The high efficiency CuInGaSe2 (CIGS) based thin film solar cells have been demonstrated by various groups across the globe. At present, the highest efficiencies are obtained using CdS buffer layers deposited by chemical bath deposition with a record efficiency of 20.9%. However, because of both environmental reasons and the fact that the CdS layer with a band gap of about 2.4-2.5 eV limits the transmittance of the short wavelength light into the absorber, development of a wide-band gap Cd-free buffer layer is currently the most pivotal topic in CIGS thin film PV technology. This thesis focuses on three parts: (i) to establish electrodeposited CuInSe2(CIS) as the absorber for the device fabrication, and (ii) to investigate the charge transport at absorber /buffer interface of the chemical bath deposited (CBD)-Zn(O,S) buffer layer, then last (iii) to develop low temperature solution based ZnSnO as alternative Cd-free buffer for CIS solar cells. Proof-of-concept devices of the electrodeposited CIS with these buffer layers yield power conversion efficiency (PCE) of ~4% for Zn(O,S)/CIS devices and 1.53% for ZnSnO/CIS devices. The first part of the thesis explores a solution based method to fabricate the CIS absorber. Electrodeposition was investigated for this purpose. Two deposition approaches (i) one-step electrodeposition and (ii) stacked elemental layer (SEL) deposition were compared. It was difficult for one step electrodeposition to achieve Cu-poor CIS as a post-selenization etching step using the toxic KCN is required. Moreover, the selenized CIS was porous with small grain size. On the other hand, SEL approach can easily control the Cu/In ratio by controlling the thickness of each layer. Selenization process, which is the most important for the SEL approach, was studied in details. And after a series of analysis and optimization, CIS with Cu/In ratio of 0.85 achieved 3.25% PCE with CdS buffer layer.
In this paper, graphene oxide is used instead of poly (3,4-ethylenedioxythiophene): poly (phenylethylenesulfonic acid) PEDOT:PSS as the hole injection layer of quantum dot light-emitting diodes. The experimental results prove that graphene oxide irradiated with ultraviolet for an appropriate time can improve the performance of the device. Compared with traditional devices, the luminance is increased by 1.9 times and current efficiency of the device is increased 2.4 times. In addition, the turn-on voltage was reduced from 2.8 V to 2.4 V. The improvement of these photoelectric properties is mainly due to the fact that graphene oxide after ultraviolet irradiation can form a good energy level structure with the anode and the hole transport layer, which is more conducive to hole injection.
Effective p-type doping is essential to enhance hole transport and balance electron-hole injection in quantum dot light-emitting diodes (QLEDs). Here, an oligothiophene material is adopted as a p-type dopant in the hole-transport layer, considering its cruciform cross-center structure, precise molecular weight, and high purity. Compared with the dopant-free counterpart, hole transport capability at the optimal doping level exhibits a significant improvement, producing a boosted external quantum efficiency (EQE) and luminance up to 20.8%, 213 439 cd m -2 , respectively, among the highest reported on the red-light emission. The work indicates the potential applications of oligothiophene material in red light-emitting devices. IntroductionQuantum dot light-emitting diodes (QLEDs), with unique physical properties such as tunable emission wavelength, light stability, and high color purity, have gained extensive research interests in the next generation of display and lighting devices in recent years. [1][2][3][4][5][6][7][8] Generally, the QLED device has a hybrid structure consisting of an organic hole-transport layer (HTL) and a metal oxide-based electron-transporting layer (ETL). However, compared with electron mobility (10 cm 2 V -1 s -1 ) in the ETL, the relatively low hole mobility (10 -3 cm 2 V -1 s -1 ) in the organic materials like poly(9,9-dioctylfluorene-co-N-(4-(3methylpropyl))diphenylamine) (TFB) and poly (9-vinylcarbazole)
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