Mechanism Of Solar Cells Research Articles

Hot carrier photocurrent induced by 0.92 eV photon energy radiation in a Si solar cell

Absorption of the below-bandgap solar radiation and direct pre-thermalizational impact of a hot carrier (HC) on the operation of a single-junction solar cell are ignored by the Shockley-Queisser theory. The detrimental effect of the HC is generally accepted only via the thermalization-caused heating of the lattice. Here, the authors demonstrate experimental evidence of the HC photocurrent induced by the below-bandgap 0.92 eV photon energy radiation in an industrial silicon solar cell. The carriers are heated both through direct free-carrier absorption and by residual photon energy remaining after the electron-hole pair generation. The polarity of the HC photocurrent opposes that of the conventional generation photocurrent, indicating that the total current across the p-n junction is contingent upon the interplay between these two currents. A model of current-voltage characteristics analysis allowing us to obtain a reasonable value of the HC temperature was also proposed. This work is remarkable in two ways: first, it contributes to an understanding of HC phenomena in photovoltaic devices, and second, it prompts discussion of the HC photocurrent as a new intrinsic loss mechanism in solar cells.

Controlling Exciton/Exciton Recombination in 2-D Perovskite Using Exciton-Polariton Coupling.

In this paper, we demonstrate that exciton/exciton annihilation in the 2D perovskite (PEA)2PbI4 (PEPI)─a major loss mechanism in solar cells and light-emitting diodes, can be controlled through coupling of excitons with cavity polaritons. We study the excited state dynamics using time-resolved transient absorption spectroscopy and show that the system can be tuned through a strong coupling regime by varying the cavity width through the PEPI layer thickness. Remarkably, strong coupling occurs even when the cavity quality factor remains poor, providing easy optical access. We demonstrate that the observed derivative-like transient absorption spectra can be modeled using a time-dependent Rabi splitting that occurs because of transient bleaching of the excitonic states. When PEPI is strongly coupled to the cavity, the exciton/exciton annihilation rate is suppressed by 1 order of magnitude. A model that relies on the partly photonic character of polaritons explains the results as a function of detuning.

Variability of temperature on the electrical properties of heterostructured CIS/Cds through SCAPS simulation for photovoltaic applications

Renewable energy research has received tremendous attention in recent years in a quest to circumvent the current global energy crisis. This study carefully selected and simulated the copper indium sulfur ternary compound semiconductor material with cadmium sulfide owing to their advantage in photovoltaic applications. Despite the potential of the materials in photovoltaic devices, the causes of degradation in the photovoltaic efficiency using such compound semiconductor materials have not really been investigated. However, electrical parameters of the materials such as open circuit voltage, short circuit current density, and fill factor have been extensively studied and reported as major causes of degradation in materials’ efficiency. Furthermore, identifying such electrical characteristics as a primary degradation mechanism in solar cells, this study work is an ardent effort that investigates the materials' electrical behavior as a cure to the degradation associated with compound semiconductor-based photovoltaic. In this study, we numerically characterized the electrical properties such as fill factor, open circuit voltage, short circuit current density, power conversion efficiency, net recombination rate, net generation rate, generation current density, recombination current density, hole current density, electrons current density, energy band diagram, capacitance–voltage, electric field strength of the heterostructured CIS/CdS compound semiconductor material using SCAP-1D. We also investigated the effect of temperature on the electrical properties of heterostructured materials. The obtained results reveal the uniformity of the total current density in the material despite the exponential decrease in the electron current density and the exponential increase in hole current density. The extracted solar cell parameters of the heterostructured CIS/CdS at 300 K are 18.6% for PCE, 64.8% for FF, 0.898 V for Voc, and 32 mA cm−2 for Jsc. After the investigation of the effect of temperature on the CIS/CdS compound semiconductor material, it was observed that the solar cell was most efficient at 300 K. The energy band gap of the CIS/CdS compound semiconductor material shrinks with an increase in temperature. The highest net recombination rate and recombination current is at 400 K, while the net generation rate and generation current density are independent of temperature. The study, on the other hand, gave insights into the potential degradation process, and utilizing the study’s findings could provide photovoltaic degradation remediation.

