Flexible a‐Si:H/nc‐Si:H tandem thin film silicon solar cells on plastic substrates with i ‐layers made by hot‐wire CVD

In this letter we report the result of an a‐Si:H/nc‐Si:H tandem thin film silicon solar mini‐module fabricated on plastic foil containing intrinsic silicon layers made by hot‐wire CVD (efficiency 7.4%, monolithically series‐connected, aperture area 25 cm²). We used the Helianthos cell transfer process. The cells were first deposited on a temporary aluminum foil carrier, which allows the use of the optimal processing temperatures, and then transferred to a plastic foil. This letter reports the characteristics of the flexible solar cells obtained in this manner, and compares the results with those obtained on reference glass substrates. The research focus for implementation of the hot‐wire CVD technique for the roll‐to‐roll process is also discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced stability of low temperature processed perovskite solar cells via augmented polaronic intensity of hole transporting layer

The commercial mass production of perovskite solar cells requires full compatibility with roll‐to‐roll processing with enhanced device stability. In line with this, the present work addresses following issues simultaneously from multiple fronts: (i) low temperature processed (140 °C) ZnO is used as electron transport layer (ETL) for fabricating the mixed organic cation based perovskite solar cells, (ii) the expensive hole transporting layer (HTL) spiro‐OMeTAD is replaced with F4TCNQ doped P3HT and (iii) the fabrication method does not incorporate the dopant TBP which is known to induce degradation processes in perovskite layer. All the devices under study were fabricated in ambient conditions. The F4TCNQ doped P3HT (HTL) based devices exhibits 14 times higher device stability compared to the conventional Li‐TFSI/TBP doped P3HT devices. The underlying mechanism behind the enhanced device lifetime in F4TCNQ doped P3HT (HTL) based devices was investigated via in‐depth electronic, ionic and polaronic characterization. The enhanced polaronic property in F4TCNQ doped P3HT HTL device ascertains its superior hole extraction and electron blocking capability; and consequently higher stability retained even after a month of ageing.

     

Development of organic light‐emitting diodes for electro‐optical integrated devices

Organic light‐emitting diodes (OLEDs) are discussed for electro‐optical integrated devices that are used for optical signal transmission. Organic optical devices including polymeric optical fibers are used for optical communication applications to realize polymeric electro‐optical integrated devices. The OLEDs were fabricated by vacuum process, i.e. the organic molecular beam deposition (OMBD) technique or a solution process on a polymeric or a glass substrate, for comparison. Optical signals faster than 100 MHz have been created by applying pulsed voltage directly to the OLED utilizing rubrene doped in 8‐hydoxyquinolinum aluminum (Alq₃), as an emissive layer. OLEDs fabricated by solution process utilizing rubrene doped in carrier‐transporting materials have also discussed. OLEDs utilizing polymeric materials by solution process are also fabricated and discussed. Moving‐picture signals are transmitted utilizing both vacuum‐ and solution‐processed OLEDs, respectively.     

Radial junction silicon solar cells with micro-pillar array and planar electrode interface for improved photon management and carrier extraction

We demonstrate radial p-n junction silicon solar cells with micro-pillar array with higher short-circuit current and open-circuit voltage than comparable planar silicon solar cells. Micro-pillar array, fabricated by RIE, acts as an effective anti-reflection coating for visible light with less than 6% reflection. Compared to devices with planar surface, devices with micro-pillar array show a 27% enhancement in short circuit current. The radial p-n junction of the micro-pillars also improves extraction probability of the photogenerated carriers, which further increases the short circuit current. Typically, micro-pillar solar cells suffer from high recombination losses at the Si/metal interface, resulting in poor V_OC. Our devices prevent these recombination losses by planarizing the Si/metal interface, leading to an open circuit voltage of 622?mV, the highest ever reported for micro-pillar solar cells. This planarized contact also reduces the series resistance associated with radial junctions, leading to series resistance of ≤0.50?Ω-cm² and fill factors up to 76.7%.     

