Critical light instability in CB/DIO processed PBDTTT-EFT:PC 71 BM organic photovoltaic devices

Self Cite

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unexpectedly clean electronic properties of these semiconductors have led to the demonstration of proof-of-concept PV devices with an initial effi ciency that outperforms competing 'next-generation' technologies such as dye-sensitized, organic, and quantum dot solar cells, and even matches that of commercially deployed PV. [ 6 ] Other attractive characteristics of hybrid perovskites include the low embedded costs of materials processing and the ability to tune the properties of the semiconductor through changes in chemical composition. [ 1,[7][8][9][10] All of these factors have led to hybrid perovskites being considered as disruptive materials in a wide range of technology applications; [11][12][13][14][15][16] interest from the materials science research community is both substantial and rapidly maturing. Realizing the technology potential of hybrid perovskites necessitates a thorough understanding of the degradation mechanisms that limit the lifetime of devices, and identifying solutions to mitigate these. This exercise is nontrivial because of the various factors that can affect device performance: light, heat, moisture, oxygen, mechanical and electrical stresses, and combinations thereof. [ 17,18 ] Currently, perovskite solar cells (PSCs) based on the "triple layer" architecture with a thick carbon back electrode have demonstrated perhaps the most stable performance characteristics of all PSCs. [ 19 ] These devices, based on the mixed cation perovskite (5-AVA) X (MA) 1− X PbI 3 (where 5-AVA corresponds to 5-ammoniumvaleric acid), have been shown to maintain their initial power conversion effi ciency (PCE) of approximately 10% over 1000 h continuous simulated solar illumination, [ 20 ] dark storage for three months at elevated temperatures and humidity (85 °C/85%) and a minimum of 7 d operation in real-world conditions (Jeddah, Saudi Arabia). [ 21 ] Although clearly commendable, it is apparent that signifi cant work is required to realize PSCs with high stability and high performance, where promising stability metrics are combined with the certifi ed PCE values for champion devices. [ 22 ] To elucidate the underlying mechanisms for degradation in hybrid perovskites and guide the development of intrinsically stable devices, considerable work has been undertaken on the archetypal semiconductor of the research fi eld: CH 3 NH 3 PbI 3 . It is known that this material (and other single or mixed-halide derivatives) are particularly sensitive to moisture, [ 1,7,17,[23][24][25][26] where the proposed chemical reactions involving water result in decomposition of the perovskite into its precursor components via The rapid pace of development for hybrid perovskite photovoltaics has recently resulted in promising fi gures of merit being obtained with regard to device stability. Rather than relying upon expensive barrier materials, realizing market-competitive lifetimes is likely to require the development of intrinsically stable devices, and to this end accelerated aging tests can help identify degradation mechanisms tha...

Section: Resultsmentioning

confidence: 87%

Oxygen Degradation in Mesoporous Al₂O₃/CH₃NH₃PbI_3‐_xCl_x Perovskite Solar Cells: Kinetics and Mechanisms

Pearson

Eperon

Hopkinson

et al. 2016

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“…[7] In organic polymer based solar cells, the burn-in period reflects an early, near to exponential photovoltaic performance roll-off. [9,10] For instance, light-induced burn-in has been experimentally and theoretically correlated, although to different extent, with photochemical reactions, [11] critical concentrations of chemical and metal impurities, [12,13] molecular weight distribution, [14] degree of crystallinity, [15] crosslinking, [16] processing additives, [17] and the formation of longlived radicals. It typically occurs so rapidly that it is generally accepted to specify the lifetime of the device post burn-in phase, i.e., neglecting the early loss in performance.…”