Design of optimized photonic-structure and analysis of adding a SiO2 layer on the parallel CH3NH3PbI3/CH3NH3SnI3 perovskite solar cells

So far, remarkable achievements have been obtained by optimizing the device architecture and modeling of solar cells is a precious and very effective way to comprehend a better description of the physical mechanisms in solar cells. As a result, this study has inspected two-dimensional simulation of perovskite solar cells (PSCs) to achieve a precise model. The solution which has been employed is based on the finite element method (FEM). First, the periodically light trapping (LT) structure has been replaced with a planar structure. Due to that, the power conversion efficiency (PCE) of PSC was obtained at 14.85%. Then, the effect of adding an SiO2 layer to the LT structure as an anti-reflector layer was investigated. Moreover, increasing the PCE of these types of solar cells, a new structure including a layer of CH3NH3SnI3 as an absorber layer was added to the structure of PSCs in this study, which resulted in 25.63 mA/cm2 short circuit current (Jsc), 0.96 V open circuit voltage (Voc), and 20.48% PCE.

Advanced analysis of internal quantum efficiency measurements using machine learning

AbstractThe internal quantum efficiency (IQE) is given as the ratio between the externally collected electron current and the photon current absorbed by the device. Spectral analysis of IQE measurements is a powerful method to identify performance‐limiting mechanisms in solar cells. It also enables the extraction of key electrical and optical parameters. However, the potential of IQE measurements is only rarely fully utilized, presumably due to the significant complexity associated with the fitting process and its sensitivity to noise. In this study, machine learning is proposed as an efficient method to extract quantitative information from IQE measurements. The extraction method is automated and easy to use, providing an array of specific device parameters. By simplifying the analytical process, the developed machine learning algorithms also extract the parasitic absorption of the antireflection coating, a key parameter that is difficult to obtain by traditional methods. Although the method has been developed for and tested on silicon solar cells, it can be adapted and applied to other types of solar cells.

A systematic review on enhancing efficiency of solar cells using upconversion

Sun is a source of abundant energy. Solar energy incident on a solar cell is converted into usable form of energy that is electrical energy. Total amount of energy incident on a solar cell is not 100% converted into electrical energy. There are several energy losses inherent in this process. The major portion of the energy is lost because of heat generation and thus leading to temperature increase of solar cell causing inevitable decrease in efficiency of solar cells. These losses and their minimization play an important role in solar to electrical energy conversion. In this paper, different loss mechanisms in solar cells and spectral correction methods are systematically discussed.

Silicon Solar Cells for Post-Detonation Monitoring and Gamma-Radiation Effects

The feasibility of using solar cells for post-detonation monitoring, and more broadly, gamma-ray monitoring, is evaluated using Monte Carlo simulations and experiments in this work. We measured the short-circuit current Isc response of commercial silicon (Si) solar cells to 137Cs and 60Co gamma rays. A clear response of both mono- and polycrystalline Si solar cells to 137Cs and 60Co gamma rays was obtained in good agreement with the simulations. Radiation effects in solar cells due to accumulated gamma-ray dose were noticed as the drop in Isc and open-circuit voltage Voc. The atomic displacement cross section of the produced secondary fast electrons and nonionizing energy loss (NIEL) concepts were revisited to understand the principal gamma-radiation damage mechanism in solar cell devices. Analytical computations of and NIEL of electrons convoluted with simulated Compton electron distributions in Si enabled a fundamental understanding of the gamma-radiation effects and recovery mechanism in solar cells, further supporting the experimental results. Different from the ionization effects in the polymer and glass layers of a solar cell/panel, displacement damage in the Si p-n layer from gamma rays or fast electrons is much less than that from massive particles, which directly affects the charge collection performance fundamental to solar cell operation.

Solar cells efficiency enhancement using multilevel selective energy contacts (SECs)

High-energy incoming photons can be absorbed and concluded to generate Hot Carriers. In normal solar cells, these carriers are scattered by electron–electron and electron-lattice mechanisms and rapidly lose extra energy and approach to conduction band energy edge. This event plus other loss mechanisms cause that the efficiency of the solar cells to be limited to 33% theoretically. If one makes the possibility for carriers that can be extracted rapidly, before scattering and releasing extra energy to the lattice, the efficiency of solar cells is enhanced considerably. This type of solar cell is named hot carrier solar cells (HCSCs). To this end and improvement the conversion efficiency, multilevel energy selective contacts (ESCs) as a new concept and new mechanism in solar cells are used. In the other words, several appropriate energy levels as carrier extraction contacts in the conduction band are introduced. Here, we use multilevel ESCs, and based on our simulation it is shown that the maximum efficiency of 75% is achievable for low bandgap materials. For a typical material such as Si, the maximum efficiency is increased to 60% using ten ESCs.

Illumination Intensity Dependence of the Recombination Mechanism in Mixed Perovskite Solar Cells.

