Charge Pair Separation Dynamics in Organic Bulk‐Heterojunction Solar Cells

Abstract

Charge pair separation in organic bulk-heterojunction (BHJ) solar cells is a complex interplay between numerous factors, such as the spatial geometry of the blend, the distribution of energetic disorder, the electric field, thermal fluctuations, and the mutual electron-hole Coulomb attraction. Insufficient separation from the interface and concomitant charge pair recombination is a main limitation in improving the PCE of organic BHJ solar cells and requires an in-depth understanding of the timescales involved. Here, a 3D kinetic Monte Carlo model of a BHJ organic solar cell is set up and the time-dependent evolution of mutual electron-hole pair distances separating from the heterojunction interface is investigated. Large fluctuations in separation times are found, in particular in dependence of the energetic disorder and the permittivity of the organic materials. At commonly observed values of energetic disorder, slight modifications of the permittivity can drastically influence the charge separation time and even outweigh orders of magnitude of geminate recombination rates, hence help to suppress geminate recombination. Thus, the results strongly support the recent trend of developing high-permittivity organic materials for solar cell applications.

Impact of Phosphorescent Sensitizers and Morphology on the Photovoltaic Performance in Organic Solar Cells

“…The presented results are obtained using our kMC tool for OSCs . We have previously extended the kMC tool of Albes et al .…”

Role of the Interface and Extraction Layer Energetics in Organic Solar Cells

“…This observation has been supported by kMC studies by some of us . Furthermore, kMC studies have provided significant insight into charge separation, − charge recombination, , and charge carrier transport in OSCs. ,,,, The importance of the detailed morphology as well as the nonequilibrium nature of charge carrier dynamics makes kMC advantageous over DD and other numerical tools.…”

“…All calculations were performed using our kMC model, which has been used successfully to analyze the role of permittivity and energetics on the charge separation efficiency and dynamics, tuning the exciton dynamics using phosphorescent sensitizers, the role of interface energetics on the open-circuit voltage, and the origin of charge transport and the importance of the morphology in dilute donor organic solar cells. ,, Here, we briefly summarize the rate models used in the kMC simulations and the system setup. Details on the Monte Carlo algorithm based on the Gillespie method can be found elsewhere. , …”

Effect of Polymer Morphology on Dilute Donor Organic Solar Cells

“…kMC is a powerful method to analyze charge transport in organic semiconductors 50−53 and full solar cell characteristics. 25,42,54,55 The used kMC model 23,31,56 accounts for the generation of excitons, exciton dynamics, and the dynamics of charge carriers. Exciton generation is modeled using a z-dependent generation rate G(z), which is obtained using a transfer matrix method.…”

“…A kinetic Monte Carlo (kMC) method for dilute donor OSCs for the analysis of the photocurrent generation by hole back-transfer and the subsequent hole transport through the fullerene matrix has been implemented. kMC is a powerful method to analyze charge transport in organic semiconductors − and full solar cell characteristics. ,,, The used kMC model ,, accounts for the generation of excitons, exciton dynamics, and the dynamics of charge carriers. Exciton generation is modeled using a z -dependent generation rate G ( z ), which is obtained using a transfer matrix method .…”