Electrical and photovoltaic properties of SnSe/Si heterojunction

Thin film of SnSe is deposited on n-Si single crystal to fabricate a p-SnSe/n-Si heterojunction photovoltaic cell. Electrical and photoelectrical properties have been studied by the current density–voltage (J–V) and capacitance–voltage (C–V) measurements at different temperatures. The fabricated cell exhibited rectifying characteristics with a rectification ratio of 131 at ±1 V. At low voltages (V<0.55 V), the dark forward current density is controlled by the multi-step tunneling mechanism. While at a relatively high voltage (V>0.55 V), a space charge-limited-conduction mechanism is observed with trap concentration of 2.3×10²¹ cm⁻³. The C–V measurements showed that the junction is of abrupt nature with built-in voltage of 0.62 V which decreases with temperature by a gradient of 2.83×10⁻³ V/K. The cell also exhibited strong photovoltaic characteristics with an open-circuit voltage of 425 mV, a short-circuit current density of 17.23 mA cm⁻² and a power conversion efficiency of 6.44%. These parameters have been estimated at room temperature and under light illumination provided by a halogen lamp with an input power density of 50 mW cm⁻².     

Photocarrier behavior in organic heterojunction photovoltaic cells

We have investigated the bias dependence of photocurrent in several organic heterojunction cells to elucidate the behavior of photogenerated charge carriers. Both the planar and planar-mixed heterojunction devices are shown to always have negative photocurrent even at large forward biases; this phenomena has been attributed to an increased driving force for carrier diffusion away from the heterointerface as the applied bias increases. In contrast, the drift current generally dominates in mixed heterojunction devices due to distributed nature of charge generation throughout the active layer, leading to a photocurrent that is highly dependent on the internal electric field. This dependence gives rise to the reversal of the photocurrent direction at high biases when compared to that at the short-circuit condition. However, the voltage yielding zero photocurrent shows appreciable wavelength dependence, which is strongly correlated to the detailed charge carrier generation profile within the active layer.     

The photoresponse of CdS/CuInSe2thin-film heterojunction solar cells

The effect of light bias on the spectral current response and spectral capacitance characteristics of CdS/CuInSe2thin-film heterojunction solar cells has been investigated. Monochromatic light bias has been used to identify specific wavelength regions responsible for the spectral behavior seen under white light bias. Variations with light or voltage bias are consistent with the effect of the field on interface recombination in both high and low CdS resistivity devices. Devices with high CdS resistivity show spectrally dependent enhancement and quenching effects very similar to those reported for CdS/Cu2S devices in which the space charge region was primarily in the CdS. It is concluded that in high CdS resistivity devices the junction behavior is controlled by the photoconductive CdS as has been established in CdS/Cu2S cells. Low CdS resistivity CdS/CuInSe2devices show none of these effects.     

CuPc/C60 bulk heterojunction photovoltaic cells with evidence of phase segregation

This work presents organic photovoltaic (OPV) device study based on cupper phthalocyanine (CuPc) and fullerene (C60) bulk heterojunction (BHJ) structure. By varying blend composition, the optimized performances were obtained in 75%-CuPc containing devices with anode buffer of either CuPc or HPCzI (1,3,4,5,6,7-hexaphenyl-2-{3′-(9-ethylcarbazolyl)}-isoindole). It was discovered by scanning electron microscopy that 75%-CuPc containing film possessed phase separation, which is beneficial to charge transport via percolation process. Additionally, electronic absorption measurement and hole only device study showed that, depending on the mixing ratio, the absorption and the hole transport ability were different. The blend film containing 75% CuPc had the largest integrated absorption with the most CuPc dimmer aggregate and the least C60 aggregate. Moreover, the 75% CuPc blend film also possessed the highest hole transport ability. Thus, the best performance of the 75% CuPc BHJ device is mainly attributed to its good carrier transport originating from phase segregation. The present work highlights the phase separation in CuPc:C60 mixing film with optimized ratio, as well as its corresponding electronic absorption and carrier transport properties, which are essential for OPV device performance. Hence, insights are inferred for further engineering of BHJ OPV devices based on small molecules.     

