High-Performance Fullerene-Free Polymer Solar Cells Featuring Efficient Photocurrent Generation from Dual Pathways and Low Nonradiative Recombination Loss
Abstract:Efficient
charge generation is a prerequisite to achieve high power
conversion efficiency (PCE) in organic/polymer solar cells (OSCs/PSCs),
which involves photoinduced electron transfer and/or hole transfer
between the donor/acceptor interface upon photoexcitation. A high
yield of charge from both processes usually requires sufficient energy
offset between the donor and acceptor for charge separation, fast
transport, and extraction for charge collection, as well as significant
absorption complementation to max… Show more
“…a) Single value (PBDB‐T:IDTIC; PBDB‐T:ITIC; PTB7‐Th:IEICO‐4F; PBDB‐T:IDTTIC; PBDBT‐2F:ITIC; PBDBT‐2F:ITIC‐4F), measured by the steady‐state transport (SST) and photocharge extraction by linear increasing voltage (p‐CELIV). b) Voltage‐dependent (PffBT4T‐2DT:FBR; PMOT40:IDIC; DR3:O‐IDTBR; DR3:ICC6; PTB7‐Th:O‐IDTBR), measured by the transient photovoltage (TPV), transient photocurrent (TPC), and charge extraction (CE) methods.…”
Section: Recombination Losses and Collection Of Free Charge Carriers mentioning
Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.
Hall of Fame ArticleAlthough the last decade has seen much progress in the field of OSC research, the development of semitransparent devices lags behind because of the lack of optimized photoactive materials. [22][23][24][25] The strong absorption of the active materials in the visible region causes difficulty when one tries to balance the power conversion efficiency (PCE) and transmittance. Encouragingly, the recent progress of fullerene-free OSCs has the potential to shed fundamental solutions to overcome these obstacles, hence becoming attractive research topics on developing narrow-bandgap nonfullerene acceptors to researchers. [26][27][28][29][30] We focus in this section on presenting recent progress in the design of narrow-bandgap organic semiconductors that have bandgaps less than 1.5 eV and have extended the boundaries of visible transparent/NIR absorbing OSC technology.
Solution-Processable Organic SemiconductorsOrganic semiconductors are carbon-based materials with an electronically delocalized π-conjugated backbone. Such π-conjugated systems are created by a linear series of overlapping p z orbitals (π bonds). [31,32] As the parallel overlap of carbon p z orbitals increases with the molecular extension, the π bonds may further spread out into π bands, and this leads to a narrower energy bandgap. The energy of the highest occupied molecular orbital (HOMO) corresponds to the topmost π band, and the lowest π* band is referred to as the lowest unoccupied molecular orbital (LUMO). The energy gap between the HOMO and the LUMO dominates optical properties. Photoexcitations result in Coulombically bound excitons (electron-hole pairs) due to the low dielectric constant (ε ≈ 2−4) characteristic of organic semiconductors. [33,34] The exciton binding energy is in the range of 0.3−1 eV, thus requiring a high interfacial area between electron donor (D) and electron acceptor (A) components to promote exciton dissociation into free carriers. [35,36] This approach has led to the development of organic bulk heterojunctions (BHJs), which are cast from blend solutions of D and A components. [37][38][39] The excitonic nature of organic semiconductors offers an advantage in wavelength-specific light harvesting applications through a delicate manipulation of energy ...
“…a) Single value (PBDB‐T:IDTIC; PBDB‐T:ITIC; PTB7‐Th:IEICO‐4F; PBDB‐T:IDTTIC; PBDBT‐2F:ITIC; PBDBT‐2F:ITIC‐4F), measured by the steady‐state transport (SST) and photocharge extraction by linear increasing voltage (p‐CELIV). b) Voltage‐dependent (PffBT4T‐2DT:FBR; PMOT40:IDIC; DR3:O‐IDTBR; DR3:ICC6; PTB7‐Th:O‐IDTBR), measured by the transient photovoltage (TPV), transient photocurrent (TPC), and charge extraction (CE) methods.…”
Section: Recombination Losses and Collection Of Free Charge Carriers mentioning
Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.
Hall of Fame ArticleAlthough the last decade has seen much progress in the field of OSC research, the development of semitransparent devices lags behind because of the lack of optimized photoactive materials. [22][23][24][25] The strong absorption of the active materials in the visible region causes difficulty when one tries to balance the power conversion efficiency (PCE) and transmittance. Encouragingly, the recent progress of fullerene-free OSCs has the potential to shed fundamental solutions to overcome these obstacles, hence becoming attractive research topics on developing narrow-bandgap nonfullerene acceptors to researchers. [26][27][28][29][30] We focus in this section on presenting recent progress in the design of narrow-bandgap organic semiconductors that have bandgaps less than 1.5 eV and have extended the boundaries of visible transparent/NIR absorbing OSC technology.
