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Graphitic carbon/n-CdTe Schottky-type heterojunction solar cells prepared by electron-beam evaporation
Abstract
We report on the analysis of the electrical and photoelectrical properties of graphitic carbon/n-CdTe Schottky-type heterojunction solar cells, which have been prepared by the deposition of transparent graphitic carbon films of nanometer thickness onto freshly cleaved n-CdTe substrates by the electron-beam evaporation technique. The presence of the electrically-active interface states at the heterojunction interface was revealed from the analysis of the dominating current transport at forward and reverse bias and from the capacitance–
voltage characteristics. The unoptimized graphitic carbon/n-CdTe Schottky-type heterojunctions possess a rectification ratio of RR = 1390 and produce a maximum short circuit current density of Jsc = 8.47 mA cm�2, an open-circuit voltage of Voc = 0.435 V and fill factor of FF = 0.37 under standard illumination conditions (100 mW cm�2 AM 1.5).
... In addition to these advantages properties and efficient application, due to it's lower cost and high absorption coefficient [45,46], it is also used in some other novel applications, such as detectors [47] and photovoltaics [48]. Various methods, such as atomic layer epitaxy (ALE) [49], pulsed laser [50], spin coating [51], electron beam evapora-tion [52], electro-deposit [53], metal-organic chemical vapour deposition (MOCVD) [50], close-space sublimation (CSS) [52], sputtering [54] and also thermal evaporation [55] have been investigated as suitable techniques for the fabrication of CdTe thin film. The third film materials used in this study, CsPbBr 3 , a member of inorganic perovskites, has attracted much interest because of large light absorption coefficient, high carrier mobility and superior stability under harsh humid and thermal environments, which are suitable for various optoelectronic devices applications [56][57][58][59], such as solar cells and photo detectors [60,61]. ...
... In addition to these advantages properties and efficient application, due to it's lower cost and high absorption coefficient [45,46], it is also used in some other novel applications, such as detectors [47] and photovoltaics [48]. Various methods, such as atomic layer epitaxy (ALE) [49], pulsed laser [50], spin coating [51], electron beam evapora-tion [52], electro-deposit [53], metal-organic chemical vapour deposition (MOCVD) [50], close-space sublimation (CSS) [52], sputtering [54] and also thermal evaporation [55] have been investigated as suitable techniques for the fabrication of CdTe thin film. The third film materials used in this study, CsPbBr 3 , a member of inorganic perovskites, has attracted much interest because of large light absorption coefficient, high carrier mobility and superior stability under harsh humid and thermal environments, which are suitable for various optoelectronic devices applications [56][57][58][59], such as solar cells and photo detectors [60,61]. ...
Recently, optical thin-films with lower reflectivity have attracted much interest for their suitability in high performance thin-film solar cells and various modern photonics devices, such as electronic display panels touchscreens, smart optical glass windows, spectacles frames, super-compact camera lenses, laser systems and optical fiber communications since lowering reflectivity coating improves the device performances. However, obtaining reduced reflectance from this arrangement remains challenging issue. As the film optical properties, such as the absorbance, reflection and transmission of particular wavelength of electromagnetic radiation can be carefully controlled by optimizing thin-film fabrication materials as well as structures, there is a lot of research scope in optimizing device reflectivity by assessing various film- and substrate materials as well as their thicknesses. Therefore, in this study, the reflectance performances of optical thin-films were characterized for obtaining lower reflectivity for various types of modern photonics applications. To obtain this, three novel optoelectronic materials InGaAs, CdTe and CsPbBr3 for film layer, three widely used substrate materials glass, Al2O3 and steel as well as various thicknesses of film layer were evaluated. Reflectance studied of the thin-films for the three film materials have been clarified that CsPbBr3 is the best among these three film materials to be used for reducing the light reflection of the thin-film. Lower reflectivity of thin-films on glass substrate suggested that glass is better than both Al2O3 and steel as substrate in high efficiency thin-film solar cells and various photonics devices. In addition, evaluation of reflectance for various film thicknesses showed that ultra-thin film layer is superior for reducing the reflection of solar energy by thin-film structure. We have therefore proposed that thin-film with the combination of CsPbBr3 based ultra-thin film layer on glass substrate would be one of the best possible solutions for reducing reflectivity of solar cells and various photonics devices, thereby for possibly increasing the performance efficiency. This research result would be very beneficial for the development of renewable energy and photonics based nanotechnology, thereby play a significant role for reducing global energy crisis and green-house gas emission concurrently and sustainably in the modern world.
... The application of carbon nanotubes for the fabrication of silicon-based solar cells combines unique electrical and optical properties of nanomaterials with the established technology of silicon photovoltaics [3]. The development of the low-cost and solvent-free technology of pencil-drawn graphite films has found its application in the fabrication of solid-state CdTe photodetectors and flexible organic electronic devices [4][5][6][7]. The electron-beam evaporation technique was used for the fabrication of the graphitic carbon/n-CdTe heterojunctions [6]. ...
... The development of the low-cost and solvent-free technology of pencil-drawn graphite films has found its application in the fabrication of solid-state CdTe photodetectors and flexible organic electronic devices [4][5][6][7]. The electron-beam evaporation technique was used for the fabrication of the graphitic carbon/n-CdTe heterojunctions [6]. Graphite films are also used in hybrid organic-inorganic perovskite solar cells [8]. ...
Nano-crystalline and amorphous films of graphitized carbon were deposited by electron-beam evaporation of bulk graphite. Structural properties and the size of graphite nanoclusters (L ≈ 1.2 - 5 nm) in the films weredetermined from the analysis of their Raman spectra. Electrical properties of the bulk nano-crystalline graphite reference sample and the deposited graphitic carbon films were measured by means of the Hall effect technique within the temperature range from 290 to 420 K. The electrical conductivity σ and Hall mobility μH of all samples exhibited exponential temperature dependences, indicating on the non-metalic behavior. Electrical properties of the amorphous graphitic carbon thin films, deposited at low substrate temperatures (620 and 750 K) wereanalyzed in the scope of the hopping charge transport mechanism via localized states. We have shown that the charge transport in the bulk and thin film (920 K) nano-crystalline graphite samples is carried out via the tunneling and thermionic emission over potential barriers at the grain boundaries.This paper contributes towards better understanding of coupling between structural and electrical properties of graphitic carbon thin films.
... To overcome this drawback, as an alternative, p-type window layer with n-type CdTe absorber can be used. Reports on n-type CdTe absorber layer used in graphitic carbon Schottky -type heterojunction [8], reduced grapheme oxide (rGO) solar cells [9] and diamond/CdTe inverted heterojunction solar cells [10] are available in literature. CZT films with n-type conductivity can improve the performance of these type of solar cells due to their favourable optoelectronic properties mentioned in the beginning. ...
... Due to high absorption coefficient of CdS, quest for high transmittance window is a frontier research field in the photovoltaic technology development. In this regard, rGO or carbon-based window layer has come to limelight [8][9][10] n-type absorber is required for these Schottky-type heterojunction solar cells. Among the samples, x ¼ 0.8 fits the need. ...
CdxZn1-xTe (CZT) films with different compositions were grown by thermal co-evaporation method, in which cadmium telluride (CdTe) and zinc telluride (ZnTe) were used as the source materials. The structural, morphological and optical properties of the films were characterized by x-ray diffraction (XRD), UV–Visible spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), photoluminescence (PL) and Raman spectroscopy. Detailed analysis has shown that films were having cubic phase with orientation along (111) plane and the optical band gap reduced from 2.17 to 1.53 eV when x was varied from 0 to 1. SEM micrographs confirmed the homogeneity of the films. Depending on the composition, Raman peaks of CZT samples were mixture of the fundamental modes of the CdTe and ZnTe and indicate “trigonal” lattice of Tellurium (Te). Analysis of PL spectra suggested tellurium as isoelectronic exciton traps, recombination of carriers in the surface traps and donor-acceptor pair present in the samples. Under electrical properties, p to n-type conversion was witnessed for x = 0.4 and above. Optical bad gap of about 1.60 eV with minimum Urbach energy (28.88 meV) and favourable electrical properties has led to conclude that CZT films with x = 0.8 may be suitable for solar cell absorber.
