Bulk Heterojunction Photoactive Layer Research Articles

Split-Second Nanostructure Control of a Polymer:Fullerene Photoactive Layer using Intensely Pulsed White Light for Highly Efficient Production of Polymer Solar Cells

Intensely pulsed white light (IPWL) treatment was tested as an ultrafast, large-area processable optical technique for the control of the nanostructure of a polymeric bulk-heterojunction photoactive layer to improve the efficiencies of polymer solar cells. Only 2 s of IPWL irradiation of a polymer:fullerene photoactive layer under ambient conditions was found to enhance significantly the power conversion efficiencies of the tested polymer solar cells to values approaching that of typical devices treated with thermal annealing. Consecutive white-light pulses from the xenon lamp induce the self-organization of the polymeric donor into an ordered structure and result in the optimized phase segregation of the polymeric donor and the fullerene acceptor in the photoactive layer, which enhances the light absorption and hole mobility and results in efficient photocurrent generation. The effects of varying the pulse conditions on device performance, including the irradiation fluence, pulse duration time, and number of pulses, were systematically investigated. Finally, it was successfully demonstrated that the IPWL treatment produces flexible polymer solar cells. The proposed IPWL process is suitable for the efficient industrial roll-to-roll production of polymer solar cells.

In situ UV-visible absorption during spin-coating of organic semiconductors: a new probe for organic electronics and photovoltaics

Spin-coating is the most commonly used technique for the lab-scale production of solution processed organic electronic, optoelectronic and photovoltaic devices. Spin-coating produces the most efficient solution-processed organic solar cells and has been the preferred approach for rapid screening and optimization of new organic semiconductors and formulations for electronic and optoelectronic applications, both in academia and in industrial research facilities. In this article we demonstrate, for the first time, a spin-coating experiment monitored in situ by time resolved UV-visible absorption, the most commonly used, simplest, most direct and robust optical diagnostic tool used in organic electronics. In the first part, we successfully monitor the solution-to-solid phase transformation and thin film formation of poly(3-hexylthiophene) (P3HT), the de facto reference conjugated polymer in organic electronics and photovoltaics. We do so in two scenarios which differ by the degree of polymer aggregation in solution, prior to spin-coating. We find that a higher degree of aggregation in the starting solution results in small but measurable differences in the solid state, which translate into significant improvements in the charge carrier mobility of organic field-effect transistors (OFET). In the second part, we monitor the formation of a bulk heterojunction photoactive layer based on a P3HT-fullerene blend. We find that the spin-coating conditions that lead to slower kinetics of thin film formation favour a higher degree of polymer aggregation in the solid state and increased conjugation length along the polymer backbone. Using this insight, we devise an experiment in which the spin-coating process is interrupted prematurely, i.e., after liquid ejection is completed and before the film has started to form, so as to dramatically slow the thin film formation kinetics, while maintaining the same thickness and uniformity. These changes yield substantial improvements to the power conversion efficiency of solar cells without requiring additional thermal annealing, or the use of solvent additives. Through these simple examples, we demonstrate that gaining insight into the thin film formation process can inspire the development of new processing strategies. The insight into the inner workings of spin-coating may be increasingly important to improving the performance or efficiency of roll-to-roll manufactured devices.

Solvent–vapor induced morphology reconstruction for efficient PCDTBT based polymer solar cells

This paper reports polymer solar cells with a 7% power conversion efficiency (PCE) based on bulk heterojunction (BHJ) composites of the alternating co-polymer, poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), and the fullerene derivative [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). As confirmed by transmission electron microscopy, solvent–vapor annealing (SVA) of the thin (70nm) BHJ photoactive layer by exposure to chloroform vapor, for a short period of time (30s) after deposition, leads to reconstructed nanoscale morphology of donor/acceptor domains, well-dispersed fullerene phase and effective photo-absorption of BHJ. Consequently, SVA-reconstructed devices with a PCDTBT:PC71BM blend ratio of 1:5 (wt%) exhibit ∼50% improvement in PCE, with short-circuit current Jsc=15.65mA/cm2, open-circuit voltage Voc=0.87V, and PCE=7.03%, in comparison to those of the 1:4 (wt%) blends with SVA treatment.

Application of sputter-deposited amorphous and anatase TiO2 as electron-collecting layers in inverted organic photovoltaics

We demonstrate the deposition of amorphous and anatase TiO2 on indium tin oxide (ITO) substrates via the process of sputtering, and the use of these materials as electron-collecting layers (ECLs) in inverted-type organic photovoltaics (OPVs). Anatase TiO2 was obtained via vacuum-annealing of as-deposited amorphous TiO2 at 300°C. No deterioration of optical and electrical properties of ITO was observed after both sputter-deposition of TiO2 and annealing process. The anatase TiO2 proved to be an effective ECL when employed in inverted OPVs using bulk heterojunction photoactive layer of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, achieving a power conversion efficiency of 3.3% (JSC=9.0 mAcm−2, VOC=0.62V and FF=0.60).

