BHJ OSCs Research Articles

Density Functional Theory Simulation of Optical and Photovoltaic Properties of DRTB-T Donor-Based Organic Solar Cells

Using the density functional theory (DFT), the influence of substitution of electron-donating (OCH3 and OH) and electron-accepting (F and Cl) groups on the peripheral thiophene units of DRTB-T donor molecule is studied. By optimizing the geometric structure, HOMO and LUMO energies, reorganization energies, optical properties, and photovoltaic properties are simulated. It is found that the ionization potential of the electron-donating derivatives (DRTB-4OCH3 and DRTB-4OH) reduces, but it increases for the electron-accepting derivatives (DRTB-4F and DRTB-4Cl) in comparison with that of DRTB-T. It is also found that the absorption spectra of the electron-donating derivatives (DRTB-4OCH3 and DRTB-4OH) get redshifted, but these get blue shifted for the electron-accepting derivatives (DRTB-4F and DRTB-4Cl) in comparison with those of DRTB-T. These changes in the electronic and optical properties of the modified structures result in higher PCE in BHJ OSCs with the blended active layer of DRTB-4F: NITI, DRTB-4Cl: NITI, in comparison with that of OSC with the active layer of DRTB-T: NITI and the highest being 15.0% in DRTB-4Cl: NITI. Our results may be expected to provide valuable insights into design optimization, leading to the fabrication of more efficient OSCs.

Structurally Complementary Star‐Shaped Unfused Ring Electron Acceptors with Simultaneously Enhanced Device Parameters for Ternary Organic Solar Cells

Ternary strategy has attracted extensive attention for bulk heterojunction organic solar cells (BHJ OSCs) owing to their potentially improved light harvesting, cascaded energy levels, and optimized film morphology of binary BHJs. Herein, three novel star‐shaped unfused ring electron acceptors (SSUFREAs), i.e., H1–3, with and without fluorine‐substituent in phenyl core or peripheral group are designed and synthesized as third components via direct C–H arylation to incorporate into PM6:Y6 BHJ films. The structure–property–performance dependence study reveals that the isotropic charge transfer, complementary star‐shape structure and light absorption, and energy‐level cascades of H1–3 with PM6 and Y6 allow ternary BHJs to have higher power conversion efficiency (PCE) compared to the PM6:Y6 binary BHJ. Among them, the ternary BHJ involving fluorine‐free H1, i.e., H1:PM6:Y6, possesses the highest PCE (16.57%) owing to the high‐lying frontier molecular orbital and the enlarged torsion angle, which enhances open‐circuit voltage, inhibits the excessive crystallization of Y6, and facilitates exciton dissociation as well as collection. The findings indicate that SSUFREAs have great potential to serve as third components to optimize morphology and improve the open‐circuit voltage of BHJ OSCs.

Layer-by-layer processing enabled alloy-like ternary organic solar cells to achieve 17.9% efficiency

Combining the ternary donor alloy strategy with the layer-by-layer spin-coating process, we have prepared OSCs with superior film morphology and thermal and light stability, with a PCE of 17.9%, higher than those of binary BHJ OSCs (15.72%).

Optimizing Device Structure of PTB7-Th:PNDI-T10 Bulk Heterojunction Polymer Solar Cells by Enhancing Optical Absorption

Using the optical transfer matrix method, we optimized the layered structure of a conventional and an inverted BHJ OSC with the active layer made of blended PTB7-Th:PNDI-T10 by maximizing the optical absorption and, hence, the JSC. The maximum JSC thus obtained from the optimised structure of the inverted OSC was 139 Am−2 and that of the conventional OSC was 135 Am−2. Simulation of the electric field distribution in both inverted and conventional OSCs showed that the formation of a single CIP was obtained in the active layer of thickness 105 nm in both OSCs. As the light incidents from the ITO side, it was found that excitons were generated more closely to ITO electrode, which favors the efficient charge transport and collection at the opposite electrodes in the inverted OSC, which produces higher JSC.

