Room-temperature deposition of interfacial contact layers for organic photovoltaics

Interfacial contact layers (ICLs) are thin films of semiconductors that can be inserted between the active organic layers and electrodes of organic photovoltaics (OPVs) to significantly improve the performance and stability of the devices. There are two types of ICL: hole transport layers (HTLs) and electron transport layers (ETLs) that require high-work-function p-type and low-work-function n-type semiconductors, respectively.¹ As a result of these work-function requirements, a high built-in potential is established across the active layer of the OPV.¹ In addition, specific carrier types can be chosen to reduce the amount of recombination in the devices (i.e., to block the minority carriers).¹ To minimize losses from contact resistance and absorption, ICLs therefore need to be good electrical conductors and to be transparent to visible light.

Metal oxides are promising candidates for ICLs because of their versatile optical and electrical properties, as well as their excellent physical and chemical stability.² However, metal-oxide ICLs are usually deposited through expensive vacuum-based techniques that are incompatible with large-area manufacturing processes.² The use of these methods therefore counters the advantage of OPVs as printable low-cost energy sources. Indeed, commonly used sol-gel methods used to deposit metal-oxide thin films usually require high-temperature post-deposition annealing to achieve the desired optical and electrical properties.³ These methods are thus incompatible with plastic substrates and organic semiconductors.

As an alternative to vacuum-based techniques, solution-processing approaches for the deposition of metal-oxide ICLs are highly desired, and in this work we demonstrate several examples of ICLs deposited from metal-oxide nanoparticle (NP) suspensions. To produce our ICLs we synthesized the metal-oxide NP suspensions with optimized optical and electrical properties. We subsequently cast the suspensions into films that required minimal post-deposition processing. With our method, we thus meet the low-temperature (i.e., room temperature), large-area processing requirements for the fabrication of flexible electronics.⁴

Zinc oxide (ZnO) and titanium dioxide are the most common materials used for metal-oxide ETLs because they are n-type semiconductors that have wide band gaps and low work functions. Recently, we have achieved independent control over the work function (4.0–4.5eV)⁵ and band gap (3.25–3.55eV)⁶ of a ZnO-NP thin film by controlling the amount of diethanolamine (for the work function) or water (for the band gap) in butanol, and by using a microwave-assisted solvothermal synthesis. Reducing the work function in this way results in a proportionally higher open-circuit voltage (V_oc) in OPVs—see Figure 1(a)—because the built-in field is increased.⁵ Furthermore, the high wetting property of the ZnO-NP suspension on organic films allows us to make OPV devices with ZnO-NP/silver top contacts. We thus achieve an equivalent performance, and enhanced ambient stability, to that of widely used calcium/aluminum contacts: see Figure 1(b).⁶

Figure 1. (a) Open-circuit voltage (V_oc) versus zinc oxide (ZnO) nanoparticle (NP) work function for organic photovoltaics (OPVs) that contain a ZnO-NP-film interfacial contact layer. Reducing the work function increases the V_oc. Blue: OPV containing a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) organic layer. Red: OPV containing a P3HT:indene-C₆₀bisadduct (P3HT:ICBA) layer. Inset: Photographs of various ZnO-NP suspensions under UV illumination. (b) Power conversion efficiency (PCE) of devices with calcium/aluminum (Ca/Al) or zinc oxide/silver (ZnO/Ag) contacts, as a function of ambient exposure time (in hours). The ZnO/Ag contacts exhibit superior ambient stability. Inset: Schematic illustration of an OPV device. ITO: Indium tin oxide electrode. PEDOT:PSS: Poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate. Al or Ag: aluminum or silver electrode.

We have also investigated both binary metal oxides and complex oxides as potential HTLs. Although molybdenum trioxide (MoO₃) is an n-type semiconductor, it is commonly used as an HTL because of its high work function (>5eV). Through a microwave-assisted solvothermal reaction, we demonstrated the first room-temperature solution-deposited MoO_x-NP (where the number x represents variable stoichiometry) HTL (1–4nm thick) in OPV devices.^{7, 8} By using an in situ oxidation reaction, we can increase the Mo⁶⁺ fraction of our MoO_x-NP films from 10 to 70%. We can thus realize improved OPV performances, and can achieve an efficiency comparable to that of an evaporated MoO₃ HTL: see Figure 2(a).⁷ To investigate other binary oxides, we developed a general catalytic oxidative method to synthesize suspensions of ultrasmall (1–2nm) NPs, e.g. of MoO₃, tungsten trioxide, nickel oxide, and cobalt oxide: see inset to Figure 2(b).⁹ We find that spin-coated films obtained from these NP suspensions exhibit high work functions (>5.1eV) and—see Figure 2(b)—produce OPV devices with excellent performances.⁹

