Fabrication and characterisations of n-CdS/p-PbS heterojunction solar cells using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cells were prepared via microwave-assisted chemical bath deposition method. A cadmium sulfide (CdS) window layer (340 nm thickness) was deposited on an indium tin oxide (ITO) glass. A lead sulfide (PbS) absorber layer (985–1380 nm thickness) with different molar concentrations (0.02, 0.05, 0.075, and 0.1 M) was then grown on ITO/CdS to fabricate a p–n junction. The effects of changing molar concentration of the absorber layer on structural and optical properties of the corresponding PbS thin films and solar cells were investigated. The optical band gap of the films decreased as the molarity increased. The photovoltaic properties (J–V characteristics, short circuit current, open circuit voltage, fill factor, and efficiency) of the CdS/PbS heterostructure cells were examined under 30 mW/cm² solar radiation. Interestingly, changing molar concentration improved the photovoltaic cells performances, the solar cell exhibited its highest efficiency (1.68%) at 0.1 M molar concentration.     

Stoichiometry controlled conversion efficiency in nanostructured heterojunction solar cell of CdS/CuInSXSe2−X grown by chemical ion exchange method at room temperature

Here in the present paper, we report on growth of stoichiometric and nonstoichiometric nanostructured heterojunction solar cell of CdS/CuInS_XSe_2-X varying X from 0 to 2 in the interval of 0.5 using cost effective, simple, chemical ion exchange method at room temperature on ITO glass substrate. The as-grown varying composition solar cells annealed at 200 °C in air and characterized for structural, compositional, optical and illumination studies. The X-ray diffraction pattern obtained from CdS/CuInS_XSe_2-X solar cell confirms the formation of CuInSe₂, CuInS_0.5Se_1.5, CuInS₁Se₁, CuInS_1.5Se_0.5 and CuInS₂ phases having tetragonal structure with varying crystallite size from 19, 19.37, 28, 33 and 20 nm respectively. The energy dispersive X-ray analysis (EDAX) confirms the expected elemental composition in the heterojunction solar cell. Optical absorbance analysis confirms composition controlled electronic transitions in the thin films while energy band gap observed to be red shifted with increase the value of X. The solar energy conversion efficiency achieved upon illuminating to 100 mW/cm² observed to be 0.27%, 0.06%, 0.17%, 0.02% and 0.23% for CuInSe₂, CuInS_0.5Se_1.5, CuInS₁Se₁, CuInS_1.5Se_0.5 and CuInS₂ respectively, which correspond for stoichiometric dependent electron-hole pair generation and separation phenomenon.     

Optoelectronic properties of chemically deposited Bi2S3 thin films and the photovoltaic performance of Bi2S3/P3OT solar cells

Thin films of bismuth sulfide (Bi₂S₃), prepared on conductive tin-doped indium oxide (ITO)-glass substrates by chemical deposition showed a variation of optical band gap with thickness: 1.8 eV for a 50 nm film (deposited at 40 °C for 30 min) to 1.5 eV for a 200 nm film deposited for 2 h. The electronegativity for Bi₂S₃ compound is 5.3 eV, as estimated from the ionization energy and electron affinity of elemental Bi and S, and thus the electron affinity of chemically deposited Bi₂S₃ film was deduced to be 4.5 eV. In the energy level analysis of ITO/Bi₂S₃/P3OT/Au structure, Bi₂S₃ was established as an electron acceptor. To produce ITO/Bi₂S₃/P3OT/Au solar cell structures, poly3-octylthiophene (P3OT), prepared in the laboratory was dissolved in toluene and was drop-cast on the Bi₂S₃ film and a gold film was thermally evaporated. Under 100 mW/cm² tungsten-halogen irradiation incident from the ITO-side, the cell using a Bi₂S₃ film with thickness of 50 nm has the highest open circuit voltage (V_oc) of 440 mV and short-circuit current density (J_sc) of 0.022 mA/cm². The addition of a CdS thin film (90 nm) between ITO and B₂S₃ would increase V_oc to 480 mV, and J_sc to 0.035 mA/cm². The role of surface morphology and optoelectronic properties of the Bi₂S₃ film in the photovoltaic performance of the junction is discussed.     

