A paper recently published in the Polymer Journal reviewed recent developments in π-conjugated polymer and oligomeric material-based indoor organic photovoltaic devices (IOPVs) for Internet of Things (IoT) applications.
Study: Indoor photovoltaic energy harvesting based on semiconducting π-conjugated polymers and oligomeric materials toward future IoT applications. Image Credit: nevodka/Shutterstock.com
IOPVs, which can generate power under ambient indoor light, have recently gained significant attention as eco-friendly self-sustainable power sources for modern IoT electronics with low power consumption, such as wireless sensors. Organic semiconductor-based lightweight IOPVs possess several advantages, including the feasibility of mass production at low temperatures, solution processability, and flexibility.
Organic semiconductor-based IOPVs are also suitable as energy harvesters, specifically in indoor lighting environments, owing to the high optical absorptivity and spectral tunability of organic semiconductors. Recently, IOPVs have achieved over 25% power conversion efficiency (PCE) with tens to hundred μW cm− 2 output power densities, which are adequate to power different IoT-compatible low-power electronics.
In this paper, the authors reviewed the recent advances in oligomeric material and π-conjugated polymer-based IOPVs.
Recent Advancements in IOPVs
The design of proper organic semiconductor materials plays a crucial role in the development of high-efficiency IOPVs. The IOPV photoactive layer typically consists of electron acceptor and donor materials, which intermix to form bulk heterojunction (BHJ) nanostructures.
The 0.1 μm thick BHJ layer performs every major function in IOPVs, including charge transport, charge separation, and photoabsorption. Thus, the composition and selection of acceptor and donor materials can critically influence the IOPV performance. The use of non-fullerene acceptors (NFAs) can significantly expand the combinations and choices of acceptors and donors in IOPVs.
IOPVs can be categorized into four groups based on the donor-acceptor combination in their photoactive layers. These include small-molecule (SM) donor–fullerene acceptor binary systems, polymer donor–fullerene acceptor binary systems, ternary and quaternary systems, and polymer donor–NFA binary systems.
Polymer Donor–Fullerene Acceptor Binary Systems
The initial polymer-fullerene BHJ systems were composed of a regioregular poly(3-hexylthiophene) (P3HT), a semiconducting polymer, and soluble fullerene derivatives such as [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). However, these systems have low open-circuit voltage (VOC) and PCE, which necessitated further alterations in donor and acceptor materials to improve the PCE and VOC.
Among all polymer-fullerene binary systems developed until now, the poly[(5,6‐bis(2‐hexyldecyloxy)benzo[c][1,2,5]thiadiazole‐4,7‐diyl)‐alt‐(5,50‐(2,5‐difluoro‐1,4‐phenylene)bis(thiophen‐2‐yl))] (PDTBTBz‐2F) donor-PC71BM acceptor system displayed the highest PCE, VOC, and output power density of 23.1%, 0.82 V, and 64.8 μW cm−2, respectively, under 1000 lx light-emitting-diode (LED) illumination.
Similarly, the poly [[6,7-difluoro[(2-hexyldecyl)oxy]-5,8-quinoxalinediyl]-2,5-thiophenediyl ]] (PTQ10) donor-PC61BM acceptor system demonstrated a PCE, VOC, and output power density of 19.9%, 0.79 V, and 35.8 μW cm−2, respectively, under 500 lx LED illumination. This system also allows IOPV upscaling in mass-production protocols, including roll-to-roll printing.
Other polymer-fullerene systems that have displayed high PCE, VOC, and output power density include poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) donor-PC71BM acceptor under 300 lx illumination from a 3000 K LED source, poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-TF/PM6) donor-PC71BM acceptor under 1000 lx illumination from a 2700 K LED source, and PCDTBT donor-PC71BM acceptor under 300 lx fluorescent lamp (FL) illumination.
SM Donor–Fullerene Acceptor Binary Systems
SM donor-fullerene acceptor systems have several advantages over polymer-based systems, including energy-level tunability, negligible batch-to-batch variations, high purity, monodispersity, and well-defined molecular structures. Multiple SM-donor-fullerene acceptor BHJ systems demonstrated exceptional indoor photovoltaic characteristics.
