One of the technical issues remaining to be solved for improvement of the performance of flexible CIGS photovoltaic cells is control of the alkali doping level in the CIGS absorber layer.  Alkali metals such as sodium (Na) are known to be used in CIGS photovoltaic cells as indispensable impurities (dopants) for enhancement of hole carrier concentration and open circuit voltage.  For improving the performance of flexible CIGS photovoltaic cells, various alkali addition methods, in which alkali compounds such as sodium selenide (Na₂Se) and sodium fluoride (NaF) are used  as alkali precursors and deposited before, after, or during CIGS layer growth, have been examined.  However, because many of these alkali compounds are very difficult to handle due to their unstable properties such as deliquescence, performance improvement of the photovoltaic cells with satisfactory reproducibility has not been achieved.

At AIST, we used alkali-silicate glass thin layers (ASTL) as alkali source materials, which are stable alkali compounds, deposited on the substrate prior to the molybdenum (Mo) back contact layer deposition, as shown in Fig. 1, and developed a technique for controlling the quantity of alkali incorporated into the CIGS absorber layer through the Mo back contact layer by controlling the deposition conditions of ASTL (the "ASTL method").  Using this technique, we can perform alkali addition with ease as well as high reproducibility, and thus, can bring about a significant improvement in the energy conversion efficiency of CIGS photovoltaic cells.  We fabricated a small-area flexible CIGS photovoltaic cell with a ceramics substrate with smooth surface using the ASTL method and achieved an active-area efficiency of 17.7%.  By using a substrate made of titanium foil, which has a slightly rough surface, we could achieve a conversion efficiency of 17.4%.  17.7% energy conversion efficiency of the flexible CGIS photovoltaic cell (see Fig. 2) is the highest value for flexible CIGS cells reported to date. 

Figure 1  Schematic illustration of a high-efficiency flexible CIGS solar cell developed in the present work. As alkali source materials, alkali-silicate thin layers (ASTL) are deposited on flexible substrates prior to sputtering of the Mo back contact layer. Al Grid electrodes as shown in Fig. 3 were formed by evaporation on the TCO layer before anti-reflection coating.

	Photovoltaic parameters Efficiency : 17.7% Open circuit voltage : 0.660 V Short circuit current density : 35.4 mA/cm2 Fill factor : 0.757
Figure 2  Current-voltage curve of the flexible CIGS solar cell fabricated on a ceramics sheet using the ASTL method. The CIGS absorber layer was grown at the substrate temperature of 550°C.

Figure 3 Flexible CIGS solar cells
(left) Ceramics substrate	(right) Polymer substrate

Polymeric materials, which are light-weight, insulating, and inexpensive, are gaining attention as suitable materials for flexible substrates.  The polymeric materials need to be processed at lower than approximately 450 °C, however, a temperature of higher than 500 °C is necessary to form a CIGS light-absorbing layer having a high energy conversion efficiency.  Furthermore, it is difficult to handle polymer-made substrates, thereby making it almost impossible to realize high energy conversion efficiency in a CIGS photovoltaic cell fabricated using a polymer substrate.  Very few researchers have reported an energy conversion efficiency of more than 12%.

Recently, Teijin has developed a polymer film that is spin-coated on a glass substrate.  After a CIGS photovoltaic cell has been formed on this polymer film, the film can be easily peeled off from the glass substrate, as shown in Fig. 3 (right).  The flexible CIGS photovoltaic cell with this polymer substrate fabricated using the ASTL method is not only easy to handle but also has improved open voltage and fill factor due to the alkali-addition effect, with an active-area efficiency of 14.7% (Fig. 4).  The CIGS absorber layer of the cell was formed at 400 °C.  This energy conversion efficiency is the highest value reported for CIGS photovoltaic cells fabricated on polymer substrates.  The use of this polymer substrate enables fabrication of flexible CIGS solar cells using a common apparatus designed for panel module fabrication and no specialized equipment such as roll-to-roll machines are required.

	(a) — with ASTL   Efficiency : 14.7%   Open circuit voltage : 0.619 V   Short circuit current density : 36.0 mA/cm2   Fill factor : 0.658 (b) — without ASTL   Efficiency : 12.2%   Open circuit voltage : 0.571 V   Short circuit current density : 35.4 mA/cm2   Fill factor : 0.604
Figure 4  Current-voltage curves from flexible CIGS solar cells fabricated on polymer substrates. CIGS absorber layers were grown at the substrate temperature of 400°C.