Heterojunction Organic Solar Cells Research Articles

Dual Additive‐Assisted Layer‐by‐Layer Processing for 19.59% Efficiency Quasi‐Bulk Heterojunction Organic Solar Cells

AbstractThe ideal vertical phase separation active layer morphology is crucial for the photoelectric conversion of organic solar cells. In this work, a layer‐by‐layer sequential deposition method is used to prepare D18/L8‐BO‐based organic solar cells and a dual additives strategy is adopted to construct the ideal active layer. Additive DIM regulates the crystallization of the D18 layer, and additive DIO induces L8‐BO to diffuse into the interior of the D18 layer to form a vertical composition distribution with large donor/acceptor interpenetrated regions. The improvement of active layer induced by DIM and DIO dual additives promote exciton generation and dissociation, shorten charge transfer distance, and improve carrier dynamics. With improved charge transport performance and suppressed carrier recombination, the short‐circuit current density and fill factor of the D18/L8‐BO quasi‐bulk heterojunction organic solar cells are improved simultaneously, and the power conversion efficiency is boosted significantly from 18.21% to 19.59%. Moreover, the improved photovoltaic performance is further verified in D18/Y6 and PM6/L8‐BO‐based organic solar cells, which implies the generalizability of the dual additive‐assisted layer‐by‐layer ‐sequential deposition method.

Heterojunction Organic Solar Cells with Efficient Charge Mobility and Separation Capabilities Studied by DFT.

The properties of the active layer materials play a decisive role in determining the power conversion efficiency of organic solar cells (OSCs). Chlorophyll and its derivatives are abundant and environmentally friendly functional organic molecular materials. Using density functional theory (DFT) and time-dependent density functional theory (TD-DFT), we have calculated the absorption spectra and their excited state properties based on optimized ground state structures. It was found that bacteriochlorin exhibits superior structural properties, a smaller energy gap and hole reorganization energy, redshifted absorption spectra, and higher hole mobility compared to the donor D18. This suggests that bacteriochlorin exhibits superior donor properties. Comparative studies between o-AT-2Cl and m-AT-2Cl showed that o-AT-2Cl had superior acceptor properties, implying that differences in substitution positions can influence the physicochemical properties of non-fullerene acceptors (NFAs). Subsequently, six bulk heterojunctions (BHJs) were constructed by combining three donors with nonfused ring electron acceptors, o-AT-2Cl and m-AT-2Cl. The bacteriochlorin-based BHJs performed well among them, with BChl3/o-AT-2Cl and BChl4/o-AT-2Cl having the largest interfacial charge separation rate. The results suggested that BHJs composed of bacteriochlorin and NFAs can improve OSCs' photovoltaic performance, providing a feasible scheme for designing efficient OSCs.

Optimizing Solar Cell Performance: Hybrid Planar‐Si/Organic Heterojunction Solar Cells Achieve 14.75% Efficiency Through Dibenzothiophene‐Spirobifluorene‐Dithienothiophene Insertion Layer Integration

Hybrid planar‐Si/organic heterojunction solar cells have garnered substantial interest due to their potential for producing cost‐effective, high‐efficiency devices. This study investigates the photophysical properties and application of dibenzothiophene‐spirobifluorene‐dithienothiophene (DBBT‐mCbz‐DBT) in enhancing the efficiency of photovoltaic devices. Utilizing ultraviolet–visible and fluorescence spectroscopy, DBBT‐mCbz‐DBT is analyzed in solutions and doped films, showing maximum absorption at 380 nm and emission at 440 nm. Notably, the photoluminescence intensity in 4,4′‐di(9H‐carbazol‐9‐yl)‐1,1′‐biphenyl films peaks at 40–50% DBBT‐mCbz‐DBT concentrations, which are selected for solar cell fabrication. Enhanced light absorption and charge transport are observed with a DBBT‐mCbz‐DBT layer on silicon, significantly improving device performance. The planar silicon/poly(3,4‐ethylene dioxythiophene):poly(styrene sulfonate) (Si/PEDOT:PSS) heterojunction solar cells with DBBT‐mCbz‐DBT exhibit a power conversion efficiency of 14.75%, demonstrating substantial gains over baseline structures. The DBBT‐mCbz‐DBT layer optimizes energy band alignment, reduces recombination losses, and enhances electron transport, improving overall device efficiency. This research underscores the potential of integrating DBBT‐mCbz‐DBT in solar cells to achieve higher performance through simple, scalable fabrication methods.

