Bulk‐Heterojunction Organic Solar Cells: Five Core Technologies for Their Commercialization

215

212

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.

Section: Doi: 101002/aenm201700770mentioning

confidence: 99%

Burn‐in Free Nonfullerene‐Based Organic Solar Cells

Gasparini

Salvador

Strohm

et al. 2017

215

212

“…[1][2][3] The active layer of an OPV device, which converts absorbed light energy into electrical energy, is usually prepared in the form of bulk-heterojunction (BHJ). With the use of nontoxic organic materials, one can fabricate light, flexible, low-cost devices, and have good performance in low light conditions.…”

Section: Introductionmentioning

confidence: 99%

Using Ultralow Dosages of Electron Acceptor to Reveal the Early Stage Donor–Acceptor Electronic Interactions in Bulk Heterojunction Blends

Cheung

et al. 2017

Tuning the donor–acceptor (D–A) weight ratio is an essential step to optimize the performance of a bulk heterojunction (BHJ) solar cell. The unoptimized regime with a low acceptor concentration is generally unexplored despite it may reveal the early stage electronic D–A interactions. In this study, PTB7:PC71BM is used to examine factors that limit the device performance in unoptimized regime. The key limiting factor is the creation of traps and localized states originated from fullerene molecules. Photothermal deflection spectroscopy is used to quantify the trap density. Starting with pristine PTB7, addition of small concentration of fullerene increases the electron trap density and lowers the electron mobility. When the D–A weight ratio reaches 1:0.1, fullerene percolation occurs. There is an abrupt drop in trap density and simultaneously a six orders of magnitude increase in the electron mobility. Furthermore, the fill factors of the corresponding photovoltaic devices are found to anticorrelate with the trap density. This study reveals that electron trapping is the key limiting factor for unoptimized BHJ solar cells in low fullerene regime.

“…[1][2][3] Exciton dissociation into free charges is the most important optoelectronic process, which is driven by the energy-level difference between donor and acceptor materials. [1][2][3] Exciton dissociation into free charges is the most important optoelectronic process, which is driven by the energy-level difference between donor and acceptor materials.…”

mentioning

confidence: 99%

Accurate Determination of the Minimum HOMO Offset for Efficient Charge Generation using Organic Semiconducting Alloys

Zhang

Liu

Zhou

et al. 2019

104

reveal the bright future of this greenenergy technology. [1][2][3] Exciton dissociation into free charges is the most important optoelectronic process, which is driven by the energy-level difference between donor and acceptor materials. [4,5] For OSCs utilizing fullerene derivatives as electron acceptors, a lowest unoccupied molecular orbital (LUMO) offset [ΔE LUMO = E LUMO(D) − E LUMO(A) ] of no less than 0.30 eV was generally proposed to guarantee efficient electron transfer. [6][7][8] However, this leads to large energy losses (>0.60 eV), as defined by E loss = E g opt − qV oc , (E g opt is the optical bandgap, V oc is the open-circuit voltage, and q is elementary charge), and limits the power conversion efficiency (PCE) to less than 12% [9][10][11][12][13][14] after decades of effort. Thus far, the development of donor (D)-acceptor (A)-type nonfullerene molecular acceptors (NFAs) with well-tunable electronic structures has opened up a great opportunity in this active topic because these NFAs have realized high PCEs over 15%. [15][16][17][18][19][20][21] To form a complementary absorption, current popular NFA OSCs are based on the combination of a widebandgap donor and a narrow-bandgap acceptor. [22][23][24][25][26][27][28][29][30][31][32][33] For such a material combination, a large ΔE LUMO always exists, which reduces the highest occupied molecular orbital (HOMO) offset [ΔE HOMO = E HOMO(D) − E HOMO(A) ] to minimize energy loss [34][35][36][37] while enhancing the light collection in near-infrared (NIR) Current research indicates that exciton dissociation into free charge carriers can be achieved in material combinations with the highest occupied molecular orbital (HOMO) offset lowered to 0 eV in non-fullerene organic solar cells. However, the quantitative relationship between the HOMO offset and exciton dissociation has not been established because of the difficulty inachieving continuously tunable HOMO offsets. Here, the binary blends of PTQ10:ZITI-S and PTQ10:ZITI-N are combined to form the positive and negative HOMO offsets of 0.20 and −0.07 eV, respectively. While the PTQ10:ZITI-S binary blend delivers a decent power conversion efficiency (PCE) of 10.69% with a shortcircuit current (J sc ) of 16.94 mA cm −2 , the PTQ10:ZITI-N with the negative offset shows a much lower PCE of 7.06% mainly because of the low J sc of 12.03 mA cm −2 . Because the tunable HOMO levels can be realized in organic semiconducting alloys based on ZITI-N and ZITI-S acceptors, the transformation of the HOMO energy offset from negative to positive values is achieved in the PTQ10:ZITIN:ZITI-S ternary blends, delivering much-improved PCEs up to 13.26% with a significant, 74% enhancement of J sc to 20.93 mA cm −2 .With detailed investigations, the study reveals that the minimum HOMO offset of ≈40 meV is required to achieve the most-efficient exciton dissociation and photovoltaic performance.