Simulation approach for optimization of ZnO/c-WSe2 heterojunction solar cells

Taking into account defect density in WSe₂, interface recombination between ZnO and WSe₂, we presented a simulation study of ZnO/crystalline WSe₂ heterojunction (HJ) solar cell using wxAMPS simulation software. The optimal conversion efficiency 39.07% for n-ZnO/p-c-WSe₂ HJ solar cell can be realized without considering the impact of defects. High defect density (> 1.0 × 10¹¹ cm^-2) in c-WSe₂ and large trap cross-section (> 1.0 × 10^-10 cm²) have serious impact on solar cell efficiency. A thin p-WSe₂ layer is intentionally inserted between ZnO layer and c-WSe₂ to investigate the effect of the interface recombination. The interface properties are very crucial to the performance of ZnO/c-WSe₂HJ solar cell. The affinity of ZnO value range between 3.7-4.5 eV gives the best conversion efficiency.     

Electrical properties of Cu4ZnSnS2/ZnS heterojunction prepared by ultrasonic spray pyrolysis

Cu₂ZnSnS₄ (CZTS)/ZnS heterojunctions have been prepared by a successive deposition of ZnS and CZTS thin films by ultrasonic spray pyrolysis technique on glass substrates. The cupric chloride concentration has been varied in the starting solution in order to investigate its influence on device properties. CZTS/ZnS heterojunctions were characterized by recording their current-voltage characteristics at different temperatures. The obtained results exhibit a good rectifying behavior of the realized heterojunction. Analysis of these results yields saturation current, series resistance and ideality factor determination. From the activation energy of saturation current we inferred that the thermal emission through the barrier height is the dominant mechanism of the reverse current rather than the defects contribution.     

Efficient oligothiophene:fullerene bulk heterojunction organic photovoltaic cells

Novel small-molecule bulk heterojunction photovoltaic (PV) cells consisting of oligothiophen (alpha-sexithiophen: 6T) and fullerene (C₆₀) have been developed. Oligothiophen is well known as a good hole-transport material, and by changing the number of thiophen rings and making chemical modifications or substitutions, its characteristics relevant to PV applications (such as carrier mobility, energy level, packing, and ordered structure) can be controlled. Thus far it has been difficult to fabricate films of oligothiophene--fullerene blends with suitable morphology by using the common co-evaporation method, because oligothiophene crystallizes easily during film deposition. The present study found that the morphology of 6T:C₆₀ blends strongly depending on the composition of 6T:C₆₀. Suitable morphology was obtained only for films deposited with the co-evaporation of excess C₆₀. It is likely that excess C₆₀ prevents the crystallization of 6T. By successfully controlling the film morphology, we were able to demonstrate good PV performance in oligothiophene:fullerene bulk heterojunction PV cells for the first time. Moreover, it was found that PV performance could be further improved by inserting a C₆₀ layer between the blend layer and exciton blocking layer.     

Radiation effect on the optical and electrical properties of CdSe(In)/p-Si heterojunction photovoltaic solar cells

The efficiency and radiation resistance of solar cells are graded. They are then fabricated in the form of n-CdeSe(In)/p-Si heterojunction cells by electron beam evaporation of a stoichiomteric mixture of CdSe and In to make a thin film on a p-Si single crystal wafer with a thickness of 100 μm and a resistivity of ～ 1.5 Ω·cm at a temperature of 473 K. The short-circuit current density (j_sc), open-circuit voltage (V_oc), fill factor (ff) and conversion efficiency (η) under 100 mW/cm² (AM1) intensity, are 20 mA/cm², 0.49 V, 0.71 and 6% respectively. The cells were exposed to different electron doses (electron beam accelerator of energy 1.5 MeV, and beam intensity 25 mA). The cell performance parameters are measured and discussed before and after gamma and electron beam irradiation.     

Efficiency of the a-Si:H solar cell and grain size of SnO2transparent conductive film

The relationship between average grain size on the surface of SnO2transparent conductive film and conversion efficiency of the a-Si:H solar cell was investigated. a-Si:H solar cells were fabricated on SnO2/glass substrates with various grain sizes. The cell structure was glass/p(SiC)-i-n/Al and the effective cell area was 4 × 10^-2cm². The reflectivity from the glass substrate was reduced to about 7 percent with increasing the grain size from 0.1 to 0.8µm, and the short-circuit current was inceased from 12 to 14mA/cm². A 7.9 percent of conversion efficiency was achieved using milky SnO2film of 0.4-µm average grain size at AM-100mW/cm².     

