Dopant-Free Hydrogenated Amorphous Silicon Thin-Film Solar Cells Using Molybdenum Oxide and Lithium Fluoride

Abstract

Toxic doping gases are usually used to produce hydrogenated amorphous silicon (a-Si:H) layers in thin-film solar cells (TFSCs). Hence, an alternative structure that avoids the use of toxic gases is desirable. In this work, we replaced both the p-type-a-Si:H and n-type-a-Si:H layers simultaneously in a normal TFSC to form a structure that is dopant-free. Molybdenum oxide (MoO3) and lithium fluoride were used as the p-type and n-type layers, respectively. The effects of the deposition method and the thickness of the MoO3 layer on the device performance were investigated. The power-conversion efficiency of the optimized hybrid solar cell reached a maximum of 7.08%, which is remarkable considering the novel structure of the dopant-free devices. The light stability of the devices with and without MoO3 was also compared: the light stability of the device with MoO3 was found to be much better than that of the device without MoO3 and with p-i-n Si layers. This was ascribed to the insignificant number of defect sites generated by the nondoping elements, which led to a less contaminated, more compact, and smoother oxide surface, resulting in an increase in the electron lifetime and improved light stability. This work opens up a new direction toward the development of a truly dopant-free device that does not involve the use of toxic gases during fabrication and provides the potential for further enhancement of the efficiency of future dopant-free solar cells.

Replacement of n-type layers with a non-toxic APTES interfacial layer to improve the performance of amorphous Si thin-film solar cells

“…This could be attributed to the low thickness and density of the LiF layer w/o APTES, which might have resulted in a poor thin-lm coverage. 5,9 The addition of APTES with LiF reduced the leakage current and improved the V oc of the devices signicantly, which indicates that the addition of APTES improved the LiF layer density by providing adequate thin-lm coverage and suppressed the charge recombination. Moreover, in terms of device stability enhancement Wang et al 30 presented that the deposition of a thin ($5 nm) APTES SAM could efficiently inhibit the diffusion of metals into adjacent dielectrics and could simultaneously improve the interfacial adhesion.…”

“…The photovoltaic technology, which converts solar energy into electricity has been extensively studied, 1,2 leading to the development of hydrogenated amorphous Si (a-Si:H) thin lm solar cells (TFSCs). 3,4 They are favorable due to their simple and inexpensive fabrication, possibility of deposition on exible/bendable substrates 5,6 and the highly transparent nature has been recently exploited for "building integrated photovoltaic (BIPV)" systems. 7,8 However, the greatest challenge in further development of high performance a-Si:H TFSCs is without (w/o) the involvement of hazardous/toxic doping gases.…”

“…7,8 However, the greatest challenge in further development of high performance a-Si:H TFSCs is without (w/o) the involvement of hazardous/toxic doping gases. 5,6,9,10 The fabrication of such devices should be carried out under extremely safe environments. In addition, the use of hazardous/toxic doping gases causes device instabilities because of carrier trapping and the formation of Si dangling bonds.…”

“…In addition, the use of hazardous/toxic doping gases causes device instabilities because of carrier trapping and the formation of Si dangling bonds. 5,6 Many alternate processes have been investigated and toxic-free layers have been utilized using various processes in the fabrication of a-Si:H TFSCs. For example, Jung et al 9 demonstrated the deposition of a V 2 O 5Àx window layer using RF magnetron sputtering as an efficient replacement of ptype silicon layer resulting in a power conversion efficiency (PCE) of about 7.04%.…”

“…For example, Jung et al 9 demonstrated the deposition of a V 2 O 5Àx window layer using RF magnetron sputtering as an efficient replacement of ptype silicon layer resulting in a power conversion efficiency (PCE) of about 7.04%. Ryu et al 5 presented the scheme for replacement of both p-and n-type layers with MoO 3 and LiF, respectively processed with thermal deposition achieving a PCE of $7.08%. Furthermore, in crystalline silicon solar cells, Battaglia et al 11 substituted the p-type Si layer with MoO 3 and achieved a PCE of 14% while, Bullock et al 12 presented a maximum efficiency of 20% by incorporating dopant-free alkali metal uorides and metal oxides for electron and hole selective carriers, respectively.…”