InP Solar Cells Research Articles

High-Efficiency Solar Cells from Extremely Low Minority Carrier Lifetime Substrates Using Radial Junction Nanowire Architecture

Currently, a significant amount of photovoltaic device cost is related to its requirement of high-quality absorber materials, especially in the case of III-V solar cells. Therefore, a technology that can transform a low-cost, low minority carrier lifetime material into an efficient solar cell can be beneficial for future applications. Here, we transform an inefficient p-type InP substrate with a minority carrier lifetime less than 100 ps into an efficient solar cell by utilizing a radial p-n junction nanowire architecture. We fabricate a p-InP/n-ZnO/AZO radial heterojunction nanowire solar cell to achieve a photovoltaic conversion efficiency of 17.1%, the best reported value for radial junction nanowire solar cells. The quantum efficiency of ∼95% (between 550 and 750 nm) and the short-circuit current density of 31.3 mA/cm2 are among the best for InP solar cells. In addition, we also perform an advanced loss analysis of the proposed solar cell to assess different loss mechanisms in the solar cell.

Characterization of a GaAs/GaAsBi pin solar cell

The structural and photovoltaic properties of the GaAs/GaAsBi pin solar cell with GaAs0.983Bi0.017 active layer are investigated by optical and electrical measurement techniques. The bandgap of GaAsBi active layer is determined to be 1.3 eV at room temperature. Current density-voltage (J-V) under AM 1.5G spectrum and spectral response measurements are carried out to determine photovoltaic properties of the solar cell. The presence of a midgap trap levels in GaAsBi active layer is identified by deep level transient spectroscopy (DLTS). J-V characteristics is analysed by using Sah-Noyce-Shockley (SNS) theory which includes the midgap trap found in DLTS measurement. The observed deviation between experimental and calculated J-V results is ascribed for metallic cluster formation at the interface between GaAs emitter and GaAsBi intrinsic active layer. Interface metallic clusters create local Schottky junction between emitter and active layers. The best fit to the experimental J-V characteristic of the solar cell is obtained by considering the presence of GaBi metallic cluster at the interface between GaAs emitter and GaAsBi active layer of the solar cell. We showed that work function of interface clusters have a significant effect on the open circuit voltage and filling factor.

Highly Photoconductive InP Quantum Dots Films and Solar Cells.

InP and InZnP colloidal quantum dots (QDs) are promising materials for application in light-emitting devices, transistors, photovoltaics, and photocatalytic cells. In addition to possessing an appropriate bandgap, high absorption coefficient, and high bulk carrier mobilities, the intrinsic toxicity of InP and InZnP is much lower than for competing QDs that contain Cd or Pb–providing a potentially safer commercial product. However, compared to other colloidal QDs, InP QDs remain sparsely used in devices and their electronic transport properties are largely unexplored. Here, we use time-resolved microwave conductivity measurements to study charge transport in films of InP and InZnP colloidal quantum dots capped with a variety of short ligands. We find that transport in InP QDs is dominated by trapping effects, which are mitigated in InZnP QDs. We improve charge carrier mobilities with a range of ligand-exchange treatments and for the best treatments reach mobilities and lifetimes on par with those of PbS QD films used in efficient solar cells. To demonstrate the device-grade quality of these films, we construct solar cells based on InP & InZnP QDs with power conversion efficiencies of 0.65 and 1.2%, respectively. This represents a large step forward in developing Cd- and Pb-free next-generation optoelectronic devices.

Indium phosphide based solar cell using ultra-thin ZnO as an electron selective layer

According to the Shockley–Queisser limit, the maximum achievable efficiency for a single junction solar cell is ~33.2% which corresponds to a bandgap (Eg) of 1.35 eV (InP). However, the maximum reported efficiency for InP solar cells remain at 24.2% ± 0.5%, that is >25% below the standard Shockley–Queisser limit. Through a wide range of simulations, we propose a new device structure, ITO/ ZnO/i-InP/p+ InP (p-i-ZnO-ITO) which might be able to fill this efficiency gap. Our simulation shows that the use of a thin ZnO layer improves passivation of the underlying i-InP layer and provides electron selectivity leading to significantly higher efficiency when compared to their n+/i/p+ homojunction counterpart. As a proof-of-concept, we fabricated ITO/ZnO/i-InP solar cell on a p+ InP substrate and achieved an open-circuit voltage (Voc) and efficiency as high as 819 mV and 18.12%, respectively, along with ~90% internal quantum efficiency. The entire device fabrication process consists of four simple steps which are highly controllable and reproducible. This work lays the foundation for a new generation of thin film InP solar cells based solely on carrier selective heterojunctions without the requirement of extrinsic doping and can be particularly useful when p- and n-doping are challenging as in the case of III–V nanostructures.