Section: Doi: 101002/aenm201700770mentioning

confidence: 99%

Burn‐in Free Nonfullerene‐Based Organic Solar Cells

Gasparini

Salvador

Strohm

et al. 2017

206

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Organic solar cells that are free of burn‐in, the commonly observed rapid performance loss under light, are presented. The solar cells are based on poly(3‐hexylthiophene) (P3HT) with varying molecular weights and a nonfullerene acceptor (rhodanine‐benzothiadiazole‐coupled indacenodithiophene, IDTBR) and are fabricated in air. P3HT:IDTBR solar cells light‐soaked over the course of 2000 h lose about 5% of power conversion efficiency (PCE), in stark contrast to [6,6]‐Phenyl C61 butyric acid methyl ester (PCBM)‐based solar cells whose PCE shows a burn‐in that extends over several hundreds of hours and levels off at a loss of ≈34%. Replacing PCBM with IDTBR prevents short‐circuit current losses due to fullerene dimerization and inhibits disorder‐induced open‐circuit voltage losses, indicating a very robust device operation that is insensitive to defect states. Small losses in fill factor over time are proposed to originate from polymer or interface defects. Finally, the combination of enhanced efficiency and stability in P3HT:IDTBR increases the lifetime energy yield by more than a factor of 10 when compared with the same type of devices using a fullerene‐based acceptor instead.

“…[45] Here, α taking the value 1 (or close to unity) indicates negligible bimolecular recombination losses (i.e., most carriers are extracted prior to recombining), [46] whereas smaller α values indicate a competition between bimolecular recombination and carrier extraction, whereby recombination yields are effectively limiting the device photocurrent. [49,50] TPC experiments have previously been used to investigate the trapping and detrapping rates in polymerfullerene, [51][52][53] "all-polymer," [54,55] and hybrid BHJ solar cells. In contrast, for optimized SM2-based BHJ thin films, the J SC versus light intensity data fit yields an α value of 0.98, indicating that carrier extraction proceeds with significantly reduced recombination losses compared to nonoptimized active layers.…”

Section: Device Testing and Characterizationsmentioning

confidence: 99%

Benzo[1,2‐b:4,5‐b′]Dithiophene–6,7‐Difluoroquinoxaline Small Molecule Donors with >8% BHJ Solar Cell Efficiency

Liang

Wang

Wolf

et al. 2017

In particular, in recent work, we showed that the development of BHJ morphologies between donor and acceptor components, [20] material processing, and optimization of thin-film morphologies via postprocessing treatments [21] is not independent of the substitution pattern and functional groups appended to SM donors. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant-group substitution promoting structural order and mediating morphological effects.In this contribution, we report on the rational pendant-group substitution and concurrent bandgap reduction in benzo[1,2-b:4,5-b′]dithiophene-6,7-difluoroquinoxaline SM donors, raising BHJ solar cell efficiencies from ≈6 to >8% with PC 71 BM, and we provide a detailed understanding of the role of the selective substitutions on material and BHJ device performance improvements. Taking advantage of the synthetic modularity of electrondeficient quinoxaline motifs, we show that the introduction of acrylate functions on the pyrazine ring is an effective approach to significantly increasing the electron affinity of the acceptor unit, in turn narrowing the optical gap of the SM donor. Importantly, our device examinations show that this discrete synthetic modification does not alter the propensity of benzo[1,2-b:4,5-b′]dithiophene-6,7-difluoroquinoxaline SM donors to order and form favorable thin-film BHJ morphologies with PCBM; carrier extraction in optimized BHJ active layers proceeds with notably suppressed geminate and bimolecular recombination, while the high open-circuit voltage (V OC ) of the BHJ solar cells is maintained to ≈1 V.In the design of symmetrical SM donors alternating donor and acceptor motifs (DADAD) setting their optical gap, two avenues have prevailed thus far: (i) swapping the donor [acceptor] units for more electron-rich [-deficient] motifs-albeit with distinctly different molecular structure [14,22] -and (ii) increasing [reducing] their conjugation length by replicating [removing] DA motifs [14] or, more commonly, by varying the number of rings involved in the main chain. [1,17] In either cases, the synthetic modifications generally impact SM self-assembly in thin films, solution-processing and device optimization procedures, [18,23] Solution-processable small molecule (SM) donors are promising alternatives to their polymer counterparts in bulk-heterojunction (BHJ) solar cells. While SM donors with favorable spectral absorption, self-assembly patterns, optimum thin-film morphologies, and high carrier mobilities in optimized donor-acceptor blends are required to further BHJ device efficiencies, material structure governs each one of those attributes. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant...