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Since the most high-performing donor polymers in polymer solar cells (PSCs) possessed the deep highest occupied molecular orbital (HOMO) level, interfacial engineering on anode contact is becoming increasingly important. Herein, we demonstrated efficient PSCs using an anionic poly(styrene sulfonate) (PSS) as an anode interfacial layer (AIL). With the formation of the dipole layer, the effective work function (WF) of indium tin oxide (ITO) electrode is significantly increased from 4.8 to 5.3 eV, providing favorable energetic alignment to the quasi-Fermi level of various donor polymers. Moreover, by incorporating cationic polyelectrolytes as a cathode interfacial layer, a pair of electric dipole layers induces a strong built-in electric field across the photoactive layer to drive efficient sweep-out of photogenerated charges. Consequently, the device with PSS AIL exhibited high power conversion efficiencies of 9.2 and 14.8% in PTB7-Th:PC71BM- and PM6:Y6-based PSCs, respectively, both of which are higher than those of the devices with PEDOT:PSS.

The photoelectric and optical characteristics of an organic solar cell (OSС) with a hole-transport layer based on a polyaniline complex and a photoactive layer with a bulk heterojunction based on P3HT polythiophene derivative and PC71BM fullerene have been studied. The results of modeling the optical properties of the functional layers and experimental measurements of the OSC characteristics enabled the optimal values of the thickness of the functional layers resulting in the maximum power conversion efficiency of the device to be identified. It has been determined that the hole-transport layer with a thickness of 30–60 nm, while having a transmission of more than 80%, effectively transferred holes to the photoanode from the photoactive layer, in which the exciton generation rate was 5.6–5.9 × 1016 cm–2 s–1. These values of the exciton generation rate at the maximum total light absorption in the device were provided by a photoactive layer with a thickness of 90–100 nm.

Recently, the bulk heterojunction (BHJ)-based photoactive layer has been favored over the planar heterojunction (PHJ)-based photoactive layer for organic photovoltaics (OPVs), owing to the evitable thickness limit of the latter induced by the low mobility of charge carriers. However, under dim indoor light conditions, the PV performance becomes immune to the resistive components of the PHJ OPVs, such as a thick photoactive layer, thereby rendering them attractive for operating indoors. In this study, we devise and demonstrate indoor OPVs with a PHJ-based photoactive layer constituting poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (ICBA). The performance of the PHJ OPVs was optimized by modulating the thickness of the ICBA layer under a 1000 lx light-emitting diode lamp. OPVs with a 65 nm-thick ICBA layer exhibited excellent indoor performance with a power conversion efficiency of 15.3 ± 0.3%, comparable to that of the reference BHJ OPVs (15.0 ± 0.2%). No noticeable performance degradation was observed by increasing the thickness of the PHJ-based photoactive layer; instead, a thick photoactive layer only enhanced the light absorption, resulting in excellent indoor performance with a higher photocurrent.

The third component played an increasingly significant role in polymer solar cells (PSCs), especially for the morphological impact. Here, we use a ternary polymer donor (TBFCl50-BDD) and organic small molecule acceptor (BTP-4F) as the main material and introduce a third component (PC71BM) to enhance the morphology of the photoactive layer and result in improved device performance. The TBFCl50-BDD:BTP-4F based PSCs only exhibited an efficiency of 10.58%, while The power conversion efficiency of ternary device (TBFCl50-BDD:BTP-4F:PC71BM) reached 13.15%. Importantly, the introduction of PC71BM can not only improve the morphology of the active layer and promote more efficient exciton transport, but also influence the molecular arrangement for better crystallization performance. This work offers an effective method for optimization of the morphology to enhance the properties of PSCs.

Copper(І) iodide (CuI) is a very efficient inorganic p-type material and has a potential application in polymer solar cells (PSCs). However, the solution-processed CuI film tends to be rough surface morphology. Herein, CuI nanoparticles (NPs) were synthesized by water decomposing the CuI N,N-dimethylformamide (DMF) dispersion and used as hole transporting layers (HTLs) of PSCs with the configuration of ITO/CuI/P3HT:PC61BM/Ca/Al. As a result, a smooth and uniform CuI film was formed by spin coating from CuI NP acetonitrile solution. The root-mean-square roughness of the film reduced from 2.25 nm for spin coating from bulk CuI acetonitrile solution to 0.65 nm for CuI NPs. The power conversion efficiency of PSCs increased from 3.10% to 3.47% for CuI NP HTL. Moreover, the CuI NPs-based device demonstrated enhanced stability. The higher efficiency and better stability of CuI NPs-based PSCs should be attributed to uniform surface morphology and reducing exciton quenching of the CuI/the photoactive layer interface. This work indicates that CuI NPs will be a novel promising hole transporting material for highly efficient and stable PSCs.

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

In the present paper, we report the studies on perovskite (CH3NH3PbI3): Zinc Oxide (ZnO) bulk heterojunction (BHJ) photoactive layer (with two different ZnO concentrations) based solar cells fabricated under ambient air. ZnO and NiOx layers are used as the electron and hole transport layers respectively. The typical cell parameters of CH3NH3PbI3: ZnO BHJ solar cells have been calculated and analysed in comparison to that of solar cell fabricated with pristine perovskite (CH3NH3PbI3) based photoactive layer. The best performing solar cell (CH3NH3PbI3:ZnO; 1 mg/ml) has demonstrated highest power conversion efficiency (PCE) of 8.97% and retained 83.82% of its original value after 30 days under ambient air.

The progress in Bulk Heterojunction-Polymer Solar Cells (BHJ-PSCs) has received much interest since the emergence of non-fullerene small molecule acceptors (NFSMAs), and power conversion efficiency (PCE) have reached up to 18%. In general, a mixer of conjugated polymer Donor and NFSM as Acceptor consists of the BHJ photoactive layer. Their optical bands and the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are of great importance to improve the resulting PSCs' efficiency and performance of the polymer solar cells. The HOMO and LUMO of Poly(4,6-bis(5-octylthiophen-2-yl)benzo[1,2-b:6,5-b':3,4-c'']terthiophene-2,8-diyl)-alt-(thiophen-2,5-diyl]-alt-(5,6-difluoro benzo[c][1,2,5]thiadiazol 4,7-diyl (denoted as P95) and NFSMAs ITIC-F, were measured using cyclic voltammetry(CV). Optical bandgap energy for each material's were calculated by UV–VIS absorbance spectra. Further charge transfer study was carried out by photoluminescence (PL) spectroscopy. It's results show that efficient electron transfer from P95 to ITIC-F and hole from ITIC-F to P95. On the basis of experimental outcomes these polymer materials can effectively produce viable organic solar cell (OSCs) devices.

