Nano-imprinted rear-side diffraction gratings for absorption enhancement in solar cells

As wafer-based solar cells become thinner, light-trapping textures for absorption enhancement will gain in importance. In this work, crystalline silicon wafers were textured with wavelength-scale diffraction grating surface textures by nanoimprint lithography using interference lithography as a mastering technology. This technique allows fine-tailored nanostructures to be realized on large areas with high throughput. Solar cell precursors were fabricated, with the surface textures on the rear side, for optical absorption measurements. Large absorption enhancements are observed in the wavelength range in which the silicon wafer absorbs weakly. It is shown experimentally that bi-periodic crossed gratings perform better than uni-periodic linear gratings. Optical simulations have been made of the fabricated structures, allowing the total absorption to be decomposed into useful absorption in the silicon and parasitic absorption in the rear reflector. Using the calculated silicon absorption, promising absorbed photocurrent density enhancements have been calculated for solar cells employing the nano-textures. Finally, first results are presented of a passivation layer deposition technique that planarizes the rear reflector for the purpose of reducing the parasitic absorption.

Effect of concentration on the performance of quantum dot intermediate-band solar cells

Implementation of a high-efficiency quantum dot intermediate-band solar cell (QD-IBSC) must accompany a sufficient photocurrent generation via IB states. The demonstration of a QD-IBSC is presently undergoing two stages. The first is to develop a technology to fabricate high-density QD stacks or a superlattice of low defect density placed within the active region of a p-i-n SC, and the second is to realize half-filled IB states to maximize the photocurrent generation by two-step absorption of sub-bandgap photons. For this, we have investigated the effect of light concentration on the characteristics of QDSCs comprised of multi-layer stacks of self-organized InAs/GaNAs QDs grown with and without impurity doping in molecular beam epitaxy.

Study of the effect of different hole sizes on mechanical strength of wafers for back contact solar cells

Drilling process on wafers to produce EWT or MWT solar cells is a critical fabrication step, which affects on their mechanical stability. The amount of damage introduced during drilling process depends on the density of holes, their size and the chemical process applied afterwards. To quantify the relation between size of the holes and reduction of mechanical strength, several sets of wafers have been prepared, with different hole diameter. The mechanical strength of these sets has been measured by the ring on ring bending test, and the stress state in the moment of failure has been deduced by FE simulation.

Damage reduction of the laser drilling process on back contact solar cells by chemical treatment

Production of back contact solar cells requires holes generations on the wafers to keep both positive and negative contacts on the back side of the cell. This drilling process weakens the wafer mechanically due to the presence of the holes and the damage introduced during the process as microcracks. In this study, several chemical processes have been applied to drilled wafers in order to eliminate or reduce the damage generated during this fabrication step. The treatments analyzed are the followings: alkaline etching during 1, 3 and 5 minutes, acid etching for 2 and 4 minutes and texturisation. To determine mechanical strength of the samples a common mechanical study has been carried out testing the samples by the Ring on Ring bending test and obtaining the stress state in the moment of failure by FE simulation. Finally the results obtained for each treatment were fitted to a three parameter Weibull distribution

ARXPS analysis of a GaAs/GaInP heterointerface with application in III-V multijunction solar cells

In this contribution, angle-resolved X-ray photoelectron spectroscopy is used to explore the extension and nature of a GaAs/GaInP heterointerface. This bilayer structure constitutes a very common interface in a multilayered III-V solar cell. Our results show a wide indium penetration into the GaAs layer, while phosphorous diffusion is much less important. The physico-chemical nature of such interface and its depth could deleteriously impact the solar cell performance. Our results probe the formation of spurious phases which may profoundly affect the interface behavior.

Structural challenges in multijunction solar cells for ultra-high CPV

The paths towards high efficiency multijunction solar cells operating inside real concentrators at ultra high concentration (>1000 suns) are described. The key addressed factors comprehend: 1) the development of an optimized tunnel junction with a high peak current density (240 A/cm2) to mitigate the non-uniform light profiles created by concentrators, 2) the inclusion of highly conductive semiconductor lateral layers to minimize the effects of the non-uniform light profiles in general, and the chromatic aberration in particular; and 3) an adequate design of reliability studies to test multijunction solar cells for real operation conditions in order to determine the fragile parts in the device and improve them. These challenges are faced by means of experimental and theoretical investigation using a quasi-3D distributed circuital model.