Suppressing Exciton‐Vibration Coupling to Prolong Exciton Lifetime of Nonfullerene Acceptors Enables High‐Efficiency Organic Solar Cells

AbstractThe limited exciton lifetime (τ, generally <1 ns) leads to short exciton diffusion length (LD) of organic semiconductors, which is the bottleneck issue impeding the further improvement of power conversion efficiencies (PCEs) for organic solar cells (OSCs). However, efficient strategies to prolong intrinsic τ are rare and vague. Herein, we propose a facile method to efficiently reduce vibrational frequency of molecular skeleton and suppress exciton‐vibration coupling to decrease non‐radiative decay rate and thus prolong τ via deuterating nonfullerene acceptors. The τ remarkably increases from 0.90 ns (non‐deuterated L8‐BO) to 1.35 ns (deuterated L8‐BO‐D), which is the record for organic photovoltaic materials. Besides, the inhibited molecular vibration improves molecular planarity of L8‐BO‐D for enhanced exciton diffusion coefficient. Consequently, the LD increases from 7.9 nm (L8‐BO) to 10.7 nm (L8‐BO‐D). The prolonged LD of L8‐BO‐D enables PM6 : L8‐BO‐D‐based bulk heterojunction OSCs to acquire higher PCEs of 18.5 % with more efficient exciton dissociation and weaker charge carrier recombination than PM6 : L8‐BO‐based counterparts. Moreover, benefiting from the prolonged LD, D18/L8‐BO‐D‐based pseudo‐planar heterojunction OSCs achieve an impressive PCE of 19.3 %, which is among the highest values. This work provides an efficient strategy to increase the τ and thus LD of organic semiconductors, boosting PCEs of OSCs.

Suppressing Exciton-Vibration Coupling to Prolong Exciton Lifetime of Nonfullerene Acceptors Enables High-Efficiency Organic Solar Cells.

The limited exciton lifetime (τ, generally <1 ns) leads to short exciton diffusion length (LD ) of organic semiconductors, which is the bottleneck issue impeding the further improvement of power conversion efficiencies (PCEs) for organic solar cells (OSCs). However, efficient strategies to prolong intrinsic τ are rare and vague. Herein, we propose a facile method to efficiently reduce vibrational frequency of molecular skeleton and suppress exciton-vibration coupling to decrease non-radiative decay rate and thus prolong τ via deuterating nonfullerene acceptors. The τ remarkably increases from 0.90 ns (non-deuterated L8-BO) to 1.35 ns (deuterated L8-BO-D), which is the record for organic photovoltaic materials. Besides, the inhibited molecular vibration improves molecular planarity of L8-BO-D for enhanced exciton diffusion coefficient. Consequently, the LD increases from 7.9 nm (L8-BO) to 10.7 nm (L8-BO-D). The prolonged LD of L8-BO-D enables PM6 : L8-BO-D-based bulk heterojunction OSCs to acquire higher PCEs of 18.5 % with more efficient exciton dissociation and weaker charge carrier recombination than PM6 : L8-BO-based counterparts. Moreover, benefiting from the prolonged LD , D18/L8-BO-D-based pseudo-planar heterojunction OSCs achieve an impressive PCE of 19.3 %, which is among the highest values. This work provides an efficient strategy to increase the τ and thus LD of organic semiconductors, boosting PCEs of OSCs.

Design and analysis of benzene fused conjugated molecules for organic Photovoltaics: A TD-DFT study on electronic and quantum chemical properties

In this work, fifteen different benzene fused conjugated molecules were designed and analyzed by comparing their properties using TD-DFT (time dependent density functional theory) with a basic set of B3LYP/6-311G(d,p) for organic photovoltaics. The core molecules used were benzodioxazole, benzodioxole, benzodithiazole, benzofurazan and benzotriazole. Optimized geometry and electronic properties such as HOMO, LUMO, band gap and quantum chemical parameters were determined. Predicted band gap ranges from 1.6 to 3.8 eV. Some compounds have a good absorption property (above 500 nm) which is suitable for our purpose. The study assists in the identification of materials with advantageous electrical, optical and photovoltaic characteristics that can serve as donor molecules in heterojunction organic solar cells. The theoretical studies also aided in the clarification of the structure–property relationships that significantly affect efficiency of the device.

