Tandem Organic Photovoltaic Cells Research Articles

Enhancing Photon Utilization Efficiency for High-Performance Organic Photovoltaic Cells via Regulating Phase-Transition Kinetics.

Efficient photon utilization is key to achieving high-performance organic photovoltaic (OPV) cells. In this study, a multiscale fibril network morphology in a PBQx-TCl:PBDB-TF:eC9-2Cl-based system is constructed by regulating donor and acceptor phase-transition kinetics. The distinctive phase-transition process and crystal size are systematically investigated. PBQx-TCl and eC9-2Cl form fibril structures with diameters of ≈25nm in ternary films. Additionally, fine fibrils assembled by PBDB-TF are uniformly distributed over the fibril networks of PBQx-TCl and eC9-2Cl. The ideal multiscale fibril network morphology enables the ternary system to achieve superior charge transfer and transport processes compared to binary systems; these improvements promote enhanced photon utilization efficiency. Finally, a high power conversion efficiency of 19.51% in a single-junction OPV cell is achieved. The external quantum efficiency of the optimized ternary cell exceeds 85% over a wide range of 500-800nm. A tandem OPV cell is also fabricated to increase solar photon absorption. The tandem cell has an excellent PCE of more than 20%. This study provides guidance for constructing an ideal multiscale fibril network morphology and improving the photon utilization efficiency of OPV cells.

Engineering of A-π-D-π-A system based non-fullerene acceptors to enhance the photovoltaic properties of organic solar cells; A DFT approach

In photovoltaic mechanization, central core alteration is an efficient tool to regulate photo-electronic features of the organic photovoltaic cells. Four fullerene free π-conjugated acceptor moieties (D1-D4) in the acceptor–donor-acceptor (A-D-A) geometries were constructed, by using quantum mechanical reproduction, from the reported acceptor molecule R with 10.27% and 13.97% solar efficiency for single and tandem organic photovoltaic cells. For the exhaustive investigation of reference molecule R and all other modified compounds through time dependent self-consistent field (TD-SCF) and density functional theory (DFT) method, MPW1PW91 functional with 6-31G (d,p) basis set was elected. Fundamental details about charge delocalization between the reference and constructed structures were obtained. Amongst all, D4 molecule elucidates the most shrunk band-gap of 2.24 eV, maximum absorption at 697 nm in chlorobenzene solvent, and a smaller binding energy of 0.47 eV that helps to effectively transmit the electron densities from bonding orbitals/HOMO to anti-bonding orbitals/LUMO. Electron and hole mobilities revealed by all constructed molecules were found to be lower when contrasted to the reference molecule R. Upon testing all modified molecules with polymer donor PTB7-Th, D3 moiety gave a higher open-circuit photo-voltage (Voc) value (1.23 eV) as compared to all other molecules, which shows that it has higher electronic charge transfer than other molecules. This current work indicates that by replacing the central part of R with some prominent donor cores, we can efficiently tune the binding energy, open-circuit photo-voltage, frontier molecular orbitals, absorption spectra, band-gap, and reorganization energy in the thus constructed structures. Thus in the future, tailored compounds ought to be suggested for the manufacture of OSCs with high solar efficiency.

Low-cost and high-performance poly(thienylene vinylene) derivative donor for efficient versatile organic photovoltaic cells

The utilization of donor materials with complex structures obviously increases the costs of organic photovoltaic (OPV) cells. Therefore, low-cost and high-performance are two issues that must be considered when designing polymer donors for the preparations of large-area OPV cells. Here, a poly(thienylene vinylene) derivative, named PTVT-BT was reported. PTVT-BT has a very simple completely non-fused molecular structure. PTVT-BT shows planar molecular structure and obvious solution aggregation effect. Besides, PTVT-BT demonstrates a high hole mobility up to the magnitude of 10-2 cm-2 V-1 s-1. The external quantum efficiency of electroluminescence of PTVT-BT is 7 × 10-3. As a result, the OPV cell based on PTVT-BT:eC9 demonstrates a power conversion efficiency (PCE) of 16.31%, which is the highest value among the poly(thienylene vinylene)- and polythiophene-based OPV cells. The tandem OPV cell with PTVT-BT as the donor of the sub-cell yields an outstanding PCE of 18.49%. Besides, the indoor OPV cell based on PTVT-BT:BTA3 exhibits a PCE of 27.30% under a light-emitting diode (LED) illumination of 1000 lux (2700 K). This study indicates that PTV-derivative is a kind of promising material for the future OPV industrialization.

