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In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Results
  • Discussion
  • Disclosures
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

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 (Jsc), 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 window of organic polymers. In this regards, two main concepts have been developed: tandem and ternary organic solar cells.7,8,9,10,11,12,13,14,15,16,17 The former are based on a complex multi-layer stack with the main challenge of designing a robust solution-processed intermediate layer.18 The latter, made of two donors and one acceptor, mixed together in a unique solution, overcomes the complexities of the tandem device architecture, maintaining the easy processability of a single junction organic BHJ solar cell.19,20,21,22,23,24,25 To date, polymers,20 small molecules,21 dyes,26 quantum dots27 and fullerene derivates,23 have been adopted as "guest" in the polymer-fullerene "host" system.

In addition to the need for donor materials with the complementary absorption, one of the key points to surpass the performance of binary cells in ternary devices is to find donor materials with compatible physical and chemical natures.20 This can prevent the formation of recombination centers, or morphological traps, that deteriorate the photovoltaic properties.28,29

Here, we report a ternary organic solar cell system processed in air that shows a pronounced sensitization effect, resulting in a power conversion efficiency of more than 4.6%. As a sensitizer, we incorporate the near infrared (NIR) polymer ADV-2 that contains 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)30 blended with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). In fact, in order to have components with a similar chemical nature in the ternary blend system, we used two polymers with the same backbone unit of indacenodithienothiophene for the host and the guest donors. We studied the aforementioned ternary system by employing various optoelectronic techniques such as current-voltage (J-V) characteristics, external quantum efficiency (EQE), photoinduced charge carrier extraction by linearly increasing voltage (photo-CELIV), charge extraction (CE), transient photovoltage (TPV) measurements and photo-induced absorption (PIA) spectroscopy.

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 mmol) and Pd(PPh3)4 (57.8 mg, 0.05 mmol) into a microwave tube under the protection of nitrogen.
  2. Perform the Stille coupling with the following procedure: 120 oC for 10 min, 140 oC for 10 min, 160 oC for 10 min and 170 oC for 40 min (microwave step-wise).
  3. Cool down the reaction to room temperature, and extract with chloroform (100 mL × 3) in a separatory funnel. Wash with deionized water (100 mL × 3) and dry with anhydrous magnesium sulfate.
  4. Using a rotary evaporator, remove the solvent under reduced pressure. Separate the mixture by adding the solute directly to a silica column (25 mm inner diameter x 300 mm length) with hexane/chloroform (from 100/0 to 0/100 in v/v) to give 0.49 g of dark-purple solid (92% yield).

3. Synthesis of ADV-2

  1. Dissolve dibromo monomer M1 (180 mg, 1 equiv) and distannyl monomer M2 (286 mg, 1 equiv) in 1 mL toluene in a microwave vial. Fix the volume of the solvent and the quantity of the monomers to 0.025 M in concentration for each polymerization procedure.
  2. Add Pd2dba3.CHCl3 (4.4 mg, 0.02 equiv) and tri(o-tolyl)phosphine (P(o-tol)3) (2.6 mg, 0.04 equiv) in the reaction mixture and stir at 120 °C under argon atmosphere for 48 h.
  3. Purify the polymer by precipitation in methanol in a beaker, and then filter through a thimble and Soxhlet extract with methanol, acetone, ethyl acetate, chloroform and dichlorobenzene (DCB) in sequential order. Use 20% in excess of the volume of the polymeric solution.
  4. Collect the DCB fraction with a rotary evaporator and remove the solvents under reduced pressure.
  5. Isolate the polymer by precipitation into methanol in a beaker. Use 20% excess in volume. Filter and finally dry under high vacuum to give ADV-2 as a blue solid in 79% yield (142.2 mg).

4. Preparation of material solution

  1. Prepare 10 mg/mL solution of indacenodithieno[3,2-b]thiophene,2,3-bis(3-(octyloxy)phenyl)quinoxaline in 1,2 dichlorobenzene (DCB).
  2. Prepare 10 mg/mL solution of ADV-2 in DCB.
  3. Prepare 40 mg/mL solution of [6,6]-phenyl C70 butyric acid methyl ester (PC71BM) in DCB.
  4. Stir all the solutions overnight on hot plate at 80 °C.

5. Preparation of bulk-heterojunction(BHJ) solar cells

  1. Mix the solutions with different composition ratios (Table 1), keeping the total solution concentration at 20 mg/mL; add 3% v/v 1-Chloronaphtalene (CN). Stir the ternary as well as binary solutions 1 hour at 80 °C.
  2. Clean the pre-structured indium tin oxide (ITO) substrates in acetone and isopropyl alcohol in an ultrasonic bath for 10 minutes each.
  3. After drying, coat the substrates with 40 nm of zinc oxide (ZnO) with a doctor-blade.30
    1. Coat the aforementioned (Table 1) active layer materials with a doctor-blade. Set the temperature of doctor blade at 80 °C, the gap between the blade and the substrate at 400 µm, use 60 µL of solution and adjust the coating speed (10 mm/s) in order to have approximately 100 nm active layer thickness.
  4. To complete the fabrication of the devices, transfer the substrate in glove-box filled with nitrogen.
  5. In an ultra-vacuum chamber, thermally evaporate 10 nm of MoOx and 100 nm of Ag through a mask with a 10.4 mm2 active area opening and under a vacuum of 2×10-6 mbar.

