Development Of Cadmium Selenide As An Absorber Layer For Tandem Solar Cells

Cadmium Selenide is a binary compound. It has a band gap of 1.7 eV. This is one of the suitable materials for an absorber layer in the top cell of a tandem solar cell. CIGS with a low Gallium content has a band gap of 1 eV suits well as an absorber layer for the bottom cell. CIGS cells have already attained an efficiency of 15% [1,2]. Since years, research has been done in developing the bottom cell. The results of the bottom cell are promising. So the fabrication of an efficient top cell in a tandem solar cell is a challenge. To achieve a high tandem efficiency of above 25 %, the top cell has to contribute at least 2/3 of the total efficiency, which necessitates the top cell to have at least 16 to 18 % efficiency [3]. Development of a defect free absorber layer is a crucial step in this process to achieve the above goals besides optimizing other layers. Selenium vacancies in CdSe make the absorber layer n-type. CdSe is deposited by closed space sublimation. Deposition of CdSe at higher substrate temperatures in comparison to the standard conditions was studied. ZnSe acts as an insulating layer. It is thermally evaporated in an Evaporation system. Copper acts as a metal contact on top of the insulator resulting in a MIS structure. Copper is also deposited by Thermal Evaporation. Devices are fabricated on different substrates like SnO2: F, AZO etc. Fabricated cells are characterized by J-V and Spectral response measurements. Devices fabricated on SnO2: F substrates show typical open circuit voltages of around 220 mV, short circuit current densities of 10.02 mA/cm2 and fill factors around 33 %. N-type CdS when deposited on SnO2: F below the absorber layer further improved Voc\u27s to around 330 mV. Annealing of these devices improved Voc\u27s to about 350 mV but Jsc\u27s remained 7.21 mA/cm2