Tailored Band Structure of Cu(In,Ga)Se2 Thin-Film Heterojunction Solar Cells: Depth Profiling of Defects and the Work Function.

An efficient carrier transport is essential for enhancing the performance of thin-film solar cells, in particular Cu(In,Ga)Se2 (CIGS) solar cells, because of their great sensitivities to not only the interface but also the film bulk. Conventional methods to investigate the outcoming carriers and their transport properties measure the current and voltage either under illumination or dark conditions. However, the evaluation of current and voltage changes along the cross-section of the devices presents several limitations. To mitigate this shortcoming, we prepared gently etched devices and analyzed their properties using micro-Raman scattering spectroscopy, Kelvin probe force microscopy, and photoluminescence measurements. The atomic distributions and microstructures of the devices were investigated, and the defect densities in the device bulk were determined via admittance spectroscopy. The effects of Ga grading on the charge transport at the CIGS-CdS interface were categorized into various types of band offsets, which were directly confirmed by our experiments. The results indicated that reducing open-circuit voltage loss is crucial for obtaining a higher power conversion efficiency. Although the large Ga grading in the CIGS absorber induced higher defect levels, it effectuated a smaller open-circuit voltage loss because of carrier transport enhancement at the absorber-buffer interface, resulting from the optimized conduction band offsets.

Halide perovskite single crystals: growth, characterization, and stability for optoelectronic applications.

Recently, metal halide perovskite materials have received significant attention as promising candidates for optoelectronic applications with tremendous achievements, owing to their outstanding optoelectronic properties and facile solution-processed fabrication. However, the existence of a large number of grain boundaries in perovskite polycrystalline thin films causes ion migration, surface defects, and instability, which are detrimental to device applications. Compared with their polycrystalline counterparts, perovskite single crystals have been explored to realize stable and excellent properties such as a long diffusion length and low trap density. The development of growth techniques and physicochemical characterizations led to the widespread implementation of perovskite single-crystal structures in optoelectronic applications. In this review, recent progress in the growth techniques of perovskite single crystals, including advanced crystallization methods, is summarized. Additionally, their optoelectronic characterizations are elucidated along with a detailed analysis of their optical properties, carrier transport mechanisms, defect densities, surface morphologies, and stability issues. Furthermore, the promising applications of perovskite single crystals in solar cells, photodetectors, light-emitting diodes, lasers, and flexible devices are discussed. The development of suitable growth and characterization techniques contributes to the fundamental investigation of these materials and aids in the construction of highly efficient optoelectronic devices based on halide perovskite single crystals.

High speed growth of MAPbBr3 single crystals via low-temperature inverting solubility: enhancement of mobility and trap density for photodetector applications.

There has been growing interest in organic-inorganic hybrid perovskites as a promising candidate for optoelectronic applications due to their superior physical properties. Despite this, most of the reported perovskite devices based on polycrystalline thin films suffer immensely from poor stability and high trap density owing to grain boundaries limiting their performance. Perovskite single crystal structures have been recently explored to construct stable devices and reduce the trap density compared to their thin-film counterparts. We present a novel method of growing sizable CH3NH3PbBr3 single crystals based on the high solubility characteristic of hybrid perovskites at low temperatures within inverse temperature crystallization. We compared both the crystallinity of perovskite single crystal structures and optoelectronic charge transport of single crystal photodetectors as a function of dissolution temperature. The performance of the photodetector fabricated with our large-scaled single crystal with high quality demonstrated low trap density, high mobility, and high photoresponse.

Broad-Band Photocurrent Enhancement in MoS2 Layers Directly Grown on Light-Trapping Si Nanocone Arrays.

