Spiro Linkage as an Alternative Strategy for Promising Nonfullerene Acceptors in Organic Solar Cells

This work focuses on developing diketopyrrolopyrrole (DPP)‐based small molecular nonfullerene acceptors for bulk heterojunction (BHJ) organic solar cells. The materials, SF‐DPP s, have an X‐shaped geometry arising from four DPP units attached to a spirobifluorene (SF) center. The spiro‐dimer of DPP‐fluorene‐DPP is highly twisted, which suppresses strong intermolecular aggregation. Branched 2‐ethylhexyl (EH), linear n‐octyl (C8), and n‐dodecyl (C12) alkyl sides are chosen as substituents to functionalize the N,N‐positions of the DPP moiety to tune molecular interactions. SF‐DPPEH , the best candidate in SF‐DPP s family, when blended with poly(3‐hexylthiophene) (P3HT) showed a moderate crystallinity and gives a J_sc of 6.96 mA cm^?2, V_oc of 1.10 V, a fill factor of 47.5%, and a power conversion efficiency of 3.63%. However, SF‐DPPC8 and SF‐DPPC12 exhibit lower crystallinity in their BHJ blends, which is responsible for their reduced J_sc. Coupling DPP units with SF using an acetylene bridge yields SF‐A‐DPP molecules. Such a small modification leads to drastically different morphological features and far inferior device performance. These observations demonstrate a solid structure–property relationship by topology control and material design. This work offers a new molecular design approach to develop efficient small molecule nonfullerene acceptors.     

Miscibility in binary blends of non-peripheral alkylphthalocyanines and their application for bulk-heterojunction solar cells

The fabrication of small-molecule bulk-heterojunction solar cells utilizing a mixed donor material composed of two types of soluble phthalocyanine derivatives with different substituent length has been studied. The power conversion efficiency (PCE) and short-circuit current density (J_sc) of the solar cells fabricated using the mixed donor material with an optimized mixture ratio reached 3.8% and 9.2 mA/cm², respectively, which were superior to those of organic solar cells utilizing each type of phthalocyanine derivative as a single donor material. The improvement of PCE and J_sc has been discussed from the viewpoints of the miscibility and carrier transport properties of the mixed donor material.     

Hydrogen dilution on an undoped silicon oxide layer and Its application to amorphous silicon thin-film solar cells

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiO_x:H) deposited on an n-μc-Si:H layer of amorphous silicon single-junction superstrate-configuration thin-film solar cells produced using 40 MHz very high frequency plasma-enhanced chemical vapor deposition. We found that undoped μc-SiO_x:H thin film under optimized hydrogen dilution conditions had high crystallinity, high conductivity, a wide optical band gap, and a high refractive index, which are advantageous properties in solar cells. However, deposition at higher hydrogen dilutions degraded the quality and optoelectronic properties of the films, because the morphology of the films changed from microcrystalline to amorphous. These results suggest that the use of an optimized undoped μc-SiO_x:H layer improves a-Si:H thin-film solar cell performance through enhancement of the short-circuit current density J_sc. The increased J_sc can be attributed to an improved light-trapping capability in the long wavelength range, between 620 and 680 nm, as demonstrated by the external quantum efficiency. This technique also allows optimal conversion efficiency to be achieved. The results demonstrated that hydrogen dilution plays a dominant role in the improvement of film quality and solar cell performance; however, the tradeoff between refractive index and conductivity must be considered.     