Hybrid solar cells of layer-by-layer thin films with a polymer/fullerene bulk heterojunction

A hybrid organic solar cell was fabricated by integrating a layer-by-layer (LbL) ultrathin polymer film with a polymer/fullerene bulk heterojunction film. The short-circuit current density (J_SC) was significantly improved by a factor of five compared to that observed for LbL-based solar cells. The open-circuit voltage (V_OC) remained at >0.7 V, which is higher than that reported for bulk heterojunction solar cells. These findings suggest that the LbL thin film can not only enhance J_SC but also tune V_OC. An optimized hybrid polymer solar cell showed the power conversion efficiency to be as high as ～1%.     

Improvement of polymer/fullerene solar cells by controlling geometry of the ITO substrate surface

We have found that the short-circuit current, J_sc, of polymer/fullerene [RR-P3HT/C₆₀] solar cells has a clear dependence on the surface roughness of the ITO/glass substrate. We prepared an ITO surface with an average roughness, R_a, of 0.7–11 nm by chemical etching. At first J_sc increases with the increase in ITO surface roughness and then gradually decreases. The maximum performance was obtained at R_a≈4 nm. J_sc is also high with a very flat surface of R_a=0.7 nm. This feature can be attributed to the trade-off between the increase in absorption light path length and film-quality deterioration.     

Computer simulation of a-Si/c-Si heterojunction solar cell with high conversion efficiency

The p-type a-Si:H/n-type c-Si (P⁺ a-Si:H/N⁺ c-Si) heterojunction was simulated for developing solar cells with high conversion efficiency and low cost. The characteristic of such cells with different work function of transparent conductive oxide (TCO) were calculated. The energy band structure, quantum efficiency and electric field are analyzed in detail to understand the mechanism of the heterojunction cell. Our results show that the a-Si/c-Si heterojunction is hypersensitive to the TCO work function, and the TCO work function should be large enough in order to achieve high conversion efficiency of P⁺ a-Si:H/N⁺ c-Si solar cells. With the optimized parameters set, the P⁺ a-Si:H/N⁺ c-Si solar cell reaches a high efficiency (η) up to 21.849% (FF: 0.866, V_OC: 0.861 V, J_SC: 29.32 mA/cm²).     

Synthesis of colloidal CuInSe2 nanocrystals films for photovoltaic applications

We synthesise 7 nm CuInSe₂ nanocrystals of low size and shape dispersion using copper(I) chloride, indium(III) chloride and selenourea as the precursors. The obtained nanocrystals are deposited on indium tin oxide (ITO) covered glass by means of layer-by-layer dip-coating. After each deposition step, a ligand exchange process with 1,2-ethanedithiol (EDT) is carried out. The new ligands interconnect the nanocrystals, which therefore become insoluble during subsequent dip-coating steps. Furthermore, the small EDT molecules replace the long insulating alkyl chains of the oleylamine stabilizing ligands used during the synthesis. I/V measurements of a 1 μm thick film sandwiched between the ITO substrate and an aluminium electrode show a current density of 1 μA/cm² at 1 V in the dark, which increases to 16 μA/cm² under illumination with white light.     

Procedure to obtain higher than 14% efficient thin film CdS/CdTe solar cells activated with HCF2Cl gas

This work reports a process used to obtain CdS/CdTe thin film solar cells with high efficiencies between 14% and 14.6%. The procedure consists of sequentially depositing by RF sputtering and close space sublimation (CSS) of several layers onto a glass substrate, configured as glass/ITO/ZnO/CdS/CdTe/Cu/Mo. The CdTe layer was deposited by CSS at low substrate temperature (500 °C) under Ar–O₂ atmosphere and the activation process is performed with a mixture of Freon gas (HCF₂Cl) and other gases.     

A novel Al and Y codoped ZnO/n-Si heterojunction solar cells fabricated by pulsed laser deposition

Al and Y codoped ZnO (AZOY) transparent conducting oxide (TCO) thin films were first deposited on n-Si substrates by pulsed laser deposition (PLD) to form AZOY/n-Si heterojunction solar cells. However, the properties of the AZOY emitter layers are critical to the performance of AZOY/n-Si heterojunction solar cells. To estimate the properties of AZOY thin films, films deposited on glass substrates with various substrate temperatures (T_s) were analyzed. Based on the experimental results, optimal electrical properties (resistivity of 2.8 ± 0.14 × 10^?4 Ω cm, carrier mobility of 27.5 ± 0.55 cm²/Vs, and carrier concentration of 8.0 ± 0.24 × 10²⁰ cm^?3) of the AZOY thin films can be achieved at a T_s of 400 °C, and a high optical transmittance of AZOY is estimated to be >80% (with glass substrate) in the visible region under the same T_s. For the AZOY/n-Si heterojunction solar cells, the AZOY thin films acted not only as an emitter layer material, but also as an anti-reflected coating thin film. Thus, a notably high short-circuit current density (J_sc) of 31.51 ± 0.186 mA/cm² was achieved for the AZOY/n-Si heterojunction solar cells. Under an AM1.5 illumination condition, the conversion efficiency of the cells is estimated at only approximately 4% (a very low open-circuit voltage (V_oc) of 0.24 ± 0.001 V and a fill factor (FF) of 0.51 ± 0.011) without any optimization of the device structure.     

