Anomalous thickness-dependence of photocurrent explained for state-of-the-art planar nano-heterojunction organic solar cells

Abstract

Due to their simple geometry and design, planar heterojunction (PHJ) solar cells have advantages both as potential photovoltaics with more efficient charge extraction than their bulk heterojunction (BHJ) counterparts, and as idealized interfaces to study basic device operation. The main reason for creating BHJs was the limited exciton diffusion length in the active materials of the PHJ: if an exciton is generated at a distance greater than its diffusion length from the hetero-interface of the PHJ, it would be very unlikely to be able to contribute to the photocurrent. Based on this argument one expects a maximum in the photocurrent of PHJs for a thickness of the active layer equal to the exciton diffusion length (~10 nm). However, in two recently developed PHJs that have appeared in the literature, a maximum photocurrent is observed for 60-65 nm of poly(3-hexylthiophene) (P3HT). In this work, we explore this anomaly by combining both an optical T-matrix and a kinetic Monte Carlo simulation that tracks the exciton behavior in the PHJs. The two systems considered are a P3HT/single walled carbon nanotube (SWNT) device, and a P3HT/phenyl-C61-butyric acid methyl ester (PCBM) device. The model demonstrates how a bulk exciton sink can explain the shifted maximum in the P3HT/SWNT case, whereas in the P3HT/PCBM case the maximum is mainly determined by PCBM molecules interdiffusing in the P3HT upon annealing. Based upon the results of this model it will be possible to more intelligently design nanostructured photovoltaics and optimize them toward higher efficiencies.

Optical simulations of P3HT/Si nanowire array hybrid solar cells

“…But, this thickness is much larger than the exciton diffusion length (approximately 10 nm) in P3HT [ 20 ]. Recently, Paulus et al have presented their experimental and theoretical results on nano-heterojunction organic solar cells, in which the maximum photocurrent occurs at 60 to 65 nm of a P3HT photoactive layer due to bulk exciton sink in P3HT [ 21 , 22 ]. Considering the P3HT/Si NWA hybrid structure has the same exciton dissociation mechanism as that proposed by Paulus et al, the thickness of the conformal P3HT thickness can be increased above the exciton diffusion length in the design of P3HT/Si NWA hybrid cells.…”

Determination of Exciton Diffusion Length by Transient Photoluminescence Quenching and Its Application to Quantum Dot Films

“…If the refractive indices are not similar, as is the case with metallic quenchers, then the Beer's-law approach is insufficient, and the transfer-matrix method should be used. 37,52 For the perfect quenching case, the effects of exciton blocking and quenching layers at x = 0 and x = L, respectively, appear as boundary conditions…”

Interface Electrode Morphology Effect on Carrier Concentration and Trap Defect Density in an Organic Photovoltaic Device

“…Since the dielectric constant of the organic semiconductor is low (2–5), the excitons generated tend to recombine unless there is a driving force for them to separate. In this case, the driving force can be provided by built-in potential offset created by choosing appropriate metal contacts. , …”

“…In this case, the driving force can be provided by built-in potential offset created by choosing appropriate metal contacts. 3,4 Practically, it is impossible to collect all photogenerated charge carriers by the bulk heterojunction at the electrode.…”