Copper Bromide as an Efficient Solution-Processable Hole Transport Layer for Organic Solar Cells: Effect of Solvents

In this work, we report copper bromide (CuBr) as an efficient, inexpensive, and solution-processable hole transport layer (HTL) for organic solar cells (OSCs) for the first time. To examine the effectiveness of the material in general, three different solvents such as acetonitrile (MeCN), dimethyl sulfoxide (DMSO), and dimethylformamide (DMF) are used for solution-processing thin-film deposition of CuBr. CuBr thin films deposited from different solvents show high transparency and no significant difference has been observed in absorption in the visible and near-IR range, whereas a slight difference has been found in the near-UV range by changing the solvents. Furthermore, two most studied combinations of the active layer such as PTB7/PC₇₁BM and PCDTBT:/PC₇₁BM are used for device fabrication with geometry of ITO/CuBr(HTL) active layer/Al. By using CuBr as a HTL in OSCs, the power conversion efficiencies (PCEs) have been achieved to up to 5.16 and 4.72% with PTB7/PC₇₁BM and PCDTBT/PC₇₁BM active layers, respectively. The CuBr film from DMF solvent shows highest PCE as compared to films deposited from DMSO and MeCN solvents. Different solvents used for HTL deposition have a major effect on the fill factor (FF), while very little difference on open circuit voltage (V _oc) and short circuit current (J _sc) has been observed. It may be mentioned here that a small difference of device parameters (PCE, FF, J _sc, and V _oc) has been observed in the devices using the HTL deposited from DMF and DMSO solvents, whereas a significant difference of the device parameters has been found in devices using the HTL from MeCN solvent.