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

Abstract

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.

Understanding the low voltage losses in high-performance non-fullerene acceptor-based organic solar cells

“…A minimal D HOMO offset of 0.5 eV is contradicting the high-performance systems reported earlier with a D HOMO offset close to 0 eV. [33][34][35][36] It should be noted that the HOMO levels reported by Karuthedath et al were measured with ultraviolet photoelectron spectroscopy (UPS), while for the reports with D HOMO E 0 eV, the energy levels were determined using cyclic voltammetry (CV). Various reports point out large discrepancies between HOMO levels derived by UPS and CV, 37,38 while others claim excellent conformity.…”

“…Several reports have shown high-efficiency OPV material combinations where the measured HOMO D À HOMO A offset (D HOMO ) between donor and acceptor is almost zero. [33][34][35][36] Considering the presented HOMO levels for D18 and Y6 suggests a D HOMO offset of 0.27 eV. Carefully reducing the D HOMO offset (without affecting the bandgaps) results in a larger HOMO D À LUMO A difference, which could potentially result in a higher V OC .…”

“…Various reports point out large discrepancies between HOMO levels derived by UPS and CV, 37,38 while others claim excellent conformity. 33,35,36 The correct determination of HOMO and LUMO levels remains controversial but the contradicting reports suggest that both methods are prone to measurement, and most notably, evaluation errors of several tenths of eV. Despite the difficulties accompanied when comparing different methods, the statement that the IQE PV is reduced significantly below a critical D HOMO offset holds true.…”

Molecular Design of Efficient Chlorine‐ and Carboxylate‐Functionalized Donor Polymers for Nonfullerene Organic Solar Cells Enabling Processing with Eco‐Friendly Solvent in Air

“…However, there is no energy offset between the HOMO levels of P(F‐BiT)‐COOBOCl(in) and IT‐4F, so if the driving force is insufficient, there may be a high probability of charge recombination. [ 34,48 ] In contrast, the HOMO level of P(F‐BiT)‐COOBOCl(out) is slightly higher (by 0.05 eV) than that of IT‐4F, which is favorable for photovoltaic performance.…”

Synthesis and characterization of a wide‐bandgap polymer based on perfluorinated and alkylthiolated benzodithiophene with a deep highest occupied molecular orbital level for organic photovoltaics

“…However, the HOMO–HOMO offset (~0.06 eV) might not be sufficient to generate efficient exciton dissociation via hole transfer at the P2FS‐ttTPD/IT‐4F interface. [ 37–42 ]…”