Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2020 Sep 18;5(39):25264–25272. doi: 10.1021/acsomega.0c03474

Initial Engineering and Outdoor Stability Assessment of “Gray/Black” Fullerene-Free Organic Photovoltaics Based on Only Two Complementary Absorbing Materials: A Tetrabenzotriazacorrole and a Subphthalocyanine

Hasan Raboui †, David S Josey †, Yin Jin †, Timothy P Bender †,‡,§,*
PMCID: PMC7542850  PMID: 33043204

Abstract

graphic file with name ao0c03474_0009.jpg

Broad absorption is a desired characteristic of materials employed in the photoactive layers of organic photovoltaic (OPV) devices. Here, we have identified tetrabenzotriazacorroles (Tbcs) as complementary absorbing chromophores and electron donors to the promising nonfullerene acceptors boron subphthalocyanines (BsubPcs). These two materials, which can be utilized as donor–acceptor pairs within fullerene-free OPVs, yield spectral coverage over the entire visible range of 300–750 nm. Oxy phosphorus Tbc derivative (POTbc) was employed as an electron donor and paired initially with multiple BsubPc derivatives having a distribution of highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels in planar heterojunction OPVs. These devices were “gray/black” due to the broad absorption across the visible spectrum. Upon screening, the partially halogenated chloro hexachloro BsubPc (Cl–Cl6BsubPc) showed the greatest promise for coupling with POTbc. The thickness ratio and total thickness of the active layer were then probed in order to identify the optical and electrical limitations on the POTbc/Cl–Cl6BsubPc-based OPV device. A maximum power conversion efficiency (PCE) of 2.13% was achieved at 60 nm total thickness of the active layer and 1 to 3 (POTbc to Cl–Cl6BsubPc) thickness ratio. Outdoor stability of the champion device was evaluated using protocols established by International Summits on OPV Stability and was found to be on par with an α-sexithiophene/Cl–Cl6BsubPc baseline OPV.

Introduction

Organic photovoltaics (OPVs) have been a very active field of research in the past two decades in pursuit of a sustainable source for energy generation. However, OPVs are often constrained by the relatively narrow light absorption range of organic materials in comparison to their inorganic counterparts. Researchers have broadly deviated from the use of fullerenes (C60 and C70) as electron-accepting materials due to their inefficient synthetic process1,2 and their unfavorable absorption in the visible spectrum.3,4 The introduction of materials that absorb broadly in the visible spectrum and of the usage of nonfullerene-based electron acceptor with high absorptivity are of great interest.

Multiple planar heterojunction OPV device engineering approaches such as tandem5−7 and cascade8−11 architectures have been proposed in order to overcome the absorption obstacles. However, these approaches require the formation of more than one exciton rectifying interface, leading to more complexed device structures. This complexity limits and challenges the understanding of the role of each material in the optoelectronic phenomenon and thus their development. Similar approaches have been addressed using ternary mixtures within bulk-heterojunction OPVs12−16 including the use of copolymers synthesized by the combination of electron-deficient and electron-rich comonomers, so-called push–pull or donor–acceptor polymers, through relatively difficult synthetic processes.17−20 The bulk-heterojunction architecture also often leads to an increased complexity of engineering the OPV systems due to the nanophase morphology. This inhibits understanding and decouples the relationship between the molecular structure of each material and the morphology of the active layer on the optics, charge transport, and performance of the device.

Recently, boron subphthalocyanines (BsubPcs, Figure 1) have shown excellent electron-accepting properties and produced planar heterojunction OPV cells with power conversion efficiencies (PCEs) beyond 8%.8,21−24 BsubPcs absorb strongly in wavelengths around 600 nm with high extinction coefficients ( ε > 70,000 L mol–1 cm–1) enabling the capture of a great fraction of the visible light spectrum.25 BsubPcs are relatively easy to synthesize from industrially produced precursors and even show the potential for room temperature synthesis.26 BsubPc-based OPV devices also exhibit greater outdoor stability when paired as electron acceptors such as α-sexithiophene (α-6T)27 in comparison to their fullerene-based counterparts,28 which displayed a burn-in effect within the first day of operation. In a similar report but employing boron subnaphthalocyanine (BsubNc) as well, α-6T was identified as the prominent degradation component in the α-6T/BsubNc/BsubPc cascade system.29 The electronic properties of BsubPcs are easily tunable. For example, control of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of BsubPcs for OPV applications was proven effective through simple peripheral halogenation.21,23,30 Additionally, BsubPcs and other phthalocyanine (Pc) derivatives1,2,31 are known to have minimal embedded energy in their chemical production, thus making them viable for ultimate mass production.

Figure 1.

Figure 1

Open in a new tab

Molecular structures and HOMO/LUMO energy levels relative to vacuum; Cl–BsubPc, Cl–Cl12BsubPc, and POTbc energy levels were retrieved from the literature.30,33,41 Cl–Cl6BsubPc energy levels are estimated to be halfway between Cl–BsubPc and Cl–Cl12BsubPc energy levels, which are consistent with the cyclic voltammetry data found in the literature.42

Now that their potential as electron acceptors is well established, the question becomes: what are additional appropriate electron donor materials to be paired with BsubPcs and what are the molecular design criteria to be considered? There are only a handful of materials that have been paired with BsubPcs. The top performing BsubPc-based OPVs are planar heterojunctions and are based on α-6T as a donor.8,22 The issue with using α-6T is that its absorbance is limited to the blue portion of the visible spectrum. Other molecular donors that have been employed include tetracene and pentacene21,30 as well as Pcs and BsubNcs.23 Polymers have been also employed as electron donors but in bulk-heterojunction architectures.24,32

