Intrinsic Layer Thickness Research Articles

Light Trapping in Hydrogenated Amorphous and Nanocrystalline Silicon Based Thin-Film Solar Cells With Ag/ZnO Back Reflectors

ABSTRACTWe report on our systematic study of light trapping effects using Ag/ZnO BRs for nc-Si:H solar cells. The texture of Ag and ZnO was optimized to achieve enhancement in photocurrent. The light trapping effect on photocurrent enhancement in solar cells was carefully investigated. Comparing to single-junction solar cells deposited on flat stainless steel substrates, the gain in Jsc by using Ag/ZnO BRs is 57% for nc-Si:H solar cells. This gain in Jsc is much higher than what has been achieved by advanced light trapping approaches using photonic structures or plasmonic light trapping reported in the literature. We achieved a Jsc of 29-30 mA/cm2 in a nc-Si:H single-junction solar cell with an intrinsic layer thickness of ∼2.5 μm. We compared the quantum efficiency of single-junction cells to the classical limit of fully randomized scattering and found that there is a 6-7 mA/cm2 difference between the measured Jsc and the classical limit, in which 3-4 mA/cm2 is in the long wavelength region. However, by taking into consideration the losses from reflection of the top contact, absorption in the doped layers, and imperfect reflection in the BRs, the difference disappears. This implies we have reached the practical limit if the scattering from randomly textured substrates is the only mechanism of light trapping. Therefore, we believe future research for improving photocurrent should be directed toward reducing (i) reflection loss by the top contact, the absorption in ZnO and at the Ag/ZnO interface, and (ii) p layer absorption.

Window layer with p doped silicon oxide for high Voc thin-film silicon n-i-p solar cells

We investigate the influence of the oxygen content in boron-doped nanocrystalline silicon oxide films (p-nc-SiOx) and introduce this material as window layer in n-i-p solar cells. The dependence of both, optical and electrical properties on the oxygen content is consistent with a bi-phase model which describes the p-nc-SiOx material as a mixture of an oxygen-rich (O-rich) phase and a silicon-rich (Si-rich) phase. We observe that increasing the oxygen content enhances the optical gap E04 while deteriorating the activation energy and the planar conductivity. These trends are ascribed to a higher volume fraction of the O-rich phase. Incorporated into n-i-p a-Si:H cells, p-nc-SiOx layers with moderate oxygen content yield open circuit voltage (Voc) up to 945 mV, which corresponds to a relative gain of 11% compared to an oxygen-free p-layer. As a similar gain is obtained on planar and on textured substrates, we attribute the increase in Voc to the higher work function of the p-nc-SiOx layer made possible by its wider band gap. These results are attained without changing the dilution ratio of the 250 nm thick intrinsic layer. We also observe an enhancement of 0.6 mA cm−2 in short circuit current density in the short wavelengths due to the higher transparency of the p-nc-SiOx layer. Finally, an initial efficiency of 9.9% for a single junction 250 nm a-Si:H n-i-p solar cell on plastic foil is achieved with the optimization of the p layer thickness, the doping ratio of the front transparent conductive oxide, and the optical properties of the back reflector.

Design and Simulation of a-Si:H/nc-Si:H Tandem Solar Cells

A kind of amorphous/nanocrytalline Si (a-Si:H/nc-Si:H) PIN type tandem solar cell of TCO/a-SiC:H(p1)/buffer/a-Si:H(i1)/nc-Si:H(n1)/tunnel junction (TJ)/nc-Si:H(p2)/ nc-Si:H(i2)/ a-Si:H(n2)/Al was optimized with numerical method. The high conductivity, high optical transmittance and wider band-gap material was selected as window layer in the cell. The buffer layer between p1 and i1 layers and the tunnel junction were designed, respectively. The thickness and band-gap of intrinsic layers in sub-cells were adjusted separately. The simulation results indicate that absorption solar spectrum by the designed cell can be expanded towards the long wave direction. The light-induced recession of present cell was restrained while the stability was improved.

