InP Solar Cells Research Articles

Research Progresses on High Efficiency Amorphous and Microcrystalline Silicon-Based Thin Film Solar Cells

AbstractThis paper reviews our research progresses of hydrogenated amorphous silicon (a-Si:H) and microcrystalline (μc-Si:H) based thin film solar cells. It coves the three areas of high efficiency, low cost process, and large-area proto-type multi-chamber system design and solar module deposition. With an innovative VHF power profiling technique, we have effectively controlled the crystalline evolution and made uniform μc-Si:H materials along the growth direction, which was used as the intrinsic layers of pin solar cells. We attained a 9.36% efficiency with a μc-Si:H single-junction cell structure. We have successfully resolved the cross-contamination issue in a single-chamber system and demonstrated the feasibility of using single-chamber process for manufacturing. We designed and built a large-area multi-chamber VHF system, which is used for depositing a-Si:H/μc-Si:H micromorph tandem modules on 0.79-m2 glass substrates. Preliminary module efficiency has exceeded 8%.

Ultrathin film, high specific power InP solar cells on flexible plastic substrates

We demonstrate ultrathin-film, single-crystal InP Schottky-type solar cells mounted on flexible plastic substrates. The lightly p-doped InP cell is grown epitaxially on an InP substrate via gas source molecular beam epitaxy. The InP substrate is removed via selective chemical wet-etching after the epitaxial layers are cold-welded to a 25 μm thick Kapton® sheet, followed by the deposition of an indium tin oxide top contact that forms the Schottky barrier with InP. The power conversion efficiency under 1 sun is 10.2±1.0%, and its specific power is 2.0±0.2 kW/kg. The ultrathin-film solar cells can tolerate both tensile and compressive stress by bending over a <1 cm radius without damage.

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.

Electrical detection of electron spin resonance in microcrystalline silicon pin solar cells

Pulsed electrically detected magnetic resonance (pEDMR) was employed to study spin-dependent processes that influence charge transport in microcrystalline silicon (µc-Si:H) pin solar cells. Special emphasis was put on the identification of the signals with respect to the individual layers of the cell structure. To do this, we systematically modulated the morphology of the highly doped n- and p-layers from amorphous to microcrystalline. By combining the information obtained from low-temperature (T = 10 K) pEDMR spectra and from the deconvoluted time evolution of spectrally overlapping resonances, we found signals from conduction band tail states as well as phosphorus donor states in samples containing an amorphous n-type layer and a resonance associated with valence band tail states in samples with an amorphous p-layer. Moreover, several signals from the intrinsic microcrystalline absorber layers could be identified. An additional resonance at g = 1.9675(5), which has not been observed in EDMR before, was found. We assign this signal to shallow donors in the Al-doped ZnO layer, which is commonly used as transparent conducting oxide in thin-film solar cells. The experimental findings are discussed in the light of various spin-dependent transport mechanisms known to occur in the respective layers of the pin structure.

Determination of organic solar cell parameters based on single or multiple pin structures

In this work we describe, apply and analyse a new method to extract the physical parameters of organic pin solar cell based on single or multiple structures with low or wide gap which can strongly improve the performance of organic solar cell from illuminated current–voltage characteristics. These parameters are the ideality factors, saturation current, photocurrent, the series and shunt resistance which are parasitic parameters, affect the illuminated I–V characteristic and efficiency. In the present study we compare the simulated results obtained by this method using MATLAB code with experimental data. The method is very convenient to use and the reasonable agreement between experiment study and calculation results confirms the model by employed a curve fitting to provide a graphical representation of the result and a statistical analysis, the root mean square error (RMSE), the bias error (MBE) and the mean absolute error (MAE) are the fundamental measures of accuracy.

Impedance and Interface Properties of Al/Methyl‐Red/p‐InP Solar Cell

An Al/methyl‐red/p‐InP solar cell was fabricated via solution‐processing method and was characterized by using current‐voltage (I-V) and capacitance‐voltage‐frequency (C-V-f) measurements at room temperature. From dark I-V characteristics, the values of ideality factor and barrier height of the device were calculated as 1.11 eV and 2.02, respectively. It has been seen that the device exhibited a good photovoltaic behavior with a maximum open circuit voltage Voc of 0.38 V and short‐circuit current Isc of 2.8 nA under only 200 lx light intensity. The barrier height and acceptor carrier concentration values for the Al/methyl‐red/p‐InP devices were extracted as 1.27 eV and 3.46 × 1017 cm−3 from linear region of its C−2‐V characteristics, respectively. The difference between Φb (I-V) and Φb (C-V) for Al/methyl‐red/p‐InP device was attributed the different nature of the I-V and C-V measurements. Also, the energy distribution curves of the interface states and their time constants were obtained from the experimental conductance properties of the Al/methyl‐red/p‐InP structure at room temperature. The interface state densities and their relaxation times of the device have ranged from 2.96 × 1012 cm−2eV−1 and 4.96 × 10−6 s at (1.11‐Ev) eV to 5.19 × 1012 cm−2 eV−1 and 9.39 × 10−6 s at (0.79‐Ev) eV, respectively. It was seen that both the interface state density and the relaxation time of the interface states have decreased with bias voltage from experimental results.