... Ideally, the work function of graphene is close to the electron affinity of CdTe, indicating a possible junction formation. According to Brus et al. [42], their graphite/n-CdTe In addition, impurity doping into graphene was also tried in back contacts. One example is the comparison among boron-doped graphene, intrinsic graphene and reduced graphene oxide (r-GO) [41]. ...
... Ideally, the work function of graphene is close to the electron affinity of CdTe, indicating a possible junction formation. According to Brus et al. [42], their graphite/n-CdTe (fabricated with the Cd-rich atmosphere) was able to acquire an efficiency of 1.36%, with a clear shunting effect. Later, Lin et al. [43] announced an unusual substrate-configured graphene/CdTe device. ...
Commercial solar cells have a power conversion efficiency (PCE) in the range of 10–22% with different light absorbers. Graphene, with demonstrated unique structural, physical, and electrical properties, is expected to bring the positive effects on the development of thin film solar cells. Investigations have been carried out to understand whether graphene can be used as a front and back contacts and active interfacial layer in solar cell fabrication. In this review, the current progress of this research is analyzed, starting from the graphene and graphene-based Schottky diode. Also, the discussion was focused on the progress of graphene-incorporated thin film solar cells that were fabricated with different light absorbers, in particular, the synthesis, fabrication, and characterization of devices. The effect of doping and layer thickness of graphene on PCE was also included. Currently, the PCE of graphene-incorporated bulk-heterojunction devices have enhanced in the range of 0.5–3%. However, device durability and cost-effectiveness are also the challenging factors for commercial production of graphene-incorporated solar cells. In addition to the application of graphene, graphene oxides have been also used in perovskite solar cells. The current needs and likely future investigations for graphene-incorporated solar cells are also discussed.
... These devices exhibited FF compared to the reported FF values range of 0.4-0.7 for some organic solar cells [54]. A PV cell system comprising graphene/CdSe and graphitic carbon/CdTe showed a PCE of 1.25 and 1.36%, respectively [55,56]. The BHJSCs fabricated using a nanocomposite without PTCDA, however, displayed a much lower PCE (0.1%), suggesting that the PTCDA molecular linker plays an important role in improving the charge generation and transfer between QDs and MWCNTs. ...
Materials and composites with the ability to convert light into electricity are essential for a variety of applications, including solar cells. The development of materials and processes needed to boost the conversion efficiency of solar cell materials will play a key role in providing pathways for dependable light to electric energy conversion. Here, we show a simple, single-step technique to synthesize photoactive nanocomposites by coupling carbon nanotubes with semiconducting quantum dots using a molecular linker. We also discuss and demonstrate the potential application of nanocomposite for the fabrication of bulk heterojunction solar cells. Cadmium selenide (CdSe) quantum dots (QDs) were attached to multiwall carbon nanotubes (MWCNTs) using perylene-3, 4, 9, 10-tetracarboxylic-3, 4, 9, 10-dianhydride (PTCDA) as a molecular linker through a one-step synthetic route. Our investigations revealed that PTCDA tremendously boosts the density of QDs on MWCNT surfaces and leads to several interesting optical and electrical properties. Furthermore, the QD–PTCDA–MWCNTs nanocomposites displayed a semiconducting behavior, in sharp contrast to the metallic behavior of the MWCNTs. These studies indicate that, PTCDA interfaced between QDs and MWCNTs, acted as a molecular bridge which may facilitate the charge transfer between QDs and MWCNTs. We believe that the investigations presented here are important to discover simple synthetic routes for obtaining photoactive nanocomposites with several potential applications in the field of opto-electronics as well as energy conversion devices.
... The non-toxicity and costeffectiveness of the production process make copper oxide an alternative to silicon, which is predominantly used for manufacturing solar panels. Recently, thin layers of graphitized carbon have been used as functional layers (windows) in inexpensive optoelectronic devices [21] owing to their unique electrical and optical properties [22,23]; thus, they could be promising candidates for use in solar cells with copper oxide. ...
In this study, we investigated the potential of copper oxide (CuO) thin films as active layers in thin-film solar cells with a Glass/ITO/Graphite/CuO/Ni structure. Furthermore, the generation rate of charge carriers was derived by modeling the optical field distribution using a transfer metric simulation. Theoretical thresholds for photovoltaic device efficiency were determined for varying active layer thicknesses by employing a normalized light intensity equivalent to that of the AM1.5 spectrum. The current-voltage characteristics are modeled by semi-empirical methods, which illustrate that the photovoltaic conversion efficiency depends on the thickness of the active layer. The highest performance of the simulated structure of the solar cell was 25.2%, obtained for the 500 nm CuO films.
... 9 Significant efforts on cadmium telluride (CdTe) have been made in structural, optical, thermal, and electrical aspects. [10][11][12][13][14][15][16][17] The present framework aims to study the dark I-V characteristics in both forward and reverse bias as a function of temperature in the range of 300-375 K and an applied voltage range between − 2 and 2 V. In this work also, the transport mechanisms are discussed in detail. ...
The main focus of this framework is the preparation of CdTe nanocrystalline thin films (similar to 120 nm) on single crystal p-Si wafers (270 mu m) with Miller index (100) using thermal evaporation. Then, the In/n-CdTe/p-Si/Al solar cell was successfully fabricated. The dark I-V characteristics for the fabricated solar cell have been determined in range of 300-375 K and an applied voltage range of - 2 to 2 V. The fabricated solar cell's behavior was thoroughly explained. As a result, the important parameters for the fabricated solar cell such as the rectification ratio RR, the junction resistance R-J, ideality factor of solar cell n, the shunt resistance R-sh, the series resistance R-s, the barrier height created at the interface between the CdTe thin film and the p-Si wafer phi(b), the energy of trap level E-t and the activation energy of carriers's recombination in the depletion region Delta E were determined. Finally, the Poole-Frenkel beta(PF) and Schottky beta(S) parameters were computed.
... Результати проведених авторами досліджень, які подано в монографії, опубліковані в працях [28], [35], [36], [42], [84], [85], [164], [170]- [178]. ...
The monograph deals with the physical properties of thin graphite films and graphene layers. The electrical, photoelectric and capacitive properties of heterojunctions based on the obtained films were studied in order to determine the dominant mechanisms of current transfer in forward and reverse biases.
... Перед початком нанесення плівок графіту, (слідуючи за технологією -«олівець-на-напівпровіднику» [12][13][14][15][16][17]), одну з поверхонь свіжосколеної монокристалічної соляної підкладки (NаCl) механічно шліфували до шорсткості Rа=0,2 мкм, R z =0,23 мкм і R mаx =1,1 мкм. Якісна графітова плівка малювалася на попередньо підготовленій поверхні соляної підкладки за допомогою чистого графітового стержня 1 мм у діаметрі та олівців з різною твердістю 2H, H, HB, B та 2В при постійній силі притиснення в 1 Н. ...
The paper presents the results of studying the structural, optical and electrical properties of thin films of graphite depending on the hardness of the rods (2H, H, HB, B and 2B) obtained by the "Pencil-on-semiconductor" method. Such studies are of great importance for the further development of highly efficient devices based on heterojunctions for electronics and optoelectronics. Typical images of the surface formed by reflected electrons (BSE) were obtained using a scanning electron microscope and shown at three magnifications (100x, 500x and 1000x). Since the cores of the studied pencils consist of mixtures of clay and graphite, a more detailed analysis of the elements that make up the cores was conducted. EDS analysis showed that the main components of the studied rods are purified graphite powder, as well as O, Al and Si, which are part of kaolin whose formula is H4Al2Si2O9, or Al2O3 • 2SiO2 • 2H2O - the main component of ordinary clay. The elemental composition of the microvolume of the studied samples was also determined. Regardless of the error that occurs when determining the composition of C and O (~ 12%), it can be argued that there is still a regularity between the graphite content and the hardness of the pencil. That is, the higher the graphite content, the softer the rod. The thickness of the graphite films was measured using the MII-4 interferometer according to the standard method. The average thickness of all investigated films was ~ 150 nm since the thickness of the films obtained by this method is mainly determined by the roughness of the surface of the salt substrate. Drawn graphite films have a higher resistivity than bulk samples (pencil rods) from which they were made. The resistance of the films increases with an increase in the hardness of pencils, due to an increase in the number of clay impurities in graphite, which is a dielectric. It was found that an increase in stick hardness leads to an increase in transmission.