Recently, we started investigating the functionality of an entire new family of molecules called tetrabenzotriazacorroles (Tbcs) in OPV devices.33 Tbcs are compounds synthesized from abundant Pc precursors34 and have similar structures, differing only by the omission of one bridging imine nitrogen within the macrocyclic structure (Figure 1).The effect of this small molecular change on optical and electrochemical properties is immense. While Pcs are strong absorbents of red light, Tbcs can capture blue (Soret band) and red (Q-band) portions of the solar spectrum simultaneously. Electrochemically, Tbc chromophores hold a higher formal charge of 3– versus their Pc analogues, which hold a 2– charge. Recently, we demonstrated the functionality of Tbcs in OPVs using phosphorus tetrabenzotriazacorrole (POTbc, Figure 1) as a representative molecule paired with conventional organic semiconductors (fullerenes and α-6T). While POTbc showed functionality both as an electron donor and as an electron acceptor, POTbc showed larger promise as an electron-donating material presumably due to its shallow HOMO level and the higher formal charge (3–) of the Tbc macrocycle. Moreover, the chemical processes to produce Tbcs are versatile and enable fine-tuning of their molecular structures; the structure of Tbc can be controlled by incorporating different elements of the periodic table into their central cavities35,36 by the axial position of the central atom37 and by peripheral substitution of the macrocycle.38 These chemical handles have the potential to improve the optoelectronic properties as well as solid-state packing of Tbcs and thus broaden their roles in OPV devices. Lastly, the Tbc synthesis proceeds in 1–2 steps from industrially relevant materials with high yields at a moderate scale (e.g., > 80% at 0.5 g scale)37 unlike many of the top-performing organic photovoltaic materials, a desired characteristic the pursuit of sustainable energy solutions.39,40

BsubPcs conveniently absorb exactly in the mid gap between the Soret and Q-bands of Tbcs (Figure 2). Both materials exhibit absorption coefficients that are in the same order of magnitude, making them ideal materials for pairing in OPV devices. A synthetic example of this concept was demonstrated by the group of Kobayashi, who synthesized multiple Tbc derivatives including BsubPc-Tbc-BsubPc trimer that absorbs throughout the entire visible spectrum (300–750 nm).38,43,44 With the established functionality of Tbcs as electron donors and of BsubPcs as electron acceptors, constructing OPV devices absorbing the entire visible portion of the solar spectrum was seen as highly possible.

Figure 2.

Figure 2

Open in a new tab

Solid-state absorption coefficient plots of POTbc33 and Cl–Cl6BsubPc.

In this paper, we assess the potential for engineering the complementary absorbing pair of POTbc as a representative example to the Tbc family and BsubPc compounds into black and fullerene-free bilayer planar heterojunction OPVs. This complementary absorption engineering approach includes: (1) scoping three BsubPc derivatives with different HOMO/LUMO energy levels as electron acceptors for optimal performance, (2) optimizing the layer thickness ratio between the donor and the optimal BsubPc acceptor, and (3) increasing the total active layer thickness while maintaining the optimal thickness ratio in an attempt to achieve %T = 0 in the visible spectrum. Optical measurements of the thicker devices are also collected to decouple the optical and electronic effects on device performance. The outdoor stability of the champion device is then examined in accordance to the protocols developed during the International Summit on OPV Stability (ISOS),45 which also represents the first stability report of a Tbc-based OPV device.

Results and Discussion

Complementary Absorption Engineering

Three BsubPc derivatives with varying levels of peripheral halogenation (Cl–BsubPc, Cl–Cl6BsubPc, and Cl–Cl12BsubPc, Figure 1) were employed as electron acceptors paired with POTbc initially in OPV cells at a 1 to 1 thickness ratio and a 40 nm total thickness (Figure 3). Three BsubPc derivatives were considered and used to establish the optimum offsets between the ionization potential of POTbc and electron affinity of BsubPc, while preserving the desired optics of the OPV device. The integration of these two materials into OPVs did result in OPV cells that were gray/black in color. This was anticipated due to their complementary absorption over the entire range of the visible spectrum and similar absorption coefficients (Figure 2). The open-circuit voltage (VOC) of the cells dropped as expected with the decreasing offset between the HOMO of the donor and the LUMO of the acceptor (Figure 1). However, the short-circuit current density (JSC) peaked when Cl–Cl6BsubPc was used as the electron acceptor. The external quantum efficiency (EQE) plots show a continuous increase of the photocurrent contribution from the POTbc as the LUMO offset between the donor and acceptor increased (Figure 3). However, the photocurrent contribution of the BsubPc layer did not correlate with the HOMO levels offset, as one would expect and was highest when Cl–Cl6BsubPc was used. Cl–Cl6BsubPc was therefore selected as the acceptor for further thickness optimization, because it exhibited superior JSC, fill factor (FF), and PCE compared to the other BsubPcs when paired with POTbc. Detailed optical characterization of POTbc and Cl–Cl6BsubPc is attached within the electronic supplementary material section.

Figure 3.

Figure 3

Open in a new tab

POTbc paired with multiple BubPcs as acceptors. (A) Averaged current density–voltage characteristic plot, (B) external quantum efficiency plot, and (C) averaged OPV characteristic parameters of POTbc (20 nm)/BsubPc (20 nm) OPV cells. The shaded area around the current density–voltage curves and error bars on the average OPV parameters are standard deviation over 11–12 pixels.

A thickness ratio optimization was conducted on the POTbc/Cl–Cl6BsubPc system; the POTbc/Cl–Cl6BsubPc thickness ratio was changed systematically while keeping the total thickness of the entire active layer constant at 40 nm (Figure 4). This was done to probe charge balance in the device since the hole mobility of POTbc is unknown. The devices with a 1 to 3 thickness ratio of POTbc to BsubPc achieved superior JSC, FF, and PCE. One implication for this optimal thickness ratio is that the electron mobility in the Cl–Cl6BsubPc layer could be higher than hole mobility in the POTbc layer. Another possibility is that the bathocuproine (BCP) layer is much more efficient in extracting electrons from Cl–Cl6BsubPc than poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) is at extracting holes from POTbc. It is noteworthy that the optimal POTbc/Cl–Cl6BsubPc ratio is the same ratio we achieved with the POTbc/C70 system using a different thickness optimization approach.33 By monitoring the EQE plots, the photocurrent contribution of POTbc showed minimal improvement when the layer thickness increased (Figure 4). This is indicative of short exciton diffusion lengths in the POTbc layer. Conversely, the photocurrent contribution of Cl–Cl6BsubPc exhibited a monotonic behavior with respect to Cl–Cl6BsubPc thickness, which is attributed to greater absorption of incident light. Insignificant variations were observed in VOC except when the POTbc/Cl–Cl6BsubPc thickness ratio was increased from 3 to 1.

