device are deposited directly onto a substrate or superstrate. The advantages of thin films are the potential ability to:
Dramatically reduce the amount of material utilized. In some cases, the active semiconductor only needs to be a fraction of a micron thick in order to absorb most of the incident sunlight.
Directly integrate into a higher voltage module, thereby eliminating much of the handling and labor necessary to produce cry‐Si modules.
For monolithically integrated thin film modules, the deposition processes and the integration processes are described in Table 1.1 and the resultant structure shown in cross section in Figure 1.2. Figure 1.2 is not drawn to scale as the thin film layers and the three different scribe lines are exaggerated in size so you can see them. In thin film devices, the semiconductors have considerably higher resistance than cry‐Si so metal grids do not work well. Instead, a Transparent Conductive Oxide (TCO) is often used on the semiconductor surface to provide a conductive path as well as serving as the AR. Because the TCO layers are not as conductive as a metal grid, thin film solar cells are usually long and skinny, so that the collected current only flows a short distance across the cell to the next cell. Monolithically integrated thin film modules often have a large number of skinny cells connected in series, resulting in higher voltage, lower current and reduced series resistance losses.
Thin film layers of materials are more susceptible to corrosion than bulk materials. When thin film layers are deposited directly in contact with glass, they may also be susceptible to ion flow in the glass [21]. This is why many thin film modules are protected in packages that are designed to keep moisture out for the lifetime of the product.
Table 1.1 Process for making monolithically integrated thin film modules.
| Step # | Step Name | Description |
| 1 | Transparent Conductive Oxide TCO | Deposit TCO layer onto the substrate |
| 2 | P1 Scribe | Scribe to remove TCO from selected areas |
| 3 | Semiconductor Deposition | Deposit the semiconductor layers that make the solar cells |
| 4 | P2 Scribe | Scribe to remove the Semiconductors from selected areas |
| 5 | Metallization | Deposit the back metallization |
| 6 | P3 Scribe | Scribe to remove the Metallization from selected areas |
Figure 1.2 Cross‐sectional drawing of monolithically integrated thin film module.
According to Mints [4], thin films only represented about 5% of total worldwide module shipments in 2018, a declining percentage from previous years. The thin‐film PV commercial market includes the following three materials.
1 Cadmium telluride (CdTe): CdTe is a well‐known semiconductor often used in high performance infrared (IR) sensors. CdTe absorbs visible light very strongly, so very thin films (1–2 μm) are sufficient to absorb most of the sunlight. Commercial CdTe modules are typically fabricated on glass with a structure similar to the cross‐sectional drawing in Figure 1.2. First Solar is by far the largest supplier of CdTe modules. First Solar now offers large area, monolithically integrated CdTe modules with efficiencies of up to 18% [22]. CdTe technology is especially attractive because of low‐cost manufacturing. In 2018, First Solar, selling just CdTe modules shipped about 3% of the world's PV modules [4].
2 Copper Indium Diselenide (CIS) and Copper Indium Gallium Diselenide (CIGS): Both CuInSe2 (CIS) and Cu(InGa)Se2 (CIGS) are ideal PV‐absorber materials. The band gap is near the optimum for absorbing the terrestrial spectrum. These materials have strong optical absorption so very thin films (~1 μm) are sufficient to absorb most of the sunlight. Grain boundaries and surfaces in CIS and CIGS are electronically benign, so simple polycrystalline films yield reasonably high efficiency PV devices without complex grain boundary passivation. Typical CIS and CIGS cells use thin CdS layers to form p‐n junctions, molybdenum for ohmic contacts to the CIS or CIGS, and transparent conductors like zinc oxide and indium‐tin oxide for contact to the CdS and to serve as an AR coating. In most cases, CIS and CIGS cells are deposited upside down, meaning that the back metal is deposited first and the cells are deposited from back to front onto the metal. So, CIS and CIGS modules often have the cells deposited onto the back glass of a glass/glass package or onto a metallic substrate that is then diced up to produce individual cells that are packaged like you would wafer‐based cells. While CIS and CIGS are attractive PV materials, it has taken a long time to move from the laboratory to commercialization. Two reasons for this appear to be the difficultly in scaling to larger sizes and larger volumes, related to uniformity of the deposited films, and sensitivity to environmental stresses. Reliability issues relate to the humidity sensitivity of the contacts and TCO necessitating hermetic sealing to achieve long‐term life. Solar Frontier from Japan is the world's largest manufacturer of CIS PV modules offering large area, monolithically integrated CIS modules with conversion efficiencies up to 15% [23]. In 2018, CIS/CIGS made up about 1% of the worldwide shipments of PV modules [4].
3 Amorphous Silicon (a‐Si): Alloys of a‐Si made of thin‐film hydrogenated silicon (a‐Si:H) can be deposited on either glass superstrates or flexible metallic substrates. Because of the low minority carrier lifetimes in doped a‐Si, p‐i‐n (p‐type, intrinsic, n‐type) cell structures are utilized rather than normal p‐n junctions. Even in undoped a‐Si, the minority carrier lifetimes are short and because of the Staebler‐Wronski effect, in which light induces additional electrically active defects, the thickness of individual layers must be minimized. Therefore, to achieve reasonable efficiencies, multi‐junctions are usually utilized. Commercial a‐Si is typically fabricated with a structure similar to the cross‐sectional drawing in Figure 1.2. The maximum reported efficiency for a‐Si modules is about 12% [17], which is less than the efficiencies of commercially available CdTe or CIGs modules. Commercial a‐Si modules have efficiencies in the 5–8% range after light stabilization. Most a‐Si products are used in consumer‐type products (watches, calculators, etc.) not for power production.
Over the years, there have been a number of efforts to use optical concentration to increase the output power that can be obtained from PV devices. Some of these efforts have resulted in laboratory systems with high efficiencies and good performance. However, to date, none have been able to establish a sustainable commercial market. For this reason, reliability of concentrator PV will not be discussed in this book.
1.3 Module Packaging – Purpose and Types
While the solar cells actually produce the electricity, the module package is important for the continued operation of the solar cells. Often the costs associated with the packaging exceed the costs of the cells themselves. Historically, it is usually