Ink formulation and low‐temperature incorporation of sodium to yield 12% efficient Cu(In,Ga)(S,Se)2 solar cells from sulfide nanocrystal inks

Solution phase deposition methods offer great potential for low‐cost photovoltaic device fabrication. We have previously developed a method for copper indium gallium disulfoselenide (CIGSSe) device fabrication based on drop‐casting copper indium gallium disulfide (CIGS) nanocrystals in a toluene or hexane‐based ink followed by chalcogen exchange in elemental selenium vapor at 500 °C. By starting with the chalcopyrite or sphaelerite phase of CIGS nanocrystals with controlled stoichiometry, superior composition uniformity can be achieved inherently. Here, we present a dramatic improvement in ink formulation using alkanethiol as the solvent, which enables the ability to create uniform nanocrystal coatings over large areas using a simple knife coating technique. In addition, we show a major improvement in device performance by a simple and low‐temperature method of incorporating sodium into the CIGSSe film based on soaking the films in aqueous NaCl solution. The addition of sodium plays an important role in improving the structural properties of the resulting CIGSSe films, where large and densely packed grain can be obtained. The improved film morphology significantly reduces recombination losses in the resulting device leading to a dramatically enhanced device performance. With the use of standard glass/Mo/CIGSSe/CdS/i‐ZnO/ITO device structure, photovoltaic devices yield total area power conversion efficiency as high as 12.0% under AM1.5 illumination without an anti‐reflection coating. Copyright © 2012 John Wiley & Sons, Ltd.     

High‐efficiency copper indium gallium diselenide (CIGS) solar cells with indium sulfide buffer layers deposited by atomic layer chemical vapor deposition (ALCVD)

This paper presents optimization studies on the formation of indium sulfide buffer layers for high‐efficiency copper indium gallium diselenide (CIGS) thin‐film solar cells with atomic layer chemical vapour deposition (ALCVD) from separate pulses of indium acetylacetonate and hydrogen sulfide. A parametric study of the effect of deposition temperature between 160° and 260°C and thickness (15–30 nm) shows an optimal value at about 220°C for a layer thickness of 30 nm, leading to an efficiency of 16·4%. Analysis of the device shows that indium sulfide layers are characterised by an improvement of the blue response of the cells compared with a standard CdS‐processed cell, due to a high apparent band gap (2·7–2·8 eV), higher open‐circuit voltages (up to 665 mV) and fill factor (78%). This denotes high interface quality. Atomic diffusion processes of sodium and copper in the buffer layer are demonstrated. Copyright © 2003 John Wiley & Sons, Ltd.     

High photo‐conversion efficiency in double‐graded Cu(In,Ga)(S,Se)2 thin film solar cells with two‐step sulfurization post‐treatment

Sulfur is extensively used to increase the bandgap of Cu(In,Ga)(S,Se)₂ (CIGSSe) solar cells and to improve the open circuit voltage (V_OC ) in order to optimize the characteristics of the devices. This study uses a sulfurization process to obtain a double‐graded bandgap profile. Selenization was carried out on Cu(In,Ga) precursors, followed by one sulfurization process or two consecutive sulfurization processes on top of the CIGSe absorber layer surface. The optimum two‐step sulfurization process provides an increase of V_OC of 0.05 V and an improvement of conversion efficiency of 1.17%. The efficiency of the 30 × 30 cm² monolithic module, which has 64 CIGS cells connected in series (aperture area: 878.6 cm²), is 15.85%. The optical and electrical properties of the phase and the work function distribution were investigated using the depth profiles of the absorber layer as a function of the sulfurization conditions. The CIGSSe thin film formed by two‐step sulfurization with a high sulfur concentration exhibits a single work function peak, better crystallinity, and higher conversion efficiency than those of the thin film formed by two‐step sulfurization at low sulfur concentration. In terms of the Raman spectra depth profile, the phase areas for the CIGSSe thin film that underwent the optimized high sulfur concentration two‐step‐sulfurization appeared to have less of Cu_2‐xSe phase than that with low sulfur concentration. Consequently, surface and interface phase analysis is an essential consideration to improve cell efficiency. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.     

