Control of Structural and Electrical Properties of Indium Tin Oxide (ITO)/Cu(In,Ga)Se₂ Interface for Transparent Back-Contact Applications

Abstract

Development of transparent-conducting oxide (TCO) back contact for Cu­(In,Ga)­Se2 (CIGS) absorber is crucial for bifacial CIGS photovoltaics. However, inherent GaO x formation at the TCO/CIGS interface has hampered the photocarrier extraction. Here, by controlling the Na doping scheme, we show that the hole transporting properties at the indium–tin oxide (ITO)/CIGS back contact can be substantially improved, regardless of the GaO x formation. Na incorporation from the glass substrate during the GaO x forming phase created defective states at the interface, which allowed efficient hole extraction from CIGS, while post Na treatment after GaO x formation did not play such a role. Furthermore, we discovered that an almost GaO x -free interface could be made by reducing the underlying ITO film thickness, which revealed that ITO/CIGS junction is inherently Schottky. In the GaO x -free condition, post-Na treatment could eliminate the Schottky barrier and create ohmic junction due to generation of conducting paths at the interface, which is supported by our photoluminescence analysis.

Flexible and Semi‐Transparent Ultra‐Thin CIGSe Solar Cells Prepared on Ultra‐Thin Glass Substrate: A Key to Flexible Bifacial Photovoltaic Applications

“…It has been already reported by other groups that a higher Na concentration facilitates the formation of a thicker GaO x interfacial layer. [ 50,52 ] Consequently, a slightly higher Na concentration was found in the CIGSe absorber on SLG and a large Ga spike was observed at the interface, as shown in Figure 9a. By contrast, a slightly lower Na concentration and a mild Ga plateau were observed in the F‐STUT CIGSe solar cell on UTG.…”

“…Among the various adverse effects of interfacial GaO x formation, an obvious phenomenon is a decrease in V oc because GaO x is usually identified as an n‐type material. [ 49,50 ] Considering the significantly higher V oc of U500 and U550 compared to S500 and S550, there is a possibility that the thickness of the interfacial GaO x layer is less in F‐STUT CIGSe solar cells on UTG compared to those on SLG.…”

Interface engineering of ultrathin Cu(In,Ga)Se2 solar cells on reflective back contacts

“…39 It is worth mentioning that in several previous studies, the presence of a Ga oxide layer at the CIGS back contact was shown to be detrimental to cell performances because of an increase of the series resistance and a current blocking behavior. 22,27,28,40,41 In this work however, the growth of a thick and rough Ga oxide layer led to a depletion of Ga in the CIGS layer close to its back contact, which results in a decreased V OC and a voltage-dependent photocurrent.…”

Is a passivated back contact always beneficial for Cu (In,Ga)Se2 solar cells?

“…20,[32][33][34][35] This will typically lead to an S-shaped light J-V curve (rollover effect) and a crossover between light and dark J-V curves. 36,37 On the other hand, some authors did not observe those non-ideal characteristics and claimed that the TCO/CIGSe back contact is quasi-Ohmic. 19,22,36,38 Recently, several publications reported that an increased back recombination is likely to lessen the impact of the back barrier potential for cells on ITO and to improve the cell performance.…”

“…36,37 On the other hand, some authors did not observe those non-ideal characteristics and claimed that the TCO/CIGSe back contact is quasi-Ohmic. 19,22,36,38 Recently, several publications reported that an increased back recombination is likely to lessen the impact of the back barrier potential for cells on ITO and to improve the cell performance. 22,36,39 This may pose a sharp contrast to previous demonstrations for cells on Mo, where a reduced back recombination is desirable.…”