High Efficiency Non-fullerene Organic Tandem Photovoltaics Based

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High Efficiency Non-Fullerene Organic Tandem Photovoltaics Based on Ternary Blend Sub-Cells Wenchao Huang, Sheng-Yung Chang, Pei Cheng, Dong Meng, Bowen Zhu, Selbi Nuryyeva, Chenhui Zhu, Lijun Huo, Zhaohui Wang, Mingkui Wang, and Yang Yang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03950 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on November 27, 2018

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High Efficiency Non-Fullerene Organic Tandem Photovoltaics Based on Ternary Blend Sub-Cells Wenchao Huang12, Sheng-Yung Chang12, Pei Cheng12, Dong Meng123, Bowen Zhu12, Selbi Nuryyeva12, Chenhui Zhu4, Lijun Huo5, Zhaohui Wang3, Mingkui Wang6, Yang Yang12* 1

Department of Materials Science and Engineering, University of California Los Angeles,

California, 90095, USA 2

California NanoSystems Institute, University of California Los Angeles, California, 90095,

USA 3

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids,

Institute of Chemistry, Chinese Academy of Sciences (ICCAS), Beijing, 100190, P. R. China 4

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, 94720,

USA 5

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of

Education, Heeger Beijing Research and Development Center, School of Chemistry, Beihang University, Beijing 100191, P.R. China 6

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Wuhan, 430070, P.R. China

ACS Paragon Plus Environment

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ABSTRACT

The application of tandem structure that integrates multiple sub-cells into one device is a promising way to realize high efficiency organic solar cells. However, current-matching among different sub-cells remains as the main challenge for organic tandem photovoltaics. Here, we provide a facile approach to achieve a good current matching via engineering the chemical composition of non-fullerene ternary blend sub-cells. For the front sub-cell, a ternary blend of PDBT-T1:TPH-Se:ITIC is selected due to its good thermal stability. The amorphous nature of TPH-Se can sufficiently suppress the unfavorable phase separation of blends during the heat treatment, enabling a sintering in the fabrication of high quality interconnecting layer. A doublejunction tandem device is fabricated with a rear sub-cell consisting of PBDB-T:ITIC. After the optimization of the chemical composition of the front sub-cell, power conversion efficiency (PCE) of double-junction tandem device increased from 10.6% using PDBT-T1:TPH-Se binary front sub-cell to 11.5% using PDBT-T1:TPH-Se:ITIC (1:0.9:0.1) ternary front sub-cell due to better current matching. In order to further enhance the light absorption in the near-infrared region, a third junction PBDTTT-EFT:IEICO-4F is introduced. The champion cell of triplejunction non-fullerene tandem solar cell achieves a PCE of 13.0% with a high open circuit voltage of 2.52 V.

KEYWORDS: Organic tandem photovoltaics, non-fullerene, ternary sub-cell, current matching, morphology


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As the world is becoming more economically advanced, the consumption of energy dramatically increases. Harnessing energy from the sun is a promising way to solve the energy crisis. Hence, organic solar cells (OSCs) have been the subject of constantly increasing research interest over the past three decades with the promise of a low cost manufacturing via continuous solution-based roll-to-roll processing.1-6 The steady advancement of certified power conversion efficiency (PCE) of OSCs over 12% has been achieved by a combination of material development and morphology optimization.7-13 However, OSCs suffer from the insufficient light absorption due to the narrow absorption spectra of semiconducting polymers. To overcome this insufficiency, two approaches are widely used in OSC community: ternary solar cells and tandem solar cells. In ternary solar cells, the use of multi-donor materials or multi-acceptor materials with complementary absorption spectra can enhance light absorption of the active layer.14-19 However, the efficiency of organic ternary solar cells is still limited by Shockley-Queisser (S-Q) theory. Organic tandem photovoltaics, on the other hand, can break the S-Q efficiency limit for a single-junction solar cell by reducing thermalization losses from hot carriers, which have higher photon energy than the bandgap (Eg).20, 21 Tandem solar cells realize the superstition of open circuit voltage of sub-cells while preserving the high short circuit current by utilizing the photoactive materials with compensatory absorption achieving a more complete coverage of the solar spectrum.8, 12, 22-27


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Two major bottlenecks for increasing the efficiency of organic tandem solar cells even further are fabrication of high quality interconnecting layer (ICL) and optimization of unbalanced short circuit current between each sub-cell. Organic tandem solar cells consist of two or more sub-cells connected in series through a thin ICL. The ICL should form an ohmic contact between two sub-cells for the efficient charge recombination and should also be robust enough to protect the underlying films from solvents during the deposition of upper layers.28 In order to fabricate the high quality ICL (e.g. MoO3/PEDOT:PSS/np-ZnO), a heattreatment is always required. However, most of high efficiency fullerene-based solar cells show rapid degradation under heat treatment due to coarsening of fullerene aggregates.29 In this study, a non-fullerene PDBT-T1:TPH-Se blend (molecular structures shown in Figure 1a) with excellent thermal stability is selected in the front sub-cell. The acceptor TPH-Se exhibits an extremely twisted three-bladed propeller configuration, which significantly suppresses the phase separation in PDBT-T1:TPH-Se blends upon heating.30 Therefore, the use of this blend is able to maintain device performance of front sub-cell when a sintering is applied during the fabrication of ICLs. In series-connected organic tandem solar cells, the short circuit current of the tandem device is largely determined by the smallest short circuit current of the sub-cells. Therefore, the optimization of current matching among different sub-cells is also important. Few of the popular methods intended to adjust the optical field distribution in tandem solar cells are tuning the active layer thickness of each sub-cell22,