A comparative study of Cd‐ and Zn‐compound buffer layers on Cu(In1−x,Gax)(Sy,Se1−y)2 thin film solar cells

This paper reports a comparative study of Cu(In,Ga)(S,Se)₂ (CIGSSe) thin‐film solar cells with CBD‐CdS, CBD‐ZnS(O,OH) and ALD‐Zn(O,S) buffer layers. Each buffer layer was deposited on CIGSSe absorber layers which were prepared by sulfurization after selenization (SAS) process by Solar Frontier K. K. Cell efficiencies of CBD‐CdS/CIGSSe, CBD‐ZnS(O,OH)/CIGSSe and ALD‐Zn(O,S)/CIGSSe solar cells exceeded 18%, for a cell area of 0.5 cm². The solar cells underwent a heat‐light soaking (HLS) post‐treatment at 170 °C under one‐sun illumination in the air; among the three condtions, the ALD‐Zn(O,S)/CIGSSe solar cells showed the highest cell efficiency of 19.78% with the highest open‐circuit voltage of 0.718 V. Admittance spectroscopy measurements showed a shift of the N₁ defect's energy position toward shallower energy positions for ALD‐Zn(O,S)/CIGSSe solar cells after HLS post‐treatment, which is in good agreement with their higher open‐circuit voltage and smaller interface recombination than that of CBD‐ZnS(O,OH)/CIGSSe solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Molecule Charge Transfer Doping for p‐Channel Solution‐Processed Copper Oxide Transistors

The doping of semiconductors plays a critical role in improving the performance of modern electronic devices by precisely controlling the charge carrier density. However, the absence of a stable doping method for p‐type oxide semiconductors has severely restricted the development of metal oxide‐based transparent p–n junctions and complementary circuits. Here, an efficient and stable doping process for p‐type oxide semiconductors by using molecule charge transfer doping with tetrafluoro‐tetracyanoquinodimethane (F₄TCNQ) is reported. The selections of a suitable dopant and geometry play a crucial role in the charge‐transfer doping effect. The insertion of a F₄TCNQ thin dopant film (2–7 nm) between a Au source‐drain electrode and solution‐processed p‐type copper oxide (Cu_xO) film in bottom‐gate top‐contact thin‐film transistors (TFTs) provides a mobility enhancement of over 20‐fold with the desired threshold voltage adjustment. By combining doped p‐type Cu_xO and n‐type indium gallium zinc oxide TFTs, a solution‐processed transparent complementary metal‐oxide semiconductor inverter is demonstrated with a high gain voltage of 50. This novel p‐doping method is expected to accelerate the development of high‐performance and reliable p‐channel oxide transistors and has the potential for widespread applications.     

Nanoparticle‐enhanced light trapping in thin‐film silicon solar cells

A systematic investigation of the nanoparticle‐enhanced light trapping in thin‐film silicon solar cells is reported. The nanoparticles are fabricated by annealing a thin Ag film on the cell surface. An optimisation roadmap for the plasmon‐enhanced light‐trapping scheme for self‐assembled Ag metal nanoparticles is presented, including a comparison of rear‐located and front‐located nanoparticles, an optimisation of the precursor Ag film thickness, an investigation on different conditions of the nanoparticle dielectric environment and a combination of nanoparticles with other supplementary back‐surface reflectors. Significant photocurrent enhancements have been achieved because of high scattering and coupling efficiency of the Ag nanoparticles into the silicon device. For the optimum light‐trapping scheme, a short‐circuit current enhancement of 27% due to Ag nanoparticles is achieved, increasing to 44% for a “nanoparticle/magnesium fluoride/diffuse paint” back‐surface reflector structure. This is 6% higher compared with our previously reported plasmonic short‐circuit current enhancement of 38%. Copyright © 2011 John Wiley & Sons, Ltd.     

