wangzhaowei

Small-molecule donors for solar cells are usually end-capped with π-systems or aliphatic chains extending the π-conjugation of the molecules's backbone. Compared with alkyl terminals, π-systems can form π−π arrangements, for example, with an aligning spherical fullerene π-system. To study the effects of two kinds of terminals on the solar cell performance, the non-alkyl, branched aromatic and electron-donating diphenylamine (DPA) and the aliphatic n-butyl (n-Bu) unit are selected as end-capping groups on a diketopyrrolo­pyrrole-based linear backbone, affording two new solution-processable small-molecule donors. Photovoltaic data indicate that by changing the end-function from n-Bu to DPA, the photocurrent significantly increases from 8.35 to 15.64 mA cm⁻² and the efficiency from 3.2 to 5.8%. Characterization of absorption, morphology, recombination, and carrier transportation clearly demonstrates that the higher photocurrent can be attributed to a higher density of the mobile carriers (i.e., free holes, in this case). The DPA end-functions enhance the light-harvesting capacity, improve the charge dissociation, and reduce the recombination loss, all of which lead to more carriers being collected by the electrode. This work demonstrates that the choice of end-function along the molecular backbone is as important to improve the cell performance as the light-harvesting backbone and the side-chains.

End-capping groups on the diketopyrrolopyrrole (DPP)-based linear backbone significantly affect small-molecule solar cell performance. Changing the end-functions from n-butyl to the non-alkyl, branched aromatic diphenylamine, the photocurrent significantly increases from 8.35 to 15.64 mA cm⁻² and the efficiency from 3.2 to 5.8%, due to enhanced light-harvesting, improved charge dissociation, and reduced recombination losses.

Efficient vacuum-deposited tandem organic photovoltaic cells (TOPVs) composed of pristine fullerenes as the acceptors and two complementary absorbing donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile for the visible absorption and 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c]-[1,2,5]thiadiazol-4-yl)methylene)malononitrile for the near-infrared absorption, are reported. Two subcells are connected by the interconnection unit (ICU) composed of electron-transporting layer/metal/p-doped hole-transporting layer. The p-doped layer in the ICU enables increasing the short-circuit current density (J _SC) of TOPVs by tuning the relative position of subcells in the tandem devices to have the maximum optical field distribution response, which is well matched with theoretical calculation. Moreover, the introduction of the doped layer benefits to the higher fill factor (FF) of the consisting subcells without losing open-circuit voltage (V _OC) even with the thick active layers. As a result, power conversion efficiency of 9.2% is achieved with higher FF of 0.62 than that of single-junction subcells (0.54, 0.57), J _SC of 8.7 mA cm⁻², and V _OC of 1.71 V using 80 nm thick active layers in both subcells.

Efficient vacuum-deposited tandem organic solar cells with fill factor higher than single-junction subcells are fabricated. A power conversion efficiency of 9.2% is obtained with a fill factor of 0.62, which is higher than those of single-junction cells.

A hybrid electrode with a combination of an ultratransparent conductive polymer (nearly 100% transparency) and a semitransparent silver grid is developed for organic photovoltaic cells. In article number 1401822, Yong Hyun Kim and co-workers report that the hybrid electrode shows an improvement of the photovoltaic performance and a potential to reduce a significant resistive power loss in large-area devices. These hybrid electrodes will contribute the development of low-cost, efficient, large-area organic photovoltaic cells.