... V). In this range, the measured I-V characteristics can be governed by the standard equation, which takes into account the effect of the series R s and shunt R sh resistance [22]: ...
PEDOT: PSS thin films are widely used as transparent coatings in flexible semiconductor devices including solar cells. However, they are not widely used as transparent coatings in combination with crystal substrates. This work shows the possibility of using PEDOT:PSS thin films as a frontal transparent conducting layer in hybrid organic-inorganic Schottky type heterojunctions of the PEDOT:PSS/n‑CdTe, which were prepared by deposition of PEDOT:PSS thin films (using the spin-coating method) on crystalline cadmium telluride substrates. The current-voltage (in a wide temperature range) and capacitance-voltage (at room temperature) characteristics of heterojunctions were measurement and analyzed. It has been established that PEDOT:PSS/n-CdTe heterojunctions have good diode properties with a high rectification ratio RR≈105, a potential barrier height φ0 = 0.95 eV, and series Rs = 91 Ohm and shunt Rsh = 5.7 × 107 Ohm resistances. Analysis of the forward branches of the I–V characteristics of heterojunctions showed that the dominant charge transfer mechanisms are determined by the processes of radiative recombination at low biases (3kT/e <V <0.3 V) and tunneling through a thin depleted layer at high biases (0.3 V <V <0.6 V). Capacity-voltage characteristics are plotted in the Mott-Schottky coordinate, taking into account the influence of series resistance, measured at a frequency of 1 MHz. Used the C-V characteristic was determined the value of the built-in potential Vc = 1.32 V (it correlates well with the cutoff voltage determined from the current-voltage characteristics) and the concentration of uncompensated donors in the n-CdTe substrate ND-NA = 8.79 × 1014 cm-3. Although the photoelectric parameters of unoptimized PEDOT:PSS/n-CdTe heterojunctions are low, their photodiode characteristics (Detectivity D*> 1013 Jones) are very promising for further detailed analysis and improvement. The proposed concept of a hybrid organic-inorganic heterojunction also has potential for use in inexpensive γ- and X-ray detectors.
... Of particular note is their use in photovoltaics, where these carbon materials are used as transparent * Author to whom any correspondence should be addressed. windows, as well as front and rear contacts in various types of barrier structures [6][7][8][9][10][11][12][13][14]. ...
Photosensitive Schottky diodes of graphite/n-Cd1−x Zn x Te were obtained by depositing thin films of graphitized carbon on crystalline substrates of n-Cd1−x Zn x Te solid solution by electron beam evaporation. Based on the analysis of the single-phonon Raman spectra, it was found that the obtained films can be considered as nanocrystalline carbon structures with crystallite sizes of La ≈ 4.8 nm. From the research on the temperature dependencies of the I–V-characteristics and frequency dependencies of the C–V-characteristics, the main parameters of the structure were determined as well as the role of surface energy states in the formation of the profile of energy zones in the contact area. The main mechanisms of the forward and reverse currents are established. Using the diffusion theory of rectification, the height of the potential barrier was calculated and found to coincide with the experimentally determined value. A model of the diode energy diagram is proposed, which accurately describes the experimental electrophysical phenomena. The photoelectric properties of the graphite/n-Cd1−x Zn x Te diodes were studied.
... The low resistivity and high transparency of graphene make it possible to use it to make cheap transparent electrodes that can be used in photodiodes and organic solar cells. Moreover, graphene is becoming increasingly attractive as a window material in heterostructural solar cells such as the Schottky diode 15 . Electronic devices based on heterojunctions are of considerable scientific and practical interest [16][17][18][19][20][21][22] . ...
... 1 The CdTe has been used in various optoelectronic devices such as photovoltaics of solar cells, 2,3 detectors, 4 and p-n junction devices. 5 CdTe nanoparticles can be grown on the substrates by a variety of methods such as thermal evaporation, 6 electron beam evaporation, 7 sputtering, 8 pulsed laser, 9 spin coating, 10 and close-spaced sublimation. 11 Thermal evaporation deposition is a promising method for depositing nanoparticles on the substrates. ...
Cadmium telluride (CdTe) nanoparticles were deposited on amorphous glass and crystal quartz as nonconductive films, indium tin oxide and fluorine doped tin oxide as transparent conducting films, and silver as a metal at 100°C under pressure of 2 × 10⁻⁵ mbar. CdTe thin films prepared by a thickness about 80 nm. The results of x-ray diffraction analysis show the grain size of preferential orientation was between 5.48 nm and 15.37 nm. Also, the preferential orientation changed from (111) for non-conducting substrates to (220) for conducting substrates. The investigation of texture coefficient (TC) has indicated the deviation of TC from unity for metals substrates is further than the other substrates. The optical properties of CdTe thin films such as the optical band gap, extinction coefficient and refractive index, real and imaginary parts of dielectric constant were investigated by ultraviolet–visible spectroscopy (UV–Vis) as a function of photon energy in the wavelength range of 600–1600 nm. These measurements indicated the increasing of optical band gap by increasing the conductivity of substrates. Scanning electron microscopy analysis used to investigate the morphology of thin films.
... Невелике значення висоти потенціального бар'єру, як правило призводить до протікання надбар'єрного струму. В області напруг (3kT/e V 0,2 В) В залежність І(V) добре описується виразом для емісійнорекомбінаційного механізму струмопереносу з урахуванням впливу послідовного і шунтуючого опорів (пряма рекомбінація носіїв заряду через енергетичні стани на поверхні розділу, яка визначається висотою потенціального бар'єру) [8]: ...
MoOx/n-Si heterojunctions were prepared by the deposition of MoOx thin films by means of the reactive magnetron sputtering technique onto silicon substrates. Current-voltage characteristics (I-V) of the prepared heterojunctions were measured at different temperatures. The temperature dependence of the height of the potential barrier and series resistance was analyzed. The energy diagram of the heterojunctions under investigation was developed. The concentration of the surface states at the heterojunction was estimated and the dominant charge transport mechanisms were deterimined at forward and reverse bias. The heterojunctions under investigation generate open-circuit voltage Voc = 0.167 V and short-circuit current Isc = 8.56 mA/CM² under illumination 80 mW/cm².
... Efforts to improve efficiency solar cells of cadmium telluride semiconductor nanoparticles deposited on substrates through various methods continued to be appropriate [8]. Cadmium Telluride nanoparticles can be deposited on the substrate using various methods such as thermal evaporation [9], pulsed laser [10], electron beam evaporation [11], sputtering [12], spin coating [13], and close-spaced sublimation [8]. Deposition by the thermal evaporation method is one of the most effective methods for deposition of nanoparticles on the substrate. ...