Figure 4.

Figure 4

Open in a new tab

Thickness ratio optimization at a constant total thickness (40 nm). (A) JV plot and (B) EQE plot of OPV devices for active layer thickness optimization. (C) Average OPV parameters changing on differential layer thickness. Colors in plots A, B, and C are corresponding to the related layer thickness outlined above. The shaded area around the current density–voltage curves and the error bars on the average OPV parameters are standard deviation over 8–15 pixels.

The total thickness of the active layer (POTbc/Cl–Cl6BsubPc) was increased beyond 40 nm at the optimal thickness ratio (1 to 3) in an attempt to achieve a %T = 0 and make an opaque OPV. Both photovoltaic and optical characterization of the devices were conducted (Figure 5). The optimal active layer thickness was determined to be 60 nm, achieving a PCE of 2.13%. The VOC and FF remained mainly invariant while the JSC reached a maximum at active layer thicknesses of 50 and 60 nm. The retention of FF is indicative of maintaining a sufficient charge balance between the POTbc and the Cl–Cl6BsubPc layers at higher thicknesses. As the JSC was the limiting factor to the PCE as the devices thickened, deeper understanding of the optics role was required. This was achieved by observing changes in the EQE and optical measurement plots. In the EQE plots, the photocurrent contribution of the POTbc from the bluest light (shortest wavelength) in the Soret band (∼350 nm) decreased continuously as the active layer thickness increased. Conversely, the photocurrent contribution of the reddest light (longest wavelength) BsubPc absorbed dropped as the thickness of the devices increased. It is unclear what is causing these drops in the EQE.

Figure 5.

Figure 5

Open in a new tab

Photovoltaic (A–C) and optical (D–F) characterization of thicker POTbc/Cl–Cl6BsubPc at 1 to 3 thickness ratio. The shaded area around the current density–voltage curves and error bars on the average OPV parameters are standard deviation over 8–15 pixels. Optical measurements were conducted on the devices without the silver electrode.

The changes in the optical properties of the OPV cells were monitored through transmission and reflection measurements of the representative cells (without the reflective Ag electrode present, Figure 5). As expected, transmission of the light through the cells decreased monotonically as the thickness of the cells increased. Unfortunately, transmission was far from the %T = 0 goal in the POTbc regions. Since the extinction coefficients of POTbc and Cl–Cl6BsubPc are very similar, this is attributed to the mismatch between the POTbc and Cl–Cl6BsubPc thicknesses required for optimal device performance. Also, the change in transmission was not only a result of absorption but also increased reflection from both layers (Figure 5). The reflection of the Cl–Cl6BsubPc layer peaked at 70 nm device thickness, while that of the POTbc layer was continuously increasing. In the absorption spectra, the Cl–Cl6BsubPc peak increased continuously in a sign that the limitation on the photocurrent generation in the Cl–Cl6BsubPc layer observed in the EQE was not caused by optics. The maximum absorption that Cl–Cl6BsubPc achieved was only 69% due to the relatively intense reflection that occurs at the surface of that same layer. The Soret band peak of the POTbc showed a maximum intensity when the active layer was 70 nm in thickness. Most surprisingly, the absorption peak of the POTbc Q-band was continuingly decreasing as the devices were thickened. This was caused by the significant reflection from both the POTbc and the Cl–Cl6BsubPc layers.

As a result of both photovoltaic and optical analyses, we concluded that the POTbc photocurrent contribution from its Q-band and Soret band using this device configuration will always be limited by the optics of the system. However, there may be some charge transport and excitonic limitations, too. On the other hand, the drop in the Cl–Cl6BsubPc peak in the EQE as the device thickness increased was not caused by optics, as absorption was continuously increasing, but was rather caused by excitonic or charge transfer limitations. The issue of imbalanced absorption between the two layers, assuming it arises from a short diffusion length and low hole mobility of POTbc, can be tackled by either a device engineering approach or a molecular design approach. Short exciton diffusion lengths can be overcome by either constructing bulk-heterojunction OPVs via thermal coevaporation or by applying an exciton-blocking layer to the donor side of the device as previously demonstrated with similar systems.10 Electron extraction from Cl–Cl6BsubPc was previously improved by replacing BCP with a mixed C60/BCP layer.23 Alternatively, the hole mobility and/or exciton diffusion length of the Tbc layer can also be via molecular design. We have recently made a significant progress in developing a synthetic process to produce silicon Tbc derivatives at a scale enabling their application in OPVs.37 This process can potentially be extended to many elements in the periodic table increasing the accessibility of Tbcs with varying molecular structures and as a result, their electronic properties.

