Sub-bandgap Region Research Articles

Influence of Interface Doping on Charge-Carrier Mobilities and Sub-Bandgap Absorption in Organic Solar Cells

P3HT:PCBM (poly(3-hexylthiophene-2,5-diyl):([6,6]-phenyl-C61-butyric acid methyl ester)-based bulk heterojunctions (BHJs) were doped by using 4-toluenesulfonic acid (TSA) as dopant. This approach was inspired by the well-known interfacial doping of the active layer via the electron-blocking layer PEDOT:PSS (poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate)) at its interface. TSA is amphiphilic, acidic, and structurally very similar to the monomeric building block of PSS. Upon TSA doping, a notable increase in the light absorption in the sub-bandgap region of pristine P3HT was observed. These features are assigned to polaron transitions within P3HT; however, the TSA impact on polaron absorption features in the BHJ is rather small. Although, for small TSA concentrations and thick active layers (∼220 nm) the fill factor of the solar cells improved dramatically with increasing TSA content in the active layer, which is discussed in terms of contact resistances at interfaces in the present paper. For 0.5% TSA concentration in the active layer solution the maximum of the power conversion efficiency was obtained. At the same time, the reproducibility of solar cell performance parameters was considerably improved.

Light trapping in ultrathin 25 μm exfoliated Si solar cells.

The optical absorption in 25-μm-thick, single-crystal Si foils fabricated using a novel exfoliation technique for solar cells is studied and improved in this work. Various light-trapping and optical absorption enhancement schemes implemented show that it is possible to substantially narrow the gap in optical absorption loss between the 25 μm Si foils and industry-standard 180-μm-thick Si wafer solar cells. An improvement of absorption by 58% in the near-infrared (740-1200 nm) range is observed for the 25 μm monocrystalline Si substrates with the use of antireflective coating and texturing. The back reflectance of the metal foil that provides mechanical support to the ultrathin Si semiconductor-on-metal foils is extracted to be ∼51.5%, based on the reflectance matching with the simulated escape reflectance in the sub-bandgap region. The back reflectance is enhanced to ∼58% by incorporating an intermediate silicon nitride layer on the back between the Si and the metal. The incorporation of Al as an improved metal reflector on top of the silicon nitride at the backside of the solar cell results in a 5.8 times enhancement in optical path length as a consequence of the improved effective back reflectance of ∼95%. A thin Si foil solar cell with an unoptimized amorphous Si/crystalline Si heterojunction with intrinsic-thin-layer design with implementation of such light-trapping schemes shows an efficiency of 13.28% with a short-circuit current density (JSC) of 35.97 mA/cm2, which approaches the JSC of industrial wafer-based Si solar cells.

Enhanced photocurrent of Si solar cell with the inclusion of a transparent indium tin oxide thin film

We investigated the enhanced photocurrents in crystalline Si solar cells with the inclusion of indium tin oxide thin film. The indium tin oxide enhances the quantum efficiency and reduces photoreflectance in the spectral region of 310–1048 nm. The enhanced photocurrent is ranged at the Si band edge for the spectral peak near 1033 nm. For the subband gap region, the photocurrent is strongly reduced by the indium tin oxide thin film, indicating that the transmission of infrared spectra was prevented by plasmonic metamaterial effect. The fabricated cell showed the short circuit current density enhanced due to the carrier excitation behavior at conduction band of the infrared spectra induced metallic indium tin oxide thin film. The enhanced shunt resistance of the pn junction is related to the blocking of Ag paste penetration into the Si emitter layer by the indium tin oxide thin film during fabrication process.

Ultrafast light induced unusually broad transient absorption in the sub-bandgap region of GeSe2 thin film

In this paper, we show for the first time that ultrafast light illumination can induce an unusually broad transient optical absorption (TA), spanning of ≈ 200 nm in the sub-bandgap region of chalcogenide GeSe2 thin films, which we interpret as being a manifestation of creation and annihilation of light induced defects. Further, TA in ultrashort time scales show a maximum at longer wavelength, however blue shifts as time evolves, which provides the first direct evidence of the multiple decay mechanisms of these defects. Detailed global analysis of the kinetic data clearly demonstrates that two and three decay constants are required to quantitatively model the experimental data at ps and ns respectively.

On the sub-band gap optical absorption in heat treated cadmium sulphide thin film deposited on glass by chemical bath deposition technique

The sub-band gap optical absorption in chemical bath deposited cadmium sulphide thin films annealed at different temperatures has been critically analyzed with special reference to Urbach relation. It has been found that the absorption co-efficient of the material in the sub-band gap region is nearly constant up to a certain critical value of the photon energy. However, as the photon energy exceeds the critical value, the absorption coefficient increases exponentially indicating the dominance of Urbach rule. The absorption coefficients in the constant absorption region and the Urbach region have been found to be sensitive to annealing temperature. A critical examination of the temperature dependence of the absorption coefficient indicates two different kinds of optical transitions to be operative in the sub-band gap region. After a careful analyses of SEM images, energy dispersive x-ray spectra, and the dc current-voltage characteristics, we conclude that the absorption spectra in the sub-band gap domain is possibly associated with optical transition processes involving deep levels and the grain boundary states of the material.