In this article, three novel and simple molecular structure with donor-acceptor (D-A) type copolymers via only head-to-head alkoxy (OR) and/or alkylthio (SR) side chains onto the bithiophene (BT) as donor units and fluorinated benzotriazole (FBTA) as acceptor unit, namely, PBTOR-FBTA, PBTOSR-FBTA and PBTSR-FBTA, were successfully designed and synthesized. The impacts of sulfur-oxygen (S⋯O) or sulfur-sulfur (S⋯S) noncovalent interactions on their physicochemical properties, molecular stacking, carrier mobility, morphologies of blend films, as well as their photovoltaic performance were deeply and systematically studied. The introduction of SR side-chains suddenly lowered the highest occupied molecular orbital (HOMO) energy levels, blue-shifted absorption, enhanced π-π stacking, as well as improved morphology of the photoactive layer blends in comparison with the reference polymer without SR side-chain. Polymer solar cells (PSCs) were fabricated to estimate their photovoltaic performance of the polymers. Under an optimized blend ratio of PBTSR-FBTA:PC71BM (1:0.8, w/w), the PBTSR-FBTA-based device exhibits a higher power conversion efficiency (PCE) of 6.25%, which is about 3.34 and 1.87 folds than that of the PBTOR-FBTA and PBTOSR-FBTA-based devices, respectively. Our research results demonstrate that the modification of the simple and low-cost SR side chains is an effective strategy to improve the photovoltaic performance of the polymers.

Carrier collection and light absorption is a couple of contradiction for polymer solar cells (PSCs) because of the limited carrier mobility and optical absorption coefficient for the organic active layers. In this article, the concept of “bridging for carriers” is introduced by embedding metal oxide (ZnO for electrons and NiO x for holes in this study) nanoparticles into the photoactive layer of PSCs to improve exciton separation, enhance carrier transport, reduce electron–hole recombination and thus facilitate carrier collection. It is found that ∼57.6% increase in the power conversion efficiency, i.e., from 1.98% to 3.12% for the optimal addition of ZnO and NiO x into the active layer of the P3HT:PC61BM blend is achieved as compared with the control case without adding ZnO and NiO x nanoparticles. In combination with the simple hydrothermal method of preparing these metal oxide nanoparticles, this study is believed to provide valuable contribution to explore facile approaches to address the contradiction of carrier collection and light absorption for PSCs by bridging for carriers in the photoactive layer.

In the past ten years, perovskite solar cells were rapidly developed, but the intrinsic unbalanced charge carrier diffusion lengths within perovskite materials were not fully addressed by either a planar heterojunction or meso-superstructured perovskite solar cells. In this study, we report bulk heterojunction perovskite solar cells, where perovskite materials CH3NH3PbI3 is blended with solution-processed n-type TiOx nanoparticles as the photoactive layer. Studies indicate that one-step solution-processed CH3NH3PbI3:TiOx bulk-heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap-induced charge recombination. Thus, bulk heterojunction perovskite solar cells by CH3NH3PbI3 mixed with 5 wt% of TiOx, which is processed by one-step method rather than typical two-step method, show a short-circuit current density of 20.93 mA/cm2, an open-circuit voltage of 0.90 V, a fill factor of 80% and with a corresponding power conversion efficiency of 14.91%, which is more than 30% enhancement as compared with that of perovskite solar cells with a planar heterojunction device structure. Moreover, bulk heterojunction perovskite solar cells possess enhanced device stability. All these results demonstrate that perovskite solar cells with a bulk heterojunction device structure are one of apparent approaches to boost device performance.

Emerging nonfullerene acceptors (NFAs) with crystalline domains enable high-performance bulk heterojunction (BHJ) solar cells. Thermal annealing is known to enhance the BHJ photoactive layer morphology and performance. However, the microscopic mechanism of annealing-induced performance enhancement is poorly understood in emerging NFAs, especially regarding competing factors. Here, optimized thermal annealing of model system PBDB-TF:Y6 (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]-thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) decreases the open circuit voltage (VOC) but increases the short circuit current (JSC) and fill factor (FF) such that the resulting power conversion efficiency (PCE) increases from 14 to 15% in the ambient environment. Here we systematically investigate these thermal annealing effects through in-depth characterizations of carrier mobility, film morphology, charge photogeneration, and recombination using SCLC, GIXRD, AFM, XPS, NEXAFS, R-SoXS, TEM, STEM, fs/ns TA spectroscopy, 2DES, and impedance spectroscopy. Surprisingly, thermal annealing does not alter the film crystallinity, R-SoXS characteristic size scale, relative average phase purity, or TEM-imaged phase separation but rather facilitates Y6 migration to the BHJ film top surface, changes the PBDB-TF/Y6 vertical phase separation and intermixing, and reduces the bottom surface roughness. While these morphology changes increase bimolecular recombination (BR) and lower the free charge (FC) yield, they also increase the average electron and hole mobility by at least 2-fold. Importantly, the increased μh dominates and underlies the increased FF and PCE. Single-crystal X-ray diffraction reveals that Y6 molecules cofacially pack via their end groups/cores, with the shortest π-π distance as close as 3.34 A, clarifying out-of-plane π-face-on molecular orientation in the nanocrystalline BHJ domains. DFT analysis of Y6 crystals reveals hole/electron reorganization energies of as low as 160/150 meV, large intermolecular electronic coupling integrals of 12.1-37.9 meV rationalizing the 3D electron transport, and relatively high μe of 10-4 cm2 V-1 s-1. Taken together, this work clarifies the richness of thermal annealing effects in high-efficiency NFA solar cells and tasks for future materials design.

The field of all-polymer solar cells (all-PSCs) has experienced rapid development during the past few years, mainly driven by the development of efficient polymer acceptors. However, the power conversion efficiencies (PCEs) of the all-PSCs are still limited by insufficient light absorption of the donor/acceptor blend and large energy loss in devices. We herein designed a polymer acceptor PYT1 constructed n-type molecular acceptor Y5-C20 as the key building block and blended it with a polymer donor PM6 to obtain an all-polymer photoactive layer. The optimized PM6:PYT1 all-PSCs achieved a record higher PCE of 13.43% with a very low energy loss of 0.47 eV and a photoresponse of up to 900 nm compared with the Y5-C20 based device with a best PCE of 9.42%. Furthermore, the PCEs of the PM6:PYT1 all-PSCs are relatively insensitive to the 1-chloronaphthalene (CN) additive contents and active layer thickness. Our results also highlight the effect of CN additive on PM6:PYT1 morphology, i.e. , charge generation, and transport find an optimized balance, and radiative and non-radiative loss is simultaneously reduced in the blend. This work promotes the development of high-performance polymer acceptors and heralds a brighter future of all-PSCs for commercial applications.