Improving photovoltaic performance of P3HT: IC60BA based organic solar cell: N-type doping effect

In this paper we report a numerical simulation study for P3HT: IC60BA based organic solar cell with Analysis of Microelectronic and Photonic Structures the simulation one dimension software (AMPS-1D). Indeed, the N-type doping concentration (ND) effect on the organic solar cell performance is done. Moreover, a combination between the P-type doping concentration (NA) and N-type one (ND) is investigated. However, due to the relationship between doping and carriers charge mobility, the effect of ND for different electron mobilities (μn) is also studied. We showed a high efficiency of 5.88% that is achieved for particular values of NA = 1017 cm−3, ND = 2 × 1016 cm−3, μp = 3 × 10−4 cm2 V−1 s−1 and μn = 7 × 10−4 cm2 V−1 s−1. Thus, we noticed that the P-type doping remains more promising than N-type one for the device performance improvement. Furthermore, the validation of the obtained results by those experimentally reported in literature is realized. In addition, the doping of other BHJ OSC devices consisting of P3HT: IC70BA is studied; an optimum efficiency of about 6.32% is reached.

Solution-processable silicon naphthalocyanine tetraimides as near infrared electron acceptors in organic solar cells

Two silicon naphthalocyanine electron acceptors, CH3O-SiNcTI and 3BSO-SiNcTI, with four strong electron-withdrawing imide groups, were designed, synthesized, and applied in bulk heterojunction organic solar cells (BHJ OSCs). They exhibited strong near-infrared absorption in 300–1100 nm, maximum extinction coefficients of up to 5.2 × 105 M−1 cm−1, deep lowest unoccupied molecular orbital energy levels of −3.92 eV, and good photothermal stability. The power conversion efficiency of BHJ OSCs based on acceptor of 3BSO-SiNcTI reached 4.22%, which is the highest value among solution-processable phthalocyanine or naphthalocyanine derivatives. The measurements of space-charge-limited current, charge recombination, charge collection capability, atomic force microscopy (AFM), and transmission electron microscopy (TEM) were carried out. It is found that BHJ OSCs based on PTB7-Th:3BSO-SiNcTI, have higher and balanced carrier mobility, less charge recombination, better charge transport, and favorable film morphology, which lead to their higher photovoltaic performance. This study indicates the great potential of SiNcTIs as near-infrared chromophores for high-performance electron acceptors in BHJ OSCs.

Operating Temperature of Nonfullerene Acceptor‐Based Bulk Heterojunction Organic Solar Cells

A comprehensive study of the operating temperature () of three nonfullerene (NF) acceptor‐based bulk heterojunction organic solar cells (BHJ OSCs), two conventional (OSC1 and OSC3) and one inverted (OSC2) structure, is presented. A quantitative analysis of the thermal power generated by photon absorption in transport layers and electrodes, thermalization of photoexcited charge carriers, tail‐state recombination, and resistive heating is conducted. For all three OSCs, the dependence of operating temperature on the voltage is simulated and it is found that OSC1 and OSC2 operate at about 320 K and OSC3 at 319 K. It is also found that the thermal power generated due to thermalization (PT) and absorption in other than the active layer () in OSC3 are smaller than those in both OSC1 and OSC2 but the thermal power generated due to the resistive heating (PR) is larger in OSC3 than in OSC1 and OSC2, leading to the net power absorbed in the active layer of OSC3 being higher than that in OSC1 and OSC2. Thus, although the operating temperature of all three cells remains in the range from 320 to 321 K, OSC3 shows a better photovoltaic performance.

A benzo[ghi]-perylene triimide based double-cable conjugated polymer for single-component organic solar cells

