Amorphous Solar Cells Research Articles

Band gap profiles of intrinsic amorphous silicon germanium films and their application to amorphous silicon germanium heterojunction solar cells

Intrinsic amorphous silicon germanium (i-a-SiGe:H) films with V, U and VU shape band gap profiles for amorphous silicon germanium (a-SiGe:H) heterojunction solar cells were fabricated. The band gap profiles of i-a-SiGe:H were prepared by varying the GeH4 and H2 flow rates during the deposition process. The use of i-a-SiGe:H with band gap profile in an absorber layer for a-SiGe:H heterojunction solar cells was investigated. The solar cell using a VU shape band gap profile shows a higher efficiency compared to other shapes. The highest efficiency obtained for an a-SiGe:H heterojunction solar cell using the VU shape band gap profile technique was 9.4% (Voc=0.79V, Jsc=19.0mA/cm2 and FF=0.63).

Effect of hydrogen doping on the properties of Al and F co-doped ZnO films for thin film silicon solar cell applications

Aluminum and fluorine co-doped zinc oxide (AFZO) thin films were prepared in Ar+H2 atmospheres by rf magnetron sputtering at room temperature. The structural, electrical, and optical properties of the prepared films were investigated using X-ray diffraction, scanning electron microscopy, atomic force microscopy, Hall-effect measurement, X-ray photoelectron spectroscopy, and ultraviolet–visible spectrometry, and their dependence on deposition atmosphere (i.e. H2/(H2+Ar) ratio) was studied. The resulting films showed a (0 0 2) diffraction peak, indicating a typical wurtzite structure, and the optimal film crystallinity was obtained with the H2/(H2+Ar) ratio of 3%. The electrical resistivity of AFZO films decreased to 9.16×10−4Ω-cm, which was lower than ZnO:Al and ZnO:F films due to double doping effect of Al and F. The resistivity further decreased to below 5×10−4Ω-cm for the AFZO film with the H2/(H2+Ar) ratio of 3%–5%. All the films regardless of hydrogen content displayed high transmittances (>92%) in the visible wavelength range. Applying the developed AFZO films as front transparent electrodes, amorphous Si thin film solar cells were fabricated and the open-circuit voltage, fill factor, and efficiency of the cell with the hydrogenated AFZO film were improved in contrast to those without the hydrogenated film.

Hydrogen dilution on an undoped silicon oxide layer and Its application to amorphous silicon thin-film solar cells

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiOx:H) deposited on an n-μc-Si:H layer of amorphous silicon single-junction superstrate-configuration thin-film solar cells produced using 40MHz very high frequency plasma-enhanced chemical vapor deposition. We found that undoped μc-SiOx:H thin film under optimized hydrogen dilution conditions had high crystallinity, high conductivity, a wide optical band gap, and a high refractive index, which are advantageous properties in solar cells. However, deposition at higher hydrogen dilutions degraded the quality and optoelectronic properties of the films, because the morphology of the films changed from microcrystalline to amorphous. These results suggest that the use of an optimized undoped μc-SiOx:H layer improves a-Si:H thin-film solar cell performance through enhancement of the short-circuit current density Jsc. The increased Jsc can be attributed to an improved light-trapping capability in the long wavelength range, between 620 and 680nm, as demonstrated by the external quantum efficiency. This technique also allows optimal conversion efficiency to be achieved. The results demonstrated that hydrogen dilution plays a dominant role in the improvement of film quality and solar cell performance; however, the tradeoff between refractive index and conductivity must be considered.

On the potential of light trapping in multiscale textured thin film solar cells

The light propagation in multiscale textured thin film silicon solar cells is studied experimentally and numerically. The short circuit current and energy conversion efficiency of multiscale textured amorphous silicon thin film solar cells are increased compared to nanoscale textured substrates. A gain of the short circuit current of 1.3mA/cm2 is achieved for the multiscale textured solar cell, resulting in short circuit current densities of 16.8mA/cm2 and energy conversion efficiency of 10.7%. The light propagation in the solar cells is determined by Finite Difference Time Domain simulations in three dimensions using realistic interface morphologies. The realistic interface morphologies of solar cells are calculated by 3D algorithms. The optical simulations reveal that the interface morphology of the back reflector of the multiscale textured solar cell has a distinct influence on the short circuit current and quantum efficiency. By tuning the optical losses of the metal back reflector, the short circuit current can be increased beyond 18mA/cm2.