In organic photovoltaics (OPVs), the mechanical contact between charge transport layers and photoactive layer can influence the electrical contact that facilitates carrier collection. Unfortunately, the mechanical contact at the interface is rarely discussed in the OPV context. Herein, we report a distinct molecular locking effect that occurs between the donor molecules in the photoactive layer and the hole transport layer (HTL). This is achieved by doping chloroplatinic acid into poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate). The "molecular lock" at the interface leads to denser distribution and more ordered assembling of PM6 donor molecules close to the HTL. Consequently, the trap-assisted recombination in the cell is greatly suppressed, and the carrier lifetime is prolonged by more than 2 times. Together with the elevated charge carrier collection probability, a high fill factor of 77% and a power conversion efficiency of 16.5% are achieved in the PM6:Y6-based OPVs. This study provides a feasible way to boost the device performance by reinforcing the interfacial interaction between the HTL and photoactive layer.

Multi-walled carbon nanotube/poly(3-hexylthiophene) (CNT/P3HT) and CNT-graft-poly(3-dodecylthiophene) (PDDT)/P3HT nanohybrids were applied in active layers to study the effects of these nanostructures on the polymer solar cell (PSC) performance. The charge-carrier dynamics and photophysics were studied in the binary and ternary systems based on P3HT, pre-developed nanostructures and/or phenyl-C71-butyric acid methyl ester (PC71BM) through the electrochemical impedance spectroscopy and morphological analyses. The weaker bimolecular recombination in the photoactive layer, consisting of CNT-graft-PDDT/P3HT nanostructures, was confirmed by short-circuit current (Jsc) and open-circuit voltage (Voc) measurements as a function of light intensity. PSC composed of P3HT:PC71BM:CNT-graft-PDDT/P3HT PSC exhibited the highest PCE of 4.18% with significantly increased Jsc and Voc.

Zinc sulphide (ZnS) nanoparticles (NPs) were synthesized by low temperature colloidal chemistry to produce stable zinc blend structure. The metallic ZnS NPs were incorporated into poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61-butyric acid methyl ester (PCBM) blend photoactive layer to improve the overall performance of organic solar cells (OSC). The newly fabricated devices have exhibited enhanced photocurrent which is likely to come from utilizing the near-field and light scattering effects due to the NPs. The short-circuit current density of the best solar cell was enhanced to as high as 15.65 mA cm−2 followed by 51% and 4.0% maximum fill-factor (FF) and power conversion efficiency (PCE), respectively. This enhancement is very comparable to those obtained from the use of expensive plasmonic gold and silver nanoparticles. The current results are encouraging to improve the performance of OSC through a facile yet cost-effective and environmentally friendly approach of metal nanoparticles synthesis.

In the present work, we have endeavored the utilization of wet-chemically synthesized copper oxide nanoparticles (CuO-NPs) as the active layer in hybrid bulk heterojunction (BHJ) solar cells. The BHJs with CuO-NPs display significantly different physics from customary BHJs, and prove a noteworthy improvement in their performance. It is noted that with the addition of CuO-NPs, the morphology of the photoactive layer endures significant changes. Incorporating CuO-NPs is an additional paradigm for BHJs solar cells which enhances the photocurrent density from 9.43 mA/cm2 to 11.32 mA/cm2 and the external quantum efficiency as well. Also the power-conversion efficiency (PCE) improved from 2.85% to 3.82% without harming the open circuit voltage and the fill factor. The enhancement in PCE achieved here makes it worthy to design high-performance organic solar cells holding inorganic nanoparticles. Keywords: Bulk heterojunction, Solar cells, Copper oxide nanoparticles, Thin films, Photo current density, External quantum efficiency

Summary To date, organic solar cells (OSCs) with the development of photovoltaic materials have realized high power conversion efficiencies (PCEs) through the solution processing strategy with bulk heterojunction (BHJ) structure, but the BHJ morphology is difficult to control in large-scale fabrication of OSCs. Herein, we report an alternative film-forming technology known as layer-by-layer (LbL). As compared to its BHJ counterpart, LbL presents many unique advantages including controllable “p-i-n” morphology, good charge transport and extraction properties, and great universality. By using the LbL-bladed coating strategy, a high PCE of 16.35% was achieved in the PM6:Y6 OSCs. Notably, a large-area solar module of 11.52 cm2 with a geometrical fill factor of over 90% exhibited an outstanding PCE of 11.86%, which represents the highest efficiency of large-area solar modules. The results may pave the way for the fabrication of the photoactive layer in the future industrial production of OSCs.

A bicontinuous interpenetrating donor–acceptor network with a suitable phase-separated domain size is crucial for fabricating highly efficient organic bulk heterojunction solar cells considering the fact that it guarantees effective exciton dissociation and smooth charge transport. For traditional binary blend bulk heterojunctions, the photoactive layer is a simple physical mixture of the donor and acceptor; further, the size of phase separation is primarily determined by the aggregation characteristics of the donor and acceptor. Moreover, it is difficult to control and stabilize it. To resolve this problem, a kind of donor–acceptor integral molecule in which the donor and acceptor units are linked by nonconjugated flexible linking units has been proposed. The energy levels and absorption spectra of the integral molecules can be easily controlled by tuning the donor and acceptor units. Furthermore, these integral molecules can form a microphase-separated bulk heterojunction of the donor and acceptor phases via a self-assembly process of the molecule, which is governed by the flexible linking unit responsible for connecting the donor and acceptor units. Since the donor and acceptor units of the integral molecules are connected by nonconjugated units, the scale of aggregation and morphological stability can be easily controlled. In this review, we first introduce the unique characteristics of three typical donor–acceptor integral molecules and then highlight their current developments. For each donor–acceptor molecule, we attempt to give a detailed summary with regard to their design and synthesis, and an in-depth understanding on the basic mechanism of molecular self-assembly and the performance comparison of SMSCs. In the end, the prospect and potential improvements in the donor–acceptor integral molecule are addressed. We believe that emerging donor–acceptor molecules can provide worthwhile opportunities for the fabrication of efficient and stable organic solar cells.

