-emitting device (OLED) in which color, grey scale and intensity can be independently achieved. The device consists of three separately contacted, red-, green-and blue-emitting OLEDs placed in a vertical stack and separated by transparent, conducting electrodes. The stacked OLED (or SOLED) is shown to efficiently generate a high-intensity optical output whose color is a linear superposition of spectra of the individual emitting elements in the device. Optical microcavity effects are found to distort the output spectrum in non-optimized SOLEDs. To our knowledge, the SOLED architecture provides the highest efficiency and resolution, as well as the simplest means for fabricating full-color displays based on small molecular weight organic materials demonstrated to date.

The thermal evolution of organic light emitting device structures and materials has been measured using X-ray specular reflectivity. Thermally induced failure of these structures is found to be due to the large thermal expansion of the hole transport layer (N,N′-diphenyl-N,N′-bis(3-methylphenyl)1,1′-biphenyl-4,4′diamine, or TPD) associated with its low glass transition temperature, suggesting a strain-driven failure mechanism (as opposed to TPD recrystallization). These results suggest the possibility of optimizing organic light emitting devices through direct in-situ measurements of their structure and stability.

Excitonic transitions responsible for low energy absorption and fluorescence in dilute solutions and thin films of the organic molecule 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) have been investigated. Combining the results of concentration-dependent solution and thin film absorption and fluorescence with results on thin film exciton diffusion lengths (presented in the following paper, Paper II), we proposed a comprehensive model for the low energy excitonic states in this archetype molecular crystal. We unambiguously identify an extended free charge transfer state at E F CT = 2.23±0.03 eV which has previously been observed to exhibit effects of quantum confinement in ultrathin layers grown under ultrahigh vacuum. This state self-traps at an energy of E ST CT = 2.11±0.04 eV due to the strong exciton-phonon coupling characteristic of this closely packed organic crystal.

Absorption, photoexcitation and internal and external luminescence efficiencies for Alq3 films grown by vacuum deposition have been measured accurately for the first time. The absorption spectrum measured at wavelength between λ = 250 and 450 nm is due to the overlap of the 1La (385 nm) and 1Bb (260 nm) transition bands found in the solution spectrum of Alq3. The efficiency of the prominent luminescence band centered at λ = 530 nm was observed to be independent of the excitation wavelength in the spectral range between λ = 250 and 450 nm. The internal quantum efficiency was found to be (32 ± 2)% independent of film thickness from 100 A to 1.35 μm, indicating a very short diffusion length for the 1La and 1Bb excited states.

We study excitons in crystalline thin films of the archetype molecular compound: 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) by measuring the photocurrent response under small electric fields ( 4 V/cm). A previously uncharacterized exciton at a wavelength of λ = 590 nm, or at 2.10 eV above the PTCDA highest occupied molecular orbital, is identified. Lower in energy than either of the previously observed charge transfer excitons at λ = 475 and 555 nm in PTCDA, the λ = 590 nm exciton is speculated to be Frenkel-like. The diffusion length of this exciton is 225 ± 15 nm, compared to 88 ± 6 nm for excitons generated between 475 and 555 nm.