Analysis molecular design of novel D-A-Di-A-D conjugated compounds for high-efficiency organic solar cells.

The work described in this section uses DFT/TD-DFT/B3LYP/6-31G (d,p) (density functional theory and time-dependent density functional theory) to study and simulate the structural, optoelectronic, and optical properties of a series of conjugated compounds based on a modular D-A-Di-A-D architecture. These compounds integrate the D donor unit (carbazole), the A acceptor unit (benzothiadiazole) and various Di donor units. Using AMPS-1D (analysis of micronic structure and photonics), work has been carried out to evaluate the photovoltaic performance of these conjugated compounds in the context of organic solar cells. The compounds show variable performance in terms of energy conversion efficiency, ranging from 7.11 to 11.70%. The addition of a PEDOT layer between the active layer and the anode results in a significant improvement in photovoltaic performance, with energy conversion efficiencies of up to 15.31%, the highest value achieved. The use of ZnO as an intermediate layer remarkably improves photovoltaic performance for all compounds, with notable energy conversion efficiencies reaching 17.13%, 17.20%, and 18%. All in all, the compounds studied present promising prospects as viable candidates for organic block heterojunction (BHJ) solar cell applications. DFT/TD-DFT/B3LYP/6-31G (d,p), these acronyms stand for the computational methods used to study the properties of compounds. DFT, for "Density Functional Theory", is a quantum computation method used to describe the electronic and structural properties of molecular systems. TD-DFT, for "Time-Dependent Density Functional Theory", is an extension of DFT that allows the treatment of optical and excitation properties. B3LYP is a density functional frequently used in DFT to calculate molecular properties. In addition, 6-31G (d,p) refers to a basic wave function used to approximate the distribution of electrons in molecules. AMPS-1D, or "Analysis of Micro and Photonic Structure", is a modeling tool for studying the photovoltaic properties of multilayer structures, particularly in the context of organic solar cells.

Versatile Self‐Assembled Molecule Enables High‐Efficiency Wide‐Bandgap Perovskite Solar Cells and Organic Solar Cells

AbstractPerovskite solar cells (PSCs) and organic solar cells (OSCs) face device efficiency losses and instability challenges with existing hole transport materials (HTMs). The development of new universal HTMs is in great demand to promote their practical applications. Herein, a versatile self‐assembled molecule (SAM) based HTM is designed for record‐high efficiency wide‐bandgap (WBG, Eg &gt;1.75 eV) PSCs, all‐perovskite tandem solar cells (TSCs) and OSCs. The SAM exhibits high transmission and a lower‐lying energy level, enabling enhanced interfacial charge transfer and suppressed non‐radiative recombination losses. SAM based WBG PSCs deliver a maximum power conversion efficiency (PCE) of 18.63% with over 90% efficiency retention after 250 h continuous work. By stacking the optimal WBG PSC and a narrow‐bandgap PSC bottom cell, the 4‐terminal all‐perovskite TSC achieves a remarkable 26.24% PCE. More importantly, this SAM based HTM exhibits impressive generality in bulk heterojunction OSCs rivalling PEDOT:PSS, with an impressive PCE of 18.84% obtained for PM6:BTP‐eC9 based devices. When scaling up the PM6:BTP‐eC9 device to 0.5 cm2 in area (0.71 cm × 0.71 cm), the SAM based OSCs afford a highest PCE of 16.33%. This work provides a perspective for the design of universal SAM based charge transport materials targeting PSCs and OSCs for facile large‐area fabrication.

