Limitations and Perspectives on Triplet‐Material‐Based Organic Photovoltaic Devices

S CT) or triplet ( T CT) character in a 1:3 ratio following the spin statistics. [8] These nearly energetically degenerate S CT and T CT excitons will recombine or separate again into FC. The recombination of S CT to the ground state (S0) can be effectively suppressed because of the high enough S CT energy (E CT ), according to the energygap law ( Figure S1a, Supporting Information). [9−12] Although the recombination from T CT to S0 is spin-forbidden, T CT can relax to the lower-lying triplet state (T1) on the material with lower S1 excitation energy (E S1 , which determines optical gap of the device, E g ) via back electron transfer (BET). [13−18] The singlet−triplet energy gap (ΔE ST = E S1 -E T1 , where E T1 is the T1 excitation energy) is usually very large (≈0.7 eV) in the organic π-conjugated molecules and polymers with strong optical absorption and emission. [19−21] If the CT driving force (ΔE CT = E S1 -E CT ) is small (as required to maximize open-circuit voltage, V OC ), the speed of T1 to thermalize back into T CT will be limited due to the large difference between E CT and E T1 (ΔE BET = E CT -E T1 , Figure S1b, Supporting Information); the decay of T1 to S0 via intersystem crossing (ISC) and tripletcharge annihilation can thus constitute a major terminal loss channel of photocurrent. [13−18] Therefore, in order to reduce both voltage loss and NG recombination, the ΔE ST needs to be minimized ( Figure 1b). [22,23] Here, we have investigated the critical role of end-group π−π stacking in reducing ΔE ST in the state-of-the-art narrowbandgap nonfullerene (NF) acceptors (ITIC, IT-4F, and Y6, [24−26] Figure 2a) by means of (time-dependent) density functional theory (TD)DFT calculations in combination with molecular dynamics (MD) simulations (see Supporting Information for details). Owing to a modest degree of intramolecular push−pull effect, these acceptors show moderate ΔE ST while large oscillator strength ( f, ≈3) in the isolated molecules. Importantly, the ΔE ST is further reduced to 0.3−0.4 eV in the end-group π−π stacking dimers that are frequently present in the films. Such small ΔE ST proves to be able to suppress the T1 decay even in the case of very low ΔE CT of 0.1−0.2 eV. Accordingly, high J SC (>20 mA cm −2 ) and fill factor (FF, 75-80%) as well as low ΔV OC (≈ 0.6 V) have been achieved simultaneously, leading to high power conversion efficiencies (PCEs) of 13-18% in the OPVs based on these acceptors. [25−40] In principle, the ΔE ST value is determined by the electron exchange energy. Normally, the S1 and T1 states are both To improve the power conversion efficiencies for organic solar cells, it is necessary to enhance light absorption and reduce energy loss simultaneously. Both the lowest singlet (S1) and triplet (T1) excited states need to energertically approach the charge-transfer state to reduce the energy loss in exciton dissociation and by triplet recombination. Meanwhile, the S1 energy needs to be decreased to broaden light absorption. Therefore, it is imperative to reduce the...

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“…[ 13 − 18 ] Therefore, in order to reduce both voltage loss and NG recombination, the Δ E ST needs to be minimized (Figure 1b). [ 22,23 ]…”

Section: Figurementioning

confidence: 99%

Reducing the Singlet−Triplet Energy Gap by End‐Group π−π Stacking Toward High‐Efficiency Organic Photovoltaics

Han

2020

Advanced Materials

108

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“…The reduced HOMO energy level could be mainly originated from the increased steric hindrance effect induced by the bulky dfppy unit on the iridium (Ir) complex. [ 1,25 ] In addition, the LUMO energy levels of the iridium (Ir) complex‐based polymers were slightly higher than that of control polymer PM6‐Ir0. Regardless of that, the energy levels of these designed polymers matched well with the acceptor Y6 (HOMO/LUMO = −5.65/−4.10 eV).…”

Section: Resultsmentioning

confidence: 99%

“…More recently, Peng and co‐workers developed an incorporated strategy of platinum (II) complexation, which can effectively regulate the crystallinity and molecular packing order of polynitrogen heterocyclic polymer donors, optimize the corresponding blend morphologies, as 10% (molar ratio related to the s‐tetrazine block) of Pt(Ph) 2 (DMSO) 2 was added to the polymer backbone of (4,8‐bis(5‐((2‐butyloctyl)thio)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐3,6‐bis(4‐octylthiophen‐2‐yl)‐1,2,4,5‐tetrazine) polymer donor (Pt0), the J sc and FF were increased from 25.10 to 26.45 mA cm −2 and from 64.9% to 76.3%, respectively, and the PCE of the Pt10‐based photoactive layer was finally improved from 13.03% (Pt0 polymer donor) to 16.35%. [ 25 ] According to these previous results, [ 1,38–44 ] it can be easily concluded that, introducing suitable third components or units into the polymer backbone of high performance photovoltaic materials is a promising approach to improve the blend microstructure, balance the above‐mentioned three competitors, and thus to increase the device performance with a higher PCE.…”

