Solution-processed Photovoltaics Research Articles

Hybrid organic–inorganic solar cells based on bismuth iodide and 1,6-hexanediammonium dication

Heavy metal toxicity and device instability are prominent limitations in the push for commercialization of photovoltaics based on low-cost, solution-processed materials. Therefore, in this study a hybrid organic–inorganic material containing trivalent bismuth and dicationic 1,6-hexanediammonium was used as the photoactive layer in solution-processed photovoltaics.

Shape-Controlled Narrow-Gap SnTe Nanostructures: From Nanocubes to Nanorods and Nanowires.

The rational design and synthesis of narrow-gap colloidal semiconductor nanocrystals (NCs) is an important step toward the next generation of solution-processable photovoltaics, photodetectors, and thermoelectric devices. SnTe NCs are particularly attractive as a Pb-free alternative to NCs of narrow-gap lead chalcogenides. Previous synthetic efforts on SnTe NCs have focused on spherical nanoparticles. Here we report new strategies for synthesis of SnTe NCs with shapes tunable from highly monodisperse nanocubes, to nanorods (NRs) with variable aspect ratios, and finally to long, straight nanowires (NWs). Reaction at high temperature quickly forms thermodynamically favored nanocubes, but low temperatures lead to elongated particles. Transmission electron microscopy studies of reaction products at various stages of the synthesis reveal that the growth and shape-focusing of monodisperse SnTe nanocubes likely involves interparticle ripening, while directional growth of NRs and NWs may be initiated by particle dimerization via oriented attachment.

Investigation of the Spatially Dependent Charge Collection Probability in CuInS2/ZnO Colloidal Nanocrystal Solar Cells

Solar cells with a heterojunction between colloidal CuInS$_2$ and ZnO nanocrystals are an innovative concept in solution-processed photovoltaics, but the conversion efficiency cannot compete yet with devices employing lead chalcogenide quantum dots. Here, we present a detailed study on the charge collection in CuInS$_2$/ZnO solar cells. An inverted device architecture was utilized, in which the ZnO played an additional role as optical spacer layer. Variations of the ZnO thickness were exploited to create different charge generation profiles within the light-harvesting CuInS$_2$ layer, which strongly affected both the external and internal quantum efficiency. By the reconstruction of these experimental findings with the help of a purely optical model, we were able to draw conclusions on the spatial dependency of the charge collection probability. We provide evidence that only carriers generated within a narrow zone of circa 40 nm near the CuInS$_2$/ZnO interface contribute to the external photocurrent. The remaining part of the absorber can be considered as "dead zone" for charge collection, which reasonably explains the limited device performance and indicates a direction for future research. From the methodical point of view, the optical modeling approach developed in the present work has the advantage that no electrical input parameters are required and is believed to be easily transferable to other material systems.

Sol–gel derived In2S3 buffer layers for inverted organic photovoltaic cells

In2S3 – a wide-gap semiconductor – has been implemented in organic photovoltaics using an all-solution based sol–gel route. Typically, indium sulfide is deployed as a buffer layer in copper–indium sulfide (CIS) photovoltaic systems as electron-selective contact on the bottom electrode. We transferred this idea to organic, solution-processed photovoltaics, exploring its potential in an inverted hybrid device structure. The optical and morphological properties of the films were investigated by UV–Vis transmittance spectroscopy and scanning electron microscopy. The optical studies showed that the In2S3 films exhibit a band gap of ∼2.25eV. The effect of In2S3 film thickness on conversion efficiency of the device was also investigated. The device with the 158±5nm of In2S3 film thickness provides the best performance with an average short-circuit current density (Jsc) of approximately 7.96±0.12mA/cm2, open-circuit voltage (Voc) of 0.609±0.007 V, fill factor (FF) of 0.49±0.014, and power conversion efficiency of 3.04±0.14%.

Exciton generation/dissociation/charge-transfer enhancement in inorganic/organic hybrid solar cells by robust single nanocrystalline LnPxOy (Ln = Eu, Y) doping.

