Efficient Bulk Heterojunction Solar Cells Research Articles

Double-site defect passivation of perovskite film via fullerene additive engineering toward highly efficient and stable bulk heterojunction solar cells

Bulk heterojunction (BHJ) perovskite solar cells (PVKSCs) are advantageous in passivating defects and facilitating electron extraction/transport. Most of the reported fullerene derivatives involved in BHJ-PVKSCs render single-site interactions, limiting their passivation effect. Herein, we developed a perfluoroalkyl and pyridine-cofunctionalized fullerene derivative (C60-PyF15), and applied it as an additive of CH3NH3PbI3 layer to construct inverted (p-i-n) BHJ-PVKSCs, affording double-site defect passivation of perovskite film and a champion power conversion efficiency of 20.10%, which is among the highest values for fullerene-incorporated inverted BHJ-PVKSCs. To elucidate the crucial role of the perfluoroalkyl group in efficiency enhancement, another analogous novel non-fluoro-substituted pyridine-functionalized fullerene derivative with the same chain length of alkyl group (C60-PyH15) was also synthesized, which afforded a champion PCE of 19.22% and is lower than that of C60-PyF15. The double-site defect passivation ability of C60-PyF15 is resulted from the coordination interaction of the pyridine moiety with Pb2+ ions and hydrogen bonding between the fluorine atom within the perfluoroalkyl group and the CH3NH3+ cations. These enable ordered orientation of CH3NH3+ and suppressed ion migration, leading to efficiency enhancement and improved ambient and thermal stabilities.

(Invited) Pyridine-Functionalized Fullerene Additive Enabling Coordination Interactions with CH3NH3PbI3 Perovskite Towards Highly Efficient Bulk Heterojunction Solar Cells

Incorporating n-type fullerene derivatives into the perovskite layer affording bulk heterojunction (BHJ) perovskite solar cells (PSCs) is an effective strategy to passivate trap states, consequently reducing the current–voltage hysteresis and improving the device performance. Herein, we report the synthesis of a novel pyridine-functionalized fullerene derivative (C60-PyP) via a 1,3-dipolar cycloaddition reaction, and the unambiguous determination of its molecular structure by X-ray single crystal diffraction. Upon incorporating C60-PyP as an additive with an optimized doping ratio of 0.13 wt% into the CH3NH3Pbl3 (MAPbI3) perovskite precursor, the regular-structure bulk heterojunction PSC devices exhibit a best power conversion efficiency (PCE) of 19.82%, which is dramatically enhanced relative to that of the control devices (17.61%). The C60-PyP additive provides heterogeneous nucleation sites, leading to the decrease of the nucleation Gibbs free energy and consequently enlarged grain size and improved crystallization of the MAPbI3 perovskite film. Besides, incorporation of C60-PyP results in enhanced crystalline orientation as confirmed by grazing-incidence X-ray diffraction (GIXRD) measurements. Meanwhile, the coordination interaction between the N atom of the pyridine moiety within C60-PyP and the Pb2+ ion within MAPbl3 leads to the passivation of the trap states of the perovskite layer, jointly contributing to the improved device performance and markedly suppressed current–voltage hysteresis. Moreover, the ambient stability of the devices is improved upon C60-PyP incorporation due to the hydrophobic nature of the C60-PyP molecule, which presumably resides at the grain boundaries.

Reduced Nonradiative Voltage Loss in Terpolymer Solar Cells.

The dissociation of hybrid local exciton and charge transfer excitons (LE-CT) in efficient bulk-heterojunction nonfullerene solar cells contributes to reduced nonradiative photovoltage loss, a mechanism that still remains unclear. Herein we studied the energetic and entropic contribution in the hybrid LE-CT exciton dissociation in devices based on a conjugated terpolymer. Compared with reference devices based on ternary blends, the terpolymer devices demonstrated a significant reduction in the nonradiative photovoltage loss, regardless of the acceptor molecule, be it fullerene or nonfullerene. Fourier transform photocurrent spectroscopy revealed a significant LE-CT character in the terpolymer-based solar cells. Temperature-dependent hole mobility and photovoltage confirm that entropic and energetic effects contribute to the efficient LE-CT dissociation. The energetic disorder value measured in the fullerene- or nonfullerene-based terpolymer devices suggested that this entropic contribution came from the terpolymer, a signature of higher disorder in copolymers with multiple aromatic groups. This gives new insight into the fundamental physics of efficient LE-CT exciton dissociation with smaller nonradiative recombination loss.