Single-component organic solar cells (SCOSCs) with high stability and simplified fabrication process are supposed to accelerate the commercialization of organic photovoltaics. However, the types of photo-active materials and photovoltaic performance of SCOSCs are still far lagging behind the bulk-heterojunction type organic solar cells (BHJ OSCs). It is still an arduous task to introduce new photo-active materials into SCOSCs, aiming to improve the efficiencies of SCOSCs. One feasible way is to construct double-cable polymers with new structures and tune conformation, morphology and mobility for the improvement in power conversion efficiencies (PCEs). Hence, in this work, we constructed a new double-cable polymer PBTT-BPTI by introducing fused core 5,7-dibromo-2,3-bis(2-ethylhexyl)benzo[1,2-b:4,5-c’]dithiophene-4,8-dione (TTDO) into the main backbone and benzo[ghi]-perylene triimide (BPTI) unit into the side chain. Both of the two units show strong electron-withdrawing property, which are expected to broaden absorption spectra and enhance intermolecular interaction. The double-cable polymer exhibited a broad absorption in the range of 300-700 nm with an optical band gap (Eg) of 1.79 eV. The PCE of PBTT-BPTI-based SCOSCs was 2.15%, which may be limited by the unconstructed efficient electron transporting channels.

Characterising Exciton Generation in Bulk-Heterojunction Organic Solar Cells.

In this paper, characterisation of exciton generation is carried out in three bulk-heterojunction organic solar cells (BHJ OSCs)—OSC1: an inverted non-fullerene (NF) BHJ OSC; OSC2: a conventional NF BHJ OSC; and OSC3: a conventional fullerene BHJ OSC. It is found that the overlap of the regions of strong constructive interference of incident and reflected electric fields of electromagnetic waves and those of high photon absorption within the active layer depends on the active layer thickness. An optimal thickness of the active layer can thus be obtained at which this overlap is maximum. We have simulated the rates of total exciton generation and position dependent exciton generation within the active layer as a function of the thicknesses of all the layers in all three OSCs and optimised their structures. Based on our simulated results, the inverted NF BHJ OSC1 is found to have better short circuit current density which may lead to better photovoltaic performance than the other two. It is expected that the results of this paper may provide guidance in fabricating highly efficient and cost effective BHJ OSCs.

Modified fullerenes as acceptors in bulk heterojunction organic solar cells - a theoretical study.

In the present study, electronic structure calculations were used to provide strategies for designing poly(3-hexylthiophene) (P3HT)-fullerene-derivative-based donor-acceptor materials for use in high-efficiency bulk heterojunction organic solar cells (BHJ OSCs). The work systematically analyses the impact of electron-donating and -withdrawing substituents on the opto-electronic properties of the fullerene structures. Parameters relating to the absorption spectra, orbital distributions, and energy ordering of the frontier molecular orbitals (FMO), the interactions between P3HT and the fullerene derivatives, and charge transfer across the interface were investigated. We found that substitution with the electron-withdrawing group NO2 enhances the electronic coupling between the fullerene and P3HT; however, it reduces the open-circuit voltage (VOC) of the OSC through lowering the LUMO energy level. Furthermore, the results show that substitution with an electron-withdrawing group (NO2) and electron-donating group (OCH3) can improve the power conversion efficiency (PCE) of the OSC, since this slightly improves the photon absorption abilities and charge transfer coupling at the interface without overly compromising VOC relative to PC61BM. Our study shows that alkyl chain modification in the PC61BM acceptor is a promising strategy for improving the performances of OSCs.

Plexiglass glove box for organic solar cells

The term Glove Box probably creates an image of a glass box with black gloves, but Glove Boxes are as varied as the applications in which they are used. It is a hermetically sealed, watertight box, built for the purpose of sensitive applications. In this paper we present the design, the implementation and the testing of a Glove Box using plexiglass of 6mm thickness. The entire control of the system is flashed on an ATmega328 microcontroller. The sensors inside the box include oxygen sensor, gas sensor, humidity and temperature sensor. The plexiglass used provides robustness for the system but being lightweight it can be easily moved in different places in a research laboratory, compared with the steel Glove Boxes which are difficult to handle. The Glove Box was tested, by filling it up with Argon (inert gas) and introducing bulk heterojunction organic solar cells (BHJ OSC) and leaving them inside for 12hours. The measurement of the OSCs after 12h revealed that their efficiency decreased with only 20% compared to those left outside of the Glove Box which had an efficiency decrease of more than 70% from their initial value.

