Elsevier

Organic Electronics

Volume 78, March 2020, 105588
Organic Electronics

Charge photogeneration and recombination in single-material organic solar cells and photodetectors based on conjugated star-shaped donor-acceptor oligomers

https://doi.org/10.1016/j.orgel.2019.105588Get rights and content

Highlights

  • Efficient single-material OSC based on conjugated star-shaped oligomers are fabricated.

  • High VOC of 1.19 V, PCE up to 1.22% (under 0.45 sun intensity) and the maximum EQE up to 24% are achieved.

  • Charge photogeneration is field-assisted and follows the Onsager model.

  • Charge recombination mechanisms are identified.

Abstract

Single-material organic solar cells (SMOSC) are attracted by their simple structure and ease of fabrication so that they are virtually free from a number of drawbacks of heterojunction organic solar cells. However, the performance of SMOSC is still low, first of all because of poor understanding of losses on the way of energy conversion from light to electricity. In this work, we present solution-processed SMOSC based on star-shaped π-conjugated donor-acceptor oligomers with triphenylamine donor (N-Ph3) and alkyl- or phenyl dicyanovinyl acceptor (DCV-R) of general formulae N(Ph-nT-DCV-R)3, where nT stands for n-oligothiophene, and study charge photogeneration and recombination in them. SMOSC demonstrate small energy losses resulting in high open-circuit voltage of 1.08–1.19 V and the power conversion efficiency up to 1.22% under illumination intensity of 0.45 sun (1.13% under one sun) with the maximum external quantum efficiency up to 24% for N(Ph-2T-DCV-Ethyl)3, which are among the highest for SMOSC based on conjugated donor-acceptor small molecules or oligomers. It was found that monomolecular recombination dominates at the short-circuit condition and the maximum power point, but at the open-circuit condition the photoinduced charges recombine nearly bimolecularly. The bottleneck in the SMOSC performance was shown to be the field-assisted charge generation perfectly described by the Onsager model in the limit of weak electric fields. The results obtained suggest that intermolecular charge delocalization in conjugated donor-acceptor molecules would be beneficial for further progress in SMOSC.

Introduction

Organic solar cells (OSC) benefit from their light weight, flexibility, potentially low cost and weak environmental impact. The performance of OSC has been dramatically increased for the recent years [[1], [2], [3]]. Сompared to conjugated polymers, conjugated small molecules and oligomers combine the advantages of a well-defined chemical structure, ease of purification, and reproducibility. This should facilitate more straightforward analysis of the structure-property relationships. Donor-acceptor small molecules and oligomers are among the most promising materials for OSC resulting in the OSC power conversion efficiency (PCE) over 10% [4].

A characteristic feature of organic semiconductors is a low dielectric constant and, as a consequence, a high exciton binding energy. The necessary driving force for separating excitons into free charges is provided by a donor-acceptor heterojunction. High-performance OSC are based on a phase-separated blend of the donor and acceptor materials, i.e., a bulk heterojunction (BHJ), which provides large-area interfaces between the donor and acceptor components, resulting in efficient exciton dissociation and high OSC efficiency. However, the BHJ morphology strongly depends on the film fabrication methods (spin-coating, spray-coating, doctor-blading, drop-casting etc.), the processing conditions such as temperature, humidity, etc., and postprocessing, specifically, the details of thermal or solvent vapor annealing protocol [5]. The BHJ morphology can easily evolve during the device operation because of thermodynamic instability [6], so that the phase separation control in the BHJ films is very difficult. Therefore, single-material OSC (SMOSC) based on donor-acceptor molecules, which contain donor and acceptor groups within the same molecule, would be of great interest.

Although the efficiency of SMOSC is still rather low as compared to blend BHJ devices, they have attracted much attention in the organic photovoltaic community for the recent years [7]. SMOSC based on donor-acceptor molecules are the simplest and most convenient object for systematic study of the effect of molecular structure on charge generation, recombination, and device properties.

