Thick‐Film Low Driving‐Force Indoor Light Harvesters

Abstract

Organic indoor photovoltaic (IPV) devices have tremendous potential for the wireless charging of internet of things (IoTs). [1-6] An ideal bulk heterojunction (BHJ) adopted for indoor light harvesters should satisfy both the requirements of device performance and fabrication feasibility. [7-12] The power conversion efficiency (PCE) of IPV cells can be optimized by adjusting donor:acceptor (D:A) materials and weight fractions for the spectral matching with the indoor light sources (e.g., fluorescence, light-emitting diode [LED], incandescent, and halogen lamps) and suitable energy levels. Together with appropriate solar cell fabrication methods, the open-circuit voltage (V oc), short-circuit current density (J sc), and fill factor (FF) values can be maximized under indoor light emissions. [13-17] Cui et al. designed a wide-gap nonfullerene acceptor (NFA) IO-4Cl and fabricated IPV devices blended with a donor polymer PBDB-TF. Such high-efficiency indoor light harvesters can achieve an optimal PCE of 26.1% under a 1000 lux (2700 K) LED illumination, with an impressive V oc of 1.10 V. [1] The operation stability under indoor light conditions was also evaluated by the same group. The PBDB-TF-based IPV devices blended with various acceptors can still maintain over 95% of their initial PCEs, indicating a slow degradation rate of functional groups in such organic materials with low-intensity indoor emissions. [18] In addition to device performance, the ease of fabrication should be considered. [19-22] A thick-film strategy can meet the requirements of current leading roll-to-roll (R2R) techniques, such as spray coating, doctor blading, screen printing, and slot die coating, because of pinholes in organic thin films can be eliminated. [23-28] A consensus of prerequisites for high-efficiency thick-film organic photovoltaic (OPV) device includes high absorbances, well-formed D:A domains and interfaces for sufficient exciton separation, and desirable charge carrier transport. [29-34] Zhang et al. fabricated a 300 nm-thick-film PffBT4T-2OD: EH-IDTBR solar cell with a high efficiency of 9.1% by optimizing device architectures to overcome the space-charge effects. [35] Recently, Ma et al. demonstrated a 12.1% PBDB-TF:BTP-4Cl BHJ device, and the thickness of active layer is impressively larger than 1000 nm. [36] To address the issue of active thickness under indoor illuminations, we hereby adopt a low driving-force P3TEA:FTTB-PDI4 BHJ model system to fabricate thick-film indoor light harvesters (Figure 1). The low driving-force BHJ solar cells have been shown to have great potential for fabricating high-performance OPV devices. [37-39] In this work, an optimized PCE of %22% was achieved with a %200 nm BHJ layer under indoor LED emissions, and this performance is comparable with the optimized

Recent progress in thick‐film organic photovoltaic devices: Materials, devices, and processing

“…Polymers with a 5,6-difluorobenzothiadiazole (ffBT) unit have experienced a long period of development in the thick-film OPV research field. [47][48][49][50][51][52][53][54] A representative class of polymer donor with thickness-insensitive characteristics is that of containing ffBT and tetrathiophene, namely FBT-Th 4 (1,4), with chemical structure shown in Figure 5A. The OPV device based on this copolymer as donor and PC 71 BM F I G U R E 5 (A) Chemical structure of part of ffBT-based polymers.…”

“…Polymers with a 5,6‐difluorobenzothiadiazole (ffBT) unit have experienced a long period of development in the thick‐film OPV research field 47–54 . A representative class of polymer donor with thickness‐insensitive characteristics is that of containing ffBT and tetrathiophene, namely FBT‐Th 4 (1,4), with chemical structure shown in Figure 5A.…”

Comparing and Quantifying Indoor Performance of Organic Solar Cells

“…First, the calculation was carried out for a wide set of data from literature [16][17][18][19][30][31][32][33][34][35] for the whole set of different LEDs presented above with a constant illuminance of 200 lux. Unfortunately, some high-performance solar cells could not be included due to lack of necessary data.…”

Recent Progress of Organic Solar Cells with Insulating Polymers

“…More importantly, the shunt resistance ( R p ) was increased by one order of magnitude due to the modification of PEIE, the large R p is associated with the performance maximization in indoor OSCs. [ 57‐60 ] In addition, Lee et al found a more simple one‐step method to fabricate the self‐assembled bilayer with PEI bottom layer and photoactive top layer. [ 61 ] During the spin‐coating process of PEI‐mixed BHJ active layer (PEI:BHJ), PEI could uniformly distribute at the bottom of the active layer and thereby influence the WF of ITO.…”

“…More importantly, the shunt resistance (R p ) was increased by one order of magnitude due to the modification of PEIE, the large R p is associated with the performance maximization in indoor OSCs. [57][58][59][60] In addition, Lee et al found a more simple one-step method to fabricate the Table 2. WF of conducting materials modified with/without insulating polymer, as independently measured by Kelvin probe in air and by UPS.…”