1,2,4-Triazoline-3,5-dione substituted perylene diimides as near infrared acceptors for bulk heterojunction organic solar cells

A series of perylene diimide derivatives (PDIs) with near infrared (NIR) absorption and different geometries were synthesized by introducing 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) and 4-butyl-1,2,4-triazoline-3,5-dione (BuTAD) to the bay position of PDIs through the Diels–Alder reaction. The compounds with various twisted three-dimensional (3D) structures were obtained through combining different connection units, and their aggregation and domain size of the donor-acceptor blends were adjusted. All of these compounds exhibit broad absorption in the range of 300–860 nm, high thermal stability (with a decomposition temperature over 350 °C), applicable lowest unoccupied molecular orbital energy levels (−3.70 ~ −3.74 eV), and appropriate solubility. Blending with a PM6 polymer donor, bulk heterojunction organic solar cells (BHJ OSCs) based on BDT-SBuTADPDI2 exhibited the highest power conversion efficiency of 2.74%. Their carrier mobility and morphology were investigated by Space-charge-limited current (SCLC) measurements and Atomic Force Microscopy (AFM) measurements respectively. This study provides a promising strategy to construct potential PDI-based acceptors with NIR absorption.

Role of DIO vis-à-vis microstructural kinetics during thermal annealing on the performance of PTB7:PC71BM organic solar cells

Thermal annealing is an important process to improve the power conversion efficiency (PCE) of bulk-hetero junction organic solar cells (BHJ OSCs) by inducing micro-structural changes within the active layer blend’s constituents leading to amelioration of charge transfer/transport dynamics and interfacial properties. This work investigates the impact of thermal annealing on the photovoltaic performance ofPoly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-Phenyl C71-butyric acidmethyl ester(PC71BM) (PTB7:PC71BM) thin film blend in the presence of DIO (1,8 –Diiodooctane) additive, at different annealing temperatures (40, 60 and 80 °C) and correlates with the performance of DIO-free devices investigated under similar annealing conditions. Our results show that DIO-containing (w/DIO)blends, annealed at moderate temperature (40 °C) for 1 h exhibit significantly improved short circuit current (Jsc)as well as fill factor (FF) and hence 25% improvement in PCE over bench dried (unannealed) DIO-containing blend films. Interestingly, thermal treatment of DIO containing blend films at a higher temperature of 80 °C results in significant reduction of FF. However, for without DIO (w/o DIO) devices, FF remains unchanged while decrease in Jsc is significant. The inferior performance of both (w/DIO and w/o DIO) the devices at higher temperatures ~80 °C emanates from an unfavorable distribution of the polymer- fullerene network, as shown by morphological and surface potential analyses. Specifically, in w/DIO devices, the decreased performance is possibly induced by the temperature driven, accelerated loss of DIO additive that produces the evolution of undesirable morphology thus deteriorating the performance.

An Alternative Approach to Simulate the Power Conversion Efficiency of Bulk Heterojunction Organic Solar Cells

An alternative approach to simulate the power conversion efficiency (ηPCE) of bulk heterojunction organic solar cells (BHJ OSCs), as a product of efficiencies of absorption (ηabs), dissociation (ηdis), and extraction (ηext), is presented. Although ηabs and ηdis do not directly contribute to the simulation of ηPCE, the approach enables us in understanding their roles in optimizing the power conversion efficiency of BHJ OSCs. This method is applied to simulate ηPCE as a function of the thickness of active layer for three different BHJ OSC structures, one with a fullerene acceptor and two with two different nonfullerene acceptors. The results are found to be in good agreement with those from the previous simulation and experimental works and are expected to be useful in optimizing the thickness of the active layer.