Recombination mechanisms in solar cells are frequently assessed through the determination of ideality factors. In this work we report an abrupt change of the value of the "apparent" ideality factor (nAP ) in high-efficiency FA0.71 MA0.29 PbI2.9 Br0.1 based mesoscopic perovskite solar cells as a function of light intensity. This change is manifested as a transition from a regime characterized by nAP ∼1.8-2.5 at low light intensities (<10 mWcm-2 ) to one characterized by nAP ∼1. This transition is equally observed in the recombination resistance extracted from open-circuit impedance measurements. We use drift-diffusion simulations with explicit consideration of ion migration to determine the origin of this transition. We find that a change ofrecombination mechanism concurrent with a modification of the concentration of ionic vacancies is the most likely explanation of the observed behaviour. In the drift-diffusion simulations we show that the apparent ideality factor is in fact affected by the ion vacancy concentration so it is not the optimal parameter to assess the dominant recombination mechanism. We argue that a procedure based on a recently derived "electronic" ideality factor obtained from the high frequency feature of the impedance spectrum is better suited to determine the recombination route that dictates the photovoltage.

In-depth understanding of catalytic mechanism in solar cells via synergistic effect of interfacial engineering and structural integration

Establishing robust yet high-efficiency counter electrode (CE) is an essential challenge for dye-sensitized solar cells (DSSCs). The CE made of carbon-based catalysts is experimentally proved to be an ideal alternative for noble Pt metal in DSSCs. Nevertheless, its reaction mechanisms remain unclear because of the lack of comprehensive understanding of its physical and chemical properties. Here, the DSSC based on the Ni3B/NiO incorporating N-doped carbon hollow spheres (N-CHS) and RGO (N-CHS/(Ni3B/NiO)/RGO) CE achieved a high power conversion efficiency (PCE) of 9.57%, which is 26.92% higher than that of the DSSC based on Pt CE. This can be attributed to: Firstly, RGO further promotes the formation of the Ni3B/NiO heterostructure to enhance electrocatalytic performance. Secondly, N-doping stimulates catalytic activity, benefited from the generation of more defects. Ultimately, the structural advantage of N-CHS/(Ni3B/NiO)/RGO nanocomposites not only lead to a high density of interfaces between the Ni3B and NiO components while maximizing its use efficiency but also synergistically accelerate the kinetics, thus boosting the PCE. This article highlights a powerful strategy of interfacial engineering and structural integration to rationally design nano-functional materials in energy conversion related fields.

Identifying Dominant Recombination Mechanisms in Perovskite Solar Cells by Measuring the Transient Ideality Factor

The ideality factor determined by measuring the open circuit voltage (VOC) as function of light intensity is often used as a means to identify the dominant recombination mechanism in solar cells. However, applying this Suns-VOC technique to perovskite cells is problematic since the VOC evolves with time in a way which depends on the previously applied bias (Vpre), the light intensity, and the device architecture/processing. Here we show that the dominant recombination mechanism in two structurally similar CH3NH3PbI3 devices containing either mesoporous Al2O3 or TiO2 layers can be identified from the signature of the transient ideality factor following application of a forward bias, Vpre, to the device in the dark. The transient ideality factor, is measured by monitoring the temporal evolution of VOC at different light intensities. The initial values of the transient ideality were consistent with corresponding photoluminescence vs intensity as well as electroluminescence vs current density measurements. Time-dependent simulations of the measurement on modelled devices, which include the effects of mobile ionic charge, show that Shockley Read Hall (SRH) recombination through deep traps at the charge collection interfaces is dominant in both devices. Using transient photovoltage measurements superimposed on the evolving VOC of bifacial devices we further show that the charge collection interface extends throughout the mesoporous TiO2 layer, consistent with a transient ideality signature corresponding to SRH recombination in the bulk of the film. This information could not be inferred from an ideality factor determined from only steady-state VOC values. The method we have developed will be useful for identifying performance bottlenecks in new variants of perovskite devices by comparison with the transient ideality signatures we have predicted for a range of possible recombination schemes.

Assessing the future prospects of solar energy conversion technologies in electronic applications

Solar energy has become one of the most important sources of renewable energy. This is evident in the fact that many organizations and individuals have resorted to renewable energy, without fear of depletion, without fear of the monthly costs of using fuel-based electricity.The study examined the mechanism of using solar energy and the mechanism of solar cells in converting sunlight into electricity in general, and thus allocating research in solar energy conversion technologies to electrical energy, and some systems of assessing of these technologies, in order to know the future status of the technology of converting solar energy, as well as to know the costs that these technologies must reach in order to be a candidate for bulk power.

