Chemical Bath Deposition of Zn_1−xSn_xO_y Films as Buffer Layers for Cu(In,Ga)Se₂ Solar Cells

Abstract

Cu(In,Ga)Se 2 (CIGSe) thin-film solar cells present an opportunity for flexible, tandem, or semitransparent applications, together with easy and cheap manufacturing for large-area modules due to milder processing temperatures compared to silicon solar cells. [1][2][3][4] This technology has been widely studied because CIGSe can be prepared by several methods that give high control over its properties, notably its tunable bandgap (1.0-1.7 eV), [5,6] and high absorption coefficient (>10 5 cm À1 ) [5,6] with a record power conversion efficiency (PCE) of 23.4% achieved by SolarFrontier. [7] A key component in CIGSe solar cells is the buffer layer, which forms the chargeseparating pn-junction with the absorber and provides a good electrical and optical interface with the front contact layers, ensuring charge carrier transport, and minimal interface recombination. CdS deposited by chemical bath deposition (CBD) is the traditional buffer material due to its electrical and structural matching with the absorber. [8] This n-type material only yields high-efficiency solar cells when it is deposited by CBD, due to the action of ammonia on the surface of CIGSe and the complete and conformal coverage over the polycrystalline CIGSe. [9] The CBD technique consists of the deposition of inorganic (oxide, sulfide, or selenide) thin films by immersion of a substrate in a precursor solution. It is based on a controlled chemical reaction (normally hydrolysis) performed typically in an aqueous solution that yields coating of the substrate (heterogeneous nucleation at the solution/substrate interface) while also resulting in a colloidal suspension (homogeneous nucleation occurring in solution). Generally, the precursor aqueous solution involves a soluble metal salt, a pH agent, and a complexing agent, used to control the hydrolysis rate.However, the relatively low direct bandgap of CdS, E g = 2.4-2.5 eV, [10] prevents high energy photons from being absorbed by the p-CIGSe. [11] In addition, the presence of toxic Cd presents health and environmental concerns. [12,13] Therefore, wider bandgap and less toxic materials have been researched to produce Cd-free CIGSe solar cells. [11,14,15] Among these materials, zinc tin oxide (Zn 1Àx Sn x O y , ZTO) has attracted much attention due to its nontoxicity, fast light responsivity, and tunable wide bandgap. CIGSe solar cells based on ZTO buffers have resulted in PCE comparable to or even higher than CdS ones. [8,11,14,16] Until now, ZTO buffer layers on CIGSe solar cells have been only fabricated by atomic layer deposition (ALD) with atomically precise resolution in vacuum conditions by sequential vapor reactions of organometallic precursors and gases. Although ALD allows excellent control over ZTO composition and assures excellent coverage, [17] it is hardly scalable, time-consuming, and expensive. In contrast, the production of record CIGSe-based solar cells makes use of the CBD to produce CdS and other inorganic thin

Variation in the Bandgap of Amorphous Zinc Tin Oxide: Investigating the Thickness Dependence via In Situ STS

“…Both ZnO and SnO 2 are well-known n-type TCOs and have been shown by Hautier et al to possess among the highest electron mobilities among all binary oxides . The arguments for a-ZTO are numerous: a-ZTO is only composed of common and abundant elements; it has been shown to possess high mobilities even in an amorphous state; and it has already been implemented in a wide range of applications such as organic light-emitting diodes (O-LEDs), photovoltaic cells, − and thin film transistors (TFTs). , It is imperative to understand how key properties such as morphology, bandgap, and conductivity are affected in the low-thickness regime as these properties often play a decisive role when it comes to selecting the compatibility of a material in a device …”

Energy Band Alignment at p–n Heterojunction Interface in CZTSSe Solar Cells with Novel ZnMgO Buffer Layer

“…At present, the alternative buffer layers include ZnS, [ 18 ] In 2 S 3 , [ 19,20 ] ZMO, [ 21–23 ] Zn(O,S), [ 24–26 ] ZnSe, [ 27,28 ] ZnSnO, [ 29,30 ] etc. Within these options, ZMO won widespread attention due to its large and variable bandgap.…”