Thermal evaporation is one of the promising methods for depositing CdTe thin films, which can obtain the thin films with the small thickness. In this work, CdTe nanoparticles have deposited on SiO2 substrates such as quartz (crystal) and glass (amorphous) at a temperature (Ts) of 150 °C under a vacuum pressure of 2 × 10⁻⁵ mbar. The thickness of CdTe thin films prepared under vacuum pressure is 100 nm. X-ray diffraction analysis (XRD) results showed the formation of CdTe cubic phase with a strong preferential orientation of (111) crystalline plane on both substrates. The grain size (D) in this orientation obtained about 7.41 and 5.48 nm for quartz and glass respectively. Ultraviolet-visible spectroscopy (UV–vis) measurements indicated the optical band gap about 1.5 and 1.52 eV for CdTe thin films deposited on quartz and glass respectively. Furthermore, to show the effect of annealing temperature on structure and optical properties of CdTe thin films on quartz and glass substrates, the thin films have been annealed at temperatures 50 and 70 °C for one hour. The results of this work indicate that the structure's parameters and optical properties of CdTe thin films change due to increase in annealing temperature.
... In recent years, methods for obtaining thin carbon layers cause considerable scientific and practical interest, due to their unique properties, good electrical conductivity, transparency, high mechanical properties, high mobility of charge carriers at room temperature 3,15 . We used a simple, low-cost and environmentally friendly method 15 for the reproducible fabrication of high quality next-generation pencil-on-semiconductor electronic, optoelectronic and photovoltaic devices by the transfer of dry drawn graphite films on arbitrary planar semicnductor substrates. ...
Graphite/n-SiC Shottky diodes were prepared by means of the recently proposed technique
based on the transferring of drawn graphite films onto the n-SiC single crystal substrate.
Current-voltage characteristics were measured and analyzed. High quality ohmic contancts
were prepared by the DC magnetron sputtering of Ni thin films onto cleaved n-type SiC
single crystal substrates. The height of the potential barrier and the series resistance of the
graphite/n-SiC junctions were measured and analysed. The dominant current transport
mechanisms through the diodes were determined. There was shown that the dominant
current transport mechanisms through the graphite/n-SiC Shottky diodes were the multi-step
tunnel-recombination at forward bias and the tunnelling mechanisms at reverse bias.
... Efforts to improve efficiency solar cells of cadmium telluride semiconductor nanoparticles deposited on substrates through various methods continued to be appropriate [7]. Cadmium telluride nanoparticles can be deposited on the substrate using various methods such as electrodeposit [8], thermal evaporation [9], sputtering [10], pulsed laser [11], spin coating [12], electron beam evaporation [13], atmospheric pressure metalorganic vapour deposition (AP-MOCVD) [14] and close-spaced sublimation [7]. Deposition by the thermal evaporation method is one of the most effective methods for deposition of nanoparticles on the substrate. ...
In this work, cadmium telluride (CdTe) nanoparticles-doped Ag was deposited on Ag wafer at 150°C and of 2 × 10⁻⁵ mbar. The thickness of thin films is 80 nm. The results of the XRD analysis show the formation of CdTe cubic phase and CdTe:Ag with a strong preferential orientation (220) at 150°C. The particle size in this orientation obtained about 13.00 nm. CdTe films were annealed at temperatures of 300 and 500°C and were placed under pressures of 1 × 10⁻⁴ and 6.5 × 10⁻⁴ mbar to investigate the effect of annealing and vacuum pressure changes on particle size, respectively. UV–vis measurements indicate the optical band gap for CdTe:Ag thin films is 1.75 eV and decreases with increasing the annealing temperature and pressure. Finally, to study the morphology of CdTe:Ag thin films, SEM analysis was done. The results revealed that variations of annealing temperature are more effective to increase the particle size than variations of pressure.
... 14 A similar PV cell system fabricated on poly(ethylene terephthalate) substrates utilized graphene/CdSe and graphitic carbon/CdTe exhibited PCE of 1.25 and 1.36%, respectively. 66,67 The PV devices demonstrated in this manuscript can be fabricated on other flexible substrates including adhesive tape, card board, and plastic using same procedures as described above. For example, we fabricated PV devices on printing transparency sheets. ...
We demonstrate fabrication and characterization of photovoltaic (PV) devices made using pencil, paper, and commonly available economical chemicals with a power conversion efficiency of ∼1.8%. The current collecting electrode of the device composed of multilayered graphene (MuLG) was hand-drawn on the cellulosic paper using an H2B pencil. CdSe quantum dots (QD) were used for charge generation, and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) as a bridging molecule to facilitate transfer of the photo-induced charges to the electrodes through MuLG. MuLG acted both as charge carrier and current collector electrode. The device fabrication and testing were accomplished in a wet lab under ambient conditions with minimum use of sophisticated instrumentation. The materials and devices were characterized using UV–visible, fluorescence, x-ray diffraction spectroscopy, and scanning and transmission electron microscopy. I–V characteristics of the PV devices fabricated on paper and polyester transparency substrates were performed using a solar simulator (AM 1.5) under ambient wet laboratory conditions. The use of pencil and paper makes the device fabrication simple, environmentally responsible, and accessible to layperson thus opening a new window for low cost PV and opto-electronic devices.
Copper oxide is one of the original semiconductor materials employed for solar cells in the early 19th century before Silicon solar cells became popular due to their abundant availability, and eco‐friendly nature. The optoelectronic parameters signify its huge potential in solar cell devices, though it's far from the theoretically predicted performance, provides tremendous scope to improve the solar cell performance by forming different heterojunctions. In this study, we investigated the copper oxide's (CuO) potential as an active layer in thin‐film solar cells theoretically with a new structure consisting of a Glass/ITO/Graphite/CuO/Ni. Furthermore, the charge carrier's generation rate and theoretical thresholds for photovoltaic device efficiency were determined for varying active layer thicknesses by employing a normalized light intensity equivalent to that of the AM1.5 spectrum. The optimized performance of the simulated structure by considering realistic optical parameters of the solar cell was ~24%, obtained for the 500 nm CuO films. The performed theoretical work can help to employ CuO and boost the performance of solar cells experimentally.
The II–VI semiconductor thin films have vast applications in optoelectronics. Cadmium telluride (CdTe) is one such material, which has proved to be useful in the fabrication of solar cells, photodetectors, etc. In the present work, CdTe thin films have been deposited on glass substrates by thermally evaporating CdTe powder under a high vacuum. The optimum film thickness for photovoltaic applications was determined by performing a detailed characterization of the films. The thermal evaporated CdTe films were found to be polycrystalline with a cubic structure. Considerable improvement in the crystallinity was observed with the increase in thickness. The lattice constant, dislocation density, strain, and stress have also been analyzed. The optical properties of the CdTe films have been observed to change with thickness. The maximum optical absorption was found in the near infrared region. The photoluminescence spectra showed a prominent peak, corresponding to the band-edge transition of the CdTe films. The electrical properties of the films were found to improve with thickness. The films yield maximum photocurrent at 820 nm. Based on the detailed characterization, the film thickness of 950 nm was found to be most suitable for photovoltaic application.
Quaternary chalcogenide Cu2ZnSnS4 (CZTS) is a promising absorber for photovoltaic cell application due to its excellent properties like natural abundancy, non-toxic material incorporation, high absorption coefficient ∼10⁴ cm⁻¹ and tuneable band gap energy of 1.4–1.6 eV. However, still there is a huge gap of efficiency between the market oriented Si based photovoltaic device and kesterite device as it is hindered by poor charge carrier mobility. To improve the carrier mobility of semiconducting CZTS material, incorporation of highly conducting material like graphene would be helpful. The incorporation of graphene into CZTS enhances the separation and transfer of charge carriers in an efficient way inhibiting recombination. In this present work, the photovoltaic performance of solution processed single phase kesterite CZTS QDs anchored on electro-chemically prepared few-layer graphene sheet (FLGS) as active layer has been reported. The QD decoration has been carried out by a simple, scalable, environment friendly solution based approach without surface modifications and post-sulphurization treatment. The prepared hybrid structure demonstrates significant increase in photocurrent and optical absorbance indicates the fast charge carrier transport behaviour. The power conversion efficiency (PCE) of fabricated hybrid CZTS QDs-FLGS photovoltaic cell with device configuration of Mo/CZTS QDs-FLGS/CdS/ZnO/Al approached ∼7.64% (for 2.5 wt% FLGS loading) with short circuit current density (Jsc) of 19.07 mAcm⁻², superior to individual CZTS QDs (∼6.11% PCE and 15.6% Jsc).