25, 31

and inserting an ultra-thin metal


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layer.32, 33 However, device performance of most OSCs is sensitive to the thickness of active layer. When the thickness of active layer is higher than its optimized value, a rapid drop of PCE is observed.34,

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The charge recombination is significantly increased in the thicker

active layer because of the nature of low mobility in organic semiconductors.34 With regards to an insertion of an ultra-thin metal layer, even though it can provide an effective connection between the two sub-cells with minimized electrical loss, the optical loss is a vital issue. Therefore, it is necessary to develop a new approach to manipulate the optical field distribution in organic tandem solar cells. In this study, we fabricate a high efficiency non-fullerene organic tandem solar cell by using ternary blend sub-cells designed for a good current matching, as absorption spectra of ternary blend can be easily tuned by varying its chemical composition. In the front sub-cell, a carefully chosen wide bandgap polymer PDBT-T1 is used,36-39 blended with a wide bandgap non-fullerene acceptor TPH-Se. As shown in Figure 1b, both PDBT-T1 and TPH-Se exhibit absorption spectra below 680 nm. In the rear sub-cell, the active layer consists of PBDBT:ITIC, whose absorption spectrum extends to 800 nm, shown in Figure S1. Tandem devices are fabricated with an architecture of ITO/ZnO/active layer 1/MoO3/m-PEDOT:PSS/npZnO/active layer 2/MoO3/Ag, as seen in Figure 1c. As a result, an efficiency of 10.6% is achieved for double junction tandem solar cells using a binary front sub-cell PDBT-T1:TPHSe. As absorption spectra of front sub-cell is optimized by introduction of an additional nonfullerene acceptor ITIC,40 the best efficiency of tandem device based on a ternary front sub-


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cell increases to 11.5%. The substitution of 10 wt.% TPH-Se by ITIC does not significantly change front sub-cells’ original morphology but enhances its light absorption and facilitates charge separation via cascade energy level, leading to a more balanced current between front and rear sub-cells. A wide bandgap polymer is usually used as the front sub-cell in a tandem device, which allows for the longer wavelength light to pass through. Among the various wide bandgap polymers, PDBT-T1 is a promising candidate with proper absorption coverage and high efficiency.36 Figure 2a compares photovoltaic performance of OSCs based on polymer PDBT-T1 blended with two different acceptors, PCBM and TPH-Se. All devices are fabricated with the same architecture of ITO/ZnO/active layer/MoO3/Ag. Device photovoltaic parameters are summarized in Table S1. Devices based on the PDBT-T1:PCBM blend exhibit an average PCE of 7.2%, short circuit current (Jsc) of 12.4 mA/cm2, open circuit voltage (Voc) of 0.85 V and fill factor (FF) of 0.68. As the fullerene acceptor PCBM is replaced by a non-fullerene acceptor TPH-Se with a better energy level alignment with PDBT-T1 polymer, the Voc increases from 0.85 V in PDBTT1:PC71BM to 1.00 V in PDBT-T1:TPH-Se. An average PCE of 8.0% is achieved in PDBTT1:TPH-Se non-fullerene solar cells. Device based on non-fullerene acceptor TPH-Se shows the increased efficiency mainly due to the enhanced Voc. Since heat treatment is usually employed during fabrication of high quality ICLs, a good thermal stability is required for the front sub-cell.29 Figure 2b compares the evolution of device performance based on two different acceptors upon annealing at 120°C. (This temperature is used for sintering ICLs.) Fullerene-based devices show a poor stability. They exhibit a rapid decrease


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in PCE, dropping to 75% of its initial PCE after only 5 min heat treatment. After heating at 120°C for 60 mins, the PCE continuously decays to 58% of its initial PCE. By contrast, there is no significant change in PCE of the non-fullerene device after 5 min heat treatment. Furthermore, PCE of non-fullerene-based device after heating at 120°C for 60 min still preserves 88% of its initial value. The morphological evolution of both fullerene and non-fullerene blend under heat treatment is investigated by using 2D grazing incident wide angle X-ray scattering (GIWAXS), as shown in Figure 2c-f (1D scattering lines extracted from 2D scattering patterns shown in Figure S2). In scattering patterns of PDBT-T1:PCBM blend before and after heat treatment (Figure 2c and d), a broad peak located along the out-of-plane direction at q= 1.7 Å-1 corresponds to polymer π-π stacking and an intensive peak at q= 0.3 Å-1 corresponds to polymer alkyl stacking. The isotropic rings at q= 1.4 Å-1 are related to the fullerene aggregation. It is noted that scattering intensities from both, polymer and fullerene, significantly increase after heat treatment (also confirmed with 1D scattering results), suggesting the rapid growth of both polymer and fullerene crystallites. An unfavorable phase separation occurs, leading to a rapid degradation of the device performance. In PDBT-T1:TPH-Se blend, due to the amorphous nature of non-fullerene acceptor TPH-Se, the crystallization of polymer PDBT-T1 is remarkably disrupted, with a relative lower polymer crystallinity. Non-fullerene acceptor TPH-Se does not exhibit any scattering patterns, indicating that most of the non-fullerene acceptor molecules dispersed in the mixed phase. Nonfullerene blends exhibit a similar microstructure before and after heat treatment with a slight increase in polymer’s scattering intensities, shown in Figure 2e and f. This highly mixed phase in


