Application of copper thiocyanate for high open‐circuit voltages of CdTe solar cells

Abstract

Copper thiocyanate (CuSCN) has proven to be a low-cost, efficient hole-transporting material for the emerging organicinorganic perovskite solar cells. Herein, we report that CuSCN can also be applied to CdTe thin-film solar cells to achieve high open-circuit voltages (V OC s). By optimizing the thickness of the thermally evaporated CuSCN films, CdTe cells fabricated by close space sublimation in the superstrate configuration have achieved V OC s as high as 872 mV, which is about 20-25 mV higher than the highest V OC for the reference cells using the standard Cu/Au back contacts. CuSCN is a wide bandgap p-type conductor with a conduction band higher than that of CdTe, leading to a conduction band offset that reflects electrons in CdTe, partially explaining the improved V OC s. However, due to the low conductivity of CuSCN, CdTe cells using CuSCN/Au back contacts exhibited slightly lower fill factors than the cells using Cu/Au back contacts. With optimized CdS:O window layers, the power conversion efficiency of the best CdTe cell, using CuSCN/Au back contact, is 14.7%: slightly lower than that of the best cell (15.2%) using Cu/Au back contact.

Solution‐processed copper (I) thiocyanate (CuSCN) for highly efficient CdSe/CdTe thin‐film solar cells

“…Figure shows the device performance for the DES‐/CuSCN‐based CdSe/CdTe solar cells with tuning the CuSCN thickness. It is reported that the sputtered CuSCN thickness can influence the CdS/CdTe devices performance remarkably, in particular, will improve the V oc , and reduce the fill factor . Without CuSCN (i.e., 0 nm, means no HTL, ERL, and Cu doping), the CdSe/CdTe device PCE is ~11%, V oc ~0.7 V, J sc ~26.5 mA cm −2 , and the Fill factor ~58%, which is similar to that of CdS/CdTe devices without CuSCN .…”

“…Figure B indicates the bandgap diagram calculated using SCAPS modeling . CuSCN thickness directly impacts the carrier transport due to its high resistivity and can play as a barrier for the carrier transport if the thickness is too thick . Figure C shows the two solvents in this work to tune the thickness of CuSCN layer.…”

“…However, the series resistance, R s , is still high and similar to that of the CdTe without CuSCN. This is expected because the high resistivity of the thicker CuSCN layer prevents the carrier transport …”

“…Meanwhile, CuSCN was also used for other dye‐sensitized solar cells (DSSC), such as chalcogenides Sb 2 S 3. As for CdTe solar cells, physical vapor deposited CuSCN thin film has also been investigated to improve the V oc for CdS/CdTe devices . Although, it is a challenge for solution‐processed CuSCN (~100 nm) to achieve a thin layer compared with the vapor process counterpart (~10 nm) in CdS/CdTe devices.…”

“…Traditionally, diethyl sulfide (DES) uses for dissolving CuSCN. However, the DES‐based CuSCN layer is difficult to get an ultrathin and smooth layer due to the high viscosity and rougher topography of DES . Recently, using aqueous ammonia (NH 4 OH) as a solvent for CuSCN can achieve a relatively thin layer (5‐10 nm) with smooth topography and successfully utilized on perovskite solar cells to obtain PCE ~17% …”