Nanosecond pulse triggered absorption modulation in a-As50Se50 chalcogenide thin films

We present strong absorption modulation (ΔA≈3.5%) in As50Se50 chalcogenide thin film, using nanosecond pulses, as a new tool to tune the optical absorption spectrum in bandgap and sub-bandgap region. Interestingly, observed change in absorption persists only for few microseconds and thus allows modulating the absorption in a very short time scale. In our experiments, As50Se50 thin films were irradiated by 532nm laser having a pulse width of 5ns (control beam) with very low energy fluency of 3mJ/cm2 to modulate the absorption spectrum of as prepared As50Se50 thin film. Modulation of the sample absorption was monitored in the time window of 1ns–200μs using a probe beam in the wavelength range of 500 to 800nm, which covers the bandgap and sub-bandgap region of the sample. Absorption changes were observed to be largest (ΔA≈3.5%) in the wavelength range 550–700nm which decays very fast. a-As50Se50 thin films were found to exhibit full reversibility in transmission post-pulse-excitation, an observation that is unusual in chalcogenide systems.

Energy Level Alignment and Sub‐Bandgap Charge Generation in Polymer:Fullerene Bulk Heterojunction Solar Cells

Using charge modulated electroabsorption spectroscopy (CMEAS), for the first time, the energy level alignment of a polymer:fullerene bulk heterojunction photovoltaic cell is directly measured. The charge-transfer excitons generated by the sub-bandgap optical pumping are coupled with the modulating electric field and introduce subtle changes in optical absorption in the sub-bandgap region. This minimum required energy for sub-bandgap charge genreation is defined as the effective bandgap.

Sub-bandgap absorption in organic solar cells: experiment and theory

Most high-performance organic solar cells involve bulk-heterojunctions in order to increase the active donor-acceptor interface area. The power conversion efficiency depends critically on the nano-morphology of the blend and the interface. Spectroscopy of the sub-bandgap region, i.e., below the bulk absorption of the individual components, provides unique opportunities to study interface-related properties. We present absorption measurements in the sub-bandgap region of bulk heterojunctions made of poly(3-hexylthiophene-2,5-diyl) as an electron donor and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as an electron acceptor and compare them with quantum-chemical calculations and recently published data on the external quantum efficiency (EQE). The very weak absorption of the deep sub-bandgap region measured by the ultra-sensitive Photothermal Deflection Spectroscopy (PDS) features Urbach tails, polaronic transitions, conventional excitons, and possibly charge-transfer states. The quantum-chemical calculations allow characterizing some of the unsettled spectral features.

Influence of Fe3+-doping on optical properties of CeO2− nanopowders

Ce1−xFexO2−y (0≤x≤0.05) nanopowders were synthesized using hydrothermal method at low calcination temperature and low doping regime. Structural and morphological characterization has been carried out by the X-ray diffraction method and non-contact atomic force microscopy. Vibrational properties were investigated by Raman spectroscopy. It was observed that the content of oxygen vacancies increased significantly with Fe doping up to 3 mol%. For higher dopant concentration, phase separation was detected. The optical properties of pure and Fe3+-doped CeO2−y samples were investigated by spectroscopic ellipsometry. Several analytical models were applied to analyze the optical absorption onset of ceria defective structure. It was found that, Cody–Lorentz model most suitably described the sub-band gap region of CeO2−y nanopowders and consequently gave more accurate band gap values, which are closer to the direct band gap transitions than to the indirect ones. The increased content of localized defect states in the ceria gap and corresponding shift of the optical absorption edge towards visible range in Fe-doped samples can significantly improve the optical activity of nanocrystalline ceria.

Electrical characterization of a-InGaZnO thin-film transistors with Cu source/drain electrodes

We analyzed the effects of Cu source/drain (S/D) electrodes on the performance of a-InGaZnO (a-IGZO) thin-film transistors (TFTs). Owing to the Cu migration, the parasitic resistance was as low as 10 Ω cm with small current transfer length. Based on the transfer characteristics, we found that VDS dependent Cu migration creates donor-like deep and tail states in the sub-bandgap region. The feasibility of Cu S/D electrodes for a-IGZO TFTs using inverter circuits indicates that fabrication of high performance circuits is possible by controlling the Cu electro-migration.