A new type of sub‐micron metal‐intrinsic semiconductor‐metal visible to near‐infrared (400–1600 nm) photodiodes based on a unique combination of radiation‐resistant functional materials: sapphire, TiN, MoOx, CdTe, Hg3In2Te6, and graphite is proposed. The promising optoelectronic characteristics are calculated in the scope of a comprehensive semi‐analytical model, based on the complementary fusion of numerical Transfer Matrix optical simulation with analytical Hecht and dark generation current equations. The findings demonstrate proof‐of‐concept next‐generation high‐performance optoelectronic devices with advanced radiation resistance. Moreover, a simple device engineering modification has revealed a significant optimization potential for considered photodiodes.
Brus V. V. Electronic processes in nanostructured carbon and carbon-based
functional materials and devices based on them.
A thesis for the scientific degree of doctor of physics-mathematical sciences in the
speciality 01.04.10 - physics of semiconductors and dielectrics. - Yuriy Fed’kovych
Chernivtsi National University, Chernivtsi, 2017.
The thesis is devoted to the investigation of reproducible fabrication and functionalization
of inorganic and organic carbon-based materials, the complex investigation
of their structural, electrical and optical properties at different external conditions
for the enhancement of their functional capabilities, the development and improvement
of efficiency and stability of electronic devices based on them.
New methods for the analysis of spectral dependences of the impedance and
quantum efficiency and the dependence of open-circuit voltage vs. light intensity of
semiconductor heterojunctions and organic solar cells were proposed. These methods
take into account the presence of interface states (interface traps and recombination
centers), series resistance and parasitic inductance.
A new branch of the low-cost and environmentally friendly electronics was
proposed basing on the concept "pencil-on-semiconductor". This is a very simple and
low-cost technique for the fabrication of photosensitive Shottky-type heterojunctions
and flexible photodetectors by drawing graphite films on semiconductor substrates
and drawing graphite/layered semiconductor nanocomposite films on regular paper,
respectively.
There was shown that the majority of charge carriers in conjugated polyelectrolyte
CPE-PyrBIm4/graphene hetero-bilayers can be dynamically changed from electrons
to holes, and vice versa, within a temperature range of 60 °C. Doping mechanisms
under consideration include charge transfer from structural units in the CPEPyrBIm4
backbone and/or field-effect doping from adjacent ionic functionalities.
The stubility of undoped graphene/silicon heterojunction solar cells was investigated.
There was shwon that the high quality passivation the Si(111) surface by
methyl groups CH3 significantly improve electrical and photoelectrical properties of
the graphene/silicon heterojunctions and solves the instability problem, which is crusial
for the hydrogen passivated silicon surface under graphene.
A new method for the determination of the concentration of free charge carrier
and the quantitative analysis of recombination losses in small molecule bulkheterojunction
solar cells was proposed basing on the spectral dependence of measured
capacitance.
A novel flexible energy harvesting device ionic-orgnaic electronic ratchet with
drawn graphite electrodes as well as a high performance ionic-organic ratchet based
on a conductive polymer with high mobility of holes were designed and fabricated.
Their working priciple is based on a charge pump.
The stability of CH3NH3PbI3 perovskite films is investigated using visible and
ultraviolet light in oxygen atmosphere and in vacuum. Illumination in O2 atmosphere
results in a swift degradation. Oxygen acts as a catalyst decomposing methylammo36
nium ions (CH3NH3+) into CH3NH2 and hydrogen. In vacuum, another degradation
mechanism is observed. Prolonged illumination of the samples with photons from
blue and UV light-emitting diodes also results in dissociation of the methylammonium
ion into CH3NH2 and hydrogen. The resulting molecules are highly mobile at
room temperature and diffuse out of the samples. The dissociation of CH3NH3
+ is accompanied
by the generation of localized defects in the band gap of the perovskite.
Perovskite solar cells have been shown to be of extraordinary radiation hardness,
considering high energetic (68 MeV) proton irradiation with doses up to 1013 p
cm−2. In this study electrical and photoelectrical properties of perovskite solar cells
with and without proton irradiation are analyzed in detail. The results reveal that proton
irradiation improves the open circuit voltage, fill factor, and recombination lifetime
of photogenerated charge carriers in perovskite solar cells. These enhancements
are mainly a result of the lower nonradiative recombination losses in the proton irradiated
devices. The proton treatment creates shallow traps, associated with the proton
induced point defects due to the displacements of atoms in the inorganic Pb–I framework,
which act as unintentional doping sources and partially compensate deep traps
originated from the photodegradation of methylammonium molecules.
Key words: graphite, graphene, polymer, perovskite, doping, impedance, current
transport, recombination.
This chapter contains sections titled: Introduction Quantum Dots and Their Properties Synthetic Methods for Quantum Dots Quantum Dot Solar Cells Challenges and Perspectives
This chapter introduces electron beam evaporation deposition, including the principle, equipments for electron beam evaporation, application, and so on. It explains some techniques for the characterization of thin films and potential applications of the electron beam technique. The chapter describes the e-type electron gun, which is mainly used for thin film deposition. The greatest advantage of the electron beam evaporation technique is the direct heating of the evaporant material in which the highest temperature of the evaporating system appears at the area of contact of the electron beam with the evaporant surface. Electron beam evaporation can be applicable to all of the metals and alloys and can also be used for compounds such as oxides and zinc sulphide (ZnS), especially high melting point materials, for thin film fabrication. The chapter discusses the characterization of thin films by atomic force microscopy (AFM) and ellipsometry, including improved ellipsometric method.
This paper reports on the both possible applications of molybdenum oxide (MoOx) thin films in combination with hole or electron conducting CdTe. The high quality ohmic contancts and strongly rectifying photodiodes were prepared by the DC magnetron sputtering of MoOx thin films onto freshly cleaved p- and n-type CdTe single crystal substrates. The analysis of DC and AC electrical properties of the MoOx/ p-CdTe ohmic contact was carried out. The dominating current transport mechanisms through the MoOx/p-CdTe heterojunction at forward and reverse bias were determined. The unoptimized heterojunction photodiode showed promising rectifying and photoelectrical characteristics for practical application in the photoconductive mode. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)
E. Singh and H. S. Nalwa, Graphene-based bulk-heterojunction solar cells: a review, Journal of Nanoscience and Nanotechnology, 15, 6237-6278 (2015).
The current highest power conversion efficiencies found for different types of solar cell devices range from 20% to 46%, depending on the nature of the photovoltaic materials used and device configuration. Graphene has emerged as an important organic photovoltaic material for photoenergy conversion, where graphene can be used as a transparent electrode, active interfacial layer, electron transport layer, hole transport layer, or electron/hole separation layer in fabricating solar cell devices. This review article briefly discusses some recent advances made in different types of photovoltaic materials, and then summarizes the current status of graphene-based bulk-heterojunction (BHJ) solar cells, including graphene-containing perovskite and tandem solar cell devices. Power conversion efficiencies currently exceed 10% for heteroatom-doped multilayer graphene-based BHJ solar cells and 15.6% for graphene-containing perovskite-based solar cells. The role of grapheme layer thickness, bending, thermal annealing, passivation, heteroatom doping, perovskite materials, and tandem solar cell structure on the photovoltaic performance of graphene-based solar cells is discussed. Besides aiming for high power-conversion efficiency, factors such as long-term environmental stability and degradation, and the cost-effectiveness of graphene-based solar cells for large-scale commercial production are challenging tasks.
KEYWORDS: Graphene, Graphene Oxide, Bulk-Heterojunction Solar Cells, Heteroatom Doping, Perovskite Solar Cells, Tandem Solar Cell Devices, Power Conversion Efficiency, Stability and Degradation of Graphene Heterojunction Solar Cells.