Section: Introductionmentioning

confidence: 99%

“…Solution‐processed polymer solar cells (PSCs) with a bulk heterojunction (BHJ) architecture, which involve mixing of donor (D)/acceptor (A) materials in the active layer, have attracted great investigation owing to their advantages such as low‐cost solution‐processing, light‐weight, and easy fabrication of flexible and semitransparent devices. [ 1–6 ] The device efficiencies have been continuously increasing in the last two decades, [ 7–9 ] enabled by crosscollaboration among material scientists, physicists, and device specialists; featured material synthesis (especially for nonfullerene acceptors (NFAs), [ 7,10–16 ] ) optimized BHJ blend processing; [ 17,18 ] and detailed understanding on the physical mechanisms. [ 19–24 ] Very recently, benefiting from the new NFA material synthesis, the power conversion efficiency (PCEs) over 16% in binary solar cells have been realized, [ 25–28 ] which make the solution‐processed BHJ PSCs viable for practical applications in the near future.…”

Section: Introductionmentioning

confidence: 99%

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Solution‐Processed Polymer Solar Cells with over 17% Efficiency Enabled by an Iridium Complexation Approach

Wang

Sun

Shi

et al. 2020

Advanced Energy Materials

129

106

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as low-cost solution-processing, lightweight, and easy fabrication of flexible and semitransparent devices. [1][2][3][4][5][6] The device efficiencies have been continuously increasing in the last two decades, [7][8][9] enabled by crosscollaboration among material scientists, physicists, and device specialists; featured material synthesis (especially for nonfullerene acceptors (NFAs), [7,[10][11][12][13][14][15][16] ) optimized BHJ blend processing; [17,18] and detailed understanding on the physical mechanisms. [19][20][21][22][23][24] Very recently, benefiting from the new NFA material synthesis, the power conversion efficiency (PCEs) over 16% in binary solar cells have been realized, [25][26][27][28] which make the solution-processed BHJ PSCs viable for practical applications in the near future.In the active layers, the BHJ morphology architecture helps the photogenerated excitons to dissociate into charge carriers at the D/A interfaces. [29,30] The free carriers diffuse/drift by the built-in electric field and can be further collected at the relevant electrodes via a continuous network of D/A materials. In the process of converting photons to a flow of electricity, efficient charge generation, fast charge carrier extraction, and less carrier recombination loss are prerequisites for realizing high performance devices. [31] However, the requirements of these three competitors (including charge generation, carrier recombination, and extraction) with regard to the interface area of the D/A (morphology) and the active layer thickness are conflicting on the one hand. [32] On the other hand, the optoelectronic properties of the D/A materials, the processing conditions, and the device structures can also greatly determine the properties of the three competitors, which in turn affect device performance. [24,31,33] Thus, finding an effective strategy or approach for simultaneously facilitate charge generation, accelerate carrier extraction, and hinder carrier recombination is very important to further improve device performance.In the organic semiconductors, the excitons of many semiconducting polymers have lifetimes in the range of 10-100 ps, [34] and the relevant diffusion lengths of 5-10 nm, [35] which generally constrain the D/A domain sizes and layer thickness in the active layers. As expected, the above-mentioned three competitors are seriously sensitive to the interface area of the nanoscale morphology, the D/A domain size, and the active layer thickness in BHJs. Many studies demonstrated that photovoltaic materials incorporated with a small amount of complexes or components can improve the relevant blend morphology and photovoltaic performance, especiallyThe commercially available PM6 as donor materials are used widely in highly efficient nonfullerene polymer solar cells (PSCs). In this work, different concentrations of iridium (Ir) complexes (0, 0.5, 1, 2.5, and 5 mol%) are incorporated carefully into the polymer conjugated backbone of PM6 (PM6-Ir0), and a set of π-conjugated polymer donors (named PM6-Ir0.5, PM...

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“…23,24 Enlarging spin-orbit coupling (SOC) by chemically or physically introducing heavy atoms into the conjugated materials has been proposed to enhance ISC rate. [25][26][27] So far, some research studies have been done on triplet material-based OPVs (T-OPVs) [28][29][30][31] and the highest PCE for small-molecule Ir complexes is 3.81%. 32 However, the voltage losses in T-OPVs were rarely investigated.…”

Section: Introductionmentioning

confidence: 99%

Investigation on voltage loss in organic triplet photovoltaic devices based on Ir complexes

et al. 2019

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Limitations and Perspectives on Triplet‐Material‐Based Organic Photovoltaic Devices

Cited by 61 publications

References 119 publications

Reducing the Singlet−Triplet Energy Gap by End‐Group π−π Stacking Toward High‐Efficiency Organic Photovoltaics

Reducing the Singlet−Triplet Energy Gap by End‐Group π−π Stacking Toward High‐Efficiency Organic Photovoltaics

Solution‐Processed Polymer Solar Cells with over 17% Efficiency Enabled by an Iridium Complexation Approach

Investigation on voltage loss in organic triplet photovoltaic devices based on Ir complexes

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