Outdoor Stability Testing

Outdoor stability of the champion device POTbc (15 nm)/Cl–Cl6BsubPc (45 nm) was also evaluated in accordance to the ISOS protocols.45 We have established previously in our laboratory the outdoor stability of BsubPcs as electron acceptors paired with α-6T and as electron donors paired with C60.27,28 The main purpose of this study is to evaluate the stability of POTbc as a photoactive material. Stability was measured against an α-6T (50 nm)/Cl–Cl6BubsPc (20 nm) baseline OPV device. The devices were fabricated, encapsulated, and initially tested for time-zero performance (T0) under an inert atmosphere. The performance of the devices was monitored in real-time under sunlight on days without precipitation, operating under a passive load corresponding to their respective maximum power point (MMP) that was determined at T0 (supplementary information). Before each day of outdoor testing, full current density–voltage measurements were collected (Figure 6). The standard ISOS protocol is to repeat this sequence until the PCE reaches 80% of its original value at T0 (designated T80). However, the POTbc OPVs exhibited a burn-in effect of 18% drop in PCE after the first day of sunlight exposure and then stabilized beyond that point (designated TS,0). The PCE of the POTbc devices were monitored until they reached 80% of their stabilized PCE value (designated TS,80). The main driving force of the burn-in effect was observed to be a 13% drop in the FF (Figure 7). The strong retention of POTbc and Cl–Cl6BsubPc peaks in the EQE indicate that the degradation is not caused by a photobleaching mechanism or other molecular degradation but is likely due to morphological or interfacial alteration, something the FF is attributable to. More specifically, the change in FF is caused by a change in the slope closer to the open-circuit condition, which is indicative of an increase in the series resistance. Surprisingly, the drop in the EQE of the POTbc device is negligible and does not correspond to the modest drop in the JSC (∼15%). It is unclear why this is the case. One explanation is that the aged device could be more sensitive to low light intensity conditions experienced during EQE testing relative to the one sunlight intensity experienced during the current density–voltage measurements. That being said, the POTbc devices exhibited stability that is on par with the α-6T baseline producing 1.28 and 1.15 kWh m–2 of energy despite the initial burn-in effect, indicating further that Tbcs are good candidates for OPV device applications.

Figure 6.

Figure 6

Open in a new tab

Outdoor stability testing of POTbc/Cl–Cl6BsubPc and baseline α-6T/Cl–Cl6BsubPc devices. (A) Normalized average PCE (left y axis) and OPV energy output (right y axis) vs insolation (accumulated irradiance), with the average solar energy conversion efficiency indicated for each set of devices. Normalized average (B) VOC, (C) JSC, and (D) FF are also shown vs insolation. POTbc/Cl–Cl6BsubPc percentages are normalized to values after the first day of sunlight exposure to decouple long-term degradation trends from the immediate burn-in effect. Amount of time spent outdoors is indicated in 10 h intervals.

Figure 7.

Figure 7

Open in a new tab

(A) J−V plot, (B) EQE plot, and (C) average OPV parameters of POTbc/Cl–Cl6BsubPc and α-6T/Cl–Cl6BsubPc OPVs before and after outdoor stability testing. Shaded area around the current density–voltage curves and error bars on the average OPV parameters are standard deviation over 2–8 pixels. T0 is before outdoor testing, TS,0 is after stabilizing following an initial burn-in effect, T80 is after reaching 80% of the PCE at T0, and TS,80 is after reaching 80% of the PCE at TS,0.

Conclusions

POTbc was paired with the complementary light absorbing material BsubPc. Three different BsubP derivatives were initially assessed at a 1 to 1 thickness ratio to produce gray/black OPV cells. Thickness ratio optimization was then conducted on the most effective pair, POTbc/Cl–Cl6BsubPc. The optimal device performance was achieved when the BsubPc layer was thicker at 1 to 3 (POTbc to Cl–Cl6BsubPc) thickness ratio. Thicker devices with the optimal ratio were then fabricated and fully characterized. Analyses of the photovoltaic performance (current–voltage and EQE plots) found a maximum PCE (2.13%) with 60 nm active layer thickness. Optical characterization (transmission, reflection, and absorption) showed that the optics of the system restricted the photocurrent contribution of the POTbc layer. Cl–Cl6BubPc, on the other hand, was not limited optically, as absorption of the Cl–Cl6BsubPc continuously increased as the devices thickened. Finally, the outdoor stability testing of the champion POTbc/Cl–Cl6BsubPc OPV device was conducted showing promise for this system. Despite the burn-in effect it exhibited, it was stable beyond the initial drop and is on par with the stability of the more known α-6T/BsubPc system. This study established Tbcs, a family of molecules potentially as big as Pcs, and BsubPcs as complementary pair of materials for OPV application due to their uniquely matching absorption profiles. Looking forward, we are continuing to explore the relatively unknown chemistry of Tbcs in order to improve both efficiency and stability of this system; more specifically, we are working to molecularly engineer Tbc molecule(s) that do not only complementarily absorb light to BsubPcs but also match the charge-carrier mobility and exciton diffusion length of BsubPcs, with the goal of engineering a stable and black OPV with %T = 0.

Experimental Section

Materials

PEDOT:PSS (Clevios P VP AI 4083, Heraeus), Bathocuproine (BCP 99.6%, Sigma Aldrich), Silver (99.99%, Angstrom), α-6T (Lumtec), and molybdenum oxide (MoOx, 99.999%, Strem) were purchased and used as received.

Synthesis of POTbc

POTbc was synthesized according to our previously reported process.3331P NMR (600 MHz, pyridine) δ −202.04. High-resolution mass spectroscopy (HRMS) calcd for C32H16N7PO, 545.1154; found, 545.1142. EA calcd for C32H16N7PO: C, 70.5; H, 3.0; N, 18.0. Found: C, 70.8; H, 3.0; N, 17.7. UV–vis (DMSO) λmax = 442 nm;

Synthesis of Cl–Cl6BsubPc and Other BsubPcs

Cl–Cl6BsubPc was synthesized as described by Morse et al.46 The resulting powder was then doubly sublimed using a train sublimation apparatus using CO2 as carrier gas. High-resolution mass spectroscopy (HRMS) calcd for C24H6N6Cl7B, 633.8567; found, 633.8571. HPLC (20% DMF, 80% acetonitrile) RT = 7.3 min (purity >99.5%). The other BsubPcs were synthesized and purified in a similar fashion.