Molecular Gelation‐Induced Functional Phase Separation in Polymer Film for Energy Transfer Spectral Conversion

A new strategy for creating the energy transfer spectral conversion thin film by using fluorophore‐functionalized molecular gelation is proposed. This is based on the facts that nanofibrillar phase separation of the self‐assembling pyrene derivative as a fluorophore is formed in a bulk polymer‐containing organic gel, and consequently that the phase‐separated nano domain in a polymer thin film is enough small to keep the transparency but also extremely high Storks shift is gained by efficient excimer formation through highly ordered stacking among the pyrene moieties. When the phase separation‐mediated functional polymer is applied as spectral conversion films (SCFs) for copper–indium–gallium–selenide (CIGS) solar cell, the SCF‐covered solar cell exhibits significant improvement of power conversion efficiency by increase of photocurrent. In this paper, the FRET efficiency and emission wavelength are also demonstrated to be thermotropically switchable since order‐to‐disordered transitions are essential characteristics of as non‐covalent low molecular assembling.     

A Site‐Specific Charge Carrier Control in Monolithic Integrated Amorphous Oxide Semiconductors and Circuits with Locally Induced Optical‐Doping Process

Amorphous oxide semiconductor (AOS) thin film transistors (TFTs) have found cutting‐edge applications in sensor technologies. To reduce manufacturing costs, sensors, analog front end, and digital signal processing circuits need to be integrated on the identical substrate. Unlike traditional silicon‐based devices, optimizations for locally controllable electrical parameters of the AOSs have rarely been investigated. Here, photoactivated combustion reduction is utilized as doping motivation for solution‐processed amorphous indium–gallium–zinc oxide (a‐IGZO) to tune their electrical performance. By controlling parameters of a‐IGZO TFTs, which can be partly doped with covering the desired area of the identical substrate, it is possible to match the particular threshold voltage for various circuits. For circuit optimization, automatic integrated circuit modeling spice is carried out to find the best match of the complementary metal–oxide semiconductor circuits. Finally, the site‐specific performance of switching TFTs, amplifiers, and ring oscillators implemented with low‐temperature solution‐processed a‐IGZO and p‐type single‐walled carbon nanotube TFTs is demonstrated. The optical‐doped a‐IGZO TFTs exhibiting a saturation mobility of >9.15 cm² V^?1 s^?1 with a locally tunable threshold voltage of ?5 – 1.5 V are realized, enabling monolithic integration of functional devices. The resultant circuits demonstrate excellent amplification of 24 dB and an oscillation frequency of 12 kHz for 7‐stage ring oscillators.     

Effects of combined additional indium deposition and potassium fluoride post‐deposition treatments on Cu(In,Ga)Se2 thin film solar cells

The effect of additional indium on copper indium gallium selenide (CIGS) thin films and solar cells was investigated with respect to potassium fluoride post‐deposition treatment (KF‐PDT) using current‐voltage, external quantum efficiency, scanning electron microscopy, X‐ray photoelectron spectroscopy, time‐resolved photoluminescence and capacitance‐voltage measurements. The cell performance, particularly open‐circuit voltage (V _oc) improved drastically by the combined treatments of additional indium deposition after CIGS growth and subsequent KF‐PDT. A Cu deficient layer at the CIGS surface increased after both treatments rather than only KF‐PDT. Photoluminescence intensity, lifetime and net carrier concentration of KF‐untreated CIGS solar cells did not change significantly by only additional indium deposition. However, they improved because of the combined treatments. Copyright © 2017 John Wiley & Sons, Ltd.     

A Facile Solution‐Doping Method to Improve a Low‐Temperature Zinc Oxide Precursor: Towards Low‐Cost Electronics on Plastic Foil

Optimization of thin‐film transistors performance is usually accompanied by an increase of the process temperature. This work presents a method to raise the field effect mobility by a factor of 3 without a change of the process parameters. The modification involves a solution doping process where an ammine zinc complex is formed in the presence of metal ions of the 13^th group, namely gallium and indium. Morphological studies, including scanning electron microscopy and atomic force microscopy, reveal the difference among the resulting films. Moreover, X‐ray diffraction results show that the doping affects the preferred orientation of the zinc oxide crystals in the resulting film. The electrical properties vary distinctly and are best for a solution doped with both gallium and indium. With a double‐layer system the performance of this new precursor exceeds field effect mobility values of 1 cm² V^?1 s^?1 after a maximum process temperature of 160 °C.     