Combining Thickness Reduction and Light Trapping for Potential Efficiency Improvements in Perovskite Solar Cells

In this contribution it is shown that the efficiency of perovskite solar cells based on CH3NH3PbI3 can be increased further by combining thickness reduction of the perovskite layer and light trapping. A physical model for the current/voltage curve of pin solar cells is used to reveal the beneficial impact of thinning on cell efficiency. If interface recombination is kept at moderate levels, the model shows that there is a potential efficiency increase above 20% relative (+3% absolute) when thickness is reduced from 500 to 200 nm, provided total light absorption is maintained. A rigorous optical model is employed to calculate light absorption on typical state–of–the–art layer stacks patterned with sinusoidal grooves on ITO coated glass. The results suggest that solar light absorption in a flat, 500 nm thick film, can be matched by a 200 nm thick perovskite layer on a sinusoidal texture, while using 300 nm leads to several sinusoidal parameter combinations delivering the same light absorption. Since the structuring step must be compatible with low cost processing, it is shown that direct laser interference patterning (DLIP) is capable of delivering +3% absolute efficiency increase, while offering a typical photovoltaic module cost reduction of 10%.

Flexible semi-transparent a-Si:H pin solar cells for functional energy-harvesting applications

We report on the fabrication and characterization of penetration-type semi-transparent hydrogenated amorphous silicon (a-Si:H) pin solar cells on flexible plastic substrates. Device-grade solar cell layers were designed and fabricated at a maximum process temperature of 150 °C, which is suitable for plastic substrate application. The solar cell layers were characterized using electrical, structural, and optical measurements, while optical and electrical measurements were carried out for the solar cells along with mechanical durability test. The flexible semi-transparent a-Si:H solar cells showed an average visible range (500–800 nm) optical transmittance of 34.2% and a power conversion efficiency (PCE) of ∼5.0%. The fabricated solar cells also showed good flexibility and durability from rigid bending cycling test. The device performance demonstrated the effectiveness of semi-transparent a-Si:H solar cells enabling low-cost, light-weight functional energy-harvesting applications.

Current-induced and light-induced macroscopic changes in thin film solar cells: Device degradation mechanism

We report on the formation of large voids in hydrogenated polymorphous silicon (pm-Si:H) PIN solar cells upon light-soaking. We could monitor in situ, the formation of macroscopic bubbles and holes during current-induced degradation of the same devices using optical microscopy. Forward bias, leading to a current-injection of 300mA/cm2, was applied to hydrogenated amorphous silicon (a-Si:H) and pm-Si:H PIN solar cells and series of optical images were taken on the same spot at various steps of current-injection. During the current-injection, the pm-Si:H PIN solar cells experience significant topological changes, which we could not detect in a-Si:H PIN solar cells. These effects were further characterized by complementary ex-situ techniques such as SEM, AFM and spectroscopic ellipsometry.

Analytical modeling of polarization effects in InGaN double hetero-junction p-i-n solar cells

We propose a new and simple analytical model taking into account of the polarization charge effects in order to assess the performance of InGaN-based PIN solar cells. The main contribution of this study consists in the derivation of new expressions of both diffusion and generation recombination currents to take into account of the polarization charges. The validity of the model is assessed through a comparison to numerical calculations, using the one dimensional self-consistent Schrödinger-Poisson equations and previously published theoretical and experimental works. The model is shown to be in a good agreement with experimental data. As expected, it is shown that the spontaneous and piezoelectric polarizations have a beneficial effect when the InGaN layer is grown on the N-face GaN/InGaN solar cells.

