High-Performance Organic Energy-Harvesting Devices and Modules

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

High-Performance Organic Energy-Harvesting Devices and Modules for Self-Sustainable Power Generation under Ambient Indoor Lighting Environments Ryota Arai,*,†,§ Seiichi Furukawa,†,‡ Yu Hidaka,†,‡ Hideaki Komiyama,‡ and Takuma Yasuda*,†,‡ Department of Applied Chemistry, Graduate School of Engineering and ‡INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § Ricoh Company Ltd., 16-1 Honda-machi, Numazu, Shizuoka 410-8505, Japan Downloaded via MIDWESTERN UNIV on February 22, 2019 at 11:12:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Organic photovoltaics (OPVs) that perform more efficiently under artificial indoor lighting conditions than they do under sunlight are attracting growing interest as they can potentially serve as ambient energy harvesters for powering low-power electronics and portable devices for the Internet of Things. Herein, solution-processed small-molecule OPVs are demonstrated to exhibit high power conversion efficiencies exceeding 16% under white LED illumination, delivering high output power densities of up to 12.4 and 65.3 μW cm−2 at 200 and 1000 lx, respectively. Increasing the open-circuit voltage (Voc) of OPVs is a critical factor for achieving higher indoor photovoltaic performance. Toward real applications, this small-molecule OPV system is adopted to fabricate six seriesconnected modules with an active area of ∼10 cm2 that are capable of generating a high output power surpassing 100 μW and a high Voc of over 4.2 V even under dimly lit indoor conditions of 200 lx. These results indicate that OPVs are promising as indoor electric power sources for self-sustainable electronic devices. KEYWORDS: energy harvesting, organic photovoltaics, small molecules, flexible modules, internet of things

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INTRODUCTION In the coming era of the Internet of Things (IoT), where information and communication systems are invisibly embedded in our living environments, wireless sensor network technologies will play a pivotal role in social and industrial advancements, including public safety, human healthcare, energy management, and automation.1 Batteries (coin cells or secondary batteries) are currently the power source of choice for operating such wireless network devices owing to their ease of installation and are still considered to be an indispensable element for developing wireless device applications; however, they require periodic replacement and maintenance. A concern within the scope of the nextgeneration IoT is the maintenance of batteries powering the implemented trillion sensor nodes, maintenance that would not be practically feasible from environmental, resource, and labor cost perspectives. In the last few years, emerging technologies that harvest energy from ambient light, heat, mechanical vibrations, and electromagnetic waves have attracted significant research interest2 because they enable low-power wireless and portable electronic devices to be completely self-sustaining without the need for battery maintenance and replacement. The most prevalent ambient energy source available in buildings (e.g., homes, offices, schools, stores, hospitals, and factories) is artificial lighting, which can be harvested using © XXXX American Chemical Society

photovoltaic (PV) devices. Even though stray sunlight is not available at all locations and at all times, ambient indoor lighting sources such as white light-emitting diodes (LEDs) and fluorescent lamps can provide sufficient energy for operating most low-power electronic devices even under dim-light conditions. The critical differences between indoor and outdoor PV applications are the light power and the spectrum of the light source available to them. Because of the considerably narrower illumination spectra that are limited to the visible wavelength region and the significantly lower power densities associated with artificial indoor lighting sources (typically 0.1−1 mW cm−2, i.e., a factor 100−1000 lower) compared to solar irradiation (AM 1.5G, 100 mW cm−2), the design of indoor light harvesters should be significantly different from that of the current solar cells. Among various PV technologies, organic photovoltaics (OPVs) and dyesensitized solar cells are now regarded as the most promising candidates for indoor energy-harvesting devices,3−18 for which a high absorption coefficient and a tunable absorption range, both characteristics of organic semiconductors, permit more effective photoabsorption and power conversion under room lighting environments. While previous studies on OPVs using Received: January 1, 2019 Accepted: February 13, 2019

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DOI: 10.1021/acsami.9b00018 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Molecular structures of BDT-nT-IDs, PTB7, and PNP.