Solution-processable star-shaped photovoltaic organic molecule with triphenylamine core and thieno[3,2-b]thiophene–dicyanovinyl arms

A new solution-processable star-shaped D-π-A molecule with triphenylamine (TPA) as core and donor unit, dicyanovinyl (DCN) as end group and acceptor unit, and 3,6-dihexyl-thieno[3,2-b]thiophene (DHT) as π bridge, S(TPA-DHT-DCN) was synthesized for the application as donor material in solution-processed bulk-heterojunction organic solar cells (OSCs). The compound exhibits broad absorption in the visible region with suitable energy levels, which are desirable for application as a donor material in organic solar cells. The OSC devices based on S(TPA-DHT-DCN) as the donor and PC₇₁BM as the acceptor (1:2, w/w) exhibited power conversion efficiency (PCE) of 2.87%, with high open circuit voltage (V_oc) of 0.96 V, short circuit current density (J_sc) of 6.80 mA/cm², and fill factor (FF) of 43.5%, under the illumination of AM.1.5, 100 mW/cm². The V_oc of 0.96 V for S(TPA-DHT-DCN) is among the top values for the solution-processed molecular-based OSCs reported so far.     

Improving the performances of all-small-molecule organic solar cells by fine-tuning halogen substituents of donor molecule

A new acceptor-donor-acceptor (A-D-A) type small molecular donor named as DRTB-CT was designed and synthesized by replacing the fluorine atoms of DRTB-FT with chlorine atoms. Such a small change plays an important role in inhibiting the strong molecular crystallinity and excessive self-aggregation in donor phase due to the larger radius of chlorine atom, which leads to optimized morphology of the active layer. Based on the same nonfullerene acceptor (F–2Cl), the optimal device of DRTB-CT exhibits a higher power conversion efficiency (PCE) of 9.05% compared with that of the fluorinated molecule (7.66%), benefiting from the increased fill factor (FF, 0.568) and short-circuit current density (J_sc, 15.07 mA cm⁻²). The results indicate that the introduction of chlorine atoms is an effective strategy in developing new small molecular donors with high device performances.     

Ternary organic solar cells with improved efficiency and stability enabled by compatible dual-acceptor strategy

To further elevate the power conversion efficiency (PCE) of organic solar cells (OSCs), ternary strategy is one of the most efficient methods via simply incorporating a suitable third component. Here, a nonfullerene small molecule acceptor MOITIC was incorporated into the state-of-art PM6:Y6 binary system to further enhance the photovoltaic performance. Detailed investigation revealed that MOITIC exhibited a good miscibility and compatibility with Y6, forming alloy-like acceptors in the ternary blends. The alloy-like phase promoted the phase separation and optimized the morphology of ternary blend, which afforded higher and more balanced carrier mobility and reduced charge recombination in devices. Moreover, the larger energy offset between PM6 and MOITIC:Y6 acceptor alloy is beneficial to enhance open-circuit voltage (V_oc) of corresponding devices. As a consequence, the optimized ternary OSC (PM6:Y6:MOITIC = 1:1:0.1) showed a significantly increased PCE of 17.1% with simultaneously enhanced V_oc of 0.882 V, short-circuit current density (J_sc) of 25.6 mA cm⁻², and fill factor (FF) of 75.7%, which has about 9% enhancement compared to the control binary PM6:Y6 (15.7%). In addition, the optimized ternary device exhibited better stability. This work indicates that ternary strategy via combining two compatible small molecule acceptors is effective to simultaneously improve the efficiency and stability of OSCs.     

Ternary Polymer Solar Cells with High Efficiency of 14.24% by Integrating Two Well‐Complementary Nonfullerene Acceptors

Ternary polymer solar cells (PSCs) are one of the most promising device architectures that maintains the simplicity of single‐junction devices and provides an important platform to better tailor the multiple performance parameters of PSCs. Herein, a ternary PSC system is reported employing a wide bandgap polymeric donor (PBTA‐PS) and two small molecular nonfullerene acceptors (labeled as LA1 and 6TIC). LA1 and 6TIC keep not only well‐matched absorption profiles but also the rational crystallization properties. As a result, the optimal ternary PSC delivers a state of the art power conversion efficiency (PCE) of 14.24%, over 40% higher than the two binary devices, resulting from the prominently increased short‐circuit current density (J_sc) of 22.33 mA cm^?2, moderate open‐circuit voltage (V_oc) of 0.84 V, and a superior fill factor approaching 76%. Notably, the outstanding PCE of the ternary PSC ranks one of the best among the reported ternary solar cells. The greatly improved performance of ternary PSCs mainly derives from combining the complementary properties such as absorption and crystallinity. This work highlights the great importance of the rational design of matched acceptors toward highly efficient ternary PSCs.     