A High-Performance Nonfused Wide-Bandgap Acceptor for Versatile Photovoltaic Applications.

Wide-bandgap (WBG) nonfullerene acceptors (NFAs) with nonfused conjugated structures play a critical role in organic photovoltaic (OPV) cells. Here, NFAs named GS-OEH, GS-OC6, and GS-ISO, with optical bandgaps larger than 1.70eV, are synthesized without using the fused ring structures. Compared with GS-OEH and GS-OC6, GS-ISO exhibits much stronger crystallinity, leading to a smaller energetic disorder and a larger exciton diffusion coefficient. GS-ISO also possesses a higher electroluminescence external quantum efficiency of 1.0×10-2 . The OPV cell based on PBDB-TF:GS-ISO demonstrates a power conversion efficiency (PCE) of 11.62% under the standard one sun illumination. Besides, the PBDB-TF:GS-ISO-based cell with effective area of 1.0cm2 exhibits a PCE of 28.37% under 2700K illumination of 500lux. A tandem OPV cell using PBDB-TF:GS-ISO as the front subcell shows an outstanding efficiency of 19.10%. Importantly, the GS-ISO-based OPV cell exhibits promising stability under the continuous illumination of simulated sunlight. This study indicates that the molecular design strategy demonstrated in this work has great superiority in developing nonfused NFAs and also that GS-ISO is a promising WBG acceptor for versatile photovoltaic applications.

A Tandem Organic Photovoltaic Cell with 19.6% Efficiency Enabled by Light Distribution Control.

Despite more potential in realizing higher photovoltaic performance, the highest power conversion efficiency (PCE) of tandem organic photovoltaic (OPV) cells still lags behind that of state-of-the-art single-junction cells. In this work, highly efficient double-junction tandem OPV cells are fabricated by optimizing the photoactive layers with low voltage losses and developing an effective method to tune optical field distribution. The tandem OPV cells studied are structured as indium tin oxide (ITO)/ZnO/bottom photoactive layer/interconnecting layer (ICL)/top photoactive layer/MoOx /Ag, where the bottom and top photoactive layers are based on blends of PBDB-TF:ITCC and PBDB-TF:BTP-eC11, respectively, and ICL refers to interconnecting layer structured as MoOx /Ag/ZnO:PFN-Br. As these results indicate that there is not much room for optimizing the bottom photoactive layer, more effort is put into fine-tuning the top photoactive layer. By rationally modulating the composition and thickness of PBDB-TF:BTP-eC11 blend films, the 300 nm-thick PBDB-TF:BTP-eC11 film with 1:2 D/A ratio is found to be an ideal photoactive layer for the top sub-cell in terms of photovoltaic characteristics and light distribution control. For the optimized tandem cell, a PCE of 19.64% is realized, which is the highest result in the OPV field and certified as 19.50% by the National Institute of Metrology.

Modeling and optimization of organic tandem photovoltaic solar cells using silver nanowires as electrode and interconnecting layer

