Using hydrogen‐doped In₂O₃ films as a transparent back contact in (Ag,Cu)(In,Ga)Se₂ solar cells

Abstract

This study evaluates the potential of hydrogen-doped In 2 O 3 (IOH) as a transparent back contact material in (Ag y ,Cu 1-y )(In 1-x ,Ga x )Se 2 solar cells. It is found that the presence of Na promotes the creation of Ga 2 O 3 at the back contact during (Ag y ,Cu 1-y )(In 1-x ,Ga x )Se 2 growth. An excessive Ga 2 O 3 formation results in a Ga depletion, which extends deep into the absorber layer.Consequently, the beneficial back surface field is removed and a detrimental reversed electrical field establishes. However, for more moderate Ga 2 O 3 amounts (obtained with reduced Na supply), the back surface field can be preserved. Characterization of corresponding solar cells suggests the presence of an ohmic back contact, even at absorber deposition temperatures of 550°C. The best solar cell with an IOH back contact shows a fill factor of 74% and an efficiency contacts are further needed in bifacial devices, which can be semitransparent if desired (eg, for "solar windows"). The upper cells in a multijunction tandem structure need to be grown on TBCs as well to allow for light propagation to the bottom cell. An advantage of these approaches over a superstrate configuration is that the required buffer layer does not have to undergo the thermal stress during absorber formation. Up to now, there are no buffer materials found, which are chemically stable at high temperatures, which results in a significant efficiency drop compared to substrate-configured solar cells. 4,5 Nevertheless, there are some requirements to a TBC as well. After the thermal stress during absorber formation, it still needs to (1) create an ohmic contact to the absorber, (2) be highly transparent, and (3) exhibit a low sheet resistance (R SH ). In the case of CIGS-based solar cells, it also needs to exhibit a certain permeability for alkaline ions, if they are supposed to be supplied from the underlying glass substrate.In the past, different transparent conductive oxide (TCO) layers have been investigated as potential TBC materials for CIGS solar cells. In this study, we use (Ag,Cu)(In,Ga)Se 2 (ACIGS) as an absorber material, which is proven to result in large grain sizes 19 and highefficiency solar cells. 20,21 The silver was added intentionally, since V OC losses for higher absorber band gap energies are less pronounced in ACIGS compared to CIGS. 21 This is in particular interesting for top cells in a tandem configuration where the optimum band gap energy E G is about 1.6 eV (two junctions).It should be emphasized that this work is supposed to act as a first potential evaluation for IOH as a TBC. Hence, the cell structure was designed for maximized efficiencies at normal front side illumination, which means that the absorber layer is about 2 μm thick and has a band gap energy of about 1.2 eV. | MATERIALS AND METHODS | Sample fabrication and configurationsThe ACIGS solar cells investigated in this study were manufactured as a stack of soda lime glass (SLG)/back contact/ACIGS/CdS/i-ZnO/AZO.As a back contact either IOH or standard Mo (DC-sp...

Bifacial Cu(In,Ga)Se2 solar cells using hydrogen‐doped In2O3 films as a transparent back contact

“…Recently, we could identify IOH as a promising TBC material for chalcopyrite solar cells, establishing an ohmic back contact to the absorber and preserving high conductivity and transmission after absorber deposition at T CIGS = 550°C . It was found that a moderate Ga 2 O 3 formation, which occurred here as well, does not deteriorate the transport properties across the back contact junction significantly.…”

“…Consequently, the (as grown amorphous) IOH films are fully crystallized before CIGS growth. It was shown earlier, that a pre‐crystallization step does not only improve the transparency of the films but might also allow for a larger Na supply from the SLG substrate during absorber deposition …”

“…Earlier results indicate that the total amount of incorporated Na significantly affects the device performance and interface properties (eg, Ga 2 O 3 formation) . In order to study the relationship appropriately, the Na supply from the glass substrate is systematically changed by a variation in IOH thickness.…”

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

“…Aluminum doped zinc oxide (ZnO:Al) is a common replacement with conductivity reaching that of ITO. The chemical stability of ZnO:Al is, however, generally lower than that of In 2 O 3 [96]. ZnO is also more reactive towards oxygen than In 2 O 3 and therefore requires a more strict control of the oxygen supply during deposition [97].…”

“…In summary, surprisingly little research has been published on transparent back contacts for kesterite solar cells and even for the more mature chalcopyrite absorbers this area is not very much explored [96,103]. In addition to the use of different passivation layers, the question of Na supply needs to be addressed.…”