Thin Film Heterojunction Solar Cells Research Articles

Open-circuit analysis of thin film heterojunction solar cells

A new technique is proposed for the calculation of the width W of space charge region and consequently the concentration of uncompensated acceptors NA–ND, which is based on the open-circuit analysis of thin film heterojunction solar cells illuminated by monochromatic light with the wavelength within photosensitivity region. The proposed method was simulated under different values of the theoretically considered parameters (ideality coefficient n, saturation current I0 and shunt resistance Rsh) of a CdS/CdTe heterojunction solar cell. The calculated values of W and NA–ND were established to be dependent on the mentioned above electrical parameters.

On quantum efficiency of nonideal solar cells

Abstract A simple approach for the calculation of the width of the space charge region (and consequently the concentration of uncompensated acceptors), which takes into account the effect of the series resistance on the quantum efficiency of anisotype asymmetrical thin film heterojunction solar cells (on the example of CdS/CdTe solar cells), was proposed. The effect of the light dependent series resistance and current transport mechanism was also taken into consideration.

Optimizing Back Surface Field of Heterojunction Thin-Film Solar Cells by Numerical Simulation

A new structure of amorphous/ polycrystalline silicon heterojunction solar cells is reported. For a-Si (n+)/poly-Si (p)/poly-Si(p+) structure, the concentration, thickness, and the defect states of BSF are discussed. It is shown that the cell performance improves with thickness between 10-15nm, concentration is of the order of 1E19cm-3, the maximum efficiency up to 20.45%.

Polymer impinged CdS/CuInSe2 solar cell

In the present paper we report, effect of conjugated polymer (polyaniline) impinging in nanostructured CdS/CuInSe2 heterojunction thin film solar cell. The heterojunction architecture for the solar cell is achieved by sandwiching the conjugated conducting polymer in n and p type of wide band gap semiconducting material by multilayer chemical deposition methods onto the ITO coated glass substrate at room temperature. The obtained multilayer thin film heterojunction of ITO/CdS/Polymer/CuInSe2/Ag has been characterized for structural, compositional, optical and solar cell characteristics by illuminating it to 100 mW/cm2 intensity light source. The X-ray diffraction pattern (XRD) confirms formation of CdS/CuInSe2 phase while on polymer impinging the crystallite size observed to be increased from 13 to 19 nm. The compositional analysis by energy dispersive X-ray spectra (EDAX) supports presence of expected elements in the heterojunction. The energy band gap calculated from absorbance spectra shows significant shift in its value from polymer and CdS/CuInSe2 band gap. I–V analysis shows increase in conversion efficiency from 0.26 in CdS/CuInSe2 to 0.55% in CdS/Polymer/CuInSe2 heterojunction upon illumination.

P-type Ca doped SrCu 2O 2 thin film: Synthesis, optical property and photovoltaic application

P-type transparent conducting oxide Sr 0.9Ca 0.1Cu 2O 2 (SCCO) films were prepared by pulsed laser deposition (PLD) technique on glass and n-Si (1 0 0) substrates. Some critical experiment factors were investigated, and it was found that well-crystallized, single phase SCCO films could be obtained with the substrate temperature higher than 350 °C, and the crystallinity was improved with the increase of laser energy fluence. Sr 0.9Ca 0.1Cu 2O 2/n-Si (SCCO/n-Si) heterojunction and prototype thin film solar cell were demonstrated respectively. Diode characteristic in the current–voltage response was observed and the ideality factor ( n) of the diode was 3.7. The obtained n value is larger than unity due to structural imperfections of the layer and interface states. A prototype thin film solar cell with the best solar cell performance exhibits an open-circuit voltage ( V oc) of 140 mV and a short-circuit current density ( J sc) of 0.072 mA/cm 2 under irradiation of AM1.5 and 100 mW/cm 2.

Electrical and optical properties of sputter-deposited cadmium sulfide thin films optimized by annealing temperature

Cadmium sulfide (CdS) is one of the most widely used materials as a window layer in heterojunction thin film solar cells. Sputtering method for the preparation of CdS thin film was employed for the mass-production of large-area deposited CdS thin films. The electrical and optical properties of sputter-deposited CdS thin films varied with the annealing temperature, which were caused by changes in phase composition, grain size, and stoichiometry of CdS thin films. The improved optical transmittance of 72.25% (at average thickness of 843.93nm) and the optical band gap energy of 2.43eV were obtained at the optimum annealing temperature of 400°C. The resistivity below 103-order Ω cm and carrier concentration above 1016carriers/cm3 are suitable for the requirements of window layers at this optimum annealing temperature.