Much research has been conducted to improve the performance of photovoltaic solar cells. Transparent conductive film and interconnection layers have a significant impact on the performance of photovoltaic cells. In this work, we analyze the experimental results obtained on tandem organic photovoltaic solar cells with simple inverted structures using silver nanowires AgNW as transparent conductive electrode (TE) and as interconnection layer (ICL) between PEDOT: PSS and ZnO. This type of contact leads to a strong ohmic contact in both sub-cells having P3HT: ICBA as the lower active layer and having PTB7: PC71BM (1: 1.5) as the upper active layer with a good complement of the absorption spectrum. To study the advantages of using AgNWs as an interconnection layer (PEDOT: PSS/AgNWs/ZnO) in tandem photovoltaic solar cells and as an anode and its impact on the performance of these organic cells, we have simulated the electrical characteristics obtained by these tandem organic photovoltaic cells using an equivalent circuit model. This model is based on a single diode model with five photovoltaic parameters. We therefore extracted all the physical parameters of the illuminated photovoltaic cell from its experimental characteristics (J–V), such as the diode saturation current density (J0), the series and shunt resistors (RS, RSh), the ideality factor (n) and the photogenerated current density (JPh). For this we have solved the analytical equations of the current density using Newton Raphson's method. The equations are derived from the single diode equivalent circuit proposed to simulate the measured current density as a function of the voltage of the manufactured tandem type organic solar cells. A good agreement was obtained between the theoretical model and the experimental electrical characteristics. This confirms that the use of AgNWs between PEDOT: PSS and ZnO as an interconnection layer in reverse geometry of these tandem devices, has improved the efficiency (PCE = 9.24%) and is proving to be an efficient recombination layer for tandem organic photovoltaic solar cells.

Enabling High-Performance Tandem Organic Photovoltaic Cells by Balancing the Front and Rear Subcells.

In tandem organic photovoltaics, the front subcell is based on large-bandgap materials, whereas the case of the rear subcell is more complicated. The rear subcell is generally composed of a narrow-bandgap acceptor for infrared absorption but a large-bandgap donor to realize a high open-circuit voltage. Unfortunately, most of the ultraviolet-visible part of the photons are absorbed by the front subcell; as a result, in the rear subcell, the number of excitons generated on large-bandgap donors will be reduced significantly. This reduces the (photo) conductivity and finally limits the hole-transporting property of the rear subcell. In this work, a simple and effective way is proposed to resolve this critical issue. To ensure sufficient photogenerated holes in the rear subcell, a small amount of an infrared-absorbing polymer donor as a third component is introduced, which provides a second hole-generation and transporting mechanism to minimize the aforementioned detrimental effects. Finally, the short-circuit current density of the two-terminal tandem organic photovoltaic is significantly enhanced from 10.3 to 11.7mAcm-2 (while retaining the open-circuit voltage and fill factor) to result in an enhanced power conversion efficiency of 15.1%.

Stacked organic solar cell increased to 15.9% efficiency

Tandem organic photovoltaic cell with reduced electrical and optical losses, has reached 15.9% power conversion efficiency.

Organic tandem solar cells with 18.6% efficiency

Tandem organic photovoltaic (TOPV) cell is one of the technologies to harvest more solar power by staking two or more OPV devices on top of each other. Recently, the highest power conversion efficiency (PCE) ever achieved was 17.3%. Herein, this paper simulates the response of 676‬ different TOPV devices that consists front and back OPV cells. For this purpose, this paper uses the best 26 single-cell OPV devices to form the TOPV front and back cells combinations. The results show that there are some new TOPVs that can exceed 17.3% efficiency limit for TOPV. Also, in this work thickness optimization was performed for these new TOPV devices with an objective of efficiency maximization. As a result, using PBDTS-TDZ: ITIC in the front cells and PTB7-Th: O6T-4F:PC71BM in the back cell gives 18.6% efficiency. Likewise, the TOPV of PBDB-T-2F:TfIF-4FIC in the front cell with PTB7-Th:O6T-4F:PC71BM in the back cell gives 18.06% efficiency.