Study of validity of the single-diode model for solar cells by I–V curves parameters extraction using a simple numerical method

The I–V measurement of solar cells is one of the most employed electrical characterization techniques in the photovoltaics research field due to the valuable information one can obtain from such a curve. Parameters like Voc, Isc and the maximum power can be easily observed at a glance. Furthermore, additional information can be extracted by a more exhaustive analysis involving the equivalent electrical circuit of a solar cell which is based on ideal electrical components with a well-defined behavior. This equivalent circuit is typically assumed to be formed by a current source in parallel with a single diode and a shunt resistor, connected to a series resistor. However, this model does not take into account the separate contributions of the different carrier transport mechanisms in solar cells; for example, carrier diffusion and recombination-generation in the depletion region. Therefore, the single diode model may not be suitable in many practical cases. In this work, a simple numerical method was developed to extract the parameters for both single diode and double diode models from experimental I–V curves of solar cells. The developed numerical algorithm was applied to extract the parameters for a published benchmark solar cell which has been used for testing this kind of algorithms. The extracted parameters using our simple method are comparable with other more sophisticated and computer power demanding algorithms. Thereafter, we applied the developed algorithm to extract the single-diode parameters from simulated “experimental” I–V curves, where two carrier transport mechanisms are present, trying to understand under what conditions the single diode approximation would provide meaningful parameters for such experimental curves. It is shown that the extracted parameters can vary strongly, particularly for the dark saturation current and ideality factor, without much variation of the root mean square error between the experimental data and the model, causing these values to be unreliable and its physical interpretation misleading. We show that the same algorithm can be applied to a double diode (two exponentials) model providing physically meaningful parameters without much computing power requirements. In summary, there is no further justification for using a single diode model to interpret the experimental I–V curves of real solar cells.

Two-dimensional modeling of TiO2 nanowire based organic–inorganic hybrid perovskite solar cells

Organo-metal halide perovskite solar cells have shown unique charms in the upgrade rate of maximum power conversion efficiency, the diversity of device architecture and facilitated fabrication process. And the clear understanding of the role of each component and the basic working mechanisms in solar cells is important for further improvement in efficiency, especially for mesoscopic perovskite solar cells. Here, a two-dimensional modeling of the TiO2 nanowire-based organic–inorganic hybrid perovskite solar cells was performed combining the optical and electrical responses to reveal the impact of the properties of TiO2 nanowire array and absorber layer. Simulation results show a great dependence of device performance on the electron concentration of TiO2 nanowires, which decided the electron field distribution inside cells, and an optimum thickness of 600nm is obtained for the TiO2 nanowires with low electron concentration. The collection of carriers is primarily within perovskite itself, and the ratio through TiO2 nanowire is less than 5%. These findings can facilitate device optimization and enhance the performance of the perovskite solar cells.

Surface passivation of boron emitters on n-type c-Si solar cells using silicon dioxide and a PECVD silicon oxynitride stack

Recombination of charge carriers is a significant loss mechanism in solar cells.

A novel approach of accelerated ageing tests for high concentration III–V multijunction solar cells through rapid irradiation/thermal cycles

Today, investigations on degradation and failure mechanisms in solar cells are often based on accelerated ageing tests (AAT), where failure effects are reproduced and quantified within a much shorter time period than the actual lifetime. This paper presents a study upon a novel approach of AAT for high concentration photovoltaic (HCPV) cells. The intended approach aims to overcome several limitations encountered in most of the AAT adopted up today, proposing the use of an alternative experimental set-up for performing nearly 400 times faster and more realistic thermal cycles, under real sun conditions (irradiation cycling), without the involvement of any environmental chamber. The cumulative impact of such very rapid irradiation/thermal cycles on the ageing of HCPV assemblies can be determined, both quantitatively and qualitatively, by evaluating specific pre-ageing and post-ageing diagnostic measurements by means of I–V characterization and scanning acoustic microscopy (SAM). The proposed irradiation/thermal cycling arrangement was successfully employed for performing accelerated ageing of both HCPV assemblies (cell receiver and heat sink) and bare HCPV cell receivers. Ultimate goal of this AAT approach is to provide the basis for a future work on the reliability analysis of the main degradation mechanisms and failure propagation in HCPV cell assemblies.