Impact of Polymer Backbone Fluorination on the Charge Generation/Recombination Patterns and Vertical Phase Segregation in Bulk Heterojunction Organic Solar Cells.

Incorporating fluorine (–F) substituents along the main-chains of polymer donors and acceptors is an effective strategy toward efficient bulk-heterojunction (BHJ) solar cells. Specifically, F-substituted polymers often exhibit planar conformations, leading to favorable packing, and electronic coupling. However, the effects of fluorine substituents on the charge generation and recombination characteristics that determine the overall efficiency of BHJ active layers remain critically important issues to examine. In this report, two PBDT[2X]T polymer analogs –poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4, 5-b′]dithiophene-thiophene] [PBDT[2H]T] and its F-substituted counterpart poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluoro-thiophene] [PBDT[2F]T]—are studied to systematically examine how –F substituents impact the blend morphology, charge generation, carrier recombination and extraction in BHJ solar cells. Considering the large efficiency differences between PBDT[2H]T- and PBDT[2F]T-based BHJ devices, significant emphasis is given to characterizing the out-of-plane morphology of the blend films as vertical phase-separation characteristics are known to have dramatic effects on charge transport and carrier extraction in polymer-fullerene BHJ solar cells. Herein, we use electron energy loss spectroscopy (EELS) in tandem with charge transport characterization to examine PBDT[2X]T-fullerene blend films. Our analyses show that PBDT[2H]T and PBDT[2F]T possess very different charge generation, recombination and extraction characteristics, resulting from distinct aggregation, and phase-distribution within the BHJ blend films.

Synthesis and Photophysical Properties of Small Molecule Donor Materials Based on Indacenodithieno[3, 2-b]Thiophene Core for Organic Solar Cells

We synthesized a small molecule donor material based on IDTT for solar cells, named C6S2ThBSeIDTT, which was obtained by IDTT-2SnMe3 and a capping fragment by a simple Stiller coupling reaction. Its structure and properties were HNMR. C6S2ThBSeIDTT was confirmed that it exhibited good solubility in conventional organic solvent. It can be found that strong intermolecular charge transfer (ICT) exists in the D-A-D-A-D conformation. It has been found to have broad and strong absorption by absorption spectroscopy with a medium wide band gap of 1.70 eV, and will well agree with fullerene acceptors such as PC60BM and PC71BM for efficient bulk heterojunction solar cells.

N‐Acylisoindigo Derivatives as Polymer Acceptors for “All‐Polymer” Bulk‐Heterojunction Solar Cells

AbstractPolymer acceptors are a class of “non‐fullerene” electron‐transporting materials that can replace [6,6]‐phenyl C61/71 butyric acid (PC61/71BM) in bulk‐heterojunction (BHJ) solar cells. In this contribution is reported the use of N‐acyl‐substituted isoindigo (IID) motifs (IID(CO)) as electron‐deficient units in the design of polymer acceptors for “all‐polymer” BHJ solar cells. While IID motifs are commonly used in the design of polymer donors for efficient BHJ solar cells with the fullerene PC61/71BM acceptors, here it is shown that IID(CO) building units used in conjunction with suitable co‐monomers represent a practical avenue for polymer acceptors with electron affinity values comparable to that of PC61/71BM (i.e., 4.2 eV).

Efficient Thick-Film Polymer Solar Cells with Enhanced Fill Factors via Increased Fullerene Loading.

Developing effective methods to make efficient bulk-heterojunction polymer solar cells at roll-to-roll relevant active layer thickness is of significant importance. We investigate the effect of fullerene content in polymer:fullerene blends on the fill factor (FF) and on the performance of thick-film solar cells for four different donor polymers PTB7-Th, PDPP-TPT, BDT-FBT-2T, and poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT). At a few hundreds of nanometers thickness, increased FFs are observed in all cases and improved overall device performances are obtained except for PDCBT upon increasing fullerene content in blend films. This fullerene content effect was studied in more detail by electrical and morphological characterization. The results suggest enhanced electron mobility and suppressed bimolecular recombination upon increasing fullerene content in thick polymer:fullerene blend films, which are the result of larger fullerene aggregates and improved interconnectivity of the fullerene phases that provide continuous percolating pathways for electron transport in thick films. These findings are important because an effective and straightforward method that enables fabricating efficient thick-film polymer solar cells is desirable for large-scale manufacturing via roll-to-roll processing and for multijunction devices.