bulk heterojunction (BHJ) structures19−21 have primarily focused on their performance as solar cells under the 1 sun condition, their unique features such as lightness, flexibility, and transparency make them highly desirable for future IoT technologies. Recent reports have revealed that semiconducting polymer-based OPVs exhibit higher power conversion efficiencies (PCEs) than silicon solar cells for measurements under white LED or fluorescent lamp illumination,5,8,9 though the OPVs typically show inferior PV performance under solar illumination. Therefore, OPVs have great potential as nextgeneration ambient energy-harvesting devices that cannot be realized using robust silicon-based solar cells. However, systematic investigations into the underlying mechanisms of power conversion and PV characteristics under indoor lighting conditions are still lacking. In addition, unlike established organic solar cell technologies, there are very few guidelines on the design of effective organic semiconductors and device architectures, which is a key issue that needs to be addressed to produce high-performance organic energy-harvesting devices. In this work, we focused on the small molecules, BDT-2TID and BDT-1T-ID (Figure 1) possessing deep-lying highest occupied molecular orbital (HOMO) energy levels,22 and evaluated their indoor PV characteristics using BHJ devices under white LED lighting conditions with illuminance ranging from 200 to 10 000 lx (lm m−2). It has been revealed that BDT-2T-ID-based OPVs tailored for indoor applications outperform analogous polymer OPVs as well as amorphous silicon (a-Si) cells, generating a notably high output power density of 12.4 μW cm−2 at 200 lx, corresponding to a 16.2% PCE. Furthermore, we demonstrated an unprecedented high performance of flexible organic energy-harvesting modules each comprising six series-connected subcells with a total active area of ∼10 cm2 that delivered a very high output power exceeding 100 μW and a large open-circuit voltage (Voc) of over 4.2 V even under dim-light conditions of 200 lx.

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Voc =

Jph yz nkT ij Jph yz nkT ijj zz ≈ lnjjj1 + lnjjjj zzzz ∝ ln(Jph ) zz j z j J0 z e J e 0 { k k {

(1)

where n is the diode ideality factor, k is the Boltzmann constant, T is the absolute temperature, e is the elementary charge, and J0 is the reverse saturation current density. Because Voc decreases with a decrease in the logarithm of Jph (which is linearly proportional to the incident light intensity), BHJ OPV systems that can provide a higher Voc under the 1 sun condition are more favorable for indoor applications. This consideration inspired us to explore emerging OPV materials that exhibit high Voc as well as PCE values under the 1 sun condition and apply them to indoor energy-harvesting devices. Given their attributes of a high Voc of ∼1 V and low energy losses in the BHJ organic solar cells, small-molecule BDT-2TID and BDT-1T-ID donors show satisfactory potential for indoor applications.22 To benchmark the performances of these small-molecule OPVs, we also tested similar polymer OPVs using a representative polymer donor PTB723 (Figure 1) in combination with a fullerene-based acceptor PNP (Nphenyl-2-phenyl[60]fulleropyrrolidine).24−26 Figure 2 depicts the absorption spectra of the respective neat films of BDT-nT-IDs, PTB7, and PNP, along with the illumination spectrum of a white LED light source that comprised a sharp blue emission from a GaN LED and broad emissions from phosphors. With optical band gaps (Eg) of 1.7−1.8 eV for BDT-nT-IDs, their optical responses indicate

RESULTS AND DISCUSSION

For OPVs utilized as indoor energy harvesters, a large Voc is a requisite factor to achieve superior PV characteristics. According to the Shockley diode equation, Voc exhibits a logarithmic dependence on photocurrent density (Jph)3,4,6,13

Figure 2. Absorption spectra of neat films of BDT-nT-IDs, PTB7, and PNP, compared with the illumination spectrum of a white LED light with a color temperature of 8500 K. B