In spite of the successful enhancement of the power-conversion efficiency (PCE) in organic bulk heterojunction (BHJ) solar cells by surface plasmon resonance (SPR), the incorporation of several tens of nanometer-sized (25–50 nm) metal nanoparticles (NPs) has some limitations to further enhancing the PCE due to concerns related to possibly transferring nonradiative energy and disturbing the interface morphology. Instead of tens of nanometer-sized metal NPs, here, dodecanethiol stabilized Au nanoclusters (Au:SR, R = the tail of thiolate) with sub-nm-sized Au38 cores are incorporated on inverted BHJ solar cells. Although metal NPs less than 5 nm in size do not show any scattering or electric field enhancement of incident light by SPR effects, the incorporation of emissive Au:SR nanoclusters provides effects that are quite similar to those of tens of nanometer-sized plasmonic metal NPs. Due to effective energy transfer, based on the protoplasmonic fluorescence of Au:SR, the highest performing solar cells fabricated with Au:SR clusters yield a PCE of 9.15%; this value represents an ≈20% increase in the efficiency compared to solar cells without Au:SR nanoclusters.

High efficiency inverted bulk heterojunction solar cells based on a mixture of conjugated polymers and soluble C70 derivatives are demonstrated through the incorporation of a dodecanethiol monolayer-protected Au nanoclusters (Au:SR) with sub-nanometer-sized Au38 cores. Because of effective energy transfer, based on protoplasmonic fluorescence, the highest performing solar cells fabricated with the Au:SR cluster yield a power-conversion efficiency of 9.15%.

To realize efficient photoconversion in organic semiconductors, photogenerated excitons must be dissociated into their constituent electronic charges. In an organic photovoltaic (OPV) cell, this is most often accomplished using an electron donor–acceptor (D–A) interface. Interestingly, recent work on MoO_x/C₆₀ Schottky OPVs has demonstrated that excitons in C₆₀ may also undergo efficient bulk-ionization and generate photocurrent as a result of the large built-in field created by the MoO_x/C₆₀ interface. Here, it is demonstrated that bulk ionization processes also contribute to the short-circuit current density (J_SC) and open-circuit voltage (V_OC) in bulk heterojunction (BHJ) OPVs with fullerene-rich compositions. Temperature-dependent measurements of device performance are used to distinguish dissociation by bulk-ionization from charge transfer at the D–A interface. In optimized fullerene-rich BHJs based on the D–A pairing of boron subphthalocyanine chloride (SubPc)–C₆₀, bulk-ionization is found to be responsible for ≈16% of the total photocurrent, and >30% of the photocurrent originating from C₆₀. The presence of bulk-ionization in C₆₀ also impacts the temperature dependence of V_OC, with fullerene-rich SubPc:C₆₀ BHJ OPVs showing a larger V_OC than evenly mixed BHJs. The prevalence of bulk-ionization processes in efficient, fullerene-rich BHJs underscores the need to include these effects when engineering device design and morphology in OPVs.

Bulk ionization is non-negligible in the operation of fullerene-rich bulk hetero­junctions (BHJs). In optimized BHJs based on the donor–acceptor pairing of boron subphthalocyanine chloride–C₆₀ (20:80 vol%), bulk-ionization in C₆₀ contributes ≈16% of the total photocurrent and increases the open-circuit voltage by 12% compared to an evenly mixed BHJ.

In less than three years, the photovoltaic community has witnessed a rapid emergence of a new class of solid-state heterojunction solar cells based on solution-processable organometal halide perovskite absorbers. The energy conversion efficiency of solid-state perovskite solar cells (PSCs) has been quickly increased to a certified value of 20.1% by the end of 2014 because of their unique characteristics, such as a broad spectral absorption range, large absorption coefficient, high charge carrier mobility and diffusion length. Here, the focus is specifically on recent developments of hole-transporting materials (HTMs) in PSCs, which are essential components for achieving high solar cell efficiencies. Some fundamentals with regard to PSCs are first presented, including the history of PSCs, device architectures and general operational principles of PSCs as well as various techniques developed for the fabrications of uniform and dense perovskite complexes. A broad range of the state-of-the-art HTMs being used in PSCs are then discussed in detail. Finally, an outlook on the design of more efficient HTMs for highly efficient PSCs is addressed.

