|
Cu(In,Ga)Se2 (CIGS) solar cells are built up by several thin films; a molybdenum back contact, the energy absorbent CIGS layer, a buffer layer (typically CdS), a highly resistive ZnO layer and finally an Al-doped ZnO top contact. Although CdS is a well working buffer layer it is being replaced by other materials in order to avoid cadmium for environmental reasons.
Here, a Zn1-XMgXO buffer layer between the CIGS and the ZnO has been deposited by Atomic Layer Deposition (ALD). Its presence introduces a small spike in the conduction band between the CIGS and the ZnO thus preventing a recombination route for the photo excited electrons. To optimize the deposition of the Zn1-XMgXO buffer layer with respect to device performance, films were deposited at different temperatures, ranging from 105 °C up to 180 °C. Measurements of solar cell performance showed that lower deposition temperatures, between 105-135 °C, offered better performance than higher deposition temperatures. Since the buffer band gap was similar for all layers, microstructural or chemical differences at the CIGS and Zn1-XMgXO buffer layer interface are likely to be responsible for the varying performance.
Detailed information of the interface region at the nanometer level was acquired by analytical TEM analysis. The Zn1-XMgXO buffer layer was studied with emphasis on elemental distribution and grain morphology at the CIGS interface. TEM images showed excellent coverage of the ALD deposited Zn1-XMgXO on the rough CIGS surface with columnar Zn1-XMgXO grains. At higher magnification, initial growth of Zn1-XMgXO was observed to vary over the CIGS surface. Elemental composition studied by EDS line scans indicated differences in Mg to Zn ratio as function of distance from the interface. An increased Mg to Zn ratio was observed in the Zn1-XMgXO buffer layer at the CIGS interface for the buffer deposited at 120 °C. Initial results suggest lateral differences in composition when mapping larger areas by Energy Filtered TEM (EFTEM). This detailed analysis will give valuable information for further improvement of the Zn1-XMgXO buffer layer with respect to solar cell performance. |