Quantitative Recombination and Transport Properties in Silicon from Dynamic Luminescence
There is now strong evidence that perovskite materials, such as MaPbI 3 , have such a band structure, 9 - 12 and that the relationship between electrochemical potential and luminescence efficiency of the cell is therefore described by the generalized Planck's radiation law, first formulated by Wurfel.
It also demonstrates why PL is such a powerful tool for the characterization of solar cells: the luminescence intensity provides direct information about the electrochemical potential within the cell; information that is difficult to access through other techniques. If we continue to assume an ideal cell, then the qFL separation will remain constant, even at voltages well below the maximum power point V MPP , despite the large current flowing out of the device. Such a device can be said to have negligible internal series resistance at V MPP , although it may still have an external series resistance; in this case, external means any component of the cell that is outside of the absorber and its interfaces with the transport layers.
In practice, devices do not follow such ideal behavior. Instead, the finite electron and hole conductivities mean that carrier transport requires some loss in potential to affect the movement of the carriers. This is reflected in the gradients of the qFLs, which appear in the transport equations: 3. To sustain a current flow in any region of the absorber, there must be a corresponding change in the local qFL separation; the magnitude of this change is dependent on the local conductivity.
Herein, we refer to such potential losses due to carrier transport within the absorber as internal voltage drops IVDs. Because IVDs are caused by current flows, devices will exhibit IVDs around their maximum power point MPP , at which there is a large current flow throughout the absorber. However, IVDs may also occur at V OC because, although there is no net current flow out of the device, there can still be large, internal, balanced electron and hole current flows within the absorber, particularly if interface recombination is significant.
The value of V OC is directly measured under an illumination intensity equivalent to one sun, whereas V int is probed by measuring the luminescence intensity from the cell by using a PL imaging setup. The different annealing regimes of the TiO 2 film may impact both the WF and the electron affinity EA of the film, and therefore, the band alignment. The luminescence intensity, and hence, the implied internal voltage of the cells, changes comparatively little, which indicates no dramatic difference in recombination dynamics.
The actual variation in V OC is much larger, and therefore, not chiefly the result of variations in recombination dynamics. The reason for the scatter in the data for cells within a set particularly the FGA set is not understood at present. Measured luminescence intensity versus V OC for three sets of three cells.
All cells were exposed to illumination of one sun. In the following discussion, we use simulations to illustrate how IVDs can form and some of the different ways they can manifest themselves. The simulations are relatively simple examples that illustrate how band misalignment can affect the shape of I—V curves. They only cover a small subset of possible scenarios. Modeling also neglects the presence of mobile ions. Its purpose is to illustrate the point that band misalignment and other factors can result in the appearance of an IVD in a cell, which causes a reduction in the voltage output, even in some cases at V OC.
Thus, care needs to be taken not to ascribe a low cell V OC immediately to high parasitic recombination. This is the result of suppressed interface recombination, through a reduction in the minority carrier hole concentration at the ETL interface. On the other hand, an increase in EA to 4. Although interface recombination does increase, this reduction is chiefly due to the appearance of a sharp drop in the electron qFL at the ETL interface; thus giving rise to an IVD.
The misalignment of the ETL with the CB of the perovskite film has resulted in a transport barrier near the ETL interface, through a reduced electron concentration. The blue arrows indicate a large electron current flow; a cross indicates termination of the current flow. This is because the electron current flow to the ETL interface is nearly the same at all bias conditions, due to the high surface recombination at that interface, so that the magnitude of the IVD is not dramatically dependent on the bias condition.
Hence, the magnitude of the IVD is slightly dependent on the bias condition. The majority carrier bottleneck is now located in the ETL, which sees no electron current flow at V OC , even in the case of high surface recombination. However, the I—V curves of the cells with high 4. The blue arrows indicate a large electron current flow.
In the above discussion we have shown, experimentally and through modeling, that poor band alignment can lead to transport barriers and IVDs in PSCs. Again, such a decoupling can only be explained by supposing an IVD. Such IVDs must be taken into account in any models that attempt to explain such transient phenomena, which is currently not the case.
The FF of a solar cell is often the most difficult parameter to optimize because it is sensitive to a range of parasitic loss mechanisms, such as resistance losses, which generally have only a minor impact on V OC or I SC. However, such high values have not been realized in practice, to date. It is therefore important to understand what is limiting the FF of practical devices. Ideality factors greater than one can have numerous origins. In the presence of trap states herein we refer to traps as defects that trap one type of carrier, causing photodoping 19 , ideality factors between one and two can result.
Shunt and series resistances can further reduce the FF of practical devices.
