Solution-processed Small-molecule Solar Cells Research Articles

Unveiling the Molecular Symmetry Dependence of Exciton Dissociation Processes in Small-Molecular Heterojunctions

Understanding the dependence of molecular symmetry on exciton (EX) dissociation could facilitate optimization of overall power conversion efficiencies (PCEs) in solution-processed small-molecule (SM) solar cells. In this work, a series of high-performance oligomer molecules DRCNnT (n = 4–8) containing different numbers of thiophene units is systematically investigated to clarify the dependence of molecular symmetry on EX dissociation dynamics. Femtosecond transient absorption spectroscopy is employed to track the evolution of photogenerated electron–hole pairs across the DRCNnT:PC71BM interfaces, and faster intermolecular charge transfer is observed in the odd-numbered (axisymmetric) molecules relative to the even-numbered (centrosymmetric) molecules. It is found that those molecules with axisymmetric structures exhibit larger local dipole (Δμge) and better interpenetrating morphology than the centrosymmetric even-numbered molecules. Furthermore, a charge separation efficiency up to ∼80% is observed, whic...

In-Depth Consideration of Vertically 3D Microstructured Bulk Heterojunction Layers via Solvent Vapor Annealing in DR3TSBDT:PC71BM Solar Cells

The physical nature of solvent vapor annealing (SVA) treatment is quite straightforward, and its application is ideal in small molecule-based bulk heterojunction solar cells. It has been suggested to rapidly achieve high-performance small-molecule photovoltaics by alternating the blends to ideally connect the crystallite morphology. However, most previous reports on SVA have shown only influences of the degree of donor/acceptor phase separation within part of active textures. Here, we investigated solution-processed small-molecule solar cells consisting of the previously developed DR3TSBDT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) after SVA in terms of the vertical gradient crystalline phase in three-dimensional (3D) active layers. These systematic studies of the vertically phase-separated morphology for 3D heterojunction structures clarified in more detail varied active blend morphology underlying SVA and showed a clear structure–property relationship in related device performance. This pro...

Photo-stability study of a solution-processed small molecule solar cell system: correlation between molecular conformation and degradation

Solution-processed organic small molecule solar cells (SMSCs) have achieved efficiency over 11%. However, very few studies have focused on their stability under illumination and the origin of the degradation during the so-called burn-in period. Here, we studied the burn-in period of a solution-processed SMSC using benzodithiophene terthiophene rhodamine:[6,6]-phenyl C71 butyric acid methyl ester (BTR:PC71BM) with increasing solvent vapour annealing time applied to the active layer, controlling the crystallisation of the BTR phase. We find that the burn-in behaviour is strongly correlated to the crystallinity of BTR. To look at the possible degradation mechanisms, we studied the fresh and photo-aged blend films with grazing incidence X-ray diffraction, UV–vis absorbance, Raman spectroscopy and photoluminescence (PL) spectroscopy. Although the crystallinity of BTR affects the performance drop during the burn-in period, the degradation is found not to originate from the crystallinity changes of the BTR phase, but correlates with changes in molecular conformation – rotation of the thiophene side chains, as resolved by Raman spectroscopy which could be correlated to slight photobleaching and changes in PL spectra.

Small is Powerful: Recent Progress in Solution‐Processed Small Molecule Solar Cells

Over the last 5 years, research on the synthesis, device engineering, and device physics of solution‐processed small molecule solar cells (SMSCs) has rapidly expanded. Improvements in molecular design and emergent device processing techniques have helped solution‐processed SMSCs overcome earlier difficulties in controlling active layer morphology, such that many systems are now at—or approaching—10% power conversion efficiency. In this review, details of the highest performing blend systems are presented in order to identify key trends and provide perspective on current progress in the field. Among the best systems, a planarized molecular structure is prevalent, which can be achieved using large fused‐ring moieties, intermolecular non‐bonding interactions, and side chain engineering. To obtain efficient devices, the highest performing systems have been optimized through the careful combination of thermal and solvent annealing procedures. Even without additional processing, some systems have been able to obtain interconnected morphologies and efficient charge generation and charge transport. Ultimately, the design of more efficient materials also requires additional understanding of the device physics and loss mechanisms. After highlighting what is known to date on processes limiting device efficiency, an outlook on the most important challenges remaining to the field is provided.

Effects of solvent additive on "s-shaped" curves in solution-processed small molecule solar cells.

A novel molecular chromophore, p-SIDT(FBTThCA8)2, is introduced as an electron-donor material for bulk heterojunction (BHJ) solar cells with broad absorption and near ideal energy levels for the use in combination with common acceptor materials. It is found that films cast from chlorobenzene yield devices with strongly s-shaped current–voltage curves, drastically limiting performance. We find that addition of the common solvent additive diiodooctane, in addition to facilitating crystallization, leads to improved vertical phase separation. This yields much better performing devices, with improved curve shape, demonstrating the importance of morphology control in BHJ devices and improving the understanding of the role of solvent additives.