Although fullerene bis-adducts have been widely used in polymer solar cells for their high LUMO energy level and good performance, the degradation behavior of this class of solar cells has not been well understood. In this paper, the performance and stability of the solar cells based on P3HT:fullerene bis-adducts, including bis-PC61BM and ICBA, were systematically investigated. Different from the P3HT:PC61BM cell, these bis-adduct based cells showed fast open circuit (VOC) and fill factor (FF) decays. The partial recovery of VOC and FF of the aged cells by renewing the MoO3/Al electrode indicated that degradation at the photoactive layer and MoO3 interface is the main reason for VOC and FF decays. The X-ray photoelectronic spectroscopy analysis confirmed that under light illumination, Mo6+ of MoO3 is partially reduced to Mo5+. By inserting a thin layer of C60, both MoO3 reduction and performance decays are slowed down, confirming that photoreduction of MoO3 by P3HT is the degradation mechanism for P3HT:bis-PC61BM cells. Finally, we found that doping a polymer:fullerene bis-adduct layer with piperazine increases the fullerene content on the surface of the photoactive layer, which consequently lowers the reduction of Mo6+ and improves the stability of the solar cells. This work gives a detailed understanding of the interfacial degradation of PSCs and provides effective solutions, which has important guiding significance for improving the stability of different types of polymer solar cells.

A novel crosslinkable acetylacetone chelate, Hf(ACBV)4, was designed and synthesized using 1,3-bis(4-vinylphenyl)propane-1,3-dione (ACBV) and hafnium chloride (HfCl4). The obtained crosslinked chelate film exhibits excellent solvent-resistance property and was successfully applied as the electron transport layer in inverted polymer solar cells (PSCs). The crosslinking property was proved by differential scanning calorimetry (DSC) measurements and the unchanged UV-visible absorption of the crosslinked film after washing with chloroform. The obtained crosslinked Hf(ACBV)4 film can effectively ameliorate the contact between ITO and the photoactive layer, facilitating the charge transfer process of the device. Moreover, the devices based on PM6:Y6 achieved an efficiency of 15.17% and exhibited excellent stability with 90% retention efficiency after 20 h of continuous illumination in air. Our work demonstrates a new approach in designing effective crosslinkable chelates as ETLs to improve the stability and efficiency of PSCs.

The innovation of photoactive layer materials is crucial for improving the power conversion efficiency (PCE) of polymer solar cells (PSCs). Herein, we report two polymer donors (PBTN-o and PBTN-p), which only differ in alkyl chains substituted at sites (6,7- or 5,8-) of the naphtho[2,3-c]thiophene-4,9-dione (NTD) unit. The single crystals of both NTD monomers demonstrate that NTD with alkyl chains at the 6,7-sites has a planar NTD skeleton, and surprisingly, NTD with alkyl chains at the 5,8-sites produces a bent NTD skeleton. The bent NTD-based polymer (PBTN-p) exhibits a more twisted conjugated backbone than PBTN-o. Our comparative studies show that PBTN-p possesses suitable aggregation properties that can optimize the photoactive layer morphology in NF PSCs. In PSCs, the optimal PBTN-o:BO-4Cl-based device shows a PCE of 11.85% with a VOC of 0.84 V, JSC of 22.41 mA cm−2, and FF of 0.63. In contrast, the optimal PBTN-p:BO-4Cl-based device exhibits a better PCE of 14.10% with the same VOC of 0.84 V, and an enhanced JSC of 24.67 mA cm−2 and FF of 0.68. This work provides a new insight into BHJ morphology optimization by side chain induced polymer main chain twisting in PSCs.

A highly efficient inverted polymer solar cell (PSC) has been demonstrated by incorporating an insulating inorganic cerium fluoride (CeF3) layer between the annealing-free zinc oxide (ZnO) electron transport layer (ETL) and the photoactive layer. The CeF3-embedded solar cell fabricated by a full annealing-free process presents an overwhelming power conversion efficiency (PCE) of up to 10.53% for the PTB7-Th:PC71BM-based blend system, which far surpasses that of the reference device with a bare ZnO interlayer (8.48%). Ultraviolet photoelectron spectroscopy revealed that the interfacial energy level alignment has been intentionally improved by forming an interfacial dipole layer at the interface between the ZnO ETL and the ultrathin CeF3 interlayer, leading to a reduced energy barrier for electron transport as well as depressed photogenerated carrier recombination. Concurrently, the addition of CeF3 also passivates the interfacial defect states of the ZnO film and ameliorates the inherent incompatibility with the hydrophobic photoactive layer. This protocol demonstrates that the utilization of the ZnO/CeF3 multifunctional interlayer affords a facile but effective strategy for achieving high-performance inverted PSCs, which does not require postprocessing thermal annealing treatments, laying a good foundation for the future developments of organic photovoltaic devices.

Inverted structure ternary polymer solar cells (PSCs) are designed by blending J61 and PffBTT2-DPPT2 as the donor and Y6 as the acceptor. J61:PffBTT2-DPPT2:Y6 ternary films can replenish the weak absorption of PffBTT2-DPPT2:Y6 binary films at medium wavelengths and broaden near-infrared wavelength absorption windows. At the J61/PffBTT2-DPPT2, J61/Y6 and PffBTT2-DPPT2/Y6 triple interface, the more efficient exciton dissociation is formed in the ternary photoactive layers due to the good compatibility among J61, PffBTT2-DPPT2 and Y6. Simultaneously, effective hole transport occurs at the blend donors due to the similar highest occupied molecular orbital (HOMO) energy levels of both J61 and PffBTT2-DPPT2. Optimized ternary PSCs have a power conversion efficiency (PCE) of 13.88%, with a short-circuit current density (JSC) of 24.04 mA cm−2, an open-circuit voltage (VOC) of 0.82 V and a fill factor (FF) of 70.4%. Approximately 10.9% and 15.7% PCE improvements are obtained compared to J61:Y6 and PffBTT2-DPPT2:Y6 binary PSCs, respectively, as a result of the simultaneous increase of the JSC and the FF of the optimized ternary PSCs. The broad and weak crystallinity peak and high crystallization temperatures of the optimized ternary photoactive layer are beneficial for better thermal stability. After a thermal annealing treatment of optimized ternary PSCs at 80 °C for 20 hours, 79% of the initial PCE was observed compared with only 72% and 70% of the initial PCE for J61:Y6 and PffBTT2-DPPT2:Y6 binary PSCs, respectively, under the same thermal stress conditions.