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Unfortunately, in practice, both series and shunt resistances may have several underlying causes in a single cell, some of which are ohmic, but some of which are not. Hence, such curves allow the impact of series resistance to be determined. This is particularly the case in the presence of mobile ions, for which the change in cell voltage at different illumination intensities will result in a rearrangement of ions, which may alter their impact on carrier transport.
Because the reconstructed I — V curve relies on measurements of V OC at different illumination intensities, a variation in the difference between V int and V OC will affect the shape of the reconstructed curve, and render the interpretation of the resulting curve ambiguous. A major benefit of suns—luminescence measurements is that they are contactless and can therefore be performed on solar cell precursors. The former is effectively known or accounted for in quantum PL yield measurements, for which the luminescence measurement system has been calibrated.
Status Solidi B , 4 pages Hirori, P. Xia, Y. Shinohara, T. Otobe, Y. Sanari, H. Tahara, N. Ishii, J. Itatani, K. Ishikawa, T.
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Murata, R. Murdey, and A. Wakamiya Angew. Handa Jpn. Sakamoto, T. Kanemitsu Nat. Zhu, H. Materials 2, 9 pages Uchida, T. Otobe, T. Mochizuki, C. Kim, M. Yoshita, K. Akiyama, L. Pfeiffer, K. West, and H. B 97, 7 pages Yumoto, H. Kanemitsu Adv. Yamaguchi, L. Kanemitsu, H. Tampo, H. Shibata, K. Lee, K. Kojima Jpn. Handa, and Y. Okano, L. Phuong, and Y. Yamada ECS J. Solid State Sci. Tarekegne, H. Hirori, K. Tanaka, K. Iwaszczuk, and P. Jepsen New J. Yarita, H. Akashi, K. Ozaki, Y. Katsuki, J. Liu, T. Nishikubo, S. Yakumaru, Y.
Hashikawa, Y. Murata, T. Saito, Y. Shimakawa, Y. Saeki, and A. Wakamiya ACS Omega 2, Phuong, I. Braly, J. Katahara, H. Hillhouse, and Y. Express 10, 4 pages Yamada, and Y. Kanemitsu Bull. Wang, W. Wang, A. Tang, H. Tsai, Z. Bao, T. Ihara, N. Tahara, Y. Kanemitsu, S. Chen, and R. Liu Angew. Evaluation of subcell power conversion efficiencies of radiation-damaged triple-junction solar cells using photoluminescence decays D.
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Hiroshige, T. Ihara, and Y. B 95, 9 pages Ihara, M. C, 5, Selected as a Hot paper DOI: Futagoishi, T. Aharen, T. Kato, T. Tada, M. Murata, A. Wakamiya, H. Kageyama, Y. Kanemitsu, and Y. Murata Angew. Sato, M. Yamashita, E. Matsubara, M. Nagai, C. Kanemitsu, and M. Ashida Appl. Handa, D. Tex, A. Shimazaki, A.
Tex, M. Imaizumi, and Y. Applied 7, 8 pages Nishihara, M. Okano, Y. Kanemitsu ECS J. Nakaike, A. Imaizumi, H. Yoshita, L. Kubota, T. Imaizumi, Y. However, the main problem of active concentrators, severely limiting their practicality, is the high cost and low angular accuracy of sun tracking apparatuses. In this paper we develop a novel active solar concentrator without any mechanical tracking. We aim to accomplish this goal through designing tunable prisms via novel chemical system comprising nanoparticles NPs , specifically gold Au nanorods and silica NPs, embedded in semi-rigid transparent sol-gel matrixes, and placed within an electrical field.
Changing the electrical field changes the partial distribution of the NPs and yields spatial gradient of refraction index, affecting the direction of the collected optical rays and allows their directing towards the PV cell according to the movement of the sun. In the paper we present the design and the realization of the first prototype as well as its preliminary experimental characterization. In order to reduce costs, the solar cell industry is aiming at producing ever thinner solar cells. Structuring the surfaces of optically thin solar cells is important for avoiding excessive transmission-related losses and, hence, to maintain or increase their efficiency.
Light trapping leading to longer optical path lengths within the solar cells is a well established field of research. In addition to this, other possible benefits of structured surfaces have been proposed. It has been suggested that nanostructures on the surface of thin solar cells function as resonators, inducing electric-field resonances that enhance absorption in the the energy-converting material.
Further, coupling of electric field resonances in periodically structured solar cells may couple with each other thereby increasing the absorption of energy. A deeper understanding of the nature of the energy-conversion enhancement in surface-structured and thin solar cells would allow to design more targeted structures.
Generally, efficiency enhancement may be evaluated by investigating the electric field and optimizing the optical generation rate. Here, we establish a model system consisting of multilayered solar cells in order to study resonances and coupling of resonances in a one-dimensional system. We show that resonances in energy-converting and non-energy converting layers exist. The coupling of resonances in the non-energy converting material and the energy-converting material is only possible for certain parameter ranges of thickness of the energy converting material and the imaginary part of the refractive index.