There has been growing research interest in realizing optoelectronic devices based on the two-dimensional atomically thin semiconductor MoS2 owing to its distinct physical properties that set it apart from conventional semiconductors. However, there is little optical absorption in these extremely thin MoS2 layers, which presents an obstacle toward applying them for use in high-efficiency light-absorbing devices. We synthesized trilayers of MoS2 directly on SiO2/Si nanocone (NC) arrays using chemical vapor deposition and investigated their photodetection characteristics. The photoresponsivity of the MoS2/NC structure was much higher than that of the flat counterpart across the whole visible wavelength range (for example, it was almost an order of magnitude higher at λ = 532 nm). Strongly concentrated light near the surface that originated from a Fabry-Perot interference in the SiO2 thin layers and a Mie-like resonance caused by the Si NCs boosted the optical absorption in MoS2. Our work demonstrates that MoS2/NC structures could provide a useful means to realize high-performance optoelectronic devices.

Influence of Gas Adsorption and Gold Nanoparticles on the Electrical Properties of CVD-Grown MoS2 Thin Films.

Molybdenum disulfide (MoS2) has increasingly attracted attention from researchers and is now one of the most intensively explored atomic-layered two-dimensional semiconductors. Control of the carrier concentration and doping type of MoS2 is crucial for its application in electronic and optoelectronic devices. Because the MoS2 layers are atomically thin, their transport characteristics may be very sensitive to ambient gas adsorption and the resulting charge transfer. We investigated the influence of the ambient gas (N2, H2/N2, and O2) choice on the resistance (R) and surface work function (WF) of trilayer MoS2 thin films grown via chemical vapor deposition. We also studied the electrical properties of gold (Au)-nanoparticle (NP)-coated MoS2 thin films; their R value was found to be 2 orders of magnitude smaller than that for bare samples. While the WF largely varied for each gas, R was almost invariant for both the bare and Au-NP-coated samples regardless of which gas was used. Temperature-dependent transport suggests that variable range hopping is the dominant mechanism for electrical conduction for bare and Au-NP-coated MoS2 thin films. The charges transferred from the gas adsorbates might be insufficient to induce measurable R change and/or be trapped in the defect states. The smaller WF and larger localization length of the Au-NP-coated sample, compared with the bare sample, suggest that more carriers and less defects enhanced conduction in MoS2.

Mie Resonance-Modulated Spatial Distributions of Photogenerated Carriers in Poly(3-hexylthiophene-2,5-diyl)/Silicon Nanopillars.

Organic/silicon hybrid solar cells have great potential as low-cost, high-efficiency photovoltaic devices. The superior light trapping capability, mediated by the optical resonances, of the organic/silicon hybrid nanostructure-based cells enhances their optical performance. In this work, we fabricated Si nanopillar (NP) arrays coated with organic semiconductor, poly(3-hexylthiophene-2,5-diyl), layers. Experimental and calculated optical properties of the samples showed that Mie-resonance strongly concentrated incoming light in the NPs. Spatial mapping of surface photovoltage, i.e., changes in the surface potential under illumination, using Kelvin probe force microscopy enabled us to visualize the local behavior of the photogenerated carriers in our samples. Under red light, surface photovoltage was much larger (63 meV) on the top surface of a NP than on a planar sample (13 meV), which demonstrated that the confined light in the NPs produced numerous carriers within the NPs. Since the silicon NPs provide pathways for efficient carrier transportation, high collection probability of the photogenerated carriers near the NPs can be expected. This suggests that the optical resonance in organic/silicon hybrid nanostructures benefits not only broad-band light trapping but also efficient carrier collection.

Epitaxial Brownmillerite Oxide Thin Films for Reliable Switching Memory.

Resistive switching memory, which is mostly based on polycrystalline thin films, suffers from wide distributions in switching parameters-including set voltage, reset voltage, and resistance-in their low- and high-resistance states. One of the most commonly used methods to overcome this limitation is to introduce inhomogeneity. By contrast, in this paper, we obtained uniform resistive switching parameters and sufficiently low forming voltage by maximizing the uniformity of an epitaxial thin film. To achieve this result, we deposited an SrFeOx/SrRuO3 heteroepitaxial structure onto an SrTiO3 (001) substrate by pulsed laser deposition, and then we deposited an Au top electrode by electron-beam evaporation. This device exhibited excellent bipolar resistance switching characteristics, including a high on/off ratio, narrow distribution of key switching parameters, and long data retention time. We interpret these phenomena in terms of a local, reversible phase transformation in the SrFeOx film between brownmillerite and perovskite structures. Using the brownmillerite structure and atomically uniform thickness of the heteroepitaxial SrFeOx thin film, we overcame two major hurdles in the development of resistive random-access memory devices: high forming voltage and broad distributions of switching parameters.