All-polymer solar cells (all-PSCs), which utilize polymer donor (PD)/polymer acceptor (PA) blend films as the photoactive layer, have attracted considerable attention and achieved substantial progress. Nevertheless, only a few PAs have led to power conversion efficiencies (PCEs) >8% in all-PSCs, and most of them are naphthalene diimide-based polymers. Herein, a series of PAs named PBTI2(xDT)-FT have been developed, which consist of a 2,5-bis(2-thienyl)thieno[3,2-b]thiophene diimide (BTI2)-alt-3,4-difluorothiophene polymer backbone, where the ratios of 2-decyltetradecyl (DT) and 2-octyldodecyl (OD) side chains on BTI2 have been varied such that x designates the percentage of BTI2 units with DT side chains (i.e. 0, 30, 50, 70, and 100). The side chain engineering aims to fine-tune the polymer chain packing and thus optimize the film morphology. When applied in all-PSCs through additive-free processing, the PBTI2(50DT)-FT-based cells yield a promising PCE of 8.32% with the well-known PTB7-Th as the polymer donor, substantially higher than those of the two parent copolymers (6.82% for PBTI2(0DT)-FT and 4.30% for PBTI2(100DT)-FT). This PCE is also one of the highest for additive-free all-PSCs. Moreover, the devices show a good stability with 90% of the initial PCE retained after 400 h. This work demonstrates the effectiveness of side chain engineering of PAs on optimizing the film morphology and improving the PCE.

Solvent additives play an important role in optimizing the morphology of the photoactive layer and improving the photovoltaic performance of polymer solar cells (PSCs). However, the toxicity of common additives to the environment limits their further application in photovoltaic cells. Herein, a non-toxic green food additive, benzyl salicylate (BS), was introduced as a novel solvent additive into inverted PSCs based on ITO/ZnO/PTB7-Th:PC71BM/MoO3/Ag. Subsequently, the effect of BS on the photovoltaic performance was studied, and the best power conversion efficiency (PCE) of 9.43% was achieved when adding 2% BS into the photoactive layer of PSCs. Furthermore, the PSCs treated by 1–4% BS all exhibited good photovoltaic performance compared to that using 3% DIO (8.56%), which demonstrated that the universality was very strong. In addition, a good solubility for fullerenes was presented on BS like DIO, which was determined by atomic force microscopy and transmission electron microscopy. The water contact angle and time-off flight secondary ion mass spectroscopy results manifested that the good solubility of high-boiling-point additive BS to PC71BM could boost the phase composition distribution at the surface and in the vertical direction. Thus, an efficient route for exciton dissociation and charge transfer could be formed when the PSC was processed via 2% BS, which could also be ascertained by space charge limited current, impedance spectroscopy. The present research findings provide useful information for realizing large-area PSC fabrication, where a greener non-toxic additive is always preferred.

A novel n-type organic small-molecule semiconductor, denoted N-IDTBR, was designed and synthesized by introducing an amine group into the extensively used non-fullerene acceptor O-IDTBR. This new material can be readily dissolved in alcohol, which allowed it to be spin-cast on top of a photoactive layer by processing with an orthogonal solvent for the construction of multilayer polymer solar cells. A poly(3-hexylthiophene):O-IDTBR blend was selected as a model system, which contains N-IDTBR as the cathode interlayer. We adopted non-halogenated anisole/2-methylanisole as the processing solvent to fabricate the active layer and achieve an impressive power conversion efficiency of 6.1%. Of particular interest is the fact that the N-IDTBR functioned not only as a single cathode interfacial layer to facilitate charge collection, but it also presented slight contribution to the light-harvesting capability of the device, thus leading to enhanced current density of the resulting devices. More importantly, the PCEs of the devices remained at 5.85 ± 0.25% as the thickness of the N-IDTBR layer varied from 5 to 30 nm, which demonstrated their great potential for future practical applications.

Open in a separate window The poor compatibility of an inorganic electron transport layer with the active layer and an ultrathin film organic material becomes a great obstacle in producing high-quality polymer solar cells with high-throughput roll-to-roll (R2R) method. Novel effective polymer solar cells had been fabricated by introducing 1, 7-disubstituted perylene diimide derivatives PDIH, PDIC, and PDIN as an electron transporting layer. It was noteworthy that PDIN could obviously improve the power conversion efficiency of solar cells that incorporated a photoactive layer composed of poly[(3-hexylthiophene)-2, 5-diyl] (P3HT) and the fullerene acceptor [6, 6-phenyl-C71-butyric acid methyl ester] (PC71BM). The power conversion efficiency varies from 1.5% for ZnO transparent cathode-based solar cells to 2.1% for PDIN-based electron transport layer-free solar cells. This improved performance could be attributed to the following reasons: the interaction between N atom in PDIN and O atom in indium tin oxide (ITO) reduced the work function of ITO, increased the built-in electric field, and thus lowered the electron transport barrier and improved the electron extraction ability of cathode, the appropriate roughness of the active layer increased the contact area with anode interfacial layer and enhanced the hole transport efficiency. These experimental results revealed that PDIN can be a promising novel effective material with a simplified synthesis process and lower cost as an electron transporting layer.

Advances in organic photovoltaic technologies have been geared toward industrial high-throughput printing manufacturing, which requires insensitivity of photovoltaic performance regarding to the light-harvesting layer thickness. However, the thickness of light-harvesting layer for all polymer solar cells (all-PSCs) is often limited to about 100 nm due to the dramatically decreased fill factor upon increasing film thickness, which hampers the light harvesting capability to increase the power conversion efficiency, and is unfavorable for fabricating large-area devices. Here we demonstrate that by tuning the bulk heterojunction morphology using a non-halogenated solvent, cyclopentyl methyl ether, in the presence of a green solvent additive of dibenzyl ether, the power conversion efficiency of all-PSCs with photoactive layer thicknesses of over 500 nm reached an impressively high value of 9%. The generic applicability of this green solvent additive to boost the power conversion efficiency of thick-film devices is also validated in various bulk heterojunction active layer systems, thus representing a promising approach for the fabrication of all-PSCs toward industrial production, as well as further commercialization.

The vertical composition distribution of a bulk heterojunction (BHJ) photoactive layer is known to have dramatic effects on photovoltaic performance in polymer solar cells. However, the vertical composition distribution evolution rules of BHJ films are still elusive. In this contribution, three BHJ film systems, composed of polymer donor PBDB-T, and three different classes of acceptor (fullerene acceptor PCBM, small-molecule acceptor ITIC, and polymer acceptor N2200) are systematically investigated using neutron reflectometry to examine how donor-acceptor interaction and solvent additive impact the vertical composition distribution. Our results show that those three BHJ films possess homogeneous vertical composition distributions across the bulk of the film, while very different composition accumulations near the top and bottom surface were observed, which could be attributed to different repulsion, miscibility, and phase separation between the donor and acceptor components as approved by the measurement of the donor-acceptor Flory-Huggins interaction parameter χ. Moreover, the solvent additive 1,8-diiodooctane (DIO) can induce more distinct vertical composition distribution especially in nonfullerene acceptor-based BHJ films. Thus, higher power conversion efficiencies were achieved in inverted solar cells because of facilitated charge transport in the active layer, improved carrier collection at electrodes, and suppressed charge recombination in BHJ solar cells.

