Correlation between Hole Accumulation and Deterioration of Device Performance in Polymer Solar Cells as Investigated by Light-Induced Electron Spin Resonance

Abstract

Light-induced electron spin resonance (LESR) study of polymer solar cells has been performed to investigate accumulated hole carriers in these devices under device operation. We analyzed clear correlation between the number of accumulated holes in regioregular poly(3-hexylthiophene) (P3HT) evaluated by LESR and the deterioration of device performance (V oc , J sc ) observed using the same device under simulated solar irradiation. The effects of hole accumulation with deep trapping levels formed in P3HTat the organic interfaces on the performance are examined by considering interfacial electric dipole layers and charge-carrier scattering by accumulated holes.

Direct Evidence of Less Charge Accumulation in Highly Durable Polymer Solar Cells Using Operando Electron Spin Resonance Spectroscopy

“…We now turn to a discussion of the mechanism which has deteriorated the device performance. When charges are accumulated in cells, the short-circuit current density is described as a function of N spin as follows , Here, e is elementary charge, N is charge density, μ is the mobility of Matthiessen’s rule, E is internal electric field, μ SC is the mobility without charge accumulation, μ CA is the mobility with charge accumulation, and c is a proportional constant. From eq , we obtain J (0) = Neμ SC E , which gives J SC ( N spin )/ J SC (0) as follows: From eq , we can know that the J SC decreases as the N spin increases because the collision time decreases due to the charge-carrier scattering by accumulated charges.…”

“…We now turn to a discussion of the mechanism which has deteriorated the device performance. When charges are accumulated in cells, the short-circuit current density is described as a function of Nspin as follows: 36,37…”

Deterioration mechanism of perovskite solar cells by operando observation of spin states

“…2c, the J SC of the cell rapidly decreased (see the inset) and then gradually increased (see the main panel) when the N spin increased under short-circuit conditions. The studies of organic solar cells have reported the J SC decrease due to charge-carrier scatterings by charge accumulation in cells 22,33,34 . When the charge-carrier scatterings occur due to hole accumulation or formation, the current density j of a solar cell may be described with a following equation based on the Matthiessen's rule with charge density (n), electric elemental quantity (e), charge mobility (μ) in the cell, internal electric field (E), charge mobility before (μ SC ), and after hole accumulation (μ HA ), and a proportional constant (c) 33,34 :…”

“…2d, the V OC decreased as the hole accumulation increased. The studies of organic solar cells have discussed that when hole accumulation occurs in p-type polymers at the interfaces between photoactive and hole-transport layers, a vacuum-level shift occurs and decreases the V OC 22,33,37 . In this study, it is expected that the V OC decrease is influenced by the shift of the vacuum level owing to accumulated holes at the interfaces between spiro-OMeTAD and gold electrode, because the perovskite layer has no charge accumulation and the electron accumulation in the metallic gold electrode may be unobservable due to the Pauli paramagnetism and the ESR detection limit as mentioned above.…”

“…In this study, it is expected that the V OC decrease is influenced by the shift of the vacuum level owing to accumulated holes at the interfaces between spiro-OMeTAD and gold electrode, because the perovskite layer has no charge accumulation and the electron accumulation in the metallic gold electrode may be unobservable due to the Pauli paramagnetism and the ESR detection limit as mentioned above. It has been argued that when charge accumulation occurs, the variation of the V OC (ΔV OC ) is related with the variation of hole accumulation N spin (ΔN spin ) using a following equation with an interfacial electric dipole length (d), the permittivity constant in vacuum (ε 0 ), a dielectric constant of spiro-OMeTAD (ε r ) 39 , an area (S) at the spiro-OMeTAD/Au interface in the device as follows 33 :…”