Enhanced electrical and optical properties of boron-doped ZnO films grown by low pressure chemical vapor deposition for amorphous silicon solar cells

In order for thin film solar cells to have high conversion efficiency, their front electrodes must have high electrical conductivity and optical transparency. The front electrode is made of transparent conductive oxide films. In this work, boron-doped ZnO films were grown using the low pressure chemical vapor deposition technique, and they were used as the front electrodes for amorphous silicon thin film solar cells. The as-grown boron-doped ZnO films have good optical properties, but their electrical properties still need to be improved for applications in thin film solar cells. This work demonstrated that the electrical properties of the as-grown boron-doped ZnO films can be significantly enhanced by annealing in hydrogen atmosphere, and at the same time their good optical properties were maintained. By using the annealed boron-doped ZnO films in amorphous silicon thin film solar cells, it was found that their conversion efficiency was remarkably increased from 7.32% to 8.92%.

Ag nanoparticle enhanced light trapping in hydrogenated amorphous silicon germanium solar cells on flexible stainless steel substrate

Silver nanoparticle (Ag NP) enhanced back reflector (BR) has been used for effective light trapping in thin-film silicon solar cells. In this letter, we demonstrate experimentally that the photocurrent density of hydrogenated amorphous silicon–germanium alloy (a-SiGe:H) based solar cells are enhanced by as much as 3.0mA/cm2 or 19.5% with medium sized (320nm) Ag NPs as compared to the specular BR. In addition, it is demonstrated that the optimized Ag NP BR is also effective in multi-junction solar cells. It is believed that it is the appropriated surface morphology of the Ag NPs that leads to the improved light-trapping. It is found that the average haze of the Ag NP enhanced BR can be improved to as much as >70% throughout the wavelength range 600–900nm. The atomic-force-microscopy measurement shows that the optimized root mean square roughness of Ag NP BR is about 35.2nm.

Light scattering effect of ITO:Zr/AZO films deposited on periodic textured glass surface morphologies for silicon thin film solar cells

Various SF6/Ar plasma-textured periodic glass surface morphologies for high transmittance, haze ratio and low sheet resistance of ITO:Zr films are reported. The SF6/Ar plasma-textured glass surface morphologies were changed from low aspect ratio to high aspect ratio with the increase in RF power from 500 to 600 W. The micro- and nano-size features of textured glass surface morphologies enhanced the haze ratio in visible as well as NIR wavelength region. Micro-size textured features also influenced the sheet resistance and electrical characteristics of ITO:Zr films due to step coverage. The ITO:Zr/AZO bilayer was used as front TCO electrode for p-i-n amorphous silicon thin film solar cells with current density–voltage characteristics as: V oc = 875 mV, FF = 70.90 %, J sc = 11.31 mA/cm2, η = 7.02 %.

Limitations of ultra-thin transparent conducting oxides for integration into plasmonic-enhanced thin-film solar photovoltaic devices

This study investigates ultra-thin transparent conducting oxides (TCO) of indium tin oxide (ITO), aluminum-doped zinc oxide (AZO) and zinc oxide (ZnO) to determine their viability as candidate materials for use in plasmonic-enhanced thin-film amorphous silicon solar photovoltaic (PV) devices. First a sensitivity analysis of the optical absorption for the intrinsic layer of a nano-disk patterned thin-film amorphous silicon-based solar cell as a function of TCO thickness (10–50 nm) was performed by simulation. These simulation results were then used to guide the design of the experimental work which investigated both optical and electrical properties of ultra-thin (10 nm on average) films simultaneously deposited on both glass and silicon substrates using conventional rf sputtering. The effects of deposition and post-processing parameters on material properties of ITO, AZO and ZnO ultra-thin TCOs were probed and the suitability of TCOs for integration into plasmonic-enhanced thin-film solar PV devices was assessed. The results show that ultra-thin TCOs present a number of challenges for use as thin top contacts on plasmonic-enhanced PV devices: (1) optical and electrical parameters differ greatly from those of thicker (bulk) films deposited under the same conditions, (2) the films are delicate due to their thickness, requiring very long annealing times to prevent cracking, and (3) reactive gases require careful monitoring to maintain stoichiometry. The results presented here found a trade-off between conductivity and transparency of the deposited films. Although the sub 50 nm TCO films investigated exhibited desirable optical properties (transmittance greater than 80 %), their resistivity was too high to be considered as materials for the top contact of conventional PV devices. Future work is necessary to improve thin TCO properties, or alternative materials, and geometries are needed in plasmonic-based amorphous silicon solar cells. The stability of ultra-thin TCO films also needs to be experimentally investigated under normal device operating conditions.