General Methods

Low-resolution mass spectra (LRMS) and high-resolution mass spectra (HRMS) were determined using Waters GC Premier using a time-of-flight spectrometer with electron ionization source (TOF-EI MS). 31P Nuclear magnetic resonance (NMR) spectra were recorded using an Agilent DD2 600 spectrometer at 23 °C in 10% CDCl3 solution in pyridine, operating at 600 MHz. Chemical shifts (δ) are reported in parts per million (ppm) referenced to triethyl phosphite (137.29 ppm). High-pressure liquid chromatography (HPLC) analysis was conducted using a Waters 2695 Separations Module with a Waters 2998 Photodiode Array and a Waters 4.6 mm × 100 mm SunFire C18 3.5 μm column. HPLC grade acetonitrile was eluted with an isocratic flow of 80/10 acetonitrile/dimethylformamide at 0.6 mL/min during operation. Optical experiments were acquired using a PerkinElmer Lambda 1050 UV/Vis/NIR spectrometer using a PerkinElmer quartz cuvette with a 10 mm-path length for solution samples and an attached 150 mm integrating sphere for solid samples to capture the scattered and reflected light.

Devices Fabrication and Characterization

All OPVs were fabricated on patterned ITO substrates (Thin Film Device Inc., Figure S3) with 145 nm ITO thickness and a sheet resistance of 15 Ω/□. The ITO surfaces were ultrasonically cleaned in detergent solution, distilled water, acetone, methanol, and finally dried in flowing nitrogen. Subsequently, ITO was treated by plasma for 5 min to remove carbon residues. After plasma treatment, a CHEMAT Technology KW-4A spin coater was used to spin a layer of PEDOT:PSS (Clevios P VP AI 4083) at 500 rpm for 10 s and 4000 rpm for 30 s on the ITO electrode. Substrates were then annealed in a temperature at 115 °C for 10 min. The device structure grown by thermal evaporation with base pressures of 10 × 10–7 torr consists ITO/PEDOT:PSS/active layer/BCP (5.5 nm)/Ag (80 nm). The BCP layer thickness for stability testing was 7–7.5 nm. Between deposition of the BCP layer and the Ag cathode, devices were transferred from the vacuum chamber to nitrogen atmosphere glove box through a gate without exposure to air to attach a shadow mask consisting a five active pixel area of 0.2 cm2 openings and then transferred back to the vacuum chamber. The mask used for stability testing comprised four 0.4 cm2-sized pixels. For encapsulation of the devices undergoing outdoor stability testing, a 50 nm layer of MoOx was deposited over the devices to serve as a physical barrier between the organic/metal layers and the epoxy. A drop of epoxy was then applied to the substrate, covered with a glass coverslip, and cured under a lamp for 20 min. Layer thickness and deposition rate were monitored using a quartz crystal microbalance (QCM). After the Ag layer was deposited, the OPVs were transferred directly from the vacuum chamber back to the nitrogen atmosphere glove box. Silver paste (PELCO Conductive Silver 187) was applied to the end Ag of electrode and ITO contact point to enhance electrical contact and left to cure for 30 min before testing. A 300 W ozone free xenon arc lamp with an Air Mass 1.5 Global filter fed through a Cornerstone 260 1/4 m monochromator and then into the glove box by way of a single branch liquid light guide was utilized as a solar simulator. The illumination levels for the test cell were kept at 100 mW cm–2. A silicon UV photodetector was used to calibrate the measurements. The current densities versus voltage (J–V) characteristics and external quantum efficiency (EQE) were recorded using a Keithley 2401 Low Voltage SourceMeter controlled by a custom LabView program and a Newport Optical Power Meter 2936-R controlled by TracQ Basic software in the nitrogen atmosphere glove box.

Stability Testing

The protocols developed for outdoor stability testing were based on the consensus protocols from the international summit on OPV stability for advanced outdoor testing (ISOS-O3)45 and follow our previous outdoor stability studies.27 Briefly, on days without precipitation from mid-May to mid-August in 2017 (Table S2), J–V curves all devices were measured under illumination of a 100 W Xe arc lamp (Oriel) with an AM1.5G standard filter in place using a Keithley 2401 Low Voltage SourceMeter. After this indoor J–V characterization, devices were taken outside and placed in direct sunlight from ∼3 hours before solar noon until ∼3 hours after solar noon. Devices were mounted on a static, custom-built apparatus on a rooftop in Toronto, Canada (latitude = 43.7o and longitude = −79.4°) with the front side oriented towards the equator, tilted at ∼43.7o. While outdoors, some devices were passively loaded with resistors corresponding to the MPP calculated from T0 voltage sweeps. The voltage output from each passively loaded device was continuously monitored at 1 Hz. Differences in how the MPP of the devices changed over device lifetimes resulted in passive load being more effective at keeping some devices at MPP than others. Nonloaded devices were held at an open-circuit while outdoors but not monitored. Substrate temperature was measured on the backside of a representative device and continuously monitored at 2 Hz while outdoors. Irradiance, relative humidity, and wind speed were monitored within ∼6 m of the testing apparatus. Irradiance was monitored continuously using a pyranometer (LP02, Hoskin Scientific Ltd.). EQEs were measured as described above, once at the beginning and once at the end of stability testing.

The protocols conform with the advanced outdoor testing (ISOS-O3) with the following exceptions: the MPP loading for the POTbc devices only meets the requirements of the basic level of testing (ISOS-O1) as the resistance at MPP deviated by more than 10%, and the resistors were not adjusted. The location of the wind speed monitoring was not quite within the recommended proximity to the testing apparatus (∼6 instead of 1.2 m) but is still reported alongside air temperature and relative humidity data in Figures S5–S8.

Acknowledgments

We gratefully acknowledge support from the Natural Sciences and Engineering Research Council (NSERC) through a discovery grant to T.P.B. This work was also supported by a Hatch Graduate Scholarship, Walter C. Sumner Memorial Fellowship, and Alexander Graham Bell Canada Graduate Scholarship to D.S.J. within the Faculty of Applied Science and Engineering (FASE) University of Toronto.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03474.

