Search

We present an optical model for thin-film silicon solar cells (both single and multijunction) with nanorough surfaces/interfaces. For these cells, the optical absorptance within each layer and the total reflectance are computed taking into account roughness, angular distribution of scattered light, thicknesses, and optical constants of all layers. In the model, we combine coherent approach, scattering theory, and Monte Carlo tracing method. Results of the model are shown to be in good agreement with the experimentally measured spectral response and the total reflectance of solar cells. Some predictions of the ultimate solar cell performance based on the model are presented as well.

The impurity photovoltaic solar cell can, in principle, increase the sunlight to electricity conversion efficiency of a conventional single junction solar cell by the introduction of optically active impurities or defects into the device. These “defects” ideally allow electrons to be excited from the valence band to the conduction band via the mid-gap defect level through the absorption of previously wasted sub-band-gap photons. In this work the maximum efficiency limits for such a device are calculated for the special case where the energy of the partly excited electron relaxes to a lower energy partly through the two-stage excitation process. This relaxation in energy by the electron when occupying the defect state is shown to give an efficiency improvement over the case where no defect relaxation occurs. In the case of silicon, an efficiency limit of 39.7% under the airmass 1.5G solar spectrum is obtained, compared to 33.0% when no defects are present and 30.5% when a defect is present but no relaxatio...

The spatially resolved evaluation of power losses in solar cells is a key issue in identifying technological and material quality problems and realistically judging their influence on solar cell performance. Up to now a spatially resolved image of leakage currents could only be obtained by lock-in thermography measuring the distribution of the dark leakage current in solar cells. However, it is well known from simulations that the dark and illuminated current paths in solar cells may differ considerably. In this article spatially resolved images of the power losses in solar cells measured under realistic operation conditions, that is at the maximum power point and under illumination, are presented. This improvement was made possible by the development of a measurement technique called illuminated lock-in thermography. The underlying physics of this technique are discussed and it is shown that this method gives an image, which is directly proportional to the local power losses in the solar cell. Different loss mechanisms as, e.g., losses in the emitter sheet resistance and due to recombination at grain boundaries could be visualized in two-dimensional images. A comparison and quantification of different loss mechanisms under operation conditions was obtained. It is found that, e.g., the losses due to recombination in the "striation rings" of Czochralski-silicon are clearly underrated as compared to losses via point-like "shunts" if a measurement of the dark characteristics of the cell is used. The comparison of dark and illuminated leakage current images gives laterally resolved experimental evidence for the differences in dark and illuminated current paths in solar cells predicted by device simulations.

In this work, a method to calculate the equivalent dose for orbits in which the main part of the damage is due to protons, is presented. The method compares the primary knock-on atom distribution and the depth dependence of damage for the cases of space-proton irradiation and laboratory irradiation of silicon solar cells, as obtained from simulations using the transport of ions in matter code. It is shown that, in order to simulate the right depth damage distribution in space for a solar cell covered by glass, it is convenient to irradiate the cell from the backside with a proton energy dependent of the glass thickness. The method is applied to predict the electrical degradation of a silicon solar cell at the end of its life in a low altitude orbit and to estimate the damage produced as consequence of the October 19, 1989 solar proton event. The results obtained are compared with predictions of the U.S. Jet Propulsion Laboratory method.

Nanostructured solar cells are too complex for standard modeling. Here, we decouple the effects at a microscopic (nm) scale from those at a macroscopic (μm) scale. The three-dimensional nanoporous geometry is simplified to a quasiperiodic nm-scale ordering of “unit cells,” each consisting of a TiO2 sphere and its p-semiconductor or dye/p-conductor shell. It is shown that, due to the periodic boundary conditions, such a unit cell is essentially field free; it is called here a “flatband cell.” The band diagram, including possible discontinuities, the Fermi levels, carrier density and recombination (interface and bulk) of these flatband cells is given accurately by a back-of-the-envelope calculation, as confirmed by numerical simulation. The unit cells are connected in a one-dimensional electrical network, which also accounts for the transport effects in the porous TiO2 network and for the contact effects at the electrodes. Results for solid-state nanostructured cells are: the interconnection geometry is dis...

This paper investigates the influence of laterally inhomogeneous cell parameters on solar cell efficiency. A universal procedure is proposed which allows the reduction of a frequency distribution of an arbitrary cell parameter to just one single value for one-dimensional (1D) device modeling. This procedure is in detail investigated for the case of lifetime and diffusion length distributions showing that the frequency distribution of local minority carrier densities ( Delta n) is the appropriate parameter for solar cell modeling. The actual position of high and low lifetime areas on the cell is found to be of minor influence if Delta n is used as an input parameter. Therefore, using the lifetime frequency distribution allows us to include the effect of spatial inhomogeneities in recombination activity in 1D solar cell modeling. Both simulated and experimental examples for the modeling procedure and the quantitative influence of spatial inhomogeneities on solar cell performance are discussed. This article presents a possibility to include laterally resolved measurements of material and process quality data in routine predictive modeling of solar cell performance.