A luminescent solar concentrator with 7.1% power conversion efficiency

The Luminescent Solar Concentrator (LSC) consists of a transparent polymer plate, containing luminescent particles. Solar cells are connected to one or more edges of the polymer plate. Incident light is absorbed by the luminescent particles and re‐emitted. Part of the light emitted by the luminescent particles is guided towards the solar cells by total internal reflection. Since the edge area is smaller than the receiving one, this allows for concentration of sunlight without the need for solar tracking. External Quantum Efficiency (EQE) and current–voltage (I –V) measurements were performed on LSC devices with multicrystalline silicon (mc‐Si) or GaAs cells attached to the sides. The best result was obtained for an LSC with four GaAs cells. The power conversion efficiency of this device, as measured at European Solar Test Installation laboratories, was 7.1% (geometrical concentration of a factor 2.5). With one GaAs cell attached to one edge only, the power efficiency was still as high as 4.6% (geometrical concentration of a factor 10). To our knowledge these efficiencies are among the highest reported for the LSC. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Three dimensional a-Si:H thin-film solar cells with silver nano-rod back electrodes

We present three dimensional (3-D) amorphous silicon (a-Si:H) thin-film solar cells with silver nano-rods as back electrodes, which are fabricated by low cost nano imprint lithography (NIL). After conformal deposition of thin metal and semiconductor layers, we can achieve a dome-shaped geometry, which is shown to be effective in reducing the reflectance at the front surface due to the graded refractive index effect. In addition, the enhancement of the diffused reflectance over a broad wavelength in this dome-shaped geometry provides light trapping due to the increase in the effective light propagation length. Using this 3-D solar cell, we achieved 54% increase in short circuit current density and 45% increase in the conversion efficiency compared to the control cells with flat Ag surfaces. This 3-D structure can be also used for improving light harvesting in various photovoltaic devices regardless of materials and structures.     

n‐type silicon solar cells with surface‐passivated screen‐printed aluminium‐alloyed rear emitter

Aluminium‐doped p‐type (Al‐p⁺) silicon emitters fabricated by means of a simple screen‐printing process are effectively passivated by plasma‐enhanced chemical‐vapour deposited amorphous silicon (a‐Si). We measure an emitter saturation current density of only 246 fA/cm², which is the lowest value achieved so far for a simple screen‐printed Al‐p⁺ emitter on silicon. In order to demonstrate the applicability of this easy‐to‐fabricate p⁺ emitter to high‐efficiency silicon solar cells, we implement our passivated p⁺ emitter into an n⁺np⁺ solar cell structure. An independently confirmed conversion efficiency of 19.7% is achieved using n‐type phosphorus‐doped Czochralski‐grown silicon as bulk material, clearly demonstrating the high‐efficiency potential of the newly developed a‐Si passivated Al‐p⁺ emitter. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An arrayed nanoantenna for broadband light emission and detection

We suggest a broadband optical unidirectional arrayed nanoantenna consisting of equally spaced nanorods of gradually varying length. Each nanorod can be driven by near‐field quantum emitters radiating at different frequencies or, according to the reciprocity principle, by an incident light at the same frequency. Broadband unidirectional emission and reception characteristics of the nanoantenna open up novel opportunities for subwavelength light manipulation and quantum communication, as well as for enhancing the performance of photoactive devices such as photovoltaic detectors, light‐emitting diodes, and solar cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The comparison of ZnO nanowire detectors working under two wavelengths of ultraviolet

Schottky-barrier ultraviolet (UV) detectors based on ZnO-nanowires (NWs) were fabricated with Pt as electrodes in this investigation. The ZnO NWs synthesized by the hydrothermal method were characterized by field-emission scanning electron microscopy (FE-SEM), Raman and PL spectroscopy. Photoelectric properties under 254 and 365 nm UV light were investigated. It is found that the photo-response properties of the devices under 365 nm UV light are better than those under 254 nm UV light, which is further illustrated by light transmission theory, energy-band diagram and absorption spectra. The results demonstrate that ZnO NWs detectors with selectivity to near-UV (NUV) light are promising candidates in photoelectric devices.     

Fabrication of three‐dimensional photonic crystals in quantum‐dot‐based materials

Controlling spontaneous emission (SE) is of fundamental importance to a diverse range of photonic applications including but not limited to quantum optics, low power displays, solar energy harvesting and optical communications. Characterized by photonic bandgap (PBG) property, three‐dimensional (3D) photonic crystals (PCs) have emerged as a promising synthetic material, which can manipulate photons in much the same way as a semiconductor does to electrons. Emission tunable nanocrystal quantum dots (QDs) are ideal point sources to be embedded into 3D PCs towards active devices. The challenge however lies in the combination of QDs with 3D PCs without degradation of their emission properties. Polymer materials stand out for this purpose due to their flexibility of incorporating active materials. Combining the versatile multi‐photon 3D micro‐fabrication techniques, active 3D PCs have been fabricated in polymer‐QD composites with demonstrated control of SE from QDs. With this milestone novel miniaturized photonic devices can thus be envisaged.     