Understanding Charge Transport in Molecular Blend Films in Terms of Structural Order and Connectivity of Conductive Pathways

The factors limiting charge carrier mobility in solution-processed small-molecule solar cells are investigated using a combination of X-ray scattering techniques and temperature dependent mobility measurements. It is found that order in the π–π stacking direction is correlated with the activation energy for transport while the connectivity of conducting domains is the primary limitation to hole transport in some systems. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

A push–pull organic semiconductor with efficient intramolecular charge transfer for solution-processed small molecule solar cells

DMF-BP-T-MMN exhibits superior intramolecular charge transfer and excellent solar cell performance with a PCE of 3.40%.

Integrated molecular, morphological and interfacial engineering towards highly efficient and stable solution-processed small molecule solar cells

High-performance small molecule OSCs with PCEs up to 6.4% are demonstrated.

Importance of domain purity and molecular packing in efficient solution-processed small-molecule solar cells.

Connections are delineated between solar-cell performance, charge-carrier mobilities, and morphology in a highperformance molecular solar cell. The observations show that maximizing the relative phase purity and structural order while simultaneously limiting the domain size may be essential for achieving optimal solar-cell performances in solution-processed small-molecule solar cells .

Photocurrent enhancement of BODIPY-based solution-processed small-molecule solar cells by dimerization via the meso position.

Three 4,4-difluoro-4-bora-3a,4a-diaza-s-indancene (BODIPY)-based small molecule donors H-T-BO, Br-T-BO, and DIMER were synthesized and fully characterized. Although modification at the meso position has a subtle influence on the light-harvesting ability, energy levels, and phase sizes, it has a striking effect on the packing behavior in solid film as two-dimension grazing incidence X-ray diffraction (2D GIXRD) and X-ray diffraction (XRD) confirm. Br-T-BO exhibits better packing ordering than H-T-BO in pristine film, which is beneficial from reinforced intermolecular interaction from halogen atoms. However, when [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is blended, no diffraction patterns corresponding to the monomeric donor can be seen from the XRD data and both H-T-BO- and Br-T-BO-based blend films give a slightly blue-shifting absorption peak with respect to their neat ones, both of which imply destruction of the crystalline structure. As for DIMER, the enhancement of the intermolecular interaction arises not only from the expansion of the backbone but the "steric pairing effect" brought on by its twisted structure. When blended with PC71BM, the diffraction patterns of DIMER are, however, kept well and the absorption peak position remains unchanged, which indicates the ordered packing of DIMER is held well in blend film. In coincidence with the fact that packing ordering improves from H-T-BO to Br-T-BO and DIMER in pristine films and the ordered packing of DIMER even in blend film, DIMER-based devices show the highest and most balanced hole/electron mobility of 1.16 × 10(-3)/0.90 × 10(-3) cm(2) V(-1) s(-1)with respect to Br-T-BO (4.71 × 10(-4)/2.09 × 10(-4) cm(2) V(-1) s(-1)) and H-T-BO (4.27 × 10(-5)/1.00 × 10(-5) cm(2) V(-1) s(-1)) based ones. The short-circuit current density of the three molecule-based cells follows the same trend from H-T-BO (6.80) to Br-T-BO (7.62) and then to DIMER (11.28 mA cm(-2)). Finally, the H-T-BO-, Br-T-BO-, and DIMER-based optimal device exhibits a power conversion efficiency of 1.56%, 1.96%, and 3.13%, respectively.

Small-molecule solar cells with efficiency over 9%

Solution-processed small-molecule solar cells with almost 100% internal quantum efficiency and a power conversion efficiency of 9% are reported. The cells make use of a donor molecule called DRCN7T and use PC71BM as an acceptor.

High-efficiency solution-processed small-molecule solar cells featuring gold nanoparticles

The effect of gold nanoparticles embedded in a hole transport layer, poly(3,4-ethylene-dioxythiophene):poly(styrenesulphonate) (PEDOT:PSS), was successfully studied based on small-molecule organic solar cells. The incorporation of nanoparticles results in a power conversion efficiency enhancement of 4.96%. This improvement was attributed to enhanced light harvesting because of the localized surface plasmon resonance (LSPR) effect.

Mobility guidelines for high fill factor solution-processed small molecule solar cells.

Analysis of measured charge-carrier mobilities and fill factors in solution-processable small-molecule bulk-heterojunction solar cells reveals that in order to achieve a high FF, the hole and electron mobilities must be >10(-4) cm 2 V(-1) s(-1) . Neat-film mobility measurements are also found to be a useful predictor of the maximum blend film mobility and FF obtained in blend film solar cells.

Temperature and light dependent diode current in high-efficiency solution-processed small-molecule solar cells

This paper reports on the detail analysis of the DC electrical and photoelectrical properties of the high-efficient (η=8.01% under standard 100mW/cm2 AM1.5 illumination) small molecule bulk heterojunction (SM BHJ) solar cells p-DTS(FBTTh2)2/PC70BM. In this SM BHJ solar cell, the dark diode current is determined by the multistep tunnel-recombination via interface states at low forward bias (V<0.65V) and the interface state assisted thermionic emission at high forward bias (V>0.65V). The effect of illumination on the diode current was also quantitatively investigated. It was observed a reduced Shockley–Read–Hall recombination via interface states at large forward bias (from the maximum power point to the open-circuit conditions). The expression of the load I–V characteristic of the illuminated high-efficient SM BHJ solar cells was derived in the presence of the light dependent series and shunt resistance.