Searching for efficient hole-transporting materials (HTMs), which are essential components for achieving high efficiencies, is one of the hottest research topics in the field of organometal halide perovskite solar cells (PSCs). Various HTMs that are being developed and applied in PSCs are summarized in detail and key issues related to developing more efficient HTMs in the future are highlighted.

The charge generation and recombination dynamics in polymer/polymer blend solar cells composed of poly(3-hexylthiophene) (P3HT, electron donor) and poly[2,7-(9,9-didodecylfluorene)-alt-5,5-(4′,7′-bis(2-thienyl)-2′,1′,3′-benzothiadiazole)] (PF12TBT, electron acceptor) are studied by transient absorption measurements. In the unannealed blend film, charge carriers are efficiently generated from polymer excitons, but some of them recombine geminately. In the blend film annealed at 160 °C, on the other hand, the geminate recombination loss is suppressed and hence free carrier generation efficiency increases up to 74%. These findings suggest that P3HT and PF12TBT are intermixed within a few nanometers, resulting in impure PF12TBT and disordered P3HT domains. The geminate recombination is likely due to charge carriers generated on isolated polymer chains in the matrix of the other polymer and at the domain interface with disordered P3HT. The undesired charge loss by geminate recombination is reduced by both the purification of the PF12TBT-rich domain and crystallization of the P3HT chains. These results show that efficient free carrier generation is not inherent to the polymer/fullerene domain interface, but is possible with polymer/polymer systems composed of crystalline donor and amorphous acceptor polymers, opening up a new potential method for the improvement of solar cell materials.

Charge generation and recombination dynamics in donor/acceptor polymer blend solar cells composed of poly(3-hexylthiophene) (P3HT) and fluorene-based copolymer (PF12TBT) are examined using transient absorption spectroscopy. Thermal annealing improves both the purity of the PF12TBT domains and the ordering of the P3HT chains. These morphological changes suppress the monomolecular recombination of charge pairs, increasing the free carrier generation efficiency up to 74%.

A series of medium bandgap polymers based on 9,10-thienylanthracene and benzothiadiazole are synthesized. The optical, electrochemical, intermolecular, and charge transport characteristics of the polymers are tuned by the introduction of various substituents. The fluorine-substituted polymer exhibits much enhanced charge generation with reduced bimolecular recombination, thereby achieving a high power conversion efficiency exceeding 8%.

Efficient conventional bulk heterojunction (BHJ) perovskite hybrid solar cells (pero-HSCs) solution-processed from a composite of CH₃NH₃PbI₃ mixed with PC₆₁BM ([6,6]-phenyl-C61-butyric acid methyl ester), where CH₃NH₃PbI₃ acts as the electron donor and PC₆₁BM acts as the electron acceptor, are reported for the first time. The efficiency of 12.78% is twofold enhancement in comparison with the conventional planar heterojunction pero-HSCs (6.90%) fabricated by pristine CH₃NH₃PbI₃. The BHJ pero-HSCs are further optimized by using PC₆₁BM/TiO₂ bi-electron-extraction-layer (EEL), which are both solution-processed and then followed with low-temperature thermal annealing. Due to higher electrical conductivity of PC₆₁BM over that of TiO₂, an efficiency of 14.98%, the highest reported efficiency for the pero-HSCs without incorporating high-temperature-processed mesoporous TiO₂ and Al₂O₃ as the EEL and insulating scaffold, is observed from PC₆₁BM modified BHJ pero-HSCs. Thus, the findings provide a simple way to approach high efficiency low-cost pero-HSCs.

A facile route for approaching high efficiency perovskite hybrid solar cells by fabrication of a bulk heterojunction structure in which perovskite materials are blended with fullerene derivatives is reported for the first time. This addresses the two major issues of imbalanced charge transporting efficiencies and significant photocurrent hysteresis behaviors in perovskite hybrid solar cells.