Silicon nanostructures for solar cell applications

Among the numerous applications of Si nanostructures in the microelectronic or photonic domains, one which could be promising concerns the use of such structures as the active layer in pin solar cells. By taking advantage of the quantum confinement of the carriers in Si nanograins whose size is lower than 8 nm, it is expected to improve the solar cell efficiency by increasing the absorption range of the solar spectrum. In this work, we report the fabrication, microstructural and optical properties of Si-rich silicon oxide (SRSO) composite layers and SRSO/SiO 2 multilayers fabricated by reactive magnetron sputtering process. This process allows monitoring either the Si nanograins size and/or the Si nanograin density through specific deposition parameters such as the hydrogen rate in the plasma, the substrate temperature, the annealing treatment. Their effects on the photoluminescent properties as well as on the absorption coefficient are discussed. The SRSO/SiO 2 multilayers absorption is higher with respect to the SRSO composite layer. Such behaviour has been attributed to a better control of the Si nanograin size.

Decoupling crystalline volume fraction and VOC in microcrystalline silicon pin solar cells by using a µc‐Si:F:H intrinsic layer

AbstractMicrocrystalline silicon thin film pin solar cells with a highly crystallized intrinsic μc‐Si:F:H absorber were prepared by RF‐plasma enhanced chemical vapour deposition using SiF4 as the gas precursor. The cells were produced with a vacuum break between the doped layer and intrinsic layer depositions, and the effect of different subsequent interface treatment processes was studied. The use of an intrinsic μc‐Si:H p/i buffer layer before the first air break increased the short circuit current density from 22.3 mA/cm2 to 24.7 mA/cm2. However, the use of a hydrogen‐plasma treatment after both air breaks without an interface buffer layer improved both the open circuit voltage and the fill factor. Although the material used for the absorber layer showed a very high crystalline fraction and thus an increased spectral response at long wavelengths, an open‐circuit voltage (VOC) of 0.523 V was nevertheless observed. Such a value of VOC is higher than is typically obtained in devices that employ a highly crystallized absorber as reported in the literature (see abstract figure). Using a hydrogen‐plasma treatment, a single junction μc‐Si:F:H pin solar cell with an efficiency of 8.3% was achieved.magnified image(© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Recombination and transport in microcrystalline pin solar cells studied with pulsed electrically detected magnetic resonance

An analysis of spin-dependent processes in microcrystalline silicon (μc-Si:H) pin solar cells is presented using pulsed electrically detected magnetic resonance (pEDMR). In this first study it is shown that by modulating the morphology of the n-type contact layer from amorphous to microcrystalline, pronounced changes in the pEDMR spectra may be observed. Due to the fact that pEDMR allows a deconvolution of the spin-dependent signals in time as well as in magnetic field domain, we were able to significantly reduce the complexity of the spectra compared to conventional EDMR. In the samples containing amorphous n-type contact layers we found signals from shallow localized conduction band tail states and phosphorous donor states. Upon replacement of this layer by its microcrystalline counterpart both signals disappeared. Possible spin-dependent transport mechanisms involving paramagnetic states in the various layers are discussed in view of sign and time evolution of the associated pEDMR signals.

Why does the open-circuit voltage in a micro-crystalline silicon PIN solar cell decrease with increasing crystalline volume fraction?

In hydrogenated micro-crystalline silicon (μc-Si:H) thin film solar cells, the open-circuit voltage (VOC) shows a decline when the crystalline volume fraction (Fc) of the intrinsic μc-Si:H layer increases from 60% to over 90%. In this article we have simulated the experimental characteristics of solar cells, having intrinsic layers of different degrees of crystallinity to understand the reasons why. In order to model all aspects of the characteristics, we had to assume (a) wider band tails, (b) a higher mid-gap defect density and (c) a lower band gap for the more crystallized material. Modeling reveals that all three factors lower the field in the volume of the device and hence VOC, due to higher photo-generated hole-trapping close to the P/I interface. The third factor brings the quasi-Fermi levels closer to the band-edges, resulting in higher free and trapped carrier densities throughout the device, with the trapped hole population particularly high at the P/I interface. We further show that VOC is higher in a crystalline silicon PN cell, in spite of a sharply reduced band gap, because the lower effective density of states at the band-edges and sharply reduced band gap defect density overcome the effect of the smaller band gap.