Research Article


were attained with the D/A ratio of 2:1 for BDT-nT-ID:PNP blends and 2:3 for PTB7:PNP blends (Supporting Information). Figure 3b displays a cross-sectional transmission electron microscopy (TEM) image of a representative small-molecule OPV based on a BDT-1T-ID:PNP (2:1, w/w) active layer, confirming that the component layers were clearly distinguishable and that their interfaces were very neat and smooth. Note here that the active layer thickness for the best-performing BDT-nT-ID-based indoor OPVs (typically 150−200 nm) is almost twice as that for corresponding organic solar cells.22 Figure 3c,d compares the current density−voltage (J−V) characteristics for the optimized OPV devices as well as commercial indoor a-Si cells measured under the 1 sun illumination (100 mW cm−2) and indoor white LED illumination with a fixed illuminance of 200 lx (76.8 μW cm−2).29 The corresponding key PV parameters, including short-circuit current density (Jsc), Voc, fill factor (FF), and PCE, of these devices under the different light sources are compiled in Table 1. While the PTB7-based polymer OPVs exhibited PCE of up to 6.7% under the standard 1 sun condition, the small-molecule OPVs employing BDT-2T-ID and BDT-1T-ID yielded lower PCEs of up to 6.1 and 3.9%, respectively, in spite of their higher Voc values reaching ∼1 V (Figure 3c). More importantly, the PCEs of these OPVs drastically increased when measured under the weaker LED lighting at 200 lx (Figure 3d and Table 1). As a result, BDT2T-ID- and BDT-1T-ID-based OPVs delivered remarkable PCEs of up to 16.2 and 12.9% under this low-light condition, generating maximum output power densities (Pout) as large as 12.4 and 9.9 μW cm−2, respectively. The PCE values of these small-molecule OPVs were enhanced by a factor of ∼3 at 200 lx, far surpassing that of the PTB7-based OPV. This observation testifies that the better-performing PV system under the 1 sun condition is not necessarily the same as the PV system performing better under indoor conditions. As presented in Figure 4, we further investigated the variations of the PV parameters as a function of incident LED light intensity (Pin). As expected, the Jsc values of these devices appeared to be nearly proportional to Pin (Figure 4a). Generally, the relationship between Jsc and Pin is characterized using the following power-law equation22,30,31

good spectral overlaps with the white LED emission. Moreover, large absorption coefficients of BDT-nT-IDs in the entire visible region suggest that small-molecule OPVs based on these donors and a complemental PNP acceptor could be better suited as indoor energy-harvesting devices than as conventional solar cells. We fabricated indoor OPVs with an inverted device structure of ITO/ZnO (30 nm)/BHJ active layer (80−250 nm)/MoO3 (10 nm)/Ag (100 nm) via solution processes,22,27,28 as illustrated in Figure 3a (see the Supporting

Figure 3. (a) Schematic of indoor BHJ OPV devices with an inverted configuration and their single-diode equivalent circuit model (Jph: photo-generated current density, Rs: series resistance, Rp: shunt resistance). (b) Cross-sectional TEM image for the fabricated device based on a BDT-1T-ID:PNP (2:1, w/w) active layer. (c,d) J−V curves for the optimized OPV devices and a-Si cell measured under (c) the 1 sun illumination (100 mW cm−2) and (d) under white LED illumination at an illuminance of 200 lx.

Information for details). For all small-molecule OPVs, each BHJ active layer consisting of a BDT-nT-ID:PNP binary blend was prepared by spin-coating from their chloroform solutions, without using any solvent additives or postdeposition treatments such as thermal annealing and solvent vapor annealing. We further optimized these BHJ OPVs by varying the donor/ acceptor (D/A) blend ratio. The best indoor PV performances

Jsc ∝ Pin α

(2)

where α is a parameter denoting the extent of bimolecular charge recombination. By fitting the Jsc data in the illuminance range of 200−10 000 lx (corresponding to Pin = 76.8−3850 μW cm−2), α values of 0.98, 0.99, and 1.0, all of which are close

Table 1. PV Parameters for the Optimized Devices under 1 sun and White LED Illumination light sourcea 1 sun

LED 200 lx

active layerb

thickness (nm)

Jsc (μA cm−2)