Simulation analysis of the thermal degradation in performance of a graphene back contacted CdS/CdTe ultrathin film solar cell is performed using Solar Cell Capacitance Simulator. The application of the graphene nanolayer on the CdTe materials is interesting because of its cheaper cost, excellent electronic properties, and optical advantages. Degradation of a CdTe thin-film photovoltaics is mostly driven from the metallic back contacts. The graphene carbon–carbon network can reduce the extra interdiffusion of the metallic ions from the metallic back contacts to the front contact through the gran boundaries. The number of graphene layers determines the resistivity of this layer at the contact. This resistivity changes under the thermal stress. Thus, we investigate the degradation of the device metrics under thermal stress and compare it with the few experimental data reported in the literature.
Carbon thin films are often called in the literature, "diamond-like carbon" films. They consist of two basic allotropic forms of carbon, which are graphite and diamond. Carbon atoms with sp 2 bonds form after deposition of a graphite-like phase. Atoms with sp 3 bonds form a diamond-like phase. Diamond-like crystallites are built into a graphite-like phase matrix. In this paper there are presented experimental results of deposition of carbon films by the magnetron sputtering method and the results of analysis of the surface and phase structures of the deposited films. The amorphous carbon films were deposited from graphite tar-gets on 316L steel substrates. The films were deposited at room temperature, in vacuum. The deposition time was 3 h; the depositions were conducted at two different distances between the substrate and the magnetron target.
This paper gives a comprehensive review about the most recent progress in synthesis, characterization, fundamental understanding, and the performance of graphene and graphene oxide sponges. Practical applications are considered including use in composite materials, as the electrode materials for electrochemical sensors, as absorbers for both gases and liquids, and as electrode materials for devices involved in electrochemical energy storage and conversion. Several advantages of both graphene and graphene oxide sponges such as three dimensional graphene networks, high surface area, high electro/thermo conductivities, high chemical/electrochemical stability, high flexibility and elasticity, and extremely high surface hydrophobicity are emphasized. To facilitate further research and development, the technical challenges are discussed, and several future research directions are also suggested in this paper.
Graphite/n-CdTe Schottky-type heterojunctions were prepared by means of a very simple and low-cost method: the drawing of graphite films on the rough surface of mechanically polished n-CdTe single crystal substrates. The unoptimized heterojunctions showed promising, rectifying, and photoelectrical parameters. The obtained results represent a good starting point for the further development of the simple and low-cost heterojunction photodiodes and gas sensors.
Electrical properties of highly rectifying Pt/InP junctions fabricated by electrophoretic deposition of Pt nanoparticles are investigated at different temperatures by the measurement of current–voltage and capacitance–voltage characteristics. The forward I–V characteristics of the junction are described by thermionic emissions theory at low forward bias (3kT/q < V < 0.2 V) and by tunnelling current transport through the narrowed space charge region at forward bias V > 0.2 V. The reverse I–V characteristics are analysed in the scope of the thermionic emission model in the presence of shunt resistance. Electrical characteristics of these diodes are sensitive to gas mixtures with a low hydrogen concentration and show an extremely fast response and recovery time.
The Schottky junction, with merits of material universality, low cost and easy fabrication, is an alternative structure for solar cells. Compared to traditional indium-tin-oxide (ITO) based Schottky junction solar cells, graphene-based ones have merits of low cost, performance stability, and are applicable to flexible devices. In this highlight, we survey the recent research on graphene-based Schottky junction solar cells, including graphene-on-silicon Schottky junction solar cells and graphene/single NW (NB) Schottky junction solar cells. The working principle of them is discussed. These works demonstrate that graphene-based Schottky junction structures are promising candidates for developing diverse novel high-efficient and low-cost photovoltaic devices. The perspective and challenge of them are also discussed and anticipated.
We report the fabrication of Resistor-Capacitor (RC) filters and field effect transistors (FETs) based on pencil drawings on paper, which contain turbostratic graphite crystallites as evidenced from Raman analysis. Pencil drawings have been employed as resistor and an ion gel, 1-butyl-3-methylimidazolium octyl sulfate mixed with polydimethylsiloxane (PDMS) as dielectric, for the fabrication of RC filters with a cut-off frequency of 9 kHz. With ion gel as gate dielectric, an ambipolar electric field effect has been obtained from the pencil-trace at low operating voltages. The carrier mobilities were found to be ∼106 and 59 cm(2) V(-1) s(-1) for holes and electrons, respectively. The mobility value showed only 15% variation among the devices tested, truly remarkable given the simplicity of the fabrication process.
A detailed analysis of the spectral dependences of the real and imaginary components of the measured impedance of a simulated silicon p-n junction is carried out within the framework of a conventional equivalent circuit (parallel R
dC
b chain and series resistance R
s). A simple technique was proposed for determining the true value of the barrier capacitance of structures with a potential barrier (without surface electrically active states) on the basis of analysis of the spectral dependence for the imaginary part of the measured impedance.
A quantitative analysis of the impedance of nonideal heterojunctions was carried out taking into consideration the effects of series resistance, shunt resistance, parasitic inductance and electrically active interface traps. A new approach is proposed to determine the energy distribution of surface state density and to calculate the actual value of barrier capacitance of heterojunctions on the basis of the analysis of their complex impedance–voltage characteristics.
We demonstrate single layer graphene/n-Si Schottky junction solar cells that under AM1.5 illumination exhibit a power conversion efficiency (PCE) of 8.6%. This performance, achieved by doping the graphene with bis(trifluoromethanesulfonyl)amide, exceeds the native (undoped) device performance by a factor of 4.5 and is the highest PCE reported for graphene-based solar cells to date. Current-voltage, capacitance-voltage, and external quantum efficiency measurements show the enhancement to be due to the doping-induced shift in the graphene chemical potential that increases the graphene carrier density (decreasing the cell series resistance) and increases the cell's built-in potential (increasing the open circuit voltage) both of which improve the solar cell fill factor.
A summary of experimental work on electrical contacts to CdTe, CdS and ZnSe is presented and recent progress of research on electrical contacts to these materials is reviewed in this paper. The surface preparation and surface characterisation prior to metallisation, interactions at the interface during contact fabrication, Schottky barrier characterisation and subsequent aging effects are considered. XPS, AES, SIMS and PL are used for surface characterisation; XPS and soft-XPS are used for interface interaction studies; I-V, C-V, DLTS and BEEM are used for Schottky barrier characterisation; and AES, GDOES and EDX profiling are used to study aging effects. The surfaces of all three materials behave in a similar way when etched in wet chemical etchants. The semiconductor cation is preferentially etched by acidic solutions and the semiconductor anion is preferentially removed by alkaline solutions. It has also been shown that the surface stoichiometry affects the Fermi level pinning position at metal/semiconductor interfaces. Furthermore, the observed Schottky barrier heights with all three materials demonstrate a multi-level pinning behaviour producing different barrier heights depending on the history of the materials used and the fabrication procedure followed. Barrier heights observed are independent of the metal work function, and their stability depends mainly on interactions occurring at the metal/semiconductor interface and are strongly related to the native defect levels within the bulk material.
Well-ordered graphite films with a thickness of a few graphene layers have been grown on Ni substrates by chemical vapor deposition (CVD) from a mixture of hydrogen and methane activated by a DC discharge. According to Auger, Raman and scanning tunneling microscopy (STM) data the CVD graphite film thickness is about 1.5 ± 0.5 nm. The graphene layers were perfectly adhered to the substrate surface except for upthrusted ridges of a few tens of nanometers in height. STM has revealed an atomically smooth surface with the atomic arrangement typical of graphite between the ridges. A difference in the thermal expansion coefficients of nickel and graphite is considered as a reason for the ridge formation.