  • Transmission, reflection, absorption spectra, and absorption coefficients of materials utilized in this study in the solid state; dimensions of glass substrates used to engineer OPVs and consider their outdoor stability; and data accumulation of OPVs in outdoor environment (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c03474_si_001.pdf (701.3KB, pdf)

References

  1. Anctil A.; Babbitt C. W.; Raffaelle R. P.; Landi B. J. Material and Energy Intensity of Fullerene Production. Environ. Sci. Technol. 2011, 45, 2353–2359. 10.1021/es103860a. [DOI] [PubMed] [Google Scholar]
  2. Anctil A.; Babbitt C. W.; Raffaelle R. P.; Landi B. J. Cumulative energy demand for small molecule and polymer photovoltaics. Prog. Photovoltaics: Res. Appl. 2013, 21, 1541–1554. 10.1002/pip.2226. [DOI] [Google Scholar]
  3. Choi J. W.; Kim C.-H.; Pison J.; Oyedele A.; Tondelier D.; Leliège A.; Kirchner E.; Blanchard P.; Roncali J.; Geffroy B. Exploiting the potential of 2-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile as an efficient donor molecule in vacuum-processed bulk-heterojunction organic solar cells. RSC Adv. 2014, 4, 5236–5242. 10.1039/c3ra47059h. [DOI] [Google Scholar]
  4. Xiao X.; Zimmerman J. D.; Lassiter B. E.; Bergemann K. J.; Forrest S. R. A hybrid planar-mixed tetraphenyldibenzoperiflanthene/C70 photovoltaic cell. Appl. Phys. Lett. 2013, 102, 073302 10.1063/1.4793195. [DOI] [Google Scholar]
  5. Shim H.-S.; Lin F.; Kim J.; Sim B.; Kim T.-M.; Moon C.-K.; Wang C.-K.; Seo Y.; Wong K.-T.; Kim J.-J. Efficient Vacuum-Deposited Tandem Organic Solar Cells with Fill Factors Higher Than Single-Junction Subcells. Adv. Energy Mater. 2015, 5, 1500228. 10.1002/aenm.201500228. [DOI] [Google Scholar]
  6. Gevaerts V. S.; Furlan A.; Wienk M. M.; Turbiez M.; Janssen R. A. J. Solution Processed Polymer Tandem Solar Cell Using Efficient Small and Wide bandgap Polymer:Fullerene Blends. Adv. Mater. 2012, 24, 2130–2134. 10.1002/adma.201104939. [DOI] [PubMed] [Google Scholar]
  7. You J.; Dou L.; Yoshimura K.; Kato T.; Ohya K.; Moriarty T.; Emery K.; Chen C.-C.; Gao J.; Li G.; Yang Y. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 2013, 4, 1446. 10.1038/ncomms2411. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Cnops K.; Rand B. P.; Cheyns D.; Verreet B.; Empl M. A.; Heremans P. 8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer. Nat. Commun. 2014, 5, 3406. 10.1038/ncomms4406. [DOI] [PubMed] [Google Scholar]
  9. Cha H.; Chung D. S.; Bae S. Y.; Lee M.-J.; An T. K.; Hwang J.; Kim K. H.; Kim Y.-H.; Choi D. H.; Park C. E. Complementary Absorbing Star-Shaped Small Molecules for the Preparation of Ternary Cascade Energy Structures in Organic Photovoltaic Cells. Adv. Funct. Mater. 2013, 23, 1556–1565. 10.1002/adfm.201201913. [DOI] [Google Scholar]
  10. Verreet B.; Cnops K.; Cheyns D.; Heremans P.; Stesmans A.; Zango G.; Claessens C. G.; Torres T.; Rand B. P. Decreased Recombination Through the Use of a Non-Fullerene Acceptor in a 6.4% Efficient Organic Planar Heterojunction Solar Cell. Adv. Energy Mater. 2014, 4, 1301413. 10.1002/aenm.201301413. [DOI] [Google Scholar]
  11. Barito A.; Sykes M. E.; Huang B.; Bilby D.; Frieberg B.; Kim J.; Green P. F.; Shtein M. Universal Design Principles for Cascade Heterojunction Solar Cells with High Fill Factors and Internal Quantum Efficiencies Approaching 100%. Adv. Energy Mater. 2014, 4, 1400216. 10.1002/aenm.201400216. [DOI] [Google Scholar]
  12. Lu L.; Chen W.; Xu T.; Yu L. High-performance ternary blend polymer solar cells involving both energy transfer and hole relay processes. Nat. Commun. 2015, 6, 7327. 10.1038/ncomms8327. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Lu H.; Zhang X.; Li C.; Wei H.; Liu Q.; Li W.; Bo Z. Performance Enhancement of Polymer Solar Cells by Using Two Polymer Donors with Complementary Absorption Spectra. Macromol. Rapid Commun. 2015, 36, 1348–1353. 10.1002/marc.201500127. [DOI] [PubMed] [Google Scholar]
  14. Xu H.; Ohkita H.; Tamai Y.; Benten H.; Ito S. Interface Engineering for Ternary Blend Polymer Solar Cells with a Heterostructured Near-IR Dye. Adv. Mater. 2015, 27, 5868–5874. 10.1002/adma.201502773. [DOI] [PubMed] [Google Scholar]
  15. Lessard B. H.; Dang J. D.; Grant T. M.; Gao D.; Seferos D. S.; Bender T. P. Bis(tri-n-hexylsilyl oxide) Silicon Phthalocyanine: A Unique Additive in Ternary Bulk Heterojunction Organic Photovoltaic Devices. ACS Appl. Mater. Interfaces 2014, 6, 15040–15051. 10.1021/am503038t. [DOI] [PubMed] [Google Scholar]