Modeling and simulation of solar cells quantum well based on SiGe/Si

In recent years, the development of quantum well solar cells QWSCs (Quantum Well Solar Cells) has generated a great deal of interest. These configurations have shown good promise to optimize the low conversion efficiency of conventional solar cells because of the high rate of absorption losses present in them. In this work, we are interested in modeling and simulation of two different structures of solar cells, a simple solar cell based on silicon Si and a quantum well solar cell SiGe/Si. When a solar cell is compared to 80 quantum well layers of Si0.8Ge0.2with a pin solar cell based on Si. The short circuit current Jsc increases from 23.55 to 37.48 mA/cm2 with a relative increase of 59.15% found. In addition, the limit of the absorption band of the lower energy photons extends from 1100 nm to 2000 nm.

Correlation between SiH2/SiH and light-induced degradation of p–i–n hydrogenated amorphous silicon solar cells

We have measured the hydrogen content ratio ISiH2/ISiH associated with Si–H2 and Si–H bonds in p–i–n (PIN) a-Si:H solar cells by Raman spectroscopy. With decreasing ISiH2/ISiH, the efficiency, short-circuit current density, open-circuit voltage, and fill factor of PIN a-Si:H solar cells after light soaking tend to increase. Namely, ISiH2/ISiH correlates well with light-induced degradation of the cells. While a single I-layer has a low ISiH2/ISiH of 0.03–0.09, a PIN cell has ISiH2/ISiH = 0.18 because many Si–H2 bonds exist in the P-layer and at the P/I interface of the PIN solar cells. To realize PIN solar cells with higher stability, we must suppress Si–H2 bond formation in the P-layer and at the P/I interface.

Nanophotonic resonators for InP solar cells.

We describe high efficiency thin-film InP solar cells that utilize a periodic array of TiO2 nanocylinders. These nanophotonic resonators are found to reduce the solar-weighted average reflectivity of an InP solar cell to ~1.3%, outperforming the best double-layer antireflection coatings. The coupling between Mie scattering resonances and thin-film interference effects accurately describes the optical enhancement provided by the nanocylinders. The spectrally resolved reflectivity and J-V characteristics of the device under AM1.5G illumination are determined via coupled optical and electrical simulations, resulting in a predicted power conversion efficiency > 23%. We conclude that the nanostructured coating reduces reflection without negatively affecting the electronic properties of the InP solar cell by separating the nanostructured optical components from the active layer of the device.

The analysis of the efficiency of solar cells

Under the circumstances of the increasing depletion of fossil fuels, renewable energy, especially solar energy, is becoming more and more important. Thus, the methods of exploring solar energy are diverse and the energy conversion efficiency keeps rising while the cost keeps decreasing. Among all the methods, using solar cell is the main way to explore solar energy. Currently, solar cells can be divided into three main categories, crystalline silicon solar cells, thin film solar cells and novel solar cells. Crystalline silicon solar cells can also be divided into monocrystalline silicon solar cell and polycrystalline silicon solar cell. These two kinds of solar cells are the most common solar cells in our daily life. The efficiency of monocrystalline silicon solar cell is already pretty high reaching 25% while polycrystalline silicon solar cell reaches 21.3%. The stability of monocrystalline silicon solar is also better than that of polycrystalline silicon solar cell. The only fly in the ointment is that the cost of monocrystalline silicon solar is a little bit higher than polycrystalline silicon solar cell which makes monocrystalline silicon solar not optimal enough. Thin film solar cell includes amorphous silicon solar cells and multi-compounds solar cells. The transformation of solar cells from solid panel to thin film is one of the most significant breakthroughs of the second generation solar cell. The amorphous silicon solar cell becomes much cheaper than both monocrystalline silicon and polycrystalline silicon solar cells. But its drawback is obvious, its efficiency only reaches 13.6% which certainly needs to be improved. As for semiconductor compounds solar cells, there are made by various semiconductor materials. Major semiconductor compound solar cells contain chemical materials such as CdTe, CuInSe2(CIS), CuInSe2 doped Ga(CIGS), GaAs, InP, and multi-junction solar cell, etc. Materials like CdTe and CIGS can surely lower the cost of solar cells and their efficiencies are pretty high reaching 21.5% and 22.3%, respectively. But Cd is a toxic element while In and Se are rare elements, their reserves are limited. Materials like GaAs and InP are much more expensive comparing to others, but their efficiencies are pretty satisfying reaching 28.8% and 22.6%, respectively. More importantly, they have rather high radiation tolerance, especially InP solar cell has the best radiation tolerance among all kinds of solar cells. Thus, despite their high costs, they can still be used in many situations such as space station. As for multi-junction solar cell, the most prominent feature of it is the high efficiency. Solar cells with two junctions, three junctions and four junctions have reached the efficiencies of 31.6%, 37.9% and 38.8%. Obviously, multi-junction solar cells are the best choice to improve energy conversion efficiency. The novel solar cell consists of organic solar cells, dye-sensitized solar cells, quantum dot solar cells and hybrid perovskite solar cells. Among all these third generation solar cells, perovskite solar cell is the most promising one due to its superior properties and rapid improvement. Its efficiency has already reached 21% in only a few years. But more work needs to be done to improve its properties such as its stability and currently perovskite solar cell still contains lead which is toxic and may need to be replaced. This paper starts from analyzing the theoretical efficiency of solar cells, then compares those theoretical efficiencies with experimental efficiencies to present the expectation of different kinds of solar cells.

