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In this study, earth-abundant thin film Cu2ZnSnSe4 (CZTSe) solar cells were fabricated by the magnetron sputtering of a quaternary compound target. Precursor films were deposited in a designed chemical composition followed by a selenization process under the mixed atmosphere of an active gas, H2Se, and a carrier gas, Ar. Due to the existence of the Se element within the film, a relatively low concentration of H2Se was utilized to mainly crystallize the precursors and also supplement Se. To explore the selenization process, the as-deposited CZTSe absorbers were annealed at different temperatures. The results of X-ray diffraction (XRD) and Raman scattering spectroscopy measurements showed that the CZTSe grains continuously grew after selenization. In this study, when the absorbers were annealed at 575 °C, the CZTSe solar cells were obtained with the highest efficiency of 11.95%, with the open circuit voltage (VOC) of 432.22 mV, short circuit current density (JSC) of 36.28 mA cm−2 and fill factor (FF) of 76.21%. The photoluminescence (PL) spectrum indicates that the peak center underwent a red-shift from the band gap (Eg) of 90 meV. The analyses of fluctuation modules indicate that the electrostatic potential fluctuation is dominant for the band tail. The investigations demonstrate that sputtering of the Cu–Zn–Sn–Se quaternary compound target via the selenization process can be an effective and simple method for the fabrication of high-performance CZTSe solar cells.

Hole transport layers (HTLs) with large work function (WF) tuning ability for good energy level alignment with deep highest occupied molecular orbital (HOMO) level donor materials are desirable for high-performance and high open-circuit voltage (VOC) organic solar cells (OSCs). Here, a novel low-temperature and solution-process approach to achieve WF tuning in HTLs is proposed. Specifically, the HTLs made from 2,3,4,5,6-pentafluorobenzylphosphonic acid (F5BnPA) incorporated graphene oxide (GO) and molybdenum oxide (MoOx) solution (representing two possible classes of HTLs where carriers transport via valence and conduction bands, respectively) offer continuous WF tuning (the tuning range as large as 0.81 eV) by controlling F5BnPA's concentration. By employing a deep HOMO donor material, OSCs using the composite HTLs can achieve improved performances with largely increased VOC (0.92 V for GO:F5BnPA versus 0.65 V for pristine GO; 0.91 V for MoOx:F5BnPA versus 0.88 V for pristine MoOx). The enhanced performance can be experimentally and theoretically explained by the decreased hole injection barrier (HIB) for GO or equivalent HIB (i.e. electron extraction barrier) for MoOx and enhanced surface recombination velocity, which contribute to eliminating S-shaped current–voltage characteristics. Consequently, the incorporation of F5BnPA can efficiently tune HTL WF for high VOC OSCs and extend HTL applications in organic electronics.

While metal oxide films are typically formed by high-temperature and sputtering processes, we report an approach with the features of a room-temperature, water-free and solution-based process for the formation of a molybdenum oxide (MoOx) film for inverted organic solar cells (OSCs) by proposing a vacuum treatment at room temperature and selecting an appropriate solvent. Remarkably, our results indicate that the vacuum treatment can introduce oxygen vacancies in the molybdenum oxide film and modify its work function for functioning as an efficient hole transport layer. To further improve OSC performances, a silver nanoparticle–molybdenum oxide (Ag NP–MoOx) composite film is prepared by the introduction of Ag nanoparticles into the solution. Evidence and explanations confirm that OSC performance enhancement is mainly due to the improvement of the electrical properties of the Ag NP–MoOx composite film. With the optimized composite film, inverted OSCs with a power conversion efficiency (PCE) of 7.94% are achieved. Through the demonstration of high performance inverted OSCs with different polymer materials, the water-free, room-temperature and solution-processed MoOx can contribute to the evolution of high performance OSCs such as inverted and tandem OSCs and other optoelectronic devices.