BDT-2T-ID:PNP BDT-1T-ID:PNP PTB7:PNP a-Sic BDT-2T-ID:PNP BDT-1T-ID:PNP PTB7:PNP a-Sic

∼175 ∼160 ∼80

13.6 (±0.7) × 10 10.9 (±0.9) × 103 13.8 (±0.4) × 103 2.9 × 103 24.2 ± 0.4 19.2 ± 0.3 19.0 ± 0.3 21.8

∼190 ∼185 ∼90

Voc (V) 3

0.94 1.02 0.77 0.85 0.75 0.84 0.57 0.63

± 0.01 ± 0.01

± 0.01 ± 0.01

FF (%)

Poutd (μW cm−2)

PCEe (%)

±2 ±1 ±3

5.8 (±0.3) × 10 3.6 (±0.3) × 103 6.4 (±0.3) × 103 5.7 × 102 12.3 ± 0.1 9.5 ± 0.4 7.3 ± 0.3 9.4

5.8 ± 0.3 3.6 ± 0.3 6.4 ± 0.3 0.57 16.0 ± 0.2 12.4 ± 0.5 9.5 ± 0.4 12.2

46 32 62 23 68 59 67 68

±1 ±2 ±1

3

Pin = 100 mW cm−2 for 1 sun illumination and 76.8 μW cm−2 for white LED illumination. bOptimum D/A ratio = 2:1 (w/w) for BDT-nTID:PNP blends and 2:3 (w/w) for PTB7:PNP blends. cA commercial indoor a-Si cell, Amorton AM-30-11, was used as a reference device. d Maximum output power density, Pout = Jsc × Voc × FF. ePCE = Pout/Pin. a

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Figure 4. Dependence of (a) Jsc, (b) Voc, (c) FF, and (d) PCE of the optimized BHJ OPVs on the incident LED light intensity, compared to the values obtained for an a-Si cell under the same lighting conditions. The straight lines in (a,b) are fittings of each of the plots.

Figure 5. (a) Photographs of the BDT-2T-ID-based six series-connected modules (total active area: 9.5 cm2) fabricated on a rigid glass substrate (left) and on a flexible PEN substrate (right). (b) Schematic of their basic module design. (c) I−V curves for the fabricated OPV modules measured under white LED illumination at 200 lx.

illumination are thus vital for achieving a higher Voc and PCE when used for indoor energy harvesting. Meanwhile, the enhancement of overall PV performance at low light intensity is illustrated by comparing the dependences of FF and PCE values on Pin (Figure 4c,d). In contrast to the behavior of Voc, the FF values of the OPVs tend to increase with decreasing light intensity. To achieve a high FF under indoor conditions, a large shunt resistance (Rp) is critical, given the lower current density compared to conventional solar cell applications, for which a small series resistance (Rs) is more favorable. Previously, Steim et al.3 studied the impacts of Rp and Rs on the performance of polymer OPVs, reporting that a higher Rp (>85 kΩ cm2) leads to a higher FF under indoor lighting conditions (500−1000 lx). Proctor and Nguyen33 reported that for OPVs with low Rp, the leakage current had a detrimental effect on the device performance under lower light intensities, leading to steep drops in FF and Voc. For the present BDT-2T-ID-, BDT-1T-ID-, and PTB7-based OPVs, the Rp values extracted from the J−V curves at 200 lx were 430, 390, and 220 kΩ cm2, respectively (see Figure 3d), which seemed to be high enough to effectively suppress the parasitic

to unity, are obtained for the BDT-2T-ID-, BDT-1T-ID-, and PTB7-based devices, respectively. The nearly linear dependence of Jsc on Pin (i.e., α ≈ 1) suggests that bimolecular charge recombination is well suppressed in these OPVs in weak LED lighting conditions. For these indoor OPVs, the Voc values were found to exhibit an obvious logarithmic dependence on Pin (Figure 4b), as rationalized by eq 1. The slopes of their fitted curves are almost identical regardless of the type of organic semiconductor, indicating that the tendency for Voc reduction upon decreasing light intensity is universal for indoor OPVs. Indeed, these three OPVs with similar diode ideality factors (n = 1.2−1.3) underwent comparable Voc reduction (0.18−0.19 V) upon changing the light source from the 1 sun to 200 lx LED light. A similar behavior on Voc reduction has been observed for some polymer-based OPV systems.6,10,13,32 More recently, Lee et al.6,12 and Yin et al.13,14 demonstrated high-performing OPVs with high Voc values over 0.7 V under weak indoor lights, utilizing D−A-structured polymers (e.g., PCDTBT) or small molecules having deeplying HOMO energy levels. Considering all of the discussion above, OPV materials that give a higher Voc under the 1 sun D



