Highly Surface-Textured ZnO:Al Films Fabricated by Controlling the Nucleation and Growth Separately for Solar Cell Applications

Abstract

Highly surface-textured ZnO:Al (AZO) thin films have been fabricated at room temperature by a two-step magnetron sputtering process and using an oxygen-deficient ZnO target with small grain sizes. The as-deposited AZO films are composed of a highly oriented seed layer and a closely packed columnar overlayer with pyramidal growth fronts, supporting a two-step mechanism of crystallite nucleation and grain growth. The structural, optical, and electrical properties of the AZO films can be tuned by the deposition conditions. The optimal two-step AZO film with a maximum root-mean-square roughness of 40.2 nm reaches a very low square resistance of 0.66 Ω/sq (ρ = 1.32 × 10−4 Ω·cm) with an average transparency of 87.9% in the range of 400−1100 nm. The maximum haze factor of the as-deposited film is 60.7% at 360 nm, and the average haze factor is 14.8%. These properties are comparable to or exceed the reported values of surface-textured SnO2- and ZnO-based transparent conducting oxide films, making our films suitable for transparent electrode applications, especially in thin-film solar cells.

Ultra‐thin (002)‐oriented Al‐doped zinc oxide transparent electrode grown on oxygen‐controlled homo‐seed layer

“…This kind of AZO ultra-thin film exhibits less defects [18], less grain boundary scattering [14] and excellent (002) orientation [11], contributing towards the improvement of electrical transport. Interestingly, the carrier concentration of the films with oxygen-controlled homoseed layer increased, which was different from the carrier concentration decrease reported in literature [31,32,25]. This may be attributed to the promoted crystalline quality of AZO films by the initial oxygen-controlled homo-seed layer, reducing the sum of electron traps.…”

“…With the oxygen-controlled homo-seed layer, the transmittance of the AZO films in the infrared range showed a drastic decrease. This decrease is caused by the increase in carrier concentration, which enhances the plasma frequency [25]. When this homo-seed layer was introduced, the ultraviolet absorption edges were blue-shifted.…”

“…Hence, it is still a challenge to fabricate ultra-thin AZO films by controlling the growth of the initial layer for promoting the structural order of the following film. In order to reach these aims, different deposition methods, such as pulsed laser deposition (PLD) [17], ion beam assisted deposition (IBAD) [18,19], ion plating with DC arc discharge [20,21], and the introduction of a hetero- [22,23] or homoseed layer [24][25][26][27] before the growth of the bulk films, have been adopted. Though significant achievements have been made, a simple and scalable sputtering process for preparing AZO ultra-thin films still needs further development.…”

Advanced solar materials for thin-film photovoltaic cells

“…A textured rough surface of the surface-textured ZnO films can improve the solar cell efficiency by allowing efficient light scattering inside the solar cell [2][3][4][5][6]. Recently, highly surface-textured ZnO:Al (AZO) thin films have been fabricated at room temperature in a two-step magnetron sputtering process using an oxygen-deficient ZnO target with small grain sizes [7]. The as-deposited AZO films are composed of a highly-oriented seed layer and a closely-packed columnar overlayer with pyramidal growth fronts, supporting a two-step mechanism of crystallite nucleation and grain growth.…”

“…The magnetron sputtering depositions of molybdenum thin films, either by DC [5][6][7][8][9][10][11][12][13][14][15] or RF [16][17][18][19][20][21][22][23][24] techniques, have been widely studied during the fabrication of CIGS cells, but few comparative and systematic investigations were reported. To meet the requirements of back contacts of CIGS solar cells, RF and dc magnetron sputtering techniques have been applied to deposit Mo thin films at the different sputtering powers in the same system with the two-step process.…”