A great deal of important and interesting advanced material is discussed in this book. Some introductory material is included in each chapter to set the stage but there is no space to consider in detail elementary concepts of semiconductors or circuits. Some familiarity is therefore assumed with analog and digital circuits, amplifiers, oscillators, modulators and gates; and with semiconductor concepts of bandgaps, mobilities, density of states, Fermi levels, doping, minority carrier diffusion and lifetime, the simple pn junction model and general ideas about bipolar and field-effect transistors. We begin, therefore, with a list of some standard equations of semiconductors. If the form and notation are not familiar some time should be spent with the introductory semiconductor books that are included in the reference list at the end of the volume.
The Third Edition of the standard textbook and reference in the field of semiconductor devices. This classic book has set the standard for advanced study and reference in the semiconductor device field. Now completely updated and reorganized to reflect the tremendous advances in device concepts and performance, this Third Edition remains the most detailed and exhaustive single source of information on the most important semiconductor devices. It gives readers immediate access to detailed descriptions of the underlying physics and performance characteristics of all major bipolar, field-effect, microwave, photonic, and sensor devices. Designed for graduate textbook adoptions and reference needs, this new edition includes: A complete update of the latest developments. New devices such as three-dimensional MOSFETs, MODFETs, resonant-tunneling diodes, semiconductor sensors, quantum-cascade lasers, single-electron transistors, real-space transfer devices, and more. Materials completely reorganized. Problem sets at the end of each chapter. All figures reproduced at the highest quality. Physics of Semiconductor Devices, Third Edition offers engineers, research scientists, faculty, and students a practical basis for understanding the most important devices in use today and for evaluating future device performance and limitations. A Solutions Manual is available from the editorial department.
Graphite/ ZnO nanorods heterojunctions were prepared by hydrothermal growth and deposition of colloidal graphite. The electrical properties of these heterojunctions were investigated at different temperatures by measuring their current-voltage (I-V) and capacitance-voltage (C-V) characteristics. The basic electrical parameters of the graphite-based junctions were estimated from I-V and C-V measurements. It was found that the I-V characteristics of the graphite/ZnO nanorods heterojunctions can be well described by a tunnel-recombination current transport mechanism via interface states. These nanostructured heterojunctions showed promising rectifying and gas sensing properties. The obtained results represent a good starting point for further development of nanostructured heterojunction diodes and gas sensors.
The charge transport in Schottky-type solar cells fabricated from graphene/ methyl-passivated silicon heterojunctions is studied in detail. The electrical device characteristics are affected by ambient temperature and illumination conditions. Moreover, the presence of deep and shallow interface states influences the current across the junction at forward and reverse bias. In the dark, thermionic emission over the potential barrier is clearly affected by the recombination via interface states, while under illumination those states become electrically inactive.
The stability of undoped graphene – silicon heterostructure solar cells was investigated. Single layer graphene was grown by chemical vapor deposition on copper foil. Prior to the transfer of graphene to the silicon wafer, the flat Si(111) surface was passivated with hydrogen or methyl groups (CH3).The conversion efficiency, , of the H terminated Si device was negligible small (0.1 %), whereas that of the CH3 passivated Si was 2and 4.2 % at 100mW (AM 1.5) and 20mW of light intensity, respectively.After 28 days in ambient atmosphere decreased only slightly to 1.5 and 3.7 %. This small change of is due to the highstability of the CH3 passivatedgraphene - Si(111) interface.
The methylated Si surface shows a high degree of chemical stability especially during the graphene transfer process.
We investigated heterojunctions of Si with large-area high-quality monolayer and multi-layer graphene, as well as thin transparent graphite. We show that by controlling the transmittance and sheet resistance of large-area graphitic electrodes, it is possible to obtain solar cells with power conversion efficiency (PCE) exceeding 3% without any doping requirements. Our calculations indicate that such junctions can form extremely robust interfaces with near-100% internal quantum efficiency. Under optimized doping conditions, power conversion efficiencies increase almost universally by a factor of 2.5. Optimized conditions for reproducibly obtaining cells with PCE > 5% are presented, with the best PCE obtained ∼7.5% with short-circuit current density exceeding 24 mA/cm2.
A detail analysis of electrical and photoelectrical properties of hybrid organic–inorganic heterojunction solar cells poly(3-hexylthiophene) (P3HT)/n-Si, fabricated by spin-coating of the polymeric thin film onto oxide passivated Si(1 0 0) surface, was carried out within the temperature ranging from 283 to 333 K. The dominating current transport mechanisms were established to be the multistep tunnel-recombination and space charge limited current at forward bias and leakage current through the shunt resistance at reverse bias. A simple approach was developed and successfully applied for the correct analysis of the high frequency C–V characteristics of hybrid heterojunction solar cells. The P3HT/n-Si solar cell under investigation possessed the following photoelectric parameters: Jsc = 16.25 mA/cm2, Voc = 0.456 V, FF = 0.45, η = 3.32% at 100 mW/cm2 AM 1.5 illumination. The light dependence of the current transport mechanisms through the P3HT/n-Si hybrid solar cells is presented quantitatively and discussed in detail.
Graphene–Si Schottky junction solar cells are promising candidates for high-efficiency, low-cost photovoltaic applications. However, their performance enhancement is restricted by strong carrier recombination and relative low barrier height. Here, we demonstrated the successful construction of high-efficiency graphene–planar Si solar cells via modification of the Si surface with a molecule monolayer as well as tuning the interface band alignment with an organic electron blocking layer. Methylated Si showed the capability to effectively suppress the surface carrier recombination, leading to a remarkable improvement of device efficiency. The recombination was further reduced by inserting a thin P3HT organic layer; the unique band alignment could prevent electron transfer from n-Si to the graphene anode so as to minimize the current leakage. These methods, along with careful control of the graphene doping level and layer number, gave rise to a power conversion efficiency (PCE) as high as 10.56%. The scalability of the devices was further investigated by studying the device area dependent photovoltaic performance.
The effect of series resistance on the apparent semiconductor junction capacitance C’ has been studied after Goodman [J. Appl. Phys. 34, 329 (1963)] in an attempt to explain the occurrence of minima in the apparent C’−2 vs voltage characteristics at high frequencies and for a large series resistance. A correction scheme has been formulated to convert C’−2 into the actual C−2 at any bias and frequency. The present model is consistent with the apparent capacitance behavior of a commercial Si Zener diode in series with a discrete resistor.
Recent Raman scattering studies in different types of graphene samples are reviewed here. We first discuss the first-order and the double resonance Raman scattering mechanisms in graphene, which give rise to the most prominent Raman features. The determination of the number of layers in few-layer graphene is discussed, giving special emphasis to the possibility of using Raman spectroscopy to distinguish a monolayer from few-layer graphene stacked in the Bernal (AB) configuration. Different types of graphene samples produced both by exfoliation and using epitaxial methods are described and their Raman spectra are compared with those of 3D crystalline graphite and turbostratic graphite, in which the layers are stacked with rotational disorder. We show that Resonance Raman studies, where the energy of the excitation laser line can be tuned continuously, can be used to probe electrons and phonons near the Dirac point of graphene and, in particular allowing a determination to be made of the tight-binding parameters for bilayer graphene. The special process of electron–phonon interaction that renormalizes the phonon energy giving rise to the Kohn anomaly is discussed, and is illustrated by gated experiments where the position of the Fermi level can be changed experimentally. Finally, we discuss the ability of distinguishing armchair and zig-zag edges by Raman spectroscopy and studies in graphene nanoribbons in which the Raman signal is enhanced due to resonance with singularities in the density of electronic states.
Since the development of the holographic notch filters during the last decade and novel detectors such as charge coupled devices, Raman spectroscopy instrumentation has evolved rapidly, so that modern spectrometers are orders of magnitude faster, present much lower signal-to-noise ratio and simplified optics than their predecessors. These advances have also had a significant effect in the construction of novel micro-Raman-dedicated spectrometers that currently use lasers with powers of the order of a few tens of mW. In the present work, a brief overview of current Raman spectroscopy technology in traditional spectrometers and of different applications of modern micro-Raman spectroscopy to the study of materials, ranging from epitaxial semiconductor thin films of interest to the optoelectronics industry, to biomaterials of interest to medical science, is presented. An emphasis is given to different acquisition modes that are possible when the excitation laser light is passed through a microscope objective and the sample is placed in a computer controlled XY stage.