Photo-induced electron transfer processes in thin-film solar cells

Power conversion efficiency of single heterojuntion and bulk heterojunction thin-film solar cells has been analyzed in terms of sequential processes of photo-induced electron transfer. Furthermore, a numerical model was developed to predict current-voltage characteristics of solar cells on the basis of photo-induced electron transfer processes. As an application of this modeling, the power conversion efficiency was analyzed for the donor/acceptor (CuPc/C60) heterojunction under various conditions. In this way, the maximum power conversion efficiency, under 1 sun AM 1.5 solar illumination, was estimated to be 6.1% for the optimum single heterojunction structure. As for the bulk heterojunction structure, a vertical lattice (alternating donor/acceptor lamellae perpendicular to a substrate) structure was proposed and the maximum power conversion efficiency was estimated to be 10.8%. It was concluded that the power conversion efficiency of the vertical lattice bulk heterojunction cell could be increased up to ca. 15% by the improvement of the hole mobility of the donor film.

An analysis of temperature dependent current–voltage characteristics of Cu 2O–ZnO heterojunction solar cells

Carrier transport and recombination mechanisms in Cu 2O–ZnO heterojunction thin film solar cells were investigated through an analysis of their current–voltage characteristics in the dark and under various illumination intensities, as a function of temperature between 100 K and 295 K. The Cu 2O–ZnO heterojunction solar cells were prepared by metal organic chemical vapor deposition of Cu 2O on ZnO films sputtered on transparent conducting oxide coated glass substrates. Activation energies extracted from the temperature dependence of the J–V characteristics reveals that interface recombination is the dominant carrier transport mechanism. Tunneling across an interfacial barrier also plays an important role in current flow and a thin TiO 2 buffer layer reduces tunneling. A high open circuit voltage at low temperature (~ 0.9 V at around 100 K) indicates that Cu 2O–ZnO heterojunction solar cells have high potential as solar cells if the recombination and tunneling at the interface can be suppressed at room temperature.

Three dimensionally structured interdigitated back contact thin film heterojunction solar cells

Three dimensionally structured thin film photovoltaic devices based on interdigitated arrays of microscale electrodes are examined by external quantum efficiency simulations, indicating considerable JSC enhancement is possible through elimination of the front contact and window layer required in planar geometry devices. Electrode parameters including, pitch, width, height, and material are modeled and experimentally probed, demonstrating experimentally and capturing in models dependence on intrinsic material properties and electrode dimensions. In contrast to analogous silicon wafer back contact solar cells where the electrodes are placed on the silicon absorber at the end of processing, in this design the semiconductor is deposited on the electrodes, taking advantage of the thin film processing already required. Electrodeposited CdS/CdTe heterojunction devices approach 1% efficiencies with simulations as well as optical measurements indicating significant potential for improvement. Suboptimal performance is attributed to unintended materials reactions that preclude annealing at the temperatures required for absorber optimization as well as the Schottky barrier formation on the nonoptimal electrode materials. The test bed structures and absorber synthesis processes are amenable to an array of deposition techniques for fabrication and measurements of three dimensionally structured semiconductors, contact materials, and photovoltaic devices subject to processing feasibility and materials compatibility.

Surface and interface porosity of polymer/fullerene-derivative thin films revealed by contrast variation of neutron and X-ray reflectivity

Contrast variation of neutron and X-ray reflections has been adapted to reveal the film in-depth (vertical) composition profiles of the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) for bulk heterojunction thin-film solar cells, with a PCBM/P3HT weight ratio of c = 0.6, 0.8 and 1.0. The X-ray scattering-length-density (SLD) profiles, extracted from X-ray reflectivity for the blend films spun-cast on Si wafer, exhibit a stratified film morphology of ca. 85 nm film thickness; the corresponding neutron SLD profiles extracted for the same films further elucidate a PCBM-enriched interfacial layer adjacent to the Si substrate. In contrast to the often assumed two-phase model, a three-phase model with porosity included as the third phase has to be used in deducing the absolute volume fractions of PCBM and P3HT from the complementary neutron and X-ray SLD profiles. In general, the thus deduced in-depth composition profiles for the blend films comprise a substantial surface layer (10–15 nm) of ca. 40% porosity, a 50 nm main layer with relatively uniform PCBM–P3HT composition, and a PCBM-enriched interface layer (∼20 nm) with ∼15% porosity. Formation of the surface porosity is related to interfacial instability occurred in a transient surface layer upon film drying. Annealing at 150 °C influences modestly the vertical phase separation of the film, but drastically activates local phase separation for formation and growth of PCBM and P3HT nanodomains, as revealed by grazing incidence small/wide angle X-ray scattering. The surface/interface porosity features (overlooked in nearly all the previous studies) and the composition-dependent vertical phase separation bear hints in advancing device performance via interfacial morphology optimization.