6. Electrical and optical characterization of solar cells

  1. J-V characteristics:
    1. Measure the J-V characteristics of the BHJ devices between -2 V to 2 V under darkness using a source measurements unit according to manufacturer's protocol. Scan the voltages between -2 V and 2 V with steps of 20 mV to record the corresponding current (I). Calculate the current density (J) by the equation J=I/A, where A is the area of the solar cells (here, 10.4 mm2).
    2. Provide illumination with a solar simulator with AM1.5G spectrum at 100 mW cm-2. Measure the J-V characteristics of the BHJ devices between -2 V to 2 V under light conditions in ambient air using a source measurements unit according to manufacturer's protocol.
      1. Scan the voltages between -2 V and 2 V with steps of 20 mV to record the corresponding current (I). Calculate the current density (J) by the equation J=I/A, where A is the area of the solar cells (here, 10.4 mm2).
  2. Absorption spectra and External Quantum Efficiency (EQE) measurements:
    1. Measure the absorption spectra of the solar cells on film with a UV-VIS spectrometer according to manufacturer's protocol.
    2. Calibrate the EQE setup with a silicon diode reference cells between 350 and 900 nm.
    3. Measure solar cells EQEs using an integrated system between 350 and 900 nm. 30
  3. Photoinduced charge carrier extraction by linearly increasing voltage (photo-CELIV):30
    1. Illuminate the devices with a 405 nm laser diode.
    2. Connect the solar cells with the oscilloscope with BNC cable. Record the current transient across an internal 50Ω resistor of an oscilloscope30.
    3. After a variable delay time (delay time reflects the time passing from the laser pulse to the start of the voltage ramp, can vary between 1υs to 1-10 ms), apply a linear extraction ramp via a function generator. Set the ramp, which is 60 µs long and 2 V in amplitude, to start with an offset matching the Voc of the cell for each delay time.
  4. Transient Photo-Voltage (TPV):31
    1. Use two lasers at 405 nm, one as background illumination and the other one for creating voltage perturbation.
    2. Connect the solar cell to an oscilloscope with BNC cable and adjust the intensity of the background illumination to reach the open circuit condition.
    3. Adjust the laser intensity pulse to keep the voltage perturbation below 10 mV, typically at 5 mV. After the pulse, the voltage decays back to its steady state value in a single exponential decay.
    4. Change intensity of background illumination in a range of 0.1 to 4 suns.
    5. Record the transient with the oscilloscope.
  5. Charge extraction (CE):31
    1. Use a 405 nm laser diode for background illumination.
    2. Connect the solar cell to an oscilloscope with BNC cable and adjust the intensity of the background illumination to reach the open circuit condition.
    3. Turn off the laser and trigger the bilateral switch in order to switch the solar cell from open-circuit to short-circuit (50 Ω) conditions within less than 50 ns.
    4. Record the transient with the oscilloscope.
  6. Photo-induced spectroscopy (PIA):
    1. Place the solar cell in a cryostat in order to reach a temperature of 10 K and a pressure of 5×10-7 mbar.
    2. Check the optical alignments for a 532 nm Nd:YAG laser (as pump) and a UV-VIS lamp light (as probe), focused on the same position on the sample.
    3. Turn off the laser and measure the sample transmittance by a standard lock-in technique under the UV-VIS lamp light which is modulated by a mechanical chopper.
      NOTE: In this method the excitation (laser PUMP) is done by using a green laser (λ=532 nm), which is modulated by a mechanical chopper. The sample is additionally illuminated by a monochromated light beam ("probe") of a white light source. First the sample's transmittance and the sample's photoluminescence is measured. The measured signal is analyzed with a lock-in amplifier using the chopper frequency as the reference signal, allowing a very precise quantification of the change in absorption induced by the pump-light. Using various detectors (Si, Ge) a broad wavelength region ranging from 400 nm up to 1800 nm can be analyzed. Varying the material composition, material dependent effects can be characterized.
    4. Turn off the lamp and turn on the laser beam, modulated by a mechanical chopper.
    5. Measure the photoluminescence of the solar cells through the standard lock-in technique.
    6. Turn on the lamp and then measure the PIA spectrum by a standard lock-in technique under modulated laser pulse.

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 Jsc 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`...

Disclosures

The authors have nothing to disclose.

Acknowledgements

This project has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under the Grant Agreement n° 607585 project OSNIRO. In addition, this project has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under the Grant Agreement no. 331389. C. L. C. acknowledges the financial support of a Marie Curie Intra European Fellowship (FP7-PEOPLE-2012-IEF) project ECOCHEM. G. P. would like to thank the Ministry of Education and Religious Affairs in Greece for the financial support of this work provided under the co-operational program "AdvePol: E850". The authors gratefully acknowledge the support of the Cluster of Excellence ''Engineering of Advanced Materials'' at the University of Erlangen-Nuremberg, which is funded by the German Research Foundation (DFG) within the framework of its ''Excellence Initiative'', Synthetic Carbon Allotropes (SFB953) and Solar Technologies go Hybrid (SolTech).

Materials

NameCompanyCatalog NumberComments
1,2-DichlorobenzeneAldrich606078solvent
1-ChloronaphtaleneAldrich970836solvent
chloroform Aldrich1731042solvent
PC71BMSolenne07099BHJ material
TolueneAldrich2036259solvent
Chloroform-dAldrich1697633solvent
trichlorobenzene Aldrich956819solvent
5,5’-bis(trimethylstannyl)-3,3’-di-2-ethylhexylsilylene-2,2’-bithiopheneAldrich143367-56-0starting material
Pd(PPh3)4 Aldrich14221-01-3catalyst
source measurements unit BoTEst
Solar simulator Oriel Sol 1ANewport
Spectrometer Lambda 950Perkin Elmer
EQE setupEnlitech
oscilloscope DSO-X 2024AAgilent Technologies 
NMR setupBruker AVANCE III 600 
GPC setupAlliance 2000 
Doctor bladeZehntner ZAA 2300
evaporatormbraun
glove boxesmbraun
Laser 405 nmTHORLABS
funtion generatorAgilent Technologies 33500B series

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