Semiconducting P3HT microstructures: fibres and tubes obtained from macroporous silicon template

Microtubes and microfibres composed of poly(3‐hexylthiophene) (P3HT) were fabricated by melt‐assisted templates using ordered macroporous silicon. We have studied the influence of the pore depth and the template type on the microstructure fabrication, where the templates were membranes or structures opened only at one end. Current– voltage (I–V) measurements demostrated that the resistivity of these P3HT microstructures was in the same order as that of homogeneous films, which allows them to be used in electronic devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of CBO and defect states of CZTSe solar cells prepared by using two-step process

We fabricated kesterite Cu₂ZnSnSe₄ (CZTSe) solar cells and studied device characteristics, where CZTSe absorbers were made by using two-step process. First, we deposited precursor CZTSe films with spin-coating or sputtering, and performed sulfurization and subsequent selenization. To complete the device, we applied In₂S₃ as a buffer layer. We obtained power conversion efficiency (PCE) of 4.18% with spin-coated CZTSe absorber and 5.60% with sputtered CZTSe absorber. Both devices showed deep defects in the bulk and strong interface recombinations near the pn junction. In addition, we observed red-kinks in the current density-voltage (J-V) curves for both devices under the filtered light illumination (>660 nm), which is attributed to large conduction band offset (CBO) between the CZTSe absorber and the buffer layer and defect states in the buffer/CZTSe absorber or in the buffer. The red-kink was also observed in CZTSe (PCE of 7.76%) solar cell with CdS buffer. Hence, to enhance the PCE with CZTSe absorber, along with suppression of deep defects which act as recombination center, optimization of CBO between absorber and buffer is also required.     

Advances in stable and flexible perovskite solar cells

Roll-to-roll (R2R) production is an innovative approach and is fast becoming a very popular industrial method for high throughput and mass production of solar cells. Replacement of costly indium tin oxide (ITO), which conventionally has served as the transparent electrode would be a great approach for roll to roll production of flexible cost effective solar cells. Indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) are brittle and ultimately limit the device flexibility. Perovskite solar cells (PSCs) have been the centre of photovoltaic research community during the recent years owing to its exceptional performance and economical prices. The best reported PSCs fabricated by employing mesoporous TiO₂ layers require elevated temperatures in the range of 400–500 °C which limits its applications to solely glass substrates. In such a scenario developing flexible PSCs technology can be considered a suitable and exciting arena from the application point of view, them being flexible, lightweight, portable, and easy to integrate over both small, large and curved surfaces.     

A localized surface plasmon resonance and light confinement‐enhanced near‐infrared light photodetector

Light manipulation is paramountly important to the fabrication of high‐performance optoelectronic devices such as solar cells and photodetectors. In this study, a high‐performance near‐infrared light nanophotodetector (NIRPD) was fabricated based on a germanium nanoneedles array (GeNNs array) with strong light confining capability, and single‐layer graphene (SLG) modified with heavily doped indium tin oxide nanoparticles (ITONPs), which were capable of inducing localized surface plasmon resonance (LSPR) under NIR irradiation. An optoelectronic study shows that after modification with ITONPs the device performance including photocurrent, responsivity and detectivity was considerably improved. In addition, the ITONPs@SLG/GeNNs array NIRPD was able to monitor fast‐switching optical signals, the frequency was as high as 1 MHz, with very fast response rates. Theoretical simulations based on finite‐element method (FEM) revealed that the observed high performance was not only due to the strong light‐confining capability of the GeNNs array, but also due to the plasmonic ITONPs‐induced hot electron injection. The above results suggest that the present NIRPD will have great potential in future optoelectronic devices application.

     

Potential‐induced optimization of ultra‐thin rear surface passivated CIGS solar cells

Ultra‐thin Cu(In,Ga)Se₂ (CIGS) solar cells with an Al₂O₃ rear surface passivation layer between the rear contact and absorber layer frequently show a “roll‐over” effect in the J–V curve, lowering the open circuit voltage (V_OC), short circuit current (J_SC) and fill factor (FF), similar to what is observed for Na‐deficient devices. Since Al₂O₃ is a well‐known barrier for Na, this behaviour can indeed be interpreted as due to lack of Na in the CIGS absorber layer. In this work, applying an electric field between the backside of the soda lime glass (SLG) substrate and the SLG/rear‐contact interface is investi‐gated as potential treatment for such Na‐deficient rear surface passivated CIGS solar cells. First, an electrical field of +50 V is applied at 85 °C, which increases the Na concentration in the CIGS absorber layer and the CdS buffer layer as measured by glow discharge optical emission spectroscopy (GDOES). Subsequently, the field polarity is reversed and part of the previously added Na is removed. This way, the J –V curve roll‐over related to Na deficiency disappears and the V_OC (+25 mV), J_SC(+2.3 mA/cm²) and FF (+13.5% absolute) of the rear surface passivated CIGS solar cells are optimized. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Luminescent solar concentrators with a bottom-mounted photovoltaic cell:performance optimization and power gain analysis