Effects of oligothiophene π-bridge length on physical and photovoltaic properties of star-shaped molecules for bulk heterojunction solar cells

The preparation of four different star-shaped donor (D)–π–acceptor (A) small molecules (N(Ph-1T-DCN-Me)3, N(Ph-2T-DCN-Me)3, N(Ph-2T-DCN-Hex)3 and N(Ph-3T-DCN-Hex)3) possessing various oligothiophene π-bridge lengths and their use in solution-processed bulk heterojunction small molecule solar cells is reported. Optical and electrochemical data show that increasing oligothiophene π-bridge length leads to a decrease of the optical band gap due to a parallel increase of the highest occupied molecular orbital (HOMO) level. Furthermore, subtle modifications of a molecular π-bridge length strongly affect the thermal behavior, solubility, crystallization, film morphology and charge carrier mobility, which in turn significantly change the device performance. Although the moderately increasing oligothiophene π-bridge length uplifts the HOMO level, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the short circuit current (Jsc) and fill factor (FF). The study demonstrates that such an approach can represent an interesting tool for the effective modulation of the photovoltaic properties of the organic solar cells (OSCs) at a moderate cost.

Efficient solution-processed small-molecule solar cells with titanium suboxide as an electric adhesive layer

We demonstrate efficient and air-stable small-molecule bulk heterojunction (SM BHJ) solar cells by introducing titanium suboxide (TiOx) between the active layer and the cathode as an electric adhesive layer. The SM BHJ solar cell with the TiOx interlayer exhibits improved power conversion efficiency of 6.4% as well as greatly enhanced device lifetime. Analysis of the dark diode characteristics and comparison of the current density-voltage characteristics upon heat annealing indicate that the TiOx interlayer improves the electrical contact between the active layer and the cathode, thereby resulting in efficient sweep-out of photogenerated charge carriers.

Linkage effects of linear D–π–A–π–D type diketopyrrolopyrrole-triphenylamine based solution-processable organic small molecule photovoltaic materials

Four novel D–π–A–π–D-type small molecules (SMs) were synthesized and their π-linkage effects were investigated. Among them, SM2 with vinylene as the π-linkage exhibited better molecular coplanarity, reaching a relatively higher power conversion efficiency (PCE) of 3.76%. SM3 and SM4 with acetylene and acrylonitrile as π-linkages exhibited relatively higher open-circuit voltages (VOC) of 0.93 V and 0.90 V, respectively, owing to their deep-lying HOMO levels. These results gave an important guide for developing new materials in solution-processed small molecule solar cells.

Enhanced efficiency of solution-processed small-molecule solar cells upon incorporation of gold nanospheres and nanorods into organic layers

The significantly enhanced performance upon incorporation of Au nanoparticles in solution-processed small-molecule solar cells is demonstrated. Simultaneously incorporating Au nanospheres into the hole transport layer and Au-silica nanorods into the active layer results in superior broadband absorption improvement in the device with a power conversion efficiency of 8.72% with 31% enhancement.

Efficient solution-processed small-molecule solar cells by insertion of graphene quantum dots.

In this work, we have demonstrated the results of several positive effects that arise from the addition of graphene quantum dots (GQDs) to solution-processed small molecule bulk-heterojunction (SM-BHJ) solar cells fabricated from a p-DTS(FBTTh(2))(2)/[6,6]-phenyl C(71) butyric acid methyl-ester (PC(71)BM). The device with an optimized ratio of GQDs exhibits increased current density and fill factor owing to 10% improved external quantum efficiency (EQE) and induction of a favorable SM-BHJ morphology. Additionally, the multiple scattering of the GQDs in the SM-BHJ leads to longer optical pathlengths according to the analysis of diffuse reflectance spectra and UV/Vis absorption spectra. The GQD inserted SM-BHJ film at the optimized concentration exhibits decreased charge transport resistance significantly by impedance measurements with effective charge extraction in the device which contributes to 15% enhancement of power conversion efficiency (PCE).

Solution-processed small-molecule solar cells: breaking the 10% power conversion efficiency

A two-dimensional conjugated small molecule (SMPV1) was designed and synthesized for high performance solution-processed organic solar cells. This study explores the photovoltaic properties of this molecule as a donor, with a fullerene derivative as an acceptor, using solution processing in single junction and double junction tandem solar cells. The single junction solar cells based on SMPV1 exhibited a certified power conversion efficiency of 8.02% under AM 1.5 G irradiation (100 mW cm−2). A homo-tandem solar cell based on SMPV1 was constructed with a novel interlayer (or tunnel junction) consisting of bilayer conjugated polyelectrolyte, demonstrating an unprecedented PCE of 10.1%. These results strongly suggest solution-processed small molecular materials are excellent candidates for organic solar cells.