Bimolecular recombination and fill factor in crystalline polymer solar cells

Fill factor (FF) is one of the important photovoltaic parameters, which is limited mainly by charge recombination in solar cells. It is still a challenging issue to realize a high FF with a thick active layer, which is required for the development of efficient bulk-heterojunction polymer solar cells. In this study, we discussed how recombination losses limit charge collection efficiency in polymer solar cells with low FF by transient optoelectronic techniques. As a result, we found that geminate recombination is almost voltage-independent, that bimolecular recombination is suppressed significantly under open-circuit condition, and that hole and electron mobilities are low and imbalanced. We therefore concluded that the low collection efficiency is caused by the bimolecular recombination of accumulated charges due to low and imbalanced mobilities.

Controllable Bulk Heterojunction Morphology by Self-Assembly of Oppositely Charged Nanoparticles

Using kinetic Monte Carlo simulations of a simple coarse-grained model, we demonstrate that the self-assembly of oppositely charged nanoparticles is a promising approach to design efficient bulk heterojunction (BHJ) solar cells. Simulations are performed starting from a random configuration of oppositely charged nanoparticles in solution for a range of concentrations. Interconnected percolated morphologies form at high nanoparticle concentrations, when the aggregate growth ceases after certain time. If only Coulombic interactions are present, the observed morphologies have very high interfacial area but too small domain size, whereas optimum values of both the interfacial area and domain size are desired for BHJ. We therefore propose and establish that an additional hydrophobic attraction between nanoparticles of same type is desired to obtain the ideal BHJ morphology. We also discuss the effects of solvent dielectric constant and the size- and charge-asymmetry of nanoparticles, which may provide addition...

Thieno[3,4‐c]Pyrrole‐4,6‐Dione‐Based Polymer Acceptors for High Open‐Circuit Voltage All‐Polymer Solar Cells

While polymer acceptors are promising fullerene alternatives in the fabrication of efficient bulk heterojunction (BHJ) solar cells, the range of efficient material systems relevant to the “all‐polymer” BHJ concept remains narrow, and currently limits the perspectives to meet the 10% efficiency threshold in all‐polymer solar cells. This report examines two polymer acceptor analogs composed of thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) and 3,4‐difluorothiophene ([2F]T) motifs, and their BHJ solar cell performance pattern with a low‐bandgap polymer donor commonly used with fullerenes (PBDT‐TS1; taken as a model system). In this material set, the introduction of a third electron‐deficient motif, namely 2,1,3‐benzothiadiazole (BT), is shown to (i) significantly narrow the optical gap (Eopt) of the corresponding polymer (by ≈0.2 eV) and (ii) improve the electron mobility of the polymer by over two orders of magnitude in BHJ solar cells. In turn, the narrow‐gap P2TPDBT[2F]T analog (Eopt = 1.7 eV) used as fullerene alternative yields high open‐circuit voltages (VOC) of ≈1.0 V, notable short‐circuit current values (JSC) of ≈11.0 mA cm−2, and power conversion efficiencies (PCEs) nearing 5% in all‐polymer BHJ solar cells. P2TPDBT[2F]T paves the way to a new, promising class of polymer acceptor candidates.

Self-Assembled Organic Materials for Photovoltaic Application.

Organic photovoltaic cells based on bulk-heterojunction architecture have been a topic of intense research for the past two decades. Recent reports on power conversion efficiency surpassing 10% suggest these devices are a viable low-cost choice for a range of applications where conventional silicon solar cells are not suitable. Further improvements in efficiency could be achieved with the enhanced interaction between the donor and acceptor components. Effective utilization of supramolecular interactions to tailor and manipulate the communication between the components in the blend is a good strategy towards this end. Literature reports suggest that the long-term stability of organic solar cells, a major hurdle for commercial applications, can also be partially addressed by generating stable supramolecular nanostructures. In this review, we have made an attempt to summarize advances in small molecule, oligomer and polymer based systems, wherein supramolecular interactions such as hydrogen-bonding, pi-pi stacking, and dipole-dipole are explored for realizing stable and efficient bulk-heterojunction solar cells.