A system for solar energy conversion using the up-conversion of sub-band-gap photons to increase the maximum efficiency of a single-junction conventional, bifacial solar cell is discussed. An up-converter is located behind a solar cell and absorbs transmitted sub-band-gap photons via sequential ground state absorption/excited state absorption processes in a three-level system. This generates an excited state in the up-converter from which photons are emitted which are subsequently absorbed in the solar cell and generate electron-hole pairs. The solar energy conversion efficiency of this system in the radiative limit is calculated for different cell geometries and different illumination conditions using a detailed balance model. It is shown that in contrast to an impurity photovoltaic solar cell the conditions of photon selectivity and of complete absorption of high-energy photons can be met simultaneously in this system by restricting the widths of the bands in the up-converter. The upper limit of the ene...

One of the major loss mechanisms leading to low energy conversion efficiencies of solar cells is the thermalization of charge carriers generated by the absorption of high-energy photons. These losses can largely be reduced in a solar cell if more than one electron–hole pair can be generated per incident photon. A method to realize multiple electron–hole pair generation per incident photon is proposed in this article. Incident photons with energies larger than twice the band gap of the solar cell are absorbed by a luminescence converter, which transforms them into two or more lower energy photons. The theoretical efficiency limit of this system for nonconcentrated sunlight is determined as a function of the solar cell’s band gap using detailed balance calculations. It is shown that a maximum conversion efficiency of 39.63% can be achieved for a 6000 K blackbody spectrum and for a luminescence converter with one intermediate level. This is a substantial improvement over the limiting efficiency of 30.9%, whi...

In the past, minimal improvements have been predicted for efficiency enhancement of solar cells using the impurity photovoltaic (IPV) effect, where optical excitation through midgap defect levels allows the use of long wavelength photons to increase the conversion efficiency of sunlight to electricity. In the present work, the principle of detailed balance is used to calculate the limiting efficiency of solar cells with the inclusion of the impurity photovoltaic effect, in the idealized case when all transitions are assumed to be radiative. Based on these calculations, the limiting efficiency of the IPV device with a large number of different defect species is determined to be 77.2%. The terrestrial performance of the IPV device is also investigated by comparing its spectral sensitivity with that of tandem solar cell designs.

Solar cells are three-dimensional objects frequently modeled as being one-dimensional for convenience. However, for more complex designs of solar cells or if the cells are only illuminated at one point, one-dimensional modeling is insufficient. Here, some conditions for reducing the complexity of multidimensional drift-diffusion simulations are investigated in realistic situations for a back-contact perovskite solar cell. The analysis investigates under what situations we may neglect vertical carrier density variation and approximate extraction currents to be linearly dependent on the vertically averaged carrier concentration. Analytic expressions for the linear relationship in both the low and high extraction velocity regimes are demonstrated, and the conditions where these approximations break down are investigated. It is shown that recombination is usually accurately modeled using only vertically averaged carrier concentrations when the distance between electrodes is many times the height and when less than half the charges that are generated recombine, although edge effects around the onset of electrodes are noted. These findings are then applied to a problem that often emerges in scanning photocurrent microscopy, a point-excited film with a laterally offset electrode. It is demonstrated that we expect the current recorded in this case to decay exponentially with the distance between excitation and electrode, with a decay constant that can be related to device parameters. The characteristic equilibration time for the system to reach this current, which can be extracted from the phase delay in a lock-in amplifier measurement, is demonstrated to increase linearly with distance. It is shown that information about the diffusion and recombination rates can be extracted from a wide variety of planar systems.

The thermophotovoltaic (TPV) efficiency is maximized by using three optimization parameters, namely, the absorber and PV solar cell temperatures and the voltage across the cell. Various combinations of spherical and (disk) plane absorbers and solar cells are analyzed. The best performance is associated with a combination plane absorber–plane solar cell. The thermal design has a significant influence on the optimum PV cell band gap. In the case of a normal thermal design, the cell temperature is usually high and depends strongly on the band gap. When a very good thermal design is considered, the optimum cell temperature is less than 30 degrees higher than the ambient temperature and decreases with an increase in the band gap. Increasing the ratio rA/rc between the absorber and PV cell radii leads to increased TPV efficiency. The optimum absorber temperature (practically) does not depend on rA/rc.