Stability of Non-Flexible vs. Flexible Inverted Bulk-Heterojunction Organic Solar Cells with ZnO as Electron Transport Layer Prepared by a Sol-Gel Spin Coating Method

In this research, inverted bulk heterojunction organic solar cells (BHJ OSC) with poly(3-hexylthiophene-2,5-diyl): (6,6)-phenyl C61 butyric acid methyl (P3HT:PCBM) as the active layer were fabricated by a sol-gel spin coating method using flexible PET and non-flexible glass as substrates. The power conversion efficiency (PCE) and the stability of the cells were investigated. According to the results, the non-flexible device showed higher short circuit current (Jsc) as well as open-circuit voltage (Voc) as compared to the flexible one so that 2.52% and 0.67% PCE for non-flexible and flexible cells were obtained, respectively. From the stability point of view, the non-flexible device maintained 51% of its initial efficiency after six weeks in a dark atmosphere, while it was about 19% for the flexible cell after four weeks. The most important reason for the higher PCE with the higher stability in the non-flexible cell can be attributed to its higher shunt resistance (Rsh) and better interlayer connections at the electron collector side.

Performance enhancement of bulk heterojunction organic solar cells using photon upconverter

In this article, we show that significant performance enhancement can be obtained by incorporating a photo upconverter layer in bulk heterojunction organic solar cells (BHJ OSCs). We systematically optimize the photon upconversion process for P3HT:PCBM, PSBTBT:PCBM, PBDTTT-C:PCBM and PTB7-Th:PCBM based BHJ OSCs using a simple numerical model followed by optoelectronic simulations. After verifying the integrated model of upconversion with experimental reports, we analyze each type of BHJ OSC incorporating photon upconversion layer having optimized spectral characteristics and estimate efficiency enhancement of ~30.8% (4.55% → 5.95%), ~24.3% (5.02% → 6.24%), ~28.9% (6.35% → 8.19%) and ~16.9% (10.55% → 12.34%) for P3HT:PCBM, PSBTBT:PCBM, PBDTTT-C:PCBM and PTB7-Th:PCBM based OSCs, respectively. We then show that effect of photon upconversion and localized surface plasmon resonance on the best performing OSC material considered in this work i.e. PTB7-Th:PCBM, is non-linear and predict a performance boost of mere ~2.3% (12.32% → 12.62%) from additional effort (due to plasmon resonance). Finally, we discuss possibility of extending the efficiency of PTB7-Th:PCBM based BHJ OSC beyond 16% using concentrated sunlight and a practically realizable upconverter layer.

Effect of cathode interface thickness on the photovoltaic parameters of bulk heterojunction organic solar cells

We investigate the role of cathode interfacial layer thickness in tuning the barrier height and charge transport in inverted bulk heterojunction organic solar cells (BHJ OSCs). The variation of cathode layer thickness significantly changed the cell parameters like open-circuit voltage (Voc) and short-circuit current density (Jsc). For better understanding of the charge transport properties in the device, photoinduced capacitance spectroscopic studies and temperature dependent studies were carried out in polythieno[3,4-b]thio phene/benzodithiophene : [6,6]-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM) based inverted BHJ OSC with ZnO as the cathode interfacial layer. These studies gave insight into the charge accumulation and barrier height at the active layer/ZnO interface. Due to the presence of trap states in the ZnO layer, charges can accumulate at the interface creating a barrier for the smooth extraction of charge carriers. The barrier height is found to increase from 0.05 to 0.13 eV as the ZnO thickness is varied from 30 to 250 nm that can be attributed to the increase in the density of trap states in the ZnO layer. Intensity-dependent studies were also performed to analyze the recombination processes in the device.

Design and chemical engineering of carbazole-based donor small molecules for organic solar cell applications

A serious of carbazole-based acceptor-donor-acceptor (A-D-A) type small molecules have been designed, synthesized and characterized for efficient bulk heterojunction organic solar cell (BHJ OSCs) applications. The synthesized molecules have an electron-rich carbazole unit acting as the electron donor (D) and an electron deficient phenylcyanovinylene unit acting as an electron acceptor to assembled A-D-A type conjugated small molecules (FHCS, NHCS, FBCS, and NBCS). The effect of chemical engineering in designed small molecules through fluorination, nitration and introducing alkyl chain has been studied in view of their optical, physio-chemical and photovoltaic properties. The NHCS and NBCS show broader absorption as compared to the FHCS and FBHS due to the strong electron withdrawing ability of the nitro groups. Interestingly, it was found that the alkyl chain did not affect the thermal and photophysical properties of these small molecules, but it has good impact on the energy level and photovoltaic properties. The FHCS and NHCS molecules showed a deep-lying HOMO energy level of − 5.21 eV, as compared to the FBCS and NBCS. NHCS and NBCS molecules showed low band gaps of 1.81 and 1.77 eV, respectively. The organic photovoltaic device fabricated with the configuration of ITO/PEDOT:PSS/donor:PC61BM/LiF/Al, exhibited the best device performance for the NBCS molecules with: the short circuit current (JSC) of 4.35 mA cm−2, open circuit voltage (VOC) of 0.75 V, fill factor (FF) of 30.07, and power conversion efficiency of ~ 1%.

