The study of silicon/poly (3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hybrid solar cells

Abstract

Polymer/silicon hybrid solar cells have been introduced to tackle the expensive fabrication problem of conventional p-n junction silicon solar cells. The idea arisen in the polymer/silicon hybrid solar cells is that by replacing the high temperature processed silicon doping layer (by diffusion) to a low temperature processed polymeric layer. By eliminating the diffusion step of the p-n junction, it has been estimated that energy input into the solar cell production process could be 35% lower. The polymer/silicon hybrid solar cells have the advantages of low equipment cost and low temperature fabrication to cut down the expensive cost in the conventional silicon solar cells. However, organic materials are less efficient than inorganic materials. The best power conversion efficiency of polymer/silicon hybrid solar cells is lower than that of conventional p-n junction silicon solar cells. Some factors have been hindering the performance of the polymer/silicon hybrid solar cells. Therefore, the aim of this work attempts to explore some methods to enhance the power conversion efficiency of the polymer/silicon hybrid solar cells. In this thesis, (i) a surface texturization structure was adopted for the silicon layer and (ii) an acid treatment was applied for the polymer layer. These approaches may be of importance in achieving better carrier separation and collection in the hybrid solar cells. Moreover, the efficiency enhancing methods are simple and low cost, that are favorable to be used without paying a huge extra cost. Firstly, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)(PEDOT:PSS) was used for the polymer layer in the hybrid solar cells because of its high work function and convenient solution processibility. The polymer/silicon hybrid solar cells with ITO/PEDOT:PSS/Silicon/GaIn architecture were fabricated. The silicon/PEDOT:PSS junctions were processed at 110 °C in air with a spin-coating and a wet etching process. Advanced equipment (high temperature and high vacuum) was not necessary. The elimination of the high temperature manufacturing step proposed a way to achieve inexpensive solar cells. Furthermore, with the aim of improving the charge collection at the solar junction, the silicon nanowires (SiNWs) array was utilized to lower the optical reflection from the shiny planar silicon surface. The power conversion efficiency (PCE) of the solar cell was improved from 1.1% to 6.67% by converting the planar silicon surface into the 390 nm height SiNWs array at the solar junction by the metal assisted electroless etching method. The optimal length of SiNWs in the experiments was around 390 nm. There was an optimal length for the SiNWs array because the polymeric PEDOT:PSS aggregated at the top portion of the long SiNWs array but could not infiltrate into the interspacing between the long SiNWs array. In the long SiNWs solar cells, poor carrier collection as well as high recombination rate suppressed the PCE to improve further. Secondly, the low conductivity of pristine PEDOT:PSS film has been a limitation for high efficiency devices. The low conductivity of PEDOT:PSS is due to the insulating PSS matrix attracted between the conductive PEDOT grains. Hence, the conductive PEDOT grains are separated by the PSS and become less interlinking. It was reported that the positive charged hydrogen atoms in the formic acid could neutralize the negative charged PSS chains. The absence of coulombic attraction between the positively charged PEDOT chains and neutral PSSH chains resulted in the phase segregation between PEDOT and PSS and hence the conductivity of PEDOT:PSS film was enhanced after the formic acid treatment. Therefore, the formic acid treated PEDOT:PSS film was used to improve the PCE of silicon nanowires/PEDOT:PSS hybrid solar cells. The formic acid treatment suppressed the reverse saturation current and reduced the series resistance of the hybrid solar cells. Finally, a 16.4% PCE enhancement was recorded after the formic acid treatment. Thirdly, this study also showed that thin silicon sheets were applicable to fabricate flexible hybrid silicon/PEDOT:PSS hybrid solar cells. Thin silicon sheets not only provided higher bending flexibility of the solar cells but also allowed less use of silicon material. The flexible 15 m silicon sheet/PEDOT:PSS hybrid solar cell yielded a PCE of 5.56%. We employed a bending test apparatus to study their bending stability. Bending cycle test showed a good recovery of the original efficiency after 5000 cycles under a bending radius of 7 mm that the estimate bending stress in the silicon sheet was around 217 MPa. The ability of bending repeatedly without varying much in the power conversion efficiency of the solar cells hold a significant importance for the development in the lightweight and rollable applications. Last but not the least, moisture is always a challenge for most of the organic materials. With the help of an environmental chamber to control the humidity level, lower relative humidity would cause slower device degradation. By comparing the devices' overall performances with the individual study of the ITO, silicon nanostructure and PEDOT:PSS layer, we confirmed that the major causes of the PCE drop in the degraded devices were due to the decrease of the PEDOT:PSS conductivity and the increase of the ITO interface resistance. The results suggested that humidity is a critical degradation factor in the hybrid silicon/PEDOT:PSS solar cells. We also demonstrated that the efficiency of the device could be almost recovered by re-depositing the fresh PEDOT:PSS layer onto a fresh ITO and recycling the silicon in the degraded device.