Window p-layer in amorphous pin solar cells using ZnO as Transparent Conductive Oxide

The main steps of p-layer development in pin thin film amorphous silicon solar cell at TEL Solar are presented. First, the implementation of a new PECVD reactor is shown to contribute to an improvement in amorphous silicon carbide (a-SiC) p-layer quality probably by reducing the porosity and leading to an increased open circuit voltage after the solar cell is subjected to light soaking. Second, an alternative high flow deposition regime for the p-doped a-SiC layer was successfully developed improving the layer purity and electrical conductivity. And third, a softer deposition regime for the very first part of the p-layer, the contact layer, which plays a crucial role in the release of Zn from the TCO, contributes to a decreased overall Zn contamination.

Updated insight into the use of μc-Si:H n-layers in a-Si:H solar cells

Although a-Si:H solar cells have great potential in different applications, enhancing their efficiency is still a challenge. An approach to increase the short-circuit current of single junction a-Si:H pin solar cells consists on using μc-Si:H in the n-layer. In this paper we compare the properties of structures with a-Si:H n-layers and a μc-Si:H one. We also developed a precise and complete simulation model for further study of the involved electrical variables. For the simulation of the structure with μc-Si:H a thermionic model must be used. The data was analyzed through the scope of recent findings which highlight the role of conductivity difference in the carrier transport of illuminated solar cells.

Towards 12% stabilised efficiency in single junction polymorphous silicon solar cells: experimental developments and model predictions

We have combined recent experimental developments in our laboratory with modelling to devise ways of maximising the stabilised efficiency of hydrogenated amorphous silicon (a-Si:H) PIN solar cells. The cells were fabricated using the conventional plasma enhanced chemical vapour deposition (PECVD) technique at various temperatures, pressures and gas flow ratios. A detailed electrical-optical simulator was used to examine the effect of using wide band gap P-and N-doped μ c-SiOx :H layers, as well as a MgF2 anti-reflection coating (ARC) on cell performance. We find that with the best quality a-Si:H so far produced in our laboratory and optimised deposition parameters for the corresponding solar cell, we could not attain a 10% stabilised efficiency due to the high stabilised defect density of a-Si:H, although this landmark has been achieved in some laboratories. On the other hand, a close cousin of a-Si:H, hydrogenated polymorphous silicon (pm-Si:H), a nano-structured silicon thin film produced by PECVD under conditions close to powder formation, has been developed in our laboratory. This material has been shown to have a lower initial and stabilised defect density as well as higher hole mobility than a-Si:H. Modelling indicates that it is possible to attain stabilised efficiencies of 12% when pm-Si:H is incorporated in a solar cell, deposited in a NIP configuration to reduce the P/I interface defects and combined with P- and N-doped μ c-SiOx :H layers and a MgF2 ARC.

Design and Analysis of Indium Gallium Nitride based PIN solar cell

Single junction solar cells are insufficient in absorbing wider range of solar spectrum, so multijunction solar cells are used to absorb wider range of solar wavelength and hence providing higher conversion efficiency. The work proposed in this paper is to build and simulate a PIN structure as a quad layer solar cell cascaded using tunnel junctions. For In x Ga 1-x N, the mole fraction values of Indium (x) are altered for the individual cell layer. The mole fraction variation leads to the change in the grading of the concentration of Indium along the depth of the structure. Variable bandgap is achieved by varying the grading, thus making the multijunction cell to be a wide energy bandgap structure. For each cell, the parameters are modeled numerically and simulated for mole fraction values (x) of 0.365, 0.525, 0.70 and 0.93. The individual cell analysis is performed and then the structure is cascaded together to form a 100 cm 2 area quad junction solar cell for which the performance analysis is obtained as open circuit voltage to be 1.17 V and short circuit current density to be 3.18 mA/cm 2 which accounts to a maximum conversion efficiency of 33.63% for 100Watt/cm 2 , under air mass AM1.5 illumination spectrum.

