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A recently suggested reciprocity relation states that the current transport efficiency from the junction to the cell terminal can be determined by differentiating luminescence images with respect to the terminal voltage. The validity of this relation is shown experimentally in this paper, by comparison with simultaneously measured electrical currents and simulations. Moreover, we verify that the method is applicable under various light concentrations and applied voltages, which allows us to investigate the cell in relevant conditions. Results evidence several kind of series resistances affecting the current transport efficiencies. We show that the relative contribution of those different resistances to the loss in current collection is a function of the illumination intensity.

Efficient semi-transparent solar cells can trigger the adoption of building integrated photovoltaics. Halide perovskites are particularly suitable in this respect owing to their tunable bandgap. Main drawbacks in the development of transparent perovskite solar cells are the high Voc deficit and the difficulties in depositing thin films over large area substrates, given the low solubility of bromide and chloride precursors. In this work, we develop a 2D and passivation strategies for the high band-gap Br perovskite able to reduce charge recombination and consequently improving the open-circuit voltage. We demonstrate 1cm 2 perovskite solar cells with Voc up to 1.73 V (1.83 eV QFLS) and a PCE of 8.2%. The AVT exceeds 70% by means of a bifacial light management and a record light utilization efficiency of 5.72 is achieved, setting a new standard for transparent photovoltaics. Moreover, we show the high ceiling of our technology towards IoT application due to a bifaciality factor of 87% along with 17% PCE under indoor lighting. Finally, the up-scaling has been demonstrated fabricating 20cm 2 -active area modules with PCE of 7.3% and Voc per cell up to 1.65V.

Under continuous-wave laser excitation in an InGaAs multi-quantum-well (MQW) structure, the carrier temperature extracted from photoluminescence rises faster for 405 nm excitation compared with 980 nm excitation, as the injected carrier density increases. Ensemble Monte Carlo simulation of the carrier dynamics in the MQW system shows that this carrier temperature rise is dominated by nonequilibrium LO phonon effects, with the Pauli exclusion having a significant effect at high carrier densities. Further, we find a significant fraction of carriers reside in the satellite L-valleys for 405 nm excitation due to strong intervalley transfer, leading to a cooler steady-state electron temperature in the central valley compared with the case when intervalley transfer is excluded from the model. Good agreement between experiment and simulation has been shown, and a detailed analysis has been presented. This study expands our knowledge of the dynamics of the hot carrier population in semiconductors, which can be applied to further limit energy loss in solar cells., Comment: 22 pages, 7 figures, submitted for publication

Hot carrier solar cells aim to overcome the theoretical limit of single-junction photovoltaic devices by suppressing the thermalization of hot carriers and extracting them through energy selective contacts. Designing efficient hot carrier absorbers requires further investigation on hot carrier properties in materials. Although the thermalization of hot carriers is responsible for a large portion of energy loss in solar cells, it is still one of the least understood phenomena in semiconductors. Here, the impact of excitation energy on the properties of photo-generated hot carriers in an InGaAs multi-quantum well (MQW) structure at various lattice temperatures and excitation powers is studied. Photoluminescence (PL) emission of the sample is detected by a hyperspectral luminescence imager, which creates spectrally and spatially resolved PL maps. The thermodynamic properties of hot carriers, such as temperature and quasi-Fermi level splitting, are carefully determined via applying full PL spectrum fitting, which solves the Fermi-Dirac integral and considers the band-filling effect in the nanostructured material. In addition, the impact of thermalized power density and carrier scattering with longitudinal optical phonons on the spectral linewidth broadening under two excitation energies is studied., Comment: 20 pages, 14 figures

We investigate numerically the impact of the electron-phonon scattering on the optical properties of a perovskite material (CH3NH3PbI3). Using non-equilibrium Green functions formalism, we calculate the local density-of-states for several values of the electron-phonon scattering strength as well as in the case of ballistic transport. We report an Urbach-like penetration of the density-of-states in the bandgap due to scattering. The density-of-states expands deeper in the bandgap with the scattering strength. We determine the electronic current contributions relative to photon absorption and photon emission. This allows to estimate the Urbach energy from the absorption coefficient below the bandgap. Values of Urbach energy up to 9.5 meV are obtained, meaning that scattering contribution to the total experimental Urbach energy of 15meV is quite important. The Urbach tail is generally attributed to the disorder of the system, leaving hope for further improvement. Yet, we show in this work that a significant contribution to the Urbach tail comes from carriers-phonon interactions, an intrinsic properties of the material which thus set a lower bound on the improvement margin. Finally, we estimate the open-circuit voltage Voc for a solar cell assuming such a material as an absorber. Voc losses increase with the scattering strength, up to 41 mV. This study then confirms that scattering of electrons in perovskite material modifies the optical behaviour.

The Shockley-Queisser model is a landmark in photovoltaic device analysis by defining an ideal situation as reference for actual solar cells. However, the model and its implications are easily misunderstood. Thus, we present a guide to help understand and avoid misinterpreting it. Focusing on the five assumptions, underlying the model, we define figures of merit to quantify how close real solar cells approach each of these assumptions.

