The Rear Contact Of Hybrid Solar Cells Based On Si/PEDOT:PSS

This thesis is based on silicon(Si)/ poly(3,4-ethylenedioxythiophene)/ poly(styrenesulfonate)(PEDOT:PSS)hybrid solar cell. Here, the effect of device performance influenced by the Si morphology and rear contact is investigated. A high doping technique has been widely used for record-efficiency crystalline Si solar cells to minimize the series resistance losses and to form a back surface field. However, it requires high temperatures(up to 1000 oC) and involves toxic gases, which may not be compatible for hybrid organic-silicon solar cells.The device based on Si/PEDOT:PSS hybrid solar cell with high performance is fabricated by controlled nanostructured Si morphology and improved rear contact. Current versus voltage, capacitance versus voltage, scanning Kelvin probe microscope(SKPM), ultraviolet photoelectron spectroscopy(UPS),transient photovoltage(TPV) and external quantum efficiency(EQE) etc. are used to analyse the work principle of the devices. The main work includes:1.The three-dimensional nanostructured Si, which is composed of short silicon nanowire array and pyramid,is further investigated. The nanostrucutred Si displays a lower reflectance below to 10% than pyramid,at the same time,exhibits less trap states than silicon nanowires,leading to a higher power conversion efficiency(PCE) than those devices based on Si nanowires array and pyramid.2. We report an ultra-thin layer of lithium fluoride(LiF) between Si and aluminum(Al) in a Si/PEDOT:PSS hybrid solar cell which results in a PCE of 11.09%. The insertion of a thin layer of LiF improves the contact between Si and Al, which decreases contact resistance from 5.4×10-1 Ω·cm2 to 2.6×10-2 Ω·cm2. Furthermore, electron transport efficiency from Si to Al is improved and charge carrier recombination is suppressed. As a result, the short circuit current density, the open circuit voltage and the fill factor are all improved in the presence of the LiF layer.3. A wide band gap organic semiconductor 8-hydroxyquinolinolato-lithium(Liq) is inserted between the rear side of Si and Al to explore the rear contact effect on the device performance. It is found that the Schottky barrier height and built-in voltage of the hybrid solar cell are both enhanced by improving the rear contact as well as reducing the charge recombination. A negative surface dipole region is built up owing to reaction of Liq and Al, which is favorable for electron collection as well as hole blocking. A remarkable open-circuit voltage of 0.609 V is obtained from this Schottky junction device, which is comparable with the Si photovoltaics based on diffused p–n junctions. With an optimized thickness of ~2 nm Liq, a high PCE of 12.2% is achieved, which results in an improvement of 29.5% in PCE compared with that of 9.4% for the device based on pristine Al.4. A high PCE of 13.7% with a device area of 0.8 cm2 has been achieved for organic-nanostructured Si hybrid solar cells by inserting a cesium carbonate(Cs2CO3) layer between Si and Al, which is realized by a solution process under low-temperature annealing(<150 ℃). The insertion of Cs2CO3 not only decreases the contact resistance, but also generates a built-in electric field on the rear electrode. The recombination rates are suppressed at the back surface due to the deflection of minority carriers.