Ultrafast Charge and Triplet State Formation in Diketopyrrolopyrrole Low Band Gap Polymer/Fullerene Blends: Influence of Nanoscale Morphology of Organic Photovoltaic Materials on Charge Recombination to the Triplet State

Abstract

Femtosecond transient absorption spectroscopy of thin films of two types of morphologies of diketopyrrolopyrrole low band gap polymer/fullerene-adduct blends is presented and indicates triplet state formation by charge recombination, an important loss channel in organic photovoltaic materials. At low laser fluence (approaching solar intensity) charge formation characterized by a 1350 nm band (in ~250 fs) dominates in the two PDPP-PCBM blends with different nanoscale morphologies and these charges recombine to form a local polymer-based triplet state on the sub-ns timescale (in ~300 and ~900 ps) indicated by an 1100 nm absorption band. The rate of triplet state formation is influenced by the morphology. The slower rate of charge recombination to the triplet state (in ~900 ps) belongs to a morphology that results in a higher power conversion efficiency in the corresponding device. Nanoscale morphology not only influences interfacial area and conduction of holes and electrons but also influences the mechanism of intersystem crossing (ISC). We present a model that correlates morphology to the exchange integral and fast and slow mechanisms for ISC (SOCT-ISC and H-HFI-ISC). For the pristine polymer, a flat and unstructured singlet-singlet absorption spectrum (between 900 and 1400 nm) and a very minor triplet state formation (5%) are observed at low laser fluence.

Making triplets from photo-generated charges: observations, mechanisms and theory

“…8 Other studies on charge recombination in low band gap polymer : fullerene donor-acceptor blends also show sub-ns triplet state formation from charges for both PDBTTT: PC 60 BM, 50 and diketopyrrolopyrrole (DPP) type polymer blends with fullerene adducts. 51,52 The formation of triplet states in thin film blends of low band gap polymers and fullerene adducts has been the focus of many studies. The great variety of polymer molecular structures, the different types of morphologies and crystalline regions, different energetics as well as the large influence of laser power applied in optical studies makes triplet formation by charge recombination difficult to pinpoint within this field.…”

Photophysics of Xanthene Dyes at High Concentrations in Solid Environments: Charge Transfer Assisted Triplet Formation

“…The same mechanism has been also reported in solution involving different moieties of the same molecule (14) and under aggregation of different dyes (15). In the context of photovoltaic materials this reaction pathway represents an important channel leading to efficiency loss and efforts have been made in order to suppress it (16,17).…”

Is it common for charge recombination to be faster than charge separation?

“…This phenomenon is different to cases involving organic sensitizers that have a high propensity for intersystem crossing (ISC), which either leads to the formation of triplet CT states, or does not allow CS if the triplet excitation energy is below the CT energy level . Such triplet CT manifolds have been known for quite some time, and they can affect the performance of organic devices …”