Continuous roll-to-roll fabrication of organic photovoltaic cells via interconnected high-vacuum and low-pressure organic vapor phase deposition systems

We demonstrate continuous roll-to-roll (R2R) fabrication of single junction and tandem organic photovoltaic (OPV) cells on flexible plastic substrates employing a system that integrates organic deposition by high vacuum thermal evaporation (VTE) and low pressure organic vapor phase deposition (OVPD). By moving the substrate from chamber to chamber and then depositing films on stationary substrates, we achieve power conversion efficiencies of PCE = 8.6 ± 0.3% and 8.9 ± 0.2% for the single junction and tandem cells, respectively. Single junction OPVs are also fabricated on a continuously translating substrate at 0.3 cm/s, to achieve PCE = 8.5 ± 0.2%. Thin films grown on translating substrates by OVPD show <3% thickness non-uniformity and 0.66 nm root mean square surface roughness, similar to that obtained by VTE. Our results suggest that R2R film deposition comprising multiple vapor deposition technologies is a promising method for rapid speed and continuous manufacturing of high quality, small molecular weight organic electronic materials.

Stacking Sequence and Acceptor Dependence of Photocurrent Spectra and Photovoltage in Organic Two-Junction Devices.

Both single-junction and tandem organic photovoltaic cells have been well developed. A tandem cell contains two junctions with a charge recombination layer (CRL) inserted between the two junctions. So far, there is no detailed report on how the device will perform that contains two junctions but without a CRL in between. In this work, we report the photocurrent spectra and photovoltage output of the devices that contains two bulk-heterojunctions (BHJ) stacked directly on top of each other without a CRL. The top active layer is prepared by transfer printing. The photocurrent response spectra and photovoltage are found to be sensitive to stacking sequence and the selection of electron acceptors. The open-circuit voltage of the devices (up to 1.09 V) can be higher than the devices containing either junction layer. The new phenomenon in the new device architecture increases the versatility of the optoelectronic devices based on organic semiconductors.

Highly efficient organic tandem solar cell with a SubPc interlayer based on TAPC:C70 bulk heterojunction.

We report a small molecule tandem organic photovoltaic (OPV) cell with a high power conversion efficiency (PCE) of 7.27%. This cell contains two subcells with an identical mixed active layer of C70:5 wt%TAPC (1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane). The performance was dramatically improved by simply inserting a thin boron subphthalocyanine chloride (SubPc) interlayer, which results in an increase of the short-circuit current and open-circuit voltage as well as a decrease of the series resistance of the tandem cell. The response of the cell only contributed from the absorption of C70. The high PCE was attributed to the high absorption efficiency of C70 and improved holes extraction efficiency at the anode due to the band bending occurs at both MoO3/SubPc and SubPc/C70:5 wt%TAPC interfaces.

Charge transport dependent high open circuit voltage tandem organic photovoltaic cells with low temperature deposited HATCN-based charge recombination layers.

Mechanisms of charge transport between the interconnector and its neighboring layers in tandem organic photovoltaic cells have been systematically investigated by studying electronic properties of the involving interfaces with photoelectron spectroscopies and performance of the corresponding devices. The results show that charge recombination occurs at HATCN and its neighboring hole transport layers which can be deposited at low temperature. The hole transport layer plays an equal role to the interconnector itself. These insights provide guidance for the identification of new materials and the device architecture for high performance devices.

Controlling the Morphology of BDTT-DPP-Based Small Molecules via End-Group Functionalization for Highly Efficient Single and Tandem Organic Photovoltaic Cells.

A series of narrow-band gap, π-conjugated small molecules based on diketopyrrolopyrrole (DPP) electron acceptor units coupled with alkylthienyl-substituted-benzodithiophene (BDTT) electron donors were designed and synthesized for use as donor materials in solution-processed organic photovoltaic cells. In particular, by end-group functionalization of the small molecules with fluorine derivatives, the nanoscale morphologies of the photoactive layers of the photovoltaic cells were successfully controlled. The influences of different fluorine-based end-groups on the optoelectronic and morphological properties, carrier mobilities, and the photovoltaic performances of these materials were investigated. A high power conversion efficiency (PCE) of 6.00% under simulated solar light (AM 1.5G) illumination has been achieved for organic photovoltaic cells based on a small-molecule bulk heterojunction system consisting of a trifluoromethylbenzene (CF3) end-group-containing oligomer (BDTT-(DPP)2-CF3) as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. As a result, the introduction of CF3 end-groups has been found to enhance both the short circuit current density (JSC) and fill factor (FF). A tandem photovoltaic device comprising an inverted BDTT-(DPP)2-CF3:PC71BM cell and a poly(3-hexylthiophene) (P3HT):indene-C60-bisadduct (IC60BA)-based cell as the top and bottom cell components, respectively, showed a maximum PCE of 8.30%. These results provide valuable guidelines for the rational design of conjugated small molecules for applications in high-performance organic photovoltaic cells. Furthermore, to the best of our knowledge, this is the first report on the design of fluorine-functionalized BDTT-DPP-based small molecules, which have been shown to be a viable candidate for use in inverted tandem cells.