Random versus periodic: Determining light trapping of randomly textured thin film solar cells by the superposition of periodic surface textures

A simple method is developed to determine the light trapping properties of arbitrarily textured solar cells with high accuracy. The method allows for determining the quantum efficiency and short circuit current density of thin film solar cells prepared on randomly nanotextured surfaces. The light trapping of the randomly textured solar cell is described by the area weighted superposition of periodically textured solar cells. The necessary input parameters for the calculations are determined by analyzing the randomly textured surfaces of the solar cells using atomic force microscopy and image processing. The analysis of the atomic force microscope images and the calculation of the quantum efficiency and short circuit current can be determined from current maps, without complex and time-consuming calculations. The calculated solar cell parameters exhibit excellent agreement with experimentally measured quantum efficiencies and short circuit current densities for amorphous and microcrystalline silicon thin film solar cells prepared on randomly textured substrates. Finally, the work contributes to a comparison of random and periodic light trapping structures.

Design of Ag nanograting for broadband absorption enhancement in amorphous silicon thin film solar cells

Abstract Light trapping is one of the key issues to improve the light absorption and increase the efficiency of thin film solar cells. The effects of the triangular Ag nanograting on the absorption of amorphous silicon solar cells were investigated by a numerical simulation based on the finite element method. The light absorption under different angle and area of the grating has been calculated. Furthermore, the light absorption with different incident angle has been calculated. The optimization results show that the absorption of the solar cell with triangular Ag nanograting structure and anti-reflection film is enhanced up to 96% under AM1.5 illumination in the 300–800 nm wavelength range compared with the reference cell. The physical mechanisms of absorption enhancement in different wavelength range have been discussed. Furthermore, the solar cell with the Ag nanograting is much less sensitive to the angle of incident light. These results are promising for the design of amorphous silicon thin film solar cells with enhanced performance.

Hydrogenated Amorphous Silicon-Based Thin Film Solar Cell: Optical, Electrical and Structural Properties

Hydrogenated amorphous silicon (a-Si:H) has been developed as an important materials in thin film-based photovoltaic technologies because of considerable cost reduction as a result of low material consumption and low-temperature process. Among the materials used for thin film solar cells, amorphous silicon is the most important material in the commercial production. Despite of these benefits, the efficiency limit for a single band gap thin film based solar cell predicted by Shockley and Queisser (i.e. ~31%) has become a matter of challenge for current research community. Considering the thermodynamic behavior of a single threshold absorber in generating electricity from solar irradiance, this limit seems inevitable, and thus a tremendous investigation is now being carried out in different dimensions such as hot carrier generation, rainbow solar cell, multiple exciton generation, multiband absorber etc. Nonetheless, so far reported efficiency (ηlab~12%) provide enough room to improve and take challenge to reach to the highest value for a-Si:H based solar cell design. Further to improve architectural design as well as engineer the materials, it is indispensable to understand the optical, electrical and structural properties of aSi:H as an active layer. Here in this article, an attempt was taken into account to focus on such characteristics that affect the overall cell efficiency.

Graphene as a transparent electrode for amorphous silicon-based solar cells

The properties of graphene in terms of transparency and conductivity make it an ideal candidate to replace indium tin oxide (ITO) in a transparent conducting electrode. However, graphene is not always as good as ITO for some applications, due to a non-negligible absorption. For amorphous silicon photovoltaics, we have identified a useful case with a graphene-silica front electrode that improves upon ITO. For both electrode technologies, we simulate the weighted absorption in the active layer of planar amorphous silicon-based solar cells with a silver back-reflector. The graphene device shows a significantly increased absorbance compared to ITO-based cells for a large range of silicon thicknesses (34.4% versus 30.9% for a 300 nm thick silicon layer), and this result persists over a wide range of incidence angles.