A 21.5% efficient Cu(In,Ga)Se2 thin‐film concentrator solar cell

Cu(In,Ga)Se₂ (CIGS) solar cells have been designed for operation under mildly concentrated sunlight. The absorber was deposited via a three‐stage evaporation process that has consistently yielded high‐performance one‐sun devices. The device structure reported here was modified by reducing the thickness of the CdS window/buffer layer to enhance the short‐circuit current at the expense of the open‐circuit voltage. Operation of the devices under optical enhancement leads to significant increases in the voltage and fill factor. At 14 suns, the open‐circuit voltage for this device was 736 mV, the fill factor was 80.5%, and the efficiency was 21.5%. This result represents the first report of a polycrystalline thin‐film solar cell with an efficiency in excess of 20%. Published in 2002 by John Wiley & Sons, Ltd.     

Perovskite‐Based Tandem Solar Cells: Get the Most Out of the Sun

Tandem solar cells (TSCs) comprising stacked narrow‐bandgap and wide‐bandgap subcells are regarded as the most promising approach to break the Shockley–Queisser limit of single‐junction solar cells. As the game‐changer in the photovoltaic community, organic–inorganic hybrid perovskites became the front‐runner candidate for mating with other efficient photovoltaic technologies in the tandem configuration for higher power conversion efficiency, by virtue of their tunable and complementary bandgaps, excellent photoelectric properties, and solution processability. In this review, a perspective that critically dilates the progress of perovskite material selection and device design for perovskite‐based TSCs, including perovskite/silicon, perovskite/copper indium gallium selenide, perovskite/perovskite, perovskite/CdTe, and perovskite/GaAs are presented. Besides, all‐inorganic perovskite CsPbI₃ with high thermal stability is proposed as the top subcell in TSCs due to its suitable bandgap of ≈1.73 eV and rapidly increasing efficiency. To minimize the optical and electrical losses for high‐efficiency TSCs, the optimization of transparent electrodes, recombination layers, and the current‐matching principles are highlighted. Through big data analysis, wide‐bandgap perovskite solar cells with high open‐circuit voltage (V_oc) are in dire need in further study. In the end, opportunities and challenges to realize the commercialization of TSCs, including long‐term stability, area upscaling, and mitigation of toxicity, are also envisioned.     

Combinatorial study of NaF addition in CIGSe films for high efficiency solar cells

We report on a sodium fluoride (NaF) thickness variation study for the H₂Se batch furnace selenization of sputtered Cu(In,Ga) films in a wide range of Cu(In,Ga) film compositions to form Cu(In,Ga)Se₂ (CIGSe) films and solar cells. Literature review indicates lack of consensus on the mechanisms involved in Na altering CIGSe film properties. In this work, for sputtered and batch‐selenized CIGSe, NaF addition results in reduced gallium content and an increase in grain size for the top portion of the CIGSe film, as observed by scanning electron microscopy and secondary ion mass spectrometry. The addition of up to 20 nm of NaF resulted in an improvement in all relevant device parameters: open‐circuit voltage, short‐circuit current, and fill factor. The best results were found for 15 nm NaF addition, resulting in solar cells with 16.0% active‐area efficiency (without anti‐reflective coating) at open‐circuit voltage (V_OC) of 674 mV. Copyright © 2013 John Wiley & Sons, Ltd.     