Charge Carrier Dynamics in Planar Heterojunction Organic Solar Cells

The ionization energy (IE) offset of a donor–acceptor pair provides the driving force for hole transfer and subsequent free charge carrier generation in low‐bandgap nonfullerene organic solar cells (OSCs). However, the interfacial energetic landscape in bulk heterojunction OSCs is determined by the materials’ electronic structure and intermolecular interactions at the donor/acceptor interface, causing local energy‐level shifts and disorder. Herein, the impact of the IE offset on the charge transfer efficiency and charge carrier dynamics is systematically evaluated by characterizing PM6/ITIC, PM6/IT‐2Cl, and PM6/IT‐4Cl planar heterojunction (PHJ) solar cells. Ultrafast spectroscopy and time‐resolved charge carrier density measurements reveal that an IE offset of about ≈0.5 eV leads to efficient hole transfer and subsequent free charge generation. Furthermore, bimolecular charge recombination and consequently triplet generation are significantly reduced in systems with high IE offset. This work underlines the importance of sizeable donor–acceptor IE offsets in PHJ nonfullerene OSCs as critical for high‐efficiency donor/acceptor material and device design.

Ternary Organic Solar Cells with Binary Additives Finely Regulated Active Layer Morphology and Improved Photovoltaic Performance

Solvent additive can be used to regulate active layer morphology and improve photovoltaic performance of organic solar cells (OSCs). Herein, the PM6:Y6:PC71BM ternary bulk heterojunction OSCs are prepared by introducing 1,8‐diiodooctane (DIO) and 1‐chloronaphthalene (CN) as binary additives. On the basis of optimizing the photovoltaic performance with CN additive, the addition of trace amounts of DIO additive further improves the molecular packing of active layer and induces Y6:PC71BM acceptor on the surface of active layer. With the optimized active layer morphology, the CN and DIO binary additives restrict carrier recombination and improve charge transport efficiently, and the prepared PM6:Y6:PC71BM ternary OSCs with binary additives demonstrate a high short‐circuit current density of 27.15 mA cm−2 and a fill factor of 76.79%, and yield an improved power conversion efficiency of 17.53%, which is higher than that of devices without additives (16.50%) and with DIO (16.79%) or CN additive (17.21%). Binary solvent additives engineering provides an effective strategy to finely regulate active morphology and boost photovoltaic performance of ternary OSCs.

Design new organic material based on triphenylamine (TPA) with D-π-A-π-D structure used as an electron donor for organic solar cells: A DFT approach

Because of the increasing scarcity of fossil fuels and the growing need for energy, it has become necessary to research new renewable energy resources. In this study, five new high-performance materials (TP-FA1F-TP - TP-FA5F-TP) of the D-π-A-π-D configuration based on triphenylamine (TPA) were theoretically investigated by applying DFT and TD-DFT methods for future application as heterojunction organic solar cells (BHJ). The influence of the modification of the acceptor (A) of the parent molecule TP-FTzF-TP on the structural, electronic, photovoltaic and optical properties of the TP-FA1F-TP - TP-FA5F-TP organic molecules was investigated in detail. TP-FA1F-TP - TP-FA5F-TP showed Egap in the interval of 1.44–2.01 eV with λabs in the range of 536–774 nm, open-circuit voltage (Voc) values varied between 0.3 and 0.56 V and power conversion efficiencies (PCE) ranging from (3-6) %. Our results also show that the donor molecules suggested in this research exhibit an improved performance compared to the recently synthesized TP-FTzF-TP, such as a lowest HOMO energy, a smaller Egap, and a greater absorption spectrum, and can lead to higher performance. Indeed, this theoretical research could lead to the future synthesis of better compounds as active substances used in BHJ.

Ambipolar Behavior of a Cu(II)–Porphyrin Derivative in Ternary Organic Solar Cells

Ternary architecture is a promising strategy to achieve high efficiency in organic solar cells. The alignment of third‐component frontier orbitals with those of donor and acceptor (D and A) is of essential importance. Herein, a new molecule VC11, consisting of a Cu(II)–porphyrin core linked to dicyanovinylene terminal acceptor units through (E)‐1,2‐di(thiophen‐2‐yl) ethene bridges, is described. The highest occupied molecular orbital and lowest unoccupied molecular orbital values of VC11 are −5.52 and −3.74 eV, respectively, lying between the corresponding values of PBDB‐T donor and Y6 acceptor, compounds used in the construction of the studied devices. The measured electron and hole mobilities reveal similar values, in the order of 10−4 cm2 Vs−1, indicating the ambipolarity of VC11, that can be used as donor or acceptor. The organic solar cells (OSCs) based on PBDB‐T:VC11, VC11:Y6, and PBDB‐T:Y6 show power conversion efficiencies (PCEs) of 10.84%, 7.89%, and 11.98%, respectively. VC11 is utilized along with PBDB‐T and Y6 to form a ternary active layer for assembling bulk heterojunction OSCs affording a remarkable PCE of 15.25%, significantly higher than those obtained for the corresponding binary devices. This enhancement of PCE originates from the additional charge separation offered by a cascaded energy‐level alignment and a fast charge transfer, due the ambipolarity of VC11.