A Novel Coaxial-Structured Amorphous-Silicon p-i-n Solar Cell With Al-Doped ZnO Nanowires

A novel coaxial-structured amorphous-silicon (a-Si) p-i-n solar cell with 1-μm-long low-temperature hydrothermally synthesized Al-doped-ZnO (AZO) nanowires was demonstrated for the first time. The conversion efficiency η increased from 3.92% to 4.27% when the intrinsic a-Si thickness was increased from 25 to 150 nm and then decreased to 3.66% when the intrinsic layer thickness was further increased to 250 nm. It was attributed to an excessively thick intrinsic a-Si layer that would decrease the internal electrical field and interfere with charge separation. With the optimum intrinsic a-Si thickness of 150 nm, the conversion efficiency increased from 4.27% to 4.73% when the AZO wire length was increased from 1 to 2 μm. Moreover, the proposed coaxial-structured solar cell exhibited a nearly 46% efficiency enhancement over a conventional a-Si thin-film solar cell.

Plasma deposition of n-SiO x nanocrystalline thin film for enhancing the performance of silicon thin film solar cells

In this paper, we firstly optimized the properties of n-SiO x nanocrystalline thin film through tuning deposition parameters by plasma enhanced chemical vapor deposition, so that we can actively control the properties of materials obtained. Secondly, we proposed using n-SiO x/Al as back reflector for amorphous silicon (a-Si:H) solar cells. Compared to Al single-layer as back reflector, adding an n-SiO x layer into the back reflector could improve the solar cell performance, which not only enhances the short circuit current density by an improvement of spectral response in the wavelength range of 550–750 nm, but also improves the open circuit voltage. With an optimized n-SiO x/Al back reflector, a-Si:H solar cells with an intrinsic layer thickness of 270 nm show 13.1% enhancement in efficiency. In addition, a-Si:H/μc-Si:H tandem solar cells with n-SiO x as intermediate reflector were also researched. As a result, it evidently balanced the current matching between top and bottom cell.

Thin crystalline silicon solar cells based on epitaxial films grown at 165 °C by RF-PECVD

We report on heterojunction solar cells whose thin intrinsic crystalline absorber layer has been obtained by plasma enhanced chemical vapor deposition at 165 °C on highly doped p-type (1 0 0) crystalline silicon substrates. We have studied the effect of the epitaxial intrinsic layer thickness in the range from 1 to 2.5 μm. This absorber is responsible for photo-generated current whereas highly doped wafer behave like electric contact, as confirmed by external quantum efficiency measurements and simulations. A best conversion efficiency of 7% is obtained for a 2.4 μm thick cell with an area of 4 cm 2, without any light trapping features. Moreover, the achievement of a fill factor as high as 78.6% is a proof that excellent quality of the epitaxial layers can be produced at such low temperatures.

Amorphous/crystalline silicon heterojunction solar cells with varying i-layer thickness

We study the effect on various properties of varying the intrinsic layer (i-layer) thickness of amorphous/crystalline silicon heterojunction (SHJ) solar cells. Double-side monocrystalline silicon (c-Si) heterojunction solar cells are made using hot-wire chemical vapor deposition on high-lifetime n-type Czochralski wafers. We fabricate a series of SHJ solar cells with the amorphous silicon (a-Si:H) i-layer thickness at the front emitter varying from 3.2 nm (0.8xi) to ~ 96 nm (24xi). Our optimized i-layer thickness is about 4 nm (1xi). Our reference cell (1xi) performance has an efficiency of 17.1% with open-circuit voltage (V oc) of 684 mV, fill factor (FF) of 76%, and short-circuit current density (J sc) of 33.1 mA/cm 2. With an increase of i-layer thickness, V oc changes little, whereas the FF falls significantly after 12 nm (3xi) of i-layer. Transient capacitance measurements are used to probe the effect of the potential barrier at the n-type c-Si/a-Si interface on minority-carrier collection. We show that hole transport through the i-layer is field-driven transport rather than tunneling.

Analysis of the Ideality Factor of a-Si:H Solar Cells

This paper analyzes the ideality factor of amorphous silicon (a-Si:H) solar cells as a function of both the thickness of the intrinsic layer and the applied voltage to the cells. The ideality factor in this work is extracted from the current/voltage characteristic that is calculated by solving the continuity and transport equations and taking into account the contributions of diffusion and drift currents for minority and majority carriers and, especially, the nonequality of mobilities and lifetimes of electrons and holes in a-Si:H solar cells.

