A Brief Review of Transparent Conducting Oxides (TCO): The Influence of Different Deposition Techniques on the Efficiency of Solar Cells

Abstract

Global-warming-induced climate changes and socioeconomic issues increasingly stimulate reviews of renewable energy. Among energy-generation devices, solar cells are often considered as renewable sources of energy. Lately, transparent conducting oxides (TCOs) are playing a significant role as back/front contact electrodes in silicon heterojunction solar cells (SHJ SCs). In particular, the optimized Sn-doped In₂O₃ (ITO) has served as a capable TCO material to improve the efficiency of SHJ SCs, due to excellent physicochemical properties such as high transmittance, electrical conductivity, mobility, bandgap, and a low refractive index. The doped-ITO thin films had promising characteristics and helped in promoting the efficiency of SHJ SCs. Further, SHJ technology, together with an interdigitated back contact structure, achieved an outstanding efficiency of 26.7%. The present article discusses the deposition of TCO films by various techniques, parameters affecting TCO properties, characteristics of doped and undoped TCO materials, and their influence on SHJ SC efficiency, based on a review of ongoing research and development activities.

Keywords: ITO thin films; TCOs; deposition techniques; optical properties; silicon heterojunction solar cells.

Figure 1

(a) Structure of an SHJ solar cell (reprinted with permission from ref. [61], copyright form Elsevier order number 5492900736345); (b) energy band diagram for an SHJ solar cell (reprinted with permission from ref. [62], copyright form AIP order number 5492901129169).

Figure 2

(a) The SHJ solar cell with flat and textured TCO layer (this figure was reprinted with permission from ref. [65]; copyright form Elsevier order number 5492910461774). (b) The energy band plot of diverse ITO work functions in rear-emitter SHJ SCs (this figure was reprinted with permission from ref. [66]; copyright form Elsevier order number 5492910771527).

Figure 3

The light–CV characteristic for SHJ SCs and the influence of the diverse densities of [Oi] on the various SC parameters (reprinted with permission from ref. [67]; copyright form Elsevier order number 5492911003623).

Figure 4

The optical characteristics (a) total transmittance, and (b) Haze ratio of ITO:Zr layers as a function of etching time. (Reprinted with permission from ref. [77]. Copyright form Elsevier order number—5492920078302).

Figure 5

The deviations of (a) TCO sheet resistance measured on glass and at the rear side of the cell, (b) oxygen partial pressure as a function of mobility and carrier density of the TCO films. (c) Comparison of carrier density and TCO sheet resistance attained on glass and p/i/glass substrates after annealing treatment at 200 °C for 30 min. (d) Correlation between the R_sheet of the TCO measured at the front side and rear side of the cell [84]. (Reprinted with permission from ref. [84]; copyright form Wiley order number 5492920290840.)

Figure 6

(a) Transmittance and (b) energy gap of ITO layers at different RF powers. (c) The J-V characteristics of SHJ PV cells for different RF powers. (d) Carrier lifetime for SHJ PV cell versus minority carrier density before and after deposition of ITO films for various RF powers. (Reprinted with permission from ref. [90]. Copyright form Elsevier order number 5492920513698.)

Figure 7

Correlation between (a) Hall mobility, resistivity, and carrier concentration and (b) optical transmittance of ITO films for different O₂ flow rates (reprinted with permission from ref. [38]; copyright form Elsevier order number 5492920712562).

Figure 8

The J-V characteristic plots of a simulated and experimental solar cell (reprinted with permission from ref. [98]; copyright form Elsevier order number 5492920955733).