Self‐Organized Anode Interlayer Enables PEDOT:PSS‐Free Structure for Thick‐Film Organic Solar Cells over 16% Efficiency

Thickness‐insensitivity of organic solar cells (OSCs) is a crucial issue for transformation from the laboratory‐scale devices to large‐area solar panels. Generally, the low carrier mobility of organic semiconductors hindering charge transport process leads to severe recombination losses and further poisons the fill factor (FF) in thick‐film OSCs. Herein, a PEDOT:PSS‐free pseudoplanar heterojunction OSC from sequential deposition method with self‐organized [2‐(9H‐carbazol‐9‐yl)ethyl]phosphonic acid interlayer is fabricated. The notable power conversion efficiency (PCE) enhancement from PEDOT:PSS‐based device of 17.0% to PEDOT:PSS‐free device of 17.8% is obtained in thin‐film devices (≈100 nm), which is correlated to the improved incident photons absorption by the absence of PEDOT:PSS. Besides, the PEDOT:PSS‐free device exhibits excellent thickness tolerance, with FF only degrades from 78.4% to 72.9% when the active layer thickness increases from ≈100 to ≈300 nm. The insights of remained FF are further traced to notably mitigated nongeminate recombination losses of PEDOT:PSS‐free structure. The strengthened charge extraction is also verified by reinforced built‐in potential and desirable and balanced mobilities. Consequently, the PEDOT:PSS‐free device with ≈300 nm active layer yields an inspiring PCE of 16.4%, much higher than that of PEDOT:PSS‐based device of 14.5%.

Effect of the Push–Pull Electronic Structure of Nonfullerene Acceptor Molecules on the Interfacial Charge Dynamics in Heterojunction Organic Solar Cells

Nonfullerene acceptor (NFA) molecules have attracted significant attention in high‐efficient heterojunction organic solar cells (OSCs) due to their push–pull electronic structures. Herein, using a universal quantum model for an organic donor/acceptor interface composed of a polymer donor and an NFA molecule, effect of the NFA molecular electron push and pull ability on the interfacial charge dynamics in the case of donor excitation is focused on. It is confirmed that energy and charge transfer coexist and compete during the interfacial charge dynamics, by which an interfacial hybrid state is created, and the quantitative correlations of the generation rates for different component states (e.g., donor exciton, acceptor exciton, and charge transfer state) with the NFA molecular electron push and pull ability are separately clarified. It is found that, by strengthening the NFA molecular electron push ability, competition between charge and energy transfer presents a nonmonotonous behavior; by strengthening the electron pull ability, charge transfer is always promoted and energy transfer is inhibited. This study provides a microscopic understanding for the interfacial charge dynamics modulated by tuning the NFA molecular push–pull electronic structure and thus sheds light on further improving the performance of heterojunction OSCs by rational designs of NFA molecules.

Rational Design of Dyes and Donor-Acceptor Type Molecules for Organic Solar Cells