Two reciprocity theorems are important for fundamental understanding of the solar cell operation and applications to device evaluation: (1) the carrier-transport reciprocity connecting the dark-carrier injection with the short-circuit photocarrier collection and (2) the optoelectronic reciprocity connecting the electroluminescence with the photovoltaic quantum efficiency at short circuit. These theorems, however, fail in devices with thick depletion regions such as p-i-n junction solar cells. By properly linearizing the carrier-transport equation in such devices, we report that the dark-carrier injection is related to the photocarrier collection efficiency at the operating voltage, not at short circuit as suggested in the original theorem. This leads to the general form of the optoelectronic reciprocity relation connecting the electroluminescence with the voltage-dependent quantum efficiency, providing a correct interpretation of the optoelectronic properties of p-i-n junction devices. We also discuss the validity of the well-known relation between the open-circuit voltage and the external luminescence efficiency. The impact of illumination intensity and device parameters on the validity of the reciprocity theorems is quantitatively investigated.

Intermediate Band Solar Cell is an advanced concept for solar energy conversion in which two low-energy photons can promote an electron to the conduction band through a so-called intermediate band. To limit recombination and preserve the photo-generated voltage, generation to- and from the intermediate band should be matched. However, all practical realizations experienced a significant voltage degradation as compared to a single junction without intermediate band. In this work, we develop a novel analytical optimization method based on Lagrange multipliers. We demonstrate that an Intermediate Band Solar Cell under solar spectrum cannot meet voltage preservation and current matching at the same time. By contrast, we show that the implementation of an energy shift (\emph{electronic ratchet}) in any of the bands allows those two criteria to be filled simultaneously. Additional insights are provided by the numerical study of the short circuit current and fill factor of the systems at stake, which show that a system with ratchet benefits from the same current increase as a standard Intermediate Band Solar Cell (same short-circuit current), while maintaining I-V properties of a single junction (same open-voltage circuit, same fill factor).

Using the non equilibrium Green functions formalism we propose a study of the electronic excitation and collection in nano-structured intermediate-band solar cell. We demonstrate that a thin tunnel barrier between the nano-objects and the host material is beneficial for both current and voltage. For the current, the confinement generated by such a thin barrier favors the intersubband optical coupling in the nano-objects and then improves the excitation-collection trade-off. We also show that a broadened density-of-state in the nano-objects increases the radiative recombination and then degrades the voltage. Using a detailed balance model we propose a broadening factor for this Voc degradation which decreases when a tunnel barrier enhances the life-time in the nano-objects.

We investigate in this letter the intrinsic properties that have limited the efficiency of nanostructured intermediate band solar cells. Those devices take advantage of intra-band transitions, which occur on narrow energy width, and present low radiative recombination efficiency. We derive the minimum requirements in terms of those two characteristics to achieve efficiencies in excess of the Shockley-Queisser limit, and show that compatible nanostructures are challenging to obtain. Especially, we evidence that currently experimentally considered materials cannot overcome the best single junction cells. In order to solve those issues, we consider devices including an electronic ratchet mechanism. Firstly, such devices are shown to be much less sensitive on the limitations of the nanostructures characteristics, so that requirements for high efficiencies can be met. Secondly, we show that quantum well devices present advantages over their quantum dots counterparts, although they have attracted much less interest so far.

Hot carrier solar cells allow potential efficiency close to the thermodynamical limit in ideal conditions. However, the feasability of such devices has not been clearly stated so far and only ideal cells were considered in previous studies. Here we develop a model with realistic energy selective contacts, carrier thermalization and absorptivity. The requirements in term of contact selectivity is investigated, showing that semi-selective contacts are not incompatible with high efficiencies. Candidates for absorbing material were synthesized and the required thermalization properties are obtained. Specific structures were designed to enhance absorption of concentrated sunlight in a small material thickness, allowing high carrier density in the absorber., Comment: 25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, Valencia : Spain (2010)

A spatially resolved electroluminescence (EL) imaging experiment is developed to measure the local sheet resistance of the window layer, directly on the completed CIGS cell. Our method can be applied to the EL imaging studies that are made in fundamental studies as well as in in-line process inspection (1-3). The EL experiment consists in using solar cell as a light emitting device : a voltage is applied to the cell and its luminescence is detected. We develop an analytical and quantitative model to simulate the behavior of CIGS solar cells based on the spread sheet resistance effect in the window layer. We determine the repartition of the electric potential on the ZnO, for given cell's characteristics such as sheet resistance and contact geometries. Knowing the repartition of the potential, the EL intensity is estimated and the experimental EL signal is fitted, which allows the determination of the window layer sheet resistance.

Hot Carrier solar cells have long been recognized as an attractive contender in the search for high efficiency photovoltaic devices but their fabrication requires solution of two important material challenges: finding materials with drastically reduced carrier cooling rates and realization of selective energy contacts to extract the photogenerated carriers. This paper is concerned with the problem of absorber phononic engineering to reduce carrier cooling rates. The physics of carrier cooling is explored and experimental data of other authors are discussed with a view to assessing the potential of state-of-the art nanostructured materials for PV conversion. A tentative initial calculation based on the thermalisation in these nanostructures and assuming all other aspects as ideal, gives 54% efficiency at 2500suns as compared to 54% for no thermalisation at all. Phononic band gap engineering to further reduce carrier cooling or bring down the threshold concentration is discussed.

Taking into account the temperature gradients in solar cells, it is shown that their efficiency can be increased beyond the Shockley-Queisser limit (J. Appl. Phys. 32 (1961) 510). The driving force for this gain is the temperature gradient between this region and its surroundings. A quantitative theory is given. Though the effect is found to be weak in conventional solar cells, it is argued that it can be substantially increased by proper choice of materials and design of the device. In particular, it is shown that the insertion of a quantum well can enhance the efficiency beyond one of the single gap cell, due to the presence of temperature jumps at the heterojunctions., Comment: Published in Special issue Physica E 14 (1-2) on Nanostructures in Photovoltaics