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leakage current. The BDT-1T-ID-based OPV showed the largest improvement of the FF values, from ∼32% under the 1 sun condition to ∼59% under the 200 lx LED condition. As can be seen from Figure 4d, the best-performing BDT-2T-IDbased OPV demonstrated rather stable PCEs over a wide illuminance range of 200−10 000 lx, retaining high values exceeding 16%. Because the intensity of indoor lights varies more than one order of magnitude depending on the conditions,29 such almost-illuminance-independent characteristics of the PCE will make the small-molecule OPVs more attractive and reliable for a wide variety of indoor applications. So far, we have demonstrated indoor PV performances for small-area cells with an active area of 6 mm2. To further explore their potential toward future real applications, we fabricated OPV energy-harvesting modules utilizing the BDT2T-ID:PNP system, with a total active area of 9.5 cm2 by connecting in series six subcells each with an area of ∼1.6 cm2 (Figure 5a,b and the Supporting Information). Here, the OPV modules were prepared using both a rigid glass substrate and a flexible polyethylene naphthalate (PEN) substrate, by adopting the same structure used for the small-area devices. The current−voltage (I−V) characteristics of the resulting OPV modules and the device parameters measured at an illuminance of 200 lx are shown in Figure 5c. Voc of both OPV modules scaled ideally with the number of the series-connected cells, achieving high Voc of 4.2−4.3 V at 200 lx. The average Voc for each subcell can be estimated as 0.70−0.72 V, indicative of a low degree of current leakage in these indoor OPV modules. Considering the expected short-circuit current (Isc ≈ 40 μA) for the area of each subcell, the glass-supported OPV module delivered a reasonably high Isc value of 39.9 μA. However, the Isc of the flexible PEN-supported module was comparatively lower at 33.8 μA, presumably due to a difference in transmittance of the substrates. It is noteworthy that the upscaling from 6 mm2 (for the small-area cells) to 9.5 cm2 (for the practical modules) caused only a negligible variation in FF from 67 to 66−69%, resulting in considerably high maximum power outputs of up to 111 μW (PCE ≈15%) at 200 lx. A short video, demonstrating ambient energy harvesting using the flexible and bendable OPV module, can be found in the Supporting Information.

Research Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b00018. Detailed experimental procedures and additional OPV data (PDF) Ambient energy harvesting using the flexible and bendable OPV module (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.A.). *E-mail: [email protected] (T.Y.). ORCID

Hideaki Komiyama: 0000-0001-5544-9305 Takuma Yasuda: 0000-0003-1586-4701 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for JSPS KAKENHI grant no. JP18H02048 (T.Y.). REFERENCES

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CONCLUSIONS We have demonstrated the viability of high-performance smallmolecule OPVs tailored for ambient energy harvesting that achieve much higher PCEs under indoor lighting conditions than they do under sunlight. The best-performing device delivered an output power density as high as 12.4 μW cm2 while achieving a remarkable PCE of 16.2% under white LED illumination at 200 lx. We have successfully developed flexible and bendable OPV energy-harvesting modules for the first time by adopting our small-molecule OPV system (see the Supporting Movie). These emerging OPV technologies offer advantages in terms of not only high PCEs in regard to artificial light sources but also ease of integration, lightweight, flexibility, and possibility of customization in shape and colors. We thus expect the present results to be a big advancement toward practical ambient energy-harvesting devices that can contribute to a wide range of applications requiring dispersive electric power sources for self-sustainable operations.34,35 E


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High-Performance Organic Energy-Harvesting Devices and Modules

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