The present invention concerns methods for producing photovoltaic material and a device able to exploit high energy photons. The photovoltaic material is obtained from a conventional photovoltaic material having a top surface intended to be exposed to photonic radiation, having a built-in P-N junction delimiting an emitter part and a base part and comprising at least one area or region specifically designed, treated or adapted to absorb high energy or energetic photons, located adjacent or near at least one hetero-interface. According to the invention, this material is subjected to treatments resulting in the formation of at least one semiconductor based metamaterial field or region being created, as a transitional region of the or a hetero-interface, in an area located continuous or proximate to the or an absorption area or region for the energetic photons of the photonic radiation impacting said photovoltaic material.

Methods for producing photovoltaic material and device able to exploit high energy photons

Multi-interface solar cells exhibit a dramatically low UV- and blue-spectrum photovoltaic performance independently of the electronic quality of their surface. A paradox can even be observed, the better the electronic passivation the poorer the conversion efficiency. The effect can be explained by solar light-induced opacity, which reduces considerably or even totally the photon penetration, into the device bulk. This opacity results from a feedback occasioned by the free-carrier absorption: better surface passivation, higher free-carrier density, stronger surface dead zone absorptance. Particularly, the total energy of the incident short wavelength beam can be absorbed in front of a carrier collection limit buried in the emitter. This limit acts simultaneously on the electronic performance, blocking free-carriers, and on the optical performance, being at the origin of an enhancement of the absorptance. As a consequence, a thin surface stratum dominates the optical functions of MIND cells through the free-carrier gas confined inside it. The main characterization methods used were reflectivity and spectral response with a varying intensity incident beam. The investigation allows modification of the free-carrier confinement using different device architectures. The results demonstrate the domination of the free-carrier optical functions on the multi-interface cell conversion

Solar Light-Induced Opacity of Mind Cells

One of the most challenging topics of today's research and development concerns efficiency of light-to-electricity
conversion, preferably on Si-derived devices. The best of the possible/imaginable solutions has to allow overcoming the
indirect Si bandgap constraints. This aim becomes realizable by transforming the hard photon-matter interaction into a
soft photon-electron-electron interaction with additional new low-energy mechanisms to allow a multistage conversion
cycle. Such effects have been observed by us within new Si-derived metamaterials obtained by multiple transformations,
leading to a nanoscale Si-layered system. In such systems, a giant photoconversion could be observed for the first time
due to hot electron interactions with active interfaces and conditioned crystalline defects transformed into Si
metamaterial units, called tectons. Today it seems to be the best way to overcome conversion shortages of the bulk, thinfilm
or any other Si-based devices. We present in this work a background and three experimental demonstrations of giant
photoconversion.

A silicon-based metamaterial for light-to-electricity conversion

The nowadays quite indispensable enhancement of PV conversion efficiency cannot be obtained without new mechanisms. The most useful of these mechanisms has to appear in the front face of the device, i.e. in the emitter, so as to allow exploitation of the energetic photons of the solar spectrum. Such an improvement can be realized through a multistage PV conversion starting by primary generation (photon absorption) followed by secondary generations (hot carrier collisions with low-energy generation centers). This cascade-like process is possible, for example, in multiinterface devices containing several emitter strata. Some of these strata assume the role of primary free-carrier generation while others do the secondary free-carrier generation. We report here investigations of new mechanisms based on I(V) curves measured on test samples with different multiinterface architectures, electronic passivations, front grids, collecting electrodes and so on. The measurements have been performed under a variable intensity incident light beam conserving always its spectral (solar) composition, except for analogous measurement cycles without a UV component. The same beam intensities with a filtered UV component complemented these investigations. The measurements have been compared with those of a weak excitation from a typical halogen lamp (relatively stable flux without a UV component). The test structures show a clear improvement of the PV conversion in the UV range induced by impact ionization within the superficial nanostratum.