A method is proposed to analyze the internal quantum efficiency spectra that are commonly obtained on solar cells by measuring their current response in monochromatic light as a function of wavelength. The efficiency values η are regarded as a function of the reciprocal y=1/α of the light absorption coefficient. The integral relation between η(y) and the charge collection probability distribution in the cell φ(z), with a simple change of variables, can be put in the form of a convolution. A transformed version of this convolution is approximately inverted to yield an explicit expression of φ(z) in terms of η(y) and its derivatives. The method is preliminarily tested on simulated data and is used to analyze published measurements obtained on a Si solar cell and a proton-irradiated InP solar cell.

Impurity photovoltaic effect is investigated in two groups of indium-doped single-crystalline silicon solar cells with n-type and p-type dopants in the base layer. The continuity equation for minority carriers is solved numerically using the charge neutrality condition and current–voltage characteristics are found. It is shown that the improvement of short-circuit current due to carrier photogeneration from the deep defect level is negligible for both groups of the cells considered. Short-circuit current increases with increasing the trap concentration and open-circuit voltage abruptly decreases for trap concentrations close to compensation by n-type dopant. However, these dependencies occur due to the increase of lifetime, the decrease of the total equilibrium carrier density, and take place even in the absence of the absorption of subgap photons. It is shown that indium is not the proper impurity for efficiency improvements of silicon solar cells due to the impurity photovoltaic effect.

A detailed balance solar energy conversion model offering a single treatment of both photovoltaic and photothermal conversion is expounded. It includes a heat rejection mechanism. The effect of multiple impact ionizations on the solar cell efficiency is reconsidered by including the constraints dictated by the first law of thermodynamics (which already exist in the model) and it improves of course the solar cell efficiency. However the upper bound efficiencies previously derived are too optimistic as they do not take into consideration the necessary increase in solar cell temperature. The cell efficiency operating under unconcentrated radiation is a few percent lower than in the ideal case (i.e., with perfect cooling). Wider band gap materials are recommended for those applications where the cell cooling is not effective. The best operation of naturally ventilated cells is under unconcentrated or slightly concentrated solar radiation. Increasing the (forced) ventilation rate allows an increase of the opti...

Temperature and doping-level dependence of CdTe solar cells is investigated, taking into account the involvement of excitons on photocurrent transport. We show that the density of excitons in CdTe is comparable with that of minority carriers at doping levels ⩾1015 cm−3. From the investigation of the dark-saturation current, we show that the product of electron and hole concentrations at equilibrium is several orders of magnitude more than the square of the intrinsic carrier concentration. With this assumption, we have studied the effect of excitons on CdTe solar cells, and the effect is negative. CdTe solar cell performance with excitons included agrees well with existing experimental results.

This article presents a systematic study on the electronic transport mechanisms of CuGaSe2-based thin film solar cells. A variety of samples with different types of stoichiometry deviations, substrates and buffer layers is investigated. We propose two transport models, namely tunneling enhanced volume recombination and tunneling enhanced interface recombination, which allow to explain the observed features for all devices under consideration. The doping level of the absorber layer turns out to be the most decisive parameter for the electronic loss mechanism. The doping is influenced by the type of stoichiometry deviation as well as by the Na content of the substrate. High doping levels result in tunnel assisted recombination. The best solar cells display the lowest tunneling rates. For these devices treatments of the absorber surface by air-annealing and/or the deposition temperature of the CdS buffer layer are decisive for the final device performance. We use the investigation of the open-circuit voltage...

Precisely how the short circuit current (JSC) is produced in a proton irradiated n+p InP/Si solar cell at very high fluence levels has been determined from combined measurements of the cell structure using electrochemical capacitance–voltage profiling and detailed analysis of the spectral quantum efficiency. Type conversion in the base region of the cell is shown to occur before an anomalous peak in the degradation curve for JSC is reached at high damage levels. The short circuit current, and hence the cell efficiency, ultimately collapse because the high absorption coefficient of InP eventually prevents the generation of electron–hole pairs close enough to the effective cell junction from being collected.

We introduce a general relationship between the effective diffusion length LQ of solar cells derived from spectral quantum efficiency Q and the effective diffusion length LJ that determines the saturation current j0=qn0D/LJ of the diode in the dark. The general relation LQ⩾LJ holds in the presence of grain boundaries and of spatially nonhomogeneous doping profiles. We find the relation LQ=LJ for solar cells without potential fluctuations at the collecting junction.

A numerical study is made of the impurity photovoltaic (IPV) effect in crystalline Si p-n junction solar cells. Application is made to In impurities whose energy level is at 0.157 eV above the valence band edge. The numerical resolution of Poisson’s equation and the continuity equations for electrons, holes and defect levels yields the current–voltage curves for different impurity concentrations Nt, from which the short-circuit current density Jsc, the open-circuit voltage Voc, and the energy conversion efficiency η is obtained. It is shown that increasing the defect concentration Nt with respect to the background donor concentration leads to an increase of Jsc but decreases Voc and η. A modified defect concentration profile in a p-n-n+ structure is suggested to extract the maximum benefit from the IPV effect.