We evaluate the resonances and the coupling of resonances in different thin-film systems and show how they affect the total absorption of energy in the energy converting layer. We show how resonances in non-absorbing layers can contribute to increasing the resonances in the absorbing layers. We optimize the parameters of the multilayered thin-film systems to achieve an increase in the amount of the absorbed energy.
The optimization is also evaluated for an experimentally realizable thin-film solar cell. Local I-V characteristics and electron beam induced current EBIC microscopy under different biases were used to probe the electrical properties and the generation patterns of individual NWs. The EBIC mapping allow to extract the doping concentration and the minority carrier diffusion lengths. Macroscopic devices based on NW arrays were fabricated by dielectric encapsulation and ITO contacting. Top view EBIC analyses were applied to probe the generation homogeneity. Richards, Bastian E.
Rapp, Ulrich W. Liquid glass, a photo-curable amorphous silica nanocomposite, recently demonstrated groundbreaking capabilities as a transparent fused silica glass that can be structured in arbitrary geometries. The ability to process high-quality glass like a polymer, including the use of 3D printing techniques, opens up new routes to integrate optical microstructures for improved light harvesting in solar module architectures.
In this contribution, we will present the direct integration of optical microstructures into transparent fused silica glass covers. This approach provides both a higher optical quality of the module encapsulation and an improved compatibility of optical microstructures with the fabrication process of common solar modules.
Despite the ideally positioned bandgap of the InGaN materials system to be used for tandem solar cells, it has been so far difficult to grow high quality epitaxial InGaN. When compared to planar InGaN absorber, such a nano-structured absorber is shown to drastically increase the efficiency of the solar cell especially. This increase is mainly due to light trapping caused by the photonic crystal and also due to the lower polarization-induced charges at semi-polar interfaces.
An antireflective coating layer is essential to obtain higher efficiency for the purpose of photovoltaic applications. A mixture of methane and argon gasses will be utilized to generate gas discharges in a vacuum chamber using a capacitively coupled radio frequency RF plasma source. As a result of the discharge, a carbon-like thin film is produced, which possesses antireflection properties.
In this work, various RF powers, gas flow rates and deposition times are studied and their effect on the antireflective properties of the produced film is investigated. Bertoni, Arizona State Univ. High-efficiency solar cell devices characterized by extremely high open-circuit voltage VOC values have shown that the traditional constraints imposed by extrinsic recombination processes will be eventually surpassed at some time in the foreseeable future.
The accurate evaluation of Auger lifetime and its temperature-dependence are thus fundamental not only for the correct interpretation of effective carrier lifetime data, but also for the simulation of device performance, especially when these are deployed in the field, where the operating conditions can greatly vary from the standard testing conditions. In this work, we present the Auger lifetime across a range of temperatures from to K and a range of injection level from 5 x to 1 x cm-3 showing that, in stark opposition with what generally accepted, a strong increment of the lifetime values happens at high temperatures.
Based on these results, we discuss the ambipolar Auger coefficient in the high injection range and propose a parameterization for its temperature dependence in agreement with a model previously presented in literature. Finally, we evaluate the intrinsic-limited implied voltage iV within the same range of injection level and temperature, and show that the evaluated strong increment of Auger lifetime counteracts the typical drop of high-efficiency solar cells performance with high temperature. Bioud, Univ. Threading dislocations TDs introduced within the Ge layer have a detrimental effect on device performances.
The goal of this research is to address the perennial need to minimize the defect density of Ge epilayers grown on a Si substrate. We seek to accommodate the effects of the lattice mismatch by introducing a porous Si interface layer to intercept dislocations and prevent them from reaching the active layers of the device. The porous Si layer is formed through dislocation-selective electrochemical deep etching and thermal annealing.
The porous layer created beneath the top Ge layer can both act as dislocation traps and as a soft compliant substrate, which displays high flexibility. Finally, we present simulation results exploring the effect of threading dislocations on device performance. Black silicon BS is a promising research area to improve optical properties of silicon solar cells. The BS obtained results in a morphology consisting of nm height pillars randomly distributed onto the silicon surface.
BS strongly enhance the surface recombination velocity, a front passivation is then necessary. Thin film solar cells made from inexpensive materials such as amorphous silicon and conductive polymer became of ultimate importance due to the urge of lowering the cost of solar cells. However, each of these materials alone still did not achieve enough efficiency to replace current bulk silicon solar cells due to multiple reasons including their limited absorption.