  16. Baran D.; Ashraf R. S.; Hanifi D. A.; Abdelsamie M.; Gasparini N.; Röhr J. A.; Holliday S.; Wadsworth A.; Lockett S.; Neophytou M.; Emmott C. J. M.; Nelson J.; Brabec C. J.; Amassian A.; Salleo A.; Kirchartz T.; Durrant J. R.; McCulloch I. Reducing the efficiency-stability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells. Nat. Mater. 2016, 16, 363–369. 10.1038/nmat4797. [DOI] [PubMed] [Google Scholar]
  17. Wang Z.; Li Z.; Zhang H.; Liu W.; Xu X.; Si C.; Cui Y.; Sun Q.; Wang H.; Shi F.; Hao Y. A Conjugated Random Copolymer of Benzodithiophene-Difluorobenzene-Diketopyrrolopyrrole with Full Visible-Light Absorption for Bulk-Heterojunction Solar Cells. Macromol. Chem. Phys. 2014, 215, 2119–2124. 10.1002/macp.201400334. [DOI] [Google Scholar]
  18. Wang W.; Chen L.; Wang G.; Zhang Z.-G.; Li Y.; Shen P. Synthesis and Optoelectronic Properties of Benzo[1,2-b:4,5-b′]dithiophene-Based Copolymers with Conjugated 2-(2-Ethylhexyl)-3,4-dimethoxythiophene Side Chains. Macromol. Chem. Phys. 2016, 217, 1586–1599. 10.1002/macp.201600076. [DOI] [Google Scholar]
  19. Wang N.; Bao X.; Yan Y.; Ouyang D.; Sun M.; Roy V. A. L.; Lee C. S.; Yang R. Synthesis and photovoltaic properties of conjugated D–A copolymers based on thienyl substituted pyrene and diketopyrrolopyrrole for polymer solar cells. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3198–3204. 10.1002/pola.27378. [DOI] [Google Scholar]
  20. Li Z.; Qiao Z.; Liu W.; Wang Z.; Wang H.; Shi F.; Hao Y. Conjugated random copolymer of benzodithiophene-difluorobenzene-diketopyrrolopyrrole-benzothiadiazole with a broad absorption range of 300–900 nm for bulk heterojunction solar cells. Mater. Lett. 2015, 139, 307–310. 10.1016/j.matlet.2014.10.122. [DOI] [Google Scholar]
  21. Beaumont N.; Castrucci J. S.; Sullivan P.; Morse G. E.; Paton A. S.; Lu Z.-H.; Bender T. P.; Jones T. S. Acceptor properties of boron subphthalocyanines in fullerene free photovoltaics. J. Phys. Chem. C 2014, 118, 14813–14823. 10.1021/jp503578g. [DOI] [Google Scholar]
  22. Josey D. S.; Castrucci J. S.; Dang J. D.; Lessard B. H.; Bender T. P. Evaluating Thiophene Electron-Donor Layers for the Rapid Assessment of Boron Subphthalocyanines as Electron Acceptors in Organic Photovoltaics: Solution or Vacuum Deposition?. ChemPhysChem 2015, 16, 1245–1250. 10.1002/cphc.201402751. [DOI] [PubMed] [Google Scholar]
  23. Cnops K.; Zango G.; Genoe J.; Heremans P.; Martinez-Diaz M. V.; Torres T.; Cheyns D. Energy Level Tuning of Non-Fullerene Acceptors in Organic Solar Cells. J. Am. Chem. Soc. 2015, 137, 8991–8997. 10.1021/jacs.5b02808. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Duan C.; Zango G.; Iglesias M. G.; Colberts F. J. M.; Wienk M. M.; Martínez-Díaz M. V.; Janssen R. A. J.; Torres T. The Role of the Axial Substituent in Subphthalocyanine Acceptors for Bulk-Heterojunction Solar Cells. Angew. Chem., Int. Ed. 2017, 56, 148–152. 10.1002/anie.201608644. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Morse G. E.; Paton A. S.; Lough A.; Bender T. P. Chloro boron subphthalocyanine and its derivatives: dyes, pigments or somewhere in between?. Dalton Trans. 2010, 39, 3915–3922. 10.1039/b922199a. [DOI] [PubMed] [Google Scholar]
  26. Fulford M. V.; Jaidka D.; Paton A. S.; Morse G. E.; Brisson E. R. L.; Lough A. J.; Bender T. P. Crystal Structures, Reaction Rates, and Selected Physical Properties of Halo-Boronsubphthalocyanines (Halo = Fluoride, Chloride, and Bromide). J. Chem. Eng. Data 2012, 57, 2756–2765. 10.1021/je3005112. [DOI] [Google Scholar]
  27. Josey D. S.; Nyikos S. R.; Garner R. K.; Dovijarski A.; Castrucci J. S.; Wang J. M.; Evans G. J.; Bender T. P. Outdoor Performance and Stability of Boron Subphthalocyanines Applied as Electron Acceptors in Fullerene-Free Organic Photovoltaics. ACS Energy Lett. 2017, 2, 726–732. 10.1021/acsenergylett.6b00716. [DOI] [Google Scholar]
  28. Garner R. K.; Josey D. S.; Nyikos S. R.; Dovijarski A.; Wang J. M.; Evans G. J.; Bender T. P. Boron subphthalocyanines as electron donors in outdoor lifetime monitored organic photovoltaic cells. Sol. Energy Mater. Sol. Cells 2018, 176, 331–335. 10.1016/j.solmat.2017.10.018. [DOI] [Google Scholar]
  29. Bormann L.; Nehm F.; Weiß N.; Nikolis V. C.; Selzer F.; Eychmüller A.; Müller-Meskamp L.; Vandewal K.; Leo K. Degradation of Sexithiophene Cascade Organic Solar Cells. Adv. Energy Mater. 2016, 6, 1502432. 10.1002/aenm.201502432. [DOI] [Google Scholar]
  30. Castrucci J. S.; Josey D. S.; Thibau E.; Lu Z. H.; Bender T. P. Boron Subphthalocyanines as Triplet Harvesting Materials within Organic Photovoltaics. J. Phys. Chem. Lett. 2015, 6, 3121–3125. 10.1021/acs.jpclett.5b01254. [DOI] [PubMed] [Google Scholar]