Polymethyl methacrylate(PMMA) plate luminescent solar concentrators with a bottom-mounted(BM-LSCs)photovoltaic(PV) cell are fabricated by using a mixture of Lumogen Red 305 and Yellow 083 fluorescent dyes and a commercial monocrystalline silicon cell. The fabricated LSC with dye concentrations of 40 ppm has the highest power gain of 1.50, which is the highest value reported for the dye-doped PMMA plate LSCs. The power gain of the LSC comes from three parts: the waveguide light, the transmitted light, and the reflected light from a white reflector, and their contributions are analyzed quantitatively. The results suggest that the BM-LSCs have great potential for future low-cost PV devices in building integrated PV applications.     

Surface passivation schemes for high‐efficiency n‐type Si solar cells

An effective passivation on the front side boron emitter is essential to utilize the full potential of solar cells fabricated on n‐type silicon. However, recent investigations have shown that it is more difficult to achieve a low surface recombination velocity on highly doped p‐type silicon than on n‐type silicon. Thus, the approach presented in this paper is to overcompensate the surface of the deep boron emitter locally by a shallow phosphorus diffusion. This inversion from p‐type to n‐type surface allows the use of standard technologies which are used for passivation of highly doped n‐type surfaces. Emitter saturation current densities (J_0e) of 49 fA/cm² have been reached with this approach on SiO₂ passivated lifetime samples. On solar cells a certified conversion efficiency of 21.7% with an open‐circuit voltage (V_oc) of 676 mV was achieved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ZnO nanorods–polymer hybrid white light emitting diode grown on a disposable paper substrate

We demonstrate intrinsic white light emission from hybrid light emitting diodes fabricated using an inorganic–organic hybrid junction grown at 50 °C on a paper substrate. Cyclotene was first spin coated on the entire substrate to act as a surface barrier layer for water and other nutrient solutions. The active area of the fabricated light emitting diode (LED) consists of zinc oxide nanorods (ZnO NRs) and a poly(9,9‐dioctylfluorene) (PFO) conducting polymer layer. The fabricated LED shows clear rectifying behavior and a broad band electroluminescence (EL) peak covering the whole visible spectrum range from 420 nm to 780 nm. The color rendering index (CRI) was calculated to be 94 and the correlated color temperature (CCT) of the LED was 3660 K. The low process temperature and procedure in this work enables the use of paper substrate for the fabrication of low cost ZnO–polymer white LEDs for applications requiring flexible/disposable electronic devices. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nanosilicon photonics

Silicon photonics is no longer an emerging field of research and technology but a present reality with commercial products available on the market, where low‐dimensional silicon (nanosilicon or nano‐Si) can play a fundamental role. After a brief history of the field, the optical properties of silicon reduced to nanometric dimensions are introduced. The use of nano‐Si, in the form of Si nanocrystals, in the main building blocks of silicon photonics (waveguides, modulators, sources and detectors) is reviewed and discussed. Recent advances of nano‐Si devices such as waveguides, optical resonators (linear, rings, and disks) are treated. Emphasis is placed on the visible optical gain properties of nano‐Si and to the sensitization effect on Er ions to achieve infrared light amplification. The possibility of electrical injection in light‐emitting diodes is presented as well as the recent attempts to exploit nano‐Si for solar cells. In addition, nonlinear optical effects that will enable fast all‐optical switches are described.     

Ultrabroadband strong light absorption based on thin multilayered metamaterials

Light absorbers have drawn intensive attention as crucial components for solar‐energy harvesting, thermal emission tailoring, modulators, etc. However, achievement of light absorbers with wide bandwidth remains a challenge thus far. Here, a thin, unprecedentedly ultrabroadband strong light absorber is proposed and experimentally demonstrated, which consists of periodic taper arrays constructed by an alumina–chrome multilayered metamaterial (MM) on a gold substrate. This MM can change from a hyperbolic material to an anisotropic dielectric material at different frequency ranges and the special material features are the fundamental origins of the ultrabroadband absorption. The absorber is quite insensitive to the incident angle, and can be insensitive to the polarization. One two‐dimensional periodic array of 400‐nm height MM tapers is fabricated. The measured absorption is over 90% over almost the entire solar spectrum, reaching an average level of 96%, and remains high (above 85%) even in the longer‐wavelength range till 4 μm. The proposed absorbers open up a new avenue to realize broadband thin light‐harvesting structures.