Effect of copper salt and thiourea concentrations on the formation of Cu2ZnSnS4 thin films by spray pyrolysis

AbstractThin films of Cu2ZnSnS4 (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been deposited by spray pyrolysis technique onto soda‐lime glass substrates held at a substrate temperature (Ts) of 643 K. The effect of copper salt and thiourea concentrations on the formation of Cu2ZnSnS4 thin films is investigated. CZTS films formed under optimized conditions are found to be polycrystalline in nature with kesterite structure. The lattice parameters are found to be a = 0.543 nm and c = 1.086 nm. The optical band gap of these films is found to be 1.43 eV. It is found to increase with decrease in copper salt concentration in the solution.

Morphologies of Self-Organizing Regioregular Conjugated Polymer/Fullerene Aggregates in Thin Film Solar Cells

In this study, we used simultaneous synchrotron grazing incidence X-ray scattering and diffraction to elucidate the overall morphologies of bulk heterojunction (BHJ) thin film (ca. 100 nm) solar cells containing phase-separated poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) domains. Specifically, the dimensions and orientation of the P3HT crystallites and the sizes of the PCBM aggregates in BHJ thin films were determined. The appropriate PCBM aggregate size and density required for an optimum performance of the film in the photovoltaics device resulted in deteriorated ordering in the out-of-plane direction, but improved the in-plane packing of the P3HT lamellae. When the P3HT crystallites and PCBM aggregates had comparable domain sizes and number densities, the interpercolated networks for electron- and hole-transport were optimized in the film. This new understanding of the underlying mechanism of carrier mobility in BHJ thin films might be crucial in improving the efficiency of future solar cells.

Regioregular Poly(3-pentylthiophene): Synthesis, Self-Assembly of Nanowires, High-Mobility Field-Effect Transistors, and Efficient Photovoltaic Cells

We report the synthesis, characterization, solution-phase assembly of nanowires, field-effect charge transport, and photovoltaic properties of regioregular poly(3-pentylthiophene) (P3PT), the first regioregular poly(3-alkylthiophene) with an odd-numbered alkyl side chain length to be so investigated. Two samples of P3PT with weight-average molecular weights of 61 800 and 77 000 and 1.4−1.5 polydispersity index have comparable solubility and processability in organic solvents as regioregular poly(3-hexylthiophene) (P3HT) but have a higher melting transition at 259 °C. X-ray diffraction of P3PT films revealed a lamellar structure with an interlayer d100 spacing of 1.51 nm and a π-stacking d010 spacing of 0.374 nm, both smaller than in P3HT. Crystalline nanowires of 16−17 nm width and aspect ratios as high as 465 were assembled from P3PT solution. Field-effect transistors fabricated from P3PT thin films showed a mobility of holes of up to 0.10 cm2/(V s), and the carrier mobility increased with molecular weight. Bulk heterojunction solar cells based on P3PT/fullerene (PC71BM) blend thin films had a power conversion efficiency of 3.70% under 100 mW/cm2 AM1.5 solar illumination in air and a maximum external quantum efficiency of 69%. Similarly illuminated solar cells based on nanocomposites of P3PT nanowires/PC71BM had a 3.33% power conversion efficiency. These results demonstrate that P3PT is an attractive polymer semiconductor suitable for applications in thin-film and nanowire-based field-effect transistors and bulk heterojunction solar cells.

Three-dimensional simulations of a thin film heterojunction solar cell with a point contact/defect passivation structure at the heterointerface

Thin film heterojunction solar cells such as those based on the chalcopyrites or amorphous silicon are often limited by interface recombination at the active heterointerface. A new strategy to overcome this limitation is described, replacing the conventional wider band gap contact material with a combination of a passivation layer plus the conventional contact in a point contact type structure. This is similar to the established method to minimize rear contact recombination in crystalline silicon solar cells. Here point contacts at the heterointerface of a CuInS2 based solar cell are modeled using the WIAS-TeSCA code. The importance of the donor defect energy level at the absorber/passivation interface is shown, and a way to improve the cell efficiency by >25% (relative) is outlined.