Optimization of the silicon subcell for III-V on silicon multijunction solar cells: key differences with conventional silicon technology

Dual-junction solar cells formed by a GaAsP or GaInP top cell and a silicon (Si) bottom cell seem to be attractive candidates to materialize the long sought-for integration of III-V materials on Si for photovoltaic (PV) applications. Such integration would offer a cost breakthrough for PV technology, unifying the low cost of Si and the efficiency potential of III-V multijunction solar cells. The optimization of the Si solar cells properties in flat-plate PV technology is well-known; nevertheless, it has been proven that the behavior of Si substrates is different when processed in an MOVPE reactor In this study, we analyze several factors influencing the bottom subcell performance, namely, 1) the emitter formation as a result of phosphorus diffusion; 2) the passivation quality provided by the GaP nucleation layer; and 3) the process impact on the bottom subcell PV properties.

Preliminary temperature Accelerated Life Test (ALT) on III-V commercial concentrator triple-junction solar cells

A quantitative temperature accelerated life test on sixty GaInP/GaInAs/Ge triple-junction commercial concentrator solar cells is being carried out. The final objective of this experiment is to evaluate the reliability, warranty period, and failure mechanism of high concentration solar cells in a moderate period of time. The acceleration of the degradation is realized by subjecting the solar cells at temperatures markedly higher than the nominal working temperature under a concentrator Three experiments at three different temperatures are necessary in order to obtain the acceleration factor which relates the time at the stress level with the time at nominal working conditions. . However, up to now only the test at the highest temperature has finished. Therefore, we can not provide complete reliability information but we have analyzed the life data and the failure mode of the solar cells inside the climatic chamber at the highest temperature. The failures have been all of them catastrophic. In fact, the solar cells have turned into short circuits. We have fitted the failure distribution to a two parameters Weibull function. The failures are wear-out type. We have observed that the busbar and the surrounding fingers are completely deteriorate

InAs/AlGaAs quantum dot intermediate band solar cells with enlarged sub-bandgaps

In the last decade several prototypes of intermediate band solar cells (IBSCs) have been manufactured. So far, most of these prototypes have been based on InAs/GaAs quantum dots (QDs) in order to implement the IB material. The key operation principles of the IB theory are two photon sub-bandgap (SBG) photocurrent, and output voltage preservation, and both have been experimentally demonstrated at low temperature. At room temperature (RT), however, thermal escape/relaxation between the conduction band (CB) and the IB prevents voltage preservation. To improve this situation, we have produced and characterized the first reported InAs/AlGaAs QD-based IBSCs. For an Al content of 25% in the host material, we have measured an activation energy of 361 meV for the thermal carrier escape. This energy is about 250 meV higher than the energies found in the literature for InAs/GaAs QD, and almost 140 meV higher than the activation energy obtained in our previous InAs/GaAs QD-IBSC prototypes including a specifically designed QD capping layer. This high value is responsible for the suppression of the SBG quantum efficiency under monochromatic illumination at around 220 K. We suggest that, if the energy split between the CB and the IB is large enough, activation energies as high as to suppress thermal carrier escape at room temperature (RT) can be achieved. In this respect, the InAs/AlGaAs system offers new possibilities to overcome some of the problems encountered in InAs/GaAs and opens the path for QD-IBSC devices capable of achieving high efficiency at RT.

Nanoimprinted diffraction gratings for crystalline silicon solar cells: implementation, characterization and simulation

Light trapping is becoming of increasing importance in crystalline silicon solar cells as thinner wafers are used to reduce costs. In this work, we report on light trapping by rear-side diffraction gratings produced by nano-imprint lithography using interference lithography as the mastering technology. Gratings fabricated on crystalline silicon wafers are shown to provide significant absorption enhancements. Through a combination of optical measurement and simulation, it is shown that the crossed grating provides better absorption enhancement than the linear grating, and that the parasitic reflector absorption is reduced by planarizing the rear reflector, leading to an increase in the useful absorption in the silicon. Finally, electro-optical simulations are performed of solar cells employing the fabricated grating structures to estimate efficiency enhancement potential.

A numerical study into the influence of quantum dot size on the sub-bandgap interband photocurrent in intermediate band solar cells

A numerical study is presented of the sub-bandgap interband photon absorption in quantum dot intermediate band solar cells. Absorption coefficients and photocurrent densities are calculated for the valence band to intermediate band transitions using a four-band k · p method. It is found that reducing the quantum dot width in the plane perpendicular to the growth direction increases the photocurrent from the valence band to the intermediate-band ground state if the fractional surface coverage of quantum dots is conserved. This provides a path to increase the sub-bandgap photocurrent in intermediate band solar cells.