Solution-Processable Organic SemiconductorsOrganic semiconductors are carbon-based materials with an electronically delocalized π-conjugated backbone. Such π-conjugated systems are created by a linear series of overlapping p z orbitals (π bonds). [31,32] As the parallel overlap of carbon p z orbitals increases with the molecular extension, the π bonds may further spread out into π bands, and this leads to a narrower energy bandgap. The energy of the highest occupied molecular orbital (HOMO) corresponds to the topmost π band, and the lowest π* band is referred to as the lowest unoccupied molecular orbital (LUMO). The energy gap between the HOMO and the LUMO dominates optical properties. Photoexcitations result in Coulombically bound excitons (electron-hole pairs) due to the low dielectric constant (ε ≈ 2−4) characteristic of organic semiconductors. [33,34] The exciton binding energy is in the range of 0.3−1 eV, thus requiring a high interfacial area between electron donor (D) and electron acceptor (A) components to promote exciton dissociation into free carriers. [35,36] This approach has led to the development of organic bulk heterojunctions (BHJs), which are cast from blend solutions of D and A components. [37][38][39] The excitonic nature of organic semiconductors offers an advantage in wavelength-specific light harvesting applications through a delicate manipulation of energy ...
“…Organic photovoltaic (OPV) materials offer unique advantages, such as low cost, synthetic variability, mechanical flexibility, transparency, lightweight, and the possibility of large‐area processing by printing and other coating techniques . Recent progress in realizing higher efficiencies in bulk heterojunction (BHJ) solar cells, comprising interpenetrating organic donor–acceptor materials, has been driven by the development of new donor materials combined with novel nonfullerene acceptor (NFA) molecules . NFAs, compared with fullerene derivatives, possess enhanced light absorption, band gap/energy level tunability, high charge carrier mobility, and promising photostability, all prerequisites for photovoltaic applications .…”
Large‐scale production of organic solar modules requires low‐cost and reliable materials with reproducible batch‐to‐batch properties. In case of polymers, their (photo)physical properties depend strongly on the polymers’ molecular weight (MW). Herein, the impact of the MW of the donor polymer poly(3‐hexylthiophene) (P3HT) on the photophysics is studied in blends with a recently developed rhodanine‐endcapped indacenodithiophene nonfullerene acceptor (IDTBR), a bulk heterojunction (BHJ) system that potentially fulfills the aforementioned criteria for large‐scale production. It is found that the power conversion efficiency (PCE) increases when the weight‐average MW is increased from 17 kDa (PCE: 4.0%) to 34 kDa (PCE: 6.6%), whereas a further increase in MW leads to a reduced PCE of 4.4%. It is demonstrated that the charge generation efficiency, as estimated from time‐delayed collection field experiments, varies with the P3HT MW and is the reason for the differences in photocurrent and device performance. These findings provide insight into the fundamental photophysical reasons of the MW dependence of the PCE, which is taken into account when using polymer‐based nonfullerene acceptor blends in solar cell devices and modules.
“…Here, we give a summary for single‐junction OSCs with power conversion efficiencies (PCEs) >11% (Figure ). Both binary and ternary blend films for the photoactive layer are included. In Figure , the data are based on over 100 devices, and the points are consecutively numbered, which correspond to their detailed device parameters listed in Table S1, Supporting Information.…”
Herein, a high‐mobility polymer (Si25) pairing a nonfullerene acceptor (O‐IDTBR) is introduced to construct active layers of organic solar cells (OSCs). The OSCs based on Si25 and O‐IDTBR with comparable bandgaps of 1.61 eV show high open‐circuit voltage (Voc) of 1.03 V. Suitable energy level offsets between the donor and acceptor as well as sufficient photon absorbance by a 400 nm thick active layer afford a notable short‐circuit current (Jsc) of 21.11 mA cm−2, indicating a significantly suppressed trade‐off between Jsc and Voc among OSCs. In addition, notable high power conversion efficiency (PCE) between 10.2% and 11.54% can be achieved with thick blend films from 210 to 560 nm, a thickness range beneficial to pin‐hole free printing. The maximum PCE of 11.54% corresponds to a 400 nm thick blend film, which is a rare thickness for high‐efficiency nonfullerene‐based OSCs. The corresponding fill factors (FFs) are between 51.59% and 53.33%. The inferior FF is due to a very low electron–hole mobility ratio, offering space for future FF elevation. The results highlight the high Voc and Jsc potentials for thick‐film nonfullerene OSCs based on a high hole mobility donor as well as looking forward to a high electron mobility nonfullerene acceptor.
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