High efficiency a-Si:H p-i-n solar cell using a SnO2/glass substrate

Dependence of conversion efficiency of a-Si:H p-i-n solar cells on the material of transparent conductive film substrates was studied. Irrespective of preparation method of the transparent conductive film, SnO2films gave an advantage over ITO films for high efficiency cell production by a parallel plate electrode type plasma C.V.D. equipment. A conversion efficiency of 7.19% was achieved by optimizing preparation conditions of the SnO2film and the a-Si:H film.     

Optimization and degradation of rubrene/C70 heterojunction solar cells

Small molecule organic solar cells (OSCs) with the structure of indium tin oxide (ITO)/molybdenum trioxide (MoO3) (5 nm)/rubrene (x nm)/fullerene (C70) (y nm)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP) (6 nm)/aluminum (Al) (150 nm) are fabricated. The thickness of active layer for the devices is investigated in details. The results show that the optimum thicknesses of rubrene layer and C70 layer are 30 nm and 25 nm, respectively. The degradation of the device is also investigated. The result indicates that the open-circuit voltage (Voc) does not change, while the short-circuit current density (Jsc), fill factor (FF) and power conversion efficiency (PCE) decrease continuously with time. The degradation can be attributed to the oxygen in ambient diffusing and infiltrating into the active materials and reacting with C70 in cells, which can result in the increase of interfacial series resistance.     

Electrical and photovoltaic properties of an organic-inorganic heterojunction based on a BODIPY dye

An organic-inorganic heterojunction based on a BODIPY dyes has been produced by forming dye thin film on n-Si. The electrical parameters of the structure have been investigated by current-voltage (I-V) and capacitance-voltage (C-V) measurements. The ideality factor, the barrier height and the series resistance values of the diode have been calculated as 2.43, 0.84 eV, and about 1.3 kΩ, respectively. The diode behaves as a non-ideal diode because of the series resistance and interface layer. The barrier height value obtained from I-V measurement has been compared with one from C-V measurement. Moreover, it has been seen that the diode is highly sensitive to the light and the reverse bias current increases about 1 × 10⁴ times at −1 V under 100 mW/cm² and AM1.5 illumination condition. The short photocurrent density (J_sc) and the open circuit voltage (V_oc), the fill factor (FF) and power conversion efficiency (η) have been determined as 3.78 mA/cm², 327 mV, 0.28 and 0.48 %, respectively.     

Water-soluble poly(3,4-ethylenedioxythiophene)/nano-crystalline TiO2 heterojunction solar cells

Nanocrystalline titanium dioxide (TiO₂) thin films were prepared by the sol-gel method and were then used to fabricate an indium-tin oxide (ITO)/nano-crystalline TiO₂/poly(3,4-ethylenedioxythiophene) (PEDOT)/Au device. The junction thus obtained shows a rectifying behavior. Their current-voltage (I-V) characteristics in dark indicate that a heterojunction at the nano-crystalline TiO₂/PEDOT interface has been created. The measured open-circuit voltage (V_oc) and short-circuit current (I_sc) for the device under illumination with 50 mW/cm² light intensity under AM 1.5 conditions (device dimension was 1 cm²) are V_oc=0.39 V, I_sc=54.9 μA/cm², the filling factor (FF)=0.429 and the energy conversion efficiency (η)=0.03%.     

GaInP2/Ge异质结外延材料特性分析

研制了一种基于Ge衬底的异质结热光伏(TPV)电池 ,并且在Ge衬底上用金属有机化学气相沉积(MOCVD)法外延单晶材料GaInP₂和GaAs,对GaInP₂/Ge异质结界面进行了断面扫描电镜(SEM)、X射线衍射(XRD)、电化学电 容电压(ECV)和光致荧光(PL)谱的测试分析,研究了基于 Ge衬底的异质结n-GaInP₂/p-Ge界面的结构、光学和电学特性,得到了高质量、宽禁带 的单晶外延层,与Ge衬底晶格匹配良好,利于更多光子进入到吸收层,为制备高效率TPV电 池打下良好基础。     