  31. Lee E.; Andrews C. J.; Anctil A. An Iterative Approach To Evaluate and Guide Fine Chemical Processes: An Example from Chloroaluminum Phthalocyanine for Photovoltaic Applications. ACS Sustainable Chem. Eng. 2018, 8230. 10.1021/acssuschemeng.7b04947. [DOI] [Google Scholar]
  32. Ebenhoch B.; Prasetya N. B. A.; Rotello V. M.; Cooke G.; Samuel I. D. W. Solution-processed boron subphthalocyanine derivatives as acceptors for organic bulk-heterojunction solar cells. J. Mater. Chem. A 2015, 3, 7345–7352. 10.1039/C5TA00715A. [DOI] [Google Scholar]
  33. Raboui H.; Thibau E. S.; Josey D. S.; Lu Z.-H.; Bender T. P. Oxy phosphorus tetrabenzotriazacorrole: firming up the chemical structure and identifying organic photovoltaic functionality to leverage its unique dual absorbance. J. Mater. Chem. A 2017, 5, 10978–10985. 10.1039/C7TA02496G. [DOI] [Google Scholar]
  34. Löbbert G.Phthalocyanines. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: 2000; Vol. 27, pp. 181–213. [Google Scholar]
  35. Fujiki M.; Tabei H.; Isa K. New tetrapyrrolic macrocycle: .alpha.,.beta.,.gamma.-triazatetrabenzcorrole. J. Am. Chem. Soc. 1986, 108, 1532–1536. 10.1021/ja00267a024. [DOI] [Google Scholar]
  36. Li J.; Subramanian L. R.; Hanack M. Substituted α,β,γ-triazatetrabenzcorrole: an unusual reduction product of a phthalocyanine. Chem. Commun. 1997, 679–680. 10.1039/A700532F. [DOI] [Google Scholar]
  37. Raboui H.; Lough A. J.; Szawiola A. M.; Bender T. P. Versatile Synthesis of Siloxy Silicon Tetrabenzotriazacorroles and Insight into the Mode of Macrocycle Formation. Inorg. Chem. 2018, 57, 5174–5182. 10.1021/acs.inorgchem.8b00181. [DOI] [PubMed] [Google Scholar]
  38. Furuyama T.; Sugiya Y.; Kobayashi N. Synthesis of a tetrabenzotriazacorrole μ-oxo dimer and investigation of its stacking effect. Chem. Commun. 2014, 50, 4312–4314. 10.1039/C4CC01115E. [DOI] [PubMed] [Google Scholar]
  39. Po R.; Bianchi G.; Carbonera C.; Pellegrino A. “All That Glisters Is Not Gold”: An Analysis of the Synthetic Complexity of Efficient Polymer Donors for Polymer Solar Cells. Macromolecules 2015, 48, 453–461. 10.1021/ma501894w. [DOI] [Google Scholar]
  40. Po R.; Roncali J. Beyond efficiency: scalability of molecular donor materials for organic photovoltaics. J. Mater. Chem. C 2016, 4, 3677–3685. 10.1039/C5TC03740A. [DOI] [Google Scholar]
  41. Morse G. E.; Gantz J. L.; Steirer K. X.; Armstrong N. R.; Bender T. P. Pentafluorophenoxy boron subphthalocyanine (F5BsubPc) as a multifunctional material for organic photovoltaics. ACS Appl. Mater. Interfaces 2014, 6, 1515–1524. 10.1021/am404179z. [DOI] [PubMed] [Google Scholar]
  42. Sullivan P.; Duraud A.; Hancox L.; Beaumont N.; Mirri G.; Tucker J. H. R.; Hatton R. A.; Shipman M.; Jones T. S. Halogenated Boron Subphthalocyanines as Light Harvesting Electron Acceptors in Organic Photovoltaics. Adv. Energy Mater. 2011, 1, 352–355. 10.1002/aenm.201100036. [DOI] [Google Scholar]
  43. Furuyama T.; Sugiya Y.; Kobayashi N. Synthesis of tetrabenzotriazacorrole-subphthalocyanine μ-oxo triad. Macroheterocycles 2014, 7, 139–144. 10.6060/mhc140488k. [DOI] [Google Scholar]
  44. Furuyama T.; Yoshida T.; Hashizume D.; Kobayashi N. Phosphorus(v) tetraazaporphyrins: porphyrinoids showing an exceptionally strong CT band between the Soret and Q bands. Chem. Sci. 2014, 5, 2466–2474. 10.1039/C4SC00569D. [DOI] [Google Scholar]
  45. Reese M. O.; Gevorgyan S. A.; Jørgensen M.; Bundgaard E.; Kurtz S. R.; Ginley D. S.; Olson D. C.; Lloyd M. T.; Morvillo P.; Katz E. A.; Elschner A.; Haillant O.; Currier T. R.; Shrotriya V.; Hermenau M.; Riede M.; Kirov K. R.; Trimmel G.; Rath T.; Inganäs O.; Zhang F.; Andersson M.; Tvingstedt K.; Lira-Cantu M.; Laird D.; McGuiness C.; Gowrisanker S.; Pannone M.; Xiao M.; Hauch J.; Steim R.; DeLongchamp D. M.; Rösch R.; Hoppe H.; Espinosa N.; Urbina A.; Yaman-Uzunoglu G.; Bonekamp J.-B.; van Breemen A. J. J. M.; Girotto C.; Voroshazi E.; Krebs F. C. Consensus stability testing protocols for organic photovoltaic materials and devices. Sol. Energy Mater. Sol. Cells 2011, 95, 1253–1267. 10.1016/j.solmat.2011.01.036. [DOI] [Google Scholar]
  46. Morse G. E.; Helander M. G.; Stanwick J.; Sauks J. M.; Paton A. S.; Lu Z.-H.; Bender T. P. Experimentally Validated Model for the Prediction of the HOMO and LUMO Energy Levels of Boronsubphthalocyanines. J. Phys. Chem. C 2011, 115, 11709–11718. 10.1021/jp1092306. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao0c03474_si_001.pdf (701.3KB, pdf)

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES