Investigation of the surface passivation mechanism through an Ag-doped Al-rich film using a solution process

Abstract

Electronic recombination loss is an important issue for photovoltaic (PV) devices. While it can be reduced by using a passivating layer, most of the techniques used to prepare passivating layers are either not cost effective or not applicable for device applications. Previously, it was reported that a low cost sol-gel derived Al-rich zinc oxide (ZnO:Al) film serves as an effective passivating layer for p-type silicon but is not effective for n-type silicon. Herein, we studied the elemental composition of the film and the interfacial structure of ZnO:Al:Ag/n-Si using TEM, XPS, FTIR, and SIMS analyses. The XPS analysis revealed that Ag-rich zones randomly formed in the film near the ZnO:Al:Ag//n-Si interface, which induced a positive charge at the interface. The maximal value of the effective minority carrier lifetime (τeff ≈ 1581 μs) is obtained for a wafer using the ZnO:Al:Ag passivating layer with RAg/Zn = 2%. The corresponding limiting surface recombination velocity is ∼16 cm s(-1). The FTIR absorption area of Si-H bonds is used to calculate the hydrogen content in the film. The hydrogen content is increased with increasing Ag content up to RAg/Zn = 2% to a maximal value of 3.89 × 10(22) atoms per cm(3) from 3.03 × 10(22) atoms per cm(3) for RAg/Zn = 0%. The positive charge induced at the interface may cause band bending, which would produce an electric field that repels the minority charge carriers from the interface to the bulk of n-Si. Two basic phenomena, chemical passivation due to Si-H bonding and field effect passivation due to the charge induced at the interface, have been observed for effective passivation of the n-Si surface. An implied Voc of 688.1 mV is obtained at an illumination intensity of 1 sun.

Recent Advances in and New Perspectives on Crystalline Silicon Solar Cells with Carrier-Selective Passivation Contacts

“…The minority carrier lifetime reached 1581.2 µs, and V oc reached 688.1 mV. This passivation was attributed to the chemical passivation of the naturally formed SiO x during the preparation of ZnO:Al:Ag, and to the field effect passivation caused by charges at the ZnO/Si interface [46].…”

“…Alternative materials are expected to completely replace doped silicon layers to form the asymmetric carrier selective heterocontacts with c-Si wafers. These alternative materials, which usually are called carrier-selective materials or passivation contact materials, include transition metal oxides [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47], organic materials [48][49][50][51][52][53], and alkali/alkaline earth metals and/or salts [30,40,[54][55][56][57][58][59][60][61]. Compared to doped-silicon layers, dopant-free carrier-selective materials open a wider optical and electrical parameter space, decoupling the optimization of different solar cell loss components.…”

“…Compared to the doped silicon films, dopant-free carrier-selective contacts consist of elementary or compound thin films, with high work functions as hole-selective contacts and low work functions as electron-selective contacts. For example, electron-selective contacts include compounds with a low work function, such as LiF x [30,32,40], MgF x [56], TiO 2 [31,40,[43][44][45] and ZnO [46,47], and alkaline metals with an extremely low work function, such as Mg [54] and Ca [57]. Hole-selective contacts mainly consist of organic films [48][49][50][51][52][53] and transition metal oxides (TMO s ), such as MoO x [30,32,[36][37][38][40][41][42]47], WO x [35,38,39] [33][34][35]38], and NiO x [31].…”

“…If both the dopant-free hole and electron-selective contacts are deposited on the front and rear sides of c-Si wafers, respectively, these novel devices are nominated as dopant-free asymmetric heterocontacts (DASH) solar cells [30,40], which have been widely investigated in recent years and will be one of the most promising key issues for further improving the performance of c-Si-based solar cells. The DASH solar cells, based on TMOs, have the following advantages [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]: (1) TMOs have a large band gap and thus low parasitic light absorption, which is beneficial for increasing the short-circuit current density of the solar cell; (2) the DASH solar cell has a very high efficiency potential of up to 28% [31]; (3) most of the TMOs can be deposited by low-temperature and low-cost techniques, such as thermal evaporation or the solution method; (4) no flammable, explosive, or toxic gases and materials are used; (5) it is easy to realize the IBC structure by multi-step masks, which avoids the complicated photolithography processes [32][33][34].…”

“…According to the research of Gerling [38], Khan [46], and Tong [42], it was found that naturally formed SiO x on the c-Si surface during the MoO x deposition by the thermal evaporation or solution method plays a key role in the c-Si surface passivation. However, the naturally formed SiO x is not compact enough to provide enough passivation.…”