Photoluminescence and photoconductivity to assess maximum open-circuit voltage and carrier transport in hybrid perovskites and other photovoltaic materials Conference Presentation Invited Paper Paper Time: PM - PM Author s : Hugh W. Hillhouse, Univ. Photovoltaic PV device development is much more expensive and time consuming than the development of the absorber layer alone. The presentation focuses on two methods that can be used to rapidly assess and develop PV absorber materials independent of device development.
These two material properties can be rapidly and simultaneously assessed with steady-state absolute intensity photoluminescence and photoconductivity measurements when combined with theory. As a result, these methods are extremely useful for predicting the quality and stability of PV materials prior to PV device development. The presentation will summarize the methods, discuss their strengths and weaknesses, and compare photoluminescence and photoconductivity results with device performance for a wide range of hybrid perovskite compositions of various bandgaps along with conventional PV materials CuInSe2, CuInGaSe2, and CuZnSnSe4.
This talk presents the use of advanced luminescence techniques to make correlations between macro and microscopic transport properties. One the one hand, hyperspectral luminescence images allows accessing to local material properties and we underline the need of a careful data treatment. On the other hand, time resolved luminescence images give access to local and global transport properties. Methods are applicable to thin film materials and devices.
We demonstrated that carrier transport and recombination in solar cells are directly accessible by measuring transient photovoltage with the picosecond temporal resolution using time-resolved photoemission spectroscopy. The time scales of charge separation and recombination in several III-V solar cells were measured as the rise and decay time of photovoltage.
A numerical simulation on carrier dynamics was performed to assess the utility of the present technique in comparison to time-resolve photoluminescence spectroscopy. We will also talk about some technical limitations we encountered at present and discuss their possible solutions. Metzger, National Renewable Energy Lab. Cathodoluminescence CL measurements can be applied to assess grain-boundary GB and grain-interior GI recombination in thin-film solar cell materials and made quantitative if we can develop CL models that account for material and measurement complexities.
https://albrasephtiotril.gq The model assumes that GB electrostatic potentials are screened by the high excess-carrier densities used in the CL measurement such that transport is governed by ambipolar diffusion. Here, we develop models to address directly GB potentials and their effects on these measurements. Simulations indicate that GB potentials can increase or decrease CL intensities relative to the zero-potential case.
However, the high electron-beam currents typically applied in CL measurements minimize the impact of GB potentials. Session PWed: Poster Session. The extension of modulated photoluminescence to low lifetime thin photovoltaic absorbers is presented. The setup uses a continuous laser and an acousto-optical modulation system for the excitation, as well as a Time Correlated Single Photon Counter with a Single Photon Avalanche Photodiode for the detection.
This allows recording low intensity fast modulated signal and extracting the phase of the first harmonic. Phase versus frequency curves are obtained, providing complementary information to pulsed techniques. In the Case of CuInGaSe2 and perovskite, they are useful for characterizing defects assisted recombination mechanisms. An ultra-broadband, nearly polarization-independent absorber is studied numerically, in which an Fe square-patch is periodically placed on Fe-SiO2 multilayer stacks. Special emphasis is placed on the determination of the patch width, since its optimum width significantly differs from that previously found in a 2D structure.
A somewhat wider width leads to a broader bandwidth than that for the 2D structure. Further calculations reveal that the proposed structure operates even for oblique incidence, although the absorption gradually decreases as the angle of incidence is increased. Comparison with a different metal-insulator structure also shows that a comparable performance is obtained for Ti-Al2O3. The absorption lines of interest are nm for Na, nm for K and nm for Cs. Excited alkali atoms return to the ground state by making two cascade emissions, which are at nm and nm for Na, nm and nm for K, and nm and nm for Cs.
Most of these emissions are readily absorbable by GaAs. Hot carrier solar cells aim at avoiding heat dissipation in solar cells. To achieve this goal and overcome the Shockley-Queisser limit, carriers must be collected before thermalization with very short collection path. However, very thin low-bandgap absorber layers require strong absorption enhancement over a broadband spectral range.
Here, we introduce arrays of asymmetric tetrahedrons for broadband absorption through multiple resonances. Preliminary experiments based on ultrathin multi-quantum wells absorbers will be presented. Mahmoud, Mohamed Farhat O. Hameed, Salah Sabry A. In this paper, a novel design of asymmetric tapered cone NWs solar cell is introduced and numerically analyzed using 3D finite difference time domain method FDTD via Lumerical software package. The particle swarm optimization PSO technique has been employed to optimize the design geometrical parameters for SC efficiency maximization. The proposed design offers an ultimate efficiency of Further, the suggested design shows a short circuit current density of This enhancement in mainly attributed to the higher order modes generated due to breaking the NWs symmetry.
In this work, we address and solve different issues related to the top cell fabrication by self-catalyzed growth of ordered GaAs1-xPx nanowires on Si We demonstrate and model the growth of ordered array of vertical and stacking-faults-free, zinc blende NWs.