CdSe Solar Cells with V_OC > 1V

An efficient, low-cost, scalable, and stable top-cell photovoltaic absorber is needed to make thin-film based tandem solar cells. Using cadmium selenide (CdSe) as an example, we highlight our methodology for rapid assessment of novel photovoltaic absorber materials and identify challenges towards making high efficiency devices.<br/>With a bandgap of 1.72eV, CdSe is well suited as a top-cell absorber in a two-junction tandem solar cell. We assess the potential of CdSe as an absorber through several measurements including external radiative efficiency (ERE), carrier density, polycrystalline grain-size and orientation. Well formulated thin films of CdSe are found to be n-type, hexagonal, and exhibit high ERE (&gt;0.2%). Photoluminescence and sub-band external quantum efficiency confirm that this absorber is capable of implied open-circuit voltages above 1.1 V. A transparent conducting oxide coated on glass forms the electron-contact to the CdSe thin-film grown on top of it. The hole-contact is formed by organic hole-transport layers used in conjunction with a bilayer of transition metal oxide and thin-gold. Solar cells with measured V_OC &gt; 1 V have been demonstrated with this device structure.<br/>Despite good V_OC, the devices made so far exhibit a limited fill factor and short-circuit current that cannot be completely accounted for by resistive or optical losses. By connecting a multitude of metrology techniques to device modeling results we conclude that the minority carrier collection is limited as the mobility and lifetime product (mu*tau) product is about 10^-9 cm²/V. This is attributed primarily to low hole mobility. Another key challenge is the unusually large ionization potential of CdSe at -6.5eV. SCAPS simulations indicate that improving the mu*tau and reducing the band-offset at the hole contact should lead to improved carrier collection. Defect signatures identified through sub-band photoluminescence measurements suggest selenium vacancies could be problematic.<br/>In conclusion, well researched p-type transparent contact layers with a deep valence-band alignment are needed as hole contacts for CdSe solar cells. Options for designing a CdSe device that can be used in a thin-film based tandem are limited by the n-type absorber and require improved minority-carrier diffusion length.