Tuning photocurrent response through size control of CdTe quantum dots sensitized solar cells

The photovoltaic performance of CdTe quantum dots (QDs) sensitized solar cells (QDSSCs) as a function of tuning the band gap of CdTe QDs size is studied. The tuning of band gap was carried out through controlling the size of QDs. Presynthesized CdTe QDs of radii from 2.1 nm to 2.5 nm) were deposited by direct adsorption (DA) technique onto a layer of TiO₂ nanoparticles (NPs) to serve as sensitizers for the solar cells. The characteristic parameters of the assembled QDSSCs were measured under AM 1.5 sun illuminations. The values of current density (J_sc) and overall efficiency (η) increase with decreasing CdTe QDs size, since the lowest unoccupied molecular orbital (LUMO) levels shifts closer to vacuum level, which causes an increase in the driving force. Consequently the electrons’ injections to the conduction band (CB) of TiO₂ NPs become faster. The maximum values of J_sc (1105 μA/cm²) and η (0.190%) were obtained for the smallest CdTe QDs size (2.10 nm). The open circuit voltages (V_oc) varies slightly with the size of the CdTe QDs, however it is only dictated by the CB level of TiO₂ NPs and the VB of the electrolyte. Furthermore, the photocurrent response of the assembled cells to ON–OFF cycles of the illumination indicates the prompt generation of anodic current.     

Bifacial transparent solid-state dye-sensitized solar cell with sputtered indium-tin-oxide counter electrode

In this study, we report a solid state dye-sensitized solar cell (SSDSC) made with a transparent ITO film as a counter electrode using the sputtering technology. For the first time, a bifacial transparent SSDSC is realized and irradiated from FTO and ITO side. The SSDSCs give short circuit photocurrent density (J_sc) of 4.16 mA cm^?2, open circuit voltage (V_oc) of 0.74 V, and fill factor (FF) of 0.64, corresponding to the photoelectric conversion efficiency of 1.96% from FTO side illumination (AM 1.5G, 100 mW cm^?2). Moreover, it is found that J_sc of SSDSCs (2.85 mA cm^?2) when irradiated from ITO side is less than that from FTO side. This result is because of the cut-off of incident photons in the blue region by the ITO film and the light screening effect by the hole transport material (HTM) absorption. Our results demonstrate the possibility of production scalable sputtering process for SSDSCs electrodes fabrication and pave the avenue for tandem design application which requires a transparent intermediate layer for interconnection.     

CdS1−yTey thin films: Production and bandgap investigation

In CdS/CdTe solar cells it is necessary to determine the efficiency limitations related with the intermixing at the interface between the CdS window layer and CdTe absorber layer. So understanding the properties of the solid solution (CdS_xTe_1?x on the CdTe side which is CdTe rich and CdS_1?yTe_y on the CdS side which is CdS rich) that is always formed in this region is essential. We produced thin films of CdS_1?yTe_y solid solution-which is CdS rich – by first producing CdS:In thin films on glass substrates by the spray pyrolysis technique and then annealing the films in nitrogen atmosphere at 400 °C in the presence of Te vapor. We are the first who produce this solid solution by this simple and low cost method. The composition and morphology of the films were determined by energy dispersive X-ray detection (EDAX) measurements and scanning electron microscopy (SEM) observations respectively. Eight values of y in the range 0 ? y ? 0.2845 were obtained. The transmittance was measured and used to investigate the optical bandgap energy by using the second derivative of the absorbance. It is found that the films show a single hexagonal phase for y ? 0.0852 and then a mixed (hexagonal and cubic) phase for 0.0997 ? y ? 0.2845. Bandgap energies in the range 2.259 ? E_g ? 2.528 eV were obtained. Urbach tailing in the bandgap was also investigated.     