Chalcogenization-Derived Band Gap Grading in Solution-Processed CuIn(x)Ga(1-x)(Se,S)2 Thin-Film Solar Cells.

Significant enhancement of solution-processed CuIn(x)Ga(1-x)(Se,S)2 (CIGSSe) thin-film solar cell performance was achieved by inducing a band gap gradient in the film thickness, which was triggered by the chalcogenization process. Specifically, after the preparation of an amorphous mixed oxide film of Cu, In, and Ga by a simple paste coating method chalcogenization under Se vapor, along with the flow of dilute H2S gas, resulted in the formation of CIGSSe films with graded composition distribution: S-rich top, In- and Se-rich middle, and Ga- and S-rich bottom. This uneven compositional distribution was confirmed to lead to a band gap gradient in the film, which may also be responsible for enhancement in the open circuit voltage and reduction in photocurrent loss, thus increasing the overall efficiency. The highest power conversion efficiency of 11.7% was achieved with J(sc) of 28.3 mA/cm(2), V(oc) of 601 mV, and FF of 68.6%.

Plasmon-Enhanced Surface Photovoltage of ZnO/Ag Nanogratings.

We investigated the surface photovoltage (SPV) behaviors of ZnO/Ag one-dimensional (1D) nanogratings using Kelvin probe force microscopy (KPFM). The grating structure could couple surface plasmon polaritons (SPPs) with photons, giving rise to strong light confinement at the ZnO/Ag interface. The larger field produced more photo-excited carriers and increased the SPV. SPP excitation influenced the spatial distribution of the photo-excited carriers and their recombination processes. As a result, the SPV relaxation time clearly depended on the wavelength and polarization of the incident light. All of these results suggested that SPV measurement using KPFM should be very useful for studying the plasmonic effects in nanoscale metal/semiconductor hybrid structures.

Mie resonance-mediated antireflection effects of Si nanocone arrays fabricated on 8-in. wafers using a nanoimprint technique.

We fabricated 8-in. Si nanocone (NC) arrays using a nanoimprint technique and investigated their optical characteristics. The NC arrays exhibited remarkable antireflection effects; the optical reflectance was less than 10% in the visible wavelength range. The photoluminescence intensity of the NC arrays was an order of magnitude larger than that of a planar wafer. Optical simulations and analyses suggested that the Mie resonance reduced effective refractive index, and multiple scattering in the NCs enabled the drastic decrease in reflection. PACS: 88.40.H-; 88.40.jp; 81.07.Gf.

Transparent conductor-embedding nanocones for selective emitters: optical and electrical improvements of Si solar cells.

Periodical nanocone-arrays were employed in an emitter region for high efficient Si solar cells. Conventional wet-etching process was performed to form the nanocone-arrays for a large area, which spontaneously provides the graded doping features for a selective emitter. This enables to lower the electrical contact resistance and enhances the carrier collection due to the high electric field distribution through a nanocone. Optically, the convex-shaped nanocones efficiently reduce light-reflection and the incident light is effectively focused into Si via nanocone structure, resulting in an extremely improved the carrier collection performances. This nanocone-arrayed selective emitter simultaneously satisfies optical and electrical improvement. We report the record high efficiency of 16.3% for the periodically nanoscale patterned emitter Si solar cell.

Wafer-scale nanoconical frustum array crystalline silicon solar cells: promising candidates for ultrathin device applications.