Low-temperature solution-processed photovoltaics suffer from low efficiencies because of poor exciton or electron-hole transfer. Inorganic/organic hybrid solar cell, although still in its infancy, has attracted great interest thus far. One of the promising ways to enhance exciton dissociation or electron-hole transport is the doping of lanthanide phosphate ions. However, the underlying photophysical mechanism remains poorly understood. Herein, by applying femtosecond transient absorption spectroscopy, we successfully distinguished hot electron, less energetic electron, hole transport from electron-hole recombination. Concrete evidence has been provided that lanthanide phosphate doping improves the efficiency of both hot electron and "less energetic" electron transfers from donor to acceptor, but the hole transport almost remains unchanged. In particular, the hot electron transfer lifetime was shortened from 30.2 to 12.7 ps, that is, more than 60% faster than pure TiO2 acceptor. Such improvement was ascribed to the facts that the conduction band (CB) edge energy level of TiO2 has been elevated by 0.2 eV, while the valence band level almost remains unchanged, thus not only narrowing the energy offset between CB levels of TiO2 and P3HT, but also meanwhile enlarging the band gap of TiO2 itself that permits one to inhibit electron-hole recombination within TiO2. Consequently, lanthanide phosphate doped TiO2/P3HT bulk-heterojunction solar cell has been demonstrated to be a promising hybrid solar cell, and a notable power conversion efficiency of 2.91% is therefore attained. This work indicates that lanthanide compound ions can efficiently facilitate exciton generation, dissociation, and charge transport, thus enhancing photovoltaic performance.

Materials interface engineering for solution-processed photovoltaics

Advances in solar photovoltaics are urgently needed to increase the performance and reduce the cost of harvesting solar power. Solution-processed photovoltaics are cost-effective to manufacture and offer the potential for physical flexibility. Rapid progress in their development has increased their solar-power conversion efficiencies. The nanometre (electron) and micrometre (photon) scale interfaces between the crystalline domains that make up solution-processed solar cells are crucial for efficient charge transport. These interfaces include large surface area junctions between photoelectron donors and acceptors, the intralayer grain boundaries within the absorber, and the interfaces between photoactive layers and the top and bottom contacts. Controlling the collection and minimizing the trapping of charge carriers at these boundaries is crucial to efficiency.

New quinoxaline and pyridopyrazine-based polymers for solution-processable photovoltaics

The recently published quinoxaline/thiophene-based polymer TQ1 has been modified on its acceptor unit, either altering the acceptor strength by incorporating a pyridopyrazine, substitution of the acceptor-hydrogens by fluorine, or substitution of the alkoxy side chain by alkyl. The changes in physical, electronic and device properties are discussed. For the polymers incorporating the stronger acceptors a decreased performance is found, where in both cases the current in the devices is compromised. Incorporation of the pyridopyrazine-based acceptor seems to result in more severe or additional loss mechanisms compared to the polymer that incorporates the fluorine atoms. A similar performing material is obtained when changing the alkoxy side chain in TQ1 to an alkyl, where the solar cell performance is mainly improved on the fill factor. It is demonstrated that the standard TQ1 structure is easily modified in a number of ways, showing the versatility and robustness of the standard TQ1 structure and synthesis.

Tuning the Vertical Phase Separation in Polyfluorene:Fullerene Blend Films by Polymer Functionalization

Achieving control over the nanomorphology of blend films of the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester, PCBM, with light-absorbing conjugated polymers is an important challenge in the development of efficient solution-processed photovoltaics. Here, three new polyfluorene copolymers are presented, tailored for enhanced miscibility with the fullerene through the introduction of polymer segments with modified side chains, which enhance the polymer's polar character. The composition of the spincoated polymer:PCBM films is analyzed with dynamic secondary ion mass spectrometry (dSIMS). The dSIMS depth profiles demonstrate compositional variations perpendicular to the surface plane, as a result of vertical phase separation, directed by the substrate. These variations propagate to a higher degree through the film for the polymers with a larger fraction of modified side chains. The surface composition of the films is studied by Near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Quantitative analysis of the NEXAFS spectra through a linear combination fit with the spectra of the pure components yields the surface composition. The resulting blend ratios reveal polymer-enrichment of the film surface for all three blends, which also becomes stronger as the polar character of the polymer increases. Comparison of the NEXAFS spectra collected with two different sampling depths shows that the vertical composition gradient builds up already in the first nanometers underneath the surface of the films. The results obtained with this new series of polymers shed light on the onset of formation of lamellar structures in thin polymer:PCBM films prepared from highly volatile solvents.