Preparation and properties of Ni/InGaN/GaN Schottky barrier photovoltaic cells

Ni/In_xGa_1−xN/GaN Schottky barrier solar cells with different In contents (x = 0.07/0.13) and two types of Schottky patterns (semitransparent current spreading layer and grid contact) are fabricated and the dependences of photovoltaic performances of these solar cells on In contents and Schottky patterns are studied. Solar cells with semitransparent contact have almost the same open-circuit voltages (V_oc) as solar cells with grid contact and exhibit a higher fill factor (FF). However, solar cells with grid contact exhibit a higher maximum output power density (P_max) than semitransparent contact due to their larger short-circuit current density (J_sc). On the other hand, V_oc and FF decrease significantly with increasing In content (beyond the decrease expected from the band gap of InGaN). By comparing the X-ray rocking curves, AFM images, dark current characteristics and spectral responsivities, it could be concluded that the deterioration of InGaN crystal quality with increasing In content is the dominant reason accounting for the strong decrease of V_oc and FF. In addition, using AMPS simulation, the band structure and ideal spectral responsivities are obtained. Comparison of the experimental and simulated results also shows that high crystal quality is a key factor to obtain high performance InGaN-based Schottky barrier photovoltaic cells.     

Interpenetrating heterojunction photovoltaic cells based on C60 nano-crystallized thin films

An interpenetrating heterojunction (IHJ) structure facilitates efficient charge separation and transport in the active layer of organic photovoltaic cells (OPVs). Additionally, the recombination of generated carriers in IHJs is reduced as these networks exhibit high carrier transport with minimal recombination sites. We have developed a simple method to fabricate nanocrystallized fullerene (C₆₀) films, which are produced by subjecting evaporated C₆₀ films to either solvent spin-coating or solvent vapor annealing (SVA). The size of the rod-shaped nanocrystals in the films were controlled by changing the solvent and annealing time.An 80-nm-diameter size nanocrystallized C₆₀ film that was fabricated using SVA with ethanol was incorporated as an acceptor material in an inverted IHJ OPV cell. Tetraphenyldibenzoperiflanthene (DBP) was evaporated onto the nanocrystallized C₆₀ film as the donor material. The power conversion efficiency of an IHJ OPV cell (ITO/TiOx/nanocrystallized C₆₀ film/DBP/MoO₃/Au) increased from 1.79% to 2.12%, when compared with the conventional PHJ OPV cell.     

Bidirectional rectifier with gate voltage control based on Bi2O2Se/WSe2 heterojunction

Two-dimensional (2D) WSe₂ has received increasing attention due to its unique optical properties and bipolar behavior. Several WSe₂-based heterojunctions exhibit bidirectional rectification characteristics, but most devices have a lower rectification ratio. In this work, the Bi₂O₂Se/WSe₂ heterojunction prepared by us has a type Ⅱ band alignment, which can vastly suppress the channel current through the interface barrier so that the Bi₂O₂Se/WSe₂ heterojunction device has a large rectification ratio of about 10⁵. Meanwhile, under different gate voltage modulation, the current on/off ratio of the device changes by nearly five orders of magnitude, and the maximum current on/off ratio is expected to be achieved 10⁶. The photocurrent measurement reveals the behavior of recombination and space charge confinement, further verifying the bidirectional rectification behavior of heterojunctions, and it also exhibits excellent performance in light response. In the future, Bi₂O₂Se/WSe₂ heterojunction field-effect transistors have great potential to reduce the volume of integrated circuits as a bidirectional controlled switching device.     

Increases in energy conversion efficiency for thin-film polycrystalline CdS/Cu2S photovoltaic cells

Thin-film polycrystalline CdS/Cu2S photovoltaic solar cells with terrestrial energy conversion efficiencies in sunlight up to 7.8 percent have been developed. The major improvements are due to the increased optical transmission and electrical contact properties of the current collection system.     

Sprayed zinc-cadmium sulfide films for Cu2S/ZnxCd1-xS heterojunction solar cells

Cu2S/ZnxCd1-xS solar cells with stable open-circuit voltages up to 0.784 V were formed on zinc-cadmium sulfide films deposited by spray pyrolysis. x was varied between zero and 0.7 by varying the spray solution composition. ZnxCd1-xS films formed Schottky diodes with evaporated chromium contacts; depletion layer width (in light) in the films increased with x, from 0.16 to 3.1 microns. Resistivity of the films increased exponentially with x. Optical band gap increased with x from 2.41 eV at x = 0 to 2.82 eV at x = 0.6.