Subbandgap absorption in polymer-fullerene solar cells

We present external quantum efficiency (EQE) studies of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenylC61-butyric acid methyl ester (P3HT:PCBM) based bulk heterojunction polymer solar cells with improved intensity resolution in the subbandgap (SBG) region, i.e., the energy range below the optical bandgaps of the pristine materials. Varying the P3HT:PCBM blending ratio, we find that in addition to a Gaussian profile an exponential tail is needed for a quantitative description of the SBG EQE spectra. While the exponential contribution can be reliably assigned to disorder effects, the SBG EQE Gaussian profile can be due to charge-transfer absorption between P3HT and PCBM or due to absorption of PCBM at the interface or in the polymer-rich phase.

Meyer–Neldel Rule and Extraction of Density of States in Amorphous Indium–Gallium–Zinc-Oxide Thin-Film Transistor by Considering Surface Band Bending

In this study, we analyzed the temperature-dependent characteristics of amorphous indium–gallium–zinc-oxide (a-IGZO) thin-film transistors (TFTs). We observed that a-IGZO TFTs obey the Meyer–Neldel rule (MN rule) at low gate-to-source voltage (VGS) and the inverse MN rule at high VGS, both of which can be explained by the statistical shift of Fermi level and electrostatic potential. Large Fermi level movement for small VGS change and the inverse MN rule, which are hardly observed for conventional amorphous TFTs, indicate that there is a very low density of state (DOS) in the sub-bandgap region for a-IGZO TFTs and the performance of TFTs is not affected by contact characteristics, respectively. By using the field-effect method and considering surface band bending, we extracted the DOS in the sub-bandgap region, the distribution of which is clearly distinguished by deep and tail states. The calculated parameters for tail and deep states were Nta = 3.5 ×1017 cm-3 eV-1, Eta = 0.18 eV, Nda = 1.6×1016 cm-3 eV-1, and σda = 0.21 eV.

Optical properties of sputter deposited transparent and conducting TiO 2:Nb films

Transparent and conducting thin films of TiO 2:Nb were prepared on glass by reactive dc magnetron sputtering in Ar + O 2. Post-deposition annealing in vacuum at 450 °C led to good electrical conductivity and optical transparency. The optical properties in the sub-bandgap region were in good agreement with Drude free electron theory, which accounts for intraband absorption. The band gap of the films was found to be in the range of 3.3 to 3.5 eV and signifies the onset of interband absorption. Electrical conductivities in the 10 − 3 Ω cm range were obtained both from dc electrical measurements and from analysis of the optical measurements.

Ho^3+-doped nanophase glass ceramics for efficiency enhancement in silicon solar cells

Currently Er(3+)-doped fluorides are being used as upconversion phosphors to enhance the efficiency of Si solar cells, to our knowledge. However, this enhancement is strongly limited owing to the small solar spectral range around 1540 nm that is used. We demonstrate that Ho(3+)-doped oxyfluoride glass ceramics are adequate to enlarge the Si sub-bandgap region around 1170 nm that can be transformed into higher-energy photons, showing an upconversion efficiency 2 orders of magnitude higher than the precursor glass. As these materials are transparent at 1540 nm, they can be used complementarily with Er(3+)-doped phosphors for the same purpose.

Temperature dependence of photocapacitance spectrum of CIGS thin-film solar cell

Deep levels in Cu(In 1 − x ,Ga x )Se 2 (CIGS) are studied by transient photocapacitance (TPC) spectroscopy by varying the Ga concentration, x, from 0.38 to 0.7. The TPC spectra of CIGS thin-film solar cells at 140 K exhibited a defect level with an optical transition energy of about 0.8 eV. The spectrum shape in the sub-bandgap region is independent of the Ga concentration. Therefore, the optical transition energy to the defect level is almost constant with about 0.8 eV from the valence band. The TPC signals for defect level are quenched by increasing temperature. The activation energy of thermal quenching is estimated to be about 0.3 eV. The thermal and optical activation processes are explained using configuration coordinate diagram.

An optical study of a-Ge20Se80−xInx thin films in sub-band gap region

The present paper reports a study of optical parameters, refractive index (n) and extinction coefficient (k), by analysing the transmission spectrum of thin films prepared from samples of Ge20Se80−xInx (x = 0, 5, 10, 15, 20) in the range 400–1500 nm. The dispersion parameters are calculated by using the Wemple–DiDomenico model. The optical band gap of the thin films is obtained by Tauc plots. The variation in the optical parameters for different thin films is explained on the basis of the presence of defect states and the change in the average bond energy of the system. The dielectric constants (εr and εi) and optical conductivity (σ) of the thin films are also reported.

