Amorphous oxide alloys as interfacial layers with broadly tunable electronic structures for organic photovoltaic cells

In diverse classes of organic optoelectronic devices, controlling charge injection, extraction, and blocking across organic semiconductor–inorganic electrode interfaces is crucial for enhancing quantum efficiency and output voltage. To this end, the strategy of inserting engineered interfacial layers (IFLs) between electrical contacts and organic semiconductors has significantly advanced organic light-emitting diode and organic thin film transistor performance. For organic photovoltaic (OPV) devices, an electronically flexible IFL design strategy to incrementally tune energy level matching between the inorganic electrode system and the organic photoactive components without varying the surface chemistry would permit OPV cells to adapt to ever-changing generations of photoactive materials. Here we report the implementation of chemically/environmentally robust, low-temperature solution-processed amorphous transparent semiconducting oxide alloys, In-Ga-O and Ga-Zn-Sn-O, as IFLs for inverted OPVs. Continuous variation of the IFL compositions tunes the conduction band minima over a broad range, affording optimized OPV power conversion efficiencies for multiple classes of organic active layer materials and establishing clear correlations between IFL/photoactive layer energetics and device performance.Solar to electrical energy conversion technologies have received great attention as abundant and sustainable resources (1–5). The diffuse nature of solar energy requires low-cost, large-area devices while maintaining high power conversion efficiency (PCE) (3). As a universal design strategy, many of the emerging thin film photovoltaic (PV) technologies such as bulk heterojunction (BHJ) organic, perovskite, quantum dot (QD), and CIGS (Cu-In-Ga-Se) solar cells are fabricated using a trilayer architecture, where light absorbers are sandwiched between two electrodes coated with various interfacial layers (IFLs) (6–9). Stringent requirements govern ideal IFL materials design. Energetically, their respective band positions should match those of the photoinduced built-in potentials to provide energetically continuous carrier transport pathways and to accommodate the maximum allowed output voltage. In recent reports, PV performance enhancement via IFL energetic tuning has been demonstrated for very specific BHJ organic, QD, and perovskite cell compositions (6, 8, 10, 11). However, true IFL energetic tunability has not been achieved and offers a challenging opportunity to optimize device performance.Fabricable from energetically diverse organic active layers, organic photovoltaics (OPVs) provide an excellent test bed for tuning IFL energetics and are the subject of this study. The basic BHJ OPV architecture contains a mesoscopically heterogeneous and isotropic, phase-separated donor–acceptor blend—a strategy to overcome the relatively short exciton diffusion lengths (∼10 nm) (2, 12, 13), sandwiched between hole-transporting (HT) and electron-transporting (ET) IFLs (2). These IFLs function to accommodate the cell built-in fields, to assist carrier extraction, and to suppress parasitic carrier recombination (2, 14–16). Indeed, studies on interfacial modification of HT electrodes verify that changing the band alignment profoundly affects OPV open circuit voltage (V_oc) and carrier lifetimes (17). Over the past decade, extensive efforts have focused on band gap engineering of OPV active layer materials, mainly by tuning the donor material highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energetics to enhance built-in potentials and maximize photon harvesting efficiency (18, 19). Consequently, compatible HT IFLs have been developed to tune their valence band minima (VBM) or HOMO energies to match that of a specific organic donor. In contrast, because of historical limitations in acceptor diversity, far less effort has been devoted to developing ET IFLs—to date, a few ET IFLs have been used in inverted OPVs, primarily TiO_x, ZnO_x, several polymers, self-assembled monolayers, and cross-linked fullerenes (11, 14–16, 20–24). Nevertheless, most of these materials have fixed band edge positions, significantly limiting their adaptability to emerging OPV materials systems with acceptors having different LUMO energies. The recent emergence of promising nonfullerene OPV acceptors—n-type small molecules and polymers having diverse electronic structures (1, 25–29)—illustrates the need for a design strategy to create ET IFLs with broadly tunable energy levels. Furthermore, moving from oxides, to polymers, to self-assembled materials incurs large, unpredictable variations in surface chemistry, compounding the challenge in energy level positioning. Also, factors such as adsorbates, interfacial dipoles, molecular orientations, and interface states confound predictable IFL/organic semiconductor interfacial energy alignment (30) and challenge precise, continuous energy level tuning for traditional organic semiconductor or vacuum deposited oxide IFLs. Thus, straightforwardly prepared ET IFL materials having broadly tunable conduction band minima (CBM) and work functions for adjusting band alignment would provide generalizable means to optimize current generation OPV performance and accommodate emerging organic donor/acceptor pairs.We report here an ET IFL design strategy using solution-processed amorphous metal oxide alloys. Continuous CBM tunability is achieved by alloying two or more electronically dissimilar oxides, realized by precursor compositional adjustment. We show that band edge energies can be dialed-in for indium–gallium oxide (a-IGO) and gallium–zinc–tin oxide (a-GZTO) (Zn:Sn = 1:1) IFLs, providing CBM tunability over 3.5–4.6 eV. The resulting films have excellent chemical/environmental stability, conformal coating, and good electron mobilities, ideal for solar cells as verified with several OPV classes where the IFL CBMs are systematically tuned to optimize performance. This includes acceptors with LUMO energies from −4.1 eV to −3.5 eV. To our knowledge, these results are the first example of band structure tuning in solution-processed amorphous oxide IFLs. Furthermore, the amorphous character enables flexible OPV fabrication by coating amorphous oxide electrodes.Note that the oxide CBM energies and acceptor LUMO energies are verified here by low-energy inverse photoemission spectroscopy (LEIPS) which provides excellent energy resolution and precision (∼0.1 eV) and negligible sample damage to organics (31, 32) (see more in SI Appendix, Figs. S2 and S3). It will be seen that OPV PV metrics closely track the IFL CBMs and can be optimized for the five BHJ OPV material sets in Fig. 1B, PBDT-BTI:PDI-CN2; PTB7:PC₇₁BM [poly[[4,8-bis[(2-ethylhexyl)oxy]-benzo[1,2-b:4,5-b'']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]-thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl-C71-butyric acid methyl ester)] (33); PTB7:ICBA (PTB7: indene-C60 bisadduct) (34); PTB7:P(NDI2OD-T2) [PTB7: poly[N,N''-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2''-bithiophene)] (35); and PDTG-TPD:PC₇₁BM (poly-dithienogermole-thienopyrrolodione: PC₇₁BM) (18).Open in a separate windowFig. 1.Tuning bottom electrode (ITO) interfacial layer (IFL) energy levels for inverted organic solar cells. (A) Inverted OPV and cross-sectional TEM image of an as-prepared ITO/a-IGO (50% Ga)/PTB7:PC₇₁BM/MoO₃/Ag solar cell. Insets show higher-resolution image and nanobeam electron diffraction pattern of a-IGO. (B) Chemical structures of the organic semiconductors used in this study and energy levels of the various inverted organic solar cell inorganic and organic components.