Enhanced photovoltaic performance of an inclined nanowire array solar cell.

An innovative solar cell based on inclined p-i-n nanowire array is designed and analyzed. The results show that the inclined geometry can sufficiently increase the conversion efficiency of solar cells by enhancing the absorption of light in the active region. By tuning the nanowire array density, nanowire diameter, nanowire length, as well as the proportion of intrinsic region of the inclined nanowire solar cell, a remarkable efficiency in excess of 16% can be obtained in GaAs. Similar results have been obtained in InP and Si nanowire solar cells, demonstrating the universality of the performance enhancement of inclined nanowire arrays.

42nd photovoltaic specialists conference

An investigation is made into the possibility of providing low resistance contacts to shallow junction InP solar cells which do not require sintering and which do not cause device degradation even when subjected to extended annealing at elevated temperatures. We show that the addition of In to Au contacts in amounts that exceed the solid solubility limit lowers the as-fabricated (unsintered) contact resistivity (R sub c) to the 10{sup {minus}5} ohm cm{sup 2} range. We next consider the contact system Au/Au2P3, which has been shown to exhibit as-fabricated R sub c values in the 10{sup {minus}6} ohm cm{sup 2} range, but which fails quickly when heated. We show that the substitution of a refractory metal (W, Ta) for Au preserves the low R sub c values while preventing the destructive reactions that would normally take place in this system at high temperatures. We show, finally, that R sub c values in the 10{sup {minus}7} ohm cm{sup 2} range can be achieved without sintering by combining the effects of In or Ga additions to Au contacts with the effects of introducing a thin Au2P3 layer at the metal-InP interface.

Evolution of Structural and Optical Properties on PIN and NIP pm-Si:H Devices During 400 Hrs of Light-Soaking

ABSTRACTPm-Si:H PIN and NIP solar cells structures grown using plasma enhanced chemical vapor deposition (PECVD) technique were analyzed during 400 hrs of light-soaking exposition. The evolution of the structural and optical properties was observed and characterized by Raman spectroscopy, spectroscopic ellipsometry. The effect observed is related to defects creation due to induced hydrogen diffusion, break of Si-H bonds and the generation of dangling bonds that causes less passivated films. The film microstructure, and therefore the optical properties varied with the exposition time. The crystalline fraction of these structures presents a slight decrease and it is observed to be between 15 to 24% for the PIN and 5 to 10% for the NIP. The optical gap increases from 1.66 to 1.68 eV for the PIN structure while for the NIP no significant change is observed during light-soaking. Hydrogen diffusion during lights soaking generates a decrease on the absorption properties of the films which in turn is expected to reduce the device efficiency during operation. In this work we show that long range motion of hydrogen during light-soaking causes a hydrogen rearrangement on the film and microstructure changes. We determined that there is not an pronounced change on the film structure during prolonged light exposition related to the stability of the pm-Si:H films. The PIN structure properties are more affected during light soaking in comparison to the NIP structure which is expected to cause less degradation of its optoelectronic properties under illumination, and a more stable device during operation.

Material and growth mechanism studies of microcrystalline silicon deposited from SiF4/H2/Ar gas mixtures

Hydrogenated microcrystalline silicon is of great interest as the intrinsic layer in thin film solar cells. It is generally accepted that optimized cells use an I-layer with a crystalline fraction ranging from 50% to 70%. We report here on the use of fully crystallized layers deposited from SiF4/H2/Ar gas mixtures. A VOC of 536 mV and a JSC of 25.4 mA/cm2 have been obtained in PIN solar cells using a fully crystallized intrinsic material. Fourier transform photocurrent spectroscopy measurements show that the films have a very low defect density down to 5.1 × 1014 cm−3. The role of H2 during deposition has been investigated: it is demonstrated that H2 is needed to remove F by forming HF molecules. When all atomic F has been removed from the plasma, the remaining atomic H can then contribute to μc-Si:H growth.