Microcrystalline silicon–germanium thin films prepared by the chemical transport process using hydrogen radicals

We propose microcrystalline silicon–germanium (μc-SiGe) as a bottom cell material of triple-junction solar cells in order to improve the conversion efficiency of thin film solar cells. The μc-SiGe thin films were prepared by the chemical transport process using Si and Ge targets exposed to hydrogen radicals. We successfully produced highly photosensitive μc-SiGe films with relatively low Ge composition by increasing the gas pressure, and observed the photovoltaic effect in pin solar cell structures. However, it was difficult to produce μc-SiGe films with higher Ge composition. We found that a small amount of argon introduction into the chemical transport process enables us to increase Ge composition at the high pressure. Moreover, the argon introduction seems effective to maintain the electrical properties in relatively high Ge composition. The results suggest that the μc-SiGe thin films prepared by the chemical transport process are one of the candidates for new photovoltaic materials.

Design and Numerical Simulation on the Optical and Electrical Behavior of a ZnO:Al Nanowire Array a-Si pin Solar Cell

AbstractOne of the parameters that limit the efficiency of a thin film solar cell, especially the a-Si and the nc-Si solar cell is the cell thickness. Although thicker film can absorb most of the sun light, the optical generated carriers will recombination through the numerous gap states in the film that obtained lower short circuit current and fill factor. In the controversy, thinner film could not absorb enough sun light that also limit the short circuit current. In this works, we utilize nanowire structure to solve the conflict between the light absorption and the carrier transport. The designed structure has ZnO:Al nanowire array on the substrate. The p-i-n a-Si solar cell structure is grown along the surface of each ZnO: Al nanowire sequentially. Under sunlight illumination, the light is absorbed in the axis direction of the nanowire. However, the carrier transport is along the radial direction of the solar cell. Therefore, the long nanowire could absorb most of the solar light. In the mean time, the thickness of the solar cell still is thin enough for photo-generated carrier transport. The dependence of short circuit current, open circuit voltage and fill factor to the length, diameter and density of ZnO:Al nanowires were simulated.

The silicon–silicon oxide multilayers utilization as intrinsic layer on pin solar cells

Silicon nanostructures are promising candidate for the intrinsic layer on pin solar cells. In this work we report on new material: silicon-rich silicon oxide (SRSO) deposited by reactive magnetron sputtering of a pure silica target and an interesting structure: multilayers consisting of a stack of SRSO and pure silicon oxide layers. Two thicknesses of the SRSO sublayer, t SRSO, are studied 3 nm and 5 nm whereas the thickness of silica sublayer is maintaining at 3 nm. The presence of nanocrystallites of silicon, evidenced by X-Ray diffraction (XRD), leads to photoluminescence (PL) emission at room temperature due to the quantum confinement of the carriers. The PL peak shifts from 1.3 eV to 1.5 eV is correlated to the decreasing of t SRSO from 5 nm down to 3 nm. In the purpose of their potential utilization for i-layer, the optical properties are studied by absorption spectroscopy. The achievement a such structures at promising absorption properties. Moreover by favouring the carriers injection by the tunnel effect between silicon nanograins and silica sublayers, the multilayers seem to be interesting for solar cells.

Deposition of new microcrystalline materials, μc-SiC, μc-GeC by HWCVD and solar cell applications

This paper describes at first the present status of solar cell efficiencies prepared by Hot Wire CVD (HW-CVD), and then preparation techniques of μc-3C(cubic)-SiC developed for innovative solar cell applications by using HW-CVD method are presented. For preparing μc-3C-SiC, monomethylsilane (MMS) and hydrogen were used for reactant gases. The high conductivity of 5 S/cm could be achieved for N doped n-type μc-3C-SiC. For p-type, as-grown Al-doped μc-3C-SiC films showed a relatively high resistivity, but on thermal annealing, the conductivity increased to the level of 1 × 10 − 2 S/cm. Monomethylgermane (MMG) and H 2 were used to prepare μc-GeC thin films. μc-GeC thin films with a carbon composition of about 7–8% showed a clear shift of absorption coefficient spectra by 0.44 eV, when compared to crystalline Ge. The pin solar cell structures in which all p,i,n layers consist of μc-SiC have been prepared for the first time. It was found that μc-3C-SiC and μc-GeC are the promising candidates as the next generation thin-film solar cell materials, but at present, the film quality is strictly limited by the residual impurity concentration of filament material Re.