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PDBT-T1:TPH-Se blend is able to effectively suppress phase separation and maintain its initial morphology during heat treatment.29 Therefore, the TPH-Se based non-fullerene blend presents better thermal stability than its fullerene-based counterpart. Considering the absolute efficiency and thermal stability, PDBT-T1:TPH-Se blend is chosen for the front sub-cell. The engineering of the active layer thickness is a widely used method for optimizing the current matching between different sub-cells. However, in some solar cells such as PDBT-T1:TPH-Sebased device, PCE is very sensitive to the thickness of the active layer. The slight increase in the thickness of the active layer may lead to a rapid degradation of the device performance, especially in FF. Here, a third non-fullerene component ITIC is introduced into the PDBTT1:TPH-Se blend to tune the optical field distribution in a tandem device. Figure 3 presents device performance of double-junction organic tandem solar cells, using non-fullerene PDBTT1:TPH-Se:ITIC ternary front sub-cell and PBDB-T:ITIC rear sub-cell. In the front sub-cell, PDBT-T1:TPH-Se:ITIC blends are deposited with a fixed PDBT-T1 concentration and varied ratios of ITIC. Active layers with different ITIC contents are deposited with a thickness of around 80 nm to achieve their optimized device performance. Figure 3a and b demonstrate the trend of average PCEs and Jsc values in double-junction tandem solar cells as a function of ITIC content (in total acceptor) in the front sub-cell. An average PCE of 10.3% is achieved in the tandem device using PDBT-T1:TPH-Se binary front sub-cell, with an average Jsc of 8.8 mA/cm2. As 10 wt.% additional component ITIC is introduced into the front sub-cell to replace TPH-Se, the average PCE and photocurrent density of tandem device monotonically increases from 10.3 to 11% and 8.8 to 9.3 mA/cm2 respectively. Figure 3c and d


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compares current density-voltage (J-V) characteristics and external quantum efficiency (EQE) spectra of the champion tandem device with 0 wt.% and 10 wt.% ITIC in the front sub-cell. The details of photovoltaic parameters are summarized in Table 1. The tandem device based on the PDBT-T1:TPH-Se binary sub-cell shows the best efficiency of 10.6%, with a Jsc of 9.2 mA/cm2, a Voc of 1.84 V and a FF of 0.63. The tandem device based on PDBT-T1:ITIC binary sub-cell exhibits a poorer efficiency due to the severe absorption overlap between front sub-cell and rear sub-cell, as shown in Figure S3. After carefully tuning the chemical composition of PDBTT1:TPH-Se:ITIC in the front sub-cell, an improvement in its efficiency is achieved by using a PDBT-T1:TPH-Se:ITIC (1:0.9:0.1) ternary blend, yielding the best value of 11.5% with an increased Jsc of 9.6 mA/cm2 and an improved FF of 0.65. As shown in EQE spectra, the increased Jsc of tandem device is attributed to the more balanced short circuit current between the front and rear sub-cells. In the tandem device based on the binary front sub-cell, the Jsc integrated from the front sub-cell is slightly lower than that from the rear sub-cell. The substitution of 10 wt.% TPH-Se by ITIC is able to enhance the light absorption of the front sub-cell in the region between 700 and 750 nm, resulting in a perfect current matching. As the ITIC content continues to increase from 10 wt.% to 50 wt.%, decreases in average PCE (from 11 to 8.6%) and Jsc (from 9.3 to 7.8 mA/cm2) are observed. As more ITIC molecules are introduced, the absorption overlap between the front sub-cell and rear sub-cell becomes large, hindering the sufficient light absorption from the rear sub-cell. The photocurrent generated from the front sub-cell is significantly higher than the one from the rear sub-cell, resulting in an unbalanced current


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matching again. As a result, both, PCE and short circuit current of tandem device, experience a rapid decrease upon using a ternary front sub-cell with high fraction of ITIC. In order to better understand the roles of the third component ITIC on fabrication of the high performance non-fullerene tandem devices, device performance and ternary blend morphology in single-junction devices are also systematically investigated, as shown in Figure 4. Figure 4a shows the average PCE and Jsc as a function of ITIC content in acceptor. As a small amount of ITIC (

High Efficiency Non-fullerene Organic Tandem Photovoltaics Based

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