Characteristics of silicon nanocrystals for photovoltaic applications

AbstractOver the last decade the progress in amorphous and nanocrystalline silicon (nc‐Si) for photovoltaic applications received significant interest in science and technology. Advances in the understanding of these novel materials and their properties are growing rapidly. In order to realise nc‐Si in the solar cell, a thicker intrinsic layer is required. Due to the indirect band gap in the crystallites, the absorption coefficients of nc‐Si are much lower. In this work we have used electrochemical etching techniques to produce silicon nanocrystals of the sizes 3–5 nm. Viable drop cast deposition of Si nanocrystals to increase the thickness without compromising the material properties was investigated by atomic force microscopy, optical microscopy, photoemission spectroscopy and optical absorption methods.

An iterative method applied to optimize the design of PIN photodiodes for enhanced radiation tolerance and maximum light response

An iterative method based on numerical simulations was developed to enhance the proton radiation tolerance and the responsivity of Si PIN photodiodes. The method allows to calculate the optimal values of the intrinsic layer thickness and the incident light wavelength, in function of the light intensity and the maximum proton fluence to be supported by the device. These results minimize the effects of radiation on the total reverse current of the photodiode and maximize its response to light. The implementation of the method is useful in the design of devices whose operation point should not suffer variations due to radiation.

High-Responsivity and High-Speed Waveguide Photodiode With a Thin Absorption Region

The characteristics of a waveguide photodiode with a thin absorption layer are reported. A responsivity of 1.08 A/W at 1.55 μm was measured using a 0.1-μm -thick absorption region with a polarization dependence of less than 0.25 dB. A thin absorption layer enables us to use a thicker intrinsic layer. Using a thicker intrinsic region of 0.9 μm and an adiabatically reduced waveguide width, a bandwidth of 42 GHz was achieved. Adopting a tapering waveguide width did not give rise to a degradation of responsivity or alignment tolerance.

New developments in the fabrication of amorphous silicon photovoltaic modules on very large 2.60 m × 2.20 m glass

AbstractThe fabrication of photovoltaic thin film modules based on hydrogenated amorphous silicon (a‐Si:H) technology on very large float glass is presented. In high volume manufacture of about 900 modules/day production present an average power of about 105 W ±5 W (initial state, 1.43 m2 modules). Light‐induced degradation (LID) of modules results in an average power of about 85±2 W after about 500 kWh of degradation. Degradation of the fill factor (FF) of current voltage curves under illumination is the main contributor to degradation reducing from about 70% to 60% (artificial light). The dependency of the power in the initial and in the degraded state on the intrinsic layer thickness of the pin‐structure is investigated. Thicker intrinsic layer results in higher initial power, however also the degradation rate is higher resulting in very similar power of all modules in the degradation state. LID is performed in in‐door conditions under metal‐halogen lamp light and in outdoor conditions with sun‐light. Under outdoor conditions the degraded state of the modules is not stable, but depends on the season, resulting in lower power in winter months and higher power in summer months. Main contributor to this effect is the change of short circuit current. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Performance of bifacial HIT solar cells on n-type silicon substrates

The performance of amorphous silicon (a-Si:H) / crystalline silicon (c-Si) heterojunction is studied, and the effects of the emitter layer thickness, doping concentration, intrinsic layer thickness, back heavily-doped n layer, interface state and band offset on the optical and electrical performance of bifacial heterojunction with intrinsic thin-layer (HIT) solar cells on ntype silicon substrates are discussed. It is found that the HIT solar cells on n-type substrates can obtain a higher conversion efficiency than those on p-type substrates by calculating the band diagrams and parameters of HIT solar cells.

Structural Optimization of Silicon Carbide PIN Avalanche Photodiodes for UV detection

The locally-enhanced electric field occurring at the etched junction surface of mesa-etched PIN avalanche photodiodes (APDs) causes early edge breakdown, resulting in a relatively lower multiplication and gain. The electric field distribution as a function of the intrinsic layer thickness and the bevel angle was investigated, and its influence on the characteristics of 4H-SiC PIN APDs, such as the breakdown voltage, the quantum efficiency, and the gain, was studied. In addition, a new structure with a field plate is proposed to diminish the edge breakdown phenomenon in mesa-etched PIN APDs.