Estructural Design of Funcionalized Porphyrins for Very Efficient (> 9%) BHJ Solar Cells

Natural photosynthetic structures utilize chlorophylls to absorb light and carry out photochemical charge separation to store light energy. Porphyrins and related derivatives, as analogues of chlorophylls, have been investigated as active materials for dye-sensitized solar cells and a record PCE of 13% has been achieved. Although porphyrins have been less studied for solution-processed BHJ OSC, impressive progress has been made in this field along the last years achieving remarkable efficiencies as high as 9.06% [1]. In this presentation, I will present our recent work tailoring porphyrin-based A-D-A [2][3] and D-A-D [4] small molecules used as donor materials in BHJ solar cells, reaching excellent efficiencies, higher than 9% with energy loss lower than 0.6 eV [5]. The influence of modification of the substitution in meso positions, role of the conjugated bridge and nature of the acceptor groups on the frontier orbital levels and band-gap as well as the photophysical and electrochemical properties will be discussed. Gao, J. Miao, L. Xiao, W. Deng, Y. Kan, T. Liang, C. Wang, F. Huang, J. Peng, Y. Cao, F. Liu, T. P. Russell, H. Wu, X. Peng. Adv. Mater. 28, 4727 (2016). Morán, S. Arrechea, P. de la Cruz, V. Cuesta, S. Biswas, E. Palomares, G.D. Sharma, F. Langa. J. Mater. Chem. A, 4, 11009 (2016).Arrechea, A. Aljarilla, P. de la Cruz, E. Palomares, G. D. Sharma, F. Langa. Nanoscale 8, 17953 (2016).Arrechea, A. Aljarilla, P.de la Cruz, M. K. Singh, G. D. Sharma, F. Langa. J. Mater. Chem. C. 5, 14259 (2017).Vartanian, S. Arrechea, R. Singhal, P. de la Cruz, F. Langa, G. D. Sharma. Submitted.

Non-Fullerene Acceptor-Based Solar Cells: From Structural Design to Interface Charge Separation and Charge Transport.

The development of non-fullerene small molecule as electron acceptors is critical for overcoming the shortcomings of fullerene and its derivatives (such as limited absorption of light, poor morphological stability and high cost). We investigated the electronic and optical properties of the two selected promising non-fullerene acceptors (NFAs), IDIC and IDTBR, and five conjugated donor polymers using quantum-chemical method (QM). Based on the optimized structures of the studied NFAs and the polymers, the ten donor/acceptor (D/A) interfaces were constructed and investigated using QM and Marcus semi-classical model. Firstly, for the two NFAs, IDTBR displays better electron transport capability, better optical absorption ability, and much greater electron mobility than IDIC. Secondly, the configurations of D/A yield the more bathochromic-shifted and broader sunlight absorption spectra than the single moiety. Surprisingly, although IDTBR has better optical properties than IDIC, the IDIC-based interfaces possess better electron injection abilities, optical absorption properties, smaller exciton binding energies and more effective electronic separation than the IDTBR-based interfaces. Finally, all the polymer/IDIC interfaces exhibit large charge separation rate (KCS) (up to 1012–1014 s−1) and low charge recombination rate (KCR) (<106 s−1), which are more likely to result in high power conversion efficiencies (PCEs). From above analysis, it was found that the polymer/IDIC interfaces should display better performance in the utility of bulk-heterojunction solar cells (BHJ OSC) than polymer/IDTBR interfaces.