For instance, the 5,5'- [[4,8-bis[5-(2-ethylhexyl)-4-hexyl-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]bis[(3',3''-dihexyl[2,2':5',2''-terthiophene]-5'',5-diyl)methylidyne ]]bis[3-hexyl-2-thioxo-4-thiazolidinone] (BTR) donor-PC71BM acceptor system displayed a PCE, VOC, and output power density of 28.1%, 0.79 V, and 78.2 μW cm−2, respectively, under 1000 lx FL illumination.
Similarly, a porphyrin-based material (P1) donor-PC71BM acceptor binary system showed a PCE, VOC, and output power density of 18.7%, 0.74 V, and 14.5 μW cm−2, respectively, under 300 lx illumination from a 3000 K LED source.
Polymer Donor–NFA Binary Systems
Although fullerene-based IOPVs have achieved more than 20% PCE in the last few years, the potential for further improvement in their PCEs is limited owing to the limited visible absorptivity and energy-level tunability of fullerene acceptors, which increase the challenges to increase the short-circuit current density (JSC) and VOC.
NFAs can improve both JSC and VOC owing to their tunable absorptivity and energy levels. Several polymer donor-NFA binary systems have demonstrated good photovoltaic properties. For instance, the PBDB-TF donor-a low-band-gap Y6-O acceptor system showed a PCE, VOC, and output power density of 30%, 0.83 V, and 110 μW cm−2, respectively, under 1200 lx illumination from a 3000 K LED source, and 29.5% PCE, 0.81 V VOC, and 62.8 μW cm−2 under 700 lx illumination from a 3000 K LED source.
Similarly, the poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-5,5'-(5,8-bis(4-(2-butyloctyl)thiophen-2-yl)dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]thiadiazole)] (D18) donor- FCC-Cl, an acceptor-donor-acceptor (A-D-A)-type NFA, the system demonstrated 29.4% PCE under 1000 lx illumination from a 2600 K LED source, and the PBDB-TF donor-FCC-Cl acceptor system showed a 27.9% PCE under 1000 lx illumination from a 2600 K LED source.
Ternary and Quaternary Systems
The addition of a third or fourth component to the binary photoactive layer to fabricate multi-component BHJ systems can improve the morphology, charge-transport properties, and photoabsorption of IOPV devices. Ternary systems contain either two donors/single acceptor or single donor/two acceptors, while quaternary systems are based on dual donors/dual acceptors.
The poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T) donor-PC71BM:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(5-hexylthienyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC-Th) acceptor ternary system displayed a PCE, VOC, and output power density of 26.4%, 0.72 V, and 73.9 μW cm−2, respectively, under 1000 lx LED illumination.
Similarly, the PM6:poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) donor- PC71BM:ITIC-Th acceptor quaternary system demonstrated 25% PCE, 0.77 V VOC, and 74.9 μW cm−2 output power density under 1000 lx illumination from a 3000 K LED source.
IOPV Modules for Practical Applications
Several state-of-the-art IOPV modules have been developed for practical use in indoor applications. Among them, the TPD-3F donor-3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (IT-4F) acceptor-based IOPV device with 20.4 cm2 effective active area demonstrated the highest 21.8% PCE with 3.21 V VOC and 818 μW output power density under 1000 lx FL illumination.
Although significant progress has been made toward the development of IOPV technologies, more research is required to address several existing challenges, including the design and application of proper organic semiconductors that allow good spectral matching with white LEDs as the incident light. Moreover, IOPVs must be evaluated under 200-500 lx illumination to obtain the real indoor photovoltaic performance of these devices. In the future, studies must be performed on IOPV upscaling, modularization, and integration to commercialize these devices.
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Hwang, S., Yasuda, T. Indoor photovoltaic energy harvesting based on semiconducting π-conjugated polymers and oligomeric materials toward future IoT applications. Polymer Journal 2022. https://www.nature.com/articles/s41428-022-00727-8