Efficient Vacuum‐Deposited Tandem Organic Solar Cells with Fill Factors Higher Than Single‐Junction Subcells

Efficient vacuum‐deposited tandem organic photovoltaic cells (TOPVs) composed of pristine fullerenes as the acceptors and two complementary absorbing donors, 2‐((2‐(5‐(4‐(diphenylamino)phenyl)thieno[3,2‐b]thiophen‐2‐yl)thiazol‐5‐yl)methylene)malononitrile for the visible absorption and 2‐((7‐(5‐(dip‐tolylamino)thiophen‐2‐yl)benzo[c]‐[1,2,5]thiadiazol‐4‐yl)methylene)malononitrile for the near‐infrared absorption, are reported. Two subcells are connected by the interconnection unit (ICU) composed of electron‐transporting layer/metal/p‐doped hole‐transporting layer. The p‐doped layer in the ICU enables increasing the short‐circuit current density (J SC) of TOPVs by tuning the relative position of subcells in the tandem devices to have the maximum optical field distribution response, which is well matched with theoretical calculation. Moreover, the introduction of the doped layer benefits to the higher fill factor (FF) of the consisting subcells without losing open‐circuit voltage (V OC) even with the thick active layers. As a result, power conversion efficiency of 9.2% is achieved with higher FF of 0.62 than that of single‐junction subcells (0.54, 0.57), J SC of 8.7 mA cm−2, and V OC of 1.71 V using 80 nm thick active layers in both subcells.

Effect of π-conjugated bridges of TPD-based medium bandgap conjugated copolymers for efficient tandem organic photovoltaic cells

Conjugated donor (D)–π–acceptor (A) copolymers, PBDT–TPD, PBDT–ttTPD, PBDTT–TPD, and PBDTT–ttTPD, based on a benzodithiophene (BDT) donor unit and thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) acceptor unit were designed and synthesized with different π bridges via Pd-catalyzed Stille-coupling. The π bridges between BDT and TPD were thiophene in PBDT–TPD and PBDTT–TPD, and 6-alkylthieno[3,2-b]thiophene in PBDT–ttTPD and PBDTT–ttTPD. The effects of the π bridges on the optical, electrochemical, and photovoltaic properties of the polymers were investigated, in addition to the film crystallinities and carrier mobilities. Copolymers with the 6-alkylthieno[3,2-b]thiophene π-bridge exhibited high crystallinity and hole mobility. Improved Jsc and FF were obtained to increase the overall power conversion efficiencies (PCE) in inverted single organic photovoltaic cells. A PCE of 6.81% was achieved from the inverted single device fabricated using the PBDTT–ttTPD:PC71BM blend film with 3 vol% 1,8-diiodooctane. A tandem photovoltaic device comprising the inverted PBDTT–ttTPD cell and a PTB7-based cell as the bottom and top cell components, respectively, showed a maximum PCE of 9.35% with a Voc of 1.58 V, a Jsc of 8.00 mA cm−2, and a FF of 74% under AM 1.5 G illumination at 100 mW cm−2. The obtained PCE of the bottom cell and FF of the tandem cell are, to the best of our knowledge, the highest reported to date for a tandem OPV device. This work demonstrates that PBDTT–ttTPD may be very promising for applications in tandem solar cells. Furthermore, 6-alkylthieno[3,2-b]thiophene π-bridge systems in medium bandgap polymers can improve the performance of tandem organic photovoltaic cells.