The second law of thermodynamics is applied to the entropy fluxes going into and out of solar power plants. It is discussed how clouds affect the entropy flux coming from the sun by redistributing the photons on a larger number of electromagnetic modes and thereby reduce the efficiency of the energy conversion in power plants. Under direct illumination the fundamental efficiency limit corresponds to the Carnot efficiency which is 94.8%. With isotropically scattering clouds the efficiency is reduced to a spectral average of 72.3%.

An attempt is made to assess the accuracy of the simplifying assumption of total transmission (total loss) of the light inside the “loss” or “escape” cone which is made in many models of optical confinement in thin-layer silicon solar cells. A closed form expression is derived for the absorption enhancement factor as a function of the refractive index in the low-absorption limit for a thin layer with a Lambertian back-reflector and is compared with the expression derived previously on the basis of the “total loss” assumption. The generation profile, needed for the evaluation of solar cell performance parameters, is also obtained and studied. More general problems involving realistic light trapping schemes and/or actual material properties are treated numerically, sometimes by means of a suitably modified ray-tracing computer program. It turns out that the error in making the above-mentioned approximation is relatively minor if the layer has a Lambertian back-reflector, whether its front surface is textured or not, but becomes more appreciable for a flat back-reflector particularly as its reflectance approaches unity.

An experimental and numerical study of a-SiGe:H based solar cells with a band gap graded i layer in the shape of a "V" is presented. The variation of the location of the band gap minimum has a strong influence on solar cell performance. Under air mass (AM) 1.5 illumination the cells show a strong increase in open circuit voltage and a distinct decrease in the fill factor when shifting the band gap minimum from the front to the rear part of the i layer. Comparisons of experimental and simulated data of the dark I/V behavior, the I/V curves under illumination and the quantum efficiency allow insights into the transport and recombination behavior within the solar cell. The simulations reveal that the position,as well as the charge state of the defects and, under illumination additionally the recharging behavior of the defect states, determine the device characteristics. (C) 1998 American Institute of Physics.

Crystalline Si1−xGex compounds offer the possibility for tuning the electronic energy band structure with the chemical composition of the alloy in order to adapt the material for devices utilizing the energy of solar photons at an optimum. We concentrate on the efficiency enhancement due to carrier multiplication by impact ionization. We calculate the internal quantum efficiency and the possible solar cell efficiency for this material system. The number of impact-generated charge carriers is obtained by a simulation of the competing carrier–carrier and carrier–photon scattering processes. These calculations show that the wave vector dependence of the scattering processes is unimportant for good agreement between theoretical and experimental quantum efficiencies in Si and Ge. Finally, we calculate solar cell efficiencies under the ideal assumption of unity collection efficiency and radiative recombination only. Impact ionization enhances the theoretical conversion efficiency by 0.5 percentage point; this i...

The Shockley’s diode equation predicts a current-voltage characteristic different from that used by Shockley and Queisser to compute the limiting efficiency of photovoltaic energy conversion under the assumptions of the detailed balance theory. The reasons for such discrepancy are discussed being the neglect of photon recycling effects in Shockley’s diode equation the main cause. This interpretation is crucial to understand the fundamentals on which the computation of the limiting efficiency of solar cells is based. Without photon recycling effects, it can be concluded that the limiting efficiency (one sun) of a gallium arsenide solar cell is 26.8% (with the sun assumed as blackbody at 6000 K) while the true figure is 30.7%, 38.7% as long as the angle of emission of photons from the cell is fully restricted.

The photoacoustic effect and a similar thermoacoustic effect with an alternating current (electroacoustic effect) are used to investigate the spatial distribution of heat sources in crystalline silicon solar cells. The use of a reference signal produced inside the same sample to normalize the acoustic signal allows background and photoacoustic cell effects to be reduced. The frequency dependence of these normalized measurements can be calculated analytically. Covering frequencies up to 10 kHz, the electroacoustic effect allows the nondestructive determination of diffusion lengths and recombination velocities at the interfaces. Volume and surface effects can thus be separated, which is useful for the analysis of high‐efficiency solar cells.

Recent work has indicated that a significant number of electrons and holes remain in the free‐exciton form in silicon at room temperature, a finding which, if supportable by experimental evidence, requires the inclusion of excitons in diode and solar cell theory. Excitons, although neutral, may contribute to device currents by diffusing to the junction region where they may be dissociated by the field. A generalized three‐particle theory of transport in semiconductors is presented. The results of application of the theory to silicon devices indicate a decrease in the dark saturation current as well as an increase in light‐generated current when excitons are incorporated in the theory so long as exciton diffusion length exceeds that of the minority carriers. The work includes suggestions for experimental methods to confirm exciton involvement and to estimate the value of the exciton‐binding parameter from spectral response measurements on solar cells.