A chemical bath process was carried out for the deposition of Te layers on CdTe films grown by the close-space vapor transport technique (CSVT) on conducting SnO2:F substrates. The Te layers were chemically-deposited on as-grown CdTe films and on previously CdCl2-treated CdTe ones. After Te deposition, the CdTe films were annealed at temperatures from 200 to 400°C. The Te layer on top of the CdTe films was studied by Raman Spectroscopy and by Scanning Electron Microscopy. The electrical resistivity of the annealed CdTe films was determined from current versus voltage measurements in a sandwich configuration, employing gold contacts on top of the CdTe modified surface. The results show that the combined effect of the Te layer on CdTe together with previous CdCl2 treatment improves the electrical properties of CSVT-CdTe films. These results are quite promising for increasing performance of CdS/CdTe solar cells.
Recent years have witnessed many breakthroughs in research on graphene (the first two-dimensional atomic crystal) as well as a significant advance in the mass production of this material. This one-atom-thick fabric of carbon uniquely combines extreme mechanical strength, exceptionally high electronic and thermal conductivities, impermeability to gases, as well as many other supreme properties, all of which make it highly attractive for numerous applications. Here we review recent progress in graphene research and in the development of production methods, and critically analyse the feasibility of various graphene applications.
We have fabricated ITO-free organic solar cells (OSCs) based on multilayer graphene (MLG) electrodes. MLG electrodes with high transparency (∼84.2%) and low sheet resistance (∼374±3 Ω/square) were obtained on glass substrates using chemical vapor deposition and a multi-transfer process. The OSCs fabricated on MLG electrodes had an open circuit voltage of 0.52 V, a short circuit current of 6.90 mA/cm2, a fill factor of 32.6%, and a power conversion efficiency of 1.17%. The performance of the OSCs fabricated on transparent MLG electrode was not comparable to that of OSCs fabricated on ITO electrodes; cost-effective MLG electrodes are a viable alternative to sputter-grown ITO electrodes for cost-efficient and flexible OSCs.
For the first time, based on the known optical constants of the materials (refractive index and extinction coefficient), calculations of optical losses in glass/transparent conducting oxide (TCO)/CdS/CdTe solar cells have been carried out taking into account reflections at the interfaces and absorption in the TCO (it can be indium tin oxide (ITO) or SnO2:F) and CdS layers. It has been shown that the losses caused by reflections at the interfaces result in lowering the short-circuit current by ∼9 % whereas absorption in the TCO and CdS layers with the typical thicknesses lead to losses of 15–16% for glass/SnO2/CdS/CdTe, and 22–24% for glass/ITO/CdS/CdTe solar cells. At 100% photoelectric conversion in the CdTe absorber layer, this corresponds to a loss in short-circuit current by ∼3 mA/cm2 due to reflection, and 4–7 mA/cm2 due to absorption at the glass/SnO2(or ITO)/CdS stack. Losses due to absorption in float glass and low-iron glasses are 3.3–3.5% and 0.6–0.7%, respectively.Graphical AbstractHighlights► Calculations of optical losses in CdS/CdTe solar cells have been carried out. ► Calculations reveal the causes of the optical losses and the possibilities to reduce them. ► Reflection losses is about 8% over a wide spectral range and increase to 10–11% at λ<400 nm. ► Absorption decreases the short-circuit current by 12–26% in ITO and 2-5% in SnO2:F at 200–500 nm thickness. ► Absorption losses in CdS at its thickness of 50–100 nm decrease the short-circuit current by 10–15%.
The differential capacitance measurement for determining the height of a Schottky barrier at a metal—semiconductor contact is based upon a number of assumptions. In practice, one or more of these assumptions may not be valid. Some of these deviations from the ``ideal case'' are examined in order to determine the effects of each upon the interpretation and validity of measurements on such contacts. Specifically, the effects of each of the following are considered: series resistance, traps in the depletion layer, effective contact area variation with depletion layer width, an insulating interfacial layer between the metal and semiconductor, semiconductor surface charge variation with bias voltage, and the reserve layer at the edge of the barrier. Experimental data for gold plated contacts to conducting cadmium sulfide single crystals are given to illustrate some of the results.
We review recent work on Raman spectroscopy of graphite and graphene. We focus on the origin of the D and G peaks and the second order of the D peak. The G and 2D Raman peaks change in shape, position and relative intensity with number of graphene layers. This reflects the evolution of the electronic structure and electron–phonon interactions. We then consider the effects of doping on the Raman spectra of graphene. The Fermi energy is tuned by applying a gate-voltage. We show that this induces a stiffening of the Raman G peak for both holes and electrons doping. Thus Raman spectroscopy can be efficiently used to monitor number of layers, quality of layers, doping level and confinement.
Optical and interface properties of the CdTe films electrodeposited on Molybdenum and Stainless Steel substrates were investigated using variable angle spectroscopic ellipsometer measurement and multilayer optical analysis. The refractive index of CdTe film obtained from the multilayer optical modeling is found to be lower than single crystal data. The Bruggeman effective medium analysis shows that the films consist of nearly 11% void due to poor crystallinity resulting in the lower refractive index. The multilayer optical model also indicates the presence of a Te rich interface between CdTe and substrate, which can be associated to the kinetics of CdTe electrodeposition that starts from nucleating Te on substrate surface followed by the formation of CdTe.
(Figure Presented) Highly conductive semitransparent graphene sheets are combined with an n-type silicon (n-Si) wafer to fabricate solar cells with power conversion efficiencies up to 1.5% at AM 1.5 and an illumination intensity of 100 mW crm-2. The Schottky junction solar cells can be extended to other semiconducting materials in which graphene serves multiple functions as active junction layer, charge transport path, and transparent electrode.
A facile method for the preparation of thin-film carbon electrodes by electron beam evaporation onto highly doped silicon is presented. The physical and electrochemical properties of these films both before and after postdeposition pyrolysis are investigated. Raman spectroscopy establishes the amorphous structure of the nonpyrolyzed carbon films and confirms the formation of graphitic carbon after pyrolysis at 1000 degrees C. Scanning force microscopy reveals the root-mean-square roughness of nonpyrolyzed films to be approximately 1 A, while pyrolyzed films exhibit an increased roughness of approximately 4 A. The electrochemical behavior of the electrodes resembles glassy carbon, with measured heterogeneous electron-transfer rate constants among the highest measured for thin carbon films. These carbon film electrodes will potentially find applications in such fields as molecular electronics and scanning probe microscopy of adsorbed species.
The theoretical capacitance of abrupt p-n heterojunctions including the effects of interface states is examined. The interface effects depend on the bulk impurity concentrations and their ratio, as well as the density and distribution of interface states. In the Ge-GaAs junctions studied, the impurity concentrations and density of interface states are such that interface effects have only a negligible influence on the capacitance of these devices. Interface states have a considerable effect on the capacitance of the Ge-Si junctions studied, however. They affect the apparent diffusion voltages obtained by extrapolating 1/C2to zero and add a significant frequency-dependent term on many diodes. The frequency-dependent term is due to the rate limited charging and discharging of interface states.
Spectral distribution of the quantum efficiency of the graphitic carbon/n-CdTe Schottky-type heterojunction solar cells. Dashed line shows the spectral distribution of the absorption coefficient of CdTe crystal
- Fig
Fig. 9. Spectral distribution of the quantum efficiency of the graphitic
carbon/n-CdTe Schottky-type heterojunction solar cells. Dashed line
shows the spectral distribution of the absorption coefficient of CdTe
crystal (Paulson and Mathew, 2004).
Semiconductor Heterojunctions
- B L Sharma
- R K Purohit
Sharma, B.L., Purohit, R.K., 1974. Semiconductor Heterojunctions.
Pergamon, New York.