Preparation and characterization of spray-deposited Cu 2ZnSnS 4 thin films

Thin films of Cu 2ZnSnS 4 (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been successfully deposited by spray pyrolysis technique on soda-lime glass substrates. The effect of substrate temperature on the growth of CZTS films is investigated. X-ray diffraction studies reveal that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643–683 K. The lattice parameters are found to be a=0.542 and c=1.085 nm. The optical band gap of films deposited at various substrate temperatures is found to lie between 1.40 and 1.45 eV. The average optical absorption coefficient is found to be >10 4 cm −1.

2D device modelling and finite element simulations for thin-film solar cells

Thin-film heterojunction solar cell devices are modelled in two dimensions, from fundamental material parameters, using the finite element method. The electrostatic potential is solved for, together with the quasi-Fermi levels, while optical absorption is calculated from n and k values of the materials used. In this implementation, all material parameters can be input as functions of spatial coordinates, which makes it very flexible when applied to materials with inhomogeneities. As an example of the model in use, it is applied to a thin-film solar cell based on a structure with a CIGS absorber layer, a CdS buffer layer and a ZnO/ZAO transparent front contact. The effects of spatial inhomogeneities in the band gap energy and in the mid-gap trap level density on device performance are simulated.

Correlation of structure parameters of absorber layer with efficiency of Cu(In, Ga)Se2 solar cell

Polycrystalline Cu(In, Ga)Se2 with Ga-content x=Ga/(In+Ga) ranging from 0.0 (CuInSe2) to 1.0 (CuGaSe2) in heterojunction thin film solar cells were grown by multi-source evaporation. Solar cells with a highest efficiency of η=15.3% need a composition of x≈0.2. At this composition, the c/a ratio of the lattice constants for the tetragonal lattice equals c/a=2, indicating ideal tetragonality. These results suggest that low electronic defect densities occur at x≈0.2, due to the smallest possible crystallographic distortion of the tetragonal lattice at this composition. Cells with high efficiencies require grain sizes above 145 nm and a high preferred orientation (P204/P220 pole density ratio) for the grains.

Sulphurization of single-phase Cu 11In 9 precursors for CuInS 2 solar cells

Using Rutherford backscattering (RBS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu 11In 9 precursors to be employed as light absorbing CuInS 2 (CIS) layers in CIS–CdS heterojunction thin-film solar cells has been investigated. The Cu 11In 9 precursor films were produced by DC-sputtering from a single-phase Cu 11In 9 target. The sulphurization at 500 or 300 °C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In 2S compound, the formation of which is favoured by the phase transition of Cu 11In 9 to Cu 16In 9 at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuS x surface phase.

The formation of hetero-junction using carbon alloys by hot-wire CVD method

The deposition and fundamental properties of a-Si 1− x C x :H thin films made using a butane gas source, and of microcrystalline films including μc-Si:H and μc-3C-SiC:H are reviewed from the viewpoint of hetero-junction formation. The durability of TiO 2 thin films upon atomic hydrogen exposure is also reviewed for suppressing the deoxidation of transparent electrodes accompanied by the use of a new microcrystalline silicon carbon alloy for a window layer. The future direction for the development of hetero-junction Si thin film solar cells by hot-wire CVD is discussed.

Current transport in ZnO/ZnS/Cu(In,Ga)(S,Se) 2 solar cell

Temperature-dependent current-voltage measurements are used to determine the dominant recombination mechanism in thin-film heterojunction solar cells based on Cu(In,Ga)(S,Se) 2 absorbers with chemical bath deposited ZnS buffer layer. The measurements are carried out in the dark and under illumination in the temperature range 200–320 K. The activation energy of the recombination under illumination follows the absorber band gap energy E g=1.07 eV of bulk Cu(In,Ga)(S,Se) 2. The thermal dependence of the diode ideality factor is described by classical Shockley–Read–Hall (SRH) recombination via an exponential distribution of trap states in the bulk of the absorber. In the dark, the current flow is dominated by tunnelling enhanced bulk recombination with a tunnelling energy E 00=18 meV. Two activation energies higher than E g, namely 1.21 and 1.40 eV, have been found. These results may be explained by dominant recombination in a region close to the surface of the Cu(In,Ga)(S,Se) 2 absorber with an enlarged band gap. Thus, the electronic loss in the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se) 2 solar cell takes place mainly in the absorber and is determined by tunnelling enhanced bulk recombination with a tunnelling energy E 00 influenced by illumination.