The solar cell does not use a pn-junction to separate electrons from holes, but uses an undoped CdS layer that is p-type inverted when attached to a p-type collector and collects the holes while rejecting the backflow of electrons and thereby prevents junction leakage. The operation of the solar cell is determined by the minimum entropy principle of the cell and its external circuit that determines the electrochemical potential, i.e., the Fermi-level of the base electrode to the operating (maximum power point) voltage. It leaves the Fermi level of the metal electrode of the CdS unchanged, since CdS does not participate in the photo-emf. All photoelectric actions are generated by the holes excited from the light that causes the shift of the quasi-Fermi levels in the generator and supports the diffusion current in operating conditions. It is responsible for the measured solar maximum power current. The open circuit voltage (Voc) can approach its theoretical limit of the band gap of the collector at 0 K and ...

The general theory developed in part I was applied to build up two models of photovoltaic conversion. To this end two different systems were analyzed. The first system consists of the whole absorber (converter), for which the balance equations for energy and entropy are written and then used to derive an upper bound for solar energy conversion. The second system covers a part of the absorber (converter), namely the valence and conduction electronic bands. The balance of energy is used in this case to derive, under additional assumptions, another upper limit for the conversion efficiency. This second system deals with the real location where the power is generated. Both models take into consideration the radiation polarization and reflection, and the effects of concentration. The second model yields a more accurate upper bound for the conversion efficiency. A generalized solar cell equation is derived. It is proved that other previous theories are particular cases of the present more general formalism.

An ideal model for quantum well solar cells is developed and is used to theoretically explore the dependence of terminal characteristics on the host cell and quantum well properties. The model, which explicitly treats carrier generation and recombination in the quantum wells, is described and compared with an analogous ideal model for bulk homojunction cells. Open‐circuit voltages, short‐circuit current densities, and conversion efficiencies are then calculated as functions of the well and barrier band gaps for ideal cells in the radiative limit, assuming air‐mass‐zero (AM0) solar illumination at a cell temperature of 300 K. Qualitative trends in these characteristics and regimes of operation are identified, the effects of non‐radiative recombination are explored, and idealized approximations used in the model are assessed. Finally, published experimental data for quantum well solar cells are surveyed and discussed, and are found to exhibit strong qualitative consistencies with predictions from the ideal ...

An approximate expression proposed by Green predicts the maximum obtainable fill factor (FF) of a solar cell from its open-circuit voltage (Voc). The expression was originally suggested for silicon solar cells that behave according to a single-diode model and, in addition to Voc, it requires an ideality factor as input. It is now commonly applied to silicon cells by assuming a unity ideality factor—even when the cells are not in low injection—as well as to non-silicon cells. Here, we evaluate the accuracy of the expression in several cases. In particular, we calculate the recombination-limited FF and Voc of hypothetical silicon solar cells from simulated lifetime curves, and compare the exact FF to that obtained with the approximate expression using assumed ideality factors. Considering cells with a variety of recombination mechanisms, wafer doping densities, and photogenerated current densities reveals the range of conditions under which the approximate expression can safely be used. We find that the expression is unable to predict FF generally: For a typical silicon solar cell under one-sun illumination, the error is approximately 6% absolute with an assumed ideality factor of 1. Use of the expression should thus be restricted to cells under very low or very high injection.

Co-optimization of the gallium and sulfur profiles in penternary Cu(In,Ga)(Se,S)2 thin film solar cell and its impacts on device performance and variability are investigated in this work. An absorber formation method to modulate the gallium profiling under low sulfur-incorporation is disclosed, which solves the problem of Ga-segregation in selenization. Flatter Ga-profiles, which lack of experimental investigations to date, are explored and an optimal Ga-profile achieving 17.1% conversion efficiency on a 30 cm × 30 cm sub-module without anti-reflection coating is presented. Flatter Ga-profile gives rise to the higher Voc × Jsc by improved bandgap matching to solar spectrum, which is hard to be achieved by the case of Ga-accumulation. However, voltage-induced carrier collection loss is found, as evident from the measured voltage-dependent photocurrent characteristics based on a small-signal circuit model. The simulation results reveal that the loss is attributed to the synergistic effect of the detrimental gallium and sulfur gradients, which can deteriorate the carrier collection especially in quasi-neutral region (QNR). Furthermore, the underlying physics is presented, and it provides a clear physical picture to the empirical trends of device performance, I–V characteristics, and voltage-dependent photocurrent, which cannot be explained by the standard solar circuit model. The parameter “FGa” and front sulfur-gradient are found to play critical roles on the trade-off between space charge region (SCR) recombination and QNR carrier collection. The co-optimized gallium and sulfur gradients are investigated, and the corresponding process modification for further efficiency-enhancement is proposed. In addition, the performance impact of sulfur-gradient variation is studied, and a gallium design for suppressing the sulfur-induced variability is proposed. Device performances of varied Ga-profiles with front sulfur-gradients are simulated based on a compact device model. Finally, an exploratory path toward 20% high-efficiency Ga-profile with robustness against sulfur-induced performance variability is presented.