PV conversion of energetic photons of the solar spectrum: Effect of the UV component

The nowadays quite indispensable enhancement of PV conversion efficiency cannot be obtained without new
mechanisms. The most useful of these mechanisms have to appear in the front face of the device, i.e. in the emitter, so
as to allow exploitation of the energetic photons of the solar spectrum. Such an improvement can be realized through a
multistage PV conversion starting by primary generation (photon absorption) followed by secondary generations (hot
carrier collisions with low-energy generation centers). This cascade-like process is possible, for example, in
multiinterface devices containing several emitter strata. Some of these strata assume the primary free-carrier generation
while others do the secondary free-carrier generation.
In this work we report investigations of new mechanisms based on I(V) curves measured on test samples with
different multiinterface architectures, electronic passivations, front grids, collecting electrodes and so on. The
measurements have been performed under a variable intensity incident light beam conserving always its spectral (solar)
composition, except for analogous measurement cycle without a UV component. The same beam intensities with a
filtered UV component complemented these investigations. The measurements have been compared with those of a
weak excitation from a typical halogen lamp (relatively stable flux without a UV component). The test structures show
a clear improvement of the PV conversion in the UV range induced by impact ionization within the superficial
nanostratum.

PV conversion of energetic photons of the solar spectrum

A continuous amorphized a-Si nanostructure with sharp a-Si/c-Si heterointerfaces is inserted in a c-Si wafer by medium-energy P ion implantation followed by a thermal treatment at 500 °C. New photovoltaic phenomena in the mesoscopically transformed material are expected and were presented recently. The simulation of the band structure taking into account the differences between the two Si phases of the MIND (multi-interface novel device) model solar cell indicates the presence of a high barrier blocking the minority carriers, which are photogenerated in the bulk superficial region of the wafer. Consequently, the collection efficiency (i.e. collected carriers versus penetrating photons) is deteriorated in UV. Collection efficiency and EBIC measurements were used to give a theoretical and experimental analysis of the causes and the consequences of such a deterioration on the photocurrent.

Minority carrier blockade in MIND model solar cells

A continuous amorphized a-Si substructure with sharp a-Si/c-Si heterointerfaces has been inserted in a c-Si wafer by P ion implantation followed by a thermal treatment at 500/spl deg/C. A simulation of the band structure taking into account the differences between the two Si phases of the MIND (multi-interface novel device) model solar cell indicates the presence of a high potential barrier blocking the minority carriers generated in the superficial region of the wafer. Consequently, the collection efficiency is deteriorated in the UV. A theoretical and experimental analysis of the causes and results of such a deterioration on the photocurrent is presented.

Electronic transport in mind model solar cells: collection efficiency in the presence of a-Si/c-Si heterointerfaces

Solar cells based on classical Si materials have reached their photovoltaic conversion limits. However, because Si conserves its attraction for any photovoltaic application, the problem of Si performance has to be addressed in another way : is there any method that might allow Si exploitation for 3/sup rd/ generation solar cells. Is there any possible transformation of this material from the optoelectronic viewpoint. The reply is, yes, it can be done by a nanoscale engineering. The innovative idea has to answer the question of how to combine simultaneously progress in material knowledge with latest device concepts. The present multiinterface design is based on a nanoscale Si-layered system. Electron microscopy and RBS studies show that the two Si phases (amorphous and crystalline) are separated by sharp interfaces and thin c-Si transition zones with new crystalline and optoelectronic properties. Active substructures and active interfaces can be controlled by bandgap, defect and stress engineering and allow considerable extesion of existing limits. The results obtained indicate an interesting path towards a practical realization of the postulated PV improvements on modified materials.

http://ieeexplore.ieee.org/iel5/8468/26685/01190560.pdf

Nanoscale Si-layered systems for 3/sup rd/ generation photovoltaics

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Papers

Minority carrier blockade in MIND model solar cells

Electronic transport in mind model solar cells: collection efficiency in the presence of a-Si/c-Si heterointerfaces

Nanoscale Si-layered systems for 3/sup rd/ generation photovoltaics