Photoactive materials for SMOSC are distinguished by the nature of the donor (polymer or small molecule) and acceptor (polymer or small molecule, fullerene or non-fullerene) units and by the way of their connection [7]. Specifically, in one approach the donor and acceptor units are covalently linked by a flexible insulating spacer. In the other approach, the donor and acceptor blocks are π-conjugated. Photoactive materials based on π-conjugated donor-acceptor molecules offer the advantage of more straightforward design and analysis of structure-property relationships. Such materials built of π-conjugated donor and acceptor blocks based on the triphenylamine core (N-Ph3) and peripheral dicyanovinyls (DCV), correspondingly, were first reported in Ref. [8]. SMOSC based on N(Ph-T-DCV)3, where T stands for thiophene, showed PCE of 0.40% [9]. By using ultrafast transient absorption spectroscopy, efficient charge photogeneration was reported in thin films of π-conjugated donor-acceptor molecules with the structure N(Ph-nT-DCV-R)3 (Fig. 1a) even without an external acceptor [10,11]. This is beneficial for the operation of SMOSC. These molecules were comprehensively studied in BHJ OSC previously [[12], [13], [14], [15], [16]] but not in SMOSC.

In this work, we demonstrate a high potential of star-shaped conjugated donor-acceptor molecules with triphenylamine donor and alkyl- or phenyl dicyanovinyl acceptor linked to each other through π-conjugated oligothiophene spacer of general formulae N(Ph-nT-DCV-R)3 for use in SMOSC. For SMOSC based on N(Ph-2T-DCV-Et)3 showing the highest PCE and external quantum efficiency (EQE), we studied charge generation, recombination and transport in details, which allowed us to conclude that the SMOSC performance is limited by the field-assisted charge generation.

Section snippets

Materials and methods

Fig. 1a shows the general chemical structure of the molecules investigated. Synthesis, optical, electrochemical, thermal properties of N(Ph-2T-DCV-Et)3, N(Ph-2T-DCV-Ph)3, N(Ph-3T-DCV-Hex)3 were described previously [[12], [13], [14]]. Synthesis and characterization of N(Ph-3T-DCV-Ph)3 are described in Supporting Information, Section 1.

Fig. 1b presents the schematic structure of SMOSC. First, glass substrates coated with patterned indium-tin oxide (ITO) layer (15 Ω/sq, Xin Yan Technology

Results and discussion

Table 1 summarizes photovoltaic parameters of the best SMOSC samples. The corresponding J-V characteristics and EQE spectra are presented in Fig. S7 and Fig. S8.

SMOSC based on N(Ph-nT-DCV-R)3 show very high VOC (1.1–1.2 V), which are among the highest for SMOSC [7]. To check the effect of the top electrode work function on VOC, we fabricated N(Ph-2T-DCV-Et)3 SMOSC without Ca (work function 2.7 eV) layer, i.e., only with Al (work function 4.2 eV) layer as a low-work function electrode. SMOSC

Conclusions

In summary, we have demonstrated single material organic solar cells (SMOSC) based on π-conjugated star-shaped donor-acceptor molecules N(Ph-nT-DCV-R)3 with PCE up to 1.22% and EQE up to 24%, which are among the highest for this class of materials. SMOSC based on N(Ph-2T-DCV-Et)3 have shown the highest PCE and EQE; moreover, they also have demonstrated VOC of 1.19 V, which is among the highest VOC for SMOSC. N(Ph-2T-DCV-Et)3 films also support the balanced charge transport for both type of

Author contributions

Artur L. Mannanov: Writing – Original Draft, Methodology, Investigation, Formal analysis, Validation.

Petr S. Savchenko: Investigation, Software, Formal analysis.

Yuriy N. Luponosov: Resourses, Writing – Review & Editing.

Alexander N. Solodukhin: Investigation, Resourses.

Sergey A. Ponomarenko: Conceptualization, Writing – Review & Editing, Funding Acquisition.

Dmitry Yu. Paraschuk: Conceptualizaion, Writing – Review & Editing, Project Administration, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank A.Yu. Sosorev for discussion of charge photogeneration mechanisms, and N.M. Surin for optical absorption spectra measurements. Materials were synthesized in the framework of Leading Science School NSh-5698.2018.3 supported by Russian Ministry of Science and Higher Education. NMR spectra registration were performed with the financial support from Ministry of Science and Higher Education of the Russian Federation using the equipment of Collaborative Access Center “Сenter for Polymer

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