Photon emission induced by elastic exciton--carrier scattering in semiconductor quantum wells

We present a study of the elastic exciton--electron ($X-e^-$) and exciton--hole ($X-h$) scattering processes in semiconductor quantum wells, including fermion exchange effects. The balance between the exciton and the free carrier populations within the electron-hole plasma is discussed in terms of ionization degree in the nondegenerate regime. Assuming a two-dimensional Coulomb potential statically screened by the free carrier gas, we apply the variable phase method to obtain the excitonic wavefunctions, which we use to calculate the 1$s$ exciton--free carrier matrix elements that describe the scattering of excitons into the light cone where they can radiatively recombine. The photon emission rates due to the carrier-assisted exciton recombination in semiconductor quantum-wells (QWs) at room temperature and in a low density regime are obtained from Fermi's golden rule, and studied for mid-gap and wide-gap materials. The quantitative comparison of the direct and exchange terms of the scattering matrix elements shows that fermion exchange is the dominant mechanism of the exciton--carrier scattering process. This is confirmed by our analysis of the rates of photon emission induced by electron-assisted and hole-assisted exciton recombinations.Comment: Thoroughly revised version of previous work. Weak and incorrect assumptions have been removed from the paper, and its scope has evolved: see abstract. This is the final version, i.e. as accepted for publication in the European Physical Journal

Understanding of the Influence of the Surface Defectivity on Silicon Heterojunction Cell Performance

International audienceThe fabrication process of silicon heterojunction (SHJ) solar cells can induce locally depassivated regions (so-called defectivity) because of transportation steps (contact with belts, trays, etc) or simply the environment (presence of particles embedded within surface thin films). This surface passivation spatial heterogeneity is gaining interest as it may hinder the SHJ efficiency improvements allowed by incremental process steps optimizations. An experimentally-supported simulation study is proposed and applied on full size M2 SHJ cells in order to understand how the local a-Si:H/c-Si interface passivation loss impacts the overall cell performance. A simulation framework (developed on ATLAS Silvaco) was first validated through comparisons with experimental results. This step allowed then to use simulations results further to explore and understand the physics behind the defectivity-induced efficiency loss. The cell performance drop due to depassivated regions was attributed to a bias-dependent minority carrier recombination current flow towards the depassivated region, which is shown to affect mainly the Fill Factor

Influence of Surface Defectivity on the Performances of Silicon Heterojunction Solar Cells

International audienc

Influence of cell edges on the performance of silicon heterojunction solar cells

International audienceFull size silicon heterojunction solar cells reach conversion efficiencies above 25%. However, photoluminescence pictures of such cells (full or cut) reveal a significant recombination activity at the cell edges. Therefore, mitigating recombination at the edges can in principle represent an interesting path to unlock higher cell efficiencies. This challenge is all the more important for cells with a high perimeter/area ratio, as achieved through the cutting of full size cells. For such technologies, the edges resulting from cutting are cleaved while the remaining edges typically feature a gap where TCO is missing to avoid front to back short-circuit. In this paper, we specify the physical mechanisms involved in the edge-induced performance losses for SHJ cells. In light of these results, we provide guidelines for the mitigation of such losses at the full-size and cut cells scale for M6 to M12 sizes such as the reduction of the TCO-free region and the c-Si bulk resistivity. Having a closer look at cut cells, we calculate the cell performance as a function of its size (from half-to sixth-cell), the size of its mother cell (from M6 to M12) and the passivation quality of the cut-edges. Our results emphasize on the interest to develop suitable repassivation schemes for cut cells to improve or even surpass the efficiency of the mother cell

Insertion of a thin highly doped crystalline layer in silicon heterojunction solar cells: Simulation and perspectives towards a highly efficient cell concept

International audienc

Investigation of selective junctions using a newly developed tunnel current model for solar cell applications

International audienceCarrier transport through a tunnel barrier was modeled and implemented in the numerical device simulator AFORS-HET, which allows calculating the tunnel current between two semiconductor layers or between a metallic contact and a semiconductor layer. Rectangular barriers have been considered, for which an exact quantum solution for the transmission probability can be derived. The implementation in the simulation program was made by approximating the tunnel-interlayer as a “membrane” which modifies the current at the semiconductor/tunnel layer interface, without the need of inserting an additional insulator layer. It is demonstrated that this approximate description of the structure allows to simulate solar cells where tunneling across an insulator plays an important role. The model is then used to investigate new hole collector designs based on a tunnel oxide. It is shown, that the tunnel layer increases the selectivity of the contact for hole extraction, such that very high power conversion efficiencies can be reached

Influence of the undoped a Si H buffer layer on a Si H c Si heterojunctions from planar conductance and lifetime measurements

International audienceIn highly efficient amorphous silicon/crystalline silicon heterojunction (a-Si:H/c-Si) solar cells, the c-Si wafer is passivated by a nanometer-thin buffer layer, which is undoped amorphous silicon. Here, we report on the systematic measurement of the passivation quality (minority carrier effective lifetime) by photo-conductance decay and of the band bending in c-Si using the planar conductance technique. The thickness of the buffer layers is varied. An analytical model to calculate the band bending in c-Si is presented; it aids in understanding the influence of the buffer layer on the band bending. We find that when the buffer layer thickness increases the passivation quality increases and the band bending decreases. Therefore, we suggest that an optimum has to be found to reach good interface defect passivation and a high band bending

Modelling infrastructure along the value chain from materials to system performance

This paper reports on progress towards the development of an infrastructure to integrate existing modelling tools, and thus allow modelling of the entire solar cell value chain, from initial material parameters to the long term energy yield of complete PV systems. Currently, the infrastructure has been developed and successfully tested from the material to the system level, for three different cell technologies using 13 different programs across 7 European institutes. Input data can be either modelled or experimentally measured. Further work will focus on comparison of the simulation results with experimentally measured system data; defining standard interface protocols to simplify the information transfer; and predicting the yearly energy output for installed PV system