In this article, we have evaluated the performance of inverted organic solar cells (OSCs) fabricated under ambient air and inert environment. Here, poly([4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) donor with [6,6]-phenyl C70-butyric acid methyl ester (PC71BM) acceptor were employed as a photoactive layer. The calculated power conversion efficiency (PCE) from current–voltage (J–V) characteristics for a device fabricated under an inert environment is 6.90% as compared to 3.96% for a device fabricated in ambient condition (25 °C, 100 kPa). The results revealed that the device processed in ambient conditions degrades the photovoltaic performance parameters, and the PCE decreased by 42.62% as compared to the devices fabricated in an inert environment. Along with this, we have also discussed in detail, the effect of the working environment on photoactive layers by Raman spectroscopy. The UV–Vis spectroscopy and Atomic Force Microscopy (AFM) techniques are presented to illustrate the optical properties and morphology of the photo-active layer respectively.

Aqueous-processed all-polymer solar cells (aq-APSCs) are reported for the first time by developing a series of water/ethanol-soluble naphthalenediimide (NDI)-based polymer acceptors [P(NDIDEG-T), P(NDITEG-T), and P(NDITEG-T2)]. Polymer acceptors are designed by using the backbones of NDI-bithiophene and NDI-thiophene in combination with nonionic hydrophilic oligoethylene glycol (OEG) side chains that facilitate processability in water/ethanol mixtures. All three polymers exhibit sufficient solubility (20-50 mg mL-1) in the aqueous medium. The P(NDIDEG-T) polymer with shorter OEG side chains is the most crystalline with the highest electron mobility, enabling the fabrication of efficient aq-APSCs with the maximum power conversion efficiency (PCE) of 2.15%. Furthermore, these aq-APSCs are fabricated under ambient atmosphere by taking advantage of the eco-friendly aqueous process and, importantly, the devices exhibit outstanding air-stability without any encapsulation, as evident by maintaining more than 90% of the initial PCE in the air after 4 days. According to a double cantilever beam test, the interfacial adhesion properties between the active layer and electron/hole transporting layers were remarkably improved by incorporating the hydrophilic OEG-attached photoactive layer, which hinders the delamination of the constituent layers and prevents the increase of series resistance, ultimately leading to enhanced durability under ambient conditions. The combination of increased device stability and minimal environmental impact of these aq-APSCs demonstrates them to be worthy candidates for continued development of scalable polymer solar cells.

A significant number of organic electronic devices rely on blends of electron-donating and electron-accepting molecules. In bulk heterojunction organic photovoltaics, the nanoscopic phase behavior of the two individual components within the photoactive layer has a major impact on the charge separation and charge transport properties. For polymer:fullerene solar cells, it has been hypothesized that an increased accessibility of the electron-deficient monomer unit in push-pull type low bandgap polymers allows for fullerene ‘docking’. The close proximity of electron donor and acceptor molecules enables more efficient charge transfer, which is beneficial for the device efficiency. With this in mind, we synthesized a series of PBDTTPD [poly(benzodithiophene-thienopyrroledione)] low bandgap copolymers with varying side chains. Solar cells were fabricated for all polymers and the device characteristics were compared. The combination of proton wideline solid-state NMR (ssNMR) relaxometry and sensitive external quantum efficiency (sEQE) measurements was shown to provide essential information on donor-acceptor interactions and phase separation in bulk heterojunction organic photovoltaics. The reduced charge transfer state absorption and the observed phase separation of crystalline PC71BM domains for the polymers containing the most accessible methyl-TPD unit indicate a diminished contact between donor and acceptor, leading to a loss in performance.

Controlling the self-assembling of organic semiconductors to form well-developed nanoscale phase separation in the bulk-heterojunction active layer is critical yet challenging for building high-performance organic solar cells (OSCs). Particularly, the similar anisotropic conjugated structures between nonfullerene acceptors and p-type organic semiconductor donors raise more complexity on manipulating their aggregation toward appropriate phase separation. Herein, a new approach to tune the morphology of photoactive layer is developed by utilizing the synergistic effect of dithieno[3,2-b:2',3'-d]thiophene (DTT) and 1-chloronaphthalene (CN). The volatilizable solid additive DTT with high crystallinity can restrict the over self-assembling of nonfullerene acceptors during the film casting process, and then allowing the refining of phase separation and molecular packing with the simultaneous volatilization of DTT under thermal annealing. Consequently, the PTQ10:m-BTP-PhC6:PC71 BM-based ternary OSCs processed by the dual additives of CN and DTT record a notable power-conversion efficiency of 18.89%, with a remarkable FF of 80.6%.

Polymer solar cells (PSCs) generally suffer energy losses due to low efficient exciton separation and carrier transfer within the active layer. In this paper, adding SnO2 nanoparticles (SNs) directly into the photoactive layer is proposed for reducing electron-hole recombination and thus facilitating electron transfer across the active layer. It is found that SN addition with appropriate amount can evidently enhance device performance with the power conversion efficiency from 2.67% to 3.39% mainly owing to the improved photocurrent density and fill factor. A systematic investigation confirms that the enhanced exciton separation and carrier transfer in the photoactive layer indeed plays the key role for performance improvement of the devices with appropriate SN addition. This work thus provides a simple strategy of reconciling the contradiction of effective carrier collection and efficient light absorption and thus improving the performance of PSCs.

Interlayers and interface engineering plays a pivotal role in achieving efficient charge extraction necessary for realizing high power conversion efficiency (PCE) in bulk heterojunction organic solar cells. In this study, we report a novel strategy of combining advantages of both fullerene derivative and non-fullerene acceptor materials by employing solution processable non-fullerene acceptor 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indan-one))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) to modify the interface between ZnO-based electron transport layer and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo [1,2-b;4,5-b'] dithiophe-ne-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carb-oxylate-2-6-diyl)] (PTB7-Th): phenyl-C71-butyric acid methyl ester-based photoactive layer. An improvement in PCE from ∼ 7.5% to ∼ 9% has been achieved for the devices with ITIC modified interlayer. Time resolved photoluminescence (PL) via time correlated single photon counting and photo-electrochemical impedance spectroscopy (photo-EIS) measurements were carried out to investigate the causes for improved PCE for the devices with ITIC modified interlayer.