Highly transparent Zn1−xMgxO/ITO multilayer for window of thin film solar cells

Texture-etched Zn1−xMgxO films were fabricated and applied as front transparent electrodes for superstrate type thin film solar cells. The Zn0.65Mg0.35O film (x = 0.35) showed optical transparency better than commercially available Asahi VU and double-textured ZnO (WT-ZnO) substrates. To provide pertinent conductivity, ITO film was coated on the texture-etched Zn0.65Mg0.35O. By employing the Zn0.65Mg0.35O/ITO substrate instead of the SnO2, we demonstrated an enhancement of quantum efficiency for amorphous silicon thin film solar cell devices, resulted in efficiency improvement from 8.92 to 9.56%. We also examined effectiveness of the Zn0.65Mg0.35O/ITO substrate for the microcrystalline silicon solar cells which delivered an efficiency of 9.73% with proper anti-reflection coating. Our experimental results suggest that the Zn0.65Mg0.35O/ITO multilayer front contact can be beneficial for reinforcing performances of silicon-based thin film solar cell devices.

Numerical investigation of a double-junction a:SiGe thin-film solar cell including the multi-trench region

We present a new approach based on the multi-trench technique to improve the electrical performances, which are the fill factor and the electrical efficiency. The key idea behind this approach is to introduce a new multi-trench region in the intrinsic layer, in order to modulate the total resistance of the solar cell. Based on 2-D numerical investigation and optimization of amorphous SiGe double-junction (a-Si:H/a-SiGe:H) thin film solar cells, in the present paper numerical models of electrical and optical parameters are developed to explain the impact of the multi-trench technique on the improvement of the double-junction solar cell electrical behavior for high performance photovoltaic applications. In this context, electrical characteristics of the proposed design are analyzed and compared with conventional amorphous silicon double-junction thin-film solar cells.

An amorphous N-type polymer based on perylenediimide and selenophene for all-polymer solar cells application

An amorphous n-type polymer (PSe-PDI) consisting of perylenediimide and selenophene units was synthesized and applied in bulk heterojunction organic solar cells as an electron acceptor. The all-polymer solar cells based on PTB7 as a donor exhibited power conversion efficiency (PCE) of 1.14%. By using a PTB7 analog with conjugated tris(thienylenevinylene) side chains (TTV7) a superior photovoltaic performance with PCE of 1.81% was achieved due to the improvement of miscibility of the two polymers. Furthermore, by using diiodooctane (DIO) as additive to increase and balance the charge carrier mobility in the TTV7:PSe-PDI blend films, the PCE was increased to 2.55% with a much higher short-circuit current density of 8.34mA/cm2. This study provides a simple and effective approach to improve the miscibility and charge carrier mobility of amorphous PDI-based all-polymer solar cells.

Light-trapping and antireflective coatings for amorphous Si-based thin film solar cells

In this paper, we study the efficiency of several types of all-dielectric, non-resonant, antireflection, and light-trapping coatings for the enhancement of photovoltaic absorption in thin-film silicon solar cells. We compare the enhancement of the photovoltaic absorption offered by a square array of nano-pillar shaped voids in the dielectric covering of the cell with that granted by a flat blooming layer, and a densely packed array of dielectric nanospheres. We optimize these coatings and show that the newly proposed nanostructure allows a significant increase of the photovoltaic absorption. The dependence of antireflection and light-trapping properties on the angle of incidence is numerically investigated, and it is shown that the array of voids keeps optimal also after averaging over the incidence angles.

Energy loss process analysis for radiation degradation and immediate recovery of amorphous silicon alloy solar cells

Performance degradation of a-Si/a-SiGe/a-SiGe triple-junction solar cells due to irradiation of silicon ions, electrons, and protons are investigated using an in-situ current–voltage measurement system. The performance recovery immediately after irradiation is also investigated. Significant recovery is always observed independent of radiation species and temperature. It is shown that the characteristic time, which is obtained by analyzing the short-circuit current annealing behavior, is an important parameter for practical applications in space. In addition, the radiation degradation mechanism is discussed by analyzing the energy loss process of incident particles (ionizing energy loss: IEL, and non-ionizing energy loss: NIEL) and their relative damage factors. It is determined that ionizing dose is the primarily parameter for electron degradation whereas displacement damage dose is the primarily parameter for proton degradation. This is because the ratio of NIEL to IEL in the case of electrons is small enough to be ignored the damage due to NIEL although the defect creation ratio of NIEL is much larger than that of IEL in the cases of both protons and electrons. The impact of “radiation quality effect” has to be considered to understand the degradation due to Si ion irradiation.