The ‘‘passivated emitter and rear locally diffused’’ (PERL) silicon solar cell structure presently demonstrates the highest terrestrial performance of any silicon‐based solar cell. This paper presents a detailed investigation of the limiting loss mechanisms in PERL cells exhibiting independently confirmed 1‐sun efficiencies of up to 23.0%. Optical, resistive, and recombinative losses are all analyzed under the full range of solar cell operating conditions with the aid of two‐dimensional (2D) device simulations. The analysis is based on measurements of the reflectance, quantum efficiency, dark and illuminated current–voltage (I–V) characteristics, and properties of the Si–SiO2 interfaces employed on these cells for surface passivation. Through the use of the 2D simulations, particular attention has been paid to the magnitudes of the spatially resolved recombination losses in these cells. It is shown that approximately 50% of the recombination losses at the 1‐sun maximum power point occur in the base of the cells, followed by recombination losses at the rear and front oxidized surfaces (25% and

Photothermal deflection is used to investigate the electronic transport in a pn silicon solar cell. The dependence of the deflection signal on an external bias voltage in the closed circuit and on additional bias illumination in the open‐circuit case is simulated successfully with a semimicroscopic approach using the expressions for the current voltage characteristics obtained from the diode theory. The experimental results on various crystalline silicon solar cells demonstrate the ability of the method for contactless all‐optical characterization of photovoltaic interfaces.

The principle of superposition forms the theoretical basis on which the comparison of illuminated and dark current‐voltage (I‐V) characteristics of solar cells depends. Two cases predicted from computer simulations where the superposition principle does not apply in silicon p‐n junction solar cells are reported. These predictions are confirmed experimentally with measurements taken on existing high‐efficiency devices, and cannot be accurately described by previous explanations for departures from this principle. The first case, which is the more important in terms of operation under 1 sun illumination, occurs in cells where recombination via defect levels (Shockley–Read–Hall recombination) with unequal electron and hole capture rates dominates the I‐V characteristics. The second case is evident at small forward voltages in almost all silicon solar cells. It is shown that the former is due to a saturation in the recombination rate, while the latter is the result of a bias‐dependent modification of the carr...

We present a systematic device model that reproduces the important features of bulk heterojunction organic solar cells. While examining the model outputs we find that one of the limiting factors in organic solar cells is the reduced built-in potential due to effective pinning of the electrodes relative to the energy gap at the bulk of the device. Having identified this as a problem we suggest a device structure that can enhance the open circuit voltage. Our detailed modeling shows that such a structure can enhance the open circuit voltage as well as the short circuit current leading to above 40% improvement in power conversion efficiency of state of the art organic solar cells.

A theoretical investigation of the impurity photovoltaic (IPV) effect for improving silicon solar‐cell efficiency is presented. The approach is better than previous analyses because of the improved treatment of generation and recombination via impurities, and because it includes the effects of optical competition and light trapping. The approach is applied to the nonmidgap, deep‐level impurity indium as the IPV effect impurity incorporated into an idealized silicon solar cell. The analysis is based on experimentally determined parameters for indium. Improvements of cell current, subgap spectral response, and energy conversion efficiency are quantified. The analysis reveals the importance of light trapping and proper selection of indium and dopant concentrations. The impurity photovoltaic effect is predicted to improve solar‐cell efficiency.

Internal collection efficiency of solar cells is calculated as a function of depletion width, minority‐carrier diffusion length, solar spectrum, and absorption coefficient. Particular attention is given to the variation of depletion width with voltage, which may reduce collection efficiency by a few percent at operating voltages and may be misinterpreted as a shunting effect. Calculations of collection‐efficiency magnitude and voltage dependence, plus comparisons with experiment, are presented for several types of solar cells. Primary emphasis is placed on polycrystalline thin‐film cells with relatively short diffusion lengths and hence potentially more pronounced collection efficiency effects.