The characteristics of anisotype CdS/CdTe heterojunction

Polycrystalline CdTe and CdS films were prepared by thermal evaporation technique with thicknesses 1.0 μm and 0.1 μm, respectively. The prepared films were deposited at substrate temperature 423 K, then annealed under vacuum at various annealing temperatures. Anisotype CdS/CdTe heterojunction has been prepared. The structure of the films was examined by X-ray diffraction. Hall measurements confirmed the conductivity types for CdTe and CdS to be p- and n-, respectively. Electrical characteristics of the junction (C–V and I–V measurements) showed that the junction was abrupt. Heat treatment (T_a) of the junction caused a decrease in the capacitance with increasing the reverse bias voltage. Also, both zero bias capacitance and built in voltage are decreased with increasing T_a. Carrier concentration around the junction was increased with increasing T_a. Transport mechanism of forward current coincides to tunneling-recombination mechanism; this was confirmed by I–V measurement.     

Thin film tandem solar cells based on CuInSe2

The CuInSe₂/(CdZn)S heterojunction is the best developed low bandgap solar cell for use in a two-junction tandem device. The potential performance of large area terrestrial systems based on this junction is reviewed. A monolithic tandem cell in which the high bandgap cell is a (CdHg) Te/CdS heterojunction deposited onto a CuInSe₂/(CdZn)S cell is being developed and progress with this system is described.     

Interface properties determined the performance of thermally grown GaN/Si heterojunction solar cells

We report the fabrication of heterojunction solar cells via the thermal chemical vapor deposition (CVD) of gallium nitride (GaN) nanostructures on clean Si substrates. GaN epitaxial layers were synthesized via the direct reaction of Ga vapor and NH₃ solution at 1050 °C. The structural and optical characteristics of the as-grown GaN layers were investigated. The effects of Si orientation (100 vs 111) and doping type (n- vs p-) on the structural and optical properties of the deposited GaN nanostructures and solar cell performance were explored. The fabricated GaN nanostructures exhibited p-type behavior at the GaN/Si interface as revealed from the Hall-effect measurements. The J–V characteristics showed rectifying behavior for the GaN/n-Si junction and Ohmic behavior for the GaN/p-Si junction. Upon illumination (30 mW/cm²), the as-deposited heterojunction solar cell devices showed conversion efficiencies of 6.18% and 3.69% for GaN/n-Si (1 1 1) and GaN/n-Si (1 0 0) heterojunctions, respectively. The growth of GaN on Si substrates in the presence of NH₃ solution has strong effect on the morphological, optical and electrical properties and consequently on the efficiency of the solar cell devices made of such substrates.     

Band gap engineering of RF-sputtered CuInZnSe2 thin films for indium-reduced thin-film solar cell application

We demonstrated the preparation and characterization of radio frequency (RF)-sputtered CuInZnSe₂ thin films for indium-reduced thin-film solar cell application. Sputtering targets composed of high-purity CuSe, InSe and ZnSe powders were employed for preparing CuInZnSe₂ films with various band gaps. Under an optimum condition, an increase of zinc concentration in the film could reduce indium approximately to 45%. The structure of the films showed a chalcopyrite phase with a predominant (1 1 2) reflection. The p-type CuInZnSe₂ films exhibited a shift of optical transmittance to a lower wavelength and the band gap could be engineered from 1.0 to 1.25 eV in proportion with increasing zinc concentration.     

Conjugated heat transfer on leading edge of a wedge-shaped concave wall internally impinged by hot jets from corrugated orifice plate

An experimental and numerical investigation is conducted to study the conjugated heat transfer performance on the leading edge of a wedge-shaped concave wall subjected to external cold flow and internal hot jets impingement. A corrugated impinging plate with an extended front-extended port inside the concave cavity is proposed for the purpose of heat transfer enhancement. The effects of corrugation length-to-diameter ratio (H_j/d) ranging from 5 to 11 and width-to-diameter ratio (W_j/d) ranging from 2.5 to 6 on the conjugated heat transfer performance are examined under some representative jet Reynolds numbers (Re_j) in the range of 7900–31,700. The results show that the corrugated impinging plate has a significant impact on improving the conjugated heat transfer performance in the vicinity of concave wall leading edge. The presence of corrugation plays two roles by reducing the jet impinging distance on one hand and aggravating the jet confinement on the other hand. Therefore, it produces more complicated jet impinging flow and convective heat transfer behaviors than the baseline case without corrugation. According to the tested results, the specified area-averaged heating effectiveness is increased approximately 6.3%–18.8% under Re_j = 7900 and 2.5%–9.4% Under Re_j = 31,700 respectively by increasing the corrugation length when W_j/d is fixed as 2.5. The specified area-averaged heating effectiveness is increased approximately 16.1%–22.1% under Re_j = 7900 and 7.7%–12.7% under Re_j = 31,700 respectively by increasing the corrugation width when H_j/d is fixed as 9. In general, the corrugation with larger length and width seems to perform the better heating effectiveness over the entire concave surface. The enhancement of heating effectiveness related to the baseline case behaves more significantly under a smaller jet Reynolds number.     