Ternary Donor-Insulator-Acceptor Systems for Polymer Solar Cells

A series of block copolymers with fixed length of the semiconductor-block poly(3-butylthiophene) (P3BT) and varying length of the insulator-block polystyrene (PS) are synthesized. These copolymers are blended with phenyl-C61-butyric acid methyl ester (PCBM) for the bulk heterojunction photoactive layers. With appropriate insulator-block length and donor-acceptor ratio, the power conversion efficiency increases by one order of magnitude compared with reference devices with pure P3BT/PCBM. PS blocks improve the miscibility of the active layer blends remarkably. The P3BT-b-PS crystallizes as nanorods with the P3BT core covered with the PS-block, which creates a nanoscale tunneling barrier between donor and acceptor leading to more efficient transportation of charge carriers in the semiconductors.

Transparent conductive electrodes of mixed TiO2−x–indium tin oxide for organic photovoltaics

A transparent conductive electrode of mixed titanium dioxide (TiO2−x)–indium tin oxide (ITO) with an overall reduction in the use of indium metal is demonstrated. When used in organic photovoltaic devices based on bulk heterojunction photoactive layer of poly (3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester, a power conversion efficiency of 3.67% was obtained, a value comparable to devices having sputtered ITO electrode. Surface roughness and optical efficiency are improved when using the mixed TiO2−x–ITO electrode. The consumption of less indium allows for lower fabrication cost of such mixed thin film electrode.

An efficient inverted organic solar cell with improved ZnO and gold contact layers

This paper presents a high efficiency (∼3.8%) inverted organic photovoltaic devices based on a P3HT:PCBM bulk heterojunction (BHJ) blend with improved electron- and hole-selective contact layers. Zinc oxide (ZnO) nanoparticle films with different thicknesses are deposited on the transparent electrodes as a nano-porous electron-selective contact layer. A thin gold film is used between the BHJ photoactive layer and the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), which improves the wettability and significantly enhances the stability of the device (>50days of air exposure). Photovoltaic device parameters such as power conversion efficiency (PCE) and external quantum efficiency (EQE) are systematically examined for inverted devices with different thicknesses of ZnO and gold layers in comparison to the non-inverted and reference inverted devices with no contact layers. The optimized organic devices with ZnO and Au contact layers show exceptional short circuit currents (in excess of 13mA/cm2), in comparison to the reference devices, which is related to increased quantum efficiency of the device observed in measured EQE experiments. These results are important for development of high efficiency and stable all-printed organic solar cells and point out the role of contact layers, in particular, ZnO conductivity and morphology in the device performance.

Calcium niobate nanosheets as a novel electron transport material for solution-processed multi-junction polymer solar cells

Solution-processed tandem polymer solar cells are demonstrated using stacked perovskite, (TBA,H)Ca2Nb3O10 (CNO), semiconductor nanosheets as an electron transport layer (ETL) within the recombination layers. Two poly(3-hexylthiophene):(6,6)-phenyl-C61-butyric acid methyl ester, P3HT:PCBM, sub-cells connected in series via a CNO–poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), Pedot:PSS, recombination layer provide a Voc of 1.02 V. The Voc is less than double that of a single junction due to leakage (dark) current. When TiOx is used as an additional hole-blocking layer within the recombination stack, the Voc is improved to 1.16 V. Although further optimization of the CNO-layer is still required, the use of CNO nanosheets within the recombination stack shows clear advantages. In addition to electron extraction from photoactive layers, the presence of defect states in the CNO nanosheets with trapped electrons facilitates the recombination of holes from Pedot:PSS, enhancing the function of the recombination stack. The robust CNO-layer can be spin-coated on top of a P3HT:PCBM bulk-heterojunction photoactive layer and is stable toward subsequent processing and heat-treatment.

Annealing Sequence Dependent Open-Circuit Voltage of Inverted Polymer Solar Cells Attributable to Interfacial Chemical Reaction between Top Electrodes and Photoactive Layers

The ability to laminate and delaminate top metal contacts during the processing and testing of inverted polymer solar cells has led us to uncover the peculiar dependence of their open-circuit voltage (V(oc)) on the annealing sequence. Specifically, thermally annealing inverted polymer solar cells having bulk-heterojunction photoactive layers after top electrode deposition above 100 °C leads to lower V(oc) compared to analogous devices with unannealed photoactive layers or photoactive layers that have been annealed prior to metal electrode deposition. This reduction in V(oc), however, can be reversed when the top electrodes are replaced. This observation is thus a strong indication that such changes in V(oc) with annealing sequence are manifestations of changes at the top electrode-photoactive layer interface, and not structural changes in the bulk of the photoactive layer. Electronic characterization conducted on the photoactive layers and metal contacts after dissection of the polymer solar cells via delamination suggests the reduction of V(oc) on thermal annealing in the presence of the metal top contacts to stem from an interfacial chemical reaction between the photoactive layers and the metal electrodes. This chemically generated interfacial layer is removed upon electrode delamination, effectively reverting the V(oc) to its original value prior to thermal annealing when the top electrodes are replaced.