Perovskite solar cells were developed very fast in the past decade, but hybrid perovskite materials with unbalanced charge carrier diffusion lengths were not fully addressed by either conventional or planar heterojunction device structures. In this study, high-performance perovskite solar cells with bulk heterojunction device structures where CH3NH3PbI2.55Br0.45 is blended with an n-type high-electron-mobility Zn2SnO4 nanoparticle as the photoactive layer are reported. Systematic studies indicate that the CH3NH3PbI2.55Br0.45:Zn2SnO4 bulk heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap density. Photoluminescence and time-resolved photoluminescence studies further demonstrate that there is an efficient photoinduced charge carrier transfer between CH3NH3PbI2.55Br0.45 and Zn2SnO4 nanoparticles. Thus, bulk heterojunction perovskite solar cells by the CH3NH3PbI2.55Br0.45:Zn2SnO4 thin film exhibit over 21.07% power conversion efficiency, which is more than 12% enhancement as compared to that (18.74%) observed from planar heterojunction perovskite solar cells by the pristine CH3NH3PbI2.55Br0.45 thin film. Moreover, bulk heterojunction perovskite solar cells possess significantly suppressed photocurrent hysteresis, dramatically enhanced device stability, and reproducibility. All these results demonstrate that high-performance perovskite solar cells can be realized through bulk heterojunction device structures.

The third-generation organic solar cells have the benefits of low-cost, solution processability, low energy payback time, environment friendliness and appropriateness for mass production via large area printing techniques. The electron transport layer improves the ohmic contact between the cathode and the photoactive layer. In this research article, we demonstrate a drop on demand inkjet printing of the zinc oxide electron transport layer and the photoactive layer blend consisting of a donor poly(3-hexylthiophene-2,5-diyl) (P3HT) and acceptor Indene C-60 bisadduct (ICBA) of an organic solar cell. Consequently, all inkjet printed solar cell achieved a power conversion efficiency of 4.7% under ambient conditions. We highlighted that drop spacing, substrate temperature and the waveform parameters such as voltage and time are the important parameters for printing. Moreover, we observed a difference in morphology between spin coated and inkjet printed devices as characterized via UV–Visible absorbance, atomic force microscopy and scanning electron microscopy. Furthermore, we applied device characterization from impedance spectroscopy to elucidate the difference in device performance. Particularly, the effective lifetime and global mobilities calculated from the impedance spectroscopy Nyquist plots at 1 Voltage bias in the dark for all inkjet-printed devices are 0.0374 μs and 57.5 × 10−3 cm2/V–S respectively, and for all, spin-coated devices are 0.0628 μs and 115.3 × 10−3 cm2/V–S respectively.

Recent developments in acceptor–donor–acceptor (A– D –A) type non-fullerene acceptors have led to substantial improvements in bulk-heterojunction polymer solar cells efficiency. The device performance strongly depends on photoactive layer morphology, as the molecular packing, donor–acceptor interface and phase separation significantly affect the charge-transfer states and charge carrier dynamics. In this review, we start with a brief introduction of the techniques most effectively utilized to characterize multiphase morphology. Then, we summarize recent progress in A– D –A type acceptors, with the emphasis on understanding the molecular structure–morphology–performance relationships. Finally, an outlook on correlating morphological characteristics with photovoltage losses is presented for further improving device performance.

Photovoltaic (PV) performance of bulk heterojunction polymer solar cells (BHJ PSCs) highly depends on the optoelectronic properties of the photoactive layer. In this regard, additives (solvent or solid) are considered as one of the versatile methods and key morphology directing agents for solution processed PV devices. Herein this contribution, we rationally utilize a new kind of pyridine-based solid additives (2-hydroxypyridine (2-DHP) and 2,4-dihydroxypyridine (2,4-DHP)) to manipulate and engineer the nanomorphology of the poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) photoactive system, that can influence the optoelectronic properties for enhanced performance in BHJ PSCs. The optical, electrical, morphological, surface, structural characteristics of the fabricated BHJ PSCs were systematically examined. The atomic force microscopy and X-ray photoelectron spectra measurements confirmed the structural reorganization, phase separation of polymer and fullerene domains resulting in the formation of well-ordered blend in the additive based devices. BHJ PSCs fabricated with P3HT:PC61BM:2-DHP and P3HT:PC61BM:2,4-DHP yielded a best power conversion efficiency (PCE) of 4.35% and 3.58%, respectively, which in contrast outperformed than the PCE obtained from P3HT:PC61BM device (3.01%). The remarkably improved performance for P3HT:PC61BM:2-DHP can be attributed to the enhanced short-circuit current density induced by the morphology optimization. Additionally, the devices fabricated with the solid additives (2-DHP and 2,4-DHP) exhibited improved air-stability due to the oxygen insensitivity of the functional groups present in the chosen additives. The pyridine and hydroxyl groups in 2-DHP and 2,4-DHP possibly form the intermolecular interactions with the photoactive components (P3HT/PC61BM), contribute to the morphology optimization and subsequent PV performance enhancement in BHJ PSCs. It is believed that our findings can inspire and trigger further advanced research toward the design and development of stable and highly efficient BHJ PSCs by rationally choosing low-cost solid additives.

Although metal oxides such as ZnO are frequently used in inverted polymer solar cells (PSCs) to serve as an electron-transporting layer, how to effectively improve its interface with the photoactive layer is still an open question. In this study, we exploited small-molecule (SM) compounds usually used as electron acceptors in PSCs to act as an interlayer on ZnO and constructed an SM/ZnO double electron-transporting layer to improve the device's photovoltaic performance. To avoid the solvent erosion to SM interlayer during the deposition of upper-layer film, spontaneously-spreading-film technique was employed to prepare the photoactive layer. We used two SM acceptors (ITIC and IDT-BOC6) with different energy levels to serve as the interlayer. Results show that devices based on ITIC interlayer, which has similar LUMO level with materials in its adjacent layers, possess a better electron-transporting ability than IDT-BOC6 based ones which lack of such an energy-level structure. Moreover, SM interlayer is beneficial to flatten the interface and improves the contact between the electron-transporting layer and the photoactive layer. Consequently, we demonstrate a new way to improve the collection of electrons in PSCs by using commonly available electron acceptors, which provides a large choosability in interfacial materials.