Imaging Techniques for Quantitative Silicon Material and Solar Cell Analysis

We present an overview of imaging techniques for analyzing different loss mechanisms in solar cells. The bulk and surface recombination, optical, series resistance, and shunt losses are investigated in more detail. It is shown that a detailed spatially resolved analysis of these loss channels is possible by combining different evaluation methods. Photoluminescence imaging is used to analyze carrier recombination in the material volume and the surfaces. Bulk recombination is investigated on wafer level by an analysis, which is able to investigate the impact of bulk lifetime and impurity concentration on cell efficiency. Variations in the production conditions of multicrystalline silicon serve as an example. Surface recombination is investigated on a cell precursor level, and the importance of detecting surface defects in cell processes featuring dielectric rear-side passivation is highlighted. For analyzing optical, series resistance, and shunt losses on final cells, a combination of photoluminescence imaging, dark lock-in thermography, and spectrally resolved light-beam-induced current is proposed. As an example, a detailed analysis of two solar cells is presented, allowing the assessment of the local and global impact of each loss mechanism on cell efficiency.

Quantum effects in silicon for photovoltaic applications

AbstractQuantum confinement effects in silicon might help to overcome the theoretical efficiency limit of ∼33% of silicon cells [Shockley and Queisser, J. Appl. Phys. 32, 510 (1961)]. The dominant loss mechanism in solar cells is the thermalization of photo‐excited carriers to the band gap. Engineering of the band gap makes it in principle possible to obtain a conversion efficiency of ∼44% [Pavesi and Turan (eds.), Silicon Nanocrystals, Fundamentals Synthesis and Application (Wiley‐VCH, Berlin, 2010)]. Band gap engineering is achieved by controlling the silicon‐nanocrystal (Si‐NC) dimension. We currently use Si‐NCs in solar cells to modify the solar spectrum in a way that the incoming light is more efficiently converted into electricity and also, to realize multiple junction cells to minimize thermalization losses. The Si‐NC energy of the band gap and the Si‐NC optical properties depend strongly on the NC size, which can be controlled by the processing parameters. Therefore, light management is possible by varying the refractive index of the Si‐NCs to improve the antireflective coating (ARC) and by conversion of absorbed high energy photons re‐emitted in the red spectral region of light. Thus, in this approach Si‐NCs are used as spectral down‐shifters and for ARC optimization. Further, NCs allow the fabrication of higher band gap solar cells, to be used on top of standard silicon cells as tandem cell elements. Due to the tunability of the band gap, it is possible to construct all silicon multilayer or tandem cells. The concepts and research results for both ideas will be presented.

Coordinated shifts of interfacial energy levels: insight into electron injection in highly efficient dye-sensitized solar cells

Coordinated shifts on the order of ∼100 meV were found in the interfacial energy levels of ruthenium dye molecules and the conduction band of nano-crystalline TiO2. The shifts were induced by potential-determining additives used in highly efficient dye-sensitized solar cells. The observation of comparable shifts of interfacial energy levels in dye/TiO2 films is beyond the conventional understanding of the varied quantum efficiency of interfacial electron injection, which had been derived under the assumption of the band shift only in TiO2. Based on general physical concepts, we advanced the understanding of the electron injection mechanism and attributed the variable electron injection efficiency to the influence of potential-determining additives on the interfacial electronic structures of both the dyes and TiO2. Our results revealed that the interfacial dipole layer affected the electronic coupling between the donor states in photo-excited dyes and the acceptor states in TiO2, which led to changes in electron injection. The present model also explains the correlation between the radiative recombination, an important loss mechanism in solar cells, of photo-excited states of the anchored dye/TiO2 in operational cells and that of free dye molecules in solution. We also found the amount of coordinated shifts to be most likely related to the number of COOX groups in ruthenium dyes and metal-free organic dyes. This relationship implies that the effect of coordinated shifts in ruthenium dyes/TiO2 is one of the most probable reasons why these dyes enable higher performance than do metal-free organic dyes, in which the latter only have relatively small coordinated shifts. This discovery provides deeper insight into the interfacial electron injection mechanism and points out the importance of the realignment effect on interfacial energy levels in highly efficient dye-sensitized solar cells.

Influence of energetic disorder on electroluminescence emission in polymer:fullerene solar cells

Electroluminescence (EL) spectroscopy and imaging can be useful techniques to analyze various loss mechanisms in solar cells, but the interpretation of the results is not trivial in solar cells made from disordered materials such as organic semiconductors. In this case the interpretation of EL measurements may be affected by the presence of a tail of localized states. Here, we study several polymer:fullerene systems and show that, despite the presence of tail states, the shape of the EL spectrum is insensitive to the applied voltage. This indicates that the emission originates mainly from mobile charges in higher lying states recombining at the polymer:fullerene interface and that most charges in deeper tail states do not contribute to the EL spectrum. The consequence of our finding is that simple models of EL emission in ideal semiconductors can be applied to polymer:fullerene solar cells and can therefore be used to evaluate the potential of different material systems in terms of recombination losses and to study resistive losses using luminescence imaging.