Carbon Dangling Bonds in Photodegraded Polymer:Fullerene Solar Cells

Abstract

conversion efficiency (PCE), [1][2][3][4][5][6][7][8] up to ≈11.5%, was achieved by, e.g., using low bandgap polymers [1][2][3][4][5][6] and developing approaches to control the active layer's morphology utilizing thermal [7] or solvent annealing, [8] mixed solvents, [4] or additives. [1][2][3][4][5][6] Low bandgap polymers improve the PCE by absorbing more sunlight, increasing the short circuit current density J sc (the value without applied bias). A larger open circuit voltage V oc (the forward bias that zeros the current) is achieved by lowering the polymer's highest occupied molecular orbital (HOMO) energy. [1][2][3][4][5] PSC degradation is the main commercialization obstacle. Hence, extensive efforts are directed at understanding degradation [9][10][11][12][13][14][15][16][17] due to, e.g., moisture and oxygen [9][10][11] and light, [11][12][13][14] although the microscopic nature of the generated defects has not been revealed. This paper demonstrates, for the first time, UV/blue photogeneration of metastable carbon dangling bonds (DBs, i.e., threefold coordinated C atoms) akin to the well-known Si DBs that pervade hydrogenated amorphous Si (a-Si:H). [18] The C DBs are revealed via the electron paramagnetic resonance (EPR) of their unpaired spin 1/2 electron after observing the signature EPR of C DBs at g = 2.0029 ± 0.0004 [19][20][21][22][23] in polymer:fullerene films. Importantly, at room temperature the DBs decay slowly in the dark, in sharp contrast to their stability in a-Si:H. [18] However, since the performance of the solar cells does not recover, it is suspected that the C DBs are passivated by, e.g., O-or OH-related groups, rather than the polymer recovering its original configuration.The paper focuses on low-bandgap polymers of alternating benzo[1,2-b:4,5-b′]dithiophene (BDT) and thieno[3,4-b]-thiophene (TT) units (PBDTTTs), [15] which exhibit PCEs up to 10% for inverted cells in polymer:fullerene bulk heterojunction (BHJ) structures. These polymers have high absorption coefficients up to ≈750 nm and high carrier mobilities. [15,16] Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b] thiophene-)-2-carboxylate-2-6-diyl)] (PBDTTT-EFT; Figure 1) has a HOMO level of −5.24 eV and a lowest unoccupied molecular orbital (LUMO) level of −3.66 eV (i.e., bandgap E g = 1.58 eV; Figure 1). [1] BHJ cells of the related PBDTTT-C:PC 70 BM (phenyl-C 70 -butyric-acid-methyl ester) degrade with heat. [17] To improve stability, understanding the defect states' nature and how they act as trapping/recombination centers is important.Intrinsic photodegradation of organic solar cells, theoretically attributed to CH bond rearrangement/breaking, remains a key commercialization barrier. This work presents, via dark electron paramagnetic resonance (EPR), the first experimental evidence for metastable C dangling bonds (DBs) formed by blue/UV irradiation of polymer:fullerene blend films in nitrogen. The DB density increases with irradiation and decreas...

Relating polymer chemical structure to the stability of polymer:fullerene solar cells

“…The photodegradation of PSCs has been recently attributed by some solely to effects triggered by charge collection layers and interfaces 49 and by others to hot carriers breaking C-H bonds at the donor/acceptor interface. 29 The latter claim required energies 44 eV, but we showed here that the monomers themselves are unstable to UV irradiation, which certainly does not involve homolytic cleavage of C-H bonds. If hot carriers were breaking apart C-H bonds, then almost no polymer would be stable, but that is not the case.…”

“…There is a lack of studies on degradation mechanisms 23,24 in general and in devices, thus, very little was done on the degradation and stability of this class of materials [25][26][27] and much less on the degradation and stability of their devices. 28,29 Semiconducting polymers generally degrade, especially under visible light illumination, resulting in steady buildup of free radical species as shown by Frolova et al 30 with studies on PTB7 and PBDTTT-CF. It is also shown that the presence of additives, in particular DIO, in the active layer of organic solar cells accelerates the degradation mechanism 31 in these devices.…”

Electron Spin Resonance Investigations on Perovskite Solar Cell Materials Deposited on Glass Substrate

“…We detected no ESR signals that were expected [17] from PC61BM (gx = 2.0060, gy = 2.0028, gz = 2.0021), or PEDOT:PSS [8] (g = 2.0037) before and after illumination. We observed no photo generated carbon dangling radicals [17] with a g-value of 2.0029 which clearly establishes that the signals we observed did not originate from the over layers. Therefore, the only source that can give rise to such signals is the underlying UV-ozone treated glass substrate.…”

Air exposure induced recombination in PTB7:PC71BM solar cells

“…The correlation between increasing EDMR signal and decreasing cell performance is in agreement with previous EPR studies which showed that the higher concentrations of paramagnetic radical species the lower the cell performance. [21][22][23] The absolute magnitude of the spin-dependent current change (in amperes) is obscured when using phase sensitive detection, as it is dependent on the eld modulation amplitude. However, for the DIO processed air exposed cell the change of the DC photocurrent (short circuit) could be measured and decreased by approximately 50 nA on resonance (using a microwave power of 200 mW).…”