A series of systematic studies have been made on both optimum design and preparation technology of high efficiency a‐Si/a‐Si/a‐SiGe triple‐junction solar cells. It has been found that current matching is one of the key factors affecting the fill factor of two‐terminal tandem solar cells, and excess carrier recombination at the p‐n interface acting as an internal electrode is dominated by the density of the minority carriers, interfacial states, and geometrical factor match of the materials at the p‐n interface. In this work, μc‐Si:H with wide band gap and high conductivity, which was prepared at low substrate temperature, was used as an n layer of a‐Si:H solar cells. In order to improve the n‐i interface property, a novel approach involving the insertion of an n−a‐Si:H buffer layer at the n‐i interface was developed. The experimental results show that the buffer layer of appropriate thickness could enhance the fill factor and open circuit voltage of a‐Si:H based solar cells. In addition, highly photosensi...

We show that capping layers of tris-(8-hydroxy-quinolinato)-aluminum Alq enable increased absorption and photocurrent in organic solar cells (OSCs) when using transparent metal films as top electrodes. Furthermore, by varying the capping layer thickness, the optical field in the OSC is tuned for selective wavelengths, opening a possibility of influencing the external quantum efficiency for specific absorber materials. It is described how a second maximum of the optical field intensity can be utilized, which is a concept significant for tandem solar cells. Indium tin oxide (ITO)-free OSCs are presented which show the influence of capping layer on efficiency, saturation, fill factor, and open-circuit voltage, with numerical calculations supporting the experimental evidence of layer-selective enhancement. Copyright 2010 American Institute of Physics.

We present the material α-ω -bis-(dicyanovinylen)-sexithiophen (DCV6T) as donor material in organic solar cells. A systematic study on the potential of DCV6T is given for different active layer concepts. DCV6T is a member of a class of acceptor-substituted oligothiophenes, which showed efficiencies of up to 3.4% and open circuit voltages (Voc) of 1.0 V, which were recently reported [K. Schulze Adv. Mater. (Weinheim, Ger.) 18, 2875 (2006)]. To verify the potential of the material (DCV6T), organic solar cells with planar heterojunctions, bulk heterojunctions, and a hybrid-planar-mixed heterojunction are investigated. The planar heterojunction solar cells of DCV6T and C60 show the highest Voc of 0.90 V. The mixed heterojunction solar cells have improved currents but a lower Voc of 0.82 V. The solar cell using the hybrid-planar-mixed heterojunction achieves the best combination of parameters. It has a Voc of 0.88 V, a short circuit current (jsc) of 5.7±0.4 mA cm-2, a fill factor of 41.6%, and a power conversion efficiency of 2.1±0.2%. © 2009 American Institute of Physics.

The high open circuit voltage is an attractive feature for the currently popular organic-inorganic hybrid perovskite solar cells. In this paper, by employing the macroscopic device model simulation, we investigate its origin for the planar heterojunction perovskite solar cells. Based on the calculated current density-voltage characteristics, it is revealed that compared to the excitonic solar cells, the fast thermal-activated exciton dissociation in the bulk due to the small exciton binding energy may improve the short circuit current and the fill factor, but its beneficial role on the open circuit voltage is marginal. The most significant contribution for the open circuit voltage comes from the reduced bimolecular recombination. In the perovskites, with the recombination prefactor many orders of magnitude smaller than that based on the Langevin's theory, the internal charge density level is significantly enhanced and the density gradient is removed, leading to the high quasi-Fermi level splitting and thus the small open circuit voltage loss. For the nonradiative recombination pathway due to the deep trap states, it may induce significant loss of open circuit voltage as the trap density is high, while for the moderately low density its effect on the open circuit voltage is small and negligible.

We have carried out modeling of the evolution of silicon solar cell performance following irradiation with high energy protons and electrons. Using the traditional semiempirical expression the solar cell output parameters of a given cell structure could be predicted for various particle energies by expressing the particle fluence as a displacement damage dose. We discuss the extent to which the evolution of different silicon solar cell structures in a radiation environment can be predicted using the software package PC-1D by taking account of the degradation of the minority carrier lifetime and majority carrier concentration in the cell.

The effect of a strained Al0.55In0.45As window layer on p/n InP solar cell performance was studied. In comparison to homojunction InP solar cells, decreased quantum efficiency in the short wavelength region of the spectrum was observed in cells having the window layer. Photoluminescence measurements of the heterojunction and light emission measurements of the solar cell under forward bias revealed an intense radiative transition, which is related to the enhanced recombination of the carriers that are photogenerated in the emitter layer. This recombination occurs between the energy levels of the triangular quantum wells formed at the type II Al0.55In0.45As/InP heterojunction, and prevents effective carrier collection in the solar cell.