Twenty-two percent efficiency HIT solar cell

We have achieved the world's highest solar cell conversion efficiency of 22.3% (V_oc: 0.725 V, I_sc: 3.909 A, FF: 0.791, total area: 100.5 cm², confirmed by AIST) by using a heterojunction with intrinsic thin layer (HIT) structure. This is the world's first practical-size (>100 cm²) silicon solar cell that exceeds a conversion efficiency of 22% as a confirmed value. This high efficiency has been achieved mainly due to improvements in a-Si:H/c-Si hetero-interface properties and optical confinement.The excellent a-Si:H/c-Si hetero-interface of the HIT structure enables a high V_oc of over 0.720 V and results in better temperature properties. In order to reduce the power-generating cost, we are now investigating numerous technologies to further improve the conversion efficiency, especially the V_oc, of HIT solar cells, with the aim of achieving 23% efficiency in the laboratory by 2010.     

Chemically and electrochemically deposited thin films for solar energy materials

Direct energy gap materials, e.g. CdTe, CuInSe₂, CuInGaSe₂, CdSe, ZnP₂ and Zn₃P₂, are the most interesting for thin-film solar cell applications. Among the various methods of preparation of these films, chemical bath deposition and electrodeposition deserve special attention because they have been shown to be inexpensive, low-temperature and non-polluting methods. Based on Pourbaix diagrams of CdS, CdTe, CuInSe₂, CdSe, etc., drawn from basic considerations, the best parameters for their electrodeposition are deduced. Theoretical considerations on the chemical-bath deposition of CdS, CdSe and Sb₂S₃ are also indicated. In particular, the role of the complexing agent and of the ligands in chemical bath deposition quality is discussed, as are the uniformity and stability of the films. The photoelectrochemical, Schottky barrier and heterojunction solar cell properties based on chemically and electrochemically deposited thin films with heteropolyacids are shown. Future trends for chemically and electrochemically deposited polycrystalline thin films are addressed. Results from very recent work done in the improvement of chemically and electrochemically deposited thin films are presented. Significant results obtained on advanced CdS/CdTe, CdS/CIS and CdS/CIGS solar cells developed by industry and by laboratory groups worldwide are indicated. Emerging low cost materials or/and less environmental hazards materials which may introduce solar cells into worldwide market are considered in the conclusion.     

The role of oxygen in CuInSe2 thin films and CdS/CuInSe2 devices

The solar cell device CdS/CuInSe₂ has been shown to require a post-treatment in air at about 200 °C to reach its state-of-the-art efficiency of close to 11%. We have shown that treating the device in a solution of chemical oxidants has the same effect as the annealing in air. The results of oxidation can be reversed by treating the device in a solution of a reducing agent (hydrazine). Several experimental techniques were employed to show that the oxidation process, whether it is chemical or air annealing, serves to introduce oxygen into the CuInSe₂ whereas the reduction process serves to remove it. The acceptance of oxygen depends on the stoichiometry of CuInSe₂, and in turn affects the electrical properties. It is this interdependence between stoichiometry and the incorporation of oxygen which determines the extent of improvement in the CdS/CuInSe₂ performance. We have also shown that the oxygen in the CuInSe₂ can be dislodged from the CuInSe₂ material upon exposure to an electron beam. We present electron beam induced current measurements on the CdS/CuInSe₂ and argue that the oxygen is being partially removed from the CuInSe₂ layer by the electron beam and hence show a buried homojunction rather than the expected heterojunction. Finally, we demonstrate how we can use the role of oxygen in this material to improve the electrical properties of the CuInSe₂ thin film and the performance of the CdS/CuInSe₂ device.