Interfacial layers are frequently used in organic solar cells performing various functions, including blocking surface recombination, improving selectivity of charge carrier extraction, modification of the work function of the contact materials, and enhancing light absorption within the photoactive layer through an optical cavity effect. The aim of this work is to investigate the origin of performance enhancement of bulk heterojunction solar cells using various electron and hole interfacial layers, with a particular focus on separating the contributions of work function modification and reduced recombination to the improvement of the open circuit voltage ( Voc). Solar cells using poly[ N-9'-hepta-decanyl-2,7-carbazole- alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]:[6,6]-phenyl C70-butyric acid methyl ester (1:4) active layers were prepared with a combination of polymeric, metal oxide, and polyelectrolyte electron and/or hole interfacial layers. Four device structures with (i) no interfacial layers (reference); (ii) only hole; (iii) only electron; (iv) both electron and hole interfacial layers were fabricated and compared using current-voltage, transient photovoltage, and charge extraction measurements. The voltage gains (Δ Voc) at matched charge density attributed to work function modification (Δ Voch or Δ Voce) are distinguished from the increase in Voc arising from increased charge carrier density. At the hole contact, Δ Voch was 0.21 V by using a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole interfacial layer, whereas Δ Voce was 0.29 V on the electron contact using a polyethoxylate imine-TiO x interfacial layer compared to reference devices. The electron lifetime also improved by orders of magnitude with the use of either electron or hole contact layers, contributing to a further 0.35-0.38 V increase in the open circuit voltage (Δ Vocrec) because of increased charge density. The increased charge carrier lifetime is proposed to originate from the larger spatial separation of the electrons and holes in the device because of the increased internal field. Using both an electron and a hole interfacial layer did not significantly increase the charge carrier lifetime compared to single interfacial layer devices; therefore, the Voc did not increase significantly. The findings presented clarify the role of interfacial layers in organic solar cells and provide new insights into using time-resolved charge extraction techniques to understand the influence of interfacial layers on the open circuit voltage.

In this work, we demonstrated the synthesis of amino functionalized MWNTs (af-MWNTs) via a simple and facile hydrothermal process and the af-MWNTs was acted as the hole transport layers (HTLs) of polymer solar cells (PSCs) based on active layers of Poly[N‑9″‑hepta‑decanyl‑2,7‑carbazolealt‑5,5‑(4′,7′‑di‑2‑thienyl‑2′,1′,3′‑benzothiadiazole)] (PCDTBT) and fullerene derivative [6,6]‑phenyl‑C71‑butyric acid methylester (PC71BM). Comparing with the control devices, devices based on af-MWNTs HLTs achieved an optimal power conversion efficiency (PCE) of 6.970%, leading to 17.6% enhancement comparing to the control device. The af-MWNTs HTLs exhibited remarkable enhancement on electrical conductivity, which was beneficial to the hole extraction and transport on interface. The enhancement of PCE was mainly attributed to the superior hole transport and extraction capabilities. Meanwhile, the af-MWNTs exhibited a suitable work function (WF) which was beneficial to hole extraction from the photoactive layer to the ITO anode. The experimental results indicated that the af-MWNTs HTLs not only presented excellent optical and electrical properties, but also played an important role for improving the efficiency of PSCs.

Performance and stability are the most important issues to be considered to propel and advance the polymer solar cell (PSC) technology to be realized in commercial market. Herein, we propose a new concept of stabilizing polymer-fullerene morphology for PSCs and simultaneously improve device performance by introducing thermally cross-linkable polymer as an additive in the photoactive layer. The cross-linker, poly[9,9-bis(6-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis(3-ethyl(oxetane-3-ethyloxy)hexyl)-fluorene] (PFN-ox), forms a three-dimensional network embedded in the polymer blend after thermal treatment and plays various roles including reducing the carrier recombination at the donor/acceptor interface, improving the electrical property of PSCs, enhancing the hydrophobicity of the photoactive layer, slightly decreasing fullerene domain size, and retarding the crystal growth of fullerenes. Those characteristics lead to the improvement of device performance from the moderate improved fill factor and stable device operation under ambient condition (>85% efficiency retention after 2 months). The stability effects of PFN-ox are not only valid in polymer-fullerene blends, but also in polymer-polymer systems, showing its general characteristics. We believe that our finding can advance the development of stable PSCs and other optoelectronic devices based on organic materials.

Ternary organic solar cells (TSCs) contain a single three-component photoactive layer with a wide absorption window, which is obtained without the need for multiple stacks. Subsequently, TSCs have attracted great interest in the photovoltaics field. Through careful selection of the three (or more) active components that form the photoactive layer, all photovoltaic parameters can be simultaneously enhanced within a TSC — a strategy that has resulted in record efficiencies for single-junction solar cells. In this Review, we outline key developments in TSCs, with a focus on the central role of the third component in achieving record efficiencies. We analyse the effects of the third component on the nanomorphology of the bulk heterojunction and the photovoltaic parameters of TSCs. Moreover, we discuss the charge-transfer and/or energy-transfer mechanisms and nanomorphology models that govern the operation of TSCs. We consider both polymer and small-molecule donors as well as fullerenes and recently developed non-fullerene acceptors. In addition, we summarize the recent success of TSCs in mitigating the stability issues of binary solar cells. Finally, we provide a perspective on the advantages of ternary blends and suggest design strategies for highly efficient and stable devices for commercial photovoltaics. Adding a third component into a binary blend is a promising strategy for simultaneously improving all photovoltaic parameters in organic solar cells. In this Review, we discuss the role of the third component in influencing the energetics, charge-carrier recombination and stability in ternary solar cells.

Silver:zinc nanocomposite (Ag:Zn BMNC) was successfully synthesized by chemical reduction method and employed in various layers of the device structure to fabricate efficient thin film organic solar cell. The photoactive layer of the solar cell is composed of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric-acidmethyl-ester (P3HT:PCBM). The nanocomposites, loaded at various layers of the device structure, appeared to have different influences on the performance of the solar cells. The optical and electrical properties of the nano-particles are found to be favourable to harvest solar radiation because of the local plasmon resonance effect (LPSR) of the metal nano-particles. The concentration of Ag:Zn nano-particle has been successfully optimized to achieve a high device performance which led to a power conversion efficiency as high as 5% under ambient laboratory condition. The x-ray diffraction (XRD), Scanning and Tunnelling electron microscopies (SEM and TEM) were employed to investigate the nature of the nano-composite.

An efficient polymer solar cell (PSC) has been demonstrated by incorporating an ultrathin interfacial insulating organic layer of polytetrafluoroethylene (PTFE) between a photoactive layer and hole-collecting buffer layer (NiOx). The photoactive layer is made with bulk heterojunction composites of poly[N-9’’-hepta-decanyl-2,7-carbazolealt-5,5-(4′,7′-di-2-thienyl- 2′,1′,3′-ben-zothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC71BM). The PTFE layer not only improves the energy level alignment by forming an interfacial dipole at the interface but also reduces the inherent incompatibility between the hydrophobic active layer and hydrophilic NiOx layer, thereby benefiting to the charge extraction and transport in the solar cells. As a result, with the NiOx/PTFE stacked structure, all the photovoltaic performance parameters are significantly improved, leading to a higher power conversion efficiency (PCE) of up to 7.11% compared to the control device without PTFE layer (PCE 5.50%). The PTFE layer provides a superior alternative to interfacial engineering of the metal oxide/organic semiconductor interface in polymer solar cells and other organic electronic devices.