Figure 2. (a) The PCE of a P3HT:PCBM OPV device as a function of Mo⁶⁺ fraction in molybdenum oxide (MoO_x)-NP films (where the number x represents variable stoichiometry). Inset: Photographs of an as-synthesized MoO_x-NP suspension, and a MoO_xsuspension after hydrogen peroxide (H₂O₂) treatment (used to increase the Mo⁶⁺ fraction). (b) Current density-voltage (J-V) response under a standard testing condition (air mass 1.5 global testing spectrum at 100mWcm⁻²) for poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC₇₁BM) OPVs with different metal-oxide hole-transport layers. Layers of evaporated MoO₃, MoO₃-NPs, tungsten oxide (WO₃)-NPs, nickel oxide (NiO_x)-NPs, and cobalt oxide (CoO_x)-NPs are shown by the black, red, orange, green and blue data, respectively. The solution-processed NP-film-OPVs exhibit device performances comparable with OPVs that include evaporated films. Inset: Photographs of the various metal-oxide NP suspensions.

Our search for highly conducting p-type oxide semiconductors for HTLs, led us to delafossite compounds. These compounds have the form Cu⁺¹X⁺³O₂, where Cu is copper, and X is aluminum, gallium, or chromium, and O is oxygen. Delafossites have previously been used as photocathodes for p-type dye-sensitized solar cells and water-splitting devices.^{10, 11} We have demonstrated a microwave-assisted synthesis of copper gallium oxide (CuGaO₂) and copper chromium oxide (CuCrO₂) NPs. In this approach, we require a much shorter reaction time (1–8 hours) compared with conventional hydrothermal methods (∼60 hours).^{12, 13} In addition, the excellent optical transparency of CuGaO₂ means that we can achieve a similar performance with a thick (about 280nm) CuGaO₂ HTL as we could with an evaporated MoO₃ (10nm-thick) HTL in thick-active-layer OPV systems: see Figure 3(b).¹² Thick HTLs such as this are more compatible with large-area and high-volume printing processes, but the large NP size—see Figure 3(a)—leads to rough CuGaO₂ films. This causes shunted device characteristics, i.e., reduced V_oc and an increased slope in the negative bias, and thus reduces the power conversion efficiency of high-performance thin-active-layer OPVs. To overcome this issue, we synthesized a suspension of sub-10nm CuCrO₂ NPs, as shown in Figure 3(a). We then cast a smooth and transparent film from this suspension to produce efficient thin-active-layer OPV devices: see Figure 3(b).¹³ Our results demonstrate that the copper-based delafossite materials are promising HTL candidates.

Figure 3. (a) Transmission electron microscope images of the delafossite NPs copper gallium oxide (CuGaO₂) and copper chromium oxide (CuCrO₂). Insets: Photographs of the corresponding suspensions of these NPs. (b) J-V response under a standard testing condition (air mass 1.5 global testing spectrum at 100mWcm⁻²) for 210nm-thick P3HT:PCBM OPVs with evaporated (evp) MoO₃and UV-ozone-treated CuGaO₂ HTLs, and for 70nm-thick PCDTBT:PC₇₁BM OPVs with UV-ozone-treated CuGaO₂ and CuCrO₂HTLs. Thick-active-layer CuGaO₂ devices exhibit efficiency as good as the evaporated MoO₃devices, but the larger NP size of the CuCaO₂increases the film roughness and leads leads to shunted device characteristics in thin-active-layer CuGaO₂devices. Switching to CuCrO₂ NP films overcame this problem.

In summary, we have shown several examples of the synthesis of metal-oxide NP suspensions. These suspensions can be used to produce thin films that function as efficient ICLs in OPV devices. We intend to continue exploring low-temperature solution-processing methods for the deposition of metal-oxide thin films. We also plan to extend their application to solar cells beyond OPV devices, by investigating hybrid perovskites, kesterite, copper indium gallium selenide, and cadmium telluride materials.

We acknowledge support from National Science Foundation (DMR-1305893), the University of Texas at Dallas, and the Texas Instruments Distinguished Chair in Nanoelectronics.