Two new A-D-A type small molecule acceptors based on C2v-symmetric dithienocyclopentaspiro[fluorene-9,9′-xanthene] core for polymer solar cells

A C_2v-symmetric core, dithienocyclopentaspiro[fluorene-9,9′-xanthene], was used as the central block for the first time to design and synthesize A-D-A type small molecule acceptors for nonfullerene polymer solar cells (PSCs), and two new small molecule acceptors of TSFX-2F and TSFX-4F were synthesized based on the C_2v-symmetric core. The two TSFX-based acceptors show high thermal stability, strong absorption in the wavelength region of 550–750 nm and appropriate energy levels. The PSCs with the broad bandgap polymer J71 as donor and TSFX-2F as acceptor demonstrated power conversion efficiency (PCE) of 9.42% with open circuit voltage (V_oc) of 0.89 V, short circuit current density (J_sc) of 15.27 mA cm⁻² and fill factor (FF) of 69.30%, while the PSC based on J71:TSFX-4F shows a PCE of 8.47% with V_oc of 0.83 V, J_sc of 15.48 mA cm⁻² and FF of 66.16%. The higher V_oc of the PSC based on J71: TSFX-2F is benefitted from the up-shifted LUMO energy level of the TSFX-2F acceptor, and its higher FF can be ascribed to the higher and more balanced hole and electron mobilities of the J71: TSFX-2F active layer. This work demonstrates that the new C_2v-symmetric building block is a promising central D-unit for the design and synthesis of new structured norfullerene acceptors for high-performance PSCs.     

Simultaneously Achieved High Open‐Circuit Voltage and Efficient Charge Generation by Fine‐Tuning Charge‐Transfer Driving Force in Nonfullerene Polymer Solar Cells

To maximize the short‐circuit current density (J_SC) and the open circuit voltage (V_OC) simultaneously is a highly important but challenging issue in organic solar cells (OSCs). In this study, a benzotriazole‐based p‐type polymer (J61) and three benzotriazole‐based nonfullerene small molecule acceptors (BTA1‐3) are chosen to investigate the energetic driving force for the efficient charge transfer. The lowest unoccupied molecular orbital (LUMO) energy levels of small molecule acceptors can be fine‐tuned by modifying the end‐capping units, leading to high V_OC (1.15–1.30 V) of OSCs. Particularly, the LUMO energy level of BTA3 satisfies the criteria for efficient charge generation, which results in a high V_OC of 1.15 V, nearly 65% external quantum efficiency, and a high power conversion efficiency (PCE) of 8.25%. This is one of the highest V_OC in the high‐performance OSCs reported to date. The results imply that it is promising to achieve both high J_SC and V_OC to realize high PCE with the carefully designed nonfullerene acceptors.     

Size-dependent polymer/CuInS2 solar cells with tunable synthesis of CuInS2 quantum dots

This paper reports the size-dependent performance in polymer/CuInS₂ solar cells with tunable synthesis of chalcopyrite CuInS₂ quantum dots (QDs) by the solvothermal method. The CuInS₂ QDs of 3.2–5.4 nm in size are fine tuned by the reaction time in the solvothermal process with the slow supply of In³⁺ ions during the crystallization, and the band gaps increased with QDs sizes decreasing according to the results from the characterization of sizes, morphologies, component elements, valence states and band gaps of CuInS₂ QDs. We fabricated MEH-PPV/CuInS₂ solar cells, and the photoactive layer of device displayed size-dependent light-harvesting, charge separation and transport ability. Moreover, the solar cells exhibit size-dependent short circuit current (J_sc) and open circuit voltage (V_oc), with higher performance in both J_sc and V_oc for smaller CuInS₂ QDs, resulting in the maximum power conversion efficiency of ca. 0.12% under the monochromic illumination at 470 nm; CuInS₂ QDs actually serve as an effective electron acceptor material for the MEH-PPV/CuInS₂ solar cells with the wide spectral response extending from 300 to 900 nm.     