Blue-Enhanced PIN Finger Photodiodes in a 0.35-$\mu{\hbox {m}}$ SiGe BiCMOS Technology

Finger photodiodes in PIN technology are introduced to enhance the responsivity for blue and ultraviolet light. A thick low doped epitaxial layer results in high responsivity and high bandwidth also for red and near-infrared light. Results of PIN finger photodiodes are compared to that of PIN photodiodes for 10- and 15-mum epitaxial intrinsic layer thickness. The cathode finger structure results in a high responsivity of 0.20 A/W (quantum efficiency 61%) for 410-nm light and a bandwidth of 1.25 GHz for 10- mum epi thickness at a reverse bias voltage of 3 V. The rise and fall times with an epitaxial layer thickness of 15 mum are below 1 ns for the wavelength range from 410 to 785 nm.

TCAD simulation of hydrogenated amorphous silicon-carbon/microcrystalline-silicon/hydrogenated amorphous silicon-germanium PIN solar cells

This study investigates the device performance of hydrogenated amorphous silicon-carbon (a-SiC:H)/microcrystalline-silicon (μc-Si)/hydrogenated amorphous silicon-germanium (a-SiGe:H) PIN thin film solar cells using Technology Computer Aided Design (TCAD) simulations. The physical parameters used in the TCAD simulations are calibrated to reproduce experimental data. The influence of the density of states (DOS) and intrinsic layer (I-layer) thickness on the performance of thin film solar cells is investigated. According to the simulation results, the highest efficiency is approximately 9% when the I-layer thickness is 4 μm.

Rectifying properties and photoresponse of CVD diamond p(i)n‐junctions

AbstractThe current–voltage characteristics and photoresponse of mesa structured {111}‐oriented homoepitaxial CVD diamond p(i)n‐junctions with different intrinsic layer thickness are investigated. When a sufficiently thick intrinsic layer is present, a rectification ratio of 108 at ±10 V could be obtained. Good rectifying diodes show a high photoresponse ratio between 210 nm (above bandgap) and 500 nm (below bandgap), making them suitable for UV detection purposes. The results are compared with similar measurements carried out on polycrystalline CVD diamond pn‐junctions.magnified image18 diamond pin mesa structures (250 μm in diameter) based on a boron‐doped/intrinsic/P‐doped CVD layer stack deposited on a 2 × 2 × 0.5 mm3 {111} Ib HPHT diamond substrate.(© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Behaviors of the third order intercept point for p-i-n waveguide photodiodes

Waveguide PIN photodiodes with different absorber thicknesses and lengths were fabricated and characterized for linearity. Device A has a thicker absorber and shorter length, resulting in a bandwidth of 20 GHz while device B reduces the absorber by half while maintaining the intrinsic layer thickness and almost doubles the length, resulting in a smaller optical overlap factor and a bandwidth of 10 GHz. Device B shows a significant enhancement in OIP3 with a record high maximum value for a PIN waveguide photodiode of 42.4 dBm at 28 mA and -4V bias compared to device A which has a maximum OIP3 of 32.7 dBm at 10 mA and -4V bias. The increased linearity in device B is attributed to the reduction in optical overlap factor and increase in device length resulting in an easing of the front facet photocurrent density and overall device heating. The DC saturation points are about 75 mA and >160 mA for device A and B at -2V bias.

Improvement of universal pin photodetectors in 0.35 µm SiGe BiCMOS technology

Universal pin photodiodes combine low capacitance with high bandwidth and high responsivity. A speed improvement is achieved for a pin finger photodiode in a high-speed 0.35 µm SiGe heterojunction bipolar transistor (HBT) BiCMOS technology. The cathode finger structure results in a high responsivity of 0.2 A/W (quantum efficiency 61%) for 410 nm light and a bandwidth of 1253 MHz at a reverse bias voltage of 3 V. Owing to the thick low doped intrinsic epitaxial layer, high bandwidths and high dynamic quantum efficiencies result for a wide spectrum of optical wavelengths from ultraviolet and blue to red and near-infrared.

The effect of the intrinsic layer on the reliability of nitride-based p–i–n photodetectors

This study successfully proves that the reliability of nitride-based p–i–n photodetectors (PDs) is highly sensitive to the thickness of intrinsic GaN layers. Results are based on i-GaN layers of 0.25 µm, 0.4 µm and 0.5 µm thicknesses. After current ageing, the p–i–n PDs with thin i-layers exhibited poor electrical strength. Increasing the thickness of the i-layer improved the electrical strength and ESD protection capability of PDs. This result is directly related to the impedance and dislocation density of the i-layer. However, the etched sidewall becomes a weak point when adopting a thicker i-layer.