Functionalized multiwall carbon nanotubes incorporated polymer/fullerene hybrid photovoltaics

Multiwall carbon nanotubes are introduced into poly(3-hexylthiophene) and [6,6] phenyl C61 butyric acid methyl fullerene, bulk heterojunction organic photovoltaic devices. Utilization of nanotubes requires chemical modifications for compatibility with solution processable photovoltaics. Better dispersions of carbon nanotubes in organic solvents are achieved by acid functionalization of tubes. Pristine and acid treated multiwall carbon nanotubes have been incorporated into the photoactive layer and better results in fill factors of 62% and efficiency of 2.3% are achieved under Air Mass 1.5 Global illumination through the use of acid treated nanotubes.

Infrared photovoltaics made by solution processing

Solution-processed photovoltaics offer a cost-effective path to harvesting the abundant resource that is solar energy. The organic and polymer semiconductors at the heart of these devices generally absorb only visible light; however, half of the Sun's power reaching the Earth's surface lies in the infrared. Flexible solar cells that harvest wavelengths beyond 1 μm were first reported in 2005. In three years they have increased over 10,000-fold in power conversion efficiency. The latest devices achieve power conversion efficiencies in the infrared of more than 4%, values comparable to those of their organic and polymer counterparts in the visible. Here we review the progress and prospects for the field, focusing on new insights into how quantum-dot solar cells operate and how these findings give guidance on optimizing these devices to their full performance potential.

Solar Cells, Photodetectors, and Optical Sources from Infrared Colloidal Quantum Dots

AbstractOptoelectronic devices made via spin‐coating of soft materials onto an arbitrary substrate enable ready integration, low cost, and physical flexibility. The use of solution‐processed colloidal quantum dots offers the added advantage of quantum‐size‐effect tuning of material bandgap. Tuning across the near‐ and short‐wavelength infrared (SWIR) spectral regions enables applications in fiber‐optic communications, night vision and biomedical imaging, and efficient solar energy collection. Here we review progress in infrared solar cells, light sensors, and optical sources based on solution‐processed materials. The latest solution‐processed photovoltaics now provide 4.2% power conversion efficiencies in the infrared, placing them a factor of three away from enabling a doubling in overall solar power conversion efficiency of visible‐wavelength solution‐processed photovoltaics. The best solution‐processed photodetectors now provide sensitivities of 1013 Jones D* (normalized detectivity), exceeding the sensitivity of the best epitaxially grown SWIR photodetectors. Infrared optical sources, both broadband light‐emitting diodes and, more recently, lasers, have now also been reported at 1.5 µm.

Solution-Processed Infrared Optoelectronics: Photovoltaics, Sensors, and Sources

The fabrication of optoelectronic devices via spin-coating onto an arbitrary substrate offers ease of integration, low cost, and physical flexibility. Reports of active optoelectronic devices operating in the IR, and made via solution-processing, emerged in the early 2000s. Here, we review progress in IR solar cells, image sensors, and optical sources based on solution-processed materials. The latest solution-processed photovoltaics (PV) now provide 4.2% power conversion efficiencies in the IR, placing them a factor of 3 away from enabling a doubling in overall solar power conversion efficiency of today's best visible-wavelength solution-processed PV. The best solution-processed photodetectors now provide sensitivities in the 1013 Jones D* range, exceeding the sensitivity of the best epitaxially grown short-wavelength infrared photodetectors. Infrared optical sources, both broadband light-emitting diodes and, more recently, lasers, have now also been reported at 1.5 mum. We review the progress and future prospects of this rapidly advancing field.

Efficient solution-processed infrared photovoltaic cells: Planarized all-inorganic bulk heterojunction devices via inter-quantum-dot bridging during growth from solution

Solution-processed thin-film organic, inorganic, and hybrid photovoltaic devices have achieved power conversion efficiencies as high as 5%. However, these devices remain limited by their capture of visible energy; more than a half of the sun’s power lies in the infrared. Herein the authors demonstrate photovoltaic devices effective across the visible and all the way out to 1700nm. Only through the use of ethanedithiol as a bridging molecule to affect interparticle linking were they able to achieve fabrication of smooth, continuous quantum dot films on rough, high-surface area transparent metal oxides. This allowed them to increase light absorption while maintaining efficient charge separation and extraction and at the same time avoiding electrical short circuits. They obtained monochromatic infrared power conversion efficiencies of 1.3%, a 50-fold gain over the previous published record of 0.025% in IR solution-processed photovoltaics. The authors demonstrate quantum size-effect tuning of device band gaps relevant to multijunction solar cells.