SF6/Ar plasma textured periodic glass surface morphologies with high transmittance and haze ratio of ITO:Zr films for amorphous silicon thin film solar cells

We report various SF6/Ar plasma textured periodic glass surface morphologies with high transmittance, haze ratio, and root mean square (rms) roughness of ITO:Zr films for amorphous silicon thin film solar cells (a-Si TFSCs). SF6/Ar plasma textured glass surface morphologies contain micro- and nano-textured features that help to scatter the light in visible and near infra-red (NIR) wavelength regions. We designed the textured glass surface morphologies with big square craters to smaller pyramids for various glass etching times from 30 to 75 min. Magnetron sputtered ITO:Zr (∼210 nm) films were deposited on textured glass surface morphologies and showed higher transmittance and haze ratio of 88.48% and 77.61%, respectively, in the visible-NIR (400–1100 nm) wavelength region. The sheet resistance and resistivity of ITO:Zr films decreased with the increase of etching time, due to high rms roughness and better step coverage. A passivation AZO (30 nm) layer was added to the ITO:Zr films, due to its better stability against hydrogen plasma exposure. The ITO:Zr/AZO films were employed as a front TCO layer and the current density–voltage (J–V) characteristics of a-Si TFSCs increased by light scattering effect, without any reduction in either the open circuit voltage (VOC) or the fill factor (FF). Relative to flat glass substrate, JSC and the efficiency of a-Si TFSCs were enhanced by 7.51% and 19.39%, respectively, for textured glass surface morphology.

Optoelectronic transport properties in amorphous/crystalline silicon solar cell heterojunctions measured by frequency-domain photocarrier radiometry: multi-parameter measurement reliability and precision studies.

A theoretical one-dimensional two-layer linear photocarrier radiometry (PCR) model including the presence of effective interface carrier traps was used to evaluate the transport parameters of p-type hydrogenated amorphous silicon (a-Si:H) and n-type crystalline silicon (c-Si) passivated by an intrinsic hydrogenated amorphous silicon (i-layer) nanolayer. Several crystalline Si heterojunction structures were examined to investigate the influence of the i-layer thickness and the doping concentration of the a-Si:H layer. The experimental data of a series of heterojunction structures with intrinsic thin layers were fitted to PCR theory to gain insight into the transport properties of these devices. The quantitative multi-parameter results were studied with regard to measurement reliability (uniqueness) and precision using two independent computational best-fit programs. The considerable influence on the transport properties of the entire structure of two key parameters that can limit the performance of amorphous thin film solar cells, namely, the doping concentration of the a-Si:H layer and the i-layer thickness was demonstrated. It was shown that PCR can be applied to the non-destructive characterization of a-Si:H/c-Si heterojunction solar cells yielding reliable measurements of the key parameters.

Uniform 3D hydrothermally deposited zinc oxide nanorods with high haze ratio

We present low cost hydrothermally deposited uniform zinc oxide (ZnO) nanorods with high haze ratios for the a-Si thin film solar cells. The problem of low transmittance and conductivity of hydrothermally deposited ZnO nanorods was overcome by using RF magnetron sputtered aluminum doped zinc oxide (ZnO:Al ~300nm) films as a seed layer. The length and diameters of the ZnO nanorods were controlled by varying growth times from 1 to 4h. The length of the ZnO nanorods was varied from 1 to 1.5µm, while the diameter was kept larger than 300nm to obtain various aspect ratios. The uniform ZnO nanorods showed higher transmittance (~89.07%) and haze ratio in the visible wavelength region. We also observed that the large diameters (>300nm) and average aspect ratio (3–4) of ZnO nanorods favored the light scattering in the longer wavelength region. Therefore, we proposed uniformly deposited ZnO nanorods with high haze ratio for the future low cost and large area amorphous silicon thin film solar cells.