Tail state limited photocurrent collection of thick photoactive layers in organic solar cells

Abstract

We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness. These blends and devices are analysed by transient optoelectronic techniques of carrier kinetics and densities, air photoemission spectroscopy of material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We conclude that, for the device series studied, the photocurrent loss with thick active layers is primarily associated with the accumulation of photo-generated charge carriers in intra-bandgap tail states. This charge accumulation screens the device internal electrical field, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail state distribution and density and increase the maximal photoactive thickness for efficient operation. Our work suggests that selecting organic photoactive layers with a narrow distribution of tail states is a key requirement for the fabrication of efficient, high photocurrent, thick organic solar cells.

Exciton and Charge Carrier Dynamics in Highly Crystalline PTQ10:IDIC Organic Solar Cells

“…Most NFA‐based solar cells show relative low effective mobilities on the order of 10 −5 cm 2 V −1 s −1 measured by the same technique, which is typically assigned to lack of continuous phases for charge transport. [ 51–53 ] In contrast, this PTQ10:IDIC blend shows a high effective mobility as 2 × 10 −4 cm 2 V −1 s −1 (Figure 1e), higher than other NFAs blends, [ 2,54–56 ] and consistent with previous reports (1.1 × 10 −3 cm 2 V −1 s −1 extracted from space charge limited current measurements from electron‐only devices for IDIC which is close to those (10 −3 cm 2 V −1 s −1 ) of fullerene acceptors [ 57 ] ). This high mobility is consistent with the reported face‐on orientation of crystallites in this blend, favorable for efficient charge transport and collection.…”

Understanding the Light‐Intensity Dependence of the Short‐Circuit Current of Organic Solar Cells

“…In the case of tail states, with increasing Urbach energies from Figure 6b–d, a low‐field zone appears in the region, where generation and recombination are high in Figure 6g–i. As reported by Wu et al., [ 55 ] electrons are extracted at the illuminated contact whereas holes diffuse to the opposite contact. At the anode however, electrons are neither created in large amounts due to the position dependent generation rate nor can they diffuse from the cathode to the anode because of the opposing electric field.…”

“…Therefore, a positive space charge builds up opposite to the illuminated contact causing a high‐field and a low‐field region. [ 55,67 ] The diffusion‐dominated low‐field regime limits the short‐circuit current density as most charge carriers recombine here (see Figure 6g–i). The band diagram in Figure 6e for a solar cell with recombination via deep traps also shows band bending similar to the case of E U = 30 meV and thereby indicates that space‐charge formation also matters for deep defects at light intensities around 1 sun.…”

“…Typical values for E U in organic solar cells can be found between 30 and 80 meV. [ 48,53–55 ] Recently, the development of materials with low Urbach energies have enabled organic solar cells with efficiencies above 16%. [ 7,56 ] These findings have highlighted the importance of tail state recombination on the solar cell performance.…”

“…These defect states in the energy gap can be a source for space charge. [ 55,57 ] When capturing an electron, an acceptor‐like defect is negatively charged. This density n T of trapped electrons is also accounted in the total charge density ρ = q ( p − n − n T ).…”