Laterally Ordered Bulk Heterojunction of Conjugated Polymers: Nanoskiving a Jelly Roll

This paper describes the fabrication of a nanostructured heterojunction of two conjugated polymers by a three‐step process: i) spin‐coating a multilayered film of the two polymers, ii) rolling the film into a cylinder (a “jelly roll”) and iii) sectioning the film perpendicular to the axis of the roll with an ultramicrotome (nanoskiving). The conjugated polymers are poly(benzimidazobenzophenanthroline ladder) (BBL, n‐type) and poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV, p‐type). The procedure produces sections with an interdigitated junction of the two polymers. The spacing between the phases is determined by spin‐coating (～15 nm to 100 nm) and the thickness of each section is determined by the ultramicrotome (100 to 1000 nm). The minimum width of the MEH‐PPV layers accessible with this technique (～15 nm) is close to reported exciton diffusion lengths for the polymer. When placed in a junction between two electrodes with asymmetric work functions (tin‐doped indium oxide (ITO) coated with poly(3,4‐ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS), and eutectic gallium‐indium, EGaIn) the heterostructures exhibit a photovoltaic response under white light, although the efficiency of conversion of optical to electrical energy is low. Selective excitation of BBL with red light confirms that the photovoltaic effect is the result of photoinduced charge transfer between BBL and MEH‐PPV.     

Bendable Solar Cells from Stable,Flexible, and Transparent Conducting Electrodes Fabricated Using a Nitrogen‐Doped Ultrathin Copper Film

Copper has attracted significant interests as an abundant and low‐cost alternative material for flexible transparent conducting electrodes (FTCEs). However, Cu‐based FTCEs still present unsolved technical issues, such as their inferior light transmittance and oxidation durability compared to conventional indium tin oxide (ITO) and silver metal electrodes. This study reports a novel technique for fabricating highly efficient FTCEs composed of a copper ultrathin film sandwiched between zinc oxides, with enhanced transparency and antioxidation performances. A completely continuous and smooth copper ultrathin film is fabricated by a simple room‐temperature reactive sputtering process involving controlled nitrogen doping (<1%) due to a dramatic improvement in the wettability of copper on zinc oxide surfaces. The electrode based on the nitrogen‐doped copper film exhibits an optimized average transmittance of 84% over a spectral range of 380 ?1000 nm and a sheet resistance lower than 20 Ω sq^?1, with no electrical degradation after exposure to strong oxidation conditions for 760 h. Remarkably, a flexible organic solar cell based on the present Cu‐based FTCE achieves a power conversion efficiency of 7.1%, clearly exceeding that (6.6%) of solar cells utilizing the conventional ITO film, and this excellent performance is maintained even in almost completely bent configurations.     

Inorganic Macroporous Films from Preformed Nanoparticles and Membrane Templates: Synthesis and Investigation of Photocatalytic and Photoelectrochemical Properties

Colloidal dispersions of titania, zirconia, tin oxide, indium oxide, and ceria have been successfully used to impregnate membrane templates and form the respective metal oxide (MO) porous films. The use of alumina and iron oxide sols in the same procedure, however, resulted in compact structures. By mixing different nanoparticle solutions before impregnation, final inorganic films containing two metal oxides, of variable metal oxide ratios, were obtained. The porous inorganic materials were analyzed in terms of surface area, pore size, film thickness, and crystallinity. The mechanism of nanoparticle infiltration and particle adsorption to the template walls is proposed based on the stability of the inorganic film and a study of the influence of either the sol concentration or washing times on the amount of inorganic substance incorporated in the hybrid material. The photocatalytic decomposition of an organic pollutant, 2‐chlorophenol, was demonstrated for the porous titania material along with the structures containing mixtures of titania with zirconia, indium oxide, and tin oxide. A ratio of 9:1 TiO₂/MO gave the highest photocatalytic activity, which was higher than the activity of Degussa P25 for the TiO₂/In₂O₃ and TiO₂/SnO₂ systems under the same conditions. The titania films have also been attached to substrates—glass or indium tin oxide (ITO) surfaces—and the photoelectrochemical properties of the porous film attained. A comparison with a spin‐coated titania film (prepared from the same colloidal dispersion) showed that the structured porous inorganic film has two times the photoelectrochemical efficiency as the spin‐coated film.     

