Light Management And Interface Control Of Silicon Based Heterojunction Photovoltaic Devices

Photovoltaics have attracted tremendous attention due to the rising concerns of energy and environment issues.Traditional silicon(Si)-based p-n junction solar cells with a high power conversion efficiency(PCE)up to 25%have been demonstrated for solar energy conversion.However,the fabrication of p-n or p-i-n junctions for efficient extraction of light-generated charge carriers usually involved high-temperature diffusion processes.The complex fabrication process led to the high cost of the photovoltaic devices.Therefore,there was growing interest in developing Si-based photovoltaic devices with lower cost and simpler processing techniques.In comparison with traditional Si p-n junction solar cells,graphene/Si(Gr/Si)heterojunction solar cells could not only inherit the advantage of Si p-n junction solar cells with high-efficiency and high stability,but also wipe off the complicated procedures of the high-temperature diffusion process.The major process for the fabrication of Gr/Si solar cells could be accomplished via a solution transfer process at room temperature in air.Therefore,Gr/Si heterojunction solar cells have been emerged as promising candidates for high-performance and cost-effective photovoltaic applications.In this dissertation,we improved the Gr/Si heterojunction solar cells from the following two aspects:(1)enhancing PCE of Gr/Si solar cells;(2)fabricating color Gr/Si solar cells and patterned Gr/Si solar cells.The main content is as follows:1.As to Gr/Si solar cells,the low utilization of the solar energy,resulting from the high reflectivity in terms of the huge variation of refractive index at air and Gr/Si interface,was a key limitation of the device performance.In this section,we transferred PMMA support layer as anti-reflection coating of Gr/Si solar cells to reduce the reflection,along with the multi-layer graphene by one step.Otherwise,the one-step transferred graphene could retain high quality by not removing the PMMA top layer,obviously enhancing the transport of photon-generated carriers.By this means,the Gr/Si heterojunction solar cell with high light harvest and high efficiency could be obtained,indicating the great potential of the polymer antireflection for high-efficiency Gr/Si heterojunction solar cells.2.As to Gr/Si solar cells,surface recombination of Si remained a key issue that limited the performance of Gr/Si heterojunction solar cells.To suppress the recombination,fabricating of a p-type inversion layer on n-Si surface through surface charge transfer doping(SCTD)method.A thin layer of the high-work function transition metal oxide(MoO3)was introduced as the surface dopant on Si wafer and a p-type inversion layer was formed by spontaneous transfer of holes from MoO3 layer to Si.The p-type inversion layer resulted in the energy band bending near the Si surface,thus effectively improving the effective barrier height and suppressing the recombination of photo-generated carriers.With an optimized thickness of MoO3 layer,a high PCE of 12.4%was achieved,manifesting the great potential of the SCTD strategy for high-efficiency Gr/Si heterojunction solar cells.3.To expand the applications of photovoltaics beyond rooftops and solar farms,aesthetically pleasing solar modules such as colored modules could find applications in building-integrated photovoltaic(BIPV)systems which formed part of the building fabric as well as generate electrical energy simultaneously.In Section IV,we introduced the fabrication of multi-color Gr/Si heterojunction solar cells by using double-layer(DL)-MgF2/ZnS stack as the anti-reflection layer.The use of DL-MgF2/ZnS rendered parasitic reflection in VIS region,yielding a sharp reflection peak with high-purity structural color.Meanwhile,the resultant multi-color Gr/Si solar cells exhibited high color purity as well as excellent device performance.Given the high efficiency and high color purity,the multi-color Gr/Si heterojunction solar cells would have important applications in diverse fields,such as in distributed generation solar projects.4.To depict the complicated figures on Gr/Si solar cells beyond the simple color-block graph realizing by DL-MgF2/ZnS,a new patterned Si wafer was introduced in Section V.The patterned Si wafer contained different size of Si inverted pyramid micro-structures.By utilizing different reflectivity of different micro-structure size,all 4-bit grayscale images could be sculptured on Si wafer by the process of photolithography-reactive ion etching(RIE)-alkali etching.In addition,the process also could be simplifed by laser etching-alkali etching,which largely decrease the proccss cost in mass production.Otherwise,DL-MgF2/ZnS could combine with micro-structures to prepare colorful image on high-performance Si solar cells.From the above,the patterned Si solar cells would have important applications in diverse fields,such as in BIPV.