Thieno[3,4-c]pyrrole-4,6-dione-3,4-difluorothiophene Polymer Acceptors for Efficient All-Polymer Bulk Heterojunction Solar Cells.

Branched-alkyl-substituted poly(thieno[3,4-c]pyrrole-4,6-dione-alt-3,4-difluorothiophene) (PTPD[2F]T) can be used as a polymer acceptor in bulk heterojunction (BHJ) solar cells with a low-band-gap polymer donor (PCE10) commonly used with fullerenes. The "all-polymer" BHJ devices made with PTPD[2F]T achieve efficiencies of up to 4.4 %. While, to date, most efficient polymer acceptors are based on perylenediimide or naphthalenediimide motifs, our study of PTPD[2F]T polymers shows that linear, all-thiophene systems with adequately substituted main chains can also be conducive to efficient BHJ solar cells with polymer donors.

Thieno[3,4‐c]pyrrole‐4,6‐dione‐3,4‐difluorothiophene Polymer Acceptors for Efficient All‐Polymer Bulk Heterojunction Solar Cells

AbstractBranched‐alkyl‐substituted poly(thieno[3,4‐c]pyrrole‐4,6‐dione‐alt‐3,4‐difluorothiophene) (PTPD[2F]T) can be used as a polymer acceptor in bulk heterojunction (BHJ) solar cells with a low‐band‐gap polymer donor (PCE10) commonly used with fullerenes. The “all‐polymer” BHJ devices made with PTPD[2F]T achieve efficiencies of up to 4.4 %. While, to date, most efficient polymer acceptors are based on perylenediimide or naphthalenediimide motifs, our study of PTPD[2F]T polymers shows that linear, all‐thiophene systems with adequately substituted main chains can also be conducive to efficient BHJ solar cells with polymer donors.

Towards improved efficiency of bulk-heterojunction solar cells using various spinel ferrite magnetic nanoparticles

A detailed study of organic solar cells (OSC) doped with various ferromagnetic and superparamagnetic (Fe3O4, ZnFe2O4 NiFe2O4) nanoparticles (MNPs) is presented. Additionally to previously used magnetite nanoparticles, various magnetic moment spinel ferrites were applied. By impedance spectroscopy (IS) analysis it is shown how the doping with various MNPs influences solar cells' performance by the charge carrier effective lifetime extension. In this regard, we introduced a convenient illustrative method to define time constants from the impedance measurements. It is also shown that, photovoltaic performance of the solar cells directly depends on the magnetic moment and alignment of the superparamagnetic single-domain MNPs. Alignment of the MNPs within the OSCs' active layer results in MNPs dipole-dipole interaction, thus further-improves photovoltaic performance due to efficient charge collection at the short-circuit condition. OSC doping with ferromagnetic MNPs showed negative influence on the device performance, however in dark conditions, devices doped with CoFe2O4 showed higher forward current presumably due to leakage current through the large MNP aggregation or electron-polaron hopping.

Incorporation of liquid crystalline triphenylene derivative in bulk heterojunction solar cell with molybdenum oxide as buffer layer for improved efficiency

ABSTRACTWe report efficient bulk heterojunction solar cells fabricated by inserting a discotic triphenylene derivative into poly (3-hexylthiophene): [6, 6]- phenyl-C61-butyric acid methyl ester. A layer of molybdenum oxide was inserted between anode and active layer. Power conversion efficiency of 2.0% was achieved for these photovoltaic solar cells containing self-organised discotic liquid crystals in the active layer under one sun condition. The influence of varying the thickness of liquid crystal layer and annealing on these solar cells was also studied. Post annealing the bulk heterojunction devices with discotic liquid crystal layer of thickness 20 nm in them yielded an open circuit voltage of 0.41 V, short circuit current density of 17.0 mA cm−2, a Fill factor of 0.35 and power conversion efficiency of 2.5%.