Organic solar cells lag behind their inorganic counterparts in efficiency due largely to low open-circuit voltages (V_oc). In this work, a comprehensive framework for understanding and improving the open-circuit voltage of organic solar cells is developed based on equilibrium between charge transfer (CT) states and free carriers. It is first shown that the ubiquitous reduced Langevin recombination observed in organic solar cells implies equilibrium and then statistical mechanics is used to calculate the CT state population density at each voltage. This general result permits the quantitative assignment of V_oc losses to a combination of interfacial energetic disorder, non-negligible CT state binding energies, large degrees of mixing, and sub-ns recombination at the donor/acceptor interface. To quantify the impact of energetic disorder, a new temperature-dependent CT state absorption measurement is developed. By analyzing how the apparent CT energy varies with temperature, the interfacial disorder can be directly extracted. 63–104 meV of disorder is found in five systems, contributing 75–210 mV of V_oc loss. This work provides an intuitive explanation for why qV_oc is almost always 500–700 meV below the energy of the CT state and shows how the voltage can be improved.

Low open-circuit voltages are one of the primary factors limiting organic photovoltaic efficiencies. This work presents a comprehensive framework for understanding and improving the voltage of organic solar cells. It explains why qV_oc is almost always 0.5–0.7 eV below the apparent CT state energy and provides an experimental method to quantify the voltage loss due to energetic disorder.

A twisted bay-substituted tetraphenyl functionalized perylenediimide monomer (TP-PDI) is synthesized. A high power conversion efficiency of 4.1% is achieved for organic solar cells incorporated a combination of TP-PDI electron acceptor and PTB7-Th electron donor, indicating that chemical modification of PDI monomers is a useful way of designing high-performance non-fullerene electron acceptors.

In article number 1401657 Maxim S. Pshenichnikov and co-workers describe pathways of initial charge generation in photovoltaic blends based on novel star-shaped molecules for solution-processed organic solar cells. The cover design shows electron and hole transfer processes that are equally important for charge separation. The results presented demonstrate how ultrafast spectroscopy, combined with quantum-chemical calculations, can shed light on ways to optimize organic solar cells. Cover art design: Nynke Kuipers, Oleg Kozlov; background photograph: Michail Pchenitchnikov.

To match the bandgap of silicon solar cells for tandem devices, Jinsong Huang and co-workers report the incorporation of Br into perovskite to increase the bandgap to 1.72 eV. In article number 1401616, the efficiency of perovskite solar cell with wide-bandgap is shown to reach 13.1% with grain size engineering. The wide-bandgap perovskite solar cells fabricated at low temperature can be directly integrated onto Si for tandem cells, which potentially increases the efficiency to 24.0%.

A unique sandwiched structure of TiO_x/Au-NPs/TiO_x is used to improve the charge transport properties of a TiO_x film via plasmonic-mediated hot carrier injection at the metal-semiconductor Schottky junction. The injected carrier helps to fill trap states and to further decrease the Fermi level of TiO_x. The combined effects dramatically enhance the perovskite solar cell performance, with a power conversion efficiency of 16.2%.

Large-area, flexible, lateral organic solar cells are demonstrated for the first time. In article number 1401317, Kilwon Cho and co-workers demonstrate that the incorporation of 1D organic semiconductor nanowires in the photoactive layer achieves efficient charge sweep-out to asymmetric electrodes in lateral solar cells. An interdigitated solar module to amplify power generation can be fabricated on a plastic substrate and mechanically deformed without loss of performance.

A surface passivation method for solution-processed ZnO films based on small molecule ethanedithiol (EDT) treatment is reported by Yizheng Jin, Baoquan Sun, Feng Gao, and co-workers in article number 1401606. The passivation using EDT treatment on ZnO effectively removes the surface defects and modulates the intragap states with the introduced intragap band. The device performances of inverted organic solar cells based on EDT-treated, solution-processed, ZnO electron-transporting interlayers are significantly improved due to reduced interface recombination and enhanced charge extraction properties.