Organic solar cells (OSCs) have received considerable attention as clean energy conversion devices because of their lightweightness, cost effectiveness, flexibility, and suitability for roll-to-roll printing. In dye-sensitized solar cells the core structure has included a dye-anchored TiO2 electrode for photoinduced charge separation, while in bulk heterojunction OSCs, the active layer of OSC devices has usually consisted of a mixture of conjugated polymers as donors and fullerene derivatives as acceptors.1,2 In particular, the power conversion efficiency (PCE) of OSCs based on fullerene derivative acceptors has reached over 10%. However, it is difficult to further increase the PCE of fullerene-based OSCs due to their inherent limitations including arduous energy-level tunability and low absorption profile. To enhance the photovoltaic performance of OSCs, nonfullerene acceptors (NFAs) with aromatic fused-ring structures have emerged rapidly in recent years to fabricate high-efficiency OPVs. Compared to traditional fullerene acceptors, NFAs possess several advantages, such as facile synthesis and high absorption profile. Most high-performance NFAs possess acceptor-donor-acceptor (A-D-A) type structures, in which fused multi-ring ladder structures are used as the D unit and 1,1-dicyanomethylene-3-indanone (IC) derivatives as the A unit.In this talk I will give an overview of our recent studies on rational design and synthesis of novel dyes and nonfullerene acceptors. In paticular, several nonfullerene acceptors possessing different D units have been designed and synthesized to address the relationship between the structure and the photophysical and photovoltaic properties of the A-D-A type nonfullerene acceptors.3-6 [1] T. Umeyama and H. Imahori, J. Mater. Chem. A (Feature Article), 2, 11545-11560 (2014).[2] T. Umeyama and H. Imahori, Acc. Chem. Res. 52, 2046-2055 (2019).[3] T. Umeyama, K. Igarashi, D. Sasada, Y. Tamai, K. Ishida, T. Koganezawa, S. Ohtani, K. Tanaka, H. Ohkita, and H. Imahori, Chem. Sci., 11, 3250-3257 (2020).[4] T. Umeyama, K. Igarashi, D. Sasada, K. Ishida,T. Koganezawa, S. Ohtani, K. Tanaka, and H. Imahori, ACS Appl. Mater. Interfaces, 12, 39236-39244 (2020).[5] T. Umeyama, K. Igarashi, Y. Tamai, T. Wada, T. Takeyama, D. Sasada, K. Ishida, T. Koganezawa, S. Ohtani, K. Tanaka, H. Ohkita, and H. Imahori, Sus. Energy Fuels, 5, 2028-2035 (2021).[6] T. Umeyama, T. Wada, K. Igarashi, K. Kato, A. Yamakata, T. Takeyama, Y. Sakamoto, Y. Tamai, H. Ohkita, K. Ishida, T. Koganezawa, S. Ohtani, K. Tanaka, and H. Imahori, ACS Adv. Energy Mater., in press.

Optical properties influence of the polarization and the temperature on heterojunction organic solar cell

We investigate the dark current-voltage properties of P3HT: PCBM based solar cell at different temperatures. The experimental data were fitted using two methods: Analytical Lambert Method, and Two Regions Method. Several electrical parameters such as: ideality factor n, series resistance Rs and shunt resistance Rsh were determined. The ideality factor for this device is bigger than unity which is discussed in terms: (i) existence of trap levels due to impurities in the band gap and (ii) the presence of tunneling conduction. It is shown that the J-V curves are driven by different effects depending on voltage and temperature ranges. By fitting the temperature dependence of the equivalent circuit’s, we extract many essential parameters such as: potential barrier (Φb), Poole Frenkel coefficient βp, mobility µ, and carrier concentration. It is shown that at low temperature the transport properties are governed by Schottky effect while at high temperature the Poole-Frenkel effect is prevailing.

High‐Efficiency Sequential‐Cast Organic Solar Cells Enabled by Dual Solvent‐Controlled Polymer Aggregation

The precise tuning of the active layer morphology to improve organic solar cells (OSCs) efficiency remains a key issue in the field of organic photovoltaics. Herein, a new solution to the above problem is provided by using the dual‐solvent modulated polymer‐assisted sequential spin‐coating method. Herein, the sequential spin‐coated OSCs based on the D18‐Cl/Y6 system are prepared for the first time and an efficiency of 16.38% is obtained, similar to that of bulk heterojunction OSCs. On this basis, the performance is further improved by using a dual solvent to balance the dissolution and crystallization of D18‐Cl, separately optimizing the morphology of the donor layer and allowing the subsequent spin‐coated Y6 solution to penetrate uniformly into the D18‐Cl framework. After the dual‐solvent treatment, the D18‐Cl (CF + CB)/Y6‐based device obtains a power conversion efficiency (PCE) of 17.33% and the D18‐Cl (THF + CB)/Y6‐based device achieves an even better PCE of 17.73%. It is worth noting that no post‐treatment is adopted here and after 2500 h of placement, the efficiency of the aforementioned devices is still 90% of the original. Thus, this work provides a simple method for tuning the film morphology to prepare efficient and stable devices, which is beneficial for future commercial production of OSCs.