Tandem small molecule organic photovoltaic cells with broad spectral response up to 1 μm and a high open-circuit voltage

We report a series-connected small molecule tandem photovoltaic cell utilizing two donors with complementary photovoltaic characteristics, lead phthalocyanine (PbPc) in the front subcell and boron subphthalocyanine chloride (SubPc) in the back subcell, to achieve both near infrared (NIR) response up to 1μm and high open-circuit voltage (VOC) of more than 1.5V in the same device. We find that the C60 layer thickness in the front subcell has a critical impact on the overall optical structure and photovoltaic performance of the tandem device. By combining transfer matrix calculations with subcell-selective spectral measurements, we are able to tune the optical field distribution inside the active layers and increase the photocurrent outputs from both subcells, leading to EQE>30% over the wavelength range 400nm<λ<900nm. This optimized tandem cell exhibits JSC=(5.5±0.1)mA/cm2, fill factor=0.54, VOC=1.53V, and a power conversion efficiency of (4.5±0.2)%.

Effect of different p-dopants in an interconnection unit on the performance of tandem organic solar cells

We report the effect of different p-dopants (ReO3, MoO3 and CuI) used in the interconnection unit (ICU) composed of electron-transporting layer (ETL)/metal/p-doped hole-transporting layer (p-HTL) on the performance of tandem organic photovoltaic (TOPV) cells. Dopants with higher charge generation efficiency resulted in higher open circuit voltage (VOC) due to the reduction of the difference between the Fermi level and the HOMO level of the p-HTL, and higher fill factor (FF) due to the efficient hole transport at the interface between Ag and p-HTL through the tunneling process.

Fluorinated Benzoselenadiazole-Based Low-Band-Gap Polymers for High Efficiency Inverted Single and Tandem Organic Photovoltaic Cells

We designed and synthesized two low-band-gap conjugated copolymers with alternating difluorinated benzoselenadiazole (DFDTBSe) and ethylhexyloxy (EH)- or octyldodecyloxy (OD)-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) building blocks. PEHBDT-DFDTBSe and PODBDT-DFDTBSe have optical band gap energies of 1.66 and 1.69 eV, respectively, and HOMO energy levels of −5.44 and −5.43 eV, respectively. The different alkyloxy side chains in the polymers affect the molecular packing and ordering in active-layer films blended with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM). The PEHBDT-DFDTBSe:PC71BM film comprises predominantly “face-on” crystal structures with short π–π stacking distances (3.69 A) while PODBDT-DFDTBSe:PC71BM has mostly “edge-on” structures according to two-dimensional grazing-incidence X-ray diffraction analysis. Bulk heterojunction solar cells were fabricated with an inverted structure of ITO/ethoxylated polyethylenimine/polymer:PC71BM/MoO3/Ag. The device fabricated using the PEHBDT-DF...

Tandem organic photovoltaics incorporating two solution-processed small molecule donor layers

We develop a partially solution-processed small molecule tandem organic photovoltaic cell using an organic/inorganic interlayer structure that provides efficient charge recombination while protecting underlying layers from degradation due to attack from solvents applied during the deposition of subsequent sub-cells. Each sub-cell consists of a functionalized squaraine (fSQ) blend donor that is cast from solution, followed by evaporation of other functional layers. The first fSQ layer is cast from chloroform, while the second is cast from a tetrahydrofuran, thereby minimizing dissolution of the relatively insoluble, underlying fullerene layer that acts to protect the first donor layer. Solvent vapor annealing increases the sub-cell performance while decreasing the damage caused by spin-coating of the second fSQ layer, both of which result from increased film crystallinity that reduces the rate of solvent penetration. The tandem cell has a power conversion efficiency of 6.2% ± 0.3% and an open circuit voltage nearly equal to the sum of the constituent sub-cells.