Previous derivations of the ideal solar cell equation based on Shockley's p-n junction diode theory implicitly assume negligible effects of photon recycling. This paper derives the equation in the presence of photon recycling that modifies the values of dark saturation and light-generated currents, using an approach applicable to arbitrary three-dimensional geometries with arbitrary doping profile and variable band gap. The work also corrects an error in previous work and proves the validity of the reciprocity theorem for charge collection in such a more general case with the previously neglected junction depletion region included.

Detailed studies of solar cell efficiency as a function of temperature were performed for quantum wire intermediate band solar cells grown on the (311)A plane. A remotely doped one-dimensional intermediate band made of self-assembled In0.4Ga0.6As quantum wires was compared to an undoped intermediate band and a reference p-i-n GaAs sample. These studies indicate that the efficiencies of these solar cells depend on the population of the one-dimensional band by equilibrium free carriers. A change in this population by free electrons under various temperatures affects absorption and carrier transport of non-equilibrium carriers generated by incident light. This results in different efficiencies for both the doped and undoped intermediate band solar cells in comparison with the reference GaAs p-i-n solar cell device.

Recombination through band tail localized states is studied analytically and by measurement of the forward-bias dark current as a function of temperature in three different organic bulk-heterojunction solar cells. The Shockley-Read-Hall mechanism is analyzed for the specific case of recombination between mobile carriers and an exponential distribution of localized band tail states. The analysis gives a simple relation between the dark current ideality factor and the band tail slope. Assumptions of the model are verified by numerical drift-diffusion modeling. Diode current-voltage measurements give good agreement with the analytical model, confirming that the band tail recombination mechanism applies to at least some organic solar cells. Deep traps provide a secondary recombination channel in some devices.

Device modeling of CH3NH3PbI3−xCl3 perovskite-based solar cells was performed. The perovskite solar cells employ a similar structure with inorganic semiconductor solar cells, such as Cu(In,Ga)Se2, and the exciton in the perovskite is Wannier-type. We, therefore, applied one-dimensional device simulator widely used in the Cu(In,Ga)Se2 solar cells. A high open-circuit voltage of 1.0 V reported experimentally was successfully reproduced in the simulation, and also other solar cell parameters well consistent with real devices were obtained. In addition, the effect of carrier diffusion length of the absorber and interface defect densities at front and back sides and the optimum thickness of the absorber were analyzed. The results revealed that the diffusion length experimentally reported is long enough for high efficiency, and the defect density at the front interface is critical for high efficiency. Also, the optimum absorber thickness well consistent with the thickness range of real devices was derived.

In photovoltaic solar cells, radial p-n junctions have been considered a very promising structure to improve the carrier collection efficiency and accordingly the conversion efficiency. In the present study, the semiconductor equations, namely Poisson's and continuity equations for a cylindrical p-n junction solar cell, have been solved analytically. The analytical model is based on Green's function theory to calculate the current density, open circuit voltage, fill factor, and conversion efficiency. The model has been used to simulate p-n and p-i-n silicon radial solar cells. The validity and accuracy of the present simulator were confirmed through a comparison with previously published experimental and numerical reports.

Precise knowledge of the series resistance is essential for failure and loss analysis as well as yield prediction of solar cell devices. In this work, a method which determines the current and photogeneration dependence of the series resistance without assuming any specific current-voltage characteristic for the internal diodes is presented. This approach is of particular interest for solar cells which cannot be described by the one- or two-diode model such as organic solar cells. Furthermore, it clarifies the difference in the series resistance values that are obtained from current-voltage curves in the dark and under illumination as well as short-circuit-current and open-circuit-voltage characteristics. Additionally, it is shown how other cell parameters, such as the shunt resistance or the current-voltage characteristic of the internal diode are determined in a consistent way. Finally, it is demonstrated that our approach can be easily implemented in a new generation of solar simulators that are based on light-emitting diodes instead of conventional light sources.

The tremendous amount of research in solar energy is directed toward intermediate band solar cell for its advantages compared with the conventional solar cell. The latter has lower efficiency because the photons have lower energy than the bandgap energy and cannot excite mobile carriers from the valence band to the conduction band. On the other hand, if mini intermediate band is introduced between the valence and conduction bands, then the smaller energy photons can be used to promote charge carriers transfer to the conduction band and thereby the total current increases while maintaining a large open circuit voltage. In this article, the influence of the new band on the power conversion efficiency for structure of quantum dots intermediate band solar cell is theoretically investigated and studied. The time-independent Schrodinger equation is used to determine the optimum width and location of the intermediate band. Accordingly, achievement of a maximum efficiency by changing the width of quantum dots and barrier distances is studied. Theoretical determination of the power conversion efficiency under the two different ranges of QD width is presented. From the obtained results, the maximum power conversion efficiency is about 70.42%. It is carried out for simple cubic quantum dot crystal under fully concentrated light. It is strongly dependent on the width of quantum dots and barrier distances.