π-Conjugated Polymers and Their Application in Organic and Hybrid Organic-Silicon Solar Cells.

The evolution and emergence of organic solar cells and hybrid organic-silicon heterojunction solar cells have been deemed as promising sustainable future technologies, owing to the use of π-conjugated polymers. In this regard, the scope of this review article presents a comprehensive summary of the applications of π-conjugated polymers as hole transporting layers (HTLs) or emitters in both organic solar cells and organic-silicon hybrid heterojunction solar cells. The different techniques used to synthesize these polymers are discussed in detail, including their electronic band structure and doping mechanisms. The general architecture and principle of operating heterojunction solar cells is addressed. In both discussed solar cell types, incorporation of π-conjugated polymers as HTLs have seen a dramatic increase in efficiencies attained by these devices, owing to the high transmittance in the visible to near-infrared region, reduced carrier recombination, high conductivity, and high hole mobilities possessed by the p-type polymeric materials. However, these cells suffer from long-term stability due to photo-oxidation and parasitic absorptions at the anode interface that results in total degradation of the polymeric p-type materials. Although great progress has been seen in the incorporation of conjugated polymers in the various solar cell types, there is still a long way to go for cells incorporating polymeric materials to realize commercialization and large-scale industrial production due to the shortcomings in the stability of the polymers. This review therefore discusses the progress in using polymeric materials as HTLs in organic solar cells and hybrid organic-silicon heterojunction solar cells with the intention to provide insight on the quest of producing highly efficient but less expensive solar cells.

Simulation of the performance of organic solar cells based on D1-BT-EDOT-BT-D2-A/PCBM structures

The analysis of microelectronic and photonic structures-one dimension (AMPS-1D) was used to study and simulate the performance of organic heterojunction solar cells based on D1 –B-Edot-B-D2-A as electron donors, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor. The organic photovoltaic cell devices using T3-B-Edot-B-T3-A/PCBM showed the improved open-circuit voltage Voc, short-circuit current density Jsc, fill factor FF, and power conversion efficiency PCE values for the optimum thickness of 120 nm and the effective state density of electrons and holes of 1021cm−3.The P3-B-Edot-B-T3-A/PCBM and P3-B-Edot-B-P3-A/PCBM based devices exhibited a power conversion efficiency (PCE) of 9.295% and 8.735%, respectively, which outperformed the corresponding T3-B-Edot-B-T3-A/PCBM, Cbz-B-Edot-B-T3-A/PCBM, F-B-Edot-B-T3-A/PCBM, and A-B-Edot-B-T3-A/PCBM based devices (7.330, 6.622, 7.226, and 7.327%). More importantly, the P3-B-Edot-B-T3-A/PCBM and P3-B-Edot-B-P3-A/PCBM -based device delivered the highest PCE of 14.432% and 15.031% respectively, when we deposit a layer of PEDOT between the indium tin oxide (ITO) and the active layer, which is a clear improvement over other results in the literature.

Generating Pure Spin Current in Heterojunction Organic Solar Cells.

We propose a mechanism for generating pure spin current in heterojunction organic solar cells, with the donor and acceptor both being degenerate ground-state polymers; thus, solitons can be formed. This mechanism contains the following steps: (i) the donor is photoexcited to create the electron-hole (e-h) pairs; (ii) the excited electrons are transferred to the acceptor; (iii) the net charges in the donor and acceptor are evolved into the localized charged solitons; (iv) the intermolecule bias is applied to drive the transferred electrons back to donor, and concomitantly, charged solitons are converted to neutral solitons. Here, the on-site Coulomb interaction plays an important role in ensuring the neutral solitons' spins in the donor and acceptor are oppositely polarized. Because spins are separated between the donor and acceptor without any charge separations, pure spin current can be formed. Our mechanism opens a new avenue for exploring potential organic spintronic devices.