A high photocurrent of 36.96 mA cm(-2) was achieved for wafer-scaled crystalline Si solar cells with hexagonal nanoconical frustum arrays at the surface. Optical simulations showed that the expected photocurrent of 10 µm thick nanostructured cells could slightly exceed the Lambertian limit.

Wafer-scale surface roughening for enhanced light extraction of high power AlGaInP-based light-emitting diodes.

A new approach to surface roughening was established and optimized in this paper for enhancing the light extraction of high power AlGaInP-based LEDs, by combining ultraviolet (UV) assisted imprinting with dry etching techniques. In this approach, hexagonal arrays of cone-shaped etch pits are fabricated on the surface of LEDs, forming gradient effective-refractive-index that can mitigate the emission loss due to total internal reflection and therefore increase the light extraction efficiency. For comparison, wafer-scale FLAT-LEDs without any surface roughening, WET-LEDs with surface roughened by wet etching, and DRY-LEDs with surface roughened by varying the dry etching time of the AlGaInP layer, were fabricated and characterized. The average output power for wafer-scale FLAT-LEDs, WET-LEDs, and DRY3-LEDs (optimal) at 350 mA was found to be 102, 140, and 172 mW, respectively, and there was no noticeable electrical degradation with the WET-LEDs and DRY-LEDs. The light output was increased by 37.3% with wet etching, and 68.6% with dry etching surface roughening, respectively, without compromising the electrical performance of LEDs. A total number of 1600 LED chips were tested for each type of LEDs. The yield of chips with an optical output power of 120 mW and above was 0.3% (4 chips), 42.8% (684 chips), and 90.1% (1441 chips) for FLAT-LEDs, WET-LEDs, and DRY3-LEDs, respectively. The dry etching surface roughening approach developed here is potentially useful for the industrial mass production of wafer-scale high power LEDs.

Printable, wide band-gap chalcopyrite thin films for power generating window applications.

Printable, wide band-gap chalcopyrite compound films (CuInGaS2, CIGS) were synthesized on transparent conducting oxide substrates. The wide band-gap and defective nature of the films reveal semi-transparent and bifacial properties that are beneficial for power generating window applications. Importantly, solar cell devices with these films demonstrate a synergistic effect for bifacial illumination resulting in a 5.4-16.3% increase of the apparent power conversion efficiency compared to the simple sum of the efficiencies of the front and rear side illumination only. We also confirmed that this extra output power acquisition due to bifacial irradiation is apparently not influenced by the light intensity of the rear side illumination, which implies that weak light (e.g., indoor light) can be efficiently utilized to improve the overall solar cell efficiency of bifacial devices.

Cocktails of paste coatings for performance enhancement of CuInGaS(2) thin-film solar cells.

To fabricate low-cost and printable wide-bandgap CuInxGa1-xS2 (CIGS) thin-film solar cells, a method based on a precursor solution was developed. In particular, under this method, multiple coatings with two pastes with different properties (e.g., viscosity) because of the different binder materials added were applied. Paste A could form a thin, dense layer enabling a high-efficiency solar cell but required several coating and drying cycles for the desired film thickness. On the other hand, paste B could easily form one-micrometer-thick films by means of a one-time spin-coating process but the porous microstructure limited the solar cell performance. Three different configurations of the CIGS films (A + B, B + A, and A + B + A) were realized by multiple coatings with the two pastes to find the optimal stacking configuration for a combination of the advantages of each paste. Solar cell devices using these films showed a notable difference in their photovoltaic characteristics. The bottom dense layer increased the minority carrier diffusion length and enhanced the short-circuit current. The top dense layer could suppress interface recombination but exhibited a low optical absorption, thereby decreasing the photocurrent. As a result, the A + B configuration could be suggested as a desirable simple stacking structure. The solar cell with A + B coating showed a highly improved efficiency (4.66%) compared to the cell with a film prepared by paste B only (2.90%), achieved by simple insertion of a single thin (200 nm), dense layer between the Mo back contact and a thick porous CIGS layer.