Wanwu Guo

Improved passivation effect at the amorphous/crystalline silicon interface due to ultrathin SiOx layers pre-formed in chemical solutions

To improve the passivation effect at a-Si:H/c-Si interface in heterojunction (HJ) solar cells, ultrathin SiOx layers with a thickness of approximately 2 nm were pre-formed on c-Si surfaces in chemical solutions. It was demonstrated that the SiOx layers pre-formed in hot de-ionized water and hydrochloric acid solutions improve effective carrier lifetime, and it is further enhanced through a post annealing process. When the thin SiOx layers were applied to HJ solar cells, increase in both Voc and Jsc was achieved, implying the improved interface quality for these HJ solar cells, as compared with the reference without the SiOx layer.

Interface processing of amorphous-crystalline silicon heterojunction prior to the formation of amorphous-to-nanocrystalline transition phase

Two effective interface optimizations, i.e., hydrogen-plasma treatment (HPT) and oxygen incorporation (OIn), have been investigated to optimize the interface structure and promote the formation of hydrogenated amorphous silicon (a-Si:H) with an amorphous-to-nanocrystalline transition phase based on the surface passivation of crystalline silicon. Both multistep HPT and intentional OIn are capable of producing a compact interface and a transition phase in the case of initial a-Si:H films deposited under different conditions. An appropriate multistep HPT is responsible for modifying the a-Si:H into a transition phase, whereas an effective OIn forms a disordered and more compact interface, as well as a crystallite-isolated and more transparent structure.

Investigation of positive roles of hydrogen plasma treatment for interface passivation based on silicon heterojunction solar cells

The positive roles of H2-plasma treatment (HPT) have been investigated by using different treatment procedures in view of the distinctly improved passivation performance of amorphous-crystalline silicon heterojunctions (SHJs). It has been found that a hydrogenated amorphous silicon thin film and crystalline silicon (a-Si:H/c-Si) interface with a high stretching mode (HSM) is detrimental to passivation. A moderate pre-HPT introduces atomic H, which plays an effective tuning role in decreasing the interfacial HSM; unfortunately, an epitaxial layer is formed. Further improvement in passivation can be achieved in terms of increasing the HSM of a-Si:H film treated by appropriate post-HPT based on the a-Si:H thickness. The minority carrier lifetime of crystalline wafers can be improved by treated films containing a certain quantity of crystallites. The microstructure factor R and the maximum intensity of the dielectric function e 2max have been found to be critical microstructure parameters that describe high-quality a-Si:H passivation layers, which are associated with the amorphous-to-microcrystalline transition phase induced by multi-step HPT. Finally, the open circuit voltage and conversion efficiency of the SHJ solar cell can be improved by implementing an effective HPT process.

Underdense a-Si:H film capped by a dense film as the passivation layer of a silicon heterojunction solar cell

Underdense hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition was used as a passivation layer in silicon heterojunction (SHJ) solar cells. By reducing the thickness of the underdense a-Si:H passivation layer from 15 nm to 5 nm, the open circuit voltage (Voc) of the corresponding SHJ solar cell increased significantly from 724.3 mV to 738.6 mV. For comparison, a widely used transition-zone a-Si:H passivation layer was also examined, but reducing its thickness from 15 nm to 5 nm resulted in a continuous Voc reduction, from 724.1 mV to 704.3 mV. The highest efficiency was achieved using a 5-nm-thick underdense a-Si:H passivation layer. We propose that this advantageous property of underdense a-Si:H reflects its microstructural characteristics. While the porosity of a-Si:H layer enables H penetration into the amorphous network and the a-Si:H/c-Si interface, a high degree of disorder inhibits the formation of the epitaxial layer at the a-Si:H/c-Si interface during pos...

Defining a parameter of plasma-enhanced CVD to characterize the effect of silicon-surface passivation in heterojunction solar cells

The hydrogen content (CH) and microstructure of a hydrogenated amorphous-silicon (a-Si:H) layer fabricated by plasma-enhanced chemical vapor deposition (PECVD) were analyzed to determine the effect of surface passivation on crystalline silicon (c-Si). The ratio of radio-frequency power to gas pressure (Cpp in WPa−1) of the PECVD system is defined as a characterization parameter. It was found that CH and the passivation of a-Si:H layers were sensitively affected by Cpp. However, CH remained almost constant at the same Cpp even though the PECVD power and pressure were widely varied. We determined that an optimal region exists in the range of 0.75 < Cpp < 1.25, where a high implied open-circuit voltage of 732 mV was obtained from a passivated a-Si:H/c-Si/a-Si:H structure, indicating that Cpp was a useful parameter for characterizing the surface passivation effect of a a-Si:H layer on c-Si.

Microstructure of hydrogenated amorphous silicon layers studied by Spectroscopic Ellipsometry for the surface passivation in heterojunction solar cells

The defect scatter interval (S_t) of a-Si:H/c-Si interface and void concentration (C_v) of a-Si:H were analyzed by Spectroscopic Ellipsometry (S_E). The passivation performance of a-Si:H layers in SHJ solar cells was not only affected by the conductivity, but more importantly, it was strongly governed by the hydrogen content (C_H) in a-Si:H layers. In addition, the microstructure deduced from SE was in perfect accordance with the results revealed by TEM technique. Then, an implied open circuit voltage (V_oc,i_m) of 732mV was obtained when S_t was about 7fs and CH around 7 at.%.

Impact of surface treatments on the passivation effect for n-type crystalline silicon in heterojunction solar cells

In a-Si:H/c-Si heterojunction solar cells, we developed two methods to improve the passivation effect at the a-Si:H/c-Si interface. (i) Chemical polish, etched c-Si wafer with HF and HNO3 mixtures to smooth the peaks and valleys of pyramids after the texturization with alkali; (ii) SiOx interlayer, formed ultrathin SiOx layers with a thickness of about 2 nm on c-Si surface after the chemical polish and standard RCA cleaning. This thin layer was formed by chemical oxidization in various hot solutions. The results demonstrated that these two methods improved the quality of a-Si:H/c-Si interface and the effective carrier lifetime. When they were applied to heterojunction solar cells, gains in Voc and Jsc were successfully achieved. The simplicity of these methods suggested the possible applications to the industry production.