Reduction of Collection Efficiency of Charge Carriers with Increasing Cell Size in Polymer Bulk Heterojunction Solar Cells

Changes in solar cell performance related to active area size were investigated using polymer bulk heterojunction devices. Cell geometry was defined by introduction of a sub‐electrode. The cells were uniform up to 16 cm². The solar cells showed little change in performance up to a cell area of 1 cm². As cell area increased above 4 cm² the power conversion efficiency dropped significantly, mostly because of fill factor (FF) drop and short circuit current density (J_sc) suppression. The changes in FF and J_sc could not be described solely by a Shockley diode equation based on an equivalent circuit model unless photocurrent collection was also considered. As cell area increased, collection efficiency deviated from unity, which further reduced device performance. That deviation is attributed to acceleration of recombination loss at low built‐in junction potentials.     

Influence of band gap of p-type hydrogenated nanocrystalline silicon layer on the short-circuit current density in thin-film silicon solar cells

The impact of the optical band gap (E_g) of a p-type hydrogenated nanocrystalline silicon layer on the short-circuit current density (J_sc) of a thin-film silicon solar cell is assessed. We have found that the J_sc reaches maximum when the E_g reaches optimum. The reason for the J_sc on E_g needs to be clarified. Our results exhibit that maximum J_sc is the balance between dark current and photocurrent. We show here that this dark current results from the density of defects in the p-layer and the barrier at the interface between p-and i-layers. An optimum cell can be designed by optimizing the p-layer via reducing the density of defects in the p-layer and the barrier at the p/i interface. Finally, a 6.6% increase in J_sc was obtained at optimum E_g for n-i-p solar cells.     

Efficient bulk heterojunction photovoltaic devices based on diketopyrrolopyrrole containing small molecule as donor and modified PCBM derivatives as electron acceptors

A new solution processable small molecule (DPP-CN) containing electron donor diketopyrrolopyrrole (DPP) core and cyanovinylene 4-nitrophenyl (CN) electron acceptor has synthesized for use as the donor material in the bulk heterojunction organic solar cells along with PCBM, modified PCBM i.e. F and A as electron acceptor. It showed a broad absorption in longer wavelength region having optical band gap around 1.64 eV. We have used PCBM, F and A as electron acceptor for the fabrication of bulk heterojunction photovoltaic devices. The power conversion efficiency (PCE) of the BHJ devices based on DPP-CN:PCBM, DPP-CN:F and DPP-CN:A blends cast from the THF solvent is 1.83%, 2.79% and 2.83%, respectively. The increase in the PCE based on F and A as electron acceptor is mainly due to the increase in both short circuit current (J_sc) and open circuit voltage (V_oc). The PCE value of the photovoltaic devices based on the blends DPP-CN:PCBM, DPP-CN:F and DDP-CN:A cast from the mixed solvents (DIO/THF) has been further improved up to 2.40%, 3.32% and 3.34%, respectively. This improvement is mainly due to the increased value of J_sc, which is attributed not only to the increase of crystallinity, but also to the morphological change in the film cast from mixed solvent. Finally, the device ITO/PEDOT:PSS/DPP-CN:A (DIO/THF cast)/TiO₂/Al device shows a PCE of 3.9%. The improved device performance could be attributed to the electron transporting and hole-blocking capabilities due to the introduced TiO₂ buffer layer.     