Role of surface state on the electron flow in modified TiO 2 film incorporating carbon powder for a dye-sensitized solar cell

An investigation of surface-related traps in nanostructured TiO 2 films modified by the incorporation of carbon powder was conducted by the potential-step chronoamperometric method. For the modification of the morphology and surface state of the nanoporous TiO 2 electrode, the incorporation of carbon into the white TiO 2 powder was accomplished. In the chronoamperometric data, all of the transients showed an initial fast phase (<1 s) followed by a slower phase which is related to the trap filling process. The trap-filling period of the carbon incorporated TiO 2 film becomes longer, as the applied negative potential increases, due to the widely distributed traps induced by the increased surface area. Furthermore, the film capacitance was derived as a function of the applied bias by integrating the current to time curves of the chronoamperometric data. The accumulated charge of the carbon incorporated TiO 2 film increases prominently in two regions. The dominant increase shown in the positive region (−0.7 to −0.9 V vs. Ag/AgCl at pH 13) of the flat band potential implies that the electron occupancy in the surface-related traps is increased. At a more negative potential (below −1.2 V vs. Ag/AgCl), electrons from the conduction band of the TiO 2 film substantially influence the total current, thereby inducing an exponential increase in the current. Therefore, it is found that most of the traps are located in the positive region of the flat band potential, since the Fermi level of the nanostructured TiO 2 film is positioned at −1.14 V vs. Ag/AgCl at pH 13. The trap sites in the sub-bandgap region of the TiO 2 film are important in the electron transport of photoinjected electrons from dye molecules and partially charge recombination with redox electrolyte in operating dye-sensitized solar cell. The influence of charge trap formed by increased surface states on the electron transport and electron transfer was investigated by photovoltage and photocurrent transient measurements.

Correlation between plasma chemistry, microstructure and electronic properties of Si:H thin films prepared with hydrogen dilution

Structural and transport properties near the amorphous to microcrystalline transition region of Si:H samples deposited from a silane–hydrogen mixture have been studied. The gas pressure during the plasma enhanced chemical vapor deposition process has been varied from 1.0 to 0.1 Torr. The defect density in the subband gap region measured by modulated photocurrent and constant photocurrent methods vary asymmetrically above and below midgap with changes in pressure. The transport properties of the electrons and holes studied by measurement of mobility lifetime product ( μτ) and diffusion length ( L D), respectively, change in the opposite direction with pressure. The sample deposited at 0.2 Torr exhibits high L D but low μτ product. Moreover, an increase of the band gap was observed with decreasing pressure. These unusual behaviors have been explained on the basis of quantum confinement effect. The changes in plasma chemistry observed by optical emission spectroscopy present an interesting perspective in understanding the evolution of the structural and electronic properties with the changes in pressure.

Analytical model for the optical functions of amorphous semiconductors and its applications for thin film solar cells

We have developed a Kramers–Kronig consistent analytical expression to fit the dielectric functions (ε 1, ε 2) of hydrogenated amorphous silicon (a-Si:H)-based alloys measured using a combination of photoconductivity, transmission and reflection, and ellipsometric spectroscopies. The alloys of interest include amorphous silicon-germanium (a-Si 1− x Ge x :H) and silicon-carbon (a-Si 1− x C x :H), with optical bandgaps ranging from ∼1.30 to 1.95 eV. The fit can be performed simultaneously throughout the following regions: (i) the sub-bandgap (or Urbach tail) region where the absorption coefficient increases exponentially with photon energy, (ii) the band-to-band onset region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-bandgap region where a Lorentz oscillator model is applicable. We describe an approach whereby, from a single accessible measure of the optical bandgap, (ε 1, ε 2) can be generated for a sample set consisting of optimum electronic quality a-Si:H-based alloys prepared by plasma-enhanced chemical vapor deposition.

The effect of mounting-induced strain and defect on the properties of AlGaAs 808 nm LDs

The effects of the mounting process on the properties of broad-area AlGaAs SQW 808 nm high-power laser bar were investigated using electroluminescence microscopy (ELM), micro-photoluminescence (μ-PL), photoluminescence microscopy (PLM) and micro-photocurrent (μ-PC) spectroscopy. Local strain and V-shaped defects were observed as a function of position by μ-PL and PLM, respectively. ELM images from each emitter were taken as a function of bar injection current, allowing the determination of the “apparent” external differential quantum efficiencies ( η ext). The results suggest that there is a strain threshold, above which η ext decreases with increased strain. μ-PC measurements show that the emitters with defects have a larger PC tail in the sub-bandgap region. The peak position of the first derivative (d I PC/d E) max of the PC spectra displays a blue shift with the increase of strain. The magnitude of the (d I PC/d E) max has a weak inverse correlation with strain. The results suggest that with the increase of strain up to threshold, defect-related non-radiative recombination centers form and accumulate. The increased non-radiative recombination results in the decrease of η ext of the emitter, i.e., the degradation of the emitters and the bar.