Fengyou Wang

Optimization of a silicon wafer texturing process by modifying the texturing temperature for heterojunction solar cell applications

In this work, a simple and effective method of two-step texturing temperature control is proposed to optimize the texturing process of commercial Cz-silicon wafers. The effective lifetime attained after passivation of the a-Si:H films is up to 1002.4 μs at a 1015 cm−3 injection level using this method, which is close to the bulk lifetime of the silicon wafers. This result shows over 50% enhancement compared with those achieved using constant texturing temperatures. Furthermore, a lower reflectivity after chemical polish treatment is achieved, and the scanning electron microscope (SEM) images demonstrate that pyramid nucleation is more homogeneous and compact. This method is found to remarkably improve the external quantum efficiency performance in the wave band of blue visible light and the fill factor of silicon heterojunction solar cells. This study provides a universal texturing process for heterojunction solar cell applications.

Increasing efficiency of hierarchical nanostructured heterojunction solar cells to 16.3% via controlling interface recombination

Silicon nanostructures show great promise for use in photovoltaic applications, owing to their enhanced light-harvesting characteristics, which allow them to form radial p–n junctions for effectively generating/separating photoexcited carriers. They are also low-cost materials and thus suitable for producing solar cells. In this study, hierarchical Si structures consisting of microscale pyramids and nanoscale pillars were fabricated through wet anisotropic texturing and reactive ion etching. Further, these substrates, which had a core–shell structure, were used to fabricate radial heterojunction Si solar cells through interface engineering with tetramethylammonium hydroxide. The substrates were pretreated with a hydrogen plasma and subsequently subjected to amorphous Si thin-film passivation. This resulted in solar cells with a markedly higher broadband wavelength; however, the electrical properties of the cells almost remained unaffected. Further, the heterojunction solar cells, which had a hierarchical nanostructure and were fabricated using as-cut Czochralski n-type Si substrates, exhibited an efficiency of 16.3%, which is the highest ever reported for such cells.

Role of hydrogen plasma pretreatment in improving passivation of the silicon surface for solar cells applications.

We have investigated the role of hydrogen plasma pretreatment in promoting silicon surface passivation, in particular examining its effects on modifying the microstructure of the subsequently deposited thin hydrogenated amorphous silicon (a-Si:H) passivation film. We demonstrate that pretreating the silicon surface with hydrogen plasma for 40 s improves the homogeneity and compactness of the a-Si:H film by enhancing precursor diffusion and thus increasing the minority carrier lifetime (τ(eff)). However, excessive pretreatment also increases the density of dangling bond defects on the surface due to etching effects of the hydrogen plasma. By varying the duration of hydrogen plasma pretreatment in fabricating silicon heterojunction solar cells based on textured substrates, we also demonstrate that, although the performance of the solar cells shows a similar tendency to that of the τ(eff) on polished wafers, the optimal duration is prolonged owing to the differences in the surface morphology of the substrates. These results suggest that the hydrogen plasma condition must be carefully regulated to achieve the optimal level of surface atomic hydrogen coverage and avoid the generation of defects on the silicon wafer.

Band alignment and enhancement of the interface properties for heterojunction solar cells by employing amorphous–nanocrystalline hierarchical emitter layers

Excellent electrical and passivation properties of p-type emitter layers are extremely important for high efficiency silicon heterojunction (SHJ) solar cells. The emitter layer should be embedded between the transparent conductive oxide (TCO) and the intrinsic amorphous silicon passivation layer, and thus the contact characteristics of p/TCO should also be carefully regulated to achieve better performance. Multifunctional p-type emitter layers combined with hydrogenated amorphous silicon/nanocrystalline silicon thin films were fabricated by the plasma enhanced chemical vapor deposition process, to meet the requirements of high conductivity and shortening of the depletion region between the p layer and TCO. Also, the p-a-Si:H film of the hybrid structure can serve as a buffer-layer to compensate the p/i band offset and a protective-layer for the intrinsic passivation films. Finally, applying this hybrid film as an emitter layer for SHJ solar cells based on low-cost commercial Cz silicon wafers, a conversion efficiency improvement of 2.6% in the solar cell photovoltaic performance has been achieved.

Perovskite/silicon-based heterojunction tandem solar cells with 14.8% conversion efficiency via adopting ultrathin Au contact*

A rising candidate for upgrading the performance of an established narrow-bandgap solar technology without adding much cost is to construct the tandem solar cells from a crystalline silicon bottom cell and a high open-circuit voltage top cell. Here, we present a four-terminal tandem solar cell architecture consisting of a self-filtered planar architecture perovskite top cell and a silicon heterojunction bottom cell. A transparent ultrathin gold electrode has been used in perovskite solar cells to achieve a semi-transparent device. The transparent ultrathin gold contact could provide a better electrical conductivity and optical reflectance-scattering to maintain the performance of the top cell compared with the traditional metal oxide contact. The four-terminal tandem solar cell yields an efficiency of 14.8%, with contributions of the top (8.98%) and the bottom cell (5.82%), respectively. We also point out that in terms of optical losses, the intermediate contact of self-filtered tandem architecture is the uppermost problem, which has been addressed in this communication, and the results show that reducing the parasitic light absorption and improving the long wavelength range transmittance without scarifying the electrical properties of the intermediate hole contact layer are the key issues towards further improving the efficiency of this architecture device.

Improved amorphous/crystalline silicon interface passivation with two-step intrinsic layers

A novel way of two-step intrinsic (i) layers growth process was described to improve amorphous/crystalline silicon (a-Si:H/c-Si) interface passivation. This effort was guided by the study of the relationship between the bulk photoelectrical properties of a-Si:H film using a variable H2-dilution gas flow ratio R = [H2/SiH4] and the passivation results of the amorphous/crystalline silicon interface. The results demonstrated that the a-Si:H/c-Si interface had more dominating influence on passivation than the bulk of a-Si:H films. However, when no epitaxial growth occurred on the a-Si:H/c-Si interface, the bulk quality of the a-Si:H film had a significant effect on passivation. The optimum two-step process was designed as follows: the initial stage of the i-layer was deposited at a lower R than the bulk to ensure that the interface remained within the amorphous phase and the second stage involved deposition using the optimum bulk a-Si:H film process to obtain the best quality of the passivated film. Although only a 5 nm thick passivated film was deposited on the polished Cz-Si wafers, the optimum effective lifetime was up to 1.7 ms. After annealing, the effective lifetime could be further increased to 2.5 ms and the corresponding implied Voc was up to 0.724 V.