Surface engineering in Cu(In,Ga)Se2 solar cells

Surface modifications of three‐stage co‐evaporated Cu(In,Ga)Se₂ (CIGS) thin films are investigated by finishing the evaporation with gallium‐free (CuInSe₂, CIS) stages of various lengths. Secondary‐ion mass spectrometry shows substantial interdiffusion of indium and gallium, smearing out the Ga/(Ga + In) profile so that the addition of a CIS layer merely lowers the gallium content at the surface. For the thinnest top layer, equivalent to 20 nm of pure CIS, X‐ray photoelectron spectroscopy does not detect any compositional difference compared with the reference device. The modifications are evaluated electrically both by temperature‐dependent characterisation of actual solar‐cell devices and by modelling, using the latest version of scaps‐1d (Electronics and Information Systems, Ghent University, Belgium). The best solar‐cell device from this series is obtained for the 20 nm top layer, with an efficiency of 16.6% after antireflective coating. However, we observe a trend of decreasing open‐circuit voltage for increasingly thick top layers, and we do not find direct evidence that the lowering of the gallium concentration at the CIGS surface should generally be expected to improve the device performance. A simulated device with reduced bulk and interface defect levels achieves nearly 20% efficiency, but the trends concerning the CIS top layer remain the same. Copyright © 2011 John Wiley & Sons, Ltd.     

Gallium gradients in Cu(In,Ga)Se2 thin‐film solar cells

The gallium gradient in Cu(In,Ga)Se₂ (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co‐evaporation processes, plays a key role in the device performance of CIGS thin‐film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X‐ray measurements. In addition, the gallium grading of a CIGS layer grown with an in‐line co‐evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth‐dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co‐evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi‐Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross‐sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper‐vacancy‐mediated indium and gallium diffusion in CuInSe₂ and CuGaSe₂ were calculated using density functional theory. The migration barrier for the In_Cu antisite in CuGaSe₂ is significantly lower compared with the Ga_Cu antisite in CuInSe₂, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co‐evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Ionic Liquid Activation of Amorphous Metal‐Oxide Semiconductors for Flexible Transparent Electronic Devices

Amorphous metal‐oxide semiconductors offer the high carrier mobilities and excellent large‐area uniformity required for high performance, transparent, flexible electronic devices; however, a critical bottleneck to their widespread implementation is the need to activate these materials at high temperatures which are not compatible with flexible polymer substrates. The highly controllable activation of amorphous indium gallium zinc oxide semiconductor channels using ionic liquid gating at room temperature is reported. Activation is controlled by electric field‐induced oxygen migration across the ionic liquid‐semiconductor interface. In addition to activation of unannealed devices, it is shown that threshold voltages of a transistor can be linearly tuned between the enhancement and depletion modes. Finally, the first ever example of transparent flexible thin film metal oxide transistor on a polyamide substrate created using this simple technique is demonstrated. This study demonstrates the potential of field‐induced activation as a promising alternative to traditional postdeposition thermal annealing which opens the door to wide scale implementation into flexible electronic applications.     

Polymer Brush Guided Formation of Conformal,Plasmonic Nanoparticle‐Based Electrodes for Microwire Solar Cells

This report explores the use of sacrificial thin polymer films prepared by surface‐initiated polymerization as a template for the fabrication of highly conformal metal nanoparticle solar cell electrodes. As a first proof‐of‐principle, the use of this method is demonstrated to prepare top electrodes on planar and microwire‐based silicon solar cell devices. These metal nanoparticle films are dual functional in that they not only mediate charge transport, but also enhance light capture due to the plasmonic scattering properties of the nanoparticles. Solar cells with a conformal silver nanoparticle‐based electrode layer show short circuit currents that are 46% higher as compared to those exhibit by devices coated with standard indium tin oxide as the electrode. It is anticipated that this methodology will contribute to novel electrode concepts in the next generation solar cells.