Indacenodithienothiophene-Based Ternary Organic Solar Cells: Concept, Devices and Optoelectronic Analysis

Summary

In this study, in addition to the synthesis of a novel polymer, we fully characterize a ternary bulk-heterojunction solar cell, with a power conversion efficiency exceeding 4.6%, with the complementary use of optical and electrical techniques.

Abstract

We report on a novel ternary bulk-heterojunction solar cell by implementing a novel conjugated polymer (ADV-2) containing alternating pyridyl[2,1,3]thiadiazole (PT) between two different donor fragments, dithienosilole (DTS) and indacenodithienothiophene (IDTT), into a host system of indacenodithieno[3,2-b]thiophene,2,3-bis(3-(octyloxy)phenyl)quinoxaline (PIDTTQ) and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM). A clear absorption contribution in the near infrared (NIR) region leads to a power conversion efficiency (PCE) exceeding 4.6% in ternary device processed by doctor blading in air, fully avoiding any thermal treatment. Current-voltage (J-V) characteristics, external quantum efficiency (EQE) spectrum, charge extraction (CE) as well as photo-induced absorption (PIA) spectroscopy reveal the higher charge carrier generation in the ternary devices compared to the reference binary cells. Despite an enhancement of about 20% in the short circuit current density (J_sc), the lower fill factor (FF) achieved in PIDTTQ:ADV-2:PC₇₁BM ternary system limits the solar cell performance. With the complementary use of photoinduced charge carrier extraction by linearly increasing voltage (photo-CELIV) and transient photovoltage (TPV) measurements, we found that the ternary cells suffer from a lower mobility-lifetime (µτ) product, adversely impacting the FF. However, the significant improvement of light harvesting in the NIR region, compensating the transport losses, results in an overall power conversion efficiency enhancement of ~7% for ternary blends as compared to the PIDTTQ: PC₇₁BM devices.

Introduction

During the last decades, the power conversion efficiency (PCE) of organic bulk-hetorojunction (BHJ) solar cells based on donor/acceptor blends surpassed the 10% threshold, mainly due to the discovery of novel materials as well as device structure engineering.^1,2,3,4,5,6 Nowadays, one of the main challenges in order to further boost the PCE of organic solar cells is to achieve better absorption match to the solar irradiance spectrum, by extending the narrow absorption windo

Protocol

1. Planning of experiment

1. Identify two donor copolymers with complementary absorption in the visible-NIR range and with suitable energy levels in comparison with the fullerene derivative acceptor (PC₇₁BM).

2. Synthesis of M1

1. Add a 10 mL freshly distilled toluene solution containing 5,5'-bis(trimethylstannyl)-3,3'-di-2-ethylhexylsilylene-2,2'-bithiophene (0.372 g, 0.5 mmol, the quantity as well as the representative mmol corresponds to 5,5'-bis(trimethylstannyl)-3,3'-di-2-ethylhexylsilylene-2,2'-bithiophene), 4,7-dibromo-[1,2,5]thiadiazolo[3,4-c]pyridine (0.295 g, 1 m

Results

 Figure 1 shows 1H and 13C NMR spectra of M1 (a-b, respectively) and ADV-2 (c-d, respectively) with their respectively list of peaks. Figure 2 shows the synthetic route for the low band gap donor-acceptor copolymer ADV-2. Figure 3 shows the absorption spectrum of ADV-2 in DCB solution and as solid. The copolymer for both cases shows a single band in the high energy region which is assigned to a localized π−π* transition and another a...

Discussion

We reported a novel ternary system with a clear contribution in the incident photon-to-current efficiency in the near IR region. A J_sc improvement of around 20% was obtained for PIDTTQ:ADV-2:PC₇₁BM (0.85:0.15:2) ternary devices compared to PIDTTQ:PC₇₁BM binary cells. However, the low FF limited the performances of the ternary BHJ solar cells.

We found that by adding ADV-2 into the host system of PIDTTQ:PC₇₁BM the μ`...

Materials

Name	Company	Catalog Number	Comments
1,2-Dichlorobenzene	Aldrich	606078	solvent
1-Chloronaphtalene	Aldrich	970836	solvent
chloroform	Aldrich	1731042	solvent
PC71BM	Solenne	07099	BHJ material
Toluene	Aldrich	2036259	solvent
Chloroform-d	Aldrich	1697633	solvent
trichlorobenzene	Aldrich	956819	solvent
5,5’-bis(trimethylstannyl)-3,3’-di-2-ethylhexylsilylene-2,2’-bithiophene	Aldrich	143367-56-0	starting material
Pd(PPh3)4	Aldrich	14221-01-3	catalyst
source measurements unit	BoTEst
Solar simulator Oriel Sol 1A	Newport
Spectrometer Lambda 950	Perkin Elmer
EQE setup	Enlitech
oscilloscope DSO-X 2024A	Agilent Technologies
NMR setup	Bruker AVANCE III 600
GPC setup	Alliance 2000
Doctor blade	Zehntner ZAA 2300
evaporator	mbraun
glove boxes	mbraun
Laser 405 nm	THORLABS
funtion generator	Agilent Technologies 33500B series