Thermal Dependence of Quantum Dot Solar Cells

AbstractVarious temperature dependent optoelectronic properties were measured for GaAs-based p-type / intrinsic / n-type (pin) solar cell devices containing 5-layers of InAs quantum dots (QDs) grown with strain-compensation layers. Curve fitting of the dark diode characteristics allowed for the temperature dependence of the saturation current and the ideality parameter to be determined. The resulting parameter values indicate high material quality. Air mass zero illuminated current density vs. voltage measurements were used to determine the temperature coefficients of the open circuit voltage, short circuit current, maximum power, and fill factor. A strong correlation between the temperature dependent quantum dot electroluminescence peak emission wavelength and the sub-GaAs bandgap spectral response was observed.

Strain-balanced MQW pin solar cells grown using a robot-loading showerhead reactor

A touch-screen controlled, robot-loading system for the Thomas Swan 7×2 flip-top showerhead reactor has been developed. The reactor has been configured for the growth of GaAs and InP materials and has been used to prepare strain-balanced MQW (SBMQW) pin solar cell material on GaAs substrates. Both material characterisation and solar cell performance for SBMQW pin cells are described.

Closed-form expression for the current/ voltage characteristics of pin solar cells

A closed-form expression for the current–voltage relationship of pin diodes and pin solar cells is obtained. The model considers drift and diffusion currents, and assumes a uniform electric field in the intrinsic layer, equal diffusion lengths for electrons and holes and a homogeneous generation rate. We show that both drift and diffusion currents must be taken into account to describe the current over a wide range of applied voltage. The inclusion of both transport mechanisms results in diode ideality factors between 1.8 at low, and 1.2 at high applied voltages. Comparisons of current/voltage characteristics and solar cell output parameters obtained from our model with experimental data of thin-film silicon solar cells show that our model accurately explains the output characteristics of pin solar cells.

Microcrystalline single and double junction silicon based solar cells entirely prepared by HWCVD on textured zinc oxide substrate

Microcrystalline silicon based single junction pin and amorphous-microcrystalline double junction pinpin solar cells were fabricated entirely by hot-wire chemical vapor deposition on textured zinc oxide substrates. The short circuit current density for microcrystalline pin cells shows an increase of 20–25% compared to unetched zinc oxide, indicating a better light trapping on textured zinc oxide substrate. The best initial conversion efficiency obtained for an all hot-wire microcrystalline p–i–n solar cell was 7.6%. Micromorph a-Si:H/μc-Si:H pinpin tandem solar cells were also investigated entirely by hot-wire CVD showing best initial conversion efficiency η=8.25%. The microcrystalline pin cells show an increase of series resistance even in the dark resulting in decrease of fill factor, however micromorph pinpin cells are stable in the dark. This instability of μc-Si:H pin cells could be mainly attributed to inter-diffusion at the TCO-p interfacial region.

Proton-induced degradation of thin-film microcrystalline silicon solar cells

This paper investigates the stability of dilution series of pin and nip microcrystalline silicon solar cells under low-energy proton irradiation (E=405keV). Variation of electrical parameters, defect-related absorption and Urbach parameter are investigated as a function of irradiation and annealing steps. Highly microcrystalline cells show a relative efficiency loss of up to 80% after proton irradiation. The efficiency loss is observed not to be completely reversible under thermal annealing. Increase of defect-related absorption and Urbach parameter is also only partially reversible. The electrical parameters (Jsc, Voc, FF) show proton-induced reductions which increase with crystallinity for both pin and nip series; short-circuit current density suffers the largest variations with relative losses of up to 65%. Defect-related absorption is shown to be low for cells of medium crystallinity, before and after irradiation.

Hole mobilities and the physics of amorphous silicon solar cells

The effects of low hole mobilities in the intrinsic layer of pin solar cells are illustrated using general computer modeling; in these models electron mobilities are assumed to be much larger than hole values. The models reveal that a low hole mobility can be the most important photocarrier transport parameter in determining the output power of the cell, and that the effects of recombination parameters are much weaker. Recent hole drift-mobility measurements in a-Si:H are compared. While hole drift mobilities in intrinsic a-Si:H are now up to tenfold larger than two decades ago, even with recent materials a-Si:H cells are low-mobility cells. Computer modeling of solar cells with parameters that are consistent with drift-mobility measurements give a good account for the published initial power output of cells from United Solar Ovonic Corp.; deep levels (dangling bonds) in the intrinsic layer were not included in this calculation. Light-soaking creates a sufficient density of dangling bonds to lower the power from cells below the mobility limit, but in contemporary a-Si:H solar cells degradation is not large. We discuss the speculation that light-soaking is ‘self-limiting’ in such cells.