ZnO-Coated TiO2 Nanotube Arrays for a Photoelectrode in Dye-Sensitized Solar Cells

In dye-sensitized solar cells, highly ordered TiO₂ nanotube arrays as a photoelectrode have higher charge collection efficiencies than a nanoparticle-based structure due to their faster charge percolation and slower recombination of electrons. Highly ordered TiO₂ nanotube arrays were grown by anodic oxidation of 0.5-mm-thick titanium foil. To increase the conversion efficiency of dye-sensitized solar cells with TiO₂ nanotube arrays, the surface of the TiO₂ nanotube arrays was modified by zinc oxide thin films. The ZnO thin film was formed by atomic layer deposition. The thin film was conformal on the inner and outer walls of TiO₂ nanotube arrays. ZnO thin film improved the short circuit current (J _sc) and open circuit voltage (V _oc) due to increasing specific surface area from particulates of ZnO thin film and increasing the surface charge induced from the isoelectric point. The power conversion efficiency of dye-sensitized solar cells with ZnO thin film on 4.5-μm-thick TiO₂ nanotube arrays was 1.43%. Microstructure and phase were observed by scanning electron microscopy, x-ray diffractometry, and transmission electron microscopy.     

Graphene Intermediate Layer in Tandem Organic Photovoltaic Cells

One strategy to harvest wide spectral solar energy is to stack different bandgap materials together in a tandem solar cell. Here, it is demonstrated that CVD grown graphene film can be employed as intermediate layer (IML) in tandem solar cells. Using MoO₃‐modified graphene IML, a high open circuit voltage (V_oc) of 1 V and a high short‐circuit current density (J_sc) of 11.6 mA cm^‐2 could be obtained in series and parallel connection, respectively, in contrast to a V_oc of 0.58 V and J_sc of 7.6 mA cm^‐2 in single PV cell. The value of V_oc (J_sc) in the tandem cell is very close to the sum of V_oc (J_sc) attained from two single subcells in series (parallel), which confirms good ohmic contact at the photoactive layer/MoO₃‐modified graphene interface. Work function engineering of the graphene IML with metal oxide is essential to ensure good charge collection from both subcells.     

Achievement of 17.9% efficiency in 30 × 30 cm2 Cu(In,Ga)(Se,S)2 solar cell sub‐module by sulfurization after selenization with Cd‐free buffer

We have achieved 17.9% efficiency in a 30 × 30 cm² Cu(In,Ga)(Se,S)₂ solar cell sub‐module prepared by selenization and sulfurization processes with a Cd‐free buffer. The development of an absorber layer, transparent conducting oxide window layer, and module design was the key focus. This permitted 1.8% higher efficiency than our last experimental result. The quantity and the injection time of the sodium were controlled, resulting in higher open circuit voltage (V_oc) and short circuit current (J_sc). In order to increase J_sc, we changed the thickness of the window layer. Boron‐doped zinc oxide was optimized for higher transmittance without reducing the fill factor. The uniformity of each layer was improved, and patterns were optimized for each module. Therefore, V_oc, J_sc, and FF could be theoretically improved on the reported results of, respectively, 20 mV, 2 mA/cm², and 1.4%. The module's efficiency was measured at the Korea Test Laboratory to compare with the data obtained in‐house. Various analyses were performed, including secondary ion mass spectroscopy, photoluminescence, quantum efficiency, solar simulator, and UV–vis spectrometry, to measure the cell's depth profile, carrier lifetime, external quantum efficiency, module efficiency, and transmittance, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Improved photovoltaic response of a near-infrared sensitive solar cell by a morphology-controlling seed layer

We demonstrate that a thin seed layer of indium phthalocyanine chloride (ClInPc) annealed under mild conditions effectively controls the morphology of both post-annealing deposited ClInPc films and ClInPc:C₆₀ mixed films, introducing the triclinic phase into the commonly monoclinic phase dominating film. ClInPc/C₆₀ planar solar cells and ClInPc:C₆₀ (1:1) planar-mixed solar cells with and without the triclinic phase were studied. Increased short circuit current (J_sc), fill factor (FF), external quantum efficiency (EQE) and internal quantum efficiency (IQE) of the devices containing triclinic phase is attributed to the enhanced absorption in the near infrared (NIR) region and decreased series resistance. The correlation between open circuit voltage (V_oc) and dark saturation pre-exponential factor (J_so) was analyzed to investigate V_oc loss upon annealing. The overall performance of device is considerably improved by introducing the triclinic phase of ClInPc.     