A benzodithiophene–thienothiophene derivative with cyano acrylate side chain: A novel donor polymer with deep HOMO level for p–n heterojunction solar cells

A new promising benzodithiophene and thienothiophene-based copolymer with cyano acrylate side chain, BDT8TT, has been designed and synthesized for efficient bulk heterojunction solar cells. The copolymer shows good solubility and film-forming ability combining with excellent thermal stability. Furthermore, BDT8TT displays a broad absorption in the near-IR region (from 300 to 1000nm). Specially, this conjugated polymer exhibits quite high open circuit voltage by a highly deep-lying HOMO level at −5.76eV. Good power conversion efficiencies are demonstrated in solar cell studies; the polymer solar cell was fabricated from the blend of the polymer as a donor and PC71BM as an acceptor without annealing and any additives.

Electron density profile at the interfaces of bulk heterojunction solar cells and its implication on the S-kink characteristics

The efficiency of a bulk heterojunction (BHJ) solar cell critically depends upon quality of its interfaces. The imperfect interfaces can lead to S-kink in the current–voltage characteristics that reduce the efficiency of BHJ solar cells. In this letter, using PCDTBT:PCBM based BHJ solar cells, we demonstrate that non-destructive X-ray reflectivity is powerful technique to estimate the electron density profile across the BHJ solar cells. A direct correlation is observed between the enhanced electron density at PEDOT:PSS/PCDTBT:PCBM interface and appearance of S-kink in J–V characteristics, which is also supported by X-ray photoelectron spectroscopy and Kelvin probe measurements.

Efficient ternary bulk heterojunction solar cells with PCDTBT as hole-cascade material

Ternary bulk hetero-junction (BHJ) architecture for organic solar cells has been developed to enhance the power conversion efficiency (PCE) by expanding the light absorption range and smoothing the energy level at the BHJ interface. In this work, we report on a ternary polymer blend solar cell with two donor materials, polythieno[3,4-b]-thiophene/benzodithiophene (PTB7), poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), and using C71-butyric acid methyl ester (PC71BM) as acceptor material. The resultant device shows an optimized PCE of 7.81% (control sample, 7.06%), with an open circuit voltage (Voc) of 0.76V, a short circuit current (Jsc) of 15.4mA/cm2 and a fill factor (FF) of 66.7%. The improved device performance is mainly attributed to the enhanced charge separation, improved hole mobility, and better film morphology. And we find that the third component PCDTBT can reduce charge recombination in the ternary blend system as well. The results bring new insight into the future development of high efficiency ternary organic solar cells.

Clustering of High Molecular Weight PCDTBT in Bulk-Heterojunction Casting Solutions

The narrow optical gap conjugated polymer poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) has been used as the electron donor material in efficient solution-processed bulk-heterojunction (BHJ) photovoltaic and photodetector devices when blended with fullerene derivatives. It was found that the solution viscosity used to form the active films could be controlled by the cooling rate of a hot solution of the materials and that fast-cooled solutions afforded more efficient bulk-heterojunction solar cells than their slow-cooled counterparts. Viscometry measurements showed that the rheological behavior of the solutions is modeled by the Martin equation for different PCDTBT molecular weights and temperatures. The Martin constant KM that describes the interpolymer interactions in solution was found to increase with polymer molecular weight and decrease with increasing temperature in an analogous manner to the Flory–Huggins interaction parameter χ. Small-angle neutron scattering (SANS) was used to show that when hot solutions of the polymer were cooled, phase separation into polymer-rich clusters and solvent-rich domains occurred. Similar phase separation was observed in the case of blend solutions. In addition, the fast-cooled solutions trapped more 70-PCBM in the polymer-rich phase, which in turn made the structure of the polymer more rodlike in the clusters. The results provide an explanation as to why fast-cooled solutions lead to devices with greater efficiency.

Interfacial Characteristics of Efficient Bulk Heterojunction Solar Cells Fabricated on MoOx Anode Interlayers.

The role of the interface between an MoOx anode interlayer and a polymer:fullerene bulk heterojunction is investigated. Processing differences in the MoOx induce large variations in the vertical stratification of the bulk heterojunction films. These variations are found to be inconsistent in predicting device performance, with a much better gauge being the quantity of polymer chemisorbed to the anode interlayer.