Spectral Dependence of the Internal Quantum Efficiency of Organic Solar Cells: Effect of Charge Generation Pathways

Abstract

The conventional picture of photocurrent generation in organic solar cells involves photoexcitation of the electron donor, followed by electron transfer to the acceptor via an interfacial charge-transfer state (Channel I). It has been shown that the mirror-image process of acceptor photoexcitation leading to hole transfer to the donor is also an efficient means to generate photocurrent (Channel II). The donor and acceptor components may have overlapping or distinct absorption characteristics. Hence, different excitation wavelengths may preferentially activate one channel or the other, or indeed both. As such, the internal quantum efficiency (IQE) of the solar cell may likewise depend on the excitation wavelength. We show that several model high-efficiency organic solar cell blends, notably PCDTBT:PC70BM and PCPDTBT:PC60/70BM, exhibit flat IQEs across the visible spectrum, suggesting that charge generation is occurring either via a dominant single channel or via both channels but with comparable efficiencies. In contrast, blends of the narrow optical gap copolymer DPP-DTT with PC70BM show two distinct spectrally flat regions in their IQEs, consistent with the two channels operating at different efficiencies. The observed energy dependence of the IQE can be successfully modeled as two parallel photodiodes, each with its own energetics and exciton dynamics but both having the same extraction efficiency. Hence, an excitation-energy dependence of the IQE in this case can be explained as the interplay between two photocurrent-generating channels, without recourse to hot excitons or other exotic processes.

Solution-processed semiconductors for next-generation photodetectors

“…Unfortunately, utilizing two mixed components in the photoactive layer significantly complicates the spectral response since both the donor and acceptor contribute to photogeneration. 28 Additional optoelectronic strategies have to be implemented in order to produce a narrowband response in OPDs, and we will return to this later in the review.…”

Modulating the Exciton Dissociation Rate by up to More than Two Orders of Magnitude by Controlling the Alignment of LUMO + 1 in Organic Photovoltaics

“…one initially photo-excited Frenkel exciton (FE) can convert to a charge transfer (CT) excited state where the hole occupies the HOMO of the donor (or a linear combination of HOMOs of many donors) and the electron occupies the LUMO of the acceptor (or a linear combination of LUMOs in case of many acceptors). These CT states evolve into free electrons and holes with a mechanism that is still controversial [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. The idea that only the lowest CT state (CT1) dominated by the HOMO(D) to LUMO(A) transition is accessible through the internal conversion process from the FE state is very useful for giving qualitative descriptions of exciton dynamics in many cases but sometimes it may be too simplified for quantitative predictions especially when one considers the relatively dense energy levels of frontier orbitals in medium to large sized π-conjugated molecules and also the structural versatility of chemical substances.…”

Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

“…The mirror process where the hole is transferred from the acceptor IP to the donor IP is equally possible, depending on which material was excited. The photoinduced electron transfer is also known as Channel I, while the photoinduced hole transfer is known as Channel II [23].…”

“…Electro-optical measurements of the external and internal quantum efficiency (EQE / IQE) are used to quantify the combined efficiencies of carrier photogeneration and extraction [23,35,109,135]. However, charge generation and charge extraction losses cannot be differentiated through such an approach.…”

“…Despite the importance of these kinetic considerations on the charge carrier separation, most studies have focused on the impact of the donor and acceptor energy levels [23,64,201] and nanoscale morphology [60,202,203,204,205,206]. For example, Gélinas et al [60] have shown that a high fullerene loading is crucial for fullerene aggregation, which assists ultrafast charge separation by enabling electron delocalization.…”