D–A–Ar-type small molecules with enlarged π-system of phenanthrene at terminal for high-performance solution processed organic solar cells

Two novel D–A–Ar-type small molecules of TPA–DPP–P and TPA(DPP–P)₂ were synthesized and characterized, in which triphenylamine (TPA), diketopyrrolopyrrole (DPP) and phenanthrene (P) were used as the donor (D) core, acceptor (A) arm, and enlarged π-system of polycyclic arene (Ar) terminal. Their absorptive, electro-chemical, thermal, and photovoltaic properties were preliminary investigated. Significantly improved photophysical and photovoltaic performances were observed for both small molecules containing the planar P terminal in comparison with those for their parent D–A-type molecule of TPA–DPP. The highest power conversion efficiency (PCE) of 3.42% and a maximum short-circuit current density (J_sc) of 9.2 mA/cm² were obtained in the solution-processed TPA(DPP–P)₂-based solar cells using [6,6]-phenyl-C-71-butyric acid methyl ester (PC₇₁BM) as acceptor. The PCE and J_sc values are 8.76 and 4.97 times higher than those of the TPA–DPP-based cells, respectively. It indicates that appending the enlarged π-system of the planar P terminal and incorporating the DPP–P arm into D–A-type small molecule are efficient approaches to improve photophysical and photovoltaic performances for its resulting molecules.     

Chlorobenzene vapor assistant annealing method for fabricating high quality perovskite films

In this study, chlorobenzene (CB) vapor assistant annealing (VAA) method is employed to make high quality perovskite films and produce high efficiency CH₃NH₃PbI_3-xCl_x perovskite solar cells. The perovskite films made by this method present several advantages such as increased crystallinity, large grain size and reduced crystal boundaries compared with those prepared by thermal annealing (TA) method, which is beneficial to charge dissociation and transport in hybrid photovoltaic device. In addition, it is found that the CB VAA method could improve the surface property of perovskite film, resulting in a preferable coverage of PCBM layer and a better interfacial contact between perovskite film and upper PCBM film. Consequently, the short circuit current density (J_sc) of the devices is significantly increased, yielding a high efficiency of 14.79% and an average efficiency of 13.40%, which is 13% higher than that of thermal annealed ones. This work not only put forward a simple and efficient approach to prepare highly efficient perovskite solar cells but also provide a new idea to improve the morphology and interfacial contact in one integration step.     

High‐Performance Pseudoplanar Heterojunction Ternary Organic Solar Cells with Nonfullerene Alloyed Acceptor

The vast majority of ternary organic solar cells are obtained by simply fabricating bulk heterojunction (BHJ) active layers. Due to the inappropriate distribution of donors and acceptors in the vertical direction, a new method by fabricating pseudoplanar heterojunction (PPHJ) ternary organic solar cells is proposed to better modulate the morphology of active layer. The pseudoplanar heterojunction ternary organic solar cells (P‐ternary) are fabricated by a sequential solution treatment technique, in which the donor and acceptor mixture blends are sequentially spin‐coated. As a consequence, a higher power conversion efficiency (PCE) of 14.2% is achieved with a V_oc of 0.79 V, J_sc of 25.6 mA cm^?2, and fill factor (FF) of 69.8% compared with the ternary BHJ system of 13.8%. At the same time, the alloyed acceptor is likely formed between two the acceptors through a series of in‐depth explorations. This work suggests that nonfullerene alloyed acceptor may have great potential to realize effective P‐ternary organic solar cells.